WO1982000191A1 - Systemes solaires de transfert de chaleur a deux phases - Google Patents

Systemes solaires de transfert de chaleur a deux phases Download PDF

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Publication number
WO1982000191A1
WO1982000191A1 PCT/US1980/000896 US8000896W WO8200191A1 WO 1982000191 A1 WO1982000191 A1 WO 1982000191A1 US 8000896 W US8000896 W US 8000896W WO 8200191 A1 WO8200191 A1 WO 8200191A1
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Prior art keywords
refrigerant
heat
liquid
pump
transfer system
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PCT/US1980/000896
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English (en)
Inventor
S Molivadas
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S Molivadas
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Publication date
Application filed by S Molivadas filed Critical S Molivadas
Priority to EP81901386A priority Critical patent/EP0055712A1/fr
Priority to AU72238/81A priority patent/AU7223881A/en
Priority to PCT/US1980/000896 priority patent/WO1982000191A1/fr
Publication of WO1982000191A1 publication Critical patent/WO1982000191A1/fr

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Classifications

    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F24—HEATING; RANGES; VENTILATING
    • F24S—SOLAR HEAT COLLECTORS; SOLAR HEAT SYSTEMS
    • F24S90/00—Solar heat systems not otherwise provided for
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E10/00—Energy generation through renewable energy sources
    • Y02E10/40—Solar thermal energy, e.g. solar towers

Definitions

  • the invention disclosed in this patent application relates to improved two-phase heat-transfer systems for absorbing heat from solar radiant energy, and for transferring and releasing this heat to a substance to be heated.
  • two-phase heat-transfer system is used here to denote a system that employs a circulating fluid to absorb heat from a heat source, at least in part by evaporation, at an essentially constant temperature, and to transfer this heat to a substance to be heated by releasing it, at least in part by condensation, at an essentially constant lower temperature.
  • the heat thus absorbed or released is called "latent heat”.
  • Two-phase heat-transfer systems can, with available fluids, solve many of the known problems of solar heating systems employing a single- phase heat-transfer system; namely, a system that employs a circulating fluid which absorbs heat from a heat source, and transfers this heat and releases it to a substance to be heated, without changing phase throughout a heat-transfer cycle.
  • a single- phase heat-transfer system namely, a system that employs a circulating fluid which absorbs heat from a heat source, and transfers this heat and releases it to a substance to be heated, without changing phase throughout a heat-transfer cycle.
  • many expired and unexpired patents have already been granted for such two-phase heat-transfer systems which use solar radiant energy as the heat source.
  • these prior-art systems fail to exploit the full potential of latent-heat transfer, in part, because they do not possess the crucial property of "self-regulation".
  • the purpose of the invention is to improve prior-art two-phase heat-transfer systems for solar heating systems by devising techniques for achieving the properties mentioned in PART II of this DESCRIPTION.
  • a "refrigerant” is defined as the heat-transfer fluid of a two-phase heat-transfer.
  • the term "refrigerant” is used to denote the function of a fluid, and not its nature; and specifically to denote the function of absorbing heat from a heat source, at least in part by evaporation, and of releasing heat to a heat sink, at least in part by condensation.
  • heat sink in the context of this definition of a refrigerant includes a sink of finite or infinite thermal capacity and, in particular, the substance to be heated, or being heated, by a two-phase heat—transfer system
  • aborber includes any means employed to absorb heat from solar radiant energy and to transfer this heat by conduction or convection, or both, to the refrigerant flowing through one or more refrigerant passages which are an integral part of the absorber, or which are in thermal contact with it; it also includes any means employed to form one or more transparent passages in which the refrigerant itself absorbs heat "directly" from solar radiant energy, where the.
  • the term "directly” is not intended to exclude reflected or refracted solar radiant energy by the concentrator (optical system) of a solar focusing collector.
  • the term “absorber inlet” refers to the inlet of the first absorber which the refrigerant of a heat-transfer system enters
  • the term “absorber out-let” refers to the outlet of the last absorber from which this refrigerant exits.
  • the term “condenser” includes any means in which a refrigerant releases heat to the substance to be heated at least in part by condensation. This substance may be a solid, or a fluid, which absorbs sensible heat, latent heat, or both.
  • self-regulation is used here to denote the capability of a two-phase, heat-transfer system to exploit the full potential of the latent-heat properties of the: refrigerant by
  • prior-art systems are only capable of exploiting the latent-heat properties of their refrigerant over a fixed narrow range of external conditions determined by the system's design.
  • the term self-regulation is used to denote the capability of a two-phase, heat-transfer system to ensure automatically that at least the four internal operating conditions cited below are satisfied for all external conditions under which this system is designed to operate: first, the refrigerant entering the collector absorber exists exclu sively in its liquid phase and has a mass flow rate large enough to maintain the amount of superheat of the evaporated portion of the refrigerant exiting the collector absorber below any preselected, positive, upper limit; second, the refrigerant vapor entering the condenser is maintained in a dry state; third, the amount of liquid refrigerant which backs up in the condenser is small enough to maintain the condenser's effective condensing surface above a preselected, positive, upper limit; and fourth, the absolute value of the difference between the saturated vapor temperature exiting the collector absorber and the saturated vapor-temperature of the refrigerant entering the condenser is maintained below a preselected, upper limit.
  • the net rate at which heat is absorbed by the refrigerant in a solar collector absorber, and hence also the evaporation rate of this refrigerant depends not only on the intensity of solar radiation; but also on the evaporation temperature and the equivalent temperature of the collector's surrounding, which can both vary over a wide range of values.
  • the heat losses of the boiler, namely of the solar-collector absorber, to its surroundings are many times greater than those of the boiler of conventional heat-transfer systems -to.its surroundings; and furthermore these heat losses increase much more rapidly in the former systems than in the latter systems for a given increment of the difference in temperature between the boiler and its surroundings.
  • the solar-collector absorber of a heat-transfer system cannot be thermally insulated from its surroundings to the same degree as the boiler of a conventional heat-transfer system without, at the same time, suffering an unacceptable decrease in the amount of solar radiant energy intercepted by the absorber surface in direct thermal contact with the refrigerant fluid.
  • the solar-collector absorber of solar heat-transfer systems is exposed directly to the "equivalent temperature" of the local outdoor surroundings; whereas the boiler of conventional heat-transfer systems is usually located in a building and is therefore at most exposed only indirectly to this equivalent temperature, namely through a building's structure.
  • the solar-collector absorber's surroundings can be much colder than those of the boiler, and consequen tly, even if this absorber were thermally insulated from its surroundings to the same degree as the boiler, its heat losses to these surroundings in cold weather would be much greater than those of the boiler.
  • the substance being heated is, at any given time, at a nearly uniform temperature.
  • a prominent example of such an application is the case where the heat-transfer system is used to heat the water in the storage tank of a domestic or service-water heating system.
  • the third internal operating condition for self-regulation becomes "the amount of liquid refrigerant actually present in the condenser, in excess of the amount which would be present if the condensate at the condenser's refrigerant exit were not subcooled, in essence, is zero".
  • this modified third internal operating condition includes the case where the condensate at the condenser's exit is subcooled by about one or two degrees Celsius. (This amount of subcooling at the condenser refrigerant exit may often be desirable or even required to avoid condensate pump cavitation.)
  • self-regulation may also be defined as the capability of a two-phase heat-transfer system to ensure automatically, for all external conditions under which the system is designed to operate, that in essence
  • the saturated temperature of the refrigerant at the inlet of the condenser is equal to the saturated vapor temperature of the refrig erant at the exit of the absorber.
  • a significant amount of subcooling, at the condenser's refrigerant outlet may be desirable.
  • a significant amount of subcooling, defined as over say 5°C, can as explained later be provided, while also achieving self-regulation, with the same techniques used to achieve self-regulation when no significant amount of subcooling is required (or desired).
  • the substance being heated may be a fluid which is, at any given time, being both preheated over a wide range of temperatures in its liquid phase and vaporized at a tempera ture near the upper end of this range.
  • a heat-transfer system is used to preheat and vaporize the working fluid of a heat engine.
  • efficient heat transfer from the refrigerant to the working fluid requires the refrigerant to be subcooled by a large amount, namely by an amount where the heat released by the refrigerant to the working fluid during subcooling may be as great as one third (or even more) of the heat released by the refrigerant to the working fluid during condensation.
  • a physically distinct means referred to here as a subcooler, in which the refrigerant is subcooled and the working fluid is preheated, from the means, referred to here as a condenser, in which the refrigerant is con densed and the working fluid is vaporized.
  • a large amount of subcooling, whether or not a separate subcooler is used, can also be provided, while also achieving self-regulation, with the same techniques used to achieve self-regulation when no significant amount of subcooling is required or desired.
  • a separate subcooling control is used, as explained later, to control automatically the amount of refrigerant subcooling.
  • the pressure of the vapor of the fluid under steady-state conditions will be equal to the saturated pressure corresponding to that temperature.
  • a collector absorber panel designed to withstand pressures corresponding to collector stagnation temperatures would be unnecessarily heavy and expensive.
  • the saturated pressure of R-114 is 439 psig at a stagnation temperature of only 290°F; whereas the maximum operating pressure of R-114 to heat water up to 160°F and 175°F is only about 100 psig and 125 psig respectively.
  • over-pressure protection denotes that the refrigerant is prevented from exceeding significantly its maximum design operating pressure under any stagnation condition. In most FRC species installations this protection is achieved by the very nature of the refrigerant circuit configuration itself. Namely, the circuit configuration allows the entire volume of liquid refrigerant to be stored below the solar-collector absorber whenever the heat-transfer system is inactive, and permits all liquid refrigerant to be removed from the collector absorber upon system de-activation solely under the influence of gravity and by the process of vapor condensation in, and migration to, the part of the refrigerant circuits below the absorber..
  • overpressure protection In NRC species installations and in some FRC species installations, liquid refrigerant is stored entirely, or in part, above the lowest level of the absorber. In this case a valve is used to prevent liquid refrigerant entering the absorber whenever the refrigerant pressure rises above a preselected upper limit. The valve is operated in a "fail-safe manner" unaffected by electrical mains or water mains failure, or by a malfunction of the heat-transfer system's electrical or pneumatic controls. I refer to this type of over-pressure protection as "fail-safe over— pressure protection".
  • the maximum stagnation temperature which can be attained by the absorber of a collector may be too high either for the refrigerant or for the absorber itself.
  • over-temperature, as well as over-pressure, protection is provided by rejecting unwanted heat to an external heat sink which will usually be water or the outdoor ambient air.
  • the most suitable refrigerant may freeze at some of the equivalent temperatures of the collector's outdoor surroundings; and furthermore this refrigerant may be of the type which expands as it changes from its liquid to its solid phase.
  • the most common example of such a refrigerant is water, which is probably the preferred refrigerant for most heating applications in the range between 125°C and 200°C.
  • the present invention provides means for protecting FRC species systems against damage caused by such a refrigerant when it freezes. 5.
  • Some refrigerants may boil at a temperature below the minimum temperature of the enclosure in which a part of a two-phase heat-transfer system is enclosed, but above some of the equivalent temperatures of the outdoor surroundings of the system's collector.
  • sub-atmospheric protection can be provided in the sense that the refrigerant pressure in all parts of the system, including the collector absorber, can be maintained above atmospheric pressure even when the equivalent temperature of the collector's surroundings falls below the refrigerant's boiling temperature at atmospheric pressure.
  • the various specific embodiments of the invention can be grouped into two general species (a) Forced Refrigerant-Circulation Species, and (b) Natural Refrigerant-Circulation Species.
  • Forced Refrigerant-Circulation species employ at least one pump to circulate the refrigerant; whereas Natural Refrigerant-Circulation species, or more briefly NRC species, rely on the net static head resulting from the combined action of the heat absorbed from solar radiation and the local gravitational field to circulate the refrigerant.
  • FRC species systems are preferred for large installations (say, with a total collector aperture greater than 100m 2 ), and NRC species systems are preferred for small installations, (say, with a collector aperture below 25m 2 ) .
  • Figure 1 illustrates in diagramatic form a typical embodiment of the invention.
  • Figure 2 shows details of Figure 1 between refrigerant inlet 9 of condenser 8 and outlet 16 of condenser pump 14 applicable to a several alternative embodiments of the invention.
  • Figure 3 illustrates a first alternative embodiment of the invention.
  • Figure 4 illustrates a second alternative embodiment of the invention.
  • Figure 5 illustrates a third alternative embodiment of the invention.
  • Figure 6 illustrates in diagramstic form a technique for obtaining an alternative signal for computing the desired effective capacity of condensate pump 14 in the case of the third alternative embodiment of the invention.
  • Figure 7 illustrates a fourth alternative form of the invention.
  • Figure 8 illustrates a sixth alternative embodiment of the invention.
  • Figure 9 illustrates in diagramatic form a technique for controlling the ratio of sensible heat to latent heat released in the case of the third alternative embodiment of the invention.
  • Figure 10 illustrates an alternative technique for controlling this ratio.
  • Figure 11 illustrates in diagramatic form a technique of overpressure and over-temperature protection applicable to several embodiments of the invention.
  • Figure 12 illustrates in diagramatic form a technique for sub-atmospheric protection in the case of the first embodiment of the invention.
  • Figure 13 illustrates an alternative technique for freeze protection in the case of the first embodiment of the invention.
  • All FRC species must satisfy, for all external conditions under which a particular system is designed to operate, at least one additional internal operating condition; namely fifth, the available Net Positive Suction Head, or more briefly NPSH, available to a pump used in the system is maintained above the required NPSH for this pump.
  • the best general mode or manner for implementing the five prerequisite conditions for achieving self-regulation in FRC species embodi ments depends on the particular solar heating application for which these embodiments are used. I distinguish and describe four general classes of FRC species embodiments. The preferred class depends on the application for which it is to be used.
  • a "principal refrigerant circuit of an FRC species system is defined as the (closed) refrigerant circuit which includes
  • a "principal refrigerant circuit" of an NRC species system is defined as the (closed) refrigerant circuit which includes (1) the absorber inlet and outlet, and
  • internal volume of the absorber denotes the total internal volume of the refrigerant passages in the absorber in which the refrigerant absorbs heat directly from solar radiant energy or indirectly through the walls of these passages.
  • preselected limit whether it refers to a lower or an upper limit does not necessarily mean that this limit has a fixed value. It may have a variable value determined by a function store in the CCU whose arguments are variables, such as evaporation rate and evaporation temperature, which are measured and supplied to the Central Control Unit (CCU) .
  • CCU Central Control Unit
  • the term "quiescent conditions” is used to refer to any set of conditions under which no radiant energy is being absorbed by the ref rigerant of a two-phase heat-transfer system, and all the, liquid- vapor interfaces of the refrigerant in this system are at the same temperature.
  • separatator includes, in particular, a common header to a number of absorber refrigerant passages which has a large enough diameter for a horizontal liquid-vapor interface to form within this header, and from which nearly dry vapor can be supplied from an outlet connected to the vapor space within this header.
  • the foregoing refrigerant passages may end
  • the first three classes of FRC species were devised for applications where no requirement exists for controlling the ratio of the amount of sensible heat released by subcooling to the amount of latent heat released by condensation from the FRC system's refrigerant to the substance being heated.
  • the first two were devised primarily for applications where the evaporated refrigerant exiting the absorber is not required or desired to be superheated; whereas the third class was devised for applications where a small amount of superheat, which can be controlled within narrow limits, is desired.
  • the fourth class of FRC species was devised primarily for applications where a requirement exists for controlling the ratio of sensible heat released by the FRC system's refrigerant during subcooling to latent heat released by this refrigerant during condensation. Furthermore, this fourth class of FRC species may, where this ratio is not controlled, also be used either to control the quality of the refrigerant vapor exiting the absorber, or to ensure that quality one vapor is supplied to the condenser, over the system's entire operating range, even when a condensate pump with no capacity control is used.
  • Each of the foregoing four classes of FRC species may use a subcooler which is physically distinct from the condenser. Employing a separate subcooler in no way affects the basic operating principles of any one of these classes. (A separate subcooler is usually preferred where the ratio of sensible heat to latent heat released to the substance being heated is significant, say over 10%.)
  • Refrigerant exclusively in its liquid phase, enters absorber 1 at 2, and vapor exits at 3, and enters separator 4 at 5.
  • the refrigerant evaporated in absorber 1 is separated from the non-evaporated refrigerant in a way which allows only dry refrigerant vapor to exit at vapor outlet 6, and only non-evaporated refrigerant to exit at liquid outlet 7.
  • the dry refrigerant vapor exiting at 6 successively enters condenser 8 at 9, exits, mostly in condensed form at 10, and enters receiver 11 at 12. Only the condensed refrigerant exits receiver 11 at 13 and enters condensate pump 14 at 15.
  • the refrigerant exits this pump at 16 and enters the refrigerant line connecting separator liquid outlet 7 to absorber inlet 2 at a mergence point 17 located between 7 and 2.
  • This refrigerant circuit identified by 2-3-5-6-9-10-12-13-15-16-17-2 as the principal refrigerant circuit
  • the refrigerant circuit identified by 2-3-5-7—17-2 as the natural—circulation, absorber, auxiliary refrigerant circuit
  • a correctly configured (natural-circulation, absorber,) auxiliary refrigerant circuit can prevent the evaporated refrigerant exiting the absorber from-being superheated and the refrigerant vapor exiting the separator-vaporoutlet from being wet, thereby satisfying the first and second internal operating conditions.
  • the volume of liquid refrigerant in this refrigerant circuit must be maintained within a lower and an upper limit for all values of the evaporation rate and the evaporation temperature over which the heat-transfer system is designed to operate. Each of these two limits varies with the evaporation rate and the evaporation temperature in a way which depends on the precise configuration of the auxiliary refrigerant-circuit.
  • the effective capacity of the condensate pump — to achieve self-regulation — must be controlled so that the volume of liquid refrigerant is maintained between the first set of limits in the auxiliary refrigerant circuit and between the second set of limits in the refrigerant circuit segment 9-15, namely between the condenser.refrigerant and the condensate pump inlet.
  • a prerequisite for this to be possible is that the total volume of liquid refrigerant present in the refrigerant circuits of the heat-transfer system be
  • limit L R1 is selec ted so that whenever the refrigerant liquid level in line segment 9-15 is above L R1 , the available NPSH to the condensate pump is greater than the required NPSH by that pump; and limit L R2 is selected so that whenever the refrigerant liquid level in this line segment is below L R2 , the liquid refrigerant which backs up in condenser 8 is small enough to maintain the condenser's effective condensing surface above a preselected, positive, upper limit.
  • the Central Control Unit here- inafter referred to as the CCU, should control the effective capacity of the condensate pump so that the liquid refrigerant level is maintained within the foregoing two limits.
  • the numeral 18 indicates a typical location of the refrigerant liquid-surface level.
  • the CCU is supplied with a measure of the difference between the levels of limits L R1 and L R2 and a measure of the height of level 18, referred to as L R with respect to a specified reference level, which may be one of the foregoing two limits.
  • the measure of L R used may, in general, be a continuous function, a multi-valued function, or in the limit a three-level function with one level indicating that L R is below L R1 , a second level indicating that L R is between L R1 and L R2 , and a third level indicating L R is above L R2
  • the CCU based on the value of the measure of L R supplied to it, generates a signal that controls the effective capacity of the condensate pump in a way that tends during system activation,
  • liquid refrigerant surface level L& will not, in general, lie between L R1 and L R2 at the time the system is activated,
  • proportional control of condensate pump capacity is preferred.
  • a sensor must' be used from which the CCU can compute a signal which is a continuous function of the level L R and increases with it, and the effective capacity of the condensate pump must be capable of being varied continuously.
  • the information represented by this signal is used in the CCU to generate a control N' pc which increases the speed N' pc of pump 14 as liquid level 18 rises, and conversely decreases the speed of pump 14 as this level falls.
  • Symbol 5 indicates that the signal is supplied by the CCU.
  • the particular criteria used to activate and de-activate the heat- transfer system depend on the particular substance to be heated, and the particular purpose for which it is to be heated. To keep the description of operating principles general, I shall refer to the control signal generated whenever these criteria are met as the activation signal. Similarly, I shall refer to the control signal generated whenever the de-activation criteria are met simply as the de-activation signal.
  • Activation and de-activation criteria include the following events: the temperature of the absorber, or of the refrigerant vapor at the absorber outlet rises above the temperature at a given point of the substance to be heated, by a preselected amount., and the temperature of the absorber, or of the refrigerant at this same outlet, falls below the temperature at the same point of the substance to be heated plus a smaller preselected amount.
  • the set of rules for controlling the condensate pump are: Condensate pump 14 is started whenever
  • liquid level 18 in receiver 11 rises above level L R1 by a presele ted amount; and it is stopped whenever
  • the pump continues to run while the activation signal is "on" and liquid level 18 remains above level L R1 . While the heat-transfer system is operating, the speed of condensate pump 14 is, in effect, controlled by level 18 of the liquid refrigerant in the receiver.
  • the pump runs at its maximum speed when liquid level 18 exceeds upper limit L R2 , and stops when this level falls below L R1 , Between these two liquid levels, the pump, as mentioned earlier, runs at a speed which increases with increasing liquid level, and decreases with decreasing liquid level By my earlier assumption, as long as level 18 remains between L R1 and L R2 ,
  • m re , m rc , and m rR are the rates at which liquid refrigerant is being evaporated in the absorber, evaporated refrigerant is being condensed in the condenser, and condensed refrigerant is exiting the rec eiver, respectively.
  • the condensate pump will return liquid refrigerant to the (natural-circulation, absorber) auxiliary refrigerant circuit at the same rate m at which condensed refrigerant exits the receiver, and therefore, using (1),
  • the level L R Since is a function of the refrigerant saturated vapor temperature , the level L R , varies with this temperature for a given value of Often this functional dependency may be acceptable, If not, it can be eliminated by the CCU if is measured and supplied to the
  • the internal volumes of the separator and receiver can be chosen so that considerable lags in the value of in response to changes in the value of can be allowed without violating the conditions that must be satisfied for self-regulation, (For example, the lag for to reach 67% of its new steady-atate value after a step-function change in m can, with, economically practicable separator and receiver volume be greater than 10 seconds for a heat-transfer system whose collector aperture is 100m 2 and whose refrigerant is R—114 — namely dichloro- tetrafl ⁇ oroethane — without violating the conditions for self-regula tion.) Furthermore, a bias error in instrumentation causes only the value of L R , for which m rp is equal to a given value of m re , to change, and does not
  • a large ratio in the present context may be any number between 25 and 500 depending on a number of factors, and in particular on the--minimum permissible quality of- the refrigerant vapor exiting the absorber at the maximum evaporation rate. Considerations which affect the choice of minimum permissible vapor quality at the absorber outlet are
  • the decrease in refrigerant average evaporation (boiling) film heat-transfer coefficient in the absorber when this quality drops below a lower limit which depends on a number of factors, including the mass flow rate per unit cross-sectional area and the particular refrigerant used. More specifically, the volume of liquid refrigerant in a given absorber, with a fixed internal volume, required to maintain the refrigerant vapor exiting the absorber in a given state — namely in a dry state with a fixed amount of superheat including zero, or in a wet state with a fixed quality — is a maximum at the lowest evaporation rate for which a Rankine heat-transfer system is designed to operate.
  • the volume of liquid refrigerant in the absorber must decrease, for a given increase in evaporation rate, by an amount which increases as the evaporation temperature, namely the saturated vapor temperature of the refrigerant in the absorber, decreases.
  • the liquid volume thus displaced from the absorber, as the evaporation rate increases, must be accommodated in other parts of the auxiliary refrigerant circuit and, in particular, in the separator.
  • the fourth internal operating condition may be violated because this liquid level is also high enough for the vapor space left between the separator inlet and the separator outlet to be small enough to cause a vapor pressure drop which results in an unacceptably large drop in saturated vapor pressure between the foregoing inlet and outlet.)
  • the liquid-level control technique described earlier does, however not allow the liquid refrigerant that cannot be accommodated in the auxriliary refrigerant circuit, without violating the conditions for self-regulation, to be stored in the receiver.
  • the increase in liquid refrigerant present in the condenser is often small compared to the amount of liquid refrigerant that must be removed from the auxiliary refrigerant circuit to preserve self-regulation as the evaporation rate increases over this range.
  • the size of the receiver is increased, and a modified condenser capacity-control technique is used, which allows the amount of liquid refrigerant that can be stored in the receiver to be changed; and, in particular, to be increased as the evaporation rate increases.
  • this modified control technique consists of varying the effective capacity of the condensate pump in accordance with.the relation where the lower levelllimit is changed between L R1 and (L R1 + ⁇ L R1 ) and where the value of ⁇ L R1 is computed by the CCU as a function of the evaporation rate, or the evaporation temperature, or both.
  • the effect of this modified control law is to cause the liquid level L R , for a given condensate-pump effective capacity, to vary with operating conditions, and thus cause the amount of liquid refrigerant stored in the receiver also to vary with these conditions.
  • the change in the amount of liquid refrigerant stored in the receiver can be made to match the net change in the amount of liquidl-refrigerant that should be present in the auxiliary refrigerant circuit and the condenser to preserve self-regulation.
  • class A FF systems employ a pump, which I shall refer to as an "overfeed pump", to produce the desired net static head in the absorber auxiliary refrigerant circuit, whereas the latter employ no such pump.
  • the separator In the refrigerant circuit configuration of an A FF system, the separator can be located at any height in relation to the absorber or the condenser: whereas, in class A FN systems, the height at which the separator cr can be located is constrained by the fact that this separator must be above the absorber inlet.
  • FIG. 3 The basic refrigerant circuit of class A FF systems is shown in Figure 3. This configuration differs from the configuration shown in Figure 1 only by the fact that outlet 7 of separator 4 — instead of being connected directly to inlet 2 of absorber 1 — is connected to overfeed ipump.21 at inlet 22, and that outlet 23 of overfeed pump 21 is connected to refrigerant line 16-2 at a mergence point 24 located anywhere on this line between absorber inlet 2 and condensate pump outlet 16, The condensate pump is controlled by any one of the two liquid-level techniques discussed under the heading "Class A FN Systems".
  • the effective capacity of the overfeed pump is — except in the unusual case where it is kept constant, or nearly constant, (by for example, using a positive, constant-displacement pump run at fixed speed) — is varied as a function of the effective capacity of the condensate pump.
  • This function can be a continuous function or a multilevel function, The choice of function depends on the desired result.
  • the effective capacity m rpo of the overfeed pump can be controlled to keep the quality of the refrigerant vapor at the absorber outlet constant, within narrow limits, over a wide range of evaporation rates and evaporation temperatures.
  • the particular technique used to control the effective capacity of the overfeed pump as a selected function of the effective capacity of the condensate pump depends greatly on the type of pump used; and also on other factors, including the tolerance within which the actual quality of refrigerant vapor at the absorber outlet is required to equal the desired quality at this outlet.
  • V c (not shown) is the voltage applied to the condensate-pump motor.
  • liquid flow transducers 25 and 26 can be used to produce signals which represent measures and of the volumetric flow rates produced by the volumetric flow rates of the condensate pump and the overfeed pump, respectively. These signals are supplied to the CCU as indicated by the symbols 27 and 28.
  • the CCU as indicated by the symbol 29, supplies overfeed pump 21 with a control signal which controls its speed N so that the overfeed I ppuump tends to satisfy the relation
  • the quality may be a fixed number or a function of any variable —
  • a low level safety switch30 is used to provide the CCU with a signal L' S1 which stops the over-feed pump if the level L S of liquid refrigerant in the separator falls below minimum preselected level L S1 which is chosen high enough to satisfy the overfeed pump's NPSH requirements.
  • the symbol 31 indicates that the signal L' S1 generated by this low level safety switch when L S falls below L S1 is available to the CCU to turn off the overfeed pump. 4.
  • Class B systems have only one refrigerant circuit, and that the refrigerant vapor exiting the absorber must always be dry if the second internal operating condition is to be satisfied.
  • Class B systems are useful where a small amount of superheat in the refrigerant vapor exiting the absorber is desirable, and the pres-ur sure drop in the absorber is high.
  • a small amount of superheat may, for example, be desirable to prevent condensation in the connecting tubing in installations where the condenser is located at a considerable distance from the absorber.
  • a high pressure drop in the absorber can often occur where several concentrating collectors are connected in series.
  • Refrigerant exclusively in the liquid phase, enters absorber 1 at 2.
  • absorber 1 In a correctly operating system, only dry refrigerant vapor exits absorber 1 at 3. This dry vapor enters condenser 8 at 9 and exits, mostly in condensed form, at 10, and then enters receiver 11 at 12. Again, in a correctly operating system only condensed refrigerant exits receiver 11 at 13, and enters condensate pump 14 at 15. The refrigerant exits pump 14 at 16, and is returned to absorber 1 at 2.
  • the absence of an absorber auxiliary refrigerant circuit has two important consequences:
  • non-evaporated refrigerant exiting the absorber would -— in contrast to Class A FN and Class A FF systems — not only violate the second internal operating condition but would also cause any condensate-capacity control technique based exclusively on the level of the liquid surface in receiver 11 to malfunction.
  • a condensate-pump, liquid-level control technique of the general ⁇ ype described earlier could be made to control the effective capacity of the condensate pump correctly, but it is usually much simpler to use instead the control techniques described next.
  • This control technique consists in measuring the actual amount of superheat at some point between the absorber outlet and the condenser inlet and using a feedback control loop to control the effective conden sate pump capacity so that the actual amount of superheat tends toward the desired amount of superheat,
  • the desired amount of superheat can be chosen to be small, perhaps as small as one degree Celsius, but cannotbe zero.
  • Any known method for measuring superheat may be used.
  • One appropriate method for measuring superheat, when the Class B system operates over a wide range of saturation temperatures is described next.
  • a temperature transducer 32 is used to measure the temperature of the refrigerant vapor at a point in line 3-9.
  • the signal generated by this transducer is, as indicated by the symbol 33 supplied to the CCU.
  • a pressure transducer 34 is located at the same point and used to measure the refrigerant pressure The signal generated by this transducer is also supplied to the CCU as indicated by the symbol 35.
  • the functional relation for the particular refrigerant used is stored in the CCU, and from this relation, and from the signals and the CCU computes the super-heat
  • the CCU compares this actual value of the super-heat with the desired value stored in the CCU and supplies, as indicated by symbol 51, the condensate pump with a control signal N' p which tends to annul the difference
  • This condensate-pump control technique is self-starting because, before system activation, the value of is determined by the temp erature of the substance to be heated, and the system would not be activated — whatever the activation criterion or criteria used — unless
  • a low-level, safety control switch 37 is used to provide the CCU with a signal L' R1 which stops the condensate pump if the level L R of liquid refrigerant in the receiver falls below a minimum preselected level L R1 , which is chosen high enough to satisfy the condensate pump's NPSH requirements.
  • the symbol 38 Indicates that the sifnal L R1 , generated by this low-level safety switch Is supplied to the CCU.
  • Class C FN systems fall into two general sub-classes: (a) Class C FN systems with no independent subcooling control, and (b) Class C FN systems with independent subcooling control.
  • the former sub-class uses two condensate-pump capacity control techniques (a,l) fixed-capacity control, and (a,2) evaporation-rate capacity control,
  • a,l condensate-pump capacity control
  • a,2 condensate-pump capacity control
  • a,2 condensate-pump capacity control
  • a,2 condensate-pump capacity control
  • a,2 condensate-pump capacity control
  • Refrigerant exclusively in the liquid phase, enters absorber 1 at 2; and, in general, wet refrigerant exits at 3 and enters separator 4 at 5.
  • this separator which also functions as a receiver for excess liquid refrigerant, the evaporated and non-evaporated refrigerant are separated in a way which allows only dry refrigerant vapor to exit at. vapor outlet 6, and only non-evaporated refrigerant to exit at liquid outlet 7
  • the dry refrigerant vapor exiting at 6 enters condenser 8 at refrigerant inlet 9; exits mostly In the condensed state at 10, and merges, with the non-evaporated refrigerant exiting separator 4 at liquid outlet 7, at point 45.
  • condensed refrigerant from condenser 8 and the non-evaporated liquid refrigerant from separator 4 are supplied to condensate pump 14 at inlet 15, and returned by this pump through outlet 16 to absorber inlet 2.
  • the refrigerant outlet 10 of condenser 8 must be at a high enough level above the free liquid surface 40 for the static head between these two levels to be greater or at least equal to the pressure drop in the condenser at the maximum design evaporation rate.
  • the system charge m is selected so that — at the minimum design evaporation rate, and at the evaporation temperature, within the design range, for which total refrigerant liquid volume is a minimum — liquid level 40 in separator 4 is above the level L S1 at which the low-level safety switch 30 supplies the CCU, as indicated by symbol 31, with turn-off signal L' S1
  • the effective capacity of the condensate p.imp is chosen just large enough to prevent the amount of superheat of the refrigerant vapor exiting absorber 1 at 3 from exceeding a preselected upper limit at the maximum design evaporation rate, over the entire range of evaporation temperatures for which the system is designed to operate.
  • the pump After system activation, using any desired activation criteria, the pump operates at constant volumetric capacity and the liquid over- feed flow rate is equal to the pump mass flow rate less the current evaporation mass rate. Therefore, the quality of the vapor at the outlet of the absorber varies with liquid density and evaporation rate mre and is given by where m rp is the mass flow rate of liquid refrigerant from condensate pump outlet 16 to absorber inlet 2, and is the volumetric pump capacity, (volumetric flow rate).
  • variable-Capacity Control A more efficient implementation of the system is to vary the volumetric flow r of the condensate pump, and hence also its effective capacity o give a controlled amount of liquid overfeed in the absorber.
  • the overfeed rate may be a selected function of the evaporation rate.
  • the overfeed rate can be made a linear function of the evaporation rate, thereby obtaining a desired constant quality at the absorber outlet which is independent of the evapo ration rate.
  • the mass flow rates m rc and m ro of the condensed and overfeed liquid refrigerant may be measured, with sufficient accuracy, by flow-rate transducers 41 and 42 in the liquid 30 lines 10-45 and 7-45, respectively.
  • the signals F' and F' generate by these transducers are supplied to the CCU as indicated by symbols
  • the refriger ant circulation pump is of a type for wKich the volumetric capacity can be easily predicted by the CCU, for example, a positive displacement pump with speed control, the pump can be controlled by a signal from the CCU so that
  • a preferable control technique is to include transducer 42 in ths system and use closed-loop control of the condensate pump so that the actual capacity
  • F tends to the desired value hich is either , or ro , for control of vapor quality or contro of overfeed flow rate, respectively.
  • the refrigerant pump is de-activated by the CCU in response to a signal L' S1 , from the low-level safety switch 31 if refrig erant level 40 in the separator falls below level L S1 . In a Class C FN . system, this would occur only because the refrigerant charge used is too small.
  • a simplified method for controlling the circulating pump capacity is to use stepped control of capacity, for exampl&stepped speed drive or stepped volumetric capacity, in which the effective capacity of the pump is always sufficiently greater than the evaporation rate for the vapor quality to be less than one at the absorber outlet.
  • the accuracy of the flow transducers 41 and 42 may be relaxed, so that less expensive transducers may be used.
  • the flow transducers may be replaced by transducers which give more indirect measurements of evaporation rate.
  • One set of alternative transducers are shown in Figure 6: namely a differential, pressure transducer 47 and a temperature transducer 6 .
  • the pressure transducer is connected to the refrigerant Inlet 9 and refrigerant outlet 10 and therefore produces a signal ⁇ p'rc , which represents the refrigerant pressure drop in the condenser.
  • the temperature transducer produces a signal , which represents the saturated vapor temperature of the refrigerant.
  • the effective capacity of the condensate pump can be controlled continuously or in steps — which in the limit may only consist of two steps — by a signal supplied by the CCU whose nature depends on the type of condensat pump used.
  • This signal designated by in Figure 6 is, as indicated. by symbol 3 , supplied by the CCU.
  • the absorber passages may consist of a single cylindrical tube, an annulus or segments of an annulus between two concentric tubes;
  • the absorber passages may consist of a tapered spiral tube; and in the case of flat-plate collectors, the absorber passages may consist of a "waffle" pattern of the type produced, for example, by the Olin Corporation ROLL BONDR process.
  • the absorber fluid passages belonging to one or more physically distinct collector modules, may be connected in series or In parallel, or both, but are interconnected by one or more headers or manifolds, so that liquid refrigerant enters at a single point, referred to as the absorber inlet.
  • the absorber fluid passages are also interconnected so that the refrigerant exits at a single point, referred to as the absorber outlet, or interconnected by one or more separator modules so that the refrigerant from all the absorber fluid passages exits in one or more separate modules, referred to collectively as the separator,
  • the vapor outlets of the separator modules are interconnected, in turn, to merge at a single point referred to as the separator vapor outlet; and the liquid outlets of these modules are interconnected to merge at a second single point referred to as the separator liquid outlet.
  • the absorber outlet of tilted absorbers namely absorbers whose axis or plane is not horizontal, must be above the absorber inlet in NRC species systems, as well as FRC species Class A FN systems.
  • both the inlet and the vapor outlet of each separator module must be above the absorber inlet.
  • This constraint on the relative levellof the absorber inlet on the one hand and the absorber outlet — or the separator module inlets and vapor outlets where a separator is used — on the. other hand does not apply to FRC species Class A FF , Class B F , and Class C FN systems.
  • Th e usually preferred basic refrigerant-circuit configuration of a Class A NN system is shown in Figure 7 .
  • the separator also fulfills the function of a receiver.
  • a receiver is usually employed in addition to a separator.
  • Refrigerant exclusively in its liquid phase, enters absorber 1 at 2. Dry or wet refrigerant vapor exits at 3, and enters separator 4 at 5. In this separator, the refrigerant evaporated in absorber 1 is sepa rated from the non-evaporated refrigerant in a way which allows only dry refrigerant vapor to exit at vapor outlet 6, and only non-evaporated refrigerant to exit at liquid outlet 7.
  • the dry refrigerant exiting at 6 successively enters condenser 8 at 9, exits mostly in condensed form at 10, and enters the refrigerant line connecting separator liquid outlet 7 to absorber inlet 2 at a point of mergence 39 located between 7 and 2.
  • I shall refer to the refrigerant circuit identified by 2-3-5-6-9-10-39-2 as the principal refrigerant circuit; and to the refrigerant circuit identified by 2-3-5-7-39-2 as the absorber auxiliary refrigerant circuit. Note that this auxiliary circuit is equivalent to the natural-circulat ⁇ on absorber auxiliary refrigerant circuit used in FRC species systems.
  • Figure 7 is intendedto show qualitatively the relative vertical distances between outlet 3 of absorber 1, and the center-lines (not shown) of separator 4 and condenser 8.
  • the line L o -L o represents the level of the liquid refrigerant surfaces in absorber 1, and in the liquid refrigerant lines between 7 and 39 and 10 and 11, when the system is quiescent and the liquid refrigerant is at a uniform temperature.
  • separator 4 and condenser 8 are located at a level above that of level L o , with the condenser outlet 10 higher than the separator outlet 6. If ports 5 and 6 of the separator or port 10 of the condenser were below level L o , the separator completely, and the condenser partially, would be filled with liquid and could not perform their functions.
  • the mean absorber L A1 liquid level (averaged over the vertical extent of the absorber) will settle at, say, level L A1 and the level of the liquid surface inj line 7-11 at a level L S1 such that the net static head (L S1 -L A1 ) balances the friction induced pressure drop by the flow of liquid and vapor refrigerant in the auxiliary refrigerant circuit 3-5-7-39-2-3.
  • the mean liquid level in the absorber falls further to say L A2 and the liquid levels in lines 7-39- and 10-39 rise higher to the levels L S2 and L C2 respectively, to balance the increased flow pressure drops and to accom odate the increase in liquid displaced from the absorber.
  • the magnitude of the static head (L S2 -L A2 ) required to offset the pressure drop around the auxiliary circuit 5-7-39-2-3-5 for a given flow rate, is a- function of the (friction-induced) pressure drop in the absorber and increases with it.
  • the minimum height of the condenser liquid outlet 10 above the vapor outlet 6 of the separator, necessary to allow the required static head (L C2 -L S2 ) to be provided — at maximim heat absorption rate — without the liquid backing up into the condenser outlet, is decreased by using condensers with smaller pressure drops.
  • the correct charge is determined by the refrigerant liquid levels L A1 , L S1 and L C1 , for a heat-jtransfer fate Q A1 equal to the minimum design operating rate, when the average temperature of the liquid refrigerant is equal to that operating temperature, ⁇ within the design range, which corresponds to a minimum liquid volume and a maximum volume of vapor, within the system for a given charge mass, (This operating temperature is determined analytically from the internal dimensions of the systemscomponents and the published tables of specific volume or density for the liquid and saturated vapor states of the particular refrigerant.)
  • the liquid level L A1 is that giving the liquid volume required in the ab absorber for the vapor at the absorber outlet to be in a given state, for example, at quality one.
  • the liquid levels L S1 and L C1 are determined relative to L A1 by the total pressure ilrops around the auxiliary and principal refrigerant circuits at this operating temperature and minimum heat transfer rate.
  • the correct charge (once determined) is inserted over a particular environmental temperature range, with the system quiescent at that temperature, and may be checked by the location offthe quiescent liquid surface line L o - L o .
  • the volume of liquid in the system will be greater so that the mean liquid levels L A1 L S1 and L C1 will, in general, be higher than before, though by differing amounts because of the different vapor and liquid densities at these temperatures, with the result that the vapor quality ⁇ exiting the absorber at 3 will be less than before and thus the first internal operating condition will continue to be met.
  • the variation in liquid volume it is convenient for the variation in liquid volume to be accommodated in the separator 4.
  • (L o )min be the minimum liquid level in the absorber under quiescent conditions at this temperature for which the vapor quality exiting the absorber at the minimum design evaporation rate is one. It then follows from the preceeding discussion that at all other evaporation rates and temperatures, within the design range, the vapor exiting the absorber will not be superheated.
  • the liquid volume which must be stored outside the absorber and the condenser is a function of the evaporation rate and the evaporation temperature.
  • V max the maximum value of this volume
  • L o max the corresponding liquid level in the separator above (L o ) max be L s .
  • This level can, in principle, be made arbitrarily small by choosing the horizontal cross-section of the separator arbitrarily large. And the static head obtained with this level can be sufficient to cause a high enough flow rate in the absorber at all evaporation rates and tempera tures for no superheat to appear at the absorber exit by choosing the cross-sectional area of the absorber refrigerant passages large enough.
  • the minimum height of the refrigerant outlet of the condenser above L S for which the third internal.operating condition . is satisfied is determined by applythe .refrigerant pressure drop in the condenser. This pressure drop is a maximum for the highest design evaporation rate and the lowest design evaporation temperature. This minimum height of the condenser refrigerant outlet can again, in principle, by made arbitrarily small by vising a condenser with an arbitrarily low maximum refrigerant pressure drop.
  • the separator liquid outlet and condenser liquid outlet can be located at arbitrarily small levels above the level (L o ) max by selecting the large enough cross-sectional areas for the absorber and condenser refrigerant passages, and a large enough volume for the separator. Note that if the separator outlet of a given refrigerant circuit configuration is at the level (L o )min at a given tilt angle, it will remain above this level for any larger tilt angle. Consequently, a given Class A NN configuration can be used over a wide range of tilt angles . 2.
  • Class B N systems differ from prior-art two-phase natural-circulation systems by the fact that they (a) use a receiver to accomodate increases in specific liquid refrig erant volume with temperature, and to prevent liquid refrigerant displaced from the absorber at high evaporation rates from backing up into the condenser through its refrigerant outlet, and may (b) use a receiver with a horizontal cross-section which changes as a function of height to help control variations in the state of the refrigerant vapor exiting the absorber with changing evaporation rate and temperature, b.
  • the principal advantage of a Class B system arises from the fact that the length of the liquid line 10-12 can be zero, and that the vertical distance between the surface of the liquid refrigerant in the receiver and the condenser refrigerant outlet can also be zero without imposing any constraints on minimum permissable refrigerant pressure drop in the condenser; whereas a zero vertical distance between the surface of liquid refrigerant in the separator of a Class A NN system and the refrigerant outlet of its condenser requires the refrigerant pressure drop in this condenser to be zero if liquid back-up in this receiver is to be prevented.
  • condensate-pump, capacity control is to ensure that the five internal operating conditions are satisfied under all external conditions in the design range. As discussed earlier, this objective is achieved by selecting the proper refrigerant charge for the system, imposing certain restraints on the system configuration, and by control ing the volume of liquid refrigerant present in the absorber and the absorber auxiliary refrigerant circuit.
  • the most appropriate type, of FRC species condensate-pump and overfeed pump capacity control depends on system size and on the refrigerant used.
  • the fourth type of control is generally the least expensive, b ⁇ t is usually not desirable for many reasons, including the resulting frequent start and stop transients which affect adversly the reliability of the pump and its driving mechanism.
  • the third type of capacity control is also usually undesirable because of a general reduction in pumping efficiency, but may be attrac tive in some small systems.
  • the second type of control is mostly appli cable to large systems and may be combined with the first type of control to obtain continuous capacity control, .
  • the first type of control is widely used and without loss of generality,will be assumed in describing in greater detail the basic principles of liquid-level condensatepump, capacity-control techniques. Clearly, these basic principles apply to any other type of pump-capacity control.
  • Variation of pump speed requires a variable speed drive. It is not relevant to the control principles described below whether the variation of speed is obtained by an engine, a variable speed ac or dc motor, a stepping motor, or a variable speed hydraulic or mechanical drive with a constant speed power source.
  • Each method of speed variation has advantages for particular applications and designs of heat-transfer systems, but the basic control principles described next are the same for all methods of speed control. 2.
  • the first is a positive displacement type in which the volume of fluid displaced per revolution (or cycle) of the pump is relatively independent of pump speed and the hydraulic head or pressure rise generated.
  • the second type is a centrifugal pump, in which the volume of fluid displaced per cycle may be very dependent on the pump speed and the hydraulic head to be overcome.
  • the control principles are the same for both types of pumps, but the volu metric flow fxinction of the centrifugal-type pump must be compensated by the CCU if approximately linear control of volumetric capacity is desired. If a positive displacement pump is used, compensation of the flow function can usually be omitted.
  • the rotational speed of the condensate pump is given by the CCU.
  • Pp. is the desired pump volumetric flow rate (m 3 /sec and is the pump volumetric function in terms of speed N the pressure rise (Pascal) and the type and temperature of the liquid refrigerant being used.
  • N the pressure rise (Pascal) and the type and temperature of the liquid refrigerant being used.
  • the pump volumetric flow function is given by
  • the desired volumetric pump flow rate is equal to the condensate flow rate s the refrigerant liquid density.
  • the condensate flow rate may be measured directly by using a flow transducer, but this is not required for controlling the refrigerant pump capacity.
  • a sensor to determine the refrig erant liquid level 18 in the receiver 11, between the condenser and the condensate pump, is sufficient to control the pump volumetric rate.
  • Figure 2 shows that a differential pressure transducer 19 is used to det ermine the liquid level 18, essentially by weighing two liquid columns, but other types of sensor could obviously be substituted. Note that pressure transducer 19 is connected between outlet 13 cr just below outlet 13, of receiver 11 and a point 20, preferrably In a horizontal leg of refrigerant line 10-12.
  • Point 20 is located at the lowest point of the circumference of the horizontal leg of line 10-12 to ensure that the line 19-20 is kept full of liquid refrigerant under all operating and non-operating conditions, and thus assure that pressure transducer 19 provides at all times a reliable measure of the difference in weights, and therefore also of the heights of the liquid columns in line 19-20 above transducer 19 and the liquid column between the level of the liquid refrigerant surface 18 in receiver 11 and the level of transducer 19 (If the lower pressure side of transducer 19 were connected to the vapor space in receiver 11 — and thus neglecting vapor weight — were used to measure directly the weight of the liquid column between surface 18 and the level of transducer 19, a heating element would have been needed to stop vapor condensing in the line connecting the lower-pressure side of transducer 19 to the vapor space in receiver 11. And clearly, liquid refrigerant in this line would cause transducer 19 to produce an erroneous signal.
  • L R is the liquid surface height and L R1 is a reference height, and is the liquid refriger ant density.
  • the CCU controls the volumetric capacity of the condensate pump to be the sum of a term proportional to signal S L and a bias signa S B . Therefore,
  • the liquid level L R in the receiver is the result of the refrigerant flow into the receiver from the condenser and out of the receiver to the pump. Therefore, the derivative L R of the liquid level is given by
  • a R is the cross section area of the receiver.
  • equation 26 gives a closed loop level control which is a stable first-order control system and has a steady-state solution the desired control law for the condensate pump volumetric capacity.
  • the steady-state liquid level L R is given by or, and the corresponding pump speed N p is given by
  • the response of the system to a step transient, such as a cloud partially obscuring the solar radiation, is exponential with a response time-constant Is given by
  • the time constant 1 may be made Independent-of the liquid--temperature by compensating the control signal S L for changes in liquid density as a function of temperature. That is, the CCU controls the proportional constant to be giving
  • the bias signal S B is generated in the CCU to control the steady- state liquid level L R so that, for a given system spatial configuration and a selected charge of a particular refrigerant, the volume of liquid refrigerant in the absorber and the absorber auxiliary refrigerant cir cuit of Class A FN and Class A FF systems can be maintained at the optimum value over the design range of saturated vapor temperatures and evaporation rates.
  • the changes in liquid density and vap nsity are compensated, for a particular system, by generating as a polyno mial function of in the CCU, which is provided with a measure of the vapor temperature , or equivalently the refrigerant liquid temperature when sub-cooling is small.
  • bias signal S B as a function of the evaporation rate m re can also be deter mined by the CCU If It is supplied with any acceptably accurate measure of m re and the functional dependence of SB on m re is also expressed as a polynomial (determined by earlier tests on the heat-transfer system) F.
  • FRC and NRC species systems may be operated either with essentially zero, or with substantial, subcooling of the liquid refrigerant.
  • a system may be operated with a moderate degree of subcooling by designing the condenser to hold a sufficient amount of condensed vapor within it and by increasing the system's refrigerant charge to supply this additional liquid volume. If a large amount of subcooling is required, it is preferable to Insert in FRC species systems.
  • the subcooler would be inserted in the line between mergence point 39 and absorber inlet 2; and in a Class B N system, between receiver outlet 13 and absorber inlet 2.
  • the mass flow rate of refrigerant through the subcooler,- if one is used, is the same as the mass flow rate through t he condenser (in the steady state). Therefore, the amount of subcooling ⁇ sbTr , obtained with these three systems, is determined by the sensible heat delivered to the substance being heated, and more specifically by where Q sb is the sensible heat transfer rate, is the specific heat of the liquid re , and m r is the refrigerant mass flow rate.
  • the product for the refrigerant exceeds the specific heat, mass flow-rate product for the substance being heated,- the subcooling temperature dr op across the subcooler is less than the temperature rise of the substance being heated in the subcooler and sufficient sensible heat transfer capacity is available in the foregoing three classes of systems.
  • the sensible heat transfer is not eontrolled independently during the operation of these three classes of systems, but- adjusts itself automatically to the sensible heat transfer load as long as sufficient sensible heat transfer capacity is available.
  • a Class C FN system should be used for liquid refrigerant subcooling under conditions when the liquid subcooling capacity in a Class A FN Class A FF , or Class B F system would be Inadequate; namely in cases where the sensible heat transfer load exceeds the sensible heat transfer capacity of the liquid refrigerant when the mass flow rate of liquid refrigerant through the subcooler ⁇ is equal to m rc .
  • a Class C FN system provides a greater sensible-heat transfer capacity than the other three classes of FRC systems because the liquid refrigerant mass flow rate through the pump and the subcooler may be arbitrarily larger than the condensation rat
  • the sensible heating load may vary
  • the control technique for a Class C system is shown in Figure 9.
  • Temperature sensors 52 and 53 measure the temperatures and at the refrigerant outlet 54 and fluid (sub stance to be heated) inlet 55 of subcooler 56, located between condensate pump outlet 16 and absorber inlet 2 and supply signals and to the CCU as denoted by symbols 57 and 58.
  • the ten ⁇ erature sensors 59 and 60 may be positioned as shown in Figure 10 at refrigerant inlet 61 and at the substance being heated outlet 62 of the subcooler 56, delivering temperature signals and , as shown by symbols 63 and 64 to the CCU respectiv ely.
  • the capacity of pump 14 would be controlled by the CCU according to the deviation of the differences measured by the sub- cooler temperature sensors from a pred etermined value ⁇ T, which may be a function of condensation temperatur . That is, the CCU would give a closed loop capacity control by commanding a pump flow rate equal to
  • G. is a fixed control gain constant, for the first or alternative arrangement, respectively.
  • the stagnation temperature, namely the no-load radiation equilibrium temperature , of some types of collectors — such as flat-plate collectors — can be replicated sufficiently accurately by a small dummy collector module.
  • the dummy collector module uses the same transparent cover or covers, absorber panel, and frequency-selective or non-selective absorber coating, as the operating collector modules.
  • the dummy collector has no refrigerant charge.
  • the absorber panel is however insulated from the surrounding air and supporting structure in a manner which replicates the rate at which the absorber panel of the operating collector modules lose heat to their surroundings.
  • the dimensions of the dummy collector module must be large enough for so-called end effects to be negligible.
  • a typical dummy collector module for a flat-plate collector would have an area of about 0.1m 2 .
  • a temperature transducer is placed in thermal contact with the dummy collector-module absorber panel.
  • the tilt angle of this panel is the same as that of the panels in the operating collector modules. I consider here only the case where the amount of superheat of the refrigerant in the absorber — which in this case consists of all the absorber panels of the flat-plate collector array — is negligible.
  • the rate at which heat is absorbed by the refrigerant in the absorber is given by where is the evaporation temperature, namely the saturation temperature of the refrigerant "in the absorber, and where Ke Is a factor which can be determined by calibration tests, and which remains fairly constant over a wide range of evaporation temperatures, Also In this case the evaporation rate m re is given by
  • This fxinction can be stored in the CCU so that the CCU can compute m re as a function of
  • All two-phase heat transfer systems are designed with a maximum pressure limit which is safely above the maximum pressure experienced during normal operation.
  • over-pressure protection may be required when the system is de-activated by error, power supply failure, certain equipment failures, or because the CCU de-activates the system in response to an indication by a sensor in the substance being heated, that this substance has reached its maximum design temperature.
  • FR.C species systems in which liquid refrigerant will not enter the absorber Inlet from the refrigerant circulation pump by gravitation al flow while the pump is not operating, the system is self-protecting in the event of a power supply failure and also for most equipment fail ures,which disable the condensate pump.
  • the system and the individual components should be sized so that the entire refrigerant charge can drain into the-Other parts of the fluid circuit below the bottom of the absorber, including the condenser, receiver, and/or piping so that none remains in the absorber in the event of malfunction.
  • This self-protection design prevents destructive pressure build-up in the absorber, even when the absorber temperature approaches stagnation temperature, and also limits temperatures in other parts of the system because there is negligible heat transfer from the superheated vapor in the absorber.
  • the refrigerant is also protected from excessively high temperatures in the stagnation condition, except for the small amount present as superheated vapor in the absorber.
  • This method is to position in the merged return line 2-39 an automatically controll ed valve 67 just ahead of the absorber inlet 2.
  • This valve 67 could be but is not limited to, an automatic two-position valve that can be operated in a so-called "fail-safe" manner. Namely, the valve would be normally kept closed in the absence of a control "signal" by, for example, a spring or weight, and would be opened by the control signal.
  • the means employed to determine whether the refrigerant has exceeded its maximum design operating pressure may either be a pressure sensor or a temperature sensor.
  • the operating pressure would-be assumed to have been exceeded if the measured temperature exceeded the refrigerant saturated vapor temperature corresponding to the maximum operating pressure.
  • pressure sensors are (a) pressure-actuated switches, in which case the valve would be actuated by an electrical signal and would probably be a solenoid valve (of the "normally-closed” kind); (b) bellows filled with a gas , in which case the valve would be actuated (closed) when the pressure exceeds the maximum operating pressure; (c) bourdon-type pressure gauge mechanisms, in which case the valve would be mechanically actuated by the movement of the bourdon tube as it straightens out, as the pressure inside it increases, and it comes into contact with the stem of the valve; and (d) crankcase-type pressure regulating valves that close on rise of pressure Examples of temperature sensors are (a) platinum resistance transducers, in which case the valve would be electrically actuated and usually be a solenoid valve; (b) bulbs containing a mixture of liquid and
  • the sensing element of the foregoing sensors would preferably be located at some point in the absorber 1 near the vicinity of the outlet and, in the case of temperature sensors would either be immersed in the refrigerant, or would be in thermal contact with the tubing containing the refrigerant or with the absorb ing surface in the vicinity of this tubing.
  • valve 67 prevents the refrigerant from exceeding its maximum design operating pressure in the event liquid refrigerant is temporarily trapped in the absorber passageways, because of equipment malfunction, or because the medium being heated reaches its maximum design temperature or pressure, and the system reduces the rate at which heat is transferred from the refrigerant to the medium being heated below the rate at which radiant heat is absorbed by the refrigerant.
  • valve 67- prevents actuation of a pres sure relief valve, or equivalent device, that would necessarily be incorporated in any practical system.
  • valve 67 would automatically prevent the liquid refrigerant from entering the absorber passage ways through inlet 2 whenever the refrigerant saturated vapor pressure exceeds a pre-selected value below, preferably by about 15 to 25%, the setting of the pressure relief valve, or below the setting of other safety means to dump the refrigerant if its pressure exceeds the safety device setting.
  • the liquid refrigerant In NRC species systems, the liquid refrigerant must also be prevented from entering the absorber through its outlet 3. To this end, sufficient volume must be provided by the refrigerant circuit outside the absorber passageways to ensure that the entire refrigerant charge can be held by the circuit and that the level of the liquid refrigerant in the circuit does not rise above the highest level of the tube from the absorber outlet to the separator outlet. This can usually be accompushed without providing a liquid refrigerant reservoir — especially in cases where the levels of the condenser and separator, liquid inlets are above the level of the absorber.
  • the entire refrigerant charge can be stored in the condenser, the liquid receiver and associated tubing, by proper sizing in design of. the system, so that no refrigerant can remain in the absorber under stagnation conditions and the system pressure is the vapor pressure of the stored liquid.
  • recovery from a stagnation condition may in some installations require that the CCU controls the system turn-on sequence to prevent a pressure surge due to the heat stored in the absorber structure.
  • the sequence is first to turn-on the pump which circulates the medium to be heated (assuming it is a fluid) through the condenser then later, after sufficient time for the condensing temperature to stabilize, to slowly increase the circulating pump speed from zero to its normal controlled speed by a control signal from the CCU.
  • the separator inlet and vapor outlet must be high enough for the net static head in the auxiliary refrigerant circuit to be zero or positive at the maximum evaporation rate and the minimum evaporation operating temperature
  • the combined over-pressure and over-temperature technique described next provides absorber and refrigerant over-temperature protection in addition to the refrigerant over-pressure and the substance over-temperature protection discussed earlier.
  • the combined protection technique uses a sensor to indicate that the maximum design operating pressure or saturated vapor temperature, or dry-bulb temperature, has been exceeded by sensing absorber or refrigerant temperature, refrigerant saturated vapor temperature, or refrigerant pressure. This sensor activates a heat rejection circuit to reject unwanted heat to an external heat sink, which will usually be water or the outdoor ambient air.
  • the means used to transfer heat from the refrigerant to the air Is any type of air cooled condenser; and, in the case where the heat sink is water, the means is any type of water cooled condenser.
  • the water for the water sink can be, or come from, any natural or man-made supply, such as the sea, a lake, an artesian well, or water from the cold-water supply of a man-made water distri bution system.
  • the air or water condenser hereinafter referred to as "heat-rejection condenser" may use either natural air and natural water circulation, respectively; or forced air and forced water circulation, respectively.
  • the heat- rejection rate, of this rejection circuit is designed so that the temperature and pressure in the two- phase heat-transfer design system remains within design limits under the highest stagnation temperature and the highest ambient temperature.
  • the combined over-pressure and over-temperature technique uses an intermediate condenser which transfers heat from the main Rankine heat-transfer system to the heat-rejection circuit, which is a small ancillary Class A NN or Class B N NRC species system.
  • the main Rankine heat-transfer system may be a Class A NN or a Class B N system.
  • a Class A NN system is illustrated in Figure 11 without loss of generality, illustrates the case where both the main and ancillary Rankine heat-transfer systems are Class A NN systems.
  • the main inlet 69 of the intermediate condenser 68 is connected to the separator outlet 6 of the main system, and the main outlet 30 of this Intermediate condenser is connected to the refrigerant inlet 9 of the main condenser 8.
  • the refrigerant of the ancillary system exits intermediate condenser 68 at 71, enters ancillary separator 72 at 73.
  • the evaporated dry refrigerant exits 72 at 74 and enters ancillary condenser 75 at 76 from which it exits, mostly in condensed form, at 77 and is returned by gravity to ancillary inlet 78 of intermediate condenser 68.
  • the non-evaporated portion of the refrigerant exits ancillary separator outlet at 79 and is also returned by gravity to 78 after flowing through mergence point 79.
  • Valve 80 which activates the ancillary heat-rejection circuit when it is open may be actuated by a pressure or a saturated temperature or a dry-bulb temperature rise in the principal refrigerant circuit, as described earlier under the heading "Over-Pressure Protection Technique", either through a bellows and mechanical actuator or by an electrical signal from the CCU, or directly by an over-pressure or over-temperature swith. If an electrically operated (solenoid) valve is used, it would be normally open, when it Is not energised, to provide fail-safe protection. 3.
  • FRC species heat-transfer systems may be designed for use in environments where the winter ambient temperatures fall below the boiling point of the refrigerant at atmospheric pressure. Because the liquid refrigerant is free to drain out of the absorber inlet, in general, a self-sustaining Rankine cycle will transfer heat, by liquid evaporation and condensation, from the lower parts of the system, where the liquid collects by gravity, to the absorber as it cools. Sustained low ambient temperatures may, therefore, result In the absorber internal temperature falling below the atmospheric-pressure boiling point of the refrigerant, so that the vapor pressure in the system becomes subatmospheric. With certain types of pumps, rotating seals may admit air to the system at subatmospheric pressure.
  • the first method consists of making heat available from an auxiliary heat source, such as a tank of the substance being heated, so that the internal temperature of the absorber does not fall below the atmospheric boiling point. This method is useful when the absorber heat-loss coefficient is small and the equivalent outdoor temperature is not far below the refrigerant boiling temperature at atmospheric pressure (say no more than 10°C) .
  • auxiliary heat may be supplied by thermosyphon flow, of the substance being heated, through the condenser which contains liquid refrigerant under quiescent conditions.
  • the second method is to configure the system so that the condensate pump (and other components requiring subatmospheric protection) are placed at a low enough level below the absorber for the vapor pressure in the system, plus the- pressure due to the quiescent static head of liquid refrigerant (which will be at minimum temperature also), to exceed the atmospheric pressure at the mechanical seal of the condensate pump.
  • This method could also protect a rotating seal in the overfeed pump of a Class A FF system if the pump is placed at a low enough level below the absorber.
  • the advantage of the second method over the first is that it does not rely on a continuing supply of heat to protect the system.
  • a liquid control valve 64 is located in the liquid line 16-17 between condensate pump outlet 16 and absorber inlet 2. (This valve would be located in liquid line 16-24 in a Class A FF system, and in liquid line 16-2 in a Class B F of a Class C NN . system.)
  • This control valve is operated by, for example, a temperature transducer 65 in thermal contact with absorber 1. The valve operates to shut off the liquid flow when the temperature sensed by 65 is below a preset value, at which the refrigerant vapor pressure is safely above atmospheric pressure.
  • valve 64 While valve 64 is closed, liquid refrigerant from vapor condensing in the absorber (and its auxiliary refrigerant circuit in the case of Class A FN and A FF systems) will be prevented from returning to the lower parts of the system. Therefore, liquid refrigerant will eventually fill the absorber (and the auxiliary refrigerant circuit, if applicable) , and the liquid-vapor interface near the absorber can be positioned in a well-insulated section of the line 6-9 (or line 3-9 or 3-5 for Class B or Class C systems, respecti vely) from which the heat-loss rate is much smaller than that from the absorber.
  • the temperature of the absorber (and its auxiliary refrigerant circuit, if applicable) may then fall further, so that it is filled with sub-cooled liquid, without appreciably affecting the temp erature and pressure at all refrigerant liquid-vapor interfaces in the system, which may be maintained above atmospheric pressure by a small heat input (such as leakage through Insulation) to the liquid remaining in the lower part of the system. 4.
  • F r e e z e P r o t e c t i o n Some FRC species systems may utilize a refrigerant such as water, which will freeze in the absorber if exposed when deactivated, to the lowest ambient temperatures which the system may encounter.
  • the first method is the same as the first method described for sub-atmospheric pressure protection in the previous section.
  • This method relies on an auxiliary source of heat, such as a hot-water reservoir, to supply the heat required, to maintain the absorber (and the absorber auxiliary refrigerant circuit, if applicable), above freezing temperatures by latent heat transfer caused by vapor evaporation from the lower parts of the system and vapor condensation in the absorber (and the absorber auxiliar circuits, if applicable).
  • liquid refrigerant fills at least part of the condenser and that the auxiliary heat be supplied to the condenser to evaporate refrigerant liquid by (a) a thermo-syphon action or (b) thermal conduction.
  • the heat lost by the auxiliary heat source, while preventing absorber circuit freezing, is replaced automatically during normal heat transfer system operation.
  • the hazard of the method is that the auxiliary heat source will become so depleted, by extended severe environments with no solar radiation available, that the heat transfer rate wil be insufficient to maintain the absorber circuit above the freezing point, when the vapor will condense directly to the solid phase and eventually block the natural circulation. After this happens, melting and freezing may generate sufficient pressure to rupture the system. Therefore, the first method would be of use where the heat loss rates from the absorber and its auxiliary exposed circuits are small enough for an auxiliary heat source to be economically practical for protecting the system fora.period In" excess of the worst (historical) environment conditions, with adequate-asafety factor.
  • the second method consists of protecting the absorber (and its auxiliary circuit, if any) by two valves which isolate the exposed portions of the system (above line L-L in Figure 13) from the lower, environmentally protected portions, thus preventing vapor condensation and the accumulation of liquid which may freeze in the exposed portions of the system while it is de-activated.
  • Figure 13 shows this method applied, for example, to a Class A FN system, but the method can obviously be applied to any other FRC species system containing a liquid receiver 11 or, alternatively, in the case of a Class C FN system a separator 4, in which liquid refrigerant is stored.
  • the absorber 1 and the absorber auxiliary refrigerant circuit 2-3-5-7-17-2 is configured so that, when the system is de-activated, the liquid refrigerant in the absorber auxiliary refrigerant circuit will drain down through the liquid line 2-17-16 and back through pump 14 into liquid receiver 11, which is able to hold all the refrigerant charge. If the pump 14 is of a type which will not permit reverse flow through It when de-activated, then pump bypass circuit 96-95-97 is opened to allow the liquid from the absorber circuit to drain down from 17 to 11, past the pump. Normally open valve 95 is opened after system de-activation and closed during system activation, by control 98 from the CCU.
  • Valve 81 In the liquid line between mergence point 17r.and 96 is located at a level which is at, or above, the level of one-way valve 94 (between condenser outlet 10 and receiver inlet 12) , and is activated by a temperature sensor 82, in thermal contact with absorber 1. Valve 81 closes when the temperature of sensor 82 falls to a preset value Tv , which is a selected amount above the temperature at which refrigerant liquid will freeze in equilibrium with refrigerant vapor (le. the triple point of the refrigerant) .
  • Tv a preset value
  • One-way valve 94 closes whenever the pressu in condenser 8 is equal to or less than the pressure in receiver 11.
  • valve 94 will close and the absorber circuit wil become isolated from refrigerant vapor and liquid in circuit 16-13-12, which is protected from freezing by a benign environmentr or additional heat sources using conventional techniques.
  • a supplemental condenser 83 In applications where the liquid refrigerant stored in the receiver is at a temperature well above the turn-off temperature for valve 81, which is usually true with small subcooling in condenser 8, a supplemental condenser 83, with inlet 84- connected to 12 and outlet 85 connected to liquid line 13-15 at 86, is used to reject heat from, the liquid refrigerant in 11 to the environment. This additional heat rejection is designed to cool receiver 11 faster than the absorber cools until valve 82 closes.
  • the heat from 83 is .rejected by a two-phase heat-transfer circuit 87-88-92-93-89-87, using a non-freezing refrigerant, in which 83 acts as the evaporator and 91 as the heat rejection condenser.
  • the heat rejection circuit is activated under control of the CCU, as indicated by symbol 90, which opens valve 89 on system deactivation, and closes 89 again when the signal from the temperature of sensor 99 indicates that the temperature of the refrigerant in the receiver has fallen below "Tv .
  • the signal from 99 is sup- plied to the CCU as indicated by symbol 100.
  • the one-way valve 94 would be located in the refrigerant liquid and vapor line 3-5 so that the condenser 8 and the evaporator 4 would be isolated from the absorber when valve 94 is closed.

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  • Engineering & Computer Science (AREA)
  • Physics & Mathematics (AREA)
  • Life Sciences & Earth Sciences (AREA)
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  • Thermal Sciences (AREA)
  • Chemical & Material Sciences (AREA)
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Abstract

Systemes ameliores de transfert de chaleur a deux phases permettant d'absorber la chaleur de l'energie solaire radiante, et de transferer cette chaleur vers une substance a chauffer. Le fluide de circulation est appele refrigerant. Les systemes de l'invention possedent la propriete decisive d'autoregulation. Le terme autoregulation est utilise pour decrire la capacite d'un systeme de transfert de chaleur a deux phases d'assurer automatiquement qu'au moins les quatres conditions de fonctionnement interne citees ci-apres soient remplies pour toutes les conditions exterieures pour lesquelles ce systeme est concu: 1) le refrigerant (2) entrant dans l'absorbeur collecteur (1) existe exclusivement dans sa phase liquide et possede un debit suffisamment important pour maintenir la quantite de chaleur de la portion evaporee du refrigerant (3) sortant de l'absorbeur collecteur (1) en-deca d'une limite superieure positive predeterminee; 2) la vapeur du refrigerant (9) entrant dans le condenseur (8) est maintenue a l'etat sec; 3) la quantite de refrigerant liquide d'appoint dans le condenseur (8) est suffisamment petite pour maintenir la surface de condensation effective du condenseur au-dessus d'une limite superieure positive predeterminee; et 4) la valeur absolue de la difference entre la temperature de la vapeur saturee (3) sortant de l'absorbeur collecteur (1) et la temperature de la vapeur saturee (9) du refrigerant entrant dans le condenseur (8) est maintenue en-deca d'une limite superieure predeterminee.
PCT/US1980/000896 1980-07-07 1980-07-07 Systemes solaires de transfert de chaleur a deux phases WO1982000191A1 (fr)

Priority Applications (3)

Application Number Priority Date Filing Date Title
EP81901386A EP0055712A1 (fr) 1980-07-07 1980-07-07 Systemes solaires de transfert de chaleur a deux phases
AU72238/81A AU7223881A (en) 1980-07-07 1980-07-07 Solar two-phase, heat-transfer systems
PCT/US1980/000896 WO1982000191A1 (fr) 1980-07-07 1980-07-07 Systemes solaires de transfert de chaleur a deux phases

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
WOUS80/00896800707 1980-07-07
PCT/US1980/000896 WO1982000191A1 (fr) 1980-07-07 1980-07-07 Systemes solaires de transfert de chaleur a deux phases

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TWI579520B (zh) * 2013-08-22 2017-04-21 財團法人工業技術研究院 熱交換器、熱機循環系統及其控制方法

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AU587005B2 (en) * 1985-01-23 1989-08-03 Hopeton George Gray Solar heating system
IL79495A0 (en) * 1985-08-02 1986-10-31 Kernforschungsz Karlsruhe Plant for generating process steam by means of solar energy in the direct evaporation mode

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US433055A (en) * 1890-07-29 tellier
US1119063A (en) * 1914-04-24 1914-12-01 Cyrus Burnap Boiler.
US2396338A (en) * 1943-02-24 1946-03-12 Honeywell Regulator Co Radiation heating and cooling system
US2933885A (en) * 1952-05-31 1960-04-26 Melba L Benedek Individually Heat storage accumulator systems and method and equipment for operating the same
US3390672A (en) * 1966-07-12 1968-07-02 Melpar Inc Solar heating device
US4052976A (en) * 1976-06-30 1977-10-11 The United States Of America As Represented By The United States Energy Research And Development Administration Non-tracking solar concentrator with a high concentration ratio
US4120289A (en) * 1977-04-20 1978-10-17 Bottum Edward W Refrigerant charged solar water heating structure and system
US4220138A (en) * 1978-01-24 1980-09-02 Bottum Edward W Refrigerant charged solar heating structure and system

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Publication number Priority date Publication date Assignee Title
US433055A (en) * 1890-07-29 tellier
US1119063A (en) * 1914-04-24 1914-12-01 Cyrus Burnap Boiler.
US2396338A (en) * 1943-02-24 1946-03-12 Honeywell Regulator Co Radiation heating and cooling system
US2933885A (en) * 1952-05-31 1960-04-26 Melba L Benedek Individually Heat storage accumulator systems and method and equipment for operating the same
US3390672A (en) * 1966-07-12 1968-07-02 Melpar Inc Solar heating device
US4052976A (en) * 1976-06-30 1977-10-11 The United States Of America As Represented By The United States Energy Research And Development Administration Non-tracking solar concentrator with a high concentration ratio
US4120289A (en) * 1977-04-20 1978-10-17 Bottum Edward W Refrigerant charged solar water heating structure and system
US4220138A (en) * 1978-01-24 1980-09-02 Bottum Edward W Refrigerant charged solar heating structure and system

Cited By (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
TWI579520B (zh) * 2013-08-22 2017-04-21 財團法人工業技術研究院 熱交換器、熱機循環系統及其控制方法

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AU7223881A (en) 1982-02-02

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