AU770892B2 - Water heater with vapour phase downward heat transfer - Google Patents

Description

Water heater with vapour phase downward heat transport.

Background to the invention.

The most common form of solar water heater comprises a flat plate collector coupled by thermosiphon heat transfer to a hot water store above the collector. While simple in operation, with no moving parts, this type of system has significant practical and economic disadvantages.

The principal disadvantage is that if the collector is roof mounted a large and heavy water store must be mounted above the collector on the roof. Typically the water store in this type of collector is 300 litres and the total mass of the collector plus water store may total 500 kilogram or half a tonne. The roof may have to be strengthened to support this mass at additional cost.

The bulk of the water store is significant and may be seen to detract from the appearance of the building.

Considerable effort has been directed towards finding means of transferring heat downwards from a roof mounted collector to a water store at ground level. The means to accomplish this usually involve controlled electric pumping of water from the collector to store. The provision of an electric pump and controller and •..*sensors is a significant additional cost and additional maintenance factor. Another possibility is the transport of heat downward in the vapour phase. A working fluid of suitable boiling 25 point is evaporated in the collector and the vapour allowed to flow down to a heat exchanger in a water store below the collector where the vapour condenses giving its latent heat to the water store. While downward heat transport by vapour phase to a store is relatively simple the recharge process by which condensed vapour is returned to the collector is much more complicated. A variety of methods to return the condensed vapour to the collector above have been developed. These include pumps, Bottum US 4,120,289 (1978) or involve a self pumping mechanism whereby the vapour pressure of the working fluid itself provides the pumping force. These self pumping systems are invariably complicated using a series of check valves and auxiliary liquid stores coupled to the collector or thermal store to provide the self pumping. An example of such a system A. Neeper, Solar Energy Vol 41, No 1, pp 91 99, 1988) is illustrated in Fig 1.

Other examples from prior art are Bohanon US 4,061,131 (1977)and Stacey US 4,357,932 (1982). The working fluid is chosen to have a boiling point intermediate between the operating temperature of the collector typically about 80°C and the desired temperature in the water store, typically about 600. The limited range of working fluids with boiling points in this range include hydrocarbons such as ethanol. The systems are sealed to prevent the escape of the working fluid. As the systems operate at pressures significantly different from atmospheric pressure care has to be exercised to prevent leakage of working fluid from the system or leakage of air into the system.

It is a first objective of the present invention to provide a method of vapour phase downward heat transfer with night time recharge which has no moving parts or valves and uses water as the working fluid and uses atmospheric pressure to provide the pumping force to recharge the collector.

The principle is illustrated in Fig 2. Solar radiation incident on tube A heats water in tube A to boiling point. The water vapour travels via insulated pipe B to condenser C in water store D. The condensing water vapour transfers the latent heat of vaporisation to the water store. The condensed water flows to receptacle E which is open to the atmosphere. Other than a fluid friction head due to vapour flow through pipe B and a small hydraulic head due to the water level in receptacle E the downward heat transfer by vapour phase occurs at essentially atmospheric pressure. Thus during transfer the collector tube A operates at very close to 100 0 C. At the end of the day when collector tube A has cooled down towards ambient temperature the vapour pressure in tube A and pipe B falls to below atmospheric pressure and the pressure difference between atmospheric pressure at the water surface in receptacle E and the water vapour pressure in collector tube A forces the condensate in E up pipe B to fully recharge A with water. More exactly, the height through which the system will recharge depends on the difference between the water vapour pressure at the condenser C and atmospheric pressure. In practice it is expected that during night time temperature stratification would result in the water temperature at the condensing coil C at the bottom of the water store not exceeding about 60 0 C. The water vapour pressure at 60 0 C is 150 mm of mercury. Thus the pressure head available to the system is 760 150 610 mm mercury equivalent to 8.3 metres or 27 feet of water. The system will not recharge if the height of pipe B exceeds this height. However the water at the bottom of the store will cool by vapour evaporation with vapour transfer to A, and condensation in A, until the temperature of the condensing coil C reaches ambient temperature, about 20°C at night time. At 20 0

C

the water vapour pressure is very small (17 mm of mercury) and the head available is 760 17 743 mm of mercury equivalent to about 10 m or 33 feet of. water. The roof of a typical Australian single level home is about 4 m above ground level and for a double level home about 7 m above ground level. Thus the available hydraulic head will accommodate most domestic dwellings.

The disadvantage of using water is the high boiling point, 100°C.

Thus the collector must be capable of operating efficiently at 100°C. With the advent of improved selective surfaces, low emissivity glazing, non imaging solar concentrators and convection suppressing covers it is becoming easier to manufacture stationary and quasi-tracking collectors that have high efficiency at 100 C.

The advantage of water as the working fluid is its ready availability, its low toxicity, its very high latent heat of vaporisation relative to other working fluids which means a relatively small mass of water is required to transfer a large amount of thermal energy. In addition its low vapour pressure at close to ambient temperature allows a water based system to be open to the atmosphere so providing atmospheric pressure as the recharge force over suitable heights for domestic use. Other systems in the prior art, for example Neeper, Solar Energy 41,1 S 30 91 99 (1988) and Stacey US 4,357,932 use hermetically sealed systems where the heat in the store is used to recharge the working liquid via a complicated set of valves (Neeper 1988) or via a fairly complicated water store (Stacey 1982).

A further advantage of water as the working fluid is the high 35 boiling point. This allows very efficient condensing transfer in the condenser in a water store. There are two useful consequences the condenser can be very simple just a short loop of copper pipe extending into the store about 20 cm; (2) heating of store water to high temperatures is possible. Most simple (unpressurised) electric water heaters store the water at Efficient energy transfer to the store at this temperature is possible with steam as the working vapour.

A typical household requires the heating of 200 litres of water from 20°C to 60*C during each day. This requires 33.6 MJ of heat energy. The latent heat in steam at 100°C is 2.27 MJ per kg. Thus, assuming no losses, the volume of water to be converted to steam in collector A and transferred via condenser C to receptacle E is 33.6/2.27 14 .8 kg of water. Thus the mass of water required in the collector system on the roof is about 20 kg. This is very much smaller than the water storage required for conventional thermosiphon heaters, about 300 kg.

The average direct solar intensity near noon in Australia is about 900 W per sq m. Thus, again assuming no losses, the energy available per square metre of collector per hour is 3.24 MJ. Over a 4 hour period 13 MJ is available. Thus, again assuming no losses, to obtain 33.6 MJ of heat energy during a four hour operating period approximately 3 sq m of collector area would be required.

If the entire 20 kg of water is thermally coupled closely to the absorbing area of the collector the response time of the system will be long. With the power input to a 3 sq metre collector area of 3 x 900 2700 W, the time to heat 20 kg of water from to 100°C is 2500 seconds or 40 minutes. During entirely clear days this will result in a small but significant loss of available ooo energy as no energy is transferred until the system reaches 100°C. However when the direct sunlight is intermittent, as on partly cloudy days, the predominant weather condition in Australia, a long delay time of the order 40 minutes will result in the collector system reaching 100°C very infrequently. Thus it is essential to thermally decouple the collector where evaporation 30 takes place from the 20 kg of water required at roof level to provide the heat transfer. In the prior art little consideration is given to the need to thermally decouple the solar collector from the roof top reservoir of working fluid. For example in Stacey US 4,357,932 (1982) the roof top reservoir is a cylinder along the top of the solar collector. The working liquid surface in contact with the vapour from the collector is large as is the volume of working fluid in the reservoir as the latent heat of vaporisation of the working fluids proposed is relatively small compared with that of water. Thus in the design proposed by Stacey most of the working fluid in the roof top reservoir will be heated to close to boiling point by condensation at the liquid surface and conduction before significant vapour transfer occurs. Thus the thermal response time of such a system is excessively long and the efficiency will be low except during entirely clear days.

It is a second objective of this invention to provide a method for achieving a short thermal response time in a solar water heater system operating with vapour phase downward heat transfer.

As the system is open to the atmosphere the volume of water contained in collector A and receptacle E will gradually decrease by evaporation from the surface of the water in container E. Thus the volume has to be made up at intervals by adding demineralised water to container E.

It is a third objective of this invention to provide a method of automatically replenishing the working fluid of a solar water heater with downward heat transfer when the working fluid is water and the vapour phase transfer line is open to the atmosphere.

Description of a solar water heater with vapour phase downward heat transport.

The solar water heater is illustrated in Fig 3. It comprises a roof 20 level collector system (the parts above the dotted line in Fig 3) which is coupled via an insulated transfer pipe 6 to a lower level heat storage system (the parts below the dotted line in Fig 3).

Solar radiation falls on the roof level collector which comprises a solar absorber 1 thermally connected to an array of boiler tubes 25 2 containing water. When the temperature of the water in the boiler tubes exceeds 100 degrees Centigrade steam forms in the boiler tubes and a mixed phase of water and steam is driven through the boiler tubes to a higher header tube 3. Steam separates from liquid water in the higher header 3 with the steam flowing to exit pipe 6 and the liquid water being returned via return pipe 4 to a lower header pipe 5 and to boiler tubes 2.

Steam exiting from higher header 3 via exit pipe 6 is driven down an insulated conduit 7 to a condenser coil 8 immersed in a lower insulated water store 9. The transferred steam condenses in condenser coil 8 transferring heat of vaporisation from the steam to the water in store 9. Condensed water flows from condenser coil 8 by gravity to water receptacle 10. Water receptacle 10 is open to the atmosphere via a small aperture 11. During times of high solar radiation steam generated in boiler tubes 2 flows via conduit 7 and condenser coil 8 to receptacle 10 thereby reducing the amount of water in the boiler tubes 2 and the return pipe 4. To maintain the water in the boiler tubes 2 at a level sufficient for efficient steam generation make up water is drawn by gravity from a high level water reservoir 12 via pipe 13. The water reservoir 12 is sized such that during the course of one day of full sunlight the quantity and level of water in reservoir 12 decreases by approximately one half as steam is generated and transferred. Typically, for a domestic size solar water heater, the volume of water required in high level water reservoir 12 at the start of the day is approximately 20 litres.

Towards the end of each day the temperature of the liquid in the roof level collector system falls towards ambient temperature.

As a consequence the water vapour pressure in the system falls below atmospheric pressure. The resulting pressure difference between atmospheric pressure at the liquid surface in receptacle 10 and the water vapour pressure in the roof level collector drives water from receptacle 10 via condenser 8 and conduit 7 into the roof level collector thereby recharging the roof level collector.

Heat may be extracted for domestic use from the lower heat store 9 by a heat exchanger coil 14 linked to the domestic hot water supply or by direct transfer if the lower water store 9 is a pressurised storage system.

The water condensate entering receptacle 10 from condenser coil S" 8 is generally above ambient temperature. As a result the total volume of water contained in the roof level collector and in receptacle 10 will gradually decrease over time due to evaporation from the liquid surface in receptacle 10 via the small aperture 11 to the atmosphere. The water volume may be replenished by manual addition of demineralised water to receptacle 10 when the level of condensate in receptacle 10 at night time falls below a specified level marked on receptacle The system as illustrated in Fig 3 and described above provides a system for vapour phase downward heat transport which has no moving parts or valves and therefore provides a method to obtain the first objective of this invention.

Absorber tubes 2, higher header 3, return pipe 4 and lower header comprise a fluid flow loop which contains only a small mass of water relative to the mass of water (approximately 20 kg) required at roof level for transfer of the required amount of energy per day. The absorber tubes 2 are thermally bonded to absorber plate 1 and carry a mixed water and steam phase when in operation. In experiments with a trial system the absorber tubes used had an small internal diameter of 3 mm. A total of 12 tubes each 1.8 m long were used to extract heat from an collector area of 3.9 square metres. The mass of water in the absorber tubes, higher header, return pipe, and lower header was about kg. Due to the relatively large specific heat of water (about times that of copper) this water dominates the thermal mass.

During the heat transfer operation this amount of water circulates rapidly around the absorber tube, header and return tube loop due to a geysering action in the boiler tubes as steam is generated. Heat transfer from the absorber tubes to the working fluid is high due to the rapid mixed phase circulation in the absorber tubes.

A second loop comprises the fluid flow loop as above, the high level water reservoir 12 and pipe 13. The high level water 2 5 reservoir is sized to contain about 20 kg of water. Using a 1.8 m •long pipe the internal diameter required is about 120 mm. The upper end of the water reservoir pipe is connected to the exit pipe 6 from the upper header 3 and the lower end of the water reservoir pipe 12 is connected via pipe 13 to the lower header pipe 5. Thus, when steam is being transferred via pipe 6 and pipe 7 the water surface in water reservoir pipe 12 is at 30 approximately 100* C and at close to atmospheric pressure. A steep thermal gradient is established in water reservoir pipe 1 2 with the upper water surface at 100C and the water at the lower S" end of the pipe at ambient temperature. Water reservoir pipe 12 is insulated and made from material of relatively low thermal conductivity such as galvanised iron to maintain a steep thermal gradient. As a consequence of the water surface in water ~reservoir pipe 12 being close to 100C the transfer of steam and heat from exit pipe 6 to water reservoir 12 is low during operation. By this method the roof level water reservoir is thermally decoupled from the absorber tube, return tube loop where heating and evaporation of water occurs. With an effective thermal mass of only 0.5 kg of water in the absorber tube/return tube loop the response time of the system is reduced in the ratio of approximately 0.5/20 40. Thus the thermal response time of the decoupled system as illustrated in Fig 3 is reduced to about 40/40 1 minute. In Fig 3 roof level water reservoir 12 is drawn directly below absorber tubes 2 and return pipe 4 for clarity of illustration. In practice water reservoir 12 lies parallel to and at the same level as absorber tubes 2 as illustrated in a plan view of the roof level collector in Fig 4. To avoid thermosiphon heat transfer from lower header 5 to water reservoir pipe 12 the lower end of connecting pipe 13 should extend about 20 cm below lower header The method of thermally decoupling the roof level water reservoir 12 from the absorber tubes 2 achieves the second objective of this invention of providing a system with a short thermal response time.

Fig 4 illustrates a variation of the invention in which the condenser pipe 8 enters at the top of the water store 9 and exits at the bottom. The advantage of this is that the water in water store 9 heats from the top down. This improves utilisation of stored energy as the drawoff from the top of water store 8 via heat exchanger 14 is from higher temperature water than the average temperature in the water store.

eeeoe Fig 5 illustrates a variation of the invention in which a second conduit 19 is added between the water in receptacle 10 and the roof top collector system. Valve means are included in conduit 19 to prevent the flow of steam from the roof top collector down conduit 19 to receptacle 10 and valve means are included at the bottom of conduit 7 to prevent the flow of water from receptacle 10 up conduit 7. As a result heat and steam transfer occur down :conduit 7 during the heating phase and recharge occurs up conduit "19 during the recharge phase. The advantage ids that the recharge height is now determined by the difference between vapour pressure in the roof top collector and atmospheric pressure 35 rather than vapour pressure in condenser 8 and atmospheric pressure.

Fig 6 illustrates a method of automatically replenishing the working fluid of a solar water heater with downward heat transfer when the working fluid is water and the vapour phase transfer conduit is open to the atmosphere. Water drawn from the domestic cold water supply passes through a demineralising water filter 16 via float valve 15 to maintain the level of demineralised water in receptacle 10. When the night time level of water in receptacle 10 falls below some specified level float valve 15 opens and demineralised water flows through filter 16 into receptacle until the water is at the specified level.

Fig 7 illustrates in a schematic view a preferred arrangement of the components of the roof level collector system in which the length of connecting pipe 6 is minimised by placing high level water reservoir between two sets of collector systems comprising absorber plates 1, boiler tubes 2 and lower header and upper header 3.

a o e m 00000*

Claims (6)

1. A passive, two stroke heating system, comprising a vaporiser connected to receive liquid working fluid from a lower header pipe and to deliver a mixed phase of working vapour and working liquid to an upper header pipe when solar radiation of sufficient intensity is incident on said vaporiser; a return pipe connected between the lower side of said upper header pipe and said lower header pipe to return working liquid gravitationally from said upper header pipe to said lower header pipe during boiling of working fluid in said vaporiser; a high level reservoir for storing working fluid primarily in the liquid state, the upper end of said high level reservoir being connected to the upper side of said upper header so as to provide unrestricted working vapour communication between said upper header pipe and said high level reservoir while gravitationally restricting transfer of working liquid from said upper header pipe to said high level reservoir, the lower end of said high level reservoir being connected to said lower header pipe via a looped conduit, said looped conduit extending below said high level reservoir and below said lower header pipe so as to deliver working liquid gravitationally to said vaporiser via said lower header pipe while preventing thermosiphon transfer of hot working fluid from said lower header pipe to said high level reservoir; a low level tank for containing a liquid mass to be heated, a condenser within said low level tank for heating said liquid mass by condensation of S-working fluid vapour therein; a receptacle to receive condensed working fluid flowing gravitationally from said condenser, said receptacle being open to the atmosphere; a conduit in 30 unrestricted fluid communication with said upper header pipe, said condenser and the working fluid in said receptacle whereby, during the heat delivery stroke, working fluid from said high level reservoir is vaporised in said vaporiser and delivered via said upper header, said conduit and said condenser to said receptacle and, during the recharge stroke, working liquid in said receptacle returns via said condenser and said conduit to refill said high level reservoir when the difference between the vapour pressure of the working fluid in said conduit and atmospheric pressure is I' sufficient to support the fluid hydraulic head between the working fluid surface in said receptacle and the top point of said conduit.

2. A heating system according to claim 1 which includes, in addition to said conduit of claim 1, now called the downward transfer conduit, a second conduit, called the upward transfer conduit, connecting said receptacle to said upper header pipe, said upward transfer conduit being provided with valve means to prevent working fluid vapour transfer from said upper header pipe via said upward transfer conduit to said receptacle and said downward transfer conduit being provided with valve means to prevent working liquid transfer from said receptacle to said upper header pipe via said downward transfer conduit.

3. A heating system according to claim 1 or claim 2 in which said vaporiser includes a plurality of tubes having restricted bores for the purpose of providing a geysering action to the working fluid therein, said tubes being connected between said lower header and said upper header.

4. A passive, two stroke heating system, comprising a solar 20 collector including a plurality of tubes in thermal connection with means for solar energy absorption, said tubes being S" connected in parallel with each other between a lower header pipe and an upper header pipe so as to obtain a working liquid from said lower header pipe and to deliver a mixed phase of working vapour and working liquid to said upper header pipe when solar radiation of sufficient intensity is incident on said solar collector; a return pipe connected between the lower side of said i-upper header pipe and said lower header pipe to return working liquid gravitationally from said upper header pipe to said lower 30 header pipe during boiling of working fluid in said solar collector; a high level reservoir for storing working fluid primarily in the liquid state, the upper end of said high level reservoir being connected to the upper side of said upper header so as to provide unrestricted working vapour communication between said upper header pipe and said high level reservoir while gravitationally restricting transfer of working liquid from said upper header pipe to said high level reservoir, the lower end of said high level reservoir being connected to said lower header pipe via a looped conduit, said looped conduit extending below said high level reservoir and below said lower header pipe so as to deliver working liquid gravitationally to said solar collector via said lower header pipe while preventing thermosiphon transfer of hot working fluid from said lower header pipe to said high level reservoir; a low level tank for containing a liquid mass to be heated, a condenser within said low level tank for heating said liquid mass by condensation of working fluid vapour therein; a receptacle to receive condensed working fluid flowing gravitationally from said condenser, said receptacle being open to the atmosphere; a conduit in unrestricted fluid communication with said upper header pipe, said condenser and the working fluid in said receptacle whereby, during the heat delivery stroke, working fluid flowing gravitationally from said high level reservoir is vaporised in said solar collector and delivered via said upper header, said conduit and said condenser to said receptacle and, during the recharge stroke, working liquid in said receptacle returns via said condenser and said conduit to refill said high level reservoir when the difference between the vapour pressure of the working fluid in said conduit and atmospheric pressure is sufficient to support the fluid hydraulic head between the working fluid surface in said receptacle and the top point of said conduit.

5. A heating system according to claims 1,2,3 and 4 in which the **working fluid is water. oo** woo•

6. A heating system according to claim 5 in which the level of working fluid in said receptacle is automatically maintained above a specified level by means of make up water drawn from the mains water supply via a demineralising water filter, the inflow of said make up water being controlled by a float valve positioned inside said receptacle. t ,j rf I -zoo 4*4*4 R~q ~ueernslc~xW UoIisij db rcho, -p 113 4 F~ \~pLti