US20160061534A1

US20160061534A1 - Latent Thermal Energy System (LTES) Bubbling Tank System

Info

Publication number: US20160061534A1
Application number: US14/828,829
Authority: US
Inventors: Peter B. Choi
Original assignee: Individual
Current assignee: Individual
Priority date: 2014-08-27
Filing date: 2015-08-18
Publication date: 2016-03-03

Abstract

For a latent thermal energy storage (LTES) system comprising phase change material (PCM) slurry, it is problematic to recover thermal energy by crystallizing out solid components from slurry mixtures. It is because the solidifying components form a solid layer of low thermal conductivity on the heat transfer surfaces making heat transfer inefficient. This invention allows an effective thermal energy recovery readily achievable by using gaseous or liquid bubbles of immiscible heat transfer fluid (HTF) in close contact with the phase change material (PCM) slurry mixtures. The circulating immiscible HTF, free of solidifying components, is used for releasing thermal energy through cold heat transfer surfaces to the heat users. A process comprising a multi-chamber LTES system has been devised for applications in the concentrated solar power (CSP) plants using a PCM binary slurry of Li₂CO₃and Na₂CO₃as a heat storage medium and CO₂gas as an immiscible HTF.

Description

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to Provisional Application No. 62/042,631, filed Aug. 27, 2014, which is incorporated by reference.

FIELD OF INVENTION

The present invention is related to the storage and release of thermal energy by using a latent thermal energy storage (LTES) containing a PCM slurry mixture, the stored thermal energy being used for generation of electricity or supplying it to consumers at a later time.

BACKGROUND OF THE INVENTION

Latent thermal energy storage (LTES) is one of the options to store thermal energy available from intermittent heat sources and release it later for electricity generators or heat users. The storage and release of thermal energy is possible because the phase change material (PCM) stores heat by melting in the form of latent heat and release it by freezing.
LTES has not yet been fully utilized in the industry, however, due to the difficulties encountered in practical applications. Some of the difficulties are: (i) low thermal conductivity of PCM; (ii) the formation of a solid layer of PCM on cold heat transfer surfaces; (iii) volume changes of fusion; (iv) sub-cooling of PCM liquid; (v) stratification of PCM solid layers; and (vi) corrosion by PCM. This invention overcomes most of these difficulties as explained in the sections of brief summary. However, items (ii) and (iv) will be discussed in detail in the following sections with examples, as they need further clarification due to their significance in practice.
The difficulty arising from forming solid layer of PCM on cold heat transfer surfaces was experienced with the PCM slurry system suggested in Pub. No. US 2010/0230075 A1 (Thermal Storage System). The system was tested as described in Report No. DOE-GO18148 for U.S. Department of Energy (Heat Transfer and Latent Heat Storage in Inorganic Molten Salts for Concentrating Solar Power Plants (2012)). In the test, it was found that the inorganic eutectic solution formed layers of PCM solid on the cold heat transfer surfaces in a heat exchanger when attempting to recover thermal energy from the PCM slurry by circulating the slurry through the equipment. Since the PCM solid layers had very low thermal conductivity, they obstructed heat transfer on the cold surfaces causing operation of the system inefficient. These test results made a conclusion that this difficulty was the bottleneck not only for the system suggested in the particular patent application but for the efficient use of PCM systems in general for the purpose of heat storage.
The difficulty arising from the sub-cooling of PCM liquid was manifested with the system suggested in U.S. Pat. No. 7,096,929 B2 (PCM System and Method for Shifting Peak Electrical Load). The system was tested as described in Report No. CEC-500-2006-026 for California Energy Commission (Phase Change Material Slurry System to Decrease Peak Air Conditioning Loads (2006)). In the test, employed was a slurry system comprising an organic PCM hexadecane having melting temperature of 18° C., water, and a surfactant. The system of the patent was expected to save electricity for air conditioning by storing cold energy during off-peak hours with the organic PCM solidifying at its melting temperature. It was further planned to store cold energy at that temperature level by utilizing geothermal energy as a cold energy source; depending on latitude, geothermal energy is available at constant temperature between 10° C. to 16° C. about 6 meters beneath the earth's surface. The test found that the PCM liquid sub-cooled and started to solidify at 9° C. to 12° C. well below the melting temperature of 18° C. of the PCM employed. Since the cold energy could not be recovered as planned, it was eventually concluded that the system of the patent should be used after an effective method was found to prevent sub-cooling of the PCM liquid.
The above two examples are just a few records about the difficulties experienced in the industry. The items of the difficulties listed above are all very important, because any single one can jeopardize the whole TES system. This invention provides the means to resolve most of these difficulties, and helps this LTES technology readily available for the energy industry.

BRIEF SUMMARY OF THE INVENTION

The LTES bubbling tank of this invention contains both a PCM slurry mixture, which comprises the crystals of solidifying components and the liquid of molten solid components, and an immiscible heat transfer fluid (HTF). For the immiscible HTF, a media either in liquid or gaseous phase can be used. The PCM slurry mixture is well mixed in a slurry tank in close contact with the immiscible HTF bubbles achieving high heat transfer between the PCM solids, PCM liquid, and immiscible HTF. Mixing is achieved due to the convective movements of the rising gaseous or liquid bubbles that are caused by the buoyant forces from the density differences.
An example of such PCM slurry is a eutectic mixture of Na₂SO₄—H₂O illustrated in FIG. 3. The binary system develops eutectic composition of 4.19 wt. % of Na₂SO₄at melting temperature of −1.27° C. The system makes liquidus line from the eutectic point to the point of 33.2 wt. % of Na₂SO₄at the temperature of 32.4° C. and solidus line from the eutectic point to the point of 44.1 wt. % of Na₂SO₄at −1.27° C. In the region between the liquidus and solidus lines, the Na₂SO₄brine phase is in equilibrium with the solid phase of Na₂SO₄.10H₂O.
Another example of such PCM slurry is a eutectic mixture of Li₂CO₃—Na₂CO₃illustrated in FIG. 4 where a eutectic composition develops with 44.3 wt. % of Li₂CO₃(55.7 wt. % of Na₂CO₃) at melting temperature of 495.8° C. This eutectic mixture exhibits liquidus and solidus lines with 100 wt. % of Li₂CO₃which melts at 723° C. and with 100 wt. % of Na₂CO₃which melts at 858° C.
Low thermal conductivity of PCM, the first difficulty listed above, poses problems where the PCM layer contacts heat transfer surfaces either when the bulk PCM is encapsulated in containers as solid balls or packed in the shell side of a heat exchanger as a solid mass. In the LTES of this invention, intimate mixing is achieved by rising immiscible HTF gaseous or liquid bubbles obviating such heat transfer surfaces. Therefore, very effective heat transfer can be achieved for the solid crystals in the slurry layer with the immiscible gaseous or liquid bubbles. The direct contact heat transfer in high degree mixing is usually the best option in heating or cooling services.
Forming a solid layer of PCM of low conductivity on cold heat transfer surfaces, the second difficulty listed above, is a problem when the liquid in the slurry mixture contacts the cold heat transfer surfaces while latent heat is released with the solid components crystallizing out. This solid layer of low thermal conductivity impedes heat transfer. In the LTES of this invention, however, withdrawal of thermal energy takes place in the slurry layer in intimate contact with the gaseous or liquid bubbles of HTF but not with cold surfaces. Also, the immiscible HTF, free of solidifying components, supplies thermal energy through the cold heat transfer surfaces to the heat users. Therefore, the problem of solid layer formation on the heat transfer surfaces is irrelevant to the system of this invention.
Volume change of fusion, the third difficulty listed above, causes a problem in the design of PCM heat storages. Normally, a volume increase of about 10 to 30% is expected during melting of inorganic PCM solids in use at service temperatures above 200° C. Localized fusion in a pocket of PCM solid mass, for example, causes an explosion due to an excessive pressure buildup. In the LTES of this invention, the heat storage tank inherently has an enough cushion to accommodate such volume changes, because the tank is provided with appreciable volume of headspace filled with an inert gas in either case when the system uses immiscible gaseous or liquid HTF.
Sub-cooling of PCM liquid, the fourth problem in the list, takes place mainly when no crystal seeds are available for the PCM solution. The sub-cooling causes thermal efficiency of the PCM heat storage system to deteriorate. In the LTES of this invention, the crystal seeds are always available in the slurry mixture as far as the system temperature is maintained below the liquidus. Especially, the slurry layer is maintained at an even temperature because the slurry is always intimately mixed by the convective movements of the liquid or gaseous HTF bubbles. This makes a close temperature control possible for the whole slurry system in the tank. Since the availability of crystal seeds is so important for successful operation of the PCM heat storages, a method to control the heat addition and withdrawal for the set point of the amount of PCM crystals in slurry is devised in an embodiment of this invention.
Stratification of PCM layers, the fifth difficulty in the list, can develop during the numerous cycling of melting and solidification processes of PCM, especially when the tank comprising the PCM solid mass is placed at a fixed location with no mixing. While the PCM storage is in use for a long time, the PCM molecules may have undergone undesirable chemical reactions becoming heavier, and sink to the bottom. The layer may also comprise the PCM solids having had the changes of physical properties such as melting temperatures. In this case, it will affect the performance of the whole heat storage system. In the LTES of this invention, however, stratification will not take place because the slurry layer is always intimately mixed.
Corrosion by PCM, the last difficulty in the list, is a problem that could lead to severe accidents with loss of life and assets. A large amount of information about corrosion has accumulated during the last century especially in the nuclear and petrochemical industries. Generally speaking, as the design of the system becomes more complex, higher is the probability of serious problems. The LTES system of this invention requires equipment units that are simple to design with an abundance of construction experiences. For example, the corrosive slurry stream is heated in a double-pipe heat exchanger of a simple design, while the non-corrosive immiscible HTF stream is cooled instead in a shell and tube heat exchanger of a complex design. Therefore, the system of this invention can be built with the highest level of safety and the minimum investment to properly handle such corrosion problems.
As has been explained above, the LTES system of this invention resolves most problems having been encountered with the PCM heat storage systems being used or under research in the energy industry. As the system of this invention can be readily scaled-up and expanded to larger capacities, it has a great potential to be used as a practical solution for thermal energy storage. This is a great first step to the solution for the problems of global warming, as the technology will help cut down on fossil fuels by utilizing renewable energy sources such as solar energy more efficiently.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing summary, as well as the following detailed description of presently preferred embodiments of the invention, will be better understood when read in conjunction with the appended drawings. For the purpose of illustrating the invention, there are shown in the drawings embodiments which are presently preferred. It should be understood, however, that the invention is not limited to the precise arrangements and instrumentalities shown.

FIG. 1 is a schematic flow chart of an embodiment of an LTES system of the present invention with immiscible liquid HTF used for generating liquid bubbles for mixing and heat transfer;

FIG. 2 is a schematic flow chart of an embodiment of an LTES system of the present invention with immiscible gaseous HTF used for generating gas bubbles for mixing and heat transfer;

FIG. 3 is a phase diagram of a Na₂SO₄—H₂O system of the prior art;

FIG. 4 is a phase diagram of a Li₂CO₃—Na₂CO₃system of the prior art;

FIG. 5 is a flowchart of an embodiment of a method of the present invention for controlling heat addition and withdrawal for a set point of slurry temperature while the solids content in phase equilibrium at the set point temperature is reported;

FIG. 6 is a flowchart of an embodiment of a method of the present invention for controlling heat addition and withdrawal for a set point of solids content while the slurry temperature in phase equilibrium at the set point of solids content is reported and used to control the heat transfer process as a control parameter;

FIG. 7 is a schematic flow chart of an embodiment of an LTES tank of the present invention comprising a draft tube with an immiscible gaseous HTF used for generating bubbles;

FIG. 8A is a schematic flow chart of an embodiment of a process of the present invention for a multi-chamber LTES system for charging and recovery of the heat from a power tower using CO₂gas as a HTF;

FIG. 8B is a schematic diagram of a physical configuration for a multi-chamber LTES system of FIG. 8A having partitioned compartment chambers separated with plain walls for low operating pressures; and

FIG. 8C is a schematic diagram of a physical configuration for a multi-chamber LTES system of FIG. 8A having cylindrical tank chambers for high operating pressures.

DETAILED DESCRIPTION OF THE INVENTION

Certain terminology is used in the following description for convenience only and is not limiting. The article “a” is intended to include one or more items, and where only one item is intended the term “one” or similar language is used. Additionally, to assist in the description of the present invention, words such as top, bottom, upper, lower, front, rear, inner, outer, right and left are used to describe the accompanying figures. The terminology includes the words above specifically mentioned, derivatives thereof, and words of similar import.
FIG. 1 illustrates an embodiment of an LTES system 100 which comprises a PCM slurry tank 101, cold liquid bubble nozzle 107, hot liquid bubble nozzle 109, heat recovery exchanger 120, cold liquid circulation pump 124, hot liquid circulation pump 143, and heat supply exchanger 140. Thermal energy from the heat sources is transferred to tank 101 by circulating immiscible HTF stream 141 as a heat transfer medium using pump 143, which is in turn heated in the heat exchanger 140 by HTF stream 144 supplied from the heat sources. This circulating hot HTF liquid stream 154 is also used to make liquid bubbles by hot liquid bubble nozzle 109. Another immiscible HTF stream 122, free of solidifying components, is cooled in heat exchanger 120 releasing sensible heat to HTF stream 126 for heat users. Pump 124 circulates liquid stream 122, and returning cold liquid stream 123 is used to generate liquid bubbles by cold liquid bubble nozzle 107. Head space 104 is filled with an inert gas, when the vapor pressure of the components in the tank is lower than the atmospheric pressure.
Still referring to FIG. 1, a swarm of immiscible liquid bubbles generated by hot liquid bubble nozzle 109 promotes mixing for slurry layer 102, while they rise to immiscible liquid layer 103. The liquid bubbles rise through slurry layer 102 owing to the buoyant forces caused by the density differences between the slurry and the immiscible liquid. The rising bubbles achieve effective heat transfer between the slurry and the liquid bubbles. In order for the thermal energy from the heat sources is charged into the tank, circulating immiscible liquid stream 141 is heated in heat exchanger 140 first and then the bubbles of heated stream 154 transfers the heat to the slurry layer in the tank. As a result, the crystals in the slurry layer are heated and melt.
Another swarm of immiscible liquid droplets are generated by cold liquid bubble nozzle 107 using the returning immiscible liquid stream 123. This immiscible liquid stream 122 transfers thermal energy to the heat users in heat exchanger 120, and then circulates back to cold liquid bubble nozzle 107 in the tank. While the liquid droplets rise through the slurry layer, they take heat from the slurry causing the PCM components to crystallize out. This is the process to recover the stored energy from the PCM solution. Since the heat transfer takes place in the slurry layer in direct contact with the immiscible liquid bubbles, a high heat transfer rate can be achieved with no barrier of solid deposits.
As the slurry layer is maintained well mixed by rising immiscible liquid bubbles, the temperature will be kept even in the slurry layer throughout the tank. The even temperature makes an accurate temperature control possible. With a slurry system in phase equilibrium, using the phase equilibrium theory, it can be made that the operating temperature can tell the content of solids, whereas, with a pure PCM solid mass, the operating temperature cannot tell the solids content but it does only whether the PCM mass is completely melted or solidified. Therefore, with a slurry system, crystal seeds can be made available always by controlling the operating temperature. For a binary slurry system, for example, the corresponding solids content can be shown alongside the operating temperature on the control panel for more accurate control, once the overall concentration of the initial charge is known. Or, the solids content in the slurry can be used directly to control the process. This operating method can be explained by the phase rule:
F=n+2−P
where F stands for the number of freedom in intrinsic properties, n number of components, and P number of phases. For a eutectic binary mixture such as shown in FIG. 4 in a region between liquidus and solidus lines, for example, n=2 for a binary mixture, P=3 for the sum of the one solid state for pure crystal, the one liquid state for the solution and the one vapor state, and then F=2+2−3=1. Therefore, a unique freedom in intrinsic property determines the state of the binary system; the slurry temperature the most sensitive to the operation of the process is chosen for the unique freedom in this invention. Setting system temperature then determines the concentrations of the liquid and solid phases. When the concentration of the initial charge is given, material balance equations are solved to find the amount of crystals in the slurry. The lever rule, an expression from the solution of the material balance equations, is an alternative method that can be directly used to find the ratio of the amounts of liquid and solid phases in lieu of the material balance calculations. Since the availability of crystal seeds is so important for successful operation of the PCM heat storages, a method is devised in this invention to ensure that a set amount of solid crystals is available as seeds in the slurry layer by controlling the heat addition and withdrawal. This control method can be further generalized for multi-component systems as explained below.
For generalization of the method to control heat addition and withdrawal for the content of PCM solids in a slurry, a system comprising n components and i phases is considered. The steps to solve the material balance equations are illustrated to find the corresponding solids content at an operating temperature. The concentrations of components 1 to n for the initial charge f and for the phases 1 to i can be expressed as follows:
Concentration of Initial Charge: X _f1 ,X _f2 ,X _f3 ,X _f4 , . . . ,X _fn
Concentration of Phase 1: X ₁₁ ,X ₁₂ ,X ₁₃ ,X ₁₄ , . . . ,X _1n
Concentration of Phase 2: X ₂₁ ,X ₂₂ ,X ₂₃ ,X ₂₄ , . . . ,X _2n
Concentration of Phase 3: X ₃₁ ,X ₃₂ ,X ₃₃ ,X ₃₄ , . . . ,X _3n
Concentration of Phase i: X _i1 ,X _i2 ,X _i3 ,X _i4 , . . . ,X _in
The phase equilibrium data provide information on the concentrations of all phases in phase equilibrium on temperature. Also, the amount W_fand the composition X_f1, X_f2, . . . , of the initial charge must be also given from the operator. Then, the material balance equations can be written as below for components 1 to n in the phases 1 to i with the unknown amounts of all phases W₁, W₂, W₃, . . . W_i.
Component 1: W ₁ ×X ₁₁ +W ₂ ×X ₂₁ +W ₃ λX ₃₁ + . . . W _i ×X _i1 =W _f ×X _f1
Component 2: W ₁ ×X ₁₂ +W ₂ ×X ₂₂ +W ₃ λX ₃₂ + . . . W _i ×X _i2 =W _f ×X _f2
Component 3: W ₁ ×X ₁₃ +W ₂ ×X ₂₃ +W ₃ λX ₃₃ + . . . W _i ×X _i3 =W _f ×X _f3
Component n: W ₁ ×X _1n +W ₂ ×X _2n +W ₃ λX _3n + . . . W _i ×X _in =W _f ×X _fn
However, the number of phases for the slurry system P′ is equal to the number of components n. That is because the phase rule must yield the freedom F=1 for the slurry systems of this invention as explained above and the number of total phases P=P′+1 where P′ stands for the number of phases in the slurry layer and 1 for the vapor phase. Therefore, the phase rule can be written as 1=n+2−(P′+1), which yields n=P′.
The existence of an immiscible HTF in the tank as a heating and cooling medium is not taken account in this derivation, as the immiscible HTF plays negligible effects on the phase equilibrium behavior of the solid and liquid phases in the PCM slurry layer in the operating conditions of the LTES systems employed. If it does, such HTF can be no longer identified as immiscible.
As W_fis given, this leads to the number of unknowns to be P′ for W₁to W_p′with the same number of independent equations n. The n independent material balance equations can then be solved for the identical number of unknowns W₁to W_p′ as shown below.
Component 1: W ₁ ×X ₁₁ +W ₂ ×X ₂₁ +W ₃ ×X ₃₁ + . . . W _p′ ×X _p′1 =W _f ×X _f1
Component 2: W ₁ ×X ₁₂ +W ₂ ×X ₂₂ +W ₃ ×X ₃₂ + . . . W _p′ ×X _p′2 =W _f ×X _f2
Component 3: W ₁ ×X ₁₃ +W ₂ ×X ₂₃ +W ₃ ×X ₃₃ + . . . W _p′ ×X _p′3 =W _f ×X _f3
Component n: W ₁ ×X _1n +W ₂ ×X _2n +W ₃ ×X _3n + . . . W _p′ ×X _p′n =W _f ×X _fn
Summing up the amounts of all solid phases gives the total amount of solids, and then subsequently the content of solids in the slurry. For further clarification, examples are given below for a binary and a ternary system.

Example 1

A binary system

- Given: X_f1, X_f2, and W_f(given from operator)
  - X₁₁, X₁₂, X₂₁, and X₂₂(given from phase equilibrium data at an operating temperature)
- Unknowns: W₁, and W₂
- Two equations can be solved for the two unknowns:

Component 1: W ₁ ×X ₁₁ +W ₂ ×X ₂₁ =W _f ×X _f1
Component 2: W ₁ ×X ₁₂ +W ₂ ×X ₂₂ =W _f ×X _f2

Example 2

A ternary system

- Given: X_f1, X_f2, X_f3, and W_f(given from operator)
  - X₁₁, X₁₂, X₁₃, X₂₁, X₂₂, X₂₃, X₃₁, X₃₂, and X₃₃(given from
- phase equilibrium data at an operating temperature)
- Unknowns: W₁, W₂, and W₃
- Three equations can be solved for the three unknowns:

Component 1: W ₁ ×X ₁₁ +W ₂ ×X ₂₁ +W ₃ ×X ₃₁ =W _f ×X _f1
Component 2: W ₁ ×X ₁₂ +W ₂ ×X ₂₂ +W ₃ ×X ₃₂ =W _f ×X _f2
Component 1: W ₁ ×X ₁₃ +W ₂ ×X ₂₃ +W ₃ ×X ₃₃ =W _f ×X _f3
Therefore, in case where the desired solids content in the slurry at the end of the heat recovery process is given, the slurry temperature to yield the solids content can be calculated indicating when to stop the cooling in the bubbling process for heat discharge. In the heat charging process, on the other hand, it is very important to keep some crystal seeds left available in the slurry layer. The seed crystals will prevent sub-cooling in the subsequent heat recovery process. For this purpose, heating must be stopped right below the liquidus temperature of the slurry system of an initial overall concentration W_f1, W_f2, . . . , and W_fn.
There are two control methods for the heating and cooling processes to achieve this goal: Method A to control heat addition and withdrawal for a set point of slurry temperature while the solids content in equilibrium at the set point temperature reported; and Method B to control heat addition and withdrawal for a set point of solids content directly as a control parameter while the slurry temperature in equilibrium at the set point of solids content is reported and used to control the process.
Method A is a control process comprising the steps illustrated in FIG. 5. Firstly, it must be assured that the PCM slurry system has a unique freedom by the phase rule so that only operating temperature can determine the state of the slurry system. Secondly, the set point of the slurry temperature must be decided. Thirdly, the phase equilibrium data for the compositions of all phases on temperature must be obtained as well as the overall amount and composition of the initial charge. Fourthly, the same number of material balance equations as for the phases in the slurry layer are solved. Fifthly, the amounts of all solid phases are added up to get the total amount of solids and subsequently the solids content in the slurry. Sixthly, the solid content is reported on the control panel that is in equilibrium at the set point temperature. Finally, the heat addition and withdrawal is controlled for the set point temperature along with the solids content in equilibrium at the temperature shown on the control panel.
Another control option Method B comprises the steps illustrated in FIG. 6. The method is a control process for heat addition and withdrawal for a set point of solids content while the slurry temperature in equilibrium at the set point of solids content is reported and used to control the process as a control parameter. Firstly, it must be ensured that the PCM slurry system has a unique freedom by the phase rule, and the slurry temperature is chosen as the intrinsic property for the unique freedom. Secondly, the set point of solids content in the slurry is decided. Thirdly, the phase equilibrium data for the compositions on temperature in all phases must be available as well as the amount and composition of the initial charge. Fourthly, by an iteration calculation, find the slurry temperature at which the set point solids content is in equilibrium. Fifthly, report this slurry temperature on the control panel. Lastly, this slurry temperature is used to control the heat addition and withdrawal for the process.
Another option to obtain the solids content at an operating temperature is to use the lever rule. As illustrated in the phase diagram of FIG. 4, with the concentration of 14 wt. % of Li₂CO₃in the initial charge as indicated by point 2014, the solids content in the slurry changes vertically following temperature 2100. At the temperature of 640° C., for example, the horizontal temperature line intersects the vertical concentration line at point S that is the thermodynamic state of the binary slurry system under consideration. The phase diagram indicates that the state S of the binary system is comprised of solid phase A and liquid phase B. The lever rule tells that the amount of the solid phase W_a, of the liquid phase W_b, and the total amount W_sof the initial charge are related to the distances between the three points AB, AS, and SB as follows:
W _a /W _s =SB/AB
W _b /W _s =AS/AB
W _a /W _b =SB/AS
The three equations above can be derived from the first material balance equation for the binary system in the previous example with W₁substituted by W_a, W₂by W_b, W_fby W_s, AB by (X₂₁−X₁₁), SB by (X₂₁−X_f1), and AS by (X_f1−X₁₁), respectively, and a constraint equation of (W_s=W_a+W_b) instead of the second material balance equation.
Therefore, once the phase diagram of the binary system and the compositions of the initial charge are given, the ratio of the amounts of the solid and liquid phases can be found and subsequently the solids content of the slurry system at an operating temperature. Accordingly, when the amount of the initial charge Ws is given, the amounts of the solid and liquid phases W_aand W_bcan be readily obtained. The lever rule is widely used for calculation of binary systems where the phase diagrams are simpler compared to those of multi-component systems.
An example of the slurry system for the embodiment in FIG. 1 is the binary mixture Na₂SO₄—H₂O with a mineral oil used as the immiscible liquid HTF. The Na₂SO₄—H₂binary system develops a eutectic composition 1001 at 4.19 wt % of Na₂SO₄and the temperature of −1.27° C., as shown in the phase diagram of FIG. 3. The initial condition for this example is chosen with the solution having the liquidus temperature of 30° C. that has a concentration of 29.1 wt % of Na₂SO₄. Now, the thermodynamic state of the system moves vertically down with the fixed overall concentration of 29.1 wt % of Na₂SO₄on concentration axis 1200 following temperature 1100. When the slurry mixture is cooled to 22.5° C., the system crystallizes out 40 wt. % (35 vol. %) of the initial charge. Due to the crystallization of the PCM component Na₂SO₄.10H₂O, the volumetric heat capacity of the slurry mixture increases by a factor of 4. During this time of heat recovery, the slurry layer is mixed with the immiscible mineral oil bubbles, and the oil HTF is heated in the slurry layer while the PCM crystallizes out. The oil HTF is re-circulated through heat recovery exchanger 120 where it releases thermal energy to heat users. When the slurry layer is heated for storing heat, on the other hand, the immiscible mineral oil HTF is re-circulated through heat supply exchanger 140 where thermal energy is transferred to oil HTF stream 154 from heat source HTF 144 and the PCM slurry in the tank is heated in turn by the bubbles of heated returning oil HTF 154. The system is working between the temperatures of 30° C. and 22.5° C. with the initial charge of an overall concentration of 29.1 wt. % of Na₂SO₄. As an example for an application to a building, the heat can be charged during daytime from solar collectors until the system temperature reaches 30° C., and this stored heat then be used during the night until the slurry temperature decreases to 22.5° C. The system is only charged with heat until its temperature reaches right below the liquidus so that the un-dissolved crystals can be used as seeds in the subsequent heat discharging step without sub-cooling.
In another embodiment of this invention, FIG. 2 illustrates LTES system 200 that uses an inert gas as immiscible HTF rather than a mineral oil. The system is comprised of slurry tank 201, cold gas bubbling nozzle 207, hot gas bubbling nozzle 209, heat recovery exchanger 220, cold gas blower 224, heat supply exchanger 240, and hot gas blower 243. Slurry tank 201 comprises slurry layer 202 and gas layer 204. Hot gaseous HTF stream 222, free of solidifying components, circulates through heat recovery exchanger 220 where the HTF is cooled while thermal energy in sensible heat is transferred to HTF stream 226 for heat users, and a cooled stream 223 returns to cold gas bubbling nozzle 207 in slurry tank 201 where gas bubbles are generated. The gas bubbles rise through the slurry layer inducing mixing to improve heat transfer. While the gas bubbles are heated in tank 201, the PCM components in the liquid phase crystallize out from the solution. For storing heat, gas stream 241 combined with a cool gas stream from other equipment circulates through heat supply exchanger 240 while being heated by HTF stream 244 from the heat sources, and a heated stream 254 returns to hot gas bubbling nozzle 209 in tank 201 to generate hot gas bubbles.
Circulating gaseous HTF streams 222 and 241 must be inert to the components of the slurry mixture, remain stable, and have as good heat transfer capability as possible in the service conditions. The CO₂gas is one of the candidates to be used along with a binary PCM slurry Li₂CO₃—Na₂CO₃at service temperatures above 600° C. The gases such as He and CO₂have been used as a coolant in nuclear power plants where service temperatures approach near 700° C. and pressures higher than 30 bar. However, unlike the nuclear reactor, the solar power tower receiver as a heat source cannot be used for both high temperatures and pressures; as the latest design, a power tower volumetric receiver can heat air stream up to above 1000° C. at pressures less than several bar and a cavity type tubular receiver heat air up to above 800° C. at pressures less than 20 bar. In thermal connection with the solar collector, a supercritical steam Rankine cycle or a supercritical CO₂Brayton cycle can generate electricity utilizing such thermal energy in a separate system at operating temperatures above 600° C. with cycle efficiencies near 50%. The major difficulties with a gaseous HTF for such applications, however, are its low heat capacity unable to provide a cushion for the variations of solar irradiation such as what is caused by clouds and the lack of a suitable heat storage medium that can be used in a dispatchable heat reservoir for storage temperatures above 600° C. The Li₂CO₃—Na₂CO₃system along with CO₂gas as an immiscible HTF resolves those difficulties, as the inorganic compounds are thermally very stable in CO₂atmosphere and also much research has been done on their corrosion characteristics at high temperatures. For example, some high nickel alloys are very promising candidates for this application, but their costs are mostly several times higher than stainless steel. However, employing a gaseous HTF such as CO₂gas obviates the necessity of the means to prevent freezing of the molten salt liquid and also of the high installation cost in case when the highly corrosive molten salts such as chloride and carbonate compounds are used as a HTF in the process. The embodiment of this invention is the first step to resolve these problems.
In an effort to improve the heat transfer efficiency with system 200 in FIG. 2, when the stored thermal energy is recovered, heat of vaporization can be utilized rather than sensible heat that was used in the process of the previous section. In that case, an immiscible HTF liquid is vaporized in the slurry layer 202, the vapor is condensed in heat recovery exchanger 220, and the condensate returns to the slurry tank 201. This alternative process is very advantageous in that the heat of vaporization can be utilized to reduce the mass flow rates of the HTF stream and also the heat transfer coefficients of the condensing vapor are usually much higher than those of a dry gas, which can reduce significantly the heat transfer area required for a given duty. In addition, pumping a condensate liquid is much easier and less costly than circulating a gas stream using a compressor. The immiscible HTF for such services is mostly organic compounds whose maximum service temperature is limited to 400° C. due to their thermal instability above the limit. For service temperatures above 400° C., there is no immiscible HTF that can be used with the molten inorganic salts other than simple inorganic gases such as He, CO₂, N₂, Ar or Air. With a gas as a HTF, therefore, a large temperature difference must be allowed to reduce the heat transfer area in order to compensate for the low heat transfer coefficients. Also, very high volumetric flow rates are necessary when a gas is used as a heat carrier utilizing only sensible heat compared to the case where heat of vaporization is used.
As explained above, an example of the slurry system that can be utilized for the embodiment in FIG. 2 is the binary eutectic system Li₂CO₃—Na₂CO₃with the CO₂gas used as an immiscible gaseous HTF. The phase diagram of the binary eutectic system is illustrated in FIG. 4. The eutectic point 2001 occurs at 44.3 wt. % of Li₂CO₃at the temperature of 495.8° C. The liquidus line in the Na₂CO₃rich side develops between the eutectic point and the point of 0% of Li₂CO₃and 858° C. the melting temperature of Na₂CO₃, and the liquidus line in the Li₂CO₃rich side between the eutectic point and the point of 100% of Li₂CO₃and 723° C. the melting temperature of Li₂CO₃. To illustrate the operation of the embodiment system, an initial condition is chosen at the liquidus temperature of 750° C. that has a concentration of 14 wt. % of Li₂CO₃(86 wt. % of Na₂CO₃). The thermodynamic state of the system moves down vertically, while discharging the stored thermal energy, with a fixed composition of 14 wt. % of Li₂CO₃on composition axis 2200 following temperature 2100. For discharging heat from the slurry by solidifying 50 wt % of initial charge, the system must be cooled to point S at 640° C. where the system comprises 50 wt. % (47 vol. %) of the initial charge as pure Na₂CO₃solid as indicated by point A and the remaining 50 wt. % as liquid as indicated by point B. For charging heat into the system, on the other hand, the slurry of a solids content of 50 wt. % is heated up to the temperature right below the liquidus temperature of 750° C. By using the LTES system in FIG. 2, thermal energy can be saved and then released by repeating this procedure. The process of this example can be used for the storage and release of thermal energy from the solar power tower, converting an energy form of low volumetric specific heat into a very stable dispachable one of high specific heat.
An embodiment in FIG. 7 shows a LTES bubbling tank with a draft tube. System 300 comprises slurry tank 301, PCM slurry layer 302, draft tube 303, heat recovery exchanger 320, cold gas blower 324, heat supply exchanger 340, and pump 343. For releasing the stored heat, gaseous stream 322, free of solidifying components, circulates through heat recovery exchanger 320 transferring the stored heat to HTF stream 326 for heat users. The cooled gaseous stream 323 returns to the tank and is introduced into the gas nozzle 307 at the bottom section of the draft tube in order to induce a vertical circulating movement of flow by the buoyant forces of gas bubbles. For storing heat, on the other hand, slurry stream 341 is circulated by pump 343 through heat supply exchanger 340 being heated by HTF stream 344 from heat sources, and then the heated stream 354 returns to the tank. The slurry in the tank circulates around the draft tube, while the gas bubbles are heated for heat recovery or cooled for heat storage in contact with the slurry flow and collect in headspace 304.
Another embodiment in FIG. 8A shows a multi-chamber LTES system for charging and discharging heat with a power tower using CO₂gas as a HTF. In FIG. 8B, a physical configuration is shown for the LTES system of FIG. 8A for low operating pressures and, in FIG. 8C, a physical configuration for high operating pressures. The system converts a form of thermal energy of low volumetric heat capacity at temperatures above 800° C. to a form of dispatchable energy of high volumetric heat capacity at temperatures above 600° C. For this purpose, the process uses CO₂gas as a HTF that is heated up to a temperature above 800° C. by a power tower receiver and contacted by gas bubbling with a binary slurry mixture of Li₂CO₃—Na₂CO₃to store the thermal energy at a maximum temperature of 750° C. the liquidus temperature of a 14 wt. % of Li₂CO₃mixture at an optimum system operating pressure. The operating temperature and pressure for the storage are determined by a trade-off study.
In order to determine the storage temperature, many factors must be considered such as thermal stability of the PCM components, compatibility of the materials of construction and their costs, maintenance and operation costs, and the temperature level required by the thermodynamic cycle system that generates electricity using the stored energy. In this embodiment, the storage temperatures have been set between 750° C. and 640° C. in order to utilize both sensible and latent heat as much as possible while supplying the heat to the thermodynamic cycle system at 700° C. at which a supercritical steam Rankine cycle or a supercritical CO₂Brayton cycle can achieve a cycle efficiency near 50%. In addition, the storage system of this embodiment provides a cushion against the variations of solar irradiation with a short response time, less than a minute to recover the normal operating temperature of 700° C., with a capability of a dispatchable storage capacity for as many hours as desired. To determine the operating pressure, on the other hand, many factors must be evaluated as well such as density of CO₂gas to increase the volumetric heat capacity, installation cost of storage tank, parasitic power consumption for the CO₂compressor, and the design of the solar power tower receiver. The higher operating pressure will increase the volumetric heat capacity of CO₂gas, while increase at the same time the installation cost of the storage tank. Most importantly, the CO₂gas as an immiscible HTF obviates the need of parasitic expenses to prevent freezing of molten salt HTF and of the high installation cost of the process facilities requiring the costly high nickel alloys that are resistant to the corrosive carbonate molten salts in case when such a corrosive molten salt liquid is used as a HTF.
During daytime while solar irradiation is available, process 400 in FIG. 8A works to supply thermal energy from power tower receiver 480 to the power block through heat recovery exchangers 470 for generation of electricity and also to LTES chambers 410, 420, 430, 440, 450 and 460 to store heat. The power block is a thermodynamic cycle system selected from a group comprising a supercritical steam Rankine cycle, a supercritical CO₂Brayton cycle, an air Brayton cycle, a conventional steam Rankine cycle, a parabolic disc with a thermodynamic cycle engine, an organic Rankine cycle (ORC) and combinations thereof. The types of thermodynamic cycle engine for the parabolic disc includes kinematic Sterling engines, free-piston Stirling engines, and Brayton turbine-alternator based engines. The CO₂gas stream then passes through recuperator 479 and air cooled exchanger 482 having a motored fan, where it is cooled to a temperature low enough for efficient compression. A CO₂gas compressor 490 circulates the gas stream at a constant flow rate required to generate the design capacity of electricity by the thermodynamic cycle system. The compressed gas is then heated again by recuperator 479 to as high temperature as possible, and sent to power tower receiver 480. Valve 492 remains closed during the charging period. Valve 412, 422, 432, 442, 452 and 462 close exit stream from LTES chamber 410, 420, 430, 440, 450 and 460, respectively, while valve 411, 421, 431, 441, 451 and 461 control the gas flow rate of the inlet stream to the respective chamber. Device 485 is installed in front of heat recovery heat exchanger 470 in order to remove the liquid particles possibly entrained in the CO₂gas stream.
In the beginning of the charging operation in the morning, all LTES chambers are at 640° C., and a portion of stream 475 from the power tower being at temperatures above 800° C. bypasses the chambers through bypass control valve 496 in order to raise the temperature of gas stream 471 to 700° C. During normal operation, the flow rates of the streams from the chambers are controlled such that stream 471 at the inlet of heat recovery exchanger 470 is maintained at 700° C. by mixing them. Also, no chambers are heated above 750° C. in order to avoid superheating. During the last period of the charging process for the last chamber, the temperature of stream 471 increases from 700° C. to 750° C. because, at that time, a colder stream below 700° C. is not available. During a disturbance of solar irradiation, the system operates normally by the system controller until the temperature of the current chamber is cooled to below 700° C., upon which time a gas flow starts to the previous chamber that is at 750° C.; the two streams from the current and the previous chambers are then mixed to make the outlet gas stream from the multi-chamber LTES system to be at 700° C. and fed into the heat recovery exchanger 470. The temperature of inlet stream 471 to the heat recovery exchanger 470 restores its normal temperature of 700° C. in a minute at the most from a temperature several degrees colder than the set point by the control of the system controller; for example, with a superficial velocity of about 0.3 m/s of the CO₂gas and a slurry depth of 15 meters, it takes about 50 seconds for the gas bubbles to rise to the ullage space. From that time on, stream 471 is maintained at the set point of 700° C. by the system controller.
The gas temperature is lowered to an economical temperature in recuperator 479 and cooled further in air cooled exchanger 482 to such a temperature as compressor 490 can operate with the optimum power consumption. The gas stream having been re-heated in recuperator 479 is then sent to power tower receiver 480.
The LTES chambers operate between 750° C. and 640° C. At the full energy charge, all chambers reach 750° C. the liquidus temperature of the binary mixture of 14 wt. % of Li₂CO₃, while at the full discharge, the slurry temperature decreases to 640° C. where the slurry reaches a crystal concentration of 50 wt. % (47 vol. %). The temperature 700° C. of the outlet gas stream from the multi-chamber LTES system is about the midpoint between 750° C. and 640° C. in terms of the heat releasing capacity of the PCM slurry. The slurry is heated only up to right below the liquidus temperature so that the crystal seeds are left available for the subsequent discharging step. The slurry is cooled only until the slurry concentration reaches 50 wt. % (47 vol. %), because that concentration is usually the highest concentration level to maintain enough fluidity for gas bubbling. The operation method to control the process during the daytime is described below.
During the nighttime while the solar irradiation is absent, the LTES chambers discharge thermal energy for the thermodynamic cycle system to continuously generate the design capacity of electricity. At this time, valves 491 and 494 are closed, and valve 492 is opened. At the start of the discharging step, all LTES chambers are at a temperature of 750° C. Therefore, the temperature of stream 471 is made to be 700° C. by mixing the streams consisting of bypass stream 476 being at a temperature much lower than 700° C. and the streams from any of the LTES chambers which are at 750° C. Each LTES chamber discharges energy until the slurry temperature reaches 640° C. where the slurry concentration becomes 50 wt. %. In a normal operation, no LTES chambers are cooled below 640° C. in order to avoid the slurry concentration increase to higher than 50 wt. %. For the last LTES chamber, once its temperature reaches 700° C., there is no other heat source to generate a hotter gas, so the temperature of the gas stream must decrease continuously until it reaches 640° C. the final design temperature in the discharging step. In the next morning with the solar irradiation available, all LTES chambers start at a temperature of 640° C. The operation method to control the process during the nighttime is described in below.
During the period when the binary system of Li₂CO₃—Na₂CO₃at a concentration of 14 wt. % of Li₂CO₃is cooled from 750° C. to 640° C. with a temperature drop of 110° C. with 50 wt. % of initial charge being crystallized, heat is discharged with a enthalpy difference on volume basis of 700 KJ/L, which is equivalent to 705 KJ/L of the solar salt when its sensible heat is discharged from 560° C. to 290° C. with a temperature drop of 270° C. Therefore, this carbonate salt binary system is a very effective PCM for TES for working temperatures above 600° C., and can be used up to 750° C. being higher than the service temperature of the solar salt heat storage system by 200° C. The CO₂gas stream from the multi-chamber LTES system is fed into the heat recovery exchanger at a constant temperature of 700° C. throughout the operation in the daytime as well as the nighttime except the last periods for charging and discharging steps.
The operation method of the process in FIG. 8A during the daytime is as follows: (i) A constant temperature of 700° C. and a constant CO₂flow rate for the feed stream to the heat recovery exchanger are maintained to generate the design capacity of electricity by the thermodynamic cycle system; (ii) In the beginning of the charging step, the bypass stream at above 800° C. and the streams from any combinations of the LTES chambers at 640° C. are mixed in order to make the feed stream to the heat recovery exchanger to be at 700° C.; (iii) In the beginning of the charging step, the first LTES chamber is charged up to 750° C. as soon as possible in preparation for the disturbances of solar irradiation; (iv) In the beginning of the charging step, once the first LTES chamber reaches 750° C., the bypass valve is closed and the whole gas flow passes through the LTES chambers which are between 640° C. and 750° C.; (v) During normal operation, a constant temperature for the gas stream to the heat recovery exchanger is achieved by mixing the gas streams from any combinations of the LTES chambers being at temperatures between 750° C. and 640° C.; (vi) During normal operation for charging, no LTES chambers are heated above 750° C. in order to avoid superheating; (vii) During the time of disturbance of solar irradiation, when the temperature of the current LTES chamber decreases below 700° C. because the temperature of the stream from the solar tower receiver is below 700° C., start the gas flow to the previous chamber that is at 750° C. and mix the streams from the current and previous ones to make the outlet gas stream from the multi-chamber LTES system to be at 700° C.; and (viii) For the last LTES chamber in the charging step, continue charging even after its temperature reaches 700° C., while the gas temperature to the heat recovery exchanger rises to 750° C. because there is no chamber whose temperature is lower than 700° C.
The operation method of the process in FIG. 8A during the nighttime is as follows: (i) A constant temperature of 700° C. and a constant CO₂flow rate for the feed stream to the heat recovery exchanger are maintained to generate the design capacity of electricity by the thermodynamic cycle system; (ii) In the beginning of the discharging step, the bypass stream being at a temperature much lower than 700° C. and the streams from any combinations of the LTES chambers at 750° C. are mixed in order to achieve 700° C. for the outlet gas stream from the multi-chamber LTES system; (iii) When the first LTES chamber reaches a temperature below 700° C., stop the bypass flow by closing the bypass valve and start the gas flow to the next chamber which is at 750° C., and then mix the two streams from the current and next ones to make the outlet gas stream from the multi-chamber LTES system to be 700° C.; (iv) In normal operation for discharging, no LTES chambers are cooled below 640° C. in order to avoid the slurry concentration increase to higher than 50 wt. %; and (v) For the last LTES chamber in the discharging step, the temperature of the gas stream to the heat recovery exchanger keeps decreasing to 640° C., because there is no chamber whose temperature is above 700° C.
A CO₂circulation compressor 490 in FIG. 8A, during the daytime, circulates CO₂gas stream 473 through power tower receiver 480, any combinations of LTES chambers 410 to 460 in parallel, heat recovery exchanger 470 for operation of the thermodynamic cycle system, recuperator 479 in both directions, and air cooled exchanger 482 while electricity is produced and the LTES chambers are charged at the same time. During the nighttime while energy is discharged, CO₂gas stream 474 passes through the LTES chambers, heat recovery exchanger, recuperator, and air cooled exchanger, but not through the power tower receiver. As the compressor is the largest consumer of parasitic power in the process, a strategy for the most economic operation of the equipment is essential for the success of this utility plant. Normally, about 10% to 15% of the gross electricity production is used for such parasitic power consumption as for pumping in CSP plants, and such parasitic consumption must be minimized as much as possible.
The operating pressure of LTES system 800 in FIG. 8A must be determined by optimizing the installation and operating costs of the system. The CO₂gas can be heated up to a temperature ranging from 800° C. to 900° C. at pressures around 20 bar at the maximum by a cavity type tubular receiver or up to over 1000° C. at pressures around a few bar by a volumetric receiver. The LTES tank chambers, however, must be designed only for limited ranges of pressure at the storage temperature of 750° C. in order to avoid an excessively high installation cost. The CO₂gas compressor, on the other hand, requires the system pressure to be higher in order to keep the volumetric gas flow rate as low as possible for more efficient power consumption. The major factors affecting the installation and operating costs of the system are as follows: (i) Lower operating pressure is more favorable for the design and the installation cost of the LTES tank chambers; (ii) Lower operating pressure is more favorable for the design and the cost of the power tower receiver; (iii) Higher operating pressure is more favorable for heat transfer due to the increased density of CO₂gas that makes the volumetric flow rate lower resulting in lower pressure drop in the heat exchangers; (iv) Higher operating pressure is more favorable for the design of transfer piping, as lower volumetric flow rates reduce friction losses; and (v) Higher operating pressure is more favorable for the design of compressor, because the lower volumetric flow rates owing to the higher pressure result in more efficient power consumption. Therefore, the optimum operating pressure must be determined by a trade-off study considering all the factors affecting the installation and operating costs of the system.
A supercritical steam Rankine cycle can be used as a thermodynamic cycle for generation of electricity using the heat supplied from the LTES system of this invention. This cycle is different from the conventional steam Rankine cycle in that enthalpy of water is increased without vaporization by heating under the pressure above the critical pressure of water (220.6 bar) with the operating temperature above the critical temperature (373.95° C.). For temperatures above 600° C., the system can perform with a cycle efficiency up to 48%. This cycle has been used in the coal fired power plants for decades for capacities larger than 400 MW, and can be used without many difficulties for CSP plants for capacities of around 100 MW.
A conventional steam Rankine cycle generates electricity by using the heat it takes while water vaporizes under the pressures lower than the critical pressure of water. At temperatures from 390° C. to 565° C., the system performs with a cycle efficiency of 37% to 42%. This system has been used for power plants for more than a century.
Another option to generate electricity by using the heat from the LTES of this invention is the supercritical CO₂Brayton cycle. With thermal energy at temperatures above 600° C., a cycle efficiency of near 50% is expected. In the temperature range from 400° C. to 700° C., the cycle efficiency of the supercritical CO₂Brayton cycle is higher than any other cycles; at the temperatures below 400° C., the steam Rankine cycle performs better and, higher than 700° C., the Helium Brayton cycle better. The supercritical CO₂Brayton cycle has been proven effective in small scale test plants, but not yet for CSP plants whose capacity starts from around 100 MW.
An organic Rankine cycle (ORC) can be utilized to generate electrical energy, mechanical energy, or both. For the thermal energy with the HTF inlet temperature of around 300° C. and outlet of 150° C., a cycle efficiency of around 18% can be expected. Depending on the process design of the power block, the ORC system can perform as a bottoming cycle.
A parabolic disc with a thermodynamic cycle engine has many advantages over other systems. Firstly, it has the highest efficiency for the solar-to-electricity performance among the CSP technologies when Stirling engines are used for electricity generation. Secondly, it can be used for capacities from kilowatts to gigawatts. Thirdly, it can be manufactured by using modular technology, making scale-up and manufacturing easier. Fourthly, it has showed the lowest water usage, because it can utilize a closed-loop cooling system such as for cars. Fifthly, it can be installed on uneven ground, making installation easier and less costly. However, for this particular system, the Sterling engines are costly, and the air Brayton cycles are investigated as an alternative. The Sterling engines use hydrogen or helium as a working fluid. An open air Brayton cycle system, on the other hand, is especially advantageous in the view point of the installation and operating costs only in that ambient air is directly fed to the compressor and the hot air from the recuperator exhausted to the atmosphere. The air Brayton micro-turbine systems have a cycle efficiency of 25% to 33% compared to around 42% of the Stirling engines. Also, electricity can be generated only during the daytime, since the thermal energy storage has been thought very difficult to be provided with. In connection with the LTES system of this invention, however, the hot CO₂gas can be used as a HTF to transfer thermal energy from the parabolic disc to the LTES system in the daytime and in reverse direction in the nighttime. Especially, the gaseous heat transfer medium enables the parabolic disc having a thermodynamic cycle engine to generate electricity in the night because the gaseous HTF never freezes unlike the molten salt liquids. Also, in this case, CO₂gas has been made available, so it can be used as a working fluid for a supercritical CO₂Brayton cycle.
The gaseous streams from the multi-chamber LTES system may need a device to remove the entrained liquid droplets. The tendency of entrainment depends on the physical properties such as the gas velocity, density and viscosity and also the liquid density and particle sizes. Therefore, a sudden change of operating conditions such as the gas flow rates and operating temperature and pressure can directly affect the possibility of entrainment. In order to ensure a safe operation, a means is needed prior to the heat recovery exchanger to trap such unexpected liquid particles.
The cylindrical tank chambers as shown in FIG. 8C can be used for this application at higher operating pressures instead of the compartment chambers having vertical plain partitions shown in FIG. 8B. Cylindrical tank reactors have been successfully used for many decades in nuclear power plants at the similar operating temperatures and pressures as for this application. Therefore, an option to increase the operating pressure of the multi-chamber LTES system to as high as the solar power tower receiver can withstand must be included in the trade-off studies. Still, a typical LTES tank of 40 meters in diameter for generation of 100 MW for a storage capacity of 7.4 hours can be readily divided into multiple cylindrical tanks while ensuring the least amount of heat losses to the atmosphere and the least friction losses owing to the shorter transfer piping.
Multi-chamber LTES system 800 of this invention in FIG. 8A for storing and recovery of thermal energy from a power tower using CO₂gas as a HTF offers many advantages in practical applications, for example: (i) The LTES system is a practical process for CSP plants converting an energy form of very low volumetric heat capacity to a dispatchable form of very high heat capacity for the power tower receiver exit gas temperatures up to over 1000° C.; (ii) The system stores and discharges thermal energy above 600° C., and up to 750° C. where the thermodynamic cycle such as a supercritical steam Rankine cycle or a supercritical CO₂Brayton cycle can be utilized to produce electricity at the cycle efficiencies close to 50%; (iii) The storage volume of this multi-chamber LTES system is more efficient by a factor of about five in terms of the electricity generation capability when compared to the two-tank system of solar salt liquid working between 390° C. to 290° C. and by a factor of about two when compared to the two-tank system of solar salt liquid working between 560° C. and 290° C.; (iv) The CO₂gas as a HTF obviates the need of freezing protection in the CSP plants for other than the LTES tank; (v) The carbonate compounds Li₂CO₃and Na₂CO₃are thermally very stable up to 1000° C. in the CO₂atmosphere; (vi) A multi-chamber LTES system makes it possible to keep heat losses to the atmosphere to the minimum; (vii) A multi-chamber LTES system makes it possible to use the least amount of material of construction for the steel walls and insulation of the partitions; (viii) Bubbling is an effective method for heat transfer in such corrosive environment obviating costly heat exchangers; (ix) For the carbonate compounds Li₂CO₃and Na₂CO₃, abundance of data on corrosion characteristics has been accumulated in the nuclear and fuel cell industries; (x) The corrosive carbonate compounds Li₂CO₃and Na₂CO₃are confined only in the LTES tank chambers; (xi) The materials being used in the process CO₂, Li₂CO₃and Na₂CO₃are readily available in bulk quantities at low prices; (xii) The process can be readily scaled up to larger capacities compared to other conventional thermal energy storages such as the heat exchanger or encapsulated PCM type, because the design and construction of this system are much simpler; and (xiii) The chemical engineering industry has extensive experiences in the design and operation for CO₂gas processing, slurry systems, three phase contacting systems, heat transfer equipment, rotating machineries and thermodynamic cycles, which are all used in this process.
The present invention may be embodied in other specific forms without departing from its spirit or essential characteristics. The described embodiments are to be considered in all respects only as illustrative and not restrictive. The scope of the invention is, therefore, indicated by the appended claims rather than by the foregoing description. All changes which come within the meaning and range of equivalency of the claims are to be embraced within their scope.

Claims

I claim:

1. A latent thermal energy storage (LTES) system comprising:

a tank; and

a layer of phase change material (PCM) slurry mixture within the tank, the PMC slurry mixture comprising more than one component in phase equilibrium; and

an immiscible heat transfer fluid (HTF) within said tank in a phase selected from a group comprising a liquid phase, a gaseous phase and a combination thereof, for charging thermal energy into and extracting thermal energy from said PCM slurry mixture;

wherein the PCM undergoes a phase change while thermal energy is charged and extracted.

2. The LTES system of claim 1, wherein the thermal energy is charged into said layer of PCM slurry up to temperatures below a solid-liquid saturation point whereby crystal seeds are available for subsequent heat discharge while preventing sub-cooling of the PCM solution.

3. The LTES system of claim 1, wherein said immiscible HTF is withdrawn from said tank and temperature varied, and returned to said tank for further heat exchange with said layer of PCM slurry.

4. The LTES system of claim 1, wherein said immiscible HTF in liquid phase is fed into said tank such that droplets evaporate into vapor and the vapor is heated while they rise in said layer of PCM slurry, whereby the vapor is condensed in an outside heat exchanger with heat of condensation released, and condensate returns to the tank.

5. The LTES system of claim 1, further comprising an apparatus for generating bubbles by breaking the stream of said immiscible HTF into small sizes being located in said layer of PCM slurry at a lower section of said tank.

6. The LTES system of claim 5, wherein the bubbles of said immiscible HTF rise in small sizes by buoyant forces within said layer of PCM slurry exchanging thermal energy between each other.

7. The LTES system of claim 6, wherein the immiscible HTF is a gaseous fluid selected from a group comprising air, He, CO₂, N₂, Ar, and combinations thereof.

8. The LTES system of claim 1, wherein the PCM slurry mixture in phase equilibrium is a eutectic system and operates between a liquidus temperature and a solidus temperature.

9. The LTES system of claim 1, wherein a thermal state of the system is controlled by a method selected from a group comprising:

a. (i) ensuring that the PCM slurry mixture has one unique freedom by the phase rule and temperature is selected for the unique freedom; (ii) deciding the set point of slurry temperature; (iii) obtaining a phase equilibrium data of the compositions on temperature for all phases in the slurry and the amount and composition of the initial charge; (iv) solving the same number of material balance equations as for the phases to find the amounts of all phases in the slurry; (v) obtaining the solids content in the slurry; (vi) reporting the solids content in equilibrium at the set point of slurry temperature; and (vii) controlling the thermal process for the set point of slurry temperature;

b. (i) ensuring that the PCM slurry mixture has one unique freedom by the phase rule and temperature is selected for the unique freedom; (ii) deciding the set point of solids content in the slurry; (iii) obtaining a phase equilibrium data of the compositions on temperature for all phases in the slurry and the amount and composition of the initial charge; (iv) finding the slurry temperature in equilibrium at the set point of solids content by solving the same number of material balance equations as for the phases; (v) reporting the slurry temperature in equilibrium at the set point of solids content; and (vi) controlling the thermal process using the slurry temperature in equilibrium at the set point of solids content as a control parameter;

c. (i) ensuring that the PCM slurry mixture has one unique freedom by the phase rule and temperature is selected for the unique freedom; (ii) deciding the set point of slurry temperature; (iii) obtaining a phase diagram of the slurry system and the amount and composition of the initial charge; (iv) finding the ratio of the amounts of solid and liquid phases from the phase diagram by the lever rule at the set point of slurry temperature and then the solids content in the slurry; (v) reporting the solids content in equilibrium at the set point of slurry temperature; and (vi) controlling the thermal process for the set point of slurry temperature; and

d. (i) ensuring that the PCM slurry mixture has one unique freedom by the phase rule and temperature is selected for the unique freedom; (ii) deciding the set point of solids content in the slurry; (iii) obtaining a phase diagram of the slurry system and the amount and composition of the initial charge; (iv) finding the slurry temperature that yields the ratio of the amounts of solid and liquid phases equivalent to the set point of solids content from the phase diagram by the lever rule; (v) reporting the slurry temperature in equilibrium at the set point of solids content; and (vi) controlling the thermal process using the slurry temperature in equilibrium at the set point of solids content as a control parameter.

10. The LTES system of claim 3, wherein the tank includes a plurality of chambers, each chamber having said PCM slurry mixture and immiscible HTF.

11. The LTES system of claim 10, wherein each chamber is sealed and thermally insulated to prevent communication of contents between each other.

12. The LTES system of claim 10, wherein the tank is thermally insulated.

13. The LTES system of claim 11, wherein the immiscible HTF transfers heat by bubbling to store thermal energy in said layer of PCM slurry in said LTES chambers and to recover thermal energy from said layer of PCM slurry in said chambers.

14. The LTES system of claim 13, wherein said immiscible HTF is circulated out of said tank to transfer thermal energy to a heat exchange system selected from a group comprising a heat recovery exchanger, a recuperator, an air cooled exchanger and combinations thereof in order to generate a form of energy selected from electrical, mechanical and a combination thereof.

15. The LTES system of claim 14, wherein said heat recovery exchanger transfers thermal energy to a thermodynamic cycle system selected from a group comprising a supercritical steam Rankine cycle, a conventional steam Rankine cycle, a supercritical CO₂Brayton cycle, an air Brayton cycle, a Sterling engine, an organic Rankine cycle (ORC) and combinations thereof.

16. The LTES system of claim 10, wherein said immiscible HTF is gaseous, the gaseous HTF circulates out of tank and is utilized to perform a thermal process selected from a group comprising carrying thermal energy from a parabolic disc having a thermodynamic cycle engine to said multi-chamber LTES system for heat storage, carrying thermal energy from said multi-chamber LTES system to said parabolic disc having a thermodynamic cycle engine for generation of electricity and a combination thereof by using a working fluid selected from a group comprising hydrogen, helium, nitrogen, air, and CO₂for operation of said thermodynamic cycle engine.

17. The LTES system of claim 1, wherein said PCM slurry mixture is selected from a group comprising Li₂CO₃—Na₂CO₃, Li₂CO₃—K₂CO₃and Li₂CO₃—K₂CO₃—Na₂CO₃and said immiscible HTF is a gaseous CO₂.

18. The LTES system of claim 10, further comprising a thermal energy source operable coupled to each chamber, wherein the immiscible HTF is a gas, wherein the system is operated to generate a predetermined power capacity by a method comprising the steps of:

a. providing a bypass gas stream from the thermal energy source at a temperature T_sto the chambers, the chambers having a temperature between a low temperature T₁and a high temperature T_h;

b. charging a first chamber to a temperature T_hprior to charging the remaining chambers;

c. immiscible HTF from one or more chambers combined to form an outlet gas stream at a temperature T_c;

d. monitoring each chamber so that each chamber is not heated to above the high temperature T_h; and

e. continue charging each chamber until the temperature of each chamber is at the high temperature T_hsuch that the outlet gas stream temperature is at the high temperature T_h.

19. The LTES system of claim 10, further comprising a thermal energy user operable coupled to each chamber, wherein the immiscible HTF is a gas, wherein the system is operated to generate a predetermined power capacity by a method comprising the steps of:

a. providing a bypass gas stream from the thermal energy user at a temperature T_oto the chambers, the chambers having a temperature between a low temperature T₁and a high temperature T_h;

b. immiscible HTF from one or more chambers combined to form an outlet gas stream at a temperature T_c;

c. monitoring each chamber so that each chamber is not cooled below the low temperature T₁; and

d. continue discharging each chamber until the temperature of each chamber is at the low temperature T₁such that the outlet gas stream temperature is at the low temperature T₁.