Latent Thermal Energy System (LTES) Bubbling Tank System

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 Li 2 CO 3 and Na 2 CO 3 as a heat storage medium and CO 2 gas as an immiscible HTF.

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 The LTES system of  claim 1 , wherein said immiscible HTF is withdrawn from said tank and temperature varied, and returned to said tank for further heat exchange with said layer of PCM slurry. 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, CO 2 , N 2 , 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 CO 2  Brayton 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 CO 2  for operation of said thermodynamic cycle engine. 
     
     
         17 . The LTES system of  claim 1 , wherein said PCM slurry mixture is selected from a group comprising Li 2 CO 3 —Na 2 CO 3 , Li 2 CO 3 —K 2 CO 3  and Li 2 CO 3 —K 2 CO 3 —Na 2 CO 3  and said immiscible HTF is a gaseous CO 2 . 
     
     
         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 s  to the chambers, the chambers having a temperature between a low temperature T 1  and a high temperature T h ;   b. charging a first chamber to a temperature T h  prior 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 h  such 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 o  to the chambers, the chambers having a temperature between a low temperature T 1  and 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 T 1 ; and   d. continue discharging each chamber until the temperature of each chamber is at the low temperature T 1  such that the outlet gas stream temperature is at the low temperature T 1 .