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Energy 93 (2015) 1523e1534
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Theoretical and experimental investigation of a closed sorption thermal storage prototype using LiCl/water N. Yu, R.Z. Wang*, L.W. Wang Institute of Refrigeration and Cryogenics and Key Laboratory of Power Mechanical Engineering, MOE China, Shanghai Jiao Tong University, Shanghai, 200240, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 6 April 2015 Received in revised form 20 July 2015 Accepted 1 October 2015 Available online xxx
A 1 kWh lab-scale sorption prototype using LiCl-water was theoretically and experimentally investigated for sorption thermal energy storage. A type of consolidated composite matrix is developed for the system by using AC (activated carbon), LiCl, expanded natural graphite treated with sulphuric acid (ENG-TSA) to increase heat transfer and SS (silica solution) to enhance mechanical strength. Thermal conductivity and permeability were measured first. A two-dimensional model considering the combined heat and mass transfer was developed to predict the sorption kinetics of the reactor. Under the operation condition of a charging temperature of 85 C and a discharging temperature of 40 C, the experimentally recovered heat is 2517 kJ, resulting a heat storage efficiency of 93%. The heat storage density is 874 kJ/kg consolidated sorbent or 2622 kJ/kg LiCl. The experimental results of the prototype were compared with the simulated results. The established two-dimensional model proves to be effective since the general evolution trends of experimental and simulated outlet fluid temperatures are in good agreement. An average gap of about 0.4 C between the experimental and simulated outlet temperature may be caused by the heat loss and the constant pressure assumption. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Thermal energy storage Sorption thermal storage Consolidated sorbent Lithium chloride-water
1. Introduction Recently, thermochemical energy storage, which includes sorption processes and chemical reactions, has emerged as an research hotspot in the filed of TES (thermal energy storage) [1,2]. Sorption TES method is regarded as an important category of processes in the scope of thermochemical TES, which stores heat in the bonds between the solid/liquid sorbent and the vaporous sorbate. The advantages of thermochemical energy storage are claimed to be substantially high energy storage densities, the ability to preserve energy for long periods with limited heat losses and combing cold storage and heat storage functions in one system [1]. Conventionally, sorption technology is applied and studied for thermal transformation applications like absorption and adsorption refrigeration/heat pumps [3]. Though the basic principles for sorption refrigeration/heat pumps and sorption TES systems are similar, the different application areas give rise to different requirements for material selection and system configurations.
* Corresponding author. Tel./fax: þ86 21 34206548. E-mail address: [email protected] (R.Z. Wang). http://dx.doi.org/10.1016/j.energy.2015.10.001 0360-5442/© 2015 Elsevier Ltd. All rights reserved.
Water is always the preferred choice of sorbate for the safety issue, thus the primary task for sorption TES is to find suitable water sorbents which should possess high storage densities and low cost. Compared with traditional water sorbents like silica gel and zeolite, sorption processes between hygroscopic salt and water vapor seem more promising due to their higher theoretic storage densities. Among all the hygroscopic salts, MgCl2 [4,5], Na2S [6], SrBr2 [7e9], MgSO4 [10,11], CaCl2 [12,13], LiBr [14,15] and LiCl [16,17] have been identified by different researchers as potential candidates. Usually, the products of hydration reactions of MgCl2, Na2S, SrBr2 and MgSO4 are assumed to be hydrates with more crystal water molecules. For example, the solid/gas hydration reaction products of MgSO4 could be MgSO4$6H2O or MgSO4$7H2O [10]. Zondag et al. [4] established an open reactor with a packed reaction bed with 17 L MgCl2 and achieved a heat storage density of 140 kWh/m3, with a discharging power of 50 W. Boer et al. [6] developed a prototype of a modular sorption cooling system using Na2S/H2O. Test results showed that a cold storage capacity of 2.1 kWh and a cooling COP of 0.56 were achieved by inputting a heat capacity of 3.7 kWh with 3 kg of Na2S in one module. Researchers in PROMES-CNRS focused their attention on SrBr2 and have tried to apply SrBr2 in closed [7] and open [8,9] sorption TES systems.
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Nomenclature AC ALi40
activated carbon activated carbon-LiCl composite prepared in 40 wt.% of LiCl solution cp specific heat, J/(kg C) df equivalent diameter of the heat transfer fluid channel ENG-TSA expanded graphite treated with sulfuric acid ESALi consolidated composite sorbent prepared with ALi40 ESD energy storage density, kJ/kg h heat transfer coefficient, W/(m2 C) Ks sorption rate coefficient, 1/s Kp Permeability of the sorbent, m2 l liquid flow direction or liquid water MH2O molar mass of water, 0.018 kg/mol m mass, kg P pressure Pa Q heat, kJ q power, kW qv volumetric flow rate, m3/s R gas universal constant, J/(mol K) Re Reynolds number S cross-section area of sorbent, m2 SS silica solution T temperature, C t time, s TES thermal energy storage u velocity, m/s
The water sorption and desorption behaviors for three commonly used hygroscopic salts, namely LiCl, CaCl2 and LiBr, have been analyzed by the authors [17], and results showed that LiCl possessed a great potential for sorption TES with its high storage density. As LiCl could be fully discharged at temperatures lower than 100 C, LiCl/H2O seems to be a good option for solar energy storage To find a proper porous matrix for LiCl, we have investigated and compared the sorption properties of silica geleLiCl composite [16] and activated carboneLiCl composite [18] in the previous research and activated carbon was regarded as a better choice. Then, A new type of consolidated composite sorbent was developed by using AC (activated carbon) as a porous host matrix to carry LiCl, mixing with expanded natural graphite treated with sulfuric acid (ENG-TSA) to increase heat transfer and adding SS (silica solution) as a binder to enhance mechanical strength [18]. The purpose of this study is to test the heat storage performance of the prepared consolidated sorbent in a lab-scale prototype designed for charging temperatures lower than 90 C. The thermal conductivity and permeability of the sorbent were measured first to obtain the basic heat and mass transfer parameters, which were prerequisite for modeling. A twodimensional model considering the combined heat and mass transfer issue was developed to predict the sorption kinetics of the reactor. Experimental results of the prototype were compared with the simulation results. 2. Material characterization 2.1. Preparation of consolidated sorbent In this work, ENG-TSA was also added to activated carbon-LiCl composite through a consolidation process. The mixture of AC,
x z
water uptake, kg/kg sorbent thickness direction
Greek symbols r density, kg/m3 ε sorbent porosity l thermal conductivity, W/(m C) m viscosity, Pa S. Subscripts and superscripts c condenser, condensation ce condenser-evaporator cha charging des desorption dis discharging e evaporation, evaporator eq equilibrium f heat transfer fluid fsr heat transfer fluid for sorption reactor fce heat transfer fluid for condenser-evaporator g gas in inlet ini initial m metal out outlet s sorbent sor sorption
ENG-TSA and silica solution was compressed into a consolidated block by a pressing machine. The obtained consolidated composite sorbents after drying were named ESALi. The detailed preparing procedures of the consolidated sorbent is shown in our previous communication [18]. Consolidated sorbents with different densities were prepared for measurements of thermal conductivity and permeability. 2.2. Thermal conductivity Thermal conductivity of these samples was tested using Hot Disk TPS (Transient Plane Source) 2500. The instrument adopts the standardized TPS (Transient Plane Source) technique for thermal conductivity with claimed accuracy better than 5%, leading to an absolute uncertainty of 0.15 W/(m K) for the consolidated sorbents (low than 3 W/(m K)). During measurement, the probe of Hot Disk was put between two identical consolidated sorbents samples and clamped tight to ensure full access between the probe and the sorbents. Tests have to be tried for several times to find the most suitable parameters of probe power and measuring time. Measured results of thermal conductivities for the 5 consolidated sorbents with varied densities are given in Fig. 1. Variation of bulk densities from 500 to 818 kg/m3 produces thermal conductivities from 2.36 to 2.76 W/(m C). The highest thermal conductivity of 2.76 W/(m C) is obtained at a bulk density of 758 kg/m3, which improves that of loose-packed AC-LiCl composite (0.14 W/ (m C)) by nearly 19 times. Falling of thermal conductivities is observed when the density is greater than 800 kg/m3. This decrease may be caused by the crush of activated carbon particles under high pressing pressures to achieve the target densities. Generally, since the mass concentrations of ENG-TSA in these samples are restricted to 12%, the influence of density on thermal conductivity is not obvious.
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With the obtained differential pressure and flow rate, the permeability could be calculated according to the following equation:
Kp ¼
mg qv Dz SDP
(2)
Nitrogen with 99.9% purity from a high pressure tank was adopted in the permeability test. A flow regulating value was used to adjust the flow rate linearly. A group of data for one sample was recorded at varied flow rates and corresponding pressure drop. The permeability for the sample was deduced by fitting this group of data. The relative error for the permeability is obtained by using the error propagation formula:
dKp dqv dDP þ K q DP p v
According to Eq. (3) and the values measured in the text, the obtained maximum relative error for permeability is less than 10%. Permeability test result of four samples with varied densities is given in Fig. 3. As predicted, higher bulk densities lead to lower permeability values. When the density increases from 400 to 600 kg/m3, the permeability drops from 1.5 1011 to 1.65 1012, meaning that permeability at 600 kg/m3 is only 10% of that at 400 kg/m3. The fact shows that the effect of bulk densities on permeability is greater than that on thermal conductivity.
Fig. 1. Variation of thermal conductivity with density.
2.3. Permeability Though high bulk densities of consolidated sorbents give rise to improvement of thermal conductivities, the corresponding consequence may be poor mass transfer conditions. Permeability Kp is an important parameter to evaluate the mass transfer ability of porous materials. The permeability of the consolidated samples was measured by using a specially designed rig shown in Fig. 2. The prepared sample was placed in a square sample holder which was made of stainless steel. A differential pressure transducer Dwyer477A-4 with a measuring range of 0e43 kPa and an accuracy of ±0.1% was used to record the pressure drop in the flow of gas through the sample and a gas flowmeter (MF5706) with a measuring range of 0e10 L/min and an accuracy of ±2% was placed before the outlet of gas pipe to measure the flow rate. The permeability was measured according to Darcy's Law:
Kp ug ¼ VP mg
(1)
where ug is vapor velocity (m/s), which could be calculate by the measured flow rate qv (m3/s) and cross-section area S; Kp is permeability (m2); mg is the viscosity (Pa s); VP is the pressure gradient along the flow direction (Pa), which could be calculated by the differential pressure DP and the thickness of the sample Dz.
Fig. 2. Permeability test rig.
(3)
3. Experimental prototype 3.1. Design of the prototype Sorption working pairs with high energy density on material level cannot guarantee efficient storage performance in practical applications. Realization of the storage potential of LiCl/H2O showed for thermal energy storage greatly relies on the efficient and compact design of sorption bed to ensure good heat and mass transfer. To confirm the performance of LiCl in real systems, a labscale module with 1 kWh heat storage capacity was designed. The experimental prototype for sorption thermal storage mainly includes two vessels: one is the sorption reactor and the other is condenser-evaporator. The picture of the prototype and the detailed structures of the sorption reactor and the condenserevaporator are shown in Fig. 4. Plate-fin heat exchangers have been widely used in automotive air conditioning systems due to higher heat exchange efficiency and
Fig. 3. Variation of permeability with density.
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Fig. 4. Structures of the experimental prototype.
smaller volumes compared with tube-fin heat exchangers. Moreover, the surface of a plate-fin heat exchanger is flat, making it easier to integrate the prepared consolidated sorbent with the heat exchanger. Thus, plate heat exchangers were used in the sorption reactor. The structure of a plate heat exchanger is presented in Fig. 4(b). The dimension of singular heat exchanger is 260 260 58 mm. Each heat exchanger has 22 parallel micro channels for heat transfer fluid. Fins between two adjacent micro channels were removed previously, leaving a valid space with a length of 200. mm, a width of 55 mm and a height of 8.5 mm. Each space between two adjacent micro channels could accommodate two blocks of the consolidated sorbent with a length of 100 mm, a width of 55 mm and a height of 7.5 mm. Consolidated sorbents were prepared using a pressing machine previously. The obtained bulky density of the sorbents is about 530 kg/m3 owing to the ability of the pressing machine. To get a good thermal contact between the sorbents and the heat exchange surfaces, the bottom of the sorbents was glued to the surface of a plate by a kind of thermally conductive adhesive with a thermal conductivity of 2 W/(m C). The space above the sorbents serve as vapor flow channel with a height of 1 mm. Each heat exchanger could accommodate 0.96 kg sorbent. As shown in Fig. 4(c), the sorption reactor consists of three identical plate heat exchangers, which are connected in series. Thus the total length for each parallel micro channel is considered to be 600 mm. About 2.88 kg consolidate sorbent. i.e. 0.96 kg LiCl, was filled into the reactor at last. To simplify the configuration of the prototype, the condenser and the evaporator are combined into one vessel, as presented in
Fig. 4 (d). Copper spiral coil is employed as the heat exchanger for both condensation and evaporation. A vacuum liquid level glass is installed at one side of the vessel to check the water level. The sorption reactor and the condenser-evaporator is connected by a flexible tube and a ball value. Both the sorption reactor and condenser-evaporator are wrapped by rubber insulation cotton.
3.2. Experimental system and uncertainty analysis Fig. 5 presents the schematic diagram of the experimental system. The temperatures of the sorption reactor and the condenserevaporator are controlled by two thermostatic bath. The inlet and outlet fluid temperatures are recorded by Pt100 temperature sensors with accuracy of ±0.15 C. Two vacuum pressure sensors with relative accuracy of 0.5% are used to measure the inside pressure during charging and discharging processes. Two turbine flowmeters with accuracy of 1% are located in fluid circuits to record the water flow rate. During the test, the flow rates of the heat transfer fluid of the sorption reactor and the condenser-evaporator were kept at 0.30 m3/h and 0.13 m3/h respectively. The detailed parameters of the measuring instruments are listed in Table 1. The heating and cooling powers for the sorption reactor and condenser-evaporator are calculated as
q ¼ rf qv cp Tf ;in Tf;out
(4)
where Tf,in and Tf,out are the inlet and outlet heat transfer fluid temperatures.
N. Yu et al. / Energy 93 (2015) 1523e1534
Fig. 5. Schematic diagram of the experimental system.
The total delivered heat is calculated by
Zt Q¼
rf qv cp Tf;in Tf ;out dt
(5)
0
Energy storage density ESD is a key parameter to evaluate the performance of storage material and is calculated as
ESD ¼
Q ms
(6)
where ms is the mass of dry sorbent. The error for the measured heating or cooling power is obtained by using the error propagation formula [19]:
vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 12 u0 2 u vq vq u@ d Tf;in Tf;out A þ dq ¼ t dqv vqv v Tf;in Tf;out rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 2 Tf;in Tf;out ðdqv Þ2 þ q2v d Tf;in Tf ;out ¼ rcp (7) The calculated average uncertainty for power measurement is about 0.078 kW. 3.3. Operation procedures Like other thermal energy storage solutions, the operation procedures of a sorption TES system can be generally divided into two processes: charging process and discharging process. The charging process consists of a desorption process in the sorption reactor and a gaseliquid condensation process in the condenser. The charging process starts when hot water flows into to the
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reactor. To distinguish sensible heat from desorption heat, the valve connecting the reactor and the condenser-evaporator keeps close at the initial temperature-increasing process. The heat added to the reactor at this stage is used to heat the sorbent from the sorption temperature to desorption temperature. When the temperature difference between the inlet and outlet fluid becomes close, the valve could be opened and the desorption process is initiated. The vapor which clings to sorbent starts to escape as the threshold binding force between sorbate and sorbent is exceeded. The vapor is channeled through a duct to a condenser at a lower temperature level, turning into its liquid state. After the charging stage, the valve between the reactor and the condenser is closed. The discharging process starts when the setting temperature of the thermostatic bath is decreased to the sorption temperature. Similar to charging process, the first process of discharging process is sensible heat recovery process, i.e., the temperature of the reactor is decreased and the sensible heat is carried away by the heat transfer fluid to meet the loads. In sorption refrigeration systems without heat recovery, the sensible heat is taken away and released to environment. Compared with sorption refrigeration, sorption thermal storage presents a distinct advantage to make use of the sensible heat. The amount of heat which could be recovered in discharging process depends on the thermal insulation condition of system. The valve is opened when the temperature difference between the inlet and outlet fluid is small and then the sorption heat recovery process starts, which includes a sorption process in the reactor and a liquidegas phase change process in the evaporator. 4. Mathematical model For particle-packed sorption beds such as silica gel and zeolite [20,21], mass transfer resistance when vapor flows through the sorbent is often neglected due to relatively low pressure drop. However, for the consolidated sorbent or more precise research on the sorption bed [22,23], the impact of mass transfer resistance on sorption kinetics may be more obvious. Thus in this study the mass transfer in the consolidated sorption bed is considered. Based on the measured thermal conductivity and permeability of the consolidated sorbent, the combined heat and mass transfer in the reactor is simulated with a two-dimensional dynamic model. Fig. 6 presents the schematic of simulation model which is simplified from the structure of the plate sorption heat exchanger. The consolidated sorbent is laid on the surface of the aluminum plate. Heat transfer fluid (water in this case) flows beneath the aluminum plate and takes the sorption heat away, obtaining a temperature increase. Here attention is only focused on the kinetic performance of the sorption reactor during discharging process. The same model could also be used to simulate the desorption kinetic performance during charging process. As three heat exchangers are arranged in series, the total length of the sorbent in the model is set to be 0.6 m. The model is built on the following basic assumptions: - Two-dimensional model is set up for the sorbent area and aluminum area and one-dimensional model is needed for the heat transfer fluid. -The porous medium is homogeneous and isotropic.
Table 1 Systematic uncertainties of measuring instruments. Instrument Pt100 temperature sensor Pressure sensor Flowmeter
Model Type A YSZK-311 LWGY-6
Range
Absolute error
50e300 C 0e30 kPa 0.1e0.6 m3/h
±0.15 C ±0.15 kPa 0.001 m3/h
Relative error e 0.5% 1%
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Fig. 6. Schematic of simulation model.
-The thermal conductivity and permeability keep constant. - The capacity of the evaporator or condenser is great enough to meet the requirement of the sorption reactor. Thus the vapor pressure above the sorbent keeps constant during the sorption process, equal to the evaporation pressure.
ε
v rg vx þ V$ rg ug þ rs ¼ 0 vt vt
The ideal gas behavior is assumed for the water vapor phase since its pressure is at low level
PMH2 O ¼ rg RT 4.1. Energy balance
(12)
The velocity of the gas ug is determined by Darcy's Law, as presented in Eq. (1)
4.1.1. Energy balance in sorbent The overall energy conservation in the porous consolidated sorbent layer is described as
v rcp T vx þ V$ rg cp;g ug ¼ V$ðls VTÞ þ rs Hs vt vt
(8)
where rg and rs are the densities of gas and sorbent respectively; cp,g is the specific heat of gas; ls is the thermal conductivity of sorbent; Hs is the differential sorption heat; x is the water uptake per kg sorbent; rcp in the first part is the total heat capacity in the control volume of the sorbent and it contains three portions (dry sorbent, gaseous phase and liquid water adsorbed by the sorbent), as shown in the following equation:
(11)
rcp ¼ rs cp;s þ xcp;l þ εrg cp;g
4.3. Equilibrium water uptake and sorption kinetics Equilibrium water uptake for the sorbent has been measured and presented in our previous research [18]. As obvious hysteresis exists between sorption and desorption, separate fitting equations have been developed for sorption and desorption. LDF (Linear driving force) model is often used [23,24] to calculate the sorption kinetic performance:
vx ¼ Ks xeq x vt
(13)
where Ks denotes the sorption rate coefficient and xeq denotes the equilibrium water uptake.
(9) 4.4. Boundary and initial conditions
where cp,s and cp,l are the specific heats of sorbent and liquid water respectively; ε represents the sorbent porosity. 4.1.2. Energy balance in metal heat exchanger The metal wall of the heat exchanger serves as a heat transfer medium between the fluid and the sorbent. The energy conservation in the metal wall of the heat exchanger is described by Fourier heat conduction equation. 4.1.3. Energy balance in heat transfer fluid The one-dimensional energy conservation equation along the fluid direction l is described as:
rl cp;l
vTf vT h Tf Tm ¼ uf rl cp;l f þ df vt vl
(10)
where rl is the density of liquid water; uf is the velocity of fluid; h is the heat transfer coefficient; df is the equivalent diameter of heat transfer fluid channel. 4.2. Mass balance The overall mass conservation in the porous consolidated sorbent layer is described as
Thermal insulation conditions are imposed on the boundaries except the upper surface of sorbent, which serves as vapor feeding surface where water vapor enters the sorbent at a constant pressure Pe. At t ¼ 0, the initial temperatures of the sorbent, metal wall and the heat transfer fluid are set to be the same, i.e., Tsor. The initial pressure in the sorbent Pini before sorption process is determined by the previous temperature and pressure conditions in the desorption process and is assumed to be uniform. After sorption process starts, the pressure at the vapor feeding surface immediately turns to be Pe. Parameters used in the simulation are given in Table 2. Based on the past experience, the sorption heat Hs was determined to be an approximate value 2600 kJ/kg. Sorption rate coefficient Ks was calculated by using LDF model (Equ. (13)) with the kinetic data measured in the Rubotherm suspension balance [16,18]. Since the flow velocity is 0.1 m/s, the Re number was calculated to be 256, indicating laminar flow state in pipes. Thus, the value of the heat transfer coefficient between the fluid and the aluminum wall h was determined to be 2602 W/(m2 C) [25]. The desorption temperature is set at 85 C. The inlet fluid temperature during sorption process is kept at 40 C. The consolidated sorbent filled into the heat exchanger has a bulk density of
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Table 2 Parameters used in simulation. Name
Symbol
Value
Tdes Tsor Tfsr,in Tfsr,out Pe Pc Pini cp cp,g cp,s cp,l cp,m
Desorption temperature Sorption temperature Inlet fluid temperature of sorption reactor Outlet fluid temperature of sorption reactor Evaporation pressure Condensation pressure Initial pressure in sorbent Specific heat capacity Specific heat of water vapor Specific heat of sorbent Specific heat of liquid water Specific heat of metal (aluminium) Bulk density of sorbent Density of metal Sorption heat Thermal conductivity of sorbent Thermal conductivity of metal Sorbent porosity Viscosity of water vapor Sorption rate coefficient Permeability of the sorbent Heat transfer coefficient between the fluid and the aluminium wall Equivalent diameter of the heat transfer fluid channel Flow velocity of the heat transfer fluid Equilibrium water uptake
85 C 30, 35, 40, 45, 50 C Tf,in ¼ Tsor
rs rm Hs
ls lm
ε
mg Ks Kp h df uf xeq
530 kg/m3. Values of thermal conductivity and permeability are obtained according to the previous experimental tests. The model is built and solved with a commercial software COMSOL® which uses finite element method.
1201 Pa 2063 Pa 200 Pa J/(kg C) 1870 J/(kg C) 932 J/(kg C) 4180 J/(kg C) 902 J/(kg C) 530 kg/m3 2700 kg/m3 2600 kJ/kg 2.6 W/(m C) 236 W/(m C) 0.7 (0.0361T-1.0108) 106 [24], Pa s 6.16 104 1/s 3.43 1012 m2 2602 W/(m2 C) 0.001 m 0.1 m/s Given in Ref. [18], kg/kg
30 C, 35 C, 40 C, 45 C and 50 C, the peak discharging powers are 1.50 kW, 1.29 kW, 1.15 kW, 0.94 kW and 0.5 kW, corresponding to maximum temperature rises of 4.0 C, 3.5 C, 3.1 C, 2.5 C and 1.3 C respectively. The total output heat capacities are 2533 kJ, 2169 kJ, 1869 kJ, 1623 kJ and 1004 kJ.
5. Results and discussion 5.2. Experimental results 5.1. Simulation results Modeling results of kinetic performance of the sorption reactor are presented in Figs. 7e9. Fig. 7 gives the temperature profiles of the sorption reactor at different times when inlet fluid temperature is 40 C. Since the length of the sorbent are much longer than the thickness, only temperature profiles near the fluid inlet and outlet are given. It is observed that the temperature increases from fluid to sorbent in the direction of thickness z. At the beginning of sorption process (t ¼ 10 s, as shown in Fig. 7(a)), the maximum temperature difference between the top surface of the sorbent and the heat transfer fluid is about 4.2 C. The difference further increases to 6.9 C at 100 s (Fig. 7(b)) when the sorption rate has arrived at its peak. After that the difference drops to 4.4 C at 1000 s (Fig. 7(c)) and then 1.1 C at 3600 s (Fig. 7(d)). In the presented area in Fig. 7, temperature variation along the fluid flow l direction isn't apparent compared with z direction. Fig. 8 describes water uptake profiles of the sorption reactor near the fluid inlet and outlet. At 10 s (Fig. 8(a)), when sorption has just started, the water uptake is around 0.05 and the maximum water uptake difference is 0.0001. At 100 s (Fig. 8 (b)), the water uptake rises to around 0.08. At 1000 s (Fig. 8 (c)), the water uptake achieves 0.28 and the maximum water uptake difference is 0.025. At the end of sorption process (3600 s, Fig. 8(d)), the final water uptake is about 0.52 and the maximum different from bottom to top is 0.025. The results suggest that a nearly uniform temperature and water uptake profiles could be guaranteed at different sorption times with the designed sorbent thickness of 0.0075 m. Variation of output power at different inlet fluid temperatures is shown in Fig. 9. The output discharging power reaches its peak value around 100s after sorption process is initiated and then the power keeps decreasing. When the inlet fluid temperatures are
The temperature profiles for inlet and outlet temperatures of sorption reactor and condenser-evaporator during charging and discharging processes are presented in Fig. 10 under the fundamental operating condition d Tdes ¼ 85 C, Tsor ¼ Tfsr,in ¼ 40 C, Tc ¼ Te ¼ Tfce,in ¼ 15 C. As mentioned before, the first sub-process of charging process is sensible heating process. The heating process takes 13 min to increase the sorption reactor temperature from 40 C to 85 C. This part of heat is consumed by sorption bed thermal mass, including the tubes, shells sorbent, adsorbed liquid water and dead water mass leaved in tubes. When the inlet and outlet fluid temperature becomes close, the valve is opened and the desorption process is initiated. The maximum temperature gap between the inlet and outlet of sorption reactor is above 4 C. The corresponding maximum temperature difference between the inlet and outlet of the condenser-evaporator is 6.8 C. The desorption process lasts for 36 min and then the valve is closed again. The first sub-process of discharging is sensible heat recovery process. The inlet fluid temperature of sorption reactor sharply decreases to 40 C in 14 min and the pressure in the reactor decreases to a low level. With the valve opened, water vapor flushes into the reactor, causing a temperature rise in the reactor and a temperature drop in the condenser-evaporator. The maximum temperature changes in the reactor and the condenser-evaporator are 2.4 C and 3.1 C respectively. Fig. 11 presents the variation of instant charging and discharging power with time, which is derived from Fig. 10. At the beginning of charging and discharging process, the power suddenly surges to higher values, owing to the large temperature gap between the inlet fluid and the reactor. The maximum powers during heating and cooling achieve 3.2 kW and 4.7 kW respectively. Compared with heating and cooling, sorption and desorption proceeds in a
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Fig. 7. Temperature profiles near inlet (left) and outlet (right) at different sorption times. Unit of horizontal and vertical axis is mm. Unit of temperature profile is C.
relative slow way, with highest instant powers of 1.5 kW and 0.86 kW, respectively. Fig. 12 shows variation of accumulated heat with time during charging and discharging processes, which is an integral result of the power curve in Fig. 11. The total input heat is 2708 kJ and the recovered heat from the reactor is 2517 kJ, resulting a heat storage efficiency of 93%. The heat loss is mainly caused by the heat released to environment. In the output heat sorption heat contributes approximately 57%, i.e. 1438 kJ. Variation of stored heat capacity with discharging temperature is shown in Fig. 13. As predicted, fewer heat could be stored at higher discharging temperature. The total output heat decreases from 3009 kJ at 30 C to 2069 kJ at 50 C, with sorption heat falling from 1751 kJ to 1278 kJ. The contribution of sorption heat keeps relatively stable, in a range of 58%e62%. Variation of stored heat capacity with charging temperature is shown in Fig. 14. It is observed that higher charging temperature gives rise to larger amount of stored heat capacity. The total heat rises from 1120 kJ at 65 C to 2790 kJ at 90 C. A sharp rise in heat
capacity from 1752 kJ to 2414 kJ is observed when the charging temperature increases from 75 C to 80 C, possibly due to the crystallization of LiCl solution and dehydration reaction of LiCl$H2O at 80 C. As predicted in our previous work, presence of crystallization and thermochemical hydration reaction could extensively increase the storage density. Variation of stored heat capacity with condensation/evaporation temperature is shown in Fig. 15. Compared with the effects of charging and discharging temperatures, the effect of condensation/ evaporation temperature on stored heat is not obvious. This may be the combined result of condensation and evaporation temperatures: increase in condensation temperature is negative for desorption process while increase in evaporation temperature is positive for sorption process. Evolutions of experimental outlet fluid temperature and pressure of the sorption reactor in LiCleH2O pressure-temperature diagram are shown in Fig. 16. The LiCleH2O pressure-temperature diagram is divided by the crystallization line into two phases:
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Fig. 8. Water uptake profiles near inlet (left) and outlet (right) at different sorption times. Unit of horizontal and vertical axis is mm. Unit of water uptake profile is kg/kg.
liquid LiCl aqueous solution and solid LiCl hydrate. The details of this diagram has been described in the authors’ previous study [17]. During the heating process, the pressure increases with temperature to as high as 8 kPa, which drops to 2.5 kPa after discharging process. It is observed that during desorption process, state of LiCl has changed from solution area to solid area, indicating LiCl solution in the porous matrix has been turned into anhydrous LiCl salt after the desorption process. After the cooling and sorption processes, anhydrous LiCl turns back to LiCl solution in which the mass concentration of LiCl is about 40%. Fig. 16 illustrates that the transformation of solution to solid salt, i.e., so-called three-phase sorption cycle, which includes both crystallization and thermochemical hydration reactions, has be realized at the given operation conditions (Tcha ¼ 85 C, Tdis ¼ 40 C, Tc¼Te ¼ 15 C). 5.3. Comparison between experimental and simulated results To validate the established model, comparison between experimental and simulated results at a charging temperature of
85 C, a condensation/evaporation temperature of 15 C and an inlet fluid temperature of 40 C are presented in Fig. 17. The general evolution trends of experimental and simulated outlet fluid temperatures are in good agreement. Compared with the peak experiential temperature rise of 2.4 C, the simulated one is 3.1 C. An average gap of about 0.4 C is observed between the experimental and simulated outlet temperature. This may be caused by two factors: the first is the heat loss between the reactor wall and environment; the second is the constant pressure assumption. In the model, vapor pressure above the sorbent is assumed constant (Pe) during the whole sorption process. However, during the experiments we found that due to the vapor transfer resistance along the flexible connecting pipe between the sorption reactor and the condenser-evaporator, the actual pressure in the reactor increased slower than expected. An experience is learned from the fact that the connecting pipe should be shorter, straighter and wider for water systems. The total experimentally collected sorption heat is 1496 kJ, which has achieved 80% of the predicted target of 1869 kJ.
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Fig. 9. Simulated discharging power.
Fig. 10. Variations of temperatures with time.
Fig. 11. Variation of input or output power with time.
Fig. 12. Variation of accumulated heat with time.
Fig. 13. Variation of stored heat with discharging temperature (Tcha ¼ 85 Tc¼Te ¼ 15 C).
C,
Fig. 14. Variation of stored heat with charging temperature (Tdis ¼ 40 Tc¼Te ¼ 15 C).
C,
N. Yu et al. / Energy 93 (2015) 1523e1534
Fig. 15. Variation of stored heat with condensation/evaporation temperature (Tcha ¼ 85 C, Tdis ¼ 40 C).
6. Conclusions To test the heat storage performance of a new type of consolidated sorbent prepared with ENG-TSA, AC, silica solution and LiCl, a 1 kWh lab-scale sorption thermal storage prototype was set up. The thermal conductivity and permeability of the sorbent were measured first to obtain the basic heat and mass transfer parameters, which were prerequisite for modeling. A two-dimensional model considering the combined heat and mass transfer was developed to predict the sorption kinetics of the reactor. The experimental results of the prototype were compared with the simulation results. The following conclusions can be addressed: The obtained bulky density of the sorbents in the sorption reactor is about 530 kg/m3. The measured thermal conductivity and permeability are 2.6 W/(m C) and 3.43 1012 m2. Under the operation condition of a charging temperature of 85 C, a condensation/evaporation temperature of 15 C and an inlet fluid temperature of 40 C, the experimentally obtained peak output power and temperature rise are 0.86 kW and 2.4 C. The
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Fig. 17. Comparison between experimental and simulated discharging power.
total input heat is 2708 kJ and the recovered heat from the reactor is 2517 kJ, resulting a heat storage efficiency of 93%., sorption heat contributes approximately 57%, i.e., 1438 kJ in the total output heat. The heat storage density is 874 kJ/kg consolidated sorbent or 2622 kJ/kg LiCl. The established two-dimensional model proves to be effective since the general evolution trends of experimental and simulated outlet fluid temperatures are similar. An average gap of about 0.4 C between the experimental and simulated outlet temperature may be caused by the heat loss and the constant pressure assumption. The total experimental collected sorption heat is 1496 kJ, which has achieved 80% of the predicted target of 1869 kJ. The designed heat storage target of 1 kWh hasn't been fully reached in this prototype. The next research goal is to develop a 15 kWh sorption TES prototype which could meet the heat demand of one single family for several days. Thus more incentive research into system modeling and design are still required. A delicate simulation model including both the sorption reactor and the condenser-evaporator should be developed to precisely predict the kinetic performance of the TES system. The design of the sorption reactor and condensor-evaporator should be modified to decease the mass transfer loss when water vapor flows between the two vessels. Acknowledgments This work was supported by the key project of the Natural Science Foundation of China for international academic exchanges under the contract No. 51020105010 and the project of the Natural Science Foundation of China under the contract No. 51206105. The support from the Ministry of Education innovation team (IRT 1159) was also appreciated. References
Fig. 16. Cycle description (Tfsr,out and Psr) in LiCleH2O P-T diagram (Tcha ¼ 85 C, Tdis ¼ 40 C, Tc¼Te ¼ 15 C).
[1] Yu N, Wang RZ, Wang LW. Sorption thermal storage for solar energy. Prog Energ Combust 2013;39(5):489e514. [2] Cot-Gores J, Castell A, Cabeza LF. Thermochemical energy storage and conversion: a-state-of-the-art review of the experimental research under practical conditions. Renew Sust Energ Rev 2012;16(7):5207e24. [3] Deng J, Wang RZ, Han GY. A review of thermally activated cooling technologies for combined cooling, heating and power systems. Prog Energ Combust 2011;37(2):172e203. [4] Zondag H, Kikkert B, Smeding S, Rd Boer, Bakker M. Prototype thermochemical heat storage with open reactor system. Appl Energ 2013;109:360e5.
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[5] Whiting GT, Grondin D, Stosic D, Bennici S, Auroux A. Zeolite-MgCl2 composites as potential long-term heat storage materials: influence of zeolite properties on heats of water sorption. Sol Energ Mat Sol C 2014;128: 289e95. [6] Rd Boer, Haije WG, Veldhuis JBJ. Determination of structural, thermodynamic and phase properties in the Na2SeH2O system for application in a chemical heat pump. Thermochim Acta 2003;395(1e2):3e19. [7] Mauran S, Lahmidi H, Goetz V. Solar heating and cooling by a thermochemical process: first experiments of a prototype storing 60kWh by a solid/gas reaction. Sol Energy 2008;82(7):623e36. [8] Michel B, Mazet N, Mauran S, Stitou D, Xu J. Thermochemical process for seasonal storage of solar energy: characterization and modeling of a high density reactive bed. Energy 2012;47(1):553e63. [9] Michel B, Mazet N, Neveu P. Experimental investigation of an innovative thermochemical process operating with a hydrate salt and moist air for thermal storage of solar energy: global performance. Appl Energ 2014;129: 177e86. [10] Van Essen VM, Zondag HA, Cot Gores J, Bleijendaal LPJ, Bakker M, Schuitema R, et al. Characterization of MgSO4 hydrate for thermochemical seasonal heat storage. J Sol Energ Eng-Trans ASME 2009;131(4):0410141e7. [11] Posern K, Kaps C. Calorimetric studies of thermochemical heat storage materials based on mixtures of MgSO4 and MgCl2. Thermochim Acta 2010;502(1e2):73e6. [12] Quinnell JA, Davidson JH, Burch J. Liquid calcium chloride solar storage: concept and analysis. J Sol Energ Eng-Trans ASME 2011;133(1). 011010e1e8. [13] Quinnell JA, Davidson JH. Heat and mass transfer during heating of a hybrid absorption/sensible storage tank. Sol Energy 2013;104(2014):19e28. s N, Luo L. Numerical dynamic simulation and [14] N'Tsoukpoe KE, Le Pierre analysis of a lithium bromide/water long-term solar heat storage system. Energy 2012;37(1):346e58.
s N, Luo L. Experimentation of a LiBr-H2O absorption [15] N'Tsoukpoe KE, Le Pierre process for long-term solar thermal storage: prototype design and first results. Energy 2013;53(1):179e98. [16] Yu N, Wang RZ, Lu ZS, Wang LW. Development and characterization of silica gel-LiCl composite sorbents for thermal energy storage. Chem Eng Sci 2014;111:73e84. [17] Yu N, Wang RZ, Lu ZS, Wang LW, Ishugah TF. Evaluation of a three-phase sorption cycle for thermal energy storage. Energy 2014;67:468e78. [18] Yu N, Wang RZ, Lu ZS, Wang LW. Study on consolidated composite sorbents impregnated with LiCl for thermal energy storage. Int J Heat Mass Transf 2015;84:660e70. [19] Taylor JR. An introduction to error analysis: the study of uncertainties in physical measurements. 2nd Revised edition. U.S: University Science Books; 1997. [20] Gong LX, Wang RZ, Xia ZZ, Chen CJ. Design and performance prediction of a new generation adsorption chiller using composite adsorbent. Energ Convers Manage 2011;52(6):2345e50. [21] Tso C, Chao CY, Fu S. Performance analysis of a waste heat driven activated carbon based composite adsorbenteWater adsorption chiller using simulation model. Int J Heat Mass Transf 2012:7596e610. [22] Dawoud B, Vedder U, Amer E-H, Dunne S. Non-isothermal adsorption kinetics of water vapour into a consolidated zeolite layer. Int J Heat Mass Transf 2007;50(11):2190e9. [23] Zhang L, Wang L. Momentum and heat transfer in the adsorbent of a wasteheat adsorption cooling system. Energy 1999;24(7):605e24. [24] Chan KC, Chao CYH, Sze-To GN, Hui KS. Performance predictions for a new zeolite 13X/CaCl2 composite adsorbent for adsorption cooling systems. Int J Heat Mass Transf 2012;55(11e12):3214e24. [25] Shah RK, London AL. Laminar flow forced convection in ducts: a source book for compact heat exchanger analytical data. Advances in heat transfer. Supplement 1. New York: Academic Press; 1978.
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Theoretical and experimental investigation of a closed sorption thermal storage prototype using LiCl/water N. Yu, R.Z. Wang*, L.W. Wang Institute of Refrigeration and Cryogenics and Key Laboratory of Power Mechanical Engineering, MOE China, Shanghai Jiao Tong University, Shanghai, 200240, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 6 April 2015 Received in revised form 20 July 2015 Accepted 1 October 2015 Available online xxx
A 1 kWh lab-scale sorption prototype using LiCl-water was theoretically and experimentally investigated for sorption thermal energy storage. A type of consolidated composite matrix is developed for the system by using AC (activated carbon), LiCl, expanded natural graphite treated with sulphuric acid (ENG-TSA) to increase heat transfer and SS (silica solution) to enhance mechanical strength. Thermal conductivity and permeability were measured first. A two-dimensional model considering the combined heat and mass transfer was developed to predict the sorption kinetics of the reactor. Under the operation condition of a charging temperature of 85 C and a discharging temperature of 40 C, the experimentally recovered heat is 2517 kJ, resulting a heat storage efficiency of 93%. The heat storage density is 874 kJ/kg consolidated sorbent or 2622 kJ/kg LiCl. The experimental results of the prototype were compared with the simulated results. The established two-dimensional model proves to be effective since the general evolution trends of experimental and simulated outlet fluid temperatures are in good agreement. An average gap of about 0.4 C between the experimental and simulated outlet temperature may be caused by the heat loss and the constant pressure assumption. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Thermal energy storage Sorption thermal storage Consolidated sorbent Lithium chloride-water
1. Introduction Recently, thermochemical energy storage, which includes sorption processes and chemical reactions, has emerged as an research hotspot in the filed of TES (thermal energy storage) [1,2]. Sorption TES method is regarded as an important category of processes in the scope of thermochemical TES, which stores heat in the bonds between the solid/liquid sorbent and the vaporous sorbate. The advantages of thermochemical energy storage are claimed to be substantially high energy storage densities, the ability to preserve energy for long periods with limited heat losses and combing cold storage and heat storage functions in one system [1]. Conventionally, sorption technology is applied and studied for thermal transformation applications like absorption and adsorption refrigeration/heat pumps [3]. Though the basic principles for sorption refrigeration/heat pumps and sorption TES systems are similar, the different application areas give rise to different requirements for material selection and system configurations.
* Corresponding author. Tel./fax: þ86 21 34206548. E-mail address: [email protected] (R.Z. Wang). http://dx.doi.org/10.1016/j.energy.2015.10.001 0360-5442/© 2015 Elsevier Ltd. All rights reserved.
Water is always the preferred choice of sorbate for the safety issue, thus the primary task for sorption TES is to find suitable water sorbents which should possess high storage densities and low cost. Compared with traditional water sorbents like silica gel and zeolite, sorption processes between hygroscopic salt and water vapor seem more promising due to their higher theoretic storage densities. Among all the hygroscopic salts, MgCl2 [4,5], Na2S [6], SrBr2 [7e9], MgSO4 [10,11], CaCl2 [12,13], LiBr [14,15] and LiCl [16,17] have been identified by different researchers as potential candidates. Usually, the products of hydration reactions of MgCl2, Na2S, SrBr2 and MgSO4 are assumed to be hydrates with more crystal water molecules. For example, the solid/gas hydration reaction products of MgSO4 could be MgSO4$6H2O or MgSO4$7H2O [10]. Zondag et al. [4] established an open reactor with a packed reaction bed with 17 L MgCl2 and achieved a heat storage density of 140 kWh/m3, with a discharging power of 50 W. Boer et al. [6] developed a prototype of a modular sorption cooling system using Na2S/H2O. Test results showed that a cold storage capacity of 2.1 kWh and a cooling COP of 0.56 were achieved by inputting a heat capacity of 3.7 kWh with 3 kg of Na2S in one module. Researchers in PROMES-CNRS focused their attention on SrBr2 and have tried to apply SrBr2 in closed [7] and open [8,9] sorption TES systems.
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Nomenclature AC ALi40
activated carbon activated carbon-LiCl composite prepared in 40 wt.% of LiCl solution cp specific heat, J/(kg C) df equivalent diameter of the heat transfer fluid channel ENG-TSA expanded graphite treated with sulfuric acid ESALi consolidated composite sorbent prepared with ALi40 ESD energy storage density, kJ/kg h heat transfer coefficient, W/(m2 C) Ks sorption rate coefficient, 1/s Kp Permeability of the sorbent, m2 l liquid flow direction or liquid water MH2O molar mass of water, 0.018 kg/mol m mass, kg P pressure Pa Q heat, kJ q power, kW qv volumetric flow rate, m3/s R gas universal constant, J/(mol K) Re Reynolds number S cross-section area of sorbent, m2 SS silica solution T temperature, C t time, s TES thermal energy storage u velocity, m/s
The water sorption and desorption behaviors for three commonly used hygroscopic salts, namely LiCl, CaCl2 and LiBr, have been analyzed by the authors [17], and results showed that LiCl possessed a great potential for sorption TES with its high storage density. As LiCl could be fully discharged at temperatures lower than 100 C, LiCl/H2O seems to be a good option for solar energy storage To find a proper porous matrix for LiCl, we have investigated and compared the sorption properties of silica geleLiCl composite [16] and activated carboneLiCl composite [18] in the previous research and activated carbon was regarded as a better choice. Then, A new type of consolidated composite sorbent was developed by using AC (activated carbon) as a porous host matrix to carry LiCl, mixing with expanded natural graphite treated with sulfuric acid (ENG-TSA) to increase heat transfer and adding SS (silica solution) as a binder to enhance mechanical strength [18]. The purpose of this study is to test the heat storage performance of the prepared consolidated sorbent in a lab-scale prototype designed for charging temperatures lower than 90 C. The thermal conductivity and permeability of the sorbent were measured first to obtain the basic heat and mass transfer parameters, which were prerequisite for modeling. A twodimensional model considering the combined heat and mass transfer issue was developed to predict the sorption kinetics of the reactor. Experimental results of the prototype were compared with the simulation results. 2. Material characterization 2.1. Preparation of consolidated sorbent In this work, ENG-TSA was also added to activated carbon-LiCl composite through a consolidation process. The mixture of AC,
x z
water uptake, kg/kg sorbent thickness direction
Greek symbols r density, kg/m3 ε sorbent porosity l thermal conductivity, W/(m C) m viscosity, Pa S. Subscripts and superscripts c condenser, condensation ce condenser-evaporator cha charging des desorption dis discharging e evaporation, evaporator eq equilibrium f heat transfer fluid fsr heat transfer fluid for sorption reactor fce heat transfer fluid for condenser-evaporator g gas in inlet ini initial m metal out outlet s sorbent sor sorption
ENG-TSA and silica solution was compressed into a consolidated block by a pressing machine. The obtained consolidated composite sorbents after drying were named ESALi. The detailed preparing procedures of the consolidated sorbent is shown in our previous communication [18]. Consolidated sorbents with different densities were prepared for measurements of thermal conductivity and permeability. 2.2. Thermal conductivity Thermal conductivity of these samples was tested using Hot Disk TPS (Transient Plane Source) 2500. The instrument adopts the standardized TPS (Transient Plane Source) technique for thermal conductivity with claimed accuracy better than 5%, leading to an absolute uncertainty of 0.15 W/(m K) for the consolidated sorbents (low than 3 W/(m K)). During measurement, the probe of Hot Disk was put between two identical consolidated sorbents samples and clamped tight to ensure full access between the probe and the sorbents. Tests have to be tried for several times to find the most suitable parameters of probe power and measuring time. Measured results of thermal conductivities for the 5 consolidated sorbents with varied densities are given in Fig. 1. Variation of bulk densities from 500 to 818 kg/m3 produces thermal conductivities from 2.36 to 2.76 W/(m C). The highest thermal conductivity of 2.76 W/(m C) is obtained at a bulk density of 758 kg/m3, which improves that of loose-packed AC-LiCl composite (0.14 W/ (m C)) by nearly 19 times. Falling of thermal conductivities is observed when the density is greater than 800 kg/m3. This decrease may be caused by the crush of activated carbon particles under high pressing pressures to achieve the target densities. Generally, since the mass concentrations of ENG-TSA in these samples are restricted to 12%, the influence of density on thermal conductivity is not obvious.
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With the obtained differential pressure and flow rate, the permeability could be calculated according to the following equation:
Kp ¼
mg qv Dz SDP
(2)
Nitrogen with 99.9% purity from a high pressure tank was adopted in the permeability test. A flow regulating value was used to adjust the flow rate linearly. A group of data for one sample was recorded at varied flow rates and corresponding pressure drop. The permeability for the sample was deduced by fitting this group of data. The relative error for the permeability is obtained by using the error propagation formula:
dKp dqv dDP þ K q DP p v
According to Eq. (3) and the values measured in the text, the obtained maximum relative error for permeability is less than 10%. Permeability test result of four samples with varied densities is given in Fig. 3. As predicted, higher bulk densities lead to lower permeability values. When the density increases from 400 to 600 kg/m3, the permeability drops from 1.5 1011 to 1.65 1012, meaning that permeability at 600 kg/m3 is only 10% of that at 400 kg/m3. The fact shows that the effect of bulk densities on permeability is greater than that on thermal conductivity.
Fig. 1. Variation of thermal conductivity with density.
2.3. Permeability Though high bulk densities of consolidated sorbents give rise to improvement of thermal conductivities, the corresponding consequence may be poor mass transfer conditions. Permeability Kp is an important parameter to evaluate the mass transfer ability of porous materials. The permeability of the consolidated samples was measured by using a specially designed rig shown in Fig. 2. The prepared sample was placed in a square sample holder which was made of stainless steel. A differential pressure transducer Dwyer477A-4 with a measuring range of 0e43 kPa and an accuracy of ±0.1% was used to record the pressure drop in the flow of gas through the sample and a gas flowmeter (MF5706) with a measuring range of 0e10 L/min and an accuracy of ±2% was placed before the outlet of gas pipe to measure the flow rate. The permeability was measured according to Darcy's Law:
Kp ug ¼ VP mg
(1)
where ug is vapor velocity (m/s), which could be calculate by the measured flow rate qv (m3/s) and cross-section area S; Kp is permeability (m2); mg is the viscosity (Pa s); VP is the pressure gradient along the flow direction (Pa), which could be calculated by the differential pressure DP and the thickness of the sample Dz.
Fig. 2. Permeability test rig.
(3)
3. Experimental prototype 3.1. Design of the prototype Sorption working pairs with high energy density on material level cannot guarantee efficient storage performance in practical applications. Realization of the storage potential of LiCl/H2O showed for thermal energy storage greatly relies on the efficient and compact design of sorption bed to ensure good heat and mass transfer. To confirm the performance of LiCl in real systems, a labscale module with 1 kWh heat storage capacity was designed. The experimental prototype for sorption thermal storage mainly includes two vessels: one is the sorption reactor and the other is condenser-evaporator. The picture of the prototype and the detailed structures of the sorption reactor and the condenserevaporator are shown in Fig. 4. Plate-fin heat exchangers have been widely used in automotive air conditioning systems due to higher heat exchange efficiency and
Fig. 3. Variation of permeability with density.
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Fig. 4. Structures of the experimental prototype.
smaller volumes compared with tube-fin heat exchangers. Moreover, the surface of a plate-fin heat exchanger is flat, making it easier to integrate the prepared consolidated sorbent with the heat exchanger. Thus, plate heat exchangers were used in the sorption reactor. The structure of a plate heat exchanger is presented in Fig. 4(b). The dimension of singular heat exchanger is 260 260 58 mm. Each heat exchanger has 22 parallel micro channels for heat transfer fluid. Fins between two adjacent micro channels were removed previously, leaving a valid space with a length of 200. mm, a width of 55 mm and a height of 8.5 mm. Each space between two adjacent micro channels could accommodate two blocks of the consolidated sorbent with a length of 100 mm, a width of 55 mm and a height of 7.5 mm. Consolidated sorbents were prepared using a pressing machine previously. The obtained bulky density of the sorbents is about 530 kg/m3 owing to the ability of the pressing machine. To get a good thermal contact between the sorbents and the heat exchange surfaces, the bottom of the sorbents was glued to the surface of a plate by a kind of thermally conductive adhesive with a thermal conductivity of 2 W/(m C). The space above the sorbents serve as vapor flow channel with a height of 1 mm. Each heat exchanger could accommodate 0.96 kg sorbent. As shown in Fig. 4(c), the sorption reactor consists of three identical plate heat exchangers, which are connected in series. Thus the total length for each parallel micro channel is considered to be 600 mm. About 2.88 kg consolidate sorbent. i.e. 0.96 kg LiCl, was filled into the reactor at last. To simplify the configuration of the prototype, the condenser and the evaporator are combined into one vessel, as presented in
Fig. 4 (d). Copper spiral coil is employed as the heat exchanger for both condensation and evaporation. A vacuum liquid level glass is installed at one side of the vessel to check the water level. The sorption reactor and the condenser-evaporator is connected by a flexible tube and a ball value. Both the sorption reactor and condenser-evaporator are wrapped by rubber insulation cotton.
3.2. Experimental system and uncertainty analysis Fig. 5 presents the schematic diagram of the experimental system. The temperatures of the sorption reactor and the condenserevaporator are controlled by two thermostatic bath. The inlet and outlet fluid temperatures are recorded by Pt100 temperature sensors with accuracy of ±0.15 C. Two vacuum pressure sensors with relative accuracy of 0.5% are used to measure the inside pressure during charging and discharging processes. Two turbine flowmeters with accuracy of 1% are located in fluid circuits to record the water flow rate. During the test, the flow rates of the heat transfer fluid of the sorption reactor and the condenser-evaporator were kept at 0.30 m3/h and 0.13 m3/h respectively. The detailed parameters of the measuring instruments are listed in Table 1. The heating and cooling powers for the sorption reactor and condenser-evaporator are calculated as
q ¼ rf qv cp Tf ;in Tf;out
(4)
where Tf,in and Tf,out are the inlet and outlet heat transfer fluid temperatures.
N. Yu et al. / Energy 93 (2015) 1523e1534
Fig. 5. Schematic diagram of the experimental system.
The total delivered heat is calculated by
Zt Q¼
rf qv cp Tf;in Tf ;out dt
(5)
0
Energy storage density ESD is a key parameter to evaluate the performance of storage material and is calculated as
ESD ¼
Q ms
(6)
where ms is the mass of dry sorbent. The error for the measured heating or cooling power is obtained by using the error propagation formula [19]:
vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 12 u0 2 u vq vq u@ d Tf;in Tf;out A þ dq ¼ t dqv vqv v Tf;in Tf;out rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 2 Tf;in Tf;out ðdqv Þ2 þ q2v d Tf;in Tf ;out ¼ rcp (7) The calculated average uncertainty for power measurement is about 0.078 kW. 3.3. Operation procedures Like other thermal energy storage solutions, the operation procedures of a sorption TES system can be generally divided into two processes: charging process and discharging process. The charging process consists of a desorption process in the sorption reactor and a gaseliquid condensation process in the condenser. The charging process starts when hot water flows into to the
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reactor. To distinguish sensible heat from desorption heat, the valve connecting the reactor and the condenser-evaporator keeps close at the initial temperature-increasing process. The heat added to the reactor at this stage is used to heat the sorbent from the sorption temperature to desorption temperature. When the temperature difference between the inlet and outlet fluid becomes close, the valve could be opened and the desorption process is initiated. The vapor which clings to sorbent starts to escape as the threshold binding force between sorbate and sorbent is exceeded. The vapor is channeled through a duct to a condenser at a lower temperature level, turning into its liquid state. After the charging stage, the valve between the reactor and the condenser is closed. The discharging process starts when the setting temperature of the thermostatic bath is decreased to the sorption temperature. Similar to charging process, the first process of discharging process is sensible heat recovery process, i.e., the temperature of the reactor is decreased and the sensible heat is carried away by the heat transfer fluid to meet the loads. In sorption refrigeration systems without heat recovery, the sensible heat is taken away and released to environment. Compared with sorption refrigeration, sorption thermal storage presents a distinct advantage to make use of the sensible heat. The amount of heat which could be recovered in discharging process depends on the thermal insulation condition of system. The valve is opened when the temperature difference between the inlet and outlet fluid is small and then the sorption heat recovery process starts, which includes a sorption process in the reactor and a liquidegas phase change process in the evaporator. 4. Mathematical model For particle-packed sorption beds such as silica gel and zeolite [20,21], mass transfer resistance when vapor flows through the sorbent is often neglected due to relatively low pressure drop. However, for the consolidated sorbent or more precise research on the sorption bed [22,23], the impact of mass transfer resistance on sorption kinetics may be more obvious. Thus in this study the mass transfer in the consolidated sorption bed is considered. Based on the measured thermal conductivity and permeability of the consolidated sorbent, the combined heat and mass transfer in the reactor is simulated with a two-dimensional dynamic model. Fig. 6 presents the schematic of simulation model which is simplified from the structure of the plate sorption heat exchanger. The consolidated sorbent is laid on the surface of the aluminum plate. Heat transfer fluid (water in this case) flows beneath the aluminum plate and takes the sorption heat away, obtaining a temperature increase. Here attention is only focused on the kinetic performance of the sorption reactor during discharging process. The same model could also be used to simulate the desorption kinetic performance during charging process. As three heat exchangers are arranged in series, the total length of the sorbent in the model is set to be 0.6 m. The model is built on the following basic assumptions: - Two-dimensional model is set up for the sorbent area and aluminum area and one-dimensional model is needed for the heat transfer fluid. -The porous medium is homogeneous and isotropic.
Table 1 Systematic uncertainties of measuring instruments. Instrument Pt100 temperature sensor Pressure sensor Flowmeter
Model Type A YSZK-311 LWGY-6
Range
Absolute error
50e300 C 0e30 kPa 0.1e0.6 m3/h
±0.15 C ±0.15 kPa 0.001 m3/h
Relative error e 0.5% 1%
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Fig. 6. Schematic of simulation model.
-The thermal conductivity and permeability keep constant. - The capacity of the evaporator or condenser is great enough to meet the requirement of the sorption reactor. Thus the vapor pressure above the sorbent keeps constant during the sorption process, equal to the evaporation pressure.
ε
v rg vx þ V$ rg ug þ rs ¼ 0 vt vt
The ideal gas behavior is assumed for the water vapor phase since its pressure is at low level
PMH2 O ¼ rg RT 4.1. Energy balance
(12)
The velocity of the gas ug is determined by Darcy's Law, as presented in Eq. (1)
4.1.1. Energy balance in sorbent The overall energy conservation in the porous consolidated sorbent layer is described as
v rcp T vx þ V$ rg cp;g ug ¼ V$ðls VTÞ þ rs Hs vt vt
(8)
where rg and rs are the densities of gas and sorbent respectively; cp,g is the specific heat of gas; ls is the thermal conductivity of sorbent; Hs is the differential sorption heat; x is the water uptake per kg sorbent; rcp in the first part is the total heat capacity in the control volume of the sorbent and it contains three portions (dry sorbent, gaseous phase and liquid water adsorbed by the sorbent), as shown in the following equation:
(11)
rcp ¼ rs cp;s þ xcp;l þ εrg cp;g
4.3. Equilibrium water uptake and sorption kinetics Equilibrium water uptake for the sorbent has been measured and presented in our previous research [18]. As obvious hysteresis exists between sorption and desorption, separate fitting equations have been developed for sorption and desorption. LDF (Linear driving force) model is often used [23,24] to calculate the sorption kinetic performance:
vx ¼ Ks xeq x vt
(13)
where Ks denotes the sorption rate coefficient and xeq denotes the equilibrium water uptake.
(9) 4.4. Boundary and initial conditions
where cp,s and cp,l are the specific heats of sorbent and liquid water respectively; ε represents the sorbent porosity. 4.1.2. Energy balance in metal heat exchanger The metal wall of the heat exchanger serves as a heat transfer medium between the fluid and the sorbent. The energy conservation in the metal wall of the heat exchanger is described by Fourier heat conduction equation. 4.1.3. Energy balance in heat transfer fluid The one-dimensional energy conservation equation along the fluid direction l is described as:
rl cp;l
vTf vT h Tf Tm ¼ uf rl cp;l f þ df vt vl
(10)
where rl is the density of liquid water; uf is the velocity of fluid; h is the heat transfer coefficient; df is the equivalent diameter of heat transfer fluid channel. 4.2. Mass balance The overall mass conservation in the porous consolidated sorbent layer is described as
Thermal insulation conditions are imposed on the boundaries except the upper surface of sorbent, which serves as vapor feeding surface where water vapor enters the sorbent at a constant pressure Pe. At t ¼ 0, the initial temperatures of the sorbent, metal wall and the heat transfer fluid are set to be the same, i.e., Tsor. The initial pressure in the sorbent Pini before sorption process is determined by the previous temperature and pressure conditions in the desorption process and is assumed to be uniform. After sorption process starts, the pressure at the vapor feeding surface immediately turns to be Pe. Parameters used in the simulation are given in Table 2. Based on the past experience, the sorption heat Hs was determined to be an approximate value 2600 kJ/kg. Sorption rate coefficient Ks was calculated by using LDF model (Equ. (13)) with the kinetic data measured in the Rubotherm suspension balance [16,18]. Since the flow velocity is 0.1 m/s, the Re number was calculated to be 256, indicating laminar flow state in pipes. Thus, the value of the heat transfer coefficient between the fluid and the aluminum wall h was determined to be 2602 W/(m2 C) [25]. The desorption temperature is set at 85 C. The inlet fluid temperature during sorption process is kept at 40 C. The consolidated sorbent filled into the heat exchanger has a bulk density of
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Table 2 Parameters used in simulation. Name
Symbol
Value
Tdes Tsor Tfsr,in Tfsr,out Pe Pc Pini cp cp,g cp,s cp,l cp,m
Desorption temperature Sorption temperature Inlet fluid temperature of sorption reactor Outlet fluid temperature of sorption reactor Evaporation pressure Condensation pressure Initial pressure in sorbent Specific heat capacity Specific heat of water vapor Specific heat of sorbent Specific heat of liquid water Specific heat of metal (aluminium) Bulk density of sorbent Density of metal Sorption heat Thermal conductivity of sorbent Thermal conductivity of metal Sorbent porosity Viscosity of water vapor Sorption rate coefficient Permeability of the sorbent Heat transfer coefficient between the fluid and the aluminium wall Equivalent diameter of the heat transfer fluid channel Flow velocity of the heat transfer fluid Equilibrium water uptake
85 C 30, 35, 40, 45, 50 C Tf,in ¼ Tsor
rs rm Hs
ls lm
ε
mg Ks Kp h df uf xeq
530 kg/m3. Values of thermal conductivity and permeability are obtained according to the previous experimental tests. The model is built and solved with a commercial software COMSOL® which uses finite element method.
1201 Pa 2063 Pa 200 Pa J/(kg C) 1870 J/(kg C) 932 J/(kg C) 4180 J/(kg C) 902 J/(kg C) 530 kg/m3 2700 kg/m3 2600 kJ/kg 2.6 W/(m C) 236 W/(m C) 0.7 (0.0361T-1.0108) 106 [24], Pa s 6.16 104 1/s 3.43 1012 m2 2602 W/(m2 C) 0.001 m 0.1 m/s Given in Ref. [18], kg/kg
30 C, 35 C, 40 C, 45 C and 50 C, the peak discharging powers are 1.50 kW, 1.29 kW, 1.15 kW, 0.94 kW and 0.5 kW, corresponding to maximum temperature rises of 4.0 C, 3.5 C, 3.1 C, 2.5 C and 1.3 C respectively. The total output heat capacities are 2533 kJ, 2169 kJ, 1869 kJ, 1623 kJ and 1004 kJ.
5. Results and discussion 5.2. Experimental results 5.1. Simulation results Modeling results of kinetic performance of the sorption reactor are presented in Figs. 7e9. Fig. 7 gives the temperature profiles of the sorption reactor at different times when inlet fluid temperature is 40 C. Since the length of the sorbent are much longer than the thickness, only temperature profiles near the fluid inlet and outlet are given. It is observed that the temperature increases from fluid to sorbent in the direction of thickness z. At the beginning of sorption process (t ¼ 10 s, as shown in Fig. 7(a)), the maximum temperature difference between the top surface of the sorbent and the heat transfer fluid is about 4.2 C. The difference further increases to 6.9 C at 100 s (Fig. 7(b)) when the sorption rate has arrived at its peak. After that the difference drops to 4.4 C at 1000 s (Fig. 7(c)) and then 1.1 C at 3600 s (Fig. 7(d)). In the presented area in Fig. 7, temperature variation along the fluid flow l direction isn't apparent compared with z direction. Fig. 8 describes water uptake profiles of the sorption reactor near the fluid inlet and outlet. At 10 s (Fig. 8(a)), when sorption has just started, the water uptake is around 0.05 and the maximum water uptake difference is 0.0001. At 100 s (Fig. 8 (b)), the water uptake rises to around 0.08. At 1000 s (Fig. 8 (c)), the water uptake achieves 0.28 and the maximum water uptake difference is 0.025. At the end of sorption process (3600 s, Fig. 8(d)), the final water uptake is about 0.52 and the maximum different from bottom to top is 0.025. The results suggest that a nearly uniform temperature and water uptake profiles could be guaranteed at different sorption times with the designed sorbent thickness of 0.0075 m. Variation of output power at different inlet fluid temperatures is shown in Fig. 9. The output discharging power reaches its peak value around 100s after sorption process is initiated and then the power keeps decreasing. When the inlet fluid temperatures are
The temperature profiles for inlet and outlet temperatures of sorption reactor and condenser-evaporator during charging and discharging processes are presented in Fig. 10 under the fundamental operating condition d Tdes ¼ 85 C, Tsor ¼ Tfsr,in ¼ 40 C, Tc ¼ Te ¼ Tfce,in ¼ 15 C. As mentioned before, the first sub-process of charging process is sensible heating process. The heating process takes 13 min to increase the sorption reactor temperature from 40 C to 85 C. This part of heat is consumed by sorption bed thermal mass, including the tubes, shells sorbent, adsorbed liquid water and dead water mass leaved in tubes. When the inlet and outlet fluid temperature becomes close, the valve is opened and the desorption process is initiated. The maximum temperature gap between the inlet and outlet of sorption reactor is above 4 C. The corresponding maximum temperature difference between the inlet and outlet of the condenser-evaporator is 6.8 C. The desorption process lasts for 36 min and then the valve is closed again. The first sub-process of discharging is sensible heat recovery process. The inlet fluid temperature of sorption reactor sharply decreases to 40 C in 14 min and the pressure in the reactor decreases to a low level. With the valve opened, water vapor flushes into the reactor, causing a temperature rise in the reactor and a temperature drop in the condenser-evaporator. The maximum temperature changes in the reactor and the condenser-evaporator are 2.4 C and 3.1 C respectively. Fig. 11 presents the variation of instant charging and discharging power with time, which is derived from Fig. 10. At the beginning of charging and discharging process, the power suddenly surges to higher values, owing to the large temperature gap between the inlet fluid and the reactor. The maximum powers during heating and cooling achieve 3.2 kW and 4.7 kW respectively. Compared with heating and cooling, sorption and desorption proceeds in a
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Fig. 7. Temperature profiles near inlet (left) and outlet (right) at different sorption times. Unit of horizontal and vertical axis is mm. Unit of temperature profile is C.
relative slow way, with highest instant powers of 1.5 kW and 0.86 kW, respectively. Fig. 12 shows variation of accumulated heat with time during charging and discharging processes, which is an integral result of the power curve in Fig. 11. The total input heat is 2708 kJ and the recovered heat from the reactor is 2517 kJ, resulting a heat storage efficiency of 93%. The heat loss is mainly caused by the heat released to environment. In the output heat sorption heat contributes approximately 57%, i.e. 1438 kJ. Variation of stored heat capacity with discharging temperature is shown in Fig. 13. As predicted, fewer heat could be stored at higher discharging temperature. The total output heat decreases from 3009 kJ at 30 C to 2069 kJ at 50 C, with sorption heat falling from 1751 kJ to 1278 kJ. The contribution of sorption heat keeps relatively stable, in a range of 58%e62%. Variation of stored heat capacity with charging temperature is shown in Fig. 14. It is observed that higher charging temperature gives rise to larger amount of stored heat capacity. The total heat rises from 1120 kJ at 65 C to 2790 kJ at 90 C. A sharp rise in heat
capacity from 1752 kJ to 2414 kJ is observed when the charging temperature increases from 75 C to 80 C, possibly due to the crystallization of LiCl solution and dehydration reaction of LiCl$H2O at 80 C. As predicted in our previous work, presence of crystallization and thermochemical hydration reaction could extensively increase the storage density. Variation of stored heat capacity with condensation/evaporation temperature is shown in Fig. 15. Compared with the effects of charging and discharging temperatures, the effect of condensation/ evaporation temperature on stored heat is not obvious. This may be the combined result of condensation and evaporation temperatures: increase in condensation temperature is negative for desorption process while increase in evaporation temperature is positive for sorption process. Evolutions of experimental outlet fluid temperature and pressure of the sorption reactor in LiCleH2O pressure-temperature diagram are shown in Fig. 16. The LiCleH2O pressure-temperature diagram is divided by the crystallization line into two phases:
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Fig. 8. Water uptake profiles near inlet (left) and outlet (right) at different sorption times. Unit of horizontal and vertical axis is mm. Unit of water uptake profile is kg/kg.
liquid LiCl aqueous solution and solid LiCl hydrate. The details of this diagram has been described in the authors’ previous study [17]. During the heating process, the pressure increases with temperature to as high as 8 kPa, which drops to 2.5 kPa after discharging process. It is observed that during desorption process, state of LiCl has changed from solution area to solid area, indicating LiCl solution in the porous matrix has been turned into anhydrous LiCl salt after the desorption process. After the cooling and sorption processes, anhydrous LiCl turns back to LiCl solution in which the mass concentration of LiCl is about 40%. Fig. 16 illustrates that the transformation of solution to solid salt, i.e., so-called three-phase sorption cycle, which includes both crystallization and thermochemical hydration reactions, has be realized at the given operation conditions (Tcha ¼ 85 C, Tdis ¼ 40 C, Tc¼Te ¼ 15 C). 5.3. Comparison between experimental and simulated results To validate the established model, comparison between experimental and simulated results at a charging temperature of
85 C, a condensation/evaporation temperature of 15 C and an inlet fluid temperature of 40 C are presented in Fig. 17. The general evolution trends of experimental and simulated outlet fluid temperatures are in good agreement. Compared with the peak experiential temperature rise of 2.4 C, the simulated one is 3.1 C. An average gap of about 0.4 C is observed between the experimental and simulated outlet temperature. This may be caused by two factors: the first is the heat loss between the reactor wall and environment; the second is the constant pressure assumption. In the model, vapor pressure above the sorbent is assumed constant (Pe) during the whole sorption process. However, during the experiments we found that due to the vapor transfer resistance along the flexible connecting pipe between the sorption reactor and the condenser-evaporator, the actual pressure in the reactor increased slower than expected. An experience is learned from the fact that the connecting pipe should be shorter, straighter and wider for water systems. The total experimentally collected sorption heat is 1496 kJ, which has achieved 80% of the predicted target of 1869 kJ.
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Fig. 9. Simulated discharging power.
Fig. 10. Variations of temperatures with time.
Fig. 11. Variation of input or output power with time.
Fig. 12. Variation of accumulated heat with time.
Fig. 13. Variation of stored heat with discharging temperature (Tcha ¼ 85 Tc¼Te ¼ 15 C).
C,
Fig. 14. Variation of stored heat with charging temperature (Tdis ¼ 40 Tc¼Te ¼ 15 C).
C,
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Fig. 15. Variation of stored heat with condensation/evaporation temperature (Tcha ¼ 85 C, Tdis ¼ 40 C).
6. Conclusions To test the heat storage performance of a new type of consolidated sorbent prepared with ENG-TSA, AC, silica solution and LiCl, a 1 kWh lab-scale sorption thermal storage prototype was set up. The thermal conductivity and permeability of the sorbent were measured first to obtain the basic heat and mass transfer parameters, which were prerequisite for modeling. A two-dimensional model considering the combined heat and mass transfer was developed to predict the sorption kinetics of the reactor. The experimental results of the prototype were compared with the simulation results. The following conclusions can be addressed: The obtained bulky density of the sorbents in the sorption reactor is about 530 kg/m3. The measured thermal conductivity and permeability are 2.6 W/(m C) and 3.43 1012 m2. Under the operation condition of a charging temperature of 85 C, a condensation/evaporation temperature of 15 C and an inlet fluid temperature of 40 C, the experimentally obtained peak output power and temperature rise are 0.86 kW and 2.4 C. The
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Fig. 17. Comparison between experimental and simulated discharging power.
total input heat is 2708 kJ and the recovered heat from the reactor is 2517 kJ, resulting a heat storage efficiency of 93%., sorption heat contributes approximately 57%, i.e., 1438 kJ in the total output heat. The heat storage density is 874 kJ/kg consolidated sorbent or 2622 kJ/kg LiCl. The established two-dimensional model proves to be effective since the general evolution trends of experimental and simulated outlet fluid temperatures are similar. An average gap of about 0.4 C between the experimental and simulated outlet temperature may be caused by the heat loss and the constant pressure assumption. The total experimental collected sorption heat is 1496 kJ, which has achieved 80% of the predicted target of 1869 kJ. The designed heat storage target of 1 kWh hasn't been fully reached in this prototype. The next research goal is to develop a 15 kWh sorption TES prototype which could meet the heat demand of one single family for several days. Thus more incentive research into system modeling and design are still required. A delicate simulation model including both the sorption reactor and the condenser-evaporator should be developed to precisely predict the kinetic performance of the TES system. The design of the sorption reactor and condensor-evaporator should be modified to decease the mass transfer loss when water vapor flows between the two vessels. Acknowledgments This work was supported by the key project of the Natural Science Foundation of China for international academic exchanges under the contract No. 51020105010 and the project of the Natural Science Foundation of China under the contract No. 51206105. The support from the Ministry of Education innovation team (IRT 1159) was also appreciated. References
Fig. 16. Cycle description (Tfsr,out and Psr) in LiCleH2O P-T diagram (Tcha ¼ 85 C, Tdis ¼ 40 C, Tc¼Te ¼ 15 C).
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