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Absorption seasonal thermal storage cycle with high energy storage density through multi-stage output
Energy 167 (2019) 1086e1096
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Absorption seasonal thermal storage cycle with high energy storage density through multi-stage output Z.Y. Xu*, R.Z. Wang Institute of Refrigeration and Cryogenics, 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 20 July 2018 Received in revised form 15 November 2018 Accepted 17 November 2018 Available online 19 November 2018
Absorption thermal storage is attractive due its small thermal loss during long term storage, which is advantageous for seasonal solar thermal storage. For the long term storage, high energy storage density is favorable to ensure a compact system. In this paper, the novel absorption seasonal thermal storage cycles with multi-stage output processes are proposed. Comparing to the conventional cycle with single stage output, larger concentration glide could be achieved by the proposed cycles under the same condition, resulting in high energy storage density. Performances of the water-LiBr absorption thermal storage cycles with double stage output and triple stage output are calculated and compared with that of the conventional single stage cycle. Energy flows, effects of temperature parameters, and working pair comparison are analyzed. For typical condition of solar thermal charging in summer and heat output in winter with output temperature of 50 C, the proposed cycles with double stage output and triple stage output have 75.4% and 82.3% less heat losses, and achieve 7.32 times and 6.78 times higher energy storage densities than the single stage cycle. The proposed absorption thermal storage cycle with multistage output could be a good option for seasonal solar thermal energy storage. © 2018 Published by Elsevier Ltd.
Keywords: Absorption Thermal storage Seasonal Multi-stage
1. Introduction Solar thermal energy has been widely used for heating, cooling, desalination and water harvesting [1e5]. However, the solar power has strong instability and intermittency with time, weather and season changes. Solar thermal storage is necessary for the stable and continuous utilization of solar thermal energy [6,7]. For the seasonal mismatch between solar energy and user demand, longterm or seasonal thermal storage is essential [8e10]. Typically, thermal storage could be classified into sensible heat storage, latent heat storage with phase change material, sorption heat storage and chemical reaction heat storage [7,8,11,12]. Sometimes, the sorption heat storage and chemical reaction heat storage are classified into thermo-chemical heat storage. Sensible heat storage stores the thermal energy in the temperature change, so material with large specific heat capacity is preferred. Water, rock and soil are the commonly used working medium. Although such method is cheap and effective, the energy storage density is low resulting in large system volume [10,11]. Latent heat storage utilizes
* Corresponding author. 800 Dongchuan Road, Shanghai, China. E-mail address: [email protected] (Z.Y. Xu). https://doi.org/10.1016/j.energy.2018.11.072 0360-5442/© 2018 Published by Elsevier Ltd.
the phase change process to store thermal energy, so the energy storage density will be much higher than the sensible heat storage. Another advantage of latent heat storage is that it could maintain a stable output temperature which is preferred under most scenarios. Different phase change materials have been developed for different working temperatures [13]. For instance, organics including paraffin waxes [14] and fatty acids [15] are used for low temperature application while molten salt [16] is used for high temperature application. Despite the different energy storage densities achieved by the sensible heat storage and latent heat storage, they cannot keep the thermal energy in a longer storage period due to the inevitable heat loss to the ambient. Effort to add the discharging barrier and make phase change material suitable for long term storage has been made [17], however, it is still under proof-of-concept stage. Different from the sensible heat storage and latent heat storage, thermo-chemical heat storage including the absorption thermal storage could convert the thermal energy into chemical potential energy, and store the energy stably in a long term. The thermochemical heat storage could further be classified into absorption thermal storage, physical adsorption thermal storage, chemical adsorption thermal storage and chemical reaction thermal storage [10,11]. Typically, binary working pair is used for such heat storage
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system, and the discharging could be triggered by physically contacting the two parts. In the absorption thermal storage system, the binary working pair consists of the solution absorbent and absorbate. Water-LiBr [18], water-LiCl [19], water-CaCl2 [20], waterNaOH [21] and ammonia-water [22,23] are commonly studied working pairs. Comparing with the other thermos-chemical heat storage methods, absorption thermal storage has the advantage of good heat transfer performance due to its fluid state. However, most of the studies on absorption thermal storage emphasize on the working pair itself. Energy storage density is the key concern of a sorption heat storage system. Cycle improvement can also be a good choice to increase the energy storage density of absorption heat storage [23,24]. In this paper, the absorption seasonal thermal storage cycles with multi-stage output process are proposed aiming at higher energy storage density through cycle improvement. In the proposed cycle, high energy storage density is obtained from large concentration glide by the multi-stage output process. First, the schematics and P-T-x diagrams of the absorption seasonal thermal storage cycles are introduced. Then, methodology for the cycle performance analysis is presented. At last, the performance of the proposed cycles is analyzed including the energy flows analysis, analysis of different temperature parameters and comparison between different working pairs. 2. Absorption seasonal thermal storage cycle with multistage output Fig. 1 shows the schematic of the proposed absorption seasonal thermal storage cycle with double stage output process and the conventional absorption seasonal thermal storage cycle with single stage output process. The proposed cycle includes two stages, i.e. stage 1 and stage 2 as shown in Fig. 1(a) and (b). Both stage 1 and
Condensation (C)
stage 2 consist of a solution tank and an absorbate tank. In the summer charging process of the proposed cycle shown in Fig. 1(a), two solution tanks absorb the heat input from external heat source, for instance, the solar thermal energy. This process generates absorbate vapor which flows into the absorbate tanks. Absorbate vapor flowing into the absorbate tanks is cooled by the ambient through either cooling water or cooling air, and condenses into absorbate liquid. With the continuous generation in solution tanks, the concentration of absorbate in solution keeps decreasing. When the concentration of absorbate decreases to a certain value, the equilibrium temperature of solution will be almost the same with the heat source temperature. The generation processes in solution tank cannot be activated by the heat source anymore, and the charging processes are over. The solution concentration under which the charging process is terminated is decided by the heat source temperature and ambient temperature together. The charging process shown in Fig. 1(a) is the same with the summer charging process of conventional cycle shown in Fig. 1(c), except that the conventional cycle only consists of one absorbate tank and one solution tank. After the charging process, the thermal energy is converted into chemical potential energy and stored in the tanks. Here in the absorption system, the chemical potential energy is reflected by the concentration of solution. The lower absorbate concentration in the solution, the higher potential amount of absorbate vapor to be absorbed by the solution. If the absorbate tank and solution tank are disconnected by valve closing, no vapor flows between the absorbate tank and solution tank, thus preventing the concentration change in solution. In this way, the chemical potential energy could be maintained with very little energy loss even for long storage period. In the winter discharging process of the proposed cycle shown in Fig. 1(b), absorbate tank 2 absorbs thermal energy from the
Evaporation (E1)
Generation (G)
Stage1
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Heat source
Absorption (A1)
Stage1
Output Vapor
Vapor Ambient Generation (G)
Condensation (C)
Heat source Vapor
Stage2
Solution tank 2
Absorbate tank 1
Solution tank 1
Absorption (A2)
Evaporation (E2)
Q
Solution tank 1
Absorbate tank 1
Ambient
Ambient
Absorbate tank 2
Vapor
Stage2
Solution tank 2
(a) Condensation (C)
Absorbate tank 2
(b) Generation (G)
Evaporation (E)
Absorption (A) Heat output
Heat source Vapor
Vapor
Ambient Absorbate tank
Solution tank
(c)
Ambient
Solution tank
Absorbate tank
(d)
Fig. 1. Schematic diagrams of the absorption seasonal thermal storage cycles. (a) Charging process of the proposed cycle in summer, (b) Discharging process of the proposed cycle in winter, (c) Charging process of the conventional cycle in summer, (b) Discharging process of the conventional cycle in winter.
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ambient and generates absorbate vapor, the vapor is absorbed by the solution in solution tank 2 thus outputting thermal energy with high temperature. The heat output from solution tank 2 is used to heat the absorbate tank 1 and generate absorbate vapor. The vapor is absorbed by the solution in solution tank 1 thus outputting thermal energy with higher temperature. In the process shown in Fig. 1(b), the thermal energy from ambient is upgraded twice for heat output, while the thermal energy from ambient is only upgraded once in the conventional cycle as shown in Fig. 1(d). During the heat output process, the absorbate concentration in solution increases gradually until the absorption temperature is equal to or lower than the required heat output temperature. Since the double stage cycle upgrades the thermal energy one more time than the conventional single stage cycle, the double stage cycle could maintain the absorption temperature higher than the required heat output temperature even when the absorbate concentration in solution is high. In this way, the double stage cycle has the potential to achieve higher energy storage density than the conventional cycle due to its better tolerance in absorbate concentration. Since the parameter changes of the proposed cycle cannot be shown from the schematic diagrams only, year round operation of the proposed cycle is shown on P-T-x diagram in Fig. 2. In this figure, the horizontal line, vertical line and parallel slash are isobaric, isothermal, and isoconcentration, respectively. The slash on the left side has higher absorbate concentration than that on the right side. Solid line and dashed line represent the liquid flow and vapor flow, respectively. In order to distinguish the different processes in the year round operation, the charging in summer, storage from summer to winter, discharging in winter and storage from winter to summer are colored in blue, black, red and orange, respectively. The parameter changes during different periods are described as follows.
C
G (Input)
Pressure
E1
A1(Output) S2
E2 A2
S1 (Heat loss)
Discharging Storage 1: summer to winter Charging Storage 2: winter to summer TA-win ter
TA-summer TOutput
TSolar
Temperature Fig. 2. Year round operation of the proposed absorption thermal storage cycle on P-T-x diagram. The cycle consists of charging in summer under heat source temperature TSolar and ambient temperature TA-summer, storage from summer to winter, discharging in winter under ambient temperature TA-winter and heat output temperature TOutput, and storage from winter to summer. The arrow “/” represents the state variation and circle “o” represents the final state of one process.
(1) The charging process in summer represents the input of thermal energy, which consists of the generation (G) and condensation (C). The generation process includes the isoconcentration preheating of solution from summer ambient temperature (TA-summer) to the saturated condition, and the isobaric heating with concentration glide till the final state decided by the solar heat source temperature (TSolar). The condensation pressure is also decided by the ambient temperature (TA-summer). This process corresponds to Fig. 1(a). (2) During the long-term storage from summer to winter, solution concentration keeps constant due to the blocked vapor flow, but solution temperature and absorbate liquid temperature cannot be maintained in a high level considering the inevitable heat dissipation to ambient. In the real cases, the temperature of stored working medium will vary with the ambient temperature, so this process is an isoconcentration cooling process till the winter ambient temperature (TA-winter). The sensible heat loss of solution (S1) comes from the temperature decrease (TSolar to TA-winter). (3) The discharging process in winter delivers heat output from vapor absorption. In the second stage, evaporation (E2) absorbs heat from the ambient (TA-winter), and the vapor evaporated from E2 is absorbed by the solution in A2. As the solution temperature is lower than the solution equilibrium temperature, part of the absorption heat will be consumed by the solution preheating, and the concentration will have a small change during the preheating. After the preheating, the absorption process is an isobaric process which delivers heat output to E1. In the second stage, the processes are similar to that in the first stage expect that the heat input to E1 comes from A2 and heat output from A1 is delivered to the user side. The heat output from A1 lasts until the absorption temperature decreases to the required heat output temperature (TOutput). During this process, the large concentration glide also makes the heat output temperature unstable. The output temperature at the initial stage of heat output will be higher than the required output temperature. This can be solved by controlling the flow rate of water to be heated. (4) During the storage from winter to summer, the solution concentration keeps constant, while the temperatures of working medium vary with the ambient. Different from the storage from summer to winter, the working medium temperatures increase with the ambient here (S2). To better illustrate the advantage of the proposed cycle, the simplified P-T-x diagrams of the proposed cycle and the conventional cycle are compared under the same condition as shown in Fig. 3. The storage and preheating processes are neglected in the simplified P-T-x diagram considering that they are sensible heat changes, while the latent heat changes including the generation and absorption are emphasized. It could be found that the proposed double stage cycle could achieve larger concentration glide than the conventional single stage cycle. This is because the double stage cycle could deliver absorption heat output higher than the required temperature (TOutput) even when the absorbate concentration in solution is high. In the absorption thermal storage cycle, larger concentration glide represents higher energy storage density [23], so the double stage cycle has the potential advantage in energy storage density which will be discussed in detail later. The other thing need to be mentioned is the crystallization risk when waterLiBr is used as working pair. Since the charging process of the proposed cycle is the same with the conventional cycle, the highest concentration in the proposed cycle is also the same with the conventional single stage cycle, which means the crystallization risk is not increased. The concentration glide is only increased from
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Discharging Storage Charging
P1
G (Input)
C
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Discharging Storage Charging
P1
C
G (Input)
E1 P3 E P2
A (Output) TA-win ter
TA-summer
TOutput
P2 TSolar
A1(Output) E2 A2 TA-winter
Concentration glide
TA-summer TOutput
TSolar
Concentration glide
Fig. 3. Concentration glide comparison between absorption seasonal thermal storage cycles on simplified P-T-x diagrams. (a) Conventional cycle with single stage output, (b) Proposed cycle with double stage output.
the discharging process. Similar to the cycle shown in Fig. 2, more absorption seasonal thermal storage cycles with multi-stage output could be constructed. Fig. 4 shows the simplified P-T-x diagram of a triple stage cycle. It has the same charging and storage processes with the single stage cycle and double stage cycle. During the discharging of the triple stage cycle, the absorption of the third stage A3 delivers heat output to the evaporation of the second stage E2, the absorption of the second stage A2 delivers heat output to the evaporation of the first stage E1, and A1 delivers heat output to the user side finally. Larger concentration glide could be achieved comparing with the double stage cycle. However, more internal heat exchange processes might also bring more irreversible losses. Whether the triple stage cycle or the double stage cycle is advantageous still depends on the specific working condition, and will be discussed later.
3. Methods for performance analysis To quantitatively investigate the performance of the absorption seasonal thermal storage cycle with multi-stage output, detailed
Discharging Storage Charging
P1
analyses are carried out through calculation. In this section, the methodology for performance calculation will be introduced, including the assumptions, parameter definitions and thermophysical property calculation. As the performance enhancement achieved by the proposed cycles is mainly decided by the thermodynamic cycle construction, the working pair selection is actually not important here. Typical absorption working pair of water-LiBr is used for the majority of the performance analysis, and cycle with ammonia-water is also analyzed for comparison. The properties of water-LiBr and ammonia-water including temperature, pressure, concentration and enthalpy are calculated from the published data [25,26]. Following assumptions are made according to the related literature on absorption systems [23,27,28]: (1) The generation pressure is equal to the condensation pressure; (2) The absorption pressure is equal to the evaporation pressure; (3) The final states of generation and absorption processes reach the equilibrium;
C
G (Input)
E1 P5
A1(Output) E2
P4
A2 E3
A3
P2 TA-winter
TA-summer TOutput
TSolar
Fig. 4. Simplified P-T-x diagram of the absorption seasonal thermal storage cycle with triple stage output.
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(4) The solution concentration keeps constant during storage process; (5) The temperatures of solution and absorbate equal to the ambient temperature during storage process. (6) The temperature approach between absorption and evaporation is 5 C. The performance calculation also follows the basic conservation [23,27,28] as shown in Eqs. (1)- (3). Eq. (1) represents that the sum of mass flow into and out of one component (mi) is equal to zero since there is no internal mass source. Eq. (2) represents that the sum of absorbate/absorbent flows into and out of one component (miXi) is equal to zero. Eq. (3) represents that the sum of energy flows into and out of one component (mihi) is equal to its corresponding external heat exchange amount (Q).
X mi ¼ 0
(1)
i
X mi xi ¼ 0
(2)
i
X mi hi ¼ Q
(3)
i
where mi, xi and hi represent the mass flow rate, concentration and enthalpy of each stream regarding one component. Parameters including the concentration glide, COP, energy storage density (ESD) and optimized mass split ratio are calculated. The concentration glide refers to the solution concentration change before charging and after charging. Considering the concentration before charging is equal to the concentration after discharging, this parameter is actually decided by the charging conditions and discharging conditions together. It could reflect the potential of energy storage density. COP is used to evaluate the energy efficiency. It is defined as the total heat output in winter divided by the total heat input in summer as shown in Eq. (4):
P Qa;i COP ¼ P i Q i g;i
(4)
where Qa,i and Qg,i refer to the absorption heat output and generation heat input, respectively. ESD is the most important parameter for energy storage system, and is defined as the total heat output in winter divided by the total mass of working medium used in the cycle as shown in Eq. (5):
P Q a;i ESD ¼ Pi i Mi
P
ms2;i i ms1;i
R2 ¼ Pi
(6)
P
ms3;i i ms1;i
R3 ¼ Pi
(7)
P P P where ms1;i , ms2;i and ms3;i refer to the working medium i i i second stage and the third stage, masses of the first stage, the respectively. For the double stage cycle, only R2 will be used, while for the triple stage cycle, both R2 and R3 will be sued. 4. Results and discussion In this section, the energy flow analysis and effect of different parameters are analyzed with water-LiBr. Charging in summer with solar thermal power and discharging in winter for heating are considered. Considering the limitations of water-LiBr working pair including crystallization and freezing, performance with ammoniawater working pair is also calculated and compared with that of the water-LiBr working pair. 4.1. Energy flow analysis To give a clear description of how energy flows during the year round operation of absorption thermal storage cycle, the heat input, heat loss, heat gained from ambient and heat output are analyzed for both the conventional single stage cycle and the proposed double/triple stage cycle. Charging in summer considers solar heat input temperature (TSolar in Fig. 2) of 90 C and ambient temperature (TA-summer in Fig. 2) of 35 C. Discharging in winter considers ambient temperature (TA-winter in Fig. 2) of 5 C and heat output temperature (TOutput in Fig. 2) of 50 C. The total heat input in each cycle (Qg) is set as 100 kJ for fare comparison. Heat losses in different stages are referred as Ql,s1, Ql,s2 and Ql,s3, respectively. Heat output in different stages is referred as Qa,s1, Qa,s2 and Qa,s3, respectively. The heat output to user in the single stage cycle, the double stage cycle and the triple stage cycle are described by Qa,s1, Qa,s2 and Qa,s3, respectively. As shown in Fig. 5, the first stage heat loss Ql,s1 from the single stage cycle is much larger than that of the other two cycles. This comes from the constant heat input setting here. For the same amount of solution, the heat loss should be equal in different cycles since they are all decided by the temperature difference between TSolar and TA-winter, but the single stage cycle has less heat input due
(5)
where Mi refers to the mass of working medium in each component. Considering that all the absorbate is absorbed into solution after discharging, the total mass of solution after discharging represents the total mass of working medium. In the proposed cycle with multi-stage output, there is an optimization issue: how to get the optimized heat output by separating the solution into different stages. Taking the double stage cycle shown in Fig. 2 as an example, if the first stage has much more solution than the second stage, then the second stage cannot provide enough heat output to the evaporation of the first stage. There will be some remaining chemical potential energy stored in the first stage when the discharging process is finished. In this case, we also include two optimized mass ratio parameters as shown in Eqs. (6) and (7):
Fig. 5. Energy flows of the absorption seasonal thermal storage cycles including the single stage cycle, double stage cycle, and triple stage cycle. TA-summer, TOutput and TA winter are 35 C, 50 C and 5 C, respectively.
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to its smaller concentration glide. In this case, the heat loss in the single stage cycle will be more serious considering the same amount of thermal energy input. It is the same reason why the Ql,s1 for double stage cycle (7.7 kJ) is larger than that of the triple stage cycle (3.6 kJ). The total heat losses of single effect cycle, double stage cycle and triple stage cycle are 65.0 kJ, 16.0 kJ and 11.5 kJ, respectively. By adopting double stage cycle and triple stage cycle, heat losses could be reduced by 75.4% and 82.3% compared with the conventional single stage cycle. As a result, the first stage heat output Qa,s1 from the single stage cycle (18.2 kJ) is much smaller than that from the other two cycles (39.3 kJ and 27.2 kJ). However, it is also noticed that the heat outputs Qa,s1 and Qa,s2 for the double stage cycle (39.3 kJ and 39.3 kJ) are larger than the heat outputs Qa,s1, Qa,s2 and Qa,s3 for the triple stage cycle (27.2 kJ, 27.2 kJ and 29.0 kJ), the triple stage cycle doesn't obtain more heat output to user as expected. This is because the solution is separated into two parts in the double stage cycle, while it is separated into three parts in the triple stage cycle. More solution is used for the heat output in each stage for the double stage cycle. Although the energy flows are analyzed based on the same heat input in Fig. 5, the three cycles actually need different amounts of working medium for the same heat input capacity. Considering the different amounts of working medium in each cycle, ESDs of 34.7 kJ/
(a)
(c)
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kg (61.2 MJ/m3), 254.0 kJ/kg (396.1 MJ/m3) and 235.5 kJ/kg (350.2 MJ/m3) are obtained for the single stage cycle, double stage cycle and triple stage cycle, respectively. Comparing with the single stage cycle, weight averaged ESD enhancements of 7.3 times and 6.8 times are achieved by the double stage cycle and triple stage cycle, respectively. Considering an area averaged heat load of 40 W/m2, 31104 MJ of heat output is necessary to fulfill the heating demand of a 100 m2 house for 3 months, and 508.2 m3, 78.5 m3 and 88.8 m3 storage volumes are needed for the single stage, double stage and triple stage cycles, respectively. The storage volume could be reduced a lot by the proposed cycles which makes the seasonal thermal storage feasible for real application. Based on the above analysis, it can be concluded that multistage output has two contrary effects on the cycle performance. On one hand, more stages bring larger concentration glide which is beneficial for storing more thermal energy and reducing the relative amount of heat losses. On the other hand, more stages also separate the solution into more parts. Larger portion of heat output is consumed internally for temperature upgrading, while the heat output to the user side will be less. Whether the multi-stage output has benefit on the cycle performance depends on the competition of these two different effects, and will have different overall impacts under different conditions. In the next few sections, the effect of temperature parameters on the cycle performance will be
(b)
(d)
Fig. 6. Effect of heat output temperature TOutput on the performance of absorption seasonal thermal storage cycles. TSolar, TA-summer and TA-winter are 90 C, 35 C and 5 C, respectively. (a) Concentration glide (b) COP, (c) ESD, (d) Optimized mass ratio.
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discussed quantitatively. 4.2. Effect of different parameters on cycle performance In this section, the effects of heat output temperature, heat source temperature, winter ambient temperature and absorptionevaporation temperature approach on system performance will be analyzed in detail. Concentration glide, COP, ESD and the optimized mass ratio will be used for performance evaluation. a. Effect of heat output temperature TOutput Fig. 6 shows the calculation results under TOutput, TSolar, TA-summer and TA-winter of 40e80 C, 90 C, 35 C and 5 C, respectively. Higher output temperature means higher grade of thermal energy output is required. When the output temperature increases, the concentration glides of single stage cycle, double stage cycle and triple stage cycle decrease in the range of 0.070e0.021, 0.158e0.058, and 0.197e0.120, respectively. When the output temperature is higher than 50 C, concentration glide of the single effect cycle reaches zero, while the double stage cycle and triple stage cycle could still maintain a relatively large concentration glide. Fig. 6 (b) shows the COP comparison. COPs of all the three cycles
decrease with output temperature, and the COP of single stage cycle decreases fastest. This could easily be explained by the small concentration glide of single stage cycle shown in Fig. 6(a). The double stage cycle has higher COP than the triple stage cycle when the output temperature is lower than 75 C due to less internal heat exchange amount. However, the triple stage cycle has the highest COP when the output temperature is higher than 75 C. This represents the advantage brought by large concentration glide is stronger than the disadvantage brought by solution separation in different stages, and this has been discussed in Section 4.1. Although COP is an important parameter for energy conversion system, the ESD is more important for the thermal storage system. Fig. 6(c) shows the ESDs of the three cycles. The double stage and triple stage cycles have higher ESDs than the single stage cycle as expected. When the output temperature is higher than 55 C, the triple stage cycle has the highest ESD. When the output temperature is higher than 50 C, the conventional single stage cycle cannot work, while the proposed double and triple stage cycle still have quite high ESD. For output temperature of 40e50 C, the double stage cycle and triple stage cycle achieve 1.22e7.32 times and 1.04e6.78 times higher ESD than the single stage cycle. For a typical heat output temperature of 50 C, it can be concluded from this section that double stage cycle is the most suitable choice. The
Fig. 7. Effect of heat source temperature TSolar on the performance of absorption seasonal thermal storage cycles. TA-summer, TOutput and TA-winter are 35 C, 50 C and 5 C, respectively. (a) Concentration glide (b) COP, (c) ESD, (d) Optimized mass ratio. c. Effect of winter ambient temperature TA-winter
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reason behind this is that the double/triple stage cycles have large concentration glide, which make them less sensitive to the decrease of concentration glide under higher output temperature. In order to achieve the optimized output, the solution needs to be separated into different stages with different masses. For the double stage cycle, the optimized mass ratio R2 decreases from 1.08 to 0.97 with increasing output temperature. For the triple stage cycle, the ratio R2 keeps almost constant around 1.08, while the ratio R3 decreases from 1.14 to 1.09. The optimized mass ratios are close to 1.00, so the even separation strategy for solution is an effective and simple method to obtain heat output close to the optimized value. b. Effect of heat source temperature TSolar Fig. 7 shows the effect of heat source temperature, i.e., the generation temperature during charging process, on the cycle performance. Performances under generation temperatures of 75e91 C are studied. As shown in Fig. 7 (a), the concentration glides increase in the ranges of 0.016e0.025, 0.061e0.134 and 0.106e0.178 for the single stage cycle, double stage cycle and triple stage cycle, respectively. As shown in Fig. 7 (b), the COP increases in the ranges of 0.07e0.27, 0.27e0.40 and 0.23e0.29 for the single stage cycle, double stage cycle and triple stage cycle, respectively. As shown in Fig. 7 (c), the ESD increases in the ranges of 11.9e57.1 kJ/kg, 95.6e264.7 kJ/kg, and 129.1e242.9 kJ/kg for the single stage, double stage and triple stage cycles, respectively. The effect of lower output temperature is almost the same with that of higher heat source temperature. This could be understood from the point view of energy grade. In the charging process, better performance could be achieved if higher grade thermal energy is inputted. In the
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discharging process, better performance could be achieved if lower grade thermal energy is outputted. As for the optimized mass ratio shown in Fig. 7 (d), the ranges of R2 and R3 here are similar to that shown in Fig. 6 (d), representing the even separation strategy for solution is still effective. Fig. 8 shows the effect of winter ambient temperature, i.e., the evaporation temperature during discharging, on the cycle performance. Performances under evaporation temperatures of 2e16 C are studied. Fig. 8 (d) proves the effectiveness of even separation strategy for solution again. As shown in Fig. 8 (a)-(c), the increases of concentration glide, COP and ESD with increasing evaporation temperature come from the same reason with the previous analysis on thermal energy grade. However, there are something different in Fig. 8 compared to the previous two figures. The concentration glides in all the three cycles increase slightly with the increasing evaporation temperature, but the effects of increasing evaporation temperature on COP and ESD are different for the three cycles. The COP and ESD of the double stage cycle and triple stage cycle are relatively steady under different evaporation temperature, while the COP and ESD of the single stage cycle decrease fast with the deceasing evaporation temperature. The single stage cycle is more sensitive to the winter ambient temperature due to its weak temperature lift ability compared to other cycles. Considering the unsteady ambient temperature, conventional single stage cycle might not be able to provide steady heat output in winter, and this could be solved by adopting the double stage cycle or triple stage cycle. Except for the external conditions, the internal heat exchange is also important for the system performance. In this case, the effect of different absorption-evaporation temperature approaches is also examined. Fig. 9 shows the COP and ESD under absorptionevaporation temperature approaches from 1 C to 9 C. Since
Fig. 8. Effect of winter ambient temperature TA-winter on the performance of absorption seasonal thermal storage cycles. TSolar, TA-summer and TOutput are 90 C, 35 C and 50 C, respectively. (a) Concentration glide (b) COP, (c) ESD, (d) Optimized mass ratio. d. Effect of absorption-evaporation temperature approach
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Fig. 9. Effect of absorption-evaporation temperature approach on the performance of absorption seasonal thermal storage cycles. TSolar, TA-summer, TOutput and TA-winter are 90 C, 35 C, 50 C and 5 C, respectively. (a) COP, (b) ESD.
there is no absorption-evaporation heat exchange in the single stage cycle, the single stage cycle is not affected. The COP and ESD of the double stage cycle and triple stage cycle decrease with increasing temperature approach. This could be explained by more internal irreversible losses under larger temperature approach. Besides, it can also be found that the ESD is more sensitive to the temperature approach than the COP. 4.3. Performance with ammonia-water working pair Typical working pairs for absorption system include water-LiBr and ammonia-water. In the previous two sections, water-LiBr is used for performance analysis. However, the water-LiBr working pair is limited by the crystallization risk under high generation temperature and freezing issue under low evaporation temperature. In the absorption seasonal thermal storage cycle, solar heat source temperature and winter ambient temperature may vary a lot due to the instable external conditions. Ammonia-water working pair could be an alternative choice when water-LiBr cannot work due to the aforementioned issues. The effects of solar heat source temperature, i.e. the generation temperature in charging process, on the performance of double stage cycle with two working pairs are compared in Fig. 10. The ESD of double stage cycle with water-LiBr increases from 95.6 kJ/kg to
264.7 kJ/kg when the generation temperature increases from 75 C to 91 C. If the generation temperature is further increased, the water-LiBr solution will have high concentration of LiBr which is easy to be crystallized. The ESD of double stage cycle with ammonia-water increases from 12.2 kJ/kg to 175.9 kJ/kg when the generation temperature increases from 75 C to 105 C. Better adaptability could be achieved by ammonia-water solution, but the ESD is lower mainly due to the lower latent heat of ammonia. The COP variation follows the similar trend with the ESD. The COPs for water-LiBr cycle and ammonia-water cycle vary in the ranges of 0.27e0.40 and 0.03e0.23, respectively. Although, the ammoniawater has better flexibility, higher ESD in water-LiBr system is acceptive. In the real operation, crystallization of water-LiBr system could actually be avoided by monitering the temperature or concentration of solution. Terminating the charging process under a certain solution temperature could be an effective option. Fig. 11 shows the results under different winter ambient temperature, i.e. the evaporation temperature in discharging. The ESD of double stage cycle with water-LiBr increases from 232.0 kJ/kg to 305.5 kJ/kg when the evaporation temperature increases from 2 C to 12 C. If the evaporation temperature is lower, the water-LiBr working pair cannot work due to freezing issue. The ESD of the double stage cycle with ammonia-water increases from 9.25 kJ/kg to 158.6 kJ/kg when the evaporation temperature increases
Fig. 10. Performance of the proposed two-stage cycles with ammonia-water and water-LiBr under high heat source temperatures. TA-summer, TOutput and TA-winter are 35 C, 50 C and 5 C, respectively. (a) COP, (b) ESD.
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Fig. 11. Performance of the proposed two-stage cycles with ammonia-water and water-LiBr under low winter ambient temperatures. TSolar, TA-summer and TOutput are 90 C, 35 C and 50 C, respectively. (a) COP, (b) ESD.
from 8 C to 12 C. The COPs for water-LiBr cycle and ammoniawater cycle varies in the ranges of 0.38e0.41 and 0.02e0.24, respectively. Similar to the previous analysis, the ammonia-water system has better flexibility but lower ESD and COP. In the real operation, the water-LiBr system is preferred for winter ambient temperature higher than 5 C considering its higher energy storage density and freezing issue. If the winter ambient temperature is lower than 5 C, ammonia-water is better.
5. Conclusion Absorption thermal storage system could store thermal energy in the form of chemical potential energy, thus avoiding heat losses during long-term storage. This makes the absorption thermal storage suitable for seasonal thermal energy storage. High energy storage density is the target of different researches on thermal storage, and previously researchers mainly focused on working pair selection. In this paper, absorption seasonal thermal storage cycles with multi-stage output are proposed to increase the energy storage density of absorption system from the cycle improvement. The proposed cycles achieve large concentration glide through the multi-stage output, which further increases the energy storage density. Detailed analyses of the proposed cycles are carried out through calculation with water-LiBr working pair. Energy flow analysis, study of different temperature parameters and comparison between different working pairs are carried out. Following conclusions can be made. (1) The proposed absorption seasonal thermal storage cycles increase the concentration glide. Large concentration glide increases the percentage of useful heating input to the total heat input, and reduces the relative amount of heat losses during long term storage. For condition with solar heat source temperature of 90 C, summer ambient temperature of 35 C, winter ambient temperature of 5 C and heat output temperature of 50 C, the proposed cycles with double stage output and triple stage output have 75.4% and 82.3% less heat losses than the conventional single stage cycle. (2) The multi-stage output processes in the proposed cycle have both advantage and disadvantage for the system performance. On one hand, it could increase the concentration glide, which is beneficial for large energy storage density. On the other hand, it will separate the stored working medium into different stages, while only one stage delivers heat output to the user side. This will reduce the energy storage density. The competition between these two effects makes
the double stage cycle and triple stage cycle suitable for different working scenarios. (3) Generally, the proposed cycle with double stage output has higher energy storage density under most studied conditions. Under the condition where TSolar, TA-summer, TOutput and TA-winter are 90 C, 35 C, 50 C and 5 C, the conventional single stage cycle, the proposed double stage cycle and triple stage cycle achieve ESDs of 34.7 kJ/kg (61.2 MJ/m3), 254.0 kJ/ kg (396.1 MJ/m3) and 235.5 kJ/kg (350.2 MJ/m3), respectively. The double stage cycle and triple stage cycle have 7.32 times and 6.78 times higher weight averaged ESDs than the single stage cycle. The proposed cycle with triple stage output has higher energy storage density under low heat source temperature, low winter ambient temperature and high heat output temperature. The conventional cycle with single stage output has lower energy storage density under most conditions. It can be concluded that the proposed absorption thermal storage cycle with multi-stage output is a good option for long term storage of solar energy. (4) The proposed cycle with water-LiBr has higher energy storage density than that with ammonia-water. However, it is limited by crystallization under high generation temperature and freezing issue under low winter ambient temperature. Since higher energy storage density is still preferred for long term storage. The crystallization risk could be controlled by monitoring the generation temperature or concentration, and freezing issue could be avoided by putting the system into warmer space such as the basement. If these two limitations cannot be avoided by such methods, ammonia-water system could be used. Acknowledgement This work is supported by the National Natural Science Foundation of China (Grant No. 51606124). The support from the Foundation for Innovative Research Groups of the National Natural Science Foundation of China (Grant No. 51521004) is also appreciated. References [1] Lewis NS. Research opportunities to advance solar energy utilization. Science. 2016;351(6271):aad1920. [2] Wang RZ, Xu ZY, Pan QW, Du S, Xia ZZ. Solar driven air conditioning and refrigeration systems corresponding to various heating source temperatures. Appl Energy 2016;169:846e56. [3] Ge TS, Wang RZ, Xu ZY, Pan QW, Du S, Chen XM, et al. Solar heating and cooling: present and future development. Renew Energy 2018;126:1126e40.
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[4] Qiblawey HM, Banat F. Solar thermal desalination technologies. Desalination 2008;220(1e3):633e44. [5] Kim H, Yang S, Rao SR, Narayanan S, Kapustin EA, Furukawa H, et al. Water harvesting from air with metal-organic frameworks powered by natural sunlight. Science 2017;356(6336):430e4. [6] Kuravi S, Trahan J, Goswami DY, Rahman MM, Stefanakos EK. Thermal energy storage technologies and systems for concentrating solar power plants. Prog Energy Combust Sci 2013;39(4):285e319. [7] Tian Y, Zhao C-Y. A review of solar collectors and thermal energy storage in solar thermal applications. Appl Energy 2013;104:538e53. [8] Xu J, Wang RZ, Li Y. A review of available technologies for seasonal thermal energy storage. Sol Energy 2014;103:610e38. [9] Pinel P, Cruickshank CA, Beausoleil-Morrison I, Wills A. A review of available methods for seasonal storage of solar thermal energy in residential applications. Renew Sustain Energy Rev 2011;15(7):3341e59. s N, Luo L. A review on long-term sorption solar [10] N’tsoukpoe KE, Liu H, Le Pierre energy storage. Renew Sustain Energy Rev 2009;13(9):2385e96. [11] Yu N, Wang RZ, Wang LW. Sorption thermal storage for solar energy. Prog Energy Combust Sci 2013;39(5):489e514. [12] Yan T, Wang RZ, Li TX, Wang LW, Fred IT. A review of promising candidate reactions for chemical heat storage. Renew Sustain Energy Rev 2015;43: 13e31. [13] Lane GA. Solar heat storage. In: Latent heat material, vol. II. CRC press; 2018. [14] Kousksou T, Jamil A, El Rhafiki T, Zeraouli Y. Paraffin wax mixtures as phase change materials. Sol Energy Mater Sol Cell 2010;94(12):2158e65. [15] Sarı A, Biçer A. Thermal energy storage properties and thermal reliability of some fatty acid esters/building material composites as novel form-stable PCMs. Sol Energy Mater Sol Cell 2012;101:114e22. [16] Zhang P, Ma F, Xiao X. Thermal energy storage and retrieval characteristics of a molten-salt latent heat thermal energy storage system. Appl Energy 2016;173:255e71. [17] Han GG, Li H, Grossman JC. Optically-controlled long-term storage and release of thermal energy in phase-change materials. Nat Commun 2017;8(1):1446. s N, Luo L. Experimentation of a LiBreH2O absorption [18] N'Tsoukpoe K, Le Pierre process for long-term solar thermal storage: prototype design and first results. Energy 2013;53:179e98. [19] Yu N, Wang RZ, Wang LW. Theoretical and experimental investigation of a closed sorption thermal storage prototype using LiCl/water. Energy 2015;93: 1523e34. [20] 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. [21] Weber R, Dorer V. Long-term heat storage with NaOH. Vacuum 2008;82(7): 708e16. [22] Xu S, Zhang L, Liang J, Du R. Variable mass energy transformation and storage
[23] [24] [25]
[26] [27] [28]
(VMETS) system using NH3eH2O as working fluid. Part 2: modeling and simulation under partial storage strategy. Energy Convers Manag 2007;48(1): 27e39. Xu ZY, Wang RZ. A sorption thermal storage system with large concentration glide. Energy 2017;141:380e8. Xu ZY, Wang RZ. Absorption refrigeration cycles: categorized based on the cycle construction. Int J Refrig 2016;62:114e36. Patek J, Klomfar J. A computationally effective formulation of the thermodynamic properties of LiBreH2O solutions from 273 to 500 K over full composition range. Int J Refrig 2006;29(4):566e78. Ziegler B, Trepp C. Equation of state for ammonia-water mixtures. Int J Refrig 1984;7(2):101e6. Herold KE, Radermacher R, Klein SA. Absorption chillers and heat pumps. 1996. Xu ZY, Wang RZ, Xia ZZ. A novel variable effect LiBr-water absorption refrigeration cycle. Energy 2013;60:457e63.
Nomenclature P: pressure (kPa) T: temperature (oC) x: concentration () m: mass flow rate (kg/s) h: enthalpy (kJ/kg) Q: power (kJ) COP: coefficient of performance () ESD: energy storage density (kJ/kg) M: mass (kg) R2, R3: optimized mass ratio (kg) Subscripts and superscripts Solar: solar heat source A-summer: summer ambient A-winter: winter ambient Output: heat output i: different streams or components a: absorption g: generation s1, s2, s3: different stages l: heat loss
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Absorption seasonal thermal storage cycle with high energy storage density through multi-stage output Z.Y. Xu*, R.Z. Wang Institute of Refrigeration and Cryogenics, 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 20 July 2018 Received in revised form 15 November 2018 Accepted 17 November 2018 Available online 19 November 2018
Absorption thermal storage is attractive due its small thermal loss during long term storage, which is advantageous for seasonal solar thermal storage. For the long term storage, high energy storage density is favorable to ensure a compact system. In this paper, the novel absorption seasonal thermal storage cycles with multi-stage output processes are proposed. Comparing to the conventional cycle with single stage output, larger concentration glide could be achieved by the proposed cycles under the same condition, resulting in high energy storage density. Performances of the water-LiBr absorption thermal storage cycles with double stage output and triple stage output are calculated and compared with that of the conventional single stage cycle. Energy flows, effects of temperature parameters, and working pair comparison are analyzed. For typical condition of solar thermal charging in summer and heat output in winter with output temperature of 50 C, the proposed cycles with double stage output and triple stage output have 75.4% and 82.3% less heat losses, and achieve 7.32 times and 6.78 times higher energy storage densities than the single stage cycle. The proposed absorption thermal storage cycle with multistage output could be a good option for seasonal solar thermal energy storage. © 2018 Published by Elsevier Ltd.
Keywords: Absorption Thermal storage Seasonal Multi-stage
1. Introduction Solar thermal energy has been widely used for heating, cooling, desalination and water harvesting [1e5]. However, the solar power has strong instability and intermittency with time, weather and season changes. Solar thermal storage is necessary for the stable and continuous utilization of solar thermal energy [6,7]. For the seasonal mismatch between solar energy and user demand, longterm or seasonal thermal storage is essential [8e10]. Typically, thermal storage could be classified into sensible heat storage, latent heat storage with phase change material, sorption heat storage and chemical reaction heat storage [7,8,11,12]. Sometimes, the sorption heat storage and chemical reaction heat storage are classified into thermo-chemical heat storage. Sensible heat storage stores the thermal energy in the temperature change, so material with large specific heat capacity is preferred. Water, rock and soil are the commonly used working medium. Although such method is cheap and effective, the energy storage density is low resulting in large system volume [10,11]. Latent heat storage utilizes
* Corresponding author. 800 Dongchuan Road, Shanghai, China. E-mail address: [email protected] (Z.Y. Xu). https://doi.org/10.1016/j.energy.2018.11.072 0360-5442/© 2018 Published by Elsevier Ltd.
the phase change process to store thermal energy, so the energy storage density will be much higher than the sensible heat storage. Another advantage of latent heat storage is that it could maintain a stable output temperature which is preferred under most scenarios. Different phase change materials have been developed for different working temperatures [13]. For instance, organics including paraffin waxes [14] and fatty acids [15] are used for low temperature application while molten salt [16] is used for high temperature application. Despite the different energy storage densities achieved by the sensible heat storage and latent heat storage, they cannot keep the thermal energy in a longer storage period due to the inevitable heat loss to the ambient. Effort to add the discharging barrier and make phase change material suitable for long term storage has been made [17], however, it is still under proof-of-concept stage. Different from the sensible heat storage and latent heat storage, thermo-chemical heat storage including the absorption thermal storage could convert the thermal energy into chemical potential energy, and store the energy stably in a long term. The thermochemical heat storage could further be classified into absorption thermal storage, physical adsorption thermal storage, chemical adsorption thermal storage and chemical reaction thermal storage [10,11]. Typically, binary working pair is used for such heat storage
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system, and the discharging could be triggered by physically contacting the two parts. In the absorption thermal storage system, the binary working pair consists of the solution absorbent and absorbate. Water-LiBr [18], water-LiCl [19], water-CaCl2 [20], waterNaOH [21] and ammonia-water [22,23] are commonly studied working pairs. Comparing with the other thermos-chemical heat storage methods, absorption thermal storage has the advantage of good heat transfer performance due to its fluid state. However, most of the studies on absorption thermal storage emphasize on the working pair itself. Energy storage density is the key concern of a sorption heat storage system. Cycle improvement can also be a good choice to increase the energy storage density of absorption heat storage [23,24]. In this paper, the absorption seasonal thermal storage cycles with multi-stage output process are proposed aiming at higher energy storage density through cycle improvement. In the proposed cycle, high energy storage density is obtained from large concentration glide by the multi-stage output process. First, the schematics and P-T-x diagrams of the absorption seasonal thermal storage cycles are introduced. Then, methodology for the cycle performance analysis is presented. At last, the performance of the proposed cycles is analyzed including the energy flows analysis, analysis of different temperature parameters and comparison between different working pairs. 2. Absorption seasonal thermal storage cycle with multistage output Fig. 1 shows the schematic of the proposed absorption seasonal thermal storage cycle with double stage output process and the conventional absorption seasonal thermal storage cycle with single stage output process. The proposed cycle includes two stages, i.e. stage 1 and stage 2 as shown in Fig. 1(a) and (b). Both stage 1 and
Condensation (C)
stage 2 consist of a solution tank and an absorbate tank. In the summer charging process of the proposed cycle shown in Fig. 1(a), two solution tanks absorb the heat input from external heat source, for instance, the solar thermal energy. This process generates absorbate vapor which flows into the absorbate tanks. Absorbate vapor flowing into the absorbate tanks is cooled by the ambient through either cooling water or cooling air, and condenses into absorbate liquid. With the continuous generation in solution tanks, the concentration of absorbate in solution keeps decreasing. When the concentration of absorbate decreases to a certain value, the equilibrium temperature of solution will be almost the same with the heat source temperature. The generation processes in solution tank cannot be activated by the heat source anymore, and the charging processes are over. The solution concentration under which the charging process is terminated is decided by the heat source temperature and ambient temperature together. The charging process shown in Fig. 1(a) is the same with the summer charging process of conventional cycle shown in Fig. 1(c), except that the conventional cycle only consists of one absorbate tank and one solution tank. After the charging process, the thermal energy is converted into chemical potential energy and stored in the tanks. Here in the absorption system, the chemical potential energy is reflected by the concentration of solution. The lower absorbate concentration in the solution, the higher potential amount of absorbate vapor to be absorbed by the solution. If the absorbate tank and solution tank are disconnected by valve closing, no vapor flows between the absorbate tank and solution tank, thus preventing the concentration change in solution. In this way, the chemical potential energy could be maintained with very little energy loss even for long storage period. In the winter discharging process of the proposed cycle shown in Fig. 1(b), absorbate tank 2 absorbs thermal energy from the
Evaporation (E1)
Generation (G)
Stage1
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Heat source
Absorption (A1)
Stage1
Output Vapor
Vapor Ambient Generation (G)
Condensation (C)
Heat source Vapor
Stage2
Solution tank 2
Absorbate tank 1
Solution tank 1
Absorption (A2)
Evaporation (E2)
Q
Solution tank 1
Absorbate tank 1
Ambient
Ambient
Absorbate tank 2
Vapor
Stage2
Solution tank 2
(a) Condensation (C)
Absorbate tank 2
(b) Generation (G)
Evaporation (E)
Absorption (A) Heat output
Heat source Vapor
Vapor
Ambient Absorbate tank
Solution tank
(c)
Ambient
Solution tank
Absorbate tank
(d)
Fig. 1. Schematic diagrams of the absorption seasonal thermal storage cycles. (a) Charging process of the proposed cycle in summer, (b) Discharging process of the proposed cycle in winter, (c) Charging process of the conventional cycle in summer, (b) Discharging process of the conventional cycle in winter.
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ambient and generates absorbate vapor, the vapor is absorbed by the solution in solution tank 2 thus outputting thermal energy with high temperature. The heat output from solution tank 2 is used to heat the absorbate tank 1 and generate absorbate vapor. The vapor is absorbed by the solution in solution tank 1 thus outputting thermal energy with higher temperature. In the process shown in Fig. 1(b), the thermal energy from ambient is upgraded twice for heat output, while the thermal energy from ambient is only upgraded once in the conventional cycle as shown in Fig. 1(d). During the heat output process, the absorbate concentration in solution increases gradually until the absorption temperature is equal to or lower than the required heat output temperature. Since the double stage cycle upgrades the thermal energy one more time than the conventional single stage cycle, the double stage cycle could maintain the absorption temperature higher than the required heat output temperature even when the absorbate concentration in solution is high. In this way, the double stage cycle has the potential to achieve higher energy storage density than the conventional cycle due to its better tolerance in absorbate concentration. Since the parameter changes of the proposed cycle cannot be shown from the schematic diagrams only, year round operation of the proposed cycle is shown on P-T-x diagram in Fig. 2. In this figure, the horizontal line, vertical line and parallel slash are isobaric, isothermal, and isoconcentration, respectively. The slash on the left side has higher absorbate concentration than that on the right side. Solid line and dashed line represent the liquid flow and vapor flow, respectively. In order to distinguish the different processes in the year round operation, the charging in summer, storage from summer to winter, discharging in winter and storage from winter to summer are colored in blue, black, red and orange, respectively. The parameter changes during different periods are described as follows.
C
G (Input)
Pressure
E1
A1(Output) S2
E2 A2
S1 (Heat loss)
Discharging Storage 1: summer to winter Charging Storage 2: winter to summer TA-win ter
TA-summer TOutput
TSolar
Temperature Fig. 2. Year round operation of the proposed absorption thermal storage cycle on P-T-x diagram. The cycle consists of charging in summer under heat source temperature TSolar and ambient temperature TA-summer, storage from summer to winter, discharging in winter under ambient temperature TA-winter and heat output temperature TOutput, and storage from winter to summer. The arrow “/” represents the state variation and circle “o” represents the final state of one process.
(1) The charging process in summer represents the input of thermal energy, which consists of the generation (G) and condensation (C). The generation process includes the isoconcentration preheating of solution from summer ambient temperature (TA-summer) to the saturated condition, and the isobaric heating with concentration glide till the final state decided by the solar heat source temperature (TSolar). The condensation pressure is also decided by the ambient temperature (TA-summer). This process corresponds to Fig. 1(a). (2) During the long-term storage from summer to winter, solution concentration keeps constant due to the blocked vapor flow, but solution temperature and absorbate liquid temperature cannot be maintained in a high level considering the inevitable heat dissipation to ambient. In the real cases, the temperature of stored working medium will vary with the ambient temperature, so this process is an isoconcentration cooling process till the winter ambient temperature (TA-winter). The sensible heat loss of solution (S1) comes from the temperature decrease (TSolar to TA-winter). (3) The discharging process in winter delivers heat output from vapor absorption. In the second stage, evaporation (E2) absorbs heat from the ambient (TA-winter), and the vapor evaporated from E2 is absorbed by the solution in A2. As the solution temperature is lower than the solution equilibrium temperature, part of the absorption heat will be consumed by the solution preheating, and the concentration will have a small change during the preheating. After the preheating, the absorption process is an isobaric process which delivers heat output to E1. In the second stage, the processes are similar to that in the first stage expect that the heat input to E1 comes from A2 and heat output from A1 is delivered to the user side. The heat output from A1 lasts until the absorption temperature decreases to the required heat output temperature (TOutput). During this process, the large concentration glide also makes the heat output temperature unstable. The output temperature at the initial stage of heat output will be higher than the required output temperature. This can be solved by controlling the flow rate of water to be heated. (4) During the storage from winter to summer, the solution concentration keeps constant, while the temperatures of working medium vary with the ambient. Different from the storage from summer to winter, the working medium temperatures increase with the ambient here (S2). To better illustrate the advantage of the proposed cycle, the simplified P-T-x diagrams of the proposed cycle and the conventional cycle are compared under the same condition as shown in Fig. 3. The storage and preheating processes are neglected in the simplified P-T-x diagram considering that they are sensible heat changes, while the latent heat changes including the generation and absorption are emphasized. It could be found that the proposed double stage cycle could achieve larger concentration glide than the conventional single stage cycle. This is because the double stage cycle could deliver absorption heat output higher than the required temperature (TOutput) even when the absorbate concentration in solution is high. In the absorption thermal storage cycle, larger concentration glide represents higher energy storage density [23], so the double stage cycle has the potential advantage in energy storage density which will be discussed in detail later. The other thing need to be mentioned is the crystallization risk when waterLiBr is used as working pair. Since the charging process of the proposed cycle is the same with the conventional cycle, the highest concentration in the proposed cycle is also the same with the conventional single stage cycle, which means the crystallization risk is not increased. The concentration glide is only increased from
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Discharging Storage Charging
P1
G (Input)
C
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Discharging Storage Charging
P1
C
G (Input)
E1 P3 E P2
A (Output) TA-win ter
TA-summer
TOutput
P2 TSolar
A1(Output) E2 A2 TA-winter
Concentration glide
TA-summer TOutput
TSolar
Concentration glide
Fig. 3. Concentration glide comparison between absorption seasonal thermal storage cycles on simplified P-T-x diagrams. (a) Conventional cycle with single stage output, (b) Proposed cycle with double stage output.
the discharging process. Similar to the cycle shown in Fig. 2, more absorption seasonal thermal storage cycles with multi-stage output could be constructed. Fig. 4 shows the simplified P-T-x diagram of a triple stage cycle. It has the same charging and storage processes with the single stage cycle and double stage cycle. During the discharging of the triple stage cycle, the absorption of the third stage A3 delivers heat output to the evaporation of the second stage E2, the absorption of the second stage A2 delivers heat output to the evaporation of the first stage E1, and A1 delivers heat output to the user side finally. Larger concentration glide could be achieved comparing with the double stage cycle. However, more internal heat exchange processes might also bring more irreversible losses. Whether the triple stage cycle or the double stage cycle is advantageous still depends on the specific working condition, and will be discussed later.
3. Methods for performance analysis To quantitatively investigate the performance of the absorption seasonal thermal storage cycle with multi-stage output, detailed
Discharging Storage Charging
P1
analyses are carried out through calculation. In this section, the methodology for performance calculation will be introduced, including the assumptions, parameter definitions and thermophysical property calculation. As the performance enhancement achieved by the proposed cycles is mainly decided by the thermodynamic cycle construction, the working pair selection is actually not important here. Typical absorption working pair of water-LiBr is used for the majority of the performance analysis, and cycle with ammonia-water is also analyzed for comparison. The properties of water-LiBr and ammonia-water including temperature, pressure, concentration and enthalpy are calculated from the published data [25,26]. Following assumptions are made according to the related literature on absorption systems [23,27,28]: (1) The generation pressure is equal to the condensation pressure; (2) The absorption pressure is equal to the evaporation pressure; (3) The final states of generation and absorption processes reach the equilibrium;
C
G (Input)
E1 P5
A1(Output) E2
P4
A2 E3
A3
P2 TA-winter
TA-summer TOutput
TSolar
Fig. 4. Simplified P-T-x diagram of the absorption seasonal thermal storage cycle with triple stage output.
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(4) The solution concentration keeps constant during storage process; (5) The temperatures of solution and absorbate equal to the ambient temperature during storage process. (6) The temperature approach between absorption and evaporation is 5 C. The performance calculation also follows the basic conservation [23,27,28] as shown in Eqs. (1)- (3). Eq. (1) represents that the sum of mass flow into and out of one component (mi) is equal to zero since there is no internal mass source. Eq. (2) represents that the sum of absorbate/absorbent flows into and out of one component (miXi) is equal to zero. Eq. (3) represents that the sum of energy flows into and out of one component (mihi) is equal to its corresponding external heat exchange amount (Q).
X mi ¼ 0
(1)
i
X mi xi ¼ 0
(2)
i
X mi hi ¼ Q
(3)
i
where mi, xi and hi represent the mass flow rate, concentration and enthalpy of each stream regarding one component. Parameters including the concentration glide, COP, energy storage density (ESD) and optimized mass split ratio are calculated. The concentration glide refers to the solution concentration change before charging and after charging. Considering the concentration before charging is equal to the concentration after discharging, this parameter is actually decided by the charging conditions and discharging conditions together. It could reflect the potential of energy storage density. COP is used to evaluate the energy efficiency. It is defined as the total heat output in winter divided by the total heat input in summer as shown in Eq. (4):
P Qa;i COP ¼ P i Q i g;i
(4)
where Qa,i and Qg,i refer to the absorption heat output and generation heat input, respectively. ESD is the most important parameter for energy storage system, and is defined as the total heat output in winter divided by the total mass of working medium used in the cycle as shown in Eq. (5):
P Q a;i ESD ¼ Pi i Mi
P
ms2;i i ms1;i
R2 ¼ Pi
(6)
P
ms3;i i ms1;i
R3 ¼ Pi
(7)
P P P where ms1;i , ms2;i and ms3;i refer to the working medium i i i second stage and the third stage, masses of the first stage, the respectively. For the double stage cycle, only R2 will be used, while for the triple stage cycle, both R2 and R3 will be sued. 4. Results and discussion In this section, the energy flow analysis and effect of different parameters are analyzed with water-LiBr. Charging in summer with solar thermal power and discharging in winter for heating are considered. Considering the limitations of water-LiBr working pair including crystallization and freezing, performance with ammoniawater working pair is also calculated and compared with that of the water-LiBr working pair. 4.1. Energy flow analysis To give a clear description of how energy flows during the year round operation of absorption thermal storage cycle, the heat input, heat loss, heat gained from ambient and heat output are analyzed for both the conventional single stage cycle and the proposed double/triple stage cycle. Charging in summer considers solar heat input temperature (TSolar in Fig. 2) of 90 C and ambient temperature (TA-summer in Fig. 2) of 35 C. Discharging in winter considers ambient temperature (TA-winter in Fig. 2) of 5 C and heat output temperature (TOutput in Fig. 2) of 50 C. The total heat input in each cycle (Qg) is set as 100 kJ for fare comparison. Heat losses in different stages are referred as Ql,s1, Ql,s2 and Ql,s3, respectively. Heat output in different stages is referred as Qa,s1, Qa,s2 and Qa,s3, respectively. The heat output to user in the single stage cycle, the double stage cycle and the triple stage cycle are described by Qa,s1, Qa,s2 and Qa,s3, respectively. As shown in Fig. 5, the first stage heat loss Ql,s1 from the single stage cycle is much larger than that of the other two cycles. This comes from the constant heat input setting here. For the same amount of solution, the heat loss should be equal in different cycles since they are all decided by the temperature difference between TSolar and TA-winter, but the single stage cycle has less heat input due
(5)
where Mi refers to the mass of working medium in each component. Considering that all the absorbate is absorbed into solution after discharging, the total mass of solution after discharging represents the total mass of working medium. In the proposed cycle with multi-stage output, there is an optimization issue: how to get the optimized heat output by separating the solution into different stages. Taking the double stage cycle shown in Fig. 2 as an example, if the first stage has much more solution than the second stage, then the second stage cannot provide enough heat output to the evaporation of the first stage. There will be some remaining chemical potential energy stored in the first stage when the discharging process is finished. In this case, we also include two optimized mass ratio parameters as shown in Eqs. (6) and (7):
Fig. 5. Energy flows of the absorption seasonal thermal storage cycles including the single stage cycle, double stage cycle, and triple stage cycle. TA-summer, TOutput and TA winter are 35 C, 50 C and 5 C, respectively.
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to its smaller concentration glide. In this case, the heat loss in the single stage cycle will be more serious considering the same amount of thermal energy input. It is the same reason why the Ql,s1 for double stage cycle (7.7 kJ) is larger than that of the triple stage cycle (3.6 kJ). The total heat losses of single effect cycle, double stage cycle and triple stage cycle are 65.0 kJ, 16.0 kJ and 11.5 kJ, respectively. By adopting double stage cycle and triple stage cycle, heat losses could be reduced by 75.4% and 82.3% compared with the conventional single stage cycle. As a result, the first stage heat output Qa,s1 from the single stage cycle (18.2 kJ) is much smaller than that from the other two cycles (39.3 kJ and 27.2 kJ). However, it is also noticed that the heat outputs Qa,s1 and Qa,s2 for the double stage cycle (39.3 kJ and 39.3 kJ) are larger than the heat outputs Qa,s1, Qa,s2 and Qa,s3 for the triple stage cycle (27.2 kJ, 27.2 kJ and 29.0 kJ), the triple stage cycle doesn't obtain more heat output to user as expected. This is because the solution is separated into two parts in the double stage cycle, while it is separated into three parts in the triple stage cycle. More solution is used for the heat output in each stage for the double stage cycle. Although the energy flows are analyzed based on the same heat input in Fig. 5, the three cycles actually need different amounts of working medium for the same heat input capacity. Considering the different amounts of working medium in each cycle, ESDs of 34.7 kJ/
(a)
(c)
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kg (61.2 MJ/m3), 254.0 kJ/kg (396.1 MJ/m3) and 235.5 kJ/kg (350.2 MJ/m3) are obtained for the single stage cycle, double stage cycle and triple stage cycle, respectively. Comparing with the single stage cycle, weight averaged ESD enhancements of 7.3 times and 6.8 times are achieved by the double stage cycle and triple stage cycle, respectively. Considering an area averaged heat load of 40 W/m2, 31104 MJ of heat output is necessary to fulfill the heating demand of a 100 m2 house for 3 months, and 508.2 m3, 78.5 m3 and 88.8 m3 storage volumes are needed for the single stage, double stage and triple stage cycles, respectively. The storage volume could be reduced a lot by the proposed cycles which makes the seasonal thermal storage feasible for real application. Based on the above analysis, it can be concluded that multistage output has two contrary effects on the cycle performance. On one hand, more stages bring larger concentration glide which is beneficial for storing more thermal energy and reducing the relative amount of heat losses. On the other hand, more stages also separate the solution into more parts. Larger portion of heat output is consumed internally for temperature upgrading, while the heat output to the user side will be less. Whether the multi-stage output has benefit on the cycle performance depends on the competition of these two different effects, and will have different overall impacts under different conditions. In the next few sections, the effect of temperature parameters on the cycle performance will be
(b)
(d)
Fig. 6. Effect of heat output temperature TOutput on the performance of absorption seasonal thermal storage cycles. TSolar, TA-summer and TA-winter are 90 C, 35 C and 5 C, respectively. (a) Concentration glide (b) COP, (c) ESD, (d) Optimized mass ratio.
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discussed quantitatively. 4.2. Effect of different parameters on cycle performance In this section, the effects of heat output temperature, heat source temperature, winter ambient temperature and absorptionevaporation temperature approach on system performance will be analyzed in detail. Concentration glide, COP, ESD and the optimized mass ratio will be used for performance evaluation. a. Effect of heat output temperature TOutput Fig. 6 shows the calculation results under TOutput, TSolar, TA-summer and TA-winter of 40e80 C, 90 C, 35 C and 5 C, respectively. Higher output temperature means higher grade of thermal energy output is required. When the output temperature increases, the concentration glides of single stage cycle, double stage cycle and triple stage cycle decrease in the range of 0.070e0.021, 0.158e0.058, and 0.197e0.120, respectively. When the output temperature is higher than 50 C, concentration glide of the single effect cycle reaches zero, while the double stage cycle and triple stage cycle could still maintain a relatively large concentration glide. Fig. 6 (b) shows the COP comparison. COPs of all the three cycles
decrease with output temperature, and the COP of single stage cycle decreases fastest. This could easily be explained by the small concentration glide of single stage cycle shown in Fig. 6(a). The double stage cycle has higher COP than the triple stage cycle when the output temperature is lower than 75 C due to less internal heat exchange amount. However, the triple stage cycle has the highest COP when the output temperature is higher than 75 C. This represents the advantage brought by large concentration glide is stronger than the disadvantage brought by solution separation in different stages, and this has been discussed in Section 4.1. Although COP is an important parameter for energy conversion system, the ESD is more important for the thermal storage system. Fig. 6(c) shows the ESDs of the three cycles. The double stage and triple stage cycles have higher ESDs than the single stage cycle as expected. When the output temperature is higher than 55 C, the triple stage cycle has the highest ESD. When the output temperature is higher than 50 C, the conventional single stage cycle cannot work, while the proposed double and triple stage cycle still have quite high ESD. For output temperature of 40e50 C, the double stage cycle and triple stage cycle achieve 1.22e7.32 times and 1.04e6.78 times higher ESD than the single stage cycle. For a typical heat output temperature of 50 C, it can be concluded from this section that double stage cycle is the most suitable choice. The
Fig. 7. Effect of heat source temperature TSolar on the performance of absorption seasonal thermal storage cycles. TA-summer, TOutput and TA-winter are 35 C, 50 C and 5 C, respectively. (a) Concentration glide (b) COP, (c) ESD, (d) Optimized mass ratio. c. Effect of winter ambient temperature TA-winter
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reason behind this is that the double/triple stage cycles have large concentration glide, which make them less sensitive to the decrease of concentration glide under higher output temperature. In order to achieve the optimized output, the solution needs to be separated into different stages with different masses. For the double stage cycle, the optimized mass ratio R2 decreases from 1.08 to 0.97 with increasing output temperature. For the triple stage cycle, the ratio R2 keeps almost constant around 1.08, while the ratio R3 decreases from 1.14 to 1.09. The optimized mass ratios are close to 1.00, so the even separation strategy for solution is an effective and simple method to obtain heat output close to the optimized value. b. Effect of heat source temperature TSolar Fig. 7 shows the effect of heat source temperature, i.e., the generation temperature during charging process, on the cycle performance. Performances under generation temperatures of 75e91 C are studied. As shown in Fig. 7 (a), the concentration glides increase in the ranges of 0.016e0.025, 0.061e0.134 and 0.106e0.178 for the single stage cycle, double stage cycle and triple stage cycle, respectively. As shown in Fig. 7 (b), the COP increases in the ranges of 0.07e0.27, 0.27e0.40 and 0.23e0.29 for the single stage cycle, double stage cycle and triple stage cycle, respectively. As shown in Fig. 7 (c), the ESD increases in the ranges of 11.9e57.1 kJ/kg, 95.6e264.7 kJ/kg, and 129.1e242.9 kJ/kg for the single stage, double stage and triple stage cycles, respectively. The effect of lower output temperature is almost the same with that of higher heat source temperature. This could be understood from the point view of energy grade. In the charging process, better performance could be achieved if higher grade thermal energy is inputted. In the
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discharging process, better performance could be achieved if lower grade thermal energy is outputted. As for the optimized mass ratio shown in Fig. 7 (d), the ranges of R2 and R3 here are similar to that shown in Fig. 6 (d), representing the even separation strategy for solution is still effective. Fig. 8 shows the effect of winter ambient temperature, i.e., the evaporation temperature during discharging, on the cycle performance. Performances under evaporation temperatures of 2e16 C are studied. Fig. 8 (d) proves the effectiveness of even separation strategy for solution again. As shown in Fig. 8 (a)-(c), the increases of concentration glide, COP and ESD with increasing evaporation temperature come from the same reason with the previous analysis on thermal energy grade. However, there are something different in Fig. 8 compared to the previous two figures. The concentration glides in all the three cycles increase slightly with the increasing evaporation temperature, but the effects of increasing evaporation temperature on COP and ESD are different for the three cycles. The COP and ESD of the double stage cycle and triple stage cycle are relatively steady under different evaporation temperature, while the COP and ESD of the single stage cycle decrease fast with the deceasing evaporation temperature. The single stage cycle is more sensitive to the winter ambient temperature due to its weak temperature lift ability compared to other cycles. Considering the unsteady ambient temperature, conventional single stage cycle might not be able to provide steady heat output in winter, and this could be solved by adopting the double stage cycle or triple stage cycle. Except for the external conditions, the internal heat exchange is also important for the system performance. In this case, the effect of different absorption-evaporation temperature approaches is also examined. Fig. 9 shows the COP and ESD under absorptionevaporation temperature approaches from 1 C to 9 C. Since
Fig. 8. Effect of winter ambient temperature TA-winter on the performance of absorption seasonal thermal storage cycles. TSolar, TA-summer and TOutput are 90 C, 35 C and 50 C, respectively. (a) Concentration glide (b) COP, (c) ESD, (d) Optimized mass ratio. d. Effect of absorption-evaporation temperature approach
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Fig. 9. Effect of absorption-evaporation temperature approach on the performance of absorption seasonal thermal storage cycles. TSolar, TA-summer, TOutput and TA-winter are 90 C, 35 C, 50 C and 5 C, respectively. (a) COP, (b) ESD.
there is no absorption-evaporation heat exchange in the single stage cycle, the single stage cycle is not affected. The COP and ESD of the double stage cycle and triple stage cycle decrease with increasing temperature approach. This could be explained by more internal irreversible losses under larger temperature approach. Besides, it can also be found that the ESD is more sensitive to the temperature approach than the COP. 4.3. Performance with ammonia-water working pair Typical working pairs for absorption system include water-LiBr and ammonia-water. In the previous two sections, water-LiBr is used for performance analysis. However, the water-LiBr working pair is limited by the crystallization risk under high generation temperature and freezing issue under low evaporation temperature. In the absorption seasonal thermal storage cycle, solar heat source temperature and winter ambient temperature may vary a lot due to the instable external conditions. Ammonia-water working pair could be an alternative choice when water-LiBr cannot work due to the aforementioned issues. The effects of solar heat source temperature, i.e. the generation temperature in charging process, on the performance of double stage cycle with two working pairs are compared in Fig. 10. The ESD of double stage cycle with water-LiBr increases from 95.6 kJ/kg to
264.7 kJ/kg when the generation temperature increases from 75 C to 91 C. If the generation temperature is further increased, the water-LiBr solution will have high concentration of LiBr which is easy to be crystallized. The ESD of double stage cycle with ammonia-water increases from 12.2 kJ/kg to 175.9 kJ/kg when the generation temperature increases from 75 C to 105 C. Better adaptability could be achieved by ammonia-water solution, but the ESD is lower mainly due to the lower latent heat of ammonia. The COP variation follows the similar trend with the ESD. The COPs for water-LiBr cycle and ammonia-water cycle vary in the ranges of 0.27e0.40 and 0.03e0.23, respectively. Although, the ammoniawater has better flexibility, higher ESD in water-LiBr system is acceptive. In the real operation, crystallization of water-LiBr system could actually be avoided by monitering the temperature or concentration of solution. Terminating the charging process under a certain solution temperature could be an effective option. Fig. 11 shows the results under different winter ambient temperature, i.e. the evaporation temperature in discharging. The ESD of double stage cycle with water-LiBr increases from 232.0 kJ/kg to 305.5 kJ/kg when the evaporation temperature increases from 2 C to 12 C. If the evaporation temperature is lower, the water-LiBr working pair cannot work due to freezing issue. The ESD of the double stage cycle with ammonia-water increases from 9.25 kJ/kg to 158.6 kJ/kg when the evaporation temperature increases
Fig. 10. Performance of the proposed two-stage cycles with ammonia-water and water-LiBr under high heat source temperatures. TA-summer, TOutput and TA-winter are 35 C, 50 C and 5 C, respectively. (a) COP, (b) ESD.
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Fig. 11. Performance of the proposed two-stage cycles with ammonia-water and water-LiBr under low winter ambient temperatures. TSolar, TA-summer and TOutput are 90 C, 35 C and 50 C, respectively. (a) COP, (b) ESD.
from 8 C to 12 C. The COPs for water-LiBr cycle and ammoniawater cycle varies in the ranges of 0.38e0.41 and 0.02e0.24, respectively. Similar to the previous analysis, the ammonia-water system has better flexibility but lower ESD and COP. In the real operation, the water-LiBr system is preferred for winter ambient temperature higher than 5 C considering its higher energy storage density and freezing issue. If the winter ambient temperature is lower than 5 C, ammonia-water is better.
5. Conclusion Absorption thermal storage system could store thermal energy in the form of chemical potential energy, thus avoiding heat losses during long-term storage. This makes the absorption thermal storage suitable for seasonal thermal energy storage. High energy storage density is the target of different researches on thermal storage, and previously researchers mainly focused on working pair selection. In this paper, absorption seasonal thermal storage cycles with multi-stage output are proposed to increase the energy storage density of absorption system from the cycle improvement. The proposed cycles achieve large concentration glide through the multi-stage output, which further increases the energy storage density. Detailed analyses of the proposed cycles are carried out through calculation with water-LiBr working pair. Energy flow analysis, study of different temperature parameters and comparison between different working pairs are carried out. Following conclusions can be made. (1) The proposed absorption seasonal thermal storage cycles increase the concentration glide. Large concentration glide increases the percentage of useful heating input to the total heat input, and reduces the relative amount of heat losses during long term storage. For condition with solar heat source temperature of 90 C, summer ambient temperature of 35 C, winter ambient temperature of 5 C and heat output temperature of 50 C, the proposed cycles with double stage output and triple stage output have 75.4% and 82.3% less heat losses than the conventional single stage cycle. (2) The multi-stage output processes in the proposed cycle have both advantage and disadvantage for the system performance. On one hand, it could increase the concentration glide, which is beneficial for large energy storage density. On the other hand, it will separate the stored working medium into different stages, while only one stage delivers heat output to the user side. This will reduce the energy storage density. The competition between these two effects makes
the double stage cycle and triple stage cycle suitable for different working scenarios. (3) Generally, the proposed cycle with double stage output has higher energy storage density under most studied conditions. Under the condition where TSolar, TA-summer, TOutput and TA-winter are 90 C, 35 C, 50 C and 5 C, the conventional single stage cycle, the proposed double stage cycle and triple stage cycle achieve ESDs of 34.7 kJ/kg (61.2 MJ/m3), 254.0 kJ/ kg (396.1 MJ/m3) and 235.5 kJ/kg (350.2 MJ/m3), respectively. The double stage cycle and triple stage cycle have 7.32 times and 6.78 times higher weight averaged ESDs than the single stage cycle. The proposed cycle with triple stage output has higher energy storage density under low heat source temperature, low winter ambient temperature and high heat output temperature. The conventional cycle with single stage output has lower energy storage density under most conditions. It can be concluded that the proposed absorption thermal storage cycle with multi-stage output is a good option for long term storage of solar energy. (4) The proposed cycle with water-LiBr has higher energy storage density than that with ammonia-water. However, it is limited by crystallization under high generation temperature and freezing issue under low winter ambient temperature. Since higher energy storage density is still preferred for long term storage. The crystallization risk could be controlled by monitoring the generation temperature or concentration, and freezing issue could be avoided by putting the system into warmer space such as the basement. If these two limitations cannot be avoided by such methods, ammonia-water system could be used. Acknowledgement This work is supported by the National Natural Science Foundation of China (Grant No. 51606124). The support from the Foundation for Innovative Research Groups of the National Natural Science Foundation of China (Grant No. 51521004) is also appreciated. References [1] Lewis NS. Research opportunities to advance solar energy utilization. Science. 2016;351(6271):aad1920. [2] Wang RZ, Xu ZY, Pan QW, Du S, Xia ZZ. Solar driven air conditioning and refrigeration systems corresponding to various heating source temperatures. Appl Energy 2016;169:846e56. [3] Ge TS, Wang RZ, Xu ZY, Pan QW, Du S, Chen XM, et al. Solar heating and cooling: present and future development. Renew Energy 2018;126:1126e40.
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Z.Y. Xu, R.Z. Wang / Energy 167 (2019) 1086e1096
[4] Qiblawey HM, Banat F. Solar thermal desalination technologies. Desalination 2008;220(1e3):633e44. [5] Kim H, Yang S, Rao SR, Narayanan S, Kapustin EA, Furukawa H, et al. Water harvesting from air with metal-organic frameworks powered by natural sunlight. Science 2017;356(6336):430e4. [6] Kuravi S, Trahan J, Goswami DY, Rahman MM, Stefanakos EK. Thermal energy storage technologies and systems for concentrating solar power plants. Prog Energy Combust Sci 2013;39(4):285e319. [7] Tian Y, Zhao C-Y. A review of solar collectors and thermal energy storage in solar thermal applications. Appl Energy 2013;104:538e53. [8] Xu J, Wang RZ, Li Y. A review of available technologies for seasonal thermal energy storage. Sol Energy 2014;103:610e38. [9] Pinel P, Cruickshank CA, Beausoleil-Morrison I, Wills A. A review of available methods for seasonal storage of solar thermal energy in residential applications. Renew Sustain Energy Rev 2011;15(7):3341e59. s N, Luo L. A review on long-term sorption solar [10] N’tsoukpoe KE, Liu H, Le Pierre energy storage. Renew Sustain Energy Rev 2009;13(9):2385e96. [11] Yu N, Wang RZ, Wang LW. Sorption thermal storage for solar energy. Prog Energy Combust Sci 2013;39(5):489e514. [12] Yan T, Wang RZ, Li TX, Wang LW, Fred IT. A review of promising candidate reactions for chemical heat storage. Renew Sustain Energy Rev 2015;43: 13e31. [13] Lane GA. Solar heat storage. In: Latent heat material, vol. II. CRC press; 2018. [14] Kousksou T, Jamil A, El Rhafiki T, Zeraouli Y. Paraffin wax mixtures as phase change materials. Sol Energy Mater Sol Cell 2010;94(12):2158e65. [15] Sarı A, Biçer A. Thermal energy storage properties and thermal reliability of some fatty acid esters/building material composites as novel form-stable PCMs. Sol Energy Mater Sol Cell 2012;101:114e22. [16] Zhang P, Ma F, Xiao X. Thermal energy storage and retrieval characteristics of a molten-salt latent heat thermal energy storage system. Appl Energy 2016;173:255e71. [17] Han GG, Li H, Grossman JC. Optically-controlled long-term storage and release of thermal energy in phase-change materials. Nat Commun 2017;8(1):1446. s N, Luo L. Experimentation of a LiBreH2O absorption [18] N'Tsoukpoe K, Le Pierre process for long-term solar thermal storage: prototype design and first results. Energy 2013;53:179e98. [19] Yu N, Wang RZ, Wang LW. Theoretical and experimental investigation of a closed sorption thermal storage prototype using LiCl/water. Energy 2015;93: 1523e34. [20] 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. [21] Weber R, Dorer V. Long-term heat storage with NaOH. Vacuum 2008;82(7): 708e16. [22] Xu S, Zhang L, Liang J, Du R. Variable mass energy transformation and storage
[23] [24] [25]
[26] [27] [28]
(VMETS) system using NH3eH2O as working fluid. Part 2: modeling and simulation under partial storage strategy. Energy Convers Manag 2007;48(1): 27e39. Xu ZY, Wang RZ. A sorption thermal storage system with large concentration glide. Energy 2017;141:380e8. Xu ZY, Wang RZ. Absorption refrigeration cycles: categorized based on the cycle construction. Int J Refrig 2016;62:114e36. Patek J, Klomfar J. A computationally effective formulation of the thermodynamic properties of LiBreH2O solutions from 273 to 500 K over full composition range. Int J Refrig 2006;29(4):566e78. Ziegler B, Trepp C. Equation of state for ammonia-water mixtures. Int J Refrig 1984;7(2):101e6. Herold KE, Radermacher R, Klein SA. Absorption chillers and heat pumps. 1996. Xu ZY, Wang RZ, Xia ZZ. A novel variable effect LiBr-water absorption refrigeration cycle. Energy 2013;60:457e63.
Nomenclature P: pressure (kPa) T: temperature (oC) x: concentration () m: mass flow rate (kg/s) h: enthalpy (kJ/kg) Q: power (kJ) COP: coefficient of performance () ESD: energy storage density (kJ/kg) M: mass (kg) R2, R3: optimized mass ratio (kg) Subscripts and superscripts Solar: solar heat source A-summer: summer ambient A-winter: winter ambient Output: heat output i: different streams or components a: absorption g: generation s1, s2, s3: different stages l: heat loss