Development of sorption thermal battery for low-grade waste heat recovery and combined cold and heat energy storage

Energy 107 (2016) 347e359

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Development of sorption thermal battery for low-grade waste heat recovery and combined cold and heat energy storage T.X. Li*, J.X. Xu, T. Yan, 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 16 April 2015 Received in revised form 27 October 2015 Accepted 28 March 2016

A promising compact sorption thermal battery is developed for low-grade waste heat recovery and combined cold and heat energy storage. Thermal energy is stored in the form of bond energy of sorption potential during the charging phase and the stored thermal energy is released in the form of heat energy and cold energy during the discharging phase. The operating principle and conceptual design of sorption thermal battery based on solidegas sorption processes is firstly introduced, and then the working performance of sorption thermal battery using consolidated composite sorbent of expanded graphite/ manganese chloride is investigated. Experimental verification showed that sorption thermal battery is an effective method to achieve combined deep-freezing cold and heat energy storage, and its working temperature can be easily adjusted by changing system pressure. The cold and heat energy densities are as high as 600 kJ/kg and 1498 kJ/kg, respectively. Moreover, energy densities of different energy storage methods and sorption working pairs are compared and analyzed. It appears that sorption thermal battery is a promising compact high-performance energy storage technology for waste heat recovery and renewable energy utilization due to its distinct advantage of high energy density and broad range of working temperature. © 2016 Elsevier Ltd. All rights reserved.

Keywords: Sorption thermal battery Heat energy storage Cold energy storage Energy density Power density Sorption working pair

1. Introduction Thermal energy storage is an effective method for adjusting the mismatch between energy supply and energy demand in renewable energy application and waste heat recovery. Advanced thermal energy storage technologies have been extensively discussed with the aim of reducing primary energy consumption by improving energy utilization efficiency [1e3]. Nowadays, high-density thermal energy storage technology is becoming urgent need in order to utilize low-grade waste heat from many industrial processes and to solve the intermittent nature of renewable energy sources. Among different energy storage methods, sorption thermal energy storage has been received much attention in recent years due to its high energy storage density when compared with conventional thermal energy storage technologies [4e6]. This storage method uses solidegas or liquidegas sorption working pair as storage material by storing thermal energy in the form of bond energy resulting from sorption process of working pair. Exergy

* Corresponding author. Tel./fax: þ86 21 34206335. E-mail address: [email protected] (T.X. Li). http://dx.doi.org/10.1016/j.energy.2016.03.126 0360-5442/© 2016 Elsevier Ltd. All rights reserved.

analysis performed by Abedin and Rosen [7] showed that sorption energy storage may be one of possible candidates for efficient compact thermal energy storage systems due to its high storage capacity. Mugnie and Goetz [8] conducted a theoretical comparison among different sorption energy storage systems for cold energy storage using liquidegas absorption, solidegas physical sorption and solidegas thermochemical sorption. The results indicated that solidegas thermochemical sorption using ammonia as sorbate provided the best storage capacity for negative temperatures, and liquidegas absorption using water as sorbate provided the best storage capacity for positive temperatures. Moreover, sorption thermal energy storage has been regarded as one promising candidate for long-term seasonal thermal energy storage because it has the ability to store thermal energy for several months at ambient temperature with only a little of energy losses in comparison with conventional sensible heat and latent heat storage methods [9e12]. Sorption thermal energy storage has some distinct advantages when compared with conventional thermal energy storage technologies: Firstly, sorption thermal energy storage has the multipurpose application for combined cold and heat storage using an energy storage device making it possible to produce useful cold and

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Nomenclatures COPc COPh Ms Pc Pe Peq Ph Pl P0 PDc PDh EDc EDh Qa-L Qcond Qcold

Coefficient of Performance for cold production Coefficient of Performance for heat production mass of sorbent [kg] condensation pressure [Pa] evaporation pressure [Pa] equilibrium pressure [Pa] high pressure of low-temperature sorbent [Pa] low pressure of low-temperature sorbent [Pa] reference pressure [1  105 Pa] power density for cold production [kW/kgsorbent] power density for heat production [kW/kgsorbent] energy density for cold production [kJ/kgsorbent] energy density for heat production [kJ/kgsorbent] adsorption heat of low-temperature sorbent [kJ] condensation heat of sorbate [kJ] useful cold production during the discharging phase [kJ] Qd-L desorption heat of low-temperature sorbent [kJ] Qevap evaporation heat of sorbate [kJ] Qheat useful heat production during the discharging phase [kJ] Qin total amount of energy storage during charging phase [kJ] Qout total amount of energy release during discharging phase [kJ] QR sorption heat of sorbent [kJ] Qs-evaporator sensible heat consumption of evaporator [kJ] Qs-bed sensible heat consumption of sorption bed [kJ]

heat for the end user simultaneously [13]; Secondly, the working temperature can be easily adjusted by changing the operating pressure of sorption working pair due to the monovariant characteristic of solidegas thermochemical sorption process, making it flexible in different working temperatures for energy storage in order to match the demand of the end user [14]; Thirdly, sorption thermal energy storage has the ability for the integrated energy storage and energy upgrade of low-temperature waste heat, thus it can work as a target-oriented heat transformer to upgrade the energy quality of low-grade thermal energy [15]. In addition, sorption thermal energy storage can be used for short-term storage and long-term storage. Its dual sorption energy storage modes for the long-term seasonal storage of solar thermal energy makes it capable of producing an appropriate heat output temperature for space heating application in winter, even at a very low ambient temperature [16]. The working performance of a sorption thermal energy storage system is mainly depended on the sorption capacity of sorbent, dynamical cycle, mass and heat transfer. A large amount of heat and mass flows are accompanied with sorption processes during the charging and discharging phases [17]. However, sorbent usually has inefficient heat and mass performance due to the low thermal conductivity and agglomeration problem of sorbent powder [18,19]. A lot of research has focused on advanced heat exchanger and composite sorbents in order to improve the performance of sorption systems. Aristov et al. [20] synthesized a selective water composite sorbent by impregnating salt hydrate into porous matrix, and they found that the addition of reactive salt can improve the sorption capacity significantly and the thermal conductivity of composite sorbent increases as function of the water uptake [21].

R0 SxCly Ta Tc Td Te Teq Tin T0 Tout

tc DHR DS DX DTa DTc DTd DTe

DTin DTout n,m

universal gas constant [kJ/(mol  C)] metal chloride adsorption temperature [ C] condensation temperature [ C] desorption temperature [ C] evaporation temperature [ C] equilibrium temperature [ C] heat input temperature [ C] ambient temperature [ C] heat output temperature [ C] cycled time [s] enthalpy of transformation [kJ/mol] entropy of transformation [kJ/(mol  C)] cycled mass of sorbate [kg] temperature difference during adsorption process [ C] temperature difference during condensation process [ C] temperature difference during resorption process [ C] temperature difference during evaporation process [ C] temperature difference during heat input process [ C] temperature difference during heat output process [ C] number of the moles of sorbate

Subscripts L/G liquidegas equilibrium line S/G solidegas equilibrium line ED energy density PD power density

Kato and co-workers [22] developed a new composite by mixing pure magnesium hydroxide with lithium bromide as reactivity enhancer and expanded graphite as heat transfer enhancer, and they observed that the composite EML can enhance hydration and dehydration kinetics, and the addition of LiBr could degrade activation energy due to its catalytic properties. Wang and co-workers [23] proposed an adsorption-based thermal battery using NaX zeolite-water working pair for the climate control of electric vehicle. They presented a general theoretical framework to determine the maximum achievable heating and cooling performance. The adsorption thermal energy storage system could enlarge the driving range of electric vehicle by reducing the energy consumption of onboard electric battery bank when compared with conventional electric-powered vapor compression refrigeration device. However, the whole energy efficiency cannot be improved by using the thermal energy storage system for the climate control of electric vehicle because the adsorption-based thermal battery uses electricity as driving heat source during the charging phase. In fact, the energy efficiency would become low when the whole energy utilization chain is considered due to the fact that adsorption thermal energy storage system usually has a COP (Coefficient of performance) lower than 0.5 (COP is defined as energy output divided by energy consumption). For this reason, adsorption thermal energy storage only exhibits its advantage when low-grade thermal energy (such as waste heat or renewable energy) is employed as driving heat source, instead of electricity. It is very important for an energy storage system to achieve a stable working temperature and controllable heat production in real application. Solidegas physical sorption usually suffers from the unstable working temperature during sorption cycles due to the

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bivariant characteristic between working temperature and pressure of sorption working pair [24]. In contrast, solidegas thermochemical sorption working pair could provide a stable heat output temperature during the discharging phase due to the monovariant characteristic of solidegas thermochemical reaction [13]. Mauran and co-workers [25] investigated a solidegas thermochemical sorption energy storage prototype using strontium bromideewater sorption working pair, and the reactive salt was implemented with expanded graphite to improve thermal conductivity and permeability. Liu et al. [26,27] developed a kind of composite sorbent made of mesoporous siliceous shale (WSS) impregnated with reactive salt (lithium chloride, calcium chloride) for an open sorption thermal energy storage system, and they found that the composite material can adsorb more water than the original ceramic material without dropping the salt solution. Shkatulov and Aristov [28] analyzed the doping effect of various salts on the dehydration dynamics of hydroxides using several kinds of salts, including chlorides, nitrates, sulphates and acetates of alkaline metals. It appears that hydroxide modification with salts is a promising way to get new advanced hydroxide-based materials for middle temperature heat storage with large heat storage density, enhanced and controllable dehydration reactivity. Recently, sorption thermal battery for combined cold and heat energy storage based on solidegas thermochemical sorption process was introduced by Li et al. [29] in order to make a distinct difference between sorption thermal energy storage and conventional sensible heat, PCM latent heat, and chemical energy storage. Thermodynamic results showed that sorption thermal battery is an effective method for the short-term and long-term storage of solar thermal energy and integrated energy storage and energy upgrade. Moreover, it can achieve cascaded thermal energy storage to enhance the versatility and working reliability of solar heat storage system by widening the working temperature range when solidegas thermochemical multilevel sorption processes with several reactive salts was employed [30]. The aim of this work is to propose and investigate a promising compact high-performance sorption thermal battery using composite sorbent for waste heat recovery and combined cold and heat storage. The operating principle of sorption thermal battery based on sorption and resorption working pairs is introduced, and the feasibility of sorption thermal battery is experimentally verified using a simple test unit. The combined cold and heat energy storage performance is investigated using a typical solidegas sorption working pair of manganese chloride-ammonia. Moreover, the energy densities of sorption thermal battery using different sorption processes for cold and heat energy storage are compared and analyzed according to solidegas physical/thermochemical sorption, liquidegas absorption, and solidegas thermochemical resorption working pairs. 2. Conceptual design of sorption thermal battery for combined cold and heat energy storage 2.1. Operating modes of sorption thermal battery for cold and heat storage The energy storage of sorption thermal battery is based on the energy conversion between the transformation of thermal energy and bond energy of sorption potential during the solid/liquidegas sorption process of working pair. The operating mode of sorption thermal battery for energy storage is shown in Fig. 1. A sorption thermal battery system usually consists of a sorption bed, a condenser, an evaporator and a storage tank. Sorbent is filled in the sorption bed as an energy storage material while sorbate is filled in the storage tank as a working gas. The proposed sorption thermal

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Fig. 1. Operating modes of sorption thermal battery system for cold and heat energy storage.

battery system has three different operating modes viz: charging phase with waste heat recovery, discharging phase for heat production, and discharging phase for cold production. During the charging phase with waste heat recovery as shown in Fig. 1a, the sorption bed is heated by thermal energy from waste heat source. The sorbate is desorbed from the sorbent inside the sorption bed and then enters into condenser. The desorbed gaseous sorbate becomes liquid sorbate inside the condenser by rejecting condensation heat to the ambient heat sink before it flows into storage tank. As a result, thermal energy is stored in the form of bond energy of sorption potential resulting from the desorption process of solid/liquidegas working pair during the course of charging phase. During the discharging phase for heat production mode, shown in Fig. 1b, the sorption bed is connected to the evaporator directly in order to adsorb the sorbate. The sorbate evaporates by absorbing vaporization heat from the ambient heat sink. The gaseous sorbate

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from the evaporator then flows to the sorption bed to react with sorbent. The sorption heat released by the sorbent provides thermal energy for end user in the course of discharging phase for heat production. During the discharging phase for cold production mode, shown in Fig. 1c, the sorption bed is firstly pre-cooled at the beginning stage before it begins to adsorb the sorbate. The liquid sorbate is cooled from ambient temperature to a low evaporation temperature by consuming a part of its vaporization latent heat. The gaseous sorbate flows to the sorption bed to react with the sorbent in which the sorption heat released is removed by the ambient heat sink. The evaporation heat absorbed by the sorbate produces useful cold for end user in the course of discharging phase for cold production.

2.2. Conceptual design of sorption thermal battery for energy storage For sorption thermal battery system, the condenser, evaporator and storage tank can be integrated into one liquid sorbate tank in order to simplify its components as shown in Fig. 2. In such situations, the working modes of the liquid tank can be interchanged alternately between condensation state and evaporation state during the charging and discharging processes. The sorption bed and the liquid tank are connected by a pipe with a gas valve, and the two components of sorption thermal battery are separated by using an insulation layer in order to avoid the heat transfer between them. The sorbent inside the sorption bed is heated by an external waste heat source to desorb the sorbate during the charging process. The desorbed gaseous sorbate flows into the liquid tank and then becomes liquid sorbate by releasing its condensation heat to the ambient heat sink. The liquid tank serves as a store vessel for the condensed liquid sorbate. During the storage period, the valve between the sorption bed and the liquid tank is closed after the completion of the sorbent-sorbate desorption process in order to separate the sorbent and sorbate of sorption working pair. Theoretically, the thermal energy can be stored in the form of sorption

Fig. 2. Conceptual design of sorption thermal battery for energy storage [37].

potential for a desirable period of time as long as the valve between the sorption bed and the liquid tank is closed. The valve between the sorption bed and the liquid tank is opened during the discharging process. The liquid sorbate evaporates by absorbing its vaporization heat. The evaporating gaseous sorbate then flows from the liquid tank to the sorption bed to react with the sorbent. For heat production mode, the vaporization heat of sorbate is supplied from the ambient heat sink, and the sorption heat released by sorbent provides thermal energy for end user application. For cold production mode, the sorption heat released by the sorbent is removed by the ambient heat sink and the evaporation heat absorbed by the sorbate produces useful cold for end user application. In order to improve the working performance of sorption thermal battery, the ground source or water source is a preferable ambient heat sink due to its relative low temperature in summer and high temperature in winter when compared with air source temperature. Both cold and heat production can be achieved simultaneously using sorption thermal battery since evaporation heat of sorbate produces useful cold for the user at a low temperature. At the same time, the sorption heat released by the sorbent provides useful heat for the other user at a relatively high temperature. In this case, the heat output temperature for heat production in the course of discharging phase is usually much lower than the heat input temperature in the course of charging phase. The combined cold and heat production mode is very suitable for long-term heat and cold energy storage.

3. Sorption thermal battery based on solidegas sorption working pairs Sorption thermal battery has different working modes for energy storage according to the solidegas physical sorption and thermochemical sorption processes. For sorption thermal battery using solidegas physical sorption working pair, the working pressure and temperature of physical sorption pair is the functional equation of two variables. Fig. 3 shows the schematic diagram of sorption thermal battery using solidegas physical sorption working pair. L/G is the liquidegas equilibrium line of sorbate and AeBeCeDeA is the physical sorption working region at different sorption capacity. The working region varies from the xmin equilibrium line to the xmax equilibrium line during both charging and discharging phases. The theoretical physisorption capacity of sorbent is dependent on the working pressure and temperature simultaneously. However, it is difficult to obtain a stable heat output temperature by controlling the operation pressure of

Fig. 3. Sorption thermal battery for energy storage using solidegas physical sorption working pair.

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Ln Peq ¼ 

DHR R0 Teq

þ

DS

(1)

R0

The working region between sorbent and sorbate is the solidegas equilibrium line. The decomposition reaction occurs when the constraining temperature/pressure is higher than the solidegas equilibrium temperature/pressure, whereas, the synthesis reaction takes place when the constraining temperature/pressure is lower than the solidegas equilibrium temperature/pressure [24]. The driving equilibrium drop is the only contributor to the chemical reaction between the sorbent and the sorbate in a thermochemical sorption system. Moreover, a higher driving equilibrium drop means a faster chemical reaction rate. For sorption thermal battery using solidegas thermochemical sorption working pair, the sorbent is heated from point D to point A by consuming a large amount of desorption heat during the charging process in order to desorb sorbate to the liquid tank by breaking the binding force of thermochemical sorption working pair. Thermal energy is converted into bond energy in the form of chemisorption potential by using the decomposition chemical reaction. During the discharging process, chemisorption potential is converted into thermal energy in the form of chemical reaction heat by using synthesis chemical reaction of the working pair. The

LnP Energy storage

S/G A

ΔTc

C

e

D

ΔTe

Te

To

S1/G

(LTR)

(HTR)

Qa-L B Energy storage

A lin e (H

TR )

ΔTa

PL

C

id-

Qd-L

D

ΔTd

Td2

ΔTin

Sol

Energy release

Q in

gas

lin e (L

TR )

Ph

S2/G

Qout ΔTout

To Ta1 Td1 Ta2 Tout

Tin

-1/ T

Fig. 5. Sorption thermal battery for energy storage using solidegas thermochemical resorption working pair.

sorbate evaporates at a low temperature and then flows to the sorption bed at an evaporation pressure of sorbate (Pe). The useful cold is produced by the evaporation heat of the sorbate while useful heat is produced by the chemical reaction heat of the sorbent. Moreover, a stable heat output temperature can be obtained during the discharging phase by controlling the operating pressure of sorbate in the liquid tank. This makes it easy to keep a good match between energy supply and energy demand. For sorption thermal battery using solidegas thermochemical resorption working pair, the liquid tank is replaced by another sorption bed in which condenser/evaporator is not required. This helps to avoid the existence of liquid sorbate in this kind of sorption thermal battery. The working principle of this kind of sorption thermal battery is based on the different equilibrium characteristics of different thermochemical sorbents. To accomplish the resorption process, at least two different sorbents are employed in a thermochemical resorption system, working as high-temperature sorbent (HTR) and low-temperature sorbent (LTR), respectively. The former has a higher equilibrium temperature than the latter at the same constraint pressure. During the charging phase, the sorbate is transferred from the LTR bed to the HTR bed due to the pressure difference between the two sorption beds. During the discharging phase, the decomposition reaction heat consumed by the LTR at a low temperature is utilized to produce useful cold (desorption cooling) while the synthesis reaction heat released by the HTR is used to produce useful heat. However, the heat output temperature of this kind of sorption thermal battery usually fluctuates during the discharging phase due to the mismatch between the decomposition reaction rate of the LTR and the synthesis reaction rate of the HTR. 4. Definition of analysis parameters for sorption thermal battery

idSol

Li

qu

Qevap

ΔTin

gas

sl id-

ga

PL

Energy release

Q in

lin

ine

Ph

L/G Qcond B

LnP

So lid -ga s

sorbate in the liquid tank due to the bivariant characteristic of solidegas physisorption. For sorption thermal battery using solidegas thermochemical sorption working pair, the relation between the working pressure and temperature is the functional equation of one variable, and thus the working temperature can be determined for a given working pressure. The monovariant characteristic of solidegas thermochemical working pair is different from the bivariant processes of absorption and physisorption working pair. In order to meet the thermal energy demand at different temperature levels, the operating modes of thermochemical sorption thermal battery can be easily interchanged by changing the working pressure of sorbate by varying the external heat source and heat sink temperatures. Thermochemical sorption thermal battery can be further divided into solidegas thermochemical sorption and resorption processes based on different operating modes. The schematic diagram of sorption thermal battery using solidegas thermochemical sorption and resorption working pairs are shown in Fig. 4 and Fig. 5, respectively. The solidegas and liquidevapor equilibrium lines are determined from the ClausiuseClapeyron equation, expressed as:

351

Ta Tc Tout

4.1. Heat consumption during the charging phase

Qout ΔTout

Td

The amount of heat consumption of sorption thermal battery during the charging phase is calculated using the following equation:

Tin

-1/ T

Fig. 4. Sorption thermal battery for energy storage using solidegas thermochemical sorption working pair.

Qin ¼ QR þ Qsbed

(2)

The first term in the right hand side is the sorption heat of sorbent. The second term is the sensible heat consumed by the

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sorption bed when it is heated from the ambient temperature to the heat input temperature, including the sensible heat of sorbent and the sensible heat of the metallic part of sorption bed during the preheating and superheating stages.

EDc ¼

4.2. Heat production during the discharging phase

4.6. Power density of sorption thermal battery

The amount of heat production of sorption thermal battery during the discharging phase is calculated using the following equation:

Power density parameter (PD), which is the amount of energy power per mass of sorbent, is defined and used to analyze the performance of the sorption thermal battery for cold and heat energy storage. Heat power density for heat production:

Qheat ¼ QR ±Qsbed

(3)

The first term in the right side is the sorption heat of sorbent. The second term is the sensible heat consumed by the sorption bed when it is heated or cooled from the working temperature to the heat output temperature, including the sensible heat of sorbent and the sensible heat of the metallic part of sorption bed during the preheating or pre-cooling stages.

PDh ¼

Useful cold production Qcold ¼ Mass of reactant MR

Useful heat production Qheat ¼ Cycle time  Mass of reactant tc  MR

(8)

(9)

Cold power density for cold production:

PDc ¼

Useful cold production Qcold ¼ Cycle time  Mass of reactant tc  MR

(10)

4.3. Cold production during the discharging phase The amount of cold production of sorption thermal battery during the discharging phase is calculated using the following equation:

Qcold ¼ Qevap  Qsevaporator

(4)

The first term in the right side is the evaporation heat of sorbate. The second term is the sensible heat consumed by the evaporator when it is cooled from the working temperature to the low evaporation temperature, including the sensible heat of sorbate and the sensible heat of the metallic part of evaporator during the precooling stage.

4.4. Energy efficiency of sorption thermal battery COP (Coefficient of performance) is defined and used to evaluate the energy efficiency for heat and cold storage of sorption thermal battery. Energy efficiency for heat production:

COPh ¼

Useful heat production Qheat ¼ Heat consumption Qin

(5)

5. Performance analysis 5.1. Clapeyron diagram of sorption thermal battery for energy storage A group of working pair of NH3, NaBr and MnCl2 is identified as a potential sorption working pair to verify the feasibility and assess the performance of the proposed sorption thermal battery. MnCl2 is used as high-temperature sorbent (HTR), and NaBr is utilized as low-temperature sorbent (LTR). The solidegas and liquidevapor equilibrium lines are calculated from the ClausiuseClapeyron Equation (1), where the enthalpy and entropy for the LTR are 35,363 J/mol and 225.2 J/(mol K), respectively, and the enthalpy and entropy for the HTR are 47,416 J/mol and 227.9 J/(mol K), respectively [31]. The Clapeyron diagrams of sorption thermal battery based on thermochemical sorption and resorption processes using the group of working pair are shown in Figs. 6 and 7, respectively. For sorption thermal battery using solidegas thermochemical sorption working pair of MnCl2eNH3, shown in Fig. 6, the operating principle of sorption thermal battery is based on the following reaction between manganese chloride and ammonia:

Energy efficiency for cold production:

COPc ¼

Useful cold production Qcold ¼ Heat consumption Qin

(6)

4.5. Energy density of sorption thermal battery Energy density parameter (ED), which is essentially the amount of energy production per mass of sorbent, is defined and used to analyze the performance of the sorption thermal battery for cold and heat energy storage. Heat energy density for heat production:

EDh ¼

Useful heat production Qheat ¼ Mass of reactant MR

Cold energy density for cold production:

(7) Fig. 6. Clapeyron diagram of the sorption thermal battery for energy storage using solidegas thermochemical sorption working pair.

T.X. Li et al. / Energy 107 (2016) 347e359



MnCl2 $2NH3 þ 4NH3 4MnCl2 $6NH3 þ DHR NH3 ðliqÞ þ DHevap 4NH3 ðgasÞ

(11)

For sorption thermal battery using solidegas thermochemical resorption working pair of MnCl2eNH3/NaBreNH3, shown in Fig. 7, the operating principle of sorption thermal battery is based on the following reaction:



MnCl2 $2NH3 þ 4NH3 4MnCl2 $6NH3 þ DHR NaBr þ 5:25NH3 4NaBr$5:25NH3 þ DHR

(12)

Fig. 6 shows the heat input temperature is 152  C during the charging phase (AeB) at a heat sink temperature of 30  C when sorption working pair of MnCl2eNH3 is used for sorption thermal battery. During the discharging phase for heat production mode (EeF), the heat output temperature is 141  C with the liquid ammonia evaporating at a temperature of 20  C to react with manganese chloride. During the discharging phase for cold and heat production mode (CeD), manganese chloride is firstly precooled and then begins to adsorb ammonia. During this process, the sorbate is cooled from 30  C to 10  C by consuming a little part of evaporation heat. The working temperatures for heat production and cold production are 112  C and 10  C, respectively. Fig. 7 shows that the heat input temperature is 124  C during the charging phase (AeB) at the same heat sink temperature of 30  C by employing a resorption working pair of MnCl2eNH3/NaBreNH3 for sorption thermal battery. During the discharging phase for heat production mode (EeF), the heat output temperature is 111  C. During the discharging phase for cold and heat production mode (CeD), the working temperatures for heat production and cold production are 73  C and 10  C, respectively. By comparing the Clapeyron diagrams of sorption thermal battery based on thermochemical sorption and resorption processes, it can be found that the former has a higher heat input temperature during the charging phase when compared with the latter at the same heat sink temperature. Moreover, the former also has a higher heat output temperature than the latter during the discharging phase at the same cold temperature. This is because the liquidegas equilibrium line of ammonia and the solidegas reaction equilibrium line of LTR have different thermodynamic characteristics at the same constraining temperatures [32], resulting to the difference in the working temperatures and pressures for the two different solidegas thermochemical processes. Generally speaking, thermochemical sorption process has advantage over thermochemical resorption process when solidegas working pair is used

Fig. 7. Clapeyron diagram of the sorption thermal battery for energy storage using solidegas thermochemical resorption working pair.

353

for sorption thermal battery. This is because the former has stable working pressure/temperature, broad range of heat and cold temperatures, and controllable flexibility the pressure of sorbate in the liquid tank when compared to the latter. 5.2. Experimental test unit for sorption thermal battery A simple experimental test unit is used to investigate the feasibility and working performance of sorption thermal battery for combined deep-freezing cold energy storage and heat energy storage. The simple test unit mainly consists of a solidegas sorption bed and a tank as shown in Fig. 8. The tank works as a condenser and an evaporator alternately during the charging and discharging processes. The sorption bed and tank are insulated in order to reduce the heat losses to the ambient. An electric-powered oil boiler is used to simulate the waste heat source equipment to drive the sorption thermal battery. The operating temperatures of sorption bed and tank are controlled respectively by using a constant-temperature oil boiler with an accuracy of ±0.5  C and a constant-temperature ethanol bath with an accuracy of ±0.01  C. Low thermal conductivity of metallic salt is the common drawback for a solidegas thermochemical sorption system [33]. In order to overcome the problem, a consolidated composite sorbent made from expanded graphite and manganese chloride was fabricated by using the similar manufacturing processes introduced in literature [34]. EG (Expanded graphite) is used as a porous additive to enhance the heat and mass transfer within the reactive salt due to its high thermal conductivity and gas permeability. The sorption bed is filled with manganese chloride/ expanded graphite composite sorbent and the tank is filled with the sorbate of ammonia. During the charging and discharging phases, the amount of sorbate desorbed or adsorbed by the composite sorbent is measured by a magnetostrictive displacement sensor positioned in the tank. The relative measuring error of the sensor is less than 0.05% of full scale. Four-wire type PT100 platinum resistance sensors were used to measure the working temperatures. The absolute error of temperature measurement is within ±0.51  C. Two pressure transducers with an accuracy of 1.5% full scale were used to measure the operating pressure. Equation (13) is used to calculate the amount of ammonia adsorbed by the composite sorbent during the discharging phase.

rðTe Þ$AEv $½LðtÞ  Lðt0 Þ

mad ðtÞ ¼ 

MR

(13)

Fig. 8. Schematic diagram of the experimental test unit for sorption thermal battery.

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where mad (t) is the amount of ammonia adsorbed by composite sorbent (kg/kgsalt); r (Te) is the density of liquid ammonia at evaporation temperature (kg/m3); AEv is the cross section area of the evaporator (m2); L (t) is the liquid level (m) inside the tank during the discharging phase at the end of synthesis reaction, and L (t0) is the liquid level (m) inside the tank at the beginning of synthesis reaction. 5.3. Sorption capacity of solidegas thermochemical sorption working pair of MnCl2/EG-NH3 The sorption capacity of MnCl2/EG composite sorbent during the discharging phase is investigated experimentally at different evaporation temperatures ranging from 0  C to 35  C. A global conversion coefficient (X), which represents the percentage of the sorbent that had reacted with the sorbate, was employed to evaluate the sorption performance of the composite sorbent. A value of X equal to 1 indicates that 1 mol of MnCl2 reacted with 4 mol of NH3 according to Equation (11). The corresponding maximum equilibrium mass concentration is 0.54 kg of ammonia per kilogram manganese chloride. In theory, the final global conversion coefficient X for the reaction must reach 1 inevitably after sufficient reaction duration when the constraining temperature/pressure is away from the equilibrium conditions of sorbent because of the monovariant characteristic of solidegas chemical reaction. However, it is usually difficult for global conversion coefficient to reach 1 during finite reaction duration for a real solidegas thermochemical sorption system due to the fact that reaction rate is strongly influenced by the heat or mass transfer of sorption bed. The sorption capacity of the composite sorbent during the discharging phase is shown in Fig. 9 where the synthesis reaction takes place from MnCl2$2NH3 to MnCl2$6NH3. The global conversion coefficient and the amount of sorption capacity increase with increasing evaporation temperature. This is because the global conversion coefficient (X) of solidegas thermochemical sorption process is mainly dependent on the driving pressure drop (Pe-Peq) between the constraining evaporation pressure (Pe) and the thermodynamic equilibrium pressure (Peq) [24]. A high evaporation temperature can produce a high driving pressure drop, thus promotes the global conversion coefficient. The global conversion coefficient increases from 0.536 to 0.995 as evaporation temperature varies from 35  C to 0  C. It is observed that the composite sorbent can reach a maximum mass concentration of 0.537kgNH3/kgsalt at an evaporation temperature of 0  C, accounting for about 99% of the maximum theoretical equilibrium mass concentration. This would be attributed to the fact that the addition of porous expanded graphite matrix can prevent the agglomeration of the reactive salt, and thus it allows an almost complete reaction to occur during the charging and discharging processes. Moreover, the global conversion coefficient and the amount of sorption capacity of the composite sorbent at evaporation temperature of 0  C are slightly higher than those at evaporation temperature of 5  C. These data are almost equal to the maximum theoretical equilibrium values of the reactive salt, implying that the sorption capacity of the composite sorbent would not increase further even if evaporation temperature becomes higher than 0  C. Although it is well-known that high evaporation temperature is beneficial to improve the global conversion coefficient and sorption capacity of sorbent, it is necessary to point out that these figures cannot always increase with increasing evaporation temperature for sorption thermal battery using thermochemical sorption working pair due to the monovariant characteristic of solidegas chemical reaction, which is distinctly different from solidegas physical sorption or liquidegas absorption used for sorption thermal battery.

Fig. 9. Sorption characteristics of solidegas thermochemical sorption working pair of MnCl2/EG-NH3 at different temperatures.

5.4. Performance evaluation of sorption thermal battery for combined deep-freezing cold and heat storage using working pair of MnCl2/EG-NH3 The performance of sorption thermal battery for combined deep-freezing cold energy storage and heat energy storage is evaluated at different production temperatures. The energy storage density and energy power density of sorption thermal battery using solidegas thermochemical sorption working pair of MnCl2/EG-NH3 are shown in Figs. 10 and 11, respectively. Fig. 10 shows cold and heat energy storage densities increase as the evaporation temperature increases. The heat storage and cold storage densities obtained are as high as 1498 kJ/kg and 600 kJ/kg at an evaporation temperature of 0  C. Moreover, heat energy density is much higher than cold energy density for a given evaporation temperature. This can be attributed to the fact that the cold is produced by the evaporation latent heat of the sorbate while the heat is produced by the chemical reaction heat of the sorbent. In general, the chemical reaction heat is usually much higher than the

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Fig. 10. Energy storage density of sorption thermal battery using solidegas thermochemical sorption working pair of MnCl2/EG-NH3.

evaporation latent heat at the same consumption of sorbate. Thus, it indicates that solidegas thermochemical sorption process is more suitable for heat storage, although this kind of process has been extensively discussed for cold production. Fig. 11 shows the energy power density of sorption thermal battery at different temperatures using solidegas thermochemical sorption pair of MnCl2/EG-NH3. Similar to energy storage density, cold and heat power densities increases with increasing evaporation temperature, with heat power density being much higher than cold power density at the same evaporation temperature. The heat and cold energy power densities are as high as 1875 W/kg and 752 W/kg at an evaporation temperature of 0  C. In fact, power density of sorption thermal battery is mainly depended on the chemical reaction rate of working pair that is related to the mass and heat transfer of the sorption bed. For a given total amount of stored energy, the higher power density means faster reaction rate and better mass and heat transfer of sorption thermal battery. Sensible heat of the sorption bed is not considered in assessing the heat storage performance of sorption thermal battery in

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Fig. 11. Energy power density of sorption thermal battery using solidegas thermochemical sorption working pair of MnCl2/EG-NH3.

Figs. 10b and 11b. This is majorly due to the fact that sensible heat can play a positive or negative role for heat storage density during the discharging phase, with its role mainly depended on the length of storage period and the insulation of the sorption bed [31]. For sorption thermal battery used for short-term energy storage with perfect insulation, the sensible heat released by the metallic part of sorption bed would play a positive contribution to heat storage density when the temperature of sorption bed at the beginning of discharging phase is higher than the heat output temperature for end user. In this case, both sensible heat storage and chemical reaction heat storage of sorption thermal battery can be used to supply useful heat, hence increasing the total heat storage density. However, for sorption thermal battery used for long-term energy storage with imperfect insulation, especially for seasonal energy storage, the sensible heat of sorption bed plays a negative role in heat storage density. This is because the temperature of sorption bed at the beginning of discharging phase may be lower than the heat output temperature for end user. In this case, in order to heat the sorption bed from the ambient temperature to the heat output

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temperature, a little part of chemical reaction heat released by the sorbent must be used to supply the sensible heat consumed by the metallic part of sorption bed before it starts to produce useful thermal energy for end user. As a result, the heat storage density decreases due to the additional sensible heat consumption. Fig. 12 shows the heat energy storage density of sorption thermal battery at different heat production temperatures using solidegas thermochemical sorption working pair of MnCl2/EG-NH3. The heat energy storage density increases with increasing cold production temperature from 35  C to 0  C for a given heat production temperature, and decreases with increasing heat production temperature ranging from 40  C to 110  C for a given cold production temperature. It is observed that the heat energy storage density at a heat production temperature of 40  C is slightly higher than that at a heat production temperature of 50  C. This means that the heat energy density obtained by the sorption thermal battery is not sensitive to the heat production temperature when heat output temperature is between 40  C and 50  C for a given cold production temperature. Heat energy storage density decreases obviously as heat production temperature increases from 70  C to 110  C, and later it becomes low when cold production temperature is lower than 15  C. For example, heat energy storage density is very low when heat production temperature is higher than 70  C at a cold production temperature lower than 30  C. The reason for the great difference in the above-mentioned heat energy storage density at different cold and heat production temperatures can be attributed to the relation between the equilibrium temperatures of sorption thermal battery using solidegas thermochemical sorption working pair of MnCl2eNH3 as shown in Fig. 13. According to the equilibrium temperatures of solidegas thermochemical sorption working pair of MnCl2eNH3, a given cold production equilibrium temperature has a corresponding heat production equilibrium temperature resulting from the monovariant characteristic of solidegas chemical reaction. A high cold production equilibrium temperature means a high heat production equilibrium temperature, whereas, a low cold production equilibrium temperature indicates a low heat production equilibrium temperature. It is impossible to achieve a heat production temperature higher than equilibrium temperature for a given cold production equilibrium temperature. This is the reason why heat energy storage density is almost equal to zero at a cold production

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Fig. 12. Heat energy storage density of sorption thermal battery using solidegas thermochemical sorption working pair of MnCl2/EG-NH3 at different heat production temperatures.

Fig. 13. Cold and heat production equilibrium temperatures of sorption thermal battery using solidegas thermochemical sorption working pair of MnCl2eNH3.

temperature lower than 30  C and a heat production temperature higher than 70  C. In addition, it can be seen that the theoretical heat production equilibrium temperature varies between 85  C and 122  C when theoretical cold production equilibrium temperature ranges between 35  C and 0  C. However, experimental results in subfigure show that the heat production temperature obtained from the experimental test unit is lower than the theoretical equilibrium temperature for a given cold production equilibrium temperature due to the driving temperature drop. 6. Energy storage density of sorption thermal battery using different kinds of sorption working pairs Solidegas thermochemical sorption working pair of MnCl2eNH3 is used as a potential working pair to assess the performance of the proposed sorption thermal battery in the aforementioned Section 5. It is worthwhile noting that a lot of different sorption working pairs can be used in sorption thermal battery for cold and heat energy storage. Energy storage density, in form of volume storage density (kJ/m3) or mass storage density (kJ/kg), is a key parameter to evaluate the working performance of thermal energy storage technologies. Usually, the volume storage density is strongly influenced by many factors, such as the packed density of stored material, the configuration of heat exchanger, and the densities of different components of composite material. However, the mass storage density is only closely related to the mass of stored material, and thus it is convenient and widely used to compare the storage capacities of different thermal energy storage materials. The mass energy storage densities of sorption thermal battery using common liquid/solidegas sorption working pairs are compared as shown in Fig. 14. For cold energy storage at subzero cold production temperature, solidegas thermochemical sorption working pair with ammonia has the best capacity among different cold energy storage technologies. In addition, sorption thermal battery using this kind of working pair can be used for deep-freezing cold application. For airconditioning cold energy storage application, liquidegas absorption working pair with water has the best capacity when compared with other cold energy storage technologies. The cold energy density obtained with liquidegas absorption pair is usually higher than 1000 kJ/kg, which is much higher than sensible heat storage and latent heat storage using PCM. Moreover, the cold energy density would become further higher by enlarging the concentration difference of salt if crystals are allowed to exist in liquidegas

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Fig. 14. Energy storage density of sorption thermal energy storage using common solid/liquid-gas sorption working pairs and different phase change materials (PCM) [2,8,10e13,15,35e59].

absorption process. Lithium chloride-water with crystal, in particular, has the highest performance among different sorption working pairs with a cold energy storage density higher than 3400 kJ/kg. In addition, solidegas physical sorption working pair with water is also a promising air-conditioning cold energy storage technology because it also has higher energy density than conventional cold energy storage methods using PCM or sensible heat of chilled water. For heat energy storage at a heat production temperature below 50  C, liquidegas absorption working pair with water has the best capacity. However, for heat energy storage at a high heat production temperature, it appears that solidegas thermochemical sorption working pair has the advantages of high energy storage density and wider working temperature range when compared with liquidegas absorption working pair, solidegas physical sorption working pair, and conventional sensible heat and latent heat storage technologies. The heat energy density obtained with solidegas thermochemical sorption heat storage is usually higher than 1000 kJ/kg, while those obtained with sensible heat storage and

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latent heat storage using PCM are usually lower than 200 kJ/kg. In addition, it seems that solidegas physical sorption working pair is also a promising technology for heat energy storage application and it can offer a distinct advantage over conventional sensible and latent heat energy storage methods. By considering combined cold and heat energy storage, it appears that sorption thermal battery using liquidegas absorption pair or solidegas physical sorption pair is more suitable for cold energy storage with air-conditioning application and heat energy storage with a relatively low heat output temperature. The two kinds of sorption working pairs usually employ environmentally friendly water as a green sorbate, making sorption thermal battery to have extensive applications for residential and industrial renewable energy and low-grade thermal energy. For deepfreezing cold energy storage and heat energy storage with a relatively high heat output temperature, sorption thermal battery using solidegas thermochemical sorption working pair has advantages of stable working temperature, broad range of heat and cold temperatures, and operation flexibility when compared with the other two kinds of sorption working pairs. This kind sorption thermal battery can be used for large-scale industrial processes and renewable energy utilization although it would suffer from high working pressure because ammonia is usually employed as sorbate. In general, sorption thermal battery exhibits more distinct advantage for heat energy storage when compared with cold energy storage. It is very important for sorption thermal battery to assure a stable production temperature during the discharging phase. For sorption thermal battery using liquidegas absorption working pair or solidegas physical sorption working pair, the heat production temperature cannot be controlled by the working pressure due to the bivariant sorption process of liquidegas absorption or solidegas physisorption. Therefore, it is difficult to obtain a stable temperature for heat production by using these two kinds of working pairs; however, they can be used for stable cold production temperature during the discharging phase. For sorption thermal battery using solidegas thermochemical sorption working pair, the production temperature can automatically be controlled by using a given working pressure due to the monovariant characteristic of solidegas chemical reaction in which working pressure and temperature is the functional equation of one variable. Therefore, the cold and heat production temperatures of sorption thermal battery can be easily adjusted by changing the working pressure of sorbate in order to meet the thermal energy demand at different heat output temperature levels. Sorption thermal battery using sorption working pairs has distinct competitive advantage over conventional sensible heat and latent heat storage technologies due to its high energy storage density whether for cold energy storage or heat energy storage. Thus, it appears that sorption thermal battery is a promising candidate for achieving compact high energy density thermal energy storage unit. 7. Conclusion Sorption thermal battery is proposed and developed for combined cold and heat energy storage. The operating principle of sorption thermal battery is based on the energy conversion between thermal energy and bond energy of sorption potential of solid/liquidegas sorption working pairs, and thermal energy can be stored in the form of sorption potential for a desirable period of time. Sorption thermal battery can be used for short-term energy storage and long-term seasonal energy storage depending on the length of inactive storage period between the charging phase and discharging phase. The feasibility and working performance of the

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proposed sorption thermal battery is investigated using a solidegas sorption working pair of EG/manganese chloride-ammonia. Experimental results showed the presented sorption thermal battery is a very effective method to achieve combined deep-freezing cold and heat energy storage simultaneously, and the cold and heat energy densities are as high as 600 kJ/kg and 1498 kJ/kg, respectively. Sorption thermal battery exhibits more distinct advantage for heat energy storage when compared with cold energy storage. The energy densities of sorption thermal battery using different working pairs are compared and analyzed for cold energy storage or heat energy storage. It appears that liquidegas absorption working pair is more suitable for air-conditioning cold energy storage and heat energy storage with a relatively low production temperature, whereas solidegas thermochemical sorption working pair is suitable for deep-freezing cold energy storage and heat energy storage with a relatively high production temperature. Data analysis showed that the advanced sorption thermal battery has a distinct advantage of high energy density and combined cold and heat storage in comparison with conventional sensible heat and latent heat storage methods. The presented sorption thermal battery is a promising compact high-density and high-performance energy storage technology for waste heat recovery and renewable energy utilization. Acknowledgments This work was supported by the National Natural Science Funds for Excellent Young Scholar of China under the contract No. 51522604. References [1] Dincer I, Rosen MA. Thermal energy storage: systems and applications. 2nd ed. Chichester, United Kingdom: John Wiley & Sons; 2011. [2] Zalba B, Marin JM, Cabeza LF, Mehling H. Review on thermal energy storage with phase change: materials, heat transfer analysis and applications. Appl Therm Eng 2003;23:251e83. [3] Kenisarin M, Mahkamov K. Solar energy storage using phase change materials. Renew Sustain Energy Rev 2007;11:1913e65. [4] Yan T, Wang RZ, Li TX, Wang LW, Ishugah TF. A review of promising candidate reactions for chemical heat storage. Renew Sustain Energy Rev 2015;43: 13e31. [5] Cot-Gores J, Castell A, Cabeza LF. Thermochemical energy storage and conversion: a-state-of-the-art review of the experimental research under practical conditions. Renew Sustain Energy Rev 2012;16:5207e24. [6] Yu N, Wang RZ, Wang LW. Sorption thermal storage for solar energy. Prog Energy Combust Sci 2013;39:489e514. [7] Abedin AH, Rosen MA. Closed and open thermochemical energy storage: energy- and exergy-based comparisons. Energy 2012;41:83e92. [8] Mugnier D, Goetz V. Energy storage comparison of sorption systems for cooling and refrigeration. Sol Energy 2001;71:47e55. [9] N'Tsoukpoe KE, Liu H, Pierres NL, Luo LG. A review on long-term sorption solar energy storage. Renew Sustain Energy Rev 2009;13:2385e96. [10] Jaenchen J, Ackermann D, Stach H, Broesicke W. Studies of the water adsorption on zeolites and modified mesoporous materials for seasonal storage of solar heat. Sol Energy 2004;76:334e9. [11] Hongois S, Kuznik F, Stevens P. Development and characterization of a new MgSO4-zeolite composite for long-term thermal energy storage. Sol Energy Mater Sol Cells 2011;95:1831e7. [12] Liu H, Edem NTK, Pierres NL, Luo LG. Evaluation of a seasonal storage system of solar energy for house heating using different absorption couples. Energy Convers Manag 2011;52:2427e36. [13] Li TX, Wang RZ, Yan T, Ishugah TF. Integrated energy storage and energy upgrade, combined cooling and heating supply, and waste heat recovery with solid-gas thermochemical sorption heat transformer. Int J Heat Mass Transf 2014;76:237e46. [14] Li TX, Li H, Yan T, Wang RZ. Performance analysis of high-capacity thermal energy storage using solid-gas thermochemical sorption method. Energy Storage Sci Technol 2014;3:236e43 [in Chinese]. [15] Li TX, Wang RZ, Kiplagat JK. A target-oriented solid-gas thermochemical sorption heat transformer for integrated energy storage and energy upgrade. AIChE J 2013;59:1334e47. [16] Li TX, Wang RZ, Kiplagat JK, Kang YT. Performance analysis of an integrated energy storage and energy upgrade thermochemical solid-gas sorption system for seasonal storage of solar thermal energy. Energy 2013;50:454e67.

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