Elsevier

Energy

Volume 113, 15 October 2016, Pages 739-747
Energy

Investigation of a 10 kWh sorption heat storage device for effective utilization of low-grade thermal energy

https://doi.org/10.1016/j.energy.2016.07.100Get rights and content

Highlights

  • A 10 kWh short-term sorption thermal energy device was developed.

  • The device was tested under conditions of transition and winter seasons.

  • The performance of the device was improved by recovering waste heat.

  • The sorption thermal energy device was compared with a 300-L hot water tank.

Abstract

Heating and domestic hot water for family houses represents a notable share of energy consumption. However, sufficient space for the installation of thermal energy storage (TES) components may not be available in family houses or urban areas, where space may be restricted and expensive. Sorption TES devices seem to be a promising means of replacing conventional TES devices and reducing the occupied space for its high energy density. In this paper, a 10 kWh short-term sorption TES device was developed and investigated. The employed composite sorbent was formed from lithium chloride (LiCl) with the addition of expanded graphite (EG). The principle of sorption TES for the LiCl/water working pair is first illustrated. This prototype was tested under conditions representative of transition or winter seasons. Under the conditions used (charging temperature Tcha at 85 °C, discharging temperature Tdis at 40 °C, condensing temperature Tc at 18 °C, and evaporating temperature Te at 30 °C), the heat storage capacity can reach 10.25 kWh, of which sorption heat accounts for approximately 60%. The heat storage density obtained was 873 Wh per kg of composite sorbent or 65.29 kWh/m3, while the heat storage density of hot water tank was about 33.02 kWh/m3.

Introduction

Energy conservation and emission reduction policies have been advocated by governments all over the world. Effective utilization of waste heat in industry and life fields or solar energy has been a research hotspot in recent years [1]. Thermal energy storage (TES) has been identified as critical in these decentralized energy systems. Among different TES methods, sorption heat storage as a category of thermochemical TES appears to be promising compared with conventional sensible TES and latent TES. Sorption TES can provide such features as high energy density and ability to conserve energy over long periods of time and can offer both heat storage and cold storage functions [2].

Considering safety, cost and availability, water is the most feasible sorbent in open and closed systems. Typical physical water sorption materials include silica gel [3], zeolites [4], silico-aluminophosphates (SAPOs) [5] and metal-organic frameworks (MOFs) [6]. However, the low sorption uptake limits the application of physical sorbents. Certain promising hygroscopic salts were recommended by researchers, including LiCl [7], [8], MgCl2 [9], CaCl2 [10], [11], MgSO4 [12], [13], SrBr2 [14], [15], and NaS2 [16]. Unfortunately, hygroscopic salts are faced with the problems of solution carryover, mass transfer obstacles, swelling and agglomeration. The novel composite sorbents, “salt in porous matrix,” were created by embedding salt in a porous host matrix [17]. Yu et al. developed silica gel-LiCl [7] and activated carbon-LiCl-expanded graphite (EG) [8] composite sorbents for TES. Zondag et al. [9] prepared composite MgCl2 with a zeolite carrier material, and results showed that the zeolite had good flow characteristics. Wu et al. [10] impregnated silica gel in 30 wt% CaCl2 solution to form a silica gel-CaCl2 composite sorbent. Hongois et al. [12] developed MgSO4/zeolite composite sorbents for TES, and results showed that the optimal mass fraction of MgSO4 in composite was 15%. Lahmidi et al. [14] used the host matrix of EG to develop composite SrBr2 sorbents. Boer et al. [16] employed fibrous cellulose to prepare a composite NaS2 sorbent. These fibers are chemically inert and thermally and mechanically stable to support the salt in composite.

Based on the outstanding sorption materials, researchers built various TES systems. A Swedish company, ClimateWell [18], has manufactured several generations of TES devices with LiCl/H2O as the working pair. Its devices combine short-term absorption thermal storage and solar cooling technologies. Zondag et al. [9] proposed an open prototype for long-term solar heat storage with MgCl2/H2O. The estimated energy storage density (ESD) was 146 kWh/m3. Zhu et al. [19] built an open TES prototype equipped with 40 kg composite silica gel-CaCl2 pellets, and the ESD remained stable at 0.264 Wh/g (264 kWh/m3). Hongois et al. [12] established a long-term TES system with a MgSO4/zeolite composite, and an ESD of 0.18 Wh/g (166 kWh/m3) was achieved. Michel et al. [20] investigated an open TES setup with SrBr2/H2O, and the ESD was in the range of 430–460 kWh/m3. Manuran et al. [15] built a large-scale TES prototype with a consolidated sorbent of SrBr2/EG: the estimated heat storage capacity was 60 kWh with 35 °C output, and the estimated cold storage capacity was 40 kWh with 18 °C output. Boer et al. [16] developed a modular adsorption cooling system employing a Na2S/H2O working pair with a cold storage capacity of 2.1 kWh.

Heating and domestic hot water of family houses represents a notable share of energy consumption. The existing products on the market include heat-pump water heaters, gas water heaters, solar water heaters and electric water heaters. In family apartments or urban areas, there is not always sufficient room for the installation of thermal storage components. In addition, space may be restricted and expensive in certain cities, such as Shanghai. Currently, the thermal storage system available on the market is hot water storage (sensible heat). Owing to the high energy density, sorption TES could be a substitute that reduces the occupied space of heat storage.

For this situation, compact sorption TES devices were researched in our lab, and a 1-kWh TES prototype was built in our previous work [21]. A hygroscopic salt, LiCl, was chosen as the high density heat storage material. After summarizing the experience and optimization, a larger-scale 10 kWh TES device was established with a charging temperature lower than 90 °C. This paper shows the system configuration and the detailed experimental performances of the device under different working conditions.

Section snippets

Description of LiCl/H2O TES principle

Fig. 1(a) displays the schematic diagram of LiCl/H2O desorption process in a phase diagram. When heated, a desorption process of LiCl sorbents in TES can be represented by the following equations:LiClweaksolutionLiClsaturatedsolution+Water(vapor)LiClsaturatedsolutionLiCl·H2O(solid)+Water(vapor)LiCl·H2O(solid)LiCl(solid)+Water(vapor)

Unlike the conventional absorption or adsorption processes, the LiCl weak solution is heated and concentrated to its saturated solution, as shown in Eq. (1). The

Configuration of the prototype

The experimental prototype for sorption thermal storage mainly includes two vessels: the sorption reactor and the condenser/evaporator. The picture of the prototype and the detailed structures of the vessels are shown in Fig. 2.

As shown in Fig. 2(b), the sorption reactor consists of 25 identical sorption bed units, which are connected by manifold tubes in parallel. Thus, a total of approximately 11.75 kg of consolidated sorbent (LiCl) was filled into the reactor. To ensure enough mass transfer

Results and discussion

In experiments, the performances of both transition conditions and winter conditions were tested, as these are the most important cases for the water heating applications of TES. The effects on performance of charging temperature, discharging temperature, evaporation/condensation temperature and flow rate were also analyzed. In addition, the heat recovery experiments to lift the evaporation temperature were conducted, and the performances were investigated.

Comparison with hot water tank

The objective of our project is to develop an energy storage device which could replace a conventional heat storage water tank and lower the occupied space. Table 2 displays the main parameters of the hot water tank and the sorption TES prototype. For a 300-L hot water tank, the practical volume is 315 L (462 L including the thermal insulation), and the mass is approximately 340 kg. The TES capacity is approximately 10.4 kWh when the temperature rise is 30 °C. Thus, the ESDV is 33.02 kWh/m3.

Conclusions

In this paper, a high energy density sorption material was developed, and a compact and efficient sorption TES prototype was built on the scale of 10 kWh. From the experimental results of the sorption TES device, the following conclusions can be made:

  • (1)

    When Tcha = 85 °C, Tdis = 40 °C, and Tc = Te = 18 °C, the heat storage capacity is 8.52 kWh, of which sorption heat accounts for approximately 60%. The total heat storage efficiency is approximately 94%. The ESDm can reach 726 Wh/kg (2612 kJ/kg).

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