Charging and discharging characteristics of absorption energy storage integrated with a solar driven double-effect absorption chiller for air conditioning applications
Introduction
Electric vapour compression systems are being used for comfort cooling in buildings both residential and commercial and other government offices and facilities. However, these systems consume large amount of energy and ultimately cause additional stress on the generation and distribution of electric systems [1]. Hence, to release this stress, an effective method of cooling is the solar air conditioning based on absorption technology [2]. The intermittent nature of solar energy is the main concern in the real-time deployment of solar driven absorption systems for air conditioning and refrigeration. Hence, energy storage is necessary to minimize the mismatch and achieve extended or continuous cooling coverage from solar driven cooling systems.
Solar thermal storage is mainly classified in to three; sensible, latent, and sorption/thermochemical heat. The most common thermal storage is the sensible heat storage compared to other thermal energy storage options [3]. The sensible heat storage is of low storage density, high heat loss, and requires large storage volumes. On the other hand, the latent heat storage has better storage density compared to sensible heat. While sorption/thermochemical heat storage is more compact due to its superior energy storage density than the sensible and latent heat storage options [4,5]. In addition, there is an increasing interest on the utilization of sorption thermal energy storage in the recent times, especially in Europe for seasonal storage focusing on space heating applications [6], [7], [8]. This is due to negligible heat losses associated with sorption storage. Depending on application requirement, cooling and heating effects can be exploited from sorption storage. Sorption energy storage (absorption or adsorption) involves a reversible physio-chemical process where the thermal energy is chemically stored. The energy is usually recovered upon supplying low-temperature heat [6].
Since absorption cooling systems are already commercial; constituting about 82% market distribution of sorption technologies [9], coupling this technology with absorption energy storage deserves further research focus. This coupling potential has been reviewed recently [10,11]. The concept of combining absorption energy storage with conventional absorption chiller begun theoretically since 1970 [12,13]. Based on geographical data of Brisbane, Australia, an integrated system consisting of basic absorption chiller, solution, and refrigerant tanks was modelled [13]. The results showed that the integrated system was thermodynamically feasible and can enhance cooling application significantly. Xu et al. [14] described the concept of absorption energy storage integral with absorption chiller as variable mass energy transformation and storage (VMETS) system. This was achieved due to the continuous mass change in the storage tanks during the charging and discharge cycles [15,16]. Dynamic simulation of the VMETS system under full and partial storage strategies was presented by the authors [14,17]. A similar system was dynamically modelled by Qiu et al. [18], replacing mechanical solution pump with a bubble pump. The results indicated that the VMETS technology could provide better storage density compared to sensible storage option.
Xu eta al. [19] investigated the efficiency of a solar absorption cooling system with refrigerant storage, in which the solar collector functions as a generator. Consequently, the working fluid (lithium bromide solution) was heated directly by the solar collector where its temperature raised to the point of saturation, generating a mixture of vapour and water. This mixture flowed into the solution tank where the vapour was separated and directed towards the condenser and the strong solution to the absorber. The results showed a COP of 0.75 and energy storage density of 368.5 MJ/m3. New hybridization of absorption chiller with reversible solid/gas sorption storage system driven by solar energy was introduced by Fito et al. [20]. The system consists of single-effect absorption cycle combined with solid/gas thermochemical storage unit driven by flat plate solar collector. The working pair of the absorption cycle was ammonia-lithium nitrate (NH3-LiNO3) and that of thermochemical storage was ammonia-strontium chloride (NH3-SrCl2) and many other ammonia-based reactive pairs. Simulation results indicated that the sorption storage unit extended the cooling demand coverage by up to 24%, which otherwise would not be possible with the absorption refrigeration alone just by increasing collector area. The authors concluded that NH3-LiNO3 is the best candidate for the absorption refrigeration system and ammonia-barium chloride (NH3-BaCl2) for the thermochemical subsystem. A simulation study on solar-driven single-effect absorption system integrated with water-lithium bromide (H2O-LiBr) absorption energy storage reported an energy storage density of 430.56 MJ/m3 [21].
Literature survey showed that several simulation-based studies on integrated H2O-LiBr chiller-absorption energy storage systems have been performed in different climatic zones around the globe. However, in the previous studies, the system integrations with absorption energy storage were based on single-effect absorption chiller. To the best of the author's knowledge, there have been fewer earlier attempts on the integration of absorption storage with double-effect H2O-LiBr absorption chillers. This is particularly important because the double-effect absorption chiller requires heat input at high temperature (up to about 180°C) [22], where the use of sensible hot storage at this temperature to drive the chiller during the period of non-available solar energy could lead to serious heat losses. Therefore, it is more suitable to integrate absorption energy storage with double-effect absorption chiller. Hence, this study aims to open a new research direction towards integrating double-effect chiller with absorption energy storage and evaluating the integrated system performance. The objective of the study is to evaluate the performance and charging and discharging characteristics of an absorption energy storage integrated with solar driven double-effect H2O-LiBr absorption chiller. Thermodynamic model of the integrated system is developed and simulated in engineering equation solver (EES) software [23].
Section snippets
System description and operations
Schematics of the integrated solar driven double-effect absorption system is shown in Fig. 1. The major system components include absorption chiller (ACH), parabolic trough collector (PTC), and absorption energy storage (AES) unit. The ACH and AES units shared the same working fluid (H2O-LiBr) and are combined through piping and control valves to complete the integrated system. The ACH considered in this work is a parallel-flow double-effect unit based on the product of Broad air conditioning
Model development
The thermodynamic model of the integrated solar driven double-effect absorption system is presented in this section. The system model consists of that of PTC, double-effect H2O-LiBr absorption chiller, and absorption storage. The model of the PTC determines the heat energy and temperature of the collector fluid required to operate the absorption chiller. Mass and energy conservation across each system component are used to develop the system model. The model of the absorption chiller/storage is
Validation
The model of the absorption chiller is first validated using catalogue information of a double-effect H2O-LiBr chiller provided by the manufacturer [24]. Some of the input parameters (Table 4) are based on the manufacturer's data. Comparison of the model results with catalogue data indicate good comparison with about 4% maximum difference between the two. The model is further validated with experimental data (Table 5). Good agreement was also observed between the experimental and the simulated
Results and discussions
Considering the weather data of a typical summer day in Dhahran, Saudi Arabia, performance of the integrated solar cooling system is evaluated first. Fig. 5 shows the solar radiation and ambient temperature data for a typical day (15th May) [47].
The initial mass of the solution in the tank is determined in such a way that crystallization in the tank is prevented, which can cause problems in pumping the solution. Using these input data, the system is simulated at different initial masses of the
Conclusions
This paper presented simulation results of a solar-assisted air conditioning system consisting of a double-effect H2O-LiBr absorption chiller with integral absorption energy storage and parabolic trough solar collector. Charging and discharging characteristics of the integral storage as well as thermodynamic performance of the cooling system are evaluated. The proposed integrated system can offer flexible operation where the charging process of the storage unit can be adjusted and controlled by
CRediT authorship contribution statement
Nasiru I. Ibrahim: Investigation. Fahad A. Al-Sulaiman: Supervision, Writing - review & editing. Aminuddin Saat: Supervision, Writing - review & editing. Shafiqur Rehman: Data curation. Farid Nasir Ani: Supervision.
Declaration of Conflict of Interest Statement
Nasiru I. Ibrahim, Fahad A. Al-Sulaiman, Aminuddin Saat, Shafiqur Rehman and Farid Nasir Ani
We, as authors have no other interest other than scientific contribution to the world.
Acknowledgments
We appreciate the award of international doctoral fellowship by the Universiti Teknologi Malaysia (UTM). We also knowledge the support of the Center of Research Excellence in Renewable Energy at King Fahd University of Petroleum & Minerals Dhahran, Saudi Arabia.
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