Optimisation of Operation of Adsorption Chiller with Desalination Function

energies Article Optimisation of Operation of Adsorption Chiller with Desalination Function Karol Sztekler Faculty of Energy and Fuels, AGH University of Science and Technology, Mickiewicza 30, 30-059 Krakow, Poland; [email protected] Abstract: The demand for electricity is growing rapidly along with economic development and increasing population. At present, its production is mainly based on non-renewable sources, which has negative impacts on the environment and contributes to global warming. A large proportion of the produced electricity is consumed by refrigeration equipment. Climate change and the progress of civilisation are additionally increasing the demand for cooling, with increasing electricity consumption as a consequence. One of the options for obtaining eco-friendly cooling is the use of adsorption chillers. These devices are powered by low-temperature heat and their operation only requires a small amount of electrical energy. The source of low-temperature heat can be, e.g., waste heat generated in many industrial processes. Its use allows one to increase energy efficiency and achieve additional financial benefits. However, adsorption chillers are characterised by low coefficients of performance. This paper presents possibilities to improve their performance. It also presents the results of tests carried out on a three-bed adsorption chiller with desalination function. The aim of the investigation was to determine the effect of the cycle time on the coefficient of performance (COP) and specific cooling power (SCP). The working pair was silica gel and water. The results confirmed the effect of the duration of adsorption and desorption on the COP and SCP of the adsorption chiller. Increasing the duration of the cycle led to an increase in the COP. Citation: Sztekler, K. Optimisation of Keywords: adsorption; chiller; desalination Operation of Adsorption Chiller with Desalination Function. Energies 2021, 14, 2668. https://doi.org/10.3390/ en14092668 1. Introduction Dynamic economic development, population growth and observed climatic changes Academic Editor: David Borge-Diez imply that the demand for cooling will be growing every year. Cooling processes are used in many industries, such as in the food industry for food production and storage, the Received: 29 March 2021 pharmaceutical industry, heavy industry and power generation [1]. Refrigeration units Accepted: 4 May 2021 Published: 6 May 2021 using thermal compression and using adsorption and desorption processes are modern alternatives to the most commonly used conventional electrically driven compressor re- Publisher’s Note: MDPI stays neutral frigeration systems. The need to seek innovative solutions for the refrigeration industry with regard to jurisdictional claims in has been dictated, among other things, by the necessity to save electricity and reduce the published maps and institutional affil- environmental impact. iations. The market for refrigeration equipment is growing steadily. Data from the Interna- tional Institute of Refrigeration (IIR) show that there are currently over 5 billion refrigeration units and heat pumps in operation globally [2]. In 2016, more than 1.6 billion air conditioning units were in use, of which more than half were located in the United States and China alone [3]. Middle Eastern countries are a particularly important market. The demand for Copyright: © 2021 by the author. Licensee MDPI, Basel, Switzerland. cooling is extremely high there due to the very warm climate. It is estimated that almost This article is an open access article 100% of households in the Middle East are equipped with air conditioning [3]. The data distributed under the terms and quoted above clearly show the scale of the use of cooling devices and, consequently, the conditions of the Creative Commons energy consumption related to their operation. In this context, it is particularly important Attribution (CC BY) license (https:// that the vast majority of the units in use are compressor systems driven by electricity [1]. creativecommons.org/licenses/by/ According to the report of the International Institute of Refrigeration in Paris, the refrigera- 4.0/). tion sector consumes approximately 20% of the world electricity production [2]. On the Energies 2021, 14, 2668. https://doi.org/10.3390/en14092668 https://www.mdpi.com/journal/energies Energies 2021, 14, 2668 2 of 20 other hand, estimates of the International Energy Agency (IEA) indicate that the share of air conditioning equipment in the global electricity demand was approximately 10% in 2016 [3]. During peak cooling demand in summer, this value can increase significantly [3]. Taking into account that global electricity production is mainly based on non-renewable sources, it can be concluded that the refrigeration sector indirectly contributes to accelerating the rate of adverse climate change [4]. Adsorption chillers can be powered by the low-temperature waste heat with temperatures in the range of 60 ◦C to 90 ◦C or by the heat from solar energy, and therefore they are characterised by very low electricity consumption [1,5,6]. Furthermore, adsorption chillers can operate at low temperatures, which additionally contributes to the reduction in thermal energy losses. Many processes in various industries emit energy which is lost to the atmosphere unless it is recovered. In thermal power plants, approximately 50–70% of the energy value of the fuel is rejected as waste heat [6]. In such cases, one of the possibilities of using waste heat is the production of cold energy in adsorption chillers [1]. The use of this type of equipment offers the possibility of creating polygeneration systems capable of producing electricity, heat and cold simultaneously [7]. In addition to reducing the negative impact of greenhouse gas emissions, energy recovery can also bring significant financial benefits [1]. A very important feature of adsorption chillers in the context of environmental care is the use of non-ozone-depleting refrigerants such as water [5]. It is a cheap, widely available, safe and the most thermally stable adsorbate which extends the applicability of adsorption chillers to water desalination [8,9]. Moreover, in addition to several previously mentioned advantages, adsorption chillers are characterised by easy and quiet operation and a small number of moving mechanical parts, which makes them especially valuable in applications where reliability of the devices is very important [8,10]. Replacing an electric compressor with a thermal compressor has the advantage of no vibration, thus opening up many new application possibilities [11]. Despite there being so many advantages of using adsorption chillers, there are also some limitations. These include the low coefficient of performance (COP) compared to conventional refrigeration systems, which is usually between 0.5 and 0.6, the low specific cooling power (SCP), and the small mass and heat flow inside the bed [8,12]. All of these are related to the large mass and dimensions of adsorption chillers [6,13]. Continuous research is being conducted to improve the efficiency of adsorption chillers so as to make them more attractive and competitive in the market. In a later part of the paper, a number of methods to improve the performance parameters of adsorption chillers are presented. One of the disadvantages of the materials used is that the highly porous adsorbents have relatively lower thermal conductivity [8]. At the same time, this is a field of searching for new solutions that take into account the balance between good thermal conductivity and sufficiently high sorption capacity [8,14]. Sorption materials that can work with water are, e.g., silica gel, zeolite and activated alumina [15]. Some very important characteristics of the sorbents are the ability to adsorb a maximum amount of adsorbate per unit mass and a low specific heat [8,15]. Particularly important from the point of view of the heat source and chiller operation is the bed regeneration temperature. This is also one of the characteristics of the sorption materials and can be different for different materials [12,15]. Furthermore, adsorbents must be non-toxic, durable, readily available and cheap [8,14,15]. The high porosity of such materials is an important characteristic from the point of view of sorption properties, but it negatively affects heat conduction [15]. An adsorption chiller with desalination function can be characterised by several key parameters. These include the coefficient of performance (COP), the specific cooling power (SCP) and the specific daily water production (SDWP) [16,17]. These indicators depend on many factors. Some of them, such as bed regeneration temperature, chilled water temperature or cycle time, can be easily modified in order to obtain the optimal values. These will vary depending on the design of the chiller, the resources available and the desired effect. This paper analyses the impacts of these factors on the COP. In addition to the above-mentioned possibilities of enhancing performance, there are also some other Energies 2021, 14, 2668 3 of 20 factors which are strictly related to the design of the chiller and the properties of the adsorbent bed. These include improving the heat exchange efficiency inside the bed and the utilisation of heat and mass recovery in the system [6,7,14]. The former method can be divided according to the three keyways of its implementation: • Improving heat transfer between the heat exchanger and the sorbent [12,18]; • Improving heat transfer in the sorbent [7]; • Using modern porous materials as sorbents [12]. A typical material that heat exchangers are made of is copper, characterised by a high thermal conductivity coefficient of 380 W/(mK) [12]. Additionally, the most commonly used adsorbent is silica gel, which has a thermal conductivity coefficient of merely 0.17 W/(mK) [12]. The significant difference in the value between these two materials means that heat transport in the boundary zone is hindered. One possible way to improve heat transfer in this area is, e.g., by coating the silica gel layer in contact with the heat exchanger surface with a binder [12].

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