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Introduction to Advancements in Non-Conventional Cooling and Thermal Storage Strategies: Technologies for More Sustainable Space Conditioning

Animesh Pal1,2, Bidyut Baran Saha1,3, and Dibakar Rakshit4

1International Institute for Carbon-Neutral Energy Research (WPI-I2CNER), Kyushu University, Fukuoka, Japan

2Department of Nuclear Engineering, University of Dhaka, Dhaka, Bangladesh

3Department of Mechanical Engineering, Kyushu University, Fukuoka, Japan

4Department of Energy Science and Engineering, Indian Institute of Technology Delhi, New Delhi, India

1.1 Background

Buildings, encompassing residential, commercial, and industrial structures, play a significant role in global energy consumption patterns. As our world becomes increasingly urbanized, understanding and optimizing the energy use of these buildings become crucial for sustainability, environmental responsibility, and economic efficiency. Space cooling is a notable driver of energy consumption, accounting for about one gigaton of CO2 emissions and approximately 5% of global energy usage in 2020. The worldwide building floor area is projected to surge by 75% from 2020 to 2050, with emerging and developing economies contributing to 80% of this growth [1]. This growth highlights a pivotal moment in implementing strategies that reduce cooling energy consumption through bio-climatic architecture and adopting passive building design approaches. Furthermore, it is observed that energy consumption for space cooling has seen a twofold increase since 2000, escalating from 1000 terawatt-hours to 1945 terawatt-hours [2]. This surge can be attributed to rising temperatures, swift urban expansion, increased accessibility and ownership of air-conditioning units, and the prevalence of inefficient cooling systems. In a scenario aiming for net-zero energy consumption, the energy required for space cooling has the potential to be cut by half by 2050 when contrasted with the existing policy trajectory.

The global temperatures are rising, and it can be observed from Figure 1.1 that the average temperature is higher for the countries in between the tropics. Thus, it is evident from Figure 1.2 that in 2020, the maximum number of air conditioners was installed in developing nations. With the rising income levels and increased affordability, the penetration of air conditioners will be more significant in underdeveloped and developing nations, and as per IEA, the number of households will double by 2050 [4]. As the world grapples with the challenges of climate change and increasing energy demands, the need for sustainable and energy-efficient space-conditioning solutions has become paramount. Conventional HVAC (Heating, Ventilation, and Air-Conditioning) systems contribute significantly to energy consumption and greenhouse gas emissions.

Figure 1.1 Global distribution of annual mean temperature.

Source: Mourshed [3]/with permission of Elsevier.

Figure 1.2 Number of homes fitted with air-conditioning across regions from 2010 to 2050.

Source: Adapted from IEA [4].

These emissions and energy demands are expected to increase further, as with the increasing income levels of developing nations, the number of air conditioners per household is expected to increase to 2.1 by 2030 compared to the current status of 0.6 [5]. With the growing concerns about climate change and the need for energy conservation, there is a pressing need to explore non-conventional cooling and thermal storage strategies.

The non-conventional (sorption) cooling technologies can be driven by thermal energy. The sorption cooling system can be simply classified into absorption (liquid–gas) and adsorption (solid–gas), which are shown in Figure 1.3. Adsorption (solid–gas) is a surface phenomenon where gas molecules, atoms, or ions adhere to the surface of a solid material. In adsorption, the adsorbate (substance being adsorbed) accumulates or concentrates on the surface of the adsorbent (the material on which adsorption occurs) rather than entering the bulk of the adsorbent. Adsorption processes can be classified into physical adsorption (physisorption) or chemical adsorption (chemisorption). Physical adsorption is relatively weak intermolecular forces that are caused by van der Waals forces between the adsorbate and the adsorbent’s surface. It is a reversible process. The adsorbate can be easily desorbed by changing temperature or pressure. However, chemical adsorption involves the formation of chemical bonds between the adsorbate and adsorbent. Chemisorption is a more potent and specific form of adsorption. These chemical bonds are typically covalent or ionic in nature. Chemisorption is usually not a reversible process and requires the breaking of chemical bonds to desorb the adsorbate. Generally, the main difference between adsorption and absorption is the sorbent pair’s nature and the cycle duration time. It is stated that the performance of the absorption cycle is higher than that of the adsorption cycle. However, the absorption cycle needs a high-temperature heat source to operate. Absorption cycles are always closed cycles. Adsorption cycles, however, can be split into open and closed cycles. Both desiccant cooling and dehumidification applications use the open cycle. Silica gel, calcium chloride, and lithium bromide are generally used as desiccants. Depending on the kind of adsorbent–adsorbate (refrigerant) used in the system and the application, closed adsorption systems can be divided into high-pressure (above atmospheric pressure) and low-pressure (subatmospheric pressure) categories. It should be noted that practically all solid adsorbents employed in adsorption cooling systems (ACSs) exhibit physical adsorption. The most commonly employed adsorbent + adsorbate pair include activated carbon (AC) + ethanol/CO2/methanol/ammonia [6–17], silica gel + water [18–21], metal–organic frameworks (MOFs) + water/ethanol/CO2 [22–26], zeolite + water/CO2 [27–29], composite + ethanol/CO2 [30–40], and AC fiber + ethanol/methanol [41–43].

Figure 1.3 Schematic diagram of the classifications of sorption systems, TES systems, and their application ranges.

Thermal energy storage (TES) systems have the potential to revolutionize space conditioning by enabling the storage of excess thermal energy for later use. These systems can help balance energy demand and supply, reduce peak load on the grid, and enhance the reliability of renewable energy sources. TES systems have been used for district heating and cooling purposes. Studies have shown that integrating a TES with a building can store thermal energy up to 93.2 MW, thereby reducing the heat requirements by 14.8% [44]. Furthermore, incorporating cold TES reduces energy imprint by reducing electricity consumption along with reducing the size of the cooling system [45]. Depending on the energy storage mechanism, the TES system can be classified into three main categories: sensible heat storage (SHS), latent heat storage (LHS), and thermochemical energy storage (TCES), as shown in Figure 1.3. SHS systems make use of specific heat capacity of materials (solid or liquid) to store heat energy without inducing a change in the material’s phase. Depending on the temperature of the heat source, commonly employed materials for these systems include rocks, metals, water, thermal oils, molten salts, and liquid metals. The heat storage capacity of these systems is directly proportional to the specific heat, temperature change, and amount of the chosen material [46, 47]. In contrast, LHS entails the process of heating a material until it reaches a critical temperature, causing it to undergo a phase transition. This transition can involve changing from a solid to a solid or a solid to a liquid, or from a liquid to a gas. When the material reaches this phase change temperature, it absorbs a significant quantity of heat energy to facilitate the transition. This absorption of heat is referred to as the latent heat of fusion or vaporization depending on the specific case, and it serves as a means to store energy. Phase change materials (PCMs) have gained attention as an innovative way to efficiently store and release thermal energy. These materials undergo a phase transition, from solid to liquid, liquid to vapor, or vice versa, at a specific temperature range. This property allows them to absorb and release heat energy during the phase change. Furthermore, the magnitude of stored energy density in latent heat is much higher than in sensible heat [48]. For solid–liquid heat storage systems, a solid material like paraffin wax or salt hydrates is employed for heat storage. In solid–solid systems, polymers or organometallics are used to store heat. In the case of liquid–gas systems, a liquid that can undergo vaporization at relatively low temperatures and pressures, such as propane, alcohols, paraffins, esters, ammonia, or water, is utilized for heat storage [46, 47]. TCES systems comprise storing and releasing...