# Evaluation of Metal-Organic Frameworks as Potential Adsorbents for Solar Cooling Applications
## 1 Introduction  
Energy consumption across different sectors is increasing continuously with households as the major contributor which is responsible for one-third of the world's total energy consumption [1]. The major portion of the energy in the household and almost 30% of the world's total energy is consumed by heating, ventilation, and air conditioning (HVAC) equipment [2, 3]. Furthermore, the energy demands for heating and cooling applications are increasing at a rapid rate and are accounting for more than 17% of worldwide consumption as can be observed from Figure 1[4]. This demand is projected to increase at a rapid rate in the future as well. To combat this increasing demand for energy, a target of 20% reduction in primary energy use by 2020 has been set by EU-28 countries [5] and to achieve this milestone different alternative energy technology needs to be introduced. Different technologies and alternative cooling systems based on thermal energy including absorption, adsorption, and desiccant cooling have been investigated in recent years. The basic operating principle of a thermal cooling system is shown in Figure 2[6]. These technologies are based on the process of dehumidification and cooling. Some adsorbent materials are used in these systems which have the potential to adsorb/absorb water molecules. Due to the thermally operated nature, the input energy is provided in the form of heat for the continuous operation of these systems. This makes the use of low-grade thermal energy sources such as solar, biomass, waste, etc., more feasible. While thermally activated cooling systems offer advantages in terms of energy consumption and CO\({}_{2}\) emissions [7; 8], despite the advantages offered by thermally driven cooling systems, it is observed that these systems are comprised of large components which make them bulky and expensive. Furthermore, most of the adsorbent used for these systems has larger water uptake capacity at higher partial pressure. Hence, it is very difficult to implement these technologies on a commercial and larger scale. These systems are available commercially but only at a small scale and only a few are installed for large scale applications [9]. Adsorption and desiccant cooling technology can be used as a great means of general cooling but, due to the above-mentioned issues, these systems have a lower coefficient of performance (COP) as shown in Figure 3[10]. The average COP of these systems is less than 1. This lower value of COP and the bulky nature of these systems are a big hurdle for its development and implementation. Concerning this, different advancements are required for adsorbent materials used for these technologies to lower their weight, cost, and required temperature of regeneration. Figure 1: Consumptions of global energy in different sectors [5]. Figure 2: Thermally activated cooling system’s working principle [4]. Metal-organic frameworks (MOFs) are a newly developed class of crystalline materials with highly porous structure and high adsorption capacity. These materials offer a wide variety of choices for its development because of individual choices of linker and metal-ligand according to the required structure and application. These materials have a high surface area which means that a small quantity of these materials can replace the bulk quantity of other porous materials for a particular application. The MOF materials are widely used for gas storage and filtration processes nowadays. In recent years, these materials have made great scientific advances due to the above-mentioned advantages. Keeping all aspects in mind, the main aim of this research work is to present a comprehensive overview of MOFs for their potential use as adsorbents for thermally activated cooling technologies. The brief introduction of adsorption-based cooling and desired behavior of adsorbent material for cooling applications is described. The paper provides a comprehensive overview of MOF-based cooling and its future developments.