# Potential Study of Solar Thermal Cooling in Sub-Mediterranean Climate
###### Abstract
There are a variety of ways to convert solar energy into useful cooling or conditioned air (cooling and dehumidification). The combination of photovoltaics with compression cooling also offers these advantages. If only looking at the cooling side, photovoltaics in combination with compression cooling could be competitive in the future. However, the great ecological advantage of solar thermal cooling systems does not lie in the cooling side, but in the multiple uses of the solar systems for water heating and heating support, which could achieve the majority of the CO2 savings [10]. Many studies have been accomplished related to thermal driven solar cooling and air conditioning. Solar thermal cooling can be mainly classified as open and closed cycles. In open cycles, operated at atmospheric pressure, the latent load (dehumidification) is handled using solid or liquid desiccants. The water vapour in the air is considered as a sorbate while the solid or liquid desiccant is considered as adsorbent or absorbent, respectively. Open processes consist of a combination of sorption dehumidification and evaporative cooling, whereby a wide variety of interconnections are possible and different sorbents are used. The most common method uses sorption rotors for dehumidification. Up to now, the rotor materials used have only been either silica gel or lithium chloride, which is incorporated into a cellulose matrix. Several studies were performed for desiccant evaporative cooling systems (DEC) applying liquid desiccants such as [11; 12] and applying desiccant wheels such as in [13; 14]. Closed processes are represented by adsorption and absorption chillers. Adsorption refrigeration systems use a solid to adsorb the refrigerant. Devices on the market use mainly silica gel as the adsorbent and water as the refrigerant [15]. The COP values of the systems are in the range of 0.65 and heat from approximately 60 \({}^{\circ}\)C can be used to provide cooling at correspondingly low re-cooling temperatures, [16]. The COP of adsorption chiller of 0.65 is obtainable only for high desorption temperatures, high cooled water temperatures, and low cooling temperatures. For the heating temperatures as low as 60 \({}^{\circ}\)C, it is drastically lower (down to 0.17-0.34) [17]. These systems have some special features due to the periodic adsorption and desorption of the sorbents. As a rule, two adsorbers are operated alternately, so that one adsorber is always available for the provision of cold. This periodic operation leads to fluctuating temperatures at all temperature levels, a constraint that must be taken into account when planning the system [18]. The main absorption chiller systems are the water-lithium bromide (H2O-LiBr) and the ammonia water (NH3-H2O) systems. The H2O-LiBr systems are generally used for air conditioning in buildings. The NH3-H2O systems are used for refrigeration applications with usable temperatures below the freezing point of water. For H2O-LiBr, single-stage absorption chillers typically achieve a COP at the nominal operating point of around 0.7-0.8 and require drive temperatures above 75 \({}^{\circ}\)C [19]. Double-stage absorption chillers have another generator-condenser pair to the components of a single-stage absorption chiller, and a higher utilization of the heat supply can be achieved. Such systems are mainly offered in the area of large cooling capacities and achieve COP values from 1.1 to 1.2. The drive temperatures required are typically above 140 \({}^{\circ}\)C [20]. The greatest market potential for solar thermal cooling lies in the international market, in countries with a high solar radiation supply and therefore also a higher need for building and commercial cooling. Large sales markets are located in China, the USA, Japan, and Southeast Asia [15]. Thermally operated chillers are currently available on the market for the output range from 5 kW cooling to the megawatt range, as well as suppliers for DEC systems with an air volume flow of 4000 m\({}^{3}\)/h and above.