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Chapter 3

Depending on the size of the combisystem installed, the annual space heating contribution can range from 10 to 60% or more in ultra-low energy Passivhaus-type buildings, and even up to 100% where a large seasonal thermal store or concentrating solar thermal heat is used.

Solar cooling

Solar cooling can be broadly categorized into solar electric refrigera- tion, solar thermal refrigeration, and solar thermal air-conditioning. In the first category, the solar electric compression refrigeration uses PV panels to power a conventional refrigeration machine (Fong et al., 2010). In the second category, the refrigeration effect can be produced through solar thermal gain; solar mechanical compression refrigeration, solar absorption refrigeration, and solar adsorption refrigeration are the three common options. In the third category, the conditioned air can be directly provided through the solar thermal gain by means of desiccant cooling. Both solid and liquid sorbents are available, such as silica gel and lithium chloride, respectively.

Solar electrical air-conditioning, powered by PV panels, is of minor inter- est from a systems perspective, unless there is an off-grid application (Henning, 2007). This is because in industrialized countries, which have a well-developed electricity grid, the maximum use of photovoltaics is achieved by feeding the produced electricity into the public grid.

Solar thermal air-conditioning consists of solar heat powering an absorp- tion chiller and it can be used in buildings (Henning, 2007). Deploying such a technology depends heavily on the industrial deployment of low- cost small-power absorption chillers. This technology is being studied within the IEA Task 25 on solar-assisted air-conditioning of buildings, SHC program and IEA Task 38 on solar air-conditioning and refrigera- tion, SHC program.

Closed heat-driven cooling systems using these cycles have been known for many years and are usually used for large capacities of 100 kW and greater. The physical principle used in most systems is based on the sorption phenomenon. Two technologies are established to produce thermally driven low- and medium-temperature refrigeration: absorp- tion and adsorption.

Open cooling cycle (or desiccant cooling) systems are mainly of interest for the air conditioning of buildings. They can use solid or liquid sorp- tion. The central component of any open solar-assisted cooling system is the dehumidification unit. In most systems using solid sorption, this unit is a desiccant wheel. Various sorption materials can be used, such as silica gel or lithium chloride. All other system components are found in standard air-conditioning applications with an air-handling unit and

Direct Solar Energy

include the heat recovery units, heat exchangers and humidifiers. Liquid sorption techniques have been demonstrated successfully.

Thermal storage

Thermal storage within thermal solar systems is a key component to ensure reliability and efficiency. Four main types of thermal energy stor- age technologies can be distinguished: sensible, latent, sorption and thermochemical heat storage (Hadorn, 2005; Paksoy, 2007; Mehling and Cabeza, 2008; Dincer and Rosen, 2010).

Sensible heat storage systems use the heat capacity of a material. The vast majority of systems on the market use water for heat storage.Water heat storage covers a broad range of capacities, from several hundred litres to tens of thousands of cubic metres.

Latent heat storage systems store thermal energy during the phase change, either melting or evaporation, of a material. Depending on the temperature range, this type of storage is more compact than heat stor- age in water. Melting processes have energy densities of the order of 100 kWh/m3 (360 MJ/m3), compared to 25 kWh/m3 (90 MJ/m3) for sen- sible heat storage. Most of the current latent heat storage technologies for low temperatures store heat in building structures to improve ther- mal performance, or in cold storage systems. For medium-temperature storage, the storage materials are nitrate salts. Pilot storage units in the 100-kW range currently operate using solar-produced steam.

Sorption heat storage systems store heat in materials using water vapour taken up by a sorption material.The material can either be a solid (adsorption) or a liquid (absorption). These technologies are still largely in the development phase, but some are on the market. In principle, sorption heat storage densities can be more than four times higher than sensible heat storage in water.

Thermochemical heat storage systems store heat in an endothermic chemical reaction. Some chemicals store heat 20 times more densely than water (at a T≈100°C); but more typically, the storage densities are 8 to 10 times higher. Few thermochemical storage systems have been demonstrated. The materials currently being studied are the salts that can exist in anhydrous and hydrated form.Thermochemical systems can compactly store low- and medium-temperature heat. Thermal stor- age is discussed with specific reference to higher-temperature CSP in Section 3.3.4.

Underground thermal energy storage is used for seasonal storage and includes the various technologies described below. The most frequently used storage technology that makes use of the underground is aquifer


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