A perspective on thermal energy storage systems for solar energy applications

The use of thermal energy storage (TES) systems is essential for solar power systems because of fluctuations in the solar energy input. Several classes of storage may be required for a single installation, depending on the type and scale of the solar power plant itself, and the nature of its integration with conventional utility systems. For heating and hot water applications, water and phase change materials (PCMs) constitute the principle storage media. Soil, rock and other solids are used as well. Water has the advantage of approximately 80% less volume than that of water for a temperature variation of 10°C, which is the difference between temperatures of a fully charged and a fully discharged storage tank. Some PCMs are viscous and corrosive, and must be segregated within the container in order to be used as a heat transfer medium. For heat storage, two PCMs must be available, unless heat pumping is employed. A variety of solids is also used; rock particles of 20 to 50 mm in size are most prevalent. Well-designed packed rock beds have several desirable characteristics for energy storage. The heat transfer coefficient between the air and the solid is high, the cost of the storage material is low, the conductivity of the bed is low when air flow is not present and a large heat transfer area can be achieved at low cost by reducing the size of particles. TES systems have also been suggested for storing thermal energy at medium (38–304°C) and high temperatures (120–566°C). For instance, systems in an oil-rock system for hot water and heat-recovery applications are examples of medium-temperature applications, while those in molten nitrate salt systems (an excellent storage medium) for steam production for process applications are for high temperatures. Oil-rock TES, in which the energy is stored in a mixture of oil and rock in a tank, is less expensive than molten nitrate salt TES, but is limited to low-temperature applications. However, this oil-rock TES has been proven successful for solar thermal applications. The selection of the type of TES depends on various factors such as the storage period (diurnal or seasonal), economic viability, operating conditions, etc. In this article, a study of TES systems (including materials) is presented particularly with solar thermal applications in mind. The evaluation of their performances, cost and economic viability, ease of installation, cleanliness, environmental impact, safety factors, technological usability and applicability are discussed.