The thermal storage system makes use of the physical and chemical properties of various substances to provide a heat bank for various purposes.

To provide more versatility for a solar thermal energy system, thermal storage can be used to holdover the heat supply for when it is required. While a large amount of heat can be produced from solar power during the day, a large demand for this solar thermal energy is at night. The thermal storage system makes use of the physical and chemical properties of various substances to provide a heat bank for various purposes. Thermal energy is generally cheaper than other energy sources, but is difficult to store for long periods of time.

One manner of storage is that of a Phase Change Material (PCM). The most common PCM in use today is ice. Consider a beverage cooled with ice. The ice has potential to absorb large amounts of thermal energy from the drink. After it melts, the ice cubes have filled their energy capacity and the drink begins to warm to room temperature. The H2O, originally in solid state, now has heat stored within and exists in liquid state. The heat can be removed and the process repeated.

PCMs are available in temperatures from -5oC to 190oC. They can store 5-14 times more solar thermal energy by volume than traditional thermal energy stores, such as rock, water and masonry. For solar heat systems, categories of PCMs used include inorganic, biological based or organic. Inorganic PCMs include hydrated natural salts in demineralized water. These salts require corrosion resistant containers and suffer from a finite life cycle. Biological based PCMs are naturally existing oils or fatty acids, non-toxic and non-corrosive with an infinite life cycle. These are expensive and flammable at high temperatures. Organic PCMs are petroleum based by-products and are toxic, flammable and expensive. The most common PCMs for a solar thermal energy system are inorganic. The energy in a PCM is stored as latent heat, a thermodynamic term referring to energy hidden within a substance; this energy cannot be observed as a change in temperature.

This is in contrast to sensible heat, energy exchanged by a system having a sole effect of a change in temperature. An increase in temperature means an increase in energy. A traditional manner of storage would be that of an insulated hot water tank. Other sensible heat storage mechanisms include high temperature oils, molten salt mixtures with low melting points or a eutectic system. A eutectic system is a mixture of chemicals that melts at a lower point than any individual chemical in the mixture; certain molten salt mixtures can be considered eutectic. This is used to tailor a substance for solar thermal energy storage.

Chemical energy can also be used to store solar power for heat purposes. This technique makes use of reversible chemical reactions whereupon heat is produced and all reagents can be recovered. Additional techniques include the heat in absorption and desorption of gases as well as hydration and dehydration applications.

Solar thermal energy systems require heat storage to be an effective installation. One such system capable of being used with nearly any thermal storage method is the SolarBeam Concentrator from Solartron Energy Systems. It uses a parabolic dish to focus the sun’s heat onto a single absorber, providing up to 14kW of heat energy in sunny weather. This heat can be stored in the manners outlined above, allowing one to take advantage of the sun’s power throughout the day.