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However, the sun never shines at night, and often does not shine during the day. Energy storage is a fundamental requirement in any serious solar energy application. It is extremely expensive to store electrical energy, even though billions of dollars have been spent improving battery and fuel cell technology. The reverse is true of thermal energy. Hot water can be stored cheaply in a thermally insulated tank. As the volume of water and energy stored increases, the cost and losses of thermal energy storage drop rapidly.

The HVAC requirements of a typical 20,000 square foot, single story commercial building in the Boston area can be stored in a cube of water approximately 8.5 feet on a side for one day of HVAC needs, 17 feet on a side for one week of HVAC needs, and 27 feet on a side for 1 month of HVAC needs.

The stored thermal energy acts as the boiler, but is larger than a conventional boiler and is charged by the heliostat array rather than fossil fuel. From the boiler on, the method of heat distribution is a conventional commercial HVAC system, using forced hot water or air to provide heating. Absorption chilling provides air conditioning.