Solar chemical

Solar chemical processes convert solar energy into chemical energy. These processes use both light (photochemical) and heat (endothermic) to drive chemical or thermochemical reactions. Solar to chemical reactions can be used to store solar energy or replace energy that would otherwise be required from an alternate source.

Electrochemical cells, commonly known as batteries, convert electrical energy into chemical energy. Solar energy can indirectly be converted into chemical energy in a system involving a photovoltaic to electrochemical cell exchange. A more direct approach involves the use of photoelectrochemical cells which use light to produce hydrogen in a process similar to the electrolysis of water.

Solar energy can also be stored thermochemically with phase change materials (PCM). Devices of this type which store latent heat can be thought of as heat batteries. Phase change materials are classified as organic (paraffins, fatty acids) and inorganic (salts, metals,alloys).
A Paraffin wax thermal storage system consists of a solar hot water loop connected to a paraffin wax tank. During the storage cycle, hot water flows through the storage tank melting the paraffin wax. The enthalpy of fusion for paraffin wax is 210-230 kJ/kg. During the heating cycle, stored heat is extracted from the tank as the wax resolidifies. These systems heat air and water up to 64°C and can reduce conventional energy use by 50%-70%.

Eutectic salts such as Glauber's salt can also be employed in thermal storage systems. Glauber's salt can store 347 kJ/kg and deliver heat at 64°C.

Concentrating solar thermal technologies can be used to drive high temperature chemical processes.
Ammonia can be decomposed into nitrogen and hydrogen at high temperatures (650-700°C). The stored gases can be subsequently recombined to generate heat or electricity via a fuel cell. A prototype system was constructed at the Australian National University.
Zinc Oxide (ZnO) can be decomposed at high temperatures (1200-1750°C). The resulting pure zinc can be marketed directly. Alternatively, the zinc can be reacted with water at (350°C) to produce ZnO and hydrogen.

Water can be directly dissociated at high temperatures (2300-2600°C). These process have so far been limited due to their high level of complexity and low solar to hydrogen efficiency (1-2%). An alternate path of research is investigating solar thermochemical cycles that can be used to dissociate water at lower temperatures. Thermochemical cycles are currently at the prototype stage.