Heat Transfer Fluid

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  • Heat can be stored for later use in a heat transfer fluid.
  • This can overcome the intermittency of solar energy to some extent.
  • Heat from other sources such as Biomass, and Waste Heat can also be stored in a heat bank, (example: waste heat from cooking is captured in a chimney and later used to heat up water)

Ethylene Glycol

  • "Pure ethylene glycol has a specific heat capacity about one half that of water. So, while providing freeze protection and an increased boiling point, ethylene glycol lowers the specific heat capacity of water mixtures relative to pure water. A 1:1 mix by mass has a specific heat capacity of about 3140 J/(kg·°C) (0.75 BTU/(lb·°F)), three quarters that of pure water, thus requiring increased flow rates in same-system comparisons with water."

Molten Salts and Metals

used in (fast) breeder nuclear reactors sodium (Na), potassium (P) or an alloy of these (NaK, pronounced "nack").

Molten Salts

Sodium nitrate - used as heat transfer fluid for large solar thermal installations (example: Andasol Power Station)


Vegetable oils can be used as heat transfer fluids. They have high thermal capacity, are often very liquid at high temperature, but may degrade over time and are expensive.

Solid Carriers

Some appropriate technology experiments have been done in the 1970-80's with sand, gravel or bricks as heat transfer items. These are heated in a solar furnace and then transferred to an insulated box for storage. Generally this was for hot box slow cooking of food, bed heating and drying boxes. Its not new some stone-age desert tribes used sun heated rocks to cook and heat at night. Proper insulated containers, heavy insulated tongs and suitable safety clearances, locks and other provisions to keep the kids out are needed. For a high flow application a cable lift or scaled down Aerial Ropeways in a protective sheath would work. Heat tolerant conveyor belts were tried in the seventies with both sand and gravel. Manual transfer with tongs or long handles will work in testing and the lowest technology application. There's nothing I can find on the web; Its all very early work.


One of the best solid carrier options is zeolites that are heated to drive off water. These store cold indefinitely if sealed properly. If water is later added the thermochemical phase change generates enough heat to cook meals and sterilise water. These are used to cook meals in army field rations. It can generate steam but most of the water in contact with the zeolite is held. Zeolitesare found as cat litter, soil conditioners etc but it would be much cheaper to buy wholesale from the bulk miners or find an unclaimed deposit. Not all commercial zeolites exhibit the thermal properties. There are also zeolite solar refrigerators. http://refrigerators-appliances.com/246008/what-year-was-the-refrigerator-invented/zeolite-solar-refrigeration.html Note: Zeolites also can store gaseous hydrogen.

Product Ecology

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