Storing Heat-PCM

The idea of a heat sink isn't new. What really spurred my interests was the amount of surplus heat generated during summer production of an over-sized system. To heat when it is cold and the available solar insolation is low, requires more collectors at different angles than those capturing hot water for a free summer shower. This extra heat is typically written off and shunted to greenhouses or swimming pools.

In designing a solar system to meet heating requirements for spring and fall equinox, the amount of summer surplus was greater than the heat I needed all winter long. I wanted to capture this heat. So began the quest of an economical way to store low grade energy for months on end!

Specific heat is a material's ability to hold energy. By definition, the specific heat of water is 1, meaning it takes 1 BTU to heat one pound of water one degree F. If you have a 5 gallon bucket of water and you raised the temperature 50 degrees F, you'd have added about 2000 BTU's.

Sand, tightly packed with all different sizes of material (fine/coarse/platelet/round) will have a specific heat of about .22. Water holds 5 times that of sand by weight. Clearly water is a better storage medium. It also has the advantage that it's easy to move that heat/energy. However water does have the disadvantage of being a liquid. It wants to leak, run, create pressure and evaporate or freeze. As a result, storage will run you about $1-$1.50/gallon minimum.

Steel has a high specific heat because of it's density, but also has much higher conductivity. This means that heat within the steel mass will be evenly distributed not allowing a core to maintain more heat but transferring this heat to the outer edges for more rapid loss. The other problem I found is that people who deal in old cars and scrap really don't take well to questions. I suspect there are some serious EPA logistics to be considered prior to the use of metal. Really, there is so much embodied energy in steel it doesn't make sense to bury.

I've heard talk of using PCM (Phase Change Materials),a substance with a high heat of fusion that attempts to change phases storing massive amounts of heat in the process. Wikipedia suggests 5-14 times the storage of other medium like water and sand. Salts are the inorganic version to nature's organic fatty acids, also a PCM. Transferring heat, controlling crystallization and getting a permit to pour a slab on top of 3' of a material not currently stocked by Home Depot are the problems here.

Considering the relation between salts and fatty acids, perhaps

is the most efficient means of storing heat is in grain. With livestock living a happily confined life in a crawl-space, humans would have only to dump grain into the pit and receive heat and fuel in exchange. No need for a beef freezer either.

Additional options here would be to install an Anaerobic Digester to convert the manure to

Natural Gas which could then be used for cooking or generating electricity. Finally, the high grade manure could then be used to fertilize plants for food production .

Perhaps the future of public housing could be doubled up with dairy's and beef yards where the people live over the animals, just like civilization pre-industrial revolution!

While these solutions are all worth further exploration, the best way to create power for the future is to not use so much now. Conservation is the easiest and most satisfying step towards reducing our consumption, so turn the lights off.

All images compliments of Googles Image Search, one of the best tools on the web for graphic conceptualization.