Friday, April 15, 2011

Thermal storage is a historic skill

15 April '11: I was chatting to Chris Wood today and we were discussing the 'patentability' of underground thermal energy storage. We both feel that the principle is so long standing, going back millenia, that this is unrealistic and unethical to do. The only thing that could be specifically patented would be the Box of Tricks that makes one particular system work - i.e. the particular set up of thermostats, switches, valves, tank and circuit design that made it different from other systems. You can as much patent underground storage as you can patent Newton's laws of motion. They are named after him, but he doesn't demand a royalty from every bicycle, car and rollerskate that is manufactured.
   I have been in Stone age caves in France, where it was clear that there were food storage larders in the coolest parts of the cave. Most of us have heard of 'Ice Houses' and if you have even visited an English country house, there is usually a small disused building nearby that is still called the Ice House - these were usually near a lake, and stored the winter ice. Ice houses have very highly insulated walls and roofs. The Persians mastered the ancient art of making and storing ice many centuries ago, and even transporting it on camels.
 Ice storage and delivery used to be a common part of the way of life in British cities, indeed, I remember taking a photograph of the demolition of the Ice Storage company building in Liverpool, in the sixties.
 There is a good entry in Wikipedia on the topic:
http://en.wikipedia.org/wiki/Ice_house_(building)
Here's an article about Ice storage in Boston, Mass:
http://www.jphs.org/locales/2004/6/2/harvesting-ice-on-jamaica-pond.html

When I read the article about underground storage at Heathrow recently, they referred to the thermal hypocausts of the ancient Romans, still visible in excavated villas and forum buildings today (hypocausts are about distribution of heat, not storage, but they are part of the big picture.).
Here's a picture of an Ice house in Kingswood, near Bristol:
http://www.kingswoodmuseum.org.uk/site.shtml
Or this one, where the house was demolished, but the Ice house remains:
http://www.1066.net/summerfields/ice-house.htm
The Findhorn Ice house, for cooling fish:
http://www.findhorn-heritage.co.uk/icehouse.htm

So back to heating - why do we not use the warm equivalent of an Ice House for our thermal storage? - a thermally insulated room, perhaps the size of a small swimming pool, filled with earth, and with pipes circulating heat from a solar panel? The reason is that the volume required to hold enough heat to provide for a house is far far more than can be stored in an insulated container unless it is to be larger than the main house. I was disappointed to discover that the theoretical thermal storage of one cubic metre of dense clay is only 0.5 kWh per degree K, and that is why you need such a deep borehole - a few thousand cubic metres per house.
   A large room sized volume would only work with a PCM material (wax or salt) and if insulated, there would be a finite limit to the amount you could inject or withdraw. Typically PCM materials have a thermal capacity of 50 kWh as they go through the phase change. That is 100 times as much! If the phase change requires a range of 4 degrees to fully change, that's 25 times better! They have a storage range above and below the phase change, that would be similar to clay or water (depending on their state).
   For house heating and DHW, you need what we have, in effect an infinite volume, but with an seasonally addressable volume of about 3,600 cubic metres. An insulated container of this size based on clay would have to be the footprint of our house and 50 metres deep, and you would have to calculate the area of solar panel precisely to ensure that the heat taken out was equal to the heat put in, as there would not be access to the infinite soil around. You could tune it by having more solar panels added later, or by having supplementary heating. Also, if the house is insulated to Passivhaus standard, the volume could be reduced.
   Unfortunately the words 'Ice House' is now adopted by bars, graphics agencies, music bands, and heaven knows what else, so Googling for 'Ice House' and finding something useful requires a lot of patience.

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