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To keep this short, a 12 x 12 x 6 foot dry sand store can achieve about 17280 Btu’s per DegF rise.(12 x 12 x 6 x 100 x .2) 7- 4 x 8 foot flats for a PSD (Perfect Solar Day) will yield about 1000 Btu’s per SqFt, tot 224 SqFt or about 224Kbtu’s. That is enough heat to raise the store 13 Degs in one day!! Allowing for a 100 degree rise in useful temperature, 70 to 170 DegF., 8 to 10 PSD’s would heat the storage to its max from bottom, once heating load was stopped. So, at even a 50% sun average, the storage is max'd in 3 weeks easily….
They could have improved the storage by using a pond liner instead of 6 mil poly, packing the sand tight then water filling until the water just broke the surface. This would have doubled the heat capacity, improving the performance, but still way, way short of any use of Summer heat for Winter.
For a system to claim an Annual style of design (As the title implies), the stored heat from non-heating season must be equal to at least 65% of the total heating season load. The system as described does not warrant the claimed title…
It is nothing more than a solar thermal system with a few weeks of heat storage capacity.
William H. Fitch III
570-752-4827
John Howe on "Storing Summer Heat for Winter"
Re; "storing summer heat for winter" ,this kind of thinking makes no sense whatsoever. A quick calculation using specific heat for sand of 0.2btullb/deg f and specific weight of 1.5 goes as follows:
A volume of 8'X8'X8' is 512 cubic feet. This quantity of sand would weigh about 48,000 pounds. If the temperature from thermal storage changed from 175 degrees to 75 degrees, the energy transferred from the storage pit to the house would be 48,000XO.2XIOO equal to about one million btu.
This is about the same heat in one tenth of a cord of wood worth $10.00 or 20 gallons of heating oil worth $60.00! This amount of fuel-energy would take care of a typical New England (or Colorado) home forabout four days .... hardly worth the infrastructure and investment.
John
----- Authors Respond:
Both Mr. Fitch and Mr. Howe are correct, the storage does not hold enough heat to last through the entire winter. In some sense, the story is about storing heat all year round for use in the winter.
Initially, we thought we would want a very large bed (70 ft x 10 ft x 10 ft). However, our modeling indicates that as the volume increases above approximately 15 m^3 (~525 ft^3), the benefit does not increase very much. This is caused by several factors, including the heat transfer coefficient of the sand which limits the rate at which heat can be added to or removed from the bed. In addition, as the summer proceeds and the bed heats up, there is less and less heat added to the bed because the temperature from the collectors must be raised higher to add heat to the bed. Both of these factors mean that you cannot add heat to the bed quickly. Part of the reason that "more panels" is better is because the bed doesn't hold that much heat. An additional benefit of the bed is that it also helps with shorter term (daily or weekly) storage in the winter - heat is collected when the sun is out and is available for use when the sun is not out.
As to adding water to the sand, this would offer some benefit in terms of heat storage capacity and in terms of heat transfer coefficient. We considered this option, but decided against if for two reasons. The first is the concern over mold developing in the bed. The second is that we put the insulation inside the liner. Moisture would lead to a breakdown of the insulation.
Dave Stets, Jim McLeskey and Marshall Sweet.
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