EcoRenovator - View Single Post - Passive Annualized Heat Storage/Annualized Geo Solar-Don't Waste That Heat!

Piwoslaw · 05-21-12, 02:46 PM

I've heard of buildings, and even communities, which store surplus solar heat in the ground to use during the winter. The up side is that it's cheap: The ground loop for the heat pump is used to "dump" heat when it is in excess. The down side is that alot of that heat is lost, especially with flowing underground water.

More efficient, and maybe not more expensive, is storing the heat in an insulated tank. How much heat you will lose over 4-8 months depends on four things:

Average ground temperature at the depth of the tank,
Temperature inside the tank,
Surface area of the tank,
Amount (effectiveness) of insulation.

1) There is not much you can do about your ground's average temperature, you might gain maybe 1-2 degrees by burying deeper.
2) By lowering the temperature inside the tank, you reduce the Delta T. This can be done by increasing the tank's size: Putting the same amount of energy (heat) into more water (mass) reduces the temperature.
3) The shape with the greatest volume to surface area ratio is a sphere, but this usually isn't easy to construct. A cylinder is next best, and is a good compromise between efficiency and technical limitations. Of course, some cylinders are better than others, but what you end up with is a trade-off between what you build/buy and what you can fit.
4) Always use the best, and the most, insulation that you can afford.

Summing up, it is possible to minimize the amount of heat that is lost though the tank's walls, so you retrieve a larger percent of the heat you put in. This in turn means that you can invest less in solar collectors, etc.
If you are already doing lots of digging, and can get a relatively cheap tank, then it may turn out to be cheaper to bury it than to drill and/or lay hundreds of meters of ground loop for a GSHP. Make the tank as large as possible, insulate it, and use it as you would use the ground loop of a heat pump.

This has been successfully done: An insulated milk tanker holding 20 tons of water at lower temperature (10-90°C) feeds a heat pump to charge a smaller indoor tank with high temperature (50-95°C) water for household use.
Milk Tanker Thermal Store with Heat Pump

Iirc user Osolemio has a similar setup, though on a smaller scale.

05-21-12, 02:46 PM	#2
Piwoslaw Super Moderator Join Date: May 2009 Location: Warsaw, Poland Posts: 961 Thanks: 188 Thanked 110 Times in 86 Posts	I've heard of buildings, and even communities, which store surplus solar heat in the ground to use during the winter. The up side is that it's cheap: The ground loop for the heat pump is used to "dump" heat when it is in excess. The down side is that alot of that heat is lost, especially with flowing underground water. More efficient, and maybe not more expensive, is storing the heat in an insulated tank. How much heat you will lose over 4-8 months depends on four things: Average ground temperature at the depth of the tank, Temperature inside the tank, Surface area of the tank, Amount (effectiveness) of insulation. 1) There is not much you can do about your ground's average temperature, you might gain maybe 1-2 degrees by burying deeper. 2) By lowering the temperature inside the tank, you reduce the Delta T. This can be done by increasing the tank's size: Putting the same amount of energy (heat) into more water (mass) reduces the temperature. 3) The shape with the greatest volume to surface area ratio is a sphere, but this usually isn't easy to construct. A cylinder is next best, and is a good compromise between efficiency and technical limitations. Of course, some cylinders are better than others, but what you end up with is a trade-off between what you build/buy and what you can fit. 4) Always use the best, and the most, insulation that you can afford. Summing up, it is possible to minimize the amount of heat that is lost though the tank's walls, so you retrieve a larger percent of the heat you put in. This in turn means that you can invest less in solar collectors, etc. If you are already doing lots of digging, and can get a relatively cheap tank, then it may turn out to be cheaper to bury it than to drill and/or lay hundreds of meters of ground loop for a GSHP. Make the tank as large as possible, insulate it, and use it as you would use the ground loop of a heat pump. This has been successfully done: An insulated milk tanker holding 20 tons of water at lower temperature (10-90°C) feeds a heat pump to charge a smaller indoor tank with high temperature (50-95°C) water for household use. Milk Tanker Thermal Store with Heat Pump Iirc user Osolemio has a similar setup, though on a smaller scale. __________________ Ecorenovation - the bottomless piggy bank that tries to tame the energy hog.