Heat-pump modification idea

[rbrowning]

I wasn't trying to encourage anyone to build a HUGE underground thermal storage facility, use the one that is already there (the ground it's self). I was just thinking that as you move further north with lower average annual temperatures there will be less heat available heat from the ground to be moved into your house and a heat pump stops being a viable option.

What I was suggesting was that in areas where the geothermal heat pumps will almost meet your needs you could help it out by adding some solar heated water tanks. They don't need to be big enough to heat your whole house all winter, but if they can keep you from having to run the furnace for another month on either end of the heating season, it could help.

In this area it used to be real popular to heat your home with fuel oil. It has fallen out of favor for natural gas or propane and there are a lot of 275 gallon tanks around that people don't want and usually pay to get rid of. I would think that they wouldn't be that hard to insulate and 2 or 3 of them should hold a goodly amount of heat.

[Piwoslaw]

I've been meaning to calculate the thermal content of a volume of water, so I'll do now.
I'll start with the 275 gallon tank, which is just barely larger than 1 cubic meter (I'll use metric units for calculations, if that's OK?). 1m3 of water weighs 1 metric ton=1 million grams.
Warming 1g of water by 1 deg.C takes 4.187 Joules (= 1 calorie, I'm approximating that this doesn't depend on the start temperature and pressure).So raising the temperature of 1m3 by 1*C will take 4.18MJ.
Let's say that the temperature of the water in the tank will have a 70*C range, warming the tank from 10*C to 80*C (50*F-176*F) will take 293 MJ = 81.4 kWh = 277818 BTU. That's also how much energy can be taken out of the tank to lower it's temp from 80*C to 10*C.

In real life:

• There will be losses through the tank walls, proportional to the temperature gradient,
• If the tank has almost perfect external insulation, then the tank's walls will also accumulate heat, adding to thermal mass,
• There will be extra solar energy being added to the tank every day,
• The heat pump also produces a certain amount of heat.

If you can keep the high temperature in the tank until cold season, then you get an extra 1kWh=3413BTU of energy per day for most of winter. The main problem here will be insulation. If you get 3 tanks, then the best thing to do would be to stack them in a pyramid, put a mess of insulation around the whole setup (with maybe smaller tanks in the open spaces) and bury it. Maybe worth trying?

[Daox]

For large format heat storage, I've heard of builders digging the basement an extra few feet deep. They lay down insulation to form a box, and fill it with sand and layer in PEX tubing. More insulation goes on top and the basement floor is poured. That large thermal mass is heated during the summer and can be drawn off for a good portion of winter. So, it is doable.