Hot Water Bottle for a House

Well, it's possible to do some back-of-the-envelope calculations using the thermal capacity of water, 4.2kJ/kg.degC, the insolation at that latitude and the total energy demand for a house there, which can be obtained from its energy bills. The larger the collector becomes, the smaller the tank.

What about the need to inhibit biological activity in a large warm water tank?

Why use a water tank for energy storage at all? Why not use batteries instead with their higher energy storage density ?

Heat pumps are current technology: the principle is 'refrigerate the garden to heat the house'. No water tank needed!

Many other energy efficiency technologies can offer a higher return-on-capital-expended [ROCE] rate. Have all these been done yet:

• Nagging users to turn stuff off instead of leaving it on standby?
• Draught exclusion?
• Top-knotch insulation?
• High efficiency lighting?
• Timeswitches?
• Double/triple glazing?
• High performance glazing?
• Occupancy detectors?
• Wastewater heat recovery?
• Rubbish-to-heat systems?
• Etc.

It has been tried or at least discussed, ages ago some guy on this site was discussing it (sorry I don't have the link).
I think some people use some salt or other and utilise the phase change to store more energy (E.G It takes a load of heat to melt it without raising the temerature too high, this energy can be recovered as it recrystalises...or something like that)
I'd have though that a ground source heat pump would be cheaper than a vast swimming pool ful of tepid algae.
Del

I've read of systems that use a heat exchanger (a long copper pipe) bored into the ground as a heat storage method.

From memory, a hole 30m deep (easily done with existing equipment) pumped with heat during the summer, could store enough energy to warm most of the winter. ffeJ

Before anyone goes charging off to try this I suggest a lot of study would be in order.

1) The basic premise is that of a GSHP

2) Copper may or may not survive in the soil.

3) 30 meters total heat exchanger length - hope it is a very small house.

In the north country with cold winters where an ASHP is not suitable the GSHP can perform well and be cost effective.

Farther south with mild winters the ASHP is far more economical.

The heat is stored in the surrounding dirt, not the pipe.

Plugging some numbers in to check for feasibility. My assumptions are - Specific heat of dirt 1.5kJ/kgC, density of earth 3000kg/m^3, Radius of warm area around pipe 2m. 10C temp rise in dirt, 100% of stored heat is recovered.

That gives ~17GJ storage, that's enough to run a 3kW heater 12hours/day for 120 days/year.

Obviously these figures aren't accurate, but they point to an idea that's not completely stupid. Yes, a lot more study is needed.

In any GSHP system the heat (or cool) is store in the media around the well or ground loop.

In my case where we hit 16 deg C water at 20 meters (water being the best case for heat transfer to the pipe) 3 wells of 135 meters depth were needed to support a 5 ton system. That could possibly even be reduced by 50% but as it would be impossible to go back and put in an additional well in the future due to the layout it would have been a bad choice to error on the inadequate side.

I liked the concept but the additional cost made no sense. A good ASHP should achieve a COP of 3 while a GSHP may achieve a COP of 4. The additional savings with the GSHP would never overcome the additional expense in locations where an ASHP will provide a decent COP.

The ground loop pipe is always recommended to be HDPE with fusion welding at connections to prevent failure. The expense of replacing wells or ground loops would be a killer.

The ground loop heat pump becomes very interesting in cold climate where the winter air is below the -12C to -15C where the air heat pump cannot extract heat from it.

My GTHP runs all year long without any supplementary heating.

We use thin ABS pipe. The closed loop operates at low pressure of a few PSI. The loop is filled with water and some glycol or ethyl alcool.

My 5 ton heat pump circulates 2.57 m3/hour of warm water. Would take a rather large tank to support the load.

I read an article in Popular Mechanics 30 years ago where a heat store was made using a large drum, (50gal oil drum?) buried in the basement. The drum was filled with glass beads and had a copper coil to supply/use the heat. Can't remember if the drum had a liquid fill but water with antifreeze would probably improve the heat transfer.

I suppose you could use crushed glass instead of beads to reduce cost. It would be interesting to hear if the idea is practical.

There was a prog on TV here ("It's not easy being green") where they used broken glass as a foundation for a greenhouse, they blew the hot air from the greenhouse through it in the day to store the heat to keep it warmer at night.
Del

Calculate the amount of usable heat either would hold - nothing to bother with.

I have calculated that my 55000l pool located in the house basement would only store a few days of heat in the Canadian winter. That is with the water going from 35C to 0.

If you let it freeze using the heat-pump, you triple the time. The problem is that it would then delay the swimming seasons by many weeks in the spring. Ice also causes mechanical problems from it expansion.

I finally installed a geothermal heat-pump with 1Km of plastic pipe buried 2 meters in the ground. It works very well. The landmass (earth) is my "free" storage container. I recommend it over any other long term storage scheme.

The heat-pump also produce most of my hot water at a fraction of the cost.

I added a secondary heat coil (nickel plated to survive the chlorine) that is used to dump the heat from the AC on the summer in the pool to keep it at a very comfortable 31C between June and September. I close the pool for the winter as it is too much trouble and we are too busy. The risk of condensation problems are also much higher even if the room is sealed from the rest of the house.