I am considering the following:
30 evacuated tubes on a new single story extension 10m x 3m fedding the ground below all Summer. Then using a heat pump in the Winter to extract the heat to UFH in the house.
What would I need to do to keep as much of the heat under the extension as possible until I wanted it out? Any ideas on whether this would be a feasible thing to do in relation to cost? I think that it would probably cost about £8000 for installing the GSHP and probably as much to retrofit the UFH.
As a possible alternative I thought about keeping the costs down and finding a means perhaps of just allowing the heat to seep upwards through the floor of the extension thus doing away with the heat pump, but only heating the extension. Obviously again the key is to keep as much of the heat underground as possible until the heating season.
Nigel
posted on 02-02-07
I think you should look at the specific heat capacity of water, work out how much heat you will need to store and then look at how big a store you need. Then you will need to think about how you are going to retain the heat in the store.
Roughly 1m3 of water hold about 1.5Kw per degree increase in temperature.
Say you need 10,000 Kw per annum at 10c above ambient you will need 666m3 of water. If you have a bigger temp diff your heat losses will be huge.
john 11668
posted on 02-02-07
I thought that GSHP relied upon the flow of the water table to replenish your source of heat as you extracted your useful heat from it.
so unless you cam create a massive highly insulated storage tank under your extension then I think your Idea is a non starter. Passive underfoor heating will be like a large storage heater. Hot at the start of the season - not a lot left by November.
Pull your "free" heat from the water table as required and heat you domestic with solar.
The KISS principle is paramount "Keep It Simple Sir"
Paul in Montreal
posted on 02-02-07
Zenbiscuit,
your scheme could work if you use a vertical borehole and store your heat deep in the bedrock - though any water flow through there will rob your store.
The alternative of storing heat in water could work - but you'd have to use a phase change system. The latent heat of fusion of ice could double the storage capacity of your system - but you'd still need hundreds or even thousands of tons stored if you wanted to extract a winter's worth of heat (depending where you're located). I did think of using solar panels to "pre charge" our GSHP borehole just after the end of the air conditioning season up to the start of the heating system, but concluded it couldn't add much capacity (we do have a vertical borehole of around 130m).
In the coldest winter days we had recently (with daytime highs of -20C) we were extracting approximately 250kwh per day from the ground source - it would take a huge solar collector to replenish that (even though it's always sunny on the coldest days). I think the most one can reasonably expect with well-oriented panels is around 400W per square metre peak - so for a 7 hour day that's probably more like 1.5kwh per m^2 - so we'd need about 160 m^2 of collectors to keep up with the load. However, it could be used to provide the backup for when the heat load is greater than the capacity of the GSHP - storing the daytime solar in hot water tanks and using that at night to smooth out the load peaks (and avoiding switching on the auxiliary resistance heating) could make a lot of sense. I doubt, however, that any GSHP system in the UK would ever be designed for less than 100% of the heat load - here we use 80% of the worst case heat load to make sure it's not oversized for airconditioning. Ah, the joys of an extreme climate.
Paul in Montreal.
Tony
posted on 02-02-07
I like the idea of interseasonal storage of heat. More power to your elbow. If you were to insulate well ie better than Passivhaus standard then is should be very easy. The less heat you need the less you will have to store. Insulating will pay you back twice!
Let us know how you get on.
Jeff Norton (NZ)
posted on 03-02-07
My favorite subject, I am building (for a client)a house with active solar heating and trying to do it on a tight budget. I agree with Tony that it will work best with a low energy demand house, the one we are building is around 4000kwh per year (10,000kwh is average for NZ), so as Paul shows becomes very hard in Canadian conditions but easier in milder ones.
We are building on dry ground with perimeter insulation down to bottom of footings, we will add some external wing insulation to increase effective storage area.
We have installed pipe work on top dry ground under sand and compacted hard fill and under concrete slab (cottage is 85 sqm), we are working on about 85 cubic metres of insulated storage (ground and hardfill). The solar collectors would need to store 2 or 3 times the yearly demand, but I would guess 30 evac tubes would achieve that. The centre of store could reach temperatures of 40 degC and reduce to outside to 20 degC.
In winter the inslab heating pipe work is connected to end of storeage pipe work to transfere heat from store to slab.
We are still trying to figure out the store losses into dry ground?
This systems has worked well in milder North Island but struggles in colder South where I live!
If any one can fill in the gaps please do.
