sizing for GSHP

Waestrel

Hello folks.

SImple question about sizing a GSHP collector bore hole. If I know roughly the conductivity of the rocks into which I am placing this, and the average ground temperature of my area, how can I calcualte the expected watts extractable per meter? I suspect its a simple calculation, and the result should be similar to whats seen here

http://www.gshp.org.uk/pdf/MIS_3005_Ground_loop_sizing_tables.pdf

Any one got any ideas?

Thanks

Condenser

Waestrel wrote: »

Hello folks.

SImple question about sizing a GSHP collector bore hole. If I know roughly the conductivity of the rocks into which I am placing this, and the average ground temperature of my area, how can I calcualte the expected watts extractable per meter? I suspect its a simple calculation, and the result should be similar to whats seen here

http://www.gshp.org.uk/pdf/MIS_3005_Ground_loop_sizing_tables.pdf

Any one got any ideas?

Thanks

That table is calculated using a 32mm single U loop and most worth their salt would use a 40mm single U. 32mm should only be used where double loops are used in one bore. It also doesn't allow for the presence of any groundwater which would enhance the performance slightly. You'll nearly always hit groundwater drilling in Ireland.


As a general rule a dry bore would return about 40-45W per linear metre and a wet bore with water at a number of levels would return around 50-60W. Again there are caveats such as how deep is the casing and will it be removed. If it isn't it will affect the performance of the cased metres. Will the borehole be grouted. Grout must be used for the entire length of the bore in a dry bore or where there is low water flow or the bore is good as useless And should be grouted down to the first ingress of water where the water flow is high and any attempt to grout would see it washed away anyway. The bore must be grouted from the bottom up to ensure no air pockets are left.

If you want to get more accurate than the above then you'd need to run a TRT (thermal response test) which is costly and normally only of any interest to systems of 60kw plus.

Waestrel

any idea how that table in the PDF is calculated?

Condenser

I'd imagine its based on TRT testing and mean averages of the return of the different rock types which you can see on the right but Ireland is geologically such a mangled mess that its very hard to determine exactly what the rock type or its make up will be from one place to another. You can literally drill one bore move a matter of meters and encounter completely different geology than you encountered in the first. I've even seen the rock type alternate between 4 different types of rocks multiple times in one bore. You can give it a fair guess and you're generally fairly right but you won't know til you're into the drilling what you're facing. Drilling records are very poor in Ireland compared to the UK and Europe.

Unless its a very large project its best to work off the averages and let the driller add a number of meters if hes uncomfortable with what hes seen (that's assuming he knows what he's doing besides just drilling a hole)