Geothermal heat pumps

[sylvester]

What are peoples views on Geothermal heat pumps? It has always seemed a great idea to me and seems to pay for itself very quickly. You just have to have enough land (or water) to put it in. It would be great to build a little community around a small field that also heated / cooled all the houses built around it.

 

More info here: http://en.wikipedia.org/wiki/Geothermal_heat_pump

 

I also noticed that new investment has been announced for a project looking at "low temperature" geothermal energy:

http://www.nzherald.co.nz/category/story.c...jectid=10515519

Note that even though this is called 'low temperature' it is higher temperature (80 to 150 degrees) than the above heat pumps (which work at 7 - 21 degrees). However they are not as high as high temperature geothermal!

[enrieb]

I remember hearing Matt Simmons talk about the potential for this over a year ago on FSN, for existing houses it could be a little too expensive to justify the cost, unless the garden was big enough for it to be done without much disruption, or if they had a pond nearby. Simmons said that the best time to do it is when laying the foundations for new homes. If it were to be made legal requirement for all new homes and buildings to have geothermal heat pumps fitted in the foundations it could become a very profitable energy infrastructure investment.

 

I always though that on a bigger scale, deep geothermal energy would have a lot of potential for the future as we don't need to invent anything special to make it work or solve any existing problems with the technology. All that is required is a high enough energy price and without cheap fossil fuels driving down the cost of energy geothermal has a good stable future. Also the drilling technology from oil wells has already provided the investment to make drilling deep holes as cost effective as possible and by the time we get to $500 oil we will have made even more advances in drilling technology.

[sylvester]

...Simmons said that the best time to do it is when laying the foundations for new homes. If it were to be made legal requirement for all new homes and building to have geothermal heat pumps fitted in the foundations it could become a very profitable energy infrastructure investment.

 

Yeah, I was thinking that it might even be possible to lay the pipes 'under' the house as the foundations are done. Many of the houses here use piles for the foundations so digging a trench between them for the pipework (at foundation laying time) shouldn't be too difficult. I guess the only hassle would be if you needed to get to the pipes to repair / replace them. I wonder what the lifetime of them is?

I suppose as long as you build in diverter valves to section off damged loops then you may loose some efficiency but the thing would still work. And by putting it under the house, at least the neighbour doesn't cut through them when you sub-divide and he puts in a driveway!

 

 

 

 

[AceofKY]

This is old, but still very good, technology now. You can generally expect 5-8 year paybacks on increased capital costs.

I have designed many geothermal wellfields for schools in KY. This is a very popular technology here; the only thing preventing near universal acceptance in residential applications is the very cheap power we have which makes air cooled heat pumps' low capital costs more attractive for small budget homes.

 

I haven't performed any calcs to support this, but I doubt if there is enough area under a house for a horizontal heat exchanger; i.e. - you can't put the pipes in the foundation and get enough heat transfer to the ground. The opposite is usually practiced around here; piping is sometimes placed in the floor slabs to heat the building. There may be some high tech heat exchanger that I'm not aware of that would do this, but you're not going to get enough heat transfer to the ground with just standard poly piping so you can expect to pay much more in capital costs.

 

For a residential application in a city, it is much more feasible to use vertical heat exchangers (which are just polyethylene pipes). One 300 foot well, by itself and not in a wellfield, can typically handle around 2 tons of cooling load. The cooling load dominates around here. A 300 foot well with piping installed will cost you about $3500. The poly piping has a very long life. I haven't heard of any piping failures yet and we've been installing geothermal since the early 80s here. Some people push the limits on pressure, however, with very deep vertical pipes.

 

The main problem that we have run across over the years is that, since the cooling load is so dominant (i.e. since we push a lot more heat into the ground in summer than we withdraw during the winter) our wellfields for larger buildings are heating up over the years and we are losing capacity. I have been addressing this by adding closed circuit evaporative coolers to the loop as this is less expensive than just adding more wells.

 

[sylvester]

This is old, but still very good, technology now.

 

Thanks for that, it was really interesting. Not sure how great a vertical bore hole would be around here due to the seismic activity? Vertical pipes might not be great for cooling either in some of the more 'thermal' areas of the islands :-)

 

So did the pumps you put in deliver heat in winter and cold in summer? I guess to do that there would need to be some sort of air flow system to pump the heat out at high level in summer and warm from low level in winter? I was originally thinking ut would all be under-floor heating but I imagine that having a cold floor in summer wouldn't help cool the room as efficiently as circularting the hot air near the ceiling out to be cooled before being 'pumped' back in near the floor?

 

Cheers

[AceofKY]

Thanks for that, it was really interesting. Not sure how great a vertical bore hole would be around here due to the seismic activity? Vertical pipes might not be great for cooling either in some of the more 'thermal' areas of the islands :-)

 

So did the pumps you put in deliver heat in winter and cold in summer? I guess to do that there would need to be some sort of air flow system to pump the heat out at high level in summer and warm from low level in winter? I was originally thinking ut would all be under-floor heating but I imagine that having a cold floor in summer wouldn't help cool the room as efficiently as circularting the hot air near the ceiling out to be cooled before being 'pumped' back in near the floor?

 

Cheers

 

I'm not sure about seismic activity, but poly pipe is very forgiving and will bend/flex long before it will crack. I never use expansion tanks on this type of piping because it will expand itself much more than other types of piping.

Yes, the heat pumps heat in the winter and cool in the summer from the same water loop and using the same air system. There is a reversing valve in the refrigeration circuit that switches automatically from one to the other based on the thermostat setpoint and mode. All of this is packaged into the water source heat pump. All the homeowner has to do is connect up the piping, electrical, and duct. Except for the geothermal water loop, it is basically a drop-in replacement for a standard central air conditioning system with air-cooled condenser.

 

There is no need to warm from low level unless you have really high ceilings. We use ceiling diffusers almost exclusively on ceilings from 8 to 12 feet without having any heating problems. You can also get ceiling diffusers with adjustable vertical throw if needed; I use these when designing HVAC systems in gymnasiums which have higher ceilings.

 

Remember, with the typical geothermal system, you don't get "hot" or "cold" water and thus it won't do any good to run it through the floor or through ceiling beams unless you use some sort of secondary chiller or heater. The ground stays at about 50-65 degrees F year round. The water loop will range from a low of around 40 degrees F in the winter to a high of around 95 degrees F (if properly designed) in the summer. You must use a heat pump (which is basically the same refrigeration cycle as a refrigerator) to push the heat into the water in the summer or extract the heat in the winter. The cheap way to do it is using the standard water-source heat pump with packaged air heat exchanger and supply fan and duct the air to the rooms in the exact same manner as the traditional central air conditioning system.

[sylvester]

Thanks for the information, very interesting!