Heating - Geothermal
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Heating - Geothermal

Star A trendy way to heat, but with a long payback for an energy efficient house.

 

I do not use Geothermal because the payback is far too long in the case of a highly energy efficient house.

If you are doing lots of excavation for your house then you could put in piping when you backfill that can be later used for geothermal heating.

Martin Holladay says...  "(For a super-insulated house it's) unwise to invest in a ground-source heat-pump system, which usually costs at least $18,000."

How it works

It uses the fact that temperatures underground are warmer in the winter than the outside air.

A Ground Source Heat Pump (GSHP) extracts heat from underground and delivers that heat at a higher temperature to your house.

While electric resistance heat is 100% efficient at converting electricity into heat, a heat pump provides at least two to three units of energy for every one unit of energy consumed.  This is often measured as the annual “coefficient of performance” or COP.  A COP of 3.5 means an effective efficiency of 350%.  Sometimes the real annual COP turns out to be only slightly higher than 2.5.  For my Pacific North West USA area with a a well implemented GSHP system a realistic COP number to use is 3.

GSHP are unique in that their reported COP efficiency typically does not include the energy of the fluid or water pump required to move the fluid through the tubes in the ground.  This electrical energy can be significant, particularly if the loop is long, the pipes are small, or the flow resistance within the heat pump unit is large.  The rated COP of ground source heat pumps doesn't include the energy required to pump fluid through the ground loop, or the energy required to power the pump/fan that distributes the heat/cool into the house.

COP = quantity of heat delivered / energy required by pump.
Convert from EER to COP by dividing by 3.412.

http://buildingscience.com/documents/digests/bsd-113-ground-source-heat-pumps-geothermal-for-residential-heating-and-cooling-carbon-emissions-and-efficiency?searchterm=gshp

Heat energy naturally flows “downhill” from high to low temperature. A heat pump is a mechanical device that takes heat at a lower temperature and “pumps” it “uphill” to a higher temperature.

All heat pumps work on the same principle, moving heat from one location to another with the help of a closed refrigerant loop, a compressor, and a heat exchanger. In winter, heat is extracted from air, water, or the earth; this heat is used to heat the house. In summer, the process can be reversed so the heat pump pulls heat from the inside and dumps it outside. A heat pump can be used with either a forced-air or hydronic distribution system.

A ground source heat pump either collects heat from the ground and pumps it to a coil inside the ductwork to provide air heating, or collects heat from the same coil in the ductwork (thereby cooling the air) and rejects it to the ground.

The efficiency of a heat pump falls as the output temperature rises.  GSHP systems are not suitable for directly replacing conventional water-based central heating systems which have been designed to operate at 60°C to 90°C because it is inefficient to make water of that temperature.  Low temperature water- based systems or underfloor heating are efficient options.  Typical efficient output water temperatures are between 35°C and 55°C.

Most water-to-air heat pumps are reversible so a forced air distribution system can readily be adapted to provide cooling as well as heating.

A reversible water-to-water heat pump coupled to an underfloor distribution system can also be designed to supply space cooling in summer.

Even with water-to-water heat pumps designed for heating only, a limited amount of ‘passive’ summer cooling can be provided by direct use of the ground loop, for example by by-passing the heat pump and circulating fluid from the ground coil.

The first aim of the space heating control circuit is to operate the heat distribution system at the lowest temperature that will still meet the required comfort conditions. This will optimize the efficiency of the heat pump.

Other heat pump examples

A refrigerator contains a heat pump that takes heat from its interior and heats a coil at the back of the fridge.  As a result the back of the fridge is much warmer than the interior of the home, and the interior is cooler.  The energy cost of doing this is the electricity to run a compressor.

An air conditioner is a heat pump that takes heat from a coil in ductwork (which cools the air that passes over it, thereby cooling the home) and pumps it to an outdoor unit (which is hot, so that the heat is released to the outdoors).

Subsidies

Depending on your area GSHPs may be eligible for the 30% tax credit with no cap on the dollar amount that can be received.

GSHP tax credit is scheduled to expire at the end of 2016.

Further reading

Good info at http://www.greenbuildingadvisor.com/blogs/dept/green-building-blog/procon-are-heat-pumps-green

Piping

Ideally keep piping at least 3 ft apart.

Never place any part of the ground loop less than 6 ft from a basement wall or crawlspace.

Smart to do multiple parallel loops with a little extra capacity.  If a loop starts to leak it can be shutoff without the system becoming unusable.

3/4" - 1.5" high-density polyethylene (HDPE) pipe.  High- density polyethylene pipe containing a circulating fluid (usually a water/antifreeze mixture).  High or medium density polyethylene pipe for horizontal ground heat exchangers.

5' is an acceptable depth for horizontal piping but greater is better.

Can place eg 4 or 6 pipes per trench as long as they are at least 1' apart.

To avoid interference between adjacent trenches there should be a minimum of 10 feet between them.

Avoid in-ground connections, ie use a continuous length of pipe.

Likely to need something like 6000 feet of pipe.

500 feet of 1" PEX costs $617 whereas 500 feet of 1" HDPE pipe is $226.

It will cost something like $3,000 for 6,000' of 1" HDPE pipe.

HDPE pipe from http://www.isco-pipe.com/products-and-services/hdpe-pipe/   scott.bevens@isco-pipe.com     sales@isco-pipe.com

HDPE joints need to be fused with special equipment.
If you have multiple circuits, they need to header into a "reverse return" manifold to insure equal flow to each circuit.
After the ground loop is built, you will need a high volume, high pressure pump to purge all of the air out of the system.
The loop circulating pump needs to be sized for the loop so that you get 3 gpm flow.

External pipework must be insulated within 5 feet of any wall, structure or water pipes, and sleeved where it enters the house.

When installing the ground heat exchanger it is important to ensure good long-term thermal contact with the ground.  Horizontal loops are usually laid on a bed of sand and then covered with a further 6" layer of sand for protection.  Care must be taken to avoid damage when backfilling, and the backfill material should be screened for rocks, stones etc.  My conclusion is that filtered non-organic soil is the best bet.

Usually best to use propylene glycol as the antifreeze.

As the mean operating temperature of the heat pump may be as low as -4°C, it is usual to add an antifreeze solution to prevent freezing to below -10°C.

Heat pump

An example unit...  Trane GEH Geothermal Heat Pump

http://www.heat-pump-pro.com/tranegeothermalheatpump.html 

Most heat pumps are designed to limit noise nuisance and vibration, for example by using anti- vibration mountings for the compressor and lining the heat pump casing with acoustic insulation.  The heat pump should not be mounted close to sensitive areas such bedrooms.

If doing radiant water floor heating then you need a water-to-water heat pump.

 

 

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