GEOTHERMAL HEAT PUMPS:  ARE THEY REALLY GREEN?

            Many individuals have asked me over the past decade about geothermal heating and cooling, and how they could go about having a system installed in their homes.  Over the past year, or so, especially after becoming a LEED® AP myself, I began to look further into the advantages and disadvantages of this technology

Geothermal heating and cooling is not really a new idea; in fact it was introduced as early as the late 1930's as an alternative to fossil fuel furnaces for home heating.  The technology worked well, but at the time it was too pricey for it to catch on rapidly.  This would change over the next several decades as advances in the technology produced more cost-effective systems that were capable of outperforming a standard fossil fuel furnace AND could supply air conditioning / de-humidification at the same time, with a SINGLE system.

When we refer to geothermal for residential or commercial use, we are really talking about Ground Source Heat Pumps (GSHP's). To fully appreciate the concept and its values, on must fully understand, at least in general terms, how GSHP's work.

Geothermal, in technical terms, actually is defined as Renewable heat energy from the earth, such as that produced by hot springs or deeply drilled wells extract actual heat from the earth.  This heat will NOT need to be boosted via technology to be used; at temperatures over that of the boiling point, it can directly heat a facility. 

Ground Source Heat Pumps work much the same way as does an item that we all are VERY familiar with -- the refrigerator.  Like the refrigerator in our kitchens, electricity is used in a GSHP, but a far more efficient rate.  A compressor is used to move heat obtained from the earth either in or out of a space, by use of refrigerants.  This compressor is linked to an earth-sourced heat transfer system. The earth, in my area outside of New York City has a steady temperature of about 55 degrees F.  When compressed under high pressure, the refrigerant will get very hot, and be able to transfer this heat away via a condenser.  In the case of the refrigerator, we move the heat out of the box area, and into the room area, while in the case of central or room air conditioning, we do the same, by taking the condenser heat, and sending it outside via the condenser.  With a Ground Source Heat Pump, or the less efficient Air-Source Heat Pump (ASHP), we do the reverse.  Here, we take the heat obtained from compressed refrigerant gases to the condenser, where we can then extract that heat for space or water heating.  We can reverse this process by sending the gas through an evaporator coil when cooling is needed.  Essentially, we are boosting the heat (or coolness) contained within the earth to meet our needs in fashion that uses up to five times less energy than straight electric heat or traditional air conditioning systems.  The energy and operating costs savings can be significant.

There are two classifications of GSHP's -- Open Loop and Closed Loop.  In an Open Loop, water is simply pumped into the heat exchanger unit, where the compressor either extracts heat from it, (heating mode) or places heat into it.  (Cooling mode)  In the Closed Loop system, a separate tubing (High Density Poly Ethylene, HDPE) system is either buried horizontally or vertically into the ground, and then filled with a non-toxic heat-transfer fluid (often food-grade ethylene glycol) that will then be pumped in and out of the GSHP heat exchanger system for the compressor to extract or reject heat.  While this gives less heat per gallon of fluid, as it is based on a transfer of heat from the ground vial the solution in the buried tubing, the efficiency is close to that of an open loop system,as a smaller pump is needed to move the heat transfer fluid than that of a well pump.  The well-pump penalty in the open loop system is offset totally by the dramatically increased heat transferability, as it carries the earth heat directly to the heat exchanger.

Most Ground Source Heat Pumps have what is called Coefficient of Performance (COP) of at least four; many over five.  This Coefficient of Performance is defined as the amount of heat, measured in British Thermal Units (BTU's) above that produced by resistive electric heat. Electric heat will produce about 3400 BTU of heat for each kilowatt-hour of electricity consumed.  Thus, if we have a heat pump with a COP of four, we will get 13,600 BTU of heat from this same amount of power consumed.  With a COP of five, it will be 17,000 BTU per kilowatt hour burned. A kilowatt-hour is defined as 1000 watts of electric power consumed for a period of one hour.

To see how this energy production compares with other standard heating systems, all we need to know is the unit cost of energy for that system, and at what efficiency that specific system is utilizing that energy.  We then compare this cost to that of what can be produced with a GSHP system at the current rate per kilowatt-hour for the electricity that will be needed to produce the equivalent heating or cooling.

Natural gas will produce, around 100,000 BTU of heat for each100 CCF of gas burned.  Number 2 Home heating Oil is about the same.  Thus, if we have even a 95% efficient furnace or boiler, we still do NOT get the total 100,000 BTU of available heat. (Only 95,000 BTU's are actually realized; the remaining five percent is lost.)

In my case, at my Upper Nyack, NY residence, I installed a GSHP system in 2005 that uses well water (that is recycled back to the ground) for my earth heat source.  My COP is 4.3.  Thus, for each 10 kilowatt-hours of electric burned, I obtain 146,200 BTU of heat.  (3400 x 10 x 4.3)  With my electric rate, I was able to purchase electricity for fewer than ten cents per kilowatt-hour last winter.  Thus for under $1.00 I produced 146,200 of fully usable heat.  Natural gas was costing around $2.00 per 100,000 BTU.  We must remember that the most efficient gas heating system is NOT 100 percent efficient.  This differential meant that I saved almost two and on-half times on my heating costs last winter at the above rate schedule. In using my existing high-capacity well as the heat source, and only needing to drill a second return well, and then install a one-pipe system for the water supply to each heat pump, my total installation costs were under $10,000.  Thus, with my energy bills costing less than $300 per month, as opposed to my neighbors that cost over $600 or more per month, I save $3,600 per year.  This means my system was able to recover the initial cost of installation in 2.78 years.  The GSHP units were comparable in cost to that of a new fossil fuel furnace, and I got both heating AND cooling from ONE system. 

My current 92% efficient boiler serves as a back-up, to be used in the event of a power failure or GSHP breakdown.  It is also available should the cost of electricity soar way beyond that of natural gas.

That said, there are many issues above and beyond my successful experience with a GSHP system. First, as I described above, we need to be aware of the cost of the energy -- both the electric and the fossil fuel, AND the total cost of the system installation.  In my case, I already had a high-capacity well capable of producing the three gallons per 12,000 BTU of heat needed.  All that I needed was to drill a single return well to recycle this water back to the aquifer, and then supply the needed piping to the GSHP units in my home.  I got off VERY easy,financially, here.

In most cases, an adequate supply of water is NOT available,and one MUST resort to other much more costly methods.  As describe above, these involve the use of a Closed Loop System.  The least costly of these, but still considerably more than that of the Open Loop, is a Buried Horizontal System, in which trenches are excavated to a depth of about eight feet, and the HDPE tubing employed with the heat-transfer fluid. The length of these trenches is dictated by the total BTU needs of the building; several thousand feet may be required, and these MUST be spaced at least 15 feet from each other.  The other and most costly of all is Vertical Bore System.  In this system, wells are bored often 400 feet deep or more per ton of BTU needed.  These must also be spaced at least 15 feet apart from each other. Thus, even an average sized residence can need ten or more bores drilled.  I have been told that the just the drilling of these bores, the placement of the HDPE tubing and then the required grouting of the wells can cost well over $50,000 ALONE for an average residential application. This does NOT include the needed interior piping.  Thus VERY FEW residential systems employ this method; the initial cost is too much and the payback are too long.  Hence we are now dealing with the Life Cycle Cost of the project.  We need to determine here, if on the basis of initial cost, whether this is best choice for the overall project needs.

There are still other questions as to just how"green" GSHP's really are.  One issue often raised is that, while they are efficient, they are NOT a true source of renewable energy.  They DO use electricity.  In my case, if this comes from the burning of dirty, bituminous coal, the carbon footprint saved by the better use of BTU is at least partly offset by the CO2 emissions from the coal burning.  Even with clean natural gas, there is issue of depleting a finite natural resource.

The above issues can be addressed by doing several things:  Purchasing power made from either solar, wind or natural hydro-power. This makes GSHP technology, in effect a renewable energy resource.  In spite of this, the LEED rating system does NOT include GSHP's as  renewable energy source in that specific area of credit; GSHP's CAN gain LEED credits in the area of Optimize Energy.

Another method that can be used to further optimize environmental performance involves a system that uses NO compressor at all.  Here, we usually mean a near-by source of water, such as a lake or storm water retention pond.  In the case of Cornell University, as reported in the September - October 2009 issue of Green Source (pp. 81-88) a Direct Water System Cooling System (DWSC) is used to draw the 39 degree Fahrenheit temperature water at depth of 250 feet, from Cayuga Lake into heat exchangers that use this water directly to cool the classrooms without a compressor.  In this case, the water is drawn into a second closed loop for central distribution purposes, prior to going into the individual classrooms, where it actually is 47 degrees Fahrenheit.  The water is then returned to the lake near the surface via a separate pipe.  This is sufficient in that climate to cool and properly dehumidify the buildings.  Their system is designed to produce 20,000 tons of cooling, and even though, at a cost of $58 million, they are saving 86% on their total cooling costs, and expect to get the initial coats recovered within 13 years.  (Most green projects aim for an initial cost recovery within five years, or less.)

Harvard University uses a system, which while using a compressor, uses a series of individual Standing Column wells drilled to depth of 1500 feet.  Here, costs were controlled, but the water must be carefully monitored temperature-wise, so that the well does not over-heat or over-cool during peak periods.  In this method, the water is drawn from the bottom, and returned at the top.

One issue often ignored is the fact that too many buildings are just not properly designed from the standpoint of energy conservation.  Their envelopes leak heat, windows are poorly placed, with regard to heat loss and gain, and control systems are either not installed or operated correctly.  I know of one GSHP contractor that will NOT install a system if the building does not meet strict duct-loss and blower door tests for heat leakage.

Another environmental threat can come when a well is drilled into different aquifer zones, in which one is found to be contaminated, while the others are not.  If this well is not promptly grouted to seal off the water flow from the adjacent aquifers, they will all become contaminated in a hurry. In Rockland County, NY, where I live, ALL wells MUST be properly grouted, and need be inspected to insure this, prior to the approval of the system.

Despite all the negatives mentioned above ALL experts agree that, when properly designed,installed and operated, GSHP's are, indeed GOOD for the environment, and are a good source of green, sustainable technology. Another benefit of GSHP's is their longevity; most compressor systems last over 25 years.  In addition, the piping systems will last the life of building, or longer.  This fact, alone, reduced the carbon footprint by eliminating the need for frequent product replacement, and the attendant disposal issue.   Added to this, is a lower need for refrigerant per system and that  GSHP's now use the more earth-friendly 410-Refrigerant or better.

The October, 2009 ASHRAE JOURNAL has a feature article (pp.24-40) that offers an insight into further increasing the effectiveness of GSHP systems by using a one-pipe system versus the standard two-pipe system commonly used in the Closed-Loop system.  In this system, the fluid is available to all GSHP's in the system, and via a series of small pumps, the fluid is then taken in and out of the main pipe, which circulates in and out of the building in one single loop.  This further reduces the initial costs, AND has been shown to improve operational efficiency, especially during off-peak hours.  This is due to the reduction in the pumping power needed, as excessive pressures are avoided.  Two pumps are used, but one really serves as a back-up, as it is only needed for 15% or less of the total operational hours.  This has the advantage of providing a redundant pump for emergency or maintenance use.  Many of these systems have been shown to receive an Energy Star score of 90 or better. 

The article goes on to discuss the importance of off-peak load management as well as the need to better educate building operators as to the proper operation of the equipment.  This leads me to the issue of building commissioning, and equally importantly post or even retro-commissioning.  The need for Integrated Building Design is paramount as well.

In summary, GSHP's can be a great asset to green and sustainable building.  They just need to be properly designed and operated as per the needs of the building.