As winter draws near and the temperature gets colder, home heating and its associated cost can become a big issue. Many people are paying a high cost for heating their homes and offices. So it may not be known by many that one of the least expensive ways to heat a building is with a geothermal heat pump.
A geothermal heat pump utilizes heat from underground to operate an efficient heating or cooling system.
So how does it work?
Let’s first explain what a heat pump is. A heat pump is a device that “transports” heat energy from one place to another place. An air conditioner is a form of heat pump. It “extracts” heat from indoors and pumps it to the outside. So on the indoor side you have cool air blowing out of the vent. On the outdoor side you have warm air blowing out of the radiator. The interesting part is that you can actually transport more heat energy out of a building than the electrical energy it takes to power the A/C. So there is an apparent efficiency of over 100%. But how is this possible? How can you get something for nothing? Well, you actually don’t. As said before, a heat pump merely transports energy from one place to another. This is not the same thing as creating something out of nothing. So in the case of a heat pump it becomes more appropriate, for semantics if nothing else, to define a Coefficient Of Performance (COP), which is: (heat energy transported)/(energy input). So for an “efficiency” of 400%, COP = 4.
Alternatively, a heat pump can be used as a heater instead of a cooler/refrigerator. This is basically taking an air conditioner and flipping it around, so that the outside part is facing indoors and the inside part is facing outdoors. With this set up you will have a heater instead of an air conditioner. And once again, you can have an apparent efficiency greater than 100%.
But there is a catch.
In order to have a high COP, you have to be operating between certain temperature ranges. So if you are using a heat pump as a heater during the winter, you cannot have an outdoor temperature that is excessively cold, otherwise your COP will go down. In fact, the COP will approach 1 for outdoor temperatures that are -18 degrees Celsius or colder. This is because it becomes increasingly difficult to extract heat from the outdoors the colder it gets. Eventually, the heat energy output becomes equal to the electrical energy input (COP = 1), and the cost of heating becomes much more expensive. So in this case a heat pump is best used during mild winter temperatures.
Similarly, if you use a heat pump as a cooler (air conditioner) during the summer, you cannot have an outdoor temperature that is excessively warm, otherwise your COP will go down. Fortunately, it never gets nearly hot enough during the summer to result in a COP approaching 1 – it would take an outdoor temperature of 50+ degrees Celsius!
This makes sense intuitively – a lower COP is the result of “pushing uphill” to a greater extent, and working against the natural direction of heat transfer – which is from hot to cold. So the greater the temperature difference you are working against, the more energy it takes and the less bang you get for your buck.
So we have a practical dilemma, especially where a heater is concerned: The more you need it, the less efficient it becomes, and the less you need it, the more efficient it becomes. But there is a way to deal with this, however. You can use the heat energy of the ground to keep the COP high. The diagram below illustrates this.
This figure shows a network of pipes running underground, to form a closed loop. If you go deep enough (5-6 m in the Ottawa region, ref: http://irc.nrc-cnrc.gc.ca/pubs/cbd/cbd180_e.html) – well below the frost line, the ground stays at a roughly constant 10 degrees Celsius, all year, with little variation. This is due to the energy of the sun which warms the earth, and maintains a consistent temperature profile year round, at a certain depth. It is this steady temperature condition that allows a geothermal heat pump to operate efficiently.
During wintertime, an anti-freeze solution, such as propylene glycol (mixed with water) – which serves as the heating medium, is pumped through the pipes and as it courses along it is heated up to roughly the (surrounding) ground temperature, which for the Ottawa area is 10 degrees Celsius. As the liquid makes its way back up (in the heated state) it enters a heat exchanger which allows this heat energy gained to be transferred to the heat pump, which then transfers the energy inside in order to heat the building. The propylene glycol, which is cooled at this point, makes its way back underground to gain heat once again from the ground, and the cycle repeats. The length of pipe running underground is proportional to the desired heating load.
During summertime the heat pump can be operated in reverse, as a cooler (air conditioner). So the operation is similar, except that instead of gaining heat from the ground, the propylene glycol (which now acts as the cooling medium) “loses” heat to the ground. As the propylene glycol makes its way back up (in the cooled state) it enters a heat exchanger which allows the heat energy of the inside of the building to be transferred to it, by way of the heat pump. This enables the building to get cooler inside. The propylene glycol, which is heated at this point, makes its way back underground to lose heat once again to the ground, and the cycle repeats. The length of pipe underground is proportional to the desired cooling load.
What makes this method so attractive is that you have a free, readily abundant source of heat available from underground, which can be utilized for high efficiency heat pumps. During the winter you use the heat pump as a source of heating and during the summer you run the heat pump in reverse and use it as a source of cooling. Since, at a certain depth underground the temperature is relatively constant year round, the COP remains high year round.
The main benefit of this method is in heating, since that is where most of the savings are realized. It can be an improvement on other heating methods which may use natural gas, heating oil, or electric heating. By the way, electric heating is one of the least efficient ways to heat a building, since it has a COP of 1.
It makes sense to use ground-source (geothermal) heating from a sustainability point of view. Using fuels such as natural gas are wasteful since that is a high quality and finite resource being used to do something that can just as easily be done with a low-grade renewable source of energy directly from the ground.
The main disadvantage of ground-source heat pumps is the initial up-front cost which can be around $2,500 per ton of capacity (ref: http://www.cogeneration.net/geothermal_Heat_Pumps.htm). Note that one ton equals 12,000 BTU/hour, or 3.5 kW.
But because the annual operating cost can be significantly less than conventional heating systems, this system can pay for itself in a few years.
The figure below shows a picture of an underground pipe network typical of such an installation.
The pipes can be placed vertically underground in a deep hole hundreds of feet deep or in large shallow trenches several meters deep. The deep hole may be the better option if you have a small property or you don’t want to dig up a large area, although this set up will cost more than a horizontal loop system. The length of pipe used is typically hundreds of feet. As a general rule, 500 – 600 feet of pipe in 250 – 300 foot trenches is required per ton of system capacity, depending on how damp the soil is (ref: http://www.earthheat.ca).
The pipes are typically made of plastic which requires longer length to reach ground temperature than metal pipes, but are lighter and more flexible and are very durable.
Here are some quick facts about geothermal heat pumps:
• Total building energy cost reduction can amount to 60% (ref: Commercial Earth Energy Systems: A Buyer’s Guide, Natural Resources Canada, 2002, page 8)
• The payback period can be 6-8 years (ref: Commercial Earth Energy Systems: A Buyer’s Guide, Natural Resources Canada, 2002, page 10)
• Typical COP of a geothermal heat pump for heating and cooling is 3-4
Cost Analysis
Before you consider replacing your current heating system with a geothermal heat pump system it is wise to calculate your energy usage to see how much you will be saving. Have someone come to your home or business and do an assessment. But you can also try your hand at a rough calculation by looking at your energy bills. If you’re using electricity for space and/or water heating then figure out how much electricity cost is tied to those particular uses, for one year. Call this amount X. So your annual savings would be equal to AS = X(1 – 1/COP).
If you’re using a heating fuel instead, the calculation becomes more involved. You first have to calculate how much heating fuel you are using annually. Your energy bills should contain this information. Let’s use natural gas in a sample calculation.
For natural gas, the unit of measurement is volume, in m3. From your energy bills, add up the total volume (in m3) of natural gas used in one year, based on your meter reading. Call this volume V.
The energy density of natural gas is about 35,000 BTU/m3 (ref: http://hypertextbook.com/facts/2002/JanyTran.shtml).
Now,
35,000 BTU = 10.26 kWh
To produce 10.26 kWh of heat energy, the amount of electrical energy input needed by the heat pump is 10.26/COP (e.g. for a COP = 3, the energy input is 10.26/3 = 3.4 kWh).
The associated electrical cost is then EC = (10.26/COP) x (cost per kWh of electricity in your area). Note that the electrical cost per kWh should be the total cost, which is: (total amount due on electricity bill)/(total kWh used based on your meter reading). In my area the electricity rate is 15 cents/kWh.
So, the annual electrical cost using a geothermal heat pump is ECxV.
Now from your energy bills, calculate the total energy cost of using natural gas for one year, taxes included. Call this NGC.
Therefore, your cost savings per year is AS = NGC – ECxV. This number may actually be negative in some cases, in which case you would be paying more for a geothermal heat pump system than with your current system.
We can now look at the payback period.
The payback period in years = { (cost of heat pump installation) – (cost of the installation you have, which is natural gas in this example) }/AS
The same basic calculation applies to other heating fuels, such as propane. Just keep track of the units used, and use the correct energy density per unit volume (an online search will give you that information). In some cases you may be using gallons instead of cubic meters (m3).
It should be mentioned that, when doing the above crude comparison of a heat pump system with your current system, you are assuming that the ductwork and other components such as blowers, thermostats, etc. are roughly the same cost for both systems. So the main difference in cost is due to the installation of the underground piping.
A typical residential installation would cost $7,500 for a 3-ton unit (ref: http://www.cogeneration.net/geothermal_Heat_Pumps.htm).
A comparable natural gas installation would cost $2,400 (ref: http://www.energyshop.com/es/info/equipcos.cfm).
Direct Exchange Geothermal Heat Pump
A variation on the geothermal heat pump is the Direct exchange geothermal heat pump. In this system the copper coil of the heat pump is placed directly in the ground and as a result exchanges heat directly with the ground. This allows for more efficient heat exchange with the soil because there is no intermediate heat exchange with a ground loop before heat exchange with the heat pump coil takes place. There is a more direct heat exchange path. This simpler design allows for a shorter length of tubing and reduced installation cost. However, the limitation of this design is that the compressor cannot be placed at a great distance from the ground coils. This can be restrictive depending on the application. As well, the cost of refrigerant can be high due to the high volume needed in the long copper coil.
Locating Heat Pump Manufacturers And Suppliers
If you’re interested in installing a heat pump in your home or business here are some websites that let you search for suppliers and manufacturers near you:
http://www.igshpa.okstate.edu/directory/directory.asp
And here are some well known heat pump manufacturers:
If they aren’t located near you, you can try inquiring to see if they have any dealers or representatives in your area. Make sure you ask about the COP as well as the approximate installation cost, as you can use it to calculate your energy savings, as shown above. And also, make sure to ask about any government rebates (incentives) which serve to offset some of the installation cost.
Additional References:
http://www.hydro.mb.ca/earthpower/faq.shtml
http://www.canren.gc.ca/app/filerepository/6F92970D42FA4713865FFB308C4A5C06.pdf
http://oee.nrcan.gc.ca/publications/infosource/pub/home/heating-heat-pump/booklet.pdf
Loved this article. I live in Maine. The energy consultant who looked at my house said geothermal wouldn’t be cost-effective, but has recommended an air-to-air heat pump. Can you explain the differences?
An air-to-air heat pump doesn’t use a ground loop. So there is no energy exchange with the ground. There is only energy exchange with the outside air. This saves a lot on excavation cost, but it’s use is limited to areas that have mild (not too cold) winters. For a heat pump to be most efficient it has to pump heat across a temperature difference that isn’t “too steep”. So in an air-to-air heat pump during the winter, you are “removing” heat from the outdoors and “pumping” it indoors. The colder it is outside the harder it is to do this. Eventually as it gets colder your COP approaches 1.
If it’s colder than about -5 deg C outside the performance of your unit will really decrease. So it’s good to have a backup heating system, like electric or gas, if it gets too cold. The specific temperature cut-off will depend on the specs of the air-to-air heat pump you’re using.
I think it is fantastic how the stimulus funds are making a difference. They are being taken advantage of all the time with the installation of geothermal heat pumps to replace high energy heating and cooling systems.