The first is the type that utilizes the high temperatures of underground rock formations in areas where there is a lot of geologic activity which is often indicated by the presence of hot springs and geysers. Places like Yellowstone immediately spring to mind. In this form of geothermal energy, water is pumped down deep wells, some as deep as 5 miles, to access rocks that have temperatures in excess of 212oF. The water is heated by the rocks and is drawn back to the surface to produce steam which can be used to drive turbines and generators to produce electricity. The source of energy in these sites comes from the earth's internal heat which is produced from the decay of radioactive elements in the rocks or the heat flow from the earth's mantle where the earth's crust is thinner.
There are a good number of locations where we can access this energy, and as the US geothermal heat flow map below shows, these red colored areas lie largely in the western portion of the US. To generate electricity from geothermal sources, one needs rock temperatures in excess of 212oF at reasonable depths. Interestingly, on close examination of the geothermal map below, one can see that there is a hotspot in NH located in the White Mountains area. At depths of 6.5 miles, rock temperatures in this area are in excess of 400oF due to the abnormally high natural radioactivity of the granite in this area. This is a source of energy we might have to tap one day.
Graphic Sources: http://www1.eere.energy.gov/geothermal/pdfs/egs_chapter_2.pdf
The other type of geothermal energy we can access comes from the dirt beneath our feet. In many areas, the temperature of the ground, 4 or more feet below the surface, is a moderate 50 to 55oF. We can utilize this property of the earth and draw heat from the ground in the winter or use the cooler ground temperatures to dump heat into during the hot summer months. This utilization of the energy in the ground beneath our feet is more correctly termed "geoexchange". Some like to call these arrangements earth exchange or ground source heat pump systems. This form of earth-based energy is very different from the "hot rock" type of geothermal energy as the source of energy in these geoexchange systems actually comes from the sun's warming of the earth surface. So, in essence, these geoexchange systems are indirectly a form of solar energy.
Because the ground temperatures are so low compared to the traditional geothermal systems which utilize the temperatures of hot rocks deep below the earth's surface to produce electricity, these geoexchange units cannot produce electricity. Instead we use the earth as a heat sink – we dump excess heat energy into the earth during the hot summers and we draw heat energy from the ground in the cold winter months to preheat the circulating air or water in our homes. But, and this is important in the understanding of these systems, it takes energy, in the form of electricity, to make these low temperature systems work. The electricity is needed to drive a device called a heat pump.
Most of us are familiar with heat pumps but we don't recognize them as such. The refrigerator in your home is a heat pump. What the refrigeration system is doing is drawing the heat from inside your refrigerator and pumping it into your kitchen. It does this through the compression and expansion of a refrigerant gas which provides the medium whereby heat is drawn out of the refrigerator and pumped into the kitchen. Because this device requires electricity to run the compressor pump and the warm air of the kitchen to work, it is referred to as an air-source heat pump. This is the same principal an air conditioner works on, which is pumping heat from inside the home into the warm outside air. With the geoexchange units we don't pump the energy into the hot summer air outside the home; instead we pump the energy into the cooler ground below our feet, which is why these units are called ground-source heat pumps. The fact that the ground is cooler than air makes these ground source units more efficient than the air-source units in typical air conditioners.
These heat pumps can also work in reverse. They can draw the heat from the air outside the house, or the ground, and pump it inside to warm up your home. You might be thinking that sounds awfully complicated when you can simply heat up the inside of your home with an electrical heater or your natural gas or oil burner. However because you are drawing energy from the moderate temperatures in the earth, you do not need as much energy as you would from an electrical or fossil fuel heater. In fact because you are drawing on the indirect solar energy stored in the earth, these heat pumps require only 25% to 35% of the energy you need to heat your home compared to an electrical heater. The ratio of energy needed to heat a space with an electrical heater to the energy used by a heat pump is termed the Coefficient of Performance, or COP, and is the basis of comparison between different heat pumps. For example, the COP value for a heat pump that requires only 25% of the energy to heat your home compared to an electrical heater is 4, calculated as follows
Coefficient of Performance = Electrical Energy required to heat home
Electrical Energy consumed by heat pump
Ground-source heat pumps range in performance with COP values of 2.5 to 4, with typical values in the 3.3 to 3.8 range. It must be noted that these values are highly dependent on proper engineering, the quality and efficiency of the mechanical device as well as the temperatures in the ground.
In geoexchange systems there are two main components, the heat pump and the circulation system that is drawing the heat from the earth. The circulation systems come in several different configurations. The most common, and the one most often used for homes, is the horizontal configuration in which the piping, containing an inert fluid, is buried in a shallow trench 4 to 6 feet deep alongside a home. The piping is made of plastic and is laid at the bottom of a trench in a slinky arrangement as shown in the figure and photo below.
These are referred to as closed loop systems as the circulation fluid, similar to antifreeze, is never directly in contact with the ground. Instead it is circulated between the heat pump and the ground through the piping. The heat pump draws the energy out of fluid, cooling it in the process and pumps the heat energy into the house. The cooled fluid is then circulated through the piping buried in the ground where it is heated up again to the ground temperature before returning to the heat pump.
Another type of closed loop configuration is a vertical system where the piping is enclosed in wells which penetrate deep into the ground as shown in the figure below. The advantage with these vertical closed loop systems is that, in these deep wells, the piping can come in contact with the ground water and water is a highly efficient medium for transferring the heat from the ground to the circulating fluid. These vertical systems also require a smaller surface footprint than the horizontal equivalents which can be crucial when space is limited or the heating loads are large, such as one might find in a school or apartment building.
A different type of system is the open loop configuration where the circulating fluid is ground water that is drawn from a vertical well and injected back into the ground via another well as shown in the diagram below.
Most geoexchange units are, energy plant wise, relatively small scale units and are designed for specifically for individual residences or facilities such as hospitals, office complexes or nursing homes. There are also fair number of vendors of these systems that have been operating for a number of years in NH. As a result, it is difficult to get good reliable data on the total installed base of geothermal energy units in New Hampshire and my best guess is that there are many thousands of these units installed in homes and buildings throughout NH. In fact, some developers are building whole housing complexes and communities that make extensive use of geothermal energy.
In my chats with geoexchange system installers and folks who have these units in their homes, I have learned the following:
- These units are expensive to install and prices for a residential system, including the well, heat pump and circulating system, range from $20,000 to $35,000
- Because the installed costs are so expensive, sometimes the units are under-designed to save on upfront equipment costs. As a result, the systems are undersized and do not work well, particularly when temperatures are colder (or hotter) than usual. In these undersized systems, there can be an enormous draw on electrical backup heating systems which then significantly diminishes the savings.
- For the well-based systems, the choice between a closed loop and an open loop system is a difficult one and can be quite site specific. Each system has its own pros and cons and each has its advocates and detractors
- After installation, the owners either love or hate them. I have heard stories where owners are disappointed that the promised savings did not materialize or where there have been issues associated with the circulating systems or heat pumps. And then there are the owners who are delighted with their units and pleased to share with me that they no longer have fossil fuel bills.
- Home Footprint: 2500 square feet
- Electricity Use: 12,000 kWh/year @$0.13 per kWh
- Oil Consumption for Heating: 800 gallons per year @ $3.75 per gallon.
- COP for Geoexchange System: 3.5
- Installed Cost: $25,000 with 30% federal tax rebate
In NH we don't have the readily accessible hot rocks and burning ground* type of geothermal energy the folks out West do, but just 4 feet down we can access the solar energy stored in the cool earth. There are lots of opportunities for us to do so and I encourage you to consider a geoexchange unit when you build a new home or you have to replace the natural gas or oil burner in your home. Yes, it is a hefty investment but forward-looking, energy-conscious folks consider a 4 to 5 year payback to be a good return on an energy project.
Next week I will be discussing a large and impressive geoexchange project in NH I have recently visited plus I will share with you some geoexchange cautionary tales. I would be interested in your experiences with geoexchange systems so be sure to share them with us in the comment box below.
Until next week, remember to turn off the lights when you leave the room.
Franklin Pierce University
(*Burning Ground – A fabulous tune by Van Morrison from his 1997 Album "The Healing Game". One of those driving songs that makes you want to roll down the window, crank up the volume and sing along.)