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What do you know about Geothermal Energy? Is a geothermal Heat Pump right for you? Could geothermal solve the energy crisis forever?
Because the Earth is a large ball of rock heated mainly by radioactive decay from within the core, and to a lesser extent by the sun’s rays, we should be able to use it to heat our homes….and in fact we do!! Geothermal heat pumps are now used in many homes. We could also use the heat from the Earth under our feet to generate power for all the foreseeable future. So what are the obstacles and why have we not yet tapped this resource fully?
Of the many ways to use the Earth as a heat source, there are two of great importance and practicality: thermal storage and geo-thermal heat pumps. Geothermal energy can be used to generate virtually unlimited power; however that is a story for another time. First, it is important to scratch the surface of the Earth, and see how the energy within can be used effectively for heating and cooling.
In many parts of the country heat (or cold) can be stored from day to night to reduce our heating and cooling bills; a principle used in the design of passive solar houses. Heavy stone or concrete slabs usually placed directly on the Earth are heated by day and give off heat by night in winter or are cooled by night and used to cool the air by day in summer. Sometimes tunnels under the earth are used to bring cooled air into buildings to replace air conditioning. Two types of control are needed to use the thermal storage aspects of passive solar energy effectively: sun blinds to control the incidence of sunlight on the store and air movement controls such as dampers and fans. Because these use little energy, this form of energy storage is almost free to operate. The cost of building the system can be quite high initially, depending on how elaborate it is and how big the store is made. In most parts of the United States, this will make life more comfortable for only about half to three-quarters of the year depending on standards of comfort. Frank Lloyd Wright used these methods at Taliesin West, near Phoenix to design buildings that need no artificial cooling or heating, but most of us live in places where at least some winter heating is needed.
To use the Earth year-round, it is important to dig deeper to access temperatures that do not change much from summer to winter. Then efficient methods of raising or lowering the temperatures must be used to reach a reliable comfort level, those being heat pumps.
The BOX – How heat pumps work
Most people think that the value of a unit of energy is just that of a kilowatt hour (kWh) or a Joule, etc. This is not true, and in order to use energy more efficiently differences between high-grade and low-grade energy must be distinguished. A unit of electricity is worth more than one of gas, oil or hot water because it is high-grade – that is, it can be used to generate very high temperatures (millions of degrees) or mechanical energy via an electric motor. This concept is known as “exergy,” which is the difference between the temperature available from an energy source and that of a large heat sink. For most practical purposes that heat sink is the Earth or the Atmosphere. All energy degrades by being used until it reaches geothermal temperatures. In a sustainable world one should be striving to be “exergy efficient” rather than simply energy efficient. That means using the lowest temperature that is effective.
Gas, oil and other fossil fuels are fairly high-grade because they can be burned to make temperatures of around 2,000oF, or used via a heat engine to make mechanical power, but at much less efficiency than electricity (40 to 50% maximum even at a large scale and only 20% in a small engine). Hot water and air are very useful for heating homes but are very low grades of heat. In fact, electric companies spend huge sums of money putting low-grade heat into the atmosphere from cooling towers when it could be used for district heating. More than 50% of European cities harness waste heat from power stations in combined heat and power systems. We should not be using a high grade source of heat such as electricity for heating homes UNLESS it is used at much more than 100% efficiency. How is this possible?
Heat pumps push energy uphill, and the less distance they push, the more efficient they are. That is, they make low-grade heat very efficiently. A typical air-to-air heat pump that may have been installed in the last 20 years is about 270% efficient when running at normal conditions. That is, for every unit of electricity it consumes it pushes 2.7 units of heat into your home. The problem with air-to-air heat pumps is that they will not work when we really need them because the outside coils that extract the heat from the air freeze! These heat pump systems then have to switch to oil, gas or electricity ‘back-up’ heat when the air temperature falls below about 37oF.
Geothermal heat pumps make use of the fact that the temperature of the Earth more than about six feet down stays around 50o F all year round. They are more properly called “geo-exchange heat pumps” because they exchange hotter water for cooler water. The heat is collected from a well drilled 300 to 600 feet down into which a sealed pipe is inserted carrying water to be heated by the Earth in winter or cooled in summer. There must be enough pipe buried in the Earth to conduct the heat needed by the heat pump. A rule of thumb is 150 feet depth per kilowatt (kW). Too little surface and the water will freeze in winter just like the air coils. However, a properly designed geothermal heat pump system will work all year round at an efficiency of around 400%.
The exact efficiency of a heat pump depends on the difference between the source temperature and the delivery temperature to the room. A good geothermal well at 50o F delivering hot water via a heat pump to a low temperature radiant floor at 90 oF (difference 40 oF) will have an efficiency of around 600%, or as we say, a Coefficient of Performance of 6. If the source is a ‘slinky’ coil buried only six feet under the ground its temperature may fall to around 40 oF in winter and if it is feeding an air delivery system at 120o F (difference 80 oF), the heat pump is doing twice as much work and the CoP will fall to around 3.
Technology note. Other heat pumps exist that do not run on electricity! Thermo-chemical heat pumps use heat to push more heat uphill!! An inexpensive source of (low-grade) heat can use more of it to create higher-grade energy.
As noted in the Box, heat pumps can be used to reduce the cost of heating a home by a factor ranging from four to six if one is currently using electric resistance heating, or by five to seven if it is oil or gas at 85% efficiency. The best choice of heat pump system can be found by using cost-benefit analysis, which for most people consists of a payback calculation since financial resources are a limitation. To calculate this figure, it needs to be determined how far ahead one is going to look, what energy costs projections look like, the capital cost of the various alternatives, and the interest rate one would have to pay on that capital. This analysis is the job of an independent energy consultant who knows about what is possible in the area and at what cost. His or her job should include the choice of delivery system as well as the heating and cooling system.
Let’s look at cooling for a moment. If a consumer has a geothermal well with water available at 50o F in summer, why not use that to cool the house at zero cost? At present this option is only offered in a few large commercial systems, but it should be considered for use in homes, which would help to avoid the cost of running the heat pump for cooling in summer. To do this requires an inexpensive way of using 50o F water to cool air and it is not possible to switch the air coils of the heat pump over to water in summer, so it would require alternative coils. New low cost plastic coils are currently under development and could make this a practical idea for geothermal systems within a few years.
In a few hydronic heat pump systems, the delivery is by water, and the direct geothermal cooling could theoretically be used but it is not usually practical. Air coils are a much better way of cooling rooms than radiators, so direct geothermal cooling is better considered in conjunction with a ventilation system.
Conclusion. Geothermal sources, amongst others, can be used to reduce the cost of heating and cooling homes by as much as 85 to 90%. There is no single right answer for every situation, and geothermal is just one solution to reducing heating and cooling costs. There are many ways that could overall reduce the use of energy in buildings to less than a quarter of the present level. Since energy used in buildings and homes is about 40% of all energy used, this represents a reduction of 30% in overall energy use. In the longer term, energy use could be reduced by another half by better building design.
To learn more about other great ways to save money on your utility bills this winter, check out GREENandSAVE.com. Or to find out specifically how much a geothermal heat pump can save you in the long run, take a look at their Return on Investment Table.
