- What is Geothermal? Energy from the Ground.
- How Does Geothermal Heating and Cooling Work?
- Types of Geothermal Loop Systems
- Efficiency and Longevity
- Tangible Benefits of Geothermal
There exists an endless form of energy – a true renewable – that resides in the earth beneath our feet and which comes from the sun. We call it Geothermal. The word comes from the Greek – yn (ge), meaning earth, and qεomoς (thermos), meaning hot. Geothermal heat in the ground is the result of two forces: heat rising from the earth’s molten core and solar energy that travels at 186,000 miles a second and reaches our earth, in the form of life-giving light, in about eight minutes’ time.
The earth is a giant solar collector and it absorbs and stores about 50% of the energy arriving on earth from the sun. That underground energy remains at a fairly constant, moderate temperature of between about 40° and 75° Fahrenheit, just below the surface, all year round. Average ground temperatures across North America range from a low of 40°F in Anchorage, Alaska to 71° in Austin, Texas.
Mankind’s ability to tap and use this heat under out feet first required the invention of something called a heat pump, which occurred in 1852, and then the application of the heat pump to actually draw the heat from the ground, first proposed in 1912. However, it wasn’t until the late 1940’s, here in the United States, before the application was field-tested, first in an office building in Portland, Oregon (1946), and then in an Ohio home (1948).
What emerged from those early experimental applications was a new form of heating and cooling technology, a very simple system that combined a heat pump located inside a building connected to an underground piping loop containing water mixed with an antifreeze fluid (glycol) and installed adjacent to the building. The system operates by transferring the heat in the ground to the building during the cold months of the year and by removing the heat in the building and returning it to the ground during the warm months of the year. This transfer of heat is called geo-exchange.
It took until the 1980s for a geothermal industry to develop and become a part of the heating and cooling marketplace. Today, that market is growing rapidly due to dedicated industry professionals and supporting organizations working to make geo a mainstream technology. Their efforts are getting a big boost because of a number of intersecting reasons including:
- The high cost of conventional heating & cooling (geothermal is highly efficient)
- A desire to reduce carbon footprints (geothermal does not burn fossil fuels)
- Geothermal is a renewable (clean & ever-present)
- Geothermal produces very comfortable and quiet heating & cooling
- Geothermal systems are durable & long lasting
- A 30% federal tax credit applies and extra state-by-state rebates are available
Geothermal is the most efficient and economical way to heat and cool homes and buildings of any kind. It does not use fossil fuels so it is very clean and environmentally friendly. Just as the energy from the sun is endless, so too is geothermal.
Are you ready to Geo?
Geothermal heating and cooling is made possible by a process called heat exchange. Geothermal, also called ground-source heat pumps use the earth’s constant temperature as an exchange medium instead of the outside air. The water circulating in the underground loop piping system exchanges heat between the ground, where absorbed heat from the sun resides (solar energy!), and a heat pump.
Geothermal Heat Pumps
Heat pumps are devices that move thermal (heat) energy opposite to the direction of spontaneous heat flow. The natural direction of heat flow is from warmer places to colder places. In the case of a heat pump, heat absorbed from a cooler space is released to a warmer one. Heat pumps don’t create heat, they pick it up and move it from one place to another. In the case of a geothermal or ground-source heat pump, heat is transferred through the heat pump from the ground and into the home or building. A refrigerator works in much the same way and contains similar components: a compressor and heat exchangers. Essentially, these devices transfer heat energy by moving heat from one place and releasing it into another. A geothermal heat pump has been called a “refrigerator on steroids” because, due to its compressor and earth source, it transfers heat much more efficiently.
During the winter, heat in the ground is carried to the heat pump. Cooled water exits the heat pump and circulates through the underground piping loop. The warmer earth releases heat energy into the water as it passes back to the heat pump. How does it do this? Inside the heat pump is a heat exchanger. A heat exchanger is a device that allows heat from a fluid (an inert liquid or gas) to pass to a second fluid without the two fluids mixing. Heat, however, is exchanged by the fluids with the hot fluid cooling down and the cold fluid warming up. In this heat exchange or transfer process, the cooled incoming water is sent back to the ground loop to pick up more heat energy. Refrigerant in the heat exchanger accepts the heat energy and it becomes a gas. The gaseous refrigerant is then sucked into a compressor in the unit where it is compressed, concentrating the heat energy, and then distributed into the interior as warm, comfortable air. Depending on the heating system in the home or building, the heat is passed through an air coil or a hydronic coil and dispersed via radiators, baseboard or in-floor (radiant) heating tubes.
In warm weather a geothermal system absorbs heat from the home or building and transfers it to the underground loop where it is then absorbed by the cooler earth. The heat pump, which has a reversing valve controlled by a thermostat, now functions in an opposite mode: the hot compressor output is sent to the returning ground loop and the cooler water returning from the earth is used as a heat sink (heat absorbing medium) to help produce cool, dehumidified air distributed via ductwork.
Geothermal loop systems are either closed or open loop, meaning they either constantly recirculate water through sealed underground pipes during the operation of a heat pump or they simply return water back to its source, as in the case of a water well, aquifer or pond. Under these two categories there are four basic types of loop systems – horizontal, vertical or well/pond closed loop systems and open loop. There is also something called a “hybrid” type system as well.
Selection of one over another type of loop system depends on climate, soil conditions, land availability, access to a well or pond, local installation costs and regulations. Let’s examine each in turn.
Commonly employed for residential installations if sufficient land is available, a horizontal-closed loop involves digging trenches at least 4 ft. deep. Piping for the loop is laid down on the floor of the trench, with two pipes laid – for example, one at 6 ft. and the other at 4 ft. Two pipes can also be laid side-by-side at 5 ft. in a 2 ft. wide trench. Pipe length totals vary quite a bit for all closed loop systems depending on design variables, but horizontal-closed loop systems usually range between 400 to 600 ft. per ton.
A newer option and one that requires less trenching (hence digging) is the closed loop Slinky Coil installation. Slinky coils are flattened, overlapped plastic piping strung out in a trench in a circular coiled loop. A slinky loop concentrates the heat transfer surface and reduces the land use requirement significantly, but large amounts of piping are required.
Residential and commercial buildings (especially schools) often use vertical systems due to land restrictions. 4” or larger diameter holes are drilled about 20 ft. apart, somewhere between 100 to 400 ft. deep. Inserted into these holes are two pipes that are connected at the bottom with a U-bend that forms the loop. The vertical loops are the connected to a trenched horizontal manifold pipe that connects to the heat pump. Vertical-closed loop systems can cost more to drill. However, depending on soil and site conditions going vertical can be the best method.
Pond/Lake Closed Loop
Properties, residential or commercial, that have an adequate body water like a pond or front a lake can connect the geothermal loop from the home or building to the water source. A supply line pipe is run underground and into the water and coiled in circles deep enough under the water’s surface to be safe from freezing. This is a very economical method because minimum land excavation is required. Slinky-type coils of pipe are used and typically weighted down with cement blocks to hold the coils on the bottom.
An open-loop system extracts heat from water that comes from a well or surface body water and is circulated through the heat pump. Once the water circulates through the system it is returned to the ground through a recharge well, or through surface discharge depending on where it came from. The body of water used should be fresh water, and the open-loop system needs to conform to local codes and regulations. Open-loop systems have advantages: they have higher efficiencies than closed-loop systems due to the fact that water conducts heat better than earth does, there is minimal digging required and minimal use of piping. If water quality is poor, there are special heat exchangers and separators that can be used to ensure dependable performance.
Standing Well Column
A standing well column is a variation of an open-loop system using deep vertical wells where water is drawn from the base of the standing columns and then returned to the loop. A standing column well system can bleed a portion of return water to the aquifer during peak heating or cooling times rather than re-injecting all of it. This bleeding cools the column during heat rejection and heats it during heat extraction.
Geothermal heat pumps can be used with other HVAC technologies to produce a “hydbrid” system especially for big cooling needs. For example a ground-source heat pump can be teamed with outdoor air using a cooling tower or a dry cooler.
The efficiency of any device is determined by output divided by input. To get above 100% efficiency more energy is being supplied than used. Geothermal heat pumps accomplish this. They are marvels of efficiency, supplying as much as 3-5 times the energy they consume. In general, they are 300-600% efficient. Compare this to a gas furnace at 98% efficiency or a condensing boiler at 95% efficiency. Because of their super high efficiencies, geothermal systems can save up to 70% on energy bills.
Geothermal systems are very reliable and durable: System life is around 25 years on average for the heat pump. The ground loop is considered a part of the building’s permanent infrastructure, similar to a home’s plumbing system. And because the loop is underground and the heat pump is located inside a home or building, unlike an air/conditioner compressor, the system is safe from corrosion and damage from extreme weather events.
A geothermal system may be almost invisible but it comes with a number of very “tangible” benefits.
- Geothermal eliminates outdoor equipment which means it’s stormproof against damage or corrosion by weather and lasts longer.
- There is no use of fossil fuel sources on-site so less pollution to the environment results. Fossil fuel costs are therefore zero. And there is no need to be concerned about flames or related accidents occurring.
- There is also no freshwater consumption in operation.
- A geothermal system operates very quietly and provides consistent, comfortable heating and cooling.
- Geothermal systems enjoy lower maintenance costs because of the absence of oil filters, nozzles, storage tanks or chimneys
- Notably longer operational life of the system: 25 years on average for the heat pump and an essentially permanent underground loop.
- Geothermal can be used to produce domestic hot water and for pool heating.
- A home equipped with geothermal will likely see its value increased
- Geothermal systems are by far the most efficient system available; a geothermal system converts a small amount of electrical energy into a large amount of heating and cooling
- Installation of a geothermal heat pump qualifies for a 30% federal tax credit (through 2016 currently) as well as various state, local and utility rebates
- Geothermal combined with a solar PV (photovoltaic) array that produces free electricity combines two renewables in one almost perfect system. This combination can reduce electric energy use to near zero, a situation called “Zero Net Energy” wherein during occasions when the solar system generates more power than a home or building is consuming (with some consumption coming from the heat pump(s), excess electricity is returned (“sold back” in essence) to the utility grid.
Interested in learning more? Visit our "Resources" page for the list of educational references.
For an introduction to geothermal electric power generation, the other use of geothermal energy, click here.