energy-savings

Geothermal Heating and Cooling: Tops in Energy Efficiency

Published on: August 5, 2013

The principle behind geothermal heating and cooling becomes clear if you’ve ever ventured into a cave on a summer day. No matter how hot it is up at ground level, the underground temperature maintains a steady level of around 55 degrees. Come back on the coldest day of the winter and you’ll discover the same phenomenon. Underground temperatures remain remarkably constant year-round.

In the United States we experience winters as low as 70 degrees below zero in Montana and summer extremes that frequently climb above 125 degrees in Death Valley. Yet, only six feet below ground, temperatures in all 48 states average between 50 to 75 degrees in all seasons. This is latent heat energy, absorbed from the sun and permanently resident in the soil and rock beneath your feet. It’s the driving force behind a geothermal heating and cooling system.

A common air-source heat pump works by moving heat from one place to another. In winter, it extracts latent heat energy in the air, concentrates it in the compressor cycle and moves it indoors to warm the home. In summer, the process reverses and heat is absorbed from inside the house and transferred outside where it’s returned to outdoor air. In a geothermal heating and cooling system, the theory’s the same, but both the source of the latent heat in winter and the place where it’s dispersed in summer is the ground instead of the air. Right upfront, several advantages that distinguish geothermal heating and cooling from conventional systems are evident:

  • Because the supply is unlimited and unvarying, geothermal energy is not in short supply or subject to changes in political policies or regulation.
  • Nobody can put a meter on the planet’s latent heat, so the underground source of geothermal heating and cooling is and always will be free.
  • No combustion is involved so carbon emissions on-site are zero. Potential hazards from byproducts such as dangerous carbon monoxide are totally eliminated.
  • Unlike a standard central air conditioner or heat pump, a geothermal heat pump has no outdoor coil and compressor unit exposed to the elements and subject to damage. This means lower maintenance costs.

Energy Savings

The above advantages of geothermal are just the prelude to the main event: energy efficiency. All sources of heating and cooling use energy to either produce heat or extract heat in some ratio to the amount of energy consumed. The efficiency of the heating phase of a geothermal heat pump is expressed by the unit’s coefficient of performance (COP) while it’s cooling efficiency is represented by the EER (Energy Efficiency Ratio).  

 

  • COP: This is the ratio of generated heat to energy consumed. With geothermal energy, you actually get out more than you put in: A geothermal system uses about 1 unit of electricity for every 5 units of heat energy in BTUs that it extracts from the ground, resulting in a COP of approximately 5 or an efficiency of 400 percent. Combustion systems, on the other hand, always lose some heat energy in hot combustion gases exhausted up the vent flue. In typical gas-fired furnaces, for every 1 unit of energy consumed only about 0.75 to 0.90 of a unit of heat is delivered by the system—an efficiency range of 75 percent up to about 90 percent maximum.
  • EER: This calculates the amount of heat energy extracted from a building in BTUs in ratio to the amount of energy consumed. A standard efficiency air-source heat pump has an EER of just over 11. However, geothermal heat pumps average EER ratings that exceed 20. This means that for every 1 kilowatt of electricity consumed, the system extracts 20,000 BTUs of heat energy from the home.

How a Geothermal System Works

While the concept of harvesting heat from the earth and dispersing it back into the ground may sound complex, the equipment is surprisingly simple.  A typical residential geothermal system consists of these elements:

  • Ground heat exchanger: Heat is exchanged with the earth through long loops of plastic tubing buried in the ground. This tubing circulates a water and glycol solution similar to anti-freeze that absorbs ground heat efficiently. The ground heat exchanger may be installed in either a horizontal configuration, utilizing rows of trenches about four feet in depth, or the tubing may be placed vertically in deep bores that extend approximately 200 to 300 feet into the ground. The latter arrangement saves space and makes geothermal possible on lots that don’t have sufficient vacant area for a horizontal installation. The total length of the tubing in the ground heat exchanger depends upon variables such as the square footage and overall energy efficiency of the home, as well as the type of soil on your property. Homeowners are often initially concerned about the integrity of plastic tubing that will be buried for years. However, these systems are proven to be long-lasting and reliable. Manufacturers of ground heat exchanger tubing commonly warranty the product for 50 years against degradation and leaks.
  • Hydronic pumps: Located in the indoor equipment cabinet, these pumps circulate the heat-absorbing fluid through the ground heat exchanger and back to the heat pump. During summer, the pumps are circulating hot fluid out to the buried tubing to disperse heat extracted from the home into the ground. In winter they are returning fluid warmed by the earth to heat the home.
  • Fluid heat exchanger: Heat energy is transferred back and forth between the ground heat-exchanging fluid and the refrigerant in the heat pump at the fluid heat exchanger. Which way it goes, depends upon the season. In winter, the heat-exchange fluid from the buried tubing transmits warmth into the refrigerant. During summer, heat conveyed by the refrigerant from the house is transferred into the heat-exchange fluid.
  • Compressor: As in any standard heat pump, vaporous refrigerant is pressurized to concentrate the molecules of heat energy and prepare it to condense into liquid. In cooling mode, after household heat is absorbed at the indoor coil the refrigerant is compressed and conveyed to the fluid heat exchanger where it condenses and transfers heat energy into the fluid circulating through the buried tubing loops. In heating mode, after absorbing ground heat from the fluid, the refrigerant is compressed and routed to the indoor coil where it condenses and releases heat energy to be dispersed through the ductwork into the home.
  • Ancillary equipment: During summer, returning all that indoor heat to the ground can be a waste of useful energy. A device in a geothermal heat pump called a desuperheater can put it to good use heating household water. Prior to entering the fluid heat exchanger to be conveyed out to the buried tubing, the fluid passes through the desuperheater that extracts the energy to pre-heat domestic water in an insulated storage tank. By raising the temperature of water before it enters the hot water heater, less energy is required to reach normal thermostat settings. In most cases, during summer the desuperheater alone can supply all of a household’s hot water requirements without need for the gas-fired heater. During winter in most climates, the desuperheater can reduce water heating costs by about half.

Green Benefits

Studies by the U.S. Department of Energy reveal that almost 40 percent of all carbon dioxide emissions in the United States emanate from heating and cooling residences and commercial buildings as well as making hot water. This is approximately the same carbon footprint created by all the automobiles and public transportation on the road today. Use of geothermal heating and cooling could reduce the amount of energy consumed by 70 percent, contributing to a commensurate decline in carbon emissions. Since geothermal systems don’t combust fossil fuels in order to generate heat, they contribute far less greenhouse gases than a standard gas- or oil-fired furnace. An average 3-ton home geothermal heating and cooling system is responsible for about one less pound of CO2 per every hour of use than a conventional HVAC system. For every 100,000 residences that adopt geothermal, over a million metric tons of carbon emissions will be eliminated during the average service life of those units. That’s equal to taking almost 60,000 cars off the road or planting over 120,000 acres of trees. In addition, the enhanced energy efficiency of geothermal systems will save homeowners approximately $500 million in energy costs during that same time frame.  

A Tax Break, Too

Though government isn’t always quick to recognize advancements in technology, legislators have embraced geothermal heating and cooling and given it a vote of confidence in the form of federal tax credits. The Energy Improvement and Extension Act of 2008 (H.R. 1424) offers a tax credit of 30 percent of the cost of purchasing and installing a residential geothermal system. The credit expires at the end of 2016, but is retroactive to include installations after December 31, 2007.

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