Building Green: Geothermal Energy for Homes

Eco-Friendly Green Houses and Construction

Studies suggest that consumers are increasingly interested in purchasing green homes or homes with green features. For example, the National Association of REALTORS®1) surveyed its members about sustainability issues. Roughly, 71% of respondents said that promotion of energy efficiency in listings was very or somewhat valuable. Approximately, 56% of them found that clients were at least somewhat interested in sustainability.

Similarly, the National Association of Home Builders2) in partnership with Dodge Data & Analytics surveyed single-family and multifamily home builders about their green building activities. About 33% of respondents reported that green building was more than 60% of their portfolios. By 2022, this number is projected to increase to nearly 50% in both the single-family and multifamily sectors. Among this group, almost 30% of multifamily builders are 'dedicated' green builders (green building accounts for more than 90% of their portfolios). Nearly, 20% of single-family builders were categorized as 'dedicated' green builders, but this group is expected to grow considerably by 2022.

Energy efficiency3) is a primary element of green home design. Renewable energy sources like geothermal energy save money, decrease demand on conventional power sources and reduce greenhouse gas emissions.

  • Facts About Geothermal Energy
  • What is geothermal energy?
Hot springs heat rising
Heat rising by means of geothermal energy under the earth

Geothermal energy4) is obtained from the fluids and rocks that lie beneath the earth's crust. It is located from shallow ground to several miles beneath the surface to the hot molten rock known as magma. Some countries have used geothermal energy for thousands of years for cooking and heating.

How is geothermal energy harnessed?

Geothermal energy is harnessed5) in three ways: geothermal power plants, direct geothermal energy, and geothermal heat pumps.

Geothermal Power Plants

Pipes can be placed deep underground to transport hot water and steam up through underground wells to generate electricity in a power plant. There are three types of geothermal power plants:

Binary Cycle Plants
Moderately hot geothermal water flows through a heat exchanger. Its heat is transferred into a liquid with a boiling point lower than water. The liquid is heated until it becomes steam that spins turbines to generate electricity.

Dry Steam Plants
Hot steam travels directly from geothermal reservoirs through pipes into generators at the power plant. The steam spins turbines to produce electricity.

Flash Steam Plants
Water between 300 and 700 degrees Fahrenheit is piped up through a well. Some of the water is turned into steam that spins the turbines to create electricity. When the steam cools and condenses into water, it is piped back into the ground.

Direct Geothermal Energy

Some areas have geothermal reservoirs or hot springs located near the earth's surface. This water can be piped up and pumped through a heat exchanger that transfers its heat into a building's heating system. The used water is returned to a well that leads to the reservoir where it is reheated and reused.

Geothermal Heat Pumps

Geothermal energy heating pipes
Geothermal energy heating pipes

The soil and water located a few feet underground remains at 50 to 60 degrees Fahrenheit year-round. This is enough warmth to heat and cool buildings.

A geothermal heat pump system uses a loop (which is a series of pipes) to circulate fluid. The loop is placed beneath the water of a lake or pond or under the ground. A heat exchanger and an electric compressor take the heat from the pipes and distribute it via a duct system through the building. During the summer months, this process is reversed to cool the building. The pipes remove heat from the building and transport it to an outdoor water source or the ground where it is absorbed.

How widespread is the use of geothermal energy?

According to National Geographic, more than 20 countries generate geothermal energy. The United States produces the largest amount of this energy. The Geysers6), which is about 70 miles north of San Francisco, is the largest geothermal power plant worldwide.

  • Geothermal Heating and Cooling for Homes
  • Geothermal Heat Pump Systems

There are four7) basic types of geothermal heat pump systems. Three of these––horizontal, pond/lake, and vertical––are closed–loop systems. The fourth is an open–loop system. The best type of system for a home depends on the available land, climate, installation costs, and soil conditions.

Closed–Loop Systems

Closed–loop geothermal heat pumps circulate antifreeze through a closed loop (typically made of plastic tubing) that is submerged in water or buried underground. A heat exchanger transfers heat between the antifreeze in the closed loop and refrigerant in the heat pump.

Direct exchange is a variation of this system. Instead of using a heat exchanger, the refrigerant is pumped through copper tubing buried underground in a horizontal or vertical configuration. Direct exchange systems need larger compressors. They are most suitable for moist soils, but they should not be used in soils that will corrode the copper tubing. These systems circulate refrigerant through the ground so local environmental laws may prohibit their use.

The loop can have a horizontal, pond/lake, or vertical configuration.

Horizontal Configuration

Horizontal installation is typically the most cost-effective for residential use, especially for new construction with adequate land. Trenches should be at least four feet deep. The most common layouts are (a) two pipes with one buried at four feet and the other at six feet or (b) two pipes placed next to each other five feet below ground in a two-foot wide trench.

The Slinky™ method makes it possible to install more pipes in a shorter trench. This decreases installation costs and allows for horizontal installation in areas that are not sufficient for conventional horizontal applications.

Pond/Lake Configuration

House by the lake warmed by geothermal heating
House by the lake warmed by geothermal heating

At sites with adequate bodies of water, the pond/lake system may be the most economical option. A supply line pipe is installed in the ground from the home to the water source. It is then coiled into circles that should be at least eight feet underground to prevent freezing. The water source must meet minimum depth, quality, and volume criteria to accommodate the system.

Vertical Configuration

Installing a vertical system involves drilling holes (about four inches in diameter) 20 feet apart and 100 to 400 feet deep. Two pipes are inserted in each hole. U–bends are used to connect them at the bottom to form loops. A horizontal pipe (i.e., a manifold) connects the vertical loops, which are placed in trenches and connected to the heat pump inside the home.

Open–Loop System

An open–loop system uses surface body or well water as the heat exchange liquid that circulates through the geothermal piping. After the water circulates through the system, it is returned to the ground through the well, surface discharge, or a recharge well.

This option is only practical in areas that have sufficient supplies of relatively clean water and all local regulations concerning groundwater discharge can be met.

Desuperheaters

A device known as a desuperheater8) can be attached to a geothermal heat pump to heat water for a home. It is a small, secondary heat exchanger that makes use of the superheated gases from the heat pump's compressor to create hot water. The water is then transported through a pipe into the home's water heater tank.

Desuperheaters can also be used with tankless water heaters. This type of water heater9) heats water directly and does not use a storage tank. After the hot water tap is turned on, cold water is transported into the unit via a pipe. Then an electric element or a gas burner heats the water. It delivers a continuous supply of hot water, but its output limits the flow rate.

During the summer, the desuperheater uses the surplus heat that would have been released into the ground. Therefore, a geothermal heat pump that runs frequently to cool a home, can heat all of its water.

Building Green Course

RealEstateCE.com's Building Green course provides more information about renewal energy sources as well as other green building principles and practices.  Course currently available in: Alabama, Connecticut, Georgia, Illinois, Indiana, Iowa, Kansas, Kentucky, Michigan, Missouri, Montana, Nebraska, Nevada, New Jersey, New York, Ohio, Oregon, Pennsylvania, Rhode Island, South Dakota, Tennessee, Texas, Utah, Virginia, Washington State, Wyoming.

Costs for Geothermal Heating and Cooling Systems

According to Home Advisor10), the national average installation cost for a geothermal heating and cooling system is $7,107. Most homeowners spend between $3,506 and $12,117 for their systems. However, equipment and excavation costs can cause total installation costs to reach more than $20,000. Geothermal HVAC systems are becoming more popular because tax incentives and improvements in technology are lowering purchase and installation costs.

  • Costs
  • Heat Pumps

Heat pumps cost11) on average about $2,500 per ton of capacity. The average home requires approximately three tons of capacity, so the unit would cost $7,500. A ton is a measure of the amount of heat that can be moved in an hour. One ton is the equivalent of about 12,000 BTUs (British Thermal Units).

The central heat pump unit accounts for most of the maintenance and repair costs, with an average cost of $350. Costs can, however, range from $50 for basic maintenance to as much as $1,700 for a new compressor.

Desuperheaters

Vendors of heat pumps may offer desuperheater options for an additional fee. For example, Heat Pump Suppliers12) provides a desuperheater option for $425 plus $195 for the desuperheater pump. Others include the price of the desuperheater in the total cost of the heat pump system. For example, Ingram's Water and Air Equipment13) sells a five-ton geothermal heat pump with a desuperheater for $5,069.

Horizontal, Pond, and Vertical Ground Looping

The average home needs about 400 to 600 feet of looping per ton of capacity. Since the average home needs about 3 tons, it would require 1,200 to 1,800 feet of looping (on average about 1,500 feet). Costs depend on the configuration:

  • Looping for a horizontal configuration costs about $800 per ton for a total cost of $2,400.
  • Looping costs for pond configurations vary depending on the location of the pond, but $800 per ton is an approximate cost ($2,400 total).
  • Looping for a vertical configuration costs approximately $1,500 per ton for a total cost of $4,500.

Advantages and Disadvantages of Geothermal Heating and Cooling

Geothermal energy has three primary advantages and disadvantages.

Advantages

Geothermal energy home infographic
Infographic of typical single-family home energy usage breakdown

It pays for itself.
Geothermal energy can reduce utility costs by 40% to 60% on average. Depending on location, the installation cost can pay for itself in approximately three years.

It has low maintenance costs.
Geothermal heating and cooling is a relatively simple and an old technology, so there are less repair and maintenance costs. Homeowners save an estimated $500 annually on maintenance costs.

It is environmentally responsible.
The greenhouse gas emissions for geothermal heating and cooling systems are 55% to 60% lower than air exchange systems.

Disadvantages

  • Geothermal heating and cooling systems have high installation costs14).
  • Installation may not be possible for housing units in cities because of small lots and incompatibility with existing heating equipment like radiators.
  • Geothermal energy is not a 100% clean energy source because the heat pump may use electricity derived from coal. Coal is not an "ecologically acceptable fuel" because it releases carbon dioxide.

Geothermal energy meets three key principles of sustainable design: eco–friendly, economical (over time), and efficient. This makes it a viable source of renewable energy for newly constructed and even older homes.

 

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