Sustainable energy under our feet?
Buildings are the largest source of energy consumption and greenhouse-gas emissions.
The American Institute of Architects is promoting goals of 50 percent carbon-emission reduction by 2010 and carbon-neutral buildings by 2030. To get there, buildings need to be designed to use sustainable energy. One sustainable-energy source with great potential is geothermal power — energy generated from heat stored in the Earth. Installations have been made in schools of all sizes:
In 1987, a 287,000-square-foot Minnesota high school was designed with a closed-loop geothermal system. Its 285 wells circulated a water/glycol mixture between deep wells into the building's heat pumps. Galvanic corrosion occurred between dissimilar pipe metals in the closed-loop system inside the building when the percentage of glycol in the water/glycol mixture dropped below the recommended levels. The contractor failed to install dielectric isolators between dissimilar pipe metals.
A partial geothermal system was installed in a 21,000-square-foot addition in 2008 using a conventional variable air volume (VAV) with standard air-handling units (AHU), four-pipe heating-cooling system, air-side heat recovery, water-to-water heat pumps, and a geothermal well field. This configuration can operate air-side economizers, and it can use the well water to cool the building directly when the ground temperature and indoor humidity allow, thus giving two sources of free cooling. When outside temperatures are cool and air-side economizers on, AHUs provide cooling without any compressors running; when the well return temperature is cool enough, the sensible cooling mode provides air conditioning without compressors operating.
Geothermal heat pumps (GHP) use 25 to 50 percent less electricity than conventional heating or cooling systems, the U.S. Department of Energy says. Geothermal heat pumps can reduce energy consumption — and corresponding emissions — up to 44 percent compared with air-source heat pumps and up to 72 percent compared with electric resistance heating using standard air-conditioning equipment.
One disadvantage has been the cost of large well systems; however, fuel costs and environmental concerns have increased interest in alternative fuels:
Centralizing equipment within the boiler room reduces long-term operations and maintenance costs.
Redesigning well points for circulating the water into the Earth have become more efficient, thus significantly decreasing the number of wells and initial costs.
A pond at least 15 feet deep with heat-exchanger plates on the bottom eliminates the need for drilling and maintaining a well field.
Geothermal saves energy and is renewable: 1 kilowatt of energy used results in 3 kilowatts of energy.