Geothermal Heat Pumps Make the Grade

HVAC is critical for a healthy and comfortable learning environment, but it’s a big operating expense for school districts. Geothermal or ground source heat pumps (GHPs) are growing in popularity throughout the country because of their ability to provide highly efficient and environmentally friendly heating and cooling. GHPs can provide educational facilities with the following benefits:

Energy savings. GHPs have energy efficiencies of up to 300% or more, reducing energy costs. Improved humidity control. Geothermal systems provide excellent humidity control, helping maintain a healthy and comfortable indoor environment. Less maintenance. GHPs have few moving parts; this reduces maintenance needs and increases system durability. Quiet operation. Heat pumps produce less noise than conventional HVAC systems, helping to create a quieter learning environment. No onsite emissions. GHPs don’t burn fossil fuels to create heat, improving safety and indoor air quality.

How Geothermal Systems Work

A geothermal heat pump system takes advantage of the fact that the ground remains at a relatively constant temperature, typically from 45°F to 75°F, a few feet below the Earth’s surface. The geothermal heat pump pulls heat out of the ground in the winter and pumps it into the ground in summer. This is easier than extracting heat from colder winter air or pushing heat out into hotter summer air, as air-source heat pumps do.

A geothermal heat pump system consists of an underground heat exchanger, a pumping or circulation system and an internal evaporator/condenser. Topography and available land area are significant issues.

System Configurations

There are four different types of GHP configurations — closed-loop (vertical, horizontal and pond) and open-loop.

• Vertical closed-loop involves drilling deep boreholes (100 to 400 feet) to install U-shaped pipe loops, making them ideal for sites with limited land or shallow rock. Vertical systems are most commonly used for schools.
• Horizontal closed-closed loop systems have pipes buried in long, horizontal trenches, generally 4 to 6 feet deep. This is often the most cost-effective option for new construction where plenty of land is available.
• Pond closed-loop systems submerge pipes pipes directly into a nearby body of water that’s at least 8-feet deep. This is often the lowest cost option if available.
• Open-loop GHPs use well or surface water directly from the source, circulates it through the heat pump, and then returns it to a separate well or body of water.

Determining the loop length requires knowing the underground temperature and thermal conductivity at planned design depths. Shorter loop lengths result in the lowest installation cost. However, a longer length results in greater heat pump capacity and energy efficiency, for the lowest operating cost. Supplemental heat rejection can be accomplished by adding a cooling tower (hybrid GHP).

Passing the Test in Educational Facilities

Gloria Marshall Elementary School in Houston, which opened in 2011, was the first school in the city to install a geothermal heat pump system. A geothermal well field with a system of tubes and valves was installed below the parking space and playgrounds. The well field helps supply hot and cold water in and out of the facility. The geothermal system has saved the district 41% in energy consumption compared to the average elementary school energy usage.