Advantages of GHPs for Schools

Introduction

Schools are an ideal application for geothermal heat pump technology. They have diverse heating and cooling needs, with conditioned spaces ranging in size from 700-800 square feet for a typical classroom, up to thousands of square feet for a gymnasium, auditorium, or cafeteria. Schools also have large unbuilt land areas -- playgrounds and athletic fields -- under which horizontal or vertical ground loops can be readily installed. Parking lots are potential sites for vertical ground loop installations.

Another attractive feature for schools is the inherent flexibility of GHP systems, which allows different temperature settings in different parts of a building based on changing occupancy and activity levels, as well as changing exposure to the sun throughout the day. This benefit becomes increasingly important as schools evolve from single-use buildings closed at the end of the school day into year-round community facilities with evening and weekend activities scattered throughout the building. The inherent flexibility of a GHP system ensures the comfort of occupied spaces while avoiding wasteful heating and cooling of unoccupied building areas.

This page describes specific advantages of GHP systems for K-12 schools and other educational institutions, in the following six categories:

Energy Benefits

For space heating, the U.S. Environmental Protection Agency has found that on a source fuel basis, accounting for all energy losses in power plant generation and utility grid transmission, GHP systems have 40% greater efficiency than air source heat pumps, nearly 50% greater efficiency than the best gas-fired furnaces, and 75% greater efficiency than oil-fired furnaces. Likewise, for space cooling, GHP systems are 30-50% more efficient than central chilled-water variable-air-volume (VAV) systems or direct-expansion rooftop units, as shown below.

Figure 1.  Electric power consumption (kW/ton) of different space cooling systems.

Kentucky Utilities commissioned studies by three engineering firms to model the life-cycle heating and cooling costs of various HVAC systems. Even when the GHP alternatives cost more to install than their conventional counterparts, the energy and maintenance savings yielded 12-19% percent life cycle savings over 20 years. The Kentucky study also compared the annual energy costs for 24 schools with different HVAC systems,and the results are plotted below.

Figure 2.  Annual energy costs of different HVAC systems in Kentucky schools.

Compared to boiler/tower systems with water loop heat pumps (WLHPs), the GHP systems yielded an annual energy savings of 16 to 26 cents per square foot of conditioned floor area. Similar energy savings were realized when compared with two-pipe unit ventilator systems, and more than 50¢ per square foot were saved in comparison with a school having a four-pipe system.

Moreover, GHP systems will continue to reliably save energy and shave peak electric loads throughout their service life, unlike conventional direct-expansion cooling systems or air source heat pumps, whose efficiency deteriorates with time. A California Energy Commission study has shown that due to weathering and contamination by pollutants and airborne dust, the standard plate-fin heat exchanger in outdoor coil equipment looses 27% of its original efficiency over the first eighteen months after installation. Thus, a 10-ton air-source heat pump effectively becomes a 7.5-ton system within two years of its installation.

GHP systems not only retain their energy benefits over time but also over all load conditions. Indeed, the part-load efficiency of a GHP system is typically better than its full-load efficiency, because the temperature of the water solution in the piping is closer to the ground temperature. This means that under part-load condtions, the water entering the heat pump units will typically be lower in the cooling mode and higher in the heating mode when compared to full-load conditions, such that the heat pump compressors do not have to work as hard.

Equipment Benefits

Once the HDPE ground-loop piping has been installed and pressure-tested, it requires no maintenance and will have a service life of at least 30-40 years. The mechanical components of a GHP system -- the water loop circulation pumps and the building heat pump units -- do require periodic maintenance, but unlike cooling towers, air-source heat pumps, or direct expansion rooftop units, geothermal mechanical components are not subject to deterioration caused by exposure to the weather. As already noted above, this enables GHP systems to retain their energy benefits and prolongs the life of the heat pumps, meaning less frequent replacement of mechanical components, reducing maintenance costs and minimizing service disruption.

Building Design Benefits

Figure 3.  Award-winning Clayton Elementary School, with clerestory windows for increased daylighting (Johnston County, North Carolina).

The selection of a GHP system over conventional alternatives can significantly benefit other building elements, enabling the use of innovative, high-performance design features. With no rooftop HVAC equipment for example, schools can more readily accomodate sloped roofs and clerestories to admit daylighting, which provides additional energy benefits and increases student performance, as has been demonstrated in North Carolina's daylit schools.

Figure 4.  Rooftop HVAC units increase building costs and limit innovative design possibilities.

GHP systems also can meet the challenge of upgrading older buildings to meet today’s ventilation requirements and other indoor air quality standards by enabling modernization without impacting outside appearance or structural design. For historic buildings, this can speed up the permitting and approval process.

With GHP systems, building structural designs can be less costly, since the need for heavy rooftop equipment is eliminated. With no roof penetrations to secure such equipment or admit piping or ductwork, and with no HVAC servicemen on the roof, the need for roof repairs also is reduced.

Finally, use of a GHP system results in more usable building space per construction dollar. Mechanical equipment rooms for GHP circulating pumps are much smaller than the rooms required for central heating and cooling plants, about one-third the size of a traditional boiler room. Furthermore, because water pipes are volumetrically more efficent than air ducts in distributing heat energy throughout a building, floor-to-roof (or floor-to-floor) heights can be reduced significantly. In one 180,000 square-foot Toronto school, these building construction cost savings amounted to more than $5 per square foot.

Health and Safety Benefits

Since geothermal heating is not combustion-based, GHP systems have none of the safety concerns associated with delivery and storage of fuel. Poor design or improper installation of a furnace or boiler, or accidental blockage of exhaust flues can cause carbon monoxide build-up, which is not a risk with geothermal heat pumps.

Figure 4.  Ground-mounted HVAC equipment, like this cooling tower, is potentially a safety hazard.

As previously mentioned, GHP systems do not have outdoor equipment, with all mechanical components located indoors and the ground loop buried or submerged out of sight. There are thus no unsafe structures or equipment that children might be tempted to explore or climb. Likewise, GHP components are not exposed to potential theft or vandalism. These features can possibly yield lower insurance premiums for both liability and equipment coverage.

Another health benefit of geothermal heat pumps is their inherent humidity control capability using hot gas reheat (HGRH), an optional feature offered on many water-source and geothermal heat pump models. The HGRH option increases indoor air quality by providing dehumidification on demand without excessively cooling the conditioned space.

In HGRH mode, the normal heat pump cooling operation converts to an air drying operation by directing high-pressure refrigerant gas from the compressor into a reheat valve, for passage through a separate reheat coil. By reheating the leaving air along a constant sensible heat line, its relative humidity is reduced. The HGRH humidity control setting on each heat pump unit can be individually adjusted from space to space, enabling greater dehumidification in spaces that need it, such as gymnasium locker rooms, indoor swimming pools, computer rooms, and in heat pump units dedicated to conditioning outdoor make-up air for meeting school ventilation requirements.

Environmental Benefits

Because they are so energy efficient, GHP systems have the least atmospheric pollution of any available heating or cooling equipment. Computer modeling of a 350-ton system for a middle school in New Jersey indicates that compared with the emissions associated with natural gas heating and electric air conditioning, a GHP system would reduce atmospheric carbon dioxide emissions by 30-40%, depending on the efficiency of the geothermal heat pump units.

The use of GHP systems also reduces the "heat island effect" of cities and densely developed suburbs. Conventional air conditioning equipment that reject heat to the air (which includes central chillers, direct expansion rooftop units, and air source heat pumps) can significantly raise ambient outdoor temperatures in their immediate vicinity. This reduces the effective capacity and energy efficiency of these systems, requiring even larger-capacity equipment to meet the building cooling load in a given climate. Since GHP systems reject heat directly into the ground or water, they do not add to the heat island effect.

Scholastic Benefits

Installation of a GHP system conveys two scholastic benefits to an educational institution. First, by providing a comfortable indoor environment throughout the school day, at all times of the year, occupants are not distracted by excessively hot or chilly room temperatures and can focus on teaching and learning. Second, a GHP system shows students how natural energy flows can be harnessed to provide sustainable heating and cooling of the building in which they work and play.

Indeed, both aspects can be combined to illustrate an important distinction between energy conservation and energy efficiency. A school can conserve heating energy by lowering thermostat settings, which reduces comfort. Alternatively, it can use energy more efficiently by installing a GHP system, which lowers energy consumption, but which at the same time improves comfort.