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Simple, Reliable and Efficient: Geothermal Heat Pump Systems
Whether you’re in charge of facilities at a small liberal arts college in the Northeast or a large luxury resort in Phoenix, a ground-source geothermal heat pump system may be the most energy efficient answer to your heating and cooling problems.
With technology no more sophisticated than a household refrigerator, relatively few breakable parts, and low operating costs, the only problem is the initial investment, which is typically significantly higher than conventional commercial HVAC systems. For those who can weather a longer payback period, however, the heat pump system can serve as a reliable and more environmentally sound investment.
Geothermal heat pump systems use the relatively stable subsurface temperature of the ground or ground water to transfer heat to the earth or extract heat from it. The greater the difference between the air and the ground temperature, the more efficient the system.
Geothermal heat pump system operating costs are lower than conventional systems, beginning with a 20 to 60 percent energy cost savings.
The heat pump technology itself is reliable and has a long life. The pumps last 20 to 30 years and require no more maintenance than a residential refrigerator might. The loop field requires no maintenance. The major maintenance cost in the system is in replacing of filters.
First costs, however, are significantly higher than conventional systems, and paybacks can range from 5 to 12 years.
The cost of the building components of the system is generally comparable to conventional systems. While there are numerous pumps and pipes in a geothermal system, there is no need for complicated temperature control systems and large high-volume fans and ductwork. Fresh air can be supplied with smaller, low-pressure systems. The heat pump costs about $800 to $1,000 per ton.
The most critical factor in determining whether a geothermal heat pump system is cost effective is the load. If there is a good balance between heating and cooling, the systems can operate cost effectively. A more cooling-dominated building can see even greater energy-cost savings. One reason is that typical HVAC systems rely on electricity to cool. Given that geothermal systems are a more efficient cooling technology, even at peak demand times, they can offer cost savings in cooling conditions compared to conventional systems.
Generally speaking, according to the U.S. Department of Energy, the cost of installing a geothermal heat pump compared to other systems is as follows: Gas heat with electric cooling — $1,700 per ton; open-loop system — $2,300 per ton; and a closed loop system — $3,000 per ton.
Cost varies significantly, depending on the experience of the designer and local costs for drilling or trenching, says John Kelly, executive director of the Geothermal Heat Pump Consortium.
Owners and utilities like the systems because they are simple and more efficient, says Greg Lampman, project manager with New York State Energy and Research Development Authority (NYSERDA), an organization that works with owners of geothermal systems in New York. That’s especially true at peak demand times, because geothermal systems don’t have to work as hard as conventional systems to dump heat into the air during the hottest time of the day.
Inside the building, geothermal systems take up less space than conventional HVAC systems. A geothermal system provides heating and cooling, eliminating the need in the building for two different systems. They are typically two-pipe systems. They do not require large central air-handling systems. They are unitary appliances that can be placed close to or within the zone to be conditioned with each zone controlled by its own thermostat. And heat from one side of the building can be extracted and moved to another side of the building, allowing for efficient simultaneous heating and cooling without reheats.
There are two types of ground-source geothermal systems, open-loop and closed-loop.
Open-loop systems use a body of surface water or ground water as a ground couple or heat sink. Open-loop systems draw water up one set of pipes for heating and cooling and discharge it through another set. These systems are more unusual, but in certain circumstances, more effective.
For instance, in some urban areas, such as New York City, an open loop system called the standing column system works well. The bedrock, the available ground water and the lack of space all make the systems effective. Deep wells are drilled and a single open pipe is installed for drawing in or discharging water. One 1,500 foot well can accommodate up to 40 tons of cooling load. A ton of heating or cooling can usually meet the needs of 400 to 1,000 square feet of space. The heating and cooling load drives the number of wells.
The more common system is the closed-loop system, which is comprised of a continuous loop of vertical or horizontal pipes placed in the ground with a liquid circulating through them. In vertical closed-loop systems, holes of 300 feet or more are drilled into the earth. A U-shaped pipe is inserted into each well through which water, usually mixed with a food-grade glycol, is circulated. A special grout fills the space in the bore hole and around the pipes. The grout not only permits efficient transfer of heat but also seals the system from contaminating the soil in the unlikely event a leak develops. Typical systems require 150 to 200 feet of bore per ton of peak load.
The horizontal closed-loop system is comprised of loops of pipes laid down 6 to 10 feet underground in trenches 2 to 4 feet apart. Because the standard system requires one acre of loop field per 24 tons of load, horizontal systems are usually used for smaller buildings.
For large, cooling-dominated commercial buildings, rising loop-field temperatures can be a concern in the summer but not necessarily a problem. Peak loop-field temperatures lag about two months behind peak atmospheric temperature. So on those unusually warm late season days, return water can be 90 degrees. But even when the air temperature is 95 degrees, properly designed systems can bring supply air temperature to 65 degrees.
It may be necessary, says Jim Bose, executive director for the International Heat Pump Association, to cool down the loop field by transferring some heat to domestic purposes or dissipate it in a fountain or other open water source.
Additionally, loop-field installation can raise the first cost of the entire system by as much as 45 percent. For vertical closed loop systems, costs for the loop field can vary from $1,200 to $2,000 per ton depending on the drilling conditions and the size of the system. For horizontal systems, the cost can vary from $800 to $1,500 per ton.
An owner will pay more for a geothermal heat pump system on a first cost basis, Kelly says, but the system will work effectively and efficiently — and quietly — creating opportunities for savings down the road.
David Kozlowski is former senior editor for Building Operating Management magazine.