Yet there is a way to provide a high level of comfort without taxing the budget or the staff. The geothermal heat pump systems can cut maintenance and energy costs in half.
How Geothermal Heat Pump Works
Geothermal heat pump systems rely on the temperature of the earth to heat and cool buildings. While there is a variety of designs, the most popular design relies on heat pumps and a closed-loop system of pipes laid under the ground or a body of water.
The efficiency and simplicity of the system is due to the closed-loop system that acts as a large heat exchanger. The loop is filled with a heat-transfer fluid, typically water or a water-antifreeze mix. When a facility requires cooling, heat is transferred from a building to the earth. When heating a facility, heat is absorbed from the earth and transferred to the building.
The system utilizes a heat pump that uses a refrigerant cycle to take low quality heat and concentrate it to high quality heat. Pumps and fans then move the heat.
The system is efficient and simple because it moves heat and does not create heat. In addition, while most mechanical cooling systems work under the same principles, geothermal systems gain efficiency because of the size of their primary heat exchanger — the loop.
At the heart of the process are two thermodynamic principles: Heat always flows from hot to cold, and the greater the difference in temperatures, the higher the rate of heat transfer.
Six to eight feet beneath the surface of the earth, the ground maintains a constant annual temperature from 45 in the north latitudes to 70 degrees in the south. Because the earth’s temperature is stable and the ambient air temperature is not, there is a seasonal differential in most climates that makes the geo-thermal process efficient.
Cost Savings
The relative simplicity of the systems is in part what makes them cost effective. Geothermal heat pump systems do not rely on boilers and cooling towers. Since reversing-cycle technology is deployed at the point of use, there are only two pipes to provide heating and cooling, not four like most conventional systems. And there are no large chillers in the mechanical room or an array of packaged units on the roof exposed to the elements and creating potential problems for the roofing system. All the critical equipment, except the ground loop — pumps, scroll compressors and fans — are inside the building, protected from the elements.
The heat pumps are generally small, which makes them more flexible in terms of design. While some manufacturers will produce 50-ton capacity pumps, most run from 2-10 tons for commercial and institutional markets. To achieve a higher tonnage capacity, multiple heat pumps are linked together. The heat pumps can be fixed in a zone distribution network that allows for point-of-use heating or cooling.
Cost Matters
There are other savings, too. Most of the piping in these systems is polyethylene plastic piping, like gas-line pipe, rather than copper or ferrous-based piping. Ductwork generally is not a part of the installation except for point of use distribution and fresh-air delivery.
There are also maintenance and operational savings. According to the American Society of Heating, Refrigeration and Air-conditioning Engineers (ASHRAE), the annual maintenance cost for a conventional boiler, chiller, cooling tower system is 65 cents per square foot. For a geothermal system that cost is 22-25 cents per square foot. The cost is lower because there is less equipment that needs attention.
Of the maintenance tasks necessary, changing filters was the highest. Less than half as many tasks involved lubrication and checking fan belts, the study notes. There are no water quality problems to worry about. You don’t have to clean or treat a cooling tower. There’s virtually no maintenance to the heat pump. About the only cost is to change filters and maybe check on the circulation pumps’ packings and on the fan belts.
The heat pumps themselves come as self-contained units that are tested at the factory. Even the refrigerant is in a sealed system that requires no checking.
Technology Evolves
Geothermal heat pump technology is energy efficient, offering energy costs about 25-50 percent less than those for a conventional system. Recent changes in the technology have made a significant difference.
The biggest change came when the industry switched from reciprocating compressors to scroll compressors. There was a sizable increase in efficiency there. Manufacturers also started using variable speed fans and compressors. There also have been improvements in reversing and expansion valves in pumps and evaporative coils.
Geothermal heat pump systems have lifespans of 22 years or more and in-ground piping can last 50 years or more. As a result of these and other changes, efficiency has improved. The coefficient of performance for these systems went from 3 to 4.5, and SEER ratings from 13-14 to 17-18. Additional changes in the technology and installation has made these systems better life-cycle investments. For instance, much of the joining of the ground pipe is done in the factory under controlled conditions, rather than in the field. When joints must be made in the field, the tools to do that have been improved. Leaks in the loop are practically unheard of.
Life-Cycle-Cost Advantage
Because a significant portion of the cost of these systems is in the installation of the buried loop, the costs for the whole system begin to decrease as the system gets bigger. Fifty tons is about the breaking point. At that point, the cost of digging more trenches or drilling more wells doesn’t continue to increase at the same rate because the contractor has already brought in his equipment and the work crews are at the site.
The first cost of the system depends on many local variables. Is there competition in the area for geothermal systems? Are well drillers readily available? What is the soil and geology of the area? These systems can more than pay for themselves over the years, avoiding costs on maintenance and roof repairs, and because equipment, principally the loop system, lasts so long, the energy efficiency is really the icing on the cake.
