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Geothermal Heat Pump Systems
Geothermal heat pump systems are gaining prominence because of the move toward more efficient, sustainable building practices and the rapidly rising costs of fuel. Their principle of operation is simple; the earth’s relatively constant ground temperature serves as a heat source in the winter and a heat sink in the summer. And, coupled with heat pump technology, they produce warm air or hot water in the winter or cool air or chilled water in the summer.
There are two basic categories of geothermal systems: open loop and closed loop systems. In open loop systems the water that is circulated through the heat pump units comes in direct contact with the earth, in closed loop systems the water runs through tubing that is buried in the ground.
One type of open loop system that is suitable for areas where the earth is made up of bedrock and where ground water yields are low is the standing column well system. Standard open loop systems require relatively large yields of reliable ground water. For example, a 100-ton capacity system requires 200-300 gallons per minute of ground water. A standing column system uses less ground water because most of the water is recirculated. The standing column well requires good bedrock to allow for a deep, uncased well, a reasonably high static water level within the well and a minimal amount of ground water flow for what is known as bleed flow. A 100 ton capacity standing column well system with a 10% bleed rate would require a ground water yield of approximately 30 gallons per minute.
Standing column wells are 1,500 foot deep, 6” in diameter with an approximate capacity of 35 tons. A 4” pipe runs in the center of the well and draws water from the bottom of the well. Water is returned to the outer ring at the top of the well. As the water in the outer ring is drawn to the bottom of the well it transfers its heat to the earth.
A typical drilling rig is the size of a cement mixing truck and about 30 feet tall. High-pressure air is used to blow out rock fragments and water during drilling. A 100 yard dumpster located next to the rig is used to collect the drilling cuttings and water. The water is allowed to settle clear and then drained. The cuttings are hauled away. Each well takes about 6 days to drill.
Each standing column well has approximately 10 times the capacity of a vertical closed loop well. This allows standing column wells to be installed on sites with limited space for well fields. Standing column well systems have been installed beneath city sidewalks to serve buildings that have no available space for traditional vertical closed loop well systems and low ground water yields that preclude the use of traditional open loop systems.
The primary advantage of a geothermal system is its highenergy efficiency and low annual energy costs. In comparative terms, geothermal systems have a cooling energy efficiency rating (EER) of 20 and over, while conventional systems have EER ratings of between 10 and 16. Other advantages are no use of fossil fuel on site, no outdoor above grade equipment and zero exterior noise pollution.
The cost of geothermal systems varies depending on the type of system used. Open loop systems typically have the lowest first cost with standing column and closed loop wells being higher in cost. Ground heat exchanger costs for standing column and closed loop wells run from $1000 per ton up to $2000 per ton. Average payback due to energy savings can be 5-7 years and even lower if incentives or rebates are available to offset first costs.
For additional information please contact us.
Written by: John Rice, PE & Robert Diemer of AKF
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