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The history of using geothermal energy as a heating source can be traced all the way back to the 3rd century BC in the Qin dynasty, where the oldest known hot spring spa was found. About a hundred years after the invention of heat pump (1852), the first direct ground-source heat pump was built by Robert C. Webber (1940s). Then in 1946, J. Donald Kroeker designed the first commercial geothermal heat pump. Geothermal heat pumps gained huge popularity ever since. “As of 2004, there are over a million geothermal heat pumps installed worldwide providing 12GW of thermal capacity.” (Wikipedia) The underground temperature remains a nearly constant between 50 and 60°F according to different latitude. A geothermal heat pump uses long loops of filled-liquid pipes buried in the ground to transfer the heat. In the winter, the heat pump moves heat from the heat exchanger into the indoor air delivery system; and in the summer, the process is reversed. Geothermal heat pump provides a efficient and safe heating and cooling system. The geothermal heating and cooling cycles can be viewed as a simple refrigeration cycle. The system contains four major components: a ground array, two heat exchangers, and a compressor. A ground array (2 in chart) is a series of energy absorbing pipes placed underground. An antifreeze mixture passes through the ground array absorbs the ground energy and goes into the first heat exchanger- evaporator (3 in chart). The pipes containing antifreeze mixture goes inside the evaporator container so the mixture will not contact the refrigerant liquid inside the evaporator. When the high temperature refrigerant inside the evaporator meets the low temperature antifreeze mixture, the temperature difference will change the phase of the refrigerant from liquid to vapor without changing its temperature. Then the vapor is collected by a compressor (4 in chart). The pressure of the refrigerant gas is increased in the compressor, and the volume is reduced, thus the temperature of the refrigerantgas increases. The heated gas then goes into the second heat exchanger condenser (1 in chart). The condenser is connected with a heating distribution system to provide heat needed by the building. The refrigerant gas inside the condenser will reverse to a liquid after transferring its heat. Then the liquid will pass through an expansion valve to the evaporator for another cycle. The cooling cycle is the opposite of the heating cycle. The refrigerant releases heat to water that flows into the ground. The coefficient of efficiency(COP) of the heat pump can be calculated by the amount of heat moved to the work required to move that heat. When measuring the cooling cycle efficiency, the performance rating of energy efficiency ratio (EER) is used. It uses a set point temperature (typically 95°F) which the system operatesat. According to the US Environmental Protection Agency(EPA), the heat pump can reduce energy consumption up to 44 percent. And the COP range for geothermal heat pump is from 4.2 to 5.2. This means that for each kilowatt consumed by the heat pump, 4 kilowatts of energy generated. Thus the cost for kilowatt hour is quartered. The evaporator and condenser are both heat exchangers, therefore they have a lot of similarities. The total heat rejected by the condenser, is Where is the mass flow rate of the refrigerant, and is the mass flow m dot mext dot rate of the antifreeze mixture. The required condenser area can be calculated from the heat transfer rate where ?Tm is the mean temperature difference between the refrigerant and the antifreeze mixture. U is the overall heat transfer coefficient. The coefficient of heat transfer depends on the type and design of the heat exchanger, Where h is the coefficient of convection. ?x is the thickness of the pipe wall that is negligible in case of thin wall condition. The fouling factor outside sometimes can be neglected for good construction. However, the fouling factor inside the pipes must be included in the calculation.Inside the refrigeration cycle, a simple metering device located after the condenser can be used to regulate the flow of the refrigerant from high pressure side into the low pressure side. The refrigerant enters the metering device as a high pressure, high temperature, subcooledliquid and leaves as a low temperature, low pressure, saturated liquid. This can ensure that refrigeration cycles can be repeated under the same condition. Because it is difficult to calculate/predict the motion of the refrigerant fluid inside the heat pump, a metering device is animportant device to help regulate the fluid. The geothermal energy is a safe and efficient. Compare to air-source heat pump,theyare quieter, last longer, and need little maintenance. Most importantly, it saves up to 25 to 50% of the electricity thus making it one of the best heating/cooling choice for homeowners.

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