The history of using geothermal energy as a heating source can be traced all the wayback to the 3rd century BC in the Qin dynasty, where the oldest known hot spring spa wasfound. About a hundred years after the invention of heat pump (1852), the first directground-source heat pump was built by Robert C. Webber (1940s).
Then in 1946, J. DonaldKroeker designed the first commercial geothermal heat pump. Geothermal heat pumps gainedhuge popularity ever since. “As of 2004, there are over a million geothermal heat pumpsinstalled worldwide providing 12GW of thermal capacity.
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” (Wikipedia)The underground temperature remains a nearly constant between 50 and 60°Faccording to different latitude. A geothermal heat pump uses long loops of filled-liquid pipesburied in the ground to transfer the heat. In the winter, the heat pump moves heat from the heatexchanger 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 refrigerationcycle. The system contains four major components: a ground array, two heat exchangers, and acompressor. A ground array (2 in chart) is a series of energy absorbing pipes placedunderground. An antifreeze mixture passes through the ground array absorbs the groundenergy and goes into the first heat exchanger- evaporator (3 in chart).
The pipes containingantifreeze mixture goes inside the evaporator container so the mixture will not contact therefrigerant liquid inside the evaporator. When the high temperature refrigerant inside theevaporator meets the low temperature antifreeze mixture, the temperature difference willchange the phase of the refrigerant from liquid to vapor without changing its temperature. Thenthe vapor is collected by a compressor (4 in chart). The pressure of the refrigerant gas isincreased in the compressor, and the volume is reduced, thus the temperature of the refrigerantgas increases. The heated gas then goesinto the second heat exchangercondenser(1 in chart). The condenser isconnected with a heating distributionsystem to provide heat needed by thebuilding. The refrigerant gas inside thecondenser will reverse to a liquid aftertransferring its heat.
Then the liquid willpass through an expansion valve to theevaporator for another cycle. The coolingcycle 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 ofheat moved to the work required to move that heat.When measuring the cooling cycle efficiency, the performance rating of energy efficiencyratio(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 reduceenergy consumption up to 44 percent. And the COP range for geothermal heat pump is from 4.2to 5.2. This means that for each kilowatt consumed by the heat pump, 4 kilowatts of energygenerated.
Thus the cost for kilowatt hour is quartered.The evaporator and condenser are both heat exchangers, therefore they have a lot ofsimilarities. The total heat rejected by the condenser, isWhere is the mass flow rate of the refrigerant, and is the mass flow m dot mext dot rate of theantifreeze mixture.The required condenser area can be calculated from the heat transfer rateWhere ?Tm is the mean temperature difference between the refrigerant and the antifreezemixture. U is the overall heat transfer coefficient.
The coefficient of heat transfer depends on thetype and design of the heat exchanger,Where h is the coefficient of convection. ?x is the thickness of the pipe wall that isnegligible in case of thin wall condition. The fouling factor outside sometimes can be neglectedfor good construction. However, the fouling factor inside the pipes must be included in thecalculation.Inside the refrigeration cycle, a simple metering device located after the condenser canbe 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 thatrefrigeration cycles can be repeated under the same condition. Because it is difficult tocalculate/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.