posted on: 2010-09-29 03:33:55
Geothermal direct expansion heat pump system wherein subterranean heat exchange tubes have an internal diameter to length ratio of between 1/4000 and 1/6000, with 5 tubes per ton of BTU capacity; where the interior air heat exchange tubes have 50% of the interior volume of the subterranean tubes.
Where the thermal expansion valve is sized to match the interior air handler capacity; where the receiver holds 75% to 95% of the total refrigerant volume; where a pump down sequence is employed on system shut down; where an oil trap is installed at the vapor line existing the subterranean heat exchange tubes; and where 4 refrigerant cut-off/isolation valves are installed for service convenience.
The compressor is usually driven by electric power, and constitutes the element which forces the refrigerant around the circulating heat transfer loop between the interior and the exterior of a building. It is also possible to include a pump for circulating refrigerant or water in a system utilizing a liquid fluid that does not change phase. Typically, electrically driven fans are provided to force air over the finned tube heat exchangers so as to reduce the amount of finned tubing necessary to effect the desired thermal transfer between the air and the refrigerant.
The recirculating refrigerant may, and generally does, undergo phase changes as part of the process. At appropriate temperature and pressure, the refrigerant can be boiled (vaporized) on the side of the loop where heat is to be extracted, and condensed back to a liquid on the side to which heat is to be released. The phase change process provides a concentrated and substantial transfer of thermal energy.
Although the idea of a heat pump is to carry thermal energy in the refrigerant at a temperature which is useful for heat transfer in a desired direction via heat exchangers, the thermal transfer efficiency across a heat exchanger is a function of difference in temperature. When the outdoor air temperature is low, it may be necessary to supplement the heat energy obtained from the outside air through the heat pump. Electrical resistance heating means or auxiliary fossil fuel burners or the like can be employed for this purpose. Heat pumps are optimally designed for particular ambient air temperature ranges and their efficiency of heating and/or cooling will be impaired if ambient temperatures occur outside of the heat pump design limits. For example, the efficiency of an air-to-air heat pump used for home heating is reduced at low outdoor temperatures and for home cooling is reduced at high outdoor temperatures.
Heat pumps are designed to operate within a range where it is always possible to achieve cooling, although cooling is not very efficient in hot areas where temperatures often exceed 100° F. On the other hand, for heating, at some minimum outdoor air temperature, the lowest temperature the evaporator can provide is so near to the ambient temperature that little if any heat transfer occurs. At this temperature (which varies with the heat pump but may be on the order of -4° C. or 25° F.), the power expended in running the compressor and an outdoor fan is generally greater than the value of the heat energy available from the outdoor air which is necessary to adequately heat the interior air of the building. In such cases, a controller associated with the heat pump, coupled to an outdoor temperature sensor, shifts the heating system to supplement the heat pump with electric resistive or fossil fuel heating when outdoor temperatures approach the minimum temperature limit, and may disable the heat pump entirely when the minimum temperature limit is reached.
Ground source/water source heat exchange healing/cooling systems typically utilize closed loops of tubing buried in the ground or placed in water, such as a lake. These closed loops may be installed in a variety of manners, including horizontal configurations or helical loops, as well as in vertical configurations usually consisting of elongated U-shaped tubes placed into holes drilled into the earth. These heat exchange loops may carry a water/anti-freeze mixture in a water source system, or a refrigerant in a direct expansion system.
Both water source systems and direct expansion systems utilize naturally occurring geothermal heat as a heat source in the winter and as a heat sink in the summer. In a water source system, water is either taken from, or circulated in, the ground, via coupled plastic tubing and a water pump, so as to effect a thermal heat exchange between the ground and the water. The water is then used to effect a thermal heat exchange with refrigerant in a heat pump system, which refrigerant is utilized to transfer heat to or from interior air, depending on operation in either the heating or cooling mode.
In a direct expansion system, the refrigerant in a heat pump system is circulated directly in the ground, typically via coupled copper tubing, so as to effect a thermal heat exchange with the earth, all absent the additional water heat exchange step inherent with a water source system, and absent the necessity to operate a water pump. The geothermally heated or cooled refrigerant is then utilized by the heat pump system to transfer heat to or from interior air. As a result of the elimination of a heat transfer step, and the elimination of a water pump, direct expansion systems are usually more efficient than water source systems.
