1. Field of the Invention
This invention relates generally to earth-embedded heat exchangers and particularly to such heat exchangers in which an aqueous primary heat transfer fluid is in thermal contact with an encapsulated material with a high latent heat of transition which is capable of undergoing a phase change under the normal operating conditions for a water-to-air heat pump system.
2. Description of the Prior Art
With the increased use of heat pumps in the heating and cooling of buildings has come the recognition that the efficiency of an air-to-air heat pump is high only when the outside ambient temperature is moderate. At ambient temperature extremes, the coefficient of performance of a heat pump falls drastically. At an ambient temperature of 32.degree. F., operation continues only at an energy loss because the evaporator must be defrosted. As a consequence, alternate heat sources which would remain essentially constant despite fluctuating ambient air temperatures have been sought.
Ground water, under favorable conditions, is one such source; and ground water-to-air heat pumps have been operated at a high coefficient of performance year-around. However, the water sources commonly used in the past have been either well-water or city water. Either source would be quickly exhausted if it were widely adopted as a heat source/sink. These drawbacks can be overcome with the use of a closed water loop in which the earth itself serves as a giant heat sink for air conditioning in the summer and as a heat source from which large amounts of heat warmer than the ambient air can be extracted in the winter. Unfortunately, attempts to utilize heat pump systems in which the heat exchange medium is circulated in a closed loop through the earth have never worked very well.
Problems with such closed loop systems arose because little of the available heat in the earth's crust can be abstracted with the heat exchangers of the prior art. These heat exchangers typically comprised long sections of copper coil. No enclosures surrounding these coils and containing phase change materials were provided. Due to the low thermal conductivity and heat capacity of the earth, the energy in the vicinity of such earth coils was rapidly dissipated when the instantaneous heating demand of a heat pump system was placed on one of them. As a consequence, the temperature of the heat transfer fluid continued to fall; and within a relatively short period of time, a second earth coil which had been idle for a lengthy time interval and which had had sufficient time to recover had to be substituted for the exhausted earth coil. Alternately, the heat pump system had to be shut down.
Such earth coils, in addition to being very expensive and costly to install because of the long sections required to effect adequate heat transfer between the earth and the heat transfer medium, also tended to separate from the surrounding earth upon freezing during winter operation and to overheat plant roots in lawns and gardens as a result of summer heat dissipation. Moreover, a substantially large open area in close proximity to the structure to be conditioned was required in which to embed the earth coils; at least one sq. ft. of open area was required for every sq. ft. of heated floor area.
A further deterrent to the widespread use of such closed loop systems was the need for custom-designed components due to the variation in soil properties from one locale to another. This obstacle has been compounded by the addition of thermal storage tanks, the most commonly employed means in the prior art for reducing the instantaneous heating or cooling demand of the heat pump system upon the earth coil.