Geothermal system utilizing supplemental ground heat from drainage fields

Abstract

Geothermal ground coils are installed and embedded in fill at the bottom of excavations, made for drainage fields associated with septic and other effluent systems, prior to the addition of the drainage field sanitary fill. This configuration eliminates the need for expensive bore holes and provides a ground heat source that is at a higher temperature than natural ground formations due to the supplemental heat imparted to the ground by the drain field effluent Fouling of the outside heat transfer surfaces of the ground coils is also avoided because of the clarifying and filtering action of the drain field fill. This provides an improved ground source heat pump system wherein the installation costs are reduced arid the capacity and efficiency of the system are improved.

Description

REFERENCES CITED [REFERENCES BY][0001]U.S. Patent Documents2,563,262August, 1951Moore 126 / 3444,448,347May, 1984Dunstan 165 / 9095,477,914December, 1995Rawlings165 / 455,533,355July, 1996Rawlings165 / 455,730,208March, 1998Barban165 / 455,738,164April, 1998Hilderbrand165 / 455,758,717June, 1998Grossman165 / 47FIELD OF INVENTION [0002] The present invention relates to heat recovery systems and methods, and more particularly pertains to a system and method for recovery of heat from the ground affected by drainage fields. BACKGROUND OF THE INVENTION [0003] Several prior art patents have utilized the earth as a source of heat and as a heat sink. Commercial geothermal energy systems are available and have been in use for several years. These systems normally utilize a water feed heat pump for both heating and cooling. The circulating fluid that feeds the heat pump flows through tubes that are buried deep in the ground and the circulating fluid uses the ground surrounding the tubes as both a heat sour...

AI Technical Summary

Benefits of technology

[0006] The present invention relates to a method of increasing the overall energy efficiency as well as reducing the installation costs of geothermal systems. The subject method comprises of installing the geothermal ground coils at the bottom of the excavations made for septic systems and similar drainage field applications to eliminate the extra cost of drilling bore holes or separate excavations. After the ground coils are placed at the bottom of the drainage field excavation, sanitary fill is used to surround and cover the coils and build up the drainage fields before the effluent distribution system is installed and covered. When the septic or other effluent system is operational, the supplemental heat from the effluent raises the temperature of the drainage field so that when heating is required, the temperature of the circulating fluid in the geothermal coils will be higher than if they were installed in natural ground formations. This higher circulating fluid temperature results in a substantial increase in energy efficiency. Also because the effluent is clarified by percolation and filtration, by the drainage field, buildup and fouling on the surface of the ground coils is avoided.

[0008] As a further enhancement of the system, in order to improve the efficiency of the system when cooling is required, one or more normal geothermal ground coils can be installed in ground which is outside the drainage field in order to take advantage of the lower temperature in the natural ground formations. These normal ground coils would preferably be in series with the drainage field coils. In the summer, when cooling is required, the circulating fluid would first go to the coils in the drainage field and then would flow into the coils outside the drainage fields, which would further reduce the temperature of the fluid in the coils before it flows to the cooling portion of the heat pump. In the winter, when heating is required, the flow of the circulating fluid would be reversed so that it would flow from the outlet of the heating portion of the heat pump to the normal ground coil in the natural ground formation where it will cause the temperature of the circulating fluid to rise. The circulating fluid would then flow into the drainage field ground coils where the temperature would further increase before flowing to the inlet of the heating portion of the heat pump. This higher circulating fluid temperature increases the capacity and efficiency of the geothermal heating system.

[0011] The liquid discharged from septic tanks is typically 5-10 degrees Fahrenheit higher than normal ground temperatures. Since the liquid distributed by the septic systems perforated pipes is subject to further exothermal bacterial digestion in the drain field, the ground temperatures under the drain fields will be effected not only by natural geothermal heat flux, but additional heating is achieved from the sensible heat from the waste liquids from the building and from the heat produced by the bacterial digestion in the septic tank and the drainage fields. Since higher moisture increases the heat conductivity of the soil, the steady flow of liquid to the septic drainage fields increases the rate of heat recovery in the ground in the vicinity of the geothermal coils. The efficiency of the geothermal system is therefore maintained during the coldest months when it is needed most. This reduces or completely eliminates the need for supplemental electric heating during the coldest months, thereby reducing or eliminating the additional cost for supplemental electric heating. The higher ground temperatures also reduce the heating costs during the entire heating season.

Problems solved by technology

In practice, the heat transfer efficiency of these systems rapidly decreases because wastewater is inherently dirty and causes a buildup of material on the heat exchange surfaces.

This fouling of the heat exchange surfaces reduces the heat transfer coefficient between the wastewater and the circulating fluid inside the coils and the amount of heat recovered by these systems drops off very quickly and renders them as being impractical.

However in actual practice, during the coldest periods of the year, the heat load obviously increases and the heat removal requirement from the ground in contact with the geothermal coils also increases but the natural heat recovery by the ground surrounding the coil is too slow to maintain the normal ground temperature.

This phenomenon reduces the ground temperature near the coil and in turn reduces the heating capacity of the geothermal system.

The energy costs increase because of the auxiliary electric heating requirements.

This greatly reduces the return on investment used to justify the geothermal system.

However if the auxiliary electric heaters are required for substantial periods the operating savings can be reduced to $500 per year resulting in only a 5% return.

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