1. Field of the Invention
The present invention relates to a borehole reservoir. More specifically it relates to a borehole reservoir for the storage, and particularly for the seasonal storage, of heat in rock or similarly dense ground, including a plurality of substantially vertical boreholes. Each borehole has a centrally located pipe which is open at the bottom thereof and which extends to the vicinity of the bottom of respective boreholes. The upper parts of the boreholes and pipes are in communication with water-supply lines and water-return lines. A pump circulates the water through the lines and the pipes and an annular passage is located between said centrally located pipes and the walls of said boreholes. The lines are connected to heat-producing devices and heat-consuming devices for alternate supply and removal of heat to and from the water circulating in the reservoir.
2. Description of the Prior Art
In borehole reservoirs of this kind the upper ends of the pipes and the boreholes are connected to collecting mains arranged on the surface of the ground, these mains being, in turn, connected to at least two heat-exchange devices and a circulation pump. The circulation circuit is connected, through one of the heat-exchangers, to a heat-producing device, such as a solar-energy collector, and, via the other heat-exchanger, also to a heat-consuming device, such as a radiator.
Heat is transferred to the ground (the rock), by circulating hot water from the heat-producing device over the borehole reservoir, and is taken back by circulating colder water over the heat-consuming device and the borehole reservoir.
In order to prevent the ingress of air, the water in the collecting mains must have at least atmospheric pressure. Thus, the pressure in the collecting mains must be higher than the pressure of the ground water. Consequently, the boreholes must be provided with water-tight linings, to prevent water in the circulation circuit from leaking out through those cracks which are ever present in the ground, even in rock which appears to be totally crack-free. This greatly increases the cost of the borehole reservoir, and when the lining is made from a plastics material it also impairs the heat-transfer effect between the water in the boreholes and the surrounding wall material, which impairment must be compensated for by increasing the number of boreholes for a given heat-transfer effect.
The object of the present invention is to provide a borehole reservoir which comprises unlined boreholes, but which nevertheless is substantially free from leakages. This object is achieved in accordance with the invention, by arranging for the boreholes to intersect a plurality of substantially horizontal boreholes, which extend radially in spoke-like fashion from a central shaft, and which are located beneath the ground-water level, the vertical boreholes and the upper parts of the pipes communicating with the central shaft and with the water-circulating means through said horizontal boreholes. In this way, the water pressure in the boreholes and pipelines will be the same as the pressure in the externally located ground water.
The horizontal boreholes can be arranged in pairs at two levels beneath the ground-water level or half as many can be arranged at one single level. In this latter case, a pipeline is laid in each horizontal borehole, to provide a feedline and a return line in each horizontal borehole. One such horizontal pipeline is suitably provided with a separate T-pipe connector at each vertical borehole to enable the vertical pipes to be readily connected to the horizontal pipelines.
Because the borehole reservoir according to the invention must include a centrally located, water-filled shaft, the shaft is suitably dimensioned to enable it to be used as a short-term accumulator. Measures are suitably taken to maintain to the greatest possible extent and in a manner known per se temperature zoning in the water in the shaft.