METHOD FOR STORING THERMAL ENERGY FOR A HEAT PUMP INSTALLATION

Abstract: Method for storing thermal energy in a combined open/closed system for improving the operation conditions of a heat pump installation employing water from e.g. a lake, a river or the like as its heat source. During the warm season, water is passed from the lake etc. through a tube system installed in bottom sediments of the lake at and/or below the level at which natural heat energy storage can occur. This results in an artificial heat energy storage in said bottom sediments. During the cold season, the lake water is passed first through said tube system to thus acquire a higher temperature and then from said tube system to said heat pump installation. Thus the water not only serves as the heat source for the heat pump but mainly serves as a heat transporting means by which heat energy is carried into the bottom sediment store in summer and is carried out of said store in winter. By optimizing the tube system layout in the bottom sediments in relation to the heating requirements typically arising during the cold season it is possible to establish a reserve of heat energy which every year is gradually consumed during the cold season and then again built up during the warm season.

Description: Method for storing thermal energy for a heat pump installation This invention is concerned with a method (combined open/closed system) for improving operation conditions for a heat pump installation employing water from a lake, pond, bay, inlet, cove, river, canal or the like as a heat source. Acccording to this method thermal energy is stored in sediments present at the bottom of these bodies of water. Energy crises during recent years have increas¬ ingly directed the attention of energy researchers to¬ wards local renewable energy resources. New, alternative energy supplying methods have been devised based on heat pump systems, and their further development is expected to make rapid progress during the next years. Whenever air, ground surface areas, ground subsurface areas or bodies of water are used as heat sources a general problem to be handled is the following: At the time when the temperature of the heat source has. dropped to its minimum the demand for heat is at its maximum, and vice versa, * that is, the demand for heat is at its minimum in the summer period, when ambient air, ground areas and bodies of water all have acquired a comparatively high temperature, Prior art methods for taking heat energy from natural sources such as subsurface or surface ground areas and bodies of water are based on the utilization of the low temperature levels that are naturally existing in^these sources during the winter season; and it is by meήns of still further lowering the inherently low temperature in the environment of these sources and artificially raising the temperature level to that required for e.g. heating buildings that prior art heat pumps have come to be used in actual practice. <5 In the bottom sediments of shallow lakes or ponds etc. heat energy will store up spontaneously during the warm season of each year, partly due to direct solar irradiation of the sediment surface and partly due to convection. A lake or pond of 5 to 8 m depth is subject to being stirred up by winds to such a substantial degree that it will not develop any temperature gradient stratification; consequently each summer its entire body of water is heated to the surface water temperature. During the summer period this heating of the water gives rise to a so-called "heat wave" in the bottom sediments. The heat wave will advance downwardly during the summer, at a slow rate; if natural conditions are prevailing the heat wave will reach a depth of about 2.5 to 3.0 m below the bottom sediment surface, depending inter alia on the composition and heat conducting properties of the sediments. Then with the onset of autumn and winter the whole lake is apt to cool down rather quickly so as to finally acquire a coherent ice surface. Once an ice surface is established the movements in the water underneath will stop, and as a result a stratified temperature gradient will now form, with cold water strata immediately under the ice layer and then progressively warmer strata as one goes farther down towards the bottom. Heat conduction in the bottom sediments is quite sluggish and so cannot keep pace with the more rapid temperature changes in the water. During the winter period the aforesaid heat wave in the bottom sediments turns in the opposite direction and moves upwardly in the bottom sediments, thereby warming the lake water from the bottom upwards. Measurements have shown that the mean temperature in the lake water is higher immediately before the ice breaks up in spring than it is immediately before the freezing stage i 'autumn/winter. This indicates that the movement of the heat wave within the bottom sediments - downward in summer, upward in winter - is very slow indeed. -- OMPI The present invention comprises a method (combined open/closed system) for improving the operation conditions of a heat pump installation employing water from a lake, pond, bay, inlet, cove, river, canal or the like as its source of heat. The method is characterized in that during the warm season water drawn from said lake etc. is passed through a tube system (the closed part) installed in bottom sediments of said lake etc. at and/or below the level at which natural heat energy storage can occur, whereby due to the thus resultant temperature increase in said bottom sediments an artificial heat energy storage is accomplished in said sediments; and that during the cold season water drawn from said lake etc. is passed first through said tube system to thus acquire a higher temperature, and then from said tube system to said heat pump installation. Thus during the warm season this method achieves an increased heat energy storage in the bottom sediments of e.g. shallow lakes, ponds, bays, inlets, coves, rivers, canals or the like; it thus creates an extra store of heat energy at a level below the natural heat energy reserve that is spontaneously built up immediately under the bottom surface of said lake or other similar body of water. This means that the heat storage level is moved further down by some 2.5 to 3 m. When during the summer season the surface water, " e.g. in Sweden, reaches a temperature around +20-25°C the water is passed through the tube system, e.g. by means of pumps, and as a^ consequence the bottom sediments of the lake, river, etc. are heated to surface water temperature about 3 to 6 below the bottom. At the end of the summer period, the extra energy store thus built up under the bottom of the lake contains 2 to 2.5 times more heat energy than the store of heat energy that can accumulate spontaneously under natural conditions. OM?I The tube system preferably consists of flexible plastics tubes, which can be laid in several rows in one or more layers in the bottom sediments. During the warm season the water passed through said tube system preferably is pumped from a water intake near the surface of said lake etc. The water leaving the tube system may be re¬ turned directly to the lake, etc. However, according to a favourable embodiment said water with somewhat decreased temperature is passed first to the heat pump installation (or the part thereof being operated during the summer) and then back to the lake etc (the open system) . In this manner the heat demand in the summer will be satisfied, and at the same time a heat energy store for the cold season will be built up in the bottom sediments. When the heat pump is to be operated during the cold season water from the lake etc. is at first passed through the tube system in the bottom sediments and is then fed from those tubes into the heat pump. During its passage through the bottom sediment tubes the water is preheated by heat from the artificially increased store of heat energy, and due to this preheating the operating conditions of the heat pump are greatly improved. It may be noted that the water of the lake etc. not only serves as the heat source for the heat pump but mainly serves as the heat transporting means by which heat energy is carried into the bottom sediment store in summer and is carried out of said store in winter. In winter ' the water to be fed to the tube system is preferably drawn from an intake arranged near the bottom of the lake, since there the water normally is warmer than it is near the surface, as explained above. In e.g. Sweden the conditions under which a heat pump installation has to operate are least favorable during the period immediately after an ice layer has formed on the body of water employed, e.g. lake. In that stage OMPI the bottom water temperature has reached its minimum, and the energy storage is just below its optimum. If now in accordance with the present invention the lake water is made to pass through the energy store the water will be preheated before it arrives at the heat pump. This preheating of the lake water is highly advantageous in creating favorable dimensioning possibilities: The heat pump, if it is expected to be fed with this type of pre¬ heated water, may be given dimensions to correspond to higher evaporation temperatures, which in turn means that capital investment costs can be reduced considerably. The dimensions for the heat pump installation and those for the energy store in the bottom sediments will be chosen such as to properly harmonize with each other. These dimensions should be optimized in each particular case so as to suit (i) heat production requirements and (ii) natural limitation factors such as for instance the type, thickness and physical properties of the bottom sedi¬ ments. . By optimizing the tube system layout in the bottom sediments in relation to the heating requirements typically arising during the cold season it is possible to establish a reserve of heat energy which every year is gradually consumed during the cold season and then again built up during the warm season. The method of this invention may be applied to any conventional heat pump installation employing water from a lake or the like as its heat source; for examplej, the heat pump may be used for heating buildings in nearby residential areas. /

Claims: " C l a i m s 1. A method for improving ' the operating conditions of a heat pump installation employing water from a lake, pond, bay, inlet, cove, river, canal or the like as its source of heat, characterized by a combined open/closed system, wherein during the warm season water drawn from said lake etc. is passed through a tube system installed in bottom sediments of said lake etc..at and/or below the level at which natural heat energy storage can occur, whereby due to the thus resultant temperature increase in said bottom sediments an artificial heat energy storage is accomplished in said sediments; and wherein during the cold season water drawn from said lake etc. is passed first through said tube system to thus acquire a higher temperature and then from said tube system to said heat pump installation. 2. A method according to claim 1, characterized in that during the warm season the water passed through said tube system is drawn from a water intake near the surface of said lake etc. 3. A method according to claim 1, characterized in that during the cold season the water passed through said tube system is drawn from a water intake near the bottom of said lake etc. y OMPI

Inventors: GARDERYD HANS OVE MAGNUS, GARDERYD, HANS, OVE, MAGNUS
Applicants: GARDERYD HANS OVE MAGNUS, MARTINELL RUDOLF HAOKAN, GARDERYD, HANS, OVE, MAGNUS, MARTINELL, RUDOLF, HAOKAN
Application year: 1983
Main classification: F24J3
IPC codes: F24D11, F28D20