Abstract:
There is provided a system for the underground storage of heat for the gradual and controlled release of same for use in agriculture, comprising a plurality of heat conduits located at predetermined depths below the surface of the soil, and means for circulating heated water through said conduits. Preferred are two-layer systems and three-layer systems of such conduits.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a novel system of heat storage, especially for use in the storage of heat for use in the heating of greenhouses or of similar structures. The invention further relates to a system of heat storage based on the utilization of heat derived from solar collectors, of geothermal heat and of waste heat, especially from industrial sources. 
     The novel system allows the gradual predetermined release of heat from the storage system for heating purposes, and specifically for the warming of the soil at a predetermined depth, where roots of plants are located. 
     STATE OF THE PRIOR ART 
     Soil warming as a medium for heat supply to winter crops was studied widely during the last decade both experimentally and theoretically, the significant investigation being carried out in the U.S.A., U.S.S.R. and in Germany (Agrotherm Project) with the most extensive experimental advances being in the Agrotherm Project (Boersma et al, A System Analysis of the Economic Utilization of Warm Water Discharges from Power Generating Stations. Oregon State Univ., Bul. No. 48, 1974). (H. Luckow, the Agrotherm Research Project, Hearing before the House of Representatives, U.S.A., Int. Activities of ERDA, U.S. Govt. Printing Office, 1977). 
     An updated list of references and discussion of the theoretical aspects of recent work can be found in the associated proposal of Merbaum and Schweitzer (A. Merbaum and S. Schweitzer, Heat Transfer Models for a Subsurface Water Pipe, Soil-warming System. Volcani Center, Agric. Eng. Inst. Research Proposal, 1979) and Shapiro (H. N. Shapiro, Simultaneous Heat and Mass Transfer in Soil with Application to Waste Heat Utilization. Proc. 6th Int. Heat Transfer Conf., Vol. 3, p. 19, Toronto, 1978). 
     These investigations were intensified lately with an objective of utilization of waste heat of power plants, geothermal heat and of solar energy. However, the available results reveal several shortcomings with regard to local applications. 
     While heat may be continuously applied at relatively shallow depths (30-40 cm) in colder climates, if applied by day under local winter conditions it may cause overheating of the root zone with attendant water losses and increase in salinity. Application of heat at night only would require costly daily storage capacity for a solar system or doubling of the pumping and transport capacity for a waste heat system based on aquifer storage (A. Nir and J. Schwarz, Survey of Thermal Energy Storage in Aquifers Coupled with Agricultural Use of Heat Under semi-arid Conditions. Proc. Thermal Energy Storage in Aquifers Workshop, Berkely, 1978). 
     Heat can be stored seasonally, from summer to winter in deeper soil or in groundwater aquifers, but this method necessitates continuous monitoring, control and transport of heat from storage to the area of application, with attendant losses in storage and complexity of equipment. 
     SUMMARY OF THE INVENTION 
     According to the present invention there is provided a heat storage system for the storage of heat obtained from solar collectors, of geothermal heat or of industrial waste heat, and wherein means are provided for the gradual predetermined release of such heat for any desired purpose. According to a preferred embodiment of the invention there is provided a stratified system for the circulation of a heated fluid at various depths below the surface of the earth. According to one preferred embodiment there is provided a two-layer system. The most preferred embodiment of the invention comprises a three-layer system. With the three-layer system a preferred embodiment comprises a pipe system located about 50± cm below the surface; the second system is located about 100 to 200 cm beneath the surface, while the third layer is located at a depth of about 3 to 10 meters below the surface of the soil. The conduits of fluids are preferably pipes, which may be made of a polyalkylene (polyethylene or polypropylene) or any other suitable material. The lowermost conduit is advantageously made of similar pipes in spiral configuration placed in well borings or of flat plate heat exchangers. Advantageously about 16 such bore holes are provided per 1000 m 2 . Pipes of about 20 to 30 mm diameter give satisfactory results. These are advantageously positioned beneath flower beds, the lowermost heat-exchangers being advantageously located in the central area of the greenhouse. 
     The general idea of the stratified storage is that heat is introduced into the soil below the vegetation area at various distances from the ground level. Heat collected on winter days is closest (the upper layer). The heat is allowed to diffuse freely to the root zone of the vegetation, reaching it during the following night, when it is really needed. 
     The heat collected in fall is stored deeper in the soil (the intermediate layer) and that collected in summer still deeper (the lower layer). In this way the upper soil is fortified in advance to have the average demand of heat supplied to it without the need for continuous monitoring and transport of heat. 
     However, if the heat demand is particularly high at some period, there is an option to transfer it from the lower layer to the upper by circulation of water between these layers. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 is a top plan primarily schematic view of the invention system in place in the soil; and 
     FIG. 2 is a vertical cross-sectional view through the system taken on the line x--x of FIG. 1. 
    
    
     DETAILED DESCRIPTION 
     Referring now in detail to the drawings, the levels A, B and C indicate deepest, intermediate and shallow layers, respectively. More in detail, the drawings show a plurality of pipes 21 arranged with spiral coils at the lower end located in the bore holes 22. Hot water is introduced at 23 by a conduit 24, and this water passes via spiral 21 and via the conduits 24&#39; and 25 into the next bore hole, via conduit 26, through spiral 27 and via conduit 28 to the exit 29. The water is thereafter recirculated to heating means, not shown. The showing of FIG. 2 is indicative of a predetermined pattern of such bore holes covering the area over which the soil is to be heated. 
     The layers comprise a plurality of parallel pipes 31 at the shallow depth, preferably 50 to 60 cm, and another layer of conduits 32 at an intermediate depth of about 120 cm below the surface of the soil. 
     Means are provided to interconnect the pipes to permit the flow of hot water to them and between the layers. To this end, pipes 31 meet at 33 and are connected to the hot water entry port 34, the same pipes 31 meeting at 35 and the water leaving via exit port 36. The pipes 32 meet at 37 which receives hot water by a conduit means 38 and 39, the same pipes meeting at exit 40. Thereafter, the water leaves via valve means 41 and exit conduit 42. The connections to the spiral pipes 21 and 27 are via 43 and 44, by means of which the direction of flow can be reversed, as indicated, in order to control the inter-layer flows of water. 
     The &#34;plumbing&#34; required to implement the system is notoriously well known, nothing more than ordinary pipe interconnections, valves, pumps, or the like, being required. Thus, the showing of FIGS. 1 and 2 are merely exemplary of many embodiments. 
     Heat is stored in protected soils, e.g. in greenhouses or under other structures in the unsaturated water zone, which is not subject to direct uncontrolled infiltration. The sources of the heat are: solar water collectors, geothermal hot waters and industrial waste heat at temperatures of 30°-80° C. 
     The heat is introduced into several horizontal layers through the use of water pipes with heat conducting walls or alternately heat exchangers, which are made of metal or plastic. 
     The selection of the depth of the storage layer is made on the basis of the following considerations: 
     (a) The upper layer (at 50±10 cm depth) delivers the heat stored by day during the following night by direct conduction to the surface and to the vegetation root zone. 
     (b) The second (intermediate) layer (at 100-200 cm depth) serves as a backup to the upper layer in case of several days of lower supply or higher demand at the upper level. Heat is stored in this layer from days to weeks before the actual demand. 
     (c) The lower layer (at 3-10 meters depth) serves as seasonal storage, e.g. stores heat from summer to winter. Heat is recovered by recirculation of water between these layers and the upper layer. 
     (d) The deposition and extraction of heat in the lower layer is controlled through an underground drip irrigation pipe placed above it. This device utilizes the extremely high sensitivity of heat conduction in unsaturated soils to their water content. Even small quantities of water applied by such drip irrigation increases appreciably the heat conduction of the soil. 
     The depth of the upper layer and the water temperature are determined by model calculation and experimental calibration to fulfill the following conditions: 
     (a) Avoid heat escape by day or at high surface temperatures; 
     (b) Not to cause overheating of the root zone; 
     (c) Not to cause excessive soil drying in the pipe area; 
     (d) Reach the root zone and soil surface at night or at low surface temperatures. 
     This determination takes therefore into account the following parameters: 
     (a) Input water temperature; 
     (b) Soil thermal properties; 
     (c) Soil water regime and hydrological properties; 
     (d) Climatic conditions; 
     (e) Physiological requirements of plants (in case of greenhouses). 
     SUMMARY OF THE INVENTION 
     The invention offers a solution for simple and inexpensive heat storage in protected soils at low temperatures. The heat is stored in stratified layers in upper soil and released according to the heat requirements of the top layers. This obviates the need for a separate storage and control mechanism. 
     EXAMPLE 
     This example is based on data for a greenhouse situated on the coastal plain of Israel, with the upper soil being sandy, and under normal winter irrigation schedule and atmospheric conditions. 
     The operational parameters are these: 
     Upper layer: 20 mm diameter polyethylene water pipes running under the flower beds at 60 cm depth. 
     Intermediate layer: Similar pipes at depth of 120 cm. 
     Lower layers: 32 mm diameter polyethylene pipes in spiral configuration, placed in 1 meter diameter wells, at depth of 4 to 7 meters in the central area of the greenhouse. Advantageously about 16 such bore holes are provided per 1000 m 2 . 
     Drip irrigation layer: A single coil of porous water pipe placed above the lower layer spiral. 
     The relevant data of average performance for this layout are (in M Joules/m 2  /day): 
     
         ______________________________________                     Energy            Energy,  released toLayer  Period    stored   Greenhouse                             Comments______________________________________Upper  daily       1-1.4  0.8-1.2  Dec.-AprilInter- November  1.5      0.3mediateLower  May         1-1.2   0.7-0.8*                             *Released in  October                    winter according                             to daily re-                             quirements______________________________________The root zone average temperature would be as follows(°C., at 25 cm depth):Heat/Month  December  January  February                             March______________________________________Unheated  15-17     15-17    15-17   16-19Heated 21-22     20-22    20-22   20-20______________________________________ 
    
     The system can be adjusted to variable weather conditions. The decisions and available actions are described in Table I. 
     
                                           TABLE I__________________________________________________________________________HEAT STORAGE DECISION ANALYSISDay           AlternativeSeasonNight    Basic Operation              or additional operation                             Comments__________________________________________________________________________1.       1.1SummerDay Store in deep layer    (3)Night         1.2              Recirculate collector                             *When root cooling              to top layer (1)*                             required2.       2.1       2.2Fall Day Store in inter-              Store in top layer (1)*                             *Early root warming required    mediate layer (2)Night         2.3            **Root cooling required              Recirculate collector              to top layer**3.       3.1       3.2WinterDay Store in top              Store in intermediate layer (2)*                             *Warm weather    layer (1)              3.3Night         Recirculate deep layer to                             **Air warming required              collector in air heating              mode**              3.4              Recirculate deep to top layer                             ***Additional root warming              (1)***         required__________________________________________________________________________ 
    
     While the invention has been described in detail above, it is to be understood that this detailed description is by way of example only, and the protection granted is to be limited only within the spirit of the invention and the scope of the following claims.