FIELD OF THE INVENTION 
       [0001]    The invention relates to the technical field of heat exchange and heat distribution systems. The invention pertains to a method for pooling thermal energy (heat) on the district scale, in which a heat exchange loop connects at least one thermal energy consumer site and at least one thermal energy producer site. 
         [0002]    The invention also pertains to a heat pooling system which can be used to carry out said method. 
       PRIOR ART 
       [0003]    In a given region, for example a township, there may be factories which often have problems with utilizing the energy produced in the form of low thermal energy heat (heat typically transported by fluids at temperatures below  150 ° C.) and residential, office or industrial premises which have energy requirements which take the form of low temperature heat (domestic or tertiary heating, drying operations, heating greenhouses, etc . . . ). 
         [0004]    Factories which do not use their surplus low level thermal energy heat, such as refineries or nuclear power stations, generally turn to:
       air coolers, which act as exchangers between the process fluids and air, intended to dissipate the heat to the air; such devices are generally provided with electric fans to force the circulation of the air;   and/or cooling towers, a type of chimney with a natural draw, into which water is sprayed in order to assist cooling. Here again, the heat is lost to the atmosphere;   and/or cooling water systems (generally glycolated water), the cold water being obtained either by heat exchange in an exchanger with seawater, river water, etc., or by refrigerator units. Here again, the heat is lost to the environment. For their part, residential, office or industrial premises which require low temperature heat:   they generate this heat themselves using combustion or electrical means. In both cases, fuels are consumed and CO 2  emissions are generated;   and/or they import steam from nearby industrial premises or from an urban heating system; again, both consume fuels in order to produce this heat.       
 
         [0010]    Thermal integration between factories and/or residential or office premises are rare because when thermal integration occurs, the two parties become dependent on each other for their respective operation. One example of this type of energy integration is described for the heating of agricultural greenhouses in http://innovagro.net/pdf/agro-industries.pdf. 
         [0011]    Constraints regarding duty to supply, penalties during outages, and in short, criticality in the event of default by one of the parties is a sticking point which generally discourages investors. As described in the example above, the fact that surplus energy is available from a neighbouring industrial site when it is alone does not absolve the beneficiary from having to invest in its own boiler in order to cover for any defaults by the nearby industrial site. 
         [0012]    Starting from an industrial heating plant, existing systems consist of supplying a hot fluid, generally water, at a temperature of about 100° C. and of recovering and returning the fluid cooled by the user. That device requires the system to have a large number of branches so that each user has the appropriate temperature at its disposal. This branching of the supply lines and having to use pipes (one for outfeed, one for return) generates large thermal losses. This problem is encountered, for example, on the website for the Compagnie Parisienne de Chauffage Urbain, http://www.cpcu.fr/La-chaleur-urbaine/FONCTIONNEMENT. 
       DESCRIPTION OF THE INVENTION 
     Summary of the Invention 
       [0013]    The invention concerns a method for pooling heat on the district scale, in which a heat exchange loop is employed which connects at least one thermal energy consumer site and at least one thermal energy producer site, said loop comprising a heat transfer fluid the flow rate of which is adjusted so that at any point of the loop the temperature differences are less than 20° C., said consumer site or sites taking the energy necessary at one point of the loop by means of heat exchange and the producer site or sites discharging the surplus energy produced by means of heat exchange. 
         [0014]    Advantageously, the temperature of the loop is kept constant using an additional heating unit. 
         [0015]    In one embodiment, a thermal battery type heat buffer store is used to store surplus thermal energy in the loop. 
         [0016]    An organic Rankine cycle may be introduced into the loop in order to utilize the surplus thermal energy in the form of electricity. 
         [0017]    Preferably, the heat transfer fluid is selected from water, aqueous mixtures, alcohols, hydrocarbons or ionic liquids. 
         [0018]    The heat transfer fluid may comprise particles of a phase change material. 
         [0019]    The invention also concerns a system for pooling heat on the district scale, comprising:
   at least one thermal energy consumer site;   at least one thermal energy producer site;   a heat exchange loop which connects said thermal energy consumer sites and said thermal energy producer sites together, and which comprises heat exchangers and a heat transfer fluid the flow rate of which is adjusted by means of a pump.   
 
         [0023]    In the system of the invention, the loop may comprise an additional heating unit. 
     
    
     
       DESCRIPTION OF THE FIGURES 
         [0024]      FIG. 1 :  FIG. 1  illustrates a known prior art system; starting from an industrial heating unit C, it consists of providing a hot fluid ( 1 ), generally water, at a temperature of about 100° C. and recovering, as the return, the fluid ( 2 ) cooled by the user, for example buildings placed in parallel B 1 , B 2 , . . . Bn. 
           [0025]      FIGS. 2 to 5  are non-limiting illustrations of the invention. 
           [0026]      FIG. 2 :  FIG. 2  represents the system of the invention for using a heat exchange loop between the issuers of the energy (for example: factories) and the energy consumers (for example: buildings). 
           [0027]      FIG. 3 :  FIG. 3  illustrates the embodiment of the invention in which pooled heat buffer stores are employed. 
           [0028]      FIG. 4 :  FIG. 4  illustrates the embodiment of the invention in which the system comprises an organic Rankine cycle. 
           [0029]      FIG. 5 :  FIG. 5  illustrates the embodiment of the invention in which the heat transfer fluid or fluids of the chemical loop comprises or comprise a phase change material in the form of capsules. 
       
    
    
       [0030]    More particularly, the system in accordance with the invention ( FIG. 2 ) consists of positioning a common loop of a heat transfer fluid between a set of energy consumers and issuers. By means of heat exchangers located on the loop lines, as a function of their needs, the consumers take the energy they need by heat exchange and the issuers discharge the surplus thermal energy they produce by heat exchange. 
         [0031]    By means of a pump P, a heat transfer fluid moves in the loop connecting the energy consumers (buildings B 1 , B 2 , B 3 , Bn) and energy issuers (factories U 1 , U 2 ). 
         [0032]    The flow rate of the heat transfer fluid is selected such that at any point of the loop, the temperature differences are small, preferably between 5° C. and 20° C. (for example, the maximum temperature difference may be 20° C., i.e. the highest temperature is equal to 80° C. and the lowest temperature is equal to 60° C.). 
         [0033]    The heat transfer fluid is any fluid which allows heat exchange in the various pieces of heat exchange equipment and is preferably selected from fluids which are in the liquid state at pressures in the range 1 to 20 bar relative, so that the cost of the lines of the loop does not become too high. Examples of heat transfer fluid which may be cited are water or aqueous mixtures or alcohols or hydrocarbons, or indeed ionic liquids. 
         [0034]    Examples of consumers which may be cited are domestic or industrial buildings to be heated, or indeed factories carrying out industrial processes which require heat, for example to carry out drying operations, for example in the agro-alimentary industry. Issuers include factories which have to dissipate heat which is lost to the atmosphere according to the prior art. 
         [0035]    The temperature of the loop is advantageously maintained, for example if the energy balance from the contributors to the loop (in this case the factories U 1  and U 2 ) is insufficient, using an additional heating system C the size of which is substantially reduced compared with the prior art industrial heating unit which was the only source of heat. This means that the overall consumption of energy in the invention is greatly reduced, since the heat is pooled within the loop and the dimensions of the additional heating unit are such that temperature differences within the loop can be smoothed out. 
         [0036]    In one embodiment of the invention, the system ( FIG. 3 ) may be equipped with one or more pooled heat buffer stores Q, termed a “thermal battery” by the firms which make it (for example, the German firm H. M Keizkörper). As an example, the storage system could use sodium acetate. Storing heat means that the temperature of the loop can be smoothed over time (for example day/night or winter/summer) or can also heat buildings when a heat supply factory is undergoing maintenance operations. 
         [0037]    In another embodiment of the invention, the system ( FIG. 4 ) can also be equipped with an organic Rankine cycle (ORC) which can be used to transform the surplus thermal energy (instead of sending it to a chiller such as an air cooler) into electrical energy if required, for example during the summer when heating requirements are lower. This electrical energy can advantageously be used to operate air conditioning. 
         [0038]    In accordance with the invention, one of the users may have need of a greater level of heat than the temperature of the hot loop (for example a need for a temperature of 120° C. for industrial agro-alimentary cooking with a loop maintained at a temperature of close to 70° C.): in this case, the user can install an additional heat pump which can be used to raise the temperature by consuming additional electricity. 
         [0039]    In accordance with the invention, the heat transfer fluid may contain solid particles encapsulating a phase change material (for example sodium acetate) and enabling the energy which can be restored with a small temperature variation to be increased. In fact, when a pure substance changes phase (state), the energy content (enthalpy) varies without a variation in temperature. The well-known example is that of water, which passes from the solid to the liquid state at  0 ° C. at atmospheric pressure, but in our case the phase change is desired at a temperature which is typically between 50° C. and 100° C. Said phase change material is preferably selected from the compounds below; the melting points are provided in brackets:
   Sodium acetate trihydrate (58° C.), partially hydrated zinc chloride =(76° C.).   Some examples of ionic liquids with a melting point in this range are:   1-butyl-3-methylimidazolium tosylate: mp=(67° C.)   1-ethyl-3-methylimidazolium hexafluorophosphate: mp=(59° C.)   1-butyl-1-methylpyrrolidinium hexafluorophosphate: mp=(85° C.)   1-butyl-3-methylimidazolium chloride: mp=(73° C.)   1-ethyl-3-methylimidazolium chloride: mp=(77-79° C.).   
 
         [0047]      FIG. 5  shows the lines (Line) for the chemical loop transporting the heat transfer fluid F with conveyed capsules containing the phase change material (liquid phase (L); solid phase (S)). The envelope E encapsulating the phase change material may be formed from a plastic such as polyethylene or polypropylene.
The invention concerns a system for exchanging heat by means of a loop in which a heat transfer fluid circulates, between a plurality of industrial and tertiary sites, each site possibly being a producer and/or consumer with the aim of reducing the energy factor on the district scale. The invention also concerns a method for pooling thermal energy on the district scale using a loop connecting at least one thermal energy consumer site and at least one thermal energy producer site, in which a heat transfer fluid circulates.