Heat storage system and method for the charging and discharging thereof

Abstract

Thermal storage system and its charging and discharging process using a heat-transfer fluid. The system includes a phase change storage material contained in a casing that is penetrated by heat exchanger tubes ( 3 ), preferably vertical ones, connected at their lower end with at least one lower chamber ( 62 ) via lower sub-chambers ( 62 ′) which include a series of injectors ( 9 ) for enabling the heat-transfer fluid to be inserted in gas form; at the upper end, these tubes ( 3 ) connect via upper sub-chambers ( 61 ′) with at least one upper chamber ( 61 ) which in turn is connected to a boiler ( 1 ) from which downpipes ( 8 ) extend via which heat-transfer fluid in liquid form circulates, located outside of the casing, connecting the aforementioned boiler ( 1 ) to the lower chamber ( 62 ), which enables the natural circulation of the heat-transfer fluid within the system.

1 . A thermal storage system, comprising:
 a phase change storage material with a thermal expansion below 20% wherein said phase change storage material is located within a casing;   a heat exchanger via which a heat-transfer fluid circulates; and   a drum; wherein:   said heat exchanger comprises heat exchanger tubes that penetrate the storage material with which they are in continuous contact;   the heat exchanger tubes are connected at their lower part with at least one lower chamber via lower sub-chambers which include a series of injectors where the heat-transfer fluid is inserted in gas form to the lower part of the heat exchanger;   in the upper part, the heat exchanger tubes connect with at least one upper chamber via upper sub-chambers;   the upper chamber is, in turn, connected to the drum located outside of the casing and above the upper chamber; and   extending from the drum is an extraction pipe for heat-transfer fluid in gas form, an entry or exit pipe for heat-transfer fluid in liquid form, and one or more recirculation downpipes via which the heat-transfer fluid in liquid form circulates, the one or more recirculation downpipes being located outside of the drum and connecting the drum to the lower chamber, enabling the natural circulation of the heat-transfer fluid within the system.   
     
     
         2 . The thermal storage system of  claim 1 , wherein the phase change storage material comprises a graphite, carbon or metal matrix infiltrated with hydroxide salts. 
     
     
         3 . The thermal storage system of  claim 1 , further comprising a recirculation pump located between the one or more recirculation downpipes and the lower chamber. 
     
     
         4 . The thermal storage system of  claim 1 , further comprising at least one valve in the one or more recirculation downpipes. 
     
     
         5 . The thermal storage system of  claim 1 , wherein the heat exchanger tubes are vertical. 
     
     
         6 . The thermal storage system of  claim 1 , wherein the heat exchanger tubes penetrate two flat plates, one lower flat plate to which the heat exchanger tubes are attached and welded and another upper flat plate to which the heat exchanger tubes are not welded. 
     
     
         7 . A charging process for a thermal storage system as recited in  claim 1 , the charging process comprising the following stages;
 insertion of the heat-transfer fluid in gas phase into the lower sub-chambers through the injectors, where the heat-transfer fluid in gas phase mixes with the recirculated heat-transfer fluid in liquid form from the drum, the heat-transfer fluid in liquid form descending via the one or more downpipes, passing through the lower chamber or chambers;   ascending circulation of the mixture of fluid in liquid and gas phase by natural circulation in the heat exchanger tubes due to the density difference between the fluid in liquid and gas phase and the liquid fluid contained in the one or more recirculation downpipes;   transfer of heat from the heat-transfer fluid in gas form contained in the mixture that ascends via the vertical thermal exchanger tubes to the phase change material housed in the casing, condensing said heat-transfer fluid and melting the phase change material;   circulation of condensed fluid collected in the upper sub-chambers and in the upper chamber up to the drum; and   extraction of saturated water via the entry or exit pipe, maintaining the constant level of the heat-transfer fluid in the drum.   
     
     
         8 . A discharging process for a thermal storage system as recited in  claim 1 , the discharging process comprising the following stages:
 supply to the drum of heat-transfer fluid in liquid form via the entry or exit pipe to maintain constant discharging conditions;   descent due to gravity of the heat-transfer fluid in liquid form from the drum via the one or more recirculation downpipes, the heat-transfer fluid passing through the lower chamber and lower sub-chambers until it enters via the lower part of the heat exchanger tubes, wherein the descending heat-transfer fluid is at a temperature below the melting temperature of the phase change material;   absorption by the heat-transfer fluid of the latent heat of crystallisation of the phase change material through thermal exchange via the heat exchanger tubes, with the phase change of said heat-transfer fluid to gas phase occurring while the phase change material crystallise;   circulation of the heat-transfer fluid, which starts changing to gas phase, up the heat exchanger tubes due to the different pressures with the fluid in liquid phase which circulates along the one or more recirculation downpipes ( 8 ), which favours the natural circulation; and   circulation of the heat-transfer fluid in gas form through the upper part of the heat exchanger tubes, the upper sub-chambers, and the upper chamber towards the drum, from which the heat-transfer fluid is extracted via an extraction pipe.