Method and System for the Utilization of an Energy Source of Relatively Low Temperature

Abstract

A method of extracting energy from an external heat source. The method comprises the steps of: a) compressing a medium in the liquid phase using an external power source to obtain a compressed liquid medium; b) heating the compressed liquid medium from step a) using heat at least partly derived from the external heat source to expand the medium and obtain it in the supercritical state; c) reducing the pressure of the heated medium from step b) to a controlled degree by applying a variable load to generate electric power of a frequency; d) converting the frequency of step c) to a desired output frequency; and e) reducing the temperature and volume of the medium from step c) to obtain the medium in the liquid phase for recycling to step a), wherein the degree of compression in step a) is controlled independently of the load applied in step c). A corresponding system is also provided.

1 . Method of extracting energy from an external heat source, comprising the steps of:
 a) compressing a medium in the liquid phase using an external power source to obtain a compressed liquid medium;   b) heating the compressed liquid medium from step a) using heat at least partly derived from the external heat source to expand the medium and obtain it in the supercritical state;   c) reducing the pressure of the heated medium from step b) to a controlled degree by applying a variable load to generate electric power of a frequency;   d) converting the frequency of step c) to a desired output frequency; and   e) reducing the temperature and volume of the medium from step c) to obtain the medium in the liquid phase for recycling to step a),   wherein the degree of compression in step a) is controlled independently of the load applied in step c) and step c) is performed by a turbine connected to an electricity generator. which is capable of controlling the load applied in the turbine.   
     
     
         2 . Method according to  claim 1 , further comprising sensing the pressure between steps a) and c) and controlling the load of step c) and/or the degree of compression of step a) at least partly depending on the sensed pressure. 
     
     
         3 . Method according to  claim 1 , further comprising sensing the pressure between steps c) and a) and controlling the load of step c) and/or the degree of compression of step a) at least partly depending on the sensed pressure. 
     
     
         4 . Method according to  claim 1 , wherein part of the heat supplied in the heating step b) is derived from step e). 
     
     
         5 . Method according to  claim 4 , wherein heat from the medium in step e) is transferred to the compressed liquid medium from step a) in a heat exchanger. 
     
     
         6 . Method according to  claim 1 , wherein step e) comprises heating a cooling medium to obtain a heated cooling medium and step b) comprises heating the compressed liquid medium using the heated cooling medium to obtain a cooled cooling medium that is recycled to step e). 
     
     
         7 . Method according to  claim 6 , wherein the heated cooling medium from step e) is compressed before it is used for heating the compressed liquid medium in step b) and the cooled cooling medium from step b) is expanded before it is recycled to step e). 
     
     
         8 . Method according to  claim 6 , wherein step b) comprises:
 b1) transferring heat from the medium from step c) to the compressed liquid from step a) in a heat exchanger;   b2) heating using heat from the heated cooling medium; and   b3) heating using heat from the external heat source.   
     
     
         9 . Method according to  claim 1 , wherein step e) comprises heating water for a district heating system. 
     
     
         10 . Method according to  claim 1 , wherein the external heat source is selected from exhaust gases, industrial cooling media, heated media from a solar collector, geothermal heat sources, ground water, sea water and fresh water. 
     
     
         11 . Method according to  claim 1 , wherein the temperature of the external heat source is less than 100° C. 
     
     
         12 . Method according to  claim 1 , wherein the temperature of the external heat source is at least 5° C. higher, such as at least 10° C. higher, than the critical point for the medium at the prevailing pressure when the heat from the external heat source is supplied. 
     
     
         13 . Method according to  claim 1 , wherein the medium is selected from carbon dioxide (CO2), ethylene (C2H4), diborane (B2H6), ethane (C2H6) and nitrous oxide (N2O). 
     
     
         14 . Method according to  claim 1 , wherein the pressure reduction during step c) is controlled to balance the pressure in the energy extraction method. 
     
     
         15 . Method according to  claim 1 , wherein the density of the medium is not decreased or decreased by less than 40%, such as less than 30%, such as less than 25%, during step c). 
     
     
         16 . Method according to  claim 1 , wherein temperature is not decreased in step c) to below a temperature which is 10° C. higher than that of the compressed medium from step a). 
     
     
         17 . Heat engine system for extracting energy from an external heat source, comprising:
 a pump for compressing a liquid medium to obtain a compressed liquid medium, said pump comprising an inlet, an outlet and an external motor capable of controlling the degree of compression of the liquid medium in the pump;   a heating arrangement connected to the external heat source for heating and expanding the compressed liquid medium to obtain the medium in the supercritical state, said heating arrangement comprising an inlet connected to the pump outlet and an outlet;   a turbine for generation of mechanical work from the medium from the heating arrangement, said turbine comprising an inlet connected to the heating arrangement outlet and an outlet;   a electricity generator connected to the turbine, said electricity generator being capable of controlling the load of the turbine such that the pressure upstream of the turbine may be controlled;   a frequency converter connected to the electricity generator; and   a cooling arrangement for reducing the temperature and volume of the medium, said cooling arrangement comprising an inlet connected to the turbine outlet and an outlet connected to the pump inlet,   wherein the compression in the pump is controllable independently of the load of the turbine.   
     
     
         18 . Heat engine system according to  claim 17 , further comprising a pressure sensor arranged for sensing a pressure of the medium at a position upstream of the turbine and a control device arranged to receive the sensed pressure from the pressure sensor and control the load of the turbine and/or the degree of compression of the medium in the pump at least partly depending on the sensed pressure. 
     
     
         19 . Heat engine system according to  claim 17 , further comprising a pressure sensor arranged for sensing a pressure of the medium at a position downstream of the turbine and a control device arranged to receive the sensed pressure from the pressure sensor and control the load of the turbine and/or the degree of compression of the medium in the pump at least partly depending on the sensed pressure. 
     
     
         20 . Heat engine system according to  claim 17 , wherein the turbine is a volumetric turbine or a reversed centrifugal pump. 
     
     
         21 . Heat engine system according to  claim 17 , wherein the turbine comprises a shaft and the electricity generator is mechanically connected to the turbine shaft. 
     
     
         22 . Heat engine system according to  claim 17 , wherein the area of the outlet of the turbine is less than 1.5 times the area of the inlet of the turbine.