Cascaded organic rankine cycle system

Abstract

A cascaded Organic Rankine Cycle (ORC) system includes a bottoming cycle working fluid is first evaporated and then superheated and a topping cycle working fluid is first desuperheated and then condensed such that a percentage of total heat transfer from the topping cycle fluid that occurs during a saturated condensation is equal to or less than a percentage of total heat transfer to the bottoming cycle fluid that occurs during a saturated evaporation.

What is claimed is: 
     
         1 . A cascaded Organic Rankine Cycle (ORC) system comprising:
 a topping cycle; and   a bottoming cycle in thermal communication with said topping cycle through a condenser/evaporator in which a bottoming cycle working fluid is first evaporated and then superheated and a topping cycle working fluid is first desuperheated and then condensed such that a percentage of total heat transfer from said topping cycle fluid that occurs during a saturated condensation is equal to or less than a percentage of total heat transfer to said bottoming cycle fluid that occurs during a saturated evaporation.   
     
     
         2 . The system as recited in  claim 1 , wherein said working fluid for said topping cycle is Siloxane MM. 
     
     
         3 . The system as recited in  claim 1 , wherein said working fluid for said bottoming cycle is R245fa. 
     
     
         4 . The system as recited in  claim 1 , wherein said bottoming cycle includes a recuperator. 
     
     
         5 . The system as recited in  claim 1 , wherein both said bottoming cycle fluid and said topping cycle fluid in the condenser/evaporator are saturated over approximately 40% of said total heat transfer. 
     
     
         6 . The system as recited in  claim 1 , further comprising a hot oil circuit in thermal communication with said topping cycle through an evaporator. 
     
     
         7 . The system as recited in  claim 1 , further comprising a cooling circuit in thermal communication with said bottoming cycle through a condenser. 
     
     
         8 . A method of operating a cascaded Organic Rankine Cycle (ORC) system in which a bottoming cycle is in thermal communication with a topping cycle comprising:
 maintaining a percent saturation for a working fluid in the topping cycle at less than a percent saturation for a working fluid in the bottoming cycle.   
     
     
         9 . The method as recited in  claim 8 , further comprising:
 utilizing Siloxane MM as the working fluid in the topping cycle; and   utilizing R245fa as the working fluid in the bottoming cycle.   
     
     
         10 . The method as recited in  claim 8 , further comprising:
 utilizing a condenser/evaporator as the thermal interface between the bottoming cycle and the topping cycle.   
     
     
         11 . The method as recited in  claim 8 , further comprising:
 operating a condenser/evaporator as a condenser for the topping cycle and as an evaporator for the bottoming cycle.   
     
     
         12 . The method as recited in  claim 8 , wherein a “knee” of the working fluid in the topping cycle flowing right to left lies to the left of the “knee” of the working fluid of the bottoming cycle flowing left to right in a normalized enthalpy plot. 
     
     
         13 . The method as recited in  claim 8 , wherein the working fluid in the topping cycle is at less than a 40% saturation for the working fluid in the bottoming cycle.