Thermal energy storage system

Abstract

A variety of energy storage and retrieval systems are described. Generally “hot” and “cold thermal reservoirs are provided. The “hot” reservoir holds both liquid and saturated vapor phase working fluid. The “cold” reservoir holds working fluid at a lower temperature than the hot reservoir. A heat engine/heat pump unit: (a) extracts energy from vapor passing from the hot reservoir to the cold reservoir via expansion of the vapor in a manner that generates mechanical energy to facilitate retrieval of energy; and (b) compresses vapor passing from the cold reservoir to the hot reservoir to facilitate the storage of energy. In some embodiments, the heat engine/heat pump takes the form of a reversible positive displacement heat engine that can act as both an expander and a compressor. To facilitate the storage and retrieval of electrical energy, an electric motor/generator unit may be mechanically coupled to the heat engine/heat pump unit.

What is claimed is: 
     
         1 . An energy storage and retrieval system comprising:
 a first thermal reservoir arranged to hold water and saturated steam in a first state;   a second thermal reservoir arranged to hold water and steam in a second state having a lower temperature than the first state; and   a reversible positive displacement steam engine arranged to,
 (a) extract energy from steam passing from the first thermal reservoir to the second thermal reservoir via expansion of the steam in a manner that generates mechanical energy to facilitate retrieval of energy from the energy storage and retrieval system, and 
 (b) compress steam passing from the second thermal reservoir to the first thermal reservoir to facilitate the storage of energy in the energy storage and retrieval system, 
   whereby water and steam serve as a working fluid for the energy storage and retrieval system.   
     
     
         2 . An energy storage and retrieval system as recited in  claim 1  further comprising an electric motor/generator mechanically coupled to the steam engine, the electric motor/generator being arranged to drive the steam engine when the steam engine is operated as a heat pump and arranged to generate electricity when the steam engine is operated as a heat engine. 
     
     
         3 . An energy storage and retrieval system as recited in  claim 1  wherein the steam engine is selected from the group consisting of a unaflow steam engine, a universal unaflow steam engine and a counter-flow steam engine. 
     
     
         4 . An energy storage and retrieval system as recited in  claim 1  wherein the first thermal reservoir includes a pressure vessel arranged to hold the working fluid in the first thermal reservoir at a pressure substantially above ambient atmospheric pressure. 
     
     
         5 . An energy storage and retrieval system as recited in  claim 1  wherein the second thermal reservoir is arranged to hold unpressurized working fluid and includes a sub-atmospheric pressure chamber that facilitates sub-atmospheric flashing of liquid water to steam and/or sub-atmospheric condensation of steam to a liquid water state. 
     
     
         6 . An energy storage and retrieval system as recited in  claim 1  wherein the second thermal reservoir includes a pressure vessel arranged to hold steam at sub-atmospheric pressures and facilitates sub-atmospheric flashing of liquid water to steam and/or sub-atmospheric condensation of steam to a liquid water state. 
     
     
         7 . An energy storage and retrieval system as recited in  claim 1  wherein the steam engine includes a crankshaft and at least one working chamber and each working chamber has an associated reciprocating piston coupled to the crankshaft and a plurality of associated valves that facilitate the introduction of steam into the working chamber and the exhaustion of steam from the working chamber and wherein the timing of the opening and closing of the valves is variable such that: (a) the steam engine can be operated in both an expansion mode and a compression mode with the crankshaft rotating in the same direction; and (b) the timing of the opening and closing of the valves relative to the crankshaft angle may be varied to facilitate altering an expansion/compression ratio of the steam engine. 
     
     
         8 . An energy storage and retrieval system as recited in  claim 1  wherein the steam engine includes a water injector for adding water to steam passing through the steam engine for compression before the compressed steam is exhausted from the steam engine to the first thermal reservoir. 
     
     
         9 . An energy storage and retrieval system as recited in  claim 1  wherein the steam engine includes:
 a plurality of sequential expansion stages; and 
 a steam separator for removing water from partially expanded steam between an associated pair of the expansion stages. 
 
     
     
         10 . An energy storage and retrieval system as recited in  claim 1  wherein a Round Trip Efficiency of the energy storage and retrieval system is at least 70 percent. 
     
     
         11 . An energy storage and retrieval system as recited in  claim 1  wherein the second thermal reservoir includes first and second stages, wherein the first stage receives and condenses steam exhausted from the steam engine after expansion by the steam engine and the second stage operates as a source of steam for compression by the steam engine. 
     
     
         12 . An energy storage and retrieval system as recited in  claim 11  further comprising:
 a heat source arranged to directly or indirectly heat working fluid in the second stage of the second thermal reservoir; and 
 a cooling unit for removing heat from working fluid in the first stage of the second thermal reservoir. 
 
     
     
         13 . An energy storage and retrieval system as recited in  claim 1  further comprising a heater for at least one of:
 superheating steam drawn from the first thermal reservoir before such steam is passed through the steam engine; and 
 reheating steam between expansion stages of the steam engine. 
 
     
     
         14 . An energy storage and retrieval system as recited in  claim 1  further comprising an electric motor/generator mechanically coupled to the steam engine, the electric motor/generator being arranged to drive the steam engine when the steam engine is operated as a heat pump and arranged to generate electricity when the steam engine is operated as a heat engine, and wherein:
 the steam engine is selected from the group consisting of a unaflow steam engine and a universal unaflow steam engine; 
 the first thermal reservoir includes a pressure vessel arranged to hold the working fluid in the first thermal reservoir at a pressure substantially above ambient atmospheric pressure; and 
 the second thermal reservoir includes a pressure vessel arranged to hold steam at sub-atmospheric pressures and facilitates sub-atmospheric flashing of liquid water to steam and sub-atmospheric condensation of steam to a liquid water state. 
 
     
     
         15 . An energy storage and retrieval system as recited in  claim 14  wherein the steam engine includes a crankshaft and at least one working chamber and each working chamber has an associated reciprocating piston coupled to the crankshaft and a plurality of associated valves that facilitate the introduction of steam into the working chamber and the exhaustion of steam from the working chamber and wherein the timing of the opening and closing of the valves is variable such that: (a) the steam engine can be operated in both an expansion mode and a compression mode with the crankshaft rotating in the same direction; and (b) the timing of the opening and closing of the valves relative to the crankshaft angle may be varied to facilitate altering an expansion/compression ratio of the steam engine. 
     
     
         16 . An energy storage and retrieval system as recited in  claim 2  wherein the electric motor/generator includes at least one motor and at least one generator that is separate from the motor. 
     
     
         17 . An energy storage and retrieval system comprising:
 a first thermal reservoir arranged to hold working fluid in a first state that includes liquid phase and saturated vapor phase work fluid;   a second thermal reservoir arranged to hold working fluid in a second state having a temperature that is lower than the temperature of the working fluid in first thermal reservoir; and   a heat engine/heat pump unit arranged to,
 (a) extract energy from working fluid vapor passing from the first thermal reservoir to the second thermal reservoir via expansion of the working fluid in a manner that generates mechanical energy to facilitate retrieval of energy from the energy storage and retrieval system, and 
 (b) compress working fluid vapor passing from the second thermal reservoir to the first thermal reservoir to facilitate the storage of energy in the energy storage and retrieval system. 
   
     
     
         18 . An energy storage and retrieval system as recited in  claim 17  further comprising an electric motor/generator arranged to drive the heat engine/heat pump unit when the heat engine/heat pump unit is operated in a manner that conveys working fluid vapor from the second thermal reservoir to the first thermal reservoir and for generating electricity when the heat engine/heat pump unit is operated in a manner that conveys working fluid vapor from the first thermal reservoir to the second thermal reservoir. 
     
     
         19 . An energy storage and retrieval system as recited in  claim 17  wherein the first thermal reservoir includes a pressure vessel arranged to hold working fluid in the first thermal reservoir at a pressure substantially above ambient atmospheric pressure. 
     
     
         20 . An energy storage and retrieval system as recited in  claim 17  wherein the second thermal reservoir is arranged to facilitate sub-atmospheric flashing of liquid working fluid to a vapor state and/or sub-atmospheric condensation of vapor working fluid to a liquid state. 
     
     
         21 . An energy storage and retrieval system as recited in  claim 17  wherein the working fluid is selected from the group consisting of:
 (a) a mixture that includes water; 
 (b) a fluorocarbon or a mixture that includes a fluorocarbon; 
 (c) ammonia or a mixture that includes ammonia; and 
 (d) a hydrocarbon or a mixture that includes a hydrocarbon. 
 
     
     
         22 . An energy storage and retrieval system comprising:
 a first thermal reservoir arranged to hold working fluid in a first state that includes liquid phase and saturated vapor phase work fluid;   a low temperature thermal energy source arranged to provide vapor phase working fluid in a second state having a lower temperature than the first state;   a condenser arranged to condense vapor phase working fluid; and   a heat engine/heat pump unit arranged to,
 (a) extract energy from working fluid vapor passing from the first thermal reservoir to the condenser via expansion of the working fluid vapor in a manner that generates mechanical energy to facilitate retrieval of energy from the energy storage and retrieval system, and 
 (b) compress working fluid vapor passing from the low temperature thermal energy source to the first thermal reservoir to facilitate the storage of energy in the energy storage and retrieval system. 
   
     
     
         23 . An energy storage and retrieval system as recited in  claim 22  wherein the heat engine/heat pump unit is a reversible positive displacement steam engine and the working fluid is water.