Field

This invention relates to devices and methods for converting thermal energy into mechanical energy. In particular, this invention relates to improving the efficiency of a heat engine.

Background

Thermal energy is often produced as a by-product in industrial and commercial processes. Waste thermal energy may be used as an input to a heat engine to produce mechanical energy, for instance as an input to an electrical generator. The efficiency of the heat engine is an important factor in determining the economic viability of building a heat engine to use waste energy as an input

A second factor affecting heat engines is that due to daily consumption patterns, it is economically advantageous to not produce electricity during periods of low consumption when electricity prices are low, and to make electricity available during periods of high consumption when electricity prices are high.

It is therefore desirable to develop a heat engine adapted to produce electricity economically during periods of high demand, thereby reducing the cost of producing backup energy in times of peak demand.

Brief Description nfrhe Drawings

In drawings which illustrate by way of example only:

Figure 1 is a schematic illustration of a thermal engine;

Figure 2 shows the dual acting piston, valves, transmission, and generator configuration of the present invention;

Figure 3 shows the dual acting piston in valve position 1;

Figure 4 shows the dual acting piston in valve position 2;

Figure 5 is a schematic illustration of the liquid nitrogen expansion engine; Figure 6 shows the piston in the actuator in position 1 of the nitrogen expander engine process;

Figure 7 shows the piston in the actuator in position 2 of the nitrogen expander engine process

Detailed Description

In an implementation, a theat engine is provided that is operable to extract energy from a thermal energy differential between a first thermal source and a second thermal source and to convert this energy into mechanical energy that can be used to generate electrical energy. In an implementation, the thermal sources are put in fluid communication with two vessels containing a gas under pressure. The thermal sources having thermal values that are different than the thermal values of the vessels. In an aspect, the higher temperature thermal source comprises waste heat from an industrial or residential process.

The thermal sources are used to alternately increase the temperature and pressure in one of the vessels and decrease the temperature and pressure in the other vessel. A pressure driven dual acting piston is moved in a single direction by the resulting pressure released by the first vessel and the suction from the second vessel. A reversal of the heating and cooling cycle causes the piston to move in the opposite direction.

According to another aspect, there is provided an apparatus for extracting a differential in thermal energy between a first thermal source and a second thermal source and converting this energy into mechanical energy is provided. The apparatus has first and second vessels that include a gas under pressure. The thermal sources have thermal values that are different than the thermal values of the vessels. The thermal sources are adapted to alternately increase the

temperature and pressure in one of the vessels while decreasing the temperature and pressure in the other vessel. A pressure driven double acting piston actuator coupled to the vessels is moved in a single direction by pressure released by the first vessel and suction from the second vessel. An apparatus for converting a differential in thermal energy between a first thermal source having a thermal conducting fluid and a second thermal source having a thermal conducting fluid, the apparatus comprising:

• a first vessel for containing a gas under pressure, the first vessel being in fluid communication with said first and second thermal sources;

• a second vessel for containing gas under pressure, the second vessel being in fluid communication with said first and second thermal sources;

a plurality of cooperating valves for alternately regulating a flow of thermal conducting fluid from the first and second thermal sources to the first and second vessels, the plurality of cooperating valves alternating between the first and second operating positions, the plurality of cooperating valves permitting a flow of thermal conducting fluid from the first thermal source to the first vessel and from the second vessel in first operating position, the plurality of cooperating valves preventing a flow of thermal conducting fluid from the first thermal source to the second vessel and from the second thermal source to the first vessel in the first operating position, the plurality of cooperating valves permitting a flow of thermal conducting fluid from the first thermal sou ce to the second vessel and from the second thermal source to the first vessel in the second operating position, the plurality of cooperating valves preventing a flow of thermal conducting fluid from the first thermal source to the first vessel and from the second thermal source to the second vessel in the second operating position;

a pressure driven piston actuator in fluid communication with the first and second vessels whereby the piston actuator is driven into reciprocating motion between a first position and a second position by alternating positive pressure and negative pressure from the first and second vessels wherein positive pressure from the first vessel coupled with negative pressure from the second vessel when the plurality of cooperating valves is in the first operating position drives the actuator to the first position and negative pressure from the first vessel coupled with positive pressure from the second vessel when the plurality of cooperating valves is in the second operating position drives the actuator to the second position. According to another aspect there is provided a method for converting a differential in thermal energy to mechanical energy comprising the following steps:

• providing first and second vessels containing a gas under pressure, the gas under pressure being of a temperature T;

• providing a first thermal source and a second thermal source, the first thermal source housing a thermal transfer fluid of a temperature above T and the second thermal source housing a thermal transfer fluid of a temperature below T.

• delivering the thermal transfer fluid from the first thermal source to the first vessel thereby raising the pressure of the gas in the first vessel;

• delivering the thermal transfer fluid from the second thermal source to the second vessel thereby lowering the pressure of the gas in the second vessel;

• delivering gas under pressure from the first vessel to a pressure activated piston actuator and applying suction from the second vessel to the pressure activated piston actuator thereby causing the pressure activated piston actuator to move in a first direction.

In an implementation, a piston driven nitrogen expander engine is provided, operative to extract heat from the heat sink and to use the extracted heat in the process of returning the liquid nitrogen to the gaseous state, and to harness the expansion of the nitrogen as it transitions from liquid to gas.

In an implementation, the heat engine and the nitrogen expender engine are arranged in an integrated fashion that creates a synergy of the two power generation methods that increases the overall efficiency of the two methods compared to the sum of the individual efficiencies on their own without being integrated.

In an implementation, a thermal engine is implemented as a piston heat engine in combination with a piston driven liquid nitrogen expander engine. The heat engine and the expander engine are combined by using the liquid nitrogen source of the expander engine as a heat sink for the heat engine, allowing the system to use a lower grade heat source and improve the Carnot efficiency of the system as a whole.

In an implementation, an apparatus for converting a differential in thermal energy between a heat source having a heating thermal conducting fluid and a heat sink having a cooling thermal conducting fluid is provided. The apparatus may comprise: a heat engine comprising: a pair of gas-filled vessels in communication with said heat source and said heat sink; a pressure driven reciprocating actuator comprising a pnematic cylinder defining a first chamber and a second chamber separated by at least one piston moveable within said pneumatic cylinder, said first chamber and said second chamber in fluid communication with said gas-filled vessels; said pair of gas-filled vessels supplying a gas comprising a working fluid to said first chamber and said second chamber of said pressure driven reciprocating actuator; and, a controller for alternating flow of the thermal energy from heat source and the heat sink between each of the pair of gas-filled vessels to alternately raise and lower pressure of said gas in the vessels to alternately transfer gas from one vessel to a one of the first chamber and the second chamber of the reciprocating actuator and transfer gas from an other of the first chamber and the second chamber of the reciprocating actuator to the other vessel to drive the actuator in reciprocating motion; and, a liquid nitrogen expansion engine comprising: a liquid nitrogen supply; the liquid nitrogen supply in thermal communication with a cooling thermal transfer fluid circulating between the heat sink and the pair of gas-filled vessels; the liquid nitrogen supply further in fluid communication with an expansion engine; wherein as the liquid nitrogen supply lowers the thermal energy of the thermal transfer fluid, liquid nitrogen is directed to the expansion engine.

In an aspect, the apparatus may further comprise: the expansion engine in thermal communication with a heating thermal transfer fluid circulating from the pair of gas-filled vessels back to the heat source; wherein the heating thermal transfer fluid raises the thermal energy of the liquid nitrogen. In an aspect, the heating thermal transfer fluid raises the thermal energy of the liquid nitrogen after it has been expanded into a gaseous state within a pressure vessel.

Tn an aspect, the expansion engine comprises at least one pressure vessel for receiving the liquid nitrogen and storing the liquid nitrogen in a gaseous state, and releasing the stored nitrogen as a second working fluid into a second pressure driven reciprocating actuator comprising a second pnematic cylinder defining a first chamber and a second chamber separated by at least one piston moveable within said second pneumatic cylinder, said first chamber and said second chamber in fluid communication with said at least one pressure vessel; the controller further operative to alternately transfer the stored nitrogen from the at least one pressure vessel to a one of the first chamber and the second chamber of the second reciprocating actuator to drive the actuator in reciprocating motion.

In an aspect, an apparatus for converting liquid nitrogen to gaseous nitrogen and harnessing the conversion to drive a piston and generator is provided. The apparatus may comprise: a means of delivering the liquid nitrogen under pressure to an expansion engine; a means of delivering the liquid nitrogen from the interchanger to expansion valves and nozzles; a means of injecting the liquid nitrogen as gaseous nitrogen into pressure vessels for storage; a means for releasing the stored gaseous nitrogen to at least one double acting piston assembly to generate work. In an aspect, a means of heating the gaseous nitrogen in the pressure vessels may be provided.

In an aspect, the liquid nitrogen source may be re-supplied using low cost energy during off-peak periods, and may be expanded through the expander engine at another time during periods of high energy consumption, improving the economics of the system. Accordingly the liquid nitrogen source further provides for an energy storage that allows for improved energy generation efficiency during peak periods of consumption. An apparatus is provided for converting a differential in thermal energy between two thermal sources into mechanical energy that can be used for a wide range of applications known to a person skilled in the art including the generation and storage of electrical energy, or storing the means to enable the production of electrical energy in the form of liquid nitrogen as a heat sink enhancer.

An embodiment is shown in Figure 1. The heat engine 1 includes a first vessel 2 and a second vessel 4. Each of the two vessels is preferably a sealed container that defines a chamber therein for containing a gas under pressure. As shown in Figure 1, the first vessel 2 defines a first chamber 3 and the second vessel 4 defines a second chamber 5. The vessels 2, 4 each contain a working gas under pressure in the chambers 3, 5.

The vessels 2, are shown in longitudinal cross-section in Figure 1. Each of the vessels 2, 4 preferably has an insulating jacket 72 for preventing thermal exchange with the ambient environment.

The first vessel 2 has a first heat exchange conduit 10 located in the first chamber 3. The conduit is preferably a tube bundle consisting of copper tubing that is adapted to conduct a heat conveying fluid. Other conduits known in the art to have favourable heat exchanging properties may also be employed in alternate embodiments. The first heat exchange conduit 10 has a first end 30 that

communicates with the exterior of the vessel 2 through an opening 31 defined by vessel 2. The first heat exchange conduit 10 has a second end 32 that communicates with the exterior of the vessel 2 through an opening 33 defined by the vessel 2.

Similarly, the second vessel 4 has a second heat exchange conduit 12 located in the second chamber 5. The second heat exchange conduit 12 is also preferably a tube bundle consisting of copper tubing that is adapted to conduct a fluid. Again, other conduits known in the art to have favourable heat exchanging properties may also be employed in alternate embodiments. The second conduit 12 has a first end 34 that communicates with the exterior of the vessel 4 through an opening 35 defined by the second vessel 4. The second heat exchange conduit 12 has a second end 36 that communicates with the exterior of the second vessel 4 through an opening 37 defined by the vessel 12.

In the implementation shown, first vessel 2 includes a first pressure sensor 102, and second vessel 4 includes a second pressure sensor 104.

The heat engine lfurther includes a thermal heat source 6, such as a waste heat source from an industrial or residential process. A thermal heat sink 8 provides the medium to transfer heat from the heat source 6 via the heat engine 1 in order to extract work. The heat sink 8 has a lower temperature than the heat source 6.

Preferably, the thermal source and thermal sink define the limits of a heat exchange circuit comprising an interchanger conduit running through them for passage of the thermal conducting fluid. The thermal conducting fluid transferring heat from the heat source 6 to the heat sink 8 via the heat engine 1 The thermal conducting fluid is preferably an environmentally suitable fluid that can operate between the temperature differentials of the operating conditions of the implementation. By way of example, the present embodiment of Figure 1 has been implemented in exemplar form using a thermal conducting fluid having an operating temperature range between about -100°C and +250°C .

The thermal heat source 6 and heat sink 8 can be any medium that is capable of storing or transferring thermal energy to or from the thermal conducting fluid. Among the examples of possible thermal sources include ambient outside air, outside soil, water heated by energy produced by natural gas combustion, wood combustion, solar energy or geothermal energy, or industrial waste heat. Sample examples of thermal sinks include, for instance, the ground, ambient outside air, or water.

A thermal fluid-conducting heat supply conduit 42 communicates between the heat source 6 and the first vessel 2. The heat supply conduit 42 further communicates between the heat source 6 and the second vessel 4. A fork 43 in the heat supply conduit 42 separates the conduit into a first branch leading to the first vessel 2 and a second branch leading to the second vessel 4. A thermal fluid-conducting sink supply conduit 38a communicates between heat sink 8 and the first vessel 2 and the second vessel 4. A fork 45 in the sink supply conduit 38a separates the conduit into a first branch 44a leading to the first vessel 2 and a second branch 44b leading to the second vessel 4. The thermal fluid- conducting sink supply conduit 38a comprises a supply path from the heat sink 8 to the first vessel 2 and the second vessel 4.

A thermal fluid-conducting sink return conduit 38b communicates between the first vessel 2 and the heat sink 8, and provides a return path from the first vessel 2 to the heat sink 8. The sink return conduit 38b further communicates between the second vessel 4 and the heat sink 8, and provides a return path from the second vessel 4 to the heat sink 8. A fork 39 in the sink return conduit 38b separates the sink return conduit 38b into a branch leading from the first vessel 2 and another branch leading from the second vessel 4.

A thermal fluid-conducting heat return conduit 40 communicates between the first vessel 2 and the heat source 6. The heat return conduit 40 further communicates between the second vessel 4 and the heat source 6. A fork 41 in the heat return conduit 40 separates the conduit into a branch leading from the first vessel 2 and another branch leading from the second vessel 4.

A first valve 14 controls the flow of fluid from the thermal unit 6 to the first heat exchange conduit 10. A second valve 26 controls the flow of fluid from the thermal unit 6 to the second heat exchange conduit 12. A third valve 22 controls the flow of fluid from the thermal unit 8 to the first heat exchange conduit 10. A fourth valve 18 controls the flow of fluid from the thermal unit 8 to the second heat exchange conduit 12. A fifth valve 16 controls the flow of fluid from the first heat exchange conduit 10 to the thermal unit 6. A sixth valve 24 controls the flow of fluid from the first heat exchange conduit 10 to the thermal unit 8. A seventh valve 28 controls the flow of fluid from the second heat exchange conduit 12 to the thermal unit 6. An eighth valve 20 controls the flow of fluid from the second heat exchange conduit 12 to the thermal unit 8. Preferably the valves are electronically operated ball or piston valves although other valves known in the art may also be employed. In an alternate embodiment, cam operated piston valves may be used, particularly in a multi-module engine, in order to ensure syn hronization of the pistons and valves. A controller 70 is operatively connected to the valves for opening and closing the valves as required. The eight valves described herein together with the controller 70 comprise a plurality of cooperating valves for alternately regulating a flow of thermal energy from the heat source 6 and the heat sink 8 to the first vessel 2 and the second vessel 4.

To avoid unnecessary heat loss during cycling between the first vessel 2 and the second vessel 4, the in valve and out valves are timed to allow the return of the respective heating and cooling fluids to their source before the incoming fluid reaches the out port of the respective vessel 2, .

Preferably, pump 46 and pump 48 pump the thermal fluids through the thermal fluid conducting conduits. The pumps 46, 8 are preferably circulating pumps.

Vessel 2 further defines an opening 53. A first pressure conduit 54 is received in the opening 53 and communicates between the chamber 3 and the exterior of the vessel 2 for delivering the working gas from the chamber 3 to the exterior and vice versa. Similarly vessel 4 further defines an opening 55. A second pressure conduit 56 communicates between the chamber 5 and the exterior of the vessel 4 for delivering gas from the chamber 5 to the exterior and vice versa.

As shown in Figure 2, the pneumatic cylinder 58 has a piston 74 moveably received therein. The pneumatic cylinder 58 defines a first chamber 106 and a second chamber 108. Each of the pressure conduits 130 and 134 preferably communicate with the first end 106 of pneumatic cylinder 58. Similarly, each of the pressure conduits 132 and 136 communicate with the second end 108 of the pneumatic cylinder 58. The piston 74 is connected to a gear 79 as shown in Figure 2, and the gear 79 is connected to a driveshaft 80 connected to a generator. A valve 50 is located at the junction of conduits 130, 132, and 54 leading from vessel 2 and the pneumatic cylinder 58 for regulating gas flow. Similarly, valve 52 is located at the junction of conduitsl36, 134, and 56 leading from vessel 4 and the pneumatic cylinder 58 for regulating gas flow. A valve 138 is located in the pressure conduit 132 between the vessel 4 and the pneumatic cylinder 58 for regulating gas flow. Similarly, valve 140 is located in the pressure conduit 132 between vessel 2 and the pneumatic cylinder 58.

Connecting driveshaft 80 is preferably coupled to the gear 79 and a transmission. The transmission can be coupled to a flywheel and generator combination.

Vessel 2 is connected to the pressure conduit 54. The first pressure conduit 54 feeds into pressure conduits 130 and 132. Valve 50 is located between the first pressure conduit 54 and the conduits 130 and 132. Similarly, vessel 4 is connected to the second pressure conduit 56. The second pressure conduit 56 feeds into pressure conduits 134 and 136. Valve 52 is located between the second conduit 56 and conduits 134 and 136. Valve 138 is located between conduit 134 and the first end of the piston actuator 58 leading to a first chamber 106. Similarly, valve 140 is located between conduit 132 and the second end of the piston actuator 58 leading to a second chamber 108.

In its operation, the apparatus reciprocates between a first valve position as shown in Figure 3and a second valve position as shown in Figure 4 thereby driving the pressure activated piston into reciprocal motion. The reciprocal motion can be transformed into mechanical energy, which in turn can drive a generator for example.

The controller 70 controls the opening and closing of the valves of the plurality of co-operating valves. To begin the cycle whereby the apparatus moves to the first operating position, the controller opens valve 14 and closes valve 26 so that warm or hot thermal transfer fluid from the heat source 6 flows through thermal fluid conduit 42 to opening 31 and into the heat exchange conduit 10 of the vessel 2. As the heated thermal transfer fluid flows through conduit 10 in the chamber 3, heat is transferred from the conduit to the surrounding gas in the chamber 3. This causes the pressure of the gas to increase. An acceptable pressure range of the gas for the purposes of the invention is approximately 150 psi to 3000 psi. The controller opens valve 16 and closes valve 24 so that the thermal transfer fluid can flow through the opening 33 through thermal fluid conduit 40 and back to the heat source 6 where the thermal transfer fluid is re-heated.

In addition to opening valve 14 and closing valve 26, the controller simultaneously opens valve 18 and closes valve 22 so that cool thermal transfer fluid from the heat sink 8 flows through conduit 44 to opening 35 and into heat exchange conduit 12 of vessel 4. As the cool thermal transfer fluid flows through the conduit 12 in the chamber 5, heat is transferred from the surrounding gas in the chamber 5 to the conduit. This causes the pressure of the gas to decrease. The controller opens valve 20 and closes valve 28 so that the thermal transfer fluid can flow through the opening 37. The thermal transfer fluid flows through thermal fluid conduit 38 and back to heat sink 8 where the fluid is re-cooled.

When maximum thermal transfer has occurred in the two vessels, the controller 70 will open the pressure valves 50 and 52. In valve position 1, Figure 3, the increased pressure in the vessel 2 will cause the gas from the chamber 3 to flow through the pressure conduit 54 to valve 50 and through conduit 130 to chamber 106 of the pneumatic cylinder 58. At the same time, the decreased pressure in the vessel 4 will cause the gas from the second chamber 108 of the pneumatic cylinder 58 to flow through the pressure conduit 136 and into the chamber 5 of the vessel 4.

The movement of the piston 74 causes the gear 79 to rotate connecting member 80 and engage the transmission and generator.

When the piston 74 has reached its maximum prescribed travel, a sensor at the end of the second chamber 108 will cause the valves 110 and 112 to close.

In valve position 2 Figure 4, when valves 110 and 112 are closed, the controller will open valves 140 and 138 causing the piston to move in the opposite direction forcing the working fluid from chamber 106 through conduit 134 and into chamber 5 of vessel 4. The cycle will repeat until the working fluid pressure in vessels 2 and 4 are at their minimum achievable differential.

During this multi-stroke operation, part of the mass of the working fluid contained in the vessels is re-distributed to the lower pressure or cooled vessel of the stage. This results in higher-pressure di ferentials than would normally be achieved if no mass transfer occurred. When the pressure achieves its minimum achievable differential in both vessels and no additional cycles can be obtained, the process will revert to the second stage. Pressure vessel 4 will then become the heated and therefore high-pressure source and vessel 2 will become the cooled and therefore low-pressure receiver of the working fluid.

The foregoing description represents a single operating module of an embodiment. In an embodiment, multiple pairs of vessels and piston actuators, which will comprise a module, may be used to attain a consistent output to the generator. In the embodiment, the modules are connected to a common driveshaft to develop higher rpms and torque to the transmission and to drive larger generators as required. The modules consisting of one pair of vessels and one piston and gear assembly can be added as needed. They can also be of different capacities to deliver versatility to the installation, depending on the demand of the application.

In this multiple module embodiment, the valves controlling the cycles between each module in valve positions 1 and 2, can be operated by a synchronized camshaft.

In a second aspect, the output efficiency of the heat engine 1 above may be enhanced by integrating the heat engine 1 with a liquid nitrogen expansion engine 200. The integrated thermal engine 300, comprised of the heat engine 1 and the liquid nitrogen expansion engine 200, provides for a system that can synergistically operate with a greater overall efficiency, than either component alone.

The liquid nitrogen expansion engine 200 of the thermal engine 300 uses liquid nitrogen to cool the output heat transfer fluid from the heat sink 8 thereby allowing the system to operate with a greater thermal differential than would be available using only the heat source 6 and the heat sink 8. The liquid nitrogen supply effectively lowers the temperature of the cold supply coming from the heat sink 8, increasing the temperature differential, and subsequent pressure differential, in the first vessel 2 and the second vessel 4.

In some aspects, the liquid nitrogen expansion engine 200 may be selectively enabled so as to operate the heat engine 1 with the greater thermal differential during periods of high electricity demand, and to disable the liquid nitrogen expansion engine ZOO during periods of low electricity demand. Depending upon the heat source 6 and the heat sink 8, and the operational needs, the heat engine 1 may continue to operate during the periods of low electricity demand, albeit at a lower efficiency. By way of example, where the heat source 6 comprises waste heat, the heat engine 1 may usefully be operated during periods of low electricity demand since the heat being supplied is effectively 'free'.

Accordingly, the liquid nitrogen supply may be replenished during periods of low electricity demand, when electricity prices are low and power is cheap, and expanded through the liquid nitrogen expansion engine 200, generating electricity and enhancing the ou tput of the heat engine 1, during periods of high electricity demand, when electricity prices are high and power is expensive.

Whereas storing liquid nitrogen and capturing its expansion is on its own an established process, the current prevailing method of using a turbine to capture the expansion does not achieve acceptable conversion efficiency to attain economic viability.

Figure 5 illustrates a method of capturing the expansion of the liquid nitrogen and allowing it to: first expand to a prescribed pressure in insulated pressure vessels; and, second, the pressurized nitrogen which is now a gas, is released to a modified piston assembly and process similar to the assembly described above for the heat engine, and illustrated in more detail in Figures 2, 3, and 4. Alternately, the expanded gas can be stored in the pressure vessels for release at a later time when demand is higher. This illustrates the versatility of the present invention over straight liquid nitrogen expansion through an expander/generator system.

Referring again to Figure 1, by way of example, a liquid nitrogen supply 170, identified as a LN dewar in the example, provides a a second, Lower, heat sink for the heat engine 1. The liquid nitrogen supply 170 reduces the temperature of the heat exchange fluid circulating through the sink supply conduit 38a. The pump 171 maintains a sufficient flow rate to ensure the heat transfer fluid does not cool below its freezing point. As the heat transfer fluid passes through the interchanger 190, it gives up enough heat to lower the temperature of the heat exchange fluid to its low temperature working limit but not the point at which it freezes. This provides the maximum temperature differential for thermal efficiency of the heat engine 1, while causing the liquid nitrogen contained in the liquid nitrogen supply 170 to expand through nitrogen supply conduit 150 to the nitrogen expansion engine 200.

Referring to Figure 5, the nitrogen expansion engine 200 is illustrated in more detail. The nitrogen expansion engine 200 includes a separate piston The nitrogen supply conduit 150 supplies a manifoldl52 which supplies insulated nitrogen pressure vessels 155 through control valves 153 in each of the vessels. Control valves 153 are of the type through which the rate of flow of the liquid nitrogen can be regulated.

The liquid nitrogen is then received by dispersion nozzles 1S4 in each of the pressure vessels, at which point the liquid nitrogen expands into gaseous form. The gaseous nitrogen is allowed to expand to a prescribed pressure.

Thermal fluid-conducting heat return conduit 400 is diverted through the pressure vessels 155 to add secondary heat to the gaseous nitrogen thereby maintaining and increasing the pressure as prescribed for storage or immediate use through the piston expander and transmission/generator assembly 157. As will be appreciated from Figure 1, thermal fluid-conducting heat return conduit 40 comprises the return path of the thermal fluid from the first vessel 2 and the second vessel 4. Accordingly, after the heat from the heat source 6 has been used in the heat engine 1, it is directed to the nitrogen expansion engine 200 to heat the gaseous nitrogen in the pressure vessels 155, and accordingly transfer further energy to the liquid nitrogen. The thermal fluid may then be directed through the thermal fluid- conducting heat return conduit 40 to return to the heat source 6 to be re-heated and supplied again to the heat engine 1 after heating.

In Figure 5, valves 160 are used to equalize the pressure between pressure vessels 155 as needed. Sensors 158 in each of the pressure vessels 155

communicate with the control module 70 to regulate the expander engine operation. Valves 159 control the release of the expanded nitrogen gas to conduit 156 and thenceforth to the expander piston 183.

Referring to Figure 6, the liquid nitrogen circuit pneumatic piston actuator is shown in position 1. The gaseous nitrogen flawing from conduit 156 is received by distribution junction 177, and flows through conduit 176.

Valves 179 and 180 are open and valves 178 and 181 are closed, causing the gaseous nitrogen flowing through conduit 176 to push the moveable piston 183 in one direction which in turn forces the gaseous nitrogen in chamber 184 to exit through valve 180.

During the operation of the expander engine in valve position 1, gear 186 is caused to rotate, transferring the mechanical energy created by the conversion of the liquid nitrogen, to connecting driveshaft 187, which in turn drives the transmission and generator 188.

Similarly, in valve position 2, illustrated in Figure 7, the gaseous nitrogen flowing from conduit 156 is received by distribution junction 177, and flows through conduit 175. Valves 178 and 181 are open and valves 179 and 180 are closed, causing the gaseous nitrogen flowing through conduit 176 to push the moveable piston 183 in the opposite direction which in turn forces the gaseous nitrogen in chamber 185 to exit through valve 181. During the operation of the expander engine in valve position 2, gear 186 is caused to rotate, transferring the mechanical energy created by the conversion of the liquid nitrogen, to connecting driveshaft 187, which in turn drives the transmission and generator 188.

Multiple piston assemblies are synchronized on a common driveshaft to enhance the steady delivery of power to the generator and facilitate the use of larger generators.

While various embodiments and particular applications of this invention have been shown and described, it is apparent to those skilled in the art that many other modifications and applications of this invention are possible without departing from the inventive concepts herein. It is, therefore, to be understood that within the scope of the appended claims, this invention may be practiced otherwise than as specifically described, and the invention is not to be restricted except by the scope of the claims.