WO2013043999A2

WO2013043999A2 - Hybrid thermal cycle with imbedded refrigeration

Info

Publication number: WO2013043999A2
Application number: PCT/US2012/056524
Authority: WO
Inventors: William R. Palmer
Original assignee: Harris Corporation
Priority date: 2011-09-22
Filing date: 2012-09-21
Publication date: 2013-03-28
Also published as: WO2013043999A3; US20130074499A1

Abstract

A cycle (100) for producing work from heat involves pressurizing a first working fluid (F1), and heating the first working fluid under pressure (102) to obtain a first vapor. A second working fluid (F2) is compressed (106) in a compressor (204a, 204b). The first vapor and the second vapor are then mixed (108) to form a third vapor (F3). Heat is thereby transferred directly between the vapors at a common pressure. The third vapor is expanded (112) to perform work. All or a portion of the third vapor is communicated (121) to a low pressure expansion zone (214, 215) where it functions as a refrigerant used to provide cooling for the third vapor, thereby facilitating the condensate of the first fluid to liquid extracted from the third vapor. Heat extracted during the condensate process is used for later performing work. The first fluid condensate is returned to the initial pressurizing step with capacity to again acquire heat that is useful for performing work.

Description

HYBRID THERMAL CYCLE WITH IMBEDDED REFRIGERATION

The invention concerns thermal energy cycles, and more particularly systems and methods for merging thermal energy cycles including multi-pass energy recirculation techniques which enable normally rejected thermal energy to be re-used in the cycle, repeatedly.

Heat engines use energy provided in the form of heat to perform mechanical work, and exhaust a portion of the applied heat which cannot be used to perform work. This conversion of heat energy to mechanical work is performed by taking advantage of a temperature differential that exists between a hot "source" and a cold "sink." Heat engines can be modeled on various different well known thermodynamic processes or cycles. Two such well known heat engine cycles include the Brayton cycle and the Rankine cycle.

A combined cycle is an assembly of two or more engines that convert heat into mechanical energy by combining two or more thermodynamic cycles. The exhaust of one heat engine associated with a first cycle is used to provide the heat source that is used in a second cycle. For example, an open Brayton cycle is commonly combined with a Rankine cycle to form a combined cycle for power plant applications. The open Brayton cycle is typically implemented as a turbine burning a fuel, and the exhaust from this combustion process is used as the heat source in the Rankine cycle. In such a scenario, the Rankine cycle is referred to as a bottoming cycle because it uses some waste heat from the Brayton cycle to perform useful work. When using high temperature sources of heat (e.g., 2000° F), a combined open Brayton cycle with a Rankine bottoming cycle can ideally be expected to provide an energy conversion efficiency as high as 60%. In the case of low temperature heat sources (e.g., 700° F) conversion efficiencies are much lower, traditionally below about 35%.

The invention concerns a method for producing work from heat. The method involves pressurizing a first working fluid, and heating the first working fluid under pressure to obtain a first vapor formed of the first working fluid. The method also includes compressing a second working fluid comprising a second vapor.

Following the heating and compressing steps, the first vapor and the second vapor are mixed to form a third vapor. As a result of such mixing, heat is transferred from the first vapor to the second vapor. During or after the transfer of heat in this step, the third vapor is expanded to perform work. After such work has been performed in the expander, at least a first portion of the third vapor is cooled to extract from the first portion a condensate of the first vapor. Notably, at least a second portion of said third vapor is communicated to a low pressure expansion zone where it functions as a refrigerant that is used to cool the first portion. The foregoing process is repeated in a continuous cycle using the condensate recovered in the cooling step, and the second working fluid which has been reconstituted from at least said first and second portions.

The invention also concerns an apparatus for producing work from heat. The apparatus includes a boiler that is arranged to heat a pressurized flow of a first liquid working fluid to form of a first vapor. At least one compressor is provided, and configured for compressing a second working fluid in the form of a second vapor. A mixing chamber or device is configured for mixing the first vapor with the second vapor from the boiler and the compressor. The resulting mixture contained in the mixing chamber comprises a third vapor. The mixing chamber arranged in this way facilitates a transfer of thermal energy directly from the first vapor to the second vapor. Notably, this transfer of thermal energy occurs exclusive of any intervening structure. The apparatus also includes an expander which is arranged to expand the third vapor for purposes of performing useful work after or during the transfer of heat from the first vapor to the second vapor. Finally, a condenser is provided which is arranged to receive at least a first and second portion of the third working fluid from the expander. The condenser includes a low pressure heat exchanger that is arranged to use the second portion as a refrigerant to cool the first portion.

According to another aspect, a method for producing work from heat can include pressurizing a first liquid working fluid, and heating the first working fluid under pressure to obtain a first vapor comprised of the first working fluid. The method also includes compressing a second working fluid comprising a second vapor, mixing said first vapor and said second vapor to form a third vapor. Heat is transferred from the first vapor to the second vapor subsequent to the mixing.

Thereafter, the third vapor is expanded to perform work, and then cooled to extract a condensate. Specifically a condensate of the first vapor is extracted from the third vapor and, as a result of such extraction, also produces a residual portion of the third vapor. The method continues by using at least a portion of one or more fluids comprising the third vapor in a low pressure expansion zone to function as a refrigerant for providing the cooling. The refrigerant can be the third vapor, the residual portion of the third vapor, or both. The process can be repeated in a continuous cycle using the condensate recovered in the cooling step, and the second working fluid at least partially reconstituted from the residual portion. The invention can also include apparatus for carrying out the foregoing process.

Embodiments will be described with reference to the following drawing figures, in which like numerals represent like items throughout the figures, and in which:

FIG. 1 is a flow chart that is useful for understanding a hybrid thermal cycle (HTC), with imbedded refrigeration.

FIG. 2 is a drawing that is useful for understanding an apparatus configured for implementing the hybrid imbedded combined cycle in FIG. 1.

FIGs. 3A and 3B are drawings that are useful for understanding an embodiment of a condenser which can be used in of the hybrid imbedded combined cycle in FIGs. 1 and 2.

FIG. 4 is a drawing that is useful for understanding a first alternative embodiment of an apparatus configured for implementing the hybrid imbedded combined cycle in FIG. 1.

FIG. 5 is a set of plots that are useful for understanding an exemplary cycle in accordance with the present invention.

FIGs. 6a and 6b are a table that is useful for understanding an exemplary cycle in accordance with the present invention, in accordance with a computer model. FIG. 7 is an alternative embodiment of the apparatus in FIG. 2.

FIG. 8 is an alternative embodiment of the apparatus in FIG. 4.

The present invention is described with reference to the attached figures. The figures are not drawn to scale and they are provided merely to illustrate the general embodiment of the invention. Several aspects of the invention are described below with reference to example applications for illustration. It should be understood that numerous specific details, relationships, and methods are set forth to provide a full understanding of the invention. One having ordinary skill in the relevant art, however, will recognize that the invention can be practiced without one or more of the specific details or with other methods. In other instances, well-known structures or operations are not shown in detail to avoid obscuring the invention. The present invention is not limited by the illustrated ordering of acts or events, as some acts may occur in different orders and/or concurrently with other acts or events. Furthermore, not all illustrated acts or events are required to implement a methodology in accordance with the present invention.

The invention concerns a Hybrid Thermal Cycle (HTC) comprising fluids Fi , F₂, and F₃, where F₃ is comprised of fluid Fi and fluid F₂ combined or mixed. Fluid F₂ is preferably selected so that it remains vaporous throughout the cycle, and is preferably comprised of a high heat rate (high heating capacity) mixture such as nitrogen and helium or nitrogen and argon. The mixture is advantageously selected to enable the F₂ fluid to (1) provide heating during a compression portion of the cycle and (2) provide cooling later during an expansion portion of the cycle. The Fi fluid is mixed with the F₂ fluid in parts of the cycle and later in the cycle the Fi fluid is separated from the F₂ fluid. The Fi fluid is a fluid construct that is advantageously selected so that it is capable of transitioning from a liquid to a vapor in some parts of the cycle, and from a vapor to a liquid during other portions of the cycle.

During a compression portion of the cycle, the Fi fluid has the capacity to act as a coolant to the F₂ fluid. This cooling function is facilitated by a large transfer of thermal energy (i.e., from F₂ to Fi) which occurs when Fi transitions from liquid to vapor state. In simple terms this means Fi has the capacity to absorb compression heat from F₂ during the compression process. This is desirable because lowering the temperature in this process reduces the compression work required, thereby making the overall cycle more efficient.

Following the compression portion of the cycle, there is an expansion part of the cycle during which F₃, comprised of a mixture of Fi and F₂, is expanded.

During this expansion, the relationship between the Fi and F₂ fluids is effectively reversed in that Fi fluid functions to support or maintain a relatively high temperature of the F₂ fluid even as the F₂ fluid tries to cool more rapidly. This characteristic or effect in the cycle is desirable as it enables the fluid mixture to perform work longer during expansion. This ability of Fi to effectively delay the cooling of F₂ essentially ends when the Fi fluid reaches a point where it transitions from a vapor back into a liquid. At the end of the expansion process, it is desirable that a portion of the Fi fluid condense out to liquid state.

In the present invention, the Fi fluid acts largely like a thermal transport device and the F₂ fluid acts much like an energy storage device. The energy stored in the F₂ fluid is increased during a compression portion of the cycle. Accordingly, the F₂ fluid will have a maximum potential to perform work when heat is added to F₂ by mixing with Fi. During expansion of the mixture, work is extracted from the composite F₃, where the F₂ portion discharges its energy. At the end of expansion, the energy contained in the Fi and F₂ fluids are effectively discharged, and at this lower energy state are ready to repeat the cycle. In some respects, the process described herein can be viewed as a refrigeration process, with the capacity to move large quantities of thermal energy to a state of kinetic energy. Uniquely, the kinetic energy formed from heat can be extracted as power. At the end of the expansion and cooling portion of the cycle, the system architecture enables residual heat to be re-disposed to the mixer, or effectively re-circulated. This is to say that heat energy that is normally rejected from traditional cycle formats, is made useful again within this closed cycle approach, affording the opportunity to use the available thermal energy to later produce power. Referring now to FIG. 1 there is provided a flowchart of method 100 that is useful for understanding an embodiment of the invention. The method 100 can begin with step 102 where heat is added to a first working fluid Fi (a liquid at this point in the cycle). The heat is added under a set pressure; for example, this step can be performed in a boiler. The heating of the first working fluid produces a pressurized vapor (Fi vapor). Concurrent with step 102, a second vapor is compressed in step 106. The second vapor is formed of a second working fluid, which can include some percentage of working fluid Fi mixed with other working fluids (where the percentage of Fi is dominantly in the vapor state). In some embodiments, prior to or during the compressing step, a liquid spray can be added to the second vapor in step 104. In some embodiments, the liquid spray can be comprised of the first working fluid Fi. The addition of the liquid spray can advantageously perform a cooling function. This cooling operation will be described in further detail as the discussion progresses. If the liquid spray is comprised of F_{l s} then the addition of the liquid spray will increase the percentage of working fluid Fi that is contained in the working fluid F₂. The cooling process described effectively holds the compression temperature lower, thereby requiring less work from the compressor, while increasing the overall flow volume as the liquid portion of Fi becomes vaporous.

In step 108, the first vapor Fi from step 102 is mixed with the second vapor F₂ from step 106. The first vapor and second vapor are mixed at approximately the same pressure. The mixture of these vapors is a new working fluid mixture, which is referred to herein as working fluid F₃. Notably, while mixing at the same pressure is presently a preferred embodiment, the invention is not limited to this approach. There may arise advantages to mixing at varied pressures under certain conditions of operation and the invention is intended to include such alternative embodiments. In step 1 10 additional heat from an external source can optionally be added to the vapor mixture of step 108. In step 1 12, work is performed by means of expansion of the vapor mixture. The expansion of the vapor mixture is facilitated by providing a pressure drop across an expansion device. In step 114, X% of F₃ is routed to a condenser where a portion of the Fi vapor contained therein is condensed out of the F₃ working fluid to obtain Fi condensate. In some embodiments, all or most of the Fi working fluid is condensed out of F₃; however, in other embodiments a relatively larger amount of Fi can be allowed to remain in F₃. The condensing process removes a portion of Fi from the X% of F₃, so that the content of the vapor exhausted from the condensing step can be represented by the expression "X% of F₃ - condensate of Fi." In step 116, a portion of the Fi condensate created in step 114 can be used in the spray cooling step 104, described above. The remainder of the Fi condensate is optionally pre -heated in step 118, after which the pre -heated Fi condensate can be used in step 118 where it is heated and pressurized to once again produce the Fi vapor so that the process can continue. While not shown, Fi can be preheated in step 116, prior to the spray operation of step 104.

Step 120 occurs in parallel with condensing step 114 and involves using Y% of F₃ as a refrigerant in the condensing step, where X% + Y% = 100%. The Y% of F₃ is used as a refrigerant by routing a percentage of the F₃ flow through a low pressure expansion zone (acting in the form of an evaporator where the Y% of F₃ is at a lower pressure) where the vapor temperature is reduced within a plurality of flow channels. The flow channels effectively function as a low pressure heat exchanger by passing the remaining X% of F₃ vapor over or past the cooler flow channels (or the exterior surface of the low pressure zone) facilitating condensation of the Fi constituent contained within the F₃ composite flow. The Fi transition from vapor to liquid then makes the liquid Fi available for use in further absorbing (and

transporting) thermal energy later in the cycle.

In step 121 the Y% of F₃ is drawn through the flow channels of the low pressure heat exchanger (LPHE). The low pressure within the LPHE can be provided by means of a high flow vacuum pump. Alternatively, if output of the expander is appreciably higher than atmospheric pressure, then the pressure drop within the LPHE can be satisfied by the use of a compressor operating at a lower pressure. The same device used in step 121 can be effectively used in step 122 to compress the Y% of F₃. Thereafter, in step 124 the Y% of F₃ and the vapor exhausted from step 114 (i.e., X% of F₃ - condensate of Fi) are each communicated to a different compressor where the mixture of the two is used to once again forming working fluid F₂. Note that the vapor exhausted from F₃ at step 114 is sometimes referred to herein as a residual portion of F₃, meaning that it is a remaining portion of the one or more fluids previously comprising F₃ that exist after the Fi condensate has been extracted from F₃. The process then continues at step 106, the F₂ working fluid is compressed, and the cycle continues.

The method 100 will now be described in further detail in relation to a heat engine 200 which is shown in FIG. 2. The heat engine 200 is capable of

implementing the method 100. However, it should be appreciated that the heat engine 200 is merely provided by way of example and is not intended to limit the invention. Many variations of heat engines incorporating the inventive methods are possible. For example the shaft comprising 205a, 205b, 205c can be continuous, but is not required to be so. Likewise other components can be substituted provided that the heat engine is capable of carrying out the various steps in method 100. Accordingly, a heat engine incorporating the inventive methods can include more or fewer components or steps and still remain within the scope of the invention.

Referring now to FIG. 2, a first liquid working fluid (Fi) is pressurized using a pump 201. The pressurized fluid is optionally communicated to a pre-heater

202 and then to a boiler 203. According to one embodiment, the boiler 203 can use as its primary heat source a supply of steam from a geothermal well. In that case, the pre-heater can use a down-line flow of residual hot water prior to returning same to the geothermal well, and after it has passed through the boiler 203, to scavenge residual heat contained in the water. Still, the invention is not limited in this regard and any other suitable heat sources can be used. The pre-heater 202 and the boiler

203 add a predetermined amount of heat to the first working fluid from heat sources applicable to the respective system design. As a result of these operations, the first working fluid is converted to a first vapor (Fi vapor). Thereafter, the vapor is communicated to a mixing chamber or mixing device 206 (which is sometimes referred to herein as a mixer). The Fi vapor will contain a certain amount of thermal potential energy (heat energy) when it enters into the mixing chamber.

Concurrent with the operations described above involving the first working fluid, a second working fluid (F₂) in the form of a second vapor (F₂ vapor) is compressed in one or more compressors 220, 204a, 204b. As noted above in relation to FIG. 1 , the second working fluid F₂ can be comprised in part of the first working fluid Fi (i.e., that portion of Fi that does not drop out as liquid condensate). The compressing operation involves the input of work into the system. In the embodiment shown in FIG. 2, a liquid spray comprising the first working fluid Fi can be added before or during the compressing operation at compressors 204a, 204b. The liquid spray can be supplied from the pressure side of pump 201 as shown in FIG. 2. The liquid spray can have the capacity to perform a cooling function by vaporizing and thereby absorbing heat from the second working fluid during compression. The liquid spray thus lowers the temperature of compression and reduces compressor work. The addition of the liquid spray will naturally increase the percentage amount of vaporous working fluid Fi contained in working fluid F₂.

Although two separate compressors 204a, 204b are shown in FIG. 2, it should be noted that two separate compressors are not really required. Compressors 204a, 204b effectively function as one compressor, but a liquid spray can

advantageously be added between a first and second stage of the compression process performed at 204a, 204b, respectively. Accordingly, two separate compressors are shown in FIG. 2 to communicate this concept more clearly. Still, it should be understood that a single compressor can be used for this purpose with the provision to inject spray between the compression stages, for purposes of absorbing the available heat within the compression process.

The compressed F₂ vapor from the compressors 204a, 204b is communicated to the mixing chamber 206. Within the mixing chamber 206, the Fi vapor from boiler 203 and the F₂ vapor from compressor 204b are combined or mixed to form a vaporous mixture of a third working fluid F₃ (F₃ vapor). Due to this mixing of the working fluids, a thermal transfer occurs between the fluids such that at least a portion of the heat associated with the Fi vapor (from boiler 203) can be transferred to the F₂ vapor (from compressor 204b). Optionally, additional heat can be provided at this point to the F₃ vapor contained in the mixing chamber. For example, the additional heat can be provided to the mixer from a source that is external to the system shown in FIG. 2.

It is not necessary for all heat transfer from the Fi vapor to the F₂ vapor to occur within the mixing chamber 206. In some embodiments of the invention, a portion of such heat transfer can occur after the F₃ vapor exits the mixing chamber. For example, in an embodiment of the invention, at least a portion of such heat transfer can continue occurring as the F₃ vapor continues through an expansion cycle discussed below. Also, it is possible for the fluids to enter the mixer at approximately the same temperature. However, as a result of the different chemical compositions of such fluids, transfer or exchange of heat as between them, can still potentially take place in a subsequent expansion cycle. Details of the expansion cycle are discussed below with regard to expander 208.

Significantly, the thermal transfer described herein occurs directly between the mixed working fluids Fi and F₂ and not across physical boundaries as would be the case if a conventional heat exchanger was used for this purpose. Consequently, the transfer of heat from the Fi vapor to the F₂ vapor can occur in a way that is substantially instantaneous, and highly efficient. In effect, this process provides a heat exchanger without the presence of walls separating the fluids that are exchanging heat (i.e., a wall-less heat exchanger).

The mixing chamber 206 can receive a vaporous fluid volumetric flow of Fi at pressure pi and a vaporous fluid volumetric flow of fluid F₂ at pressure p₂, where pi and p₂ are substantially the same pressure. In an embodiment of the invention, the volume of the mixing chamber is not restrictive with respect to the flow of fluid Fi and F₂. Accordingly, the volume of the mixing chamber can be selected to be V_FI+V_F2=V_F3 where V_FI is the volumetric flow rate of fluid F_ls V_F2 is the volumetric flow rate of fluid F₂, and V_F3 is the sum of the volumetric flow rate of fluid Fi + fluid F₂ at a near constant pressure. Still, the invention is not limited in this regard and the volume of the mixing chamber 206 could be increased or decreased, thereby providing the potential to change the flow velocity and having affect on the pressure of the third working fluid F₃ (F₃ vapor).

The third vapor F₃ is communicated under pressure from the mixing chamber 206 to expander 208 for performing useful work. Well known conventional expander technology can be used for this purpose, provided that it is capable of using a pressurized vapor to perform useful work. For example, the expander can be an axial flow turbine, custom turbo-expander, vane expander or reciprocating expander. Advantageously the expander 206 will be selected by those skilled in the art, to provide the highest conversion efficiency based on the specific properties of F₃ delivered to the expander, and for a particular embodiment of the cycle. Still, the invention is not limited in this regard. After such work is performed by the expander 208, the F₃ working fluid is communicated from the expander to a condenser 212 and evaporator 214 as hereinafter described.

At the condenser 212, condensate is extracted from one portion (X%) of the working fluid F₃, while another portion (Y%) of the working fluid F₃ is used as a refrigerant to facilitate the extraction of the condensate, where X% + Y% = 100%. This step makes use of a low pressure flow zone that functions in a manner that is similar to a conventional evaporator used for cooling. The arrangement involves positioning such an evaporator 214 within a condenser 212 to form a heat exchange system. The arrangement essentially acts as a low pressure heat exchanger within the condenser portion of the overall cycle and enables the F₃ fluid disposed within the low pressure zone to act like a refrigerant, where it is capable acquiring heat from within the condenser 212. A dominant (X%) portion of the F₃ fluid is further allowed to pass over the exterior surface of the low pressure zone, accelerating the condensate rate of the Fi fluid contained within the F₃ fluid mixture. The Fi portion therefore drops out as a liquid in the form of condensate, is collected in the condenser as Fi fluid liquid, and is available for reuse.

According to a preferred embodiment, the low pressure zone described above is provided in a plurality of flow channels 215 of the evaporator 214. According to one aspect of the invention, one or more flow restrictors, expansion valves or throttles 216 can be used to effect cooling of the working fluid Y% of F₃ in the evaporator. Expansion valves, flow restrictors and other types of throttling means are well known in the art and therefore will not be described here in detail. However, it should be understood that the pressure within the flow channels 215 is at a lower pressure than the environment within the condenser 212. Cooling of the working fluid Y% of F₃ is accomplished as the vaporous F₃ working fluid is drawn through the expansion valve 216 by means of the low pressure compressor or vacuum pump 220. As will be understood by those skilled in the art, the expansion of the vaporous F₃ working fluid lower will lower its temperature. This reduction in temperature allows the Y% of F₃ working fluid in flow channels 215 to then draw heat from the X% of F₃ working fluid (in vaporous state) which surrounds and circulates past the flow channels 215 and/or exterior of the evaporator. The foregoing process results in extraction of Fi condensate from the X% of F₃ working fluid as shown.

When the X% of F₃ is communicated to the condenser 212, it is passed first to an inlet chamber 217 which receives the incoming working fluid. As indicated by arrow 218, the X% of working fluid F₃ is forced to flow around the exterior of the flow channels 215 as it passes from the inlet chamber 217 to the outlet chamber 219. As it passes over the exterior of the flow channels 215, heat is drawn away from the X% of F₃ and passed to the Y% of F₃ contained inside the flow channels. This results in cooling of the X% of F₃ and extraction of Fi condensate (liquid) as shown. Since there is some amount of Fi condensate that has been removed from the X% of F₃, the content of the vapor exhausted from the expansion outlet chamber 219 can be described as "(X% of F₃) - (Fi condensate)" This vapor is also sometimes referred to herein as a residual portion of F₃, meaning that it is a remaining portion of the one or more fluids previously comprising F₃ that exist after the Fi condensate has been extracted from F₃. This vapor exhausted from the outlet chamber 219 is thereafter communicated to an inlet of the compressor 204a. The Y% of F₃ exhausted from the evaporator 214 is communicated first to the low pressure compressor 220, and then to the one or more compressors 204a, 204b. At compressor 204a, the Y% of F₃ and the "(X% of F₃) - (Fi condensate) are combined together in a mixture with optional Fi liquid spray to form F₂. The compressed F₂ vapor is thereafter communicated to compressor 204b where additional spray may be added. Notably in some

configurations this can be a single compressor with a single method of spray. From compressor 204b the fluid F₂ is communicated to mixing chamber 206 so that the cycle can continue.

The performance of the condenser is reliant on many factors, including the properties of the constituent fluids, the flow rates of the fluids, the ratios of the fluids, the condenser pressure and temperature, and hardware or apparatus physical configuration. These are all common variables that are well understood by those skilled in the art of condenser designs.

Referring now to FIGs. 3A and 3B, there are shown two different views of an evaporator 300 in which the flow restrictions or expansion valve (or valves) 216 is combined with the flow channels 215 so the two elements are integrated with one another. An inner diameter of the flow channels can be tapered so that it is larger at an outlet end of each flow channel (shown in FIG. 3B), as compared to an inner diameter at an inlet end shown in FIG. 3A. When arranged in this manner, the flow channels function essentially as an expansion valve or throttle. In such an

embodiment, the location of the expansion valve 216 can be thought of as residing at the inlet plane 302 of the tapered tubes. The arrows in FIGs. 3A and 3B show the direction of flow of the X% of F₃ and the Y% of F₃ described above in relation to FIG. 2. Of course the invention is not limited to the evaporator arrangement in FIGs. 3 A and 3B. For example separate expansion valves and flow channels can be used instead. Alternatively, any other suitable arrangement of flow channel sizing and tapering can be used for carrying out the methods described herein.

In the present invention, the condenser 212 is advantageously configured to convert to a condensate portion of the Fi working fluid contained in the F₃ vapor, but does not condense the second vaporous portion of working fluid F₂. In other words, the residual portion comprised of the second working fluid remains in a vaporous state. Those skilled in the art will appreciate that this can be accomplished by choosing the first and second working fluid to have different physical properties.

The heat engine 200 and the associated cycle can be optimized based on the temperatures and pressures of the working fluids internal to the cycle, in concert with selection of the most appropriate chemical configurations of the fluids. For example, the first and second working fluid can are comprised of chemical compositions such that the Fi portion transitions between a liquid and vapor and the F₂ portion remains dominantly vapor (it being understood that a portion of liquid may reside in a vapor stream when the vapor stream is saturated). However, the invention is not limited in this regard, and it is also possible to operate the cycle with many chemically unique constructs comprising different mixing ratios thereof. The limit of the design might be having a single chemical fluid that performs both Fi and F₂ functions adequately. In such an embodiment, the percentage of condensation, or dropout rate of fluid Fi within condenser 212 is managed to effectively let some portion (or percentage) of the fluid pass through the cycle in a vapor state (so it can be re-used as part of F₂) while permitting the remaining portion to condense to a liquid state (so that it can be re-used as Fi).

Those skilled in the art will recognize that the pumping, heating, expanding and cooling processes of steps 102, 1 12 respectively, are analogous to those which are performed in a conventional Rankine cycle. Accordingly, it is convenient to sometimes refer to this portion of the cycle as the Rankine portion of the cycle, or more simply the Rankine cycle portion. Similarly, the process of compressing the second vapor in step 106, heating in step 108, expanding in step 1 12, and subsequent cooling in step 1 14 are analogous to those which are performed in a conventional closed Brayton cycle. Accordingly, it is convenient to sometimes refer to this portion of the cycle as the Brayton cycle portion, or the Brayton portion of the cycle.

Notably, there are some aspects of the cycle in FIGs. 1 and 2 that are common to both the Rankine portion of the cycle, and the Brayton portion of the cycle. These common portions exist where the Brayton and Rankine portions of the cycle overlap. In FIG. 1 , the common steps would involve the mixing step (108), the option of adding heat to the third vapor (1 10), the expanding step (1 12), and the condensing (and therefore cooling) step (1 14). These process steps in FIG. 1 would be performed in the mixing chamber 206, the expander 208, and the condenser 212 of FIG. 2. In fact, it can be seen that the F₂ fluid acts in the capacity of a refrigeration cycle imbedded within the overall cycle construct. Overall there is a portion of a Rankine cycle combined with a Brayton cycle and integrated with a refrigeration cycle. As such, it is convenient to sometimes refer to that portion of the cycle where the working fluids are mixed, as the imbedded cycle portion.

The method described with respect to FIGS. 1 and 2 has advantages over conventional systems. For example, the method provides high thermal transfer rates which are made possible by having the working fluid be the heat exchanger (or act in the capacity of a heat exchanger). A further advantage is gained in the present invention by selecting the fluid chemical properties, temperatures and pressure such that the latent heat of vaporization is used for thermal exchange purposes. In particular, the boiler 203 can at least provide the first working fluid (Fi) with thermal energy equal to the latent heat of vaporization for such working fluid. Those skilled in the art will appreciate that liquid to vapor transformations provide very high thermal capacity.

The high thermal transfer rate accomplishes two important objectives.

First, the direct mixing process eliminates the need for the addition of heat exchanger hardware that represents additional costs associated with purchase, real-estate and maintenance. Second, the direct mixing increases the thermal transfer efficiency, effectively enabling the cycle to operate with advantageously near instantaneous thermal transfer. By enabling these higher thermal transfer rates between the working fluids, and allowing them to work together, it is possible to extract more useful energy from the mass flow rate of the combination than would otherwise be possible using the same thermal (temperature) reference. Consequently, more of the thermal potential energy contained in the first fluid is available to perform work in the expander 208 when the mixed fluids act in unison within the expansion process. In the present invention, a large quantity of heat energy is extracted from the thermal source by means of vaporization. The latent heat of vaporization provides a useful method for converting very large quantities of heat in the liquid into kinetic energy residing in the vapor F . Still, the ability of the vaporous fluid to perform work and therefore create power is constrained by the overall volume that is created (relative to the heat that is consumed in creating that volume). In order to overcome this limitation, it is advantageous to have a large volume second fluid F₂ that facilitates the process of producing the actual power.

In traditional systems, providing a large volume fluid in vaporous form presents a challenge. In particular, vaporous fluids tend to be difficult to heat by heat exchanger means, as such devices are governed by principles of convection. The present invention overcomes the limitations of the prior art, and facilitates improved efficiency by moving large quantities of thermal potential energy to the first working fluid Fi (in the Rankine portion of the cycle), and then transferring this thermal energy directly to the second working fluid (in the Brayton portion of the cycle) by mixing the first and second working fluids. Notably, a Brayton cycle and a Rankine cycle each has the capability to convert thermal energy to power at a relatively low efficiency (typically <15% assuming 325° F is the heat source temperature).

However, by combining the methods of these independent cycles, where the best features of each is utilized, it is possible that the resulting efficiency can be considerably higher.

Referring now to FIG. 4, there is shown an alternative embodiment of the invention in which a heat engine 400 operates in a manner similar to that described above with respect to the FIGs. 1 and 2. However, the embodiment in FIG. 4 includes certain additional features that provide the opportunity to increase thermal absorption (i.e., higher heat transfer rates) for drawing heat from the F₃ fluid flow that is passing over the flow channels 215 or exterior of the evaporator 214. In this embodiment a spray of liquid is added at or near the inlet portion of the low pressure zone of the low pressure heat exchanger (212, 214, 216), within the condenser portion of the overall cycle. Notably, the liquid used in this example is the Fi fluid provided from pump 201 ; but it should be understood that the invention is not limited to use of Fi fluid. Instead, the use of Fi fluid is merely shown as one possible example and other suitable fluids can be used for this purpose, where the unique fluids can be separated in the condensate process.

The foregoing arrangement has the advantage of providing a larger quantity of (non vaporous) liquid, to the fluid flow stream. As a result of adding the liquid in this way, there is a larger percentage of latent heat capability within the overall flow combination, within the low pressure zone. Stated differently, this means that there is an increased potential for thermal absorption for purposes of pulling heat from the F₃ fluid flow that is passing over the exterior of flow channels 215.

In the example shown, the Fi fluid is added at the expansion valve 216. If an evaporator used is similar to the evaporator 300 shown in FIGs. 3A and 3B, then the Fi fluid can be added at or near the inlet plane 302, just before the Y% of F₃ enters into the tapered flow channels 215. Notably, the introduction of the liquid does not have to be specifically a "spray" method. As one example, a venturi could be placed near the openings of the flow channels for this purpose, and would also be considered as functionally equivalent to spraying or providing a spray for purposes of the present invention. In addition it may be desirable to cool the portion of the liquid fluid (Fi in this example) prior to introducing it into the evaporator. These liquid cooling methods are common in industrial applications and are well known by those skilled in the art.

In the heat engines shown in FIGs. 2 and 4, the Fi spray fluid

communicated to the compressors 204a, 204b and/or expansion valve 216 can be supplied directly from pump 201 as shown. However, in an alternative embodiment, the Fi spray can be advantageously passed through a pre-heater 202 before being used for these purposes. Notably, the pre-heater 202 can provide thermal energy supplied from any source of heat that is either rejected from the boiler 203 or is otherwise deemed not useful for the operation of boiler 203. Accordingly, the pre-heater can make use of waste heat available from a thermal source associate with the heat engines shown in FIGs. 2 and 4, or can be waste heat from adjacent thermal processes that are common in many industrial plants and factories. For example, in one embodiment pre-heater 202 could utilize a return line on a geothermal well (e.g., geothermal water which has already released a substantial portion of its thermal energy in boiler 203) that may be too cool to provide boiling of the Fi fluid. Such return line can still be sufficient to provide useful thermal energy for purposes of elevating the temperature of the Fi fluid that later could be boiled. In the case of the Fi spray, it is not required that Fi fluid reaches its boiling point.

Accordingly, if the pre-heater simply raises the temperature by some amount prior to the liquid being used as a spray, then the heat energy has not been wasted, rather it has been made available for further use within the cycle. Alternatively, the pre-heater 202 can comprise a fluid path, chamber or jacket that is configured to facilitate the absorption of thermal energy from the compressor 204a, 204b. In such an

embodiment, the Fi fluid is circulated around a portion of the compressor 204a, 204b to absorb thermal energy that is naturally produced by the compressor during the process of compressing F₂. Consequently, the heat energy from the compressing step has not been wasted, but rather it has been made available for further use within the cycle.

From the foregoing, it will be understood that the heat engines 200, 400 use the coefficient of performance (CoP) of the Y% of F₃ working fluid to improve the cooling performance of the condenser 212 (heat removal from the X% of F₃ working fluid), and reissues the thermal energy to the F₂ working fluid at the compressor 204a. Consequently, the rejected heat from both the Rankine cycle and Brayton cycle portions removed by the condenser can be used again later in the Brayton cycle portion, thereby effectively delivering thermal energy to the second working fluid before it enters the compressor 204a and later mixing chamber 206. The normally rejected heat therefore has the capability to be recycled internally, providing the capacity to improve overall cycle efficiency as the cycle repeats itself.

The foregoing description provides one possible method by which the F₃ working fluid can be cooled, and a component thereof used to absorb heat that is useful in the cycle. However, it should be appreciated that the invention is not limited to the particular methods described herein. Instead, other suitable methods known now or in the future can be used for this purpose. All that is required is that, as part of the cycle, the third working fluid, or a portion thereof, is used as a coolant or refrigerant to provide a mechanism for transferring heat as part of the process. For example, FIG. 7 shows an alternative embodiment of the apparatus in FIG. 2 and FIG. 8 shows an alternative embodiment of FIG. 4. The embodiments in FIGs. 7 and 8, are similar to FIGs. 2 and 4 except that the F₃ fluid flow in the area of the condenser 212 is modified. In particular, 100% of the F₃ fluid in FIGs. 7 and 8 flows around the exterior surfaces of evaporator 214 which operates as a heat exchanger to cool the F₃ fluid. This cooling produces Fi condensate and a residual portion of F₃ consisting of one or more fluids (usually some mixture of Fi and F₂). This residual portion (F_r) can be expressed as F_r = F₃ - Fi condensate.

After flowing around the exterior of the evaporator, Y% of this residual portion is diverted and used as a refrigerant in the low pressure evaporator 214 to facilitate the extraction of the condensate from F₃. This fluid can be represented as Y% of F_r or Y% of (F₃ - Fi condensate). Taken to the limit, this design approach could involve the Y% being as much as 100% of the residual portion. In this embodiment all of the F₃ flows over the exterior of the evaporator for condensing F_{l s} and is thereafter communicated to the inlet of the evaporator low pressure zone. In this case all of the residual portion is used at a lower pressure to absorb thermal energy from the F₃ flow, thereby having the capacity to further increase thermal transfer rates without the need for splitting the flow. In such an embodiment, 100% of the Y% portion is provided to the low pressure pump 220 and 0%> of the X% portion is delivered to compressor 204a.

As in FIGs. 2 and 4, this refrigeration is facilitated by use of a low pressure flow zone that functions in a manner that is similar to a conventional evaporator used for cooling. The Y% (F₃-Fi condensate) functions as a refrigerant within the evaporator, where it is capable acquiring heat from within the condenser 212. The F₃ fluid passes over the exterior surface of the evaporator 214, accelerating the condensate rate of the Fi fluid contained within the F₃ fluid mixture. The Fi portion therefore drops out as a liquid in the form of condensate, is collected in the condenser as Fi fluid liquid, and is available for reuse. Still, the invention is not limited to the arrangements shown in FIGs. 2, 4, 7 and 8.

Referring now to FIG. 5, there is shown a graphical representation which is useful for understanding the condition of F₂ as it passes through the process described in FIGs. 2 and 4. In this example, Fi is water, and F₂ is a mixture comprised of nitrogen, helium and water vapor. Included in this drawing are several lines plotted to help in the understanding of the invention. Saturation line 502 is a reference line which shows the saturation temperature for pure water at the stated pressure. At the saturation temperature for a given pressure, cooling the vapor further will result in condensation, and heating further will result in maintaining the water in a vapor state (i.e., above the saturation line).

In FIG. 5, line 506a represents the F₂ fluid mixture as it moves through the compression portion of the cycle up to reference point 514. At reference point 514, the F₂ fluid is mixed with the FI fluid and is thereafter identified as F₃. The behavior of the F₃ fluid mixture is represented by line 506b. As noted above, F₂ is comprised of a fluid mixture containing nitrogen, helium and water. Since Fi is water in this example, mixing of Fi and F₂ will result in a fluid F₃ which is also a mixture of nitrogen, helium, and water. The difference between the two fluids F₂ and F₃ is that F₃ will have a higher percentage of water vapor as compared to F₂.

In order to understand the behavior of the F₂ and F₃ fluid mixtures, it is helpful to separately consider the theoretical behavior of the constituent components of each of those fluids. Accordingly, FIG. 5 includes plots showing the theoretical behavior of the helium and the water components of F₂ and F₃ as though they were acted upon separately and apart from the fluid mixture during compression and expansion. To this end, line 510a shows the theoretical behavior of the helium component of the F₂ mixture and line 504a represents the water component of the F₂ mixture. Similarly, line 510b shows the theoretical behavior of the helium component of the F₃ mixture and line 504b represents the water component of the F₃ mixture. The significance of these plots is discussed below in further detail. The heat capacity rate is heat transfer terminology which is used in thermodynamics and other forms of engineering. The term denotes the quantity of heat that a flowing fluid of a certain mass flow rate is able to absorb or release per unit temperature change per unit time. Helium is an example of a fluid with a relatively high capacity heat transfer rate. This means that, during the compression process 500a, helium has the potential to increase in temperature more rapidly and more substantially as compared to the remaining components (e.g., water) contained in the F₂ mixture. This difference in heat capacity rate is apparent when comparing the line 510a (representing the helium component of F₂) to line 504a (representing the water vapor component of F₂) over the compression portion of the cycle. The helium gets hotter much faster and to a greater extent as compared to the water. Notably, these lines 504a, 510a represent the behaviors of the fluids as described in an unmixed state.

In the context of the invention, this tendency of the helium toward rapid and substantial heating when compressed provides heat to the adjacent constituents (e.g., water) in F₂. The result during the compression portion 500a of the cycle is that the helium tends to raise the overall temperature of the F₂ mixture. This increase in temperature is apparent from the upward slope of line 506a in the area denoted by arrow 512. Line 506a represents the combined response of the constituent

components (helium and water) to compression.

Reference point 514 represents the point in the cycle where Fi is mixed with F₂ and the mixture becomes what we refer to herein as F₃. In relation to FIGs. 2 and 4, reference point 514 would generally correspond to the mixing chamber 206. The mixing process can cause the temperature of the fluid mixture at 518 to change in a step-like fashion as shown. For greater clarity, a plot of Fi is not included in FIG. 5. However, it can be assumed in this example that Fi is hotter than F₂, so that the temperature of the fluid mixture will increase in a step-like fashion at 518. Still, the invention is not limited in this regard and Fi can in some scenarios have a temperature which is equal to or slightly lower than F₂. Once Fi is added to F₂, the resulting fluid mixture is F₃, which is represented by line 506b. Notably, the F₃ mixture represents a larger thermal mass as compared to F₂ because F₃ also includes Fi. The larger thermal mass cools more slowly as compared to the cooling that F₂ would have experienced in the absence of the addition of Fi. Accordingly, the constituent components (the helium and water) as represented by lines 510b and 504b) will cool at a slower rate during expansion as compared to the rate which would have existed in the absence of the additional fluid (Fi). This concept is illustrated in FIG. 5 by the larger negative slope associated with lines 504b, 510b as compared to line 506b, in the area between reference points 514 and 522. The addition of the Fi fluid to the F₂ mixture tends to move the point 522 (where F₃ intersects the saturation line 502) so that it is further to the right in FIG. 5. This slowing of the cooling rate is advantageous during expansion because it allows F₃ to flow for a longer period of time to perform work before it begins transitioning back to a liquid state. Notably, once F₃ transitions to liquid, it is less useful for performing work.

The temperature of the F₃ mixture will tend to follow the saturation line 502 as it gives up latent heat. This is shown in the portion of line 506b in the area indicated by reference arrow 524. Note that to the right of reference point 522, the temperature profile of the F₃ mixture tends to track the profile of the water constituent saturation line 504b. This tracking is due to the fact that the F₃ mixture contains water and the saturation line represents the transition point of water. Notably, however, the helium constituent of F3 (as represented by line 510b in the area indicated by arrow 518) tends to cool rapidly and substantially with decreasing pressure. This cooling of the helium component at first has relatively little effect upon the F₃ mixture due to the much lower mass of helium compared to water.

However as the expansion process continues and the pressure of the helium

approaches 10 psia, the cooling tendency of helium pulls a portion of the water below the saturation line at 520, resulting in condensation. In one embodiment of the invention, this condensation can occur using the condenser arrangement as shown in FIGs. 2 and 4. In another embodiment of the invention this condensing can occur using the condenser arrangement shown in FIGs. 7 and 8. Still, it should be realized that the cooling effect provided by the helium and nitrogen can be utilized in other condenser arrangements where its tendency for rapid and substantial cooling can be applied to draw thermal energy from the water constituent.

In FIG. 5, the lines to the right of the mixing point 514 represent cooling profiles that are useful in demonstrating how the desired performance can be accomplished with this invention. Further, in this view the unconstrained helium line would not follow the saturation line, but would have a continuous slope. In FIG. 5 it is being shown in this manner, inclusive of the influence of the water portion, so that those unfamiliar with this new mixed fluid approach, can better understand the overall interaction of the fluids and how the internal heating and cooling process is possible.

It should be noted that the functionality show in FIG. 5 is a representative example, and similar effects can be achieved using alternative fluid combinations. In one embodiment, the water in the example could be replaced with methanol, pentane, or ammonia to achieve the same phenomena at lower temperatures. In such a scenario, helium could still be used as a constituent of F₂, or as an alternative, argon, or neon could be used instead. Still, the invention is not limited to these particular fluids.

An example of the operation of heat engine 400 will now be provided in order to more completely understand the present invention. In particular, a simplified computer model representing the operation of an exemplary heat engine 400 is shown in FIG. 6. As illustrated therein, this computer model uses water as the first working fluid. The second working fluid is a mixture of helium, nitrogen and water vapor. The temperature, pressure, and mass flow rates of the first and second working fluid are provided in FIG. 6 relative to the locations of the various system components in FIG. 4. Still, it should be appreciated that the invention is not limited to these working fluids and/or to the temperatures, pressures and/or mass flow rates that are stated in this example.

From the foregoing it will be understood that the integrated refrigeration cycle of the present invention has many advantages, particularly when used to absorb available energy from locations within the cycle, or adjacent to the cycle, and thereafter facilitate use of this available energy within the overall cycle. Conventional systems absent such regenerative heat transfer processes have far less capacity to use such available energy because the temperature differentials are generally too small to provide a benefit, and most often the energy is deemed non-useful and/or rejected from the cycle as waste heat.

The condensed first working fluid Fi in the form of liquid that is produced by the refrigeration cycle using Y% of F₃ can also have other uses. In some embodiments, the condensed first working fluid Fi can be further cooled by conventional means, and then used for cooling at least a portion of the F₃ within the condenser. This can be accomplished by either spraying the incoming flow of F₃ or providing the Fi directly to the low temperature heat exchanger (evaporator) at 302 of FIG. 3 A as has been previously described. Notably, spraying the flow of F₃ entering the condenser with the cooler Fi liquid is a method of accelerating the overall condensate rate out of F₃ while using a fluid that is available within the cycle.

Those skilled in the art will appreciate that overall system performance of the various embodiments will naturally be governed by a variety of different variables. For example, such variables can include, without limitation:

(1) the chemical properties of the first working fluid F_ls where Fi may be comprised of multiple chemical constituents (some percentage water and some percentage methanol, as an example)

(2) the chemical properties of the second working fluid F₂, where F₂ may be comprised of multiple chemical constituents (some percentage helium, some percentage nitrogen, some percentage water vapor as an example)

(3) the mass flow rate of the first working fluid Fi;

(4) the mass flow rate of the second working fluid F₂;

(5) temperature at each point in the cycle;

(6) pressure at each point in the cycle;

(7) density/volume at each point in the cycle;

(8) enthalpy of each fluid and mixed fluid states; (9) the ratio of specific heat (C_p/C_v) of each fluid and mixed fluid states.

In general, the cycle described in FIGs. 1-5 will be configured based on the nature or type of thermal source available for providing thermal energy. From there, appropriate fluid mixtures are selected and modeled using computer simulation tools. The operation and control of the cycle is then fine tuned around desired control points of temperature and pressure within the apparatus of the system configuration chosen. These temperatures and pressures are dominantly controlled by altering the fluid flow rates of the individual working fluids of certain fluid combinations at select points within the cycle.

In the inventions described herein, the mixing ratio of the first and second working fluids in the mixing chamber can be either static or dynamic. Static fluid mixing involves mixing the first and second working fluids in the mixing chamber 206 at a fixed rate. In other words, a fixed mass flow rate is used for each working fluid under set conditions of temperature and pressure. In such an embodiment, the dynamics of each working fluid remains at a near steady state, with the input thermal energy set at a near constant rate, and a constant or substantially constant output (shaft mechanical energy).

The ability to dynamically alter the flows and mixtures facilitates control of the cycle relative to deviations in heat addition and/or power extraction. Dynamic fluid mixing involves mixing of the first and second working fluids in the mixer / heat transfer chamber 206 at variable rates. For example, such dynamic mixing might be implemented for purposes of controlling the operational dynamics of such an engine system. In such a dynamic fluid mixing implementation, the state conditions of temperature, pressure, and mass flow for each working fluid may be fluctuating dynamically as a function of fluctuations or changes within the operating cycle. In those instances where the input (thermal) energy is being changed in conjunction with load levels (i.e., power output levels), it can be more appropriate in some

embodiments to change the relative mass flow rates of the constituent first and second working fluids rather than changing the gross overall flow rate as a set mixture (or fixed mixture ratio). Still, the invention is not limited in this regard, and the gross overall flow rate can be changed with a fixed mixture ratio. Alternatively, the mixture ratio and the overall flow rate can be changed.

Dynamic control of the mixing ratios can be controlled by any suitable means. For example, the flow rate of Fi can be controlled by the pump rate or pump speed 201 , and valves that control spray rates and feed rates to the boiler. In addition, the flow rate of vapor from the condenser and/or evaporator can be controlled by adding vaporous fluids to the line leaving the condenser or removing vaporous constituents by means of releasing or pumping them out of the cycle flow.

Additionally, the flow of F₂ can be controlled by altering the speed of the compressor 220, 204a, and/204b. By having these methods of fluid control, the flow rates of one or both of these working fluids, and the ratio of the first working fluid and the second working fluid can be selectively varied. Consequently, the mixed fluid state can be optimized to provide the desired conditions in the mixing chamber and later in the expander.. Those skilled in the art will appreciate that locations of valves and preferred ranges of mixing ratios under various conditions will depend on a variety of system specific considerations. These can include the chemicals comprising the first and second working fluids. The third working fluid comprising mixed first and second fluids would be configured on a system specific basis based on heat rates, temperature, pressure and chemical properties of the fluids and fluid combinations.

In very general terms, the ratio of first working fluid Fi to second working fluid F₂ contained in F₃ would be about 1/3 (one-third) to 2/3 (two-thirds) in an embodiment of the invention. The ratios may in various embodiments also extend over a range. The range can extend from a first arrangement having 1/5 first working fluid to 4/5 second working fluid, to a second arrangement having 2/3 first working fluid and 1/3 second working fluid. Still, the invention is not limited in this regard. It is possible in the extreme case to operate the system using only one fluid composition. In this case a single working fluid is configured to operate both as the first working fluid and the second working fluid. In such an embodiment, the invention relies on carefully managing the liquid to vapor transition at the different locations within the physical apparatus comprising elements of the system. This would include boiler, expanders, compressors, pumps and condensers.

Fluid selection for operating the configuration of thermodynamic cycles described herein is based on many inter-related factors. The conditions of operating temperature and pressure are important in selection of the chemical makeup of the fluid. It is also desirable to choose a boiling point of the first working fluid which is appropriate relative to the source temperature of available heat. For example, if the source of heat is geothermal with a temperature of 350° F, it is desirable to have a first working fluid that has the capability to absorb heat from the source thru vaporization of the selected working fluid at a rate that is commensurate with both the heat rate of the source and the size of the desired system. It is further desirable to select the first working fluid so that it can re-condense to liquid in the condenser. For example, propane can be advantageously chosen as a first working fluid in some embodiments since it changes from a liquid to a vapor at lower temperatures than other working fluids, such as pentane (assuming the same operating pressure). Fluid choices are also governed by the latent heat capacity of a given working fluid.

Use of some fluids as the first working fluid can also be a disadvantage in the present invention. For example, working fluids with extremely low volumetric expansion potential may not be the best choice for use in the present invention. Also, certain fluids that have higher boiling points and good volumetric expansion capabilities may only operate at temperatures that are above the source temperature. Accordingly, such fluids would be ruled out for a lower temperature source, but may perform well for another configuration with a higher temperature thermal source. There may be fluids that that perform well in the Rankine cycle portion of the cycle, but may not perform well in the expansion portion of the cycle. Accordingly, it is important to select and match the fluid capability with the characteristics of the thermal energy source.

Those skilled in the art will appreciate that the invention is not limited to the particular working fluids or fluid compositions described herein. Instead, any suitable combination of optimized fluids and/or fluid compositions can be selected for a particular application based on available heat source, temperature differentials between heat sources and fluid vaporization rates, and other similar design considerations.

In general, the first and second working fluids should be selected such that they work in concert with one another. In particular, the more rapid cooling of the second fluid (as compared to the first fluid) during the expansion process can facilitate the exchange of energy from the first fluid to the second fluid. This leaves the first fluid very close to the vapor to liquid transition point as it approaches the end of the expansion cycle. As the first working fluid condenses, it is therefore separated from the second working fluid and can be collected in the condenser. This unique fluid capability provides the means to tune the thermal take-up rates (heat

addition/vaporization) and additionally the drop-out rates (condensate rates) of the fluids in operation.

As noted above, a liquid spray fluid can be used to cool the second working fluid in the compressors 204a, 204b. The liquid spray fluid can be added to the second working fluid immediately before or concurrently with compression of the second working fluid in compressor 204a, 204b. In some embodiments it may be advantageous to select the liquid spray fluid to be a unique working fluid that is different from the first and second working fluid. However, in a preferred

embodiment, the liquid spray fluid is advantageously selected to be comprised of the first working fluid as shown in FIGs. 2, 4, 7 and 8. Still, the invention is not limited in this regard and other working fluids can also be chosen as the spray fluid where they can be separated in the condensate process.

Notably, the above-described liquid spray techniques can reduce the compressor work required to create a given volume of working fluid at a specified pressure. In particular, a liquid spray introduced into the compressed second working fluid within the compressor 204a, 204b can facilitate transfer of thermal energy from the second working fluid to the liquid spray fluid, which has a lower temperature and has additional capacity to absorb heat. The transferred thermal energy from the compressed second working fluid provides the latent heat of vaporization energy required for the liquid spray fiuid, causing the liquid spray fluid to transition to a vapor. By using this technique, the physical apparatus in FIGs. 2, 4, 7 and 8 can produce a larger volumetric flow of vapor to the mixing chamber 206, without further increasing the compressor temperature, thereby resulting in less required compressor work.

It can be noted that a higher temperature of un-cooled flow could have a larger volumetric flow than the cooled variant, however this volume would

additionally have a lower overall density (lower gross mass flow rate without the spray) and this lower density flow, for equivalent net energy, may have a less desirable influence on expander performance causing the overall cycle efficiency to drop.

When spray cooling is used, the liquid spray fluid is converted to a vaporous fluid, and therefore is available to perform work. More particularly the mixture of the vaporized spray fluid and second vapor will be available to perform work at the design pressure and at a lower temperature as compared to using the second fluid alone without spray cooling. This naturally assumes that the selected pressures used in the compressor 204a, 204b are adequate to provide temperatures high enough to cause the liquid to vapor transition of the spray fluid and that the rate of spray is commensurate with the heat of compression that is available. A key note that is understood in the field of thermodynamic fluid transformation, is that as the pressure increases, so too does the temperature that is required to result in the liquid to vapor transition. It is therefore important to select the chemical comprising the liquid spray fluid such that it has the potential to transition to a vapor under the conditions of temperature and pressure of the compression process selected.

In the embodiments shown in FIGs. 2, 4, 7 and 8, the temperature of the first working fluid is generally controlled by selecting the source temperature of the boiler and/or controlling the flow rate of the first working fluid. The pressure of the first working fluid supplied to the boiler 203 is controlled by pump 201. Accordingly, the initial pressure levels in the Rankine cycle portion can be controlled by the pump 201. The low pressure compressor 220 may control the vacuum or relatively low pressure provided within the evaporator 214 in concert with throttle design of 216.

The pressure of working fluid F₂ is generally controlled by the operation of compressors 204a, 204b. Accordingly, compressors 204a, 204b are preferably designed to raise the pressure of the second working fluid F₂ to a suitable level for providing to the mixing chamber 206 a pressurized flow of the second working fluid at approximately the same pressure as the first working fluid. Specific designs of the mixing chamber would allow for or enable deviations in pressures between these fluids, and are intended to be included within the scope of the present invention.

The temperature of the second working fluid exiting the compressor 204a, 204b is most appropriately controlled by the mass flow rate and type of liquid spray fluid that is incorporated for a specific design configuration. Increasing the mass flow rate of the liquid spray fluid added to the second working fluid at compressor 204a, 204b will act to lower the temperature of the combined fluids. The limit of this temperature lowering capability is when there is remaining liquid spray fluid leaving the compressor (i.e., liquid spray fluid that has not been vaporized). Any such residual liquid spray fluid exiting the compressor possesses little or no capacity to provide work in the system and in some cases may further reduce the potential for optimized flow later in the cycle. The conversion of the liquid spray to vapor both reduces compressor temperature and increases the volumetric fluid flow at an effectively lower temperature.

In a preferred embodiment, the combination of temperature and pressure of the vapor leaving the compressor 204a, 204b is sufficient to maintain the spray fluid in a vapor form as it proceeds to the mixing chamber 206. If the temperature is too low and/or the pressure is too high relative to the mass flow rate of the spray, then a portion of the spray may undesirably remain in liquid state. Notably, some small portion of the liquid spray fluid that is not completely converted from liquid to vapor is considered acceptable for specific applications of the invention in some cases. Such liquid spray fluid may later vaporize in the mixer with the addition of heat from Fi leaving the boiler 203. Therefore in some cases, the small portion of un- vaporized fluid will not overly influence system performance.

The pressure, temperature and construct of the heated working fluid mixture F₃ leaving the mixing chamber 206, are key in establishing the performance capability of the cycle. These factors include the constituent mass flow rates and therefore establish the parameters for the expansion rate and the design requirements of the expander 208. It is further understood that the expander performance is highly dependent on the energy content and expansion profile of the F₃ flow. It would be understood by those skilled in the art that the volumetric flow rate, density, pressure, and temperature can be used to establish the performance characteristics and therefore best expander design. These parameters can be established and controlled within the cycle construct for a broad range of applications where the cycle is designed around the available thermal source temperature and heat rate.

The cycle described herein is an improvement over other cycles because it posses the inherent capacity to use larger quantities of the available source energy (i.e., heat energy) effectively over a broad range of temperatures. There is no compromise to the integrity of basic thermodynamic principles or processes in the overall cycle configuration. In fact, each of the separate process steps described in the cycle herein represents a well understood thermodynamic process. Specifically, the invention incorporates a Rankine cycle, a Brayton cycle and a refrigeration cycle concurrently. The dynamics of each cycle portion can be traced to similar processes occurring independently in conventional systems. Still, this invention goes beyond such conventional methods and systems because they do not combine process steps in the manner described herein, and therefore do not achieve the same results.

Computer models have been developed to evaluate the details of each of the cycle processes, the relationships across the individual cycle boundaries, and the hardware that supports each specific process. These models facilitate evaluation of the cycle as a whole. The cycle advantageously involves improving thermal to power conversion efficiencies at lower source energy temperatures. This approach is afforded by re-use of normally rejected thermal energy by means of re-circulation and incorporation of methods of using larger quantities of available the low temperature thermal resources. Accordingly, the computer models involve an evaluation process that incorporates iterative calculations within the performance simulations to account for the recycled energy. Such modeling and simulation has validated this unique method of using latent heat energy through appropriate placement of vapor state transitions. The resulting method and apparatus is capable of converting a greater portion of available thermal source energy to work than is currently understood to be possible using conventional means known today.

All of the apparatus, methods and algorithms disclosed and claimed herein can be made and executed without undue experimentation in light of the present disclosure. While the invention has been described in terms of preferred

embodiments, it will be apparent to those of skill in the art that variations may be applied to the apparatus, methods and sequence of steps of the method without departing from the concept, spirit and scope of the invention. More specifically, it will be apparent that certain components may be added to, combined with, or substituted for the components described herein while the same or similar results would be achieved. All such similar substitutes and modifications apparent to those skilled in the art are deemed to be within the spirit, scope and concept of the invention as defined.

Claims

1. A method for producing work from heat comprising:

heating a pressurized flow of a first working fluid to form of a first vapor; compressing a second working fluid in the form of a second vapor;

forming a mixture of said first and second working vapor to form a third working fluid;

transferring thermal energy directly from said first vapor to said second vapor in said third working fluid, exclusive of any intervening structure;

expanding said third working fluid to perform useful work after or during said transferring; and

using a second portion of said third working fluid as a refrigerant to cool a first portion of said third working fluid in a condensing step.

2. The method according to claim 1, wherein said condensing step further comprises separating a condensate of said first vapor from said first portion of said third vapor.

3. The method according to claim 1, further comprising spraying a liquid into a flow of said second working fluid before or during said compressing to reduce a temperature of said second working fluid during said compressing relative to a temperature without said spraying.

4. A method for producing work from heat comprising:

transferring thermal energy directly from said first vapor to said second vapor in said third working fluid, exclusive of any intervening structure; expanding said third working fluid to perform useful work after or during said transferring;

cooling said third working fluid to extract a condensate and a residual portion of said third working fluid from which said condensate has been extracted; and

using at least a portion of one or more fluids comprising said third working fluid as a refrigerant to perform said cooling.

5. The method according to claim 4, wherein said refrigerant is said third working fluid.

6. The method according to claim 4, wherein said refrigerant is comprised of said residual portion.

7. The method according to claim 6, wherein said refrigerant is exclusively comprised of said residual portion.

8. An apparatus for producing work from heat comprising:

a boiler configured for heating a pressurized flow of a first working fluid to form of a first vapor;

at least one compressor configured for compressing a second working fluid in the form of a second vapor;

a mixing chamber configured for mixing said first vapor with said second vapor to form a third vapor and to transfer thermal energy directly from said first vapor to said second vapor, exclusive of any intervening structure;

an expander configured for expanding said third vapor to perform work after or during said transferring; and

a condenser configured to receive said third working fluid from said expander, said condenser including a low pressure heat exchanger configured for using one or more fluids comprising said third working fluid, as a refrigerant to cool said third working fluid.

9. The apparatus according to claim 8, wherein said refrigerant is comprised of said third working fluid.

10. The apparatus according to claim 8, wherein said condenser is configured to produce as a result of said cooling a condensate and a residual portion of said third working fluid, and said refrigerant is comprised of said residual portion.

11. The apparatus according to claim 10, wherein said refrigerant is exclusively comprised of said residual portion.