Patent Application
20130312414
1. Statement of the Technical Field
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.
2. Description of the Related Art
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. Examples of typical thermal cycles are the Brayton Cycle, the Rankine Cycle and Refrigeration Cycles.
A combined cycle is an assembly of two or more of these processes that convert heat into mechanical energy, by combining the 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%.
Embodiments of the invention concern a method for producing work from heat in a continuous cycle. The involves communicating a first flow of a first working fluid to a low pressure boiler. A pressure of the low pressure boiler is set below a vaporization pressure of the first working fluid at the temperature of a low temperature thermal source. The first flow of the first working fluid is heated in the low pressure boiler using the low temperature thermal source to form a first flow of first working fluid vapor. A second flow of the first working fluid is provided to a high pressure boiler to produce a second flow of first working fluid vapor at a pressure higher than the low pressure boiler. The high pressure boiler uses a high temperature thermal source to heat the second flow of first working fluid. The high temperature thermal source has a temperature higher than the low temperature thermal source. The method continues by compressing a second working fluid in vaporous form, and then forming a third working fluid by mixing the first flow of first working fluid vapor, the second flow of second working fluid vapor, and the second working fluid which has been compressed. Thereafter, thermal energy is transferred directly between one or more of the second working fluid, the first flow of first working fluid vapor and the second flow of first working fluid vapor, exclusive of any intervening structure.
The invention also includes a system for producing work from heat in a continuous cycle. The system includes a low pressure boiler having an internal pressure below a vaporization pressure of a first working fluid at the temperature of a low temperature thermal source. The boiler is configured to heat a first flow of the first working fluid using the low temperature thermal source to form a first flow of first working fluid vapor. The system also includes a high pressure boiler configured to heat a second flow of the first working fluid using a high temperature thermal source. The high temperature thermal source has a temperature higher than the low temperature thermal source. The high temperature boiler produces a second flow of the first working fluid vapor at a pressure higher than the low pressure boiler. A compressor is provided to compress a second working fluid in vaporous form. A mixing chamber is provided to form a third working fluid by mixing the first flow of first working fluid vapor, the second flow of second working fluid vapor, and the second working fluid which has been compressed. The mixing chamber is also configured to facilitate a transfer of thermal energy directly between one or more of the second working fluid, the first flow of first working fluid vapor, and the second flow of first working fluid vapor, exclusive of any intervening structure.
Embodiments will be described with reference to the following drawing figures, in which like numerals represent like items throughout the figures, and in which:
The invention is described with reference to the attached figures. The figures are not drawn to scale and they are provided merely to illustrate the instant 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 readily 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 operation are not shown in detail to avoid obscuring the invention. The 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 invention.
The invention concerns a Hybrid Thermal Cycle (HTC) comprising fluids F1, F2, and F3 where F3 is comprised of a mixture of fluids F1 and F2. The F1 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. A low pressure boiler is advantageously used in part of the cycle to enable the F1 fluid to scavenge heat which is not otherwise useful in traditional engine approaches. The low pressure boiler further has the potential to absorb residual energy from locations within the overall cycle construct. Consequently, heat energy that is normally not useful or simply rejected from traditional cycle formats, is made useful within this closed cycle approach, affording the opportunity to use a higher quantity of the available thermal energy to produce power.
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. Other combinations could include hydrogen or argon, and in the other cases vapors similar to nitrogen alone are viable. Within a portion of the cycle the F2 fluid first provides heating during a compression portion of the cycle and second provides cooling later during an expansion portion of the cycle.
The F1 fluid is mixed with the F2 fluid in parts of the cycle to form the F3 fluid. Later in the cycle the F1 fluid is separated from the F3 fluid. Following a compression portion of the cycle during which F2 is compressed, there is an expansion part of the cycle during which F3, comprised of a mixture of F1 and F2, is expanded. During this expansion, the F1 fluid functions to support or maintain the temperature of the F2 fluid, preventing it from cooling more rapidly than without the latent heat that available from the F1 fluid. If the F2 fluid were expanded without the portion of the F1 fluid it would cool more rapidly, having less capacity to perform work. This characteristic or effect in the cycle is desirable as it enables the fluid mixture F3 to perform work for a longer period of time during expansion. This ability of F1 to effectively delay the cooling of F2 essentially ends when the F1 fluid reaches a point where it transitions from a vapor back into a liquid. At the end of the expansion process, at least a portion of the F1 fluid condenses out of the F3, leaving a residual portion, which is F2.
One aspect of the invention concerns the basic idea that it is possible to get more power out of a thermal source if energy is extracted at two (or more) thermal energy extraction points. The thermal extraction points are defined by thermal and pressure points that are on the working fluid saturation line. These are unique temperatures and pressures where the working fluid can have a phase transition from a liquid to a vapor. By arranging for the system to extract energy across a working fluid phase transition, it is possible to enable a larger quantity of thermal energy extraction from the thermal source. Most thermal extraction or exchange systems use a single boiler at a set output temperature (this temperature is controlled by the thermal input to the boiler.) However, it is possible to split the thermal extraction points—and by doing so it is possible to acquire a larger quantity of useful energy from the heat (thermal) source. It is therefore possible to generate a larger quantity of power from that source of heat, (with the heat of vaporization being extracted from the source at separate & unique temperature & pressure points along the working fluid saturation line.
Because of the use of latent heats to extract power from a thermal source, the quantity of power derived from the source can be considerably larger relative to the power extraction capability using only one vapor transition point (i.e., the working fluid boiling point). This process enables extracting much larger quantities of useful thermal energy and therefore enabling the production of larger quantities of useful work from a thermal source, as when compared to using only one boil off point. Notably, traditional calculations for overall cycle thermal efficiency may show little or no improvement, but when measuring the efficiency of extracting power from the same source as is the benchmark standard today—there is a considerable improvement in the extraction efficiency. The inventive cycle construct enables the thermal energy extracted at the two or more unique extraction points to be valuable in the production of power.
Referring now to
In step 104, a second flow F1(2) of the first working fluid F1 is provided in liquid form to a high pressure boiler. In some embodiments, this step can be preceded by a pre-heating step that is not shown in
The process continues in step 108 with the formation of a third working fluid F3. The F3 working fluid is formed as a mixture of working fluids F1(1), F1(2) and the compressed F2. Within the F3 mixture, thermal energy is transferred in step 110 directly between F1(1), F1(2) and the second working fluid F2, exclusive of any intervening structure. In other words, the fluids are able to directly exchange or share the thermal energy they contain. Subsequently, the third working fluid is expanded in step 112 to perform useful work concurrently with or after the transference of heat described in step 110. The expansion of the vapor mixture is facilitated by providing a pressure drop across an expansion device, capable of extracting useful work (power).
The process in
As an alternative to these conventional condensing methods, the liquid F1 can be used directly as part of the condensing process, or an internally powered condensate method can be used. An example of such an internally powered condensate method is shown in
As a result of the condensing process in step 114, the F1 working fluid (F1 condensate) will be separated from the F3 working fluid, thereby leaving a residual portion of F3 from which the condensate has been extracted. This residual portion of F3 is F2. Thereafter, the continuous cycle is repeated using the first working fluid (F1 condensate) recovered in the condensing step and the second working fluid F2. More particularly, a first flow of the first working fluid F1 recovered as condensate in step 114 is once again provided to the low pressure boiler as F1(1). A second flow of the first working fluid F1 recovered as condensate is provided in step 118 to the high pressure boiler as F1(2). Finally, in step 120, F2 working fluid (which is comprised of the residual portion of F3 produced in step 114) is provided to be compressed again in step 106.
The method 100 will now be described in further detail in relation to components that form an effective heat engine 200 which is shown in
The heat engine 200 makes use of a high temperature thermal source 225 and a low temperature thermal source 227. The “high temperature” nomenclature which is used to describe thermal source 225 is intended to emphasize that such thermal source is at a higher temperature as compared to the temperature of low temperature thermal source 227. Although thermal source 225 will have a higher temperature compared to low temperature thermal source 227, it should be appreciated that high temperature thermal source 225 can actually have a relatively low temperature as compared to those temperatures which are normally used to provide efficient operation of a conventional heat engine. For example, in some embodiments, the high temperature thermal source 225 may actually only have a temperature of about 800° F. or less. In other embodiments, the high temperature thermal source 225 can have a temperature of about 400° F. or less. The ability to efficiently utilize such sources of heat is a significant advantage of the present invention.
Suitable choices for working fluids F1 and F2 will be described below in further detail. Still, given the anticipated temperatures for thermal source 225, 227, it can be advantageous to select the working fluid F1 to be a low vapor state formulation to facilitate vaporization of such working fluid at relatively low temperatures. Examples of such low vapor state formulations can include fluids such as methanol or pentane.
A high pressure boiler 203 can use as its primary heat source a supply of steam from the high temperature thermal source 225. For example, the high temperature thermal source can be a geothermal well or waste heat from some high temperature process or other power generation system. The low temperature thermal source can be a thermal source that is entirely independent of the high temperature thermal source 225. However, it can be advantageous to select the low temperature thermal source 227 to be a down-line flow from the high temperature thermal source 225, after such flow has provided a portion of its thermal energy to the high pressure boiler 203. This concept is illustrated in
The high pressure boiler 203 will have a higher pressure compared to low pressure boiler 207. However, it should be appreciated that high pressure boiler 203 can actually have a relatively low pressure as compared to those operating pressures which are normally used to provide efficient operation of a conventional heat engine. For example, in conventional heat engines, high pressure boilers typically are understood as boilers that operate in the range of 1000 to 3000 psi. In contrast, the high pressure boiler 203 can operate at a pressure in the range of 300 psi or less. Still, the invention is not limited in this regard and the actual operating pressure in the high pressure boiler 203 and low pressure boiler 207 can vary in accordance with the available heat source and other design conditions.
Referring again to
A second flow F1(2) of a first liquid working fluid F1 is also pressurized using the pump 201. The pressurized fluid is communicated to the high pressure boiler 203 which is maintained at a relatively high temperature as determined by high temperature thermal source 225. The high pressure boiler 203 will add a predetermined amount of thermal energy to the F1(2) working fluid. As a result of these operations, the F1(2) working fluid is converted to a vapor (sometimes referred to herein as F1(2) vapor). Thereafter, the F1(2) vapor is optionally communicated to an expander 209 where the thermal energy contained in the F1(2) vapor is optionally used to perform work. As explained above in relation to
Within the mixing chamber 206, the F1(1) vapor, F1(2) vapor are mixed with a vaporous flow of working fluid F2 which has been compressed in compressor 204. These three separate vaporous fluid flows comprised of F1(1), F1(2) and F2 are combined or mixed to form a vaporous mixture which is referred to herein as third working fluid F3 (or F3 vapor). Due to this mixing of the working fluids, the transfer of thermal energy between the fluids is facilitated. In some embodiments, additional thermal energy can optionally be provided from an independent source to the F3 vapor contained in the mixing chamber 206. For example, the additional thermal energy can be provided to the mixer from a source that is external to the system shown in
It is not necessary for all thermal energy transfer between the F1(1), F1(2) and F2 vapor to occur within the mixing chamber 206. In some embodiments of the invention, a portion of such 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 transfer can continue occurring as the F3 vapor continues through an expansion cycle discussed below. Also, it is possible for the F1(1), F1(2) vapor to the F2 vapor fluids to enter the mixer at approximately the same temperatures and pressures. However, as a result of the different chemical compositions of such fluids, transfer or exchange of thermal energy 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 F1(1), F1(2), F2 and not across physical boundaries such as a thermally conductive wall of a heat exchanger (as would be the case if a conventional heat exchanger was used for this purpose). Consequently, the transfer of thermal energy between the F1(1), F1(2), and 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 physical walls separating the fluids that are exchanging heat (i.e. a wall-less heat exchanger).
The mixing chamber 206 receive a vaporous fluid volumetric flow of F1(1) at pressure p1(1), a vaporous fluid volumetric flow of fluid F1(2) at pressure p1(2), and a vaporous fluid volumetric flow of fluid F2 at pressure p2. In some embodiments, the pressures p1(1), p1(2) 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 fluids F1(1), F1(2) and F2. Accordingly, the volume of the mixing chamber can be selected to be VF1(1)+VF1(2)+VF2=VF3 where VF1(1) is the volumetric flow rate of fluid F1(1); VF1(2) is the volumetric flow rate of fluid F1(2); VF2 is the volumetric flow rate of fluid F2, and VF3 is the volumetric flow rate of VF1(1)+VF1(2)+VF2 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 an affect on the pressure of the third working fluid F3.
The vaporous third working fluid F3 is communicated under pressure from the mixing chamber 206 to expander 208 for performing useful work. A rotating output shaft 205c of expander 208 can be connected directly or indirectly to the rotating output shaft 205a of expander 209 as shown in
The condenser 212 can be any device capable of condensing a working fluid from its vapor state to its liquid state. As is well known in the art, condensing is commonly performed by cooling the working fluid under designated states of pressure. As will be appreciated by those skilled in the art, this cooling process will generally involve a release of heat contained in the third working fluid F3. In the most commonly used configurations, the cooling process is accomplished by using a heat exchanger to move the heat from the F3 fluid to the ambient environment external to the heat engine (thermal cycle). This simple configuration is shown in
In an alternative embodiment a heat engine 300 can operate in a manner similar to that described above with respect to heat engine 200. However, in place of the conventional condenser arrangement, the heat engine 300 utilizes a heat exchange process that is internal to the heat engine and is therefore an integral part of the thermal cycle. As illustrated therein, the F3 working fluid which exits expander 208 is communicated to a compressor 230. The compressor 230 performs work to increase the pressure of the F3 working fluid as it enters the first condensing chamber 317 of condenser 312. A heat exchanger 314 within the condenser 312 forms a heat exchange system. A flow 318 of the vaporous F3 fluid is allowed to pass over the exterior surfaces of the heat exchanger 314. This cools the F3 fluid and thereby facilitates the condensing of the F1 fluid contained within the F3 fluid mixture. The F1 portion therefore drops out as a liquid in the form of condensate, is collected in the condenser as F1 fluid (liquid), and is available for reuse. This process leaves a residual portion of the F3 working fluid 322. Residual portion 322 is a remaining portion of the one or more fluids previously comprising F3 that exist after the F1 condensate has been extracted from F3.
The residual portion 322 passes into a second condenser chamber 319 and is later used for purposes of cooling the heat exchanger 314. More particularly, a low pressure zone is provided in a plurality of flow channels 315 of the heat exchanger 314. One or more flow restrictors, expansion valves or throttles 316 can be used to effect cooling of the residual portion of the F3 working fluid as it passes into the heat exchanger. 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 appreciated that the expansion valve (or other type of throttling means) would be sized and configured to accomplish the desired cooling, within the fluid capabilities of the flow provided.
It should be understood that the pressure within the flow channels 315 is at a lower pressure than the environment within the second condenser chamber 319. Cooling of the residual portion of the F3 working fluid is accomplished as the vaporous residual portion of the F3 working fluid is forced through the expansion valve 316 by means of compressor 230 and transitions from a higher pressure state to a lower pressure state. As will be understood by those skilled in the art, the expansion of the vaporous residual portion of the F3 working fluid will lower its temperature. This reduction in temperature allows the residual portion of the F3 working fluid in flow channels 315 to then draw heat from the flow 318 of F3 working fluid (in vaporous state) which surrounds and circulates past the flow channels 315 and/or exterior of the heat exchanger. The foregoing process results in extraction of F1 condensate from the F3 working fluid as shown. The residual portion of F3 subsequently enters a third chamber 321 of condenser 312 where it is used as the second working fluid F2.
It will be appreciated that in
In
Based on the foregoing disclosure it will be understood that the condenser process used for the present invention can be effected using the conventional condenser arrangement shown in
The temperature and pressure conditions inside the condenser 212, 312 are chosen so that the F1 constituent part of F3 is converted to a condensate within the condenser, while the second working fluid F2 is not condensed. In other words, a residual portion of F3 will remain in a vaporous state. This residual portion of F3 is later used as the second working fluid F2. Those skilled in the art will appreciate that this condensing process applied for purposes of separating F1 from the residual portion of F3 can be accomplished by choosing the first and second working fluid to have different thermal properties.
In conventional Rankine cycle heat engines, the condensers can often be the most constraining part of the system. This is because it is necessary to completely condense 100% of the vapor that comprises the working fluid. Because of the difficulty in accomplishing 100% condensate, many systems will bleed a portion of the working fluid outside of the main system. This bleed process may include directly releasing working fluid to the atmosphere or to an independent reservoir that is outside of the primary condenser of the system. A key advantage of the current invention is the ability to not require a full condensate process. Accordingly, condenser 212/312 may not actually condense 100% of F1 from the vaporous F3 mixture. Advantageously, this condition is acceptable for purposes of the present invention, and a portion of F1 can be permitted to remain mixed within a residual portion of F3. For purposes of the present description, any portion of the residual F1, either in the vapor or liquid form (carried in the fluid stream), after the condensing process, therefore remaining in F2), can be considered a constituent of F2 by design. With the F1 condensate and the residual portion of F3 available within the condenser in this way, the process has essentially returned to its starting point, with the first working fluid F1 in a liquid form, and the second working fluid F2 in a vaporous state. Thereafter, the entire process described above can be repeated in a continuous cycle.
The invention has been described herein with respect to a high pressure boiler 203 and a low pressure boiler 207. As such, two boil-off points are established and used to more efficiently perform work utilizing the available thermal energy from the heat source. Although only two boilers are included in the embodiments shown herein, it should be understood that the invention is not limited in this regard. It is contemplated that additional boilers can be used to provide additional boil off points, where each boiler operates at a predetermined temperature and pressure to transport heat to a corresponding flow of the first working fluid. For example, a system could utilize a low pressure boiler, medium pressure boiler, and high pressure boiler. More generally, at least a third flow of the first working fluid can be communicated to a third pressure boiler to produce a third flow of first working fluid vapor at a pressure different from the low pressure boiler and the high pressure boiler. The third pressure boiler will use a thermal source having a temperature different as compared to the low temperature thermal source and the high temperature thermal source. Systems with four or more boilers are possible and may be desirable under certain operating conditions as will be apparent to those skilled in the art. The resulting vapor from each boiler can be subsequently transported to the mixing chamber where a transfer of thermal energy can occur as previously described.
An example of the operation of exemplary heat engine 300 will now be provided with reference to
F1(1)=2 lbs/s
F1(2)=2 lbs/s
F2=10 lbs/s (7 lbs/s nitrogen, 1 lb/s helium, 2 lb/s methanol)
F3=14 lbs/s
For this example we assume that the high temperature heat source temperature is 310° F. and that the heat added to working fluid F1(2) in the high pressure boiler 203 is 1072 kW. This produces high pressure F1(1) vapor at the output of the high pressure boiler 203 at a temperature of 280° F. and a pressure of 130 psia. This high pressure vapor is used to perform 34 kW of work in expander 209. The vapor exits the expander 209 at relatively low pressure and temperature (T=226° F., p=65 psia).
An outflow of the heating process at high pressure boiler 203 provides the low temperature thermal source 227 at a temperature of about 250° F. The low temperature thermal source is used to provide thermal energy to the low pressure boiler 207 where 1,042 kW is added to the F1(1) working fluid. This produces low pressure F1(1) vapor at the output of the low pressure boiler (T=226° F., p=65 psia). An outflow of the heating process at low pressure boiler 207 is returned to the source, such as a geothermal well, at a temperature of about 170° F.
Concurrent with the foregoing vaporization steps, the F2 working fluid is compressed in compressor 204. The F2 working fluid is also heated as a result of being compressed. In the exemplary model, the F2 working fluid enters the compressor 204 at a temperature of 165° F. and a pressure of 65 psia. The compressor uses 978 kW of power to perform the compressing operation, which results in the F2 working fluid exiting the compressor at a temperature of 390° F., and a pressure of 17 psia. The process continues as described with F1(1), F1(2), and F2 being mixed in the mixing chamber 206 to form F3 working fluid. The F3 working fluid is subsequently delivered to the expander 208, where it can perform 1,418 kW of work. As noted above, an output shaft of expander 208 can be connected directly or indirectly to compressor 204, so that a portion of this work output can be used to drive compressor 204. After the F3 working fluid has been used to perform useful work, it exits the expander 208 at relatively low pressure and temperature (T=149° F., p=14 psia) and is communicated to compressor 230. Compressor 230 uses 103 kW of power to pressurize the F3 working fluid before the working fluid enters the condenser.
The condensing process effectively results in the separation of the F1 working fluid from the F2 working fluid as previously described. The F2 vapor produced by the condenser 312 has a relatively low temperature and pressure (T=165° F., p=17 psia). The F1 working fluid is pressurized by liquid pump 201 for transport to the low pressure boiler 207 and the high pressure boiler 203. Approximately 41 kW of power is consumed by pumping (e.g. pump 201) and by other mechanical systems losses. These losses are simplistically represented in
Given the foregoing assumptions, a tabular representation of the model in
Notably, computer modeling shows that a total of 330 kW of power can be extracted from the relatively poor quality thermal source in this example. In contrast, a conventional Rankine cycle utilizing the same heat source geothermal vapor at the same temperature (310° F.), and using only a single boiler, could only extract about 136 kW. In this regard it may be noted that the efficiency for such a conventional Rankine Cycle under these conditions would be 12.7%. Accordingly, the present invention has the potential to produce a substantially larger amount of power by extracting larger quantities of thermal energy from same, relatively poor quality thermal source (i.e., relatively low temperature) as compared to more conventional commercial power systems.
The heat engine 200 and its associated cycle described in
The heat cycle described with respect to
The cycle and associated apparatus described with respect to
In the present invention, a large quantity of heat energy is extracted from the high temperature thermal source 225 by means of vaporization. The latent heat of vaporization provides a useful method for converting very large quantities of heat in the F1 liquid into kinetic energy residing in the vapor. 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 working fluid (F2) that facilitates the process of producing the actual power.
Providing a large volume second working fluid in vaporous form presents another 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 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 F1 (in the Rankine portion of the cycle), and then transferring this thermal energy directly to the second working fluid F2 (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 less than 15% assuming 400° 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 cycle efficiency can be increased.
In the embodiments shown in
In general, the cycle described in
In the embodiments shown in
In
The pressure, temperature and construct of the heated working fluid mixture F3 entering the expander is key for 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 provide the basis for the 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 mixing ratio of F1 and F2 contained within the F3 flow can be either static or dynamic. Static fluid mixing involves mixing the F1 working fluid (i.e., F1(1), F1(2)) with the F2 working fluid 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 the working fluids 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 F1 and F2 rather than changing the gross overall flow rate as a set mixture (or fixed mixture ratio). If the mass flow rate of F1 is changed, this change can be implemented by varying the flow rate of F1(1), F1(2) or both. Still, the invention is not limited in this regard, and the gross overall flow rate of F3 can be changed with a fixed mixture ratio. In yet a further alternative embodiment, the mixing ratio of F1 and F2, and the overall flow rate of F3, can be changed.
In very general terms, the ratio of first working fluid F1 to second working fluid F2 contained in F3 would be about ⅓ (one-third) to ⅔ (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 ⅕ first working fluid to ⅘ second working fluid, to a second arrangement having ⅔ first working fluid and ⅓ second working fluid. Still, the invention is not limited in this regard.
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 F1 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 F1 so that it can re-condense to liquid in the condenser efficiently relative to the condensate cooling that is available. For example, F1 can be chosen to be propane 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). This configuration would be appropriate where there is a large supply of a cooling medium (such as cold water) that is readily available. Fluid choices are also governed by the latent heat value and heating capacity of the constituent working fluids.
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 some parts of the cycle, but may not perform well in the other parts of the cycle. Accordingly, it is important to select and match the fluid capability with the characteristics of the thermal energy source.
The first and second working fluids, and ratios thereof, should also 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.
In one example of the invention, F1 can be water, and F2 can be a mixture comprised of nitrogen, helium and water vapor. Since F1 is water in this example, mixing of F1 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. Still, the foregoing is merely one example of fluids that could be selected for use as F1, F2 and F3. 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, hydrogen or neon could be used instead. Still, the invention is not limited to these particular fluids.
The various cycles described herein are an improvement over other cycles because they 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. 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 having the capability to use a larger quantity of the source energy over a given temperature range and in addition is capable of re-using a portion of normally rejected thermal energy by means of re-circulation. 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.
Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. It will be further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein.
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.
