Abstract:
A piston and chamber system to be used in connection with a heat concentrator such as those used for converting low grade thermal energy into useful energy. The example apparatus disclosed herein includes a heat engine floating piston which inhibits condensation normally associated with thermodynamic cycles which run at or near vapor saturation. The resulting improvement allows increased efficiency for lower temperature systems.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention is directed toward apparatus used in thermal concentrators. More specifically to large piston designs for machines used in converting low grade heat into useful energy, mechanical work and the like. 
       BACKGROUND 
       [0002]    Three major technologies are currently being used for concentrating solar power generation to produce useful work; the parabolic trough, the power tower, and the sterling dish. The costs of generating electricity from these power sources are high. All three require a high working temperature, which creates problems with maintenance and seal failure rates. 
         [0003]    With these technologies, the solar radiation is concentrated at the time of collection requiring a high working temperature at the point of collection in order to enter the chamber as superheated steam. This higher temperature generally leads to higher thermal losses, which typically forces the use of more expensive and complicated collectors and thermal storage units. This constraint leads to higher costs for construction of these devices. 
         [0004]    With the advent of low temperature solar concentrators such as those disclosed in patent application Ser. No. 11/387,405, it is desirable to minimize condensation from saturated vapors associated with thermodynamic cycles in the heat engine cycle which run at or near the phase change point. Such improvements increase the efficiency and total power output of these systems. 
         [0005]    It is illustrative to compare the ideal heat engine cycle described presently to a typical Carnot cycle. A Carnot cycle is a cycle that undergoes two isothermal reversible processes and two adiabatic reversible processes. However, the present heat engine cycle differs from the typical Carnot cycle in several unique ways. 
         [0006]    A typical Carnot cycle includes an isentropic compression process during which wet steam, which consists of steam and liquid, is compressed to saturated liquid. The heat engine cycle of this embodiment includes an isentropic compression process during which wet steam, which consists of steam and liquid, is compressed until the liquid evaporates to leave only saturated vapor. 
         [0007]    The next process is adding energy to the cycle. In the Carnot cycle the energy added, typically in the form of heat, isothermally evaporates the liquid until only saturated vapor remains. In the present cycle, only saturated vapor is present at the beginning of the energy addition process. In the present cycle, energy is added by isothermally adding mass, of saturated vapor, to the system. 
         [0008]    A typical Carnot cycle also includes an isentropic expansion process that starts with saturated vapor and condenses to form a wet steam combination of vapor and liquid. The heat engine cycle of this embodiment also includes an isentropic expansion process during which saturated vapor is condensed to form a mixture of vapor and liquid. 
         [0009]    The Carnot cycle&#39;s final process removes heat isothermally from the wet steam to obtain the same ratio of vapor and liquid as at the beginning of the cycle. The final process of the present invention isothermally removes heat and liquid to obtain the same ratio of vapor and liquid as at the beginning of the cycle. 
         [0010]    In the isentropic compression process the typical Carnot cycle starts with wet steam and ends with saturated liquid, whereas the present cycle starts with wet steam and ends with saturated vapor. The disclosed process is relatively unintuitive because condensation from a vapor to a liquid is commonly associated with a compression process. 
         [0011]    In the present cycle, the compression process must result in saturated vapor to maintain constant entropy as required by the isentropic nature of the process. In the present embodiment, only approximately 12.5% of the wet steam mixture is liquid at the beginning of the compression process. At the beginning of the process, the specific entropy of the liquid is approximately 0.53 kJ/kg-° K. and the specific entropy of the vapor is approximately 8.32 kJ/kg-° K. At the end of the compression process, the specific entropy of the liquid is approximately 1.31 kJ/kg-° K. and the specific entropy of the vapor is approximately 7.36 kJ/kg-° K. Quantitatively, an algebraic calculation equating total entropy at the beginning and end of the compression process with a single unknown of the amount of mass that changes between phases provides the result of vapor at the end of the cycle. Qualitatively, it can be seen that the relatively low percentage of liquid in the system at the beginning of the process drives the process to produce vapor. Because the majority of the system initially consists of high entropy vapor, converting all of the vapor to liquid at approximately 16% of the specific entropy can not be a constant entropy process. However, if the process produces vapor at approximately 88% of the initial vapor specific entropy, constant entropy can be maintained, with the approximately 13.9 times increase in the liquid to vapor entropy balancing the approximately 12% drop in the specific entropy of the initial vapor mass. 
         [0012]    In a typical Carnot cycle that has a high initial percentage of liquid, the process is suboptimal. In this case, using the same starting and ending entropy values, the specific entropy of the majority of the mass, which is liquid, increases by a factor of approximately 2.5, if the final result is liquid. The mass of vapor that condenses drops in entropy by a factor of approximately 6.4 to balance out the increase in entropy of the liquid. The small drop in entropy of the initial vapor reduces the useful work which can be done by the system. 
         [0013]    Therefore, it can be seen by one skilled in the art that there remains a large incentive to maintain as much liquid in the vapor phase as possible at the end of the process. By reducing the number of surfaces inside the chamber, including the piston head, where condensation can occur, this new cycle can be enabled with greater efficiencies as shown above. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]      FIG. 1A  is an exemplary layout of a concentrator system. 
           [0015]      FIG. 1B  is an exemplary schematic block diagram of the system of  FIG. 1A . 
           [0016]      FIG. 2  is a cutaway perspective view of an exemplary thermal collector subsystem using two reservoirs. 
           [0017]      FIG. 3  is a cutaway perspective view of an alternate thermal collector subsystem. 
           [0018]      FIG. 4  shows a sectional view of a concentrator heat engine side with piston in top dead center of stroke. 
           [0019]      FIG. 5  shows a sectional view of a concentrator heat engine side with piston in bottom dead center of stroke. 
           [0020]      FIG. 6  shows a detail left side section one embodiment of one embodiment of a piston gap to liquid connecting rod interface. 
           [0021]      FIG. 7   a  shows a plan schematic view of a piston head with a heater coil in an Archimedes configuration. 
           [0022]      FIG. 7   b  shows a plan schematic view of a piston head with a heater coil in a Fermats&#39; configuration. 
           [0023]      FIG. 7   c  shows a plan schematic view of a piston head with a heater coil in a serpentine configuration. 
           [0024]      FIG. 7   d  shows a plan schematic view of a piston head with a heater in a disk configuration. 
           [0025]      FIG. 7   e  shows a plan schematic view of a piston head with a heater coil an inverted Fermats&#39; configuration. 
           [0026]      FIG. 8  shows a through sectional view of the piston head shown in  FIG. 7   d.    
           [0027]      FIG. 9  shows an alternate embodiment exemplary floating piston. 
           [0028]      FIG. 10   a  shows an alternate embodiment exemplary floating piston. 
           [0029]      FIG. 10   b  shows an alternate embodiment exemplary piston wall unit. 
           [0030]      FIG. 11   a  shows an exemplary thermal wall profile where the piston is at top dead center. 
           [0031]      FIG. 11   b  shows an exemplary thermal wall profile where the piston is at bottom dead center. 
           [0032]      FIG. 12  shows an exemplary P-V diagram for an embodiment of a heat engine cycle. 
           [0033]      FIG. 13  shows an exemplary P-V diagram for an alternate embodiment of a heat engine cycle. 
       
    
    
     DESCRIPTION 
       [0034]    The present system utilizes a dual loop U, or other suitably shaped heat actuated liquid piston heat pump, where one leg contains part of a heat engine section and the other leg contains part of a heat pump section. But as those skilled in the art will note, it can apply to any method or apparatus which runs a thermodynamic cycle at or near the condensation point of a vapor. 
         [0035]    These floating pistons are usually constructed from a solid material, for example, aluminum, steel, or other suitable material. They should be designed to withstand the conditions of temperature and pressure found in the system. 
         [0036]    The heat engine section operates using a thermodynamic cycle and draws the heat energy from a natural or waste heat source, such as, but not limited to, solar energy. Fluid, typically water, in the liquid or steam form, is transferred between the solar collectors and the heat engine as part of the heat engine loop. 
         [0037]    The heat pump loop contains the heat pump described above coupled with a device which does useful work, such as a steam turbine, which can be connected with a conversion device such as, but not limited to, an electrical generator. Water, in the form of superheated steam, is transferred from the output of the heat pump, to the inlet of the steam turbine, through the steam turbine, and from the steam turbine exhaust back to the inlet of the heat pump. 
         [0038]    It is preferred that the input heat be at a temperature of at least 60° C. higher than the ambient temperature. The method can be used with a temperature differential lower than this, but possibly at a reduced efficiency. 
         [0039]      FIG. 1A and 1B  show exemplary embodiments of a system  10  that generates electricity using a hot and a cold source.  FIG. 1B  is an exemplary schematic block diagram of the system  10 . The system  10  utilizes a heating device  100  that heats a fluid  15 , which is then pumped by a hot pumping device  200  to a hot thermal storage device  250 . The system  10  also utilizes a cooling device  300  that cools a fluid  20  of the same material as the heating fluid  15 , which is pumped by a cold pumping system  400  into a cold thermal storage device  450  after it is cooled. 
         [0040]    The lower portion of the concentrator  700  can be constructed above or below grade and is filled with fluid, such as, water, and includes a liquid connecting rod  716 . A heat engine floating piston  704  floats on the top of the liquid connecting rod  716  in one vertical leg, forming a heat engine expansion chamber  708  between the heat engine floating piston  704  and the concentrator wall  702 . A heat pump floating piston  706  floats on top of the liquid connecting rod  716  in the other vertical leg, forming a heat pump expansion chamber  712  between the heat pump floating piston  706  and concentrator wall  702 . 
         [0041]    The fluid  15  from hot thermal storage device  250  is transferred to concentrator  700  and the cold fluid  20  from cold thermal storage device  450  is used to transfer heat from the concentrator  700 , which cools the concentrator  700 . The cold fluid  20  from the concentrator  700  may also be transferred to the cold thermal storage device  450 . 
         [0042]    The concentrator  700  heats a fluid  714  to a higher temperature than that of the fluid  15  stored in the hot thermal storage device  250 . This high temperature fluid  714  is then transferred into an electric converter  600 , which in one embodiment is a steam turbine, similar to the type used in a conventional steam power plant. The fluid  714  rejected from fluid to electric converter  600  is returned to the concentrator  700  where both the temperature and pressure of the fluid  714  are increased. The concentrator  700  is driven or actuated by the heat from hot thermal storage device  250 . 
         [0043]    In one embodiment of the disclosed system  10 , the heat concentration is done near the time of use, such as high electric demand, rather than at the time of collection. It will be understood by one skilled in the art that many different variations and configurations of elements shown in  FIGS. 1A and 1B  may be used while still using the heat actuated dual loop liquid piston heat pump and steam turbine method and apparatus disclosed herein. 
         [0044]    The pumping means  200  shown in  FIG. 1B  may include any type of pump, which is available commercially in various styles. 
         [0045]    The thermal storage device  250  may be any type of reservoir capable of holding water at approximately 100° C. and approximately an atmospheric pressure of 0.1 MPa. The thermal storage device  250  may minimize the heat loss from the reservoir and substantially prevent entry of air into the reservoir. 
         [0046]    As shown in  FIG. 3 , an exhaust valve  722  and a piping system  732  connects the heat engine expansion chamber  708  to a heat exchanger chamber  726 . The exhaust valve  722  may be controlled to turn on and off at the appropriate points in the cycle. As described below, a heat exchanger  724  is enclosed in the heat exchanger chamber  726 . The heat exchanger  724  may be a standard heat exchanger as commonly known by persons of ordinary skill in the art. The heat exchanger  724  may be cooled using fluid  20  from the cold thermal storage device  450 . A piping system  733  and a return pump  730  connects the heat exchanger chamber  726  and the heat engine expansion chamber  708  to pump water back into heat engine expansion chamber  708  in the form of a mist at the appropriate point in the cycle. 
         [0047]    A piping system  735  and a pumping device  734  are connected to the bottom of heat exchanger chamber  726 . The fluid  710  is pumped from the heat exchanger chamber  726 , reheated in the heating device  100 , and then returned to the hot thermal storage device  250 . 
         [0048]    A detailed view of a heat engine expansion chamber  708  and heat pump floating piston  706  are shown in  FIG. 4  which shows the piston  500  at top dead center, and  FIG. 5  which shows the piston  500  at bottom of stroke. The drawings focus on the heat engine  790  side of the concentrator  700 , which is only partially shown. But by analogy it can also be applied to the piston on the heat pump  792  side. This system is designed to minimize condensation on the piston head and chamber for reasons discussed in the background portion of the specification. In this embodiment, the piston  500  comprises a piston top  502  which is integrally coupled with a piston head conduit  510  which circulates vapor at the conditions found at the inlet valve  718  (not shown). 
         [0049]    The piston head conduit  510  can take any of several forms as shown in  FIG. 7   a,b,c,d  sufficient to assure a good thermal transfer between the piston head conduit  510  and the piston head  502 . The conduit is preferably standard ¼ inch copper or aluminum tubing suitable for plumbing applications. 
         [0050]    The piston side(s)  504  extend between the piston top  502  and the piston bottom  506 , and are predetermined in length to extend approximately the length of the stroke. If the zone is too short then efficiency is decreased. If too long then the over all size of the system increases and the cycle time per stroke. The diameter of the cylinder created by the piston sides  504  should be approximately 4 to 20 mm smaller than the diameter of the concentrator wall  702 . Those skilled in the art will appreciate that the final determination of this diameter be a function of the desired piston to wall gap  520  for the creation of steam in the piston to wall gap  520  during the upstroke of the piston  500  as shown in  FIG. 4  and are dependent upon the final sizing of the system. The piston top  502 , piston side  504 , and the piston bottom  506  should be constructed in such a way as to be water tight. 
         [0051]    Internal conduit  512  are integrally connected with the piston head conduit  510  and extend between the piston top and bottom where it flows through an interface seal  514  which serves to allow a water tight seal between the internal conduit  512  and the piston bottom  506 . Below the piston bottom  506 , flexible tubing  516  should be used to allow for movement of the piston. The flexible tubing  516  then exits the concentrator wall  702  through a separate interface seal  514  in order to assure integrity of the system. 
         [0052]    Alternate Embodiment 
         [0053]    As shown in  FIG. 10   a,  heat engine floating piston  704  has a piston top member  760 , which includes the bottom wall of heat engine expansion chamber  708 . Beneath the piston top member  760  is a layer of piston insulation  762 . The piston insulation  762  should be sufficient reduce heat loss through the piston top member  760 . The density of the piston insulation  762  may also play a role in determining the depth at which heat engine floating piston  704  floats. Beneath the piston insulation  762  is a piston sealing member  764 , which serves to seal the cavity formed by the piston sealing member  764  and the piston top member  760 . A plurality of piston vertical supports  766  may run between the piston top member  760  and the piston sealing member  764 , to support them against operational pressure. In this embodiment, the piston top member  760 , the piston sealing member  764 , and the piston vertical supports  766  are made of aluminum. These members together form a piston top assembly  759 . 
         [0054]    The piston top assembly  759  is connected to a piston structure  768 . A plurality of piston wall units  770  are fastened to a circumference of the piston structure  768 , providing a thermal barrier between the heat engine expansion chamber wall  709  and the part of liquid connecting rod  716  that is inside the heat engine floating piston  704 . 
         [0055]    An example of the piston wall unit  770  is shown in more detail in  FIG. 10   b.  The piston wall unit  770  includes a wall member  772 , which may be constructed of die cast aluminum, and a sealing plate  778 . The wall member  772  may be a single unit including an outer wall  774  and a series of supporting ribs  776 . The sealing plate  778  may also be an aluminum sheet welded to the wall member  772  to form a substantially airtight seal. Additionally, the interior of the piston wall units  770  may be air or a vacuum to reduce heat transfer. Still further, the interior of piston wall units  770  may be filled with a closed cell water resistant material. It will be appreciated by persons skilled in the art that any suitable thermal barrier may be used between the heat expansion chamber wall  709  and the heat engine floating piston  704 . A seal material  780 , such as rubber, nylon, or other suitable material, may be placed between the piston wall units  770  during assembly to substantially prevent the flow of water from the gap between the heat engine expansion chamber wall  709  and the heat engine floating piston  704  to the interior of the heat engine floating piston  704 . 
         [0056]    The heat engine floating piston  704  may provide a small gap, sufficient to inhibit heat transfer, approximately 2 mm in this example, between the outer surface of heat engine floating piston  704  and the inner surface of concentrator wall  702 . This gap may influence the efficiency of the system, as discussed below. 
         [0057]    To facilitate the efficiency of the system, a gap seal  522 , as shown in  FIG. 6 , may be added to keep liquid from the liquid connecting rod  716  from entering the piston wall gap  520 . The gap seal  522  should be designed to fit around the lower perimeter of the piston  500  near the piston bottom  506  as shown in  FIG. 4 and 5 . 
         [0058]    The gap seal  522  may also be oriented such that any excess pressure by steam generated during the upstroke will be relieved across the gap seal  522  and into the mass of the liquid connecting rod  716 , similar to the action of a flapper valve. In one embodiment, the gap seal  522  can extend around the perimeter near the bottom of the piston side  504  and with a fixed end  528  attached to the piston  500  and the free end  530  able to move relative to the concentrator wall  702 . 
         [0059]    Thermodynamic Cycles 
         [0060]    A flowchart representative of an example process to implement the system of  FIGS. 1A and 1B , is shown in  FIG. 12 . In this example, the process and/or machine readable instructions comprise a program for execution by a processor, controller, or similar computing device as described above. Generally speaking, the process acquires and stores thermal energy via one or more heat collectors  100 . The acquired and stored thermal energy is provided to a concentrator  700 , which includes a heat engine to drive a heat engine piston through various thermodynamic processes. The heat engine transfers energy to the heat engine floating piston  706  via a liquid connecting rod  716  of the concentrator  700 . Such energy transfer to the heat pump floating piston  706  further delivers energy to an electrical generating unit  600  to produce electricity. 
         [0061]    In view of  FIG. 12  and starting with the liquid connecting rod  716  and the heat engine floating piston  804  at top dead center, the inlet valve  718  is opened allowing the flow of steam from the hot thermal storage device  250  into the heat engine expansion chamber  708 . In the ideal cycle, this flow occurs in an isothermal, isobaric, and isentropic manner. This section of the cycle is labeled as Process  1 , Isothermal Expansion in  FIG. 12 . At the beginning of Process  1 , the heat engine fluid  710  may be a saturated vapor at approximately 364° K. and approximately 0.072 MPa and the heat engine expansion chamber. The heat engine  790  supplies work to the liquid connecting rod  716  during this phase of the cycle. 
         [0062]    After the liquid connecting rod  716  has moved down to expand the heat engine expansion chamber  708 , the inlet valve  718  is closed starting Process  2 , Isentropic Expansion. Process  2  is expansion of the heat engine fluid  710  in the heat engine expansion chamber  708  along the saturation curve. At the beginning of Process  2 , the heat engine fluid  710  is still saturated vapor at approximately 364° K. and approximately 0.072 MPa. As the heat engine expansion chamber  708  expands, the pressure and the temperature of the heat engine fluid  710  drop and a part of the heat engine fluid  710  begins to change from the vapor and/or steam phase to liquid water. As the heat engine expansion chamber  708  continues to expand, the temperature and the pressure continue to drop and additional steam is changed to liquid water. In this example, the temperature of both the steam and the liquid phase drops at the same rate as the heat engine expansion chamber  708  expands. The heat engine supplies work to the liquid piston during this phase of the cycle. Controlling the temperature of both the steam and the liquid phase to drop at the same rate may be accomplished using several different methods. In this example, the concentrator wall  702  in the region of the heat engine, and the piston  500  are maintained at a temperature above the saturation point so the liquid water will have no surface on which to condense and will basically form a fog or liquid suspended in vapor. 
         [0063]    When the piston  500  reaches the bottom of the stroke, Process  3  begins as the heat engine exhaust valve  810  is opened, connecting the heat engine expansion chamber  708  to the condensation chamber  812 . In this example, the temperature and pressure in the condensation chamber  812  is lower than temperature and pressure in the heat engine expansion chamber  708  when the heat engine exhaust valve  810  opens. Additional condensation in the condensation chamber  812  occurs, causing the temperature and the pressure in the heat engine expansion chamber  708  to rapidly drop which is shown as condensation at constant volume in Process  3 . In practice, the volume changes slightly during Process  3 , but the change in volume is minimal. 
         [0064]    As the heat engine floating piston  804  begins its upward stroke due to inertial forces of the system  10 , Process  4 , Isothermal Compression starts as shown in  FIG. 12 . At the beginning of Process  4 , the heat engine fluid  710  is a mixture of liquid and vapor at approximately 300° K. and approximately 0.0038 MPa and the heat engine expansion chamber  708  is at a volume of approximately 1.71 m 3  in one embodiment of the system. The heat engine expansion chamber  708  begins to decrease in volume, compressing the heat engine fluid  710 . As the steam begins to compress, the temperature and the pressure rise incrementally and the steam will begin to condense in the condensation chamber  812 . Sufficient heat is transferred out of the system  10  through the condensation process so that this process proceeds isothermally. The liquid piston supplies work to the heat engine during Process  4 . At the end of Process  4 , the heat engine fluid  710  is at approximately 301° K. and approximately 0.0038 MPa. 
         [0065]    After the proper amount of heat and mass have been transferred during Process  4 , the exhaust valve  810  is closed, isolating the heat engine expansion chamber  708  from the condensation chamber  812 . Closing the exhaust valve  810  causes the start of Process  5 , Isentropic Compression. At the beginning of process  5 , the heat engine fluid  710  includes a mixture of liquid and vapor at a temperature of approximately 300° K. and a pressure of approximately 0.0038 MPa. As the heat engine floating piston  804  continues upward, compression of the heat engine expansion chamber  708  is continued. The heat engine expansion chamber  708  contains a mixture of liquid and steam at this point in the cycle. During Process  5 , the liquid evaporates and the heat engine fluid  710  becomes a saturated vapor at a temperature of approximately 364° K. and a pressure of approximately 0.072 MPa. When the heat engine floating piston  804  reaches the top of its stroke, the process repeats in an iterative manner. 
         [0066]    It can be noted that all four heat engine processes occur on the saturation line. The processes that are isentropic are only isentropic when both the liquid and vapor phases are considered. 
         [0067]    Condensation can be prevented by maintaining the temperature of the heat engine expansion chamber wall  709  and the heat pump expansion chamber wall  713  at or above the saturation temperature for the highest pressure point in the cycle. This is also applicable to the top face of the heat engine floating piston  704  and the heat pump floating piston  706 . As shown in  FIG. 10   a  and  10   b,  using an adequate amount of insulation behind the wall and below the top of the piston, heat transfer losses may be lowered. 
         [0068]    The thermal mass of the wall of the heat engine expansion chamber  708  will normally be much higher than the thermal mass of the combination of that part of the liquid connecting rod  716  which is located between the heat engine expansion chamber  708  and the heat engine floating piston  704  and the outer wall of the heat engine floating piston  704 . It may be advantageous to reduce the mass of the liquid connecting rod  716  and the heat engine floating piston  704  in the described area. When the liquid connecting rod  716  is at the top stroke of the heat engine, the liquid at the top of the liquid connecting rod  716  between the heat engine floating piston  704  and the heat engine expansion chamber wall  709  may be at a slightly lower temperature than the adjacent section of the heat engine expansion chamber wall  709 . Heat will flow from the heat engine expansion chamber wall  709  into the adjacent element of the liquid connecting rod  716 . As the liquid connecting rod  716  begins to drop, this same element will now be adjacent to a lower and colder section of the heat engine expansion chamber wall  709 . Heat will flow from the element of liquid connecting rod  716  into the adjacent element of the heat engine expansion chamber wall  709 . Due to the differences in thermal mass, this typically will cool the element of the liquid connecting rod  716  and slightly raise the temperature of the element of the heat engine expansion chamber wall  709 . This process may continue as the liquid connecting rod  716  continues to drop, until the same element of the liquid connecting rod  716  is completely cooled by the time that it reaches the bottom of the stroke. 
         [0069]    The process is reversed on the upward stroke of the liquid connecting rod  716 . As the piston  500  is pushed upward, it moves into the zone adjacent to a warmer element of the heat engine expansion chamber wall  709 . This will continue as the liquid connecting rod  716  rises, with the result that the element of the liquid connecting rod  716  will be nearly at the maximum temperature of the heat engine expansion chamber wall  709  when it reaches the top stroke. 
         [0070]    With the process described herein, only a very small amount of heat is added to the system during each stroke because almost all of the heat required to heat the portion of liquid connecting rod  716  in the gap between the heat engine floating piston  704  and the heat engine expansion chamber wall  709  is recycled between the element of liquid connecting rod  716  and the heat engine expansion chamber wall  709  during the cycle. 
         [0071]    A similar process occurs for the outer wall of the heat engine floating piston  704 , with the outer wall transferring heat back and forth through the liquid connecting rod  716  to the heat engine expansion chamber wall  709  during each cycle. Having steam or vapor in the piston to wall gap  520  increases efficiency for the system by minimizing heat transfer between the concentrator  702  and the piston  500  by achieving a lower thermal mass and relying upon vapor phase transfer as opposed to liquid wetting and conduction, it also provides a buffer between the piston bottom  506  which is constrained in temperature by the liquid connecting rod, and the piston top  502  which is bounded by the heat carried by the piston head conduit. The result being reduced condensation around each piston head and chamber. An advantage of minimizing the condensation around the piston head and chamber, is to increase the efficiency of the system by keeping the thermodynamic cycle at or near the condensation point of the working fluid. A further advantage is that steam entering the chamber need not be superheated to inhibit condensation.  FIGS. 11   a  and  11   b  illustrate how reduction of condensation around a piston head and chamber is achieved. 
         [0072]      FIGS. 11   a  and  11   b  show an exemplary thermal wall profile, illustrating how heat transfer is minimized between the piston side  504  and the concentrator wall  702  of the heat engine  790 . The same profiles may apply to the piston and wall on the heat pump  792  side, by analogy.  FIG. 11   a  shows the profile for a piston  500  at top dead center of the heat engine expansion chamber  708 .  FIG. 11   b  shows the profile for a piston  500  at bottom dead center of the heat engine expansion chamber  708 . In both profiles, T 1  represents the temperature of the boundary condition of the liquid connecting rod  716 . T 2  represents the temperature of the boundary condition of the heat engine expansion chamber  708 . T SAT  represents the saturation temperature of the liquid connecting rod  716  whereby condensation is vaporized. T 2  is greater than T 1  in both profiles because the temperature inside the chamber  708  is constantly maintained at or near its initial temperature by chamber wall conduits  526  and piston head conduits  510 . The profiles plot T 1  and T 2 , as measured along the piston side  504 , from the piston bottom  506  to the piston top  502 . 
         [0073]    The optional housing structure  524  can be made of any structural or insulative material designed to retain the heat near the chamber  708  wall. The structure  524  is lined with chamber wall conduits  526 , which vaporize the liquid connecting rod  716 , as the liquid makes contact with the structure  524 . The temperature of the structure  524  is higher than that of the concentrator wall  702  because of the conduits  526 . T SAT  is achieved at the piston top  502  in both  FIGS. 11   a  and  11   b.  Thus, condensation reduction, i.e. vaporization, occurs inside the heat engine expansion chamber  708 , regardless of how the position of the piston  500  changes relative to the chamber  708 . A difference in how T SAT  is achieved between  FIGS. 11   a  and  11   b,  is the rate at which T 2  is achieved in relation to T 1  along the piston side  504 . 
         [0074]    In  FIG. 11   a,  the piston  500  is located at top dead center of the chamber  708  and the liquid  716  between the piston  500  and the structure  524  is vaporized because of the conduits  526 . The temperature along the piston side  504 , quickly approaches T SAT  because each point along the piston side  504  is adjacent to the conduits  526 , which are maintained at the same temperature as T SAT . Accordingly, vaporization occurs in the piston to wall gap  520 . 
         [0075]    In  FIG. 11   b,  the piston is located at bottom dead center of the chamber  708  and the concentrator  702  is cooler than the piston side  504 . The temperature along the piston side  504  resembles a more linear relationship than that in  FIG. 11   a,  in part because of the lack of conduits in the concentrator  702 , which has a cooler surface relative to the structure  524  and piston side  504 . T SAT  is still achieved where the piston side  504  meets the piston top  502 , because of how the temperature is maintained in the chamber  708  with conduits  526  and  510 . When the piston  500  moves from top dead center to bottom dead center, vapor is created inside the piston to wall gap  520 , inhibiting heat transfer from the piston side  504  to the concentrator  702  because the intermediate vapor has a temperature at or above T SAT . 
         [0076]    Internal Spray Embodiment 
         [0077]    The heat engine floating piston  804  and the heat pump floating piston  706  are constructed to reduce the thermal mass exposed to the heat engine expansion chamber wall  709 . 
         [0078]    As shown in  FIG. 9 , the heat engine floating piston  804  has a piston top member  814 , which includes the bottom wall of the heat engine expansion chamber  708 . The piston top member  814  is connected to a piston outer wall  816 , which is designed such that the height substantially matches the length of the stroke. In this example, the piston outer wall  816  is formed from rolled and welded aluminum sheet approximately 1.5 mm thick. The piston inner wall  818  is also formed from rolled and welded aluminum sheet approximately 1.5 mm thick. The gap  820  between the piston outer wall  816  and the piston inner wall  818  provides a thermal barrier between the concentrator wall  802  and the part of liquid connecting rod  716  that is inside of the heat engine floating piston  804 . The heat engine floating piston  804  is designed and built to provide a small gap, approximately 2 mm, between the outer diameter of the heat engine floating piston  804  and the inner diameter of the concentrator wall  802 . A piston seal  822  may be located near the top of the heat engine floating piston  804  to minimize condensation and evaporation effects from the concentrator wall  802 . 
         [0079]    An exhaust valve  810  may connect the heat engine expansion chamber  708  to a condensation chamber  812 . The exhaust valve  810  can be controlled to turn on and off at the appropriate points in the cycle. A spray system  824  may be located in the condensation chamber  812 . When the exhaust valve  810  is opened, liquid from liquid connecting rod  716  is sprayed into the condensation chamber  812  to cause condensation of the heat engine fluid  710 . Heat is removed from the liquid connecting rod  716  either by using a conventional heat exchanger or by circulating fluid through the liquid connecting rod  716  and cooling the fluid, for example, at night using cooling device  300 . 
         [0080]    Although certain example methods, apparatus, and articles of manufacture have been described herein, the scope of coverage of this patent is not limited thereto. On the contrary, this patent covers all methods, apparatus and articles of manufacture fairly falling within the scope of the appended claims either literally or under the doctrine of equivalents.