Abstract:
Methods and systems are disclosed to convert low temperature thermal energy to electricity. An example apparatus disclosed herein includes an electrical generating unit to receive heat energy to produce electricity, a concentrator including a heat engine, a liquid piston operatively coupled to the heat engine, and a heat pump operatively coupled to the liquid piston, the heat engine adapted to collect thermal energy, and the heat pump operatively coupled to the electrical generating unit to provide heat to the electrical generating unit. The example apparatus disclosed herein also includes a heat engine floating piston disposed in the heat engine, a heat pump floating piston disposed in the heat pump, and wherein the heat engine floating piston and the heat pump floating piston oscillate.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
   This application is a non-provisional application claiming priority from U.S. Provisional Application Ser. No. 60/664,480, filed Mar. 23, 2005, entitled “Utility Scale Method and Apparatus to Convert Low Temperature Thermal Energy to Electricity” and incorporated herein by reference in its entirety. 

   FIELD OF THE INVENTION 
   The present invention is directed toward a thermal concentrator inputting low grade thermal energy and outputting useful energy. More specifically converting heat from solar power into electricity, mechanical work and the like. 
   BACKGROUND 
   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. With these technologies, the solar radiation is concentrated at the time of collection requiring a high working temperature at the point of collection. 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. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1A  is an exemplary layout of a concentrator system. 
       FIG. 1B  is an exemplary schematic block diagram of the system of  FIG. 1A . 
       FIG. 2  is a perspective view of one embodiment of a heating device. In this example a solar collector. 
       FIG. 3  is a schematic view of an array of solar collectors connected to a thermal reservoir. 
       FIG. 4  is a cutaway perspective view of an exemplary hot thermal storage. 
       FIG. 5  illustrates an alternative embodiment of  FIG. 3  using two thermal reservoirs. 
       FIG. 6  shows example components of a heat actuated dual loop heat pump. 
       FIG. 7A  shows a side view of a through section of an exemplary floating piston. 
       FIG. 7B  shows a section detail of an exemplary piston wall unit. 
       FIG. 8  shows a schematic view of an exemplary heat exchanger unit. 
       FIG. 9  shows a schematic view of a heat pump loop for converting energy. 
       FIG. 10  shows an exemplary thermal wall profile. 
       FIG. 11  shows an exemplary level control configuration. 
       FIG. 12  shows example components of a heat actuated dual loop concentrator for an alternate embodiment. 
       FIG. 13  shows a P-V diagram for an embodiment of a heat engine cycle. 
       FIG. 14  shows an exemplary heat engine piston for an alternate embodiment. 
       FIG. 15  shows a P-V diagram for an embodiment of a heat pump cycle. 
       FIG. 16  illustrates a flow diagram of an example process to convert thermal energy to electricity. 
       FIG. 17  illustrates a flow diagram of an example process for a heat engine cycle. 
       FIG. 18  illustrates a flow diagram of an example process for an isothermal expansion of the heat engine cycle of  FIG. 17 . 
       FIG. 19  illustrates a flow diagram of an example process for an isentropic expansion of the heat engine cycle of  FIG. 17 . 
       FIG. 20  illustrates a flow diagram of an example process for a constant volume condensation of the heat engine cycle of  FIG. 17 . 
       FIG. 21  illustrates a flow diagram of an example process for an isothermal compression of the heat engine cycle of  FIG. 17 . 
       FIG. 22  illustrates a flow diagram of an example process for an isentropic compression of the heat engine cycle of  FIG. 17 . 
       FIG. 23  illustrates a flow diagram of an example process for a heat pump cycle. 
       FIG. 24  illustrates a flow diagram of an example process for an isentropic compression of the heat pump cycle of  FIG. 23 . 
       FIG. 25  illustrates a flow diagram of an example process for an isothermal compression of the heat pump cycle of  FIG. 23 . 
       FIG. 26  illustrates a flow diagram of an example process for an isentropic expansion of the heat pump cycle of  FIG. 23 . 
       FIG. 27  illustrates a flow diagram of an example process for an isothermal expansion of the heat pump cycle of  FIG. 23 . 
       FIG. 28  is a block diagram of an example processor system that may be used to implement portions of the system. 
   

   DESCRIPTION 
   The present system utilizes a dual loop U, partial square, or other suitably shaped heat actuated liquid piston heat pump, where one vertical leg contains part of a heat engine loop and the other vertical leg contains part of a heat pump loop. Persons of ordinary skill in the art will appreciate that the heat pump described herein is sometimes referred to as a compressor. The top of the vertical legs contain steam. The bottom of each vertical leg and the horizontal portion contains liquid water, on top of which is typically a floating piston usually constructed from a solid material, such as, for example, aluminum or steel. 
   The system operates at or near resonance. The resonance occurs between the kinetic energy of the mass of the liquid water and pistons, the potential energy due to gravity or hydraulic head, and the potential energy stored in the steam at the top of each vertical leg. Among other advantages, resonance allows the steam to enter the heat engine with little or no throttling. 
   The heat engine section operates using a thermodynamic cycle and draws the heat energy from a natural or waste heat source, typically from 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. 
   The heat pump loop contains the heat pump described above and a steam turbine, which is connected to and drives 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. 
   Steam and liquid water reservoirs are typically used between the solar collectors and the heat engine. Steam reservoirs are also typically used between the heat pump and steam turbine to even the flow of steam from the reciprocating heat actuated liquid piston heat pump. 
   Both of the loops may operate at or below atmospheric pressure. This feature, in combination with placement of the heat actuated liquid piston heat pump below grade, may allow the use of low cost materials, such as concrete. 
   While the equipment and method described herein to generate electricity are described in terms of solar power, it could be used with other sources of heat. For example, the system could be used with low grade heat from a geothermal source. It is preferred that the heat is available 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. 
     FIG. 1A  is an exemplary embodiment 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. 
   This embodiment utilizes two thermal storage systems (hot and cold)  250 ,  450 , but alternative systems which use multiple thermal storage systems or no thermal storage systems can also be used. This embodiment uses solar as the heat source. Because solar energy is intermittent, the system may work more efficiently if thermal storage is utilized. 
   If a continuous heat source, such as geothermal or industrial waste heat is utilized, the thermal storage system  250  may be eliminated. 
   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 . 
   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 . In an exemplary embodiment, the fluid  15  stored in the hot thermal storage device  250 , the cold thermal storage device  450  and the fluid  716  used in concentrator  700  and in the electric converter  600  are all water in either liquid or steam form. 
   In one embodiment of the disclosed system  10 , the heat concentration is done near the time of use, rather than at the time of collection. It will be understood by one of ordinary skill 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. 
   In one example, it will be understood that the system  10  can operate at any time, such as, for example, during periods of high electricity demand rather than continuously during the 24 hour period, in which case fewer solar collectors and heat storage may be required for the same peak output level. 
   The pumping means  200  shown in  FIG. 1B  may include any type of pump, which is available commercially in various styles. 
   The thermal storage device  250  may be any type of reservoir, such as, for example, a reservoir capable of holding water at approximately 100 C and 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. In this embodiment, the thermal storage device  250  is constructed from concrete  251  and an insulator  252  as shown in  FIG. 4 . 
   The cold thermal storage device  450  can be a similar type of tank as the hot thermal storage device  250 . In this example, the cold thermal storage device  450  may store water in the liquid and vapor form at approximately 37 C. and 0.0062 MPa. 
   As shown in  FIG. 6 , the concentrator  700  in this example includes a heat actuated liquid piston heat pump  792 . The concentrator  700  includes a concentrator wall  702 , which forms an internal U shaped chamber. The concentrator wall  702  may be constructed with substantially the same diameter along the length of the tube. In this example, the concentrator wall  702  is constructed of concrete and is approximately 150 mm thick. The inside diameter of the U tube is approximately 10 m. The vertical legs are approximately 35 m long and the horizontal leg is approximately 10 m long. Additionally, in this example, the height of the vertical heat engine  790  vertical leg  709  is 0.3 m lower than the vertical heat pump leg  713 . 
   The upper portions of the vertical legs  709 ,  713  may be made from a different material than the lower part of the U tube if desired. For example, the top 10 meters may be constructed of steel. 
   The lower portion of the concentrator  700  is filled with fluid, such as, water, and includes a liquid piston  716 . A heat engine floating piston  704  floats on the top of the liquid piston  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 piston  716  in the other vertical leg, forming a heat pump expansion chamber  712  between the heat pump floating piston  706  and concentrator wall  702 . The heat engine expansion chamber  708  may be filled with heat engine fluid  710 . The heat pump expansion chamber  712  may be filled with a heat pump fluid  714 . 
   The construction of the heat engine floating piston  704  and the heat pump floating piston  706  may be constructed such as to minimize the thermal mass exposed to the heat engine expansion chamber wall  709 . 
   As shown in  FIG. 7A , 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  may be a sufficient thickness and insulating value to reduce the heat losses 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 the 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 . 
   The piston top assembly  759  is connected to a piston structure  768 , which, in this example, is approximately 10 meters tall. 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 piston  716  that is inside the heat engine floating piston  704 . 
   An example of the piston wall unit  770  is shown in more detail in  FIG. 7B . 
   The heat engine floating piston  704  may provide a small gap, such as, for example, approximately 2 mm, between the outer diameter of heat engine floating piston  704  and the inner diameter of concentrator wall  702 . This gap may influence the efficiency of the system, as discussed below. 
   As shown in  FIG. 8 , 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. 
   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 . 
     FIG. 9  shows components used in a heat pump cycle of the heat pump  792 . In this example, the heat pump expansion chamber  712  is connected to an ambient pressure chamber  550  with a piping system  750  that contains heat pump pressure valve  752 . The ambient pressure chamber  550  is connected to the inlet of the fluid to electric converter  600 , which in this case is a standard steam turbine. The outlet of fluid to electric converter  600  is connected to vacuum chamber  560 . The vacuum chamber  560  is connected back to the heat pump expansion chamber  712  through a piping system  751  that contains a heat pump vacuum valve  754 . 
     FIG. 11  shows an actuator  736  and a partial sealing device  738 , a plurality of which may extend, at least partially, around the circumference of liquid piston  716 . In this example, the actuator  736  controls the gap (g) between the heat engine expansion chamber wall  709  and the heat engine floating piston  704  at one elevation. Activation of one or more sealing devices  738  closes the gap (g) for a particular circumferential span of the liquid piston  716 , thereby impeding fluid flow between the liquid piston  716  and the expansion chamber  712 . Activation and deactivation of the partial sealing devices  738  has the effect of a proportional flow control valve. 
   Persons of ordinary skill in the art will appreciate that the aforementioned example apparatus and processes below may be controlled by a processor, a controller, and/or similar computing device(s). Various processes may be executed by machine readable instructions and/or programs. The programs may be embodied in software stored on a tangible medium such as, for example, a flash memory, a CD-ROM, a floppy disk, a hard drive, a digital versatile disk (DVD), or a memory associated with the computer. Persons of ordinary skill in the art will readily appreciate that the entire program and/or parts thereof could alternatively be embodied in firmware or dedicated hardware in a well known manner (e.g., it may be implemented by an application specific integrated circuit (ASIC), a programmable logic device (PLD), a field programmable logic device (FPLD), programmable logic controller (PLC), personal computer (PC), discrete logic, etc.). Also, some or all of the machine readable instructions represented by flowcharts, discussed below, may be implemented manually. Further, persons of ordinary skill in the art will readily appreciate that many other methods of implementing the example machine readable instructions described below may alternatively be used. For example, the order of execution of various function blocks may be changed, and/or some of the blocks described may be changed, substituted, eliminated, or combined. 
   In operation, one embodiment of the system disclosed in this patent converts solar energy into electricity. Throughout this disclosure, energy and power levels and calculations generally refer to average levels over a 24-hour period. This differs from the typical practice of describing solar energy equipment in terms of peak power. 
   It will be appreciated that any number of hot thermal storage devices  250  may be utilized, including, for example, a pair of hot thermal storage devices as shown in  FIG. 5 . In this type of system, fluid  710  may be pumped out of the first hot thermal storage device  250 A by a first pump  270 , through the heating device  100 , and into the second hot thermal storage device  250 B. Then, at the time of use, the fluid  710  is drawn from the second hot thermal storage device  250 B by a second pump  272 , evaporated in an evaporation chamber  248 , which cools the remaining liquid  710 , and then is pumped back to the first hot thermal storage device  250 A by a third pump  274 . 
   The cooling system may operate in an analogous manner and, alternatively, in this example, the system  10  utilizes the heating device  100  at night as the cooling device  300 . This eliminates the need for the additional cost of a separate cooling device  300  and provides the additional advantage of preventing freezing of the cooling device  300  when ambient temperature is below 273 K. 
   A flowchart representative of an example process to implement the system of  FIGS. 1A and 1B , is shown in  FIG. 16 . 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.  FIG. 16  is an example process  2600  for converting thermal energy to electricity. Generally speaking, the process  2600  acquires and stores thermal energy (block  2605 ) via one or more heat collectors  100 , see  FIG. 1 . 
   An example thermodynamic cycle for the heat engine loop operates in the following manner, which is substantially different than a typical Carnot or Rankine cycle. Referring to  FIGS. 6 ,  8  and  13 , the liquid piston  716  and the heat engine floating piston  704  at top dead center, the inlet valve  718  is opened allowing the flow of fluid  710 , e.g., 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. In the actual cycle, the fluid  15  of the thermal storage device  250  may cool slightly. This section of the cycle is labeled as Process  1 , Isothermal Expansion, in  FIG. 13 . At the beginning of Process  1 , the heat engine fluid  710  comprises a saturated vapor. The heat engine supplies work to the liquid piston during this phase of the cycle. 
   After the liquid piston  716  has moved down to expand the heat engine expansion chamber  708  (block  2810 ), inlet valve  718  is closed (block  2905 ), starting Process  2 , Isentropic Expansion as shown in  FIGS. 17 and 19 . This volume change can be sensed using any suitable device, including, for example, a commercially available sensor, such as a laser distance measurement sensor. Alternatively, the beginning of Process  2  may be determined by measurement of process parameters such as temperature and pressure. 
   Controlling a temperature drop of both the steam and liquid phases at the same rate may be accomplished using several different methods. In this example, the concentrator wall  702  and the heat engine floating piston  704  are maintained at a temperature above the saturation point, as described in the analysis of thermal losses section, so that the liquid water will have no surface on which to condense and will basically form a fog or liquid suspended in vapor. 
   When the heat engine floating piston  704  reaches the bottom of the stroke, the heat engine exhaust valve  722  is opened see  FIG. 8 , connecting the heat engine expansion chamber  708  to the heat exchanger  724  located in the heat exchanger chamber  726 . In an ideal cycle, the heat exchanger vapor  728  in the heat exchanger chamber  726  will be the same temperature and pressure as the heat engine fluid  710  in the heat engine expansion chamber  708 . 
   As the heat engine floating piston  704  begins its upward stroke, Process  3 , Isothermal Compression, starts. At the beginning of Process  3 , the heat engine fluid  710  is a mixture of liquid and vapor at approximately 310.degree. K and 0.0062 MPa and the heat engine expansion chamber  708  is at a volume of approximately 1876 m.sup.3. The heat engine expansion chamber  708  begins to decrease in volume, compressing the heat engine fluid  710  and the heat exchanger vapor  728 . As the steam begins to compress, the temperature and the pressure rise incrementally and the heat exchanger vapor  728  will begin to condense on the heat exchanger  724 . Sufficient heat is transferred out of the system through the heat exchanger  724  so that this process proceeds isothermally. In the ideal cycle, a quantity of water is transferred from the heat exchanger chamber  726  so that the process also proceeds isentropically on a specific entropy basis. The total entropy decreases because heat and mass is transferred out of the system in this process. The liquid piston supplies work to the heat engine during Process  3 . 
   After the proper amount of heat and mass have been transferred during Process  3 , the exhaust valve  722  is closed, isolating the heat engine expansion chamber  708  from the heat exchanger chamber  726 . In the ideal cycle, condensation and heat transfer in the heat exchanger chamber  726  would stop at this point, but in the actual cycle condensation and heat transfer can be allowed to proceed while the exhaust valve  722  is closed. Closing the exhaust valve  722  causes the start of Process  4 , Isentropic Compression. At the beginning of Process  4 , the heat engine fluid  710  includes of a mixture of liquid and vapor at a temperature of approximately 310.degree. K and a pressure of approximately 0.0062 MPa and the heat engine expansion chamber  708  has a volume of approximately 1443 m.sup.3. As the heat engine floating piston  704  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  4 , the liquid evaporates and the heat engine fluid  710  becomes a saturated vapor. This is different from a typical compression process in which the mixture of liquid and vapor is compressed with the resultant fluid including saturated liquid. This difference is explained in subsequent paragraphs. 
   In the actual process, the liquid water of Process  4  may need to be added back into the heat engine expansion chamber  708  from the heat exchanger chamber  726  to reach the proper conditions at the beginning of Process  1 . This can be done using the return pump  730  shown in  FIG. 8 . The amount of fluid  710  to be added back to the heat engine expansion chamber  708  can be determined by process conditions of the heat engine fluid  710  measured in the heat engine expansion chamber  708  or by other process parameters. 
   It is illustrative to compare the ideal heat engine cycle described in the prior sections to a typical ideal Carnot cycle. A Carnot cycle is a cycle that undergoes two isothermal reversible processes and two adiabatic reversible processes. By this definition, the ideal heat pump cycle disclosed herein is a form of a Carnot cycle because it has two isothermal reversible processes and two adiabatic reversible processes as can be easily seen in  FIG. 16 . However, the present heat engine cycle differs from the typical carnot cycle in several unique ways. 
   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 a isentropic compression process during which wet steam, which consists of steam and liquid, is compressed until the liquid evaporates to leave only saturated vapor. 
   The next process in both the Carnot cycle and the present heat engine cycle is a process of 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. 
   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. 
   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. 
   The most distinct and unique difference between the two cycles occurs in the isentropic compression process where 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. 
   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 all vapor result. 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 cannot be a constant entropy process. However, if the process produces all vapor at approximately 88% of the initial vapor specific entropy, constant entropy can be maintained by converting the liquid to vapor, 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. 
   In a typical Carnot cycle that has a high initial percentage of liquid, the process is reversed. 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. In the typical Carnot case, where the initial state is primarily liquid, the process can not end in vapor and maintain constant entropy because the majority of the mass would be increasing in entropy by a factor of approximately 13.9. The small drop in entropy of the initial vapor cannot offset such a large increase. 
   Another unique characteristic of the ideal cycle disclosed here is that the average specific entropy of the mass of both liquid and vapor during the heat engine cycle is constant throughout the cycle. The average specific entropy is always equal to the specific entropy of the vapor added to the cycle during the energy addition process. This is possible because low specific entropy liquid is removed from the system during the heat removal process. As heat is removed, high entropy vapor condenses to low entropy liquid, which has the effect of lowering the average specific entropy. However, at the same time, low entropy liquid mass is removed from the system, which raises the average specific entropy of the remaining mass, and offsets the previous effect. 
   It is not necessary to evaporate all of the liquid water at the end of Process  4 . Some liquid water may remain in the heat engine expansion chamber  708  at this point without substantially changing the cycle. 
   An additional exemplary thermodynamic cycle for the heat engine loop of this embodiment operates in the following manner, which is substantially different than a typical Carnot or Rankine cycle, but is similar to a typical nineteenth century steam engine cycle. Refer to  FIGS. 12-13 , and the flowcharts of  FIGS. 17-22 .  FIG. 17  illustrates additional detail of an example process of the heat engine to heat pump energy transfer (block  2615 ) described above in view of  FIG. 16 . The heat engine  790  position is checked by, for example, a laser distance measurement sensor to determine whether the heat engine piston  804  is at a top-stroke position (block  2705 ). If the heat engine piston  804  is not at the top-stroke position (block  2707 ), then the example process waits until the top-stroke position occurs. Persons of ordinary skill in the art will appreciate that, prior to steady-state harmonic operation of the concentrator pistons, the system may be initiated in a known state and/or pre-determined piston positions. For example, the fluid  710  (e.g., steam) may be injected into the concentrator  700  (e.g., the heat engine  790  side or the heat pump  792  side) to position the piston(s)  804 ,  706  into a known starting location and/or cycle the pistons  804 ,  706  through several strokes to get the system started. During steady-state harmonic operation of the heat engine  790 , the heat engine chamber  708  may proceed through isothermal expansion (block  2710 ), isentropic expansion (block  2715 ), constant volume condensation (block  2720 ), isothermal expansion (block  2725 ), and isentropic expansion (block  2730 ). The process of  FIG. 17  repeats in a harmonic manner when the heat engine piston  804  returns to the top-stroke position (block  2705 ). 
   In view of  FIG. 18  and starting with the liquid piston  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  (block  2805 ). In the ideal cycle, this flow occurs in an isothermal, isobaric, and isentropic manner. In the actual cycle, the fluid  15  of the hot thermal storage device  250  will cool slightly during each cycle, but this can be ignored for the purposes of understanding the cycle. This section of the cycle is labeled as Process  1 , Isothermal Expansion in  FIG. 13 . At the beginning of Process  1 , the heat engine fluid  710  may be a saturated vapor at approximately 364.degree. K and approximately 0.072 MPa and the heat engine expansion chamber  708  has a volume of approximately 0.046 m.sup.3. The heat engine  790  supplies work to the liquid piston  716  during this phase of the cycle. 
   After the liquid piston  716  has moved down to expand the heat engine expansion chamber  708  from approximately 0.046 m.sup.3 to approximately 0.717 m.sup.3 (block  2810 ), the inlet valve  718  is closed (block  2905 ), starting Process  2 , Isentropic Expansion (block  2715 ), as shown in  FIGS. 17 and 19 . This volume change can be sensed using a commercially available sensor, for example, a laser distance measurement sensor. Alternatively, the point may be determined by measurement of process parameters such as the temperature and the pressure. 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.degree. K and approximately 0.072 MPa, but the volume of the heat engine expansion chamber  708  has expanded from approximately 0.046 m.sup.3 to approximately 0.717 m.sup.3. 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  802  and the heat engine floating piston  804  are maintained at a temperature above the saturation point, as described in the analysis of thermal losses section, so the liquid water will have no surface on which to condense and will basically form a fog or liquid suspended in vapor (block  2910 ). At the end of Process  2 , the heat engine fluid  710  is at approximately 340.degree. K and approximately 0.027 MPa and the volume of the heat engine expansion chamber  708  is approximately 1.71 m.sup.3. 
   When the heat engine floating piston  804  reaches the bottom of the stroke (block  2915 ), Process  3  begins (shown in  FIG. 19 ) as the heat engine exhaust valve  810  is opened (block  3005 ), 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. This is shown as Process  3 , condensation at constant volume. In practice, the volume changes slightly during Process  3 , but the change in volume is minimal compared to the other processes. At the end of Process  3 , the heat engine fluid  710  is saturated vapor at approximately 301.degree. K and approximately 0.0038 MPa and the volume of the heat engine expansion chamber  708  is approximately 1.71 m.sup.3. 
   As the heat engine floating piston  804  begins its upward stroke (block  3010 ) as shown in  FIG. 20  due to inertial forces of the system  10 , Process  4 , Isothermal Compression, starts as shown in  FIG. 21 . At the beginning of Process  4 , the heat engine fluid  710  is a mixture of liquid and vapor at approximately 301.degree. K and approximately 0.0038 MPa and the heat engine expansion chamber  708  is at a volume of approximately 1.71 m.sup.3. 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 , Isothermal Compression. At the end of Process  4 , the heat engine fluid  710  is at approximately 301.degree. K and approximately 0.0038 MPa and the volume of the heat engine expansion chamber  708  is approximately 0.646 m.sup.3 (block  3105 ). 
   After the proper amount of heat and mass have been transferred during Process  4 , the exhaust valve  810  is closed (block  3110 ), 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, as shown in  FIG. 22 . At the beginning of process  5 , the heat engine fluid  710  includes a mixture of liquid and vapor at a temperature of approximately 301.degree. K and a pressure of approximately 0.0038 MPa and the heat engine expansion chamber  708  has a volume of approximately 0.646 m.sup.3. 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.degree. K and a pressure of approximately 0.072 MPa (blocks  3210  and  3205 ). When the heat engine floating piston  804  reaches the top of its stroke, the process repeats in an iterative manner, as shown in  FIG. 17 . 
   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. The entropy of each individual phase is not constant. 
   Heat Pump Cycle 
   A Carnot cycle is a cycle that undergoes two isothermal reversible processes and two adiabatic reversible processes, the internal heat pump cycle of heat pump fluid  714  inside of the heat pump expansion chamber  712  consists of a Carnot cycle. 
   This description of operation is illustrated in  FIG. 23  and starts at the point where the heat pump floating piston  706  of  FIG. 12  is at the bottom stroke, which occurs at the same instant in time that heat engine floating piston  804  is at the top stroke. The heat pump fluid  714  is a superheated vapor throughout all four processes of the heat pump cycle. The heat pump cycle processes may include, but are not limited to, an Isentropic compression process (block  3305 ), an Isothermal compression process (block  3310 ), an Isentropic expansion process (block  3315 ), and an Isothermal expansion process (block  3320 ). 
   Process  1 , as shown in  FIG. 24 , is isentropic compression as the heat pump floating piston  706  starts at the bottom of the stoke (block  3405 ) with the heat pump vacuum valve closed (block  3410 ), and begins upward travel. The heat pump fluid  714  starts Process  1  as shown in  FIG. 15  at approximately 376.degree. K and approximately 0.0193 MPa and the heat pump expansion chamber  712  has a volume of approximately 1.71 m.sup.3. As the heat pump floating piston  706  travels upward, the heat pump fluid  714  is compressed isentropically to a temperature of approximately 612.degree. K and approximately 0.15 MPa, which is slightly above atmospheric pressure. 
   When the heat pump expansion chamber  712  reaches a volume of approximately 0.38 m.sup.3 (block  3415 ), the heat pump pressure valve  752  is opened (block  3420 ) connecting the heat pump expansion chamber  712  to the pressure chamber  550  as shown in  FIG. 9 . This is the start of Process  2 , as shown in  FIG. 15 , which is an isothermal process of ejecting steam from the heat pump expansion chamber  712  to the pressure chamber  550  (block  3505 ) as shown in  FIG. 9 . Because the pressure chamber  550  is substantially larger than the heat pump expansion chamber  712 , the process is idealized by assuming that the temperature and pressure in the pressure chamber  550  remain substantially unchanged during Process  2 . In practice, multiple individual units, typically 18, of the concentrator  700  would be running out of phase to each other, so that a somewhat continuous flow of steam would be provided to the pressure chamber  550 . In addition, a continuous flow of steam would be withdrawn from the pressure chamber  550  to the fluid to electric converter  600 , which in this embodiment is a 650 kW steam turbine/generator set. As Process  2  begins, the heat pump expansion chamber  712  and the pressure chamber  550  are both at approximately 612.degree. K and approximately 0.15 MPa. The heat engine floating piston  804  continues upward until the volume of the heat pump expansion chamber  712  is approximately 0.046 m.sup.3, at which point the heat engine floating piston  804  is at the top of stroke (block  3510 ). The temperature of the heat pump expansion chamber  712  remains at approximately 612.degree. K and approximately 0.15 MPa. 
   At the top of stroke, the heat pump ambient pressure valve  752  is closed (block  3605 ) and Process  3  begins, as shown in  FIGS. 15 and 26 . Process  3  is isentropic expansion. Process  3  continues (block  3610 ) until the heat pump expansion chamber  712  reaches a volume of approximately 0.22 m.sup.3, at which point the heat pump vacuum valve  754  is opened (block  3615 ). 
   This starts Process  4 , as shown in  FIG. 37 , which is an isothermal injection of steam from the vacuum chamber  560  to the heat pump expansion chamber  712  (block  3705 ). Process  4  is idealized for this discussion for reasons identical to those described for Process  2 . The heat pump expansion chamber  712  starts and ends Process  4  at a temperature of approximately 376.degree. K and approximately 0.0193 MPa. When the heat engine floating piston  804  reaches the bottom stroke (block  3710 ), the heat pump vacuum valve  754  is closed and Process  1  begins again. 
   The heat pump fluid  714  flows from the heat pump expansion chamber  712  to the pressure chamber  550  to the fluid to electric converter  600  where it undergoes an isentropic expansion process in the fluid to electric converter  600 . It enters fluid to the electric converter  600  at a temperature of approximately 612.degree. K and approximately 0.15 MPa and exits at a temperature of approximately 376.degree. K and approximately 0.0193 MPa, which are the same conditions as those of the pressure chamber  550  and the vacuum chamber  560 . 
   It will be easily understood that all of the thermodynamics conditions described above are simply one set of many that may be selected without changing the nature of the thermodynamic cycles. 
   Operation of Liquid Piston 
   The mass of the liquid piston  716  and the floating pistons may play a key role in the operation of the system  10 . For example, the total mass affects the resonant frequency of the system and, therefore, may have a major influence on the cycle time of the system  10 . Streeter&#39;s Fluid Mechanics shows the physical response of a liquid filled U tube in section 12.1, “Oscillation of Liquid in a U tube.” The physics of the present system  10  are closely related to that described by Streeter, but differ because the present example system  10  uses a closed U tube and applies a driving force. The system  10  is essentially in resonance between the kinetic energy of the mass of the liquid piston  716 , the heat engine floating piston  704  and the heat pump floating piston  706 , the gravitational potential energy of the vertical legs of liquid piston  716 , the heat engine floating piston  704  and the heat pump floating piston  706  and the potential energy stored in the heat engine fluid  710  and the heat pump fluid  714 . The inlet valve  718  is opened and closed at the proper times to apply and remove the force of the heat engine in phase with the natural frequency of the system  10 . 
   A throttling valve may not be required on the inlet valve  718 , which removes any associated losses, because of the nature of the system  10 . An energy balance is achieved between energy put into the system  10  in the form of work provided by the heat engine loop and the energy taken out in the form of work done in the heat pump loop as well as losses. 
   The theoretical efficiency of the heat actuated dual loop heat pump with the thermodynamic conditions of this exemplary embodiment is approximately 15.3% versus a Carnot efficiency of approximately 16.9%. The additional losses are believed to be related to the manner in which the mass that enters and exits the cycle compared to a Carnot cycle, which uses heat flow into and out of the system. 
   This efficiency calculation is only for the heat actuated dual loop heat pump and does not include heat losses in the heat pump, solar collection losses, or losses in the steam turbine  600 . The steam turbine  600  may run at an efficiency of approximately 83%. The high efficiency of the steam turbine  600  is common because the closed heat pump to steam turbine cycle does not involve any rejection of heat. 
   Analysis of Thermal Losses During Heat Engine and Heat Pump Cycle 
   Condensation of the heat engine fluid  710  onto the heat engine expansion chamber wall  709  of the heat engine expansion chamber  708  may cause a decrease in the efficiency of the heat engine  790 . Condensation of the heat pump fluid  714  onto the heat pump expansion chamber wall  713  of the heat pump expansion chamber  712  may cause a decrease in the efficiency of the heat pump  792 . Boiling of the liquid piston  716  from the heat pump expansion chamber wall  713  of the heat pump expansion chamber  712  into the heat pump fluid  714  may reduce the quality of the heat pump fluid  714 . Boiling of the liquid piston  716  into the heat engine expansion chamber  708  during the compression stage typically has a lower impact, because the heat engine fluid  710  is saturated, and boiling occurs during this process as a normal part of the cycle. 
   There may also be heat transfer losses through the heat engine expansion chamber wall  709  and the heat pump expansion chamber wall  713 . However, as long as condensation or boiling doesn&#39;t occur, these losses are typically not significant. Boiling typically does not occur above the top of the liquid piston at the upper end of stroke, because there is no liquid present to boil. Condensation above this point 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 . By using an adequate amount of insulation behind the wall and below the top of the piston, heat transfer losses may be lowered. In an exemplary embodiment, described herein, the wall would be maintained at a temperature of at least approximately 373.degree. K. 
   Losses where the liquid piston intermittently contacts the heat engine expansion chamber wall  709  and the heat pump expansion chamber wall  713  during the oscillating stroke are additional potential losses in the heat actuated dual loop liquid piston heat pump system. There are several methods to reduce the losses, including pumping liquid into and out of the system  10  and various methods involving insulation and low thermal mass. One method is described in more detail in the following paragraphs. 
   The losses in the system  10  may be lowered by eliminating or reducing condensation and boiling from the heat engine expansion chamber wall  709  and the heat pump expansion chamber wall  713  during the cycle. The discussion will initially refer to the heat engine expansion chamber wall  709 , with differences that apply only to the heat pump expansion chamber wall  713  being discussed later. At any point in the cycle, vapor in the heat engine expansion chamber  708  may not condense onto a surface if the temperature of the surface is above the saturation temperature. At any point in the cycle, the liquid in the liquid piston  716  will not boil if the temperature of the liquid and the adjacent section of heat engine expansion chamber wall  709  are below the saturation temperature. Therefore, this method may reduce losses by maintaining the heat engine expansion chamber wall  709  of the heat engine expansion chamber  708  at an approximate temperature gradient as shown in  FIG. 10 . The wall temperature at the level of the top of liquid piston  716  and the bottom of the heat engine expansion chamber  708  may be maintained at the saturation temperature of the heat engine fluid  710  at the same point in the cycle. This temperature gradient can be maintained by an external heating device along the length of the wall or by designing the wall in a manner which naturally maintains the gradient. A complication may arise because this temperature is not the same for the compression and expansion strokes, as is discussed several paragraphs below. 
   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 piston  716  which is located between the heat engine expansion chamber  708  and the heat engine floating piston  704  and the outer wall of heat engine floating piston  704 . It may be advantageous to reduce the mass of the liquid piston  716  and the heat engine floating piston  704  in the described area. This can be accomplished in any suitable manner, including, for example, by manufacturing the heat engine expansion chamber  708  and the heat engine floating piston  704  to dimensions and tolerances which provide a small gap between the heat engine expansion chamber  708  and the heat engine floating piston  704  and provide a thin wall on the heat engine floating piston  704 . A gap of around 2 mm is used for this example. The wall of the heat engine floating piston  704  can be manufactured as shown in  FIG. 7B  with a wall thickness of around approximately 2 mm. 
   When the liquid piston  716  is at the top stroke, the liquid at the top of the liquid piston  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 piston  716 . As the liquid piston  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 piston  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 piston  716  and slightly raise the temperature of the element of the heat engine expansion chamber wall  709 . This process may continue as the liquid piston  716  continues to drop, until the same element of the liquid piston  716  is completely cooled by the time that it reaches the bottom of the stroke. 
   The process is reversed on the upward stroke of the liquid piston  716 . As the element of the liquid piston  716  begins to rise, it will be adjacent to a warmer element of the heat engine expansion chamber wall  709 . Heat will typically flow from the adjacent element of the heat engine expansion chamber wall  709  into the element of the liquid piston  716 , raising the temperature of the liquid piston  716 . This will continue as the liquid piston  716  rises, with the result that the element of the liquid piston  716  will be nearly at the maximum temperature of the heat engine expansion chamber wall  709  when it reaches the top stroke. 
   This process may substantially increase the efficiency of the system. If heat was added to the element of the liquid piston  716  at the top of stroke and rejected at the bottom of stroke, approximately an additional 5% heat would need to be added to the system during each stroke, even with a gap between the heat engine floating piston  704  and the heat engine expansion chamber wall  709  that was one tenth the of the size described herein. 
   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 piston  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 piston  716  and the heat engine expansion chamber wall  709  during the cycle. 
   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 piston  716  to the heat engine expansion chamber wall  709  during each cycle. 
   In order to accomplish work with the heat engine, the saturation temperature and pressure is typically lower for a given volume during the compression stroke when compared to the expansion stroke. If the height of the top of the liquid piston  716  in the gap between the heat engine floating piston  704  and the heat engine expansion chamber wall  709  relative to the top surface of the heat engine expansion chamber wall  709  is constant throughout the entire cycle, boiling will occur in the compression stroke or condensation will occur in the expansion stroke on the heat engine expansion chamber wall  709 . This has the effect of lowering the efficiency of the system. 
   As a result, the system  10  may be, alternatively, operated at a higher efficiency if the height of the top of liquid piston  716  is varied relative to the height of the heat engine floating piston  704  during the cycle. One method of accomplishing this is shown in  FIG. 11 . During the compression stroke, the partial sealing device  738  is moved towards the heat engine floating piston  704  by the actuator  736 . This slows down the flow of water into the gap between the heat engine floating piston  704  and the heat engine expansion chamber wall  709 , lowering the top surface of the liquid piston  716 . As the heat engine floating piston  704  nears the top of stroke, the partial sealing device  738  is moved away from the heat engine floating piston  704  by the actuator  736 , allowing liquid to rise in the gap between the heat engine floating piston  704  and the heat engine expansion chamber wall  709 , relative to the top surface of the heat engine floating piston  704 . 
   The height of the liquid between the heat engine floating piston  704  and the heat engine expansion chamber wall  709  can be sensed using a variety of sensors, for example, a pressure transducer. The height of the liquid can then be controlled by providing the necessary gap (g) between the partial sealing device  738  and the heat engine floating piston  704  at each point in the cycle. This allows the top of the liquid to be at the correct point on the heat engine expansion chamber wall  709  to maintain the temperature of the top of the liquid at the saturation temperature for both the expansion and the compression stroke. 
   The situation is similar, but reversed, on the heat pump side of the system. Again, a thermal gradient is maintained on the heat pump expansion chamber wall  713 , corresponding to the saturation temperature for a corresponding volume of the heat pump expansion chamber  712 . In this case, the top of liquid piston  716  relative to the top of heat pump floating piston  706  is maintained at a higher level during the compression stroke and a lower level during the expansion stroke. 
   In addition, the heat pump  792  uses superheated steam rather than saturated steam, so the temperature of the heat pump fluid  714  is above the saturation temperature. However, as long as unwanted condensation and evaporation are avoided, the heat transfer coefficients are low enough that the heat losses are minimal. 
   It will be appreciated by persons of ordinary skill in the art that there are various methods to minimize heat losses of the apparatus disclosed herein, while still utilizing the concept of a heat actuated dual loop liquid piston heat pump. 
   It should be noted that the desired temperature gradient on the heat pump expansion chamber wall  713  may be different than that on the heat engine expansion chamber wall  709 . 
   An alternate embodiment of the concentrator  700  utilizing different operating parameters for the heat engine and heat pump loops is described below. In this example, the heat pump loop operates above atmospheric pressure during part of the cycle. 
   The concentrator  700  in this embodiment includes a heat actuated liquid piston heat pump. As shown in  FIG. 22 , it is constructed in the general form of a U, square, or other suitable shape. The U tube includes a concentrator wall  802 , which forms an internal U shaped chamber. The concentrator wall  802  is constructed with substantially the same diameter along the length of the tube. In this example, the concentrator wall  802  is constructed of aluminum and is approximately 3 mm thick. The inside diameter of the U tube is approximately 1.5 m. The vertical legs are approximately 3 m long and the horizontal leg is approximately 1 m long. The height of the vertical heat engine leg is 1.5 m higher than the vertical heat pump leg. 
   The lower portion of the internal cavity is filled with water in the liquid form, which includes the liquid piston  716 . In this embodiment, the liquid piston  716  has a volume of approximately 9 cubic meters and a mass of approximately 9,000 kg. The heat engine floating piston  804  floats on the top of the liquid piston  716  in one vertical leg, forming the heat engine expansion chamber  708  between the heat engine floating piston  804  and the concentrator wall  802 . The heat pump floating piston  706  floats on top of the liquid piston  716  in the other vertical leg, forming the heat pump expansion chamber  712  between the heat pump floating piston  706  and the concentrator wall  802 . The heat engine expansion chamber  708  is filled with the heat engine fluid  710 . The heat pump expansion chamber  712  is filled with the heat pump fluid  714 . 
   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 . 
   As shown in  FIG. 14 , 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 approximately 1 meter tall. 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 piston  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 . 
   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 piston  716  is sprayed into the condensation chamber  812  to cause condensation of the heat engine fluid  710 . Heat is removed from the liquid piston  716  either by using a conventional heat exchanger or by circulating fluid through the liquid piston  716  and cooling the fluid, for example, at night using cooling device  300 . 
     FIG. 9  shows the components that are used in the heat pump cycle. The heat pump expansion chamber  712  is connected to the pressure chamber  550  with the piping system  750  that contains the heat pump pressure valve  752 . The pressure chamber  550  is connected to the inlet of the fluid to electric converter  600 , which in this example is a 650 kW steam turbine connected to a 650 kW generator. The outlet of fluid to electric converter  600  is connected to a vacuum chamber  560 . The vacuum chamber  560  is connected back to heat pump expansion chamber  712  through a piping system that contains a heat pump vacuum valve  754 . 
   The present system  10  discloses a unique combination of a dual loop heat actuated liquid piston heat pump, where the heat can be supplied by a natural source such as solar energy, and where the hot vapor, typically steam, output from the heat pump loop is fed into a steam turbine-generator combination and the lower pressure vapor from the turbine exhaust is fed back into the heat pump. 
   The present system  10  also discloses a unique natural heat source heat actuated liquid piston heat pump where both the heat engine and the heat pump operate at near atmospheric pressures or below, allowing the apparatus to be constructed below grade or underground using low cost materials, such as concrete which have a high compressive strength, but much lower tensile strength. 
   The present system also discloses a unique thermodynamic cycle for the heat actuated liquid piston heat pump. The unique cycle, which pertains to the heat engine end of the apparatus, uses a combination of steam and liquid water, and cools both the steam and liquid water during the expansion phase of the cycle. 
   The thermal and pressure concentration is done at the time of use, not at the time of collection. As a result, the hot thermal storage is at atmospheric pressure or below and the temperature of the thermal storage is much lower than conventional solar concentration systems. This also allows the thermal storage chambers to be constructed using low cost, high compressive strength materials such as concrete. It also allows the use of water as the thermal storage medium. 
   In one embodiment, the only liquid in the system is water, which is non-hazardous and non-polluting. The solid materials used are also non-hazardous and non-polluting. 
   Because the concentration is done at the time of use rather than the time of collection, the solar energy can be collected using low cost, low temperature flat plate collectors. These collectors can be manufactured from a combination of low cost plastics, concrete, and standard insulation, all of which can be easily manufactured in large volumes at relatively low cost. 
     FIG. 28  is a block diagram of an example computer system  3800  capable of implementing the apparatus and methods disclosed herein. The computer system  3800  can be, for example, a server, a personal computer, a personal digital assistant (PDA), or any other type of computing device. 
   The computer system  3800  of the instant example includes a processor  3810 . For example, the processor  3810  can be implemented by one or more Intel.®. microprocessors from the Pentium.®. family, the Itanium.®. family, the XScale.®. family, or the Centrino.®. family. Of course, other processors from other families are also appropriate. 
   The processor  3810  is in communication with a main memory including a volatile memory  3812  and a non-volatile memory  3814  via a bus  3816 . The volatile memory  3812  may be implemented by Synchronous Dynamic Random Access Memory (SDRAM), Dynamic Random Access Memory (DRAM), RAMBUS Dynamic Random Access Memory (RDRAM) and/or any other type of random access memory device. The non-volatile memory  3814  may be implemented by flash memory and/or any other desired type of memory device. Access to the main memory  3812 ,  3814  is typically controlled by a memory controller (not shown) in a conventional manner. 
   The computer system  3800  also includes a conventional interface circuit  3818 . The interface circuit  3818  may be implemented by any type of well known interface standard, such as an Ethernet interface, a universal serial bus (USB), and/or a third generation input/output (3GIO) interface. 
   One or more input devices  3820  are connected to the interface circuit  3818 . The input device(s)  3820  permit a user to enter data and commands into the processor  3810 . The input device(s) can be implemented by, for example, a keyboard, a mouse, a touch screen, a track-pad, a trackball, isopoint and/or a voice recognition system. 
   One or more output devices  3822  are also connected to the interface circuit  3818 . The output devices  3822  can be implemented, for example, by display devices (e.g., a liquid crystal display, a cathode ray tube display (CRT), a printer and/or speakers). The interface circuit  3818 , thus, typically includes a graphics driver card. 
   The interface circuit  3818  also includes a communication device such as a modem or network interface card to facilitate exchange of data with external computers via a network  3824  (e.g., an Ethernet connection, a digital subscriber line (DSL), a telephone line, coaxial cable, a cellular telephone system, etc.). 
   The computer system  3800  also includes one or more mass storage devices  3826  for storing software and data. Examples of such mass storage devices  3826  include floppy disk drives, hard drive disks, compact disk drives and digital versatile disk (DVD) drives. 
   As an alternative to implementing the methods and/or apparatus described herein in a system such as the device of  FIG. 28 , the methods and/or apparatus described herein may alternatively be embedded in a structure such as processor and/or an ASIC (application specific integrated circuit). 
   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.