T ^{he present disclosure is directed, in general, to electric utility systems, and more specifically, to an apparatus and method for producing and storing electricity. BACKGROUND}

A ^{s an example of hourly fluctuations in electricity prices, FIGURE 1 shows the price of electricity at the National Grid PLC utility in the State of New York. (Source:} _{https://www.nationalgridus.com/niagaramohawk/business/r ates/5_hour_charge.asp) The price fluctuates with supply and demand—sometimes wildly. Value may be created by storing inexpensive electricity produced in the early morning and releasing it in the afternoon when the price peaks.}

F _{urthermore, the modern electrical grid includes increasing amounts of renewable energy – often made from solar and wind– which are available intermittently and unpredictably. The ability to store electricity helps stabilize the grid as these energy sources come on and go off line.}

^SUMMARY

A ^{ccording to a first embodiment of the present disclosure, a configurable system for storing and producing electricity includes a heat source, a heat sink, a first heat exchanger, a first compressor/expander, a thermal storage medium, and a second heat exchanger. In a first configuration, where the system stores electricity as thermal energy, the system is configured to pump refrigerant through the first heat exchanger to transfer heat from the heat source to the refrigerant; compress the heated refrigerant in the first compressor; and, in the second heat exchanger, transfer heat from the compressed heated refrigerant to the thermal storage medium. In a second configuration, where the system generates electricity from thermal energy, the system is configured to pump refrigerant through the second heat exchanger to transfer heat from the thermal storage medium to the refrigerant; expand the heated refrigerant in the first expander, thereby generating electricity in a generator; and, in the first heat exchanger, transfer heat from the expanded heated refrigerant to the heat sink.}

A ^{ccording to a second embodiment of the present disclosure, an apparatus for use in a system for storing and producing electricity includes a first heat exchanger, a first compressor/expander, and a second heat exchanger. In a first configuration, where the apparatus stores electricity as thermal energy, the apparatus is configured to pump refrigerant through the} _{first heat exchanger to transfer heat from a heat source to the refrigerant; compress the heated refrigerant in the first compressor expander; and, in the second heat exchanger, transfer heat from the compressed heated refrigerant to a thermal storage medium. In a second configuration, where the apparatus generates electricity from thermal energy, the apparatus is configured to pump refrigerant through the second heat exchanger to transfer heat from the thermal storage medium to the refrigerant; expand the heated refrigerant in the first expander, thereby generating electricity in a generator; and, in the first heat exchanger, transfer heat from the expanded heated refrigerant to the heat sink.}

A _{ccording to a third embodiment of the present disclosure, a method is provided for storing and producing electricity in an apparatus that includes a first heat exchanger, a first compressor/expander, and a second heat exchanger. The method includes, during a first period of time, storing electricity as thermal energy by pumping refrigerant through the first heat exchanger to transfer heat from a heat source to the refrigerant; compressing the heated refrigerant in the first compressor; and, in the second heat exchanger, transferring heat from the compressed heated refrigerant to a thermal storage medium. The method further includes, during a second period of time, generating electricity from thermal energy by pumping refrigerant through the second heat exchanger to transfer heat from the thermal storage medium} ^{to the refrigerant; expanding the heated refrigerant in the first expander, thereby generating electricity in a generator; and, in the first heat exchanger, transferring heat from the expanded heated refrigerant to the heat sink.}

A ^{ccording to a fourth embodiment of the present disclosure, a method is provided for storing and producing electricity in an apparatus that includes a first heat exchanger, a first compressor/expander, a second heat exchanger, a second compressor/expander, and a third heat exchanger. The method includes, during a first period of time, storing electricity as thermal energy by pumping refrigerant through the first heat exchanger to transfer heat from a heat source to the refrigerant, compressing the heated refrigerant in the first compressor, in the second heat exchanger, transferring heat from the compressed heated refrigerant to a thermal storage medium, expanding refrigerant from the second heat exchanger in the second expander, and transferring expanded refrigerant from the second expander to the first heat exchanger. The method further includes, during a second period of time, generating electricity by pumping} _{refrigerant through the second heat exchanger to transfer heat from the thermal storage medium to the refrigerant; expanding the heated refrigerant in the first expander, thereby generating electricity in a generator; in the first heat exchanger, transferring heat from the expanded heated refrigerant to the heat sink; pumping refrigerant from the first heat exchanger through the third heat exchanger to transfer heat from the second heat source to the refrigerant; and transferring heated refrigerant from the third heat exchanger to the second heat exchanger.}

B _{efore undertaking the DETAILED DESCRIPTION below, it may be advantageous to set forth definitions of certain words and phrases used throughout this patent document: the terms “include” and “comprise,” as well as derivatives thereof, mean inclusion without limitation; the term “or,” is inclusive, meaning and/or; the phrases “associated with” and “associated therewith,” as well as derivatives thereof, may mean to include, be included within, interconnect with, contain, be contained within, connect to or with, couple to or with, be communicable with, cooperate with, interleave, juxtapose, be proximate to, be bound to or with, have, have a property of, or the like.} ^{BRIEF DESCRIPTION OF THE DRAWINGS}

F ^{or a more complete understanding of the present disclosure and its advantages, reference is now made to the following description taken in conjunction with the accompanying drawings, in which like reference numerals represent like parts:}

F ^{IGURE 1 is a graph of recorded prices of electricity over the course of two days;}

F ^{IGURE 2 shows a system for storing and producing electricity;}

F ^{IGURES 3 and 4 show ratios of heat engine work output to heat pump work input; FIGURES 5A and 5B show graphs of temperature and entropy for Carnot cycle devices operating as a heat engine and heat pump, respectively;}

F ^{IGURE 6 shows a first heat pump according to the disclosure;}

F ^{IGURE 7 shows a second heat pump according to the disclosure;}

F ^{IGURE 8 shows a first heat engine according to the disclosure;}

F ^{IGURE 9 shows a third heat pump according to the disclosure;}

F ^{IGURE 10 shows a fourth heat pump according to the disclosure;}

F ^{IGURE 11 shows a second heat engine according to the disclosure;}

F ^{IGURE 12 shows a fifth heat pump according to the disclosure;}

F ^{IGURE 13 shows a third heat engine according to the disclosure;}

F ^{IGURE 14 shows a fourth heat engine according to the disclosure;}

F _{IGURE 15 shows a compressor model; and}

F _{IGURE 16 shows an expander model. DETAILED DESCRIPTION}

T _{here are numerous approaches to storing grid electricity, including conventional rechargeable batteries, flow batteries, pumped water storage, and compressed-air storage. Another option is heat pump storage. When electricity prices are low, heat from a low- temperature body is pumped to a high-temperature body. Conversely, when electricity prices are high, heat is released from the high-temperature body to a heat engine that producing electrical power while rejecting waste heat to a low-temperature body.}

T _{he use of heat pump storage was recognized by Cahn et al. in US Patent 4,110,987 (Sep. 5, 1978). The inventors described a system that employs a high-temperature body of low- vapor pressure hydrocarbon stored at elevated temperatures (100 to 315oC). The inventors show that when heat is pumped from a body above 80oC, the engine produces more electrical power than was invested in the heat pump by a factor of up to 1.365. In essence, the system is over 100% efficient, which on the surface appears to violate the First Law of Thermodynamics, when} ^{in fact, the system does not violate the First Law. A modest amount of electricity is needed to pump heat when the temperatures of the two bodies are close together. In contrast, a much larger amount of electricity is produced when the heat engine rejects heat to the ambient environment.}

I ^{n a conceptual manner, Figure 2 illustrates a system including elements described by Cahn et al. There are three bodies at cold (Tc), medium (Tm), and hot (Th) temperatures. By investing work Whp, Heat Pump 1 transfers thermal energy from the medium-temperature body to the high-temperature body. When Heat Engine 1 transfers thermal energy from the high- temperature body to the low-temperature body, work Weng is produced, which can be used to} m _{ake electricity. Because there is a temperature difference between the bodies at medium and low temperatures, there is also the opportunity to produce work from Heat Engine 2.}

A _{system with one heat pump and two heat engines can adopt the following strategy: i) when electricity prices are low, run Heat Pump 1 ; ii) when electricity prices are moderate, run Heat Engine 2; and iii) when electricity prices are high, run Heat Engine 1. Other strategies also are available.}

G _{reater economic value may be realized when the same piece of hardware is versatile enough to function in all three modes (Heat Pump 1 , Heat Engine 1 , Heat Engine 2), which is an objective of this invention.}

H _{eat Engine 1 can be modeled as a Carnot heat engine, the most efficient heat engine allowed by nature, as shown in Equation 1 :}

w ^{here W is work, Q is heat, η is efficiency, and T is temperature.}

T ^{he efficiency ηeng accounts for inherent irreversibilities that prevent a real heat engine} f _{rom achieving the full work output calculated by the Carnot equation.}

H _{eat Pump 1 can be modeled as a Carnot heat pump, the most efficient heat pump allowed by nature, as shown in Equation 2:}

^{The efficiency ηhp accounts for inherent irreversibilities that prevent a real heat pump from achieving the full heat output calculated by the Carnot equation.}

T _{he ratio of work output from Heat Engine 1 compared to the work input to Heat Engine 2 is calculated as shown in Equation 3:}

F ^{IGURES 3 and 4 show the ratio Weng/ Whp for various temperature combinations assuming the efficiency of Heat Pump 1 and Heat Engine 1 are each 85% of Carnot.}

G ^{enerally, the Carnot heat engine and heat pump are viewed as theoretical constructs that cannot be built as practical hardware. In fact, practical Carnot cycle devices can be built that operate within the phase envelopes shown in FIGURES 5A and 5B. FIGURES 5A and 5B show graphs of temperature and entropy for Carnot cycle devices operating as a heat engine and heat pump, respectively. FIGURE 5A shows a Carnot cycle device operating as a heat engine and Figure 5B shows a Carnot cycle device operating as a heat pump. The shaded area represents work associated with each machine if it were reversible. This figure clearly shows that Heat Engine 1 can produce more work than is required by Heat Pump 2. A challenge to achieving the Carnot cycle shown in FIGURES 5A and 5B is that compressors and expanders must be able to handle two-phase flow. Examples of such machines are gerotor compressors and expanders, and screw compressors and expanders. It is also important to regulate the ratio of liquid to vapor} p _{roperly. In the case of the engine, the compressor is fed with a large quantity of liquid and a small quantity of vapor. If there is excess liquid, the compressor can be damaged because liquid is incompressible.}

O _{ption 1– Batch System}

F _{IGURE 6 shows a schematic view of a first heat pump 600 according to the disclosure. The heat pump 600 operates as a batch system for storing electricity. The heat pump 600 is operating in a“charging” portion of the cycle where electrical energy is converted to thermal energy. Heat pump 600 includes a heat source at near-ambient temperature and a thermal storage medium that comprises a large tank of hot fluid.}

A _{s shown in FIGURE 6, liquid refrigerant is pumped from a low-temperature storage tank 610 into a low-temperature heat exchanger 620. Liquid water 605 is pumped through a packed column that contacts ambient air. Alternatively, liquid water could be sourced from a well, so it would be at ground temperature. The water provides the heat of evaporation needed} ^{to convert the liquid refrigerant into vapor. Additionally, a small stream 615 of liquid refrigerant is sent to the low-temperature heat exchanger 620 to provide sensible heat so that its temperature is nearly identical to the vapor. Rather than employing water from a packed tower or well, it is also possible to use a finned heat exchanger that uses ambient air as the heat transfer fluid rather than liquid water.}

I ^{n all systems according to the disclosure, to achieve better performance and lower cost, it is preferable to have good heat transfer coefficients in the heat exchangers. In some embodiments, this goal may be achieved by coating the heat exchanger surfaces with a hydrophobic coating, such as nickel Teflon.}

A ^{compressor system 623 is powered by an electric motor 625, which compresses the low-pressure refrigerant to an elevated pressure. In a preferred embodiment, the compressor system 623 has multiple stages. In the heat pump shown in FIGURE 6, system valves are operated to configure the compression stages of the compressor system 623 to operate in series, which is preferable because of the large pressure ratio. During the compression, atomized liquid refrigerant may be added to the compressor to remove superheat, which makes the compression more efficient. Preferably, the vapors exiting the compressor are saturated. However, the residence time in the compressor is short, so the vapors are likely to exit with some remaining} _{superheat. To remove the superheat, the high-pressure vapors are sent to a liquid contactor 630, illustrated as a packed tower with liquid refrigerant flowing downward and the superheated vapors flowing upward. Alternatively, the superheated vapor could be contacted with a fine mist of atomized refrigerant.}

T _{he high-pressure vapors enter a high-temperature heat exchanger 635 where they condense against fluid that circulates through a large thermal storage tank 640. The resulting high-pressure liquid refrigerant is stored in a high-temperature storage tank 645.}

T _{o ensure there is enough high-temperature liquid in the system, a pump can transfer refrigerant from the low-temperature side to the high-temperature side. The pumped liquid can be added to the contactor 630 and/or the high-temperature storage tank 645.}

F _{IGURE 7 shows a second heat pump 700 according to the disclosure. The heat pump 700 is also operating in the“charging” portion of the cycle; however, in the heat pump 700, a medium-temperature heat source 707 is used to evaporate and warm the low-temperature liquid refrigerant. Examples of medium-temperature heat sources include, but are not limited to: low- grade waste steam from oil refineries or chemical plants, waste heat from power plants, waste hot water from food processing operations, solar collectors, and solar ponds. The thermal storage medium is identical to the previous configuration, a large tank 740 of hot fluid. In the} ^{heat pump 700, system valves are operated to configure the compressor system 723 to operate the compressor stages in parallel. The parallel compressor stages configuration takes advantage of the low pressure ratio and provides extra volumetric capacity.}

F ^{IGURE 8 shows a first heat engine 800 according to the disclosure. The heat engine 800 is operating in a discharge cycle, which operates in reverse of the heat pumps 600 and 700 shown in FIGURES 6 and 7, respectively. A high-temperature refrigerant is pumped from a high-temperature storage tank into a high-temperature heat exchanger where it vaporizes.}

R ^{efrigerant from storage tank 845 is pumped through high-temperature heat exchanger 835, where it is heated by hot fluid from storage tank 840. The resulting high-temperature vapor refrigerant flows to an expander system 823, which is coupled to a generator 825. Preferably, the expander system 823 is the same compressor system used in the charging cycle and the generator 825 is the same motor used in the charging cycle. Because of the large pressure ratio, system valves are operated to configure the expander system 823 to operate its expander stages in series. The discharge from the expander system 823 is directed to a low-temperature heat exchanger 820 where the refrigerant condenses against a heat sink, such as cooling water 805 that circulates through a cooling tower or water produced from a well. In other embodiments, the} _{refrigerant condenses in a heat sink comprising a finned heat exchanger that rejects heat to ambient air. In still other embodiments, another suitable heat sink is used to condense the refrigerant. The condensate exiting the low-temperature heat exchanger 820 is stored in a low- temperature storage tank 810.}

N _{ote that FIGURES 6, 7, and 8 show the same system—i.e., the same piece of hardware—reconfigured by operation of system valves, to operate as a heat pump charging from a low-temperature source (FIGURE 6), a heat pump charging from a medium-temperature source (FIGURE 7), and a heat engine (FIGURE 8). The compressor/expander system (623/723/823) operates as a compressor powered by an electric motor when the system is operating as a heat pump and as an expander powering an electric generator when the system is operating as a heat engine. The compressor/expander system (623/723/823) is adapted to handle two-phase refrigerant, that is, refrigerant in both the liquid and vapor phases. Examples of such machines are gerotor compressors and expanders, and screw compressors and expanders. The same electrical device (625/725/825) operates as a motor when the system is operating as a heat pump and as a generator when the system is operating as a heat engine. A single heat source/sink may be used (e.g., 605/805, FIGURES 6 and 8) or separate heat source and heat sink (e.g., 707, FIGURE 7, and 805, FIGURE 8).} ^{Examples of refrigerants that may be used in systems according to the disclosure including, but are not limited to standard refrigerants used in air conditioning and refrigeration systems. For the purposes of analysis below, ammonia will be used as the refrigerant because it has a high latent heat of vaporization and it is inexpensive.}

T ^{he fluid circulating through the large storage tank 640, 740, or 840 could be nearly any liquid, including, but not limited to: water, salt water, aqueous solutions of glycol, alcohol, and} h _{ydrocarbon. Preferably, the fluid is water at a temperature of 100} ^o _{C, or less. For the purposes of analysis below, water will be used as the fluid. If storage volume is an issue, the storage tank can incorporate a substance that undergoes a phase change at the temperature of interest. For example, catechol melts at 105oC. The phase-change material could be contained in hollow spheres located in the large thermal storage tank.}

C _{ompressor Analysis}

F _{IGURE 15 shows a compressor model 1500. The fluid exiting the compressor 1500 is assumed to be saturated and the compression is isentropic, therefore:}

w ^{here M is mass, T is temperature, P is pressure, H is heat, and S is entropy.}

C _{ompression work is given by:}

E ^{xpander Analysis}

F _{IGURE 16 shows an expander model 1600. The expander operation is given by the equation:}

C _{ycle Recovery}

T ^{his equation produces results that are similar to those presented in FIGURES 3 and 4.} A _{mmonia flow rate to produce 1 MW}

_{Ammonia Volume}

A _{mmonia volume to store 1 MWh at 90oC.}

W ^{ater Volume}

W _{ater volume to store 1 MWh with 20} ^o _{C temperature difference.}

O ^{ption 2– Continuous System}

F ^{IGURES 9, 10, and 11 show third and fourth heat pumps and a second heat engine, respectively, according to the disclosure, that are Carnot cycle devices. Compared to Option 1 , the primary advantage of Option 2 is that a large inventory of refrigerant is not required. This benefit comes at the price of additional hardware that is required to recycle the refrigerant.}

F _{IGURE 9 shows a schematic view of a third heat pump 900 according to the disclosure. The heat pump 900 operates using a substantially closed refrigerant system and, therefore, does not require the large storage tanks 610 and 645 of the heat pump 600 described with reference to FIGURE 6. The heat pump 900 is operating in a“charging” portion of the cycle where electrical} ^{energy is converted to thermal energy. Heat pump 900 includes a heat source at near-ambient temperature and a heat sink comprises a large tank of hot fluid.}

A ^{s shown in FIGURE 9, liquid refrigerant 903 flows from an expander system 955 (to be discussed below) into a low-temperature heat exchanger 920. Liquid water 905 is pumped through a packed column that contacts ambient air. In other embodiments, liquid water could be sourced from a well, so it would be at ground temperature. The water provides the heat of evaporation needed to convert the liquid refrigerant into vapor. Additionally, a small stream 915 of liquid refrigerant is sent to the low-temperature heat exchanger 920 to provide sensible heat so that its temperature is nearly identical to the vapor. Rather than employing water from a packed tower or well, it is also possible to use a finned heat exchanger that uses ambient air as the heat transfer fluid rather than liquid water.}

A ^{compressor system 923 is powered by an electric motor 925, which compresses the low-pressure refrigerant to an elevated pressure. In a preferred embodiment, the compressor system 923 has multiple stages. In the heat pump shown in FIGURE 9, system valves are operated to configure the compression stages of the compressor system 923 to operate in series, which is preferable because of the large pressure ratio. During the compression, atomized liquid} _{refrigerant may be added to the compressor to remove superheat, which makes the compression more efficient. Preferably, the vapors exiting the compressor are saturated. However, the residence time in the compressor is short, so the vapors are likely to exit with some remaining superheat. To remove the superheat, the high-pressure vapors are sent to a liquid contactor 930, illustrated as a packed tower with liquid refrigerant flowing downward and the superheated vapors flowing upward. Alternatively, the superheated vapor could be contacted with a fine mist of atomized refrigerant.}

T _{he high-pressure vapors enter a high-temperature heat exchanger 935 where they condense against fluid that circulates through a large thermal storage tank 940. The resulting high-pressure liquid refrigerant is received by an expander system 955. In a preferred embodiment, the expander system 955 has multiple stages. In the heat pump shown in FIGURE 9, system valves are operated to configure the expansion stages of the expander system 955 to operate in series. As described above, liquid refrigerant 903 from the expander section 955 flows into the low-temperature heat exchanger 920. Work produced by the expander system 955 is used to operate a generator 950. In other embodiments, the shafts of the compressor and expander are coupled mechanically, which allows the expander to help drive the compressor and thereby reduces conversion losses in generator 950.} ^{FIGURE 10 shows a fourth heat pump 1000 according to the disclosure. The heat pump 1000 is also operating in the“charging” portion of the cycle; however, in the heat pump 1000, a medium-temperature heat source 1007 is used to evaporate and warm the low-temperature liquid refrigerant. The thermal storage medium is identical to the system described with reference to FIGURE 9, a large tank 1040 of hot fluid. In the heat pump 1000, system valves are operated to configure the compressor system 1023 to operate the compressor stages in parallel. The parallel compressor stages configuration takes advantage of the low pressure ratio and provides extra volumetric capacity.}

F ^{IGURE 11 shows a second heat engine 1100 according to the disclosure. A high- temperature refrigerant is pumped from a high-temperature storage tank into a high-temperature heat exchanger where it vaporizes.}

R ^{efrigerant from compressor system 1155 (discussed below) passes through heat exchanger 1135, where it is heated by hot fluid from storage tank 1140. The resulting high- temperature vapor refrigerant flows to an expander system 1123, which is coupled to a generator 1125. Preferably, the expander system 1123 is the same compressor system used in the charging cycle and the generator 1125 is the same motor used in the charging cycle. Because of the large pressure ratio, system valves are operated to configure the expander system 1123 to operate its} _{expander stages in series. The discharge from the expander system 1123 is directed to a refrigerant storage tank 1110 and to a low-temperature heat exchanger 1120 where the refrigerant condenses against a heat sink, such as cooling water 1105 that circulates through a cooling tower or water produced from a well. In other embodiments, the refrigerant condenses in a heat sink comprising a finned heat exchanger that rejects heat to ambient air. In still other embodiments, another suitable heat sink is used to condense the refrigerant. The condensate exiting the low-temperature heat exchanger 1120 is mixed with liquid and vapor refrigerant from the storage tank 1110 to form refrigerant mixture 1145 and passed through the compressor system 1155, which is powered by motor 1150. Alternatively, the shafts of the compressor and expander can be coupled mechanically, which allows the expander to help drive the compressor and thereby reduces conversion losses in generator 1125.}

A _{s mentioned earlier, the refrigerant mixture 1145 comprising both vapor and liquid could be problematical if excess liquid is fed to the compressor, because liquid is not compressible.}

N _{ote that FIGURES 9, 10, and 11 show the same system, reconfigured by operation of system valves, to operate as a heat pump charging from a low-temperature source (FIGURE 9), a heat pump charging from a medium-temperature source (FIGURE 10), and a heat engine} ^{(FIGURE 11 ). The compressor/expander systems (923/1023/1123 and 955/1055/1155) operate as compressors powered by electric motors or as expanders powering electric generators, depending upon whether the system is operating as a heat pump or as a heat engine. The compressor/expander systems are adapted to handle two-phase refrigerant, that is, refrigerant in both the liquid and vapor phases. The same electrical devices (925/1025/1125 and 950/1050/1150) operate as motors or generators, depending upon whether the system is operating as a heat pump or as a heat engine. A single heat source/sink may be used (e.g., 905/1105, FIGURES 9 and 11 ) or separate heat source and heat sink (e.g., 1007, FIGURE 10, and 1105, FIGURE 1 ).}

O ^{ption 3– Modified Continuous System}

F ^{IGURES 12, 13, and 14 show a fifth heat pump and third and fourth heat engines, respectively, according to the disclosure. FIGURE 12 shows a heat pump 1200 that is very similar to the heat pump 1000 described with reference to FIGURE 10. Operation of the two heat pumps is identical. Both heat pumps charge from a medium-temperature source. However, expander system 1255 includes a first expander stage that powers a first generator 1253 and a second expander stage that powers a second generator 1250.}

F ^{IGURE 13 shows a third heat engine 1300 according to the disclosure. Much of the} _{heat engine 1300 operates like the heat engine 1100 described with reference to FIGURE 11. Two significant differences may be noted.}

F _{irst, liquid refrigerant from storage tank 1310 is added to the recycling refrigerant system as an input to a second expander stage of expander system 1323. Second, only a second compression stage of compressor system 1355 is used. A first compression stage of the compressor system 1355 is replaced with a liquid pump 1360 and a heat exchanger 1365. This configuration can minimize or prevent problems associated with feeding excess liquid to the compressor system 1355. The final pressurization in the system 1300 occurs using the second compression stage of the compressor system 1355, fed with liquid and vapor refrigerant from heat exchanger 1365 and vapor refrigerant from the storage tank 1310.}

F _{IGURE 14 shows a fourth heat engine 1400 according to the disclosure. To avoid potential problems in system 1300 with feeding excess liquid to the second compression stage of the compressor system 1355, the pump 1460 of the system 1400 accomplishes full pressurization of the refrigerant coming from the heat exchanger 1420 and therefore bypasses altogether the second compression stage of the compressor system 1455. One negative aspect of this approach is that the liquid will not be fully saturated and hence will put an additional sensible load on the high-temperature heat exchanger 1435. The sensible load does not produce vapor, so the impact} ^{is to lower efficiency slightly. Despite this drawback, given the simplicity and ease of control, the system 1400 is still an attractive option.}

O ^{ne further difference between the system 1300 and the system 1400 is that the expansion stages of the expander system 1423 are operated in parallel.}

A ^{gain, FIGURES 12, 13, and 14 show the same system, reconfigured by operation of system valves, to operate as a heat pump charging from a medium-temperature source (FIGURE 12), and as heat engines operating in two different discharge modes (FIGURES 13 and 14). The system includes compressor/expander systems that operate as compressors powered by electric motors or as expanders powering electric generators, depending upon whether the system is operating as a heat pump or as a heat engine. The compressor/expander systems are adapted to handle two-phase refrigerant, that is, refrigerant in both the liquid and vapor phases. The system includes electrical devices that operate as motors or generators, depending upon whether the system is operating as a heat pump or as a heat engine.}

I ^{t should be understood at the outset that, although example embodiments are illustrated above, the present invention may be implemented using any number of techniques, whether} _{currently known or not. The present invention should in no way be limited to the example implementations, drawings, and techniques illustrated below. Additionally, the drawings are not necessarily drawn to scale.}

M _{odifications, additions, or omissions may be made to the systems, apparatuses, and methods described herein without departing from the scope of the invention. The components of the systems and apparatuses may be integrated or separated. Moreover, the operations of the systems and apparatuses may be performed by more, fewer, or other components. The methods may include more, fewer, or other steps. Additionally, steps may be performed in any suitable order. As used in this document, "each" refers to each member of a set or each member of a subset of a set.}

T _{o aid the Patent Office, and any readers of any patent issued on this application in interpreting the claims appended hereto, applicants wish to note that they do not intend any of the appended claims or claim elements to invoke paragraph 6 of 35 U.S.C. Section 112 as it exists on the date of filing hereof unless the words "means for" or "step for" are explicitly used in the particular claim.}