energies

Concept Paper Isobaric Expansion Engines: New Opportunities in Energy Conversion for Heat Engines, Pumps and Compressors

Maxim Glushenkov 1, Alexander Kronberg 1,*, Torben Knoke 2 and Eugeny Y. Kenig 2,3

1 Encontech B.V. ET/TE, P.O. Box 217, 7500 AE Enschede, The Netherlands; [email protected] 2 Chair of Fluid Process Engineering, Paderborn University, Pohlweg 55, 33098 Paderborn, Germany; [email protected] (T.K.); [email protected] (E.Y.K.) 3 Chair of Thermodynamics and Heat Engines, Gubkin Russian State University of Oil and Gas, Leninsky Prospekt 65, Moscow 119991, Russia * Correspondence: [email protected] or [email protected]; Tel.: +31-53-489-1088

Received: 12 December 2017; Accepted: 4 January 2018; Published: 8 January 2018

Abstract: Isobaric expansion (IE) engines are a very uncommon type of heat-to-mechanical-power converters, radically different from all well-known heat engines. Useful work is extracted during an isobaric expansion process, i.e., without a polytropic gas/vapour expansion accompanied by a pressure decrease typical of state-of-the-art piston engines, turbines, etc. This distinctive feature permits isobaric expansion machines to serve as very simple and inexpensive heat-driven pumps and compressors as well as heat-to-shaft-power converters with desired speed/torque. Commercial application of such machines, however, is scarce, mainly due to a low efficiency. This article aims to revive the long-known concept by proposing important modifications to make IE machines competitive and cost-effective alternatives to state-of-the-art heat conversion technologies. Experimental and theoretical results supporting the isobaric expansion technology are presented and promising potential applications, including emerging power generation methods, are discussed. It is shown that dense working fluids with high thermal expansion at high process temperature and low compressibility at low temperature make it possible to operate with reasonable thermal efficiencies at ultra-low heat source temperatures (70–100 ◦C). Regeneration/recuperation of heat can increase the efficiency notably and extend the area of application of these machines to higher heat source temperatures. For heat source temperatures of 200–600 ◦C, the efficiency of these machines can reach 20–50% thus making them a flexible, economical and energy efficient alternative to many today’s power generation technologies, first of all organic Rankine cycle (ORC).

Keywords: isobaric expansion; alternative engine; unconventional engine; low-grade heat; waste heat; hydraulic output; heat-driven pump; regenerator; ORC

1. Introduction Heat-to-electricity conversion machines (engines) operating on Rankine cycle account for the lion’s share of worldwide power generation. The higher the temperature of the heat source, the more efficient, economical and compact is the engine. Vice versa, efficiency drops while cost and footprint rise with decreasing heat source temperature. For temperatures below 300 ◦C, water steam turns to be inefficient as a working fluid, and hence, alternative fluids with lower boiling temperatures must be applied. Such modified cycles are called Organic Rankine Cycles, or ORC [1,2]. When the heat source temperature is below 100 ◦C, the increased cost of ORC cycle installation makes the process uneconomical. A similar trend is valid for power range (i.e., size of the machines); large-scale Rankine cycle plants are the most efficient and economical, whereas mini-scale (less than 50 kW) and

Energies 2018, 11, 154; doi:10.3390/en11010154 www.mdpi.com/journal/energies Energies 2018, 11, 154 2 of 22 micro-scale (less than 1 kW) plants are unavailable on the market because of their high cost and low efficiency [3]. As a result, low-temperature heat and small-scale applications are not covered by the state-of-the-art technologies. The heart and most expensive (up to 69% of capital cost [3,4]) and complex part of a Rankine cycle plant is the expansion machine, e.g., a turbine or a piston-based expander. The expansion machine converts energy of a hot, compressed steam (vapour) into mechanical energy. The development of new types of expansion machines and the improvement of the today’s well-known expanders is one of the main research directions in the Rankine cycle-based and ORC-based power generation [5,6]. An expansion machine, extracting work during polytropic (ideal case: isentropic) gas or vapour expansion is a key element of all state-of-the-art heat-to-power converters (heat engines)—gas or steam turbines, piston engines, etc. However, there exists another, less known, class of heat engines. In this class, there is no polytropic gas/vapour expansion accompanied by notable pressure fall. The Worthington direct-acting steam pump [7] and the Bush compressor [8] represent this type of engines. By convention, they can be called isobaric expansion (IE) machines, although, in some cases, the actual expansion process they perform can substantially deviate from isobaricity. In essence, the main distinguishing feature of these machines is the absence of a polytropic gas/vapour expansion stage in expansion machine, resulting in a simple, low-cost design. In addition to power generation, these engines are able to pump liquids and compress gases using heat directly, without any intermediate stages such as shaft power/electricity generation. In spite of all these advantages, the technical application of these machines is very limited, mainly because of their low efficiency. Several thermo-compressors that employ the principle of isobaric expansion have been constructed and investigated in the 1970s, but their thermal efficiencies were rather disappointing. The compressor of Air Products and Chemicals achieved a thermal efficiency of not more than 5.4% at a hot temperature of over 550 ◦C (9% of Carnot efficiency) [9], compressors with hot temperatures above 625 ◦C built by McDonald Douglas corporation and Aerojet/Nimbus operated with 14.4% (20% of Carnot efficiency) and 18% (28% of Carnot efficiency) respectively [10–12]. In this paper, we propose significant changes to the original concepts of the Worthington steam pump and the Bush thermo-compressor, aiming to increase their efficiency to the levels comparable or even superior to modern heat engines, above of all ORC power plants. These modifications include the application of dense working fluids with high thermal expansion at high process temperature and low compressibility at low temperature instead of gases, new concepts for efficient heat regeneration/recuperation and further technical improvements. We provide experimental results obtained in a lab-scale unit to prove the technical feasibility of the proposed concepts. Moreover, thermodynamic simulations for a number of selected working fluids and operating conditions were carried out in order to display the capability of improved IE machines. Finally, some further technical modifications and potential application areas including emerging power generation methods are discussed.

2. Isobaric Expansion Machines

2.1. Worthington Direct-Acting Steam Pump The most known examples of isobaric expansion machines are so-called Worthington’s or direct-acting steam pumps [7] invented and patented by Henry Worthington around 1841. Remarkably, such types of machines are still being produced and operated as boiler feed pumps, ship emergency pumps, etc., without any significant changes in the original design. The basic principle of a so-called single-acting steam pump is shown in Figure1. The pump consists of steam cylinder 1 and pumping cylinder 2, with steam piston 3 and a pumping piston 4 inside, correspondingly. Energies 2018, 11, 154 3 of 22 Energies 2018, 11, 154 3 of 22

Energies 2018, 11, 154 3 of 22

Figure 1. Scheme of a single-acting Worthington steam pump. 1—steam cylinder, 2—pumping Figure 1. Scheme of a single-acting Worthington steam pump. 1—steam cylinder, 2—pumping cylinder, cylinder, 3—steam piston, 4—pumping piston, 5—connecting rod, 6—steam inlet valve, 7—steam 3—steamoutlet piston, valve,4 —pumping8—liquid inlet piston, valve, 95—liquid—connecting outlet valve. rod, 6—steam inlet valve, 7—steam outlet valve, 8—liquid inlet valve, 9—liquid outlet valve. The pistons are rigidly connected by rod 5 and move together as a unit. The steam cylinder is equipped with steam inlet/admission valve 6 and outlet/exhaust valve 7 shown at the top. The pistons are rigidly connected by rod 5 and move together as a unit. The steam cylinder Accordingly,Figure 1. theScheme pumping of a single-actincylinder hasg Worthington inlet/suction steam valve pump. 8 and 1outlet/discharge—steam cylinder, valve2—pumping 9. The inlet is equippedand outletcylinder, with valves 3 steam—steam of the inlet/admissionpiston, steam 4 —pumpingcylinder are piston, valveforcedly 5—connecting 6 actuated; and outlet/exhaust the rod, valves 6—steam of the inlet valve pumping valve, 7 7 shown —steamcylinder atare the top. Accordingly,self-acting.outlet the valve, pumping 8—liquid cylinder inlet valve, has 9—liquid inlet/suction outlet valve. valve 8 and outlet/discharge valve 9. The inlet and outlet valvesWhen the of inlet the valve steam of the cylinder steam cylinder are forcedly opens, steam actuated; enters thethe cylinder valves and ofthe moves pumping the piston cylinder The pistons are rigidly connected by rod 5 and move together as a unit. The steam cylinder is are self-acting.performing pumping stroke in the pumping cylinder. The valve remains open during the whole equipped with steam inlet/admission valve 6 and outlet/exhaust valve 7 shown at the top. Whenstroke, the i.e., inlet steam valve enters of the the steam cylinder cylinder at constant opens, pressure steam and enters does the not cylinder expand andin the moves cylinder. the piston Accordingly,Therefore, during the pumping the whole cylinder stroke, thehas pressuinlet/suctionre in the valve steam 8 andcylinder outlet/discharge remains constant. valve 9. The inlet performing pumping stroke in the pumping cylinder. The valve remains open during the whole stroke, and outletSuch avalves process of canthe steambe defined cylinder as an are isobaric forcedly expa actuated;nsion. At the the valves end of the pumpingstroke, the cylinder inlet steam are i.e., steamself-acting.valve enters closes theand cylinderthe outlet atvalve constant opens. The pressure steam exhausts and does from not the expand cylinder inand the the cylinder. piston moves Therefore, during theback wholeWhen under the the stroke, inlet pressure valve the of pressureof liquidthe steam entering in cylinder the steamthrough opens, cylinder the steam inlet enters valve remains the cylinderpumping constant. and cylinder. moves Therefore,the piston Suchperformingthe areciprocating process pumping can motion be stroke defined of the in piston the as anpumping unit isobaric is accompanied cylinder. expansion. The by valve pumping At theremains end and ofsuctionopen the duringstroke, of a liquid the the whole in inlet the steam stroke, i.e., steam enters the cylinder at constant pressure and does not expand in the cylinder. valve closespumping and cylinder. the outlet valve opens. The steam exhausts from the cylinder and the piston moves Therefore, during the whole stroke, the pressure in the steam cylinder remains constant. back underThe the pump pressure uses ofa very liquid simple entering valves throughactuation system the inlet without valve a thecut-off pumping mechanism cylinder. and has Therefore,no crankSuch gear a and process no flywheel. can be defined as an isobaric expansion. At the end of the stroke, the inlet steam the reciprocatingvalveDepending closes motionand onthe theoutlet of thediameters valve piston opens. of unit the The steam is accompaniedsteam and exhausts pumping byfrom pistons, pumping the cylinder the discharge and and suction the pressure piston of amoves of liquid the in the pumpingbackliquid cylinder. under to be thepumped pressure can ofbe liquidequal to,entering lower throughor higher the than inlet the valve steam the pressure. pumping The cylinder. pump can Therefore, operate Thetheat pumpvery reciprocating high uses temperatures a motion very simple of the(up piston to valves 400 unit °C), actuation is pump accompanied liquids system byin withoutapumping very broad aand cut-off rangesuction mechanismof of viscosities a liquid in and andthe has no crank gearpumpingcompress and no cylinder.gases. flywheel. The pump uses a very simple valves actuation system without a cut-off mechanism and has no DependingDirect onacting the pumps diameters are very of simple, the steam reliable and and pumping cheap. They pistons, are probably the discharge the oldest machines pressure of the crankon the gear market and [13]. no flywheel. liquid to beDepending pumped canon the be diameters equal to, of lower the steam or higher and pumping than the pistons, steam the pressure. discharge The pressure pump of can the operate A remarkable feature of direct acting◦ pumps is that they pump liquids directly by the use of heat at veryliquidwithout high to temperatures interpositionbe pumped can of (up electricitybe equal to 400 to, and lower C),without pumpor hi agher long liquids than sequence the in steam of a veryenergy pressure. broad transformations The range pump ofcan typical viscosities operate for and compressatconventional very gases. high temperaturesRankine cycle (up instal to 400lations °C), suchpump as liquids conversion in a veryinto broadrotary range motion of viscositiesand electricity and Directcompressgeneration. acting gases. pumps are very simple, reliable and cheap. They are probably the oldest machines Direct acting pumps are very simple, reliable and cheap. They are probably the oldest machines on the marketObviously, [13]. any direct acting pump can also be used as a heat-to-shaft or electrical power on the market [13]. A remarkableconverter. We featurebelieve that of the direct simplest acting way pumps to design is such that a theyprocess pump is to use liquids a hydraulic directly circuit by with the use of a hydraulicA remarkable motor. featureA functional of direct scheme acting of pumps such a ispower that they plant pump with liquidsa single-acting directly pump by the is use shown of heat in heat without interposition of electricity and without a long sequence of energy transformations withoutFigure 2. interposition of electricity and without a long sequence of energy transformations typical for typicalconventional for conventional Rankine Rankine cycle instal cyclelations installations such as conversion such as into conversion rotary motion into and rotary electricity motion and electricitygeneration. generation. Obviously,Obviously, any direct any direct acting acting pump pump can alsocan bealso used be used as a heat-to-shaftas a heat-to-shaft or electricalor electrical power power converter. converter. We believe that the simplest way to design such a process is to use a hydraulic circuit with We believea hydraulic that the motor. simplest A functional way to scheme design of such such aa processpower plant is to with use a asingle-acting hydraulic circuitpump is with shown a hydraulicin motor. AFigure functional 2. scheme of such a power plant with a single-acting pump is shown in Figure2.

Figure 2. Scheme of a Worthington pump-based power plant producing shaft power. 1—feed water pump, 2—heat recuperator, 3—evaporator, 4—condenser, 5—high-pressure hydraulic accumulator, 6—low-pressure hydraulic accumulator, 7—hydraulic motor.

Figure 2. Scheme of a Worthington pump-based power plant producing shaft power. 1—feed water Figure 2. Scheme of a Worthington pump-based power plant producing shaft power. 1—feed water pump, 2—heat recuperator, 3—evaporator, 4—condenser, 5—high-pressure hydraulic accumulator, pump, 26—low-pressure—heat recuperator, hydraulic3—evaporator, accumulator, 7—hydraulic4—condenser, motor.5 —high-pressure hydraulic accumulator, 6—low-pressure hydraulic accumulator, 7—hydraulic motor. Energies 2018, 11, 154 4 of 22

EnergiesWater2018, 11is ,pumped 154 by feed water pump 1 through recuperative heat exchanger (recuperator)4 of 222 and heater (evaporator, boiler) 3 where it is heated and turned to steam. The steam is supplied to the steamWater cylinder is pumped of the direct by feed acti waterng pump pump performing 1 through recuperative the isobaric heatworking exchanger stroke (recuperator) of the piston. 2 andThe heaterexhausted (evaporator, steam cools boiler) down 3 where and turns it is heated back into and liqu turnedid in to recuperator steam. The 2 steam and condenser is supplied 4. to The the energy steam cylinderof the liquid of the discharged direct acting from pump the pumping performing cylinder the isobaric (ideally, working it is a strokehydraulic of the oil) piston. is converted The exhausted to shaft steampower coolsin a hydraulic down and circuit turns consisting back into of liquid two hi ingh- recuperator and low-pressure 2 and condenser hydraulic 4. accumulators The energy of5 and the liquid6 and hydraulic discharged motor from the7 connected pumping to cylinder a load. (ideally,The pressure it is a in hydraulic high-pressure oil) is converted accumulator to shaft 6 is almost power inequal a hydraulic to the maximum circuit consisting delivery pressure of two high- of the and pump low-pressure. The pressure hydraulic in the low-pressure accumulators accumulator 5 and 6 and hydraulic5 must be motor sufficient 7 connected to move to the a load. piston The of pressure the pump in high-pressure back, displacing accumulator the spent 6 is steam almost from equal the to thecylinder. maximum delivery pressure of the pump. The pressure in the low-pressure accumulator 5 must be sufficientSuch toa movepower the plant piston has of much the pump in common back, displacing with conventional the spent steam Rankine from cycle the cylinder. plants (boiler, condenser,Such a feed power water plant pump) has being, much however, in common mu withch simpler conventional and cheaper, Rankine since cycle inexpensive plants (boiler, mass- condenser,market hydraulic feed watermotors pump) are used being, instead however, of turbines. much simpler and cheaper, since inexpensive mass-market hydraulic motors are used instead of turbines. 2.2. Bush Thermocompressor Based Expansion Machines 2.2. BushThe Thermocompressorisobaric expansion Based principle Expansion can be Machines implemented without the feed pump necessary for the WorthingtonThe isobaric pump-based expansion power principle plant can shown be implemented in Figure 2. without Such a pumpless the feed pump converter necessary can be for built the Worthingtonbased on the pump-basedingenious concept power of plant the so-called shown in Bush Figure compressor,2. Such a pumplessor thermo-compressor, converter can invented be built basedby V. onBush the ingeniousin 1939 [8]. concept Later ofon, the various so-called ve Bushrsions compressor, of thermo-compressor or thermo-compressor, were proposed invented and by V.extensively Bush in 1939 investigated [8]. Later on,[14–19]. various The versions main direct of thermo-compressorion of the research were and proposeddevelopment and extensivelyefforts was investigatedusing the thermo-compressor [14–19]. The main as a direction source of of mechanical the research power. and development efforts was using the thermo-compressorA conceptual sketch as a source of a ofthermo-compressor-ba mechanical power. sed power unit is shown in Figure 3. The converterA conceptual consists sketchof cylinder of a thermo-compressor-based 1 filled with gas, with well-sealed power unit ispiston shown 2 inside, in Figure so3 .that The the converter piston consistssubdivides of cylinderthe internal 1 filled volume with of gas, the withcylinder well-sealed into two—hot piston and 2 inside, cold—parts so that shown the piston in rose subdivides and blue thecolours, internal correspondingly. volume of the Both cylinder parts into are two—hotcommunicated and cold—parts through heater shown 3, regenerator in rose and 4 blueand colours,cooler 5 correspondingly.connected in series. Both The parts regenerator are communicated plays here through the same heater role 3,as regenerator the recuperator 4 and of cooler the direct 5 connected acting inpump series. shown The regeneratorin Figure 2. playsThe piston here the can same be set role in reciprocating as the recuperator motion of theby directmeans acting of sealed pump rod shown 6 and inlinear Figure actuator2. The 7. piston can be set in reciprocating motion by means of sealed rod 6 and linear actuator 7.

Figure 3.3. SchemeScheme of of Bush Bush thermo-compressor-based thermo-compressor-based shaft-power shaft-power unit. unit.1—cylinder, 1—cylinder,2—piston, 2—piston,3—heater, 3— 4heater,—regenerator, 4—regenerator,5—cooler, 5—cooler,6—connecting 6—connecting rod, 7—linear rod, 7—linear actuator, actuator,8—diaphragm 8—diaphragm unit, 9 —non-returnunit, 9—non- valves,return valves,10—high-pressure 10—high-pressure accumulator, accumulator,11—low-pressure 11—low-pressure accumulator, accumulator,12—hydraulic 12—hydraulic motor. motor.

The converter is equipped with diaphragm unit 8, the main part of which is a polymerpolymer flexibleflexible diaphragm separating the working fluidfluid and hydraulic oil in the closed-loop hydraulic circuit. In fact, fact, the diaphragm plays a role of the pumping piston in the direct-drive direct-drive steam steam pump pump shown shown in in Figure Figure 22.. The hydraulic circuit is similar to that shown in FigureFigure2 2.. TheThe circuitcircuit consistsconsists ofof twotwo checkcheck valvesvalves 9,9, high- andand low-pressurelow-pressure hydraulic hydraulic accumulators accumulators 10 10 and and 11, 11, and and hydraulic hydraulic motor motor 12 connected 12 connected to a loadto a (e.g.,load (e.g., an electric an electric alternator). alternator).

Energies 2018, 11, 154 5 of 22

In operation, the linear actuator sets the piston into a reciprocating motion. Moving down, the piston displaces some part of the working fluid from the cold to the hot part of the cylinder. The working fluid passes the cooler, the regenerator and the heater and heats up. Pressure in the converter rises, and a part of the working fluid is displaced to the diaphragm unit. The diaphragm transmits the pressure to hydraulic oil which flows through the discharge check valve, high-pressure accumulator and hydraulic motor to the low-pressure accumulator. The hydraulic motor converts the oil energy to shaft power. Moving up, the piston displaces the hot, compressed working fluid through the heater, regenerator and cooler back to the cold part of the cylinder. The pressure in the cylinder drops down. As soon as it becomes lower than the charge pressure in the low-pressure accumulator, oil forces the working fluid from the diaphragm unit back to the cylinder. Afterwards, the cycle repeats. The regenerator plays a very important role: it stores heat from the previous cycle and uses it to preheat the working fluid in the next cycle increasing thermodynamic efficiency of the heat conversion. Theoretically, in case of perfect regeneration possible for ideal gases (working fluid returns to the hot space of the converter with the temperature of the hot space), the efficiency of the converter can be equal to the Carnot cycle efficiency. Low-power (less than 1 kW) machines can have a very simple design, without the external heater-regenerator-cooler loop. In this case, the upper part of the cylinder is heated, while the lower one is cooled. A small gap between the piston and cylinder plays the role of the regenerator. In some cases, the regenerator can also be placed inside the piston. Most of the thermo-compressors were built according to these two schemes. Hydraulic power output permits the heat-to-mechanical-energy converter to be improved. In particular, it makes the actuation of the piston very convenient: some part of the hydraulic power produced is used for the actuation by means of any type of hydraulic mover, e.g., reciprocating (hydraulic cylinder) or rotary hydraulic motor equipped with a crank mechanism [20]. There also exist some advanced versions of the thermo-compressor with a self-oscillating piston [21]. In this case, the compressor does not need any actuator and, most important, it does not need any sealing; leakages and seal friction losses are totally eliminated. In spite of the attractiveness of the isobaric expansion machines, all attempts to develop a ready-to-market thermo-compressor have failed. It is commonly assumed that the thermo-compressor represents a particular type of the Stirling engine [22] with typical disadvantages [23,24] inherent in all Stirling engines. First of all, the thermo-compressor is very sensitive to the so-called dead volumes. Indeed, the heater, cooler, regenerator and all connection pipelines have internal volumes, which are detrimental to the efficiency and the power density. A reason is that heating of the working fluid in the heater results not only in useful work generation but also in substantial compression of working fluid in the dead volumes. This harmful effect increases rapidly with the machine power. The power (i.e., volume) of the engine is proportional to the third power of the size, whereas the heat exchanger area is proportional only to the second power of the size. In practice, this means that the internal volume of the heat exchangers grows much faster than the heat exchangers area. The bigger the size of the machine, the more difficult is the heat supply. The second drawback is the low thermal expansion of gases. The power density of the machine is proportional to the difference between the maximum and the minimum cycle pressures. A high pressure difference can be obtained only at a very high temperature of the heat source and/or at a high initial pressure of the gaseous working fluid in the machine. Both conditions require expensive, heat resistant alloys and thick walls worsening heat exchange. Sealing of gases at high pressure is difficult and can cause additional friction losses. Reaching higher power density by increasing the cycle frequency results in enhanced flow and frictional losses, generation of frictional heat and wear of sliding/rubbing parts. Energies 2018, 11, 154 6 of 22

EnergiesOne 2018 of, 11 the, 154 ways to overcome these drawbacks is to use fluids with high thermal expansion6 of at22 a high cycle temperature and low compressibility at a low cycle temperature. Fluids that undergo a phasephase changechange fromfrom liquidliquid toto gasgas duringduring thethe cyclecycle asas wellwell asas supercriticalsupercritical fluidsfluids nearnear thethe criticalcritical pointpoint belong toto thisthis category.category. ForFor brevity,brevity, wewe callcall these these fluids fluids dense dense working working fluids. fluids. Since liquidsliquids have have much much lower lower compressibility compressibility than than gases, gases, the negative the negative influence influence of dead of volume dead filledvolume with filled a liquid with cana liquid be reduced can be reduced considerably. considerably. During the phasephase change,change, thermalthermal expansionexpansion of densedense workingworking fluidsfluids atat practicallypractically interestinginteresting operating pressures of 10–10010–100 bar isis significantlysignificantly higherhigher (one-two order ofof magnitude)magnitude) than that for gases. As a result, the heatheat sourcesource temperature,temperature, initialinitial pressurepressure andand speed/frequencyspeed/frequency of the cyclecycle requiredrequired forfor thethe samesame specificspecific powerpower cancan bebe decreaseddecreased inin thethe machinemachine withwith aa densedense workingworking fluid.fluid. Boiling and condensationcondensation processes together withwith higherhigher thermalthermal conductivityconductivity of liquidsliquids comparedcompared to gasesgases facilitatefacilitate heat transfer, becausebecause ofof thethe decreaseddecreased heatheat transfertransfer resistanceresistance onon thethe workingworking fluidfluid side.side. InIn addition,addition, sealingsealing ofof liquidsliquids isis muchmuch simplersimpler asas comparedcompared toto gases.gases. The higher thermalthermal conductivityconductivity and and heat heat capacity capacity of of liquids liquids facilitates facilitates the the removal removal of the of heatthe heat of friction of friction and thusand reducesthus reduces wear wear of rubbing of rubbing parts. parts. Despite the fact fact that that the the direct-acting direct-acting pumps pumps us useded liquid liquid (water) (water) from from the thevery very beginning, beginning, the thefirst first attempts attempts to use to liquid use liquid in Bush-type in Bush-type machines machines started started not until not the until seventies the seventies of the last of thecentury last century[14,25–27].[14 Unfortunately,,25–27]. Unfortunately, this interesting this interesting development development was not followed was not up followed in spite of up encouraging in spite of encouragingtheoretical data theoretical and expectations data and expectations [22]. We may [22 as].sume We may that assume low thermal that low efficiency thermal efficiencyand technical and technicalcomplexity complexity of the engines of the used engines were used the werereasons. the reasons. As inin thethe case case of of gaseous gaseous working working fluids, fluids, it is alsoit is possiblealso possible to realise to realise self-oscillating self-oscillating piston designs piston fordesigns machines for machines with dense with working dense working fluids [ 28fluids–31]. [28–31]. A functional A functional scheme scheme of one of one such of self-driven such self- machinesdriven machines is shown is inshown Figure in4 Figure. 4.

Figure 4.4. Scheme of Bush-type self-driven machine; A—hot part, B and C—cold parts with thethe displacer inin intermediateintermediate ((leftleft)) andand twotwo extremeextreme positions.positions.

For simplicity, the diaphragm unit and the hydraulic circuit are not shown; so the machine is For simplicity, the diaphragm unit and the hydraulic circuit are not shown; so the machine is depicted as a pump for the liquid used as the working fluid. The pump consists of a differential depicted as a pump for the liquid used as the working fluid. The pump consists of a differential cylinder with a differential piston inside. The piston divides the internal volume of the cylinder into cylinder with a differential piston inside. The piston divides the internal volume of the cylinder into three parts: heated (hot) part A and two cold parts B and C. The machine is equipped with an external three parts: heated (hot) part A and two cold parts B and C. The machine is equipped with an external cooler, regenerator and heater. In non-operating condition, the piston is maintained in the middle cooler, regenerator and heater. In non-operating condition, the piston is maintained in the middle position inside the cylinder by means of a spring, shown in the lower part of the cylinder. position inside the cylinder by means of a spring, shown in the lower part of the cylinder. The lower part of the cylinder has three fluid ports communicated by a manifold. The manifold The lower part of the cylinder has three fluid ports communicated by a manifold. The manifold is is equipped with two self-acting (suction and discharge) non-return valves. There is a passage inside equipped with two self-acting (suction and discharge) non-return valves. There is a passage inside the piston allowing the piston to operate as a valve: in the lower and upper positions of the piston, the piston allowing the piston to operate as a valve: in the lower and upper positions of the piston, the passage communicates the part B of the cylinder with the part C. The parts A and B are always the passage communicates the part B of the cylinder with the part C. The parts A and B are always communicated through the cooler, regenerator and heater. communicated through the cooler, regenerator and heater. When the working fluid is heated and turns into vapour, pressure in the parts A and B rises forcing the piston down and compressing the spring. In addition, the piston displaces the working fluid from the cold part B through the cooler, regenerator and heater to the hot part A causing a further rise of pressure and forcing the piston down. The liquid from the part C is displaced into the manifold and then discharged through the lower non-return valve to the delivery line. When the

Energies 2018, 11, 154 7 of 22

When the working fluid is heated and turns into vapour, pressure in the parts A and B rises forcing the piston down and compressing the spring. In addition, the piston displaces the working fluid from the cold part B through the cooler, regenerator and heater to the hot part A causing a further rise of pressure and forcing the piston down. The liquid from the part C is displaced into the manifold and thenEnergies discharged 2018, 11, 154 through the lower non-return valve to the delivery line. When the piston7 of reaches 22 the lower position, it stops and communicates parts A, B and C, equalising the pressure inside the cylinder.piston During reaches thisthe lower stage, position, some amountit stops and of commu liquidnicates from theparts part A, B A and is C, also equalising forced the to thepressure manifold and dischargedinside the cylinder. through During the valve this stage, to the some delivery amount line. of Afterwards,liquid from the the part spring A is pushesalso forced the to piston the up, disconnectingmanifold and the discharged parts B and through C. Moving the valve up, theto the piston delivery displaces line. Afterwards, the working the fluid spring back pushes from the the hot part Apiston to the up, cold disconnecting part B. Going the parts through B and the C. regeneratorMoving up, the and piston the cooler,displaces the the vapour working turns fluid into back liquid from the hot part A to the cold part B. Going through the regenerator and the cooler, the vapour turns and the pressure in the parts A and B drops below the pressure in the part C, manifold and suction into liquid and the pressure in the parts A and B drops below the pressure in the part C, manifold line. Liquidand suction from line. the Liquid suction from line the enters suction the line part enters C, forces the part the C, piston forces the up piston and expands up and expands the spring. the Inspring. the upper position, the passage in the piston communicates the parts B and C, again equalising the pressure.In the Liquid upper position, from the the suction passage linein the and piston the communicates manifold enters the parts the B part and BC, and again then equalising the part A throughthe thepressure. cooler, Liquid regenerator from the and suction heater, line heats and up the and manifold turns intoenters vapour. the part At B the and same then time,the part the A spring pullsthrough the piston the cooler, down; regenerator the piston and disconnects heater, heats parts up Band and turns C, into the pressurevapour. At rises the same again time, starting the the next cycle.spring pulls the piston down; the piston disconnects parts B and C, the pressure rises again starting the next cycle. 3. Experimental Investigation 3. Experimental Investigation We studied the operation of a self-driven small-scale Bush-type engine/pump with R134a We studied the operation of a self-driven small-scale Bush-type engine/pump with R134a refrigerant as working fluid using the experimental set-up shown in Figure5. The basic principle of refrigerant as working fluid using the experimental set-up shown in Figure 5. The basic principle of operation of this machine is shown in Figure4. Due to low power the machine does not need external operation of this machine is shown in Figure 4. Due to low power the machine does not need external heat exchangers.heat exchangers. Side Side surface surface of of the the cylinder cylinder wallwall is sufficient sufficient to to supply supply and and reject reject heat heat when when working working fluidfluid flows flows through through the the annular annular gap gap between between thethe cylindercylinder and and displacer. displacer. A part A part of the of theannular annular gap in gap in betweenbetween heated heated and and cooled cooled parts parts plays plays the the rolerole of regenerator. To To enhance enhance heat heat exchange, exchange, the side the side surfacesurface of displacer of displacer is well-developed is well-developed by by means means ofof plurality of of small small grooves. grooves.

FigureFigure 5. Scheme 5. Scheme of of the the experimental experimental setup. setup. 1—IE1—IE engine, engine, 2—heating2—heating jacket, jacket,3—cooling3—cooling jacket, 4— jacket, 4—diaphragmdiaphragm unit, unit, 55—non-return—non-return valves, valves, 6—high-pressure6—high-pressure accumulator, accumulator, 7—low-pressure7—low-pressure accumulator, accumulator, 8—throttle valve, PT—pressure transmitters, FT—flow transmitter. 8—throttle valve, PT—pressure transmitters, FT—flow transmitter.

The setup included IE machine 1 with a cylinder inner diameter of 30 mm and a displacer stroke Thelength setup of 40 included mm provided IE machine with heating 1 with jacket a cylinder 2 (heater) inner and diametercooling jacket of 30 3 (cooler). mm and Hot a displacer water with stroke lengthtemperature of 40 mm varying provided between with heating 50 and 90 jacket °C circulating 2 (heater) in and the coolingheating, jacketupper 3jacket (cooler). was used Hot wateras the with temperatureheat source. varying The lower between part of 50 the and cylinder 90 ◦C was circulating cooled with in the cold heating, (25 °C) water upper circulating jacket was through used a as the heat source.cooling Thejacket. lower Pressure part ofpulses the cylindergenerated was by cooled the engine with coldwere(25 transmitted◦C) water through circulating a rubber through a coolingdiaphragm jacket. Pressure of diaphragm pulses unit generated 4 to hydraulic by the oil enginecirculating were in transmitteda hydraulic circuit through which a rubber included diaphragm two non-return valves 4, two high- and low-pressure 160 mL hydraulic accumulators 6 and 7 and needle of diaphragm unit 4 to hydraulic oil circulating in a hydraulic circuit which included two non-return throttling valve 8. The valve imitated a load throttling the oil flow from the high-pressure to the low- valves 4, two high- and low-pressure 160 mL hydraulic accumulators 6 and 7 and needle throttling pressure accumulator. The pressures before and after the throttle valve as well as pressure inside the cylinder were measured by pressure transmitters PT (Sendo JF302 5MPa gauge, Ninghai Sendo Sensor Co., Ltd, Hangzhou, China with 0.5% accuracy and 4 ms response time).

Energies 2018, 11, 154 8 of 22 valve 8. The valve imitated a load throttling the oil flow from the high-pressure to the low-pressure accumulator. The pressures before and after the throttle valve as well as pressure inside the cylinder were measured by pressure transmitters PT (Sendo JF302 5MPa gauge, Ninghai Sendo Sensor Co., Ltd., Hangzhou, China, with 0.5% accuracy and 4 ms response time). Independently,Energies 2018, 11, 154 the pressures were controlled by 6 MPa Empeo manometers with 1.6%8 of 22 accuracy (are not shown) installed in parallel with the pressure transmitters. The oil flow rate was measured Independently, the pressures were controlled by 6 MPa Empeo manometers with 1.6% accuracy by gear flow transmitter FT (Profimess VM-01.0.1 with accuracy of ±2%) installed in the line after (are not shown) installed in parallel with the pressure transmitters. The oil flow rate was measured the throttleby gear valve. flow Alltransmitter transmitters FT (Profimess had 4–20 VM-01.0.1 ma output with accuracy to avoid of ±2%) influence installed of in cable the line lengths. after the Signal of the transmittersthrottle valve. were All processed transmitters and had displayed 4–20 ma output by 12 to bitsavoid PicoLog influence 1000 of cable data lengths. acquisition Signal of system the with 1 MS/s samplingtransmitters rate. were Temperatureprocessed and displayed of hot and by cooling12 bits PicoLog water 1000 were data measured acquisition by system two 3with mm 1 shielded K-typesMS/s thermocouples, sampling rate. pre-calibrated Temperature of hot in theand rangecooling of water 0–100 were◦C. measured by two 3 mm shielded K- types thermocouples, pre-calibrated in the range of 0–100 °C. The machine demonstrated a self-starting ability and very stable, repeatable operation. The results The machine demonstrated a self-starting ability and very stable, repeatable operation. The for two differentresults for hottwo different water temperatures hot water temperatures are shown are shown in Figure in Figure6. 6.

R134a, 65 oC R134a, 90 oC 17 5 27 5 16 4.5 25 4.5 23 15 4 4 21 14 3.5 3.5 19 13 3 17 3 12 2.5 15 2.5 11 2 13 2 Pressure, bar Pressure, bar 11 10 1.5 1.5 Flow rate, l/minFlow rate,

Flow rate, l/min 9 9 1 1 7 8 0.5 5 0.5 7 0 3 0 567891011121314151617181920 5 6 7 8 9 1011121314151617181920

Time, s Time, s Pressure inside the engine Pressure inside the engine Pressure in high-pressure accumulator Pressure in high-pressure accumulator Pressure in low-pressure accumulator Pressure in low-pressure accumulator Flow rate Flow rate (a) (b)

Figure 6. Examples of pressure change and pumped flow rate in the self-driven Bush-type engine; Figure 6. Examples of pressure change and pumped flow rate in the self-driven Bush-type engine; working fluid: R134a; temperature of the cooler: 25 °C; temperature of the heater: 65°C (a) and ◦ ◦ ◦ working90°C fluid: (b R134a;). temperature of the cooler: 25 C; temperature of the heater: 65 C(a) and 90 C(b).

As can be seen, the machine can pump liquid with a considerable pressure differences (up to 20 As can be seen, the machine can pump liquid with a considerable pressure differences (up to bar) even at such low temperature differences. At higher temperatures, the generated pressure 20 bar) evendifference at such can reach low temperaturetens and hundreds differences. of bar, depending At higher on temperatures,the working fluids. the The generated measured pressure differencepower can reachwas up tens to 20 and W. The hundreds power was of bar,calculated depending as the product on the workingof the measured fluids. flow The rate measured of the power was up tohydraulic 20 W. The oil and power the pressure was calculated difference asover the the product throttling of valve. the measured The heat supply flow to rate the ofengine the hydraulicwas oil and the pressurenot measured difference in the presented over the setup, throttling so that valve.the thermal The efficiency heat supply could to not the be engine evaluated was directly. not measured Some estimates of the efficiency were made by replacing the heating jacket with an aluminium cast in the presented setup, so that the thermal efficiency could not be evaluated directly. Some estimates electric heater. It was found that the thermal efficiency could be as high as 50% of the Carnot of the efficiencyefficiency even were without made careful by replacing thermal insulation the heating of the jackethot engine with parts an and aluminium without other cast measures electric heater. It was foundto decrease that theheat thermal losses. The efficiency efficiency couldwas estimated be as high as a ratio as 50% of the of engine the Carnot power efficiencyto electric power even without careful thermalsupplied insulationto the heater. of Both the the hot efficiency engine and parts powe andr strongly without depend other on measures the temperatures, to decrease the load heat losses. The efficiency(resistance was to estimatedthe oil flow) asand a the ratio amount of the of the engine working power fluid in to the electric engine. power The estimated supplied efficiency to the heater. and power are very promising taking into account the temperatures and size of the investigated Both theengine. efficiency and power strongly depend on the temperatures, the load (resistance to the oil flow) and theA amountremarkable of thefeature working of the engine fluid inis the ability engine. to Theadapt estimated to the load: efficiency at a given andheat powersource are very promisingtemperature, taking into the accountengine can the deliver temperatures the same hydrau andlic size power of the at different investigated combinations engine. of pressures A remarkableand flow rates feature depending of on the the engine load. When is the the abilitypressure toin the adapt high-pressure to the load: accumulator at a given increases, heat source temperature,the engine the engineimmediately can delivergenerates the a higher same pressure hydraulic and powera lower flow at different rate. Such combinations a self-adaptation of is pressures very useful for practical implementation. and flow rates depending on the load. When the pressure in the high-pressure accumulator increases, the engine immediately generates a higher pressure and a lower flow rate. Such a self-adaptation is very useful for practical implementation. Energies 2018, 11, 154 9 of 22

4. Efficiency of Isobaric Expansion Machines with Dense Working Fluids

4.1. Thermodynamic Modelling To estimate the efficiency of the machines described in the previous sections, thermodynamic models where developed for both the Worthington-type machine and the Bush-type engine. Here, the basic equations used for the calculation of the engines efficiencies are presented.

4.1.1. Worthington-Type Engine The thermal efficiency is defined as the work produced during a full cycle in relation to the supplied heat: W η = (1) QH In a simplified model of the Worthington-type single-acting engine (cf. Figure2), the cycle starts with vaporised working fluid entering the steam cylinder. The pressure increases up to the maximum cycle pressure at constant (minimum) volume of the fluid in the steam cylinder. We assume the initial volume in the steam cylinder to be equal to zero, therefore isochoric pressurisation does not apply and pressure in the steam cylinder rises to the maximum value immediately. As soon as the maximum pressure is established, further supply of steam results in expansion of the fluid in the steam cylinder and, consequently, in expansion work. When reaching the maximum volume, the pressure inside the steam cylinder is reduced by a throttled exhaust of working fluid at constant steam cylinder volume. Afterwards, the steam cylinder volume is reduced back to its minimum while working fluid leaves the steam cylinder at constant low pressure. The expansion-compression work performed by the piston during this process is:   WE = Phigh − Plow ∆VE (2)

In this equation, Phigh and Plow are the pressures during the expansion and compression processes, and ∆VE is the change of steam volume in the cylinder. The pumping work, assuming a reversible process with negligible changes in kinetic and potential energies, can be determined as:     WP = mtot h Phigh, TP,out − h(Plow, TC) (3) where mtot is the total mass of the working fluid pumped per cycle, h(P, T) is the enthalpy of the working fluid per unit mass, TC is the temperature at the inlet of the pump and TP,out is the discharge temperature of the pump. The discharge pump temperature can be determined assuming an isentropic pump operation. The net value of the work produced during the cycle is found as:

W = WE − WP (4)

The working fluid is heated up in a heater from the pump discharge temperature (usually slightly above the temperature of the cooler) to the desired inlet temperature of the steam cylinder. This heat transfer process can be assisted by a recuperative heat exchanger, recovering a part of the heat the working fluid exhausted from the steam cylinder. In this case, the duty of the heater depends on the performance of the recuperator, i.e., on the temperature of the working fluid at the outlet of the recuperator TR,out:      QH = mtot h Phigh, TH − h Phigh, TR,out (5)

In order to determine TR,out from the energy balance of the recuperator, the inlet temperature of the countercurrent stream TR,in coming from the steam cylinder is necessary. It was assumed that the pressure of the steam exhausted from the steam cylinder decreases from the high cycle pressure Phigh to the low cycle pressure Plow in a throttle located before the recuperator for technical reasons. Energies 2018, 11, 154 10 of 22

The exhaust valve of the steam cylinder may play the role of the throttle. This pressure reduction process was considered as a Joule—Thompson expansion. Neglecting the changes in kinetic and potential energy, the temperatures in the cylinder and at the inlet of the recuperator are related by an equation ensuring no enthalpy change in this process:

h = (PE, TE) = h(Plow, TR,in) (6)

The calculated temperatures TR,in and TE were always above the saturation temperature; therefore, no condensation was taken into account in Equation (6). Finally, temperature and pressure in the cylinder, PE and TE or before the exhaust valve during the pressure reduction process and backstroke of the piston were determined from the energy balance for the adiabatic process: ρ(T, P)dh = dP (7) where ρ(T, P) is the fluid density. Although the amount of working fluid in the cylinder reduces the energy, the balance equation (Equation (7)) is the same as that for adiabatic expansion of a fluid of constant mass. Equations (2)–(7) together with an equation for the recuperation efficiency (see e.g., Equation (11) below) permit calculation of the amount of heat supplied by the heater QH and ultimately the thermal efficiency (Equation (1)).

4.1.2. Bush-Type Engine The useful work generated during the cycle is: I I I W = P dVtot = P dVH + P dVC (8) where the total engine volume Vtot consists of the volumes of the hot space, VH, the cold space, VC and the regenerator VR which does not change during engine operation. The interdependence of temperatures and volumes of the individual engine compartments and the pressure is expressed by the equation for the total mass of the working fluid:

mtot = ρ(P, TH)VH + ρ(P, TC)VC + ρ(P, TR)VR (9)

The heat supply to the hot part is determined from the energy balance of this space, neglecting changes of kinetic and potential energies: I QH = PdVH + ∆QH (10)

∆QH is the energy required to heat the displaced working fluid from the regenerator outlet temperature (or low cycle temperature TC, if there is no regenerator) to the high cycle temperature TH. A method to estimate the performance of the regenerator is presented in the subsequent section. With Equations (8) and (10), the thermal efficiency η = W can be evaluated. QH The modelling of the Bush-type engine is explained in more detail in [32].

4.2. Estimation of the Regeneration/Recuperation Efficiency The performance of the recuperative and regenerative heat exchangers (or recuperator and regenerator) in both considered engine types strongly affects the efficiency of the respective engine. As there is no functional difference between regeneration and recuperation, we will use the term regenerator for both these types of heat exchangers. In an ideal regenerator, the heat removed from the working fluid during its cooling would be completely returned to the working fluid during heating. This, however, is unrealistic because of Energies 2018, 11, 154 11 of 22 thermodynamic and technical limitations of actual heat transfer processes. For instance, different heat capacities of the working fluid during heating and cooling make it impossible to achieve perfect regeneration. This is particularly true for cycles in which dense working fluids are evaporated at a high pressure (and temperature) and condensed at a lower pressure. The heat of condensation Energies 2018, 11, 154 11 of 22 cannot be used to provide the latent heat for evaporation in such a case. In addition, a positive driving temperaturehigh pressure difference (and between temperature) the hotand andcondensed cold streamat a lower in pressure. a heat exchanger The heat of condensation must be maintained. cannot A reasonablebe used to provide value of the this latent temperature heat for evaporation difference in for such a reala case. heat In exchangeraddition, a positive needs to driving be found in accordancetemperature to desired difference performance between andthe hot available and cold space.stream in To a estimateheat exchanger the regeneration must be maintained. efficiencies for the, compositeA reasonable curves [33 value] for of the this heating temperature and cooling difference processes for a real areheat determined. exchanger needs They to be give found the in amount accordance to desired performance and available space. To estimate the regeneration efficiencies for of energy required to obtain a certain temperature change over the investigated temperature range. the, composite curves [33] for the heating and cooling processes are determined. They give the The regenerationamount of potentialenergy required canbe to obtain found a bycertain shifting temperature the heating change curve over the (receiving investigated heat) temperature sufficiently far to maintainrange. a minimumThe regeneration temperature potential difference can be found between by shifting the cooling the heating curve curve (providing (receiving heat) heat) and the heating curve.sufficiently Figure far to7 showsmaintain examples a minimum of temperature such composite difference curves. between The the dashed cooling red curve line (providing illustrates the heating curveheat) and resulting the heating in acurve. minimum Figure temperature7 shows examples difference of such composite of 0 K which curves. represents The dashed the red theoreticalline maximumillustrates of regeneration. the heating curve The resulting solid red in a line minimu is anm exampletemperature for difference a positive of 0 minimumK which represents temperature the theoretical maximum of regeneration. The solid red line is an example for a positive minimum difference.temperature The regeneration difference. efficiencyThe regeneration is defined efficiency as the is defined ratio of as heat the toratio be of recovered heat to be inrecovered the regenerator in to the totalthe energyregenerator tobe to the supplied total energy to the to engine:be supplied to the engine: Q η = QR =R +R (11) ηR QQRH (11) QR + QH

Figure 7. Example of composite curves during isobaric cooling and heating at different pressures; Figure 7. Example of composite curves during isobaric cooling and heating at different pressures; (QC)’, (QR)’ and (QH)’ are shown for the theoretical maximum of regeneration, whereas (QC), (QR) and (Q )0,(Q )0 and (Q )0 are shown for the theoretical maximum of regeneration, whereas (Q ), (Q ) and C (RQH) correspondH to a real situation with the positive minimum temperature difference. C R (QH) correspond to a real situation with the positive minimum temperature difference. The higher the minimum temperature difference is, the lower is the regeneration efficiency. The regeneration efficiency of the Bush-type engine can be higher than that of the Worthington-type The higher the minimum temperature difference is, the lower is the regeneration efficiency. engine, as the condensation in the Bush-type engine occurs not only during the isobaric process, but The regenerationalso during efficiency the isochoric of process the Bush-type at decreasing engine pressure. can be Hence, higher compared than that to the of Worthington-type the Worthington-type engine, asprocess, the condensationa larger portion of in the the heat Bush-type of condensation engine can occurs be recovered. not only during the isobaric process, but also during the isochoric process at decreasing pressure. Hence, compared to the Worthington-type process,4.3. a larger Simulation portion Results of the heat of condensation can be recovered. The models of Worthington-type and Bush-type engines together with the approach to estimate 4.3. Simulationthe regeneration Results efficiency described above (Equations (1)–(11)) were implemented in a MATLAB environment to study the influence of different operating pressures and working fluids on the The models of Worthington-type and Bush-type engines together with the approach to estimate efficiency of both engine types. The simulations were performed for two distinctive cases: (1) the the regeneration efficiency described above (Equations (1)–(11)) were implemented in a MATLAB

Energies 2018, 11, 154 12 of 22 environment to study the influence of different operating pressures and working fluids on the efficiency of both engine types. The simulations were performed for two distinctive cases: (1) the simplest case without regeneration and (2) a regenerative operation, assuming that a minimum temperature difference of 5 K in the regenerator is realised. Dead volumes were not taken into account, even though they may have a negative effect on the efficiency of Bush-type engines. Different working fluids were investigated. Thermodynamic properties of the working fluids were obtained from the NIST Standard ReferenceEnergies Database 2018, 11,23. 154 Results obtained for water, propane, pentane, R32 refrigerant12 of and22 carbon dioxide are discussed below. simplest case without regeneration and (2) a regenerative operation, assuming that a minimum ◦ The minimumtemperature cycledifference temperature of 5 K in the (temperature regenerator is ofrealised. the heat Dead sink) volumes was were chosen not taken as T intoC = 40 C for ◦ all the workingaccount, fluids even though except they carbon may dioxide.have a negative For carbon effect on dioxide, the efficiency it was of chosen Bush-type as 30engines.C, i.e., below the criticalDifferent temperature working of fluids 30.98 were◦C investigated. in order to Thermo guaranteedynamic its properties liquid state of the at working low temperature fluids were and any pressure throughoutobtained from the the cycle. NIST TheStandard maximum/high Reference Data cyclebase 23. temperature Results obtained was variedfor water, from propane, 50 ◦C to 600 ◦C pentane, R32 refrigerant and◦ carbon dioxide◦ are discussed below. for carbon dioxideThe minimum and from cycle 50 temperatureC to 300 (temperatuC for there of other the heat working sink) was fluids. chosen The as TC low = 40 pressure°C for all Plow was chosen slightlythe working above fluids the except saturation carbon pressuredioxide. For for carbon each dioxide, fluidat it was low chosen temperature as 30 °C, i.e., to excludebelow the undesired boiling at lowcritical cycle temperature temperature, of 30.98 but°C in not order lower to guarantee than 1.0 its bar, liquid depending state at low on temperature which of theand twoany pressures is higher. Thepressure difference throughout between the cycle. the The high maximum/high and low pressure cycle temperature∆P was variedwas varied in afrom range 50 of°C 1.0to to 50 bar. 600 °C for carbon dioxide and from 50 °C to 300 °C for the other working fluids. The low pressure For R32 refrigerantPlow was chosen and slightly carbon above dioxide, the saturation the pressure pressu differencere for each fluid range at waslow temperature extended to exclude 200 bar. Selected ◦ results areundesired shown in boiling Figures at low8–12 cycle. Figure temperature, 13 shows but resultsnot lower for than carbon 1.0 bar, dioxide depending at veryon which high of (up the to 600 C) heat sourcetwo temperature pressures is higher. to estimate The difference a potential between of the this high technology and low pressure for high-temperature ∆P was varied in a range applications. The dash-and-dotof 1.0 to 50 line bar. inFor each R32 refrigerant graph indicates and carbon the dioxide, Carnot the efficiency. pressure difference range was extended to 200 bar. Selected results are shown in Figures 8–12. Figure 13 shows results for carbon dioxide at For bothvery typeshigh (up of to engines, 600 °C) heat stepwise source temperature increase in to theestimate efficiency a potential is observed of this technology at certain for high- temperatures of the heattemperature source. Theseapplications. heat The source dash-and-dot temperatures line in each correspond graph indicates to the the Carnot saturation efficiency. temperatures in subcritical area.For The both verytypes lowof engines, efficiencies stepwise at increase temperatures in the efficiency below is theobserved saturation at certain temperature temperatures are found for engineof operation the heat source. with workingThese heat fluidsource being temperatur in liquides correspond state. to the saturation temperatures in subcritical area. The very low efficiencies at temperatures below the saturation temperature are found At firstforsight, engine theoperation results with for working non-regenerative fluid being in operationliquid state. of the two engine types under comparison are almost equivalent.At first sight, For the R32 results and for CO non-regenerati2, better efficienciesve operation are of obtainedthe two engine for the types Worthington-type under machine.comparison These results, are almost however, equivalent. are overestimated, For R32 and CO as2, better the assumption efficiencies are of obtained incompressible for the liquids, used in theWorthington-type calculation machine. of the Worthington-typeThese results, however, engine, are overesti failsmated, for theseas the twoassumption fluids of under the incompressible liquids, used in the calculation of the Worthington-type engine, fails for these two investigatedfluids conditions. under the investigated conditions.

15% 15%

10% 10% Efficiency Efficiency

5% 5%

0% 0% 40 80 120 160 200 240 280 320 40 80 120 160 200 240 280 320 ° Temperature TH ( C) Temperature TH ( °C) without regeneration 1 bar 2 bar 5 bar Plow = 1.0 bar ∆P = ° with regeneration TC = 40 C 10 bar 25 bar 50 bar

(a) (b)

Figure 8. Efficiency of regenerative Worthington-type machine (a) and Bush-type machine (b) Figure 8. Efficiencyoperated with of water. regenerative Worthington-type machine (a) and Bush-type machine (b) operated with water.

Energies 2018 11 Energies 2018, , 11, 154, 154 13 of13 22 of 22 Energies 2018, 11, 154 13 of 22

20% 20% 20% 20%

15% 15% 15% 15%

10% 10% 10% 10% Efficiency Efficiency Efficiency Efficiency

5% 5% 5% 5%

0% 0% 0% 0% 40 80 120 160 200 240 280 320 40 80 120 160 200 240 280 320 40 80 120 160 200 240 280 320 Temperature TH ( °C) 40 80 120 160 200 240 280 320 ° Temperature TH ( °C) Temperature TH ( C) Temperature T ( °C) P = 1.17 bar without H Plow = 1.17 bar without ∆P = 1 bar 2 bar 5 bar Tlow= 40 °C regeneration ∆P = 1 bar 2 bar 5 bar C regeneration 10 bar 25 bar 50 bar TC = 40 °C with regeneration with regeneration 10 bar 25 bar 50 bar (a) (b) (a) (b) Figure 9. Efficiency of regenerative Worthington-type machine (a) and Bush-type machine (b) FigureFigure 9. 9.Efficiency Efficiency of regenerativeof regenerative Worthington-type Worthington-type machine machine (a) and (a) Bush-typeand Bush-type machine machine (b) operated (b) operated with pentane. withoperated pentane. with pentane.

30% 30% 30% 30%

25% 25% 25% 25%

20% 20% 20% 20%

15% 15% 15% 15% Efficiency Efficiency Efficiency Efficiency 10% 10% 10% 10%

5% 5% 5% 5%

0% 0% 0% 0% 40 80 120 160 200 240 280 320 40 80 120 160 200 240 280 320 40 80 120 160 200 240 280 320 40 80 120 160 200 240 280 320 Temperature TH ( °C) Temperature TH ( °C) Temperature TH ( °C) Temperature TH ( °C) P = 13.8 bar ∆P = 1 bar 2 bar 5 bar low without regeneration 1 bar 2 bar 5 bar Plow = 13.8 bar without regeneration ∆P = TC = 40 °C with regeneration 10 bar 25 bar 50 bar TC = 40 °C with regeneration 10 bar 25 bar 50 bar (a) (b) (a) (b) Figure 10. Efficiency of regenerative Worthington-type machine (a) and Bush-type machine (b) Figure 10. Efficiency of regenerative Worthington-type machine (a) and Bush-type machine (b) Figureoperated 10. withEfficiency propane. of regenerative Worthington-type machine (a) and Bush-type machine operated with propane. (b) operated with propane.

Energies 2018, 11, 154 14 of 22 Energies 2018,, 11,, 154154 14 of 22

35% 35% 35%

30% 30% 30%

25% 25%

20% 20%

15% 15%

Efficiency 15%

15% Efficiency Efficiency Efficiency

10% 10% 10%

5% 5% 5%

0% 0% 0% 40 80 120 160 200 240 280 320 40 80 120 160 200 240 280 320 40 80 120 160 200 240 280 320 Temperature T ( °C) Temperature TH ( °C) Temperature TH ( °C) H Temperature TH ( °C) 1 bar 2 bar 5 bar without regeneration ∆P = Plow = 25.03 bar without regeneration 10 bar 25 bar 50 bar low with regeneration 10 bar 25 bar 50 bar TC = 40 °C with regeneration 100 bar 150 bar 200 bar TC = 40 °C 100 bar 150 bar 200 bar (a) (b)

FigureFigure 11. 11. EfficiencyEfficiency of ofregenerative regenerative Worthington-type Worthington-type machine machine (a) (anda) andBush-type Bush-type machine machine (b) operated(b) operated with with refrigerant refrigerant R32. R32.

35% 35%

30% 30%

25% 25%

20% 20%

15% 15% Efficiency Efficiency Efficiency Efficiency

10% 10%

5% 5%

0% 0% 30 70 110 150 190 230 270 310 30 70 110 150 190 230 270 310 Temperature TH ( °C) Temperature TH ( °C) Temperature TH ( °C) Temperature TH ( °C) 1 bar 2 bar 5 bar ∆P = 1 bar 2 bar 5 bar Plow = 72.86 bar without regeneration low without regeneration 10 bar 25 bar 50 bar TC = 30 °C with regeneration TC = 30 C with regeneration 100 bar 150 bar 200 bar (a) (b) Figure 12. Efficiency of regenerative Worthington-type machine (a) and Bush-type machine (b) FigureFigure 12. 12. EfficiencyEfficiency of ofregenerative regenerative Worthington-type Worthington-type machine machine (a) (anda) andBush-type Bush-type machine machine (b) operated with carbon dioxide. operated(b) operated with with carbon carbon dioxide. dioxide.

EnergiesEnergies2018 2018, 11,, 15411, 154 15 of 2215 of 22

Plow = 72.86 bar without regeneration

TC = 30 °C with regeneration 50%

40%

30%

20% Efficiency

10%

0% 50 100 150 200 250 300 350 400 450 500 550 600

Temperature TH ( °C) ∆P = 50 bar 100 bar 150 bar 200 bar

FigureFigure 13. 13.Efficiency Efficiency of of regenerative regenerative Bush-type mach machineine operated operated with with carbon carbon dioxide dioxide in high in high temperaturetemperature range. range.

OperationOperation without without regeneration regeneration is is particularly particularly interesting interesting for low-temperature for low-temperature applications. applications. For instance, an engine with propane as working fluid (Figure 10) can generate a 10 bar pressure For instance, an engine with propane as working fluid (Figure 10) can generate a 10 bar pressure amplitude at a heat source temperature of roughly 70 °C with an efficiency of nearly 5%. With amplitude at a heat source temperature of roughly 70 ◦C with an efficiency of nearly 5%. With pentane, pentane, a heat source temperature◦ of 58 °C is sufficient to generate a 1 bar pressure amplitude with a heatan source efficiency temperature of 4.3% (Figure of 58 9).C It isis sufficientworth noting to generatethat the Carnot a 1 bar efficiency pressure for amplitude this process with conditions an efficiency of 4.3%is 5.4%. (Figure Thus,9). the It is relative worth to noting the Carnot that theefficiency Carnot is efficiency above 80%. for this process conditions is 5.4%. Thus, the relativeWith to the the heat Carnot regeneration, efficiency the is higher above the 80%. temperature is, the more pronounced is the influence ofWith regeneration the heat regeneration,for both types theof engines, higher theand temperature the Bush-type is, engine the more outperforms pronounced the Worthington- is the influence of regenerationtype engine for for both all working types of fluids engines, under and investigation. the Bush-type engine outperforms the Worthington-type engine forWith all workingand without fluids regeneration, under investigation. water demonstrates a very poor performance for both types of enginesWith and in withoutthe studied regeneration, temperature water range. demonstrates For Worthington-type a very poor performanceengines, the influence for both typesof of recuperation in case of water is almost negligible, whereas for propane, the efficiency can be doubled engines in the studied temperature range. For Worthington-type engines, the influence of recuperation at high temperatures. For Bush-type engines, the regeneration in operation with propane can increase in case of water is almost negligible, whereas for propane, the efficiency can be doubled at high the efficiency almost fivefold. temperatures.At very For low Bush-type temperatures, engines, the influence the regeneration of the regeneration in operation is poor. with Effective propane heat regeneration can increase the efficiencyfor dense almost working fivefold. fluids in low temperature range can be problematic. A reason is that heat released duringAt very condensation low temperatures, of working the fluid influence at low temperature of the regeneration and pressure is poor. cannot Effective be used heat to evaporate regeneration for denseit at higher working temperature fluids in lowand temperaturepressure of the range isobaric can expansion be problematic. stage. Moreover, A reason isthe that latent heat heat released duringconstitutes condensation a considerable of working part fluidof the at whole lowtemperature amount of energy and pressuresupplied cannotduring heating be used and to evaporate released it at higherduring temperature cooling. The and latent pressure heat of of the water isobaric condensa expansiontion, for stage. example, Moreover, accounts the for latent about heat 90% constitutes of the a considerablewhole heat part released of the during whole the amount cooling of from energy 300 supplied °C to 40 °C during at normal heating pressure. and released Poor regeneration, during cooling. resulting in the low efficiency, explains a relatively limited area of application of water-based The latent heat of water condensation, for example, accounts for about 90% of the whole heat released Worthington pumps. during the cooling from 300 ◦C to 40 ◦C at normal pressure. Poor regeneration, resulting in the low For other fluids, the fraction of latent heat at studied process conditions is around 50%; therefore, efficiency,a considerable explains part a relatively of heat can limited be regenerated. area of application of water-based Worthington pumps. For otherUnfortunately, fluids, the the fraction incomplete of latent regeneration heat at is studied typical processalso for conditionsworking fluids is around in a supercritical 50%; therefore, a considerablestate near the part critical of heat point. can In bethis regenerated. case, the problem is caused by a difference in heat capacities of the workingUnfortunately, fluid at different the incomplete pressures regeneration used during its is heating typical and also cooling for working in the regenerator. fluids in a supercritical state near the critical point. In this case, the problem is caused by a difference in heat capacities of the working fluid at different pressures used during its heating and cooling in the regenerator. In contrast to Worthington-type machines, in Bush-type machines, both the evaporation and the condensation of working fluid partially take place at variable pressure. The evaporation occurs Energies 2018, 11, 154 16 of 22 simultaneously with a pressure rise; the condensation is accompanied by a drop of pressure. Such a regime is favourable for the regeneration. As it can be seen, even with water (Figure8), the regeneration is much more pronounced resulting in an efficiency rise. Efficiency in case of propane (Figure 10) can compete with that of ORC [34].

5. Discussion

5.1. General The term “isobaric expansion” is essentially used as a term rather than an exact characterisation of the process, just to distinguish from isentropic expansion processes. As a matter of fact, the process pressure may vary, depending on the load; for instance, in case of gas compression, pressure changes during the cycle. Isobaric expansion machines represent a peculiar type of heat-to-mechanical energy converters, which are characterised by the absence of a gas expansion stage. Practically all well-known today’s heat engines (piston IC-engines, gas, steam and ORC turbines, etc.) fall into a different group, performing polytropic gas expansion.

5.2. Bush- and Worthington-Type Engines—Results and Comparison Bush-type and Worthington-type engines are two examples of simple and inexpensive machines performing isobaric expansion cycle. At high temperatures, the Bush-based machines outperform the Worthington-based ones. In a low-temperature range, where the influence of heat recuperation is negligible, a choice between these two types is not evident. At first glance, the Worthington-based machines are more complex because of the valve system, piston and valves sealing and the need for a pump. However, they are totally insensitive to the dead volume. This means an unlimited choice of types and sizes of heat exchangers and, therefore, an ability to supply heat in any form from any possible source. The Bush-type machines are simpler, since they do not require a valve system or a separate pump. Moreover, they can be made self-driven. Self-driven machines need neither an actuator and nor connection rod sealing. As a result, both leakages of working fluid and seal friction losses are eliminated. In addition, self-actuation is more energy efficient. Nevertheless, the actuated machines can be preferable in some cases. They are easy to control and a synchronization of several machines is very simple. All the designs of Bush-type machines known from the literature concern applications at relatively small power (less than 1 kW), with heat addition and rejection through the surface of the engine cylinder and a regenerator located in the displacer or in the gap between the displacer and the cylinder. Medium- and large-scale machines require more powerful heat exchangers being placed outside the cylinder. Ten to twenty years ago such engines with external heat exchangers were not feasible due to too large internal volume of the then existing heat exchangers. However, a recent revolution in the field of very compact heat exchangers [35,36] makes it possible to build medium-scale and even large-scale engines with external heat exchangers. State-of-the-art microchannel/printed circuit heat exchangers permit the heater, regenerator and cooler to be combined into a compact single integral unit with minimal internal (dead) volume. A further integration of such a composite heat exchanger into the piston-cylinder unit is an option to decrease the dead volume of the machine even more. Bush-type machines also can have great potential in high-temperature application; no need of high-temperature sealing makes that especially attractive. This potential is also supported by the high efficiencies obtained for carbon dioxide (Figure 13). The found efficiencies at low temperature differences were above 80% of the Carnot efficiencies. These results are very encouraging, taking into account the simplicity and low price of the machines for harvesting ultra-low grade heat, e.g., waste and geothermal heat sources, low temperature solar energy, such as solar ponds, etc. [37,38]. Energies 2018, 11, 154 17 of 22

for harvesting ultra-low grade heat, e.g., waste and geothermal heat sources, low temperature solar Energies 2018, 11, 154 17 of 22 energy, such as solar ponds, etc. [37,38].

5.3.5.3. Potential ImprovementsImprovements

5.3.1.5.3.1. Regeneration AA veryvery promising promising way way to improveto improve the regenerationthe regeneration for both for typesboth oftypes engines of engines is to use is an to approach use an successfullyapproach successfully employed inemployed cryogenics, in namelycryogenics, the replacement namely the of replacement single-component of single-component working fluids withworking multi-component fluids with multi-component ones with different ones boiling with temperatures different boiling [39,40 temperatures]. By this means, [39,40]. boiling By andthis condensationmeans, boiling of and the condensation working fluid of take the working place at variablefluid take temperatures. place at variable As atemperatures. result, both boiling As a result, and condensingboth boiling someand condensing part of the some working part fluidof the is working possible fluid in the is possible regenerator in the rather regenerator than in rather the cooler than andin the heater. cooler In and other heater. words, In theother heat words, of condensation the heat of of condensation a high-boiling of componenta high-boiling can component be used for can the evaporationbe used for the of low-boiling evaporation components, of low-boiling and components thus, this part, and of thethus, latent this heatpart ofof thetheworking latent heat fluid of canthe beworking regenerated. fluid can be regenerated. TheThe expected effect depends depends on on the the mixture mixture comp composition,osition, initial initial temperature temperature and and pressure pressure as aswell well as ason on the the temperature temperature and and pressure pressure ratios. ratios. Taking Taking into into account account the the fraction fraction of latent heat in the totaltotal amountamount ofof heatheat supplied,supplied, thethe resultingresulting riserise inin efficiencyefficiency cancan bebe remarkable.remarkable. However,However, selectionselection and/orand/or design design of of approp appropriateriate multicomponent working fluids fluids re requirequire very serious research efforts.efforts.

5.3.2.5.3.2. Dead Volumes AtAt firstfirst glance,glance, thethe veryvery impressiveimpressive efficienciesefficiencies ofof Bush-typeBush-type enginesengines might look too optimistic, sincesince thethe deaddead volume volume of of the the machines machines has has not not been been taken taken into into account. account. In fact,In fact, the greaterthe greater part part of this of deadthis dead volume volume arises arises due to due internal to internal volumes volumes of the heater, of the coolerheater, and cooler regenerator. and regenerator. A harmful A influence harmful ofinfluence the heat of exchanger the heat internalexchanger volumes internal can volumes be reduced, can ifbe necessary, reduced, by if decreasingnecessary, theby energydecreasing density the ofenergy the machines. density of For the instance, machines. a machineFor instance, with a a ma workingchine with volume a working (displacement) volume of(displacement) one liter and of a cycleone liter frequency and a cycle of 1 Hz frequency generates of the1 Hz same generates power the as a same machine power with as aa displacementmachine with of a twodisplacement liters and aof cycle two frequencyliters and ofa cycle 0.5 Hz. frequency To deliver of this0.5 power,Hz. To bothdeliver machines this power, require both heat machines exchangers require with heat the sameexchangers internal with volume. the same However, internal in casevolume. of the Howeve machiner, with in case the of lower the speedmachine and with higher the displacement, lower speed theand dead higher volume displacement, of the heat the exchangersdead volume related of the to he theat exchangers total volume related of the to machine the total is volume twice as of low. the Decreasemachine ofis powertwice as density low. appearsDecrease to of be power unimportant density in appears view of theto be generally unimportant high power in view density of the of thesegenerally machines high power compared density to other of these engines. machines compared to other engines. Unfortunately,Unfortunately, the influence influence of of the the regenerator regenerator dead dead volume volume cannot cannot be minimised be minimised in the in same the sameway. The way. volume The volume of the regenerator of the regenerator is directly is directlyproportional proportional to the volume to the displaced volume displaced in the machine. in the machine.A possible A solution possible solutionto reduce to the reduce regenerator the regenerator dead volume dead volume is to replace is to replace the regenerator the regenerator by a byrecuperative a recuperative heat heatexchanger exchanger in a duplex in a duplex arrangement arrangement as shown as shown in Figure in Figure 14. In 14 this. In scheme, this scheme, two twoidentical identical machines, machines, working working in counter-phase, in counter-phase, share share a common a common recuperator. recuperator. When When one one machine machine is in is inthe the heating heating process, process, the the other other machine machine provides provides heat heat during during its itscooling cooling process process (and (and vice vice versa). versa). In Incontrast contrast to toa regenerator, a regenerator, the the heat heat is is here here directly directly transferred transferred to to the the working working fluid fluid and and the recuperator sizesize isis nono longerlonger proportionalproportional toto thethe volumevolume displaceddisplaced inin thethe machine.machine.

FigureFigure 14.14. Duplex Bush-type engine, consisting of two individualindividual enginesengines coupledcoupled byby aa commoncommon recuperativerecuperative heatheat exchanger.exchanger.

Energies 2018, 11, 154 18 of 22

Energies 2018, 11, 154 18 of 22 5.3.3. Non-Isothermal Heat Supply 5.3.3. Non-Isothermal Heat Supply The temperature of the heating agent in the heater decreases due to heat exchange with the The temperature of the heating agent in the heater decreases due to heat exchange with the working fluid. This is especially important in the case of indirect heat supply by means of different working fluid. This is especially important in the case of indirect heat supply by means of different heat transfer fluids (heat carriers) such as pressurised water, heat transfer oils or liquid metals. heat transfer fluids (heat carriers) such as pressurised water, heat transfer oils or liquid metals. In Infact, fact, the the indirect indirect isothermal isothermal heating heating requires requires an an infinite infinite circulation circulation rate rate of ofthe the heat heat carrier carrier whereas whereas non-isothermalnon-isothermal heat heat supply supply makes makes itit possiblepossible to decr decreaseease the the circulation circulation rate rate to to economically economically viable viable levels.levels. The The higher higher this this temperature temperature decrease,decrease, the more more efficient efficient and and economical economical is isthe the use use of ofthe the heat heat source.source. Therefore, Therefore, heat heat supply supply to to the the enginesengines cancan be highly non-isothermal. non-isothermal. FigureFigure 15 a15a shows shows a simplified a simplified layout layout of an isobaricof an isobaric expansion expansion machine machine with the with heater, the regenerator heater, andregenerator cooler combined and cooler in onecombined integral in printed one integral circuit printed heat exchanger. circuit heat Vertical exchanger. black, Vertical red and black, blue red lines signifyand blue here lines working signify fluid, here heatworkin carrierg fluid, and heat cooling carrier agent and channelscooling agent correspondingly. channels correspondingly.

FigureFigure 15. 15.Bush-type Bush-type engine enginedesign design for isothermal ( (aa)) and and non-isothermal non-isothermal heat heat supply supply (b, (cb):, c1):—heater,1—heater, 2—regenerator,2—regenerator,3—cooler, 3—cooler,4 —regenerative4—regenerative heater.heater.

A highly non-isothermal heat supply implies a high temperature difference between the hottest andA coldest highly parts non-isothermal of the heater. heat As supplya result, implies during athe high backstroke temperature (when difference the piston between moves upwards) the hottest andthe coldest hot working parts offluid, the heater.displaced As from a result, the hottest during pa thert backstrokeof the cylinder (when and the heater, piston heats moves up the upwards) heat thecarrier hot working at the lower, fluid, colder displaced part of from the theheater hottest shown part in Figure of the cylinder15a. The heat and heater,is carried heats away up instead the heat carrierof being at the absorbed lower, by colder the regenerator. part of the heaterThis leads shown to a inpoor Figure efficiency 15a. The of the heat machine is carried and awayan inefficient instead of beinguse absorbedof the heat by source. the regenerator. This leads to a poor efficiency of the machine and an inefficient use of the heatThe problem source. can be solved by means of a different design of the heat exchanger shown in Figure 15b.The Here, problem the heater can is be divided solved into by several means parts of a differentwith regenerative design ofsections the heat in between. exchanger The shown bigger in Figurethe temperature 15b. Here, difference, the heater the is more divided heater/regenerator into several parts pairs withare necessary. regenerative sections in between. The biggerA limiting the temperature (and the most difference, interesting) the case more of heater/regenerator such a design is shown pairs in arethe necessary.Figure 15c, where the heaterA limiting and regenerator (and the mostare merged interesting) into a case single of suchunit, acombining design is showntheir functions. in the Figure This regenerative15c, where the heaterheater and is a regenerator special type areof heat merged exchanger into a which single can unit, also combining accumulate their heat functions. of both the This working regenerative fluid heaterand heat is a specialcarrier. type of heat exchanger which can also accumulate heat of both the working fluid and heatAt carrier.low frequency operation typical for isobaric expansion engines, a non-stationary, periodic supply of heat carrier can be used in combination with such a heat exchanger. This is particularly At low frequency operation typical for isobaric expansion engines, a non-stationary, periodic convenient for the duplex (two engines working in counter phase) engine layout. The heat carrier is supply of heat carrier can be used in combination with such a heat exchanger. This is particularly pumped through the heat exchanger only during the working stroke (i.e., during a half cycle, when convenient for the duplex (two engines working in counter phase) engine layout. The heat carrier is the piston moves downwards). During the backstroke, the flow of heat carrier is diverted to the pumped through the heat exchanger only during the working stroke (i.e., during a half cycle, when the adjacent engine. This method is ideally suited for actuated machines, where the actuators of the pistonpistons moves and downwards).the heat carrier During diverting the valve backstroke, can be synchronised. the flow of heat carrier is diverted to the adjacent engine.The This reasoning method isdiscussed ideally suitedabove foris eq actuatedually valid machines, for engine where coolers. the actuators of the pistons and the heat carrier diverting valve can be synchronised. 5.4.The Hydraulic reasoning Output discussed and Applications above is equally valid for engine coolers.

5.4. HydraulicThe hydraulic Output output and Applications of the machines under consideration may have a clear advantage for many applications in comparison to state-of-the-art energy conversion technologies. This is especially importantThe hydraulic for pumping output and of compression, the machines resulting under in consideration elimination of several may have energy a clear conversion advantage stagesfor many applications in comparison to state-of-the-art energy conversion technologies. This is especially

Energies 2018, 11, 154 19 of 22 important for pumping and compression, resulting in elimination of several energy conversion stages together with the corresponding equipment (electric generator and motor, power transmission, etc.). As a result, numerous sources of low-grade and waste heat can be used directly and cost-effectively for pumping and compression. One of the most interesting applications, especially for the Bush-type machines, is the pumping of liquids which can also be used as working fluids. Such pumps can be applied for pumping of liquefied gases, light hydrocarbons, heat transfer fluids, etc. Since these machines do not need lubrication, they can be of great interest also in cryogenic applications. The convenience to operate between any desired pressure levels makes them a very promising heat-driven alternative to conventional refrigerant compressors in vapour compression refrigerating machines. The conversion of the hydraulic power output of the machines to shaft power by means of an oil-hydraulic loop outlined above is just one out of many possible options. It was selected because of the availability of low-cost, off-the-shelf industrial hydraulic components. The main part of the hydraulic loop—a positive displacement hydraulic motor—can reach an efficiency of above 90% in a broad power range and shows excellent energy density and reliability. A broad spectrum of low- medium- and high-speed hydraulic motors is available in a power range from several tens of watts to several hundred kilowatts. An obvious advantage of oil-hydraulic output is the possibility to generate shaft power at the most convenient speed and torque without an additional transmission. Moreover, one engine can drive several motors with different speeds and torques. Vice versa, one hydraulic motor can be driven by several engines as well. In the latter case, these engines can operate at different temperatures of the heater and cooler and even use different working fluids. A direct conversion of the working fluid energy to shaft power, without transmitting the pressure to oil, is also of a great interest. Unfortunately, hydraulic motors for liquids with low viscosity and poor lubricating ability are missing on the today’s market. Perhaps the experience gained in the area of water-hydraulic power utilisation systems can be used for a development of such motors. A so-called “pump as turbine” (PAT) technology uses rotodynamic (centrifugal) pumps as turbines, i.e., one of the types of hydraulic motors in micro- and pico-hydro power as well as a reverse osmosis energy utilisation [41,42]. As the oil-hydraulic motors, centrifugal pumps are inexpensive mass-market machines with relatively high maximum efficiencies (up to 80–85%). Today, such a reverse operation of centrifugal pumps with water is well studied. The operation with other fluids in the turbine mode requires further investigations. The main disadvantage of centrifugal pumps/turbines is a remarkable drop of efficiency with a decreasing power range and in a part-load operation.

5.5. Emerging Technologies Although it is very convenient to use hydraulic motors in combination with a rotary electric generator in case of electricity generation, alternative approaches are possible. Examples include a direct conversion of pressure to electricity by means of a piezoelectric pressure-to-electricity transmitter [43–45], different electrokinetic effects associated with electricity generation by forcing liquids through membranes [46,47], electrohydrodynamic electrostatic generators [48], reverse electrowetting [49], etc. All these methods make it possible to eliminate electric alternators, as well as to decrease the number of moving parts of the machines.

5.6. Final Remarks It is worth noting that in this paper, only simple basic schemes of isobaric expansion engines and very limited experimental and theoretical data are presented. As in the case of Rankine cycle plants, numerous modifications of the schemes and working fluids are possible. Energies 2018, 11, 154 20 of 22

6. Conclusions Isobaric expansion is a very promising energy conversion technology. Isobaric expansion cycles show excellent efficiency (up to 80% of the Carnot efficiency) at ultra-low heat source temperatures (as low as 70–100 ◦C). At higher temperatures, heat regeneration increases the efficiency remarkably, up to 30–35% at 300 ◦C. The efficiency at 600 ◦C can reach as high as 50%, showing a tendency to rise further with temperature. Replacement of single-component working fluids with multi-component mixtures could potentially result in a further efficiency rise in all temperature ranges, due to improved heat regeneration. The elimination of both polytropic gas expansion process and machine results in the substantial simplification of the engine design and cost. Moreover, some of these engines (self-driven types) do not need seals; this eliminates any leakage and frictional losses. All this together with a wide choice of output speed/torque without gearboxes can make the isobaric expansion technology a simple and low-cost alternative to the present ORC-based plants and other state-of-the-art energy conversion systems based on the conventional heat engines. The potential area of application of isobaric expansion machines is not limited to power generation. They can serve as very efficient, simple and cost-effective heat-driven pumps and compressors. A remarkable feature in this case is the ability to generate very high pressure differences even at relatively low temperatures.

Acknowledgments: The experimental results presented in this publication were made possible by NPRP grant # 8-547-2-222 from the Qatar National Research Fund (a member of Qatar Foundation). The findings achieved herein are solely the responsibility of the authors. Author Contributions: Maxim Glushenkov and Alexander Kronberg proposed and developed the technology and engine designs, invented the presented technical solutions, worked out the theoretical background and performed experiments. The modelling as well as the analysis of the computational results was performed by Torben Knoke and Eugeny Y. Kenig. All the authors participated in writing the paper. Conflicts of Interest: The authors declare no conflicts of interest.

Nomenclature h Specific enthalpie (J/kg) m Mass (kg) P Pressure (bar) Q Heat (J) V Volume (m3) W Work (J) Greek Symbols η Thermal efficiency (-) ρ Density (kg/m3) Subscripts C Cold, Cooling E Expansion chamber (steam cylinder) H Hot, Heating min Minimum out Outlet P Pump R Regenerator, Recuperator tot total

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