SE1550274A1

SE1550274A1 - Energy conversion system and method

Info

Publication number: SE1550274A1
Application number: SE1550274A
Authority: SE
Inventors: Brohall David; Forslund rjan
Original assignee: Greel Ab
Priority date: 2015-03-06
Filing date: 2015-03-06
Publication date: 2016-09-07
Also published as: ZA201706680B; EP3265655A1; EP3265655A4; US20180045077A1; WO2016144233A1

Abstract

The present invention relates to an energy conversion system for converting thermal energy to mechanical energy, comprising an evaporator, an expander, a condenser, a first tank, and a second tank. The energy conversion system further comprises flow control devices for controlling flow or working fluid between the evaporator, the expander, the condenser and the tanks, and a control unit for controlling operation of the energy conversion system by controlling the flow control devices. Each of the tanks has an outlet connected to an inlet of the evaporator, and an inlet connected to the condenser as well as to an outlet of the evaporator. Hereby, some of the pressurized vapor state working fluid flowing from the outlet of the evaporator can be used for pressurizing liquid state working fluid supplied from one the tanks to the evaporator. This configuration of the energy conversion system provides for improved energy conversion efficiency.Publication fig: Fig 2

Description

ENERGY CONVERSION SYSTEM AND METHOD Field of the lnventionThe present invention relates to an energy Conversion system and to a method of controlling such an energy conversion system.

Backqround of the lnvention lt is known to convert thermal energy to mechanical energy by meansof energy conversion systems using non-ideal versions of the Rankine cycle.According to an example of such a prior art energy conversion system 100,referring to fig 1, an electric pump 101 is used for increasing the pressure ofliquid state working fluid before the working fluid is evaporated by evaporator102, to vapor state working fluid. The evaporated vapor state working fluid isprovided to an expander 103, such as a turbine, whereby thermal energystored as pressure is converted to mechanical energy. The expanded vaporstate working ﬂuid is cooled and condensed, by condenser 104, to liquid stateworking fluid. Following condensation in the condenser 104, the liquid stateworking fluid flows to a buffer tank 105, and from the buffer tank, the liquidstate working fluid is again supplied to the pump 101. ln high temperature applications, the (electrical) energy consumption ofthe pump 101 used for increasing pressure of the liquid state working fluid isoften considered to be negligible, due to the high efficiency of the Rankinecycle for such applications.

For lower temperature applications, in which a so-called organicRankine cycle, or ORC cycle, is sometimes used, the efficiency is typicallylower, which means that the (electrical) energy consumption of the pump 101may be significant in relation to the output power from the expander 103. lt would be desirable to provide for more efficient conversion of thermalenergy into mechanical energy, and in particular to provide an energyconversion system based on the Rankine cycle allowing the use of less electrical energy for increasing the pressure of liquid state working fluidprovided to the evaporator.

Summaryln view of above-mentioned and other drawbacks of the prior art, it is an object of the present invention to provide an improved energy conversionsystem and method.

According to a first aspect of the present invention, it is thereforeprovided an energy conversion system for converting thermal energy tomechanical energy, comprising: an evaporator for evaporating liquid stateworking fluid to vapor state working fluid through supply of heat, theevaporator having an evaporator inlet for receiving liquid state working fluidand an evaporator outlet for output of vapor state working fluid at a firstpressure; an expander for expanding vapor state working fluid and convertingexpansion into mechanical energy, the expander having an expander inletconnected to the evaporator outlet for receiving vapor state working fluid atthe first pressure and an expander outlet for output of vapor state workingfluid at a second pressure lower than the first pressure; a condenser forcondensing vapor state working fluid to liquid state working fluid by cooling,the condenser having a condenser inlet connected to the expander outlet forreceiving vapor state working fluid and a condenser outlet for output of liquidstate working fluid; a first tank having a first inlet ﬂuid flow connected to thecondenser outlet, a second inlet fluid flow connected to the evaporator outlet,and an outlet fluid flow connected to the evaporator inlet; a second tankhaving a first inlet fluid flow connected to the condenser outlet, a second inletfluid flow connected to the evaporator outlet, and an outlet fluid flowconnected to the evaporator inlet; a first flow control device for controlling flowof working fluid from the outlet of the first tank to the evaporator inlet; asecond flow control device for controlling flow of working fluid from thecondenser outlet to the first inlet of the first tank; a third flow control device forcontrolling flow of working fluid from the evaporator outlet to the second inletof the first tank; a fourth flow control device for controlling flow of working fluid from the outlet of the second tank to the evaporator inlet; a fifth flow controldevice for controlling flow of the working fluid from the condenser outlet to theinlet of the second tank; a sixth flow control device for controlling flow ofworking fluid from the evaporator outlet to the second inlet of the second tank;and a control unit connected to each of the flow control devices, for controllingoperation of the energy conversion system.

The evaporator may be any device capable of evaporating the workingfluid (transitioning the working fluid from liquid state working fluid to vaporstate working fluid). For example, the evaporator may be a part of or beconnected to a solar heating system, a combustion-based heating system, ora heat accumulator etc.

The expander may be any device capable of expanding vapor stateworking fluid and converting the expansion of the vapor state working fluid tomechanical energy. The expander may, for instance, comprise a turbine. lt should be noted that the first and second inlets of the first tank maybe provided as a common inlet of the first tank and/or that the first andsecond inlets of the second tank may be provided as a common inlet of thesecond tank.

The control unit may advantageously comprise processing circuitrywhich may include at least one microprocessor and a memory. The memorymay contain a set of instructions for the microprocessor, and themicroprocessor may control the different flow control devices in the energyconversion system based on the set of instructions. ln particular, the set ofinstructions may specify a scheme of sequentially controlling the flow controldevices to allow or restrict flow of working fluid past the respective flowcontrol device. ln this manner, the energy conversion system may betransitioned between operational states when various state transitionconditions are fulfilled.

The present invention is based upon the realization that a more energyefficient conversion, based on the Rankine cycle, of thermal energy tomechanical energy can be achieved by using vaporized working fluid forpressurizing liquid state working fluid supplied to the evaporator. lt has further been realized that vaporized working fluid can conveniently be used forpressurizing the liquid state working fluid supplied to the evaporator byproviding two tanks, where one of the tanks supplies the pressurized liquidstate working ﬂuid, and the other tank receives liquid state working fluidfollowing evaporation, expansion and condensation. To keep the energyconversion process going, the function of the tanks may be alternated, so thatthe working fluid originating from the first tank is provided to the second tank(following evaporation, expansion and condensation) until a predeterminedcondition has been fulfilled, after which working fluid from the second tank isprovided to the first tank (following evaporation, expansion andcondensation). ln embodiments of the energy conversion system according to thepresent invention, it may not be necessary to use a pump to keep the energyconversion process going. lt may not even be necessary to use a pump to getthe energy conversion process started, but the energy conversion systemaccording to embodiments of the present invention may be configured toautomatically start when heat is supplied to the working fluid via theevaporator. This ability to start energy production without supply of electricalenergy is often referred to as a “black start”.

To provide for efficient flow of working fluid in the energy conversionsystem according to embodiments of the present invention, the condensermay advantageously be arranged at a higher vertical level than the first andsecond tanks. Moreover, the expander may advantageously be arranged at ahigher vertical level than the condenser.

Furthermore, the first and second tanks may advantageously bearranged at at least approximately the same vertical positions. ln each tank, the first and second inlets (or the common inlet formingthe first and second inlets) may be arranged at a higher vertical level than theoutlet.

The energy conversion system according to embodiments of thepresent invention may be controlled to convert thermal energy to mechanical energy by transitioning between operational states based on a fixed schedule, so that the control unit maintains the flow control devices in a firstconfiguration of 'open' and 'closed' during a predetermined period of timebefore transitioning the flow control devices to a second configuration of'open' and 'closed'.

To improve the efficiency and adaptability of the energy conversionsystems to, for example, variations in the supply of thermal energy from theevaporator, the energy conversion system may, according to variousembodiments, further comprise at least one state sensor for sensing apresent state of the energy conversion system, and the control unit mayadditionally be connected to the at least one state sensor and configured tocontrol the flow control devices based on a signal from the at least one statesensor.

The at least one state sensor may be configured to sense at least oneprocess-related parameter of the energy conversion system, such as one orseveral of pressure, temperature and liquid state working fluid level.

Advantageously at least one of the working fluid pressure, temperatureor (interface) level may be sensed in each of the first and second tanks. Thisallows the control unit to transition the energy conversion system betweenoperational states based on, for instance, the pressure or liquid level(interface between liquid state and vapor state working fluid) in the respectivetanks.

According to various embodiments, the control unit may be configuredto alternate the energy conversion system between a first operational state inwhich each of the first, third and fifth flow control devices is controlled to allowflow of working ﬂuid past the respective flow control devices; and each of thesecond, fourth and sixth flow control devices is controlled to prevent flow ofworking fluid past the respective flow control devices; and a secondoperational state in which each of the second, fourth and sixth flow controldevices is controlled to allow flow of working fluid past the respective flowcontrol devices; and each of the first, third and fifth flow control devices iscontrolled to prevent flow of working ﬂuid past the respective flow control devices.

Hereby, working fluid will alternatingly follow a first flow path from thefirst tank and sequentially through the evaporator, the expander, and thecondenser to the second tank, and a second flow path from the second tankand sequentially through the evaporator, the expander, and the condenser tothe first tank. When following the first flow path, some of the vapor stateworking fluid leaving the evaporator is used for pressurizing the first tank, andwhen following the second flow path, some of the vapor state working fluidleaving the evaporator is used for pressurizing the second tank.

As was discussed further above, the control unit may transition theenergy conversion system from an operational state when a respectivepredetermined condition is fulfilled. Such a predetermined condition mayadvantageously be selected in such a way that the control unit keeps theenergy conversion system in the above-mentioned first operational state untilthe first tank substantially only contains vapor state working fluid, and keepsthe energy conversion system in the second operational state until the secondtank substantially only contains vapor state working fluid.

After having supplied liquid state working fluid from the first or secondtank, that tank may contain vapor state working fluid at an elevated pressure.For an efficient energy conversion process, it may be desirable to reduce thepressure in the 'emptied' tank (the tank having supplied liquid state workingfluid to the evaporator).

For efficient use of the thermal energy stored in the emptied tank, theenergy conversion system may advantageously further comprise a pressureequalization conduit directly connecting the first tank and the second tank;and a seventh flow control device for controlling flow of working fluid directlybetween the first tank and the second tank.

Hereby, the stored thermal energy in the ”emptied” tank may be usedfor preheating the (liquid state) working ﬂuid and increasing pressure in theother tank.

To provide for efficient transfer of heat from the remaining working fluidin the 'emptied' tank to the liquid state working fluid in the other tank, the pressure equalization conduit may advantageously be connected to the first tank in a bottom portion of the first tank, and to the second tank in a bottomportion of the second tank. Hereby, efficient heat transfer between the hotvapor state working ﬂuid from the 'emptied' tank and the liquid state workingfluid in the other tank can be achieved.

According to various embodiments, the energy conversion system ofthe present invention may advantageously comprise at least one additionaltank connected to the evaporator and condenser in the same way as theabove-mentioned first and second tanks are fluid flow connected to theevaporator and condenser. Through the provision of one or several additionaltanks, variations in the output of mechanical energy (or electrical energyconverted from the mechanical energy) can be reduced. By providing at leastfour tanks, such as two sets of the above-described first and second tanks,embodiments of the energy conversion system may be operated practicallycontinuously.

According to a second aspect of the present invention, there isprovided a method of controlling an energy conversion system according toembodiments of the first aspect of the present invention. The methodcomprises the steps of: (a) controlling the flow control devices to allow flow ofworking fluid from the outlet of the first tank, through the evaporator, theexpander and the condenser to the first inlet of the second tank, whileallowing flow of vapor state working fluid from the evaporator into the secondinlet of the first tank; (b) releasing vapor phase working fluid (under pressure)from the first tank; (c) controlling the flow control devices to allow flow ofworking fluid from the outlet of the second tank, through the evaporator, theexpander and the condenser to the first inlet of the first tank, while allowingflow of vapor state working ﬂuid from the evaporator into the second inlet ofthe second tank; and (d) releasing vapor phase working fluid (under pressure)from the second tank. ln step (b) and step (d), respectively, the vapor phase working fluidmay be released (allowed to flow out of the tank to reduce pressure in thetank) through any suitable conduit. For instance, the pressurized vapor phaseworking fluid may be allowed to flow back towards the condenser.

Alternatively or in combination, at least some of the pressurized vapor phaseworking fluid may be passed to an auxiliary expander to be converted tomechanical energy.

To provide for an efficient energy conversion, step (b) may comprisereleasing vapor phase working fluid from the first tank to the second tank; andstep (d) may comprise releasing vapor phase working fluid from the secondtank to the first tank.

As was mentioned further above in connection with embodiments ofthe first aspect of the present invention, continuous energy conversion can beprovided for using an energy conversion system further comprising a thirdtank and a fourth tank, by adding the following steps: (e) contro||ing the flowcontrol devices to allow flow of working fluid from the outlet of the third tank,through the evaporator, the expander and the condenser to the first inlet ofthe fourth tank, while allowing flow of vapor state working fluid from theevaporator into the second inlet of the third tank; (f) releasing vapor phaseworking fluid (under pressure) from the third tank; (g) contro||ing the flowcontrol devices to allow flow of working fluid from the outlet of the fourth tank,through the evaporator, the expander and the condenser to the first inlet ofthe third tank, while allowing flow of vapor state working fluid from theevaporator into the second inlet of the fourth tank; and (h) releasing vaporphase working fluid (under pressure) from the fourth tank.

To provide for further improvements in the uniformity over time of themechanical (or electrical) energy output from the energy conversion systemaccording to embodiments of the present invention, the different tanks may beemptied, filled and pressure equalized asynchronously. ln particular, the flowcontrol devices may be controlled to allow flow of working fluid from each ofthe tanks from a substantially full state to a substantially empty state of thetank, through the evaporator, the expander and the condenser, to at least twoother tanks.

This emptying of a tank into at least two other tanks may take place sequentially. Furthermore, the flow control device(s) allowing flow into one of the destination tanks may be 'closed' before the flow control device(s)allowing flow into another one of the destination tanks is 'opened'. ln the case of an energy conversion system having four tanks, theamount of working ﬂuid in the energy conversion system may be adapted sothat the total liquid volume of working fluid in the tanks is, at any time, about1.5 times the volume of one of the tanks. ln summary, according to various embodiments the present inventionrelates to an energy conversion system for converting thermal energy tomechanical energy, comprising an evaporator, an expander, a condenser, afirst tank, and a second tank. The energy conversion system furthercomprises flow control devices for controlling flow or working ﬂuid betweenthe evaporator, the expander, the condenser and the tanks, and a control unitfor controlling operation of the energy conversion system by controlling theflow control devices. Each of the tanks has an outlet connected to an inlet ofthe evaporator, and an inlet connected to the condenser as well as to anoutlet of the evaporator. Hereby, some of the pressurized vapor state workingfluid ﬂowing from the outlet of the evaporator can be used for pressurizingliquid state working fluid supplied from one the tanks to the evaporator. Thisconfiguration of the energy conversion system provides for improved energy conversion efficiency.

Brief Description of the Drawinqs These and other aspects of the present invention will now be describedin more detail, with reference to the appended drawings showing an exampleembodiment of the invention, wherein: Fig 1 schematically shows a Rankine cycle based energy conversionsystem according to the prior art; Fig 2 schematically shows an energy conversion system according to afirst embodiment of the present invention; Figs 3a-d schematically illustrate different operational states of theenergy conversion system in fig 2; and Fig 4 schematically shows an energy Conversion system according to asecond embodiment of the present invention.

Detailed Description of Example Embodiments ln the present detailed description, various embodiments of theapparatus and method according to the present invention are mainlydescribed with reference to energy conversion systems comprising two orfour tanks. Furthermore, pressure sensors and level sensors are shown tosense the pressure and level in each tank. lt should be noted that this by no means limits the scope of the presentinvention, which equally well includes, for example, energy conversionsystems comprising another number of tanks. Furthermore, the energyconversion process may be controlled using parameters other than sensedpressure and level in the tanks. For instance, the energy conversion may becontrolled using preset time durations for each operational state, or otherprocess parameters may be sensed, such as the temperature and/or theenergy output by the expander or a generator which may be connected to theexpander.

Fig 2 schematically illustrates an energy conversion system 1according to a first example embodiment of the present invention. Referring tofig 2, the energy conversion system 1 comprises an evaporator 2, a primaryexpander 3, a condenser 4, a first tank 5, a second tank 6, an auxiliaryexpander 7, and a control unit 8 for controlling operation of the energyconversion system 1. As is indicated in fig 2, the energy conversion systemfurther comprises a first pressure sensor 9 for sensing the pressure in the firsttank 5, a first level sensor 10 for sensing the liquid level in the first tank 5, asecond pressure sensor 11 for sensing the pressure in the second tank 6,and a second level sensor 12 for sensing the liquid level in the second tank 6.

As is also shown in fig 2, the evaporator 2 has an evaporator inlet 14and an evaporator outlet 15, the primary expander 3 has an expander inlet 16and an expander outlet 17, the condenser 4 has a condenser inlet 18 and acondenser outlet 19, the first tank 5 has an inlet 20 and an outlet 21, the 11 second tank 6 has and inlet 22 and an outlet 23, and the auxiliary expander 7has an auxiliary expander inlet 24 and an auxiliary expander outlet 25.

The various parts of the energy conversion system 1 in fig 2 are fluidflow connected by conduits schematically indicated as solid lines in fig 2, andcontrol lines (which may be wired or wireless) are indicated by dashed lines.Flow of working fluid through the conduits can be controlled using flowcontrolled devices, here in the form of controllable valves 30a-i.

An example of a suitable working fluid is the Genetron® R-245fa fromHoneywell. The skilled person will realize that this is merely an example, andthat there is a large number of commercially available working fluids that maybe suitable for different embodiments depending on various factors, such asthe thermal power provided by the evaporator etc.

By controlling the states ('open' or 'closed') of the valves 30a-i, thecontrol unit 8 can control the energy conversion system to differentoperational states for achieving a sustained conversion of thermal energy,supplied to the working fluid circulating through the conduits of the energyconversion system 1 by the evaporator 2, to mechanical energy provided bythe expander 3.

This will now be illustrated for the relatively simple first embodiment ofthe energy conversion system 1 of fig 2 with reference to figs 3a-d.

To more clearly illustrate the different operational states of the energyconversion system 1 in fig 2, the schematic illustrations in figs 3a-d onlyinclude parts of the energy conversion system 1 that are 'active' in theparticular operational state.

Referring first to fig 3a, this schematic illustration only includes the flowpath from the outlet 21 of the first tank 5 sequentially via the evaporator 2, theprimary expander 3, and the condenser 4 to the inlet 22 of the second tank 6.To define this flow path, the control unit 8 (not shown in fig 3a) controls valves30a, 30c and 30g to 'open'. ln the operational state schematically shown in fig3a, the remaining valves in fig 2 are 'closed'.

With reference to fig 3a, liquid state working fluid flows from the outlet21 of the first tank 5 to the inlet 14 of the evaporator 2. ln the evaporator 2, 12 thermal energy is supplied to the liquid state working fluid, which evaporatesto vapor state working fluid. The vapor state working fluid is provided from theevaporator outlet 15 to the expander inlet 16 and to the inlet 20 of the firsttank 5. The vapor state working fluid provided to the inlet 20 of the first tank 5increases the pressure of the liquid state working fluid supplied from the outlet21 of the first tank 5 to the evaporator inlet 14. The vapor state working fluidsupplied to the expander inlet 16 is expanded and the expansion convertedby the expander 3 to mechanical energy. ln the exemplary case when theexpander 3 is provided in the form of a turbine, the expansion is converted torotation, which can be used to drive a generator. After leaving the expander 3through the expander outlet 17, the expanded vapor state working fluid iscondensed to liquid state working fluid in the condenser 4, before beingsupplied to the second tank 6 through the inlet 22 of the second tank 6.

Fig 3a schematically shows the beginning of the illustrated operationalstate, with the first tank 5 being almost filled with liquid state working fluid andthe second tank 6 being almost empty (or rather the level of liquid stateworking fluid being low). As the illustrated operational state is maintained, theliquid level in the first tank 5 will decrease, while the liquid level in the secondtank 6 increases. When the first tank 5 is 'empty' or almost 'empty' (onlycontains vapor state working fluid), the control unit 8 controls the energyconversion system 1 to a new operational state for pressure equalization. ltmay be determined by the control unit 8 that the first tank 5 is ”empty” oralmost ”empty” based on signals provided by one or both of the first pressuresensor 9 and the first level sensor 10.

Referring now to fig 3b, this schematic illustration only includes the flowpath from the outlet 21 of the first tank 5 to the outlet 23 of the second tank 6.To define this flow path, the control unit 8 (not shown in fig 3b) controls valve30d to 'open'. ln the operational state schematically shown in fig 3b, theremaining valves in fig 2 are 'closed'. As is shown in fig 3b, the first tank 5now has a low liquid level (is 'empty'), while the second tank 6 has a highliquid level (is 'full'). 13 With reference to fig 3b, hot vapor state working ﬂuid at high pressure,such as about 20 bar, is released to the second tank 6 through the out|et 23of the second tank 6. The hot vapor state working fluid bubbles through therelatively cool liquid state working fluid in the second tank 6, heats the liquidstate working ﬂuid in the second tank 6, partly condenses and increases thepressure in the second tank 6. The transfer of vapor state working fluid fromthe first tank 5 to the second tank 6 stops when the pressure is equalized inthe system formed by the first 5 and second 6 tanks. The pressure in thesecond tank 6 (and in the first tank 5) may then be about 5 bar, and the liquidstate working fluid stored in the second tank has been preheated.

Turning to fig 3c, this schematic illustration only includes the flow pathfrom the out|et 23 of the second tank 6 sequentially via the evaporator 2, theprimary expander 3, and the condenser 4 to the inlet 20 of the first tank 5. Todefine this flow path, the control unit 8 (not shown in fig 3c) controls valves30f, 30b and 30h to 'open'. ln the operational state schematically shown in fig3c, the remaining valves in fig 2 are 'closed'.

With reference to fig 3c, liquid state working fluid flows from the out|et23 of the second tank 6 to the inlet 14 of the evaporator 2. ln theevaporator 2, thermal energy is supplied to the liquid state working fluid,which evaporates to vapor state working fluid. The vapor state working fluid isprovided from the evaporator out|et 15 to the expander inlet 16 and to theinlet 22 of the second tank 6. The vapor state working ﬂuid provided to theinlet 22 of the second tank 6 increases the pressure of the liquid state workingfluid supplied from the out|et 23 of the second tank 6 to the evaporatorinlet 14. The vapor state working fluid supplied to the expander inlet 16 isexpanded and the expansion converted by the expander 3 to mechanicalenergy. ln the exemplary case when the expander 3 is provided in the formof a turbine, the expansion is converted to rotation, which can be used todrive a generator. After leaving the expander 3 through the expander out|et17, the expanded vapor state working fluid is condensed to liquid stateworking fluid in the condenser 4, before being supplied to the first tank 5through the inlet 20 of the first tank 5. 14 Fig 3c schematically shows the beginning of the illustrated operationalstate, with the second tank 6 being almost filled with liquid state working fluidand the first tank 5 being almost empty (or rather the level of liquid stateworking fluid being low). As the illustrated operational state is maintained, theliquid level in the second tank 6 will decrease, while the liquid level in the firsttank 5 increases. When the second tank 6 is 'empty' or almost ”empty” (onlycontains vapor state working ﬂuid), the control unit 8 controls the energyconversion system 1 to a new operational state for pressure equalization. ltmay be determined by the control unit 8 that the second tank 6 is “empty' oralmost 'empty' based on signals provided by one or both of the secondpressure sensor 11 and the second level sensor 12.

The final pressure equalization operational state before the energyconversion system 1 is again back to the initial configuration shown in fig 3a,is shown in fig 3d. As can be seen by comparing fig 3b and fig 3d, theconfiguration is the same. The only difference is that the direction of fluid flowbetween the first 5 and second 6 tanks is now from the second tank 6 to thefirst tank 5.

A full 'main' energy conversion cycle of the energy conversionsystem 1 in fig 2 has now been described with reference to figs 3a-d. Tosimplify the description and make the explanation clearer, there has so farbeen no reference to the auxiliary expander 7 in fig 2. The purpose of theauxiliary expander 7 is to use the thermal energy remaining in an 'empty' tankfollowing the pressure equalization process described above with reference tofig 3b and fig 3d. Considering, for example, the state of the energy conversionsystem 1 following the pressure equalization process described withreference to fig 3b. The control unit 8 may then control valves 30a, 30b, and30c to their 'closed' states, and control valve 30e to its open state. Vaporstate working ﬂuid remaining in the first tank 5 can then be expanded by theauxiliary expander 7. Hereby, the pressure in the first tank 5 can be reducedfurther, and more energy can be converted to mechanical energy.

As can readily be understood from the above process description, theenergy conversion device 1 in fig 2 will not supply mechanical energy (or electrical energy converted from the mechanical energy) continuously due tothe pressure equalization processes described with reference to fig 3b andfig 3d. By adding further tanks, such as one more set of two tanks, a pressureequalization between two tanks in a first set of two tanks can be timed to takeplace while liquid state working fluid stored in a first tank in a second set oftwo tanks is used for energy conversion as described above with reference tofig 3a and fig 3c.

A second embodiment of the energy conversion system according tothe present invention is schematically shown in fig 4. The energy conversionsystem 50 in fig 4 differs from the energy conversion system 1 describedabove with reference to fig 2 and figs 3a-d in that a second set of twotanks 51 and 52 has been added. These added tanks 51 and 52 areconnected to each other as well as to the other parts of the energy conversionsystem 50 (the evaporator 2, the primary expander 3, the condenser, and theauxiliary expander 7 using controllable valves in exactly the same way as wasdescribed above with reference to fig 2. To avoid cluttering the drawing and todispense with an unnecessarily lengthy description, pressure and levelsensors, inlets and outlets and controllable valves associated with the addedtanks 51 and 52 have been omitted from the drawing.

Furthermore, the control unit 8 is configured to control thesecontrollable valves associated with the added tanks 51 and 52 in the sameway as the controllable valves 30a-i associated with the first 5 and second 6tanks were controlled to transition the energy conversion system 1 in fig 2between operational states.

Example According to an example embodiment of the inventive method, theenergy conversion system according to the above-described secondembodiment may be controlled asynchronously to provide for a uniformoutput of mechanical (or electrical) energy from the energy conversionsystem. 16 Time Tank 1 Tank2 Tank3 Tank4 5 Full, outlet open Empty, 2:nd expander Half full, filling Empty, outlet open10 3/4, outlet Empty, condensing 3/4, filling Empty, outlet closed15 Half full, outlet Empty, filling 7/8, preheated Empty, preheating20 1/4, outlet 1/4, filling Full, preheated Empty, preheating25 Empty, outlet open Half full, filling Full, outlet open Empty, 2:nd expander30 Empty, outlet closed 3/4, filling 3/4, outlet Empty, condensing35 Empty, preheating 7/8, preheated Half full, outlet Empty, filling 40 Empty, preheating Full, preheated 1/4, outlet 1/4, filling Empty, 2:nd 45 expander Full, outlet open Empty, outlet open Half full, filling 50 Empty, condensing 3/4, outlet Empty, outlet closed 3/4, filling 55 Empty, filling Half full, outlet Empty, preheating 7/8, preheated 60 1/4, filling 1/4, outlet Empty, preheating Full, preheated 65 Half full, filling Empty, outlet open Empty, 2:nd expander Full, outlet open 70 3/4, filling Empty, outlet closed Empty, condensing 3/4, outlet 75 7/8, preheated Empty, preheating Empty, filling Half full, outlet 80 Full, preheated Empty, preheating 1/4, filling 1/4, outlet ln the table above, the tanks are denoted by numbers from left to rightin fig 4. Furthermore, the transition between operational states of the energyConversion system is determined by a predetermined duration of the differentsteps. When a tank is referred to as being 'empty', this means that the tankonly contains vapor phase working fluid. A tank that is 'preheating' isconnected to a tank that is 'preheated', and pressure equalization asdescribed above with reference to fig 3b and fig 3d takes place.

The person skilled in the art realizes that the present invention by nomeans is limited to the preferred embodiments described above. On thecontrary, many modifications and variations are possible within the scope ofthe appended claims. For example, many other operational sequences ofemptying, filling, and pressure equalizing the tanks are possible and may bebeneficial depending on application and configuration of the energyConversion system. ln the claims, the word "comprising" does not exclude other elementsor steps, and the indefinite article "a" or "an" does not exclude a plurality. Asingle processor or other unit may fulfill the functions of several items recitedin the claims. The mere fact that certain measures are recited in mutually 17 different dependent claims does not indicate that a combination of thesemeasures cannot be used to advantage. Any reference signs in the claimsshould not be construed as Iimiting the scope.

Claims

1. An energy Conversion system for converting thermal energy tomechanical energy, comprising: an evaporator for evaporating liquid state working fluid to vapor stateworking fluid through supply of heat, said evaporator having an evaporatorin|et for receiving liquid state working fluid and an evaporator outlet for outputof vapor state working ﬂuid at a first pressure; an expander for expanding vapor state working ﬂuid and convertingexpansion into mechanical energy, said expander having an expander in|etconnected to said evaporator outlet for receiving vapor state working fluid atsaid first pressure and an expander outlet for output of vapor state workingfluid at a second pressure lower than said first pressure; a condenser for condensing vapor state working ﬂuid to liquid stateworking fluid by cooling, said condenser having a condenser in|et connectedto said expander outlet for receiving vapor state working fluid and acondenser outlet for output of liquid state working fluid; a first tank having a first in|et fluid flow connected to said condenseroutlet, a second in|et fluid flow connected to said evaporator outlet, and anoutlet fluid flow connected to said evaporator in|et; a second tank having a first in|et fluid flow connected to said condenseroutlet, a second in|et ﬂuid flow connected to said evaporator outlet, and anoutlet fluid flow connected to said evaporator in|et; a first flow control device for controlling flow of working fluid from theoutlet of the first tank to the evaporator in|et; a second flow control device for controlling flow of working fluid fromthe condenser outlet to the first in|et of the first tank; a third flow control device for controlling flow of working fluid from theevaporator outlet to the second in|et of the first tank; a fourth flow control device for controlling flow of working fluid from theoutlet of the second tank to the evaporator in|et; 19 a fifth flow control device for controlling flow of said working ﬂuid fromthe condenser outlet to the inlet of the second tank; a sixth flow control device for controlling flow of working fluid from theevaporator outlet to the second inlet of the second tank; and a control unit connected to each of said flow control devices, forcontrolling operation of said energy conversion system.

2. The energy conversion system according to claim 1, furthercomprising at least one state sensor for sensing a present state of saidenergy conversion system, wherein said control unit is further connected to said at least one statesensor and configured to control said flow control devices based on a signal from said at least one state sensor.

3. The energy conversion system according to claim 1 or 2, whereinsaid control unit is configured to alternate said energy conversion systembetween: a first operational state in which each of said first, third and fifth flowcontrol devices is controlled to allow flow of working ﬂuid past the respectiveflow control devices; and each of said second, fourth and sixth flow controldevices is controlled to prevent flow of working fluid past the respective flowcontrol devices; and a second operational state in which each of said second, fourth andsixth flow control devices is controlled to allow flow of working fluid past therespective flow control devices; and each of said first, third and fifth flowcontrol devices is controlled to prevent flow of working fluid past therespective flow control devices.

4. The energy conversion system according to claim 3, wherein saidcontrol unit is configured to keep the energy conversion system in said firstoperational state until said first tank substantially only contains vapor stateworking fluid, and to keep the energy conversion system in said second operational state until said second tank substantially only contains vapor stateworking fluid.

5. The energy conversion system according to any one of thepreceding claims, further comprising: a pressure equalization conduit directly connecting said first tank andsaid second tank; and a seventh flow control device for controlling flow of working fluid directly between said first tank and said second tank.

6. The energy conversion system according to claim 5, wherein saidpressure equalization conduit is connected to said first tank in a bottomportion of said first tank, and to said second tank in a bottom portion of saidsecond tank.

7. The energy conversion system according to any one of thepreceding claims, further comprising: a third tank having a first inlet ﬂuid flow connected to said condenseroutlet, a second inlet ﬂuid flow connected to said evaporator outlet, and anoutlet fluid flow connected to said evaporator inlet; an eighth flow control device for controlling flow of working fluid fromthe outlet of the third tank to the evaporator inlet; a ninth flow control device for controlling flow of working fluid from thecondenser outlet to the first inlet of the third tank; and a tenth flow control device for controlling flow of working fluid from theevaporator outlet to the second inlet of the third tank, wherein said control unit is additionally connected to said eighth, ninthand tenth flow control devices.

8. The energy conversion system according to any one of thepreceding claims, further comprising a generator connected to said expanderfor converting said mechanical energy into electrical energy. 21

9. A method of controlling an energy Conversion system according toany one of the preceding claims, said method comprising the steps of: (a) controlling the flow control devices to allow flow of working fluidfrom the outlet of said first tank, through said evaporator, said expander andsaid condenser to the first inlet of said second tank, while allowing flow ofvapor state working fluid from said evaporator into the second inlet of saidfirst tank; (b) releasing vapor phase working fluid from said first tank; (c) controlling the flow control devices to allow flow of working fluidfrom the outlet of said second tank, through said evaporator, said expanderand said condenser to the first inlet of said first tank, while allowing flow ofvapor state working fluid from said evaporator into the second inlet of saidsecond tank; and (d) releasing vapor phase working fluid from said second tank.

10. The method according to claim 9, wherein: step (b) comprises releasing vapor phase working fluid from said firsttank to said second tank; and step (d) comprises releasing vapor phase working fluid from said second tank to said first tank.

11. The method according to claim 10, wherein vapor phase workingfluid is controlled to flow directly between a bottom portion of said first tank and a bottom portion of said second tank.

12. The method according to any one of claims 9 to 11, wherein saidsteps (a) to (d) are performed repeatedly in sequence.

13. The method according to any one of claims 9 to 12, for controllingan energy conversion system further comprising a third tank and a fourthtank, the method further comprising the steps of: 22 (e) controlling the flow control devices to allow flow of working fluidfrom the outlet of said third tank, through said evaporator, said expander andsaid condenser to the first inlet of said fourth tank, while allowing flow of vaporstate working fluid from said evaporator into the second inlet of said thirdtank; (f) releasing vapor phase working fluid from said third tank; (g) controlling the flow control devices to allow flow of working fluidfrom the outlet of said fourth tank, through said evaporator, said expanderand said condenser to the first inlet of said third tank, while allowing flow ofvapor state working fluid from said evaporator into the second inlet of saidfourth tank; and (h) releasing vapor phase working fluid from said fourth tank.

14. The method according to claim 13, wherein said flow controldevices are controlled to allow flow of working fluid from each of said tanksfrom a substantially full state to a substantially empty state of said tank,through said evaporator, said expander and said condenser, to at least twoother tanks.

15. The method according to claim 14, wherein the flow to said at least two other tanks is controlled to take place sequentially.