Patent Publication Number: US-2018045077-A1

Title: Energy conversion system and method

Description:
FIELD OF THE INVENTION 
     The present invention relates to an energy conversion system and to a method of controlling such an energy conversion system. 
     BACKGROUND OF THE INVENTION 
     It is known to convert thermal energy to mechanical energy by means of 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 of liquid state working fluid before the working fluid is evaporated by evaporator  102 , to vapor state working fluid. The evaporated vapor state working fluid is provided to an expander  103 , such as a turbine, whereby thermal energy stored as pressure is converted to mechanical energy. The expanded vapor state working fluid is cooled and condensed, by condenser  104 , to liquid state working fluid. Following condensation in the condenser  104 , the liquid state working fluid flows to a buffer tank  105 , and from the buffer tank, the liquid state working fluid is again supplied to the pump  101 . 
     In high temperature applications, the (electrical) energy consumption of the pump  101  used for increasing pressure of the liquid state working fluid is often considered to be negligible, due to the high efficiency of the Rankine cycle for such applications. 
     For lower temperature applications, in which a so-called organic Rankine cycle, or ORC cycle, is sometimes used, the efficiency is typically lower, which means that the (electrical) energy consumption of the pump  101  may be significant in relation to the output power from the expander  103 . 
     It would be desirable to provide for more efficient conversion of thermal energy into mechanical energy, and in particular to provide an energy conversion system based on the Rankine cycle allowing the use of less electrical energy for increasing the pressure of liquid state working fluid provided to the evaporator. 
     SUMMARY 
     In view of above-mentioned and other drawbacks of the prior art, it is an object of the present invention to provide an improved energy conversion system and method. 
     According to a first aspect of the present invention, it is therefore provided an energy conversion system for converting thermal energy to mechanical energy, comprising: an evaporator for evaporating liquid state working fluid to vapor state working fluid through supply of heat, the evaporator being arranged to receive liquid state working fluid and output vapor state working fluid at a first pressure; an expander for expanding vapor state working fluid and converting expansion into mechanical energy, the expander having an expander inlet connected to the evaporator for receiving vapor state working fluid at the first pressure and an expander outlet for output of vapor state working fluid at a second pressure lower than the first pressure; a condenser for condensing vapor state working fluid to liquid state working fluid by cooling, the condenser having a condenser inlet connected to the expander outlet for receiving vapor state working fluid and a condenser outlet for output of liquid state working fluid; a first tank having a first inlet fluid flow connected to the condenser outlet, a second inlet fluid flow connected to the evaporator for receiving vapor state working fluid from the evaporator, and an outlet fluid flow connected to the evaporator for providing liquid state working fluid to the evaporator; a second tank having a first inlet fluid flow connected to the condenser outlet, a second inlet fluid flow connected to the evaporator for receiving vapor state working fluid from the evaporator, and an outlet fluid flow connected to the evaporator for providing liquid state working fluid to the evaporator; a first flow control device for controlling flow of working fluid from the outlet of the first tank to the evaporator; a second flow control device for controlling flow of working fluid from the condenser outlet to the first inlet of the first tank; a third flow control device for controlling flow of working fluid from the evaporator to the second inlet of the first tank; a fourth flow control device for controlling flow of working fluid from the outlet of the second tank to the evaporator; a fifth flow control device for controlling flow of the working fluid from the condenser outlet to the inlet of the second tank; a sixth flow control device for controlling flow of working fluid from the evaporator to the second inlet of the second tank; and a control unit connected to each of the flow control devices, for controlling operation of the energy conversion system. The condenser is arranged at a higher vertical level than each of the first tank and the second tank; and the evaporator is at least partly arranged at a lower vertical level than each of the first tank and the second tank. 
     The evaporator may be any device capable of evaporating the working fluid (transitioning the working fluid from liquid state working fluid to vapor state working fluid). For example, the evaporator may be a part of or be connected to a solar heating system, a combustion-based heating system, or a heat accumulator etc. 
     The expander may be any device capable of expanding vapor state working fluid and converting the expansion of the vapor state working fluid to mechanical energy. The expander may, for instance, comprise a turbine. 
     It should be noted that the first and second inlets of the first tank may be provided as a common inlet of the first tank and/or that the first and second inlets of the second tank may be provided as a common inlet of the second tank. 
     The control unit may advantageously comprise processing circuitry which may include at least one microprocessor and a memory. The memory may contain a set of instructions for the microprocessor, and the microprocessor may control the different flow control devices in the energy conversion system based on the set of instructions. In particular, the set of instructions may specify a scheme of sequentially controlling the flow control devices to allow or restrict flow of working fluid past the respective flow control device. In this manner, the energy conversion system may be transitioned between operational states when various state transition conditions are fulfilled. 
     The present invention is based upon the realization that a more energy efficient conversion, based on the Rankine cycle, of thermal energy to mechanical energy can be achieved by using vaporized working fluid for pressurizing liquid state working fluid supplied to the evaporator. It has further been realized that vaporized working fluid can conveniently be used for pressurizing the liquid state working fluid supplied to the evaporator by providing two tanks, where one of the tanks supplies the pressurized liquid state working fluid, and the other tank receives liquid state working fluid following evaporation, expansion and condensation. To keep the energy conversion process going, the function of the tanks may be alternated, so that the working fluid originating from the first tank is provided to the second tank (following evaporation, expansion and condensation) until a predetermined condition has been fulfilled, after which working fluid from the second tank is provided to the first tank (following evaporation, expansion and condensation). 
     In embodiments of the energy conversion system according to the present invention, it may not be necessary to use a pump to keep the energy conversion process going. It may not even be necessary to use a pump to get the energy conversion process started, but the energy conversion system according to embodiments of the present invention may be configured to automatically start when heat is supplied to the working fluid via the evaporator. This ability to start energy production without supply of electrical energy is often referred to as a “black start”. 
     To provide for efficient flow of working fluid in the energy conversion system according to embodiments of the present invention, the condenser may advantageously be arranged at a higher vertical level than the first and second tanks. Moreover, the expander may advantageously be arranged at a higher vertical level than the condenser. 
     Furthermore, the first and second tanks may advantageously be arranged at at least approximately the same vertical positions. 
     In each tank, the first and second inlets (or the common inlet forming the first and second inlets) may be arranged at a higher vertical level than the outlet. 
     The energy conversion system according to embodiments of the present 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 first configuration of ‘open’ and ‘closed’ during a predetermined period of time before transitioning the flow control devices to a second configuration of ‘open’ and ‘closed’. 
     To improve the efficiency and adaptability of the energy conversion systems to, for example, variations in the supply of thermal energy from the evaporator, the energy conversion system may, according to various embodiments, further comprise at least one state sensor for sensing a present state of the energy conversion system, and the control unit may additionally be connected to the at least one state sensor and configured to control the flow control devices based on a signal from the at least one state sensor. 
     The at least one state sensor may be configured to sense at least one process-related parameter of the energy conversion system, such as one or several of pressure, temperature and liquid state working fluid level. 
     Advantageously at least one of the working fluid pressure, temperature or (interface) level may be sensed in each of the first and second tanks. This allows the control unit to transition the energy conversion system between operational states based on, for instance, the pressure or liquid level (interface between liquid state and vapor state working fluid) in the respective tanks. 
     According to various embodiments, the control unit may be configured to alternate the energy conversion system between a first operational state in which each of the first, third and fifth flow control devices is controlled to allow flow of working fluid past the respective flow control devices; and each of the second, fourth and sixth flow control devices is controlled to prevent flow of working fluid past the respective flow control devices; and a second operational state in which each of the second, fourth and sixth flow control devices is controlled to allow flow of working fluid past the respective flow control devices; and each of the first, third and fifth flow control devices is controlled to prevent flow of working fluid past the respective flow control devices. 
     Hereby, working fluid will alternatingly follow a first flow path from the first tank and sequentially through the evaporator, the expander, and the condenser to the second tank, and a second flow path from the second tank and sequentially through the evaporator, the expander, and the condenser to the first tank. When following the first flow path, some of the vapor state working fluid leaving the evaporator is used for pressurizing the first tank, and when following the second flow path, some of the vapor state working fluid leaving the evaporator is used for pressurizing the second tank. 
     As was discussed further above, the control unit may transition the energy conversion system from an operational state when a respective predetermined condition is fulfilled. Such a predetermined condition may advantageously be selected in such a way that the control unit keeps the energy conversion system in the above-mentioned first operational state until the first tank substantially only contains vapor state working fluid, and keeps the energy conversion system in the second operational state until the second tank substantially only contains vapor state working fluid. 
     After having supplied liquid state working fluid from the first or second tank, that tank may contain vapor state working fluid at an elevated pressure. For an efficient energy conversion process, it may be desirable to reduce the pressure in the ‘emptied’ tank (the tank having supplied liquid state working fluid to the evaporator). 
     For efficient use of the thermal energy stored in the emptied tank, the energy conversion system may advantageously further comprise a pressure equalization conduit directly connecting the first tank and the second tank; and a seventh flow control device for controlling flow of working fluid directly between the first tank and the second tank. 
     Hereby, the stored thermal energy in the ‘emptied’ tank may be used for preheating the (liquid state) working fluid and increasing pressure in the other tank. 
     To provide for efficient transfer of heat from the remaining working fluid in 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 bottom portion of the second tank. Hereby, efficient heat transfer between the hot vapor state working fluid from the ‘emptied’ tank and the liquid state working fluid in the other tank can be achieved. 
     In embodiments, the evaporator may comprise: a first evaporator unit fluid flow connected to the expander inlet to provide vapor state working fluid to the expander; and a second evaporator unit fluid flow connected to the second inlet of the first tank and to the second inlet of the second tank. 
     Through the provision of separate evaporator units for feeding the expander (the first evaporator unit) and for pushing liquid phase working fluid out of the first and second tanks (the second evaporator unit), the efficiency of the energy conversion system can be improved even further. 
     At least the second evaporator unit may advantageously be arranged at a lower vertical level than the first and second tanks. In particular, at least an inlet of the second evaporator unit may be arranged at a lower vertical level than the respective outlets of the first and second tanks to facilitate flow of liquid state working fluid from the tanks to the second evaporator unit. 
     According to various embodiments, the energy conversion system of the present invention may advantageously comprise at least one additional tank connected to the evaporator and condenser in the same way as the above-mentioned first and second tanks are fluid flow connected to the evaporator and condenser. Through the provision of one or several additional tanks, variations in the output of mechanical energy (or electrical energy converted from the mechanical energy) can be reduced. By providing at least four tanks, such as two sets of the above-described first and second tanks, embodiments of the energy conversion system may be operated practically continuously. 
     According to a second aspect of the present invention, there is provided a method of controlling an energy conversion system according to embodiments of the first aspect of the present invention. The method comprises the steps of: (a) controlling the flow control devices to allow flow of working fluid from the outlet of the first tank, through the evaporator, the expander and the condenser to the first inlet of the second tank, while allowing flow of vapor state working fluid from the evaporator into the second inlet 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 of working fluid from the outlet of the second tank, through the evaporator, the expander and the condenser to the first inlet of the first tank, while allowing flow of vapor state working fluid from the evaporator into the second inlet of the second tank; and (d) releasing vapor phase working fluid (under pressure) from the second tank. 
     In step (b) and step (d), respectively, the vapor phase working fluid may be released (allowed to flow out of the tank to reduce pressure in the tank) through any suitable conduit. For instance, the pressurized vapor phase working fluid may be allowed to flow back towards the condenser. Alternatively or in combination, at least some of the pressurized vapor phase working fluid may be passed to an auxiliary expander to be converted to mechanical energy. 
     To provide for an efficient energy conversion, step (b) may comprise releasing vapor phase working fluid from the first tank to the second tank; and step (d) may comprise releasing vapor phase working fluid from the second tank to the first tank. 
     As was mentioned further above in connection with embodiments of the first aspect of the present invention, continuous energy conversion can be provided for using an energy conversion system further comprising a third tank and a fourth tank, by adding the following steps: (e) controlling the flow control 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 of the fourth tank, while allowing flow of vapor state working fluid from the evaporator into the second inlet of the third tank; (f) releasing vapor phase working fluid (under pressure) from the third tank; (g) controlling the flow control 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 of the third tank, while allowing flow of vapor state working fluid from the evaporator into the second inlet of the fourth tank; and (h) releasing vapor phase working fluid (under pressure) from the fourth tank. 
     To provide for further improvements in the uniformity over time of the mechanical (or electrical) energy output from the energy conversion system according to embodiments of the present invention, the different tanks may be emptied, filled and pressure equalized asynchronously. In particular, the flow control devices may be controlled to allow flow of working fluid from each of the tanks from a substantially full state to a substantially empty state of the tank, through the evaporator, the expander and the condenser, to at least two other 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’. 
     In the case of an energy conversion system having four tanks, the amount of working fluid in the energy conversion system may be adapted so that the total liquid volume of working fluid in the tanks is, at any time, about 1.5 times the volume of one of the tanks. 
     In summary, according to various embodiments 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. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other aspects of the present invention will now be described in more detail, with reference to the appended drawings showing an example embodiment of the invention, wherein: 
         FIG. 1  schematically shows a Rankine cycle based energy conversion system according to the prior art; 
         FIG. 2 a    schematically shows an energy conversion system according to a first embodiment of the present invention; 
         FIG. 2 b    schematically shows an energy conversion system according to a second embodiment of the present invention; 
         FIGS. 3 a - d    schematically illustrate different operational states of the energy conversion system in  FIG. 2 ; and 
         FIG. 4  schematically shows an energy conversion system according to a third embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS 
     In the present detailed description, various embodiments of the apparatus and method according to the present invention are mainly described with reference to energy conversion systems comprising two or four tanks. Furthermore, pressure sensors and level sensors are shown to sense the pressure and level in each tank. 
     It should be noted that this by no means limits the scope of the present invention, which equally well includes, for example, energy conversion systems comprising another number of tanks. Furthermore, the energy conversion process may be controlled using parameters other than sensed pressure and level in the tanks. For instance, the energy conversion may be controlled using preset time durations for each operational state, or other process parameters may be sensed, such as the temperature and/or the energy output by the expander or a generator which may be connected to the expander. 
       FIG. 2 a    schematically illustrates an energy conversion system  1  according to a first example embodiment of the present invention, and  FIG. 2 b    schematically illustrates an energy conversion system  1  according to a second example embodiment of the present invention. 
     Referring first to  FIG. 2 a   , the energy conversion system  1  comprises an evaporator  2 , a primary expander  3 , a condenser  4 , a first tank  5 , a second tank  6 , an auxiliary expander  7 , and a control unit  8  for controlling operation of the energy conversion system  1 . As is indicated in  FIG. 2 , the energy conversion system further comprises a first pressure sensor  9  for sensing the pressure in the first tank  5 , a first level sensor  10  for sensing the liquid level in the first tank  5 , a second 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  14  and an evaporator outlet  15 , the primary expander  3  has an expander inlet  16  and an expander outlet  17 , the condenser  4  has a condenser inlet  18  and a condenser outlet  19 , the first tank  5  has an inlet  20  and an outlet  21 , the second tank  6  has and inlet  22  and an outlet  23 , and the auxiliary expander  7  has an auxiliary expander inlet  24  and an auxiliary expander outlet  25 . 
     The various parts of the energy conversion system  1  in  FIG. 2  are fluid flow connected by conduits schematically indicated as solid lines in  FIG. 2 , and control lines (which may be wired or wireless) are indicated by dashed lines. Flow of working fluid through the conduits can be controlled using flow controlled devices, here in the form of controllable valves  30   a - i.    
     An example of a suitable working fluid is the Genetron® R-245fa from Honeywell. The skilled person will realize that this is merely an example, and that there is a large number of commercially available working fluids that may be suitable for different embodiments depending on various factors, such as the thermal power provided by the evaporator etc. 
     The second embodiment of the energy conversion system  1  in  FIG. 2 b    differs from the first embodiment described above with reference to  FIG. 2 a    in that the evaporator  2  is shown to include a first evaporator unit  2   a  and a second evaporator unit  2   b . As is schematically shown in  FIG. 2 b   , the first evaporator unit  2   a  is arranged to receive liquid phase working fluid from the first tank  5  and the second tank  6 , and to provide vapor phase working fluid to the expander  3 , and the second evaporator unit  2   b  is arranged to receive liquid phase working fluid from the first tank  5  and the second tank  6 , and to provide vapor phase working fluid to second inlets  20 ,  22  of the first tank  5  and the second tank  6 , respectively. The second evaporator unit  2   b  is arranged at a lower vertical level than the outlet  21  of the first tank  5  and the outlet  23  of the second tank  6 . As is schematically indicated in  FIG. 2 b   , the second evaporator unit  2   b  may comprise an evaporator container and a heater to evaporate working fluid in the evaporator container. As is also schematically shown, liquid phase working fluid enters the evaporator container at a relatively low vertical level, and vapor phase working fluid exits the evaporator container at a relatively high vertical level. The heat supplied to the second evaporator unit may emanate from any suitable heat source, such as steam, thermal oil, or electricity. The first  2   a  and second  2   b  evaporator units may advantageously be supplied with heat from the same heat source. 
     By controlling the states (‘open’ or ‘closed’) of the valves  30   a - i , the control unit  8  can control the energy conversion system to different operational states for achieving a sustained conversion of thermal energy, supplied to the working fluid circulating through the conduits of the energy conversion system  1  by the evaporator  2 , to mechanical energy provided by the expander  3 . 
     This will now be illustrated for the relatively simple first embodiment of the energy conversion system  1  of  FIG. 2 a    with reference to  FIGS. 3 a - d   . Corresponding control is equally applicable to the second embodiment of the energy conversion system  1  illustrated by  FIG. 2   b.    
     To more clearly illustrate the different operational states of the energy conversion system  1  in  FIG. 2 a   , the schematic illustrations in  FIGS. 3 a - d    only include parts of the energy conversion system  1  that are ‘active’ in the particular operational state. 
     Referring first to  FIG. 3 a   , this schematic illustration only includes the flow path from the outlet  21  of the first tank  5  sequentially via the evaporator  2 , the primary 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. 3 a   ) controls valves  30   a ,  30   c  and  30   g  to ‘open’. In the operational state schematically shown in  FIG. 3 a   , the remaining valves in  FIG. 2 a    are ‘closed’. 
     With reference to  FIG. 3 a   , liquid state working fluid flows from the outlet  21  of the first tank  5  to the inlet  14  of the evaporator  2 . In the evaporator  2 , thermal energy is supplied to the liquid state working fluid, which evaporates to vapor state working fluid. The vapor state working fluid is provided from the evaporator outlet  15  to the expander inlet  16  and to the inlet  20  of the first tank  5 . The vapor state working fluid provided to the inlet  20  of the first tank  5  increases the pressure of the liquid state working fluid supplied from the outlet  21  of the first tank  5  to the evaporator inlet  14 . The vapor state working fluid supplied to the expander inlet  16  is expanded and the expansion converted by the expander  3  to mechanical energy. In the exemplary case when the expander  3  is provided in the form of a turbine, the expansion is converted to rotation, which can be used to drive a generator. After leaving the expander  3  through the expander outlet  17 , the expanded vapor state working fluid is condensed to liquid state working fluid in the condenser  4 , before being supplied to the second tank  6  through the inlet  22  of the second tank  6 . 
       FIG. 3 a    schematically shows the beginning of the illustrated operational state, with the first tank  5  being almost filled with liquid state working fluid and the second tank  6  being almost empty (or rather the level of liquid state working fluid being low). As the illustrated operational state is maintained, the liquid level in the first tank  5  will decrease, while the liquid level in the second tank  6  increases. When the first tank  5  is ‘empty’ or almost ‘empty’ (only contains vapor state working fluid), the control unit  8  controls the energy conversion system  1  to a new operational state for pressure equalization. It may be determined by the control unit  8  that the first tank  5  is ‘empty’ or almost ‘empty’ based on signals provided by one or both of the first pressure sensor  9  and the first level sensor  10 . 
     Referring now to  FIG. 3 b   , this schematic illustration only includes the flow path 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. 3 b   ) controls valve  30   d  to ‘open’. In the operational state schematically shown in  FIG. 3 b   , the remaining valves in  FIG. 2 a    are ‘closed’. As is shown in  FIG. 3 b   , the first tank  5  now has a low liquid level (is ‘empty’), while the second tank  6  has a high liquid level (is ‘full’). 
     With reference to  FIG. 3 b   , hot vapor state working fluid at high pressure, such as about 20 bar, is released to the second tank  6  through the outlet  23  of the second tank  6 . The hot vapor state working fluid bubbles through the relatively cool liquid state working fluid in the second tank  6 , heats the liquid state working fluid in the second tank  6 , partly condenses and increases the pressure in the second tank  6 . The transfer of vapor state working fluid from the first tank  5  to the second tank  6  stops when the pressure is equalized in the system formed by the first  5  and second  6  tanks. The pressure in the second tank  6  (and in the first tank  5 ) may then be about 5 bar, and the liquid state working fluid stored in the second tank has been preheated. 
     Turning to  FIG. 3 c   , this schematic illustration only includes the flow path from the outlet  23  of the second tank  6  sequentially via the evaporator  2 , the primary expander  3 , and the condenser  4  to the inlet  20  of the first tank  5 . To define this flow path, the control unit  8  (not shown in  FIG. 3 c   ) controls valves  30   f ,  30   b  and  30   h  to ‘open’. In the operational state schematically shown in  FIG. 3 c   , the remaining valves in  FIG. 2 a    are ‘closed’. 
     With reference to  FIG. 3 c   , liquid state working fluid flows from the outlet  23  of the second tank  6  to the inlet  14  of the evaporator  2 . In the evaporator  2 , thermal energy is supplied to the liquid state working fluid, which evaporates to vapor state working fluid. The vapor state working fluid is provided from the evaporator outlet  15  to the expander inlet  16  and to the inlet  22  of the second tank  6 . The vapor state working fluid provided to the inlet  22  of the second tank  6  increases the pressure of the liquid state working fluid supplied from the outlet  23  of the second tank  6  to the evaporator inlet  14 . The vapor state working fluid supplied to the expander inlet  16  is expanded and the expansion converted by the expander  3  to mechanical energy. In the exemplary case when the expander  3  is provided in the form of a turbine, the expansion is converted to rotation, which can be used to drive a generator. After leaving the expander  3  through the expander outlet  17 , the expanded vapor state working fluid is condensed to liquid state working fluid in the condenser  4 , before being supplied to the first tank  5  through the inlet  20  of the first tank  5 . 
       FIG. 3 c    schematically shows the beginning of the illustrated operational state, with the second tank  6  being almost filled with liquid state working fluid and the first tank  5  being almost empty (or rather the level of liquid state working fluid being low). As the illustrated operational state is maintained, the liquid level in the second tank  6  will decrease, while the liquid level in the first tank  5  increases. When the second tank  6  is ‘empty’ or almost ‘empty’ (only contains vapor state working fluid), the control unit  8  controls the energy conversion system  1  to a new operational state for pressure equalization. It may be determined by the control unit  8  that the second tank  6  is ‘empty’ or almost ‘empty’ based on signals provided by one or both of the second pressure sensor  11  and the second level sensor  12 . 
     The final pressure equalization operational state before the energy conversion system  1  is again back to the initial configuration shown in  FIG. 3 a   , is shown in  FIG. 3 d   . As can be seen by comparing  FIG. 3 b    and  FIG. 3 d   , the configuration is the same. The only difference is that the direction of fluid flow between the first  5  and second  6  tanks is now from the second tank  6  to the first tank  5 . 
     A full ‘main’ energy conversion cycle of the energy conversion system  1  in  FIG. 2 a    has now been described with reference to  FIGS. 3 a - d   . To simplify the description and make the explanation clearer, there has so far been no reference to the auxiliary expander  7  in  FIG. 2 a   . The purpose of the auxiliary expander  7  is to use the thermal energy remaining in an ‘empty’ tank following the pressure equalization process described above with reference to  FIG. 3 b    and  FIG. 3 d   . Considering, for example, the state of the energy conversion system  1  following the pressure equalization process described with reference to  FIG. 3 b   . The control unit  8  may then control valves  30   a ,  30   b , and  30   c  to their ‘closed’ states, and control valve  30   e  to its open state. Vapor state working fluid remaining in the first tank  5  can then be expanded by the auxiliary expander  7 . Hereby, the pressure in the first tank  5  can be reduced further, and more energy can be converted to mechanical energy. 
     As can readily be understood from the above process description, the energy conversion device  1  in  FIG. 2 a    (or  FIG. 2 b   ) will not supply mechanical energy (or electrical energy converted from the mechanical energy) continuously due to the pressure equalization processes described with reference to  FIG. 3 b    and  FIG. 3 d   . By adding further tanks, such as one more set of two tanks, a pressure equalization between two tanks in a first set of two tanks can be timed to take place while liquid state working fluid stored in a first tank in a second set of two tanks is used for energy conversion as described above with reference to  FIG. 3 a    and  FIG. 3   c.    
     A third embodiment of the energy conversion system according to the present invention is schematically shown in  FIG. 4 . The energy conversion system  50  in  FIG. 4  differs from the embodiments of the energy conversion system  1  described above with reference to  FIGS. 2 a - b    and  FIGS. 3 a - d    in that a second set of two tanks  51  and  52  has been added. These added tanks  51  and  52  are connected to each other as well as to the other parts of the energy conversion system  50  (the evaporator  2 , the primary expander  3 , the condenser, and the auxiliary expander  7  using controllable valves in exactly the same way as was described above with reference to  FIG. 2 . To avoid cluttering the drawing and to dispense with an unnecessarily lengthy description, pressure and level sensors, inlets and outlets and controllable valves associated with the added tanks  51  and  52  have been omitted from the drawing. 
     Furthermore, the control unit  8  is configured to control these controllable valves associated with the added tanks  51  and  52  in the same way as the controllable valves  30   a - i  associated with the first  5  and second  6  tanks were controlled to transition the energy conversion system  1  in  FIGS. 2 a - b    between operational states. 
     Example 
     According to an example embodiment of the inventive method, the energy conversion system according to the above-described third embodiment may be controlled asynchronously to provide for a uniform output of mechanical (or electrical) energy from the energy conversion system. 
     
       
         
           
               
               
               
               
               
             
               
                   
               
               
                 Time 
                 Tank 1 
                 Tank 2 
                 Tank 3 
                 Tank 4 
               
               
                   
               
             
            
               
                  5 
                 Full, outlet open 
                 Empty, 2:nd expander 
                 Half full, filling 
                 Empty, outlet open 
               
               
                 10 
                 ¾, outlet 
                 Empty, condensing 
                 ¾, filling 
                 Empty, outlet closed 
               
               
                 15 
                 Half full, outlet 
                 Empty, filling 
                 ⅞, preheated 
                 Empty, preheating 
               
               
                 20 
                 ¼, outlet 
                 ¼, filling 
                 Full, preheated 
                 Empty, preheating 
               
               
                 25 
                 Empty, outlet open 
                 Half full, filling 
                 Full, outlet open 
                 Empty, 2:nd expander 
               
               
                 30 
                 Empty, outlet closed 
                 ¾, filling 
                 ¾, outlet 
                 Empty, condensing 
               
               
                 35 
                 Empty, preheating 
                 ⅞, preheated 
                 Half full, outlet 
                 Empty, filling 
               
               
                 40 
                 Empty, preheating 
                 Full, preheated 
                 ¼, outlet 
                 ¼, filling 
               
               
                 45 
                 Empty, 2:nd 
                 Full, outlet open 
                 Empty, outlet open 
                 Half full, filling 
               
               
                   
                 expander 
               
               
                 50 
                 Empty, condensing 
                 ¾, outlet 
                 Empty, outlet closed 
                 ¾, filling 
               
               
                 55 
                 Empty, filling 
                 Half full, outlet 
                 Empty, preheating 
                 ⅞, preheated 
               
               
                 60 
                 ¼, filling 
                 ¼, outlet 
                 Empty, preheating 
                 Full, preheated 
               
               
                 65 
                 Half full, filling 
                 Empty, outlet open 
                 Empty, 2:nd expander 
                 Full, outlet open 
               
               
                 70 
                 ¾, filling 
                 Empty, outlet closed 
                 Empty, condensing 
                 ¾, outlet 
               
               
                 75 
                 ⅞, preheated 
                 Empty, preheating 
                 Empty, filling 
                 Half full, outlet 
               
               
                 80 
                 Full, preheated 
                 Empty, preheating 
                 ¼, filling 
                 ¼, outlet 
               
               
                   
               
            
           
         
       
     
     In the table above, the tanks are denoted by numbers from left to right in  FIG. 4 . Furthermore, the transition between operational states of the energy conversion system is determined by a predetermined duration of the different steps. When a tank is referred to as being ‘empty’, this means that the tank only contains vapor phase working fluid. A tank that is ‘preheating’ is connected to a tank that is ‘preheated’, and pressure equalization as described above with reference to  FIG. 3 b    and  FIG. 3 d    takes place. 
     The person skilled in the art realizes that the present invention by no means is limited to the preferred embodiments described above. On the contrary, many modifications and variations are possible within the scope of the appended claims. For example, many other operational sequences of emptying, filling, and pressure equalizing the tanks are possible and may be beneficial depending on application and configuration of the energy conversion system. 
     In the claims, the word “comprising” does not exclude other elements or steps, and the indefinite article “a” or “an” does not exclude a plurality. A single processor or other unit may fulfill the functions of several items recited in the claims. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope.