Abstract:
Heat engine with at least two cylinder-piston units, each containing an expansion fluid, which stands under a prestressing pressure and which changes its volume in the case of a change of temperature and thus moves the piston, elements for the individually controllable supply of heat to the expansion fluid of each cylinder-piston unit, and a control means controlling the heat supply elements to allow each expansion fluid to alternately heat up and cool down and thus move the pistons, wherein a common prestressing fluid acts on the pistons of all cylinder-piston units in order to exert a common prestressing pressure on the expansion fluids, the control means is fitted with a pressure gauge for the prestressing pressure, and the control means controls the heating and cooling phases of the heat supply elements in dependence on the measured prestressing pressure in order to hold the prestressing pressure within a predetermined range.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a National Phase application of International Application No. PCT/AT2008/000469 filed Dec. 22, 2008 which claims priority to Austrian Patent Application No. A 7/2008, filed Jan. 3, 2008. 
     BACKGROUND OF THE INVENTION 
     The present invention relates to a heat engine, in particular for low-temperature operation for the utilisation of solar heat, waste heat from biological or industrial processes or the like, with at least two cylinder-piston units, each containing an expansion fluid, which stands under prestressing pressure and which changes its volume in the case of a change of temperature and thus moves the piston, elements for the individually controllable supply of heat to the expansion fluid of each cylinder-piston unit, and a control means controlling the heat supply elements to allow each expansion fluid to alternately heat up and cool down and thus move the pistons. 
     Such a heat engine is known from U.S. Pat. No. 5,916,140. Effective expansion fluids frequently require a specific prestressing pressure in order to show a significant coefficient of expansion in the desired operating temperature range. An example of this is liquid carbon dioxide, which changes its volume by about 2.2-fold when heated from 20° C. to 30° C. at a pressure of approx. 60-70 bar. 
     U.S. Pat. No. 5,916,140 discloses different variants to place the expansion fluid in the cylinder-piston units under the required prestressing pressure. On the one hand, metal or gas springs are proposed to prestressing the pistons in the direction of the expansion fluid. However, no prestressing pressure independent of the piston movement can be reached with such a distance-dependent spring force. On the other hand, a mechanical coupling of two cylinder-piston units by means of a crankshaft or by opposed cylinder assembly is described, so that the respectively extending piston maintains the prestressing pressure on the expansion fluid of the retracting piston. However, such a rigid coupling requires that the heating and cooling phases are about equal in length, since otherwise a piston that retracts too slowly will hinder the extending one, which is detrimental to efficiency, or a piston that extends too slowly will generate too little prestressing pressure to assure operation. 
     It is proposed in U.S. Pat. No. 5,916,140 as a solution to the last-mentioned problem to accelerate the cooling phase by removing heat as rapidly as possible, so that it is always shorter than the heating phase. However, this is scarcely realisable in practice, since highly variable heat supply is to be expected especially when using solar heat. Thus, for heating liquid carbon dioxide from 20° C. to 30° C. at midday, for example, a heat supply temperature of 70° C. and thus a temperature difference of 40-50° C. may be available, whereas for cooling from 30° C. to 20° C. there is only a temperature difference of 15-25° C.—even in the case of forced cooling with cold water at 5° C.—, and as a result a cooling phase that is about double the length of the heating phase is to be expected. On the other hand, in the morning and evening hours the temperature level of the solar plant may also only amount to 30°-40° C., for example, and as a result of which a heating phase that is longer than the cooling phase may even be expected. 
     SUMMARY OF THE INVENTION 
     Therefore, the aim set by the invention is to provide a heat engine of the aforementioned type, which always achieves a good efficiency even with highly fluctuating heat supply. This aim is achieved according to the invention in that a common prestressing fluid acts on the pistons of all cylinder-piston units in order to exert a common prestressing pressure on the expansion fluids, the control means is fitted with a pressure gauge for the prestressing pressure, and the control means controls the heating and cooling phases of the heat supply elements in dependence on the measured prestressing pressure in order to hold this within a predetermined range. 
     This enables a variable dynamic coupling of the cylinder-piston units to be achieved. Control of the piston movement dependent on the prestressing pressure prevents a deterioration in the efficiency of the engine as a result of an unnecessarily high prestressing pressure, while always assuring the necessary prestressing pressure for the expansion fluid. As a result, a constantly optimum operation is achieved even under changing ambient conditions. 
     A particularly advantageous embodiment of the heat engine according to the invention has at least three cylinder-piston units, and is distinguished in that the control means increases the number of cylinder-piston units, which are in the heating phase at a point in time, in relation to the number of cylinder-piston units, which are in the cooling phase at the same point in time, if the prestressing pressure drops below the predetermined range, and reduces same if the prestressing pressure exceeds the predetermined range. This allows the operation to be adapted to particularly highly fluctuating ambient conditions. For example, in the low-temperature morning or evening hours of a solar plant an about equal number of cylinder-piston units can be operated in the heating and cooling phases, while in the midday heat few rapidly heating cylinder-piston units are countered by many slowly cooling cylinder-piston units. 
     According to a further feature of the invention, the control means can also reduce or extend each individual heating and/or cooling phase for fine adjustment in order to hold the prestressing pressure within the predetermined range. 
     In principle, any fluid known in the art with an appropriately substantial coefficient of thermal expansion can be used as expansion fluid. It is particularly favourable if—as known from U.S. Pat. No. 5,916,140—the expansion fluid contains liquid carbon dioxide and the prestressing pressure is higher than or equal to the condensation pressure of carbon dioxide at the operating temperature. Because of its high coefficient of thermal expansion at room temperature, liquid carbon dioxide is particularly suitable for operation of the heat engine in the low-temperature range for the utilisation of solar heat, waste heat from biological or industrial processes or the like. Moreover, carbon dioxide formed from combustion processes can thus be fed to a beneficial secondary treatment process, in which it does not have any environmentally unfriendly greenhouse effect. The heat engine according to the invention therefore also makes a contribution to environmentally favourable CO 2  sequestration in the sense of a carbon dioxide capture and storage process (CSS). 
     The prestressing fluid can also be of any desired type, e.g. compressed air. However, it is particularly preferred if the prestressing fluid is hydraulic fluid, which provides a non-positive and reliable pressure coupling. In this case, the hydraulic circuit of the prestressing fluid is preferably fitted with a flexible intermediate reservoir, so that short-term pressure fluctuations during switchover operations or in the case of individual reductions or increases of the heating and cooling phases necessary for control can be temporarily absorbed. 
     The action of the prestressing fluid on the pistons can occur in a wide variety of ways, e.g. by mechanically coupling separate hydraulic prestressing cylinders to the cylinder-piston units. The pistons of the cylinder-piston units are preferably configured as double-action pistons, the expansion fluid acting on one side thereof and the prestressing fluid acting on the other side thereof, which results in a particularly simple structure. 
     The decoupling of the work performed by the cylinder-piston units can also be achieved in any desired manner known in the art, which takes into consideration the generally non-sinusoidal courses and different phase positions of the reciprocating movements of the individual pistons, e.g. by means of free-wheeling hubs, planetary gear trains, ratchet/pawl mechanisms etc. A hydraulic decoupling of the movement work, in which each cylinder-piston unit drives a working piston and all working pistons act on a common working fluid of a hydraulic load, is particularly favourable. 
     A preferred embodiment of the invention is distinguished in that the heat supply elements have a heat exchanger, through which a heat transfer medium flows and which is provided with a shut-off valve controlled by the control means. The time points and durations of the heating phases can be preset by simply opening and closing the shut-off valves, and the cooling phases then result between these. 
     The cooling phases can be accelerated if the heat supply elements preferably also comprise elements for forced cooling of the expansion fluids in the cooling phases. It is particularly favourable for this purpose if the heat transfer medium stands under pressure in the heating phase and the forced cooling elements have a controllable pressure release means for each heat exchanger. As a result, the heat transfer medium can be simultaneously used as coolant by it causing cooling as a result of pressure release. 
     The pressure release means preferably comprises a vacuum intermediate reservoir, which can be switched to the heat exchanger by means of a controllable switch valve, as a result of which a sudden release and therefore a particularly rapid cooling can be achieved. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention shall be explained in more detail below on the basis of exemplary embodiments illustrated in the attached drawings. 
         FIG. 1  is a basic circuit diagram of a heat engine of the invention with four cylinder-piston units; 
         FIGS. 2   a  to  2   c  are time diagrams relating to the control of the heat supply elements and the thus resulting piston movements of the engine of  FIG. 1 ; and 
         FIG. 3  is a circuit diagram of a practical embodiment of a heat engine according to the invention with two exemplary cylinder-piston units. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  shows a heat engine  1  with four cylinder-piston units  2 - 5 . Each cylinder-piston unit  2 - 5  has a cylinder  6 , in which a piston  7  can move between a retracted position (shown at  2 ) and an extended position (shown at  5 ). 
     The space in the cylinder  6  to the left side of the piston  7  is completely occupied by an expansion fluid  8 . The expansion fluid  8  has a high coefficient of thermal expansion and expands during heating in order to move the piston  7  from the retracted position into the extended position, or contracts during cooling to move the piston  7  back again. 
     In the shown example, the expansion fluid  8  is liquid carbon dioxide (CO 2 ), which has a condensation pressure of approx. 65 bar at room temperature. Liquid CO 2  has a thermal expansion of about 2.2-fold in the range from 20° C. to 30° C. Mixtures of liquid carbon dioxide with other substances could also be used as expansion fluid  8  instead of pure carbon dioxide. 
     To keep the CO 2  as expansion fluid  8  in its liquid state, the piston  7  is subjected to or prestressed in the direction of the expansion fluid  8  with a prestressing pressure p v  higher than or equal to the condensation pressure. 
     As shown in  FIG. 1 , the prestressing pressure p v  is exerted by a prestressing fluid  9 , which directly acts on the side of each piston  7  remote from the expansion fluid  8 . The prestressing fluid  9  is preferably a hydraulic oil and circulates in a hydraulic circuit  10  common to all the cylinder-piston units  2 - 5 . The prestressing fluid  9  displaced when a piston  7  extends (arrow  11 ) thus maintains the prestressing pressure p v  on the expansion fluids  8  of the retracting pistons  7  (arrows  12 ). The retraction movement of the pistons  7  is thus supported in the cooling phase and the pressure is prevented from dropping below the condensation pressure in the cooling phase. 
     The hydraulic circuit  10  is fitted with a flexible intermediate reservoir  13 , e.g. a pressure tank with gas filling means  14  and/or with a flexible membrane  15  in order to buffer short-term pressure fluctuations. 
     The heating of the expansion fluids  8  in the cylinder-piston units  2 - 5  is conducted by means of controllable heat supply elements  16 - 20 . In the shown example, the heat supply elements  16 - 19  comprise a heat exchanger  16  for each cylinder-piston unit  2 - 5 , which is in contact with the expansion fluid  8  in a thermally conductive manner and in which a heat transfer medium  17  circulates. The heat transfer medium  17  is heated by a solar panel  18  in a heat transfer circuit  19  (return pipes not shown in  FIG. 1  for reasons of clarity). 
     The heat exchangers  16  can be of any type known in the art. They are preferably fitted with heat pipes to promote the heat exchange and for rapid and uniform distribution of the supplied heat in the expansion fluids  8 . 
     Each heat exchanger  16  is provided with a controllable shut-off valve  20 . The shut-off valves  20  are alternately and intermittently opened by a central control means  21  in order to alternately heat and cool each cylinder-piston unit  2 - 5 , and thus alternately expand and contract the expansion fluids  8  in the cylinders  6  and ultimately move the pistons  7  back and forth, wherein the piston movements are synchronised by means of the prestressing fluid  9  of the hydraulic circuit  10 . 
     The control means  21 , e.g. a microprocessor, operates the shut-off valves  20  in dependence on a measured value of the prestressing pressure p v , which it receives from a pressure gauge  22  connected to the hydraulic circuit  10 . In this case, the control target of the control means  21  is to keep the prestressing pressure p v  in the hydraulic circuit  10  within the predetermined range. This is achieved primarily by controlling the number of the cylinder-piston units  2 - 5  currently in the heating phase at a specific time point in relation to the number of the other cylinder-piston units  2 - 5  that are currently in the cooling phase at this time point, as will now be explained in more detail on the basis of  FIG. 2 . 
     The switching signals e 2 -e 5  of the control means  21  for opening the shut-off valves  20  are respectively recorded in the upper time diagrams of  FIGS. 2   a - 2   c  and the movements or paths s 2 -s 5  of the pistons  7  of the cylinder-piston units  2 - 5  resulting therefrom are plotted in relation to time t in the lower time diagrams. 
       FIG. 2   a  shows a first operating state of the heat engine  1  for ambient conditions, in which the cooling phase of the expansion fluid  8  is about three times as long as the heating phase, e.g. because the temperature of the heat transfer medium  17  is high and causes a rapid heating. The shut-off valves  20  are respectively opened cyclically for about a quarter of the stroke periods. As may be seen, at a specific time point one cylinder-piston unit  2 - 5  is always located in the heating phase and three others are in the cooling phase, i.e. the ratio of expanding cylinder-piston units  2 - 5  to contracting cylinder-piston units  2 - 5  amounts to 1:3 here. 
       FIG. 2   b  shows a second operating state of the heat engine  1 , in which the shut-off valves  20  are respectively opened cyclically for half a stroke period. The ratio of cylinder-piston unit  2 - 5  in the heating phase to cylinder-piston units  2 - 5  in the cooling phase amounts to 2:2 here, which takes into account heating and cooling phases of about equal length, e.g. because of reduced heat supply. 
     If, for example, the temperature of the heat transfer medium  17  decreases even further and the heating phase is thus extended even further, the control means  20  moves into the third operating state of  FIG. 2   c , in which the ratio of cylinder-piston units  2 - 5  in the heating phase to cylinder-piston units  2 - 5  in the cooling phase amounts to 3:1. 
     The respective operating state of  FIG. 2   a ,  FIG. 2   b  or  FIG. 2   c  is set by the control means  21  in dependence on the prestressing pressure p v : if the prestressing pressure p v  drops below a predetermined lower limit p min , in particular the condensation pressure of the expansion fluid  8  at the current operating temperature, the ratio of cylinder-piston units  2 - 5  in the heating phase to cylinder-piston units  2 - 5  in the cooling phase increases successively, e.g. 1:3→2:2→3:1; if the prestressing pressure p v  exceeds a predetermined upper limit p max , in particular the condensation pressure plus a hysteresis threshold, then this ratio decreases successively, e.g. 3:1→2:2→1:3. 
     It is understood that the discussed control can be extended to any desired numbers of cylinder-piston units  2 - 5 , e.g. to 3, 5, 6, 7, 8, 12, 24 etc. cylinder-piston units. The more cylinder-piston units there are available, the more finely stepped the control can be. 
     For the fine control, the control means  21  can additionally reduce or extend each individual heating or cooling phase, e.g. by shifting the beginning t 1  of a heating phase and/or the beginning t 2  of a cooling phase or by changing the duration t 2 −t 1 . If heating or cooling phases of different cylinder-piston units  2 - 5  overlap one another briefly in this case to a higher or lower ratio than that selected by means of the primary control (1:3, 2:2, 3:1), corresponding short-term pressure fluctuations of the prestressing pressure p v  can be temporarily absorbed by means of the intermediate reservoir  13  in the hydraulic circuit  10 . 
     It must be mentioned at this point that in a greatly simplified embodiment of the heat engine  1 , which only comprises two cylinder-piston units and thus only allows the single ratio 1:1, the control means  21  can also only perform the last-mentioned control with corresponding restriction with respect to the usable operating conditions. 
       FIG. 3  shows a specific configuration and further development of the heat engine  1  of  FIG. 1 , wherein for reasons of clarity only two cylinder-piston units  2 ,  3  are shown as representative and the control means  21  with its measurement and control lines is not shown. However, it is understood that the embodiment shown in  FIG. 3  can be extended to any desired number of cylinder-piston units. 
     According to  FIG. 3  a pump  23  pumps heat transfer medium  17 , e.g. refrigerant R 123 from Hoechst, from a reservoir  24  via a pipe  25  to the solar panel  18 , from there via pipe  19  and shut-off valves  20  to the heat exchangers  16  and from there back to the reservoir  24  via switch valves  26  and a return pipe  27 . In the operating state shown in  FIG. 3  the right shut-off valve  20  is currently open and the left shut-off valve  20  is closed, so that the right cylinder-piston unit  3  is in the heating and expansion phase and the left cylinder-piston unit  2  is in the cooling and contraction phase. 
     For acceleration of the cooling phases the heat supply elements  16 - 20  also comprise elements for forced cooling of the expansion fluids  8  here. The forced cooling elements can be an optional feed path  28  for non-heated heat transfer medium  17 , for example, in order to feed this into the heat exchanger  16  in the cooling phases via shut-off valves  20  configured as multiple-way valves. Alternatively, separate heat exchangers could be used for a separate cooling medium (not shown). 
     The forced cooling elements preferably comprise a controllable pressure release means, as shown, which after the shut-off valve  20  closes relieves the heat transfer medium  17  that is still under the transport pressure of the pump  23  in a heat exchanger  16  via the shut-off valve  26  to a vacuum intermediate reservoir  29 . The vacuum in the vacuum intermediate reservoir  29  is created via a suction pipe  30  of a venturi ejector  31 , which is continuously fed with heat transfer medium  17  in the circuit by the pump  23  via a pipe  32 . As a result of the sudden expansion of the heat transfer medium  17  after the shut-off valve  26  is opened, the heat transfer medium  17  evaporates and thus cools the expansion fluid  8  via the heat exchanger  16 . 
     In the embodiment of  FIG. 3 , the flexible intermediate reservoir  13  of the hydraulic circuit  10  can be selectively switched to the hydraulic circuit  10  via its own shut-off valve  33 . The power of the cylinder-piston units  2 ,  3  is mechanically transferred via piston rods  34  to working pistons  35 , which act on a common working fluid  36 , e.g. hydraulic oil, that circulates in a hydraulic load circuit  37  via check valves  38 . 
     The working pistons  35  can be integrated into the cylinders  6  of the cylinder-piston units  2 ,  3 , so that these have three operating zones: a reaction zone  39 , in which the expansion fluids  8  operate, a prestressing zone  40 , in which they are coupled via the prestressing fluids  9 , and a working zone  41 , in which the movement is decoupled via the working fluid  36 . The working pressure of the working fluid  36  substantially corresponds to the thermal expansion-related reaction pressure of the expansion fluid  8  minus the prestressing pressure p v  of the prestressing fluid  9 . 
     The invention is not restricted to the represented embodiments, but covers all variants and modifications that fall within the framework of the attached claims. Thus, for example, a larger number of cylinder-piston units could also be actuated synchronously group by group in several groups to reduce the switching and control expenditure. In this case, the cylinders  6  of a synchronous group of cylinder-piston units could also share a common heat exchanger  16  and/or a common expansion fluid  8 .