Abstract:
A device for conversion of thermodynamic energy into electrical energy includes a piston/cylinder unit ( 16 ), a generator ( 18 ), and a controller ( 14 ). The piston/cylinder unit ( 16 ) includes a pressure cylinder ( 24 ) and a piston ( 26 ) arranged in the pressure cylinder ( 24 ) and linearly movable by a change in volume of a working medium. The generator ( 18 ) includes a coil ( 22 ) and a magnet ( 20 ). The magnet ( 20 ) is coupled to the piston ( 26 ) such that a linear movement of the piston ( 26 ) effects a linear movement of the magnet ( 20 ) within the coil ( 22 ). The controller ( 14 ) controls the working stroke of the device as a function of at least one measured process parameter.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is the United States National Stage of Patent Cooperation Treaty Application No. PCT/EP2007/010368 filed on Nov. 29, 2007, which claims priority to German Patent Application 10-2005-056-349.2 filed on Nov. 29, 2006, the disclosures of which are incorporated herein by reference in their entireties. 
     BACKGROUND OF THE INVENTION 
     The present general inventive concept relates to a device for conversion of thermodynamic energy into electrical energy. 
     There is a large variety of ways of generating electric power from mechanical energy of motion involving different efficiencies. In most cases, generators are used which transform a rotary motion. However, such periodically operating machines achieve an acceptable efficiency only as of a specific minimum frequency (rotational speed). When the energy supply is not constant, the conversion of energy is inefficient. A decrease in the supply of energy results in lower rotational speeds which are outside of the optimum operating range of the machine. This is often compensated for by transmissions, which, however, results in a reduction of the overall efficiency and in higher costs. Switching the machine off and on again does not satisfactorily solve this problem because of the associated losses. 
     A totally different approach for the generation of power from energy of motion is found in the publication: H. Polinder, M. E. C. Damen, F. Gardner, “Design, modeling and test results of the AWS PM linear generator”; in European Transactions on Electrical Power, 2005 (vol. 15), pp. 245-256. In this concept, the pressure originating from ocean waves is converted into mechanical energy of motion and transformed into electric power with the aid of a linear generator. But in this case too, the efficiency is most likely subject to strong fluctuations in particular in the conversion of pressure into motion, due to the non-constant supply of energy. The working cycle of the device is imposed by the natural wave formation and wave motion and can not be varied. In addition, the high demands on the dimensional relationships of the pressure cylinder used and of the linear generator as caused by the dynamic wave motions are assumed to result in a lower total energy transformation efficiency. 
     A further problem resides in that irregular voltage pulses generated by a linear generator (individual sporadic pulses of different amplitudes and durations) are not suitable for feeding into an electric power grid. Until now, this problem has been solved by first converting the voltage pulses into a direct voltage. For feeding into an electric power grid, this direct voltage is converted into a suitable alternating voltage by a grid-synchronous inverter. These measures, however, turn out to be very involved and are difficult to put to practice on a large scale. In addition, this type of conversion has an adverse effect on the efficiency. An alternative solution provides for the generation of rotational energy from the voltage pulses. A subsequent transformation and inversion of the output voltage results in an alternating voltage suitable for feeding. In this case too, the relatively high expenditure and the inevitable losses are disadvantageous. 
     SUMMARY OF THE INVENTION 
     It is the object of the present general inventive concept to provide a low cost device for generating electric power which is as simply structured as possible and which operates exclusively at a high efficiency. 
     To achieve this object, a device is proposed which has a piston/cylinder unit which includes a pressure cylinder and a piston that is arranged in the pressure cylinder and is linearly movable by a change in volume of a working medium, a generator which includes a coil and a magnet, the magnet or the coil being coupled to the piston such that a linear movement of the piston effects a linear movement of the magnet relative to the coil, and a controller which controls a working stroke of the device as a function of at least one measured process parameter. The operation of the device according to the present general inventive concept is not subject to any periodic working cycle, but is based on a controlled sequence of individual working strokes, so that each working stroke can proceed with optimum energy conversion efficiency. On the basis of a continuous evaluation of the measured process parameter, the controller specifies the time sequence of the equivalent working strokes. In comparison with periodically operating machines, such as, e.g., known two-stroke engines (Stelzer engine, Stirling engine), the working stroke length is not proportional to the cycle frequency. In the ideal case, in the device according to the present general inventive concept the energy conversion process is always performed with the same efficiency, irrespective of how many times it is carried out per unit of time. 
     Advantageous and expedient further developments of the device according to the present general inventive concept are specified in the dependent claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and/or other aspects and advantages of the present general inventive concept will be apparent and more readily appreciated from the following description of the embodiments and from the accompanying drawings, to which reference is made and in which: 
         FIG. 1  illustrates a schematic structure of a system for generating electric power according to an embodiment of the present general inventive concept; 
         FIG. 2  illustrates a schematic structure of a linear part of a system in a first operating position according to an embodiment of the present general inventive concept; 
         FIG. 3  illustrates a schematic structure of a linear part in a second working position according to an embodiment of the present general inventive concept; and 
         FIG. 4  illustrates a schematic structure of a system for generating electric power according to an embodiment of the present general inventive concept. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present general inventive concept will be described below, taking a system for generating electric power as an example. The system illustrated in  FIG. 1  comprises a thermodynamic part  10  having a working medium and a linear part  12  and a controller  14  acting on both parts. 
     The main components included in the linear part  12  (see also  FIGS. 2 and 3 ) are, on the one hand, a single-stroke “engine” having a linear expansion means in the form of a piston/cylinder unit  16  and, on the other hand, a linear generator  18  having a magnet  20  and a coil  22 . The piston/cylinder unit  16  essentially consists of a pressure cylinder  24  and a piston  26  which is displaceable therein and is coupled to the magnet  20  of the linear generator  18 . A first working chamber  28  of the pressure cylinder  24  is formed on the side of the piston  26  facing away from the magnet  20 , and a second working chamber  30  is formed on the side of the piston  26  facing the magnet  20 . 
     The main components of the thermodynamic part  10  are essentially constituted by a pump  32 , a heat exchanger  34 , an optional heat accumulator  36 , and a condenser  38 .  FIGS. 2 and 3  illustrate that the thermodynamic part  10  of the system is coupled to the linear part  12 , more precisely to the single-stroke engine by means of two ducts  40 ,  42 . The two ducts  40 ,  42 , which are connected to the heat exchanger  34  (or more generally to a heat reservoir of higher temperature) and to the condenser  38  (or more generally to a heat reservoir of lower temperature), respectively, lead to the two variable working chambers  28 ,  30 , respectively, of the pressure cylinder  24 . The four ports  44 ,  46 ,  48 ,  50  coupling the ducts  40 ,  42  to the working chambers  28 ,  30  may be selectively opened or closed by the controller  14 . 
     The mode of operation of the entire system will now be described below. Basically, the system operates according to the following principle: At first, thermal energy (heat energy) is converted into thermodynamic energy (steam pressure) in a thermodynamic cycle. The steam pressure is transformed into mechanical energy of motion (kinetic energy) in the single-stroke engine. The energy of motion is finally transformed into electrical energy by means of the linear generator  18 . 
     The working medium is heated by supplying thermal energy and vaporizes, which leads to a strong expansion in volume of the working medium. For the heat exchanger  34 , solar panels are used, for example, which absorb heat by insolation and give it off to the working medium which flows past and vaporizes due to being heated. When a cooling agent is used as the working medium having a lower boiling point than water, an estimated efficiency of n&gt;20% is achievable for this partial process. The contraction in volume of the working medium by cooling and condensation, which is required for the cyclic process, is effected in the condenser  38  in colder surroundings. The liquid working medium is compressed by means of the pump  32  and supplied to the heat exchanger  34  again. 
     In particular when an ORC process (Organic Rankine Cycle) is provided as thermodynamic cycle, the working medium employed is preferably a medium suitable for use in such an ORC process, e.g., R245fa or a synthetic working medium which is specially designed for the application described and which has good thermal transfer characteristics and also distinguishes itself in that in the working medium no partial vacuum is produced in relation to the ambient pressure in the required ORC temperature range, since the ORC efficiency is reduced due to air penetrating because of a partial vacuum, which is difficult to avoid technically in the long run. Furthermore, prior to expansion only a superheating of the vaporized gas which is as low as possible should be necessary since the energy added during superheating only slightly raises the ORC energy yield. 
     The heating/vaporization of the working medium is time-independent as related to the overall process and is not subject to any permanent minimum requirements. Basically, a cyclic process which is only based on heating and cooling of the working medium (without vaporizing and condensing) is also conceivable; in this case, however, the overall efficiency would be distinctly lower. Basically, other fluids such as, e.g. hydraulic oil, or gases may also be used as the working medium for the thermodynamic part  10 . 
     As illustrated in  FIG. 2 , from the higher temperature heat reservoir, the expanding working medium reaches the first working chamber  28  of the pressure cylinder  24  via the first duct  40 . To this end, the controller  14  opens the port  44  and closes the port  46 . At the same time, the controller  14  closes the port  48  of the second duct and opens the port  50 . This exerts a force F stroke  on the piston  26 , which results in a movement of the piston  26  to the right (according to the illustration in the Figures), with work being performed. This process, which terminates after a relatively large stroke of the piston  26 , constitutes a “normal” working stroke of the engine. 
     The regulation of the volume (intake volume) flowing into the working chamber  28  as realized by means of the controller  14  as a function of the available medium pressure or the usable volume of expansion allows a very high efficiency in the conversion of the thermodynamic energy into mechanical energy of motion at an estimated efficiency of roughly η=95%. The very high efficiency is also enhanced by the use of a smoothly running piston/cylinder unit  16  optimized for the above-mentioned requirements and having low frictional and low thermal losses, so that high expansion speeds can be realized. 
     Along with the piston  26 , the magnet  20 , directly coupled by means of a rigid piston rod  52 , of the linear generator  18  moves within the coil  22 , so that a voltage pulse is induced within the coil  22 . Thus, no provision is made for a prior conversion of the linear piston movement into a rotary motion, for which reason the conversion into electrical energy by means of the linear generator  18  is very efficient at an estimated efficiency of roughly η=90%. As illustrated in  FIG. 1 , the magnet  20  may also be connected to the piston  26  by means of a joint  55 . The joint  55  absorbs transverse forces which are caused by mounting tolerances. In fact, a linear motion in the pressure cylinder  24  and in the linear generator  18  on precisely one axis is possible only theoretically. 
       FIG. 3  illustrates the contradirectional working stroke following the working stroke described above. The controller  14  closes the open ports  44 ,  50  and opens the closed ports  46 ,  48 , so that an oppositely directed piston force −F stroke  and a movement of the piston  26  to the left is produced. This results in a voltage pulse of opposite sign. 
     The two previously described working strokes are totally independent of each other (in particular in terms of time); that is, no periodic stroke sequence fixed in advance is provided as in the case of known multi-stroke engines. Rather, an individual working stroke is initiated depending on the situation, i.e. only if specific criteria are satisfied (particularly a sufficient pressure of the working medium) does the controller  14  provide for carrying out a working stroke by opening and, respectively, closing the ports  44 ,  46 ,  48 ,  50 . It depends on the current position of the piston  26  which one of the two working strokes (normal or contradirectional) is carried out. 
     The special feature in operating the piston/cylinder unit  16  employing the working medium provided by the thermodynamic part  10  of the system resides in that the controller  14  knows the pressure of the working medium both in the higher temperature heat reservoir and in the lower temperature heat reservoir and initiates a working stroke only when the pressure differential between the two heat reservoirs is so great that the lower heat reservoir is able to receive the quantity, ejected in a full piston stroke, of the working medium located in the working chamber  28  (normal working stroke) or  30  (contradirectional working stroke). This ensures that always the full piston stroke is utilized, which enhances the efficiency in the conversion of the mechanical energy of motion into electrical energy as described later. 
     The dimensions of the piston stroke and the piston surface of the piston/cylinder unit  16  and the dimensions of the magnet  20  and the coil  22  of the linear generator  18  are matched with each other. With regard to the amounts of energy to be transferred and the overall energy transfer efficiency, it has turned out that a piston/cylinder unit  16  having a comparatively large stroke (long stroke cylinder) is suited best. 
     A special form of the linear generator  18 , which may also be functionally modeled on a combination of a crankshaft and a rotating generator that is loss-free during start-up and shut-down, such as, e.g., the RMT generator, makes provision that upon each stroke in the pressure cylinder  24 , a rotor of the rotating generator performs a rotation of 180 degrees and remains in this position until the contradirectional working stroke is carried out owing to a process criteria that has been satisfied and the rotor is then either rotated further as far as to the initial position or is rotated back. 
     As already indicated, the thermodynamic cycle and the single-stroke engine are controlled using a multitude of suitable sensors (pressure, temperature, filling level sensors, etc.) and the controller  14 , which may include a plurality of subordinate controlling means. The controller  14  continuously monitors the overall situation taking into consideration all relevant process influencing variables (thermal energy supply, pressure and temperature of the working medium and of the surroundings, filling levels, etc.). For achieving an optimum overall efficiency, the controller  14  performs a variety of process controls such as, e.g., adjustments of the filling levels, velocities of flow of the working medium, amount of energy/volume of expansion of a working stroke, stroke frequency, size of the stroke, stroke length, etc. Under certain circumstances, the controller  14  may discontinue the energy conversion process altogether if, based on the sensor data, it may be expected that this will lead to a higher overall energy conversion efficiency. 
     An essential advantage of the system according to the present general inventive concept will be apparent from the following consideration. When the thermal energy supply is low, the theoretically possible efficiency in the thermodynamic conversion of thermal energy into mechanical energy is reduced by the Carnot efficiency:
 
η Carnot =1 −T   OUT   /T   IN ,
 
     with T IN : temperature of the working medium in the higher temperature heat reservoir, and T OUT : temperature of the working medium in the lower temperature heat reservoir. 
     A reduction in the flow velocity of the working medium in the solar panels as caused by the controller  14  results in higher T IN  and in lower rates of flow per unit of time. By a combination of a reduction in the flow velocity and a reduction in the clock frequency of the situation-controlled linear generator  18 , it is thus achieved that even in case of a low thermal energy supply the overall efficiency of the conversion of thermal into electrical energy remains constant. 
     The optional, thermally insulated heat accumulator  36  (pressure accumulator) of the thermodynamic part  10  of the system is adapted to (intermediately) store heated/vaporized working medium over a longer period of time. This is useful in particular in the case of a non-uniform thermal energy supply (e.g. in the case of a varying insulation) and, to a certain extent, allows an energy conversion that is independent of the period of time of the thermal energy supply without any substantial deterioration in the efficiency. In this way, in particular minimum start-up quantities may be ensured in order to permit a timing with respect to the working strokes of the pressure cylinder over a minimum period of time in this manner. A further development of the thermodynamic part  10  of the system provides for the use of a plurality of working media (cooling agents) having different boiling temperatures. Depending on the instantaneously maximum attainable temperature of the medium, the different boiling temperatures of the cooling agents allow the cooling agent or the mixture of two (or more) cooling agents to be used by which instantaneously the highest efficiency is achieved in the thermodynamic cycle. In particular, a mixture suitable for a Kalina cycle may be used, e.g. an ammonia-water mixture. To separate cooling agent mixtures again, where required, in this case a separator stage is provided in the condenser  38 . 
     Instead of the solar panels, other means for thermal energy supply from regenerative heat sources (e.g. thermal spring) may also be used. Furthermore, the otherwise unutilized waste heat of technical apparatus or systems may be exploited by means of suitable heat exchangers  34 . 
     Using the measures described above, a constant total efficiency of η total , &gt;15% should be achievable for the conversion of the thermal energy into electrical power. 
     The transformation of the irregular voltage pulses generated by the linear generator  18  into an alternating voltage suitable for feeding into an electric power grid is effected in that each individual voltage pulse is directly transformed into a grid-synchronous alternating voltage. To this end, provision is made for a direct coupling of the output of the linear generator  18  to the input of an inverter  54 . (In  FIG. 1  an additional filter and rectifier unit  56  is indicated between the linear generator  18  and the inverter  54 , which is employed in an alternative embodiment to be discussed later.) This type of transformation requires: 
     1. The voltage pulses are (distinctly) longer than the reciprocal of the mains frequency to be generated and are in a voltage range required by the inverter  54  as the input voltage. 
     2. The electric power grid to be fed must be able to take up sporadically generated mains power. In its simple form, this type of voltage transformation is therefore not suitable for self-contained power supply systems. 
     Even in case of a rapidly varying input power, the employed inverter  54  generates, in a wide input voltage range, an output power having a constant grid-synchronous alternating voltage at a high efficiency. When there is no or too low an input voltage, the inverter  54  discontinues the transformation. As soon as the input voltage has exceeded a threshold value again, the inverter  54  continues its work and immediately feeds the grid-synchronous alternating voltage (at low losses) into the grid again. 
     According to a further development of this aspect, idle times of a generator or fluctuations in feeding into the grid may be at least partially compensated for by an arrangement of several generators having working strokes offset in time. The generators may either each be connected in parallel to an inverter to form generator/inverter pairs or may all be coupled to the same inverter at low cost, which, however, leads to a lower efficiency. 
     According to the embodiment illustrated in  FIG. 1 , at the output of the linear generator  18  a number of voltage pulses are emitted per unit of time, dependent on the currently prevailing power throughput. The output of the linear generator  18  is coupled to the input of an inverter  54  by means of a filter and rectifier unit  56  which converts the pulses into direct voltage usable for the inverter  54 . The output of the inverter  54  is coupled to the electric power grid to be fed, so that the inverter  54  continuously transforms into an alternating voltage suitable for feeding into the electric power grid. 
     Even in case of a “low energy throughput”, a continuous conversion of thermal energy into an alternating voltage suitable for feeding into the electric power grid takes place. The efficiency of the conversion depends only on the temperature levels of the thermodynamic cycle (preferably an ORC cycle), but not on the quantities of heat converted per unit of time. The filter and rectifier unit  56 , which converts the pulses into direct voltage usable for the inverter  54 , is dimensioned such that, even in case of the required minimum energy throughputs, it converts the resultant low frequency of voltage pulses into a direct voltage level which the inverter  54  can transform into an alternating voltage suitable for feeding into the electric power grid without any further significant losses or interruptions. 
     A different application of the present general inventive concept is illustrated in  FIG. 4 . In this case, the linear generator  18  is not coupled to an inverter for an electric power grid, but to a generator of suitable battery charging voltages and currents (battery charger)  60 , e.g., for lithium-ion or nickel-cadmium batteries for automobiles. 
     It is further possible to couple the linear generator  18  to a voltage generator in order to thus perform an electrolysis for the release of hydrogen. 
     Although a few embodiments of the present general inventive concept have been illustrated and described, it will be appreciated by those skilled in the art that changes may be made in these embodiments without departing from the principles and spirit of the general inventive concept, the scope of which is defined in the appended claims and their equivalents.