ORC FOR TRANSFORMING WASTE HEAT FROM A HEAT SOURCE INTO MECHANICAL ENERGY AND COMPRESSOR INSTALLATION MAKING USE OF SUCH AN ORC

Abstract

An Organic Rankine Cycle (ORC) device and method for transforming waste heat from a heat source containing compressed gas into mechanical energy. The ORC includes a closed circuit containing a two-phase working fluid, the circuit including a liquid pump for circulating the working fluid in the circuit consecutively through an evaporator which is in thermal contact with the heat source; through an expander like a turbine for transforming the thermal energy of the working fluid into mechanical energy; and through a condenser which is in thermal contact with a cooling element. The ORC determines the mechanical energy generated by the expander. A control device regulates the fraction of the working fluid entering the expander based on the determined mechanical energy such that the mechanical energy generated by the expander is maximum.

Claims

1-17. (canceled)

18. An Organic Rankine Cycle (ORC) for transforming waste heat from a heat source containing compressed gas into mechanical energy, the ORC comprising a closed circuit containing a two-phase working fluid, the circuit comprising a liquid pump for circulating the working fluid in the circuit consecutively through an evaporator which is in thermal contact with the heat source; through an expander like a turbine for transforming the thermal energy of the working fluid into mechanical energy; and through a condenser which is in thermal contact with a cooling element, wherein the ORC is equipped with a determiner of the mechanical energy generated by the expander and a control device that regulates the vapor fraction of the working fluid entering the expander, whereby the control device will regulate the aforementioned vapor fraction based on the determined mechanical energy such that the mechanical energy generated by the expander is maximum and whereby the expander is of any kind suitable to accept a mixture of liquid and gaseous working fluid.

19. The ORC according to claim 18, wherein the control device will regulate the vapor fraction of the working fluid entering the expander, by varying the working fluid flow through the pump and/or by varying the working fluid flow through the expander.

20. The ORC according to claim 18, wherein the control device will regulate the vapor fraction of the working fluid entering the expander in a continuous manner.

21. The ORC according to claim 19, wherein the control device will regulate the vapor fraction of the working fluid entering the expander, by switching repeatedly between two control algorithms, whereby the first control algorithm consists of varying the working fluid flow through the pump until the mechanical energy generated by the expander is at a local maximum and the second control algorithm consists of varying the working fluid flow through the expander until the mechanical energy generated by the expander is at a further optimized maximum.

22. The ORC according to claim 19, wherein the variation of the working fluid flow through the expander is realized by a by-pass over the expander, by a varying of a speed of the expander, by slide valves and/or lift valves, by varying a swept volume of the expander or by a varying of the oil injection of the expander.

23. The ORC according to claim 19, wherein the variation of the working fluid flow through the pump is realized by a by-pass over the pump, by a varying of a speed of the pump, by a varying of a swept volume of the pump or by a varying of the on-off frequency of the pump.

24. The ORC according to claim 18, wherein the vapor fraction of the working fluid entering the expander is between 10% and 99% mass fraction.

25. The ORC according to claim 18, wherein the expander is a volumetric expander or that the expander is a screw expander.

26. The ORC according to claim 18, wherein a working fluid is used which comprises a lubricant or which acts as a lubricant.

27. The ORC according to claim 18, wherein a working fluid is used of which the boiling temperature is lower than 900 C, preferably is lower than 600 C.

28. The compressor installation comprising a compressor element for compressing a gas and a cooler for cooling the compressed gas, wherein the compressor installation comprises the ORC according to claim 18, whereby the cooler is integrated in an heat exchanger which also integrates the evaporator of the ORC for heat transfer between the cooler and the evaporator.

29. The compressor installation according to claim 28, further comprising a multistage compressor installation with at least two compressor elements connected in series for compressing a gas and at least two coolers acting either as an intercooler between two compressor elements or as an aftercooler for cooling the gas compressed by the last stage compressor element, whereby the compressor installation comprises an ORC with at least one evaporator, whereby each above-mentioned coolers is integrated in an heat exchanger which also integrates at least part of the evaporator of the ORC.

30. The compressor installation according to claim 29, wherein the evaporator of the ORC is composed of a plurality of evaporators or evaporator parts, each evaporator or evaporator part being integrated together with an intercooler or with an aftercooler in a heat exchanger, the evaporators or evaporator parts of the ORC being fluidly connected in series or in parallel in the ORC circuit.

31. The compressor installation according to claim 30, wherein the evaporators or evaporator parts are connected in parallel and are configured to divide the flow of the working fluid coming from the pump into separate flows through the evaporators or evaporator parts.

32. The compressor installation according to claim 31, wherein the configuration to divide the flow of the working fluid over the evaporators or evaporator parts comprises a three way valve or by a restriction and/or a valve.

33. The compressor installation according to claim 28, wherein the compressor element or compressor elements are oil free air compressor elements.

Description

[0033] FIG. 1 schematically represents a single stage compressor installation making use of an ORC system according to the invention;

[0034] FIG. 2 schematically represents a multi stage compressor installation according to the invention;

[0035] FIGS. 3 to 4 represent different embodiments of the multi stage compressor installation according of FIG. 2.

[0036] The compressor installation 1 represented in FIG. 1 comprises a compressor element 2 with an inlet 3 and an outlet 4 and driven by a motor 5 for compressing a gas flow Q and a cooler 6 for cooling the compressed gas before it is supplied to a net 7 of consumers of compressed gas.

[0037] The afore-mentioned gas can be for example air or nitrogen. However, the invention is not limited thereto.

[0038] The compressor installation 1 further comprises an ORC 8 according to the invention wherein the above-mentioned cooler 6 is integrated in an heat exchanger 9 which also integrates the evaporator 10 of the ORC 8 for recovering the waste heat of the compressed gas used as a heat source 11 and transforming said heat into useful mechanical energy by means of an expander 12 of the ORC 8, for example a turbine driving an electrical generator 13 as shown in the example of FIG. 1.

[0039] The ORC 8 comprises a closed circuit 14 containing a two-phase organic working fluid with a boiling temperature below the temperature of the heat source 11, i.e. the compressed gas, the working fluid being continuously circulated around in the circuit 14 by means of a liquid pump 15 in the direction as indicated with arrows F.

[0040] The working fluid is made to flow consecutively through the evaporator 10 which is in thermal contact with the heat source 11; then through the expander 12 and finally through a condenser 16 before being launched again by the pump 15 for a next cycle in the circuit 14.

[0041] The condenser 16 is, in this example, in thermal contact with a cooling element 17 of a cooling circuit 18 which, in the example of FIG. 1, is represented as a supply of cold water W taken from a tank 19 to circulate through the condenser 16 by means of a pump 20.

[0042] According to the invention, the ORC 8 is equipped with means 21 for determining the mechanical energy generated by the expander 12.

[0043] These means 21 can be for example a Power meter or Power sensor.

[0044] The ORC 8 is further equipped with a control device 22 that can regulate the vapour fraction of the working fluid entering the expander 12.

[0045] Normal operation of the ORC 8 according to the invention is that the control device 22 will regulate the afore-mentioned vapour fraction based on the determined mechanical energy by the means 21 such that the mechanical energy is maximum.

[0046] In the example of FIG. 1 and according to a preferred characteristic of the invention, the control device 22 will regulate the vapour fraction of the working fluid entering the expander 12, by varying the working fluid flow through the pump 15 and by varying the working fluid flow through the expander 12.

[0047] It is of course also possible that the control device 22 will only regulate the expander 12 or the pump 15.

[0048] In this case however, the control device 22 will regulate the vapour fraction of the working fluid entering the expander 12 by switching repeatedly between two control algorithms.

[0049] A first control algorithm consists of varying the working fluid flow through the pump 15 until the mechanical energy generated by the expander 12 is at a local maximum.

[0050] The second control algorithm consists of varying the working fluid flow through the expander 12 until the mechanical energy generated by the expander 12 is at a further optimize maximum.

[0051] The control device 22 will vary the working fluid flow through the expander 12 or the pump 15, i.e. vary the expander 12 or pump 15 capacity, and at the same time determine the mechanical energy generated by the expander 12, i.e. determine the ORC power output, and will select the expander 12 or pump 15 capacity for which the determined the ORC power output is at a maximum.

[0052] After the first control algorithm, the ORC power output will be optimized in function of only the pump 15 capacity. This means that the ORC power output will be at a local maximum.

[0053] By applying the second control algorithm, the ORC power output will be optimized in function of the expander 12 capacity, such that an optimized maximum can be reached.

[0054] By switching again to the first control algorithm, the ORC power output will be optimized again in function of the pump 15, such that changes in operating conditions can and will be taken into account.

[0055] Such changes in operation conditions are: changes in the temperature of the compressed air to be cooled, changes in the flow of the compressed air, changes in ambient temperatures, changes in cooling water flow, changes in cooling water temperature or changes in heat exchanger efficiency.

[0056] By applying such a regulation, the control device 22 will regulate the vapour fraction of the working fluid entering the expander 12 in a continuous manner, such that changes in operating conditions can be readily acted upon.

[0057] In this way, a maximum ORC power output can be guaranteed under all operating conditions.

[0058] In order to vary the working fluid flow through the expander 12, several options are possible.

[0059] The expander 12 capacity can be varied by means of varying the speed of the expander 12, as in the present example or by means of a by-pass over the expander 12, by means or slide valves and/or lift valves, by varying swept volume of the expander 12 or by means of varying the oil injection of the expander 12.

[0060] Also to vary the working fluid flow through the pump 15, several options are possible.

[0061] The pump 15 capacity can be varied by means of varying the speed of the pump 15, as in the present example or by means of a by-pass over the pump 15, by means of varying swept volume of the pump 15 or by means of varying the on-off frequency of the pump 15.

[0062] According to a preferred embodiment of the invention, the vapour fraction of the working fluid entering the expander 12 is between 10% and 99% mass fraction. It is of course also possible that the vapour fraction of the working fluid entering the expander 12 is kept between different limits, for example between 20% and 95% mass fraction or between 40% and 90% mass fraction.

[0063] The expander 12 can be any kind of expander 12 capable of generating mechanical energy by expansion of a two phase fluid supply, i.e. a mixture of liquid and gaseous working fluid. Preferably, a volumetric expander 12 like a screw expander 12 or a mechanical cylinder or the like which can accept a mixture of liquid and gaseous working fluid.

[0064] The compressor element 2 can also be of any kind, in particular an oil free air compressor element 2.

[0065] It is also clear that the cooling of the condenser 16 can be realized in other ways than in the example of FIG. 1, for example by blowing ambient air over the condenser 16 by means of a fan or the like.

[0066] Preferably a working fluid is used of which the boiling temperature is lower than 90 C. or even lower than 60 C., depending on the temperature of the available heat source 11, i.e. the temperature of the compressed gas to be cooled.

[0067] An example of a suitable organic working fluid is 1,1,1,3,3-pentafluoropropaan. The working fluid could be mixed with a suitable lubricant for the lubrication of at least part of the moving parts of the ORC 8. Alternatively, the working fluid itself could act as a lubricant, meaning that a working fluid is chosen which has lubricating properties.

[0068] In FIG. 2 a multistage compressor installation 1 according to the invention is represented with in this case two compressor elements, a first stage compressor element 2 and a last stage compressor element 2 respectively, which elements 2 and 2 are driven via a gearbox 23 by a single motor 5 and are connected in series for compressing a gas in two incremental pressure stages.

[0069] The compressor elements 2, 2 can also be of any kind, in particular an oil free air compressor elements.

[0070] The installation 1 is provided with a intercooler 6 for cooling the gas compressed by the first stage compressor element 2 before it is supplied to the next element 2 and an aftercooler 6 for cooling the gas compressed by the last stage compressor element 2 before it is supplied to the net 7.

[0071] Each of the above-mentioned coolers 6 and 6 is integrated in an heat exchanger 9 and 9, which also integrates part of the evaporator 10 of the ORC 8.

[0072] In the example shown, the ORC comprises two evaporators 10 and 10 connected in series in the circuit 14, although it would not be excluded to have only one evaporator 10 of which a part 10 is in thermal contact with the intercooler 6, whilst another part 10 is in thermal contact with the aftercooler 6.

[0073] Also in this case the control device 22 will be regulated according to the same method as in FIG. 1.

[0074] In that case the same advantages apply as in the single stage compressor element of FIG. 1.

[0075] FIG. 3 gives another example of a multistage compressor installation 1 according to the invention which differs from the embodiment of FIG. 4 in that the evaporators 10 and 10 are connected in parallel instead of in series but still with the same advantages.

[0076] FIG. 4 illustrates an alternative of the installation 1 of FIG. 3 comprising additionally an three way valve 24 in order to split the flow of the working fluid coming from the pump 15 into two suitable separate flows through the evaporators 10 and 10.

[0077] Instead of using a three way valve 24 one or two restrictions or a combination of a restriction and a valve could be used in the branches of parallel circuit connecting the evaporators 10 and 10.

[0078] The present invention is in no way limited to the form of embodiments described by way of an example and represented in the figures, however, such an ORC according to the invention for transforming waste heat from a heat source into mechanical energy and of a compressor installation making use of such an ORC can be realized in various forms without leaving the scope of the invention.