WO2009045117A2

WO2009045117A2 - A method of utilising low- and medium-temperature heat sources and media and a system for utilising low- and medium-temperature heat sources and media

Info

Publication number: WO2009045117A2
Application number: PCT/PL2007/000085
Authority: WO
Inventors: Wladyslaw Novak; Aleksandra Borsukiewicz-Gozdur; Aleksander Andrzej Stachel
Original assignee: Politechnika Szczecinska
Priority date: 2007-10-02
Filing date: 2007-12-27
Publication date: 2009-04-09
Also published as: PL210568B1; WO2009045117A3; PL383471A1

Abstract

The method of utilising low- and medium-temperature heat sources and media according to the invention is characterised by their use to preheat the working fluid in the lower cycle of a binary power plant comprising two working cycles thermally coupled with one another by at least one heat exchanger, whereas the working fluid in the lower cycle is a substance with a low evaporation enthalpy and a relatively high preheating enthalpy, preferably an organic fluid, while a high-temperature source of heat is used to preheat, evaporate and superheat the working fluid in the upper cycle. An organic fluid with the critical temperature higher, by 15 K - 20 K at best, than the temperature of the utilised heat source or medium is selected for the lower cycle. The system according to invention including two working cycles, is characterised by the connection of the preheater (10) of the lower cycle (6) to a flux of a low- or mid-temperature heat medium or source (16) from one or more sources, whereas the working fluid in the lower cycle is a substance with a low evaporation enthalpy and a relatively high preheating enthalpy, preferably an organic fluid, while the boiler (3) of the upper cycle is supplied with high- temperature heat. In another version of the invention, the system has the third cycle (9) with the lowest range of working temperatures, which is thermally coupled with the second working cycle (6) via a condenser/evaporator exchanger (8) or an exchanger of the condenser/evaporator- preheater (20) type.

Description

A method of utilising low- and medium-temperature heat sources and media and a system for utilising low- and medium-temperature heat sources and media

The subject of this invention is a method of utilising low- and medium-temperature heat sources and media and a system for utilising low- and medium-temperature heat sources and media by means of work through using them at a binary power plant.

Literature cites methods for utilising low- and medium-temperature sources, like geothermal water or heat media such as oil, flue gas and waste water by way of heat or work. Most frequently these sources and media are used by way of heat to heat pools, houses etc. By way of work, they were used in steam power plants with a low-boiling fluid. A method for utilising geothermal water as a source of co-feeding a preheater of the working fluid (water) in a traditional water steam power plant is known from a publication by Bruhn M.: Hybrid geothermal-fossil electricity generation from low enthalpy geothermal resources: geothermal feedwater preheating in conventional power plants, Energy 2002; 27: 329-346. A method for using low-temperature waste heat and converting it into electricity and a heat medium at a temperature higher than the original temperature of the heat medium is known from the Japanese patent description JP 1174801. This effect was achieved by using a low-boiling fluid which circulates in a closed circuit. The energy of the flux of fluid which acts as the heat medium is transferred in the evaporator to an organic fluid which is then compressed to raise its temperature. Downstream of the compressor the fluid flux is split, with one part of the flux being fed to a turbine driving the electricity generator in the power plant circuit, while the remaining part is directed to a heat exchanger supplying heat receivers. In this patent the word binary is used in the meaning of the 'combined' generation of electricity and heat. A solution for utilising low and medium enthalpy heat sources and transforming the heat into electricity is known from the patent description GB2162583. This effect has been achieved by using the so-called power plant cascade, which means that the heat medium supplies the evaporator of the first power plant, where it causes the working fluid in the first circuit to evaporate, then that heat medium supplies the evaporator of the second power plant of lower parameters, then of the third and the following. Having given off heat in all evaporators, the heat medium supplies the preheaters of particular power plants in parallel. The patent description RU 2140545 shows an opportunity for utilising geothermal heat sources of high parameters, i.e. geothermal steam. The power plant is built of repeatable modules, every one of which constitutes the system of a binary geothermal water - organic fluid power plant. However, every module is supplied with one and the same source of heat. The Russian patent description SU 1377420 presents a binary power plant with water in the upper cycle and another substance in the lower cycle. This solution also calls for the use of a second source of heat, for example solar heat, but this energy is used to evaporate the fluid, and the two cycles are combined in a row of heat exchangers which play the role of water condensers and organic fluid preheaters. These exchangers are also partially supplied with steam from extractions of the high-pressure turbine.

A system for utilising high-temperature heat generated by burning waste is known from the Japanese patent description JP 2000145408. This description depicts the system of a binary power plant with water as the working fluid in the upper cycle, and with an organic fluid in the lower cycle. However, in this solution the entire power plant is supplied from a single source, the fluid is preheated as a result of regenerative preheating with steam from extractions of the turbine. A system of a power plant for utilising low- and medium-temperature sources or media, containing a single-fluid closed cycle with a preheater, an evaporator and a turbine is known from literature. However, the efficiency of such a power plant is very low, so it was never widely used. A system of a power plant with an organic fluid, in which the evaporator and the preheater of the lower organic cycle are supplied with steam from extractions of the steam turbine, is known from a publication of Angelino G., Invernizzi C, Molteni G.: The potential role of bottoming Rankine cycles in steam power stations, Proc. Instn. Mech. Engrs. 1999; VoI 213, part A: 75-81. This solution is based on a single-source supply of the power plant.

The method of utilising low- and medium-temperature heat sources and media according to the invention through their use by way of work is characterised by their use in binary power plants including two working cycles that are thermally coupled with one another through at least one heat exchanger. The essence of the method is that low- and/or medium-temperature heat sources or media are used to heat the working fluid in the lower cycle of the power plant, which is a substance with a low evaporation enthalpy and a relatively high preheating enthalpy, ideally an organic fluid. Possible fluids would be: R 245ca, R245fa, R227ea, isobutane, butane. It is best to select such organic fluids for the lower cycle which have a critical point higher than the temperature of the utilised heat source or medium, the best difference being 15 K to 20 K. Organic fluids have favourable thermodynamic characteristics. They cannot, as a rule, replace water in the cycle of a steam power plant within the entire temperature range. This is due to the relatively low values of critical parameters of organic fluids and other operational and chemical characteristics, such as the temperature of their thermal breakdown. Water is used as the working fluid in the upper cycle, where it is heated by a fossil fuel or another high-temperature source of heat, for example heat from burning biomass or waste. The upper and lower cycles are coupled by a heat exchanger of the condenser/evaporator type in which the working fluid of the upper cycle is condensed and the working fluid of the lower cycle evaporates.

The method according to the invention facilitates a very effective utilisation of significant amounts of low- and/or medium-temperature heat sources and media, for example waste, geothermal, solar and other heat, which is used to preheat the fluid in the lower cycle, as a result of the relatively high preheating enthalpy. This also leads to increasing the amount of fluid in the lower cycle as a result of the low evaporation enthalpy of that fluid in this cycle. The method according to the invention contributes to a significant gain in the power plant capacity compared to the capacity of a single-fluid, water steam power plant, and yields an efficiency greater than of other installations.

System according to the invention including two working cycles - the upper made up of a boiler, superheater, preheater, turbine, pump and an evaporator/condenser type of a heat exchanger which couples the upper cycle to the lower cycle, which in turn comprises a preheater, pump, turbine and condenser- is characterised by the connection of the preheater of the lower cycle to the flux of the medium or the source of low- and/or medium-temperature heat from one or many sources. The working fluid in the lower cycle is a substance with a low evaporation enthalpy and a relatively high preheating enthalpy, preferably an organic fluid. The boiler of the upper cycle is supplied with high-temperature heat, e.g. generated by burning fossil fuel, biomass or waste, which preheats, evaporates and superheats the working fluid of the upper cycle.

Another variation of the system is characterised by the presence of the third cycle with the lowest range of working temperatures, which is thermally coupled with the second working cycle via a condenser/evaporator exchanger or in another version, via an exhanger of the condenser/evaporator-preheater type. The third cycle in the first version is made up of a preheater, a condenser/evaporator exchanger, a turbine-generator, a condenser and a pump. In the second version, the third cycle including a condenser/evaporator exchanger, a condenser, a pump and a decompression valve instead of a turbine-generator. As a result of installing a decompression valve, the working flux is not transformed into electricity, but dissipated into the environment, so there is no reason for using an additional preheater to improve the efficiency and raise the capacity of this cycle. Another variation of the system is characterised by the presence of the third cycle with the lowest range of working temperatures, thermally coupled with the first working cycle via a condenser/evaporator exchanger, while the second cycle is thermally coupled with the first cycle by a condenser/evaporator exchanger supplied with steam from the turbine extraction. This solution is applied when there are two available fluxes of media carrying waste, geothermal or other heat of different temperatures, and those temperatures of both of them are to be reduced to the same value.

In all three cycles the working fluids are selected so that they would meet the current requirements as to their environmental, operational and physiological characteristics. In the first, upper cycle, the working fluid must have a high critical point: water is such a fluid. The working fluid in the second, the middle cycle, must be characterised by an evaporation enthalpy that is low compared to the evaporation enthalpy of water, and a preheating enthalpy sufficiently high compared to its evaporation enthalpy so that a large flux of the fluid in the middle cycle can be achieved. The fluid of the third, lower cycle has a sufficiently high enthalpy of evaporation at low temperatures, as a result of which the flux of fluid circulating in the lower cycle is reduced, with the consequent reduction of the flux of heat evacuated into the environment. What is also important is the thermal stability within a wide temperature range. These are the features of organic substances. In the first, upper cycle, the high-temperature heat is efficiently transformed into electricity. The job of the second, middle cycle is the high efficiency transformation of medium-temperature heat into electricity, with the ability to utilize large amounts of that type of heat medium. The application of the third, lower cycle, enables a significant reduction of the size of the steam power plant condenser by reducing the flux of heat evacuated into the environment. Electricity could also be generated in this cycle, but the profitability of such operation must be analysed in every individual case. The upper cycle is supplied only with energy obtained by burning fuel in the boiler, while the medium and lower cycles are fed with one or many fluxes of geothermal and/or waste heat media and/or steam from the extraction of the water steam turbine, or other media, or in the last resort may be additionally supplied with energy from the boiler.

A ternary power plant provides a significant increase in the capacity and improves the efficiency of the power plant compared to traditional solutions. The application of the third, lower cycle, enables a significant reduction of the size of the steam power plant condenser by reducing the flux of heat evacuated into the environment. The solution envisaged in this invention leads to a significant capacity increase and improves efficiency compared to the capacity and efficiency of a water steam power plant.

The invention is illustrated in examples and in figures of drawing, where Fig. 1 shows an conceptual diagram of the system of a binary power plant with a multi-source supply, Fig. 2 shows the block diagram of a steam power plant whose third, lower cycle is thermally coupled with the second cycle and equipped with a turbine-generator, Fig. 3 presents the block diagram of a steam power plant whose third, lower cycle is thermally coupled with the second cycle and equipped with a decompression valve, Fig. 4 shows a block diagram of a steam power plant whose second and third cycles are thermally coupled with the first cycle, and the second cycle is supplied with steam from the turbine extraction of the first cycle. Example 1

The power plant comprises the upper cycle 1, in which water is the working fluid, and the lower cycle 2, in which R245ca, with an evaporation enthalpy lower than that of water, is the working fluid. As water is a so-called wet fluid, it is routed to the superheater 7 before it is directed to the steam turbine 3. After the working fluid of the upper cycle leaves the turbine, it is condensed in the condenser/evaporator exchanger 4, and then pumped to the appropriate high pressure by the pump 5. Upper cycle working fluid is preheated and evaporated in the boiler 6. The working fluid of the lower cycle is evaporated in the condenser/evaporator exchanger 4 and routed in the following order: to the turbine 8, then the condenser 9, then the pump 10 and then to the fluid preheater 11. The upper cycle is supplied only with energy obtained by burning fuel in the boiler, while the lower cycle is supplied with one or many fluxes that carry geothermal heat 12 with the temperature of 100°C. 1 kg/s of water circulates in the upper cycle, which corresponds to 13.76 kg/s of the R245ca fluid. The total efficiency of the binary power plant in this case amounts to 38,63% and is close to the efficiency of a conventional power plant with the same working assumptions. This allows 5,22 kg/s of the geothermal water flux to be utilised. Utilising water of the same temperature in a single-fluid steam power plant with an organic fluid would allow the capacity of 45 kW to be achieved with the thermal efficiency of 14,74%, which represents only 8,85% of the capacity increase in the binary cycle. Example 2

The power plant consists of three cycles of working fluids: the first - teh upper cycle 1, the second - the middle cycle 6, the third - the lower cycle 9. After the working fluid - water - of the upper cycle 1 leaves the turbine-generator 2, it is condensed in the condenser/evaporator exchanger 4, and then pumped to the appropriate high pressure by the pump 5. Upper cycle working fluid 1 is preheated, evaporated and superheated in the boiler 6. The working fluid of the middle cycle 6 is evaporated in the condenser/evaporator exchanger 4 and routed in the following order: to the turbine-generator 7, then the condenser 8, then the pump 11 and then to the fluid preheater 10. The lower cycle 9 is thermally coupled with the middle cycle 6 via the evaporator/condenser exchanger 8. The fluid of the lower cycle 9, having evaporated in the exchanger 8, is routed to the turbine-generator 12 and then to the condenser 13 cooled with the cooling medium - air 17. After condensing, the working fluid is routed to the circulating pump 15 and then to the preheater 14. The upper cycle is supplied only with energy obtained from burning fuel in the boiler 3, while the middle cycle 6 and the lower cycle 9 are supplied with the flux of medium carrying geothermal heat 16 and 18.

The table below details calculation results of ternary power plants presented in example 1 with various combinations of working fluids in the upper, medium and lower cycles and with various condensation/evaporation temperatures in exchangers 4 and 8 as well as 4 and 20. These results are referenced to a traditional water single-fluid power plant with the capacity of 32,1 MW and the efficiency of 37,02%. It was assumed that all power plants work in the same conditions of heat supply and evacuation. Table

The table of example calculations presented above indicates that the working efficiency of a ternary power plant is influenced, apart from the selection of organic fluids of the middle and lower power plants, also by the temperatures assumed in the condenser/evaporator exchangers, which should in every individual case be adjusted to the temperatures of the upper and lower heat sources and the working fluids selected.

Example 3

A power plant as in example 2, but the decompression valve 19 is installed in the third, the lower cycle 9 instead of the turbine-generator 12. As a result of installing the decompression valve 19, the working flux is not transformed into electricity, but dissipated into the environment, so there is no reason for using the additional preheater 14 to improve the efficiency and raise the capacity of this cycle. Such a solution allows the surface of the condenser 13 to be reduced significantly.

The upper cycle is supplied only with energy obtained from burning fuel in the boiler 3, while the middle cycle 6 is supplied with the flux of medium carrying geothermal heat 16.

Example 4

Power plant as in example 3, except that the middle cycle 6 is supplied with a flux of heat medium, i.e. steam from the extraction of steam turbine 2 or 7.

Example 5

The power plant consists of three cycles of working fluids: the first - the upper cycle 1, the second - the middle cycle 6 thermally coupled with the first cycle 1 via the condenser/evaporator exchanger 4 fed with a flux of steam 22 from the exhaust of the turbine 2, the third - the lower cycle 9 thermally coupled with the first cycle 1 by the condenser/evaporator exchanger 8.

Claims

Patent claims

1. The method of utilising low- and medium-temperature heat sources and media through using them by way of work, characterize by using them to preheat the working fluid in the lower cycle of a binary power plant comprising two working cycles thermally coupled with one another by at least one heat exchanger, whereas the working fluid in the lower cycle is a substance with a low evaporation enthalpy and a relatively high preheating enthalpy, preferably an organic fluid, while a high-temperature source of heat is used to preheat, evaporate and superheat the working fluid in the upper cycle.

2. The method as claimed in claim 1 characterize by the selection, for the lower cycle, of an organic fluid with the critical point higher, by 15 K - 20 K at best, than the temperature of the utilised heat source or mediu,.

3. The method as claimed in claim 1 characterize by using water as the working fluid in the upper cycle.

4. The system for utilising low- and medium-temperature heat sources and media by way of work, comprising two working cycles, the upper consisting of a boiler, a preheater, a turbine, a pump and an evaporator/condenser exchanger which couples the upper cycle with the lower cycle, which in rum consists of a preheater, a pump, a turbine, a condenser, characterize by the connection of the preheater (10) of the lower cycle (6) to the flux of the medium or the source of low- and/or medium-temperature heat (16) from one or many sources, whereas the working fluid of the lower cycle is a substance with a low evaporation enthalpy and a relatively high preheating enthalpy, preferably an organic fluid, while the boiler (3) in the upper cycle is supplied with high-temperature heat.

5. The system as claimed in claim 4 characterize by the presence of the third cycle (9) with the lowest range of working temperatures, which is thermally coupled to the second working cycle (6) via an exchanger of the condenser/evaporator (8) or of the condenser/evaporator-preheater (20) type.

6. The system as claimed in claim 4, characterize by the presence of the third cycle (9) with the lowest range of working temperatures, thermally coupled with the first working cycle (1) via the condenser/evaporator exchanger (8), while the second cycle (6) is thermally coupled with the first cycle (1) by the condenser/evaporator exchanger (4) supplied with steam (22) from the extraction of turbine (2).

7. The system as claimed in claim 5 or 6, characterize by the presence of the third cycle (9) made up of the preheater (14), the condenser/evaporator exchanger (8), the turbine- generator (12), the condenser (13) and the pump (15).

8. The system as claimed in claim 5, characterize by the presence of the third cycle (9) made up of the condenser/preheater-evaporator exchanger (20), the decompression valve (19), the condenser (13) and the pump (15).

9. The system as claimed in claim 5 or 6 characterize by the high critical point of the working fluid in the first cycle (1), while in the second cycle (6), the working fluid has a low evaporation enthalpy compared to the evaporation enthalpy of water, and by the sufficiently high preheating enthalpy of the fluid compared to the evaporation enthalpy, while in the third cycle (9) the working fluid has a sufficiently high evaporation enthalpy at low temperatures.

10. The system as claimed in claim 5 or 6 characterize by using water as the working fluid in the first cycle (1), while the working fluids in the second cycle (6) and the third cycle (9) are organic fluids.