Circuit arrangement of an ORC process with two hydraulically separated circuits to improve efficiency.

Abstract

Circuit design of an ORC process with two hydraulically separated circuits and different working fluids for the stepwise generation of electrical current. The aim is to increase efficiency. The circuit arrangement is used for generating electricity from waste heat of any kind and thermal solar energy with thermal storage.

Description

[0001] The state of the art is the use of ORC technology for generating electricity from industrial waste heat, solar thermal energy with thermal storage, geothermal energy, etc.

[0002] In all applications, maximum efficiencies of 20% are to be achieved. An ORC cycle is always used in all applications.

[0003] The aim of the invention is to generate electric current in stages using two hydraulically separated circuits and different working media.

[0004] The mode of operation is described in the circuit arrangement of the Fig. 1. A CHP unit 1 operated with biogas with a power output of 500 KW has two waste heat sources: the cooling water and the exhaust gases.

[0005] The cooling water enters the preheater 2, and the exhaust gases enter the evaporator 3. The working fluid in the first circuit is ethanol, stored in the reservoir 5. It is pumped by the working fluid pump 6 into the preheater 2, from there into the evaporator 3, and finally into the turbine set 4 for expansion. So far, the process is identical to single-stage ORC systems. The working fluid pumps 6 and 12 are speed-controlled to regulate the evaporation rate according to the load. The control variable is the electrical power in percent. The control loop is not shown in the circuit diagram. For other types of waste heat, the control variable is the temperature. However, fluctuations can be temporarily compensated for with a thermal storage unit.

[0006] According to the invention, instead of a cooler, an evaporator 7 is connected downstream of the turbine 4, which takes over the condensation process and thus eliminates the need for a cooler. The liquefied working fluid flows into the storage tank 5, thereby closing the cycle.

[0007] The design of evaporator 7 must ensure a complete condensation process. The working fluid of the second circuit is isopentane, which has a different vapor pressure curve than ethanol. The working fluids must be determined by a specialist depending on the temperature range and application.

[0008] The second working fluid is pumped from the reservoir 11 by the working fluid pump 12 into the evaporator 7 and enters the superheater 8, and from there into the second ORC turbine 9. A control valve 14 is installed in the inlet of the superheater 8, forming a fixed-value control loop with the measuring and control point 13. The inlet of the superheater 8 is connected upstream of the turbine 4, and the return is connected downstream of the turbine 4.

[0009] After expansion in turbine 9, a cooler 10 is connected which liquefies the working fluid and thus transfers it to the storage tank 11. This completes the second circuit.

[0010] The efficiency of CHP unit 1 is assumed to be approximately 30%, corresponding to 500 kW, resulting in 70% waste heat. Of this, approximately 20% is generated in the first ORC stage, which corresponds to about 230 kW. The second stage generates approximately 10% of the remaining waste heat, corresponding to about 90 kW. The overall gross efficiency of this circuit configuration is approximately 50%.

[0011] Thermoelectric generators (TEGs) can be installed on the condenser of cooler 10, with the condenser representing the hot side and the fan side the cold side. Due to the resulting temperature difference, a voltage is generated according to the Seebeck effect.

[0012] By connecting the TEGs in series and parallel, a ready-to-use DC voltage can be generated. Approximately 4% of the condensation heat can be generated as DC power; in this example, that would be about 13 kW. Reference symbol list: 1 Biogas CHP unit 2 preheaters 3 evaporators 4 Turbine set 1st circuit 5 storage containers, 1st circle 6 Working fluid pump 1st circuit 7 evaporators, 2nd circuit 8 superheaters, 2nd circuit 9 Turbine set 2nd circuit 10 Dry coolers 11 storage containers, 2nd circuit 12 Working fluid pump 2nd circuit 13 Temperature measurement and control 14 Control valve

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