Abstract
Organic Rankine Cycle (ORC) is a proven technology in the field of waste heat recovery and present days ORC is extensively being used in exploiting biomass, geothermal and solar energy. The overall performance of ORC depends on the expander, making it a core component of the system. Generally, these expanders are classified into velocity type and displacement type. The velocity type expanders find their applications in large scale power generation and are not preferred in small scale power applications as their rotational speeds exponentially increase with a decrease in expander power output. As a result, the displacement type expanders are best suited for small scale power generation in ORC. Yet, till date expanders capable of producing power at small scale are not commercially available in the market for ORC application. As an effect commercially available scroll compressors are modified to work as expanders in ORC systems. The present study aims to examine the feasibility of using one such scroll compressor as an expander in ORC with and without modification. A test rig was developed to test the compressor running in reverse as expander using compressed air, before and after modifications. The scroll machine was tested for operating conditions consisting of pressure varying from 0.5 bar to 4.5 bar and the load varying from 0.2 kg to 2.2 kg for a constant airflow rate. The configurations tested were, scroll compressor with the suction port as inlet, modified scroll compressor with the suction port as inlet and modified scroll compressor with discharge port as an inlet. Based on the experimental test data obtained it is observed that, in all three configurations, for various loading conditions at given inlet air pressure there exists a maximum power generation point and a further increase in the load at given pressure has a negative effect on power output. Also, a significant increase in speed is observed from 300 to 4250 rpm at no load condition with increasing inlet air pressures. Maximum power of 210 W was achieved at a load of 1.1 kg with the inlet pressure of 4.5 bar for modified scroll machine when the discharge port was used as an inlet. Finally, it is recommended to use a modified scroll machine with discharge port as inlet as it gives more power when compared with other configurations for the same operating conditions.
Highlights
In present days increased consumption of fossil fuels has caused environmental problems, ozone layer depletion, air pollution, acid rain and global warming [1]
The velocity type expanders find their applications in large scale power generation and are not preferred in small scale power applications as their rotational speeds exponentially increase with a decrease in expander power output
The results obtained from the tests conducted show that the pressure has a positive effect on speed for all three configurations as shown in figure 7 but, the configuration of modified compressor with discharge port as inlet has the highest speed in the range of 4200 rpm at 4.5 bar pressure whereas, the other two configurations had the speed in the range of 4000 rpm
Summary
In present days increased consumption of fossil fuels has caused environmental problems, ozone layer depletion, air pollution, acid rain and global warming [1]. The criteria for market survey mainly include temperature range, pressure range, volume ratio, type of the working fluid and modification capability [8] With all these factors taken into account, a scroll compressor of an automobile air conditioning system manufactures by the company Mitsubishi shown in figure 3 was selected. When the conventional compressor is made to work straight the refrigerant enters the compressor through the suction port and leaves through discharge port after compression but, to use it as expander the flow has to be reversed i.e., the high-pressure refrigerant has to be sent through the inlet This high-pressure refrigerant expands itself by imparting energy to orbiting scroll with which shaft power can be obtained, but after removal of a non-return valve (generally named as modified scroll expander), either suction or discharge port can be used as an inlet.
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