Experimental analysis and comparison between CO₂ transcritical power cycles and R245fa organic Rankine cycles for low-grade heat power generations

In this study, experimental investigations were conducted on two different test rigs to investigate and compare the performances of CO₂ transcritical power cycles (T-CO₂) and R245fa organic Rankine cycles (ORC) for low-grade heat power generations. Each test rig consisted of a number of essential components including a turboexpander with a high speed generator, finned-tube air cooled condenser, liquid pump and plate-type gas generator/evaporator. The exhaust flue gases from an 80 kWe micro-turbine CHP unit were utilised as heat sources for both T-CO₂ and R245fa ORC power generation systems and hot thermal oil flow was applied commonly as a heat transfer medium. Both test rigs were fully commissioned and instrumented from which comprehensive experimental investigations were carried out to examine the effects of various important operational parameters on system performance. These include working fluid mass flow rate and heat source input etc. at constant heat sink (ambient) parameters. Results showed that with a fixed heat source input, the turbine power generation and overall efficiency of the R245fa ORC or T-CO₂ system could be improved significantly at higher working fluid mass flow rates. Quantitatively, when the CO₂ and R245fa mass flow rates increased respectively from 0.2 kg/s to 0.26 kg/s and from 0.23 kg/s to 0.27 kg/s, the corresponding turbine power generation increased by 88.2% and 27.3% while the respective turbine overall efficiency enhanced by 35.4% and 7.5%. On the other hand, the turbine power generation and overall efficiency of the R245fa ORC or T-CO₂ system increased variably with higher heat source input when the working fluid mass flow rate is fixed. In percentage, when the heat source inputs of the T-CO₂ and R245fa ORC systems increased respectively from 52 kW to 60 kW and 61 kW to 68 kW, the corresponding turbine power generation increased 47.7% and 63% while the respective turbine overall efficiency enhanced 8.65% and 1.08%. In addition, the cycle point temperatures and pressures of both systems revealed similar increments at higher working fluid mass flow rates or at higher heat source inputs. Furthermore, heat transfer analyses of both CO₂ gas generator and R245fa evaporator can be used to set up efficient controls of working fluid superheating at the heat exchanger outlet. The test results and analyses are essential in evaluating and comparing both systems’ operations at different operating conditions, design structures and components, and can significantly contribute towards optimal component selections and system performance controls.