| Title | New Concepts for R245fa Direct Orc Application in High Temperature Process Heat Systems |
|---|---|
| Authors | H. Abbas, B. Habib, M. Farid |
| Year | 2016 |
| Conference | New Zealand Geothermal Workshop |
| Keywords | ORC, innovative concept, gas, R245fa refrigerant, evaporator, finned tube, skin temperature, decomposition, porous media, CFD, ANSYS/CFX. |
| Abstract | The application of non-flammable refrigerants (R245fa) for direct heat extraction from high temperature heat source (above 380°C) is normally limited due to the risk of refrigerant decomposition, where a decomposition threshold of 250°C was reported. The option is to incorporate indirect heat extraction loop using heat transfer fluid. This option is associated with an additional cost and complications compared to that encountered in direct heat extraction. In this paper, two new concepts were proposed for direct heat extraction. The concepts were studied analytically and computationally. A reference design of highly finned tube evaporator was used in both options. In the first, a harvested high temperature gas (up to 470°C) was mixed with different portions of intake air to provide adequate temperature control of the gas inlet temperature. Three analytical studies were developed and several CFD models were generated. More than twenty different operating conditions were assessed. With this concept the significant fluctuations in the harvested gas temperatures was mitigated and the workable zone was determined. No considerable increase was found in the resulting pressure drop. However, overall performance showed slight decrease in thermal duty compared to that obtained with high intensity temperatures of the harvested gas. Applying this eliminates the risk of the refrigerant reaching localized decomposition temperature. In the second concept, open pore metal foam was implemented inside the tubes at effective locations where high risk of refrigerant decomposition could occur. Evaporator performance associated with the harvested gas was compared. Significant improvement in thermal performance was achieved resulting in considerable reduction in the size of the heat exchanger and elimination of hot zones associated with refrigerant decomposition. The penalty was a significant increase in pressure drop, which strongly increased with an increase in mass velocity. Implementing porous media at effective locations is the key parameter in this novel concept where the pressure drop was controlled under acceptable level. |