Organic rankine cycle with solution circuit for low-grade heat recovery

Abhinav Krishna, Purdue University

Abstract

Increasing interest in utilizing low-grade heat for power generation has prompted a search for ways in which the power conversion process may be enhanced. A novel Organic Rankine Cycle with Solution Circuit (ORCSC) using Carbon Dioxide / Acetone as the working fluid pair was studied for this purpose. A thermodynamic simulation model was developed and an experimental test stand was built to serve as a proof of concept for the technology. The thermodynamic model showed that the ORCSC using Carbon Dioxide / Acetone as the working pair offers no significant efficiency improvements over a conventional Organic Rankine Cycle (ORC) using only Carbon Dioxide as the working fluid. Furthermore, the ORCSC with a Carbon Dioxide / Acetone working pair has significantly lower performance than an ORC using conventional working fluids such as pentane or R245fa. This may render the ORCSC unattractive since the low-temperature heat sources mean that the theoretical (Carnot) efficiency limit is itself relatively low, and achieving cycle efficiencies as close to the Carnot limit as possible is necessary for ensuring the economic feasibility of the technology. However, the ORCSC was found to have significantly lower working pressures than an ORC, provides the ability to use temperature glide to match the temperature profiles of the source and sink fluids and facilitates intrinsic capacity control. This may lead to higher overall system efficiencies when coupled with sources that have varying heat input temperatures or loads. More application-specific studies that address the nature and capacity of the source and sink streams are required to identify where this ability may be most advantageous. The experimental tests showed good agreement with the simulation data when all the boundary conditions were matched. However, the efficiencies of the system were generally poor and many of the expected trends were skewed due to design shortcomings and the use of equipment that was not optimized for the ORCSC system. Isolation of individual parameters was an acute challenge due to the number of variables that need to be tightly controlled during system operation. Nevertheless, the experimental results provided a validation of the simulation model. The simulation model was expanded to include a parametric study of the various components on the overall system performance. It showed that the ORCSC is particularly sensitive to the performance of the expander and pump. Amongst the heat exchangers, the performance of the absorber had the greatest impact on the overall system performance. It is clear from this study that a range of practical considerations need to be taken into account and weighed together with the thermodynamic analysis when evaluating the feasibility of ORCSC technology. The ORCSC offers some potential practical advantages which may outweigh the added cost and complexity of these systems in certain applications. However, the maturity of the technology and associated body of literature is limited, and further work needs to be pursued in this area before widespread adoption of the technology is possible.

Degree

M.S.M.E.

Advisors

Garimella, Purdue University.

Subject Area

Mechanical engineering|Sustainability|Energy

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