Selection of Working Fluid in Organic Rankine Cycle Integrated into Combined Cycle Cogeneration Plant

Abstract

열병합발전 (Cogeneration)의 효율을 증가시키기 위해 유기랭킨사이클 (Organic Rankine cycle)을 접목시키는 연구가 이루어지고 있으며, 지역난방수를 유기랭킨사이클 열원으로 사용할 경우에 성능과 경제성 면에서 타당하다고 알려져 있다. 본 연구에서는 작동유체의 물성치가 유기랭킨사이클과 열병합발전 하부사이클 (bottoming cycle)에 미치는 영향을 분석하고, 작동유체에 따른 유기랭킨사이클 터빈 크기를 비교하였다. 본 해석결과를 바탕으로 향후 주어진 설계 상황에 적절한 작동유체를 제안하는 것을 최종 목표로 하였다. T-s 선도 기울기, 안정성, 환경적인 부분을 고려하여 R-123, R-245fa, R-245ca 작동유체를 선정하였다. 작동유체의 물성치 중 기화잠열이 유기랭킨사이클과 복합열병합발전 성능에 영향을 크게 미쳤다. R-123은 하부사이클 출력 38.49 MW, 유기랭킨사이클 출력 1.95 MW로 출력 부분에서 가장 낮게 나타내었다. R-245ca는 유기랭킨사이클 터빈 출구에서 20.5 m3 /s의 가장 많은 체적유량을 나타내어 터빈 크기가 증가하는 단점을 가지고 있으나, 하부사이클 출력 38.68 MW, 유기랭킨사이클 출력 2.61 MW로 출력 부분에서는 가장 높게 나타내었다. R-245fa는 R-245ca를 작동유체로 사용하였을 때보다 전체 출력을 250 kW 낮게 나타내었지만, 유기랭킨사이클 터빈 출구에서 14.96 m3/s의 가장 적은 체적유량을 나타내어 작은 크기의 터빈을 사용할 수 있는 가능성을 나타내었다.|In order to increase the efficiency of cogeneration, researches have been conducted to combine organic Rankine cycle (ORC) and it is known that it is feasible in terms of performance and economic feasibility when operating ORC with district heating as heat source. In this study, the effects of working fluid properties on ORC and cogeneration bottoming cycle were analyzed and the size of ORC turbine according to working fluid was compared. Based on the results of this numerical analysis, the final aim was to propose an appropriate working fluid for the given design situation. The working fluids of R-123, R-245fa and R-245ca were selected in consideration of the T-s diagram slope, environmental effect and safety. The latent heat of vaporization in the properties of the ORC working fluid had a significant effect on the ORC and cogeneration bottoming cycle performance. The R-123 produces the lowest overall cycle output of 38.49 MW of bottoming cycle output and 1.95 MW of ORC output. The R-245ca has the disadvantage of the increased turbine size by showing the largest volume flow rate of 20 m3 /s at the ORC turbine outlet. However, the bottoming cycle output was 38.68 MW and the ORC output was 2.61 MW, which was the highest output. The R-245fa indicated the possibility of using a compact turbine size because of the smallest volume flow of 14.96 m3/s at the ORC turbine outlet, although the overall output is 250 kW lower than the case of R-245ca.; In order to increase the efficiency of cogeneration, researches have been conducted to combine organic Rankine cycle (ORC) and it is known that it is feasible in terms of performance and economic feasibility when operating ORC with district heating as heat source. In this study, the effects of working fluid properties on ORC and cogeneration bottoming cycle were analyzed and the size of ORC turbine according to working fluid was compared. Based on the results of this numerical analysis, the final aim was to propose an appropriate working fluid for the given design situation. The working fluids of R-123, R-245fa and R-245ca were selected in consideration of the T-s diagram slope, environmental effect and safety. The latent heat of vaporization in the properties of the ORC working fluid had a significant effect on the ORC and cogeneration bottoming cycle performance. The R-123 produces the lowest overall cycle output of 38.49 MW of bottoming cycle output and 1.95 MW of ORC output. The R-245ca has the disadvantage of the increased turbine size by showing the largest volume flow rate of 20 m3 /s at the ORC turbine outlet. However, the bottoming cycle output was 38.68 MW and the ORC output was 2.61 MW, which was the highest output. The R-245fa indicated the possibility of using a compact turbine size because of the smallest volume flow of 14.96 m3/s at the ORC turbine outlet, although the overall output is 250 kW lower than the case of R-245ca.