System Improvement and Thermodynamic Optimization of an Organic Rankine Cycle Based Geo-plant in an Oilfield in China

Abstract

With respect to an actual geo-plant in China, the authors have identified four critical measures. They are, removing the plate heat exchangers (PHEs), installing an internal heat exchanger (IHE), adding a preheater, and replacing the working fluid, are proposed, aiming to improve the system performance. Starting from the original system, the four measures are implemented in a step-wise manner. The modified system is compared with the original one by performing the energy and exergy analyses. The results indicate that the four measures are effective to the system improvement, and the detailed findings are as follows: (1) Heat source directly transferring heat to the working fluid is preferable, especially for the low- and medium-grade geothermal water. (2) Installing an IHE reduces the power consumption of the cooling pump. (3) Installing an IHE or adding a preheater can enhance the specific enthalpy at the evaporator inlet, thereby improving the system performance. (3) Isopentane exhibits the most attractive performance and its flammability can be suppressed by mixing it with R245fa. (4) The optimal R245fa mass fraction recommended in the mixture is 0.45 in engineering applications. Overall, the modified system is shown to be able to improve the net power output from 270 to 458.3kW (an increase of about 70%). The conclusions made in this study are not limited to the specific geo-plant considered, and might be extended to the design and operation of other geo-plants alike.