Optimization of waste heat utilization in oil field development employing a transcritical Organic Rankine Cycle (ORC) for electricity generation

This study describes the low temperature waste heat exploitation during enhanced crude oil development providing a 600-900 kW continuous electric power supply under deteriorating and oscillating thermal boundary conditions. The aim of this project was to optimize the heat-to-power conversion process by maximizing the net power output employing a transcritical Organic Rankine Cycle (ORC) with R134a as working fluid. Volatile supercritical thermo-physical fluid properties demand a review of heat transfer and expansion procedures. In order to design a comprehensive and dynamic unit configuration, a flexible cycle layout with an adjustable working fluid mass flow is required. The optimization developed a positive heat exchange/pressure correlation for the net power output with reasonable cycle efficiencies of around 10% employing moderate device sizes. Changing ambient temperatures from a winter night with 10 °C to a summer day with 28 °C ambient temperature, on the US West Coast, leads to net power differences of up to 200 kW for an identical cycle configuration. Positive and negative effects result from these temperature oscillations while the heat sink temperature decreases, as does the bottom pressure. Extended available temperature and pressure differences lead to higher power outputs and cycle efficiencies. Considering higher ambient temperatures, the efficiency and power increase per heat transfer performance increase is higher than at low ambient temperatures. For this reason a "large" heat exchanger is more beneficial under warm ambient conditions than under cold ones. In addition, "large" configurations do not easily run the risk of subcritical cycle operation unlike small configurations. It is therefore important to examine the ORC machine performance over a full range of ambients. Considering a likelihood of heat source temperature degradation, an aggressive cycle design does not pay off. The selection of larger heat transfer devices almost equalizes the net power output compared to smaller ones after a heat source temperature deterioration of ∼ 6 K. Sensitivity analyses in the way presented here are an indispensable tool for achieving optimal cycle layout and operation.