Organic Rankine Cycle Design and Feasibility Analysis

Photo of Brandon W. Beck, Thomas A. Browder, and Stephen J. Swan Students: Brandon W. Beck, Thomas A. Browder, and Stephen J. Swan

Sponsor: Chevron

Date: Spring 2010

A critical aspect of the ORC system design is integration with the existing co-generation unit. This requirement poses several key constraints that effect design, analysis, and operation of the ORC. First, the system must be adapted to the local climate. Bakersfield, CA falls within an arid region without a readily accessible cooling water source. Thus, condensing must be performed using ambient air cooling, which lowers the thermodynamic efficiency and increases the complexity of the system. This design requirement causes the flow rate and power output to vary with ambient temperature. Next, as described in the cycle schematic displayed below, the "fuel" or energy source, for the ORC is the low-temperature exhaust gas. This exhaust gas enters our system at a pressure of 103.3 kPa. We may not induce an exhaust pressure drop of more than 2 kPa in the heat exchange process within our system, or it will stagnate. In addition, for every 1 kPa of exhaust pressure drop, there is a 0.42% power loss in the existing gas turbine. The exhaust flow rate also changes with ambient temperature: hotter weather yields lower flow rates and less available energy. These factors combined create a strong variance in ORC system output based on the ambient temperature.

Growing energy demand, volatile fuel prices, and increasingly strict regulation of power plant emissions have led companies to investigate means to make their systems more efficient. One of the best established efficiency enhancement techniques is waste heat utilization, which is common in co-generation and combined cycle power plants throughout the world. However, even the most efficient co-generation plants release a meaningful amount of energy into the atmosphere as low-temperature (≈ 200-300 °C) exhaust gas, because traditional waste heat recovery technology is uneconomical below 370 °C. This type of thermal energy is typically called low-quality, because of the inevitably low efficiency associated with its conversion into a useful product. One promising heat recovery technology, which could allow viable recovery of this low-quality energy, is an Organic Rankine Cycle (ORC). An ORC operates on the same principles as a traditional steam Rankine cycle used in coal and nuclear power plants, but utilizes an organic working fluid that boils at much lower temperatures in order to effectively utilize a low-temperature heat source. This project seeks to recover the maximum amount of energy currently exhausted from a co-generation unit with an 85 MW gas turbine in Bakersfield, CA to produce electricity using an ORC.

The first key step in ORC design is working fluid selection. We developed thermodynamic and heat transfer models to compare thermal conversion efficiency and power producing capability of various organic fluids. In addition, we considered factors such as flammability, toxicity, availability, and cost in our selection process. Based on these metrics, we selected Refrigerant 123 (R123) as our working fluid. We then contacted Chevron's preferred manufacturers to identify industrially available equipment to design the system. The key components for system design and implementation are a centrifugal pump, heat exchangers for the evaporating and condensing processes, a turbo-expander, and a generator. We obtained footprint, cost, and efficiency data for each component from the vendors, which we used to refine our system model, develop a plant layout, and determine the sensitivity of each component to the viability of the system. We learned from this process that the heat exchangers were the most significant component of the system from a performance, footprint, and cost perspective. Based on our refined model, we found the cycle net electric output and system conversion efficiency over our range of ambient temperatures. Based on these data and average daily temperatures for Bakersfield, CA, we calculated the average yearly electricity output of the plant, which was just over 50 GWh, at an average electric conversion efficiency of about 14%. This means our system would produce an additional 7% of electric output for the plant over the course of the year. Considering a MARR of 10%, the break even cost of electricity produced would be $0.12/kWh.

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