(118a) Exergy Analysis of Modular Organic Rankine Cycle (ORC) Power Plants

This research investigates thermodynamic and economic feasibility of using modular Organic Rankine Cycle (ORC) power plants for generation of electricity from geothermal fluids or waste heat coming from engine exhaust gases or cement plants. Analysis of market demand for ORC power plants is also performed. Thermodynamic characteristics of different heat sources are analyzed and a set of design conditions maximizing ORC unit performance for the most promising applications is determined. A thermodynamic model of ORC power plant using recuperator and wet mechanical-draft cooling tower was developed. Four hydrocarbon working fluids were selected for detailed modeling to determine their thermodynamic performance for a year-round operation. For a heat source at 150 °C, isopentane provides the best thermodynamic performance. Isopentane and n-pentane were proved to be unable to fully utilize low outdoor temperatures, which resulted in a significant decrease in annual energy output. Use of n-butane reduces the size and cost of heat exchangers when compared to isobutene, but not enough to compensate for the decrease in net power output.

Thermoeconomic optimization is the main feature of the design procedure. This method is based on exergy analysis and allowed us to find the optimal power plant configuration and size of the heat exchangers. Traditional approaches to design and optimization of power plants focus on maximizing fuel utilization efficiency. Our approach is different and is centered on cost-optimal design of each component to minimize the levelized cost of electricity (LCOE) generated by the power plant.

As the first stage of thermoeconomic optimization, a detailed economic analysis is made for four different applications: geothermal plants using water from conventional hydrothermal or former oil and gas reservoirs, as well as waste heat recovery plants coupled with diesel engines or clinker coolers in cement plants. Case-specific parameters, such as interest rates, project lifetime, load factor or type of steel used for heat exchangers were specified for each application. Costs of single components are described by non-linear equations in which a key characteristic parameter that scales with component cost is used as a variable. Exergy flow rates in each of the systems are calculated and costs are assigned to these streams in order to identify cost-ineffective processes.

Analysis has shown that, depending on the type of application, the levelized cost of electricity ranges from 5.07 to 7.55 U.S. ¢(2008)/kWh, making modular ORC power plants thermodynamically and economically feasible. While the majority of commercially available ORC waste heat recovery units do not incorporate recuperators, the cost-optimal design for each application considered in this study included this component. In geothermal power plants, use of regenerative heat exchange lowers the utilization efficiency, if based on incoming exergy of heat source. However, recuperator also reduces the size of preheater and evaporator made out of costly stainless steel and lowers LCOE. In waste heat recovery applications the LCOE is somewhat lower than for the best design without the regenerator. In the same way as in the geothermal unit, the regenerator increases the thermal efficiency of the cycle. In waste heat applications, decreased efficiency of heat recovery has a limited impact.

Finally, sensitivity analysis shows the impact of heat source characteristics and macroeconomic variables on levelized cost of power. As expected, economic performance of geothermal units is highly sensitive to temperature and flow rate of geothermal water.