Advanced Organic Vapor Cycles for Improving Thermal Conversion Efficiency in Renewable Energy Systems

The Organic Flash Cycle (OFC) is proposed as a vapor power cycle that could potentially increase power generation and improve the utilization efficiency of renewable energy and waste heat recovery systems. A brief review of current advanced vapor power cycles including the Organic Rankine Cycle (ORC), the zeotropic Rankine cycle, the Kalina cycle, the transcritical cycle, and the trilateral flash cycle is presented. The premise and motivation for the OFC concept is that essentially by improving temperature matching to the energy reservoir stream during heat addition to the power cycle, less irreversibilities are generated and more power can be produced from a given finite thermal energy reservoir.;A theoretical investigation on the OFC is conducted using the aforementioned Helmholtz-explicit equations of state for 10 different aromatic hydrocarbon and siloxane working fluids for intermediate temperature finite thermal energy reservoirs (∼300°C). Results showed that aromatic hydrocarbons to be the better suited working fluid for the ORC and OFC due to less "drying" behavior and also smaller turbine volumetric flow ratios resulting in simpler turbine designs. The single flash OFC is shown to achieve utilization efficiencies that are comparable to the optimized basic ORC (∼0.63) which is used as a baseline. It is shown that the advantage of improved temperature matching during heat addition was effectively negated by irreversibilities introduced into the OFC during flash evaporation. Several improvements to the basic OFC are proposed and analyzed such as introducing a secondary flash stage or replacing the throttling valve with a two-phase expander. Utilization efficiency gains of about 10% over the optimized basic ORC can be achieved by splitting the expansion process in the OFC into two steps and recombining the liquid stream from flash evaporation prior to the secondary, low pressure, expansion stage.;For low temperature thermal energy reservoirs (80-150°C) applicable to non-concentrated solar thermal, geothermal, and low grade industrial waste heat energy, alkane and refrigerant working fluids possess more appropriate vapor pressures. The optimized single flash OFC was again shown to generate comparable power per unit flow rate of the thermal energy reservoir than the optimized basic ORC. With some of the previously proposed design modifications though, the OFC can produce over 60% more power than the optimized ORC. For low temperature applications, the minimum temperature difference between streams in the heat exchanger, or pinch temperature, becomes an important design parameter. Reduction of the pinch temperature even slightly can yield significantly higher gains in power output, but will also increase required heat exchanger surface area and subsequently capital costs.;A high-level design of a liquid-fluoride salt (NaF-NaBF4) cooled solar power tower plant is presented; liquid-fluoride salt is used rather than current molten nitrate salts to increase the receiver temperature and subsequently allow for higher efficiency gas power cycles to be used. Graphite or direct energy storage in the salt itself is proposed. The power block component of this heliostat-central receiver plant is a combined cycle system consisting of a topping Brayton cycle with intercooling, reheat, and regeneration and a bottoming low-temperature modified OFC. The combined cycle is designed with dry cooling in mind, such that operation in desert climates are more suitable. The combined cycle design is shown to increase power block efficiencies by 6%-8% over the Brayton cycle with intercooling, reheat, and regeneration alone. An estimated 30% annual average total solar-to-electric conversion efficiency is possible with this system design, which is comparable to some of the most efficient high temperature solar power tower designs to date. Theoretically, power block efficiencies over 60% are possible; however, emission losses from the isothermal central receiver would limit the plant's operational temperature range. Results show that for high efficiency solar power towers to be realized, high temperature non-isothermal, or partitioned, receivers operating efficiently above 1000°C are necessary. Other potential areas of renewable energy system integration for the OFC include a co-generation solar thermal-photovoltaic system that employs highly concentrated, densely packed photovoltaic cells using single-phase or two-phase cooling. The thermal energy absorbed by that coolant could then be used as the working fluid in a separate OFC to further produce power in co-generation with the concentrated photovoltaics. (Abstract shortened by UMI.)...