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

  • Author(s): Ho, Tony
  • Advisor(s): Greif, Ralph
  • Mao, Samuel S
  • et al.
Abstract

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. In this study, modern equations of state explicit in Helmholtz energy such as the BACKONE equations, multi-parameter Span-Wagner equations, and the equations compiled in NIST REFPROP 8.0 were used to accurately determine thermodynamic property data for the working fluids considered. Though these equations of state tend to be significantly more complex than cubic equations both in form and computational schemes, modern Helmholtz equations provide much higher accuracy in the high pressure regions, liquid regions, and two-phase regions and also can be extended to accurately describe complex polar fluids. Calculated values of saturated liquid and vapor density and vapor pressure were then compared to values listed in the NIST Chemistry WebBook to ensure accuracy for the temperature range of interest. Deviations from the NIST WebBook were typically below 1%; a comparison of first law efficiencies for an ideal basic Rankine cycle yielded less than 0.4% difference between calculations using the Helmholtz-explicit equations of state and NIST REFPROP. Also by employing the BACKONE and Span-Wagner equations, the number of potential aromatic hydrocarbon and siloxane working fluids that are appropriate for high and intermediate temperature applications is expanded considerably.

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 (~300oC). 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. Results show that the proposed enhancements had a more pronounced effect for the OFC using aromatic hydrocarbon working fluids (5-20% utilization efficiency improvement) than for siloxane working fluids (2-4%). The proposed modifications were aimed towards reducing irreversibility in flash evaporation; it was observed for siloxanes that the primary source of irreversibility was due to high superheat at the turbine exhaust because of the highly "drying" nature of the fluid. Though an order of magnitude analysis, results showed that the OFC and ORC to require similar heat transfer surface areas.

For low temperature thermal energy reservoirs (80-150oC) 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 1000oC 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.

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