A new thermodynamic cycle has been developed for the simultaneous production of power and cooling from low-temperature heat sources. The proposed cycle combines the Rankine and absorption refrigeration cycles, providing power and cooling as useful outputs. Initial studies were performed with an ammonia-water mixture as the working fluid in the cycle. This work extends the application of the cycle to working fluids consisting of organic fluid mixtures. Organic working fluids have been used successfully in geothermal power plants, as working fluids in Rankine cycles. An advantage of using organic working fluids is that the industry has experience with building turbines for these fluids. A commercially available optimization program has been used to maximize the thermodynamic performance of the cycle. The advantages and disadvantages of using organic fluid mixtures as opposed to an ammonia-water mixture are discussed. It is found that thermodynamic efficiencies achievable with organic fluid mixtures, under optimum conditions, are lower than those obtained with ammonia-water mixtures. Further, the refrigeration temperatures achievable using organic fluid mixtures are higher than those using ammonia-water mixtures.
Hydrogen yield of conventional biomass gasification is limited by chemical equilibrium constraints. A novel technique that has the potential to enhance the hydrogen yield by integrating the gasification and absorption reactions has been suggested. The method involves gasification of biomass in presence of a CO2 sorbent. Ethanol was used as the model biomass compound and CaO was the representative sorbent. Equilibrium modeling was used to determine the product gas composition and hydrogen yield. The analysis was done using ASPEN PLUS software (version 12.1) and the Gibbs energy minimization approach was followed. The effects of temperature, pressure, steam/ethanol ratio, and CaO/ethanol ratio on product yield were investigated. Three case studies were conducted to understand the effect of sorbent addition on the hydrogen yield. Thermodynamic studies showed that the use of sorbents has the potential to enhance the equilibrium hydrogen yield of conventional gasification by ∼19% and reduce the equilibrium CO2 content of product gas by 50.2%. It was also found that the thermodynamic efficiency of sorbent-enhanced gasification (72.1%) was higher than conventional gasification (62.9%). Sorbent-enhanced gasification is a promising technology with a potential to improve the yield and lower the cost of hydrogen production.
A combined power and cooling cycle is being investigated. The cycle is a combination of the Rankine cycle and an absorption refrigeration cycle. Evaluating the efficiency of this cycle is made difficult by the fact that there are two different simultaneous outputs, namely power and refrigeration. An efficiency expression has to appropriately weigh the cooling component in order to allow comparison of this cycle with other cycles. This paper develops several expressions for the first law, second law and exergy efficiency definitions for the combined cycle based on existing definitions in the literature. Some of the developed equations have been recommended for use over others, depending on the comparison being made. Finally, some of these definitions have been applied to the cycle and the performance of the cycle optimized for maximum efficiency. A Generalized Reduced Gradient (GRG) method was used to perform the optimization. The results of these optimizations are presented and discussed.
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