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The world's energy supplies will continue to be 'pressured as the population grows and the standard of living rises in the developing world. A move by the rest of the world toward energy consumption rates on par with the United States is most probably unsustainable. An examination of population trends, current energy utilization rates, and estimated reserves shows that a major worldwide transition to renewable resources is necessary in the next 100 years. This paper examines one possible scenario of how energy usage and renewable power generation must evolve during this time period. As the global standard of living increases, energy consumption in developing nations will begin to approach that of the developed world. A combination of energy conservation and efficiency improvements in developed nations will be needed to push the worldwide energy consumption to approximately 200 million Btu per person per year. Fossil fuel resources will be exhausted or become prohibitively expensive, necessitating the development of renewable energy resources. At this projected steady state population and energy consumption, the required contribution of each type of renewable resource can be calculated. Comparing these numbers to the current renewable capacities illustrates the enormous effort that must be made in the next century. [DOI: 10.1115/1.4001574] Keywords: sustainability, sustainable development, renewable energy, fossil fuel, energy demand, energy consumption, energy projections

Waste heat driven ammonia/water Kalina cycles have shown promise for improving the efficiency of electricity production from low-temperature reservoirs (T < 150°C). However, there has been limited application of these systems to utilize widely available, disperse, waste heat streams for smaller scale power production (1-10 kWe). Factors limiting increased deployment of these systems include large, costly heat exchangers, and concerns over safety of the working fluid. The use of mini- and microchannel (D <1 mm) heat exchangers has the potential to decrease system size and material cost, while also reducing the working fluid inventory, enabling penetration of Kalina cycles into these new markets. However, accurate methods of predicting the heat and mass transfer in microscale geometries must be available for designing and optimizing these compact heat exchangers. In the present study, the effect of different heat and mass transfer models on the calculated Kalina cycle condenser size is investigated at representative system conditions. A detailed heat exchanger model for a liquid-coupled microchannel ammonia/ water condenser is developed. The heat exchanger is sized using different predictive methods to provide the required heat transfer area for a 1 kWe Kalina system with a source and sink temperature of 150°C and 20° C, respectively. The results show that for the models considered, predicted heat exchanger size can vary by up to 58%. Based on prior experimental results, a nonequilibrium approach is recommended to provide the most accurate, economically sized ammonia/water condenser. [ABSTRACT FROM AUTHOR]

A novel thermal management approach is explored, which uses supercritical carbon dioxide (SCO2 ) as a working fluid to manage extreme heat fluxes in electronics cooling applications. In the pseudocritical region, SCO2 has extremely high volumetric thermal capacity, which can enable operation with low pumping requirements, and without the potential for two-phase critical heat flux (CHF) and flow instabilities. A model of a representative microchannel heat sink is evaluated with single-phase liquid water and FC-72, two-phase boiling R-134a, and sC02. For a fixed pumping power, sC02 is found to yield lower heat-sink wall temperatures than liquid coolants. Practical engineering challenges for supercritical thermal management systems are discussed, including the limits of predictive heat transfer models, narrow operating temperature ranges, high working pressures, and pump design criteria. Based on these findings, sC02 is a promising candidate working fluid for cooling high heat flux electronics, but additional thermal transport research and engineering are needed before practical systems can be realized. [ABSTRACT FROM AUTHOR]