Improved thermal energy utilization through coupled and cascaded cooling cycles
Brown, Ashlie M.
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Limited worldwide energy supplies demand the improved utilization of thermal energy, which is the dominant form of all primary energy sources used today. Large quantities of waste heat are routinely exhausted wherever thermo-mechanical energy conversion occurs, providing an opportunity to improve utilization. Two waste-heat-driven cycles are analyzed: an absorption/compression cascade cooling cycle and a coupled Rankine/compression cycle. The absorption/compression cascade provides an environmentally-sound option for a common approach to thermal energy recovery: the use of absorption cycles for cooling applications. To achieve cooling at temperatures below 0ºC, ammonia-water is the overwhelming choice for the working fluid. However, concerns about the toxicity and flammability of ammonia sometimes limit its application in sensitive arenas. In this study, a lithium bromide-water absorption cycle is coupled with a carbon dioxide vapor compression cycle to realize the benefits of high-lift cooling without the concerns associated with ammonia. This cycle utilizes a waste heat stream at temperatures as low as 150°C to provide cooling at -40°C. The topping absorption cycle achieves a coefficient of performance (COP) of about 0.77, while the bottoming cycle achieves a COP of about 2.2. The coupled Rankine/compression cycle provides a mechanical expansion and compression approach to achieve thermally activated cooling, again driven by waste heat. The power produced in the turbine of the Rankine cycle is directly coupled to the compressor of a vapor-compression cooling cycle to generate cooling to be utilized for space-conditioning. The refrigerant R245fa is used throughout the cycle. Even with low grade waste heat sources, a Rankine cycle efficiency of about 11-12 percent can be achieved. When coupled to the bottoming compression cycle with a COP of about 2.7, this yields an overall waste heat to cooling conversion efficiency of about 32 percent at nominal conditions.