• Energy Research

Abstract The need for high efficiency energy systems is of vital importance, due to depleting reserves of fossil fuels and increasing environmental problems. Industrial operations commonly feature the problem of rejecting large quantities of low-grade waste heat to the environment. The aim of this work is to develop methods for the conceptual screening and incorporation of low-temperature heat upgrading technologies in process sites. The screening process involves determination of the best technology to upgrade waste heat in process sites, and the combination of waste heat source and sink temperatures for a technology. Novel simplified models of mechanical heat pumps, absorption heat pumps and absorption heat transformers are proposed to support this analysis. These models predict the ratio of the real performance to the ideal performance in a more accurate way, than previous simplified models, taking into account the effect of changing operating temperatures, working fluids non-ideal behaviour and the system component inefficiencies. A novel systems-oriented criterion is also proposed for conceptual screening and selection of heat pumps in process sites. The criterion (i.e. the primary fuel recovery ratio) measures the savings in primary fuel from heat upgraded, taking into account power required to drive mechanical heat pumps and missed opportunities for steam generation when absorption systems are used. A graphical based methodology is also developed for applying the PRR in process sites and applied to a medium scale petroleum refinery. Results show that applying the PRR yields 9.2% additional savings in primary fuel compared to using the coefficient of performance to screen and incorporate heat pumps.

Abstract Thermodynamic cycles such as organic Rankine cycles, absorption chillers, absorption heat pumps, absorption heat transformers, and mechanical heat pumps are able to utilize wasted thermal energy in process sites for the generation of electrical power, chilling and heat at a higher temperature. In this work, a novel systematic framework is presented for optimal integration of these technologies in process sites. The framework is also used to assess the best design approach for integrating waste heat recovery technologies in process sites, i.e. stand-alone integration or a systems-oriented integration. The developed framework allows for: (1) selection of one or more waste heat sources (taking into account the temperatures and thermal energy content), (2) selection of one or more technology options and working fluids, (3) selection of end-uses of recovered energy, (4) exploitation of interactions with the existing site utility system and (5) the potential for heat recovery via heat exchange is also explored. The methodology is applied to an industrial case study. Results indicate a systems-oriented design approach reduces waste heat by 24%; fuel consumption by 54% and CO2 emissions by 53% with a 2 year payback, and stand-alone design approach reduces waste heat by 12%; fuel consumption by 29% and CO2 emissions by 20.5% with a 4 year payback. Therefore, benefits from waste heat utilization increase when interactions between the existing site utility system and the waste heat recovery technologies are explored simultaneously. The case study also shows that the novel methodology can select and design optimal solutions for waste heat exploitation which are technically, economically and environmentally feasible from a range of technology options, heat sources and end-uses of recovered energy.