In general, the building sector accounts for about 40% of the final energy consumption and 36% of greenhouse gas emissions. In order to meet international emissions targets, there is a need for more efficient heating and cooling systems to reduce electricity requirements and improve system efficiency and reliability, as these systems, comprised more than 80% of residential heating use in most countries and especially in cold countries like Canada. Advanced heating, cooling, and power technologies in buildings have the potential to reduce electricity and fossil fuel usage through increased utilisation of renewable energy sources, thermal storage, micro-cogeneration and overall more efficient systems. One notable system is the organic Rankine cycle (ORC), a thermodynamic system that is capable of recovering low-temperature heat from various heat sources like solar, geothermal or low-grade thermal energy sources to cogenerate both heat and power. Of particular interest is its combination with a heat pump (HP), with which it would be able to more efficiently deliver heating and cooling, thus reducing electricity usage and pollutant emissions generation.

 For the purpose of developing efficient and lower emissions heating and cooling systems, this paper focuses on considerations for the novel combination of a HP and ORC. In this state-of-the-art review, the possibilities and benefits of combined HP-ORC systems have been investigated and discussed. The information has been obtained from a survey of available literature on the subject. Previous studies have examined a variety of combinations, including directly-coupled units, reversible combination units, parallel configurations and indirectly-coupled units. After these configurations are defined, their performance is discussed in detail. Considerations for optimising the overall architecture of these hybrid systems are identified using these same sources, touching upon heat source and sink selection, operating temperatures, thermal energy storage, expander/compressor units, control strategies, selection of working fluids, and managing more variable seasonal temperatures. Additionally, experimental studies pose more practical obstacles and areas requiring further research, and serve to highlight experimental techniques that can be applied in this area of research.

Based on the research, it was found that regional conditions such as temperatures, annual weather and energy prices have the largest impact on the economic feasibility of these systems, and partially dictate the selection of the overall system configuration. Also identified was the importance of a higher temperature differential between the heat source and heat sink, further highlighting the importance of proper heat source and sink selection, working fluid selection as well as thermal storage to maintain this difference. Additional optimisation on an individual component level and with advanced or predictive control strategies have a smaller impact than the previous revelations, and should be evaluated economically as they may not be viable for the given system. Finally, from the studies explored, some areas for future research were recommended, including a comparison of technology configurations to understand the optimal system for different regions, a sensitivity analysis to understand key system factors for optimisation and design, an investigation and testing of currently available units and applicable systems, and the identification of novel use cases.

*Keywords:*

Heat Pump (HP), Organic Rankine Cycle (ORC), Micro-combined heat, cooling and power, micro-co/trigeneration, Building application


Heat Pump-Organic Rankine Cycle Hybrid Systems for Co/tri-Generation Applications: A State-of-the-Art Overview

Division / Sponsor:
Advanced Energy Systems Division (AESD) & Solar Energy Division (SED)

Paper Proceedings: https://asmedigitalcollection.asme.org/ES/ES2020/volume/83631

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