Performance of working fluid mixtures in an ORC-CHP system for different heat demand segments

Organic Rankine cycle (ORC) power systems are being increasingly deployed for waste heat recovery and conversion to power in several industrial settings. In the present paper, we investigate the deployment of working-fluid mixtures in ORCs operating in combined heat and power mode (ORC-CHP) with shaft power provided by the expanding working fluid and heating provided by the cooling-water exiting the ORC condenser. Using the flue gas from a refinery boiler as the waste-heat source and with working fluids comprising normal alkanes, refrigerants and their subsequent mixtures, the ORC-CHP system is demonstrated as being capable of delivering over 20 MW of net shaft power and up to 15 MW of heating, leading to a fuel energy savings ratio (FESR) in excess of 20%. Single-component working fluids such as pentane appear to be optimal at low hot-water supply temperatures. Working-fluid mixtures become optimal at higher temperatures, with the working-fluid mixture combination of octane and pentane giving an ORCCHP system design with the highest efficiency. However, in most CHP applications, the fluctuation of heat demand can determinate a discharge of heating, in particular when a waste-heat source makes profitable the system operation also in only electricity mode, and if thermal storage options are not considered. For this reason, the influence of heat demand intensity on the global system conversion efficiency and optimal working fluid selection is also explored.