Potential and Cost Effectiveness of a Reversible High-temperature Heat Pump/ORC Unit for the Exploitation of Industrial Waste Heat

orc2019 Tracking Number 23

ORC units have demonstrated their potential for the exploitation of waste heat sources in industry, with many applications even at low-temperatures of 100 oC. At the same time, high-temperature heat pumps have been developed for various industrial sectors to exploit the waste heat thus producing useful heat, and subsequently recycling the heat quantities instead of rejecting them to the ambient. These two components can be combined within a single unit, by reversing the heat pump cycle for operation as a heat-to-power unit, resulting to an ORC. This is achieved with few modifications, such as the addition of a pump for ORC mode and minor auxiliaries. The resulting reversible high-temperature heat pump/ORC can then exploit the low-temperature industrial waste heat and produce either electricity (ORC operation) or upgraded heat (heat pump operation), according to: (1) the real-time needs for process heating and electricity, and (2) the economic benefits derived from each operating mode, which depend on the local energy prices and fluctuations. The current work investigates the potential of this reversible concept at industrial settings. Focus is given on the exploitation of low-temperature waste heat of 100 oC, in order for the heat pump to produce useful heat with a temperature lift of up to 50 K, which is close to its upper temperature limit, while at ORC mode the thermal efficiency is 4.5%. The maximum heat pump capacity is 1000 kW, defining accordingly the maximum ORC capacity, representing the actual sizes in small/medium-scale industries, and considering a variable operating load. Various scenarios are examined that lead to different operational hours of each mode, according to the temporal profile of the heating load and the heat pump capacity. The results of this analysis show the energy production of each mode, as well as of the whole reversible unit. To quantify the benefits of this reversible unit, a cost analysis is implemented, using average energy prices for electricity and gas of medium-sized EU industries, subjected to a sensitivity analysis. Various scenarios are considered, showing that the payback period of the high-temperature heat pump only can be as short as 5 years, and of the complete reversible unit lower by about 20% in some conditions. Therefore, the reversibility option except from introducing superior flexibility on operation, it also enhances the cost-effectiveness and fully justifies the more complex configuration.