Reversible high-temperature heat pump/ORC for waste heat recovery in various ships: A techno-economic assessment

Abstract

An alternative waste heat recovery solution for ships is examined, relying on a reversible unit. This unit switches its operation between an organic Rankine cycle (ORC) for electricity production and a high-temperature heat pump (HTHP) for steam generation, using the refrigerant R1233zd(E), with the compressor/expander operating with a similar volume ratio that favors the performance. The waste heat of the engine's cooling water is exploited, when the ship is both at sea and at port, increasing the capacity factor. The performance of each mode has been examined at reference conditions in relation to the heat exchangers' sizing for the same compressor/expander, showing that the evaporator should be as large as possible to maximize the efficiency. The costs have been then assessed and compared with HTHP-only and ORC-only solutions, leading to the cost-optimal sizing that minimizes the discounted payback period (PBP). The reversible unit has been then up-scaled to match the steam needs and waste heat availability of several ship types, calculating the net fuel savings and PBP. The latter becomes much shorter than the one of the individual solutions, and ranges from 3.5 to 5 years, reduced to less than 4 years in case the fuel prices increase by 20%.