Waste heat upgrading with high-temperature heat pumps for assisting steam generation in ships: Performance, cost and emissions benefits

An alternative waste heat recovery technology is examined based on a high-temperature heat pump (HTHP) for heat upgrading in ships. This solution exploits the low-temperature heat of the cooling water of marine engines at 85 degrees C with two integration options, for either preheating the water before entering the auxiliary (steam) boiler or directly generating 6-bar steam for a vessel's needs. An electric-driven heat pump cycle is used with an economizer for increasing the Coefficient of Performance (COP) and the heating capacity, while reducing the discharge temperature of the compressor. An ultra-low GWP refrigerant (R1233zd(E)) with a high enough critical temperature is considered. The sizing parameters of the HTHP are examined with the use of a validated numerical model attempting to find the best match between high performance and short discounted payback period (PBP). The latter is estimated with appropriate cost correlations that relate the component sizing with the equipment cost, in order to calculate the total capital cost. The operating costs on the other hand depend on the net fuel savings, i.e. fuel savings minus the fuel required to generate the electricity for the heat pump, the fixed operating cost such as for maintenance and service and the fuel price. The net emissions savings of the HTHP are also examined in a variety of ship types and sizes, based on their typical specific factors. The results show that the proposed solution leads to significant net fuel savings especially in oil tankers and cruise ships, reducing the boiler's fuel consumption by up to 2.5%. But due to the low COP of the steam generation HTHP case of around 2, a very long PBP is achieved of over 20-30 years, whereas the preheat case always leads to a very short PBP of 2-6 years in all ship types, even though its capacity is low. Moreover, the net total emissions savings are significant especially for the preheat case, although additional fuel is consumed for producing electricity to drive the HTHP, while NOx and CO emissions are increased with the steam generation HTHP.