Performance and economic evaluation of a reversible high-temperature heat pump/ORC for waste heat recovery in the marine sector

The prevailing technology for waste heat recovery in ships is the ORC, which can exploit a variety of waste heat sources to produce electricity, and thus reduce the fuel consumption of the generator engines. The main challenge is to increase the capacity factor and exploit the highest possible amount of the various waste heat sources on-board, preferably of elevated temperature, in order to reach a high electric power output. The exploitation of the thermal content of the exhaust gases usually introduces severe constraints, since heat is extracted from this gas flow first from the engine’s turbocharger and then by the exhaust gas boiler (economizer) for steam production in the case of the main engine. Moreover, the main engine in a typical merchant ship operates for about two thirds of the year, which might not be enough to secure the ORC cost-effectiveness when only its low-grade heat source is exploited. On the other hand, the generator engines operate for an even larger duration, but with a much lower capacity. With the aim to increase the capacity factor (operating days/year), an advancement of the ORC technology is examined here, which exploits low-temperature waste heat sources. Such configuration is based on a reversible cycle, operating either as an ORC or a high-temperature heat pump (HTHP). The heat pump mode produces steam, replacing the use of auxiliary boilers, which mostly operate when the ship is at port. When the ship is at sea, no steam is needed by those boilers, because the steam generation is accomplished by the exhaust gas economizer (exhaust gas boiler), and then the unit reverses its operation to ORC mode for electricity production. This flexible unit is examined in terms of performance for producing saturated steam at 6 bar/158 ⁰C (heat pump mode) and electricity (ORC mode), when using an ultra-low global warming potential (GWP) refrigerant. A detailed techno-economic study has then been conducted for a variety of longdistance ships, according to their typical boiler capacity that is matched to the heat pump capacity and days at sea/port, leading to the estimation of the net fuel savings and discounted payback period. The latter becomes short in the range of 3-4 years for the reversible unit when the fuel price is increased.     «

The prevailing technology for waste heat recovery in ships is the ORC, which can exploit a variety of waste heat sources to produce electricity, and thus reduce the fuel consumption of the generator engines. The main challenge is to increase the capacity factor and exploit the highest possible amount of the various waste heat sources on-board, preferably of elevated temperature, in order to reach a high electric power output. The exploitation of the thermal content of the exhaust gases usually i...     »