Waste heat recovery for marine application
Dredging and offshore vessels are generally powered by diesel or dual-fuel engines. These engines are used as they are robust and have a high power density. However, during the fuel combustion process, these engines also emit greenhouse gases and harmful emissions. Additionally, a significant amount of energy is rejected in the form of heat during their operation and not used for power generation.
Engine manufacturers optimise their engine design to the improve engine efficiency (less waste heat) and reduce emissions. However, the engine efficiency improvement is limited by the Carnot theorem. This limits the efficiency improvement of the in-cylinder processes and reducing the heat transfer to the cooling water is challenging due to the almost adiabatic in-cylinder process (Klimstra, 2006). Internal combustion engines generally waste 50% or more of the valuable fuel energy as heat through the exhaust gas, cooling water loops (both low temperature and high temperature) and radiation.
Effective use of the waste heat flows helps to improve the overall system efficiency and reduces the environmental impact of marine diesel/dual-fuel engines. Generally, this heat is recovered from the exhaust gas and/or cooling water with an economiser and thermal oil system. The recovered heat may be used to heat the vessel's accommodation and enclosed working areas, as heating for the fresh water maker and to heat the fuel in the bunker tanks if needed. Steam as heat transfer medium is often only used in case of larger heat demands. It is used aboard cruise vessels for the heating, ventilation and air-conditioning (HVAC) system, swimming pool, galley, laundry services, or for ships operating in the Arctic region to de-ice the ship. Steam is also used for turbine-driven cargo pumps in case of LNG carriers.
The energy transition results in a change to more sustainable cleaner fuels. These fuels may not require heating or a much smaller amount than the traditionally used heavy fuel oil (HFO). Therefore, this heat may be used to increase the system efficiency by using a waste heat recovery system. Waste heat can be converted it into electric power, which in turn reduces the vessel’s fuel oil consumption, GHG emissions and harmful emissions. The most suitable WHR technology depends on the waste heat quality (i.e. temperature). The second law of thermodynamics states that energy is not only defined by its available quantity but also by its quality. The quality of energy is expressed using the thermodynamic concept of exergy (Moran, 2010). Singh and Pedersen (2016) consider the exhaust gas as the waste heat flow with the highest energy recovery potential, due to its high temperature and mass flow. Approximately 75% of the total waste heat of a marine diesel engine that can be converted into work is in the exhaust gas.
The literature discussed in this article focuses on maritime application of WHR systems. Other industries, such as the automotive and the chemical industry have performed considerable research on the topic with advanced cycles, which may become relevant in the future. However, these will not be discussed in this article. Singh and Pedersen (2016) provide a comprehensive overview of waste heat recovery system technologies suitable for maritime applications. They have a focus on three options: thermoelectric generators, turbocompounding and bottoming cycles.
Thermoelectric generators are solid-state devices and utilise advanced materials to convert heat directly into electricity through the Seebeck effect (Uyanık et al., 2022). Despite the potential benefits of this solid-state technology, such as its simplicity and reliability, it has not yet been placed on the market due to its low efficiency (about 2%).