## 2 Options of Waste Heat Recovery from a Naval Engine  
In this study a dual fuel engine (model 6L50 DF by Wartsila) for ship propulsion has been considered. This engine model is a six cylinder in-line, it produces it 5.7 MW with efficiency of about 49%, operating at 514 rpm. It is very flexible from the fuel point of view: it can switch almost instantaneously from gas (LNG) to liquid diesel (LFO-HFO), and vice versa, allowing the choose of the most suitable solution. Because of its complexity, a dual-fuel engine costs up to 30% more than a traditional diesel engine, but it allows an annual saving in term of fuel equal to 5-8% [5].  
The benefits mentioned above can still be enhanced by recovering the thermal energy mainly available in the exhaust gases and in other engine's fluids, which is discharged to the environment in the traditional propulsion plants.  
The heat balance of a Wartsila 6L50 DF engine shows about 11.5 MW of fuel input and a power output of 5.7 MW, at maximum load (Figure 1). The remaining energy is rejected to the environment by the exhaust gas (about 3.1 MW available at 400 \({}^{\circ}\)C) and the cooling circuits (2.4 MW in LT and HT circuits).  
The cooling circuit is split in two parts, according to the temperature level: a high temperature circuit (HT, at about 90 \({}^{\circ}\)C) and a low temperature circuit (LT, at 50 \({}^{\circ}\)C). It is worth nothing that the amount of energy rejected by the engine is about equally split between the exhaust gas and the two cooling circuits HT + LT together (56% and 44%, respectively, at the maximum engine load), but the exergy content of the exhaust gas is three times higher than that of the HT + LT circuits (74% and 26%, respectively, at the maximum engine load). Therefore, it can be inferred that a first selection of possible energy recovery can be based primarily on the availability of high temperature heat sources, namely the exhaust gas and possibly the HT water circuit.  
To introduce the feasibility of the heat recovery, a brief description of the cooling circuits and of exhaust gas circuit of the engine W6L50 DF is necessary (Figure 2).  
Figure 1: Energy balance of the engine W6L50 DF [1].  
Figure 2: Simplified schematic of the cooling circuits and exhaust gas of the engine W6L50 DF.  
The water of the high temperature fresh-water circuit (HT CIRCUTI) flows through the cylinder jackets, the cylinder heads and the first stage of the air-cooler, from the engine inlet (IM) to the engine outlet (UM). The water in the low temperature fresh-water circuit (LT CIRCUTI) cools the second stage of the air-cooler and then the lubricant oil. The control of the HT circuit is based on the temperature of the water leaving the engine. A three-way thermostatic valve set at 91 \({}^{\circ}\)C re-circulates part of the flow (RIC) to keep the water inlet temperature at the right level. The remaining part of the water is then cooled in a central cooler, dissipating heat into the sea water. A heat exchanger, high temperature waste heat recovery (HT WHR) allows the recovery of thermal energy from the HT cooling circuit. The object of this recovery is to allow the maximum power production from the bottom cycle, without preventing the proper operation of the temperature control loop, therefore the extracted thermal power shall be equal to the engine thermal production, at each different load. The water at the outlet of the HT WHR returns to the engine inlet (RIT), with a possible integration from the LT circuit (REINT). In this way, the regulation system of the engine cooling guarantees, for all engine loads, a constant temperature in the high temperature fresh water circuit, while the temperature of the low temperature fresh water circuit does not exceed 50 \({}^{\circ}\)C, but it strong depends on the engine load and on the see water temperature. Therefore, an energy recovery from the LT circuit has to be regarded as not feasible in practice.