Thesis scope is to plan arrangement onboard a ship allowing waste heat recovery for the engine generated heat that will be used in an onshore application.
Waste heat to be recovered under the scope of this thesis is namely Heat to coolants and Heat in exhaust gases. The principles are based on the fact that marine diesel engines are losing energy as heat to exhaust gases and to cooling water. The efficiency of diesel and gas engines ranges between 42-50%, depending on the engine type.
(Wärtsilä Oyj. Sustainability) (Wärtsilä Oyj, 2020). The Sankey diagram of a ship’s energy use by DNVGL in picture 8 below indicates the use of energy and the waste streams on a ship very well.
Picture 8 Ship energy use Sankey diagram, DNVGL (DINOPOULOS, 2014)
A significant amount of heat is generated by the machinery plant on a ship. American Bureau of Shipping has stated it very well on their energy efficiency advisory document Ship Energy Efficiency Measures - Status and Guidance “While modern diesel engines are very efficient, with greater than 50 percent of the energy generated by the combustion of fuel oil being converted to mechanical energy, they still generate a large amount of waste heat when running at full load. The heat is removed from the engine in many forms. About 5 percent of the engine’s total energy production goes to the engine
cooling water system and about 25 percent is contained in the exhaust gas. In both these forms the heat is useful as a heat source for other systems.
For many years it has been common to use the heat from the main engine high temperature cooling system to generate fresh water and the heat on the exhaust gas to generate steam for heating. As the size of the ship and it’s engines increase, the amount of exhaust heat available increases much more rapidly than the demand for steam for heating. This is because the primary uses for the steam are heating oil tanks and accommodation spaces.” (American Bureau of Shipping (ABS), 2020, pp. 45-46).
In pictures 9 to 11 below the system principle is indicated in heat balance diagrams showing the balance between utilized energy and wasted energy in normal ship installation. Further a system installation case is shown where the waste heat is recovered and utilized onshore with increased utilization rate and increased system efficiency.
Picture 9 Typical heat balance of a marine Diesel engine
Picture 10 Diagram indicating the energy waste and utilization percentage
Picture 11 Diagram indicating the reduced wasted energy streams and increased
Picture 12 System PI diagram
Picture 12 shows the arrangement for recovering the waste heat generated by the ships engine system PI diagram. Indicating the systems main components onboard and onshore and the relations between them. The same principle of using engine cooling water and exhaust gases as energy recovery sources is commonly used in diesel powered combined heat and power plant solutions. This is illustrated in the schematic picture of CHP power plant using diesel generators in picture 13. The major difference is that the heat is first stored onboard in thermal storage tank during the operating the ship at the sea and when the ship is in the port the same is discharged onshore to be used as district heating energy.
Picture 13 Schematic picture of diesel CHP system with the main (Wärtsilä Energy, 2019, pp. 4-5)
3.1 Operational functions of the system in Phase 1 loading cooled district heating water onboard
In phase 1 the ship is in port and connected to the district heating system. Cool district heating water is transferred from the heat accumulator or directly from the district heating system return line to an Energy recovery ballast tank 1 (thermal storage tank) onboard in temperature of 40C.
Picture 14 Phase 1 - Loading cooled district heating water onboard the ships energy
3.2 Operational functions of the system in Phase 2 charging heat to district heating water by waste heat onboard
In Phase 2 the ship is operating at sea using the power plant engines at the MCR.
Onboard the district heating water is pumped from the Energy recovery ballast tank 1 through HT heat exchanger where the T is 10C and the district heating water leaving the heat exchanger is at 55C. After HT heat exchanger the water is lined through a steam powered heater (heat exchanger). T over the steam heater is 43C and the district heating water leaving the heat exchanger is at 98C.
Picture 15 Phase 2 - Charging heat to district heating water by waste heat onboard, the ship is in operation – Cooling water for ME is pumped from Energy recovery ballast tank no. 1 through ME HT heat exchanger and steam heating heat exchanger. Hot water is returned to Energy recovery ballast tank no. 2 at 98°C
The heated district heating water is drained to Energy recovery ballast tank 2 and the mass of district heating water will accumulate in the tank ready to be discharged to shore at 98C with the energy content of 222070MJ i.e. 61,68MWh.
3.3 Operational functions of the system in Phase 3 discharging heat laden district heating water to shore heat accumulator
In phase 3 the ship is back in port and connected to the district heating system. The mass of heated district heating water is discharger from Energy recovery ballast tank 2 to shore Heat accumulator tank 2 at 98C. At the same time Cool district heating water is transferred from the heat accumulator or directly from the district heating system return line to an Energy recovery ballast tank 1 onboard in temperature of 45 C.
Picture 16 Phase 3 - Discharging heat laden district heating water to shore, the heat accumulator Ships Energy recovery ballast tank no. 2 is discharged by 1000m3 of heated district heating water at 98°C
The same cycle is repeated once the ship leaves the port and returns the next day. The energy accumulated onboard by heating a mass of district heating water by T of 53C is used onshore to district heating purposes for example powering the peak power demand of the district heating system. Annual energy content of 81055 GJ i.e. 22515 MWh.