For the sake of this thesis a theoretical onboard energy recovery system description has been designed. The system has the necessary elements required for a system recovering waste heat from a diesel engine onboard a ship, for storing the energy in a mass of water onboard and transferring the mass of water onshore where the energy may be utilized as a district heating energy source.
Picture 17 Shipboard system PI diagram 4.1 System onboard
The system onboard a ship has the following features: Main diesel engine as a waste heat source. The waste heat will be recovered from HT-water via additional heat recover / cooling circuit that will cool the engine HT-water using the low temperature district heating water as cooling medium and exhaust gas via exhaust gas economizer. The heat is transferred from the emitting systems to the district heating water in way of HT water and steam produced from the excess heat of the exhaust gases.
4.2 System main components
• Pumps for circulating cooling water and feed water, corresponding to numbers 4
& 5 in the patent
• Heat exchangers (primary / secondary fluid), corresponding to numbers 6 & 7 in the patent
• Two insulated tanks of 1000m3 capacity, one for low temperature water and one for high temperature water. To be used as Energy recovery ballast tanks with fluid used for storing recovered heat, corresponding to number 8 in the patent.
There will be two tanks in the system. One that is filled with the low temperature water in the port from the district heat recovery system. The other tank where the same water is accumulated once it is circulated through the HT-water heat exchanger as cooling medium and by steam heating in the steam heat exchanger, hence added heat. The water in the later tank will be additionally heated with the installed steam or hot water slings to top up the temperature.
Tanks to be located in a way minimizing the need for additional ballast water for stabilizing and trimming the ship
• Main engine HT-water cooling to DH water. Normal LT-cooling system to be provided as a back up and for use in other than MCR conditions
• Discharge pump and piping capacity to be adequate for discharging one tank volume in one hour i.e. 1000m3/h
• Supply piping for the charging of the 1000m3 of low temperature DH water onboard
• Automation for controlling and operating the system
• In Picture 18 an example diagram for multiple main engines from Wärtsilä 31 project guide is shown. Drawing is shoving the Heat recovery system in the HT cooling circuit: item 4E03 Heat recovery heat exchanger, item 4P19 Circulating pump and item 4V02 Temperature control valve (heat recovery). System in the
the waste heat recovery for the purposes in the scope of this thesis. The system allows proven, easy to use design for the utilization of HT water waste heat.
Picture 18 Example diagram for multiple main engines (Wärtsilä. Marine Solutions, 2019, pp. 9-4).
Pictures 19 and 20 illustrates basic energy from exhaust gases configuration for general type of ship. Illustration is shown to indicate the fact that energy recovery from exhaust gases is common place onboard ships and a topic of number of energy efficiency development plans in shipping. It also indicates that the PI configuration for exhaust gas economizer plant is very simple.
Picture 19 Energy from exhaust gases (Alfa Laval Corporate AB, 2020, p. 10)
Picture 20 Typical arrangement for exhaust gas economizer in a ship ME installation.
(Osaka boilers, 2020)
Pictures 21 and 22 are illustrating typical steam heating arrangements that could be used for the heating of the district heating water onboard with ∆T of 53°C using the flow type application for heating the water.
Picture 21 Typical arrangement of steam heating system utilizing plate heat exchanger.
(Endress+Hauser, 2020)
Picture 22 Typical arrangement of steam heating system utilizing shell and tube heat exchanger. (Endress+Hauser, 2020)
4.3 Onshore system
System onshore is arranged with the thermal storage tanks located as close to the pier as possible and the manifolds on the pier close to the ships mooring position. The same is illustrated in the picture 23. The ship will connect to the manifolds that have a low temperature line and high temperature line with insulated hoses. The low temperature line is to provide the ship with low temperature technical water for cooling medium and the high temperature line is to receive heated water with added energy content respectively. The manifold area is the custody transfer point where the ship discharges the heated water and the energy content and the mass of the water are controlled. This is done in way of measuring the temperature and the volume of the water discharged.
The density of the water will remain the same during the process onboard, allowing the total energy transfer to be calculated using these parameters.
Picture 23 Onshore system PI diagram
The manifolds are connected to the local district heating system and lead to the thermal storage tank(s).
Recovered heat energy may be stored in a thermal storage tank allowing the usage of the energy at the time when the energy demand is the highest (morning time) allowing
highest benefits and possibly highest energy selling prices from the system. Generally the benefit in adding thermal storage tanks to district heating system is the ability to produce heat steadily avoiding and smoothening production peaks to make the heat production more efficient. Conventionally thermal storages have been charged during low energy demand time, at nights for example and discharged during high energy demand times. Other benefits of the thermal storage tanks for the district heating system is the enhanced reliability and added redundancy of the system as contingency for pipeline or equipment damages.
Thermal storage systems are commonplace in district heating systems and are very well established technologies dating several decades back in time. Many of the main district heating producers are using them. In the temperature range (<100°C) in consideration in this thesis a thermal storage tank is a simple insulated steel structure due to the system being operated in atmospheric pressure. Below in picture 24 is a drawing illustrating the principle design of a thermal storage tank for water.
Picture 24 Energy storage tank principle drawing (KARA, 1987, p. 48)
4.4 System main losses
System losses are not part of the scope of this thesis and are merely introduced for information. The system losses should be studied in detail in project planning phase, when the location and actual operational details are available for defining the system and equipment design. Few main loses are listed in this paragraph with the common ways to mitigate the effect of the same.
• Conventional losses through the hull structures – Mitigated by insulation in way of ship internal structures. Double bottom arrangement towards the sea or accepting losses due to oversupply of heat in the prime movers.
• Conventional losses through the thermal storage tanks structures – Mitigated by insulation in way of tank structures.
• Heat losses in piping – insulation to mitigate.
• Piping losses – Mitigated by using the shortest piping routes, most simplistic piping schematics and good building workmanship avoiding bends in pipelines.
• Pumping losses – Mitigated by using the most energy efficient pumping arrangements and use of adjustable speed pumps.
• Operational losses lowering the efficiency of the storage system – Mitigated in the design phase by design optimization and good operating practices.