One of the solutions Imtech Marine Group (the Netherlands) can offer is the Green Hybrid Propulsion System. Incorporated into a tugboat it will provide a Green Hybrid Tug that will be environmentally friendly, energy efficient and powerful when needed. A tugboat has a very wide range of power demands for different operational tasks. It needs full power when towing or pushing another ship. When sailing high speeds, a large amount of power is needed. But most of the operational time the tugboat will be maneuvering or sailing at low speeds.
In a conventional propulsion system the diesel engines will be chosen on maximum power. But this maximum power will be used not more than 5% of the total operational time. The rest of the time, the tugboat will be using less power, generated at a less energy efficient working point of the diesel engines. To overcome this problem diesel-electric propulsion is a great step forward in saving energy. The supply of power is more in equilibrium with the demand for power than in conventional propulsion systems. With two or three diesel-generator sets, the choice of which sets to run and which sets to stop depends on the demand for power.
To even further close the gap between power demand and power supply, energy storage is needed. An energy storage system build with high-energy lithium-ion battery banks can store the efficiently generated energy when not needed. The diesel-generator sets can run either very efficiently, storing a surplus of energy into the batteries or not run at all. When extra power is needed or the diesels are not running at all, the energy can be taken from the batteries to provide power for the propulsion system.
With large direct diesel engines the storage of excess energy is done with a shaft generator and a lithium-ion battery system. The parallel hybrid propulsion system can be extended by coupling diesel-generator sets to the energy storage system and to the shaft generator. The main direct diesels can be stopped by the use of a clutch. A calculated choice can be made in the amount of direct diesel power, the amount of diesel-electric power and the amount of battery energy storage installed to provide a very efficient propulsion system.