POWER AND ENERGY SYSTEM
(◎: Head of the Group)
The Power and Energy System Research & Development Group has been engaged in research and development of marine SCR technology. SCR (Selective Catalytic Reduction) technology is an exhaust gas aftertreatment technology that reduces NOx in exhaust gas. NOx is reduced by using a catalyst and a reducing agent (ammonia gas or urea water). NOx reacts with ammonia (NH3) on the catalyst as shown in the figure below, and is decomposed into nitrogen and water (see the figure below).
So far, we have conducted a performance evaluation test using a 4-stroke marine diesel engine equipped with an SCR, and have developed an SCR system for field tests. In addition, we identified many technical issues through many long-term actual ship tests.
Overview of SCR
Traditionally, ships have been powered by high-efficiency diesel engines with a thermal efficiency of more than 50%. On the other hand, in other modes of transportation, thermal efficiency of gasoline engines in cars is about 35% and the thermal efficiency of jet engines in aircraft is about 30 percent. The power source of a ship is extremely effective in using energy as compared with other power systems. However, due to social issues such as the recent environmental problems represented by global warming and the fluctuation of oil prices, further reduction of fuel consumption is required in the shipping field. Against this background, in order to reduce global warming gas emitted from ships, next-generation hybrid systems for ships that make the most of the features of high-efficiency diesel engines and use electric energy and natural energy are drawing attention.
Because the operation of the automobile hybrid system is based on frequent repetition of starting and stopping. it is possible to save energy by using the energy recovered at stopping (regenerative energy) at the time of start-up. In addition, the engine and the electric motor can be used together when a large torque is required such as when starting the vehicle, so fuel consumption can be reduced by reducing the maximum output of the engine. On the other hand, ships operate at a constant speed most of the time, so we cannot expect the use of regenerative energy like automobiles. In addition, since ships operate near the maximum output of the engine (actually about 80% of the maximum output), it is not appropriate to reduce the output of the engine. For these reasons, out Research Group is studying a system that makes the best use of the characteristics of high-efficiency diesel engines by making effective use of electrical energy.
Basic Concept of Marine Hybrid System
Reducing air pollutants emitted from ships is a serious issue. The Power and Energy System Research and Development Group has been engaged in research and development of combustion technology that can handle a variety of fuels for marine diesel engines. As a result, we developed a hybrid injection system (HIS) that combines a conventional mechanical fuel injection device with a small electronically controlled fuel injection device (using a common rail for automobiles) in order to cope with poorly ignitable fuel. By performing injection (assist injection) with a small electronically controlled fuel injection device at appropriate timing, we confirmed effects such as improvement of ignitability, reduction of smoke generation, and reduction of NOx generation.
Overview of Hybrid Injection System
The features of this hybrid injection system are as follows.
The Stirling engine is an external combustion engine, which obtains drive power by expanding and contracting the gases inside by heat sources of different temperature. Its theoretical thermal efficiency is high. And Since it is an external combustion engine, it can use any high temperature heat source.
Our Group has been conducting research and development of Stirling engines that utilize the exhaust heat in exhaust gas emitted from marine diesel engines. Based on the results obtained, we developed a heat recovery system using a Stirling engine and installed it aboard the electric propulsion ship "Tsuruyomaru" in October 2011. This Stirling engine can generate about 3kW of electricity from exhaust gas at a temperature of about 300 degrees Celsius as the heat source. A control system for starting and stopping power generation has also been developed at the same time, so that heat recovery power generation is always performed when the ship is in operation.
Today, most marine engines are diesel engines. Diesel engines are at a high technological level in terms of reliability, fuel efficiency, durability, load response, etc. On the other hand, the large amount of emissions of environmentally hazardous substances such as NOx, SOx, and soot has become an issue.
In recent years, there has been an increasing international sentiment toward the reduction of such hazardous substances emitted from ships, and research and development of environmental load reduction technology for ships has been promoted in various fields. Under these circumstances, marine gas engines that can significantly reduce the emission of environmentally hazardous substances are drawing attention. A gas engine is an engine that uses natural gas as fuel, and has the environmentally-friendly features that the fuel is clean and it does not easily generate environmentally hazardous substances when burned.
Our Group is promoting research on the use of gas engines in ships. Using a gas engine with a power generation output of 400kW, we are conducting research on the effects of differences in fuel gas components on engine performance, research on systems that detect abnormal combustion, research on exhaust gas recirculation and hydrogen-mixed combustion, etc.