to railway transportation or marine transportation, which is more efficient), improvement of distribution systems, reform of urban structures, etc. Besides, it is quite essential to improve fuel economy for motor vehicles, and therefore the promotion of relevant supporting technologies would have significant, immediate energy-saving effects.

Hybrid vehicles use two or more distinct powers or fuel sources, such as a combination of an engine and a motor, and can be characterized by a very high energy-saving performance and a significant reduction of exhaust gas. Among the widely used hybrid system configurations for passenger cars is that which combines gasoline engines with electric motors, wherein the battery is charged not by an external power source but by the engine's rotation, and a vehicle can run on just the engine, just the battery or a combination of both. A combination of a diesel engine and an electric motor is also seen in some commercially marketed passenger cars. These hybrid vehicles use high-level control technology. Also, battery technology plays an important role.

Electric vehicles and natural gas vehicles had many requirements and difficulties for commercialization and wide utilization, including the battery-recharging system, installation of charge stations for user convenience and the necessity of frequent recharging or refilling. On the other hand, hybrid vehicles do not require any new facilities or frequent recharging, and are user-friendly, allowing people to easily enjoy the energy-saving benefits.

Although the prices of hybrid vehicles are relatively higher than conventional cars, hybrid vehicles will reduce fuel consumptions substantially (which means substantial reduction in $ CO_{2} $ emissions) and also improve fuel economy significantly.

In addition, conventional vehicles with a gasoline or diesel engine also benefit from technologies. Their fuel economy has been improved considerably with the introduction of the improved fuel-injection system, control system, adoption of a continuously variable transmission, etc.

### g. Facility-wide maintenance systems

In recent years, much attention has been paid to BEMS (Building Energy Management System) and HEMS (Home Energy Management System). Those systems intend to achieve comprehensive energy saving for an entire building or house, where a building-wide status of energy supply and demand is monitored to integrally promote comprehensive, efficient operation of equipment and facilities inside the building.

## (2) Energy-saving technologies for energy suppliers

Technologies to be used by energy suppliers include the efficiency improvement technology used by power plants as well as the energy shift from conventional energy sources to new sources of low $ CO_{2} $ emissions.

This section will focus on solar energy, wind energy and biomass energy, all of which are renewable, relatively familiar to consumers, and are currently at the diffusion stage in commercial and residential, and transport sectors. Those energies have come into widespread use partly because of the enforcement of the Law Concerning Special Measures for Promotion of the Use of New Energy (RPS Law), which binds electric utility companies to the requirements to use the designated amount of new energies, such as solar power and wind power.

### a. Solar energy technologies

One of the major forms for the use of solar energy is the power-generation system using solar cells.

Solar cells convert solar energy directly into electricity. Sunlight collected on a rooftop is used to generate electricity, and this process causes no $ CO_{2} $ emissions. In recent years, solar cells have been becoming increasingly commonplace among ordinary households.

Here, key technologies include the conversion device from sunlight to electricity and control systems for electricity utilization.

Also, the level of efficient use of solar heat has been improved, thanks to emerging technologies, such as solar heat collection and heat accumulation.

### b. Wind energy technologies

Wind power is considered to have a large potential for effective utilization as an energy source, including the utilization of ocean wind. Although there are some disadvantages, such as instable power or voltage, the use of wind power has been increasing. Since the efficiency of electricity generation can be increased when wind blows stronger and a wind turbine is larger, there is an upsizing tendency for wind turbines. Also, small-sized wind-power generators, which have an advantage of user-friendliness while having a