Hydro power is an indirect form of solar power. The heat from the sun evaporates water from the sea, which falls as rain and finds its way into the rivers and ultimately back to the sea. Hydro power is harnessed by means of mechanical turbines.
Hydro power is a well-established form of renewable energy, providing about 16% of the world electricity supply in 2003. Large-scale hydroelectric stations are found in countries where hydro power resources are abundant. In terms of hydroelectric generating capacity, Canada, China, Brazil, the U.S.A and Russia are the leading countries and they together accounts for over 50% of the total hydroelectric generating capacity of the world. In countries like Norway, Iceland, and Canada, hydroelectricity accounts for more than half of the total electricity supply of the country.
Hong Kong does not have rivers with sufficient flow and head for large-scale hydroelectric generation. On the other hand, there are certain locations in the territory where it may be possible to install rather small hydroelectric installations.
Hydro power can be harnessed by using the energy in flowing water to turn a turbine, which then drives an electric generator to produce electricity. The energy in water is proportional to the flow rate (in cubic metres per second) and the head difference of the falling water.
Based on the installed capacity, hydroelectric stations can be divided into micro hydro, mini hydro, small hydro and large hydro systems. There are no common definitions for micro, mini, small or large hydro. It varies from country to country. The definitions according to the International Energy Association are as follows:
The potential for developing large hydro is not present in Hong Kong. Therefore this website mainly focuses on the small-scale hydroelectric systems.
Depending on their configurations, hydroelectric station can be divided into 2 major types. i.e. hydroelectric dam type and the run-of-the-river type.
Hydroelectric dam type
This type of hydroelectric station involves the construction of a dam to store water in a reservoir to create the water head difference. Water flows by gravity through a channel (called penstock) to reach a turbine at a lower level to make it spin. The spinning turbine then drives an electric generator to produce electricity.
Although hydroelectric dams are usually built for large hydro stations of hundreds and even thousands of mega watts in capacity, they are also built for small hydro stations.
Hydroelectric dams can store water, and then release the water for power generation when needed. Thus this type of system is more suitable than other intermittent renewable energy sources to provide peak load power. Besides, dams also serve other practical functions of flood control, irrigation and navigation.
However, there are concerns over the construction of hydroelectric dams. Constructing a dam will cause flooding of the upstream areas. Besides, dams block the way of migrating fish such as salmons. A fish ladder can be built to allow salmons to migrate upstream.
This type of hydroelectric station is normally located on swift flowing streams or rivers, and extracts the energy from the water as it passes through the station. Run-of-the-river stations are usually built on small dams or diversion weirs, and in some occasions these structures are not required.
Water collected by the diversion weir falls by gravity through a penstock to drive a turbine at a lower level, which is coupled to a generator to produce electricity. The water then joins back to the normal river channel. Since such systems do not involve significant amount of water storage, they have much smaller impact on the surrounding environment than the hydroelectric dam systems.
Run-of-the-river systems are usually mini or micro hydro systems, and small hydro systems in the lower end of the capacity range. Since this type of system does not store a large amount of water, the electricity output will be affected by the seasonal flow of the river.
Turbines with different shapes of the runners (the turning part of a turbine that interacts directly with the flowing water) and different sizes are used in hydroelectric systems. The major turbine types include the Francis turbines, propeller turbines and Pelton turbines.
This is the most common turbine type in hydroelectric stations. The Francis turbine is a radial-flow turbine with water flowing in a radial direction inward over the curved runner blades toward the centre of the turbine. Francis turbines are suitable for hydroelectric systems with water heads between 2 metres to 200 metres, and the efficiency can be over 90%.
Above: Typical Francis turbine
The propeller turbine is an axial-flow turbine with propeller-like runner having three to six runner blades depending on the design water head. It operates like a boat propeller but in a reverse mode. Propeller turbines are usually applied in systems with water heads between 2 metres and 30 metres. One type of propeller turbine called the Kaplan turbine has adjustable blade pitch, and it can achieve high efficiency under varying power output conditions.
Above: Typical propeller turbine
Pelton turbines (Pelton wheels)
A Pelton turbine consists of a set of buckets or cups mounted around a hub. Pelton turbines are not immersed in water. Instead, a Pelton turbine operates in air with the wheel driven by jets of high pressure water hitting the buckets or cups. Kinetic energy of the water jets is transferred to the turbine. Pelton turbines are applied in sites with large water heads of over 250 metres.
Above: Typical Pelton turbine
Pico hydro turbines are small water turbines with capacities of a few hundred watts to 5 kW.
Pico hydro turbines can be used to harness hydro power from streams, rivers, irrigation canals and waterfalls, and require only a constant water supply with a very low water head. Pico hydro turbines are usually axial-flow turbines. Pico hydro turbines are portable, and are suitable for village applications in developing countries. Apart from electricity generation, some pico hydro turbines, such as the water mill, can also be used to provide mechanical power for various applications such as grinding flour and irrigation.
A set of interesting pico hydro turbines using "Turgo wheels" is being advertised on the Internet. Although EMSD do not have actual experience with such turbines, they are introduced here for illustration of the technologies of pico hydro turbines. The Stream Engine and Water Baby are turbines designed by the Canadian company Energy Systems and Design (http://www.microhydropower.com/). Both the Stream Engine and the Water Baby use Turgo wheels to harness the water power. According to the information on the website, the former can produce power at water head as low as 2 metres while the latter can produce power at flow rate as low as 0.18 liters per second.