A three month consultation on a short list of five possible Severn tidal power schemes has been launched. This consultation will be used to help the government decide whether or not to support tidal power development in the Severn Estuary. The Severn Tidal Power Feasibility Study Consultation was launched on 26 January and will close on 23 April.

The different kinds of power stations

The term hydropower in fact covers different methods to generate electricity from water. In all cases the existence of height differences is the key starting point, leading to a fast current. The most widespread form of hydropower is a station at the end of a dammed mountain lake reservoir. Besides this most common construction, electricity is generated in pumped-storage power plant, river stations and from seawater thanks to the existence of tides and waves. These are all discussed below.

Hydropower station

Hydropower stations have turbines that are driven by flowing or falling water. The larger the height differences and the flow rate of the river, the large the capacity of the power stations. These stations are generally located in dams in rivers, mainly in mountainous areas. In many cases, there is a reservoir behind the dam. Reservoirs in France, Norway, Austria and Switzerland deliver substantial proportions of the domestic demand for electricity.

Hydropower stations can have large capacities, with 100 - 300 megawatt being most common. In comparison, the largest offshore wind farm in Europe that is fully operational has a capacity of 165.5 megawatt. The review "Largest (offshore) wind farms in Europe" provides more details. Large hydro projects can reach high capacities. Grand Coulee in the United States has a capacity of almost 7,000 megawatt. To put this in perspective, this capacity can be compared to the combined capacity of some seven combined cycle gas-fired power stations, or several nuclear power plants. The infamous Three Gorges Dam in China, expected to be fully operational in 2009, has a planned capacity of about 18,000 megawatt.

A pump storage station is in fact a way to store electricity and release it at times when the demand is highest. In off-peak hours water is pumped up to a reservoir located at a higher altitude. During peak hours, when demand for power and the price are high, the water is released from the reservoir and the falling water drives the turbines. This principle is mainly applied in rivers, but can be used in other situations as well. Sometimes, deep coal mine shafts are mentioned as a possibility. In the process of pumping up and releasing water, about 70% - 85% of the energy needed to pump up water is released when the water returns downwards. This seems a waste of energy at first sight. Still, pump storage stations are a useful means of storing a large amount of electricity, which is otherwise very expensive.

This station can be a pure pump storage that only moves water between two reservoirs. There also exist combined pump storage stations that also generate their own electricity from conventional hydropower, using the natural movements of water.

River stations are located in less hilly landscapes and use the current of the water to drive turbines. The principle is very comparable to that of conventional hydropower stations. River stations generate electricity because large amounts of slow moving water in relatively flat terrain pass through their turbines. A river station does not have a reserve to absorb fluctuations in the flow rate.

The difference between low and high tide makes tidal electricity generation possible. In most locations, the difference between low and high tide in the open sea is just several decimeters. Certain coastal areas have funnel-shaped bays, which have much more substantial tidal movements. During high tide, the elevated water is captured behind a dam to be released during the subsequent low tide. As usual, the water drives turbines that generate electricity. Scientists in the United Kingdom have calculated that an underwater tidal station can reach a capacity of 1,200 megawatt. The cost of construction of such a station is currently still very high, and the construction would be uneconomical without large subsidies.

Waves also possess large amount of energy. In theory at least, it is possible to capture this energy and convert some of it into electricity. However, engineers have not designed a cost-efficient method to use wave energy on a large scale. Still, several concepts have been developed already. Some installations are floating; others are attached to the seabed. One of the often-mentioned concepts uses a closed roof chamber, the lower part of which is placed in the sea. The waves cause the water level in the room to fluctuate. This in turn causes large pressure differences of the air above the water surface. The high pressure, as a result of rising water levels, is used to drive a turbine. The Israeli firm SDE announced that it has designed a system that generates electricity from waves with a capacity up to 1 megawatt.

As discussed in the introduction, roughly 15% of Europe's electricity is generated in hydro-electric power plants. There are large regional differences. Norway (98.8%), Iceland (82.7%), Austria (62.1%), Switzerland (54.1%) and Sweden (41.2%) are the countries with the highest shares of hydro in their power generation mix. In absolute terms, France, Spain and Italy are also important producers of hydropower. The table below summarises the generation of hydropower in 2004 for eight European countries where hydro is an important source of electricity.

In general, hydropower is considered a green form of electricity: it is CO2 neutral and no harmful pollutants are released in the generation process. These are important advantages of hydroelectricity. Still, it is important to put hydro in perspective and look at the constraints:

The power annually generated from hydro fluctuates heavily. The main cause is unpredictable rainfall. In 2005, this happened in Spain and led to a huge increase in the demand for natural gas as an alternative for power generation. Dry weather - implying low hydro generation - in California contributed to the famous electricity crisis. The limited reliability of hydro causes the need for rerserve capacity or the possibility to import electricity from other areas.

Hydropower is a cheap way of generating power, but in many countries the full potential is almost captured and few extension possibilities exist. This means that the EU targets for green electricity (see the review about the feasibility of EU targets) in many cases cannot be met by increasing hydro generation. While hydropower is a green source of electricity, building the power plants and reservoirs involves a lot of (fossil) energy and has a large impact on the landscape. The construction of dams can endanger fish species, cause permanent damage to river ecosystems and change the landscape in a radical way. This explains why many nature conservation organisations protest against the construction of new days, in spite of the environmental advantages of avoiding carbon emissions.

Hydropower is an important source of energy in Europe and generates a sizeable part of Europe's total electricity demand. Although possibilities to extend the hydro portfolio by building new large dams are limited, there are other forms of hydropower (wave and tidal energy) that are potentially important in the future. In the short and medium term Europe will have to rely mostly on wind and biomass to meet its aspirations for renewable energy.