Smallscale hydro

Hydropower has a history going back at least 2000 years. The Doomsday book records 5000 waterwheels. One of the earliest hydroelectric schemes and first in the world to power a private home was installed by the First Lord Armstrong in the 1880s at Cragside in Northumberland. This means, of course, that it is not exactly at the cutting edge of progress. The expansion of the National Grid sounded the death knell for many small hydroschemes. However, it is increasingly now being perceived as an important source of clean electricity, devoid of the environmental penalties associated with large-scale hydropower. It has minimal impact on the environment, resulting in almost zero emissions of SO2, CO2 and NOx. Nor does it cause acidification of water; on the contrary, it can oxygenate rivers and streams.

A life-cycle comparison with coal and combined cycle natural gas makes the point (see Table 10.1).

Renewable energy systems have 31 times less impact on the environment than fossil-based energy, with one kWh produced by small-scale hydro being 300 times less polluting than the dirtiest of them all, lignite.

Another method of comparison is to award ecopoints to the various technologies. These are points of environmental penalty. The factors considered include global warming, ozone depletion, acidification, eutrophication of water, heavy metal pollution, emission of carcinogens, production of winter and summer smog, industrial waste, radioactive waste and radioactivity and depletion of energy resources.

Table 10.2 compares small hydro with other energy sources on this basis.

According to a European Union White Paper A Community Strategy and Action Plan for the Future: Renewable Sources of Energy (November 1997), 'only about 20% of the economic potential for small hydro power plants has so far been exploited . . . An additional installed capacity of 4500 MW of small hydro plants by 2010 is a realistic contribution which could be achieved.'

Table 10.1 Life cycle emissions g/kWh




Small hydro




Natural gas combined cycle








Table 10.2 Comparative eco-penalty points

Fuel Ecopoints

Fuel oil 1398

Coal 1356

Nuclear 672

Natural gas 267

Small hydro 5

Source: APPA (Spain) Study on Environmental Impact of the Production of Electricity.

The Community guidelines also recognize the need to internalize the external costs of electricity generation. Investments in renewables are deemed to be equivalent to environmental investments.

Having said all this, there is a sting in the tail. The European Commission does not consider that small hydro should receive support as a source of renewable energy. This is because it reckons that large-scale hydro can produce electricity at market prices. Small hydro suffers because of this blanket perception. It also reveals that the Commission is still driven by market rather than environmental considerations since small hydro is one of the cleanest of all technologies as stated above.

As for defining small-hydro, the EU regards 10 MW as the demarcation line. There are two ways of extracting energy from situations capable of providing a head of water. They both depend on geographical/geological characteristics and the location of a suitable water source.

• Mountainous country provides a high hydraulic head of water which makes a highspeed impulse turbine the appropriate conversion technology.

• River valleys usually only create opportunities for a low head of water, generally less than 20 m. In this case water is diverted to a pipe (penstock) or channel (leat) leading to a crossflow turbine like an updated water wheel or a Kaplan turbine which has variable blades. Turbines are now available which can exploit as little as 2-3 m head of water.

The essential components of a hydro scheme are:

• an adequate rainfall catchment area

• a weir or dam to provide a suitable head of water

• alternatively, a river with a suitable drop in level to enable water to be diverted to a penstock or leat to be delivered to a turbine at the right speed and in the right quantity

• a turbine, generator and electrical connection

• a tailrace to return water to the river.

One of the concerns about this technology is that it can deplete fish stocks. Some reports implicate small hydro in the destruction of salmon fishing in a part of Spain. The remedy is a fish channel suitable for even the smallest fish incorporating an acoustic guidance system to prevent fish entering the forebay of the plant. An alternative is an electric fish screen which was first tested at a 500 kW installation at Deanston, Scotland.

In the UK the most abundant small hydro potential is in Scotland. An example of a community initiative to harness small hydro energy is the 230 kW scheme at Loch Poll within the North Assynt Estate in Sutherland. The Assynt Crofters' Trust constructed a dam to raise the

Figure 10.1 Ffestiniog small hydroplant, Tanygrisiau, North Wales

level of the loch. Water channelled along a penstock drives a turbine-driven generator which produces around 1.32 GWh of electricity per year. The Trust has a 15-year contract to sell to the grid. This is the most northerly hydroelectric project in the UK and has been the basis of a study into the provision of small hydro serving remote communities.

A 200 kW turbine has been constructed at a weir on the River Thames. This location is important since it will raise the profile of small hydro within the metropolitan field of influence.

How discreet small hydro can be within an area of outstanding beauty such as the Snowdonia National Park is illustrated by the Garnedd power station near Dolwyddelan, North Wales. It is a run-of-river scheme which taps the energy from the Tyn-y-ddol river. Water is fed into a penstock pipe from a small pond 1 km from the plant and with a static head of 102 metres.

A Turgo impulse turbine drives a synchronous generator producing up to 600 kW and an average of 2.3 GW/h per year. The power station is operated and monitored remotely.

Similarly discreet is the small hydro plant at Tanygrisiau, which discharges its tail race into the lake which serves the pump storage for Ffestiniog power station (see Fig. 10.1).

One of the most cost effective ways of installing small hydro is to restore the infrastructure created for industrial water power, usually in eighteenth- and nineteenth-century mills.

A successful run-of-river low-head project has been in operation at Blantyre Mill on the River Clyde since 1995. With an output of 576 kW it supplies the grid under contract. The site has a 200-year tradition of exploiting water power for a cotton mill later to become a sawmill. Salmon have been returning to the Clyde and so a by-pass channel for the fish is incorporated into the scheme.

Occasionally older hydropower plants have been abandoned as grid-delivered power became economically attractive. Now things have gone full circle and hydroinstallations are being refurbished. An example is the East Mill in Belper, Derbyshire, a low head run-of-river scheme linked to a weir.

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