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Hydropower schemes have considerable appeal as a source of renewable energy across the world – while their multi-disciplinary nature poses great engineering challenges. This briefing sheet provides information on the status of hydropower in the UK and worldwide.
For thousands of years mankind has used water power on a small scale, as in mills for grinding corn, and such use continues in many parts of the world although it has largely ceased in the UK. Hydropower, the term generally used for the generation of electricity from water power, was developed from the late nineteenth century. The first hydro-electric power station is said to have been built by the industrialist William Armstrong at Cragside, Northumberland, UK, in 1870.
Power can be generated where water falls through a height, known as the “head” (measured vertically). The power, P, generated in kW, is calculated as follows:
P = hQgρ
(where h = head in metres; Q = flow rate in m3/s; g = gravitational constant; ρ = overall efficiency of the installation)
Therefore, a flow of 1m3/s falling through 1m vertical distance can theoretically generate just under 10kW of electricity. The highest efficiency achievable with modern plant is more than 90%.
A useful rule of thumb to estimate the potential of a site, in MW, is P = hQ/120, incorporating g and an efficiency of 0.85.
As a source of renewable energy, hydropower has considerable appeal, especially when part of a multi-purpose scheme. However, capital costs tend to be several times that of the cheapest thermal power plant. Part of the attraction is that fuel costs are zero and other ongoing costs are small. The multi-disciplinary nature (e.g hydrology, hydraulics, geology; and geotechnical, civil, structural, electrical and mechanical engineering) means that hydropower poses some of the greatest challenges of any engineering projects. Large schemes especially can also have significant sociological and environmental impacts which may be hard to predict; because of their potential impacts, large schemes are often controversial. All aspects, even for small schemes, warrant careful study of their feasibility.
The installed capacity of hydropower worldwide is listed by the International Hydropower Association to have been 1096GW in 2016, excluding pumped storage. Estimated world annual hydropower electricity generation was 4,102TWh in 2016 (IHA, 2017). The energy generated represents 2.5% of total primary energy supply and 16.6% of world electricity generation (Enerdata, 2017).
IHA draws attention to the multiple benefits that hydropower can offer. A large proportion of the untapped resources are in regions where new development has the greatest potential to affect people’s lives positively but, IHA stresses, it is crucial to ensure sustainability. A Hydropower Sustainability Assessment Protocol has been established by a range of international NGOs, financial institutions, governments and the industry. It promotes sustainable development and provides a way to assess performance (HSAP, 2011).
The proportion of a country’s total electricity production generated by hydropower depends on its geographical characteristics and the extent to which its potential has been developed. For example, Paraguay generates almost 100% of its energy from hydropower but approximately 50 countries generate little or no energy from hydropower. Combining hydropower with flood control, irrigation, water supply, navigation and recreation can make such schemes economically feasible.
The 2017 edition of the Digest of United Kingdom Energy Statistics (DUKES) contains UK Government statistics on the Energy sector and was published on 27 July 2017.
In the UK, the total installed capacity in 2015 was 1.58GW, about 2.0% of the total UK generating capacity, most of which is in large schemes in Scotland (Gov.UK National Statistics, Plant capacity: United Kingdom DUKES 5.6). The energy produced in 2015 was 6.3 TWh which was about 1.9% of the UK's total electricity production (Gov.UK National statistics, DUKES 5.1). By comparison, large thermal power stations in the UK are typically 2.0GW and produce in the order of 10TWh per year of energy. Most of the hydropower capacity was built in the 1950s and 1960s, absorbing much of the larger scale opportunities in the UK.
Incentives for developing renewable resources have contributed to a resurgence in interest in new projects and in refurbishing old schemes, even some old watermills. The figures quoted above (world and UK) are for "primary power” or “natural flow” only. They do not include the pumped storage schemes, such as Dinorwig and Ffestiniog in North Wales with capacities of 1,728MW and 360MW respectively, and Foyers (300MW) and Cruachan (400MW) in Scotland.
Why doesn’t the UK generate more hydropower?
Land in the west and north of the UK is at a comparatively high elevation and receives plenty of rain, but the remainder is generally low lying and has less rainfall. This is the reason that the majority of the country’s hydropower is generated in the north of Scotland. Even there, average elevation is low and catchment areas small by comparison with continental mountainous regions, so the potential for economic development of hydropower is relatively small. By comparison, Norway and Switzerland generate 95% and 56% of their electricity from hydropower respectively, and the installed capacities of the largest schemes in China and Paraguay are measured in thousands of megawatts.
The UK does however have significant untapped potential tidal and wave energy resources if the technological, economic and environmental challenges can be overcome.
Types of hydropower schemes
Hydropower schemes can be classified as run of river, storage or impounding or pumped storage schemes. Tides can also be used to generate power, either by forming a barrage across a bay or estuary with a substantial tidal range or by using the kinetic energy of the flow where the velocity is high.
Sizes of schemes vary considerably. The smallest run of river schemes (of which there are many in rural communities around the world or that are privately owned) range from just a few kilowatts. At the other end of the scale, the Three Gorges Dam in China has an installed capacity of 22,500MW.
There are also several types of plant, in two main generic groups, reaction and impulse, that can be used to convert the head (potential energy) of the water into electrical energy. The most widely used machines are Francis turbines (reaction type) which operate over a wide range of head and can have efficiencies in excess of 90%. Pelton wheels (impulse type) tend to be used for the highest head schemes. Several other types of machine are used, many of which may be cheaper to manufacture but are less efficient. For example, some low head schemes use Archimedean screws. Impounding tidal projects may be equipped with Francis turbines; kinetic tidal schemes can have a propeller type or vertical or horizontal turbine, or reciprocating hydrofoils.
Run-of-river schemes abstract the flow, or a proportion of it, available in the river at an intake, conveying it by canal or tunnel (known as the headrace) at a gentle gradient until a useable head (from just a few metres to some hundreds of metres) is available above the river. To be economic, the gradient of the river may be of the order of 5% while that of the headrace may be of the order of 1/1000 or less. A pipeline or shaft is used to deliver the flow to the power station. Run of river schemes have the advantage that they do not change the flows in the river except over the length of the scheme. Conversely, their output varies and is restricted by the flow available at any time. The efficiency of turbines falls as the flow reduces below the design capacity and generation stops completely at low flows; the economics can be very sensitive to variations in rainfall and intermittency is an issue for plants not connected to a grid.
In a storage or impounding scheme, the head is created or augmented by the construction of a dam. Water in the reservoir created by the dam can be released to match diurnal or seasonal variations in demand, still limited of course by the total inflow. Energy output may be restricted in dry years through lack of inflow and because, as the level of water in the reservoir is drawn down, the head and thus the output of electricity is reduced.
The potential impacts of impounding schemes are numerous and are mentioned further under Concerns (see section 3.3).
Pumped storage schemes take advantage of surplus electricity at off peak times from ‘base load’ stations (particularly nuclear) or increasingly from renewable sources such as wind turbines. Most use Francis type turbines which perform the dual role as pumps with only a small loss of efficiency to pump water from a lower reservoir to an upper reservoir. In generation mode, the water is discharged through the turbines back to the lower reservoir. The water is used repeatedly requiring a relatively small in-feed to make up for evaporation and any leakages.
Even though capital costs tend to be high and the overall efficiency of a scheme may be not much more than 70%, pumped storage schemes remain (in 2015) the dominant way of storing substantial amounts of energy in a form that is readily convertible to electricity (though other technologies such as compressed air energy storage and batteries are beginning to offer alternatives). They perform a valuable function in balancing supply and demand, providing a reserve of generating capacity to respond to changes in load or sudden failure of other generating plant or transmission links, and for frequency control. Especially if held in “spinning reserve”, that is spinning in air and synchronized to the grid, turbines can be brought on load within a few seconds.
Pumped storage schemes ideally require large upper and lower reservoirs separated by as much vertical but little horizontal distance as possible. Very few sites provide these attributes naturally; typically the upper reservoir is substantially artificial with little or no natural catchment, tends to be small and is therefore subject to significant change in water level. The lower reservoir is usually larger and the changes in level are therefore less. Occasionally the lower ‘reservoir’ may be a large river or the sea. The lower reservoir may be isolated from the wider environment to prevent harmful influences. For example the river which previously flowed into Llyn Peris, which was adapted to form the lower reservoir for the Dinorwig scheme, is now diverted around it in a tunnel to avoid its natural flow being affected by the scheme.
Tidal power offers great potential but presents significant technological and environmental challenges. It is entirely predictable but its cycle does not match that of demand, and output diminishes significantly during periods of neap tides. Its capital cost exceeds most other types of generation. Hence it is still very much in the developmental stage with the exception of a few projects including La Rance (240MW) near St Malo in France built in the early 1960s, the Bay of Fundy in Canada (20MW, 1984) and the Sihwa Lake tidal power plant in Korea (254MW, 2011), which are all barrage schemes.
Tidal barrage schemes are potentially economic in the relatively few places worldwide where tidal ranges are the highest. For example, spring tide ranges are 14.5m and 13.5m at the Bay of Fundy and La Rance respectively. A barrage across the River Severn estuary, where the spring tide range at about 11m is also exceptionally large, has been studied and proposed for many years; it could provide up to about 5% of the UK's electrical energy but remains highly contentious, both environmentally and economically. A 320MW tidal lagoon in Swansea Bay was given Government approval in June 2015; the developer aims to start construction in 2018 (Tidal Lagoon Power, 2017).
Kinetic or dynamic generation schemes operate where tidal stream velocities are highest which tends to be where there are constrictions between land masses such as in estuaries (e.g. The Humber) or between islands (e.g. around the Orkney Islands). The highest tidal velocities are often in the same regions as the highest tidal ranges, because they are generated by the general progression of the tides around the Earth being impeded by land masses, but are not precisely coincident. Worldwide potential “could exceed 120GW” (TidalEnergyToday, 2017) – not substantial relative to other sources.
Kinetic tidal generation technology is still very much in the developmental stage with a limited number of prototype or demonstration schemes in operation around the world.
They have the advantage of low environmental impact, though potentially presenting a hazard to shipping and unknown effects on fish and marine mammals, but of course they operate necessarily in the most hostile marine environments so are expensive and difficult to install and maintain, and it is difficult to achieve adequate reliability. In early 2015, construction of Phase 1a (6MW) of the 398MW MeyGen project started in the Pentland Firth off the north coast of Scotland. The first of four 1.5MW turbines of Phase 1a started operation in November 2016 (Atlantis, 2017).
Another related technology, though not strictly hydropower in the way defined at the beginning of this briefing, is wave energy. This technology is also at the developmental stage with a variety of devices being tested on coastlines exposed to ocean waves with the highest energy intensity.
Tidal and wave power is estimated to be able to meet up to 20% of the UK's electricity needs (DECC, 2013ii). At present, there is no identifiable feasible and economic path to the development of such a major contribution though some development is likely as technology improves and the drive for renewable sources increases.
That there are few schemes in operation illustrates that tidal and wave power is, to date, challenging and marginally economic.
The technically exploitable capability of hydropower plants in the UK is about 14TWh annually. The development of more large natural flow schemes is unlikely because the best sites have already been developed. Because of the UK's topography, hydropower cannot be a major contributor to the national requirement for primary energy but there is a remaining viable potential of 850-1550MW in small scale resources (Gov.UK, 2013i). Existing channels leading to abandoned mills, or weirs which exist to allow navigation on rivers, provide some opportunities for small schemes in lower lying areas. Further pumped storage schemes are likely to be developed as intermittency on the grid increases with the growth of renewables. The extent of development of tidal projects (impounding and kinetic) will depend on overcoming the technological, environmental and economic challenges.
Concerns over hydroelectric plants
Hydroelectric power projects, particularly impounding schemes, have a wide range of potential impacts. Projects that were built before the present emphasis on environmental and social impacts might not be promoted nowadays, or they might have been designed better to mitigate harmful effects. Impacts can include displacement of people, their homes and livelihood, destruction of habitat and fertile land, unsightly mud banks at low reservoir water level, siltation (reducing effective storage volume), and loss of water through evaporation from the large surface area of the reservoir.
In the UK, siltation is less significant where vegetation limits sediment loads, and evaporation losses are not high. Siltation is important in 'young' mountain ranges with high erosion rates; evaporation is important in hot climates.
If vegetation is not removed before inundation, there can be a significant release of methane, a much more potent greenhouse gas than carbon dioxide. There can be rapid changes in the flow in the river downstream when generation starts or ceases unless the reservoir discharges directly into another reservoir, or to the sea. Dams are a barrier to the migration of fish. For very large schemes there may be unpredictable micro-climate changes.
At least some of the disadvantages can be mitigated. Limiting drawdown of a reservoir still allows generation of much of the available energy as the majority of the volume of water is in the upper range of the reservoir where the area of the water surface is greater and the head is the maximum.
"Compensation water" is released to prevent flows in rivers drying up completely and "freshets" of increased water flow are sometimes released to simulate a more natural flow variation in the river. Fish passes are built to allow migratory fish to pass dams and reach their breeding grounds in the headwaters upstream of the reservoirs.
On the positive side, many reservoirs have acquired a considerable amenity value, in some instances being designated SSSIs because of the habitat provided for wildfowl, while others provide flood protection for communities downstream. Many are used for recreational facilities and for the development of tourism.
The reputation of schemes perceived as damaging may have enhanced opposition to new projects, possibly inhibiting reasonable progress in developing countries seeking to improve the health and standard of living of their populations. In the UK, there is a similar risk that the extent to which environmental impacts can be addressed will not be recognised, and that schemes which would have a beneficial impact overall meet considerable opposition.
Hydropower can and should be an important part of strategies for reducing carbon emissions. However, for the reasons outlined above, all hydropower projects require thorough study and planning in order to evaluate their technical and economic feasibility, and environmental and social impacts. For example, careful hydrological, geotechnical, environmental and other studies are necessary; the size (installed capacity) of the scheme needs to be optimised and the likely energy production determined.
Costs, including debt, and financial benefits must be analysed carefully. Sensitivities to inaccuracies in all assumptions, especially forecast rainfall and flow, and costs, must be tested. It has been suggested that large hydropower dams will be too costly in absolute terms and take too long to build to deliver a positive risk-adjusted return (Ansar et al, 2014) though this has been contested byICOLD (ICOLD, 2014) and others. Thorough risk assessment and management is essential, addressing the wide range of variables to which projects may be subject.
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