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Flow gauging a potential burn

Typical hillside burn

What makes a good hydro system?

The capacity to generate electricity from water flow is determined by two factors: the available head (the difference in height of the water levels between the intake site and the turbine site) and the available flow.

Head

The absolute difference in water levels is known as the gross head. However, the head available to generate electricity will be slightly lower. There will be a proportion of the potential energy of the gross head which is lost in the system due to friction losses. Additionally, the design of the intake structure, turbine house and outfall require utilising some head to enable efficient water flow. The head that remains for generation is known as the net head. This is typically roughly around 90% of gross head but varies by scheme type. The technology currently available requires a minimum gross head of at least 1.5 metres, but at this level a considerable flow of water would be required to generate only a small amount of electricity.

Flow

Higher flow rates mean greater generation. However, a burn that has a strong flow when it rains is not necessarily a good burn for a hydro scheme. More important is the average flow rate over the year. Flow measurement throughout the year is assessed utilising specialist prediction software and via flow measurements taken on site. Flows are measured in cubic metres per second (m3/s or cumecs) or, for smaller schemes, litres per second (l/s). One cumec is 1000 litres per second. To meet legal environmental considerations, nearly all schemes will require some flow to be left in the watercourse (known as compensation flow), so not all the flow is available for generation.

As a rough rule of thumb:

Net head in metres x flow in cumecs x 7.5 = approximate electrical output of turbine in kW

For instance a scheme with 150m net head (circa 160-165m gross head) and an average flow rate of 0.4 cumecs (400 l/s) would mean a turbine of around 450 kW.

For small and medium hydro systems, high head systems are usually more economically viable than low head schemes. A low head scheme will require a lot more water to generate an equivalent power (and therefore return). This higher flow will require relatively large-scale civil engineering and larger pipe, turbine, etc. This increases the build cost relative to higher head, smaller flow systems. This does not mean that all low head schemes are unviable, particularly where there is existing infrastructure (i.e. a weir and/or lade) that can be utilised.

Schemes are more likely to be financially viable if the gradient is steep, giving a relatively short penstock length (one of the significant costs of any scheme) for a given power output. Other considerations are the nature of the terrain for access of the civil engineering machines and proximity of grid connection.

SEPA Abstraction License

All new hydro electric schemes will require an abstraction licence issued by the Scottish Environment Protection Agency (SEPA). This applies even if the water will be returned to the river only a few metres downstream. SEPA consider the grant of a licence balancing the benefits of a hydro electric scheme against potential environmental impacts. They have issued guidance on the factors that are taken into consideration when considering these applications. Key positive factors include:

A hydro scheme will also require planning permission. This permission will include for the building of the intake structure and the turbine house and for route under which the penstock is buried.

British Hydro Power Association Scottish Land & Estates Renewable Energy Assurance Ltd