Head and flow
The amount of renewable energy generated from a hydroelectric scheme will vary greatly and is subject to two main determinants: the available head (difference in height of the water levels between the intake site and the turbine site) and available flow of the river. The head and the flow of a river dictate the type of scheme (low or high head) and thus the scale of civil engineering required, the type of turbine installed and effectively the build cost.
A low head scheme typically relies on significant volume and flow of water to generate power- classified as a scheme with less than 15m of head. Low head projects can cost considerably more than high head schemes due to the civil engineering work involved. However, they can create an opportunity to utilise existing redundant weirs which can considerably reduce construction costs.
High head schemes use the natural downward flow of river to utilise kinetic energy carried by water. Typically, water is diverted from a river via an intake weir and through a penstock (pipe) where there is a significant drop in elevation. The water then passes through a turbine generator. As high head schemes do not require a huge dam or reservoir structure, they can be relatively economical to install and have minimal environmental impact over larger schemes.