Hydroelectric plants work by converting the potential energy from water at height into electrical energy. This is achieved through water powering a turbine – using the rotational movement to transfer energy through a shaft to an electric generator.
The two basic classifications of hydro electric generators are ‘High head’ and ‘Low head’. Head refers to the height from which the water drops before reaching the turbine. Therefore ‘low head’ refers to mills and generators in large rivers with great volumes of water that meander through the lowland valleys. ‘High head’ is used to describe those systems that use only a small amount of water but are able to use the water once it has dropped from a great height.
To capture this potential energy in a controlled form, some or all of the water in a natural waterway can be diverted from a watercourse through an intake and into a pipe which will transport the water downhill. The pipe is smooth bored compared to the rough stream bed. There is far less friction loss in the pipe and this saved friction is the energy that is used to drive the turbine. In the turbine house at the bottom of the system the water can be directed in a focussed jet under pressure onto a turbine wheel. The rotation of the turbine and the generator, to which it is attached, convert the energy into electricity that can be exported to the national grid.
Micro hydro is typically defined as the generation of electricity from a few hundred watts up to 100kW.
How to calculate power potential
Hydro power is a mature and well-understood technology that offers many advantages over other renewable energy:
- High efficiency and high power density
- Long system lifetime (up to 50 years)
- Predictable energy outputs
- Excellent load factor characteristics
The particular technologies required for generation differ from site to site according to various site characteristics and these are outlined below. The fundamental elements which make up the basic power generation equation are explained in below. However, there are several other factors which will reduce the actual power that can be generated at any site. There are multiple factors that reduce your potential energy during conversion. These include head loss in pipes, efficiency of turbines, loss in cables, and loss in inverters.
In order to simplify the calculation at the planning stage of a hydroelectric installation, these efficiency losses are assumed to amount to 50% of the ideal calculation.
Simple power calculations can then be calculated from the flowing variables:
- Q (flow): this is the amount of water that can be abstracted from a given point in a stated period of time. It is usually measured in m³/s or l/s. The abstraction limit if often limited to the annual mean flow of the river (see below for additional note on abstraction limits).
- H (head): This is the vertical distanced that water drops from the source to the turbine. It is measured in meters, m.
- Gravity constant: Also known as acceleration due to gravity, it is represented by the letter ‘g’ and for the purposes of this calculation can be regarded as a constant of 10m/s2
- System efficiency: Overall system efficiency.
- Power: The potential output of any site is usually expressed in Kilowatts (kW)
The basic power of a system can be expressed by the equation:
- Power (in kW) = Q (in m³/s) x H (in m) x g (in m/s2 x Efficiency (as a fraction)
Example: At the abstraction point a watercourse has an annual mean flow of 0.03m3/s. The vertical height difference from the intake point to the turbine house is 100m. Assume a 50% system efficiency. Hydro system potential in this case would be about 0.030 x 100 X 10 x 0.5 = 15 kW.
In determining the limits of abstraction from a water course the Environment Agency (EA) will not allow any installation to abstract all of the water in a river or stream. The EA have regulations and standards to protect both the local flora and fauna in the depleted reach and also the greater riparian system. Consequently the Environment Agency will place a restriction on the water that can be abstracted.
Stream flows are usually measured in terms of Q% values. A Q rate of Q75 represents the stream flow that is in the steam for at least 75% of the year. In most cases the EA will set an initial amount of water that must remain in the water course at all times; this is known as the Hands off Flow (HOF). No abstraction can take place while the stream flow is less than the HOF. The HOF is usually set between Q85 and Q95.
The EA will then set the abstraction regime above the HOF. Current guidance is that abstraction can be 100% of flow above the HOF but only up to a limit equivalent to Qmean.
Hydro scheme’s intake dam/weir designs and automatic flow regulators ensure that the agreed abstraction regime is maintained.
Due to the seasonal variations in flow maximum abstraction will often only be achievable for 25% of any year. High head turbines maintain good efficiencies down to about 10% of their maximum design flow, because of this the seasonal flows mean that typical high head hydro scheme will not produce significant levels of power for about 20% of any year.