Renewable Technologies: Hydroelectricity
The Earth’s surface is two-thirds covered by water. Harnessing the potential energy stored within it is a fantastic way to produce clean, renewable energy, and it is one that has been used for thousands of years.
What is Hydroelectricity?
Quite simlpy the oldest method by which renewable energy has been harnessed by the human race. The first water wheels were used well over 2000 years ago, and the technology has since been refined to become very efficient in the production of electricity
The potential energy stored in a body of water held at a given height is converted to kinetic energy (movement energy) which is used to turn a turbine and create electricity. Due to recent improvements in technology, ‘micro’ hydro schems (those under 100kW) have become viable means of producing electricity. There is enough energy in a small stream to produce electricty.
In a similar way to solar power, the system can be on or off grid, and the source of the hydro must be relative close to the site of power usage. If the system is grid connected, the surplus may be sold back to the grid. Off grid systems are usually connected to a battery bank and an inverter. Seasonal changes in water flow must in this latter case be take into account. Installing a micro hydro system can supply enough electricity for a whole household, and if grid connected electricty bills can be ‘negative’, although initial capital costs can be high. An oft-proposed solution is a community hydro project which can yield similar benefits at a lower cost.
The amount of electricty generated depends on a number of factors, not least the speed and quantity of water in the flow, and the efficiency of the turbine. You can calculate the power contained in water using the following formula:
P = H x Q x g x e
Where P is power measured in kW, H is head (height) measured in metres, Q is flow rate in cubic metres, g is the gravitational constant (~9.8m/s), and e is the efficiency factor (usually ~0.5, i.e. 50%). A small turbine on a hill stream with a flow of 15 litre/sec and a head of 15m will therefore generate about 1kW, enough for the basic needs of a house. This is a basic calculation and at all times a proper feasibility study should be carried out by a micro-hydro consultant who can give high quality advice.
The components of a typical hydro system are as follows:
- An intake, often incorporated into a weir to diverts ome of the flow from the water course
- A penstock pipe to convey the water to the turbine
- A powerhouse, in which the turbine and generator convert the water’s energy into electricity
- An outflow through which the water is released back to the river or stream
- Underground cables or overhead lines to transmit the electricty to it’s point of use. These must be short enought to minimise ‘voltage drop’
What will it cost?
Not including civil works (assuming an existing pond or weir), you can expect to pay around £10,000 in fixed costs and £2000 per kW on top of that. This may sound expensive, but if the sytem is grid connected it can be sold back to the grid. The payback for a small scale hydro station in such cases can be as little as 7 or 8 years.
Although there are visual and noise issues with hydro, these can be mitigated very easily. Maintaining the river’s ecology is paramount, and care has to be made not to divert the flow of the river more than necessary for the system, and certainly no more than that with which the river’s ecology can cope. Part of the fixed costs outlined above include the price of an extraction licence which needs to be obtained from the Environment Agency or SEPA (Scotland).