Hydroelectricity

Hydroelectric power captures the potential energy contained in the fall of water from burns, rivers and lochs towards the sea. It has been used for well over a century so is a very mature technology. It is gaining popularity as the value of electricity rises and environmental and energy security concerns encourage the return to the local and renewable energy sources of the pre-industrial age. Hydro schemes can be very cost-effective and long-lasting. The main obstacles to micro hydro schemes are high initial costs and the necessity to comply with environmental protection regulations and not to conflict with other uses of a watercourse such as fishing.

How does a micro hydro scheme work?

Hydro power works by directing the pressure of falling water to drive a turbine attached to a generator. The amount of energy produced depends on how far the water is falling (the head) and the volume of water passing through the intake (the flow rate). A head of 2 to 3m is a typical minimum starting point. The important requirements for a micro hydro scheme are a fairly reliable water flow and a reasonably steep drop from a dam or weir to a turbine site near a building.

A power-head-flow graph can be used to estimate the yield from your site:

There are 4 main components of a micro hydro scheme:

  • Intake – water is collected into the pressure pipe (penstock) at an intake in a weir or dam which must withstand the forces of flash floods and prevent debris from entering the pipe
  • Penstock  - pressure builds in the pipe as it passes downhill due to the weight of water above it. The pipe, which is normally buried, must be strong enough to handle the pressure and big enough to minimise pressure losses due to friction. It must be very long-lasting, usually made of polyethylene but sometimes mild steel or glass-reinforced plastic (GRP)
  • Turbine – this is bolted to the bottom end of the penstock in the turbine house. The turbine ‘runner’ converts the water pressure to rotational force either by being struck by a jet (high head systems) or spinning within the flow of water (low head systems). The rotating shaft then turns a generator to produce alternating current which can be used in the building or fed into the national grid
  • Tailrace – this gently returns the de-pressurised flow to the watercourse in a way which does not cause erosion, either via a pipe or an open channel. 

Assessing the energy resource of your hydro scheme

Hydro consultants can estimate the likely energy output from a hydro scheme using commercial software, and this can be tested by on site flow gauging. Alternatively on site gauging over an extended period, ideally one or two years, can be effective when rainfall records for the period are compared with long term averages. Flow records can be used to generate a flow duration curve which can then be used to choose the size and type of turbine (eg. Pelton, Turgo, Cross-flow, Kaplan, Francis or Archimedes Screw), the diameter of the penstock and the likely kW output for the grid connection and CAR license applications.

Once the maximum power and likely annual energy production can be calculated, capital costs can be estimated and a financial feasibility assessment carried out.

Obtaining planning consent and grid connection

Before you can install even the smallest hydro scheme you must obtain:

  • Planning permission from your local planning authority
  • Controlled Activities Regulations (CAR) License from SEPA (www.sepa.org.uk) for construction, water abstraction and discharge and, unless you use all the power on site, or cannot export more than about 3.6kW per phase*,
  • Grid connection from the Distribution Network Operator (ScottishPower or SSE in Scotland).

Find out more at: http://www.sepa.org.uk/water/hydropower.aspx

British Hydro Association www.british-hydro.org 

Micro Hydro Association: www.microhydroassociation.org

*Generators exporting up to 3.6kW per phase can submit a G83 connection declaration after the scheme has been installed and commissioned to the required engineering safety standards.

Costs & Incentives

It is difficult to generalise the cost of hydro systems as every site is unique. Whilst the turbine and generating equipment may be similar at two sites, pipe runs and required civil works can vary greatly, impacting costs.

The capital costs of small hydro schemes are in the region of £4,000-£10,000 per kW capacity. Higher output sites tend to have a lower cost per kilowatt of capacity. High head schemes tend to cost less per kW as they involve controlling much lower volumes of water compared to, for example, schemes on former mill sites where the civil works (weir, lade, etc) are not very well preserved. All hydro installations should have long working lives and low on-going maintenance costs.

Whilst the economics at the smallest scales have proved challenging in the past, the arrival of feed-in tariffs means that that more domestic-scale schemes have become feasible.

If your system is less than 15kW all the electricity you generate will, at 2012/13 tariff rates, earn 21p/kWh from the generation Feed In Tariff and every surplus unit you sell to the grid will earn at least 4.5p. Some FiT providers will simply assume that 50% of generation is exported, thereby removing the need for an export meter. Off-grid systems will still earn the full generation tariff.

For the latest Feed-In Tariff rates visit www.ofgem.gov.uk/fits .