CPD Programme
> English Region Anglian
North West Midlands Southern
South West Thames
North East
England Total Wales
Wales Total
Number of Sites 126 284 157 36
322 125 318
1368
Number of Sites 324
Figure 1: Potential sites for hydro power generation in England and Wales (Source: Hydropower Resource Assessment Report, DECC October 2010)
infrastructure is not wholly dependent upon output capacity: a 5kW system may only cost 50% more than a 2kW system.3
Power generated Hydro <=15kW
Hydro >15-100kW FITs tariff
19.9p/kWh 17.8p/kWh
Hydro >100kW-2MW 11.0p/kWh Hydro >2-5MW
4.5p/kWh Figure 2: Generation FIT for hydro installations
Micro-hydro system components Smaller systems will not visibly have all the individual components, however their function will be present albeit in a consolidated form.
Intake/weir Ideally, the intake is sited at a point in the waterway where both the flow and streambed are stable – solid bedrock, constant flow and a low gradient on a straight section where there is reduced chance of erosion and sediment deposition. Depending on the size of the installation this could be a simple sink, a diverted inlet using a weir or a bifurcated waterway. The intake would have some coarse ‘filter’ with some sort of grating to reduce the intake of flotsam, flora and fauna, this is often referred to as a ‘trash rack’.
Canal/pipeline and forebay tank This section provides a diverted track for the water that will be used to generate the power. As the water passes through the canal (known as a ‘headrace’ or ‘water race’) any drop in height will not contribute to the available power and so is a loss of potential generation. The forebay tank will allow the water to settle, sediment to drop out and provide a location for a finer trash rack. Excess water (not used for generation) flows freely from the forebay directly back through channels down to the waterway and helps to purge silt so that it
56 CIBSE Journal February 2011
does not accumulate and enter the generation equipment. Many micro-hydro systems do not utilise the canal/forebay section as they pass water directly from the intake into the ‘Penstock pipe’.
Penstock pipe The penstock pipe is one of the most important elements of the system to optimise the availability of the water’s power. Typically made of HDPE, steel or UPVC, this provides a closed pathway from high level (providing the ‘head’) for the water to enter the powerhouse and generation equipment. As with any pipe carrying liquid the frictional losses will be related to the pipe diameter and the roughness of the internal surfaces. When the penstock pipe is sized it is a balance of pipe cost and pipe diameter, with a rough rule of thumb that the pipe is sized so that the frictional losses amount to no more than 10% - 20% of the available head – the lower the head loss, the more power will be generated.
Powerhouse The ‘powerhouse’ may be nothing more than a mounting frame for weatherproof generation equipment, or it could be a substantial building housing larger scale generation and electrical conditioning and control equipment. The functioning parts of the powerhouse can even be totally submerged to reduce noise breakout. However, it would typically be sited above the point where flooding might be expected, also taking into account foreseeable changes in the path of the river that may take place due to erosion. There should be good access for construction and maintenance – the powerhouse may be some distance from the main waterway to satisfy all of the required criteria.
Tailrace Leading from the powerhouse, this channel
Potential (kW) 4, 920 – 13,370 32,000 – 37,700 18,000 – 32,400 1,100 – 2,600
20,000 – 29,400 16,200 – 30,120 27,330 – 39,810
119,550 – 185,400 Potential (kW)
26,730 – 63,000
calms the flow and reintroduces the water back into the main waterway.
The turbine Normally housed in the powerhouse, the turbine converts the flow of water into shaft rotation that (directly or indirectly) drives the electrical generator or alternator. The choice of turbine will depend on the net head and the available water flow rate, together with the required running speed for the electrical equipment. The range of seasonal flow rates may also affect the selection. Turbines are split into two principal groups
– impulse and reaction. Pelton, Turgo and Banki Michell turbines are commonly- used impulse-type turbines in micro-hydro installations with nozzles directing the water flow onto the turbine. A small Pelton turbine is simple to manufacture, relatively cheap and has good efficiency and reliability. Individual Pelton or Turgo ‘cups’ may be simply cast (or cheaply mass produced in plastic). Pelton turbines are generally more suited to high-head, low-flow rate applications and Turgo turbines more suited to lower-head, higher-flow rate applications. To adjust for variations in available water flow, these turbines are adapted by changing nozzle sizes or by using adjustable nozzles. The efficiency of both the Turgo and Pelton wheel depends on size and manufacture but is likely to be between 70%+ and 90%+. The Banki Michell cross-flow turbine
will not reach the efficiencies of the Pelton or Turgo turbines (although may well reach 80%) but it is cheap to produce, simple to maintain and has a reasonably constant efficiency (down to 50% design flow) and is ideally suited to low-head applications. A chart comparing the performance of common impulse turbines is shown in Figure 3. Reaction turbines sit in the full flow of
the water delivered by the penstock pipe and are either propeller machines (such as the ‘Kaplan’) that are suited to low-heads and high-flow rates; or they are similar to a centrifugal pump acting as a turbine (such as the ‘Francis’) that are most suitable for mid-range flows and heads (flow properties between those ideal for the Kaplan and the Pelton).
Available power and energy The static-head (or gross-head) is calculated from the vertical distance in metres between the water intake of the system (normally the entry for the Penstock) and the point where the water enters the generator. For reaction turbines the static head includes the vertical
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