Guide to Mini Hydro Development



Mini-hydro: a step-by-step guide

This Guide is designed to assist anyone in the Pakistan who is planning to develop a small-scale hydro-electric scheme.

It has been prepared by the Rehan Energy in order to support and encourage new developments in this sector.


The term used in this Guide will be ‘Mini-hydro’, which can apply to sites ranging from a tiny scheme to electrify a single home, to a few hundred kilowatts for selling into the National Grid.


The Guide will explain:


  • ·         The basic concept of generating power from water
  • ·         The purpose of different components of a scheme
  • ·         The principle steps in developing a project
  • ·         The technology involved
  • ·         Where to go for help and sources of funding

Each section of the Guide is listed in the menu titles across the page.  You can either follow the Guide through from page to page, or pick out the topics of interest from the drop-down menus.  Each page can be printed out on A4 by selecting the printer-friendly icon in the corner of each page.

Why mini-hydro ?

Small-scale hydropower is one of the most cost-effective and reliable energy technologies to be considered for providing clean electricity generation.


In particular, the key advantages that small hydro has over wind, wave and solar power are:

  • A high efficiency (70 – 90%), by far the best of all energy technologies.
  • A high capacity factor (typically >50%), compared with 10% for solar and 30% for wind.
  • A high level of predictability, varying with annual rainfall patterns.
  • Slow rate of change; the output power varies only gradually from day to day (not from minute to minute).
  • It is a long-lasting and robust technology; systems can readily be engineered to last for 50 years or more.

It is also environmentally benign. Small hydro is in most cases ‘run-of-river’; in other words any dam or barrage is quite small, usually just a weir,  and little or no water is stored. Therefore run-of-river installations do not have the same kinds of adverse effect on the local environment as large-scale hydro.


Hydropower basics – Head & Flow

Hydraulic power can be captured wherever a flow of water falls from a higher level to a lower level.  This may occur where a stream runs down a hillside, or a river passes over a waterfall or man-made weir, or where a reservoir discharges water back into the main river.The vertical fall of the water, known as the “head”, is essential for hydropower generation; fast-flowing water on its own does not contain sufficient energy for useful power production except on a very large scale, such as offshore marine currents.  Hence two quantities are required: a Flow Rate of waterQ, and a Head H. It is generally better to have more head than more flow, since this keeps the equipment smaller.


The Gross Head (H) is the maximum available vertical fall in the water, from the upstream level to the downstream level.  The actual head seen by a turbine will be slightly less than the gross head due to losses incurred when transferring the water into and away from the machine.  This reduced head is known as the Net Head.


Sites where the gross head is less than 10 m would normally be classed as “low head”.  From 10-50 m would typically be called “medium head”.  Above 50 m would be classed as “high head”.


The Flow Rate (Q) in the river, is the volume of water passing per second, measured in m3/sec.  For small schemes, the flow rate may also be expressed in litres/second where 1000 litres/sec is equal to 1 m3/sec.



Hydropower basics – Power & Energy

Energy is an amount of work done, or the ability to do work, measured in Joules.  Electricity is a form of energy, but is generally expressed in its own units of kilowatt-hours (kWh) where 1 kWh = 3,600,000 Joules and is the electricity supplied by 1 kW working for 1 hour.Power is the energy converted per second, i.e. the rate of work being done, measured in watts (where 1 watt = 1 Joule/sec. and 1 kilowatt = 1000 watts).


Hydro-turbines convert water pressure into mechanical shaft power, which can be used to drive an electricity generator, or other machinery. The power available is proportional to the product of head and flow rate. The general formula for any hydro system’s power output is:


                                                P = h r g Q H   



  • P is the mechanical power produced at the turbine shaft (Watts).
  • h is the hydraulic efficiency of the turbine.
  • r is the density of water (1000 kg/m3).
  • g is the acceleration due to gravity (9.81 m/s2).
  • Q is the volume flow rate passing through the turbine (m3/s).
  • H is the effective pressure head of water across the turbine (m).

The best turbines can have hydraulic efficiencies in the range 80 to over 90% (higher than all other prime movers), although this will reduce with size.  Micro-hydro systems (<100kW) tend to be 60 to 80% efficient.


If we take 70% as a typical water-to-wire efficiency for the whole system, then the above equation simplifies to:

(kW) = 7 ´ (m3/s) ´ (m)

 Hydropower basics – Main Elements

The main figure illustrates a typical small hydro scheme on a medium or high head. Click on the picture for a dynamic presentation of the elements of the scheme. Other possible layouts are discussed in Section 2.4.The scheme can be summarised as follows:

  • Water is taken from the river by diverting it through an intake at a weir.
  • In medium or high-head installations water may first be carried horizontally to the forebay tank by a small canal or ‘leat’.
  • Before descending to the turbine, the water passes through a settling tank or ‘forebay’ in which the water is slowed down sufficiently for suspended particles to settle out.
  • The forebay is usually protected by a rack of metal bars (a trash rack) which filters out water-borne debris.
  • A pressure pipe, or ‘penstock’, conveys the water from the forebay to the turbine, which is enclosed in the powerhouse together with the generator and control equipment.
  • After leaving the turbine, the water discharges down a ‘tailrace’ canal back into the river.


Hydropower basics – Different Site Layouts

In practice, sites that are suitable for small-scale hydro schemes vary greatly.  They include mountainous locations where there are fast-flowing mountain streams and lowland areas with wide rivers. In some cases development would involve the refurbishment of a historic water power site.  In others it would require an entirely new construction. This section illustrates the four most common layouts for a mini-hydro scheme.


A variation on the canal-and-penstock layout for medium and high-head schemes (Section 2.3) is to use only a penstock, and omit the use of a canal.  This would be applicable where the terrain would make canal construction difficult, or in an environmentally-sensitive location where the scheme needs to be hidden and a buried penstock is the only acceptable solution.


For low head schemes, there are two typical layouts.  Where the project is a redevelopment of an old scheme, there will often be a canal still in existence drawing water to an old powerhouse or watermill.  It may make sense to re-use this canal, although in some cases this may have been sized for a lower flow than would be cost-effective for a new scheme.  In this case, a barrage development may be possible on the same site.


With a barrage development, the turbine(s) are constructed as part of the weir or immediately adjacent to it, so that almost no approach canal or pipe-work is required.


A final option for the location of new mini-hydro turbines is on the exit flow from water-treatment plants or sewage works.  This application is growing in popularity with PAKISTAN water companies.





There are a few pieces of essential information that need to be obtained when a new site is being considered for hydro generation.


1.       Firstly, one has to identify whether there is a significant energy resource.  This involves estimating or measuring the flow and available head, and estimating what annual energy capture would result.


2.       If the potential output of a scheme is attractive, then one needs to be certain that permission will be granted to use all of the land required both to develop the scheme and to have the necessary access to it.

3.       Finally, there needs to be a clear destination for the power: is there a nearby load that needs to be supplied, or is there a convenient point of connection into the local distribution network?


These issues are explored in more detail in the next sections:



Obtaining Flow Data

The Environment Agency measures the flow in most significant rivers and streams in the PAK, and data from the 1300 gauging stations can be obtained from the Centre for Ecology and Hydrology in Wallingford. Data for 200 sites is available over the internet, at: These records can be used to assess stream flow at the proposed site, as long as due allowance is made for the actual site location in relation to the gauging station (upstream or downstream).


If no data is available, it is also possible to use hydrological methods that are based on long-term rainfall and evaporation records, and on discharge records for similar catchment areas. This allows initial conclusions to be drawn on the overall hydraulic potential without taking actual site observations. It is advisable to follow this up with site measurements once the project looks likely to be feasible.


The reference books included in the bibliography offer a number of more or less sophisticated methods both for estimating the hydrology of a catchment area and for measuring the flow in streams.

The most accurate and reliable flow measurement method is to install a measuring weir, as summarised below.

3.2.2   Measuring weirs

A flow measurement weir has a rectangular notch in it through which all the water in the stream flows.  It is useful typically for flows in the region of 50-1000 l/s. The flow rate can be determined from a single reading of the difference in height between the upstream water level and the bottom of the notch (see Figure). For reliable results, the crest of the weir must be kept ‘sharp’ and sediment must be prevented from accumulating behind the weir.


The formula for a rectangular notched weir is:


Q = flow rate (m3/s)

Cd = the coefficient of discharge

L = the notch width (m)

h = the depth of the weir crest below upstream water level (m)

g = acceleration due to gravity (9.81m/s2)


If Cd is taken, typically, as 0.6, then the equation becomes:


               Q = 1.8 (L – 0.2h) h1.5


Since stream flow varies both from day to day and with the season, measurements should ideally be taken over a long period of time, preferably several years.


Measuring Weir

3.2.3  Flow Duration Curve

There are two ways of expressing the variation in river flow over the year: the annual hydrograph and the Flow Duration Curve or FDC, as illustrated below.


The annual hydrograph is the easiest to understand, since it simply shows the day-by-day variation in flow over a calendar year.  However, the FDC is more useful when calculating the energy available for a hydro-power scheme.


The FDC shows how flow is distributed over a period (usually a year). The vertical axis gives the flow,  the horizontal axis gives the percentage of the year that the flow exceeds the value given on the y-axis.


Hence, for example, the FDC can immediately indicate the level of flow which will be available for at least 50% of the year (known as Q50).  The flow exceeded for 95% of the year (Q95) is often taken as the characteristic value for minimum river flow.


FDCs are often very similar for a region, but can be affected by soil conditions, vegetation cover, and to a lesser extent by catchment shape.  They are also modified by man-made reservoirs, abstractions and discharges.


A flatter FDC (characterising a heavily spring-fed river) is preferable to a steeply sloping one, and means that the total annual flow will be spread more evenly over the year, giving useful flow for a longer period, and less severe floods.

Compensation Flow

A portion of the flow, termed the compensation flow, will need to by-pass the scheme for environmental or aesthetic reasons.  In abstraction schemes, where water is diverted from the main course of the river, this compensation flow is needed to maintain the ecology and aesthetic appearance of the river in the depleted stretch.


The amount of compensation flow will depend on site-specific concerns, but a reasonable first estimate will lie between the Q90 and Q99 values of river flow.


Head measurements

The head of water available at any one site can be determined by measuring the height difference between the water surface at the proposed intake and the river level at the point where the water will be returned.


A number of reference books can provide details of basic survey techniques to measure or estimate the available head. The most common methods are summarised as follows.


An initial estimate for a high-head site (> 50m) can be taken from a large-scale map, simply by counting the contours between the inlet and discharge points: the distance between contours on standard Ordnance Survey maps is 10 m.


Altimeters can also be useful for high-head pre-feasibility studies. Surveying altimeters in experienced hands will give errors of as little as 3% in 100m. Atmospheric pressure variations need to be corrected for, however, and this method cannot be generally recommended except for approximate readings.
The use of a Dumpy level (Theodolite or builder’s level) is the conventional method for measuring head accurately and should be used wherever time and funds allow. Such equipment should be used by experienced operators who are capable of checking the calibration of the device.

Head measurement by Dumpy level

Low-head schemes

An important factor on low head schemes is that the gross head is not a constant but varies with the river flow. As the river fills up, the tailwater level very often rises faster than the headwater level, thus reducing the total head available. To assess the available gross head accurately, headwater and tailwater levels need to be measured for the full range of river flows.

 Preliminary power and energy calculation

3.4.1             Design Flow


It is unlikely that schemes using significantly more than the mean river flow (Qmean) will be either environmentally acceptable or economically attractive.  Therefore the turbine design flow for a run-of-river scheme (a scheme operating with no appreciable water storage) will not normally be greater than Qmean.  The exception would be a scheme specifically designed to capture very high winter flows, which is very rare in mini-hydro applications.


The greater the chosen value of the design flow, the smaller proportion of the year that the system will be operating on full power, i.e. it will have a lower ‘load factor’.

3.4.2             Load Factor


The ‘load factor’ is a ratio summarising how hard a turbine is working, expressed as follows:


Load factor (%)     =      Energy generated per year (kWh/year)                   

                                  Installed capacity (kW) x 8760 hours/year


A first estimate of how load factor varies with design flow is given as follows:


Design Flow Qo

Load Factor



0.75 Qmean


0.5 Qmean


0.33 Qmean


3.4.3             Rated Power

The peak power P can be estimated from the design flow Q0 and head H as follows:


P(kW) = 7 ´ Qo(m3/s) ´ H(m)

3.4.4             Energy Output

The annual energy output is then estimated using the Load Factor (LF) as follows:


Energy (kWh/year) = P (kW) ´ LF ´ 8760


There is clearly a balance to be struck between choosing a larger, more expensive turbine which takes a high flow but operates at a low load factor, and selecting a smaller turbine which will generate less energy over the year, but will be working flat out for more of the time i.e. a higher load factor.  The load factor for most mini-hydro schemes would normally fall within the range 50% to 70% in order to give a satisfactory return on the investment.


Most turbines can operate over a range of flows (typically down to 20-40% of their rated flow) in order to increase their energy capture and sustain a reduced output during the drier months.


Use of the Land

No project can proceed unless you have the right to utilise all the land in question. It is also important to establish how contractors will access the different parts of the scheme with the necessary equipment, and to confirm that these routes will be available.It is therefore wise to approach the relevant land-owners at an early stage to establish any objections to the proposed scheme and to negotiate access.  Since water courses often form property boundaries, the ownership of the banks and existing structures may be complex.  Failure to settle this issue at an early stage may result in delays and cost penalties later in a project.

Leasing agreements will need to be drawn up which establish the right to use the necessary land areas and also to define the responsibilities of the tenant in maintaining it.  For example, the operator of a scheme may be required to take on the maintenance liability of an existing weir and mill leat as part of the agreement allowing him to install a turbine in the old mill.



Grid-connect or stand-alone

It is important to determine at the outset what the value of the electricity generated by the scheme will be, i.e. to whom the power will be sold.The electricity generated by a scheme may be used at the point of generation, in place of electricity supplied by the local electricity company. Alternatively it may be exported via the local distribution network by agreement with the Distribution Network Operator (DNO).


It is nearly always financially advantageous to consume as much of the power as possible on site, and only export the surplus into the network.


If the scheme is to produce power for export to the local network, there should be early discussions with the DNO who will specify the system protection and metering equipment, and will also provide an estimate of connection costs and the best location for feeding into their system.




4.1.1             Getting Professional Help


Any developer should seek independent professional advice before committing significant finance to the design and construction of a small-scale hydro scheme.


The involvement of professionals in a small-scale hydro development can range from preliminary site assessment, through the conducting of a feasibility study, to a full ‘turnkey’ service, handling every aspect of a development.  In addition, there are several companies that lease, develop and operate sites as a business activity, and can provide a full skills and finance package.

4.1.2             Preliminary Site Assessment


An experienced hydro professional should be able to indicate whether a site is worth considering further, on the basis of an initial site visit and discussions with the developer and others.  Preliminary investigations of this type will typically require no more than 2-3 days’ work and will cost between £300 and £1000.  A minor investment at this stage could save much greater expense and potential complications later in the development process.


The main issues that should be considered in a preliminary investigation are:


  • The existence of a suitable waterfall or weir and a turbine site
  • A consistent flow of water at a usable head
  • The likely acceptability of diverting water to a turbine
  • Suitable site access for construction equipment
  • A nearby demand for electricity, or the prospect of a grid connection at reasonable cost
  • the social and environmental impact on the local area
  • land ownership and/or the prospect of securing or leasing land for the scheme at a reasonable cost
  • an initial indication of design power and annual energy output


The accuracy of the information may only be plus or minus 25%, however, this should be sufficient for deciding whether to proceed to a more detailed feasibility study.



A feasibility study uses accurate data and looks closely at costs.  It can take the project forward from the initial idea to a final design that will support applications for project finance and the necessary licenses.  It is therefore wise always to employ a professional to conduct the feasibility study and the detailed design work.The cost of a full feasibility study carried out by an independent consultant depends on its scope and on the specific characteristics of the site, but would typically be £5,000-£10,000.


For a domestic-scale scheme (i.e. less than 30 kW), a detailed feasibility may not be affordable, and a less detailed Pre-feasibility Study may prove sufficient.  This would cover the same basic ground but use approximate data analysed less extensively.  It should be possible to commission a pre-feasibility study for less than £4000.
The following essential tasks should form components of a feasibility study:

  1. Hydrological Survey.  Typically, a hydrological survey would produce a flow duration curve.  This would be based on long-term records of rainfall and/or flow data, together with a knowledge of the catchment geology and soil types.  This long-term information might be backed up by short-term flow measurements.   The study should also include an estimate of the required compensation flow.
  2. System design. This would include a description of the overall project layout, including a drawing showing the general arrangement of the site.  The prominent aspects of the works should be described in detail, covering:
    – Civil works (intake and weir, intake channel, penstock, turbine house, tailrace channel, site access, construction details)
    – The generating equipment (turbine, gearbox, generator, control system)
    – Grid connection
  3. System costing.  A clear system costing would include a detailed estimate of the capital costs of the project, subdivided into:
    – Civil costs
    – The cost of grid-connection
    – The cost of electro-mechanical equipment
    – Engineering and project management fees
  4. Estimate of energy output and annual revenue.  This would summarise the source data (river flows, hydraulic losses, operating head, turbine efficiencies and methods of calculation) and calculate the output of the scheme in terms of the maximum potential output power (in kW) and the average annual energy yield (kWh/year) converted into annual revenue (£/year).

An additional task, which may form part of the main feasibility report but is often undertaken separately, is the environmental assessment of the scheme, discussed in Section 4.3.


Environmental impact

4.3.1             The Environmental Statement.


Some form of environmental assessment is essential when it comes to applying for planning permission and environmental licenses.


Under the Town and Country Planning (Assessment of Environmental Effects) Regulations 1988, the planning application for any development that the planning authority considers likely to have a significant impact on the environment must be accompanied by an Environmental Statement.  This document provides an assessment of the project’s likely environmental effects, together with any design, construction, operational and decommissioning measures that are to be taken to minimise them.  It would typically cover such issues as flora, fauna, noise levels, traffic, land use, archaeology, recreation, landscape, and air and water quality.


The Environment Agency may also require a report assessing the environmental effects of the development. If the planning authority has asked for an Environmental Statement, this may meet the requirements of the Environment Agency.  However, the Environment Agency may ask for environmental information even if the local planning authority does not.  Such information might cover water use, water quality, fisheries, river ecology, flood defence, nature conservation and public recreation issues.   The Environment Agency should be consulted at an early stage and will provide guidance on what is required.


Specialist environmental consultants may be employed if project complexity merits their involvement. However, general hydro consultants with an appropriate track record may also undertake an assessment of this type.

4.3.2             Fisheries

Hydro-installations on rivers populated by migrating species of fish, such as salmon or trout, are subject to special requirements as defined in the Salmon and Freshwater Fisheries Act.


Migratory fish must not be ingested into the turbine (so the mesh of the trashrack must be fine enough), and there must be a water passage by-passing the hydro-plant at all times so that fish can migrate up or downstream. To allow fish to pass upstream sometimes requires the construction of a ‘fish ladder’, which is usually a series of pools one above the other, with water overflowing from the higher ones to the lower ones, so that fish can jump up from one pool to the next.

Planning and licenses

5.1              Whom to consult

Informal and formal consultation should underpin every stage of a development and may be handled either by the developer or by a hydro professional.  Consultation will be tailored to each individual development.  Some sites, for instance, may not be located on fishing rivers and therefore consultation with fisheries bodies or angling clubs would be limited.  Similarly, where a site does not require planning permission, there is no need for detailed consultation with the relevant planning authorities.


The bodies listed in the table below should be approached, as appropriate, at the outset of a development, and contact should be maintained throughout.  Full consultation will ensure that any problems are identified at an early stage, and this may prevent the incurring of unnecessary expenditure.

Body to be consultedPurpose of ConsultationThe Environment Agency (England and Wales)
Scottish Environmental Protection Agency (SEPA)To ensure that the site is acceptable

To establish a design that is acceptable

To identify the permissions required

To discuss and agree an acceptable river operating regime (i.e. amount and timing of abstractions)


Relevant planning authorityTo ensure that the site is acceptableTo establish a design that is acceptable, especially where construction work is neededTo identify permissions required Fisheries bodies or those with an interest in fisheries (e.g. angling clubs).
Scotland: the District Salmon Fisheries BoardTo address possible concerns at the design stageStatutory environmental bodies e.g. English nature and the Countryside Commission; Scottish Natural HeritageTo address potential environmental impacts at the design stageLandownersTo address ownership, access and leasing issues, way-leaves for cablesTo address possible objections to development Regional Electricity Company (REC)If an electricity connection is required, to establish any design constraints and connection costsIf appropriate, to enter negotiations for electricity sale


Planning issues

Planning aspects of hydro developments are the responsibility of the local planning authority in England and Wales.  Planning permission will be required for most hydro developments.  A possible exception is the refurbishment of an existing scheme, where there is no ‘change of use’.The planning department will indicate whether planning permission   is required and also whether other related procedures, such a Building Regulation Approval or the submission of an Environmental Statement, are necessary.


As well as giving advice on how to make an application and on the fee charged, the planning department will also suggest who should be consulted, indicate sensitivities to development, and outline measures that might be taken to make developments more acceptable.  An early approach to the planning department is recommended so that any uncertainties can be clarified and a good working relationship established.


The primary issues of concern to the planners are likely to be:


  • The visual appearance of the scheme, including the powerhouse and penstock in particular
  • Potential noise impacts on nearby residents
  • Disturbance during the construction phase, both to local residents and disrupting traffic
  • Preservation of structures of historical importance


On environmental issues, the planners will normally take advice from their statutory consultees, such as the Environment Agency and English Nature.  They will also be able to advise on whether the scheme warrants a public display for the purpose of presenting the project to local people and helping allay any concerns.


It is sometimes advisable to apply for outline planning permission in the first instance, in which the main elements of the scheme can be agreed but without the completion of the final design.  This means that the overall planning process will be longer, but allows feedback received during the outline planning process to be accommodated more easily into the final design and therefore reduces the risk of the full planning application being rejected.





Costs and economics

6.1              Investment Costs

Small hydro costs can be split into four segments :

1. Machinery
This group includes the turbine, gearbox or drive belts, generator and the water inlet control valve.
Generally speaking, machinery costs for high head schemes are lower than for low head schemes of similar power.  High head machines have to pass less water than low head machines for the same power output and are therefore smaller. They also run faster and thus can be connected directly to the generator without the complication of gearbox or belts.



2. Civil Works
This includes the intake, forebay tank and screen, the pipeline or channel to carry the water to the turbine, the turbine house and machinery foundations, and the tailrace channel to return the water to the river.

The Civil Works are largely site-specific. On high head sites the major cost will be the pipeline; on low head sites probably the water intake, screens and channel.




3. Electrical Works
The electrical system will involve the control panel and control system, the wiring within the turbine house, and a transformer if required, plus the cost of connection to the electricity.  These costs are largely dependent on the maximum power output of the installation. The connection cost is set by the local electricity distribution company..






4. External Costs
This could encompass the engineering services of a professional to design and manage the installation, plus the costs of obtaining a the licences, planning permission, etc.
For a 100kW small hydro installation, the costs could range as follows:-


Low head

High head





60 – 120

30 – 60

Civil works

30 – 100

30 – 80

Electrical works (no grid connection)

15 – 30

15 – 30

External costs

10 – 30

10 – 30


115 – 280

85 – 200


Generally, the cost per kilowatt of new schemes increases as size reduces, due to economy of scale and the fact that any scheme has a certain fixed cost element which does not greatly change with size of scheme.

6.2              Running Costs

6.2.1             Leasing

If part of the land is leased, then there will be an annual rent to pay. It can be beneficial to tie this rent into the revenue from the scheme, so that the landlord also has an incentive for the turbines to be operating.  Schemes which lease all the land should expect to pay no more than 4% of annual revenue as rent, and the lower you can negotiate the better!

6.2.2             Metering

Larger schemes currently require half-hourly metering to be installed, which has to be monitored by an independent meter-reading company, although this requirement may change in the future.  There is an annual charge to pay for this service, currently in the range £350 – £1000 / year.

6.2.3             Rates

Hydroelectric schemes are subject to business rates unless they are seen as being part of a domestic property.  The rateable value is constantly under review, and the correct value for 2004 is £9 per kW installed which, when multiplied by the Unified Business Rate (0.46 in 2004), gives the annual sum to be paid.

6.2.4             Maintenance and Servicing

Modern, automated equipment requires very little maintenance.  The cost of routine inspections and an annual service should come to no more than 1-2% of the capital cost of the scheme.  As the machine ages, there will eventually be extra costs associated with replacing seals and bearings, a new generator, refurbished sluice gates, etc., but these should not occur for at least 10 years.

6.2.5             Insurance

Although hydro plant is generally very reliable, the following insurances are recommended (and may be required by financiers):

    • Material damage insurance against the cost of repairing damage to the works caused by fire and ‘special perils’ such as explosions, storms, flooding, impact and malicious damage
    •  Business interruption insurance against profit loss caused by fire or special perils damage
    • Public and employer’s liability insurances, which are required by law;  a minimum indemnity of £5 million is recommended.


Maximising the revenue from your scheme

Operators of ‘clean’ electricity plant can generate revenue by selling:

  • The electricity itself
  • Levy Exemption Certificates
  • Renewable Obligation Certificates


If the electricity generated by a hydro-scheme is sold directly to an electricity company, then the price offered for the electricity itself is relatively small  – in the range 2.0 – 2.5 p/kWh on average over the year.


Alternatively, if there is a substantial electrical load close to where the power is being generated (e.g. factory or office complex), it will be more beneficial to use the hydropower to feed that load, so displacing electricity that would otherwise be bought in from the grid at perhaps 3.5 – 5.5 p/kWh.


For smaller schemes, some electricity companies are willing to enter into a special contract which will balance the energy generated against the energy consumed on site on an annual or quarterly basis.


Furthermore, business customers who would otherwise have to pay an extra 0.43 p/kWh for the Climate Change Levy will make that additional saving on any hydroelectricity they buy from the scheme.

Levy Exemption Certificates (LECs)

Green electricity which is sold into the grid will generate Levy Exemption Certificates (LECs) which can be sold on to business customers to enable them to avoid paying the full Climate Change Levy.  The LECs can be sold for up to 90% of the Levy value.


Electricity generated from renewable sources can be used to obtain Renewables Obligation Certificates (ROCs) which all the supply companies need in order to prove they are meeting the governments targets for renewable energy. ROCs have a market value in the range 3p – 4.5p per kWh which will vary over time depending on how well these companies are doing in meeting their targets. The concept and trading of ROCs is explained in the diagram.

Ofgem issues ROCs to all registered generators of green electricity.

They can sell them either to ROC traders or directly to the electricity supply companies who need them

to meet their Renewable Energy Quota.

Electricity Traders

There are several options on who to approach to obtain the maximum income for your scheme.  Not only will any one of the main electricity supply companies (Powergen, npower, etc.) make an offer for your output (including the ROCs, LECs, etc.), it is also possible to approach a range of specialist electricity trading companies which focus purely on getting the best price for renewable energy schemes.


The BHA will be able to advise on which companies are offering the best deal for mini-hydro generation.



8.1 OverviewAll hydro turbines convert the energy from falling water into rotating shaft power, but there is often confusion as to which type of turbine should be used in different circumstances.

The selection of the turbine depends upon the site characteristics, principally the head and flow available, plus the desired running speed of the generator and whether the turbine will be expected to operate in reduced flow conditions.

8.1.1 Classification

Turbines can be crudely classified as high-head, medium-head, or low-head machines, as shown in the table below.


Electricity generation usually requires a shaft speed as close as possible to 1500rpm to minimize the speed change between the turbine and the generator. Since the speed of any given type of turbine declines with head, low-head sites need turbines that are inherently faster under a given operating condition.


Turbines are also divided by their principle of operation and can be either impulse or reaction turbines.


The rotor of the reaction turbine is fully immersed in water and is enclosed in a pressure casing. The runner blades are profiled so that pressure differences across them impose lift forces, just as on aircraft wings, which cause the runner to rotate.


In contrast an impulse turbine runner operates in air, driven by a jet (or jets) of water.


There are 3 main types of impulse turbine in use:  the Pelton, the Turgo, and the Crossflow (or Banki) turbines. The two main types of reaction turbine are the propeller (with Kaplan variant) and Francis turbines.


The approximate ranges of head, flow and power applicable to the different turbine types are summarised in the chart below (up to 500kW power). These are approximate and depend on the precise design of each manufacturer.


Impulse and Reaction Turbines

Turbine Type   Head Classification  

High (>50m)

Medium (10-50m)

Low (<10m)










Multi-jet Pelton

Multi-jet Pelton





Francis (open-flume)



Francis (spiral case)






Head-flow ranges of small hydro turbines


Impulse Turbines

The Pelton Turbine consists of a wheel with a series of split buckets set around its rim; a high velocity jet of water is directed tangentially at the wheel.  The jet hits each bucket and is split in half, so that each half is turned and deflected back almost through 180º.  Nearly all the energy of the water goes into propelling the bucket and the deflected water falls into a discharge channel below.





The Turgo turbine is similar to the Pelton but the jet is designed to strike the plane of the runner at an angle (typically 20°) so that the water enters the runner on one side and exits on the other. Therefore the flow rate is not limited by the discharged fluid interfering with the incoming jet (as is the case with Pelton turbines). As a consequence, a Turgo turbine can have a smaller diameter runner than a Pelton for an equivalent power.









The Crossflow turbine has a drum-like rotor with a solid disk at each end and gutter-shaped “slats” joining the two disks.  A jet of water enters the top of the rotor through the curved blades, emerging on the far side of the rotor by passing through the blades a 2nd time.  The shape of the blades is such that on each passage through the periphery of the rotor the water transfers some of its momentum, before falling away with little residual energy.




Reaction Turbines

Reaction turbines exploit the oncoming flow of water to generate hydrodynamic lift forces to propel the runner blades. They are distinguished from the impulse type by having a runner that always functions within a completely water-filled casing. All reaction turbines have a diffuser known as a ‘draft tube’ below the runner through which the water discharges.  The draft tube slows the discharged water and reduces the static pressure below the runner and thereby increases the effective head.

Propeller-type turbines are similar in principle to the propeller of  a ship, but operating in reversed mode. Various configurations of propeller turbine exist; a key feature is that for good efficiency the water needs to be given some swirl before entering the turbine runner.  With good design, the swirl is absorbed by the runner and the water that emerges flows straight into the draft tube.  Methods for adding inlet swirl include the use of a set of guide vanes mounted upstream of the runner with water spiralling into the runner through them. Another method is to form a “snail shell” housing for the runner in which the water enters tangentially and is forced to spiral in to the runner.


When guide vanes are used, these are often adjustable so as to vary the flow admitted to the runner. In some cases the blades of the runner can also be adjusted, in which case the turbine is called a Kaplan.  The mechanics for adjusting turbine blades and guide vanes can be costly and tend to be more affordable for large systems, but can greatly improve efficiency over a wide range of flows.


The Francis turbine is essentially a modified form of propeller turbine in which water flows radially inwards into the runner and is turned to emerge axially. For medium-head schemes, the runner is most commonly mounted in a spiral casing with internal adjustable guide vanes.


Since the cross-flow turbine is now a less costly (though less efficient)alternative to the spiral-case Francis, it is rare for these turbines to be used on sites of less than 100 kW output.


The Francis turbine was originally designed as a low-head machine, installed in an open chamber without a spiral casing.  Thousands of such machines were installed in the PAK and the rest of Europe from the 1920s to the 1960s.  Although an efficient turbine, it was eventually superseded by the propeller turbine which is more compact and faster-running for the same head and flow conditions.  However, many of these ‘open-flume’ Francis turbines are still in place, hence this technology is still relevant for refurbishment schemes.





A significant factor in the comparison of different turbine types is their relative efficiencies both at their design point and at reduced flows. Typical efficiency curves are shown below.
An important point to note is that the Pelton, Crossflow and Kaplan turbines retain very high efficiencies when running below design flow; in contrast the efficiency of the Francis turbine falls away sharply if run at below half its normal flow, and most fixed-pitch propeller turbines perform poorly except above 80% of full flow.

 Part-flow efficiencies


The control panel is the black box which monitors the operation of the hydro scheme.  The main functions of the control panel are to:

  • Start up and shut down the turbine
  • Synchronise the generator with the local network
  • Monitor the upstream water level and ensure it is maintained above its minimum value
  • Operate the flow-control valve to the turbine to match the availability of water
  • Detect faults and activate warning or shut-down sequences


For grid-connected schemes, the control panel must conform to the G59 recommendations for the connection of embedded generators.  However, very small plant, less than 3.7 kW per phase, only needs to comply with a reduced set of standards defined by the new G83 recommendations.


For schemes which are not connected to the local network, but operate in isolation, the control system will ensure that both the voltage and frequency of the generator remain within the allowable ranges regardless of the load being applied.


On larger plants supplying three phase power, it is usual for the control panel to have the following displays:


  • a voltmeter with a selector switch to read the voltage between phases and the line voltage,
  • an ammeter on each phase to measure current
  • a frequency meter
  • a kilowatt meter, for the instantaneous power
  • a kilowatt-hour meter, for the energy generated over a period
  • a power factor meter


TerminologyAbstraction LicenceAuthorisation granted by the Environment Agency to allow the removal of water from a source (permanently or temporarily)Capacity factorThe ratio of energy output per year to the maximum output if the system runs at full rated capacity all year round.Compensation FlowThe flow which must be left in the river at the point of abstraction, for ecological purposes.Fish Ladder (or Fish Pass)A structure consisting of a series of overflow weirs which are arranged in steps that rise about 30cms in 3 to 4m horizontally, and serve as a means for allowing migrant fish to travel upstream past a dam or weir.Flow Duration CurveA graph showing the percentage of time that the flow at a particular gauging station equals or exceeds certain values.ForebayAn open tank for slowing down the incoming flow and settling out silt and gravel before the flow passes into the penstock.Gauging StationA site where the flow of a river is measured.Gross HeadThe difference between the upstream and downstream water levels.HeadraceThe channel that forms the inlet to a turbine.Impoundment LicenceThe authorisation granted by the Environment Agency to allow the obstruction or impeding the flow of water.Installed CapacityThe total maximum output (kW) of the generating units in a hydropower plant.Kilowatt (kW)Unit of power, equal to 1000 wattsKilowatt hour (kWh)Unit of electrical energy, equal to the electricity supplied by 1 kW working for 1 hour.  1 kWh = 3,600,000 JoulesLeat or LadeAn open channel that conveys water at a shallow gradient from a river channel where sufficient head has been gained for a turbine to be installed. (Also sometimes called Goit or Contour Canal).Net HeadThe pressure head available to the turbine after friction losses through the intake and trash rack.OutputThe amount of power (or energy depending on definition) delivered from a piece of equipment, station or system.PenstockA pipe (usually steel, concrete or plastic) that conveys water under pressure from intake to turbine.Sluice GatesA vertical shaft slide gate, which can be operated either manually or by electric motors (there are other types).SpillwayA controlled discharge of excess flow back into the river.TailraceThe channel that takes flow away from the turbine outletTrashrackA protective screen that prevents large brances, tree trunks and other debris from entering and damaging the turbine.  It usually consists of vertical bars spaced between 30-100 mm apart.  The screen is typically cleaned by an automatic rake which removes the debris, either to a platform or to be flushed into the river.TurbineA machine converting the speed and/or pressure of flowing water into rotational energy.WeirA low dam which is designed to provide sufficient upstream depth for a water intake while allowing flow to pass over its crest.