Anatomy of a Small Scale Hydropower system - RETS Project

Anatomy of a Small Scale Hydropower system
Article submitted by N Packer, Staffordshire University, UK, June 2011.
Overview
Solar energy evaporates seawater producing clouds and, when droplets attain enough mass, gravity
attracts the water back to the ocean as rain. Alternatively if this rain falls over high ground and is
channelled into fast moving rivers and streams we have the opportunity to extract some of its
energy by separating some of the flow for passage through water turbine/generator sets to produce
electricity.
Definitions vary but small-scale hydropower can be sub-divided into mini (

From the forebay the water enters a pipe (5), commonly called the ‘penstock’, to commence the
final leg of its journey down to the hydro-turbine. If a site has difficult topography or is
environmentally sensitive the canal is sometimes omitted and an extended penstock carries water
from the river bifurcation direct to the powerhouse.
The hydro-turbine and electrical generator are housed in the powerhouse (6).
[In a barrage scheme arrangement the headrace and penstock are dispensed with and the
powerhouse is effectively in-line with the river or stream.]
After passage through the turbine the water is returned to the river or stream via a calming channel
(7) known as the ‘tailrace’.
Power and Energy output
The two most important parameters associated with the power output of a hydropower system are
flow and head. The flow is the volumetric passage of water Q(m 3 /s) through the turbine and the
head H(m) is the vertical distance the water travels from supply point (i.e. the penstock entry) to
turbine discharge.
Hydro-turbines are reasonably efficient fluid power to mechanical (rotating shaft) power converters
having efficiencies in the range 70 – 90%. Overall efficiency will depend on the electrical generator
efficiency but values in the range 50 – 70% are common.
For the purposes of illustrating the influence of head and flow, a simplified expression for the design
electrical power output P(kW e ) of a hydropower plant operating at an overall efficiency of 60%
efficiency would be:
P = 5.9 x Q x H (kW e )
Clearly maximising the head (H) and design flow rate (Q) maximises the power output and energy
generated.
To maximise energy capture at times of reduced river flow, most hydro-turbines are able to operate
at flow rates below their design value. The power output will, at these times, be below the design
discussed above. An estimate of the annual energy output will then depend on an ‘equivalent’ full
power output over the year. This variation is taken care of by parameter called the capacity factor.
Capacity factors for hydro plants tend to be in the range 40-70%.
Using the above example, for a hydro-turbine with a capacity factor of 55% the estimated annual
electrical energy output E (kWh e /annum) would be:
E = 5.9 x Q x H x 0.55 x 8760
= 28426 x Q x H (kWh e /annum)

Final thoughts
Head is more or less fixed and is best determined by onsite measurement or less accurately, for
preliminary feasibility studies, by topographical mapping.
The available flow rate through the turbine on the other hand is a variable dependant on a range of
factors.
Hydrographs (flow versus time) and flow duration curves (flow versus percentiles or time exceeded)
are commonly used to determine the ‘flashiness’ (flow variability) of a river or stream.
Above all, the fraction of flow diverted for energy generation must not leave the depleted region
(‘reach’) of the river ecologically damaged and consequently extensive local hydrological and
biological studies should be carried out prior to scheme sizing.
Units & Abbreviations
Volume: m 3 – cubic metres Volume flow rate: Q – m 3 /s Note 1m 3 = 1000litres
Head: metres (a vertical distance)
Energy: kWh (kilowatt-hour)
Power: W – Watt kW (kilowatts) – thousands of watts MW (Megawatts) – millions of watts
Further suggested reading and research
A Beginner’s Guide to Energy and Power, N Packer, Staffordshire University, UK
RETS articles, February 2011.
www.esha.be
www.microhydropower.net
www.british-hydro.org
www.cat.org.uk
www.environment-agency.gov.uk

Neil Packer is a Chartered engineer and Senior lecturer at the Faculty of Computing, Engineering and
Technology, Staffordshire University, UK. He has been teaching thermo-fluid and environmental engineering
for nearly 20 years and acts as a Low Carbon Consultant providing a range of energy services to business,
industry and local authorities.
Contact details:
Faculty of Computing, Engineering and Technology
Staffordshire University
Beaconside, Stafford, ST18 0AD
Tel 01785 353243 email n.packer@staffs.ac.uk
This information was presented as part of the Renewable Energies Transfer System Project (RETS) funded by
INTERREG IVC through the European Regional Development Fund. The project time line is January 2010 to
December 2012. For more information and to take part in our online community visit: http://www.retscommunity.eu/