Hydropower (Small-scale)

While water resources are valued for human health and for sustaining food production, the energy contained in moving water such as rivers or tides can also be harnessed to create energy through hydropower or mechanical uses. Hydropower schemes can either be small-scale or large scale, depending on the local conditions and the energy demand, and represent a renewable energy source that can be implemented wherever there is running water. Globally, 1.4 billion people lack access to electricity, with an additional 1 billion having only intermittent access (UNDP 2012). As running water is a resource that is globally available and renewable, harnessing its power to generate electricity can provide a sustainable source of energy to improve livelihoods and increase working productivity. Particularly in rural or developing areas, small-scale hydropower can represent a locally available, reliable source of energy where no other energy generation is feasible.

Regardless of size, the principle behind hydropower generation is the same: that the power of moving water can be harnessed to produce energy. The key elements for hydropower generation are “head” and “flow.” “Head” refers to the height of the gradient over which the water falls, while “flow” refers to the volume of water per unit time. To maximise energy production, both head and flow should be high. In other words, greater amounts of energy are generated when a large volume of water flows over a steep gradient. Hydropower plants generate electricity by directing water through a turbine, which in turn drives an electric generator. The electricity produced can either used directly, or fed into an electricity grid.

Adapted from SINGH (2009) and GAIUS-OBASEKI (2010)

Hydropower plants are classified according to their energy production capacity, expressed in megawatts. While large scale hydropower plants can produce well over 100 MW, small hydropower plants generally produce less than 10 MW. Based on energy production capacity, small-scale hydropower production is broken into four size categories of pico- (<5 kilowatts), micro- (5-100 kW), mini- 100 kW-1 MW), and small (1-10 MW).

However, classifications vary from country to country as there is currently no common consensus among countries and hydropower associations regarding the upper limit of small-scale hydropower plant capacity. For instance, some European Union countries like Portugal, Spain, Ireland, Greece and Belgium accept 10 MW as the upper limit for small-scale hydropower installed capacity, while others place the maximum capacity from 3 to 1.5 MW. Outside the EU, this limit can be much higher, as in the USA (30 MW) and India (25 MW).

Because hydropower generation depends on only two factors (water flow and head) to produce energy, small-scale hydropower can be implemented in any system where these factors are met. This can include natural systems such as streams or rivers, but can also include any water network where pipes provide water flow over a decline (head). Hydropower can therefore also be integrated into many stages of the drinking water or wastewater network (KUCUKALI 2011). Most small hydropower schemes fit into two main categories: run-of-river systems, or integrated in existing water infrastructure.

Run-of-River Schemes

Adapted from SINGH (2009)

In run-of-river systems, river water is diverted by a weir through an opening in the river side (the ‘intake’) into a channel. A settling basin is built in to the channel to remove sand and silt from the water. The channel follows the contour of the area so as to preserve the elevation of the diverted water. The channel directs the water into a small reservoir/tank known as the ‘forebay’ from where it is directed on to the turbines through a closed pipe known as the ‘penstock’. The penstock essentially directs the water in a uniform stream to the turbine at a lower level. The turning shaft of the turbine can be used to rotate a mechanical device (such as a grinding mill, oil expeller, or wood lathe), or to operate an electricity generator. When electricity is generated, the ‘power house’ where the generator is located, transfers the electricity to a step-up ‘transformer’ which is then transmitted to the grid sub-station or to the village/area where this electricity is to be used. Once electricity is produced, the water flow is returned back to the river. For this reason, small hydropower plants are considered a renewable, non-polluting and environmentally source of energy.

Most small hydropower plants are “run-of-river” schemes. The power is generated only when enough water is available from the river/stream. When the stream/river flow reduces below the design flow value, the generation ceases, as the water does not flow through the intake structure into the turbines.

Integrated in Existing Water Infrastructure

Small-scale hydropower plants within existing water infrastructure have four main advantages compared to those which utilise rivers and streams (KUCUKALI 2011):

• All civil works are already present, which can reduce the investment cost for new infrastructure by about 50%.
• Facilities have no significant environmental impacts, and will have a guaranteed discharge (and production rate) throughout the year.
• The generated electricity can be used in the water supply system, with excess electricity sold to the government.
• There will be no land acquisition or significant operating costs.

Small-scale hydropower can be implemented in the following ways within existing drinking water or wastewater infrastructure (SAN BRUNO et al. 2010):

• Within a drinking water network: Excess pressure in pipes is used to generate hydropower by replacing pressure breakers with turbines.
• Within an irrigation network: Implementation is similar to within a water supply network, but special considerations need to be taken to produce energy outside the irrigation season.
• Before wastewater treatment: Wastewater is diverted through a turbine before it enters a wastewater treatment plant. A trash rack should be installed to remove solids before wastewater passes through the turbine.
• After wastewater treatment: Before being discharged to the environment, wastewater is passed through a turbine to generate electricity.
• Within a runoff collection system: Implementation is similar to within a water supply network, but consideration needs to be taken to remove particles that may be suspended in stormwater before it passes through the turbine.
• On a reserved flow or compensation discharge: In large hydropower schemes or in water works, water is often discharged according to national laws. A turbine can be installed to generate hydropower from this discharge.
• On a fish pass system: In order for fish to migrate past obstacles such as dams, a fish pass systems are often installed. To allow fish to find the pass, an “attraction discharge” is necessary. A turbine placed at this discharge can make use of the difference in water level between the upstream basin and fish pass entrance to generate hydropower.
• In a navigation lock or dam: Locks and dams regulate water levels. Turbines can be installed that utilise fluctuating water levels for hydropower generation during the filling and emptying of the locks.
• In a desalination plant: The process of reverse osmosis to separate water from dissolved salts through semi-permeable membranes necessitates high pressures (from 40 to 80 bars). The high-pressure residue of liquid water containing salt can be passed through a turbine in order to recover part of the energy used for the initial compression.
• In a cooling or heating system: Cooling or heating systems can produce excess pressure that can be recovered by hydro-turbines to produce energy.

Hydropower is a non-consumptive water use. Therefore, there are possibilities to link hydropower to other uses, such as irrigation in agriculture. This can reduce the investment costs for individual users, thus expanding the possibilities for income generation and development.

Adapted from UN-HABITAT (2012)

Because of their geographical versatility and relatively low investment costs, the development of small hydropower could be key in spreading access to electricity in rural and developing areas. Access to electricity can have many positive development impacts, which can be regarded for each of the Millennium Development Goals.

For more information on the Millennium Development Goals with regard to water and sanitation, see Access to Water and Sanitation, and Water Sanitation and Development.

The construction of small-scale hydropower systems depends on a number of variables such as site characteristics, power plant size, and location, with cost generally ranging from $1,000 - $20,000 USD/kW. Maintenance costs are relatively small in comparison to other technologies. Given a reasonable head, small-scale hydropower is a concentrated energy source. It is a long-lasting and robust technology, and the life of systems can be as long as 50 years or more without major new investments (SINGH 2009).

Assessments of the feasibility of small hydropower in rural areas have shown that even if population density is sparse, micro- and pico- hydropower may be cost-effective solutions due to the low cost of distribution and minimal effects on the environment (WILLIAMS & PORTER 2006). Integrating small-scale hydropower scheme as part of an existing water or sanitation infrastructure can reduce the investment costs while simultaneously producing valuable electricity that can be used by the facility or by an electricity grid.

• As most small hydropower schemes are run-of-river or integrated within an already existing water infrastructure, the effect on the local environment is minimal. Run-of-river systems return water to streams, thus minimising the impact on stream ecosystems, while systems integrated into existing infrastructure merely make use of existing water flows.
• Small hydropower is a non-polluting, renewable energy source. Unlike large hydropower schemes, small hydropower does not necessitate a reservoir, and therefore does not lead to methane production, a potent greenhouse gas.
• Small hydropower schemes take up little space. Because no reservoir is created, there is minimal impact on nearby communities with respect to displacement.