Small hydropower here refers to hydroelectric power plants below 10MW installed capacity. Hydroelectric power plants are power plants that produce electrical energy by driving turbines and generators thanks to the gravitational force of falling or flowing water. Through the natural water cycle mainly evaporation, wind and rain, the water is then brought back to its original height. It is thus a renewable form of energy. Small-scale hydro power may be a useful source for electrification of isolated sites and may also provide an extra contribution to national electrical production for peak demand.
Small hydro power uses the flow of water to turn turbines that are connected to a generator for the production of electricity. Small hydro is divided into further categories depending on its size, such as mini- (less than 1000kW), micro-hydro (less than 100kW) and pico-hydro (less than 5kW) (EHSA 2005); the definitions may vary according to manufacturers and countries, as there is no internationally accepted definition of small hydropower. In China, small hydropower refers to capacities of up to 25 MW, in India of up to 15 MW and in Sweden ‘small’ refers to up to 1.5 MW. However, a capacity of up to 10 MW is a generally accepted norm by the European Small Hydropower Association (ESHA), the European Commission, and UNIPEDE (International Union of Producers and Distributors of Electricity).
In general, there are three different configurations of hydro power plants
- run-of the river
- pumped storage.
If there is a water storage possibility, through an existing or newly built dam, then the power plant is a storage power plant. If there is the possibility to return the water to the upper reservoir through pumping then it is a pumped storage power plant. These are rare for small scale hydropower plants. Run-of the river power plants use the flowing water to generate power, without needing changes to the river flow. Mini-, micro- and pico- power plants generally have no dam and are therefore run-of the river power plants. After use, the water used in small-hydro plants is returned to its natural course.
There are two factors that determine the amount of power that can be produced: the head (i.e. the height of the water drop) and the flow rate; the higher the head the smaller the flow rate needed to produce the same amount of electricity. The overall production capacity depends on the seasonal and yearly differences in water availability.
Depending on the head height and the amount of flow, different types of turbines can be used (see table). There are mainly two types of turbines: impulse and reaction turbines.
[media:image:1] Impulse turbines have the runner (the turning part of the turbine) operated in air, and the whole process takes place at atmospheric pressure. This kind of turbine is more tolerant to particles in the water, the access to working parts is easier compared to reaction turbines, there are no parts that work under pressure and they have a better part-flow efficiency (ESHA, 2005). Nonetheless, these turbines cannot be used at all sites as they require a high head, a part from cross-flow turbines that are able to operate up to about 4m head. Cross-flow turbines are a type of impulse turbine, which has several advantages: they can be used for a wide range of head heights and power classes and can be produced very easily e.g. by cutting long pipes into strips (ESHA, 2005). [media:image:2] Reaction turbines are fully immersed in water and are enclosed in a pressure casing. This increases the complexity of the system and complicates maintenance; therefore these systems are not well suited for areas where access to maintenance may cause difficulty. [media:image:3]
The head height and flow of water available determine the amount of power that can be generated. When planning a hydropower plant attention needs to be paid to the seasonal and yearly differences in water availability. In particular for run-of the river power plants, the flow of water needs to be above a certain minimum all year round to be able to produce electricity all year round.
Engineering & Infrastructure requirements
Micro- and pico- hydro power plants are best suited for isolated areas where there is no electricity grid Off the grid power plants require local load controlling to stabilise frequency and voltage of supply. They have the advantage that they are generally designed for single households or small villages and can be developed with local materials and labour. For small pico-hydro turbines the turbine/generator set can be bought as a module “off the shelf”, whereas from micro-power plants upwards the turbines are especially designed for the location.
Starting from mini-hydropower plants upwards in size, conventional engineering approaches are used. The size of the machinery is then such that road access is advisable Mini-hydro power plants are most often grid connected.
Small hydro power plants generally have no form of water storage.
In order to proceed with a small hydropower scheme, it is necessary to obtain the right to utilize all the land concerned and it is important to find out how contractors will access the different areas of the hydropower scheme with the necessary equipment. It is therefore wise to approach the relevant land-owners at an early stage in order to identify any objections to the proposed project and to negotiate access to the land. Since water courses frequently define property boundaries, ownership of the banks and existing structures may be complex. Failure to settle these issues at an early stage may result in delays and in cost penalties later on in the project (The British Hydropower Association, 2005).
The world wide estimated amount of installed small hydro capacity was 85GW at the end of 2008: 65GW were estimated to be in developing countries of which 60GW in China (REN21, 2009). The overall technical potential for small hydro is estimated to be between 150 and 200GW (IEA 2008).
[media:image:4] Small hydro already provides electricity to a large number of households in developing countries where other technologies would be more difficult to install. Over the last 30 years the technology has been used and installed extensively in China, but also in Nepal, Vietnam and several South American countries, among others; the market for Africa is expected to grow (ESHA, 2005). Hydropower plants represent 27% of all CDM projects requesting validation (UNEP Risoe). In absolute numbers this implies 1351 projects and an installed capacity of 44'995MW (UNEP Risoe). Currently 553 projects are registered.
Small scale hydro is a mature technology. It can profit from technology development done for large scale hydropower turbines.
Specifically for small scale hydro, expected technical improvements include developments in lower head turbines, in-stream turbines and turbines which have a lower impact on fish populations (IEA, 2008). In addition, reduction in operation and maintenance (O&M) are expected.
Further in the future, the development of hybrid systems that combine hydro with wind or even with hydrogen production for energy storage could be expected (IEA, 2008); particularly in combination with hydrogen this development is still at R&D stage.
Substituting traditional fuels by a the switch to electricity can reduce air pollution, improve health and decrease social burdens, e.g. from collecting firewood. The electricity can be used to increase income generating activities, in particular it can improve irrigation, crop processing and food production (ESHA, 2005). The income generating activities may provide more jobs to the rural communities.
In a study for the UK Department for International Development (Fulford et al., 2000), the poverty reduction potential of small hydropower has been found to be significant, particularly in low-income countries. Micro-hydro has also been found to be a relatively efficient method of poverty reduction, in terms of costs per person moved across the poverty line. In addition, the estimates of poverty reduction from micro hydro are systematically understated, as they exclude a range of very difficult to measure, but important effects, including savings from no longer having to carry firewood, kerosene or other fuel, improved education through the availability of electric light and improved health and agricultural production from drinking and irrigation water which is made available by channels originally built for micro-hydro schemes.
Small scale hydro, being a renewable energy also has the advantage of reducing dependency from fossil fuels on the macro-economic level, if the country imports fossil fuels.
Depending on what forms of energy use the hydro powered system substitutes, the decrease in air pollution and GHG emissions differs, but given that small hydropower is virtually CO2 neutral, it is expected to be a significant improvement compared to conventional electricity production in terms of emissions of GHG and air pollutants.
In contrast to large hydro-power installations, the environmental impacts on the ecosystems are limited. Small hydro power plants require limited changes to the flow of the river and therefore the existing ecosystem can continue to function as before; improvements in this field in particular related to the development of “fish-friendlier” turbines, are nonetheless expected (IEA, 2008).
A hydropower plant runs practically CO2 free. The main emission source is the production of the components and the transport to the location of the power plant.
Small hydro projects, similarly to larger plants, have a wide range of levelized cost of electricity (LCOE) due to its sensitivity to the plant capacity factor and the cost of capital. The range can be from a minimum of 35 USD/MWh up to and possibly exceeding 230 USD/MWh (IEA, 2012). IRENA (2012) suggests that the ranges are similar, but that LCEO for very small projects can run as high as 270 USD/MWh.
Considering that some power plants have rather large upfront investment costs it is important to verify if the hydrology in the region will remain constant over time.
The capital requirements for small hydro plants depend on the effective head, flow rate, geological and geographical features, the equipment (turbines, generators, etc.) and the required civil engineering works. Making use of existing constructions such as weirs, dams, storage reservoirs or ponds can significantly reduce environmental impacts and costs. In general, sites with low heads and high flows need a greater capital outlay as they require larger civil engineering works and turbine machinery to handle the larger flow of water. If, however, the system can have a dual purpose, such as power generation as well as flood control, power generation and irrigation, or power generation and drinking water purposes, the payback period can be shortened.
In addition, special attention should be paid to the cost of using water (water charges and/or concession fees) as well as to the administrative procedure necessary to obtain the water and building licenses. Operation and maintenance costs may represent 1.5-5% of the project costs (ESHA, 2005).
Generally, on a cost per kilowatt basis, larger small hydropower projects tend to be cheaper due to economies of scale and the sunk costs of any scheme, irrespective of its size.
[This information is kindly provided by the UNEP Risoe Centre Carbon Markets Group ]
Project developers of hydro projects (excluding run of river projects) in the CDM pipeline mainly apply the following CDM methdologies: ACM2 “Consolidated baseline methodology for grid-connected electricity generation from renewable sources” for large-scale projects and AMS-I.D. “Grid connected renewable electricity generation”  for small-scale projects
CDM projects based on hydro represent 27.4% of all CDM projects in the pipeline and, as such, are the most common project type in the pipeline. The geographical distribution of hydro projects is concentrated around Asia and in particular China [media:image:6] Excluding run of the river projects, the CDM pipeline contains 522 hydropower projects as of March 2011. Out of these 261 projects are registered and for a 101 projects CERs have been issued.
Example CDM project:
The British Hydropower Association, 2005. A Guide to UK Mini-hydro Developments. Available at http://www.british-hydro.org/mini-hydro/download.pdf 
ESHA, 2005. Small Hydropower for developing Countries, European Small Hydropower Association, IT Power.
Fulford, D., A. Gill, P. Mosley, 2000. Micro-hydro generation as instrument of poverty reduction: Asian achievement and African potential. Reports to DFID, Reading University
IEA, 2008. Energy Technology Perspectives 2008 - Scenarios and Strategies to 2050. IEA/OECD, Paris.
IEA, 2012. Technology Roadmap: Hydropower. IEA/OECD, Paris.
Khennas, S. and Barnett A., 2000. Best Practices For Sustainable Development Of Micro Hydro Power In Developing Countries, Final Synthesis Report Contract R7215. London Economics & de Lucia Associates, Cambridge, Massachusetts, USA and The World Bank.
REN21, 2009. Renewables Global Status Report. Renewable Energy Policy Network for the 21st Century, Copyright Deutsche Gesellschaft für Technische Zusammenarbeit (GTZ) GmbH.
UNEP Risoe. CDM/JI Pipeline Analysis and Database. Available at: http://cdmpipeline.org/