Cover Image
close this book Local Experience With Micro-Hydro Technology
close this folder B. Development of hydropower resources
View the document 4. BIG OR SMALL HYDRO?


a) Big Hydropower

b) Small Hydropower

c) Summary of Conclusions

Definition: big: all plants with a capacity of more than 1000 kW (1 MW) small: generic term for all plants with 1000 kW or less capacity and specific term for the range from 501 to 1000 kW. mini: plant capacity from 101 to 500 kW micro: all plants with a capacity of 100 kW or less

There is considerable argument about this question on different levels in government, in industry and among the general public. An answer can probably be found more easily, if a second question is asked: application in which context? Before an answer is attempted it is worthwhile looking at the specific characteristics of each and at basic differences between the two groups of plant size. a) Big Hydropower

Big hydropower stations are of a nature that requires a good infrastructure such as roads (during construction) and access to a big market, resulting in long high-tension grid systems and an extensive distribution system. It serves a great number of individual consumers and supplies power to electricity-intensive large industry.

Big plants are usually owned and operated by big companies or state enterprises. The skill requirements in management, administration, operation and maintenance are considerable. Unit cost of energy generation is relatively low and there are pronounced economies of scale involved. This is due to a decrease in specific investment cost with rising plant size, and the probability of higher load factors with a larger number of consumers. A problem is peak demand; big numbers of consumers tend to have their maximum individual demand during the same time-interval, which results in a largely uncontrollable peak of demand that must be met with increased capacity, such as standby installations and high cost pumped-storage.

Transmission and distribution costs are considerable and may exceed 30 % of generating cost The OECD average in 1975 was more than 7,5 % of generated energy lost in transmission and distribution. In the same year, 35 % of total investment in the electricity supply industry within OECD was for transmission and distribution. There is a direct relationship between relatively low losses and a high level of investment in transmission. In developing countries, where generally less sophisticated equipment is used, the share of investment is lower but losses are higher, e.g. from an average of 15,7 % in the ESCAP region to about 25 % in Indonesia and more than 28 % in Nepal . High tension transmission costs per unit are further a function of line length and energy consumption per capita. And, if a country is fully electrified, line length is a function of population density and the deqree of urbanization. With this it becomes clear that high population density and high consumption per capita reduce the required transmission length, while low consumption and low population density result in considerably longer transmission lines and higher costs. Thus, a big load, concentrated on a small area, is the most economic to supply.

From the engineering point of view, big hydro power calls for sophisticated technology in manufacturing electro-mechanical equipment, and high standards of feasibility studies, planning and civil construction activities, because the risks involved are great. Long-term flow data are a necessity and gestation periods are long. It is possible to apply computer design technology and highly specialised fabrication technology to achieve very high performance efficiencies that may reach 96 % in the case of hydraulic turbines. Needless to say, this process brings about very high cost, which however may be justified because of the large scale, where equipment cost is generally a relatively small fraction of total cost.

Big-scale hydropower stations require careful environmental considerations. Artificial lakes may change an entire landscape and inundate sizeable areas of arable land. Positive aspects are flood controlling capability and the creation of new recreational sites (boating, fishing, camping) although it is obvious that the benefits for recreation do not rise in proportion with size. Another negative effect that should not be neglected in tropical areas is the possibility of water-borne diseases spreading by large storage reservoirs. b) Small Hydropower

Small hydropower, on the other hand, implies decentralisation. Energy produced is usually supplied to relatively few consumers nearby, mostly with a low-tension distribution network only. Because of its size, supervision of consumption is possible. The potential exists to diversify consumption in a planned way, and peaking problems are relatively less severe. Small stations lend themselves to local individual or cooperative ownership, operation and administration.

Small hydro is very often mentioned in connection with high unit-specific cost. This is an argument which may be put in its proper place by the following statement: "Traditional economic reasoning against the development of very small hydro plants is beginning to weaken, not only because of the rising price of oil which affects the relative costs of all forms of hydropower, but also because of experience gained in the operation of very large enterprise. It is a common proposition that the larger the number of units, the smaller the unit cost. It is easy to understand how capital and running costs for a farm, a factory or a power station, do not rise in proportion to the output, and therefore the more units produced, the more the fixed and running costs can be shared. Nevertheless, against all the economic theories, many small factories and businesses can still operate successfully in competition with the giants for a number of reasons, which apply equally to small power stations. As unit size falls, the involvement of operators with their work becomes more intimate, and therefore is likely to result in a higher efficiency and better maintenance. Additionally, labour costs for a large firm may be higher than for a small enterprise."

Apart from this, there is still another issue; small hydro has in the past usually been treated like big hydro. What can often be seen are elaborate construction works in reinforced concrete, an oversize "luxury" powerhouse and highly optimised electromechanical equipment, all carefully miniaturised and with a very high standard of safety, operational reliability and performance. All this was sometimes preceded by detailed and costly pre-feasibility, feasibility and planning stages. OLADE reports that in some projects in Latin America, costs for preparatory stages were running as high as 50 % of total project cost. It is clear that such a way of doing things - following the idiom "as good as possible" -runs contrary to all efforts at cost reduction.

The nature of small hydro installations usually needs none or little of all this. If an approach of "as good as necessary" is adopted and carried through consistently, things may very well look quite different. Generally, hydrological data for small hydro schemes are scarce or not available anyhow. Even the most elaborate of studies cannot disprove this fact. Experience shows that a station based on apparent minimum-flow for its maximum output - which may be established with relatively few measurements, the advice of local people and intelligent estimates - will be acceptably reliable. High safety standards in construction works are often not necessary, even the rupture of a small dam would not usually threaten human life, and money-wise the risks are smaller anyway if initial costs are kept down. This makes it possible to use mainly local materials and local construction techniques, with a high degree of local labour participation.

On the equipment side, standards of voltage and frequency fluctuations and of reliability of supply can usually be lowered - involving considerable savings - without reducing overall benefits of a scheme in proportion. A decrease in conversion efficiency brings about considerable savings, while the amount of reduced generating capacity remains small in absolute terms. The same is true to some extent for losses in the penstock. Higher losses may be accepted in exchange for a cheaper (smaller diameter) penstock. The aim of such considerations is a trade-off between available potential, required generating capacity, acceptable technical standards and cost. The result should be a fairly low-cost installation with simple construction works and equipment that has an acceptable reliability.

Environmental impacts due to small hydro stations are generally negligible or are controllable because of their size. Often they are non-existent. At the same time, flood controlling capability can not be credited to small hydro to a great extent. Road accessibility is a must only for sizes at the upper end of the scale, and schemes can be put into operation in a relatively short time. Many examples could be cited where construction time was a few months only.

A last characteristic attributable to small hydropower stations is that it is often possible to use mechanical power directly to operate all kinds of machinery. This does away with all the sophistication of electricity generation and constitutes the most economical and low-cost power use thinkable. At the other end of the scale of sophistication, small hydropower stations may well be suited to supply power into a big grid-system if one is nearby and if its cost can be attributed to big power stations. While the input is probably very small in relation to the overall system capacity, it still provides additional, saleable energy. Each 650 kWh of electricity supplied represents the equivalent of 1 barrel of oil (If oil is converted into electricity at an efficiency of 38%).

After looking at the salient features of big and small hydropower, it is now possible to conclude this chapter with a summarised answer to the question asked before.

c) Summary of Conclusions


· large centralised power demand; large-scale industry, cities, urban areas

· international, national and regional grid-systems

· big corporations or state enterprises employing highly-skilled and well paid staff

· depends on long term assessment of potential, long planning and construction periods involving sophisticated technology

· depending on potential it can make a sizeable contribution to a nation's commercial energy requirements

· its share of total potential is perhaps 90 % based on the known, reasonably developable potential.


. decentralised, small power demand; small industry, individual farms and enterprises, rural communities

· low tension distribution networks and eventually sub-regional micro-grid systems

· individual, co-operative or communal ownership with semi-skilled labour requirements and co-operative administration

· short gestation period with local materials and skills applicable

· depending on potential, it can make a considerable impact on the quality of rural life, which is clearly over-proportional to the amount of energy supplied.

· its share of the total known potential is on the order of 10 %. Since very few hydrological data exist for small rivers and watersheds, there are good prospects that additional potentials can be identified, particularly in developing countries.