Simple water-wheels have been used already in ancient times to relieve man of some forms of hard manual labour. Much later, but long before the advent of the steam engine, the art of building large water-wheels and the use of considerable power capacities was highly developed. The use of this natural energy resource became even easier and more widespread with the invention of the waterturbine in the early 1800's. The first small industries emerged soon after in many regions of Europe and North America, powered by water turbines. In Switzerland, a country with abundant hydropower resources, industrialization began in the first half or the nineteenth century, entirely based on hydropower. In the canton of Zurich alone, comprising an area of less than 2000 km 2, more than 450 installations were operating by the year 1900, with capacities from less than one to 450 HP and also about 40 turbines with capacities greater than 450 HP.
In later years, when cheap oil became available worldwide, interest in hydro power was lost to a great extent in many areas, but today the situation is different again. Governments, policy-makers, funding and lending agencies and sundry institutions and individuals take a growing interest. This led -and still does -to the reassessment of many projects once found not feasible; the identification of new sites and potentials, and a number of other activities related to hydro development.
An international commission established by the World Energy Conference in 1974, worked out, inter alia, an objective analysis of the world hydraulic resources.
Fig. 2: World Total Installed and Installable Capability
Source: NRECA, Small hydroelectric powerplants, Washington 1980
The results of these studies show that hydropower developed so far is around 17 % of the potential considered reasonably developable (the theoretical energy in global runoff is more than eight-fold).
Figure 2 illustrates the hydropotential in various regions of the world and the amounts developed, under construction or planned, and the amount remaining. The total developable capacity amounts to 2,2 million MW and has -at a 50 % plant factor - a theoretical yearly production potential of nearly 10 million GWh of electrical energy. The same amount of electrical energy in thermal plants with oil as fuel would require approximately 40 million barrels of oil per day.
If this is compared to the world consumption of pertroleum products, which amounted to around 70 million barrels per day in 1980, it becomes evident that hydropower resources are very substantial indeed. For developing countries, who together possess almost 60 %(Calculated from fig. 2) of the installable potential, e.g. the equivalent of about 24 million barrels of oil per day, the magnitude is striking. All these countries together consumed 2,54 million barrels of oil equivalent per day, to produce electricity from carbonic fuels (oil, gas and coal) during 1980.
The graphical presentation of continent-wise potentials in fig. 2 does of course not show how distribution is within the regions and over time. There are two main factors that determine the generating potential at any specific site: the amount of water flow per time unit and the vertical height that water can be made to fall (head). Head may be natural due to the topographical situation or may be created artificially by means of dams. Once developed, it remains fairly constant. Water flow on the other hand is a direct result of the intensity, distribution and duration of rainfall, but is also a function of direct evaporation, transpiration, infiltration into the ground, the area of the particular drainage basin, and the field-moisture capacity of the soil. Runoff in rivers is a part of the hydrologic cycle in which -powered by the sun - water evaporates from the sea and moves through the atmosphere to land were it precipates, and thence returns back to the sea by overland and subterranean routes.
Fig. 3: World Distribution of River Runfoff in mm/year
Source: AMBIO, Vol. 3, No. 3-4, 1974: The Global Freshwater Circulation
Area-wise distribution of river runoff (in mm/year) in fig. 3 gives an indication of the geographical situation of hydro resources in the various parts of the world. It appears that regions around the aequator, Central America and parts of South-East Asia, northern Europe and North America have higher than average runoffs. In large parts of northern Africa (Sahel, Sahara), Arabia, Central Asia, Australia and western North America, as well as southern Africa and America, runoff is far below average. These areas are of little or no interest in the context of hydropower potential. For areas with average and higher runoff, the short-, medium-and long-term variations of flow are of prime interest. It is this local pattern that determines the availability of water to generate power in relation to time and duration. Such variations are subject to the weather regime, i.e. seasons, and a multitude of other factors such as those already mentioned. Generally speaking, perennial rivers with slight flow variations are the most suitable for hydropower development. High runoff variations, on the other hand, make harnessing more difficult, and extremes such as only seasonal runoff and floods impose serious economic and technical constraints on possible utilisation.
Perhaps the most particular characteristic is that no two potential sites are alike. Topography, flow regime and volume of the river concerned, together with the geological condition of the site are variables that make each installation unique. It is also true that hydro resources must be harnessed where the potential exists. In situations where likely consumers are far away from the generating site, transmission costs are considerable. Before electricity generation came into use, all activities relying on hydropower were situated adjacent to or near the generating site, because only mechanical power transmission was possible. Therefore it is obvious that the economic value of hydro potential varies considerably in different environments.
The lack of accurate long-term hydrological data and to some extent topographical maps of insufficient detail place a severe constraint on hydropower development, mainly in developing countries. It is a specific characteristic that reliable predictability of possible firm power capability is only possible with accurate runoff data over a very long period (>30 years).
Unlike the technologies associated with many new and other renewable energy sources, equipment associated with hydropower is well developed, relatively simple, and very reliable. Because no heat (as e.g. in combustion) is involved, equipment has a long life and malfunctioning is rare. Experience is considerable with the operation of hydropower plants in output ranges from less than one kW upto hundreds of MW for a single unit.
Hydro plants are non-consuming generators of power. Once water has passed through the turbine, it is available again (although at a lower elevation) for other uses. It is a non-polluting technology which, however, may have some negative environmental impacts. From the energy conversion point of view, it is a technology with very high efficiencies, in most cases more than double that of conventional thermal power plants. This is due to the fact that a volume of water that can be made to fall a vertical distance, represents kinetic energy which can more easily be converted into the mechanical rotary power needed to generate electricity, than caloric energies.
The fact of high capital intensity in hydropower development has not favoured this resource during the time of cheap oil. Now this disadvantage is relatively smaller and outweighs in many instances variable (and probably rising) fuel costs of thermal plants, due to relatively low and stable operating costs, which are largely insensitive to outside inflation and other factors.
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
THE CONTEXT FOR BIG HYDROPOWER STATIONS:
· 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.
THE CONTEXT FOR SMALL HYDROPOWER STATIONS
· 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.