River flows hydropower and elevated water storage

The kinetic energy of water flowing in rivers or other streams constitutes an energy source very similar to that of ocean currents. However, rather than being primarily wind driven or caused by differences in the state of the water masses themselves, the river flows are part of the hydrological cycle depicted in Fig. 2.61. Water vapour evaporated into the atmosphere is transported and eventually condensed. It reaches the ground as water or ice, at the elevation of the particular location. Thus, the primary form of energy is potential. In the case of ice and snow, a melting process (using solar energy) is usually necessary before the potential energy of elevation can start to transform into kinetic energy. The origin of many streams and rivers is precisely the ice melting process, although they are joined by ground water flows along their subsequent route. The area from which a given river derives its input of surface run-off, melt-off and ground water is called its "drainage basin".

The flow of water in a river may be regulated by means of dam building, if suitable reservoirs exist or can be formed. In this way the potential energy of water stored at an elevation can be transformed into kinetic energy (e.g. driving a turbine) at the times most convenient with respect to utilisation.

Figure 3.57. Power duration curves for power in currents at Halskov Rev, Denmark, based on a summer and a winter month of 1972. Thin, dashed lines indicate the monthly average power and the percentage of time during which it is available.

An estimate of the potential hydro-energy of a given geographical region could in principle be obtained by hypothetically assuming that all precipitation was retained at the altitude of the local terrain and multiplying the gravitational potential mg by the height above sea-level. According to Fig. 2.61, the annual precipitation over land amounts to about 1.1 x 1017 kg of water, and taking the average elevation of the land area as 840 m (Sverdrup et al., 1942), the annually accumulated potential energy would amount to 9 x 1020 J, corresponding to a mean energy flux (hydropower) of 2.9 x 1013 W.

Collection of precipitation is not usually performed as part of hydropower utilisation, but rather the natural processes associated with soil moisture and vegetation are allowed to proceed, leading to a considerable re-evaporation and some transfer to deeper lying ground water which eventually reaches the oceans without passing through the rivers (see Fig. 2.61). The actual run-off from rivers and overground run-off from polar ice caps comprise, on average, only about 0.36 of the precipitation over land, and the height determining the potential energy may be lower than the average land altitude, namely, that given by the height at which the water enters a river flow (from ground water or directly). The minimum size of stream or river which can be considered useful for energy extraction is, of course, a matter of technology, but these general considerations would seem to place an upper limit on the hydro-energy available of roughly 3 x 1020 J y-1, corresponding to a power average of below 1013 W (10 TW).

If, instead of using average precipitation and evaporation rates together with average elevation, the geographical variations of these quantities are included, the result is also close to 3 x 1020 J y-1 or 1013 W. These figures are derived from the integral over all land areas, dw pot f , x J „

—— = I (r - e) gz dA, dt ^ land area where r and e are the rates of precipitation and evaporation (mass of water, per unit of area and time), g is the gravitational acceleration and z is the height above sea level. The observed annual mean precipitation and evaporation rates quoted by Holloway and Manabe (1971) were used in the numerical estimate.

Geographical distribution of hydropower resources

A different estimate of hydropower potential is furnished by counts of actual rivers with known or assumed water transport and falling height. According to such an estimate by the World Energy Conference (1974, 1995), the installed or installable hydro-generation capacity resource at average flow conditions may, in principle, amount to 1.2 x 1012 W, for both large installations and smaller ones and down to "micro-hydro" installations of around 1 MW. On the other hand, it is unlikely that environmental and other considerations will allow the utilisation of all the water resources included in the estimate. The World Energy Conference (1995) estimates 626 GW as a realistic reserve (including an already installed capacity producing, on average, 70 GW).

Figure 3.58 gives an idea of the geographical distribution of the hydropower resources on a national basis. The largest remaining resources are in South America. The figures correspond to average flow conditions, and the seasonal variations in flow are very different for different regions. For example, in Zaire practically all the reserves would be available year round, whereas in the USA only 30% can be counted on during 95% of the year.

Figure 3.58 (above). Hydropower potential average production (W m-2), including existing plants, plants under construction, and planned and contemplated installations, both for large- and small-scale power schemes (based on data from World Energy Council, 1995).

Figure 3.58 (above). Hydropower potential average production (W m-2), including existing plants, plants under construction, and planned and contemplated installations, both for large- and small-scale power schemes (based on data from World Energy Council, 1995).

Figure 3.59 (below). Seasonal variation in the power associated with the water flow into Norwegian hydropower reservoirs for a typical year (1985) and a "dry year" (1990) (based on Andersen, 1997; Meibom et al, 1999).

Figure 3.59 (below). Seasonal variation in the power associated with the water flow into Norwegian hydropower reservoirs for a typical year (1985) and a "dry year" (1990) (based on Andersen, 1997; Meibom et al, 1999).

Figure 3.59 gives seasonal variations (for two years) in the flow into the existing hydropower reservoirs in Norway, a country where the primary fill— ing of reservoirs is associated with the melting of snow and ice during the late spring and early summer months.

Environmental impact

The environmental impact of non-regulated hydro-generation of power is mainly associated with preventing the migration of fish and other biota across the turbine area, but the building of dams in connection with large hydro facilities may have an even more profound influence on the ecology of the region, in addition to introducing accident risks. For large reservoirs, there has been serious destruction of natural landscapes and dislocation of populations living in areas to be flooded. There are ways to avoid some of the problems. Modular construction, where the water is cascaded through several smaller reservoirs, has been used, e.g. in Switzerland, with a substantial reduction in the area modified as a result. The reservoirs need not be constructed in direct connection with the generating plants, but can be separate installations placed in optimum locations, with a two-way turbine that uses excess electric production from other regions to pump water up into a high-lying reservoir. When other generating facilities cannot meet demand, the water is then led back through the turbines. This means that although the water cycle may be unchanged on an annual average basis, considerable seasonal modifications of the hydrological cycle may be involved. The influence of such modifications on the vegetation and climate of the region below the reservoir, which would otherwise receive a water flow at a different time, has to be studied in each individual case. The same may be true for the upper region, for example, owing to increased evaporation in the presence of a full reservoir.

Although these modifications are local, they can influence the ecosystems with serious consequences for man. An example is provided by the building of the Aswan Dam in Egypt, which has allowed water snails to migrate from the Nile delta to the upstream areas. The water snails may carry parasitic worms causing schistosomiasis, and this disease has actually spread from the delta region to Upper Egypt since the building of the dam (Hayes, 1977).

It is unlikely that hydropower utilisation will ever be able to produce changes in the seasonal hydrological cycle, which could have global consequences, but no detailed investigation has yet been made. Such a study could proceed along the same lines as the investigation of the influence of deforestation, shown in Fig. 2.93.

3.4.3 Tides

The average rate of dissipation of tidal energy, as estimated from the slowing down of the Earth's rotation, is about 3 x 1012 W. Of this, about a third can be accounted for by frictional losses in definite shallow sea regions, bays and estuaries, according to Munk and MacDonald (1960).

In order to understand the concentration of tidal energy in certain coastal regions, a dynamic picture of water motion under the influence of tidal forces must be considered. The equations of motion for the oceans, (2.60) and (2.61), must be generalised to include the acceleration due to tidal attraction, i.e. pidd/m where an approximate expression for the tidal force is given in (2.67). Numerical solutions to these equations (see e.g. Nihoul, 1977) show the existence of a complicated pattern of interfering tidal waves, exhibiting in some points zero amplitude (nodes) and in other regions deviations from the average water level far exceeding the "equilibrium tides" of a fraction of a metre (section 2.D). These features are in agreement with observed tides, an example of which is shown in Fig. 3.60. Newer data based on satellite measurements can be followed in near-real time on the internet (NASA, 2004).

Figure 3.60. Tidal range H (difference between highest and lowest level in m) of the semidiurnal lunar tides in the North Sea and adjacent straits (based on Kiilerich, 1965;

Figure 3.61. Examples of the time-development of tidal amplitudes at two different locations for the same month of 1936 (based on Defant, 1961).

The enhancement of tidal amplitudes in certain bays and inlets can be understood in terms of resonant waves. Approximating the inlet by a canal of constant depth h, the phase velocity of the tidal waves is Ut = (gh)1/2 (Wehausen and Laitone, 1960), and the wavelength is

= TUt, where Tt is the period. For the most important tidal wave, Tt equals half a lunar day, and the condition for resonance may be written

where i is an integer, so that the length L of the inlet is a multiple of a quarter wavelength. For i = 1, the resonance condition becomes L = 34973h1/2 (L and h in metres). Bays and inlets satisfying this condition are likely to have high tidal ranges, with the range being the difference between highest and lowest water level. An example of this is the Severn inlet near Bristol in the UK, as seen from Fig. 3.60. Cavanagh, Clarke and Price (1993) estimate the total European potential to be 54 GW or about 100 TWh y-1, of which 90% is in France and the UK.

As discussed in section 2.D, the tides at a given location cannot be expected to have a simple periodicity, but rather are characterised by a superposition of components with different periods, the most important of which being equal to one or one-half lunar or solar day. As a function of time, the relative phases of the different components change, leading to regularly changing amplitudes, of which two different patterns are shown in Fig. 3.61. The upper one is dominated by the half-day period, while the lower one is dominated by the full-day period.

If the water level at high tide, averaged over an area A, is raised by an amount H over the level at low tide, then the potential energy involved is

Wtidal = pw H A g H, and the maximum power which could be extracted or dissipated would, as an average over a tidal period Tt, be

Based on measured tidal ranges, and on an estimate of the area A of bay or inlet which could reasonably be enclosed by a barrage with the purpose of utilising the energy flow (3.31), a number of favourable sites have been identified, as shown in Fig. 3.62. These include only localities with considerable concentration of tidal energy, considering that if the tidal range decreases, the area to be enclosed in order to obtain the same power quadratically increases, and the length of barrage will have to be correspondingly greater. For the same reason, sites of adequate tidal range, but no suitable bay which could be enclosed by a reasonably small length of barrage have been excluded. Of course, the term "reasonable" rests on some kind of economic judgement, which may be valid only under given circumstances. It is estimated that 2-3 GW may be extracted in Europe, half of which at costs in the range 10-20 euro-cents (or US cents) per kWh (Cavanagh et al., 1993, using costing methodology of Baker, 1987), and 20-50 GW in North America (Sorensen and MacLennan, 1974; Bay of Fundy Tidal Power Review Board, 1977).

Figure 3.62. Tidal range for selected locations and the estimated average tidal power for each bay in a number of cases where utilisation has been proposed (based on King Hubbert, 1969; Gray and Gashus, 1972; Sorensen and MacLennan, 1974).

Environmental impacts may arise from utilisation of tidal power. When the La Rance tidal plant was built in the 1960s, the upper estuary was drained for water during two years, a procedure that would hardly be considered environmentally acceptable today. Alternative building methods using caissons or diaphragms exist, but in all cases the construction times are long and careful measures have to be taken to protect the biosphere (e.g. when stirring up mud from the estuary seabed). Generally, the coastal environment is affected by the building and operation of tidal power plants, both during construction and to a lesser extent during operation, depending on the layout (fish bypasses etc., as known from hydropower schemes). Some fish species may be killed in the turbines, and the interference with the periodic motion of bottom sand may lead to permanent siltation problems (and it has at la Rance).

The total estimated power of about 120 GW at the best sites throughout the world may be larger than what can be considered economic, but smaller than the amount of tidal energy actually available. It is still 12% of the above-

mentioned estimate of the total dissipation of tidal energy in the vicinity of land, and it is unlikely that all the coastal sites yielding a total of 1000 GW would be suitable for utilisation, so the order of magnitude is likely to be correct. The 2000 GW remaining relative to the tidal power derived from astronomical data (Munk and MacDonald, 1960) presumably becomes dissipated in the open ocean.

The maximal tidal power believed to be accessible, as well as the total resource estimate, is about 10% of the corresponding figures for hydropower, discussed in section 3.4.2. Tidal variations are coupled to river run-off and sea level rise due to global greenhouse warming (Miller and Douglas, 2004).

Getting Started With Solar

Getting Started With Solar

Do we really want the one thing that gives us its resources unconditionally to suffer even more than it is suffering now? Nature, is a part of our being from the earliest human days. We respect Nature and it gives us its bounty, but in the recent past greedy money hungry corporations have made us all so destructive, so wasteful.

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