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October 19, 2020
Hydropower plants can be constructed in a variety of sizes and with different characteristics. In addition to the importance of the head and flow rate, hydropower schemes can be put into the following categories
» Run-of-river hydropower projects have no, or very little, storage capacity behind the dam and generation is dependent on the timing and size of river flows.
» Reservoir (storage) hydropower schemes have the ability to store water behind the dam in a reservoir in order to de-couple generation from hydro inflows. Reservoir capacities can be small or very large, depending on the characteristics of the site
and the economics of dam construction.
» Pumped storage hydropower schemes use off-peak electricity to pump water from a reservoir located after the tailrace to the top of the reservoir, so that the pumped storage plant can generate at peak times and provide grid stability and flexibility
These three types of hydropower plants are the most common and can be developed across a broad spectrum of size and capacity from the very small to very large, depending on the hydrology and topography of the watershed. They can be grid-connected or form part of an isolated local network. run-of-river technologies In run-of-river (ROR) hydropower systems (and reservoir systems), electricity production is driven by the natural flow and elevation drop of a river. Run-of-river schemes
have little or no storage, although even run-of-river schemes without storage will sometimes have a dam.12 Run-of-river hydropower plants with storage are said to have “pondage”. This allows very short-term water storage (hourly or daily). Plants with pondage can regulate water flows to some extent and shift generation a few hours or more over the day to when it is most needed. A plant without pondage has no storage and therefore cannot schedule its production. The timing of generation from these schemes will depend on river flows. Where a dam is not used, a portion of the river water might be diverted to a channel or pipeline (penstock) to convey the water to the turbine.
Run-of-river schemes are often found downstream of reservoir projects as one reservoir can regulate the generation of one or many downstream run-of-river plant. The major advantage of this approach is that it can be less expensive than a series of reservoir dams because of the lower construction costs. However, in other cases, systems will be constrained to be run-of-river because a large reservoir at the site is not feasible. The operation regime of run-of-river plants, with and without pondage, depends heavily on hydro inflows. Although it is difficult to generalise, some systems will have relatively stable inflows while others will experience wide variations in inflows. A drawback of these systems is that when inflows are high and the storage available is full, water will have to be “spilled”. This represents a lost opportunity for generation and the plant design will have to trade off capacity size to take advantage of high inflows, with the average amount of time these high inflows occur in a normal year. The value of the electricity produced will determine what the trade-off between capacity and spilled water will be and this will be taken into account when the scheme is being designed. Hydropower schemes with reservoirs for storage
Hydropower schemes with large reservoirs behind dams can store significant quantities of water and effectively act as an electricity storage system. As with other hydropower systems, the amount of electricity that is generated is determined by the volume of water flow and the amount of hydraulic head available.
The advantage of hydropower plants with storage is that generation can be decoupled from the timing of rainfall or glacial melt. For instance, in areas where snow melt provides the bulk of inflows, these can be stored through spring and summer to meet the higher electricity demand of winter in cold climate countries, or until summer to meet peak electricity demands for cooling. Hydropower schemes with large-scale reservoirs thus offer unparalleled flexibility to an electricity system.The design of the hydropower plant and the type and size of reservoir that can be built are very much dependent on opportunities offered by the topography and are defined by the landscape of the plant site.
However, improvements in civil engineering techniques that reduce costs mean that what is economic is not fixed. Reduced costs for tunnelling or canals can open up increased opportunities to generate electricity. Hydropower can facilitate the low-cost integration of variable renewables into the grid, as it is able to respond almost instantaneously to changes in the amount of electricity running through the grid and to effectively store electricity generated by wind and solar by holding inflows in the reservoir rather than generating. This water can then be released when the sun is not shining or the wind not blowing. In Denmark, for example, the high level of variable wind generation (>20 % of the annual electricity production) is managed in part through interconnections to Norway where there is substantial hydropower storage.
Pumped storage hydropower technologies Pumped hydro plants allow off-peak electricity to be used to pump water from a river or lower reservoir up to a higher reservoir to allow its release during peak times. Pumped storage plants are not energy sources but instead are storage devices. Although the losses of the pumping process contribute to the cost of storage, they are able to provide large-scale energy storage and can be a useful tool for providing grid stability services and integrating variable renewables, such as wind and solar. Pumped storage and conventional hydropower with reservoir storage are the only large-scale, low-cost electricity storage options available today. Pumped storage represents about 2.2 % of all generation capacity in the United States, 18 % in Japan and 19 % in Austria.
Pumped storage power plants are much less expensive than lead-acid and Li-ion batteries. However, an emerging solution for short-term storage are Sodium Sulphur (NaS) batteries, but these are not as mature as pumped hydro and costs need to be confirmed. However, pumped storage plants are generally more expensive than conventional large hydropower schemes with storage, and it is often very difficult to find good sites to develop pumped hydro storage schemes. Pumped hydropower systems can use electricity, not just at off-peak periods, but at other times where having some additional generation actually helps to reduce grid costs or improve system security. One example is where spinning reserve committed from thermal power plants would be at a level where they would operate at low,inefficient loads. Pumped hydro demand can allow them to generate in a more optimal load range, thus reducing the costs of providing spinning reserve. The benefits from pumped storage hydropower in the power system will depend on the overall mix of existing generating plants and the transmission network. However, its value will tend to increase as the penetration of variable renewables for electricity generation grows.
The potential for pumped storage is significant but not always located near demand centres. From a technical viewpoint, Norway alone has a long-term potential of 10 GW to 25 GW (35 TWh or more) and could almost double the present installed capacity of 29 GW.
Hydropower capacity factors
The capacity factor achieved by hydropower projects needs to be looked at somewhat differently than for other renewable projects. For a given set of inflows into a catchment area, a hydropower scheme has considerable flexibility in the design process. One option is to have a high installed capacity and low capacity factor to provide electricity predominantly to meet peak demands and provide ancillary grid services. Alternatively, the installed capacity chosen can be lower and capacity factors higher, with potentially less flexibility in eneration to meet peak demands and provide ancillary services.13
Analysis of data from CDM projects helps to emphasise this point. Data for 142 projects around the world yield capacity factors of between 23 % and 95 %. The average capacity factor was 50 % for these projects .