Hydropower A Reliable Renewable Energy Source: Hydropower is a renewable energy source based on the natural water cycle. Hydropower is the most mature, reliable and cost-effective renewable power generation technology available.
Hydropower schemes often have significant flexibility in their design and can be designed to meet base-load demands with relatively high capacity factors, or have higher installed capacities and a lower capacity factor, but meet a much larger share of peak demand.
Hydropower is the largest renewable energy source, and it produces around 16 % of the world’s electricity and over four-fifths of the world’s renewable electricity. Currently, more than 25 countries in the world depend on hydropower for 90 % of their electricity supply (99.3 % in Norway), and 12 countries are 100 % reliant on hydro.
Hydro produces the bulk of electricity in 65 countries and plays some role in more than 150 countries. Canada, China and the United States are the countries which have the largest hydropower generation capacity.
Hydropower is the most flexible source of power generation available and is capable of responding to demand fluctuations in minutes, delivering base-load power and, when a reservoir is present, storing electricity over weeks, months, seasons or even years.
One key advantage of hydropower is its unrivalled “load following” capability (i.e. it can meet load fluctuations minute-by-minute).
Although other plants, notably conventional thermal power plants, can respond to load fluctuations, their response times are not as fast and often are not as flexible over their full output band.
In addition to grid flexibility and security services (spinning reserve), hydropower dams with large reservoir storage be used to store energy over time to meet system peaks or demand decoupled from inflows. Storage can be over days, weeks, months, seasons or even years depending on the size of the reservoir.
As a result of this flexibility, hydropower is an ideal complement to variable renewables as, when the sun shines or the wind blows, reservoir levels can be allowed to increase for a time when there is no wind or sunshine. Similarly, when large ramping up or down of supply is needed due to increases or decreases in solar or wind generation, hydro can meet these demands.
Hydroelectric generating units are able to start up quickly and operate efficiently almost instantly, even when used only for one or two hours. This is in contrast to thermal plant where start-up can take several hours or more, during which time efficiency is significantly below design levels.
In addition, hydropower plants can operate efficiently at partial loads, which is not the case for many thermal plants.
Reservoir and pumped storage hydropower can be used to reduce the frequency of start-ups and shutdowns of conventional thermal plants and maintain a balance between supply and demand, thereby reducing the load-following burden of thermal plants. Hydropower is the only large-scale and cost-efficient storage technology available today.
Despite promising developments in other energy storage technologies, hydropower is still the only technology offering economically viable large-scale storage. It is also a relatively efficient energy storage option.
The system integration capabilities of hydropower are therefore particularly useful for allowing the large-scale large penetration of wind and other variable power sources.
Systems with significant shares of large-scale hydro with significant reservoir storage will therefore be able to integrate higher levels of variable renewables at low cost than systems without the benefit of hydropower. Hydropower can serve as a power source for both large, centralized and small, isolated grids.
Small hydropower can be a cost-competitive option for rural electrification for remote communities in developed and developing countries and can displace a significant proportion of
diesel-fired generation.
In developing countries, another advantage of hydropower technology is that it can have important multiplier effects by providing both energy and water supply services (e.g. flood control and irrigation), thus bringing social and economic benefits.
Hydropower is generally CO2 -free in operation, but there are GHG emissions from the construction of hydropower schemes, from silting in the reservoirs and from the decomposition of organic material (predominantly an issue in tropical regions). Hydropower schemes can have an important spatial and visual footprint.
One of the greatest challenges with the development of hydropower is ensuring that the design and construction of hydropower projects is truly sustainable.
This means that, in addition to an economic assessment, proper social and environmental impact assessments must be conducted and if there are negative impacts on local populations, ecosystems and biodiversity, these issues need to be mitigated in the project plan.
In the past, this is an area where hydropower has had a poor track record in some cases. Some of the more important impacts that need to be considered and mitigated include changes in river flow regimes, water quality, changes in biodiversity, population displacement and the possible effects of dams on fish migration.
Although hydropower technologies are mature, technological innovation and R&D into variable-speed generation technology, efficient tunnelling techniques, Hydropower projects account for an estimated half of all “certified emissions reduction” credits in the CDM pipeline for renewable energy projects.
These can be direct (e.g. CO2 emissions from construction vehicles) or indirect (e.g. the CO2 emissions from the production of cement). The International Hydropower Association has a “hydropower sustainability assessment protocol” that enables the production of a sustainability profile for a project through the assessment of performance within important sustainability.
Hydropower Technologies
Hydropower has been used by mankind since ancient times. The energy of falling water was used by the Greeks to turn waterwheels that transferred their mechanical energy to a grinding stone to turn wheat into flour more than 2000 years ago. In the 1700s, mechanical hydropower was used extensively for milling and pumping.
The modern era of hydropower development began in 1870 when the first hydroelectric power plant was installed in Cragside, England. The commercial use of hydropower started in 1880 in Grand Rapids, Michigan, where a dynamo driven by a water turbine was used to provide theatre and store front lighting.
These early hydropower plants had small capacities by today’s standards but pioneered the development of the modern hydropower industry. Hydropower schemes range in size from just a few watts for pico-hydro to several GW or more for large-scale projects. Larger projects will usually contain a number of turbines, but smaller projects may rely on just one turbine.
READ: 2019 HYDROPOWER STATUS REPORT
The two largest hydropower projects in the world are the 14 GW Itaipu project in Brazil and the Three Gorges project in China with 22.4 GW. These two projects alone produce 80 to 100 TWh/year. Large hydropower systems tend to be connected to centralised grids in order to ensure that there is enough demand to meet their generation capacity.
Small hydropower plants can be, and often are, used in isolated areas off-grid or in mini-grids. In isolated grid systems, if large reservoirs are not possible, natural seasonal flow variations might require that hydropower plants be combined with other generation sources in order to ensure continuous supply during dry periods.
Hydropower transforms the potential energy of a mass of water flowing in a river or stream with a certain vertical fall (termed the “head”). The potential annual power generation of a hydropower project is proportional to the head and flow of water.
Hydropower plants use a relatively simple concept to convert the energy potential of the flowing water to turn a turbine, which, in turn, provides the mechanical energy required to drive a generator and produce electricity.
The main components of a conventional hydropower plant are:
Dam
Most hydropower plants rely on a dam that holds back water, creating a large water reservoir that can be used as storage. There may also be a de-silter to cope with sediment build-up behind the dam.
Intake, penstock and surge chamber
Gates on the dam open and gravity conducts the water through the penstock (a cavity or pipeline) to the turbine. There is sometimes a head race before the penstock. A surge chamber or tank is used to reduce surges in water pressure that could potentially damage or lead to increased stresses on the turbine.
Turbine
The water strikes the turbine blades and turns the turbine, which is attached to a generator by a shaft. There is a range of configurations possible with the generator above or next to the turbine. The most common type of turbine for hydropower plants in use today is the Francis Turbine, which allows a side-by-side configuration with the generator.
Generators
As the turbine blades turn, the rotor inside the generator also turns and electric current is produced as magnets rotate inside the fixed-coil generator to produce alternating current (AC).
Transformer
The transformer inside the powerhouse takes the AC voltage and converts it into higher-voltage current for more efficient (lower losses) long-distance transport.
Transmission lines
Send the electricity generated to a grid-connection point, or to a large industrial consumer directly, where the electricity is converted back to a lower voltage current and fed into the distribution network. In remote areas, new transmission lines can represent a considerable planning hurdle and expense.
Outflow
Finally, the used water is carried out through pipelines, called tailraces, and re-enters the river downstream. The outflow system may also include “spillways” which allow the water to bypass the generation system and be “spilled” in times of flood or very high inflows and reservoir levels.
Hydropower plants usually have very long lifetimes and, depending on the particular component, are in the range 30 to 80 years. There are many examples of hydropower plants that have been in operation for more than 100 years with regular upgrading of electrical and mechanical systems but no major upgrades of the most expensive civil structures (dams, tunnels).
The water used to drive hydropower turbines is not “consumed” but is returned to the river system. This may not be immediately in front of the dam and can be several kilometres or further downstream, with a not insignificant impact on the river system in that area.
However, in many cases, a hydropower system can facilitate the use of the water for other purposes or provide other services such as irrigation, flood control and/or more stable drinking water supplies. It can also improve conditions for navigation, fishing, tourism or leisure activities.
The components of a hydropower project that require the most time and construction effort are the dam, water intake, head race, surge chamber, penstock, tailrace and powerhouse. The penstock conveys water under pressure to the turbine and can be made of, or lined with, steel, iron, plastics, concrete or wood.
The penstock is sometimes created by tunnelling through rock, where it may be lined or unlined.
The powerhouse contains most of the mechanical and electrical equipment and is made of conventional building materials although in some cases this maybe underground. The primary mechanical and electrical components of a small hydropower plant are the turbines and generators.
Turbines are devices that convert the energy from falling water into rotating shaft power. There are two main turbine categories: “reactionary” and “impulse”. Impulse turbines extract the energy from the momentum of the flowing water, as opposed to the weight of the water.
Reaction turbines extract energy from the pressure of the water head. The most suitable and efficient turbine for a hydropower project will depend on the site and hydropower scheme design, with the key considerations being the head and flow rate.
The Francis turbine is a reactionary turbine and is the most widely used hydropower turbine in existence. Francis turbines are highly efficient and can be used for a wide range of head and flow rates.
The Kaplan reactionary turbine was derived from the Francis turbine but allows efficient hydropower production at heads between 10 and 70 metres, much lower than for a Francis turbine. Impulse turbines such as Pelton, Turgo and cross-flow (sometimes referred to as Banki-Michell or Ossberger) are also available.
The Pelton turbine is the most commonly used turbine with high heads. Banki-Michell or Ossberger turbines have lower efficiencies but are less dependent on discharge and have lower maintenance requirements. There are two types of generators that can be used in small hydropower plants: asynchronous (induction) and synchronous machines.
Asynchronous generators are generally used for microhydro projects. Small hydropower, where a suitable site exists, is often a very cost-effective electric energy generation option. It will generally need to be located close to loads or existing transmission lines to make its exploitation economic.
Small hydropower schemes typically take less time to construct than large-scale ones although planning and approval processes are often similar. Large-scale hydropower plants with storage can largely de-couple the timing of hydropower generation from variable river flows.
Large storage reservoirs may be sufficient to buffer seasonal or multi-seasonal changes in river flows, whereas smaller reservoirs may be able to buffer river flows on a daily or weekly basis.
With a very large reservoir relative to the size of the hydropower plant (or very consistent river flows), hydropower plants can generate power at a near constant level throughout the year (i.e. operate as a base-load plant).
Alternatively, if the scheme is designed to have hydropower capacity that far exceeds the amount of reservoir storage, the hydropower plant is sometimes referred to as a peaking plant and is designed to be able to generate large quantities of electricity to meet peak electricity system demand.
Where the site allows, these are design choices that will depend on the costs and likely revenue streams from different configurations.
Hydropower Classification By Type
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 services.
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. 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 generation to meet peak demands and provide ancillary services.
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.
Large and Small Hydropower Schemes
A classification of hydropower by head is interesting because it is this that determines the water pressure on the turbines, which, together with discharge, are the most important parameters for deciding the type of hydraulic turbine to be used.
However, generally speaking, hydro is usually classified by size (generating capacity) and the type of scheme (run-of-river, reservoir, pumped storage). Although there is no agreed definition, the following bands are typical to describe the size of hydropower projects:
- Large-hydro: 100 MW or more of capacity feeding into a large electricity grid.
- Medium-hydro: From 20 MW to 100 MW almost always feeding a grid.
- Small-hydro: From 1 MW to 20 MW usually feeding into a grid.
- Mini-hydro: From 100 kW to 1 MW that can be either stand-alone, mini-grid or grid connected.
- Micro-hydro: From 5 kW to 100 kW that provide power for a small community or rural industry in remote areas away from the grid.
- Pico-hydro: From a few hundred watts up to 5 kW (often used in remote areas away from the grid).
However, there is no agreed classification of “small” and “large” hydro and what constitutes “small” varies from country to country. A given country’s definition of what is a “small” hydropower system is often important because it can determine which schemes are covered by support policies for small hydro and which are covered by those (if any) for large hydro.
Small hydropower plants are more likely to be run-of-river facilities than are large hydropower plants, but reservoir (storage) and run-of-river hydropower plants of all sizes utilise the same basic components and technologies.
The development of small hydropower plants for rural areas involves similar environmental, social, technical and economic considerations to those faced by large hydropower. Local management, ownership and community participation, technology transfer and capacity building are basic issues that will allow sustainable small hydropower plants to be developed.
Small hydropower plants have been used to meet rural electrification goals in many countries. Currently there is 61 GW of small hydropower capacity in operation globally. China has been particularly successful at installing small hydropower projects to meet rural electrification goals and 160 TWh was produced from 45 000 small hydro projects in China in 2010.
Hydropower Resource
The overall technical and economic potential for hydropower globally is available from some literature sources. However, the accuracy of these estimates is open to debate. In many cases country-level estimates of technical or economic potentials have been calculated using different criteria and combining these results means the totals are not directly comparable.
Efforts to improve the mapping of the global hydropower resource are ongoing, but further work is required and should be encouraged. However, taking into account these uncertainties, it is clear that the hydropower resource is very large, with many parts of the world being fortunate enough to have large resource potentials.
Virtually all regions have some hydropower resources although these resources are sometimes concentrated in a small number of countries and are not always located adjacent to demand centres. The total technical hydropower resource potential depends on a number of critical assumptions in addition to average inflows into a catchment area.
However, despite the uncertainty around the calculations, the estimated technical potential for hydropower is as much as 15 955 TWh/year or 4.8 times greater than today’s production of hydropower. Estimates of the economically feasible hydropower capacity are not comprehensive enough to provide global estimates.
What the economically feasible hydropower potential is for a given country is a moving target. The cost of alternative generation options, which sets the limit at which the LCOE of a hydropower project would be economically feasible, as well as the costs of developing hydropower projects (e.g. through advances in civil engineering, cost reductions for equipment), will
change over time.
The very high ratio of economic to technically feasible resources for some countries tends to suggest that only hydropower resources that have already been examined in detail have been included in the analysis. In other cases, the reason is that the country does have very economic hydropower resources.
Further work to better characterise the hydropower resource under standard definitions would help improve the comparability of resource estimates between countries and with other renewable power generation options. The efforts underway to achieve this should be encouraged.
Africa remains the region with the lowest ratio of deployment-to-potential, and the opportunities for growth are very large. However, in Africa complicated competing priorities and concerns mean that hydropower development is not straightforward.
The impact of hydropower development on local populations, their impacts on water use and rights, as well as issues over the biodiversity impacts of large scale hydropower developments, mean that significant planning, consultation and project feasibility assessments are required.
This is often required to take place in consultation with countries downstream, given the importance of Africa’s rivers to the water supply of each country.
Only once all major concerns are addressed can projects move to the detailed design phase and look to secure financing. The critical issue in Africa, and other regions, of the allocation of water rights between countries and different users within countries can be a significant delaying factor in getting project approval and funding.
Growing populations and increasing water scarcity in some regions mean that these issues are complex and potentially divisive, but, without agreement, development is unlikely to move forward.
