Most people understand the electrical energy historically produced by large hydropower dams. There are however multiple types of flowing water that have the ability to drive turbines to produce electricity from generators. Over the past millennium water has been used to turn wheels to grind grain and then to power various mechanical devices starting in the 1800’s finally leading to electric power generation in 1878. Hydropower is widespread and currently provides 85% of the world’s renewable energy – Norway gets ~99% of its electricity from hydropower. The largest hydropower plant in the world is the Three Gorges Dam in China at 80-100 TWh per year (enough for 70-80 million households). The world has ~1308 GW of installed hydropower capacity which generated ~4306 TWh in 2019. Africa has the smallest installed capacity at 37 GW but fortunately it has very large potential to produce hydropower to meet the needs of a growing population trying to improve living standards and economies. It has been estimated by IRENA that the world has up to 15,000 TWh potential per year – almost four times current capacity.
Three general types of hydropower facilities have been identified: (1) impoundment (e.g. dams); (2) diversion (e.g. run-of-river); and (3) pumped storage (PSH). Facilities range in size from (1) large hydropower (>10 MW); (2) small hydropower (<10 MW); and (3) micro hydropower (<100 kW).
Diversion (run-of-river) hydropower is typically on a smaller scale and operates without interfering with the river flow, so it is very environmentally friendly with less impact on ecosystems and populations. The small scale nature means that it is generally suitable for distributed locations closer to smaller and or remote users.
One of the best Energy Storage Systems is Pumped Storage Hydropower (PSH) and today it accounts for ~94% of installed global energy storage capacity at 158 GW but capacity only grew by 304 MW in 2019 which is not enough to keep up with energy storage requirements as the penetration of intermittent renewables like Solar PV and Wind increased. Where topography permits, PSH needs to grow significantly.
Dams have been used by humans for millennium since the Egyptians first constructed gravity dams ~5,000 years ago. Since then dams have become larger and more complex with today’s largest hydropower facilities producing electricity up to thousands of MW each. The record is China’s Three Gorges Dam with ~22,500 MW electricity. Of the 71# largest dams in the world today, almost one third are located in China. The largest dam in Africa is Ethiopia’s Grand Renaissance with ~6,450 MW planned. These large hydropower facilities cost billions of dollars and can take up to a decade to build and operate. Then they rely on suitable hydrographic conditions to fill (and keep filled) the reservoirs to start (and continue to) efficiently producing electricity.
The pictures below are of Namibia’s largest dam called Neckartal. It began construction in 2013 and was completed in 2018 for a cost of ~R5.7 billion (~US$3 billion). Due to drought conditions, the dam was finally filled two years later this year as shown. The primary role for this dam is agricultural irrigation for this semi-arid region, but there is also a small hydropower plant of ~3 MW (scaled according to the irrigation outflows) – in areas of more rain or more river flow, electrical capacity could be substantially ramped with this size of dam and reservoir.
One of the challenges for large hydropower facilities is to consider the environmental and social impacts. Large hydropower reservoirs can flood substantial land which may have critical habitats for natural species and human populations. So not all locations would be suitable. Careful environmental impact assessments would be needed to identify risks and potential mitigations. Important agricultural land, land of historic or social importance, or essential transportation links may be affected. Then once a dam is in place, there could be environmental impacts on the water impounded and the subsequent downstream flows. Sediment flows would be restricted and may deposit behind the dam with potential adverse impacts both upstream and downstream (less transport of nutrients). Water quality may be impacted with cyanobacterial blooms, spread of water-associated diseases, depleted oxygen levels (hypoxia), and invasive species. All these risks need to be considered and mitigated where possible. GE has developed aerating turbines that send small bubbles of oxygen into the water through low-pressure points in the turbine during production of electricity.
Large hydropower has an important role in improving access to electricity in the developing world, but should be considered as part of a range of solutions due to these technical and economic challenges. There will be excellent locations with good source of funding / finance and a grid ready to utilise the electricity especially to replace non-renewable energy sources. There are also existing dams used for flood control and/or agricultural irrigation which could be modified to generate electricity thereby not creating any new environmental or social impacts (e.g. the US has 2,200 dams producing electricity but ~85,000 that do not – the DOE estimates ~1,800 of these could be modified to generate electricity). But the ability to have distributed power generation capacity like small hydropower is important to open up more potential locations where electricity can more quickly produced with less funding requirements and potentially less environmental impacts.
Small-scale hydropower plants may be characterised by head height, discharge (flow-rate) and capacity:
- Large flow-rate and small head characterises large run-of-the-river plants equipped with Kaplan turbines, a propeller type water turbine with adjustable blades;
- Low discharge and high head features are typical of mountain-based dam installations driven by Pelton turbines, in which water passes through nozzles and strikes spoon-shaped buckets arranged on the periphery of a wheel;
- Intermediate flow-rates and head heights are usually equipped with Francis turbines, in which the water comes to the turbine under immense pressure and the energy is extracted from the water by the turbine blades
An interesting technology is Turbulent’s micro-hydro water turbines available from 5 to 70 kW as shown below (and up to 100-200 kW). Cost is a function of turbine size, site conditions, and availability of local contractors, but it can range from under 100,000€ to over 400,000€ which is over 5,000€/kW to around 2,000€/kW.
Pumped Storage Hydropower
PSH relies on topology (height between upper and lower reservoirs) and water availability. Historically fresh water was used in closed systems, but recent studies have considered open coastline installations where the water is readily available seawater. Some facilities resemble conventional large dams, whilst other locations are able to use the topography to separate the reservoirs. The use of underground reservoirs such as disused mine facilities is also being investigated. PSH helps balance conventional Renewables intermittency and mitigate the need for conventional power generation (e.g. peaking power plants) or large capacity, long-duration Energy Storage Systems.
IRENA has stated that global PSH needs to double from ~160 GW today to 325 GW over the next few decades (N.B. at least) to help provide the energy storage needed with increased market penetration of Renewables. Large scale PSH range in size from the world’s largest Bath County Pumped Storage Station, Virginia, USA at 3,003 MW capacity down to the tenth place Raccoon Mountain Pumped Storage Plant, Tennessee, USA at 1,652 MW capacity. Many more PSH however are in the hundreds of MW size range.
Small scale PSH have also been constructed including small decentralised PSH in the range of 2 to 50 MW with flows up to 10 m3/s. Micro PSH (also called µ-PHES) have also been investigated down to 10 kW and flows of ~0.1 m3/s.
Pumped Storage Hydropower (PSH) is also an excellent Energy Storage System for Microgrids where conditions are right. Increased penetration of Renewables like Solar PV and Wind have the challenge of intermittency. When excess daily electricity exists, these Renewables can work together with a PSH to pump water to an upper reservoir where it can then be released back down to produce electricity when there is a shortage of Solar and Wind Renewables energy. Vattenfall has adopted this hybrid solution for the Geesthacht PSH in Germany (below).
IRENA estimated some key data for hydropower in 2015 which is probably still relevant. Development of more economic technologies for small-capacity and low-head applications has helped reduce costs for small scale hydropower. Large scale hydropower costs are very dependent on site conditions and logistical challenges of materials and equipment. The distance to transmission lines is also a significant cost factor. The total installed cost for large scale hydropower generally ranges from $1000-3500/kW with average site conditions and locations. The total installed cost for small scale hydropower (1-10 MW) can be somewhat cheaper, but averages in the same range. Very small scale hydropower can cost more per kW based on infrastructure costs that do not scale down as much. Hydropower systems have minimal maintenance so OPEX is low (e.g. O&M ~ 1-4% CAPEX per year). Diversion (e.g. run-of-river) hydropower is usually an economic solution with less large infrastructure and only small reservoirs (pondage) required, so its costs are on the low end of these ranges. Average CAPEX cost is ~30-35% facility, ~20-30% transmission and distribution, and turbines between 20-50%. Remote community locations may benefit from reduced transmission costs.
These cost appear in line with more recent IRENA cost estimates:
- “The LCOE of large-scale hydro projects at high-performing sites can be as low as USD 0.020/kWh, while average costs were of the new capacity added in 2019 was slightly less than USD 0.050/kWh.”
- “For large hydropower projects the weighted average LCOE of new projects added over the past decade in China and Brazil was USD 0.040/kWh, around USD 0.080/kWh in North America and USD 0.120/kWh in Europe.”
- “For small hydropower projects (1-10 MW) the weighted average LCOE for new projects ranged between USD 0.040/kWh in China, 0.060/kWh in India and Brazil and USD 0.130/kWh in Europe.”
- “The total installed costs for the majority of hydropower projects commissioned between 2010 and 2019, range from a low of around USD 600/kW to a high of around USD 4 500/kW.”
Opportunities to implement hydropower energy solutions exist in many countries but, based on the need for increased access to electricity, we should look at Africa. A geospatial assessment of small-scale hydropower potential in Sub-Saharan Africa has shown over 270 GW of energy potential. “With favourable hydrological conditions, hydropower offers a relatively low levelized cost, continuous generation without storage requirements, and the ability to operate in both isolated (e.g. microgrids) and interconnected (e.g. utility grid) modes.”
Meanwhile large hydropower projects have continued to be developed across the continent, but with various significant challenges including cost (e.g. increases), schedule (e.g. delays), regulatory (i.e. permitting and power purchase agreements), political (e.g. with downstream neighbouring countries), social (e.g. environmental issues with local communities), and funding/finance. Large dam projects include and are underway in Nigeria (Mambilla, Zunguru), Sierra Leone (Bumbuna), Equatorial Guinea (Sendje), Madagascar (Sahofika), Namibia (Neckartal), Cameroon (Nacktigal, Grand Eqeng), Ethiopia (Grand Renaissance), and DRC (Inga III). Not all large hydropower projects will be completed and the associated electrical transmission infrastructure would also need to be developed.
For these reasons, small scale hydropower energy solutions need to continue to be implemented across the continent to more quickly improve access to electricity in order to raise living standards and help provide clean water, improved sanitation, and economic stimuli for communities and commerce / industry. Where topography permits, linking Renewable energy solutions of Solar PV and Wind Power with Pumped Storage Hydropower could also provide high capacity, long duration Energy Storage.