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Hydro power: good or bad?

By Dave Elliott

Hydroelectric power is still the largest source of renewable electricity, generating nearly 17% of all global electricity, with over 1,200GW of large and small capacity in use around the world, the small plants (under 10MW) making up over 10% of the total. Most projects involve the construction of large dams and reservoirs, but smaller run-of-the-river schemes are also common, operating on river flows, without reservoirs. More capacity of all types is being added all the time. For example, Brazil recently installed an extra 4GW, and some see hydro use expanding significantly worldwide in the years ahead. Others are less sure, and point to a range of environmental and strategic problems.

The positive case is strong. Hydro supplies nearly all of the electricity on the grids of many developing countries, for example nearly 100% in Albania, Angola, Bhutan, Burundi, Costa Rica, D R Congo, Lesotho, Mozambique, Nepal, Paraguay, Tajikistan and Zambia, as well as 60-90% in 30 other developing countries. In addition, it provides nearly all the electricity in Norway, most of it in Iceland, and around 60% in Austria, Canada, New Zealand and Sweden.

Hydro projects are expensive to build but once built they last a long time, hundreds of years, with just occasional replacement of turbines, and can generate power very cheaply, often being the cheapest source on the grid. Although big hydro dam projects involve a lot of concrete, which is a carbon intensive material (it takes a lot of energy to produce it), their long operating life means that the net associated emissions per MWh output over their full life, due to this embedded energy/carbon debt, is very low, lower than any other energy supply option, including other renewables, with Energy Return on Energy Invested ratios of 200:1 or more for some projects.

Hydro plants can generate firm continuous output, using the water running downstream or stored in large reservoirs, but they are climate and weather dependent. The amount of water available at any particular location and time for run of the river schemes, or to top up hydro dam reservoirs, can vary. Indeed, with decreased rainfall in some areas in recent years, due to climate change, output from some hydro plants has fallen, sometimes significantly. But otherwise they are usually a very reliable large-scale energy source.

Hydro reservoirs can be used for pumped storage. Extra water can be pumped uphill using cheap off-peak power or surplus power from wind and PV generation, to be converted back into power when needed, using the hydro plant’s turbines. There is around 130GW of pumped storage capacity around the world, with more being added in response to the growth of the use of variable renewables – it is a major grid-balancing option. And in general, having hydro on the grid can help with balancing, adding value to variable renewables.

However, despite all these positives, there can be problems, for example with some large hydro projects. In addition to major potential social disruption when dams are built and reservoirs behind them are flooded, biomass coming downstream can be trapped and rot to produce climate-changing methane gas. In some locations with a lot of biomass on river banks and high temperatures, these emissions can sometimes make a hydro project a larger source of net greenhouse gas than a coal-fired power plant of the same generation capacity.

The debate on methane emissions rumbles on, with conflicting options being expressed. The industry side says it’s not a big issue, or only significant at some sites and, even if so, it should be possible to avoid or reduce the biomass collection problem. Some say that it doesn’t matter anyway, since the biomass would have rotted in any case without the dam, though it’s not clear if that would be so efficient downstream as when trapped together in a reservoir sump. The debate continues, but this overview isn’t bad.

Less open to debate is the fact that major dam failures can be catastrophic, drowning many people downstream. A major hydro disaster in China in 1975 led to 26,000 dead from flooding, and 145,000 dead from subsequent famine and epidemics. There have been many other smaller disasters around the world, with thousands killed, making hydro one of the worst energy supply options in terms of deaths/MWh, arguably on a par with, or worse than, nuclear, although perhaps not with coal, when the latter’s massive air pollution and climate-change-related impacts are included. Better design, management and maintenance of hydro projects may be able to reduce the risks, but at the same time large hydro projects are also potential targets for terrorist or military attacks. Though that’s true of many things – nuclear plants being an especial worry with, given their complexity and sensitivity, cyber-attacks being the latest fear. Some hydro projects also seem to have major engineering and siting problems – and they certainly all need careful maintenance.

Smaller hydro projects may be less of a problem on all counts. Although it is site-specific, small hydro in general is usually seen as less economically viable than large projects, at least in generation cost terms, but small projects are faster to build and so easier to finance, and can offer local social and economic benefits. Small run of the river projects (without reservoirs) will avoid some of the environmental problems, as may some smaller dam/reservoir projects, and most environmental groups favour small hydro, while opposing large schemes. Certainly there are issues with very large schemes like the huge new 6GW project in Ethiopia, with a potential for local problems and conflicts. The vast 40GW Inga project in Central Africa is even more worrying – who benefits, at what local cost

Big projects like this are unlikely to benefit most people in the region – most of the energy will go to large industrial users and cities, whereas the vast bulk of the population in rural Africa is not on the grid. Smaller local hydro, solar or wind projects would arguably be more use to them. But aid agencies often like big very visible projects and they also attract foreign investment – there is money to be made, with big construction companies involved and lucrative contracts. It’s a question of development policy.

So on one hand we have giant projects like the Three Gorges scheme in China, at 22GW, and on the other mini and even micro hydro projects – globally there is around 150 GW of hydro projects under 10MW, most of them in China, in all over 48,000 small projects. Arguably, we could do with more like that and less like these proposed new large ones.

Hydro currently accounts for over half of global renewable capacity, and most of this capacity will continue to run into the far future – it is a long-lived technology. However, it remains unclear whether new large plants should go ahead. Would that matter in energy terms? It would clearly reduce the total potential renewable resource available, although that loss could be partly balanced by focusing instead on large numbers of small hydro projects.

In terms of net expansion, wind is coming up fast behind, expected to pass 800GW by 2021. PV solar is also accelerating fast; it is expected to reach 400GW soon, and solar thermal has already achieved more than that. With, in addition, wave and tidal power likely to make multi-GW scale contributions in the 2020s, and geothermal also potentially able to make similar inputs, the so-called “new renewables” may soon eclipse existing large hydro, even leaving out modern biomass, currently with over 100GW of generation plant installed. With small hydro also building up, it may be that large hydro projects can be avoided without too much loss, although some will no doubt argue that a blanket ban is unwise – given the right policy framework, some big projects may be developed that are socially and environmentally acceptable. The debate continues.

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