The oldest form of renewable electrical generation is Hydroelectric Power (HEP). If you take only the electrical generation aspect of this technology then it is clean with no pollution at all – after all it is simply moving water down a gradient over a water wheel that directly spins a turbine shaft generating electricity. But, like any technology it has full process problems. The biggest drawback to HEP is of course the requirement of continuously moving water in amounts necessary to spin the turbine shaft. On the east coast it rains enough that reservoirs constructed behind dams, like the Tennessee Valley Authority project, not only provided flood control for whole watersheds but each dam serves as an HEP plant. In the west, the aridity of the region meant that water capture behind strategically located dams would capture run off (from winter snow and rain) to serve as reservoirs for irrigation and HEP plants. Another major drawback for large scale HEP is that the necessary water must come from a major river. Over the last 100 years or so, nearly all the major rivers in the world have been damned for HEP – there really are no major rivers that can currently be used without major disruptions of local communities and ecosystems. Indeed, the construction of dams themselves disrupts local ecosystems and because of the natural forces of erosion also condemns the dams to a life-time activity of about 100 years or less. Silt fills in the area behind the dam and eventually fills in the reservoir.
Besides the sedimentation problem, large dams have as many pros as they do cons. The pros are that HEP provides clean, non-polluting electricity; flood control; Water storage for irrigation and drinking water, and also opportunities for water recreation (e.g. boating and fishing). The cons are reduced river flow and sedimentation needs for habitat downstream of a dam; capture of sedimentation behind the dam instead of being spread along the rivers course; alteration and disruption of aquatic life – this often happens because of temperature pollution where cold water releases from the lower portion of the dam reservoir shocks aquatic life in the lower river; disruption of migrating aquatic species (e.g. Salmon, eels) although fish ladders along the side of a dam can ease this problem; Large scale permanent flooding of arable and recreational land (e.g. Completed in 2003, the largest dam in the world, China’s Three Gorges Dam across the Yangtze River, displaced over 1 million people and submerged Farmland, archaeology, and viable habitat); and one rarely mentioned is the potential of catastrophic flooding from Dam failure.
While over 98% of worldwide rivers are already dammed, in some areas like the Western USA, the amount of water needed to maintain the reservoirs has been declining since the dams were build and because of increased needs for irrigation and drinking water, the storage capacity needed to run the HEP is also diminishing. This is further exacerbated by droughts many already arid regions are experiencing.
Pump Storage Schemes (PSS)
In hilly or mountainous areas, using excess/unused electricity can pump water uphill through sluice tubes from one lower reservoir to an upper reservoir. This serves as a form of energy storage and currently worldwide is one of the largest ‘grid storage systems’ available. When immediate electricity is needed, as in peak-use times, the water from the upper reservoir can be released back down the sluices to spin electric turbines as in regular HEP. The only difference is that water is in a closed system with the upper and lower reservoirs being linked as storage and usage of electricity. Obviously, in arid areas, evaporation and other minor leakages means the reservoirs needs topping up a little on occasion. In wet rainfall areas, the upper reservoir is naturally topped up by runoff. The amount of electricity that can be generated varies with the size of the system, but it is not unusual for capacity to reach 3000 MW in a single system – roughly equivalent to 3 large fossil fuel stations. In 2017, the USA obtained over 22 GW of power from Pump Storage schemes. One of the big problems with PSSs is that they must be located in areas that are usually of natural beauty (i.e. Mountains), which upsets many advocates of outdoors protection. Options to overcome this are to use areas that are designated as ‘brownfields’ – they are often disused mining areas or areas that have been heavily disturbed from previous activity. This would stop impacts on scenic or ecologically sensitive areas.
Smalls and Micro scale HEP
In many countries where rain is regular and sufficient, individuals or small groups of houses clustered together can take advantage of small-scale HEP generation. In areas with riparian rights to running water (if you live by a river you can use it) a minor part of the water flow can be diverted through a small sluice or penstock and into a micro HEP turbo generator before being returned to the river. If the water is low flow then a Pelton wheel generator may be needed. While small scale sounds good, it does require a continuous and reliable water source. Isolated rural communities can capitalize on the small scale HEP. In arid areas, Prior Appropriation Doctrine of water rights makes this option all but impossible. Add to that the likelihood of insufficient water capacity in arid areas and small scale becomes only a reality for wet areas with enough vertical drop height to convert the potential energy to kinetic energy with enough efficiency to warrant investment.
HEP has been a great renewable energy option for over a century, but while it generates up to 17% of the worlds electricity it has almost no option for expansion. Building new dams is expensive and disruptive for a growing human population where arable land is now a premium for agriculture. The choice of food versus electricity is becoming a real one in many parts of the world. In many areas where dams have long been a fixture, they are now being removed to allow the rivers to flow naturally again. The sedimentation problem of dams restricts their lifespan and cleaning out or rebuilding dams is economically prohibitive at this time. PPSs are still options, but social and environmental pressures will always be a stumbling block.