When you think of hydropower in the US, the first image that’s likely to come to mind is that of the Hoover Dam. As well as being the largest dam in the world at the time of completion in 1935, leading to the creation of Lake Mead, to this day America’s largest reservoir, the project was perhaps the most vivid example of President Franklin D. Roosevelt’s New Deal in action.
As many as 20,000 men made jobless by the Great Depression descended on southern Nevada to work on the project, which was pioneering both in terms of its sheer scale and the construction techniques it employed.
The US today has around 2,500 hydropower plants, five of which have a higher total capacity than the Hoover Dam. Together these dams produce 51% of the country’s total renewable energy output and as of 2012 made it the fourth largest hydropower player in the world, behind China, Brazil and Canada.
While the 78GW of conventional- and 22GW of pumped-storage hydropower is a considerable contribution to the grid, some believe the country’s hydropower potential has been far from fully realised.
NPD represent great untapped potential
In 2012, a team at the Oak Ridge National Laboratory in Tennessee, which is owned by the US Department of Energy, published an exhaustive report titled 'An assessment of energy potential at non-powered dams in the United States'. The report, which caused quite a stir, examined the electricity generating potential of 54,000 of the country’s more than 80,000 non-powered dams which were built for tasks such as irrigation or ensuring stable navigation depths of waterways.
It found that the 100 dams with the most potential could contribute around 12GW of clean energy to the mix, equivalent to a 15% increase in current hydropower output. The top ten, all found on the Ohio, Mississippi, Alabama and Arkansas rivers and their tributaries, could alone account for 3GW and the top 597 NPDs had the potential to generate more than one MW of power each. Eighty-one of the 100 largest are owned by branches of the US Army engineering corps, including each of the top ten.
“Many of the monetary costs and environmental impacts of dam construction have already been incurred at NPDs,” says Boualem Hadjerioua, the study’s principal investigator.
“So adding power to the existing dam infrastructure can be achieved at lower cost, with less risk, and in a shorter timeframe than development requiring new dam construction. The abundance, cost and environmental favourability of NPDs combined with the reliability and predictability of hydropower, make these dams a highly attractive source for expanding the nation’s renewable energy supply.”
Generation figures are a good guide, but more research needed
Deciding whether an NPD has electricity generating potential is an uncertain science. The team immediately discounted dams under 5ft in height and those that might be considered auxiliary dams (sharing the same headwater as another, larger dam). The team then went about judging the individual potential of the remaining dams through the equation potential hydropower generation (in Mwh) equals average flow over an hour times gross head for hydropower generation times generating efficiency (an assumed 0.85).
The Department of Energy acknowledge that these figures are really just a guide. Water flow measurements need to be carried out over a longer period so as to determine the characteristics of wet, dry and normal years and the report assumes that all water passing a facility can be converted into electrical energy and that hydraulic head is constant, which is often not the case.
Hadjerouia also stresses that site-specific environmental factors, such as fish migratory routes and the economic rationale for retrofitting, are all factors to be considered by the dam’s owners.
“The assessment provides preliminary information for stakeholders, who can further evaluate the potential to increase hydropower production at NPD sites,” he says. “Developers could use the information provided in the assessment to focus on more detailed analysis of sites that demonstrate a reasonable potential for being developed.”
NPD transformation starting to speed up
In the years since the report was released, a number of developers have embarked on NPD retrofitting projects. The first major project in the wake of its publication broke ground in 2014, with electricity wholesaler Missouri River Energy Services looking to turn the Red Rock Dam near Des Moines, Iowa, from a flood control dam into a hydroelectric power station. Expected to be active in 2018, the plant will generate 36.44MW and could produce around 20MW more when water levels are high.
“This is an expensive resource, but it’s going to last 100 years,” says Missouri River Energy Services CEO Tom Heller. “You can’t build coal, you can’t build nuclear. The only other solution is natural gas, and we are not strong believers that natural gas is a good investment for power supply.”
According to the Energy Information Administration, 300MW of electricity generation capacity is expected to come online from retrofitted non-powered dams in 2016. This is compared to just 126MW between 2006 and 2015, suggesting that the projects started post-2012 are finally coming to fruition.
In July, NPD conversion was given another boost by the Department of Energy when it announced it was making $9.8mn of funding available for up to twelve projects that can produce “innovative technologies that will reduce capital costs and deployment times for pumped-storage and non-powered dam retrofits”.
“Adding power to these facilities can often be achieved at lower costs and in shorter timeframes than development requiring new dam construction,” said the Office of Energy Efficiency and Renewable Energy in a press statement. “However, the majority of these non-powered dams have challenges such as low heads, low/varying inflows, and environmental considerations that existing technologies are not able to overcome in a cost-effective way.”
Known as HydroNext, the grant scheme is looking for “innovative turbine/generator units that can operate at low heads and across a variety of flows can be incorporated with modular, standard designs to improve technical performance and reduce civil works costs and deployment times”.
The government is currently accepting project proposals and intends to start distributing funds this fiscal year. Although details on the bids have not been revealed, technologies such as the SLH turbine by California-based Natel give a good indication of what to expect. Unlike conventional low-head turbines, which are shaped like boat propellers or wheels, the SLH’s blades move more like the caterpillar tracks of a tank, a design that has proved more efficient in a series of low-head pilot projects.
At the same time as HydroNext, the authors of the influential energy department report are working on the 'Hydropower vision report', a detailed roadmap for the development of hydropower in the US with contributions from more than 200 experts.
While the 12GW that non-powered dams could contribute is a drop in the ocean of the country’s overall generation capacity, which in 2014 stood at 1,068GW, it could see hydropower become a bigger energy source than wind. It also serves as a reminder that sustainability is not necessarily about building anew, but thinking about what’s already there in a new way.