Tildy Bayar, Associate Editor, Renewable Energy World
September 10, 2013 | 10 Comments
Large pumped storage plants typically feature a wide, shallow reservoir, which holds a large volume of water. This means that, as the water empties out, the overall depth of the reservoir does not change significantly during a typical operational cycle and the pressure at the turbine inlet doesn’t change that much. In a plant where the reservoirs are deeper – at Glyn Rhonwy the planned reservoirs will be 50 metres deep – but hold much smaller volumes, one effect of using the stored water is an appreciable impact on the available head as the water level falls, creating what’s called a variable head (defined in metres) as water is pumped from one reservoir to the other.
At Glyn Rhonwy the head will be highly variable according to Holmes – from 270 to 220 metres. This “can pose some operational challenges” in making the water’s output come out predictably at 50 MW. “You might need to choke down [turn the turbine down] at the start and boost up at the end somehow,” Holmes said. “If the head varies the pressure inside the turbine, you’re not running at optimal consideration. You’ll have to slow it down in order to maintain the best operational profile.” QBC is researching technological fixes for sites with very variable heads, but according to hydropower experts no such fixes currently exist.
Holmes believes there is room for a varied mix of energy storage technologies in the UK. Because pumped storage is limited by the availability of suitable locations, even smaller facilities won’t work in urban areas. While he points to chemical batteries as well as flywheel energy storage as more suitable for many densely populated regions, “in terms of proven technology nothing really beats pumped storage,” he says. “It’s fast, reliable, energy-dense and cheap.” But, he says, “all technologies have a place – depending on where the demand is, where the connections are, and how much cheaper those technologies are over time.”
QBC has received a £200,000 (€236,000), one-year R&D grant from the Department of Energy and Climate Change and is looking into different types of pumped storage, such as seawater storage (of which there is currently only one plant in the world, Japan’s Okinawa Yanbaru, but the UK’s Severn Estuary has also been proposed for this purpose) and low-head storage “where you haven’t got a mountain but just a hill – is it still worth doing?”, says Holmes. The company is also investigating the possible use of potable water reservoirs for pumped storage.
The Time is Right for Pumped Storage
According to National Grid’s report Operating the Grid Beyond 2020, as more wind power is connected to the UK’s grid (up to 32 GW according to the report), the amount of uncertainty in demand forecasting is set to almost double, from 3 to 5 percent. With increasing uncertainty, flexible power delivery will be crucial. In addition, electricity market regulator Ofgem is currently seeking industry views on a proposal to mandate charges for its grid balancing mechanism, with the results to be announced this month. Rather than the current method of averaging the costs associated with grid balancing, the highest cost could be charged, affecting energy trading behaviour in the UK and increasing the value of balancing power plants. “This could change the economics for pumped storage in the UK, and could be the start of a renaissance,” Holmes said.
QBC says it is aiming for the £100 million site to be operational by 2017, and while the company was founded to undertake the Glyn Rhonwy project, “we don’t plan to be just a one-hit wonder,” says Holmes. “We’re looking at number of sites, which tend to be brownfield – some bigger and some smaller, some are in Wales and some aren’t. We think the market is big enough for some more.”
For a live discussion of hydropower as an energy storage asset as well as new developments within the North American hydropower market, register for our upcoming webcast, The HydroPower Opportunity, on Wednesday, September 25.
Lead image: The Glyn Rhonwy site, courtesy of Quarry Battery Company