Peak Demand

16 February 2017

The fourth and final unity of the Ingula Pumped Storage Scheme recently went into commercial operation. Most readers will be aware of the need for schemes of this type, but it is interesting to sit down and think about the sheer importance of the end-use of what tunnel engineers are building.

Getting the world to kick its dirty energy habit is not just about building more nuclear power plants, further developing photovoltaic technology or even the heavy taxing of undesirable fuel sources.

One of the biggest challenges to keeping the network green is the enormous variance in daily energy demand. Some events cause such an incredible strain on the network that they must be planned for a long time in advance. As an example, British Gas releases UK energy usage statistics during major sporting events. A relatively major event, New Zealand vs. France in the 2011 Rugby World Cup final caused demand to hit 420MW, while England vs. West Germany in the 1990 Football World Cup saw a surge to 2,800MW.

The problem is that a lot of green energy sources are temperamental and often out of sync with peak energy demand. There is no solar generation after sunset, for example. The green sources that do work consistently, i.e. nuclear generation, take a long time to start up and shut down, so they are not the best suited for hitting peak demand. Although they are a very good option for meeting the base demand if the price is right. To meet peak energy demand, we rely on rapidly activating but dirty fossil fuels. The question for green energy then is how to bring peak generation in line with peak consumption. The answer: storage.

Dr Steven Novella who hosts the science and critical thinking podcast ‘The Skeptics’ Guide to the Universe’ recently tackled the various options (see episode 597). Aside from the familiar pumped storage, Novella highlighted recent discussion of cryogenic storage. This uses energy to liquefy air, compressing it 700 fold. The subsequent expansion into gas turns a flywheel.

In pure theory the various methods can be judged on their ‘round trip efficiency’ (RTE), which is the percentage of energy lost after energy is put into storage then sent back into the grid. An article on by energy manager and engineer Tom Lombardo on Engineering.com quotes numbers from HPS, a designer of cryogenic storage systems, which claims for its product “a 50% RTE, which they hope to increase to 80%. By comparison, batteries are 60 to 70% efficient, pumped hydro is 75% to 85% efficient, and compressed air energy storage is 45% efficient.” As it stands for energy storage, particularly with the limitations of batteries, a tunnelled solution is king