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Monday. 10 April 2017

Old Coal Mines Could Have A Future In Green Energy Storage

Germany continues its transition from coal to renewables thanks to some clever engineering

Energiewende (energy transition). That’s the name of the German government’s ambitious goal to transform their energy landscape over the next few decades. By 2025, they want 35-40% of their electricity to come from renewable energy sources. By 2035, they’re targeting 55-60%. And by 2050, they hope to hit at least 80% renewable energy, coupled with an overall reduction in energy consumption of 25% (compared to 2008).

To get anywhere near this goal requires a huge investment in wind and solar energy generation, as well as a step up in their use of biomass and hydropower, and improving the overall efficiency of natural gas power plants. So far, signs are good, at least in terms of their energy mix. In 2015, renewable energy made up 32.5% of Germany’s total electricity demand.  On one day in 2016, renewable technologies generated 55 GW of energy – that was 87% of Germany’s electricity demand on that day. As reported in Quartz at the time, there was so much electricity available, “Power prices actually went negative for several hours, meaning commercial customers were being paid to consume electricity.”

Alongside the environmental argument for renewables, there are also economic reasons a region might want to move away from coal and oil. A 2015 report from Bloomberg New Energy Finance showed that in Germany, coal and gas were more expensive than onshore wind – $106 and $118 versus $80/MWh – and the same was true in the UK. In China, coal was still cheap in 2015, coming in at just $44/MWh. But solar power there was cheaper than gas ($109 versus $113/MWh). With China now taking a leading role in the fight against climate change, the prices of renewables are likely to drop further.

The main argument laid against renewable energy generation is that it is often intermittent – the sun doesn’t always shine, and the wind does occasionally die down (though, living in Wellington, I question that second one). And this doesn’t really match with what the traditional electricity grid was designed to handle. Coal-fired power plants burn coal to produce steam, which is then used to spin huge electricity generators. These steam-powered, endlessly spinning generators do more than simply spit out a stream of AC electricity, though. They actually offer a physical stability to the system. It’s a bit like a spinning top – once you get it moving, it keeps on spinning. With something the size of a national grid, you might have hundreds or thousands of spinning turbines, all connected to each other. The benefit of this is that, if one power plant goes offline, the grid doesn’t suddenly shut down. Because they have so much rotational energy, the generators keep spinning for a little while, giving the grid managers a chance to redirect power from elsewhere, and avoid plunging a city into blackout.

Though wind turbines also produce energy via spinning turbines, if it’s a calm day, they just won’t spin. And solar panels produce DC electricity, and even then, that’s only when the sun is in the sky. How do you balance supply and demand, and keep everything stable, when you add these technologies into your grid? The only real option is to store the energy when it’s produced, and then send it back to the grid when it’s needed. And it’s in energy storage that old coal mines could play a major role.

Germany started to produce coal in the mid-1700s, and continued to rely on the fuel right through to the 1960s – the output of its many mines reached a peak in 1957. But as oil-burning and nuclear-powered plants**  grew in popularity between 1960s and 1980s, the demand for coal began to decline. By the turn of the millennium, just a handful of the country’s coal mines were still in operation. In 2007, the government committed to end all subsidies for coal mining by 2018 – a move seen as the final death knell for the industry. So, like a number of other sites, the Prosper-Haniel hard coal mine in North-Rhine Westphalia will cease coal production next year. But unlike other sites, Prosper-Haniel will continue to produce power – hydroelectric power, that is.

Pumped Hydroelectric Storage (PHS) has been used all over the word for many decades. These power plants rely on two water reservoirs that are separated vertically. Cheap, off-peak, renewable electricity is used to pump water into the higher reservoir. During peak times, the water is released back into the lower altitude reservoir, where is drives turbines to generate electricity. And this cycle of storing and releasing energy when needed can be repeated again and again, providing a backup source of energy, and stabilizing Germany’s increasingly ‘green’ grid. The water pressure and flow rate, as well as the efficiency of the turbines and motors define the power output of a hydroelectric plant, but ultimately, the amount of energy that can be stored depends on the height difference between the two reservoirs and the total volume of water involved.

This means that hydroelectric plants require a specific geography in order to function, but in the new Prosper-Haniel plant, they’ll be making use of its existing underground network. The lower reservoir will be made up of more than 25 km of mine shafts that reach up to 1200 meters deep, with the upper reservoir at the surface. They’re not the first to use flooded mine shafts, underground caves or even oceans as reservoirs – a recent paper from Applied Energy (£) pointed to Okinawa Yanbaru in Japan, amongst others. There’s also another project in Queensland, Australia that hopes to make use of an abandoned gold mine. But these projects have historically had trouble getting off the ground, as deftly reported in ArsTechnica. This one seems to on the right road, though. The North-Rhine Westphalia state government have been working with the mining operator, the mining service provider, and engineers at the University of Duisburg-Essen on this project.

The hope is that as well as providing a 200 MW energy storage facility – enough to power up to 450,000 homes, according to the DUE website – the new plant will provide new jobs for the community that has relied on coal production for more than 50 years. Despite what some (read: Donald Trump) might have you believe, the coal industry isn’t going to make a comeback. For environmental, health, logistical and economic reasons, it just can’t compete with renewables any longer. But if we’re smart, rather than representing the energy industry past, coal mines could become very important in securing its future.

** As an aside, Germany’s relationship with nuclear energy has always been complicated, but following the Fukushima disaster of 2011, then Environment Minister Norbert Rottgen announced that the country’s 17 remaining plants would be shut down by 2022. They’re still on target to hit that particular goal.

Source : Forbes

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