Future Grid

Solar needs less diesel and more storage


28 July 2015 by Mike Stone

Relying on diesel generation alone to back up solar for off-grid networks is making less and less sense.


Younicos Storage

Image: Younicos


Battery energy storage may still be too expensive to replace diesel entirely as backup to solar-powered networks. But there is a case being made for diesel to take more of a back seat - which will allow more renewable energy into the mix.

Philip Hiersemenzel, spokesperson for the German energy storage project developer Younicos, is keen to point out that battery storage should be seen as complementary to diesel backup and a way to reduce the overall system’s reliance on diesel. In fact, without storage, says Hiersemenzel, renewables will not have the opportunity to reach their optimal level in an off-grid scenario.

“In far and away most of the hundred-plus cases we’ve looked at, the optimal renewable share leading the lowest cost per kWh is above 30%. Since you simply can’t get to that while you still have a diesel (gen-set) running, because of the diesel’s must-run requirement, you need the battery,” he comments.

The must-run requirement for big diesel generators is the key point here. Simply stopping and starting a heavy-duty gen-set whenever there’s a threat of a cloudy day is not an option if you want a long life for your investment. And if you do that, you aren’t going to want too much solar to swamp your system.

If instead you invest in energy storage to mop up excess solar energy, and use a more modest diesel set-up power (typically 20-40% of the overall energy produced, says Hiersemenzel), your operating costs will fall. So if the case is so strong, why isn’t there more storage being sold?

Hiersemenzel points to a lack of trust in energy storage technologies as being one of the big current barriers to storage making inroads to replacing diesel. He insists that when Younico's project on the island of Graciosa in the Azores comes online these doubts will be dispelled. To do so, it will have to fulfil its goal of increasing the energy generated by renewables from 15% today to a projected 65%.

Island micro-grids are the low-hanging fruit of the solar-plus-storage market. The costs of importing diesel by tanker are high – so solar can make big economic sense. Adding more renewables, such as bio-fuels and wind, to the mix, as on Graciosa, helps add stability to the system.

Tim Hennessey, COO and president of US energy storage company Imergy, also identifies “various parts of Africa or anywhere the grid is unreliable,” as currently being economically viable for solar-plus-storage over solar plus diesel. In fact, he said: “Battery storage solutions make sense in every emerging market. There is no emerging market where energy prices are low.”

Imergy has recently committed to delivering 1,000 vanadium redox flow batteries to be used by SunEdison as a solar-plus-storage offering in India, over the next three years. SunEdison says its goal is to install 5,000 systems over the next five years, with the intention of bringing power to 10 million people. According to Imergy, diesel backup will not be part of that deal.