Energy storage - a key to the green transition

Green Transition 26 October 2016 Flemming Ulbjerg

Giant pools of hot water can help control the supply of solar and wind energy – and thus help fulfil the ambitions of the Paris climate agreement.

12 min

The road from the Paris Agreement to a low-carbon society could go through the small Jutland town of Vojens. This might seem an unlikely detour at first, but a look at two central publications from the International Energy Agency (IEA) will show its plausibility.

In “IEA Energy Technology Perspectives” the agency zooms in on how to keep global warming below 2 degrees Celsius – a de facto fulfilment of the pledge politicians made in Paris – compared to the business-as-usual scenario of 6 degrees warming. Take electricity, for example. Today fossil fuels dominate the energy mix with a 68% share, but this must be effectively reversed by 2050, when renewables should account for roughly the same share.

Achieving this reversal, however, greatly depends on having the ability to store renewable energy, says experts from Yale University and Standard & Poor’s. The credit rating agency calls storage “the final piece in the global energy transition puzzle”.

Professor Steven Cohen, Executive Director at the Earth Institute, Columbia University, USA, puts it this way:

“Renewable energy is central to the sustainable economy. And because solar and wind power are intermittent, storage, energy efficiency and smart grid technology are critical to the increased use of renewable energy,” he says.

A continuous source of fuel

Here’s the challenge: Running a commercially viable power plant requires a continuous source of fuel. That is no problem with coal, gas or uranium isotopes, but the supply of solar or wind energy cannot be controlled despite its infinite abundance. It may be cloudy or calm – or even night time, when energy demand is low.

Electricity grids cannot function without a balance between supply and demand. Imbalances cause voltage fluctuations, and money is also an issue – heating, for example, will be too cheap in summer, when demand is down, and too expensive in winter.

This is why storage plays a crucial role in the necessary energy evolution, says Maria van der Hoeven, Executive Director of the IEA, in the foreword to another publication from the agency, “Technology Roadmap on Energy Storage”.

“Energy storage technologies can help to better integrate our electricity and heat systems and can play a crucial role in energy system decarbonisation,” she writes.

Two world energy records

One of the best practice cases the IEA promotes in its roadmap is the consumer-owned Marstal district heating system in Denmark, where, with Ramboll’s help, hot water from solar heating is being stored in a covered pit.

The Marstal storage system had some initial sealing problems, as birds and cats were able to get into the pit, and at that stage the technology still needed subsidies.

Today, however, the pit storage system has been secured and further developed by Ramboll elsewhere in Denmark – which brings us to Vojens. The consumer-owned district heating system there now holds two world energy records: a 70,000- m2 solar heating plant and a 200,000-m3 heat storage – a giant pool of sorts, 13 metres deep and 610 metres in circumference – in an old gravel pit. What is more, the pool is commercially viable under Danish conditions.

It takes about five months to fill the pit to its maximum of 200 million litres. “The floating cover makes it possible to store the hot water for the Danish winter season when consumers turn on their radiators,” says Flemming Ulbjerg, Senior Consultant at Ramboll Energy.

The large-scale investment, which will increase the share of solar heating to 50% of the annual heat demand, provides consumers with annual savings of 10-15% on their heating bills, and the plant saves 6,000 tonnes of CO2 per year.

The next stage will be to use electric boilers and heat pumps to harvest the cheapest electricity and generate the remaining 50% heat with electricity from wind energy – an integration already established by the district heating company in a similar, but slightly smaller pit in the nearby town of Gram.

Thermal storage will become more common

“This type of energy storage is still rare, but that’s about to change,” says Sven Werner, Professor of Energy Technology at Halmstad University in Sweden and one of the world’s leading experts on district heating and cooling.

“It’s inevitable that thermal storage will become more common in order to increase the efficiency of our heating systems,” he says.

Like Sven Werner, Brian Vad Mathiesen, Professor of Energy Planning at Aalborg University, believes we have to move away from a sole focus on the electricity sector to a look at the energy demands in the heating, cooling and transport sectors as well. Combining various utility systems can help provide the cheap storage needed for electric vehicles, hot water and cold water.

“The sole focus on one grid is a problem. We need to focus more strongly on district heating grids in combination with the power grid. If we combine heat and power grids, we can use cheap heat storage to reduce the fossil fuel-based production of heat and power by using excess wind or solar power for heat production. Heat storage can even enable seasonal storage of renewable heat such as in the covered pits in Vojens and Gram. Whether we have a colder climate or a hotter climate like those in the Middle East or parts of Asia, water storage and the combination of utility grids is key to cost-effectiveness,” Brian Vad Mathiesen says.

Written by Michael Rothenborg.