Are you buying certified green power? Then perhaps you have wondered whether you are actually getting what you are paying extra for. Because how do you tell electricity apart?

Green, sustainable and renewable energy is in and has been for years. Land-based and offshore wind farms are popping up everywhere, and house owners across the globe are looking to invest in solar power, where deployment has soared since the early 2000s. The worldwide growth of solar cell power, for instance, has averaged 40 per cent per year since 2000 and reached a total capacity of 253 terawatt hours (TWh) in 2015.

That is a lot of green power from a considerable number of different places flowing into a grid infrastructure designed and optimised for centralised power generation – the fossil-fuel based system that the European power grid relied on in the past. So how do you distinguish where the power is coming from?

Postdoctoral Fellow Mirko Schäfer is working on exactly that problem in a project funded by the Carlsberg Foundation.

"Think of it as a big pool that everyone is putting power into and taking it from again. Once you put power into the pool or take power out, there’s no telling where it came from. You can’t put a stamp on the power to show you where it originated and who profits from it. It’s very hard to distinguish who’s actually using certain parts of the grid infrastructure," he says.

Complicating the complicated

The ever-increasing number of micro-contributors – such as a single family house with solar cells or a single wind turbine – makes an already complex grid system even more complicated. And it becomes still more complex because, even though the pool analogy may sound simple, it is actually far from reality.

"You have a specific grid infrastructure and there are limits on it. Only a certain amount of power can be transmitted because, otherwise, the infrastructure might break down. An example is the grid between northern and southern Germany, where the infrastructure is not large enough to accommodate the flow of power required," explains Mirko Schäfer.

So in a world in which an increasing number of private individuals, companies and publicly owned energy farms contribute to a failing grid, who is paying? And how can you be sure that you are actually getting the ‘certified green power’ that you might be paying for explicitly if energy cannot be marked?

The solution is flow tracing and, if we return to the pool analogy, it is just like adding colour to the water.

"You can’t distinguish one drop of water from the next. But what you can do is ‘colour’ it at its origin. You can trace the energy through the grid by looking at its origin, the origin’s data and flow pattern. That way you can tell who’s injecting how much at what place and what time. The result is a pool of different colours, and that way you can distinguish the flow algorithmically," says Mirko Schäfer.

Ensuring the right price

An example could be offshore wind farms, where a large percentage of the generated energy is not used by the country that creates it – the country where the wind farm is located. Flow tracing can then be used to allocate some of the investment cost and the cost of producing the energy to the countries that are actually buying the energy from the particular farm.

This way the cost of the future power grid – which is dependent on long-range power transmission, storage, back-up power generation units, etc. – can be more fairly distributed between those who actually use the power and the general public, who up until now have usually borne the cost of grid inefficiencies.