Hydrogen and oxygen, possess potential energy when they exist divided as separate gasses. This energy is released as they combine chemically into water, e.g. in a fuel cell.
Through an electrolytic process water can again be decomposed into the two gasses by applying electric energy – similar to charging a battery
Electrolytic splitting of water into hydrogen and oxygen is a way to store energy on a grand scale. E.On is going to take the first expensive steps to gain practical experience in this area.
By Kent Krøyer
One of the largest energy suppliers of Europe, the German E.ON, has decided to work on one of the great barriers for renewable energy - the general problem of storing energy on an industrial scale. E.ON’s chosen solution is to store it through the electrolysis of hydrogen. However this is not a mature technology and it involves a large investment and a long term commitment.
E.ON is taking this step in order to bring down the cost of future energy storage plants.
E.ON is building a pilot plant in the German city of Falkenhagen, about 30 miles east of Berlin. The electrolysis capacity will be 360 cubic meters of hydrogen per hour, using 2 MW. This amounts to 30 MWh stored per day, according to a press release by Hydrogenics Corporation, the Canadian turnkey supplier.
An artistic impression of the coming power-to-gas pilot plant at Falkenhagen, Germany. Graphics: E.ON
Renewables put pressure on storage capacity
The general lack of proper storage mechanisms keeps coming up as a major obstacle for a large share of renewable energy in the grids because of the fluctuating nature of the power production from solar panels and wind turbines. They only produce power when sun shines or the wind blows.
As long as the share of solar or wind power is small the electrical energy can be fed into the grid easily, but as the percentage grows larger the power fluctuations also grow, until the point where the grid becomes unstable.
The need to increase the share of renewable energy has become very real after the German government’s decision to close all nuclear power plants by 2022, influenced by the Japanese nuclear disaster last year. Germany has a goal of getting a third of its energy consumption from renewable sources in 2020.
“The use of renewable energy sources will increasingly bring the power grid to the limits of its capacity. E.ON is therefore investing in the development of technologies to store large energy volumes. In this respect, power-to-gas is a promising solution for the future energy supply system,” says Klaus-Dieter Maubach, the person responsible for technology and innovation and member of E.ON’s Board of Management, in a press release.
Use of electric surplus for water-splitting
Power-to-gas storage works by using surplus electrical power to split water into its constituents, hydrogen and oxygen, and then using the hydrogen as the energy medium, which is better suited for large scale energy storage than the current battery technologies.
This is untested on a larger scale and the best practice needs to be established.
“The traditional alkaline electrolysis working at 100 degree Centigrade has an energy efficiency of about 60 percent. However, the less mature SOEC technology, being developed at e.g. DTU, and working at 6-800 degrees, may achieve efficiencies above 80 percent, and in principle close to 100 per cent,” says professor Søren Linderoth, Head of Department of Energy Conversion and Storage at the Technical University of Denmark (DTU).
None of mentioned electrolysis technologies need expensive platinum as a catalyst.
The gas grid used as hydrogen storage
Another challenge for the project is to have a facility to store the hydrogen once it is extracted from water. Hydrogen is very difficult to store and distribute because the small molecules are hard to contain.
E.ON will simply feed the hydrogen into the existing natural gas distribution network rather than storing it in a huge pressure bottle. There is a limit to the allowed percentage of hydrogen in the distribution network due to regulations, currently two percent by volume at 55 bar. This is, however, just the beginning and it means that the first hydrogen will be used as a substitute for natural gas.
To overcome the two percent barrier the next step is to convert the hydrogen into synthetic gas or bio-methane, E.ON says in a press release. That way the existing natural gas network can be utilized 100 percent as energy storage.
Stored energy – not stored electricity – is the goal
Producing natural gas from wind electricity is friendly to the environment but the economics is questionable. This is because natural gas is not a scarce resource and will not be for decades, now that new shale gas reservoirs are being discovered regularly all over the world.
“This is not the way to look at it. The problem that needs to be solved is the efficient conversion and storage of renewable energy as gas. Once this is done, the next step is to use the gas for the transportation sector which is otherwise hard to convert to sustainable energies,” Søren Linderoth says.
In principle, the electrically produced natural gas could be converted back to electricity to close the circle. This could be done with gas turbine driven generators or fuel cells.
A simple splitting of electrolytic water: Hydrogen bubbles will appear at the cathode and oxygen bubbles at the anode when direct current is applied at the electrodes. Graphics: Wikipedia
“On a small scale, less than 10 MW say, fuel cells are more efficient than gas turbines in producing electricity. Micro-CHPs are now being demonstrated with electrical efficiencies of about 60%”, Søren Linderoth says. A micro-CHP is a consumer-sized system that generates heat and electrical power combined.
The E.On pilot project will provide a power substation, metering station, hydrogen pipeline and natural gas grid access station. The first production of hydrogen from Falkenhagen is expected in 2013.
“The project will enable E.ON to gain a greater understanding of the technical and regulatory challenges involved, and to acquire practical experience for future multiple or larger installations. The knowledge and experience will allow E.ON to define sound business models and place us in a strong position to win business in countries looking for proven, cost-effective ways to store renewable energy,” the company says.