More and more electricity produced by solar and wind plants will be fed into the power grid. The grid will therefore have to handle large amounts of such electricity, which can fluctuate sharply in line with the weather. Researchers from Siemens and Munich's Technical University are developing solutions that can prepare the grid for this flood of green power.
In a pilot project near Fürth, Germany, Siemens researchers are using inverters that intervene in the power network, thereby enabling more renewable energy to be fed into the grid.
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Dark blue is the new color of an increasing number of roofs. That’s because ever since Germany’s Renewable Energy Sources Act was passed in 2000 — and was then used as a model for similar subsidy legislation in around 50 other countries — more and more roofs have been transformed by photovoltaic units into small blue power plants.
Scientists from Siemens and the Technical University of Munich (TUM) are addressing the challenges associated with this development, including the danger of local grid overloads. They are also searching for new approaches that can make power grids smarter and able to accommodate large amounts of photovoltaic electricity.
That’s easier said than done, as most grids today are designed to transport electricity generated at large coal or gas-fired power plants. Such facilities supply electricity to the highvoltage network. The power then flows into the medium and finally into the low-voltage grid, the one that serves consumers. This hierarchical principle has functioned well up until now — but it’s not adequate for a future characterized by numerous small-scale electricity producers.
The share of solar power in the global energy mix is still relatively low, but experts agree that it could increase by a factor of 50 over the next 20 years. If that happens, it would put tremendous pressure on grid stability and voltage. “Such changes might not only damage important and expensive components like transformers but also negatively affect the functionality and lifespan of other electrical equipment and appliances,” says Dr. Michael Metzger from Siemens Corporate Technology (CT). Metzger and Professor Rolf Witzmann from the Department of Electrical Energy Supply Networks at TUM are therefore working to find a solution to these problems.
Their first step was to analyze the situation. They calculated how much photovoltaic electricity could be generated in Germany if all suitable roofs and open areas were fitted with photovoltaic units. The result they came up with was 161 to 188 gigawatts. Pilot photovoltaic facilities currently supply only around ten percent of that amount — or 18 gigawatts maximum. A second calculation showed how costs could skyrocket if such a step were taken. Upgrading the grid in just a single village to accommodate the potential increase would cost between €140,000 and €200,000. That’s because today’s transformers are designed solely for a specific voltage range and they overload if this range is exceeded. Electricity distribution cables could also be damaged by too much voltage, leading to a potential proliferation of short circuit events.
Mastering Reactive Current. In view of this challenging scenario, a pilot project near Fürth, Germany indicates that there are less expensive alternatives (see also article "No Longer a One-Way Street"). Researchers there are integrating inverters into the grid. Normally, inverters transform direct current from photovoltaic units into alternating current and adjust it to the frequency of the power network. However, a new development from Siemens enables inverters to also draw so-called reactive current from the grid and thus assume a control function. In other words, more electricity could be fed into the grid without having to implement costly expansion projects. Reactive current is generated by devices like motors that continually build up and break down magnetic fields. In this way, they draw current at regular intervals and then immediately feed it back into the grid.
Another challenge associated with renewable energy sources is their fluctuating output. Wind and solar power facilities do not continuously generate the same amount of electricity because winds speeds change, clouds cover the sun, and it gets dark at night. Scientists from Siemens and TUM are therefore looking into electricity storage units that take in surplus power and then return it to the grid when it is needed. A variety of concepts for such storage units already exist (see article "Second Wind for Hydrogen")and (Pictures of the Future, Fall 2009, Switching on the Vision). “The main thing that concerns us now is the question of how big such storage units should be to ensure that the pressures being placed on the grid are reduced in the most effective and least expensive manner possible,” says Witzmann. To do this, scientists are simulating the year 2005 in a kind of slow-motion sequence that uses weather data to depict the interactions between environmental conditions, photovoltaic facilities, and consumers. The results, which are expected by late 2011, will enable them to compare electricity production with demand.
Data Transfer in Milliseconds. In another project, Dr. Dragan Obradovic from Siemens CT and Professor Sandra Hirche from TUM are trying to determine the fastest possible way to offset fluctuations in electricity infeeds. “We’re developing control strategies that enable all of the plants in a grid to communicate with one another,” Obradovic explains. At the moment, only large power plants exchange information, but problems can often be foreseen many hours in advance — for example, when a power plant component needs to be replaced. A status report every ten minutes is sufficient for addressing smaller fluctuations. However, because photovoltaic electrical output depends on wind and weather, an outage can occur much more spontaneously than is the case with other sources. And while it may not be a big deal if only one unit goes down, failure on a regional scale can result in a blackout. In this case, all of the surrounding plants and storage units have to kick in. “We believe that in the future, the type of information required will have to be exchanged in just milliseconds,” says Obradovic.
Obradovic and other researchers are incorporating the results of the project into a laboratory network that is now being set up by CT researchers at a Siemens center in Erlangen. This network will not only test solutions for individual problems such as local voltage surges; it will also feature a small-scale grid of the future complete with photovoltaic plants, power consumers, and electricity storage units. At that point, in addition to simulating interactions, it will be possible to test everything under real conditions.