Sönke Ingwersen, Contributor
June 20, 2013 | 13 Comments
LONDON -- The key to further wind sector success may be the ability to provide baseload power. New solutions based on lithium batteries can deliver convincing foundations to balance wind's variability.
For more than two decades, practitioners have been developing control solutions to make operating wind energy plants more efficient. One of the latest projects commissioned by a Chinese operator was dedicated to energy storage. In China, feed-in regulations stipulate that 10 percent of the power installed from wind energy plants must be available via energy storage at any time for two hours.
Norbert Hennchen, managing director of Freqcon in Walsrode, Hannover, Germany says lithium batteries possess the greatest technological potential with regards to saving energy, and therefore the ability of renewable energy sources to provide baseload power.
The topic of storing renewable energy is currently on trend and is being discussed and implemented on many fronts. Energy density plays a big part in this. Pumped storage plants deliver approximately 0.5 kWh/m3 from an assumed drop height of 200 metres; in contrast, hydrogen plants deliver 500-2300 kWh/m3. With the latter, however, the efficiency of the energy supply is comparably low. Lithium battery systems are right in the middle, achieving approximately 125-400 kWh/m3 - and up to 2500 kWh/m3 under laboratory conditions. When on the subject of efficiency, Hennchen's eyes light up: "Such energy storage achieves values between 90 percent and 95 percent," he says.
Sophisticated Control Technology Ensures Sustainability
One reason is that when loading and discharging the cells, power dissipation is only around 1 percent. Each cell which is built to a standard version has a voltage of 3.2 V and delivers around 1000 Ah. The cells themselves measure 560 mm x 356 mm x 130 mm. If you connect 300 of these in a row, there will be a voltage of around 1000 V. This voltage feeds water-cooled inverters, like the ones the Chinese plant construction firm are developing and building.
"We don't build these power stacks under 1 MW anymore," explains a physicist working on the project. The energy industry doesn't need extreme power density, he continues; it is much more interested in storing large amounts of energy. For the project in China, Freqcon has developed a system which delivers 5 MW for two hours with the help of 1280 lithium cells. But control technology is an important factor in the long-term stability of this solution. In Hennchen's experience, "If you remain around 10 percent away from the maximum when loading and discharging the batteries, you can achieve double the number of cycles."
According to researchers, with 7000 as an assumed number of cycles and with daily loading and discharging of the batteries, they could have a service life of 20 years. Moreover, it must be ensured that individual cells are not overloaded. Since all cells behave in the same way at under 90 percent of load capacity, then beyond that, resistors must be individually installed in order to ensure that further charging proceeds consistently. "In order to achieve this, we have to monitor each individual cell metrologically, for example," the physicist explains. For this purpose, each cell contains specially developed measurement technology, which measures the temperature, voltage and balancing.
Energy Storage Requires Fast Data Speeds
In a cabinet with one of the respective decentralised peripherals, there are around 40 individual battery modules. Data from the peripheral travels to the higher-level control via Profinet. One battery block stores 1 MWh of electrical energy; therefore, for the project in China, five blocks will suffice. To control these five blocks, due to the high data speed necessary, the Profinet protocol is used rather than the more typical internet (TCP/IP) protocol. Energy supplier requirements with regard to reaction times for storage systems are extremely high. "A reaction time of 20 microseconds and an 80 percent system availability after 60 microseconds is not unusual," Hennchen reports.
Energy reaches the batteries and the medium voltage grid via specially-developed water-cooled inverter systems (IGBT). A PC also takes over the control of the inverter. Hennchen explains: "With a reaction time of 250 microseconds, this solution is as quick as we need for our frequency converter."
Communication here also takes place via Profinet. Data transfer from battery storage to the PC takes place in real-time mode, while from the PC to the individual inverter takes place in IRT (Isochronous Real Time) mode. At this point real-time ability is required, and synchronisation must be extracted using time stamps.
In both computer systems there is an integrated web server which makes remote service possible. "That is also an important feature, without which the wind energy industry would no longer work," says Hennchen.
Factors for Success: Consistency in Automation Technology
Battery units are connected electrotechnically via a central control cabinet. A circuit breaker takes over with overload and short-circuit protection. The control electronics are supplied by a 24 V power supply. Visualisation is undertaken by open architecture software. "We can see great advantages in this consistent concept based on the model of Totally Integrated Automation (TIA)," Hennchen adds. For him, consistency and efficiently functioning complete systems are the key to sustainable success.
Work on Hennchen's next project will be closer to home. He intends to develop a microgrid as part of a research project, which consists of 1.5 MW of installed wind energy, 2.5 MW of solar energy, and energy storage of 10 MWh.
With this, many remote places on earth could be supplied with clean energy in a decentralised and stable way – for example in Tibet, where there is no central energy supply grid but many small grids fed by diesel generator sets. Hennchen sees a great future in the efficient storage of electrical energy, which he wants to actively shape: "With high-tech battery systems, wind energy amongst others can provide base load power efficiently and economically."
Sönke Ingwersen is director of the sales and support centre, wind at Siemens Industry Automation.
Lead image: Wind turbines via Shutterstock
Additional image: Each lithium battery pack cell can deliver up to 1000 Ah. Courtesy of Siemens