Artikel Saving for a Rainy Day
Electric Power Storage Systems, Energy, Achema 2012
The world is becoming more plugged in. Energy researchers predict that electricity generation worldwide will rise by two-thirds between now and 2030. Global population growth and the electrification of previously undeveloped regions are not the only factors which are driving the increase in demand. Electricity grids in the industrialized world are expected to supply more and more power. Electromobility is one of the mega issues which are driving this trend. The German government has set a target of 6 million electric cars on German roads by 2030.
The problem is that electricity is a transient form of energy. When electricity is generated, there must be corresponding demand in the grid. If too much electricity is generated, the line frequency increases. Frequency variations can result in major power outages. Increased use of renewables such as solar and wind power makes the problem even more difficult to manage. This became very noticeable last spring when the grid in Eastern Germany threatened to shut down. The turbines in Eastern Germany, driven by strong winds, generated up to 12 GW of electricity which was fed into the grid. Regional demand, however, is only a fraction of that level.
Back in 2006, high electricity output from European wind turbines triggered a chain reaction that had serious consequences. The wind force was strong in Northern Germany late in the evening of November 4th. Shortly after 10 P.M., an overhead power line above the Ems River was shut down for safety reasons while a cruise ship was being launched. At the time, the power line was carrying 10 GW of wind-generated electricity to Southern and Western Europe. To stabilize the line frequency, part of the grid in Western Germany, Belgium, Northern and Southern France, Northern Italy and Spain were temporarily shut down. Some areas of Spain were affected by a blackout for up to two hours.
The Power Grid Plays a Key Role
Renewable energy is also having a noticeable effect on the electric power markets. At times, payment is offered to customers for using excess wind-generated electricity late at night. Solar power is also part of the equation. On the bright afternoon of July 16th, 2011, the price of electricity in the middle of the day on the EEX power exchange fell to the same level as the cheap night rate. The reason for the fall in price was the high amount of PV power in the grid. German PV systems now supply up to 13 GW of electricity (the equivalent of 10 nuclear power stations) to the grid during good weather conditions. Not only that, the portion of renewables in the electric power generation mix continues to increase. The catastrophe in Fukushima in particular has forced a rethink in some industrialized nations (most notably Germany). As a result, we can expect to see a massive expansion of wind, PV and solar power generation.
Currently, balancing supply and demand is a tightrope act. A new approach is needed to the problem. Some of the options are:
- Further grid expansion to enable better distribution of excess electricity.
- Demand management using smart grids, for example automatically switching on user loads to compensate for oversupply.
- Use of energy storage systems to manage excess or insufficient supply.
Grid expansion is the cheapest option, and it is also the fastest. However, public opposition to new overland lines in the highly-developed industrialized nations continues to increase. Smart grids, where for example domestic washing machines turn on automatically in the middle of the night when excess wind-generated electricity happens to be available, are still in their infancy, and the same applies to energy storage.
To gain a clearer picture of the many different storage technologies which are available, it is useful to distinguish between grid stabilization and local electricity storage. Thermal storage systems, especially those which make use of process heat in the chemical, pulp & paper and other process industries, are becoming increasingly important.
Electricity can be converted into chemical, potential, kinetic or electromagnetic energy. Each has its advantages and disadvantages. Super caps have a low energy density, but they can supply electricity very quickly to stabilize the grid and compensate for fluctuations. Other technologies are used to store electrical energy for varying amounts of time.
Grid Electric Storage: from Pumped Water to the Mega Battery
Pumped-storage hydroelectricity is currently the most wide-spread form of grid electric storage. At times when cheap electricity is available, large amounts of water are pumped from a lower elevation to a reservoir at a higher elevation. During periods of peak demand the water flows downhill again, driving turbines and generators to produce electricity. The 33 pumped-storage hydroelectric stations in Germany have a peak generation capacity of 6.7 GW and can store roughly 40 GWh of energy. By way of comparison, maximum pumped-storage hydroelectric capacity in the US is 21.5 GW.
CAES (Compressed Air Energy Storage) is based on a similar approach. Compressed air is stored in underground caverns. Gas turbines convert the energy back into electricity during periods of peak demand. Only two of these systems exist worldwide, one in Huntdorf, Germany (290 MW) and the other in McIntosh, Alabama, USA (110 MW). Both of these plants have been supplying power to the grid for a number of years. RWE, General Electric, Züblin and DLR have joined forces on a project to enhance the efficiency of this technology by utilizing compression heat.
The Stassfurt compressed air storage plant (RWE, DLR) is scheduled to go into operation in 2013. The 90 MW plant will have a storage capacity of 360 megawatt hours. The gas turbine on the ADELE (adiabatic compressed air energy storage) Project in Strassfurt is designed to operate without fuel combustion. Heat generated when the air is compressed is stored and recovered when the air is decompressed. This technique increases overall efficiency from roughly 55 % to as much as 70 %.
Network Storage so far Uneconomic
The reason why relatively few grid electric storage systems are currently in use is primarily economic. Coal or gas fired plants and nuclear power stations are cheaper to operate. "Pumped-storage hydroelectricity and CAES are only commercially viable if the difference between the night and peak rates is at least 3 ct per kWh," explained Prof. Dirk Uwe Sauer from the RWTH Aachen University. Sauer pointed out that expansion of the grid is always cheaper than grid electric storage.
However in the longer term, the development and use of storage technologies will be a necessity. In contrast to conventional power generation, wind and solar power are not always available. The ratio of fluctuating power (wind and solar) to predictable power (conventional power stations) is currently around 1:5. However, the German Environment Ministry expects that the ratio will be about 1:1 by 2030. In addition, storage technologies (both for electric and thermal energy) hold the key to efficient utilization of energy in the power generation and process industries. With this in mind, in the spring of 2011 the German Economics and Environment Ministry decided to make €200 million of funding available for energy storage technology research over the coming years to accelerate the pace of development. Other countries including China and the US are also providing financial support for storage technology research. $158 million has been allocated to energy storage technologies as part of the US government stimulus package. The US Energy Storage Association estimates that this funding will generate roughly $780 million of investment in storage technology. In addition to grid electric storage, the programme also includes decentralized solutions in electromobility and battery technologies.
A promising approach is the methanation of carbon dioxide through the use of electrolytically generated hydrogen. The main reason: The required natural gas infrastructure is already there. The poor, however, seems to be the total balance of energy: Only 28 percent of the originally applied current can be generated by reconversion of methane. However, if users manage to use the heat oft he reconversion process in combined heat and power plants, up to 80 percent of the original energy can be used.
Government officials and energy experts expect that electric cars will play a major role in the future energy mix. For one thing, countries hope to reduce their dependence on imported oil. In addition, electric cars attached to charging stations can be used to store renewable energy and feed the electricity back into the grid during periods of peak demand (Vehicle to Grid, V2G). This distributed energy concept could also solve the problem of grid instability.
Battery Technology: R&D and Investment in the Chemical Industry
The chemical industry is a key player. Chemical producer BASF recently announced plans to invest more than €100 million over the next five years in R&D and production startup for new battery materials. Part of the money will be spent on a production facility for advanced cathode materials in Elyria, Ohio. More than $50 million will be invested in the new plant, and production of cathode materials for high-performance lithium-ion batteries is expected to get underway in the middle of 2012. "We are developing innovative storage technologies because energy from renewable resources is not available 24/7/365, especially at the latitude where we live," explained Dr. Andreas Kreimeyer who is responsible for research on the BASF Executive Board.
Evonik, a BASF competitor, is also working on battery systems. The company has joined forces with other partners to build the world‘s largest lithium-ceramic battery. By using a special combination of ceramics and high molecular weight ion conductors, the consortium is attempting to increase energy density and maximize cycle life. The electric storage unit is being built at a power station in the Saar region, and it will have a storage capacity of around 700 kWh. If this storage unit were to charge and discharge every fifteen minutes, it could theoretically supply enough electricity for 4,000 households. Plans are already in place to increase the capacity to 10 MW. "We are leveraging our lithium-ion expertise to penetrate a completely new market," said Dr. Klaus Engel, Executive Board Chairman at Evonik. "For the first time, lithium-ceramic technology will enable us to separate generation and consumption at a viable cost. We will be able to stabilize fluctuations in the grid caused by solar and wind generation, and that will increase overall efficiency. As is the case in the vehicle sector, this market will be worth billions," claimed Engel. Experts predict that the market for state-of-the-art energy storage systems will eventually exceed €10 billion.
In Germany alone, demand for state-of-the-art storage systems will be in the upper 3-digit MW range. The goal of the 3-year LIB 2015 research initiative, which is receiving funding from the Federal Ministry of Education and Research (BMBF), is to develop mega batteries for stationary applications. Under the umbrella of the LIB 2015 Lithium Ion Battery Innovation Alliance, an industry consortium consisting of BASF, Bosch, Evonik, LiTec and VW has allocated €360 million to lithium-ion battery research and development. BMBF will be providing €60 million of funding for this field of research.
Scalable electrochemical energy storage systems in the 100 kW to 5 MW range will be needed to store large amounts of energy. This is an area where redox flow batteries appear to have very significant potential. With this type of battery, electricity is stored as chemical energy in redox pairs which are held in external tanks. Electricity is generated in a separate reactor while electrolyte is continuously circulated to the electrodes from the storage tanks. The direction of the electrolyte flow is reversed for charging. Tank size provides storage scalability, and efficiency can be as high as 80 %. At the Fraunhofer ICT institute in Pfinztal, a prototype redox flow battery has been developed to test various electrode materials, membranes and electrolytes. The long-term goal is to build a 20 MWh battery system which is capable of supplying electricity to around 2,000 households when renewable energy is unavailable.
There are a number of other battery technologies besides the ones described above. Some are already operational (e.g. sodium sulfide batteries), but most are still at the research stage. Energy density is a major factor to consider when assessing the future energy potential. The energy density of NaS batteries is 150 kWh/m3 compared to 70 kWh/m3 for lead batteries. Li-ion batteries have even higher energy density (350 kWh/m3).
Hydrogen Storage - High Energy Density, Big Challenges Ahead
Hydrogen storage provides even higher energy density than batteries. The basic idea is to use excess electricity to electrolytically decompose water. The hydrogen is then stored in underground caverns. As much as 350 kWh per cubic meter can be stored at pressures up to 350 bar, a factor of more than 100 compared to compressed air storage. The first challenge is to improve electrolysis techniques (efficiency roughly 75 %). Moreover, to generate electricity with the stored hydrogen, advances in power station technology (steam turbine efficiency approximately 60 %) will be necessary, and that is anything but trivial.
Nevertheless, hydrogen storage offers very significant potential. Hydrogen is not only a source of stored energy, but it is also used to synthesize a large number of chemicals including methane and methanol as well as many basic and special chemicals. Hydrogen is used in fuel cells and hydrogen-powered vehicles. Up to this point, natural gas has been the principle source of hydrogen in the chemical industry. Hydrogen produced with eco-friendly electricity could be a viable CO2-free option. Siemens, among others, is looking for ways of optimizing water electrolysis for commercial applications. New membrane materials and current collectors made of porous sintered metal at the electrodes ensure that changes in the current supply are anticipated within milliseconds and the nominal output and operating pressure can increase. Plans are in place to build a 300 kW demonstration container based on Proton Exchange Membrane technology by 2012.
The results will also provide an input to the CO2RRECT project (CO2-Reaction using Regenerative Energies and Catalytic Technologies), a joint research undertaking involving the academic community and Bayer, RWE and Siemens, which is looking at ways of utilizing carbon dioxide as a resource with the aid of renewable energy.
Summary: The energy revolution is politically decided. Now is the time to provide appropriate technical solutions to compensate the fluctuations in the electricity supply from renewable sources. Solutions are required from the process industry, the chemistry, and the measurement and control technique.
Facts for decision Makers - Storage of Electricity
- Electrical energy can be converted into chemical, potential, kinetic or electromagnetic energy. Each of these options has its own advantages and disadvantages.
- Primarily pumped storage plants are currently in use as a web store.
- Compressed air storage systems store energy in the form of compressed air in caverns. At peak times the stored energy is converted back to electricity in gas turbines.
- BASF and Evonik are working on advanced batteries, which can store electricity directly.
Innovative energy carriers and energy storage are a key issue of the upcoming trade show Achema in Frankfurt, Germany (18th to 22nd of June 2012).
Autor: Armin Scheuermann, Editor in Chief
Ausgabe:S1/2012 Achema Messeguide