In spite of its critical importance to modern technologies and infrastructures, electricity has remained a transient resource that has virtually no shelf-life. Once electricity is generated from an energy reservoir, which can supplied by any type of energy sources, the electric current has to be used instantaneously by an electricity consuming device, or be lost otherwise. The only way to resolve this inherent disadvantage of electricity is to have energy storage systems that can save the electricity for later use.
Historically speaking, energy storage has been a recent issue for modern electricity grids since traditional energy sources, such as coal, oil and gas, could be consumed rapidly to balance the electricity supply with the demand of the grid. Mobile and ready-to-use energy source have provided a working, albeit inefficient, solution to this problem, which has been practiced in the energy industry for a long time. Nevertheless, the advent of renewable energy technologies, electric vehicles, and increased efforts to boost efficiency, have lead to more and more improvements to the inherent supply-demand issues of modern electricity infrastructures. Information technology products, for instance, have the capability to gather and analyze data on energy usage to more intelligently manage supply and demand of the electricity grid, while transient energy storage technologies have enabled more effective balancing of various types of loads. The combination of these two technologies have created modern smart-grid systems that manage the electricity infrastructure more efficiently and create a more secure energy infrastructure.
The rise of energy storage technologies has not only benefitted modern electricity grids by enabling smart-grid systems, but it has also enabled the rise of distributed energy generation and off-grid energy systems that can operate independently from the larger grid. The ability to go off-grid is especially important for regions that have historically been deprived of energy access from the traditional grid. Energy access is a considerable issue in many developing countries, as can be seen from the map below depicting rural energy access across the world:
Rural communities in developing countries in Africa, Latin America and South-East Asia have historically not have access to an electricity grid to support their energy needs. The advent of off-grid energy technologies, however, has enabled these areas to actually leap off-grid with self-reliant microgrid systems that can supply remote communities with basic energy needs. Recently, microgrid systems powered by renewable energy sources, especially solar power, have become a more and more popular tool to address energy access challenges. Given the decreasing cost of solar modules on a global scale, the main challenge with renewable microgrid systems remains energy storage, which enables the community microgrids to become self-reliant power systems.
The critical importance of energy storage technologies for both modern, developed energy infrastructure and energy access in underdeveloped areas of the world prompts a closer look at what energy storage technologies currently exists and how they are applied.
Pumped Hydro Energy Storage
Pumped hydro energy storage, i.e. pumping water up a mountain, is the godfather of energy storage systems. In a pumped hydro system, electricity is used to pump water up an elevation, commonly a mountain, and the water is then released down the same elevation to turn a turbine that generates electricity. Pumped hydro still accounts for over 90% of global energy storage systems given its simple and efficient nature leading to 70%-85% of energy recovery efficiency depending on the nature of reservoir.
The major disadvantage of pumped hydro storage is limited application, mostly due to geographical and economic constraints. Suitable water reservoirs are necessary for effective pumped hydro facilities, as substantial amounts of water and sizable changes elevation are required. Moreover, a 1000 MW pumped hydro facility can cost between $1 billion and $2 billion dollars, which makes large-scale pumped hydro only economically feasible for large-scale storage projects in modern grids. The traditional pumped hydro scheme, however, may soon be challenged by innovative approaches to redesign a traditional technology. Gravity Power’s hydro storage designs, for example, allow for greater geographical and economic flexibility by building their pumped hydro systems underground. Genereally speaking, pumped hydro storage will presumably continue to be attractive for modern grids that require large storage capabilities to balance their electrical loads.
Japan is one of the world’s most prominent users of pumped hydro storage due to country’s favorable topography. Japan can currently store ~10% of its electricity generation capacity using pumped hydro. The United States and Europe, on the other hand, can store ~5% and 2.5% of their respective generating capacities using pumped hydro systems.
Compressed Air Energy Storage
Compressed air storage systems store energy by compacting air and thereby putting it into a higher energy state. When the energy is needed, the compressed air can then be expanded to turn a turbine that generates the electricity. During the ‘idle phase’ of this process the compressed air is often stored in a tank or in suitable underground rock formations, such as slat fields or depleted gas fields. The process of air compression also generates heat, which can be re-used to raise the temperature of the air during the expansion process. The energy recovery efficiency of compressed air storage can range between 50% and 70% depending on how efficiently heat is used during the electricity generation process. Light Sail Energy, a Silicon Valley startup company, is aiming to reach efficiencies above 70% through efficient heat capture.
The United States and Germany have historically been global leaders in the development of compressed air energy storage facilities. The two countries account for all currently installed compressed air storage capacity, which totals less than 1 GW, as well as future development plans of compressed air energy storage facilities. Navigant Research estimates that worldwide compressed air storage capacity will grow to 11.2 GW by 2023.
Thermal Energy Storage
Thermal energy storage systems have recently experienced significant growth for large-scale energy storage applications, particularly due to the success of molten-salt energy storage system. Molten salt energy storage systems store the energy through heat transfer, meaning that a salt is heated to a liquid state using electricity or excess heat from the energy generation process. The conversion of electricity to heat occurs with 100% efficiency, meaning that the energy recovery efficiency of a molten salt system is dictated by the heat capture and heat transfer capabilities of the system, as well as the efficiency of the subsequent generation process for electricity recovery. Generally, molten salt systems have ~70% energy recovery efficiency.
Molten salt energy storage systems are already applied in solar thermal power plants, giving solar thermal plants the ability to store the energy they generate during the day. The current industrial application of molten salt technology will catalyze further improvements in the technology, as well as cost reductions, that may make these storage systems feasible for microgrids in remote areas.
Electrochemical Energy Storage
Electrochemical energy storage is achieved by using electricity to initiate a chemical reaction. The chemical reaction requires energy input and can then be reversed to release the stored energy. The most common electrochemical energy storage systems are batteries and supercapacitors, which are further described here. Batteries are ubiquitous in today’s electronic devices, including laptops, cell phones and even electric vehicles. Presently, the dominating battery technology is Lithium-Ion which has also been considered for grid storage applications. Researchers are also continuously working on new battery chemistries that may be used for grid storage applications, as well as electronic devices and electric vehicles, in the future. Microgrids that combine solar power and battery storage are already being built today, including a 2.5 MW Green Mountain Power facility in Rutland, Vermont.
Another form to electrochemically store energy are fuel cells. In a fuel cell storage system electricity would be used to generate a chemical fuel, such as hydrogen, that can then be converted back into electricity by the fuel cell. Hydrogen fuel cells are the most commonly applied technology and the ability to synthesize hydrogen from water makes hydrogen fuel cells attractive for energy storage application, especially for microgrids in remote areas. When the chemical reaction is reversed, hydrogen and oxygen combined in the fuel cell to make water, releasing energy in the process. Fuel cells, however, can also operate with non-hydrogen fuels, such as natural gas, and can thereby serve as an enhanced version of many modern generator systems.
As seen from the figure above, the various electrochemical storage technologies can serve different energy storage. While fuel cells, from a technological perspective, would be a good fit for larger scale grid storage applications, batteries would be a better fit for smaller systems that require faster responses, such as microgrids or electric vehicles. Yet, when designing the overall system, further factors, such as cost and reliability, have to be taken into account to ensure that the system performs as expected during its lifecycle.
The energy storage technologies described here only represent a fraction of present research and development efforts in energy storage systems. Energy experts across academia and industry have realized the critical role that storage will play in the future of the energy sector as a whole. The transformative nature of storage will also continue to enable future innovations in the energy field, which will ultimately change the way the energy game is played. The energy system of the future will include a diverse set of energy sources and will likely be accompanied by an equally diverse set of energy storage technology that meet different needs.
Cross-posted from BERC Blog, published online by the Berkeley Energy & Resources Exchange, a network of UC Berkeley scholars and industry professionals.