Renewable energy is a topic which has gained significant traction in recent years. Unlike fossil fuels, which are finite and contribute to environmental degradation, renewable energy provides a cleaner, healthier, and more sustainable path forward for meeting our energy needs. Energy storage systems refer to technologies that store energy for later use, allowing for a more flexible and reliable energy supply from renewable sources such as solar and wind.
There are a wide variety of energy storage systems that enhance the power generation capabilities of renewable power plants. The most familiar system may be hydropower, with over 95% of today’s energy grid storage being held by pumped hydropower. When electric demand is low, “turbines pump water to an elevated reservoir using excess electricity. When electricity demand is high, the reservoir opens to allow the retained water to flow through turbines and produce electricity” . Thanks to its performance, pumped hydropower has dominated the energy grid storage market for years. However, other emerging technologies are gaining notoriety, including compressed air energy storage, which will be the topic of today’s blog.
Have you ever wondered what happens to the air when you blow up a balloon? Well, some clever people have figured out how to use that air to store electricity. It’s called compressed air energy storage (CAES), and it’s basically like having a giant balloon underground that you can fill up with air when you have extra electricity and let it out when you need more. Sounds simple, right? Well, not quite. Some challenges are involved, like keeping the air from getting too hot or cold, and making sure it doesn’t leak or explode. But if done right, CAES systems can help us use more renewable energy sources like wind and solar, and reduce our dependence on fossil fuels.Compressed air energy storage is a technology that stores excess electricity as compressed air in underground reservoirs or containers. When electricity is needed, the compressed air is heated and expanded to drive a turbine and generate power. CAES can help balance the supply and demand of electricity, especially from intermittent renewable sources like wind and solar. There are two main types of CAES: diabatic and adiabatic. Diabatic CAES dissipates some of the heat generated during compression to the atmosphere and uses natural gas or other fuels to reheat the air before expansion. Adiabatic CAES stores the heat of compression in a thermal storage system and uses it to reheat the air without additional fuel. Adiabatic CAES has higher efficiency and lower emissions than diabatic CAES but also higher costs and technical challenges.
CAES has several advantages over other energy storage technologies, such as batteries or pumped hydro. It can store large amounts of energy for long durations at relatively low costs. It can also provide ancillary services to the grid, such as frequency regulation, voltage support, spinning reserve, and black start capability. Moreover, it can reduce greenhouse gas emissions by displacing fossil fuel power plants during peak demand periods.
However, CAES also faces some limitations and barriers. It requires suitable geological formations or containers to store compressed air safely and reliably. It may also have environmental impacts on land use, water resources, noise pollution and wildlife habitats. Furthermore, it faces regulatory uncertainties and market challenges regarding cost recovery, revenue streams, and grid integration.
The cost of compressed air energy storage depends on several factors, such as the type of CAES system, the size and location of the storage facility, the availability and price of electricity and fuel, and the market conditions and revenue streams. According to a report by Siemens Energy , the capital cost of a diabatic CAES plant ranges from $400 to $500 per kW, while the capital cost of an adiabatic CAES plant ranges from $800 to $1000 per kW. The operating cost of a diabatic CAES plant depends mainly on the fuel price, while the operating cost of an adiabatic CAES plant depends primarily on the electricity price.
The levelized cost of energy (LCOE) is a metric that compares the pros and cons of different energy technologies over their lifetime. It is calculated by dividing the total overlooked costs by the total discounted energy output. The LCOE of CAES varies depending on the assumptions and scenarios used in the analysis. According to a study by EPRI , the LCOE of diabatic CAES ranges from 6 to 13 cents per kWh, while the LCOE of adiabatic CAES ranges from 9 to 18 cents per kWh.
Turbomachines in Compressed Air Energy Storage Technology
Two main types of turbomachines are used in CAES: compressors and turbines. Compressors increase the pressure and temperature of a fluid by reducing its volume, while turbines decrease the pressure and temperature of a fluid by increasing its volume. In CAES, compressors are used to compress air during the charging phase, while turbines are used to expand air during the discharging phase.
The performance and efficiency of turbomachines depend on several factors, such as the design, operation, control, and integration of the components. Some of the main challenges and opportunities for improving turbomachines in CAES are:
- Design optimization: The design of turbomachines should consider the specific requirements and constraints of CAES systems, such as variable load conditions, low-temperature operation, high-pressure ratios, multi-stage configuration, and integration with thermal energy storage. Advanced design methods and tools can help optimize the geometry, aerodynamics, thermodynamics, mechanics, materials, and manufacturing of turbomachines for CAES applications.
- Operation control: The operation of turbomachines should be controlled to ensure optimal performance and reliability under different scenarios. For example, variable-speed drives can help adjust the speed of compressors and turbines according to the load demand. Active clearance control can help reduce leakage losses by minimizing the gap between rotating and stationary blades. Smart sensors can help monitor the condition of turbomachines and detect faults or anomalies.
- Integration with thermal energy storage: The integration of turbomachines with thermal energy storage (TES) can enhance the efficiency and flexibility of CAES systems. TES can store or recover heat from compressors or turbines during compression or expansion processes, respectively. This can reduce or eliminate fuel consumption for reheating air before expansion (in diabatic CAES) or improve round-trip efficiency by avoiding heat losses (in adiabatic CAES). Different types of TES systems can be used for CAES applications such as sensible heat storage (e.g., packed-bed), latent heat storage (e.g., phase change materials), or thermochemical heat storage (e.g., metal hydrides).
As technology advances, the efficiency of compressors and turbines will improve with updated designs. Such enhancements will take CAES to new heights as it continues to scale the abilities of green energy plants .
In conclusion, CAES is a promising technology that can support renewable energy integration, reduce greenhouse gas emissions, and decrease fossil fuel consumption. Turbomachines play an essential role in these systems by enabling energy conversion between electrical and mechanical forms. However, designing and optimizing turbomachinery and the entire system can be challenging. Fortunately, AxSTREAM and AxSTREAM System Simulation can help address these technical challenges.
Interested in learning more about compressed air energy storage systems? Join our upcoming webinar which discusses strategies to solve challenges in renewable energy storage with a focus on compressed air-energy storage systems, taking place April 6th. Register here.
Compressed Air Energy Storage – an overview | ScienceDirect Topics
CAES | Thermo-Mechanical Energy Storage | Siemens Energy Global (siemens-energy.com)
Compressed Air Energy Storage – Energy Storage | CTCN (ctc-n.org)