Energy storage supports the large-scale integration of renewables onto the grid, increases the effectiveness of traditional energy systems and distributed energy systems, and is a provider of safe and economical energy. Energy storage has been viewed as a key component of the energy revolution and has seen extensive national support as an emerging technology.
Compressed Air Energy Storage (CAES) is one technology that has captured the attention of the industry due to its potential for large scalability, cost effectiveness, long lifespan, high level of safety, and low environmental impact.
Recently, the Chinese Academy of Sciences Institute of Engineering Thermophysics (IET) Energy Storage R&D Center published an article in the international journal Energy on some of their recent findings in CAES research. The research used computational fluid dynamics to create a three-dimensional numerical model of the internal flow field of a compressed air turboexpander. Researchers studied the variation in aerodynamic efficiency and wear of the turboexpander in relation to the turbine tip clearance and expansion ratio. The results found that, under the condition that the efficiency of the turboexpander is low, the optimal range of the tip clearance and the operating range of the turboexpander can be significantly reduced.
From Slow Growth to Leading Technology
As assistant director of the IET Dr. Chen Haisheng shared with China Science Daily, CAES technology originates from traditional gas turbine energy storage technology. During low energy use periods, the system’s electric motor will drive an air compressor to compress air and store it in a container, thereby converting electric energy into internal energy in the form of compressed air. During peak energy use periods, the compressed air will be released from the container and combine with a fuel in a combustor where it will ignite, driving a turbine that will generate power.
However, as Dr. Chen explained, traditional CAES energy storage technology relies on gas storage caverns, fossil fuels, and has relatively low efficiency, among other drawbacks.
IET Lead Engineer Ji Lu told reporters that IET has worked over 10 years to achieve breakthroughs in critical technologies for CAES systems of a scale of 1~10MW. In 2013, IET deployed a 1.5MW new model CAES demonstration project in Langfang, and in 2016 released the world’s first, and currently still only, 10MW new model CAES demonstration project in Bijie, Guizhou, with an efficiency rate of 60.2%. It is currently the highest efficiency CAES system in the world.
The new model CAES systems are characterized by three major technological innovations. First, the systems use thermal storage technology to capture and reuse the heat that is generated during air compression, thereby eliminating the need to burn fossil fuels to generate heat. Second, the system eliminates the need for a gas storage cavern by relying on liquefied compressed air or high-pressure gas during the compression stage. Third, the system uses highly efficient compression, expansion, and supercritical heat storage and heat transfer methods in an optimized system, thereby increasing the overall efficiency of the complete system.
As assistant researcher Dr. Wang Xing stated, “the turboexpander is the core power generation device of the system, its efficiency and operating characteristics have a decisive factor on the overall operations of the system.”
As researchers explained, their most recent research was focused on developing CAES systems that can meet the environmental conditions of China’s western regions, optimizing the turboexpander according to such conditions. Although western China possesses abundant wind and solar resources that make the region suitable for CAES systems, the high concentration of dust and particles in the air present dangers for turboexpanders, the core component of any CAES system.
As Wang Xing and other researchers discovered, increasing the clearance between turbine blades and the housing can decrease wear, though increasing such clearance also causes flow loss. The complex internal flow pattern of the turboexpander also means that the movement of dust will likewise be complex, necessitating thorough research into and proper design of the flow field to optimize the resistance to wear of the components. To solve these problems, researchers coupled the Navier-Stokes equation and the Tabakoff & Grant erosion model to create a three-dimensional gas-solid multiphase flow model of the turboexpander. The model was used to measure the extent of wear of the components. The results showed significant wear to the trailing edge of the guide vane, leading edge of the moving vane, the hub, and the casing. Researchers recommended measures such as increased filtration and the applications of an anti-wear coating to improve performance of the system.
According to Ji Lu, following intense foundational research and breakthroughs in critical technologies, IET completed development of its 1~10MW new model CAES system, creating “the world’s first, largest, and most efficient” system of its kind. Researchers spent 10 years bringing the technology to its current state, first beginning research in 2005, achieving technological breakthroughs in 2009, and establishing the first 10MW demonstration project in 2016.
In 2017, IET begin research into a 100MW-scale CAES system. Research of the prototype system is expected to be complete in 2020 and will have a rated efficiency of approximately 70%. Once complete, the demonstration project will be the largest scale and highest efficiency CAES energy storage station in the world. Developers hope that the project will help stimulate the growth of China’s CAES technology and industry.
The Trillion RMB Value Chain
What is the value of constructing energy storage systems, particularly CAES energy storage systems?
According to Dr. Chen, current electric power systems are host to five major value chains: raw materials, power generation, transmission, distribution, and use. “The electricity market has many of the same elements as a retail consumer market in that we have raw materials, production, shipping, distribution, and consumption, the only element that is missing is storage” said Dr. Chen.
As Dr. Chen explained, the electric power demand at the end-user side fluctuates frequently, while the peak and off-peak price gap continues to increase over time. To ensure needs are met, generation and grid infrastructure must be built which can handle the maximum peak demand. Power is then generated according to the real-time conditions and needs of the end-user side. During off-peak periods, generators are turned off or operate at a low load level, leading to a low utilization rate of generation capacity and grid capacity. Moreover, the dynamic balance of such a system is risky. For example, India, South Korea, the United States, and the United Kingdom have all experienced large-scale power outages in recent years. The addition of energy storage provides the system with a buffer that can act as an effective solution for minimizing the risk of power loss or shortage.
According to Dr. Chen, as of the end of 2018, China’s operational energy storage capacity totaled 31.2GW, close to 1.6% of the country’s total power installation, but lower than the average global total of 2.7%. According to International Energy Agency predictions, by 2050, China’s installed energy storage capacity will be above 200GW, approximately 10% to 15% of the country’s total installed power capacity. Growth of this size will lead to a trillion RMB industry.
Energy Storage: Supporting the Energy Revolution
Aside from the ability to help tackle fluctuations in the power load, energy storage is also a valuable tool for the support of renewable energy integration into the grid and the development of distributed energy resources.
According to Dr. Chen, energy storage technology can store the intermittent, unstable, and often unreliable energy produced by renewable power generators, releasing it later in a stable and controllable manner. Energy storage can also help limit curtailment from wind and solar generators, allowing energy that would normally go unused to be stored and sent to the grid at a later time.
Distributed energy is viewed as one component of the safe, efficient, and low carbon energy system of the future. However, in comparison to a large-scale grid network, distributed energy systems have many issues, such as large load fluctuations, system control difficulties, high fail rates, and others.
As Dr. Chen stated, energy storage technology can serve as a resource for load balancing and backup power, addressing many of the above issues by providing a reliable and stable energy source. Because of this, energy storage has been called the “supporting technology of the energy revolution.”
By acting as a voltage regulator, CAES can help turn unstable and low-quality renewable and distributed energy into high quality, usable energy, thereby providing huge benefit to other innovative new energy sources.
The Future of CAES
As Dr. Chen explained to reporters, development of CAES from the current stage will require continued focus on performance, demonstrations, and power pricing mechanisms.
First, increasing system performance also means decreasing costs. Large scale systems will be the trend for future CAES, and are also the pathway to high performance systems at low cost.
Second, there are currently only a few demonstrations projects for new model CAES systems, and those that exist are small in scale, unable to meet the needs for further technological development. Therefore, critical support is needed from government, private industry, and academic researchers to bring greater investment to new project demonstrations that will promote new technology applications and commercialization.
Finally, other large-scale energy storage technologies have not yet enjoyed the same two-part power price mechanism as pumped hydro storage. As Dr. Chen states, “If we can do more to show the benefit of CAES to the electricity system and create a reasonable power price system, we will be able to provide an effective stimulus to CAES and other new energy storage technologies that will support industry development and widespread applications.”
Author: Chi Han, China Science Daily Translation: George Dudley