One of the emerging seed areas within the CMI program involves the development of energy storage technologies to complement existing efforts on carbon reduction, capture and storage during energy generation. Craig Arnold and colleagues have taken a approach involving the assessment and optimized usage of existing technology and the development of new technologies to meet emerging demands for energy storage that are compatible with modern forms of energy generation. Their focus is on the development of hybrid energy storage systems that are able to provide optimal levels of storage for different energy generation situations. Important considerations include energy and power density (energy and power per unit mass or volume), cycle life (how many times batteries can be recharged), capacity fade (decay in battery capacity over time), and general safety. Such issues are critical for stationary power applications as well as portable platforms such as electric vehicle, military, or consumer electronics applications.


Integrating existing storage technology with intermittent power sources

In the area of assessment and optimization, the Arnold Group has been working to detail the relevant metrics of energy storage, including the response time or power density (i.e. how fast a given technology can store energy and how fast it can release that energy) and the energy density (i.e. how much energy can be stored) for a number of existing technologies including pumped hydro, compressed air, mechanical flywheels, batteries, and supercapacitors. Given a better understanding of the benefits and limitations of existing technologies, the team has begun to examine methods of integrating multiple storage technologies that have the ability to work synergistically to provide more stable and higher efficiency storage.

In experimentally approaching this topic, attention has been focused on the relevant model system of integrating storage with wind and solar power generation. The integration of energy storage with alternative conversion technologies presents a number of important challenges to overcome. For one thing, fluctuations in output power have multiple time scales whereas most energy storage technologies have an optimal charging/discharging rate. Solar and wind exhibit large scale diurnal cycles which require many hours worth of storage to provide reliable off-grid power throughout a 24 hour period. But on top of this, there are seasonal cycles as well as short duration, large intensity intermittencies. For instance, in the case of wind, the output power scales with the wind velocity to the third power, thus relatively small gusts (or calms) can produce significant increases (or decreases) in output power over short times. Therefore, it is not only necessary to store excess energy in order to level out fluctuations, but it is necessary to optimize the energy storage technology so that it can function efficiently over the many different time scales that it will experience over its lifetime. A non-optimized or unmatched storage device leads to excessive loss and a decrease in charge storage.

The Arnold Group’s initial results on this topic have focused on short time storage (seconds to minutes) using electrochemical energy storage such as batteries and capacitors. This regime is critical for handling fluctuating output as well as filling a gap for longer time storage such as compressed air or pumped hydro to ramp up. The team’s experiments are performed at the laboratory scale but with an eye toward understanding the effects of scaling these technologies to larger sizes and capacities.

Arnold and colleagues find that for different electrochemical systems, the storage efficiency as defined by the amount of energy stored divided by the amount of energy generated depends on the power (rate of energy generation) as well as the state of charge in the system. Current data indicates an increase in efficiency with increasing power up to a maximum beyond which it rapidly decreases. These results imply that as one tries to integrate electrochemical storage with a fluctuating source, the efficiency of the process will similarly fluctuate if only a single, monolithic energy storage device is used. Additional studies are ongoing to assess different chemistries and voltage/current regimes.


Increasing the lifetime of energy storage systems

The second key challenge for energy storage integration is the lifetime of the storage devices. Currently most alternative energy technologies have lifetimes of many years, e.g., solar cells are manufactured to last more than 20 years on average. In contrast most traditional electrochemical storage systems last significantly shorter and over time, they gradually lose capacity. Therefore, in order to provide reliable storage, it is necessary to develop systems with high integrated capacity, i.e. those that can last for long times without significant loss in capacity. As it turns out, this is not only affected by the fundamental materials and chemistry of the storage system, but how the device is controlled and implemented in the field. For instance, deep discharge/ overcharge or rapid charge/discharge can lead to degradation in the battery, causing capacity fade and premature failure.

The Arnold team’s efforts have focused on the effects of mechanical degradation to battery materials during the charging/discharging cycles. In some cases, the mechanical degradation is obvious, such as in plating-type systems where dendrite growth leads to shorting of the battery or fracture and failure of the electrodes. However in other cases, more subtle effects that occur during electrochemical cycling can lead to damage. For instance, in lithium-ion batteries during charging and discharging, lithium ions pass from one electrode to the other through the electrolyte and porous polymer separator, and are intercalated, or inserted, into the crystal lattice at the electrode. This causes expansion and contraction which leads to mechanical fatigue and can contribute to significant dynamic stress on the internal components of the battery.

The researchers have found that the build up of stress can result in significant changes to the separator materials, such as pore closure, which has a direct influence on the cycle life and capacity of the system. By improving the mechanical stability of these materials, they seek to improve the cycle life of existing batteries and supercapacitors, thereby decreasing the need for replacements and decreasing the overall cost for implementation.