Off-design performance and operation strategy of expansion process in compressed air energy systems

As a way of reducing environmental pollution and emission of greenhouse gases as well as enhancing sustainability, development of alternative renewable energy sources have been encouraged to reduce the overdependence on fossil fuels. This has led to the development of several renewable energy sources such as solar and wind energy. Interestingly, storage of the generated renewable energy has, however, remained the biggest challenge in the sector due to unreliable storage technologies. Therefore, the development of more efficient energy storage technologies is highly desirable.

Among the available energy storage techniques, compressed air energy storage that operates under off-design conditions is generally durable, cost-effective and flexible and thus a promising storage facility. Recent studies have shown that environmental conditions like pressure and ambient temperature can consequently result in the off-design operation of the system. Alternatively, in order to optimize the energy storage of the compressed air systems, a lot of emphasizing has been on the effects of the dynamic characteristics. Unfortunately, the effects of the performance of the components constituting the off-design systems as well as the optimizing system regulation methods have not been fully explored to date.

To this note, Chinse Academy of Sciences researchers led by Professor Haisheng Chen from the Institute of Engineering Thermophysics recently investigated the multistage expansion process involved in thermal storage-based compressed air energy storage systems. In particular, off-design performance and characteristics, as well as the optimization methods, were further explored. The work is currently published in the research journal, International Journal of Energy Research.

In brief, the research team commenced their experimental work (Fig.1) by first exploring the off-design performance of the compressed air energy storage systems. Next, an off-design model based on the multi-stage expansion of the thermal-based compressed air energy storage system was developed. Fundamentally, two main operation models, equal power ratio and optimizing variable stator vane rotation angle, were utilized. The operation of the systems under the two aforementioned conditions were analyzed and compared. Eventually, the relationship between some of the parameters including inlet pressure mass flow rate ratio and was examined.

The author observed that the thermal storage temperature and the mass flow rate ratio linearly increased with the output power ratios. This was for a particular inlet pressure for both the equal power ratio and variable inlet guide vanes operating modes. However, at larger mass flow ratios, optimizing variable stator vane rotation angle operation exhibited a significant increase in energy efficiency as compared with the equal power ratio mode. Consequently, at larger mass flow ratios, optimizing variable stator vane rotation angle operation mode exhibited remarkable variation in the expansion ratio with opposite changing trends. In addition, for variable stator vanes, a stronger linear relationship was noted between the rotation angle and the mass flow rate for a given inlet pressure.

In summary, for the first time Huan Guo and colleagues successfully presented the off-design performance and optimization strategies using two operation modes. The various relationships were obtained through a polynomial fitting form. In general, the study will advance renewable energy storage technologies for sustainability.