A Hybrid Energy Storage System comprising a Small-Scale Compressed air Energy Storage System and a Battery

Abstract

The thesis investigates the control and component sizing of a stand-alone hybrid alternative energy storage system (HES) comprising a small-scale compressed air energy storage (SS-CAES) and a battery for renewable energy application such as a photovoltaic (PV) system. These systems can be considered as an eco-friendly power generation system since both energy source and storage systems have the benefit of having a small environmental footprint.

The thesis starts with developing the simulation models of an SS-CAES system and its maximum power point tracking (MPPT) controller. MPPT speed controller was designed by using state-space averaging and small signal analysis. The controller was also compared with a maximum efficiency tracking (MEPT) controller. The results show that the maximum power point (MPP) is close to the maximum efficiency point and the loss of efficiency is small around 1-3%. But the MPP is much easier to implement and less sensitive to parameter uncertainty.

The MPPT controller shows a good performance to control the SS-CAES operating at the MPP. However, the SS-CAES has a very narrow maximum efficiency peak. A significant drop of efficiency results from a small deviation from the MPP. The battery was added to resolve this issue by buffering fast load changing while enabling the SS-CAES to operate at the MPP. A bidirectional converter is used to connect the battery to the load and maintain a constant output voltage; a cascaded PI voltage controllers is used. The SS-CAES combining with the battery is so called the hybrid system. The hybrid system is compared to a SS-CAES only system in constant voltage mode. The output power of the proposed hybrid system is matched with the load demand perfectly and the output voltage reaches the reference faster than one of the SS-CAES. By using the MPPT controller, the efficiency of the SS-CAES in hybrid system is greater than that in voltage mode.

A typical house in the Southern region of the UK is used as a case study to determine the component’s sizes of the HES and PV systems under two scenarios: the HES only system supplies the power to the load demand for a day and the HES system connected in parallel to the PV system supplies the power to the load demand. A sizing method is proposed by using the difference between the generated and demand powers during a day. The criteria is that the excess energy is greater than the deficit one. For the HES only system supplies the power to the load demand, the difference of total excess and deficit energies is considered to determine the power rate of the air motor which is available in commercial companies, while the battery size is estimated from the deficit. The results show that the 270 W of the air motor and the 1500 Whr-battery were the selected sizes, which generate sufficient power for this demand and meet the criteria.

For another scenario, the sizing method mentioned above is used to determine the components’ sizes of the whole system. 41 PV panels are the selected number, which provide the total excess energy greater than the deficit one while it supplies the load demand. The excess energy is stored in the 2.8 m3 air tank within 7 hour at 20 bar by the 720 W of compressor. The deficit one is used to determine the air motor size, which is used when the PV is unavailable. The 353.8 W of the air motor is selected to supply to the load for 17 hours. In case of peak demand, the 900 Whr-battery is required since the PV and CAES system are unable to meet the load. The sizing method developed in this work can be used as the guideline for other load profile, e.g. the shop or office building.

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ORCID for Suleiman Sharkh: orcid.org/0000-0001-7335-8503

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