Optimal sizing and control of supercapacitors for cost-effective hybridization of battery-alone energy storage systems

Abstract

With growing effects of global warming, batteries have a crucial role to play in supporting global effort to reach carbon neutrality. However, batteries remain prone to early failure. Operational causes of premature deterioration are high current intensity, abrupt current fluctuations, repetitive charge-discharge cycles, deep discharge, and thermal stress. An effective way to mitigate stress sustained by the battery, and therefore increase its service life, is association of batteries and supercapacitors (SC) into battery-SC hybrid energy storage systems (HESS). Despite considerable interest enjoyed among the research community, the adoption of battery-SC HESS is still marginal in practice due to the relatively high cost of SC. As a result, most batteries are still implemented as stand-alone energy storage systems, or battery-alone energy storage systems (BESS).

Given the continuous advancement in the manufacturing economics of SC, in one hand, and the market domination of BESS, in the other hand, close attention should be paid to the upgrade of BESS into battery-SC HESS. The study of contemporary literature shows that such a conversion has been scarcely investigated before. Most control strategies reported in the past were also found unsuitable for retrofit applications, due to significant modifications required on the existing control infrastructures. In view of technical, economic, and environmental implications of such modifications, this may raise concerns for stakeholders.

Regarding sizing of battery-SC HESS, it was found that previous investigations focusing on their control used arbitrary SC size to test the performances of controllers. On the other hand, studies interested in the economic sizing of this HESS usually failed to fully assess the costs and benefits over the entire lifespan of the energy system. Moreover, other shortcomings such as a fixed service life for each ES equipment regardless of the operating conditions, overlooking of certain cost components and economic parameters (inflation and escalation rate of electricity price) were also found in some studies. This usually results in misleading assessments of costs and benefits associated with battery-SC HESS.

In an attempt to address the above gaps in the current literature, this thesis presents two control strategies aimed at achieving trouble-free retrofit of BESS with SC, a preliminary investigation on the impact of spatial arrangement on the thermal stress sustained by the battery and SC cells, and finally an SC sizing model for cost-effective hybridization of BESS.

Primarily designed for hybrid renewable systems (HRS) originally equipped with BESS and controlled by a receding horizon control (RHC), the first control scheme consists of a hierarchical RHC of the SC-retrofitted HRS. Depending on the characteristics of the power management unit (PMU) running the previous RHC scheme, no or little modifications are required to integrate the SC into the existing infrastructure. Besides the reduction in electrical stress sustained by the battery, the proposed control framework also increases the amount of energy supplied by intermittent renewable resources and the power stability at the point of common coupling (PCC).

The second control model is built around a fuzzy logic controller unit to assist in retrofitting any BESS with SC. Thermoelectric management of batteries is realized through sharing of low-frequency current components between the two ES devices, besides the SC’s full supply of high-frequency current components. Improvement in battery service life is demonstrated. Compared to the previous controller, specially designed for retrofitting of BESS-equipped power plants controlled by RHC, this thermoelectric controller is intended for implementation on any existing BESS.

The preliminary study conducted on the influence of spatial arrangement of battery-SC HEES cells on the thermal management of batteries demonstrates that the proximity between a battery cell and an SC cell can effectively contribute to the cooling process of batteries, thanks to thermal interaction between them. Accordingly, adequate arrangement of ES cells can offer a passive assistance to the above controller in achieving thermoelectric management of batteries.

Finally, the thesis introduces a life cycle cost (LCC)-based optimization model that assists in properly sizing SC intended for retrofit in existing BESS. The possibility offered to stakeholders to take informed decisions about the economic opportunity of such an upgrade is also demonstrated.