Hybrid and Dual Chemistry Battery for Energy Storage Applications

Abstract

Energy storage is predicted to play an increasing role in a renewable energy future. There is a wide range of storage technologies available, with different technical characteristics, to assist the renewable transition. Hybrid options are also possible to improve the technical and economic performance of storage systems. The technology proposed in this work is the directly connected hybrid lead-acid and Li-ion battery storage system, which makes use of Li-ion's high-performance characteristics and the low-cost, lower specifications of lead-acid cells.

The work presented in this thesis answers the following key questions, which also summarise the novelty of this research:
•What are the hybrid characteristics of directly coupled, hybrid Li-ion (NMC) and lead-acid (VRLA) systems?
•Can the instantaneous hybrid behaviour be modelled using equivalent circuits?
•How do hybrid battery systems perform over time in real-world applications?
•What storage applications are best suited for hybrid lead-acid and Li-ion systems, and what are the associated techno-economic parameters?

To answer these questions, I have built and tested hybrid systems, developed battery models, and monitored the first commercial hybrid lead-acid and Li-ion system installed in the UK. The lab analysis, done for five domestic-size hybrid systems of 24V and 48V, provides details about the system efficiency and energy flows in a hybrid configuration. The round-trip efficiency drops from a maximum of around 94-95% in the first charge/discharge stages, when only Li-ion strings are active, to 82-90%, depending on the lead-acid strings' depth of discharge.

The most important parameters in the round-trip efficiency function are the kWh capacity ratio between the two chemistries, the energy available and the charge/discharge current. The energy transfer between the strings, caused by the transient currents, is negligible when only Li-ion is active, and increases with the lead-acid depth of discharge.

A hybrid equivalent circuit battery model was built to predict the experimental results. The model approximates the dynamic effects of energy transferred between the strings with an accuracy of 90%, except when the lead-acid strings are discharged to 10-30% depth of discharge. The overall efficiency, the total energy discharged, and the Li-ion energy available for independent cycling can be predicted with above 90% accuracy.

The demonstrator project data shows that the hybrid system is stable over time, the average operating round-trip efficiency in real-world applications is 90%, and the lead-acid degradation in hybrid configurations is around 1.3% per year. The energy transfer between the strings, due to different dynamic time constants of the two chemistries, depends on the charge stopping points, and to a lesser extent, on the discharge interruption depth of discharge. For the analysed system, the average Li-ion to lead-acid energy transfer during the charging process is 13 kWh, 5.5% of the total charged energy. The average lead-acid to Li-ion energy transfer during discharge is 5.2 kWh, 2.47% of the total discharged energy.

The final techno-economic analysis shows that the hybrid battery system can be used for renewable storage applications, dedicated to serving residential, commercial, industrial, and off-grid EV charging load profiles, but it is not suitable for front-of-the-meter applications, which operate in frequency response or balancing markets. The cost-saving potential for using hybrid Li-ion and lead–acid systems varies with the load profile, storage operational strategy, and renewable overgeneration design choices. The total system cost reduction can be up to 26.1% when compared with full Li-ion solutions. However, for the cost-optimised energy solutions, the CAPEX reduction using hybrid storage is between 1.4% and 12.7%, depending on the percentage cost of storage as a share of the total energy system. The minimum cost ratio between Li-ion and lead-acid, beyond which the hybrid storage system is no longer justified, is between 1.1 and 1.5.

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Identifiers

ORCID for Andrei - Dascalu: orcid.org/0000-0003-3072-1864

ORCID for Andrew Cruden: orcid.org/0000-0003-3236-2535

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

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