An isolated microgrid needs a battery energy storage system able to deliver a…

PHYSICS · Multi-part · Fermi University Physics · Electricity, Magnetism & Electromagnetism · Earth, Climate & Energy Physics · Electric current and circuits

Energy from Physics, Ch Storing energy: cells, rotors, air, heat and hydrogen

Problem

An isolated microgrid needs a battery energy storage system able to deliver a constant

5\,\mathrm{MW}

of electrical power for

3\,\mathrm{h}

during a supply interruption, a delivered energy of

15\,\mathrm{MWh}

. ((a)) Real lithium-ion packs, once packaging and thermal management are included, deliver about

180\,\mathrm{Wh\,kg^{-1}}

(against

376\,\mathrm{Wh\,kg^{-1}}

of active material for the bare cobalt oxide-graphite couple). Estimate the mass of pack needed to supply the required energy. ((b)) The pack is built from cells rated

3.7\,\mathrm{V}

open-circuit,

4.0\,\mathrm{Ah}

, with internal resistance

15\,\mathrm{m\Omega}

each. Discharging the pack fully over

3\,\mathrm{h}

corresponds to a

C/3

rate. Find the discharge current drawn from each cell and the pack's round-trip efficiency at this rate, taking the charge and discharge currents equal. ((c)) Hence find the total electrical energy that must be drawn from the grid to recharge the battery after it has supplied the

15\,\mathrm{MWh}

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