= Solution
A <battery> stores electrochemical free <energy>. Representative pack specific <energy> is of order $0.1$-$0.25$ kWh/kg, with electrical <round-trip efficiency> roughly $85$-$95\%$. It responds rapidly and can be modular, but <energy> capacity requires more active material, and ageing depends on <time>, <temperature> and cycling.
<Pumped storage> stores <gravitational potential energy>. At a $400$ m head with $89\%$ generation efficiency, useful <energy> is $\eta gh\simeq0.97$ Wh/kg of water. Its low mass-specific <energy> is acceptable when inexpensive water and large reservoirs are available. Storage <volume> and generator rating can be chosen substantially independently, making multi-hour service practical, but suitable sites and civil works are major constraints.
<Hydrogen> stores chemical <energy>, with a lower heating value around $33$ kWh/kg of <hydrogen> before allowing for tank <mass>. <Electrolysis>, compression or other storage processing, and conversion back to electricity can reduce electrical <round-trip efficiency> to roughly $25$-$45\%$, depending on the route. Separate tanks or caverns can make long-duration <energy> capacity less expensive than adding electrochemical cells, but containment, infrastructure, leakage and conversion equipment matter.
The appropriate comparison includes <power> rating, <energy> duration, cycle count, system <mass> or area, and whether <heat> or electricity is the final useful product. A large fuel heating value does not imply a high electricity-storage efficiency.
Back to article page
