Design methodology of a hybrid micro-scale fuel cell-thin-film lithium ion source

Emerging ad-hoc wireless sensor nodes and other micro-scale applications demand long operational lives, small form factors, and total integration, which are next to impossible to fully achieve with conventional battery technologies. Efficient, power-moded, fully integrated systems inherently demand high peak-to-average power ratios (PAPRs), as in wireless sensor applications where telemetry is a power-consuming function with low duty-cycle operation. Lithium ion batteries (Li-Ion), while conforming to micro-scale dimensions and supplying moderate power densities, cannot store enough energy to sustain extended lifetimes, which is where fuel cells (FCs) excel. Although various control strategies for energy flow between batteries and FCs have been proposed in the past, none of them superimpose the severe constraints of a micro-scale system on the design, where volume, energy, and power are scarce and the performance of the MEMS FCs degrade with time. This paper presents a hybrid micro-scale MEMS FC-thin-film Li-Ion source and proposes a design methodology for the same wherein volume, energy, and power are optimized for peak-power and extended-lifetime performance. The FC is ultimately used to both charge and supply the load asynchronously, depending on the state of the load, while the Li Ion mostly functions as a power cache. System simulations of a multi-sensor wireless system load show and validate how peak-power, average-power, duty-cycle, and frequency performance are achieved and how they relate to lifetime.