  We apply a fluid-reservoir model to study the Age-of-Information (AoI) of
update packets through energy-harvesting transmitters. The model is closer to
how energy is stored and depleted in reality, and can reveal the system
behavior for different settings of packet arrival rates, service rates, and
energy charging and depletion rates. We present detailed results for both
finite and infinite transmitter buffers and an infinite energy reservoir, and
some indicative results for a finite reservoir. The results are derived for the
mean AoI in the case of an infinite transmitter buffer and an infinite
reservoir, and for the mean peak AoI for the remaining cases. The results show
that, similar to a system without energy constraints, the transmitter buffer
should be kept to a minimum in order to avoid queueing delays and maintain
freshness of updates. Furthermore, a high update packet rate is only helpful in
energy-rich regimes, whereas in energy-poor regimes more frequent updates
deplete the energy reservoir and result in higher AoI values.
