18-12-2012, 04:32 PM
Design of a Solar-Harvesting Circuit for Batteryless Embedded Systems
Design of a Solar-Harvesting Circuit.pdf (Size: 1.49 MB / Downloads: 81)
Abstract
The limited battery lifetime of modern embedded systems
and mobile devices necessitates frequent battery recharging
or replacement. Solar energy and small-size photovoltaic (PV)
systems are attractive solutions to increase the autonomy of
embedded and personal devices attempting to achieve perpetual
operation. We present a batteryless solar-harvesting circuit that
is tailored to the needs of low-power applications. The harvester
performs maximum-power-point tracking of solar energy collection
under nonstationary light conditions, with high efficiency
and low energy cost exploiting miniaturized PV modules. We
characterize the performance of the circuit by means of simulation
and extensive testing under various charging and discharging
conditions. Much attention has been given to identify the power
losses of the different circuit components. Results show that our
system can achieve low power consumption with increased efficiency
and cheap implementation. We discuss how the scavenger
improves upon state-of-the-art technology with a measured power
consumption of less than 1 mW. We obtain increments of global
efficiency up to 80%, diverging from ideality by less than 10%.
Moreover, we analyze the behavior of supercapacitors. We find
that the voltage across the supercapacitor may be an unreliable
indicator for the stored energy under some circumstances, and this
should be taken into account when energy management policies
are used.
INTRODUCTION
THEINTEREST in supply circuits that harvest energy from
the surrounding environment for powering embedded systems
has been increasing over the last years [1]–[4]. Thanks
to the progress in low-power design, research has greatly reduced
the size and the power consumption of distributed embedded
systems, and the autonomy of these systems can be
further increased by energy-harvesting techniques. Nowadays,
small solar panels suffice to ensure continued operation, and
several photovoltaic (PV) harvesting circuits have been recently
proposed for this purpose [5], [6].
RELATED WORK
Several techniques for the MPPT of PV arrays have been presented,
and the number of proposed implementations has grown
significantly in recent years. Techniques vary in many aspects,
such as complexity, cost, or accuracy of the tracking method.
Large-scale PV power systems are out of the scope of this paper,
but a detailed survey with the great majority of articles presented
on MPPT can be found in [12] and [25].
Concerning energy harvesters using small and microscale PV
modules, [11] presents a cost-effective MPPT system that can be
directly integrated onto solar arrays. The authors focus on the
issue that it is more cost effective to design high-efficiency lowpower
MPPT systems in order to scale down the PV array and
storage devices, resulting in a lower cost system that is suitable
to be utilized on wider application scenarios (e.g., distributed
embedded systems and WSNs).
SUPERCAPACITOR ANALYSIS
The occasionally unexpected behavior of supercapacitors and
its influence on the performance of an energy scavenger have
never been investigated in the context of PV harvesters for lowpower
embedded systems. In fact, supercapacitors are usually
employed in the power supplies of the driver and logic circuits
to allow them to operate in the absence of the primary power
source. Conventional ESDs, such as batteries and aluminum
electrolytic capacitors, must often be replaced during the lifetime
of a product. Supercapacitors do not suffer from aging
effects, and they do not experience irreversible chemical reactions.
Moreover, they do not suffer from dry-up problems, unlike
aluminum electrolytic capacitors. These features allow supercapacitors
to fill the gap between conventional capacitors and batteries
and to be an attractive solution as energy reservoirs for
low-power distributed embedded systems.