Introduction - Energy Harvesting Autonomous Sensor Systems - page 24

Current
Sensor

Accelerometer

Power Controller

Wireless
Transceiver

Supercapacitor

Regulator

Step-Up
Converter

Sensor

Te l o s

450 mW Solar Cell

Solar Panel

100 F Supercapacitor

Lithium Polymer Battery

(a) Prometheus sensor node (b) Everlast prototype system

Harvesting Unit

Wind
Speed
Sensor
Input

Reservoir
Capacitors

Light Sensor

Wind
Generator

Control/Charger
Unit

Light
Sensor
Input

Battery

Solar
Panel

Solar Panel

Power Output

(c) AmbiMax solar panel with light
sensor

(d) AmbiMax board with supercapacitors

FIGURE 1.17
Examples of a solar EH system. (From X.F. Jiang, J. Polastre, and D.E. Culler, “Perpetual envi-
ronmentally powered sensor networks,” 4th International Symposium on Information Processing in
Sensor Networks (IPSN) , pp. 463-468, 2005 [33]; F.I. Simjee and P.H. Chou, “Efficient charging of
supercapacitors for extended lifetime of wireless sensor nodes,” IEEE Transactions on Power Elec-
tronics , vol. 23, no. 3, pp. 1526-1536, 2008 [34]; C. Park and P.H. Chou, “AmbiMax: Autonomous
energy harvesting platform for multi-supply wireless sensor nodes,” 3rd Annual IEEE Communi-
cations Society on Sensor and Ad Hoc Communications and Networks (SECON) , vol. 1, pp. 168-177,
2006 [35].)

supercapacitor. This means that, especially for low supercapacitor voltages,
the solar panel generates much less power than its maximum power P MPP .
Hence, an efficient solar energy harvesting (SEH) system should be able
to adapt the electrical operating point of the solar panel to the given light
condition so that P MPP is always maintained. For solar panels of a few square
centimetres, particular care has to be taken in order not to waste the few mil-
liwatts generated by the solar panel. Everlast [34] is an example that uses the
fractional short-circuit current technique (see Figure 1.17b ) to achieve maxi-
mum power point tracking P MPPT . This technique is easy, simple, and cheap to
implement. The voltage V MPP is estimated based on the open-circuit voltage
V oc of the solar panel, which is measured periodically by momentarily shut-
ting down the power converter that is connected to the solar panel. However,
the drawback with this technique is the transient drop of power during that
time where no energy is harvested.