Hybrid Energy Harvesting System - Energy Harvesting Autonomous Sensor Systems - page 160

TABLE 5.2
Characteristics of Indoor Energy Sources

Energy Source Characteristics Comments

Solar Intermittent Illumination from artificial lighting during

office hours

Wind Continuous Air circulation from air conditioner and

electric fans

Thermal Continuous/intermittent Thermal gradient between body, machine

heat, and ambient

Vibration Intermittent Vibration from machine and human

motion during walking, running, etc.

for visual function and air circulation for cooling purposes. During the nor-
mal operation of these electrical appliances and human movements, some
stray energies are generated for EH. The availability of the artificial energy
sources seen in Table 5.2 is largely dependent on the operational schedules of
the electrical appliances and the lifestyles of the human beings. Some energy
sources, such as solar and vibration, are only available according to the in-
termittent operating profile of the artificial lighting, machine operation, and
human movement. Conversely, the wind and thermal energy sources exist in
continuous form as the air conditioners and machineries of indoor industrial,
commercial, or biomedical environment are operated at a duty cycle close to
100% of the time.
Apart from the issue of availability of energy sources, another concern is
power harvesting throughput. The power densities of various EH technolo-
gies per unit square centimetre or cubic centimetre are recorded in
Table 5.3 . Under indoor conditions, it can be observed that all the artificial
energy sources become very weak. The power density of a solar panel, for
example, with efficiency of less than 10% under indoor solar irradiance of 10
W/m 2 cm 2 as compared to 10 mW/cm 2 under the outdoor

standard testing condition (STC). Table 5.3 shows that the average power that
can be harvested by all the artificial energy sources is 10 to 100 times lower
than that of the outdoor ambient energy sources. As such, these weak and

is only 100 W /

TABLE 5.3
Performance of Energy Harvesters under Indoor Conditions

Energy Power Densities

Harvester Indoor Condition Outdoor Condition

cm 2 at 10 W/cm 2 cm 2

Solar panel 100 W / 10 mW / at STC

W/cm 2 3.5 mW/cm 2

Wind turbine generator 35 at < 1 m/s at 8.4 m/s

at 5 ◦ C gradient at 30 ◦ C gradient W/cm 2 3.5 mW/cm 2 Thermoelectric generator 100

W/cm 3

Electromagnetic 4 at human motion-Hz

W/cm 3

generator 800 at machine-kHz