Introduction - Energy Harvesting Autonomous Sensor Systems - page 17

10 4

Advanced Flywheels

5

2

Supercapacitors

10 3

Conventional
Flywheels

Methanol

5

H 2 Ice

Gasoline

Ni/Zn Lithium Ion

2

10 2

5

Zn-Air

LiM/FeS 2

Pb-Acid

2

H 2 Fuel Cell

10

5 10 2
Specific Energy, Wh/kg

10 3 5 10 2 2 5 23

FIGURE 1.13
Ragone plot for comparing the energy storage technologies and their power density versus energy
density characteristics. (From J.W. Tester, “Energy transfer and conversion methods,” Sustainable
Energy Lecture Notes, Topic on Energy Storage Modes, MIT, Cambridge, MA, 2005 [22].)

in Figure 1.13 , the peak power densities of supercapacitors are well above
1000 W/kg, whereas the power densities of all types of batteries are in the
range of 60 to 200 W/kg, and fuel cells are even lower, which is below 100
W/kg. Hence, for burst power operation, supercapacitors are a better choice
than batteries and fuel cells. Conversely, batteries have much higher energy
storage capacities than the supercapacitors. This means that batteries can de-
liver electrical power for a longer period of time as compared to supercaps.
Referring to Figure 1.13 , it can be seen that the peak energy densities of all
types of batteries are in the range of 20 to 200 Wh/kg, whereas the power den-
sity of a supercap is below 10 Wh/kg. For sustaining the extended operational
lifetime of wireless sensor nodes, solely relying on a supercap might not be
suitable due to its very low energy density as compared to the rest of the en-
ergy storage devices. Research to increase the energy storage density of both
batteries and supercap has been conducted for many years and continues to
receive substantial focus [24]. While these technologies promise to extend the
lifetime of wireless sensor nodes, they cannot extend their lifetime indefinitely.
Among these nonrenewable energy systems or sources, the rechargeable/
alkaline battery is one of the most popular methods for powering the great