RF Energy Harvesting for Ubiquitous, Zero Power Wireless Sensors

Table 1

Comparison between available ambient sources [4].

Energy sources/parameters	Solar energy	Thermal energy	Ambient RF energy	Piezoelectric energy
Power density	100 mW/cm^2	60 μW/cm^2	0.0002–1 μW/cm^2	Vibration	Push button
				200 μW/cm^3	50 μJ/N
Output	0.5 V (single Si cell); 1.0 V (single a-Si cell)	—	3-4 V (open circuit)	10–25 V	100–10000 V
Available time	Daytime (4–8 Hrs)	Continuous	Continuous	Activity dependent	Activity dependent
Weight	5–10 g	10–20 g	2-3 g	2–10 g	1-2 g
Pros	Large amount of energy; well-developed technology	Always available	Antenna can be integrated onto frame; widely available	Well-developed technology; light weight	Well-developed technology; light weight; small volume
Cons	Needs large area; noncontinuous; orientation issue	Needs large area; low power; rigid and brittle	Distance dependent; depending on available power source	Needs large area; high variable output	High variable output; low conversion efficiency