Earlier this year, a company called Betavolt made news by announcing a nuclear battery that can go on for 50 years before needing a recharge. Technology is marching further ahead. In the future, at least for small electronic devices, you wouldn’t ever need to charge the batteries — they would self-charge from (man-made) energy in the atmosphere. 

There is plenty of energy around us — only we don’t quite know how to tap it well. There is sunlight, which we are now harnessing. Then we have heat and vibrations — scientists are trying to figure out how to make use of them. However, the problem is that these energy sources are not available everywhere or always. 

Yet another ambient energy source is electromagnetic (EM) radiation. Now, EM is a broad spectrum, extending from radio waves at one end to gamma rays at the other, and with infra-red, visible light, ultraviolet and X-rays in between. While all these are energetic, scientists have trained their sights on radio waves — perhaps because they are now practically omnipresent, thanks to human activity. 

Radio frequency (RF) signals are generated by the millions of devices we now use all the time — including Wi-Fi routers, radio and television broadcasting stations, and mobile networks. You can capture these ambient RF signals and convert them into alternating current. The rest is then routine stuff — a rectifier converts the AC into direct current, which can go into a battery for use by devices such as wireless IoT sensors (especially those deployed in hard-to-reach areas), wearable electronics (including medical implants), and other appliances such as smart light switches and security systems in buildings. 

Effortless wireless

Dr Sumit Som, Director of Variable Energy Cyclotron Centre (VECC), a unit of the government’s Atomic Energy Department, in Kolkata, describes ‘radio frequency energy harvesting’ (RFEH) as a “promising technology for dynamic recharging of wireless devices”.

Dr Som explains that RFEH has “numerous distinct advantages” — it can work in any location with a strong RF signal, which is practically everywhere; it is not affected by lack of sunlight or weather conditions; and it can work just as fine in indoor spaces, without needing specialised transmitters. 

Since the batteries are dynamically recharged by RF, they can be small and, in turn, the electronic device can be smaller too. 

Above all, it is green — no carbon footprint. 

That said, it is important to know that RFEH is still a technology in the making, though there is little doubt it will be within reach in future. The fundamental challenge to be cracked is the ‘power conversion efficiency’. Researchers are stretching themselves to improve antenna and rectification efficiency. RFEH technology is getting better by the day but, by all accounts, it needs to improve more. Prof Manash Sarma of Gauhati University says in a scientific paper that researchers at his institute have developed a ‘transmission gate-based system’ that is “capable of generating output power at a low level of input with good conversion efficiency”. However, despite the “strong performance”, there is still “an opportunity for improvement”.

Connected devices

A look at various research papers shows that RFEH will prove to be a good technology for building smart cities, where there is a growing demand for connected devices and sensors. Powering IoT sensors is one area where RFEH will help big time. You can have millions of sensors without worrying about how to power them. A smart city would need sensors for monitoring air quality, ambient temperature and humidity, monitoring infrastructure (such as integrity of bridges and buildings), smart meters for utilities, regulating transport and parking, surveillance, waste management, and so on. 

In India, RF technologies spin out of the Department of Atomic Energy’s labs such as Bhabha Atomic Research Centre (BARC) and VECC. The latter uses RF in particle accelerators — namely to accelerate and control the motion of charged particles (such as protons). Thanks to its expertise with RF, VECC has developed quite a few societal applications for RF technologies. 

For example, RF can be used in drying agricultural produce. Here, RF systems generate electromagnetic waves that cause ‘dipole rotation’ in the water molecules in agricultural produce — the water molecules spin, producing heat, and vaporise. This method is said to be faster and more uniform than conventional air or sun drying. 

“RF technology is used in a variety of important fields and will always remain in high demand,” says Dr Som.