Battery Maintenance Free System Revolutionizes Air Quality MonitoringFluctuating Photovoltaic Cell Output Caused Headaches
As air pollution becomes an increasingly serious issue around the world*1, ascertaining the air quality*2 where you are is the first step to preventing negative health effects from atmospheric pollutants. The United States Environmental Protection Agency publishes Air Quality Index (AQI) reports for major US cities that quantify air quality on the basis of the concentrations of nitrogen oxides (NOx), ozone, carbon monoxide, particulate matter, and other pollutants*3. However, air quality monitoring systems typically require a lot of power to function, and the need to either connect them to power cable or replace their batteries periodically limits the locations in which they can be installed.
*1 According to the WHO, atmospheric pollution resulted in 3.6 million premature deaths in 2018.
*2 Quantified in terms of the concentrations of gases in and around buildings and structures.
The Challenge of Obtaining Stable Power from Photovoltaic Cells
Major semiconductor manufacturer Renesas Electronics (hereinafter Renesas) manufactures a gas sensor IC known as the ZMOD4510 which enables to measure the concentrations of gases that contribute to unhealthy outdoor air quality, and outputs AQI values based on measurements of ozone and NOx on the level of parts-per-billion.
By combining the ZMOD4510 with the ultra-low power firmware running selective Ozone and the ultra-low power RE microcontroller (MCU) that utilizes Renesas’ proprietary silicon-on-thin-buried-oxide (SOTB™) technology*4, Renesas also developed an Air Quality Monitoring system (AQM) for outdoor applications that can monitor AQI continuously using the minuscule currents generated by photovoltaic cells. However, while this innovation removed the need to lay power cables or replace batteries, fluctuations of output from photovoltaic cells during cloudy weather and rain would prevent the AQM from functioning reliably.
*4 SOTB Process: a proprietary transistor technology of Renesas that incorporates Silicon On Insulator (SOI) technology. (SOTB is a registered trademark of Renesas in the US and in other jurisdictions.)
Finding a Power Storage Device that Could Store Electricity from Energy Harvesting
The capacitors which are traditionally used to store power generated through energy harvesters, have high rates of self-discharge, making it difficult for them to store energy for an extended period. Conventional rechargeable batteries, on the other hand, are able to store energy for long periods, but their high resistance causes them to output low levels of current, making them unable to power AQMs. Renesas needed a storage device which can be used in harsh environments, can store the minuscule and fluctuating currents produced by photovoltaic cells, long-lasting, and requires minimal or no maintenance works.
Because the output of energy harvesters varies with the weather and other conditions, on their own, these devices cannot be used to reliably power sensor systems.
Capacitors’ high rates of internal discharge makes them difficult to store energy for extended periods.
Conventional rechargeable batteries present many issues, too. They are unable to power AQMs or store minuscule currents, and have short lives. The use of such batteries in mass-produced sensing systems also significantly adds manufacturing costs.