Abstract
In this study, an energy harvesting chip was developed to scavenge energy from artificial light to charge a wireless sensor node. The chip core is a miniature transformer with a nano-ferrofluid magnetic core. The chip embedded transformer can convert harvested energy from its solar cell to variable voltage output for driving multiple loads. This chip system yields a simple, small, and more importantly, a battery-less power supply solution. The sensor node is equipped with multiple sensors that can be enabled by the energy harvesting power supply to collect information about the human body comfort degree. Compared with lab instruments, the nodes with temperature, humidity and photosensors driven by harvested energy had variation coefficient measurement precision of less than 6% deviation under low environmental light of 240 lux. The thermal comfort was affected by the air speed. A flow sensor equipped on the sensor node was used to detect airflow speed. Due to its high power consumption, this sensor node provided 15% less accuracy than the instruments, but it still can meet the requirement of analysis for predicted mean votes (PMV) measurement. The energy harvesting wireless sensor network (WSN) was deployed in a 24-hour convenience store to detect thermal comfort degree from the air conditioning control. During one year operation, the sensor network powered by the energy harvesting chip retained normal functions to collect the PMV index of the store. According to the one month statistics of communication status, the packet loss rate (PLR) is 2.3%, which is as good as the presented results of those WSNs powered by battery. Referring to the electric power records, almost 54% energy can be saved by the feedback control of an energy harvesting sensor network. These results illustrate that, scavenging energy not only creates a reliable power source for electronic devices, such as wireless sensor nodes, but can also be an energy source by building an energy efficient program.
Highlights
The coming decade will see a widespread deployment of sensors throughout a variety of environments
The wireless sensor node can sense the harvested energy lowering the threshold by current sensing resistor (CSR), as shown in Figure 1(b).During this period, the current status and measured data can be stored in the non-volatile flash memory of the signal processor
The energy harvesting chip was taped out in standard CMOS processes, and the microstructures were fabricated for the chip embedded transformer by post MEMS process
Summary
The coming decade will see a widespread deployment of sensors throughout a variety of environments. Most energy harvesting systems, either commercial products [16] or experimental prototypes [17,18,19,20], developed for mobile computing and WSNs play the role of auxiliary power sources for extending operational lifetime and rechargeable batteries are still necessary for the devices In such a case, battery lifetime and charging from an unsteady energy source, such as indoor lighting, limits the number of allowable charge cycles, which are considerable issues facing the applications. The power supply module consists of a solar cell, a capacitor, and an energy harvesting chip to provide a battery-less solution, and it is expected to be a sustainable and renewable power source for WSNs. The sensor nodes equipped with the chip based power supply were fabricated for performance tests, including sensor accuracy, response time, and signal deliveries of wireless communication. The WSNs were employed for air conditioning control in a convenience store, and the effects were discussed in detail in order to illustrate the potential application and the advantages of an energy harvesting power source
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