Solar-Supercapacitor Harvesting System Design for Energy-Aware Applications Moeen Hassanalieragh, Tolga Soyata, Andrew Nadeau, Gaurav Sharma University of Rochester, Dept. of Electrical and Computer Engineering, Rochester, NY 14627 {m.hassanalieragh,tolga.soyata,andrew.nadeau,gaurav.sharma}@rochester.edu Abstract—Supercapacitors are an emerging choice for energy buffering in field systems and their use in solar-powered field systems has been the focus of recent research. Supercapacitors offer advantages compared to rechargeable batteries for energy buffering due to their energy charge/discharge efficiency as well as environmental friendliness. Additionally, a supercapacitor- based system permits an energy-aware operation due to its superior energy-predictability. This paper describes a circuit for solar/supercapacitor energy harvesting, which includes power and voltage measurements, voltage regulation circuit and RS232 communication capability with the host embedded processor. A complete system is prototyped and its operation is discussed in terms of design parameters. I. I NTRODUCTION Recent research focused on powering Wireless Sensor Net- works [1] and field systems using supercapacitors [2] due to their potential in providing a batteryless power supply for embedded systems, thereby yielding a much longer opera- tional lifetime. Although these two type of systems do not take advantage of one of the most important high power density feature of supercapacitors, which is the backbone of other high-powered applications such as industrial elevators or electric vehicles [3]–[5], another equally important feature of the supercapacitors are taken advantage of by both of these applications : energy efficiency and their superior energy pre- dictability by observing the supercapacitor terminal voltage, V SC and predicting the remaining energy as E = 1 2 CV 2 SC . Although a rich body of circuit references exist for the building blocks of an energy harvesting system, a complete harvesting system design for incorporating energy awareness into higher system levels is not readily available which details relevant design and runtime issues. In this paper, we intro- duce a microcontroller-based energy harvester design which receives its energy from multiple solar panels, harvests energy by using a DC-DC converter and stores the harvested energy in two blocks of supercapacitor. This battery-less harvester is intended to power field systems with an embedded CPU, such as Nexus 7, with a target overall system power consumption of 0.5–10W. We demonstrate results on a prototype we built, shown in Figure 1. The primary goal of our design is to create a solar harvesting platform that not only buffers energy to sustain operation, but provides the embedded processor with enough information to make intelligent decisions to take advantage of remaining energy. We detail multiple design issues relevant to such a field system harvester and elaborate on each issue. Fig. 1. Our prototype solar/supercapacitor harvesting system along with supercapacitor reservoirs and the embedded processor. This paper is organized as follows: In Section II, we provide background information on solar panels and supercapacitors. Section III is where we introduce our proposed system, in Section IV details of the harvesting circuit is presented followed by evaluations on our prototype in Section V. We draw conclusions in Section VI. II. BACKGROUND AND RELATED WORK A. Solar Energy In recent years photovoltaic (PV) cells have gained much interest to increase the autonomy of embedded systems. The output characteristics of a PV cell varies non-linearly with en- vironmental conditions such as temperature and irradiation [6], [7]. Also, the power gained from a PV cell greatly depends on its operating point, i.e we need to keep it at its optimum operating point by demanding sufficient amount of energy. When the demanded current from a PV cell is high, its terminal voltage drops to a very small value. This current is denoted by the short circuit current I Solar = I SC . When there is no current demand from the PV cell, its terminal voltage increases to the open circuit voltage. At both of these extreme cases no