Alessandro Liberale

Single-Variable Optimization Method for Evaluating Solar Cell and Solar Module Parameters

This paper presents a simple and accurate method to assess the single-diode model parameters of an illuminated and dark solar cell (SC) and its extension to a photovoltaic solar module (PVSM). The proposed method is based on the acquisition of experimental data related to the v -i characteristics of illuminated or dark SC/PVSM at fixed climatic conditions. The method presented in a previous work is generalized, the effects of measurement uncertainty of the results are discussed, and its application to different technologies are shown. A micropower monocrystalline SC, a monocrystalline SC, a polycrystalline PVSM, and an amorphous PVSM have been characterized using direct insulation in an outside environment with a correlation coefficient up to the noise limit.

Microbial fuel cells for direct electrical energy recovery from urban wastewaters.

Application of microbial fuel cells (MFCs) to wastewater treatment for direct recovery of electric energy appears to provide a potentially attractive alternative to traditional treatment processes, in an optic of costs reduction, and tapping of sustainable energy sources that characterizes current trends in technology. This work focuses on a laboratory-scale, air-cathode, and single-chamber MFC, with internal volume of 6.9 L, operating in batch mode. The MFC was fed with different types of substrates. This study evaluates the MFC behaviour, in terms of organic matter removal efficiency, which reached 86% (on average) with a hydraulic retention time of 150 hours. The MFC produced an average power density of 13.2 mW/m3, with a Coulombic efficiency ranging from 0.8 to 1.9%. The amount of data collected allowed an accurate analysis of the repeatability of MFC electrochemical behaviour, with regards to both COD removal kinetics and electric energy production.

An Interface Circuit for Low-Voltage Low-Current Energy Harvesting Systems

In this paper, we present an interface circuit designed to match low-power and low-voltage harvesters to generic electronic loads. The interface circuit consists of a power management unit, a supply regulation unit, and an energy storage module. The electric power delivered by the harvester is stored in a low leakage capacitor, converted to match the load electrical characteristics, and used cyclically to supply the load during fixed time-windows. The interface circuit is compatible with dc harvesters with a current of at least 2 μA at 0.5 V and exhibits an efficiency of 65% in the 1 μW-1 mW range. The supply regulation unit is a two-stage, self-starting boost circuit that steps-up the 0.5-V input voltage to 3 V. To test the interface circuit, an autonomous wireless sensor node has been realized; it exploits the little electric power delivered by a 385 μm × 245 μm photovoltaic harvester to sense and transmit information about the environment wirelessly. The harvester is implemented with a custom 0.35-μm BCD SOI chip. The system has been designed to be low cost, fully autonomous and smaller of 9 cm3.

Theoretical and Experimental Analysis of an MPP Detection Algorithm Employing a Single-Voltage Sensor Only and a Noisy Signal

In this paper, a maximum power point (MPP) detection algorithm for photovoltaic (PV) systems is introduced, which uses the experimental information obtained from a single-voltage sensor, measured on a capacitor load, either linked at the output of a solar cell (SC), a PV module, or a PV string. The voltage signal is naturally affected by the noise which has a relevant effect on the process necessary for MPP determination, such as voltage first- and second-order derivatives. The aim of this study is to demonstrate the technical feasibility of a maximum power point tracker (MPPT) based on the present MPP detection algorithm employing a single-voltage sensor acquiring a signal affected by the significant noise. Theoretical evaluation, numerical simulations, and experimental measurements are carried out. Excellent agreement between the theoretical and experimental behavior is observed. Conditions for correct MPP detection are shown and good performances are obtained.

Direct MPPT Algorithm for PV Sources With Only Voltage Measurements

This paper presents a direct maximum power point tracking method, based on an easy and robust way of identifying the maximum power point (MPP) of a photovoltaic (PV) source that needs the measurement of the PV generator voltage only. The algorithm accurately detects the MPP and can rapidly track it in presence of irradiance variations, with no erratic behavior. An interface for a PV inverter that employs the proposed algorithm is described. The converter, called double capacitor interface (DCI), is designed to charge the dc link of the inverter and follow the MPP of the PV generator. Numerical simulations of the system operation, in different irradiance conditions, are discussed and evaluation of the tracking efficiency is reported. Finally, experimental results of a prototype breadboard built to test on the field the MPP tracking capability are presented.

A wireless irradiance-temperature-humidity sensor for photovoltaic plant monitoring applications

In this paper we present the project and a first concept circuit of a wireless sensor station designed to collect and send information about the environment conditions of a photovoltaic (PV) power plant, in order to monitor on-site specific phenomena of interest. The system is battery-free and uses a 4 mm2 PV harvester chip as electronic supply. It is equipped with temperature, irradiance and humidity sensors and with a MicroController Unit (MCU) for data treatment and wireless management. The light irradiance sensor discriminates the contribute of direct and diffusive light, accounting for air mass index effects. The PV harvester is integrated using a 0.35 μm BCD SOI technology while sensors, the MCU and a 802.15.4 transmitting module are placed in a external Printed Circuit Board (PCB). The goal of this project is to realize a 1 cm2 autonomous fully-integrated device representing the single low-cost node of a Wireless Sensor Network (WSN) to be installed in each PV module of a solar power plant.

A 300-mV Low-Power Management System for Energy Harvesting Applications

In this paper, a power management system (PMS) for low-voltage and low-power harvesters is presented. The PMS consists of an energy storage module and a supply regulation (SR) unit. The energy provided by the harvester is first stored in a capacitor CTN. When enough energy is accumulated in CTN, the SR block is enabled and transfers the charge from CTN to an output capacitor COUτ at a higher voltage. The storage process is controlled by a 300-mV voltage supervisor. The voltage stepup process is realized through a two-stage self-starting switching converter, which is able to automatically reconfigure itself after the cold start to improve conversion efficiency during the steady state. Correct system operation is ensured with harvesters that deliver at least 8 μA at 300 mV. The PMS exhibits an average steadystate charge transfer efficiency of 55% in the 2.5 μW-1 mW input power range. The circuit is built with no need for programmable devices and is battery free, low cost, and small sized. A prototype was realized and tested with two different low-voltage harvesters, namely a microbial fuel cell and a photovoltaic cell.

An improved ultra-low-power wireless sensor-station supplied by a photovoltaic harvester

In this work we describe a system optimized to use the few microwatts generated by a 4 mm2 photovoltaic energy harvester to acquire, process and wirelessly transmit information about the environment temperature and light irradiance. The energy needed for the described power-expensive operations is obtained by charging a buffer capacitor while the system is in a low-power-consumption state. A voltage level detector senses the charging status of the capacitor and wakes-up the sensing station every time enough energy is stored. Thus, the environmental parameters are monitorized and transmitted with an asynchronous and intermittent strategy. The optimization work related to this paper has persued the target of minimize the unerasable continuous power consumption of the voltage detector during the charging of the capacitor. We reached the goal of reducing it from the 50 μW of the previous version of the system to the current 5 μW, with a significant usability improvement. Simulations and experimental evidences demonstrate that our station can work totally battery-free when just illuminated with 100 W/m2 of light irradiance, transmitting data with a 802.15.4 compliant protocol and potentially occupies a volume of 1 cm3.

Energy harvesting system for wireless body sensor nodes

In this paper a photovoltaic energy harvesting system for wireless body area network applications is presented. The device is able to store the energy produced by the harvester in a storage element, acquire data from a generic body sensor and transmit the information wirelessly to an external receiver. The system is supplied by a thin-film photovoltaic energy harvester, which is capable of ensuring a proper functioning of the system even in indoor lighting conditions, when the incident power is limited. The energy is accumulated in a supercapacitor, and, once enough energy has been stored, the data from the sensor are collected and transmitted to the receiver. The interface exhibits negligible power consumption during the charge of the capacitor. Wireless transmissions can be performed through a Bluetooth® or Zigbee® module to a smartphone or a personal computer, which is used as main receiver. The harvesting system proposed in this paper is fully autonomous, since no battery is required even for the start-up, and is designed to be coupled to a variety of sensors which can be worn by the patient.

Ultra low voltage supervisor for energy scavenging systems

In this paper, a voltage supervisor (VS) for low-voltage low-power energy harvesting systems (EHSs) is presented. The task of a VS is to change the logic state of its output when a conveniently scaled fraction of its supply voltage crosses a fixed threshold. The presented VS includes an ultra-low-voltage ultra-low-power reference, a voltage comparator, and an output latch. The circuit was specifically designed for energy scavenging applications that make use of a low-voltage source with a very limited current capability. Correct operation of the VS is ensured with a supply voltage as low as 300 mV. Current consumption is lower than 1 μA. An EHS was built to test the VS. The energy provided by the low-voltage harvester is first stored in a buffer capacitor, CIN; then, when the voltage across CIN reaches a predetermined voltage level, the VS enables a step-up circuit, which transfers the charge from CIN to an output capacitor at a higher voltage. Experimental results from a prototype are provided and discussed.