Elsevier

Applied Energy

Volume 102, February 2013, Pages 1295-1302
Applied Energy

Characterization of photovoltaic modules for low-power indoor application

https://doi.org/10.1016/j.apenergy.2012.07.001Get rights and content

Abstract

Photovoltaic energy generation is envisaged as an efficient, natural and valuable energy source not only for outdoor but also for indoor applications, even if it is often difficult to give an adequate description of the indoor illumination conditions, and till now no (international) norms regulate the characterization of solar cells under the particular indoor conditions.

In this work commercially available photovoltaic modules have been measured under artificial light conditions using an appositely designed experimental set-up, and under mixed natural/artificial illumination in real indoor conditions. These modules are based on completely different physical and chemical concepts, from semiconducting amorphous pin junctions to photoelectrochemical cells based on TiO2 nano-particles, to semiconducting polymers, to crystalline silicon micro-spheres. Aim of this work is to compare their performances in view of possible indoor applications for low-power devices.

Highlights

► Characterization of commercially available third generation photovoltaic modules. ► Characterization performed under artificial light conditions only. ► Characterization performed under mixed natural/artificial illumination conditions. ► Some modules present a decrease in their performances under fluorescent tubes. ► Si–μsph and DSC modules are usable for low-power devices in real indoor conditions.

Introduction

Low consumption electronic devices such as wireless phones, personal organizers, tablet computers, remote controllers and sensors, are undergoing a continuous growth. All these devices need power, and generally the power needs are increasing together with the functionality of the device and decreasing with the progress and the improvements in the integrated circuits and in the batteries technologies. Moreover, all these devices usually work under indoor conditions, given by a mixture of natural and artificial light. Even if it is often difficult and quite impossible to give an adequate description of the indoor illumination conditions, photovoltaic energy generation is envisaged as an efficient, natural and valuable energy source not only for outdoor but also for indoor applications.

The photovoltaics market for terrestrial applications is nowadays dominated by crystalline silicon (both in mono and poly-crystalline form), which generally gives the best compromise between costs and performances: in July 2011 in Europe, the modules reached costs of around 1.2 €/Wp and efficiencies as high as 20% under direct solar illumination [1]. However it is well known that, under non-direct radiation, amorphous thin films, nanostructured materials and also polymers [2] can give better absorption of the light with respect to crystalline solids. For this reason the so called “second and third generation” solar cells should be more appealing for usage under both direct and diffuse radiation conditions: the “second generation” solar cells are made by thin films of amorphous silicon, cadmium telluride, copper indium gallium selenide, etc.; while the dye sensitized solar cells (fabricated using a nanostructured TiO2 film) [3], the organic solar cells (made with polymers) [4] and also the recently proposed solar cells obtained using silicon micro-spheres [5] are often identified as the “third generation” or “emerging concepts” in photovoltaics [6].

Recently, great attention has been devoted to test PV solar modules under year-round typical weather conditions [7], to simulate different operating conditions such as solar intensities [8] and partial shadowing [9]. Nevertheless, the characterization of market available photovoltaic cells and modules is generally performed under Standard Testing Conditions: 1000 W/m2 (1 sun), direct normal irradiance intensity with Air Mass 1.5 (AM1.5) spectral composition and temperature of 25 °C. In practice, no solar cell experiences such conditions; however they are useful for establishing valuable comparisons. It must be taken into account that these conditions do not represent indoor environments, where illumination intensities are always lower, typically in the range 0.05–5 mW/cm2, depending on the light sources and lightening conditions, and the spectral composition diverges from AM1.5, due to the presence of reflected or diffused light and due to the presence of artificial light sources, window glazing and filtering effects.

Till now no (international) norms regulate the characterization of solar cells under the particular indoor conditions, and furthermore, the indoor light intensity levels are usually given in terms of photometric rather than radiometric units, which renders difficult the comparisons.

Some papers have been published on the modeling of lightening conditions available in indoor situations such as offices, or domestic ambient, through the study of the variations in intensity and color of the daylight [10], or using Computer Aided Design software for ray-tracing in 3-dimensional sceneries [11], or simulating and measuring the user presence in different months of the year [12]. For outdoor conditions the solar spectrum is varying with the weather, with the day-time and with the year period; for indoor conditions such effects can be less important, however all the results are strongly dependent on the artificial light sources which are used. Other works have been made for simulating the solar cells performances under low irradiance conditions [13], [14], [15], [16]: aim of these works was to obtain models for calculating important parameters such as the short circuit current and the efficiencies of different kinds of solar cells exposed to different indoor conditions, in particular to different artificial lamps. Also, some papers have suggested and evaluated the utilization of photovoltaics for low-power indoor devices such as remote sensors [17], wireless computer mouses [18], organic light emitting diodes [19], smart clothing [20] and also emergency street lights [21].

In this work commercially available photovoltaic modules have been measured under artificial light conditions using an appositely designed experimental set-up, and under mixed natural/artificial illumination in real indoor conditions. These modules are based on different physical and chemical concepts, from semiconducting amorphous pin junctions to photoelectrochemical cells based on TiO2 nano-particles, to semiconducting polymers, to crystalline silicon micro-spheres. Aim of this work is to compare their performances in view of possible indoor applications for low-power devices.

The paper is organized as follows. Section 2 is devoted to experimental, explaining the used units, light sources and PV modules, together with the measurements apparatuses. In Section 3 the theory about radiometric and photometric units and the correlation between the short circuit current density and the quantum efficiency of a solar cell are explained. In Section 4 the performances of the different PV modules obtained under artificial light illumination are presented and discussed and the results of the two best-performing PV modules under artificial and natural daylight illumination are reported. Section 5 is devoted to the conclusions.

Section snippets

Artificial light sources

For the present work different lamps have been used: one incandescent lamp (model 005002-ST1340 from Orbitec), one halogen lamp (model FMW-FG 15057 from Eiko) and one low-consuming fluorescent lamp (model 2050 from Duralux). Their spectral irradiances have been measured by means of a StellarNet EPP2000-UVN spectrometer and a Molectron EPM1000 wattmeter. The illuminance has been measured by a luxmeter (model HD2102.2 from DeltaOhm).

Available solar modules for indoor applications

Different commercially available solar modules have been

Radiometric and photometric units

When evaluating the light generated by different sources, from the natural sunlight to the artificial lamps, two different units are generally used: one radiometric and one photometric. Radiometric units refer to the power (in watt) of the total spectrum, while photometric units make use of the lux, and refer to the sensitivity of the human eye, described by the CIE international standard curve V(λ) which practically limits the response only to the visible region (360–760 nm). The “irradiance” E

Spectra of artificial light sources and of solar cells quantum efficiencies

In Fig. 1 the measured spectral irradiances of the different lamps are reported. In order to compare the same lighting conditions, the irradiance values have been obtained to an illumination of 1000 lux, measuring the irradiance of each lamp with a thermopile-based wattmeter, and normalized assuming equal to 1 the irradiance of the fluorescent lamp at 612 nm. The examined sources exhibit very different spectral behaviors and are also covering different wavelength regions. The fluorescent lamps

Conclusions

Commercially available photovoltaic modules based on completely different physical and chemical concepts, from semiconducting amorphous pin junctions to photoelectrochemical cells based on TiO2 nano-particles, to semiconducting polymers, to crystalline silicon micro-spheres have been measured under artificial light conditions using an appositely designed experimental set-up. The Si–μsph and the CIGS behavior is strongly reduced – at the same illuminance level – when fluorescent lamps are used

Acknowledgments

Authors would like to thank G. Mina for the technical support with the experimental set-up, A. Virga for the help with spectroscopy measurements, Ascent Solar and Solaronix for kindly providing the modules, T. Meyer for the helpful discussion and all the modules manufacturers for kindly giving the IPCE data of their solar cells.

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