12th International Conference on DEVELOPMENT AND APPLICATION SYSTEMS, Suceava, Romania, May 15-17, 2014

978-1-4799-5094-2/14/$31.00 ©2014 IEEE

A study on light energy harvesting from indoor environment

The autonomous sensor nodes

Aurel Chirap, Valentin Popa, Eugen Coca, Dan Alin Potorac Department of Computers, Electronics and Automatics

Stefan cel Mare University of Suceava Suceava, Romania aurel@eed.usv.ro

Abstract—Micro-level energy sources from the ambient

environment can be a viable alternative to increase the life of the sensor nodes from a WSN network. Harvesting energy from the environment - particularly solar energy, is a technique that can support the operation of the sensor nodes. The power extracted from the luminous radiation is extremely variable, depending on day/night alternation, atmospheric conditions, temperature, geographical location, indoor or outdoor environment - which makes the energy harvested insufficient for use as an exclusive power source to supply sensor nodes. A special case is the indoor environment, in which the solar radiation is strongly reduced, compared to the outside environment, and artificial lighting levels are usually below 1000 Lux. We made experiments on an indoor solar cell using illumination from three different sources: fluorescent, incandescent, LED. Detailed measurements, performance comparisons, and conclusions are presented.

Keywords—amorphous solar cells; indoor energy harvesting system; autonomous sensors node

I. INTRODUCTION The energy produced by renewable natural sources (solar

energy, wind power, wave), as well as other forms of energy derived from artificial sources (thermal jet engines, machinery and equipment, radio and television transmitters) can be harvested and used to support the operation of the sensor nodes. Energy harvesting is the physical process by which the energy is captured from the environment and transformed into usable electricity.

In recent years there has been a growing interest of researchers for developing energy harvesting systems at the micro level, from a variety of sources: light, vibration, heat, radiofrequency waves [1]. Kansal et al. [2] formulated the theoretical concept of a neutral energy and proposed a model of energy management for a system of harvesting solar energy from the external environment. The platform Heliomote has been used for evaluation. It the integrate Mica2 sensor node. Harvesting system that feed the Prometheus platform designed by Jiang et al. [3] it was composed of a solar panel, a supercapacitor as primary energy buffer and a LiPolymer battery as secondary buffer energy. Another harvesting system

has been proposed by Simjee and Chou [4] (Everlast platform), which used a single buffer, a supercapacitor of 100F, and a MPP-tracking circuit. Subsequently, in order to increase the availability of energy, has been developed the concept of harvesting energy from multiple sources and have been carried out the so-called hybrid harvesting energy systems [5], [6], [7], [8]. However, there are a small number of investigations concerning the use of solar cells for harvesting systems in indoor environment [9],[10].

In this paper is presented a study concerning the harvesting of energy from natural light radiation and/or artificial in the indoor environment. The main objective is to use the energy harvesting subsystems based on mini-photovoltaic panels to support the operation of the permanent regime of electronic devices with low power consumption such as sensor nodes.

Fig. 1. The general architecture of an autonomous sensor node powered by multiple sources

There are two main problems associated with energy harvesting of energy from indoor environment: i) regardless of the source, natural or artificial, the luminous intensity is diminishing, usually below 1000 lux. Therefore, the power harvested is much smaller than in direct solar illumination in outdoor conditions; and ii) availability of energy sources is dependent on environmental conditions inside.

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II. LIGHTING CONDITIONS AND PHOTOVOLTAIC DEVICES The measurement principles for terrestrial photovoltaic

devices are defined in the IEC60904-3 standard. One of the main parts of the standard testing conditions (STC-Standard Test Condition) is the spectral distribution (Fig. 2) of the irradiance known as AM1.5G spectrum, assuming an intensity /*of 1000Wm2. The definition of these spectral data is based on realistic atmospheric and environment parameters[11].

Fig. 2. Previous (Ed. 1 (1989)) and new (Ed.2 (2008)) data of the IEC60904-

3 spectral distribution. The ratio between the spectral smoothed in 10nm intervals, is plotted in the insert [11].

A. Lighting conditions in indoor environment The indoor environment can be illuminated by solar

radiation (indirect) or artificial sources, typical fluorescent lamps, incandescent or LED lamps. The indoor spectrum is given by the superposition of artificial light and sunlight. The indoor intensity is in the order of 1 mW/cm2 as the outdoor intensity is about of 0.1 W/cm2 [12]. In Fig. 2, one can see the spectrum of radiation from fluorescent lamps with white light, compared to solar radiation spectrum spectral area perceived by the human eye.

The effectiveness of a system of harvesting energy, for indoor environment, primarily depends on the performance of the transducer, typically a photovoltaic device. Therefore, the determination of the conditions of lighting is important to the proper characterization of a source of energy, and is useful for choosing the type of photovoltaic device.

B. Photovoltaic Devices A photovoltaic module (PV) is a device consisting of

several solar cells connected in series or in parallel, which has the ability to transform light energy directly into electricity because result of the photoelectric effect.

The solar cells are classified according to the material employed: amorphous silicon, crystal silicon, and compound semiconductor solar cells. Other classifications can be made depending on the substrate (glass, stainless steel, film) or environment of use (indoors, outdoors).

Fig. 3. Radiant spectrum of light source and spectral sensitivity of solar cells [13].

Analyzing the characteristics of Fig. 3, one can find that the amorphous silicon solar cells are suitable for use in indoor environment, as they are the most susceptible to artificial light. However, they have a low efficiency (≈ 12.5%), compared with multicristalin Silicon cells (20.4%) or with monocrystalline silicon cells (25%) [12].

III. SUBSYSTEMS ARCHITECTURE AUTONOMOUS SENSORS NODE Considering the sensor node architecture at subsystems

level based on modularity of the hardware platform (Fig. 4), a more efficient optimization of subsystems provides a higher degree of design flexibility. For example, if the subsystem of harvesting energy is adequate to the application environment, it should be able to ensure power supply for any type of platform node. This implies the existence of technical specifications relating to the platforms developers.

Fig. 4. The modular architecture of autonomous sensor node

A. Subsystem Energy Harvesting (SEH ) Energy has always been the major problem, which directly

influenced the lifetime of the sensor nodes and implicitly of networks of sensors. Research on this issue were directed, in the first instance, on the optimization of network-level protocols[14]. The next step was the development of ultra-low power technology [15], technology that provides support for the development and expansion of sensors networks applications. Current researches are oriented on the techniques of harvesting energy from the micro level, and finding the best practical solutions.

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Although, the results of laboratory research are promising, the costs are too high for them to be applied to at a larger scale. Generic scheme for a subsystem of harvesting light energy from indoor environment is presented in Fig. 5. The followings are the characteristics of functional blocks.

Fig. 5. Functional blocks of energy harvesting subsystem

1) Power generation Choosing the type of cells is a complex problem because it

depends on the advancement of technology and the availability of commercial devices. Sanyo Semiconductor is the leading producer of solar cells with amorphous silicon (Amorton series), and in 2012, CYMBET Corporation released the CBC-PV-02 model [16]. For comparison have been selected four models of solar cells of medium size, with amorphous silicon.

In TABLE I. are presented the characteristics for selected cells. The Model AM-1815 was tested under conditions of illumination from three different light sources (fluorescent, incandescent lamps and LEDs).

2) Power storage In general, a node of sensors may not work directly with the

energy harvested. For this reason, it is necessary to store the energy when possible or there is an excess of energy. They present drawbacks such as limited charge cycles, memory effect for NiCd, limited range of temperature and in addition, special circuits are necessary for recharging and monitoring. For example, the limit cycles of loading is 300 for a NiMH battery and a maximum of 1000 for a Li-ion battery, with compliance to the usage conditions [17].

TABLE I. INDOOR PRODUCTS AMORPHOUS SILICON SOLAR CELLS

Model

Typical operating characteristics

FL-200lux FL-50lux External dimensions(mm)

AM-1454 1.5V/ 31.0µA 1.4V/ 7.75µA 41.6 x26.0

AM-1513 1.8V/ 15.0µA 1.6V/ 3.75µA 55.0x13.5

AM-1815* 3.0V/42.0µA 2.6V/10.5µA 58.1x48.6

CBC-PV-02 0,8V/110µA - 67.3x26.7

*solar cell use experimental setup

In TABLE II. are presented the output characteristics for amorphous silicon cells illuminated by an artificial source of fluorescent light, with 200-lux luminance, at 25°C temperature.

TABLE II. OUTPUT CHARACTERISTICS OF INDOOR USE AMORPHOUS SOLAR CELLS

Model Open-circuit voltage

Short-circuit current

Maximum power output

Light source

AM-18xx 0.63 V/cell 17.0 uA/cm2 7.0 µW/cm2 FL

200 lux CBC-PV-02 0.6 V/cell 15.8 µA/cm2 6.0 µW/cm2 FL

200 lux

Recent technologies for battery with thin film solid electrolyte (TFB, Thin-Film Battery) [18], [19] led to increased electrical performance such as energy density and power density, but also the life time, the number of recharging cycles (≥105), the use in a wide range of temperatures, reduced volume and practicability in various forms. Due to the special properties of the TFB, it can bee used in combination with supercapacitor for efficient energy storage.

Supercapacitors are suitable for storing energy because they combines the propriety of batteries to store energy with the ability to provide instant power, specific of capacitors [20], [21]. In addition, it supports a large number of charge/discharge cycles (≈5x105), does not require special circuits for load and duration of use is greater than 10 years. In TABLE III. are presented the advantages and the disadvantages of devices used for energy storage.

TABLE III. ENERGY STORAGE DEVICES

Energy Storage Comparisons

Conventional Batteries Supercapacitors Thin Film Batteries

+ High discharge current

+ High energy density

+ Inexpensive

- Limited life

- Reaplacement labor cost

- Unsafe, polluting

- Form factor

+ Peak power delivery

+ Long life

+ Inexpensive

+ Form factor

- High leakage

- Low energy density

- High temp degradation

+ High discharge current

+ High energy density

+ Near zero leakage

+ Long life / Permanent

+ From factor

+ Safe / Eco-Friendly

+ Broader temp range

- Disadvantage + Advantage

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3) Power management Increasing the global efficiency of all energy harvesting

systems can be achieved by techniques of tracking the maximum power point (Maxim Power Point) [22].

Most of the MPP techniques applicable to large-scale are not appropriate for harvesting systems at micro level. The conclusion of the study effectuated in [23], show that the application of FOC (Fractional Open Circuit voltage) tracking method of MPP for a solar cell has efficiency if the illumination is greater than 504 Lux.

IV. TEST CONDITION AND RESULTS The AM-1815 model was selected for testing. The

measurements were made during the night so as not to be influenced by natural light.

There were three types of sources used for illumination.

a) Fluorescent lamps with cold light Fluorescent lamps used for lighting the indoor environment

are TLD 36W/54 type.

a) Incandescent lamp The incandescent lamp, with variable luminous intensity,

was fixed on a support at a height of 75 cm from the solar cell and the light meter. The used bulb was TUNGSRAFLEX R63 230V/60W.

b) LEDs lamp with cold light. The LEDs lamp was set on a telescopic support. The used

lamp was R50/7W type with luminous intensity of 6000 lux.

Fig. 6. Output characteristics for AM-1815, artificial light, such as fluorescent (FL), incandescent (IL) and LED light, is used indoors.Voc and Isc measured values are represented by solid line, Vmpp and Impp calculated values are represented by dashed Impp

Fig. 7. The instruments used for measurements

The luminosity was measured with a PeakTech 5025 light meter with range between 0-2000 lux (Fig. 7). Open-circuit voltage (VOC) and short-circuit current (ISC) were measured with digital instruments AMPROBE 38XR-A. The measurement and calculated results are presented in Fig. 6.

V. CONCLUSION Energy harvesting technology shows great potential as a

promising approach to powering for autonomous wireless sensor nodes.

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This paper presents a preliminary study concerning the possibility of using indoor light radiation as an energy source for the autonomous sensor nodes. A modular approach offers the possibility of optimization at subsystems level.

Energy transducer is the first element of which depends the efficiency of energy harvesting subsystem. As a result, the assessment of environmental conditions and the choice of an appropriate transducer are the first steps that should be made. For the storage and the management of energy were given only general considerations. A detailed presentation will be made in a future paper.

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