ECS 300/310 Solar Panel - Design Considerations
Contents
1. see data sheet for more details For applications that need more energy like e g bidirectional room controllers Smart Acknowledge see AN 501 larger solar panels like ECS 310 eight serial cells 5 x 2 cm 10 cm are required For application examples requiring even larger solar panels also consult AN 311 autarkic motion sensors with EnOcean Note The indoor solar panels ECS 300 310 can deliver under sunlight an open circuit volt age unloaded gt 6 V To limit the STM 300400 input supply voltage below 5 V by acci dentally longer direct sunlight exposure its VDD and VDDLIM terminals must be connected together see STM 300 400 user manuals respectively appropriate Dolphin V4 Core De scriptions By the way longer exposure of indoor amorphous silicon cells in direct sunlight i e hundreds of hours gt 10 000 Ix leads to gradual degradation of their efficiency by up to 25 due to the so called Staebler Wronski effect light induced degradation process es Conversely at such light levels actually hundred times higher vs typical indoor levels the delivered power will be still more than enough in this case too Disclaimer The information provided in this document describes typical features of the EnOcean radio system and should not be misunderstood as specified operating characteristics No liability is assumed for errors and or omissions We reserve the right to make changes without prior notice For the latest docu2. _Operating point 8 lop voltage 1 cell Vop1 Voci Vop2 Voc2 Vop3 Voc3 Fig 7 poorest single cell in the chain bottleneck Means a single shadowed cell in the chain can cause a total failure Partially shaded cells lead correspondingly to output current decrease Make sure that in worst cases i e poor light or full sunlight the delivered output voltage current remains within the specifications of the powered electronics e g provide appropiate limitation General Recommendations Do not forget that these are only rough estimations depending on many variables First carefully check the specific environmental conditions e g typical use case define worst case application requirements and add an additional reserve of 20 by the solar panel dimensioning Make sure that the full solar cell active area is light exposed i e espe cially the peripheral ones are not obstructed by the device case cover design Verify your assumptions by measuring the embedded real solar panel performance in worst case Generally consider 25 Ix as a lowest brightness limit for designing ambient light powered devices Below this limit solar cell efficiency drops dramatically EnOcean www enocean com Subject to modifications Christian Bach Sep 2015 Page 2 3 e APPLICATION NOTE 207 Green Smart Wireless enocean ECS 300 310 Solar Panel Design Considerations Calculation Example An EnOcean STM 300 ba
3. APPLICATION NOTE 207 Solar Panel Design Basics Relative output current 1000 Outdoor range Indoor range BE 100000 Illumination ix Fig 1 Spectral sensitivity characteristics of solar cells Las x Crystalline ri silicon Ralatve spectral sansiinity oi 300 500 700 900 Wavelength nim Voc normalized V 6 Green Smart Wireless enocean One of the most common energy harvesters to day is the solar cell Solar radiation represents the largest energy resource of the terrestrial eco system The discovery of the photovoltaic effect opened entirely new perspectives on the use of solar light for electricity generation Unfortunately only about 0 1 to 0 2 of the sunlight level is available indoors Fig 1 Important factors for the right solar panel choice are the specific light source and application energy requirements EnOcean offers an indoor solar panel family ECS 300 310 EnOcean Converter Solar in two di mensions ECS 300 small and ECS 310 medium optimized for its typical applications This appnote is a short introduction to indoor use case of solar panels as energy harvesters for typical EnOcean applications For more details regarding typical indoor light sources and levels as well as meas urement recommendations consult EnOcean AN 201 and especially AN 406 Solar cell efficiency depends on matching be tween solar cell technology cell spectral sens
4. akage currents These effects become more considerable the smaller the single area cell and lower the light intensity Brightness is the term that describes how intense a light source is perceived by the human eye and Is measured in lux Ix As a rough conservative estimation indoors and for small area amorphous solar panels a few cm an operating current in the range 10 uA cm 200 Ix FL can be considered This value can be roughly linearly extrapolated us ing a de rating factor for lower illumination and or smaller area That corresponds e g to 4 5 uA cm 100 Ix Another extrapolation approach area x brightness is shown in Fig 6 Additionally amorphous cells deliver indoors nominal power densities of around 4 uW cm 200 Ix or lt 2 uW cm 100 Ix i e about 4 5 uA x 0 35 V For best performance the solar panel output voltage should be near the required application operating voltage The amount of solar panel output voltage mainly depends on the number of serial connected cells Fig 7 while the max imal delivered current capacity is limited by the Green Smart Wireless enocean Dependence cell area Dependence light intensity Increase cell area Increase light intensity lse Current Decrease area Decrease light intensity 4 F Cs Chart approx constant output current 2 om 400 Ix 4 emt 200 Ix a a 4 i 200 400 GO SLM IHN lux Fig 6 Current
5. itivi ty Fig 2 and source radiation spectrum Fig 3 Technologically there are two major solar cell types for outdoor and indoor applications Crystalline silicon solar cells also called for out door reach best efficiency under outdoor sun light peak sensitivity at 800 nm Amorphous thin film silicon cells also called for indoor with peak spectral sensitivity at 550 nm similar to the human eye one are actually spe cialized for poor daylight especially indoor light Fig 4 output voltage comparison While conventional crystalline solar cells are about two times more efficient by optimum day light conditions outdoors amorphous cells beat them by far at poor indoor light levels and win on the 24 hour energy cycle because they can also efficient use poor artificial light and early morning or evening light Also because of larger bandgap amorphous silicon solar cells typically have higher open circuit voltages up to 0 8 V cell APPLICATION NOTE 207 ECS 300 310 Solar Panel Design Considerations Solar cells work as light dependent current generators While their open circuit output voltage depends only relatively slightly on il lumination level over decades their short circuit current varies directly proportional to light intensity and solar cell area Fig 5 The delivered power is however not quite linear slightly reduced due to its voltage depend ence and internal peripheral le
6. mentation visit the EnOcean website at Www enocean com EnOcean www enocean com Subject to modifications Christian Bach Sep 2015 Page 3 3
7. sed room sensor should transmit environment data every 100 se conds The ambient brightness is 100 Ix continuously What dimensions are needed for its solar panel Current requirement STM 300 current consumption can be determined from the STM 300 user manual 3 11 see also AN 209 This is about 160 uC 160 uAs for each trans mission including an external sensor circuit and external storage capacitor leakage cur rent By one wake up and transmission every 100 seconds a storage capacitor recharge current of gt 1 6 uA during the next 100 seconds is needed for the next transmission 1 6 uA x 100 s 160 pAs A 1 cm single solar cell delivers an operational current of about 4 5 uA 100 Ix at lt 2 uW see considerations above therefore half of 1 cm cell will be enough to deliver 2 uA 0 35 V Voltage requirement STM 300 needs a supply voltage between gt 2 6 V start up and 5 V Therefore at 100 Ix eight single cells 0 35 V 100 Ix serially connected are needed These two conditions voltage and current requirements 100 Ix give the minimal solar panel area needed for the required application eight cells x 0 5 cm 4 cm An amorphous silicon solar panel consisting of eight single solar cells connected serial with an overall area of 4 cm is therefore needed EnOcean s smallest ECS 300 solar panel with the dimensions 3 5 x 1 28 cm 4 cm consisting of eight serial amorphous cells would fulfill these requirements
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