Wireless sensor nodes are autonomous devices which sense parameters and communicate them wirelessly. Their independent operation precludes the use of wired power supplies, and energy harvesting is becoming an attractive alternative to batteries. This paper presents the circuit design and embedded software of a photovoltaic power harvesting system for indoor wireless sensor nodes, which delivers energy-awareness and improved efficiency levels

Harris, N.R.

Weddell, A.S.

White, N.M.

English

Southampton (e-Prints Soton)

AN EFFICIENT INDOOR PHOTOVOLTAIC POWER HARVESTING SYSTEM FOR ENERGY-AWARE WIRELESS SENSOR NODES A.S. Weddell*, N.R. Harris, N.M. White Pervasive Systems Centre, School of Electronics and Computer Science, University of Southampton, UK  *Corresponding author: Tel: +44(0)23 8059 4996 Fax: +44(0)23 8059 2901 Email: asw05r@ecs.soton.ac.uk  Abstract: Wireless sensor nodes are autonomous devices which sense parameters and communicate them wirelessly. Their independent operation precludes the use of wired power supplies, and energy harvesting is becoming an attractive alternative to batteries. This paper presents the circuit design and embedded software of a photovoltaic power harvesting system for indoor wireless sensor nodes, which delivers energy-awareness and improved efficiency levels.  Keywords: photovoltaic energy harvesting, wireless sensor networks, energy management   INTRODUCTION  Wireless  sensor  nodes  are  autonomous  devices which  sense  parameters  and  communicate  them wirelessly  (generally  via  radio  protocols  such  as IEEE  802.11-2007  [1]  or  ZigBee  [2]).  As  they operate without cabling, they must be self-sufficient for  their  energy  provision.  Conventionally,  such devices  are  powered  by  non-rechargeable batteries, but recent advances in energy harvesting and  storage  technologies  now  mean  that  nodes can be designed to operate indefinitely from energy ‘harvested’ from their environment.   A common form of energy harvesting technology is from  light  via  photovoltaic  (PV)  modules,  but  low indoor  light  levels  mean  that  power  tracking circuitry  designed  for  outdoor  use  is  often  too power-hungry  to  operate  indoors.  Conventionally, indoor  PV  energy  harvesting  circuits  will  simply consist of a diode between the PV module and a battery or capacitor, which inhibits the reverse flow of  energy  from  the  store  when  the  cell  is  not exposed to sufficient light [3]. This arrangement is particularly  inefficient  when  charging  a supercapacitor from empty, as in this situation the PV  module  is  forced  to  operate  far  from  its maximum power point (MPP) voltage.  In  order  for  sensor  nodes  to  function  effectively, they  must  be  protected  against  out-of-tolerance supply  voltages  and  have  an  awareness  of  their energy  status.  Transceivers  and  microcontrollers used in wireless sensing applications will typically have  a  very  low  sleep  current  (around  1µA)  but draw large “bursts” (of around 25mA) when active. For this reason, energy harvested indoors from PV modules  and  other  devices,  which  is  normally  of the order of 1mA, must be ‘buffered’ in batteries or supercapacitors.  A photovoltaic energy conversion circuit has been reported  in  a  previous  publication  by  the  authors [4]. This paper presents an extension to this work, which  permits  the  nominal  power  of  the  module, and the light level to which the cell is exposed, to be deduced from the open-circuit voltage of the PV module. We present the developments to the circuit and  the  embedded  software,  along  with  the theoretical  work  undertaken,  to  deliver  this functionality  on  a  Texas  Instruments  CC2431 system-on-chip microcontroller and transceiver.  Energy-aware  network  algorithms  such  as  those proposed  in  [5]  balance  the  importance  of messages against the energy status of the node. With  indoor  PV  energy  harvesting,  delivering  this awareness, along with high efficiency, is non-trivial but  has  been  achieved  and  is  described  in  this paper.  Circuit  extensions  to  protect  the  node hardware  against  under-  and  over-voltage  events have also been developed, and are described later.  PV MODULE CHARACTERISTICS  y = 0.2571ln(x) + 2.912844.14.24.34.44.54.64.74.80 200 400 600 800 1000Open-circuit Voltage (V)Illuminance (lux)Datasheet VocLog. (Datasheet Voc) Figure 1. Logarithmic fit of Voc against illuminance, for Schott Solar OEM 1116929 Indoor PV module.  Amorphous silicon (a-Si) photovoltaic cells have a relatively  high  efficiency  at  low  light  levels, compared to other types of cell. This makes them particularly  suited  to  use  indoors.  The  I-V characteristic  of  PV  cells  is  described  by  the  Shockley solar cell equation [3], and shown in (1). Here, kB is the Boltzmann constant, T is absolute temperature, q is electronic charge, V is the voltage across the PV terminals, I0 is the diode saturation current, and Iph is the photogenerated current.   Figure 2. The developed photovoltaic energy harvesting circuit, which can be interrogated by the microcontroller.    − − = 1 0T B k eqVph e I I I   (1)  The datasheet parameters for a Schott Solar OEM indoor  solar  cell  [6]  have  been  plotted  and  a logarithmic trend line has been added, as shown in Fig. 1. In this instance, the open-circuit voltage is approximated to the light level by equation (2).    9128 . 2 ) ln( 2571 . 0 + = V oc E V   (2)  Where EV is the illuminance level in lux, and can be generalized  as  equation  (3),  where  A  and  B  are parameters  to  be  found  for  the  individual  PV module.    B E A V V oc + = ) ln(   (3)  This equation can be rearranged, as shown in (4), to determine the level of illuminance from Voc.    AB VVoce E−=   (4)  A  useful  property  of  silicon  PV  cells  is  that  their MPP  voltage  (Vmpp)  is  related  to  their  Voc  by parameter  k.  This  parameter  is  typically  between 0.70  and  0.80  (5),  and  can  be  determined  for individual modules [3]. This property can be used by simpler maximum power point tracking circuits. Furthermore,  the  current  obtained  from  the  PV module at its MPP (Impp) is related to its illuminance by parameter m, as shown in (6).    oc mpp kV V =   (5)   V mpp mE I =   (6)  Table 1. Parameters found for Schott Solar OEM 1116929 Indoor PV Module.  Parameter  Description  Value k  Ratio between Voc and Vmpp  0.71 m  Ratio between EV and Impp  5.4x10-7 A  Natural log fit of Voc-EV  0.2571 B  Natural log fit of Voc-EV  2.9128  The  nominal  power  (Pnom)  obtained  from  the  PV module can be estimated by means of (7), which makes use of the parameters outlined above and summarized in Table 1.   ( )   ⋅ ⋅ = =−m e k V I V P AB Voc mpp mpp nomoc  (7) There are certain limitations to this model, in that the parameter k only holds if the module is under uniform  illumination  (i.e.  no  partial  shadowing). Secondly, parameter m holds only for relatively low levels of current. While it varies by less than 1% in the range 100-1000 lux, it can be expected to vary more widely in brighter deployment environments. However,  the  situation  of  interest  to  this investigation  (indoor,  artificially-lit  environments) means that this simplification is acceptable. Lastly, the equation for nominal power assumes that the cell is operated at its maximum power point and the conversion circuitry is 100% efficient. The following section gives an idea of the efficiency of the circuit and  other  complications,  but  the  result  given  for nominal power should be sufficient to give an idea of the energy harvesting status of the node, for use by energy-aware algorithms.  For  SPICE  simulation  of  the  power  conditioning circuit, a model was developed for the PV module used in this investigation.  This was adapted from [7], and is shown in Fig. 3.    Figure 3. SPICE model for Schott Solar OEM 1116929 Indoor PV Module.  CIRCUIT DESIGN AND OPERATION  A circuit for indoor PV energy harvesting has been developed  (Fig.  2),  which  consists  of  a  modified buck-boost converter circuit with additional isolation and  metrology  circuitry.  During  normal  operation, this  circuit  acts  to  maintain  a  constant  voltage across  its  input  terminals  in  order  to  keep  the photovoltaic  module  at  a  voltage  set  by  R4.  The photovoltaic  module  is  maintained  at  this  fixed voltage  (in  the  range  of  maximum  power  point voltages of the cell at expected light levels) in order to keep its operation near its indoor MPP voltage. This  simplification  is  acceptable  as  the  normal range of indoor light levels is 100-1000 lux and in this range the power loss due to voltage clamping has been determined to be less than 3%.  The combination of JFET J1 and MOSFET M1 act to disconnect the load from the photovoltaic module when a ‘high’ signal is raised on the Vctrl line. This permits the open-circuit voltage (Voc) of the cell to be  measured.  However,  a  complication  of  this circuit  is  that,  as  it  is  based  on  a  buck-boost converter, the input and output stages do not share a  common  ground  (effectively  the  ground  of  the input  is  the  Vcc  of  the  output).  For  this  reason,  a high-impedance resistor divider comprised R1 and R2  is  used  to  bring  the  Voc  signal  into  the  range which  can  be  interpreted  by  the  microcontroller. The output from this divider is fed through a unity gain buffer around IC1 to provide the required low-impedance input to the ADC of the microcontroller. Resistors R3 and R4 act to ensure that the circuit operates normally on start-up, when supercapacitor C2  has  built  up  insufficient  voltage  for  the microcontroller to become active, and the switching converter is required to operate normally.  The Voc of the module can be calculated by means of (8), where A is the ratio of R1 to R2, and VµC is the voltage at which the microcontroller is operating (i.e. the voltage across supercapacitor C2).    ( ) ( ) c meas oc V A V V µ − + × = 1   (8)  R6 and D1 provide a reference voltage input to the micropower  comparator  IC2.  R7  and  R8  provide hysteresis  to  the  system.  In  normal  operation (when  Vctrl  is  low),  C1  acts  as  a  small  buffer capacitor. When the voltage across C1 exceeds the threshold set by R5, comparator IC2 gives a ‘low’ output  which  switches  MOSFET  M2  ‘on’  and permits current to flow through D2 and L1. After a very short period, the voltage across C1 will have dropped sufficiently to cause IC2 to turn M2 ‘off’, and for energy to be transferred from L1 through D3  to  be  stored  in  supercapacitor  C2  (note  the polarity of this component).    Figure 4. Photograph of the developed system.  SPICE simulations indicate that this circuit runs at over 70% efficiency with the PV module described earlier  under  normal  indoor  lighting.  Although  the absolute  efficiency  of  the  circuit  has  not  been tested, practical tests comparing the circuit shown in Fig. 4 against a conventional diode-only system show  a  30%  improvement  in  node  start-up  times under normal office lighting.  The voltage on supercapacitor C2 is monitored by the microcontroller to determine the energy stored, and used to estimate its remaining lifetime.  Additionally,  not  shown  in  Fig.  2,  is  under-  and over-voltage protection circuitry. This is important in wireless sensor node applications as under-voltage events  can  cause  the  microcontroller  to  turn  on erroneously in an undefined state and drain power from  the  store,  and  overvoltage  situations  can cause permanent damage to the node. Over- and under-voltage  protection  circuitry  has  been implemented with the aid of voltage detector ICs.    (a)  (b)  Figure 5. Under-voltage and over-voltage protection circuits, as implemented in demonstration system.  As shown in Fig. 5(a), a 2.0V voltage detector IC feeds  through  to  digital  MOSFETs  M3  and  M4 which  interrupt  the  supply  connection  of  the microcontroller, should the input voltage fall below 2.0V. The voltage detector has built-in hysteresis to mitigate  issues  with  repeated  switching. Overvoltage protection can be implemented in two ways: the system implemented in the demonstrator is  shown  in  Fig.  5(b),  in  which  a  3.3V  voltage detector  turns on a  digital MOSFET which drains excess  current  through  R9  when  the  threshold  is crossed. Alternatively, it would be equally feasible for the output from this detector to be fed back to the  Vmeas  input  of  the  circuit  in  Fig.  2,  thus disconnecting  the  cell  and  preventing  further energy from being switched through.  EMBEDDED SOFTWARE DESIGN  The  embedded  software  of  the  sensor  node incorporates  an  “energy  stack”,  a  concept introduced in [8] and shown in Fig. 6 as part of a unified  stack.  This  abstracts  the  energy management  functions  of  the  node  in  order  to promote modularity and adaptability. The physical functions  to  ‘interrogate’  the  photovoltaic  module and obtain a raw voltage reading are implemented in  the  “physical  energy”  layer.  The  “energy analysis”  layer  hosts  the  mathematical  functions that which allow an estimate of the energy resource to  be  calculated.  The  “energy  control”  layer processes  requests  for  data  from  the  application, and presents the results to it in a standard fashion.    Figure 6. Energy and sensing stacks, as implemented in the demonstrator system.  In order to determine Pnom, it is necessary to use the equation given at (1). The Taylor series expansion for ex is given by (9), which simplifies to (10). Taylor expansions  are  suitable  for  implementation  in microcontrollers  as  they  provide  relatively  simple approximations  for  a  range  of  mathematical functions.    K + + + + + =! 4 ! 3 ! 2 ! 114 3 2 1 x x x xex   (9)      K + + + + + =24 6 214 3 2 x x xx ex   (10)  The  function  for  ex  implemented  in  our  8-bit microcontroller calculates its value up to 10 terms. The accuracy and effectiveness of the function is limited firstly by the maximum size of long integers (32 bits, thus giving a limit of 12!, or 12 terms), and secondly by the precision of floating-point numbers (also  32  bits)  and  their  operations,  meaning  that accuracy is potentially lost at each iteration.  The final component in the system is a balancing algorithm  which  controls  message  transmission dependent  on  the  calculated  energy  status.  This allocates  a  “power  priority”  to  the  status  of  the energy  subsystem,  meaning  that  the  application layer  can  make  high-level  decisions  without  a detailed knowledge of the circuitry.  The system has been tested and evaluated in an office  environment  with  a  mix  of  natural  and fluorescent lighting. From cold-starting, the system steadily  charged  the  supercapacitor  energy  store C2 until the system voltage reached approximately 2.1V,  at  which  point  the  microcontroller  became active  and  started  transmitting  with  a  30s  sleep period. At its peak level of harvesting the system reported  a  nominal  power  level  of  3.5mW,  but  a more typical level was around 1mW.  As  the  voltage  across  C2  increased,  the  sleep period  of  the  system  decreased  in  steps,  until  it was reporting its  parameters every 1s. When the system  voltage  reached  3.45V,  the  overvoltage protection  circuit  was  observed  to  restrict  the voltage to between 3.35-3.45V.  CONCLUSIONS  An  efficient  circuit  for  harvesting  energy  indoors from  a  PV  module  has  been  demonstrated.  The circuit  permits  the  open-circuit  voltage  of  the  PV module  to  be  determined  by  a  simple  8-bit microcontroller,  which  is  then  transmitted  via  a 2.4GHz  ISM  band  transceiver.  The  system  is completely  self-powered,  and  incorporates  under- and over-voltage protection circuitry to regulate the operation of the sensor node.  The  performance  of  the  PV  module  has  been modeled and an algorithm to determine the nominal power  of  the  module  from  its  Voc  has  been developed, implemented, and tested. Limitations to the  scheme  have  been  described,  but  have  a minimal impact on the indoor deployment scenario described in this paper. The system analyses the energy stored on the supercapacitor and uses this to  determine  its  reporting  frequency.  This  work forms part of a complete  scheme for the energy-aware  operation  of  wireless  sensor  nodes harvesting energy from their indoor environment.  ACKNOWLEDGEMENTS  The work reported on in this paper was undertaken as  part  of  the  Data  Information  Fusion  Defence Technology  Centre  (DIF  DTC)  Phase  II  ‘Adaptive Energy-Aware  Sensor  Networks’  project,  funded jointly by the UK Ministry of Defence and General Dynamics UK.  REFERENCES  1.  IEEE Standards Association, IEEE Std 802.11-2007, New York, 2007. 2.  ZigBee  Standards  Organization,  ZigBee Specification, San Ramon, 2008. 3.  T.  Markvart,  editor;  Practical  Handbook  of Photovoltaics:  Fundamentals  and  Applications, Oxford 2006. 4.  A.S.  Weddell,  G.V.  Merrett,  N.R.  Harris,  B.M.  Al-Hashimi, Measurement+Control, 41(4) pp. 104-108. 5.  G.V.  Merrett,  N.R.  Harris,  B.M.  Al-Hashimi,  N.M. White;  Sensors  and  Actuators  A:  Physical,  142 (2008), pp 379-389. 6.  Schott  Solar  GmbH,  ASI®-OEM  Solarmodules  for Indoor, Putzbrunn, 2006. 7.  S.G.  Hageman,  Electronics  Design  News,  May 1992, pp. 220. 8.  G.V.  Merrett,  A.S.  Weddell,  N.R.  Harris,  B.M.  Al-Hashimi,  N.M.  White,  “A  Structured Hardware/Software  Architecture  for  Embedded Sensor Nodes”, Proc. ICCCN 2008 (in press). 

A Structured Hardware/Software Architecture for Embedded Sensor Nodes”,

Electronics Design News,

GmbH, ASI®-OEM Solarmodules for Indoor,

ZigBee Standards Organization, ZigBee Specification,

An efficient indoor photovoltaic power harvesting system for energy-aware wireless sensor nodes

An efficient indoor photovoltaic power harvesting system for energy-aware wireless sensor nodes

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