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  • Author Author: ralphjy
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Solar SPEC - Design Concept

ralphjy
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Solar Power Devices

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The initial Block Diagram for the SPEC:

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First pass at component selection:

1) Solar cells - AOSHIKE Monocrystalline Silicon Solar Cells in Epoxy Package

    2V @ 160mA

    50mm x 50mm x 2mm

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   These are compact, inexpensive cells which are reasonably robust mechanically.

   Use cells for both power and irradiance sensor.

 

2) Microcontroller Board - Sparkfun ESP8266 Thing

    80mm x 30mm x 5mm

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   Features:

     ESP8266 microcontroller

     A/D Converter

     I2C Interface

     LiPo Battery charger

     802.11n Wifi

 

3) Temperature Sensor (x2)

    TMP102 Digital temperature sensor with I2C interface

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4) LiPo Battery

    3.7V @ 1000mAh

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5) Mechanical housing and mounts

    Need a watertight and temperature tolerant box to house all the components except for the remote temperature sensor.

    Must have cutouts for the solar cells.

    Probably try to 3D print a custom unit.

    Needs to allow mounting with the solar cells in the same plane as the solar panels being monitored.

    Remote temperature sensor needs to mount to bottom of one of the solar panels (possibly just use adhesive).

 

Design considerations:

 

Power:

The ESP8266 has an average current consumption of 80mA @ 3.3V.  802.11n transmit current is typically 135mA and receive current is typically 62mA.  Standby is 0.9mA.

 

The solar cells are rated at 2V @ 160mA, but I do not have a detailed spec sheet.  I'm assuming that this is under standard test conditions.  I plan to use 3 cells in series to provide 6V @ 160mA.  I'll use a series diode to drop the voltage closer to 5V for the input to the Thing board.  I'll need to do some characterization to ensure that I won't exceed the 6V maximum of the board input.  I plan to use a 1000 or 1200mAh battery to provide power when there is no sunlight, so I think I'll be okay except for possibly a string of days with poor total irradiance (winter months).  Again, I'll need to do some characterization.  I can add another string of cells in parallel at the expense of a larger housing.

 

I'll need to have power management incorporated in the program (control the frequency and duration of WiFi activity, standby when there is no irradiance).  I don't think I'll need the deep sleep mode but that could be an option at night.

 

Irradiance sensor:

Since I don't have a detailed spec for the solar cell, I'll need to generate the I-V curves.  I'll determine the MPP voltage and current under standard test conditions (STC - 1000W/meter^2 and 25C) and use that to select the load resistor for the solar cell.  I'll need to scale the load resistor voltage as the A/D input range is only 0-1V.   The A/D reading will provide the voltage and current (voltage/load resistance) that are needed for the irradiance computation.

 

I plan to use a Daystar Irradiance meter as a reference check.

 

Possible additions:

1) OLED display to display irradiance and temperature locally without requiring WiFi connectivity.

2) External power switch - the Thing board has a power switch, I would just have to wire this to an external switch.

  • in reply to Jan Cumps

    Hi power or not, the basic cell still only outputs 1.2 volts across the  p-n junction.

    So you just run the solar cell to the A/D converter with high value resistors to get the correct scaling and you have an automatic light sensor.

     

    DAB

  • in reply to ralphjy

    When the sun is out, your solar cell will output a positive voltage, just like your light sensor.

    So the solar cell can be used as a light sensor as well as your source of power.

    You just need to run it into an A/D converter with the appropriate voltage scaling.

     

    Saves you needing a sensor.

     

    DAB

  • in reply to genebren

    Hi Gene,

     

    Thanks for the info.  I'll have to admit that I did not consider the optimizations that you've suggested.  And I really hate to admit that I had downloaded the MCP73831 spec but had not gotten around to reading it.  I'll probably need to change that 2K resistor in any case.  Great feedback, I'll let you know what I try.  I'll be gone until the middle of the month, so I won't get to implementing until after that.

     

    Ralph

  • Very interesting update to your solar powered devices project.  You mention MPP (maximum power point), but I don't think that you can get the 'thing' board to control the charge rate for your battery.  On periods of partial sunlight, the charger will load your solar panel down to the point were the voltage drops below the charging threshold and no charge will occur.  The trick with simple Li-ion charger chips (like the MCP73831) is that you have to be able to control the charge rate to match the solar panel's available energy (MPP).  There are a couple of ways to do this.  One is to use a programmable resistor (or digital potentiometer) in place of the fixed (2K) resistor (increasing resistance as the solar panel's voltage drops).  It is also possible (and a bit trickier) to create a closed loop system to trim the charge setting, inject current into the 'PROG' proportionally to the voltage on the solar panel (as the voltage drops, inject current to effectively lower the charge rate), to decrease the charge charge so the solar panel's voltage increases (to stay about the charge threshold voltage).

     

    Good luck,

    Gene

  • in reply to DAB

      wrote:

     

    ...

    I am curious, why did you not just use the solar cell as your indicator for sunlight?

    ...

    This may be possible. But the solar cells are high power. The signals they give would have to be transformed into something that's appropriate for a micro controller.  And the high power in the solar circuit would have to be isolated.

    A separate cell that doesn't have a connection with the power side may be cheaper and easier to keep safe?