Small IoT Gadget, Solar Energy Harvesting and Possible Heat Problems

[Kalvin]

Hi,

Here is a mechanical and temperature related problem I cannot solve:

I am in a process of starting developing a small IoT gadget which will measure some soil parameters (temperature and moisture) and use a Wifi to send the measurement data a few times in a day into the "cloud". The gadget will use solar energy for its operation, and the harvested solar energy will be stored in a rechargeable 4.2V Li-battery. There will be a 5cm x 5cm solar panel used for harvesting the energy.

The solar panel and the rest of the electronics including the Li-battery will be housed in a single plastic enclosure: width=6cm x depth=6cm x height=3cm, the 5cm x 5cm solar panel facing to the sun through the 6cm x 6cm front plastic window. The enclosure needs to be weather proof (winter with snow, summer with rain, temperature -30C ... +45C) and it needs to last for some years, if possible.

So far the design seems to be quite simple as there are quite a few suitable IoT devboards available. However, the temperature effect of the direct sunlight and the generated heat by the sun is my main concern and how it affects the Li-battery. It is now winter here in Finland, so I cannot make any real measurements. I have tried to find some actual data and measurements on how the direct sun exposure affects the internal temperature of a small, closed plastic enclosure.

Selecting a reflective (white) color has a profound effect on the heat load the sun will provide. Unfortunately the black solar panel (5cm x 5cm) needs to be placed inside the enclosure with the electronics and the Li-battery, behind a transparent plastic front panel. The plastic enclosure will not have any holes in order to keep it weather-proof. The electronics inside will not generate any heat in practical terms, but the small 25 cm² solar panel will absorb heat from the sun, and that heat will increase the internal temperature. At this point no heat sinking is possible.

These are the numbers I have right now: During the summer time the sun will provide approximately 1 kW/m^2, the temperature in the sun maybe 30C - 40C, the solar panel size inside the enclosure is 25 cm² facing directly to the sun, the enclosure size is 6cm x 6cm x 3cm and the thickness of the enclosure is 3mm, on a hot summer day the wind is typically non-existent.

Does someone have any links or suggestions for calculating the guesstimates for the temperature rise when the device is in the direct sunlight for a few hours, or would someone have even hands-on experience and/or measurement data for similar situation? I already have a bad feeling that I will be in a trouble with the excessive heat ...

Br,
Kalvin

[Kalvin]

With the help of this article http://www.engineeringtoolbox.com/conductive-heat-transfer-d_428.html I was able to come up with the following quick & dirty approximation for the temperature increase inside the enclosure:

Code: [Select]

Sun power = 1000.0 W/m^2
Panel width = 0.05 m
Panel depth = 0.05 m
Panel area = 0.0025 m^2
Panel heat power = 2.5 W
Enclosure width = 0.06 m
Enclosure depth = 0.06 m
Enclosure height = 0.03 m
Enclosure thickness = 0.003 m
Enclosure thermal conductivity k = 0.035 W/(m*K)
Enclosure area = 0.0144 m^2
Temperature increase inside enclosure = 14.880952381 Celcius

The temperature increase inside the enclosure of 15 Celcius compared to the enclosure's surface temperature makes sense to me. In real life the temperature increase inside the enclosure compared to the ambient temperature in "no wind" conditions may be a somewhat higher, like 20 - 25 Celcius.

What do you think, does this make sense?

Python source code:

Code: [Select]

#!/usr/bin/python
#
# Enclosure temperature increase when the closed enclosure
# is placed into the direct sunlight.
#

# Dummy object container.
class Container(object):
    pass

# Sun properties in SI units
sun = Container()
sun.power = 1000.0 # W/m^2
print "Sun power = " + str(sun.power) + " W/m^2"

# Solar panel window dimensions in SI units
panel = Container()
panel.width = 0.05 # meters
panel.depth = 0.05 # meters
panel.area  = panel.width * panel.depth # m^2
panel.power = sun.power * panel.area # Heat power from sun into panel
print "Panel width = " + str(panel.width) + " m"
print "Panel depth = " + str(panel.depth) + " m"
print "Panel area = " + str(panel.area) + " m^2"
print "Panel heat power = " + str(panel.power) + " W"

# Enclosure dimensions and material properties in SI units
enclosure = Container()
enclosure.width = 0.06      # meters
enclosure.depth = 0.06      # meters
enclosure.height = 0.03     # meters
enclosure.thickness = 0.003 # meters
enclosure.k = 0.035         # W/(m*K) Thermal conductivity
enclosure.area =  2.0 * (enclosure.width * enclosure.depth)  # m^2 Top and bottom
enclosure.area += 2.0 * (enclosure.width * enclosure.height) # m^2 Front and back sides
enclosure.area += 2.0 * (enclosure.depth * enclosure.height) # m^2 Left and right sides
print "Enclosure width = " + str(enclosure.width) + " m"
print "Enclosure depth = " + str(enclosure.depth) + " m"
print "Enclosure height = " + str(enclosure.height) + " m"
print "Enclosure thickness = " + str(enclosure.thickness) + " m"
print "Enclosure thermal conductivity k = " + str(enclosure.k) + " W/(m*K)"
print "Enclosure area = " + str(enclosure.area) + " m^2"
delta_temperature = panel.power * enclosure.thickness / (enclosure.k * enclosure.area)
print "Temperature increase inside enclosure = " + str(delta_temperature) + " Celcius"


[Ian.M]

That sounds like you are FUBARed.  I don't think there's any Lithium battery technology that can operate safely and reliably over the -30C ... +60C, including charging below 0C and at elevated temperatures.   Lead Acid is more forgiving so consider using a SLA 'cyclon' cell.

[Kalvin]

That sounds like you are FUBARed.  I don't think there's any Lithium battery technology that can operate safely and reliably over the -30C ... +60C, including charging below 0C and at elevated temperatures.   Lead Acid is more forgiving so consider using a SLA 'cyclon' cell.

What I have read, the Li-battery should not be charged when the temperature is below 0 Celcius. That is not problem, as there won't be practically any sunlight available during winter period if the device is covered by the snow. The device may disable the charging circuit if the internal temperature is 0 Celcius or below.

Charging in the elevated temperatures may also be disabled by the device when the internal temperature is above 45 Celcius.

In my understanding the heat is the enemy here. Yes, you may be correct that I am FUBARed. 

Edit: The Li-battery has nice properties like high energy density and low self-discharge rate which would allow compact design and make it possible to operate the device over winter when solar energy is not available. However the extreme temperatures are problem. I probably need to look at some other rechargeable battery chemistry.

[xani]

Supercaps have more forgiving temperature range (-40C to +70C is not uncommon), but that would mean it would have to be much bigger for same capacity and you'd probably have to include DC/DC power converter in your design to utilize most if its capacity. Also the question is, do you still need sensing when there is snow/ice on the device ?

You might want to look into hybrid solution; have solar cell feeding into supercap for "main" power supply and on top of that "backup" non-rechargeable battery with right temperature range (and some sensing to alert on bad battery). That way for majority of the time you'd be running off solar power and for times when it is impossbile, battery will provide backup

[Kalvin]

Thanks xani! Actually, I am considering to use a supercapacitor in parallel with the rechargeable lithium battery in order to help the battery in the freezing temperatures.

Also the question is, do you still need sensing when there is snow/ice on the device ?

I want to monitor soil properties also over winter time.

[GreggD]

I have some super caps. Samwha Green-Cap 500F 2.7V 35mm diameter, 61mm tall. Even after soaking them for 24 hours these still leak down to 1/2 voltage in 2-3 days. I also think four of these are very roughly equal to one AA, without a Batteroo.

[Kalvin]

Murata has supercaps which looks pretty promising:

http://www.murata.com/~/media/webrenewal/products/capacitor/edlc/techguide/electrical/c2m1cxs-053.ashx?la=en

- Specified from -40 Celcius ... to +70 / +80 Celcius
- Leakage current < 1uA (Fig. 33)

As the leakage current is said to increase exponentially as the temperature rises, the leakage current should not be a problem at low temperatures:
http://www.eetimes.com/document.asp?doc_id=1279362

[james_s]

I have a weather station which is solar powered with a battery backup. The way it works is it has a standard CR123 lithium primary cell for the backup, and a super capacitor like the sort used for memory backup that is charged by the solar panel. I haven't looked closely but I suspect there is a simple diode isolating each source. There is a commercial version of almost exactly what you are proposing to build that works the same way. No need to reinvent the wheel when you can copy part of a time tested design.