I’m quite new in electronic projects and so my basics are quite limited. I've been reading a lot regarding this topic (refer to the title) and I’m currently planning to buy the suitable Solar Panel, Charger and battery for my project. The price is expensive, so I need to ensure I make the right choices. Note : the values are mostly rough estimation for me the purpose of understanding. Actual correct values will be recalculated later on.
(1) My project
I assume the RPi's power consumption is ~5.25 W or ~1200mA with Wi-Fi dongle, camera, and IR sensor connected at GPIO.
Thus, in short: - power to the RPi should be at 5V regulated, 2A - power consumption is ~5.25 W. - runs on battery solely during night time - runs on battery with Solar Panel charging during day
How to proceed in calculating suitable Solar Panel, Solar Charger and Battery?
(2) This is an example for similar project, but the calculation is not presented in details. I’m building something similar, but for only a 24 hours operation (a single day). This example covers 5-6 days without sunlight which I don’t intend to do, for now.
From http://goo.gl/bfDTfp “…I used a 50 watt solar panel, a 20 Ah Lithium battery, and a charge controller that was made for that particular battery. I also used a USB car charger to convert the 12 volts from the battery down to 5 volts… this setup is working perfectly. I've had several cloudy days with no issues. The Pi keeps on running! I also see the controller charging the battery (and running the Pi) when there is fairly low light, so I think the 50 watt panel was the way to go…”
3) Some choices that I can get locally. Are these suitable? Solar Panel - goo.gl/DKshT4 This one gives 12V 20W Battery - goo.gl/Ja9wdu 12AH, 12V I need to understand the calculation involved in order to justify these. Any help?
Update : I came out with this after further reading. Can anyone with knowledge do a check on this? is my understanding correct?
- Determine the energy consumed in a 24 hour period.
RPi usage (load) = 5.25W * 24 Hour = 126 watt-hours
- Add in the Fudge Factor for energy consumed (50% losses)
Lets multiply the total 24 hour load energy by 1.5 to account for several factors such as the system efficiencies, including wiring and interconnection losses as well as the efficiency of the battery charging and discharging cycle, and allowing extra capacity for the system to recharge the batteries after they have been drained.
New load = 126*1.5 = 189 watt-hours
- Determine Solar Insolation in Hours (disregard 9 hours of sunlight stated earlier)
Most solar map data are given in terms of energy per surface area per day (kWh/m2/day) which can be read directly as "Sun Hour Day” (hours of a day with sunlight). Kuala Lumpur receives in average around 6.0 kWh/m2/day.
- Determine the size of the Solar Panel
The size of the solar panel array is determined by having the adjusted daily load divided by the Sun Hour Day. So for Kuala Lumpur;
189 /6.0 = 31.5 watts
- Determine Battery size
All batteries will last substantially longer if they are shallow cycled (discharged only by about 20% of their capacity) whereas deep discharge means that a battery is discharged by as much as 80% of its capacity. Let’s assume we want the battery for the system discharged by 80% of its capacity. The battery daily load (the daily energy taken from the battery) should be more or the same as the energy consumed in the 24 hours period. Add an additional fudge factor for battery of about 50% to account for the efficiency of the battery discharge, the battery's capacity is available after use, and the loss in efficiency due to temperature. We have;
189*1.5 = 283.5 watt hours
This is the battery capacity, which is usually given in ampere-hours so it must be divided by the nominal voltage; 283.5 / 12 = 23.625 Amp Hours