A recent storm with horrific winds took down our electrical power. The outage was widespread, straining the power company to get everyone back online. We were without power for almost 3 days - no heating, no hot water, no refrigeration, no range, no microwave, no lighting. No TV, no WiFi, and no hot coffee in the morning. And we were the lucky ones; many were blacked out for over a week. Time for a backup plan.
My first thought was to buy a gas-powered generator. Plenty of backup power but with one problem; it's gas-powered. Big, heavy, noisy, lots of exhaust, and not exactly eco-friendly. And it uses fuel which may be scarce when, for instance, zombie hordes rule the land. I wanted something that would provide efficient power indefinitely, without burning up the gas. Something mobile and quiet I could use while camping as well as emergency backup. Something like a small solar power plant.
Features
- Sun Tracking Sensor
- Renogy 50W Monocrystalline Solar Panel
- Renogy 30A PWM Charge Controller
- 85AH Deep Cycle Gel Batteries
- 2000W DC-AC Power Inverter With 3 AC Outlets
- Bussmann 150A Resettable Circuit Breaker
- Servo Motor for Azimuth Axis
- Linear Actuator for Elevation Axis
- Arduino UNO Controller
- Adafruit Motor Shield v2
- Backlit LCD Voltage Meter
- Adjustable LED Bar Work Light
- 12VDC Switched Outlet With 15A Resettable Circuit Breaker
- USB Charging Outlet
- Durable Battery Box With Carrying Handles
Schematic
Solar Panel With Sun Tracking Sensor
After a little research I found that a solar panel tracking the sun from morning through evening is much more efficient than a fixed panel. Estimates range from 35% to 50% higher efficiency using dual-axis tracking. The expense to implement sun-tracking can get quite high however, especially for larger systems. Given that only one medium-sized solar panel was going to be used for this project, I decided sun-tracking was needed to get maximum efficiency. Besides, tracking the sun makes for a more fun project.
A servo motor geared for high-torque controls the azimuth positioning of the solar panel, and a 12VDC linear actuator controls the elevation. Both of these were purchased from Servo City along with the aluminum tubing, brackets, and mounting hardware. The servo and linear actuator are driven by an Arduino UNO microcontroller using the Adafruit Motor Shield, and powered by the deep cycle Gel batteries.
The sun-tracking sensor is made from a cheap 6-LED flashlight. I picked up a pack of 4 of these for less than 6 bucks at the local hardware store. I removed the LEDs from one of the flashlights, cut some tracks, then soldered in 3 LDR photo sensors arranged in a triangle. These are separated by "shades" cut from a black vinyl plastic sheet that were shaped and welded together using a heat gun as shown in the photos.
The LDRs are connected via a 4-conductor cable to analog input pins A0, A1, and A2 of the Arduino; azimuth is determined by the light levels sensed by two of these side-by-side, and elevation is determined by averaging the levels of those two and comparing to the light sensed by the third LDR positioned below them. This arrangement allows the Arduino to orient the face of the solar panel towards the direction with the most light by driving the servo and linear actuator. At night, the Arduino automatically positions the solar panel to face east in anticipation of the morning sunrise. The end of a clear polypropylene test tube protects the LDR photo sensors, with a small hole drilled in the bottom to help prevent it from fogging.
The Arduino communicates with the Adafruit Motor Shield using I2C over pins A4 (SDA) and A5 (SCL). The servo motor is controlled using an output from the Arduino on pin 9. The linear actuator has a built-in potentiometer for indicating its position. One end (yellow wire) of the pot resistor is connected to +5v. The other end is tied to the wiper (white and blue wires) which are then connected to pin A3 through a 10k resistor, forming a voltage divider allowing the Arduino to sense the position of the actuator.
The metal bracket covering the base of the solar unit is a custom piece I created with eMachineshop. It has slots that fit snugly around the screw heads on the legs, allowing it to securely snap into place.
Low-Cost 6-LED Flashlights
Hacking The Flashlight
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Battery Power Pack
An Attwood Power Guard 27 box was the perfect size to house the two 12V deep cycle Gel batteries; one 50Ah and one 35Ah wired in parallel for a total of 85Ah. These are kept charged by aRenogy 30A PWM Charge Controller connected to the Renogy 50W monocrystalline solar panel via a 20 foot cable with MC4 connectors, as shown in the video.
A 2000W DC-AC power inverter is connected to the batteries through a Bussmann 150A resettable circuit breaker. The power inverter provides 120VAC to three outlets and a USB outlet handy for charging cell phones, GPS trackers, and tablets.
The Renogy Charge Controller and the power inverter are bolted directly to the sides of the Attwood battery box to keep everything together, making it easy to carry.
A sunlight-readable backlit LCD meter is also mounted to one side of the battery box, along with rocker power switch, a sealed marine 12VDC outlet, and an adjustable LED bar work light.
I deliberately avoided mounting any components to the lid of the battery box so it could be removed freely, allowing the 20ft cables that connect to the solar panel unit to be stored in the box as shown in the video.
Wiring
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Mobility
I'm happy with the compactness of the battery power pack. Housing 85Ah at 12VDC for 1KWh of power, a 2000W power inverter, a charge controller, a voltage meter, an LED light bar, multiple outlets with circuit breakers, as well as all cables in such a small space is extremely convenient for portability.
However...this thing is HEAVY! At 58 lbs. I won't be taking it backpacking anytime soon. But the handles do make it fairly easy to carry even if getting a tad winded, and I wouldn't hesitate to throw it in the back of my Jeep to take on a camping trip. And it is still very easy to move and set up for emergency backup at the house. Although I would put it on wheels if I had to lug it more than a couple hundred yards - maybe in a wheeled cooler or on a luggage wheeler. Still, beats dragging around a heavy gas generator with fuel hands down.
The solar panel unit is much lighter at 15.2lbs but a bit more awkward to carry and would take a fair amount of space in the back of the Jeep. I've considered modifying the design to allow the panel to be easily removed, and collapsible or removable legs would probably also help make it easier to transport. I wouldn't call it fragile, but obviously it's not as rugged as the battery power pack.
One other complaint: the screw terminals on the Renogy Charge Controller are under-sized. They're almost too small to capture the #10 MC4 cable reliably, and I always make sure to have a screwdriver on hand to tighten up when needed.
