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From Storm to Inspiration

August 17, 2015
From Storm to Inspiration

It all began with an intense storm with powerful winds that knocked out his family’s electrical power.  One day without power turned to two and then almost three full days without any power – no hot water, no refrigerator, no heating, no range, microwave, WiFi and no hot coffee.  “We were the lucky ones; many were blacked out for over a week,” Schulz said.

So Kurt thought, “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.”

How long did it take to build?

I spent about six weeks on this project, working (playing?) mostly weeknights and weekends. I spent a day writing up a list of features and drawing rough sketches, so I had a pretty good idea of what I wanted from the start. I then ordered the mechanical and electronic parts from ServoCity and Adafruit. The Renogy solar panel, charge controller, cables, batteries, battery box, and nearly everything attached inside and outside the battery box were purchased from various vendors on Amazon. While waiting for parts to come in, I started on the Arduino programming for sensing sunlight and controlling the servo and linear actuator. Much of the basic code was hammered out by the time the parts arrived.

What difficulties/challenges did you run into while designing/building? What was frustrating? What went well?

My background is in embedded controllers and programming, so the electronics and electrical came together without much difficulty. The big learning curve for me was on the mechanical side, with the two biggest challenges being 1) sizing and fitting the linear actuator and 2) creating a strong “base” stable enough to support a moving solar panel structure.

Using PVC pipe and foam project board, I threw together a mock up of the linear actuator to get an idea of the stroke length. I’m glad I did this; I found I needed a much shorter stroke (2″) than what I had originally thought. I wasn’t exactly sure how much thrust was required either, and eventually settled on 115 lbs. Maybe overkill, but I figured better too much than too little, especially considering another solar panel might be added later for more charging power.

Project Description 1

My rough sketches had a sleek, stout base with a low center of gravity and telescoping legs spread out in a tripod arrangement. Once I got the servo motor and mechanical parts in, I quickly realized this wasn’t going to happen. The frame for the servo motor somewhat limited my choices for what I might consider to be an elegant but sturdy mounting base. I ended up using various angle brackets to cobble together a five-legged stand. It turned out functional, but barely so and is the one thing I really want to improve on. It’s not as transportable as I’d like and ground stakes or sandbags are needed to keep it upright on a windy day.

Project Closeup 1

Project Closeup 2

Favorite part about this project?

I’m very happy with how the sun direction sensor and 2-axis solar panel control turned out. I’m also satisfied with the amount of power provided by the compact battery unit, given its intended use. The system is also easily scalable; batteries can be added in parallel along with a second solar panel to increase the amount of available energy.

Let’s take a look at his parts list and instructions….

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

Solar Power Plant Features

Arduino/Motor Shield

Kurt went with a tracking panel instead of a fixed panel due to its efficiency (estimates ranging from 35% to 50% higher efficiency using dual-axis tracking). Sun-tracking units can definitely cost more, especially if you’re building a large system. Kurt went with a medium sized sun-tracking unit.

A servo motor geared for high-torque controls the azimuth positioning of the solar panel, and a 12VDC linear actuator controls the elevation. Both were purchased from ServoCity along with the aluminum tubing, brackets, and mounting hardware. The servo and linear actuator are driven by an ArduinoUNO microcontroller using the Adafruit Motor Shield, and powered by the deep cycle Gel batteries.

Project Closeup 3

Project Closeup 4

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 Kurt created with eMachineshop. It has slots that fit snugly around the screw heads on the legs, allowing it to securely snap into place.


Parts closeup

parts closeup 2

Parts closeup 3

Parts closeup 4

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 a Renogy 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 closeup 1

Wiring closeup 2

Solar Power Plant Closeup

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 weighing in at 58 lbs. The handles do make it fairly easy to carry and still great to take on a family 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.

A big shout out to Kurt for sharing his project and info. from his blog.  Be sure to check out his work!

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