Sunday, May 3, 2009

Solar powered Mac Mini

A little backgound
I live in a ~750 sq. foot apartment with my girlfriend and dog, and my utility bill was ~$100. That seemed a little high, so I went to work on bringing down the bill. I first replaced all the light bulbs with either compact florescent or L.E.D bulbs as well as set my Mac to turn off at night and back on in the morning, on a schedule. These two actions alone, brought the bill down to ~$75. This was nice, and we've been living with this bill since Christmas, but I knew we could do better. I'm sure that most of the bill is made up of the dryer and my server that I run 24 hours a day, but things like the laptops we both have, the phone/camera chargers, and my Mac could be run completely off of solar power.

What you'll need
Before this project, I knew nothing about solar power, batteries, how to measure power consumption, or how to hook it all up. After some research, I was able to piece it all together, and I now have a working solar system.

So, how do you build it? It all breaks down to the following materials:

1. Solar panels
2. A charge controller
3. A battery or batteries
4. A power inverter
5. Wiring

First things first, the panels. There are a ton of places to get panels, but since I'm new to this, and I love Amazon, I felt I probably couldn't go wrong just getting them online at Amazon. The first thing I did was search Amazon for "Solar Panels". I got a ton of hits, that ranged from 5 watts, to a few hundred watts. I wasn't sure how many watts I needed, so I didn't just want to buy anything. I took a look at the Google, and figured out how to calculate the watt/hours your electronic uses. The quick and dirty is this:

Take a look at the back of your equipment for a tag that has info about the power output. You'll see something like "writing... 100v ~ 2.5a ...more writing". To figure out the watt/hours, you take the Volts x the Amps. So the above calculation, would be 100v X 2.5a = 250w. Now add up all the watts from all the appliances you want to run, and then you'll know how much power you need to run your equipment. I figured my Mac takes around 40Watts and if I wanted to run it for 8 hours, I need to be able to make 320Watts of power. I was conservative and figured I'd only have about 5 - 6 hours of good sun a day. That means I could probably get away with a 60w system. So I went back to Amazon, and I after browsing, decided on a Sunforce 60w kit. This kit came with the panels, and it also came with an inverter, charger controller, and wiring. And best of all, I got it for $300.

So, all I needed now, was a battery. You'd think I could just buy any old car battery, and I'd be set. This is probably true, but there are some things to take into consideration. First, and most important is safety. You need to remember, that standard lead acid batteries will leak flammable gases when charged as well as become pretty hot. That being said, if you have a dry, ventilated, fire-walled area, you can build a nice battery cluster for a good price. This is not the case for me, so I needed to explore other avenues. If you plan on running your system in your house, you'll probably want to look into agm sealed batteries. These batteries don't leak gases, but do still get hot. There is a premium to pay for no gases though, and if you can use standard lead acid batteries, you should.

Because I practice agile, I believe in building a vertical slice of the system before I build in bells and whistles. Because of this, I went with a simple charge controller, and I am starting with a one battery system. As you search for batteries, you'll notice that there are a lot to choose from and they range from pricey to expensive. I decided on a Duracell powepack 600 (for $130) not because it's the best battery for the job, but because it is not just a battery (it had a built in 600w inverter), and if I decided that I don't have room for my panels and system, I'd be able to use my battery for many other things. Plus, when I have my vertical slice complete and have work out the kinks, I'll build a true battery cluster.

Putting it all together

The first thing I did was lay all my panels out:

4 15W solar panels

You'll want to build a frame to hold your panels. The Sunforce kit came with a PVC frame, but you could easily build this yourself.

PVC rack/stand for my solar panels

Next I attached the panels to the frame.

4 15W solar panels

I then laid out all the wires, and examined the mess.

Various wiring for my solar panel generator kit

It's important to keep in mind that you need to maintain a 12v system. That means when adding panels, or batteries to the system, everything needs to be in parallel (positive to positive, negative to negative). As long as you keep that in mind, you should be pretty safe and not damage anything. My panels came with a four way connector that wires them in parallel, but when I add more panels, I'll need to figure out how to splice the new panels in. Also important to note, is that the inverter that came with the panels, is only a 200w inverter. If you plan on running anything bigger than a laptop computer, you may want to think about investing in a bigger inverter. I am using a 600w inverter.

Once my batter came in, I just needed to hook it all up. I first charged my battery from AC over night, to get it topped off.

The battery and inverter

Then I mounted the solar panels on the outside of my second story apartment balcony.

I used 10 inch house clamps and hung the panels from the top rail. This allows them to swing out and catch more sun.

I also bundled all the wiring with zip ties.

Here is what they look like from the ground.

My solar panels are installed

Here is what they look like making free power.

My charge controller

I've been running the system for about a week now, and I love it. I charge my phone on it every day, run my laptop off it every night, and I've had the TV and Mac hooked up to it. I'll be rearranging my entertainment system, so that the Mac can use the battery everyday as well. I've run the battery all the way down once (we've had a few days of rain, that didn't allow the battery to charge all the way), but otherwise it's charged and ready to go everyday when I get home. When using my laptop, I get about 7 hours off the battery. When using the TV, I get about 3 hours.

The future of the system

The next thing I will be doing, is building a Tweet - a - watt and attaching it to the inverter, so see exactly how much power I'm using from the sun.

A few months down the road, I'd like to buy a two or three stand alone lead acid batteries, and build a cluster and put it in a box on the balcony. After that, I'll probably buy a few more panels, as I'd like to ultimately have a 100w system.