Wednesday, February 25, 2009

Day 28 - The Last Math Blog and Installation

Thanks for sticking through the math in the last post, we have a little more to do today and I promise the blogs for next week will be calculation-free (except multiplying recipes and the magic of baking). For me, it was a really interesting experience to be in a mixed-level math class, sitting all day working out equations, and be truly enjoying myself.

Step 4: The Charge Controller, the Inverter, and the Wires

CHARGE CONTROLLER - function is to maintain the batteries at the proper level of charge and to protect them from overcharging. When a solar panel is bringing in power it will first go to run the loads on the system (lights, fans, etc) and then any extra will go to charge the batteries for use later.
Power = 100W (1 panel) divided by 12V = 8.3 Amps
Of course, we could use a 12V/12 Amp charge controller, but that wouldn’t give us much room to control for sudden increases in voltage, etc. The max voltage is taken into consideration by the manufacturer, so really a 12amp CC really has a max voltage of about 22 or so. On farm we have a currently unused 12V/36amp Charge controller, so, we will use what we have. A charge controller will run between $100-250, or more depending on the size and quality.

INVERTER – Because the PV system runs in DC, you must use an inverter if you would like to power AC devices. You need to keep in mind the maximum (peak) watts needed at one time. Our client, Chef Keith, needs 86 watts of AC power at one time if he were running all of his devices at one time. That is NOT very much because this system is already maximized for efficient appliances and lights. Much like the CC, we have a 12V/150W Inverter, which will cover the 86watts needed and give some head room just in case he introduces a new device. The cost of an inverter is dependent on the shape of the sine wave. If it is a modified sine wave it will be cheaper, but if it is too choppy it will not have an even current.

WIRES – I am not going to go into this calculation because it turned out to be a huge pain. Because we are only using a 12V panel and a small CC, we cannot increase the voltage to decrease the amount of current, so there will be a lot current and thus, a lot of resistance. Here is one calculation:
The distance from the panel to the CC is 30 feet
12Gauge wire has a resistance of 1 ohm drop per every 650feet, so the drop over 30 feet is about 0.05 ohm.
If we have to keep voltage at 12, the max current is about 16Amps.
Power loss = I2R
162x0.05 = 12.8W, it’s a 220W system, so that is about a 6% loss. 6% is too much loss, we had to move up to 8gauge wire in order to get low enough resistance.

Step 5: Installation

We mounted the one panel on the top of a very long board, anchored to the yurt platform. Installation would have been better if it hadn’t been a rainy day, so whenever the downpours got worse we went inside the yurt to take care of installing the batteries into the floor of the platform. I didn’t get to do much by myself because of the number of people involved in the task, but it was good to see how to hook up the big 6V batteries into two series strings and then hooking those in parallel.

Can I put one together for you with this one-week tutorial? No, but I have a much greater understanding of the components.

Moreover, learning about solar energy has been a great lesson in the excesses of our modern lifestyle. When you list every appliance, light, AC adapter that you plug in, you start to realize just how much energy your home consumes. Do we need an inversion blender, a mixer, a food processor, a magic bullet, and a small chopper in every kitchen?

Do you really need a curling iron?

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