Solar Energy – DC vs AC

DC and AC in Solar Panel Systems


DC Current


Photovoltaic solar electric panels, or PV modules, generate electricity by converting sunlight directly into power. The PV modules generate direct current, or DC electricity. DC electricity is what you’ll typically find in batteries. Direct current flows in one direction (more about this vs. AC below). DC is typically hard to transmit over long distances because the voltage can’t be easily increased. The source generation needs to be matched to the load and the distance from the load. In a long transmission line, the voltage drop is current * resistance or (I *R). Wire has a fixed resistance so you need a high starting voltage to overcome the voltage drop.


DC Current Example


For example, say you have a motor that requires 1 Amp at 12 Volts. Let’s say you’re using 12 guage wire that has a resistance of 1.5 ohms per 1000 ft. If you’re close to the light, the resistance of the wires is negligible so your starting voltage can be 12V. If however, you’re a mile away then the resistance is noticeable – 1.5 * (5280ft/1000ft) = 7.92 ohms. Now the drop in the wire is 7.92 Volts (I * R = 1.0 Amps * 7.92 ohms = 7.92V) and you’ll need to start off with 12V + 7.92V = 19.92V or almost 20V to power that same motor


The Problem with DC Electricity


At the beginning of the electrical revolution, many different types of generators were needed to match various loads such as lights, motors and railway systems. You can see that this can quickly get out of hand. An interesting wikipedia article discussing the argument between Edison (DC proponent) and Tesla (AC proponent) can be found at this History of Electrical Power Transmission link. Is there a solution to this type of problem? Luckily there is and it comes in the form of AC electricity.


AC Electricity


Alternating Current is typically referred to as AC electricity. This is what you find in your house power outlets. In today’s AC systems, the movement of electrical charge changes direction many times per second. AC systems have the advantage of being easier to increase or decrease in voltage – on other words transform. A device called a transformer is used to convert a high voltage/low current into a low voltage/high current or vice-versa. If the voltage doubles, the current will be cut in half and on the other hand, if the voltage is cut in half, the current will double. This is an example, transformers can actually convert AC power in any ratio – not just ½ and 2 (ie. 4:3, 12:1, etc). For efficiency reasons, the typical AC waveform is a sine wave as shown below





 


The Advantage of AC with Power Transmission


Remember how the voltage drop in a wire is I * R? Well, if the current is 1/10 of the DC example, we’ll only get 0.792V of drop (0.1A * 7.92 ohms = 0.792V). In this case, the voltage would be 120V while the current would be 0.1A. Just before the motor, we can place a 10:1 (10 to 1) transformer which would convert 120V to 12V and the 0.1A to 1A. We have the same result 12V at 1A, but we’re getting less loss in the wire. Additionally, the wire will heat up less as a result of the lower current (the 7.92V drop is dissipated as heat).


AC Discussion and Solar Power


You may be wondering at this point, why this is important. I wanted to discuss why AC power is used throughout the home. I also wanted to point out that the electricity that is produced by solar panels isn't directly compatible with the power in your home. I'll have additional posts are articles in my newsletter to discuss these issues.


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Chad Decker