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Organic Rankine Cycle

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Back to the Basics

Work in

Progress Therese Peffer

There are no affordable perfect conductors. Wire conducts pretty well, but even copper wire has a certain resistance. If a wire is too small for the amount of current it’s carrying, then it will heat up! I think of a traffic jam — too many electrons trying to move at the same time.

Part of figuring out how to hook up my photovoltaic panel to the battery was figuring out the size of wire to use! My PV panel produces about 3 Amperes of electrical current at about 17 Volts. I think of the voltage as the force that causes the electrons to move. Time to do a little math! A fellow named Ohm discovered a relationship between voltage, current and resistance which can be expressed as Resistance = Voltage divided by Current, or R = E/I. If you have current flowing through a bit of wire, you can measure the voltage drop across it, and figure out the resistance!

Reducing the heat

Here’s another equation: the amount of power (in watts) is equal to the voltage times the current. Low voltage systems such as my 12 Volt system, carry a lot of current compared to 117 volts ac from the utility for the same amount of power. If you look at the cords around the house you’ll notice that your cords for appliances are much smaller than the battery jumper cables. If I used a small wire for my 12 Volt system, some of the power that the panel is producing turns into heat in the wire instead of reaching the battery. More math here: Power (lost to heat) equals the voltage loss times current, P= EI. (Aside: Voltage loss equals current times wire resistance, E=IR, so substitute IR for E, and Power equals current squared times resistance, P=I2R. For a constant resistance (a piece of wire), if you double the amount of current, you quadruple the power lost to heat!)

The resistance of the wire should be as low as possible so the voltage loss will be as low as possible. The PV panel may produce 17 Volts, or on a sweltering summer day, the panel may only produce 15 Volts. For the panel to charge the battery, the voltage produced by the panel has to be larger than the voltage of the battery. This wasn’t obvious to me when I first started my system, but it makes sense! The electrons flowing from the panel have a hard job; jump-started by the sun, they have to have enough force (voltage) behind them to overcome the resistance of the connections, the wire, the regulator, and finally, the battery voltage. If the voltage of the current when it reaches the battery is only 12 Volts, then the battery won’t get charged.

So the idea is to find wire with low resistance. The total resistance of the wire depends on the type of metal, the

Trains and orchestras need conductors to guide their way, and well, electrons need them too! I’m

still putting together a small system for the trailer I live in. I figured out my loads, bought a 60 Watt photovoltaic (PV) panel to produce electricity, and have a car battery to store it in (discussed in last issue, HP #32). Now what? I need to connect the two and plug in!

What is a conductor? I have a very basic understanding of electricity. I know that there are negatively charged particles called electrons revolving around the nucleus of all atoms. Some materials are made up of atoms that let their electrons wander around, moving from atom to atom. If we can induce a current, that is, a continuous movement or flow of these electrons in a material, we call that material a conductor. For example, metal is a good conductor of electricity — copper in particular, but also aluminum. Metal wires are typical conductors.

If you look around your home, you may see many cords, for the telephone, for lights, for a radio, extension cords, or maybe battery jumper cables. Essentially all the cords are doing the same thing: conducting electricity, but in different ways.

One size does not fit all

One difference is the size of the wire. Sometimes I think of wires as freeways for electrons. A small wire, say #20 AWG (American Wire Gauge), could be a single lane road, but a larger wire such as #10 AWG would be a one way 4 lane road, and #0000 AWG is an Los Angeles interchange of 7 lanes and more stacked up!! Lower numbers correspond to larger diameter wire. The bigger the cross section of the wire, the more capacity the wire has to carry these electrons. It is called ampacity; electrical current is measured in Amperes.

86 Home Power #33 • February / March 1993

©1993 Therese Peffer

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