A few more theoretical questions that linger in my mind

Re: A few more theoretical questions that linger in my mind

quique wrote: »

In reviewing the NABCEP study guidelines, some doubts came up:

1) I understand electricity can be compared to water in a system. And that has helped me understand some things. Now as for Voltage and Current, Im trying to understand IV Curves. I am thinking of voltage as a differential that must be “overcome” in order for current to flow. This makes me understand that at low voltages, currently flows freely but as voltage gets higher, the difference is so large that electrons cannot flow anymore and thus current drops. Is this correct?

Think of electricity as a source of power, as in a hydrological system... Not just the delivery of water as the product.

Pressure * Volume ~ Voltage * Current

If you need a little amount of power at the hydraulic (remote) motor/tool, then a small hose with low pressure and low volume is just fine.

However, if you need a lot of power at the remote motor, you need to increase either pressure or flow or both...

You can have a 10x larger hose cross section (more flow)--But the hose is big and bulky.

You can have 10x higher pressure, smaller hose, but must be able to withstand the extra pressure.

Or you can have ~3.2x the pressure and 3.2x the volume... And of the above three options will give you your 10x amount of power at the remote motor.

Voltage = I*R = Current * Resistance

Voltage is the result of flow and the amount of resistance. It is not that low voltage is different than high voltage--It is simply the result of things moving through a "restriction".

Of course, voltage is a "potential"... The battery has 12 or 24 etc. of potential. Resistance (really impedance--another topic) will simply let you calculate the rate of flow (Amperes are basically electrons per second rate of flow--Of course there are a heck of a lot of electrons in a chunk of wire One ampere of current represents one coulomb of electrical charge (6.24 x 10¹⁸ charge carriers) moving past a specific point in one second.).

2) I understand that a diode is a component that allows current to flow in one direction but not in the other. Ive seen questions about "why solar panels act like diodes". Why is that and why is it important?

It is not that solar cells accidentally behave like diodes... They actually are diodes that happen to generate electricity when exposed to light. They are PN Junctions--Just like any other diode.

And they are optimized to generate power from sunlight (i.e., very big cell exposed to light--With electrical terminals on the front and back of the cell to better capture the resulting current flow--potential).

Because of the way solar cells are designed, they just are not really good diodes for use in electronic applications.

3) Back to IV curves, there are 2 important effects to understand, temperature and irradiance. With a temperature change, the IV curve shifts from right to left, meaning V is mostly affected. Whereas a with an irradiance change, the IV curve shifts from top to bottom, meaning mostly I is affected. I know that when its sunrise, voltage starts reading, even though the sun is not overhead, I just don’t know why. So that is in line with the fact that irradiance affects current and not so much V, since voltage is quite high even when the angle of irradiance is not ideal. But I would like to know, conceptually, why. Likewise for the Temperature, I know hot temperatures affect solar panel output but Im not sure why greater temperatures affect current by dropping it. If the panel gets hotter, won't electrons flow faster? Unless shunts or bypasses have something to do with it.

It is the physics of the materials (silicon in this case). Don't combine/mix the effects together... It is easier to think of them as separate "things".

First, the electrons (or probably really positive "holes") are sort of moved by the photons from the light--I.e., they are swept across the PN junction.

The basic voltage across that PN junction is Vmp~0.5 volts for a silicon based cell. And it (sort of) takes one photon to move one hole across that junction (actually efficiency is much less for lots of reasons--I am not a physics).

So, you can see why the amount of sunlight affects the current (basically current is directly poprotional to amount of sun light--and linear to ~10% or so--I.e., 2x the sunlight, 2x the available current). Of course, there are minimum thresholds or other effects (like leakage current, sufficient energy to go across the PN junction, etc.) that mean low light will result in almost no current on a typical solar panel.

Temperature has two major effects. One effect is to drop the Vmp of each cell (by upwards of 20% at high panel temperatures in full sun, no wind) or Vmp~0.4 volts.

If you have an MPPT (maximum power point tracking) controller (MPPT solar charge controllers, and probably all Grid Tied Inverter), remember that Power=Voltage*Current. As voltage falls, the useful power falls too.

PWM controllers (less expensive solar charge controllers) do not "care" about Vmp-array (as long as it is >17.5 volts for a 12 volt battery bank to account for temperature, wiring resistance, etc. losses)--It only passes current.

That is why you do not "care" about temperature deratings for PWM controllers--Other than if the Vmp-array-hot falls so much, there is not enough voltage to recharge the battery bank (and overcome other voltage drops in the system).

And there is a second effect where current efficiency actually rises with temperature (hot panel, more available current)--But this effect is something like 1/3rd or 1/10th of the voltage effect--So it is usually ignored as being so small.

Lastly, there is the tilt of the panel... More or less, the first order effect is the Cosine Error... Simply, as you tilt the panel from the sun, there is less surface area exposed... Cosine 0 degrees = 1.0 (100%) or full available power.

Cosine of 10 degrees = 0.984 = 98% of rated solar power--Virtually 100% of sunlight

Cosine of 60 degrees = 0.5 = 50% of available solar power...

Another effect of off angle sun--Reflections. As you tilt the panel away from the sun, it begins to act more like a mirror--And less light hits the panels themselves. There are surface finishes for the glass and solar cells that can make reflections less--And increase the efficiency of off angle sun.

Combine all of that together--Then you get your actual solar output--Not your theoretical output.

4) Im not clear about shunt and series resistance and how its important.

May need to know more about where these terms are used... Series resistance is simple... A small wire has less copper cross section, a higher resistance, and as you pump more current through it, higher voltage drop (and more losses).

Remember that there are other power equations:

Power = V*I = I²R = V²/R

If the resistance of the wire is fixed--If you double the current, the power loss goes up by 4x (2²). Other secondary effects, hot copper wire has higher resistance, so even more losses as the wiring heats up.

5) In the NABCEP guidelines we are asked to distinguish between solar radiation, solar irradiance, solar irradiation and solar insolation. Im assuming solar radiation is what they refer to the concept of EM radiation from the sun. Solar irradiance should be the 1000 W/m2 as a standard, power measurement. Solar irradiation would be the amount of energy fallen over a period of time such as 1000 W / m2 x 6 hours = 6 kW/m2 a day. But Im not so sure about solar insolation?

There are lots of ways of talking about the amount of energy from the sun and what can be collected... Other countries use MJ/Day or MJ/Month, MJ/year etc. Sometimes it is just unit conversion or just a term used when filling out forms/using a spread sheet to size a system/predict output. Usually, paying attention to how the term is used in a specific application will tell you want you need to know (I am terrible with terms/English).

6) We are also asked to determine the operating point on a given IV curve given the electrical load? I can follow the curve and sort of see how I affects V as I mentioned in #1, but I don't how loads would affect it. I do understand that power output changes from an unloaded circuit to a loaded one, but in some cases, doesn't having a load pulling more current actually make the module produce more?

It depends on the type of loads... There are constant resistance, constant current, variable impedance, etc... It is actually a major part of the course work for any electronics/electrical engineering degree.

The simple way to look at IV curves--In theory, you put a source IV curve over a load IV curve, and where they cross is your operating point.

You have seen IV curves for solar panels... Pay attention to what changes the IV curve of a load... For example the slope of the IV curve for the load becomes flatter with less resistance (the slow of the line is the resistance in an IV curve).

The slope of an "ideal" solar panel for an IV curve is a vertical line (i.e., same amount of current for any output voltage until some maximum output voltage is reached). A real solar panel curve has some slope to it (internal panel resistance, the limiting voltage of Vmp, Voc, temperature, etc.).

It depends on what "produce more" means... With MPPT controllers, the "produce more" is more Power... Power=Voltage*Current.

Some solar panel IV curves will also plot the V*I curve too... And you will see a peak Pmp at Vmp of the panel.

Look at it another way, maximum current is Isc--Short circuit current. However, voltage is zero, so P=0v*Ia=zero watts. Similar, at Voc, current is zero amps, and the power output is zero watts.

7) One paragraph says that PV modules are excellent battery chargers given their IV curve, but I have not idea why?

Basically, an ideal battery will hold 12 volts at any current... If you connect a 24 volt battery to charge a 12 volt battery--It will be a variation of a dead short. The 24 volt battery will output 100s to 1,000s of amperes to keep 24 volts. And the 12 volt battery will absorb 100s to 1,000s of amps to keep 12 volts... Who loses--You with cherry red electrical wiring and/or melted/burning/exploded batteries.

An ideal current source will maintain a fixed current at any output voltage... So a Imp=10 amp solar panel will output 10 amps between zero volts and ~Vmp volts. No smoking wiring, no puddle of lead and sulfric acid on the flow.

Voltage and current sources (battery=voltage source; solar panel=current source) are another major topic for electrical

-Bill