A few more theoretical questions that linger in my mind

Solar Expert Posts: 259 ✭✭
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?

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?

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.

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

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?

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?

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

8) What do they mean by PV modules being limited current sources?

9) Grounded conductors are like the black-negative conductor in a PV source circuit, grounding conductors are the actual green wire that connects to the grounding electrode conductor and this latter would be the actual rod in the ground, right?

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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?

Nope. Voltage is electrical potential which is used to overcome resistance and thus allow current flow. Current-based power sources (PV's) tend to confuse this because they flow current regardless of resistance. In a way they are 'backwards' using current to facilitate Voltage against resistance (the opposite of a Voltage-based power source).
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?

Because PV's are semi-conductors: photons hit the P-N junction and push electron across in one direction only (DC). Diodes and PV's both have limits as to how much Voltage they can 'hold at bay' before they reverse-conduct. This is why on some (but not most) installations a blocking diode is used to prevent battery Voltage from flowing backward through the panel at night and discharging the battery. Think about Zener diodes; specifically designed to allow current to flow backwards through them at a specific Voltage level, thus affording a method of Voltage regulation.
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.

The temperature affect is straightforward for conductors: cold = lower resistance, hot = higher resistance. Change the resistance, change the Voltage. Much like a battery's resistance changes as a function of its Voltage, only the other way around. It take very little light to achieve the PV's approximate 0.5 Volt per cell potential, just as you can have 1.5 Volts from an AAA battery or a D cell. Current is another matter: with increased irradiance more photons are bombarding the cell and increasing the electron flow = greater current. Electrons flow at the same speed for the most part. Bypass diodes are used to protect segments of the panel from blocking current flow in the case of shading (conductivity decreases in low light).
4) Im not clear about shunt and series resistance and how its important.

It is confusing because the resistance is a part of the power source. Just as you don't want to try and measure the resistance of a battery with an Ohm meter, the PV resistance has to be viewed as 'theoretical' or a function of the V and I. The cells you can buy on the Internet to build your own panels are typically ones rejected by manufacturers because their shunt resistance was not in line with the needs of the product: if you have one cell with a resistance of 'X' in series with one that has a resistance of '1.2X' it is like putting an AAA battery in series with a D cell; the two may have the same Voltage but will not respond the same in terms of current. (For batteries you'd be looking at discharge current, for PV you'd be looking at charge current). It is possible to have a cell so far off in relation to the other cells that it will reverse-conduct and contribute nothing to the output. Many a DIY panel builder has encountered this and wondered why the number of cells adds up to 'V' Volts but the actual output is less.
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?

I didn't define these terms, but my understanding is that insolation refers to the over-all exposure of a panel or array during a given day, whereas irradiance is at a particular point in time. Radiation would be the actual amount of light falling on the panel or array. So you have 1000 Watts per meter squared (solar radiation) being picked up by a panel that is 16% efficient (irradiance) and over the course of the day it receives 'W' Watt hours (insolation). Again, I didn't write the definitions - just trying to interpret the terms.
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?

Application of R to I/V perhaps? Normally we deal with fixed V and set or variable R providing alterations to I. With a PV under full illumination I has a maximum in respect to R and allows V to vary from 0/R = Isc to Voc/infinite R = 0I.
7) One paragraph says that PV modules are excellent battery chargers given their IV curve, but I have not idea why?

This is just someone's opinion, according to battery manufacturers who still have a good deal of trouble coping with the bell curve of current available from PV. Most battery charge specs are on a basis of constant current to a V point, but panels inevitably provide rising current (or at least potential) as the sun angle changes while at the same time battery Voltage climbs. Curiously in practice this actually seems to work better than the AC powered charger's 'full current from the start, tapering as Voltage rises' approach. I have no idea why, but I'm not the only one who has observed this.
8) What do they mean by PV modules being limited current sources?

Self-limiting current sources; they can produce no more than their Isc (under maximum sun; certain factors such as snow reflection can increase the irradiance causing the Isc to go higher than rated.
9) Grounded conductors are like the black-negative conductor in a PV source circuit, grounding conductors are the actual green wire that connects to the grounding electrode conductor and this latter would be the actual rod in the ground, right?

More wars and arguments are fought over ground than anything else. No circuit has ground unless it is actually wired to Earth at some point. Normally for DC circuits negative is referenced as ground, but a few use positive ground. Wire colour is a matter of convention: in DC black for negative, red for positive, green or bare for ground. The same is not true of AC where black is the 'first hot' red is typically the 'second hot' and white is neutral which is usually bonded to ground.

New NEC rules do not allow negative to be bonded to ground, as DC must follow the code for ground fault. Not everyone agrees with those rules.
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
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
Re: A few more theoretical questions that linger in my mind

8 ) What do they mean by PV modules being limited current sources?

The available current of the solar panel is proportional to the amount of sunlight, regardless of load. That is the definition of a Current Source. If you short a solar panel, you cannot get 10-100x the amount of current.

Batteries are voltage sources... They have stored energy internally... They can let out a little current for a number of hours, or a very high amount of current for seconds-minutes.

Arc welders are current sources (just enough current for the arc to weld, not too much to turn everything into melted metal).

For most people, they are used to voltage sources in daily life (i.e., your utility grid is a voltage source). They use current sources, but don't recognize them as such (such as the alternator in your car--Much of analog electronics uses current sources inside the integrated circuits and such, your florescent/CFL/LED lights all use some form of current source to limit the current/power going to the light source, etc.).
9) Grounded conductors are like the black-negative conductor in a PV source circuit, grounding conductors are the actual green wire that connects to the grounding electrode conductor and this latter would be the actual rod in the ground, right?

Make that a separate question... Do not connect negative (black) leads with Ground (really versions of safety grounds). In many devices today, we do ground negative bus/terminals (like automobiles). However, that is not the same as safety "green wire" grounds (we ground the white wire "neutral" in North American homes--That is an Alternating Current ground--+/- polarity does not mean anything--This is a "hot" vs "neutral" vs "return" vs Safety Ground question).

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset