Physicist answering solar related questions

[e350_ambo_camper]

Hello All,

I am a physicist with 25+ years experience in PV, energy storage, thermal & related fields. I am also the owner of Bogart Engineering. I am happy to answer any questions on these topics as they relate to off-grid, boondocking & related topics. I will not make specific product recommendations (including my own), but try to offer un-biased, science based responses.

- Kedar

 

[jadmt]

is true if I have a potato, foil from a stick of gum and paper clip i will be able to jump start my vehicle? Just kidding look forward to learning as I need to get going on a small portable solar set up for the camping season.

 

[e350_ambo_camper]

is true if I have a potato, foil from a stick of gum and paper clip i will be able to jump start my vehicle?

Absolutely! if this is what you have under the hood -

 

[dwh]

Welcome to ExPo!

Q: Assuming that a PWM would work (i.e., not so great a spread between Vbat and Vmp that an MPPT controller would become a requirement just to handle the voltage mismatch), at what point does it become "worth it" to use MPPT?

Obviously, that's a somewhat subjective judgement call, so I'm asking for your judgement. Is it worth bothering with MPPT for 100w of solar? 200w? 300w?

Where's the point at which the gains become significant enough?


(This is an honest request for an opinion, it is not a leading question or a trap. I am aware of your products, have read the extensive documentation and am not against PWM.)

 

[emtmark]

What is the simplest explanation of monocrysteline vs amorphous. I really really like the fold up “blanket” style solar. the price for the amorphous are out of hand and the cheaper versions are monocrystaline are significantly more affordable. What am I giving up by getting the cheaper version?


Sent from my iPhone using Tapatalk

 

[e350_ambo_camper]

Welcome to ExPo!

Q: Assuming that a PWM would work (i.e., not so great a spread between Vbat and Vmp that an MPPT controller would become a requirement just to handle the voltage mismatch), at what point does it become "worth it" to use MPPT?
Obviously, that's a somewhat subjective judgement call, so I'm asking for your judgement. Is it worth bothering with MPPT for 100w of solar? 200w? 300w?
Where's the point at which the gains become significant enough?
(This is an honest request for an opinion, it is not a leading question or a trap. I am aware of your products, have read the extensive documentation and am not against PWM.)

My standard response for this PWM vs MPPT question is: If you're chasing the Sun (camping in relatively warmer areas during the winter), PWM ~MPPT. If you're chasing snow (Aspen, CO in Jan), MPPT >> PWM. Most people don't have true appreciation of how panel/cell temperature affects the voltage (Voc & Vmp) of a panel. On paper, MPPT harvests 15-25% more power than PWM. However, under typical use conditions, the panels heat up so much that the 20% advantage is lost.

Both lead acid & lithium battery voltages rise quickly in the early stages of charging. So, even if a MPPT controller harvests more power the early stages of charging, this advantage is soon lost as the battery fills up.

The cost differential between PWM & MPPT controllers is ~$100, which will buy an additional panel & provide 100% more power/current.

edited my response for accuracy; edits in red.

 

[e350_ambo_camper]

What is the simplest explanation of monocrysteline vs amorphous. I really really like the fold up “blanket” style solar. the price for the amorphous are out of hand and the cheaper versions are monocrystaline are significantly more affordable. What am I giving up by getting the cheaper version?

simple explanation - amorphous silicon (a-Si) cells lack the orderly crystal structure mono-crystalline (c-Si) cells. This lack of order makes the photon -> electron conversion process less efficient; also inefficient is the process of collecting those electrons. However, a-Si cells can be made by relatively efficient deposition processes. This makes it possible to make commercially viable ($/Watt) a-Si cells. a-Si cells have 1/3 the efficiency of c-Si and hence, require 3X area to produce he same power.

 

[dwh]

What is the simplest explanation of monocrysteline vs amorphous. I really really like the fold up “blanket” style solar. the price for the amorphous are out of hand and the cheaper versions are monocrystaline are significantly more affordable. What am I giving up by getting the cheaper version?

I think you've got that backwards. Monocrystalline is generally more expensive (and more efficient) than amorphous.

 

[e350_ambo_camper]

Obviously, that's a somewhat subjective judgement call, so I'm asking for your judgement. Is it worth bothering with MPPT for 100w of solar? 200w? 300w?
Where's the point at which the gains become significant enough?

There's really no upper limit to PWM's advantage for flat-mounted rooftop systems. Under these conditions, the panels tend to run hot and MPPT's theoretical edge is lost. Some connect panels in series under the mistaken belief that boosting the input voltage somehow recovers the temperature induced lost power. Or, use larger (250W+) residential panels which operate at higher voltages.

We have been recommending 100 W panels with lower Voc/Vmp. These panels generally have a higher Isc and PWM controllers deliver this to the battery. In addition, installing solar panels on camper rooftops is like playing Tetris and smaller panels allow to you play this well.

 

[e350_ambo_camper]

I think you've got that backwards. Monocrystalline is generally more expensive (and more efficient) than amorphous.

emtmark is correct. China has flooded the market with inexpensive mono-crystalline panels, driving the price down. Amorphous panel makers now target niche markets where size, weight, portability is important and have priced their products accordingly. Several large scale a-Si panel makers for utility scale applications have ceased operations.

 

[Rando]

My standard response for this PWM vs MPPT question is: If you're chasing the Sun (camping in relatively warmer areas during the winter), PWM > MPPT. If you're chasing snow (Aspen, CO in Jan), MPPT > PWM. Most people don't have true appreciation of how panel/cell temperature affects the voltage (Voc & Vmp) of a panel. On paper, MPPT harvests 15-25% more power than PWM. However, under typical use conditions, the panels heat up so much that the 20% advantage is lost.

Both lead acid & lithium battery voltages rise quickly in the early stages of charging. So, even if a MPPT controller harvests more power the early stages of charging, this advantage is soon lost as the battery fills up.

The cost differential between PWM & MPPT controllers is ~$100, which will buy an additional panel & provide 100% more power/current.

I am not sure I understand this argument. In terms of power production, there are vanishingly few scenarios where PWM > MPPT. It is true that if your cell temperature is high enough that Vmp is reduced to near Vbat then PWM and MPPT are essentially equal. But under almost all other circumstance, particularly the non-ideal circumstances often encountered in mobile installations such as partial shading, then MPPT out performs PWM. Looking at the spec for one of the most common 12V panel for mobile/RV installations (the Renogy 100W panel):

You would need cell temperatures around 100C to drop Vmp down to typical bulk charging voltages around 13.xV where PWM and MPPT would be equal, which would require an unusual set of circumstances.

The claims that that MPPT beats PWM by 30% are dubious as it is only true under a very limited set of circumstances and the gains are usually less, but the same can be said for the claim that PWM matches or outperforms MPPT, it is only under a very limited set of circumstances that this is true.

The point is that price differential between quality MPPT and PWM controllers has largely disappeared, making it hard to see a reason to go with PWM.

 

[dwh]

emtmark is correct. China has flooded the market with inexpensive mono-crystalline panels, driving the price down. Amorphous panel makers now target niche markets where size, weight, portability is important and have priced their products accordingly. Several large scale a-Si panel makers for utility scale applications have ceased operations.

Huh. Learn something new every day.

So the answer to your question entmark is that you don't lose anything going with the mono - they're more efficient. And if they are now cheaper as well...

 

[luthj]

Huh. Learn something new every day.

So the answer to your question entmark is that you don't lose anything going with the mono - they're more efficient. And if they are now cheaper as well...

Maybe not for long. Economies of scale and government subsidies have driven down the price of the mono cells. However, they are set to rise over the next couple years. As grid scale solar installs increase we will likely see amorphous or other lower efficiency but low cost cells types increase dramatically.

 

[e350_ambo_camper]

I am not sure I understand this argument. In terms of power production, there are vanishingly few scenarios where PWM > MPPT. It is true that if your cell temperature is high enough that Vmp is reduced to near Vbat then PWM and MPPT are essentially equal. But under almost all other circumstance, particularly the non-ideal circumstances often encountered in mobile installations such as partial shading, then MPPT out performs PWM. Looking at the spec for one of the most common 12V panel for mobile/RV installations (the Renogy 100W panel):

You would need cell temperatures around 100C to drop Vmp down to typical bulk charging voltages around 13.5V, which would require an unusual set of circumstances.

The claims that that MPPT beats PWM by 30% are dubious as it is only under a very limited set of circumstances and the gains are usually less, but the same can be said for the claim that PWM matches or outperforms MPPT, it is only under a very limited set of circumstances that this is true.

The point is that price differential between quality MPPT and PWM controllers has largely disappeared, making it hard to see a reason to go with PWM.

Panels don't have to reach 100C; charging performance suffers when the panel voltage drops to 15 Volts. Panel/cell temperatures often reach 70C even in mild conditions. After all, the cells are designed to absorb as much sunlight as possible, but convert less than 20% in to electricity. Add to this temperature induced loss, the power consumed by the controller. Most inexpensive MPPT controllers are *not* 97% efficient, but around 90%; PWM are by design more efficient. This difference is quite apparent when you compare the heat sink size of MPPT & PWM controllers. This makes PWM comparable to MPPT under typical use conditions ( I have edited my initial response to reflect this).

The real advantage of PWMs for rooftop camper applications is reliability. With PWM controllers, we recommend running individual wire pairs from the rooftop to the controller's location, where they are combined in to a single pair. In most MPPT installs the panels are wired in series. If a panel is shaded or fails, the remaining panels continue to deliver power. When panels are connected in series, as they often are for MPPT controllers, shading, poor MC4 connections, etc can have a significant effect on overall performance.

 

[e350_ambo_camper]

From a paper written by Victron's CTO (https://www.victronenergy.com/uploa...Which-solar-charge-controller-PWM-or-MPPT.pdf)

PWM & MPPT are with in 5% of each other under typical use conditions. not included is the controller power consumption which can be 7-10W/100W.