Went solar

[orezok]

I believe they are talking ambient.

 

[beppington]

The school district I work for out here in the desert looked into solar. After all, what could be better than 330+ cloudless days. We talked to the sales people from several different solar companies and were given glowing reports on how much power we would produce.

We then had our professional engineers evaluate. A different story for us. It turns out that PV panels are most efficient at 59 degrees F. As the temperature climbs the cells produce less and less electricity and at about 110 degrees they about stop performing. Heat within the PV cell itself increases the internal resistance to a point that voltage diminishes to a minimum. Not a consideration for many, but it just didn't fit our environment.

The only other item which they considered a maintenance item was the inverters. They predicted a 7 to 8 year life based upon actual experience.

Currently there are several huge commercial solar power plants in development west of Blythe CA. It's interesting to note that they all will be solar concentrators and not PV.

That implies that there's no solar energy to be had when it's 110 outside. Sure seems counterintuitive to me, but I'm no expert.

 

[Phils]

That implies that there's no solar energy to be had when it's 110 outside. Sure seems counterintuitive to me, but I'm no expert.

We've had plenty of 110 degree days this summer and there was no noticible reduction in power harvested from our array. Daily KWH totals is what I check when I return home from work.

Yes, high temps reduce the power the array will produce (nowhere near the estimate mentioned above) but on those days the sun shines longer to make up for it.

It's those week-long stormy periods that are the only problem I haven't yet figured out a way around, so far the propane bill just rises with the generator use during those times. Adding more panels won't help if there's no sunshine.

Phil

 

[ShenandoahJoe]

If you're doing long-range planning, here's something I heard from the chief engineer of a pretty-big airport. TVA told him he should expect his cost of electricity to basically double in the next five years. Don't know if that applies to residential customers, as well. But that's where I'm going to be starting my baseline, when I decide whether to switch over.

 

[orezok]

That implies that there's no solar energy to be had when it's 110 outside. Sure seems counterintuitive to me, but I'm no expert.

Think of an electric resistance heater. The hotter the wire gets the more internal resistance. If you put a clamp meter on a heater and turned it on, the amperage would spike until the wire got hot (a matter of fractions of a second). As the wire heats, it's internal resistance increases until it reaches a balance that is the output of the heater i.e. 1500 watts. That's why wire lowered to cryogenic temperatures has virtually 0 resistance.

The same condition occurs internally in a PV cell.

Maybe what I should have stated is that at high temperatures (and it reaches well into the 120's here), the power produced was less than necessary to achieve the return on investment that we required. This was effectively 0 return on investment, not 0 power.

Sorry for the confusion.

 

[eepete]

The industry warranties for degradation tend to be 90% of rated value at 10 years, and 80% of rated value at 25 years. My panels (Sharp ND-216U1F) have a 25 year limited warranty. Their rating is 216 Watts with an initial tolerance of +10/-5%.

The data sheet for the panel also has temperature coefficients for the panels. The panels have their ratings at 25 degrees C. The panel I have has a derating of -0.485% per degree C. The data sheet does not state if this temperature is ambient or the surface of the cells. I'll go out tomorrow and measure my ambient vs. cell surface temperature so we can put a number on that.

So at 77 degrees F (25 C), the panel in "full light" makes 216 Watts of power. At other temperatures:
90F makes 208 Watts
100F makes 203 Watts
110F makes 197 Watts
120F makes 191 Watts
448F makes no power (and I suspect the plastic on the back is melting )

The temperature derating is linear, and I suspect it's only good for real planet tempuratures not the extrapolated zero power out point :laughing:.

Note also that as the temperature goes down, the output goes up. So:
50F makes 226 Watts
32F makes 242 Watts
0F makes 260 Watts

And again, there is probably a limit on this end too. Temperature has other effects on copper interconnections used. Temperature cycling is probably also a prime means of wear out, even though it takes decades. For here in North Carolina, I'll see an output range between 242 and 200 watts for my 216 watt rated panels. The "110 degree and they stop performing" doesn't make a lot of sense, and I suspect there is other criteria applied to the system performance that makes that temperature a cut off point (as orezok mentions).

ShenandoahJoe's post points out the difficulty of make decade long or more guestimates on payback periods, a point I danced around on a previous post.

BTW, in the last 24 hours I made 86% of the energy my house needed, and made money to boot! Lower temps means lower HVAC use, this is great time of year!

Pete

 

[sandman2234]

Interesting reading, thanks to all posters.
David from jax

 

[RobertN]

Reminds me of dark current noise on image intensified CCD's. The photonic emission microscopes I used to work with had peltier cooling, with liquid cooling of the peltier. Overall affect was -65C on the CCD. When first powering the system, at 25C, noise on the camera was terrible. As the CCD cooled down, the current, and hence noise, minimized. -65C was as cool os we could go without going cryogenic.

We were detecting photonic emissions for failure sites on IC's, while in a dark enclosure. The image intensified CCD could pick up very small photonic emissions, common to a number of leakage types on memory and microprocessor chips.

emmi

The data sheet for the panel also has temperature coefficients for the panels. The panels have their ratings at 25 degrees C. The panel I have has a derating of -0.485% per degree C. The data sheet does not state if this temperature is ambient or the surface of the cells. I'll go out tomorrow and measure my ambient vs. cell surface temperature so we can put a number on that.

So at 77 degrees F (25 C), the panel in "full light" makes 216 Watts of power. At other temperatures:
90F makes 208 Watts
100F makes 203 Watts
110F makes 197 Watts
120F makes 191 Watts
448F makes no power (and I suspect the plastic on the back is melting )

The temperature derating is linear, and I suspect it's only good for real planet tempuratures not the extrapolated zero power out point :laughing:.

 

[eepete]

Yeah, as temp goes up you get more recombination of electron hole pairs at the junction. Noise and leakage current, both temperature dependent, impact the device in a function dependent way. I'm also guess that the junction voltage goes up as the temp goes down, just like any semiconductor junction. That's probably another factor in the thermal derating for the PV panels. The data sheet had thermal derating for power, voltage, and current (I just sited power). They did not have a voltage vs. temp curve, either because most consumers might not know what that's about or because at the end of the day it's just a big honking PN junction.

Cool story , didn't know photonic emissions were a leakage detection mechanism. I had a buddy who back in the days of EEPROMS (like the 2704) spent months working on methods of detecting how many electrons _per year_ leaked out of the floating gate. They tweaked the fab process and got it down into the single digits! I know a few more "you wouldn't believe what someone can measure" stories.

Be it electronics or mechanical stuff, it's amazing how thousands of people have spent decades just quietly understanding and improving stuff. I think a lot of people focus on the big break throughs and don't focus on the effect of lots of little 1 to 2 percent improvements.

Pete

 

[MossRoad]

We've had plenty of 110 degree days this summer and there was no noticible reduction in power harvested from our array. Daily KWH totals is what I check when I return home from work.

Yes, high temps reduce the power the array will produce (nowhere near the estimate mentioned above) but on those days the sun shines longer to make up for it.

It's those week-long stormy periods that are the only problem I haven't yet figured out a way around, so far the propane bill just rises with the generator use during those times. Adding more panels won't help if there's no sunshine.

Phil

When you run your generator are you powering the house or charging the batteries, or both?