Year 2 with our solar (PV) system

Tomorrow is "DAY 200" for our panel's installation. Here's our stats & totals:

16 panels on one inverter: 2,834KWh
20 panels on one inverter: 4,122KWh

Both Totals for 200 days: 6,956KWh

Our surplus on Meter Reads: 2,310KWh


To calculate our KWh usage ... I'm thinking I have to subtract the surpluss 2,310KWh from total 6,956KWh PV generation ... right?

If so, that'd mean we're burning through 4,646KWh of power over 200 days, or 23Kwh per day ... or 697KWh/month.

Hey! If that's correct, we've REALLY cut back from a few years ago. We used to average 965KWh/month over a 2yr period, prior to the PV install. Got rid of a guzzler fridge, added Low E windows throughout ... +more insulation, CFL's ... front load washer ... It's looking like it really helped.

BTW, at some point, I think it was Icarus who mentioned there may be a graph floating around somewhere, that shows solar panel efficiency increases/decreases that turns on higher/lower temperatures. I'm still trying to find that. There was a bit of discussion about how much or little effect cleaning panels can (or can not) have. Well, Google just threw out some WAY cool charts. Check it:

Solar Panel Cleaning Analysis (Final Draft)

Man! Those Google folks . . . I've got panel envey :madgrin:

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As Hill found out, thanks to Googles efforts, cleaning does work. Keep the panels clean, MORE POWER!!!

 

Even better than keeping them clean, keep then COLD! My small off grid system, at -40 with lots of refection off of snow and ice will put out ab out double the name plate rating.

I have looked through many sources and I can't find a generic table that shows the power curve relative to temperature. (I know I have one somewhere!)

PV specs are based on a 25C operating temperature. (77F) It doesn't take long for the average roof top solar array to exceed this by a lot. Temps over 65C (~150F) for a roof top array is not uncommon.

Even at -40, in full sun my panels will heat enough to melt snow, just by absorbing heat from the sun.

Icarus

 

Yep, you are indeed correct, There are days when the system lowers its output due to thermal issues. This also applies to the Inverters.
In the pic I posted there is a line that indicates when its operating in reduced output mode due to heat issues!
Its always one issue or another, but I do love the system!!!

 

-40 ... and still melting snow. Well THAT creeps me out. So ... one company seeks to take advantage of all those black solar panels. It looks VERY interesting:

http://www.sundrumsolar.com/files/SunDrum_Total_Energy_Data_Sheet_Rev_C.pdf

I've never heard of the company. Anyone else? It looks pretty interesting! ... cooling the panels for greater efficiency, while capturing heat for hot water.

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Nice, I was wondering if a set up like that would work, using a fluid to draw heat from the PV panel for hot water.

At mid-latitudes the decking below shingles will reach 170+ F in summer. I would expect a PV panel to be just as bad or worse with respect to heat gain. So I wonder if a secondary heat exchanger in addition to the water cooling might pay out during peak solar months? For this I'm thinking along the lines of some coils in the ground/pool of water/etc. with a 3 way valve and/or serial arrangement after the water heater tank.

 

This has been discussed extensively on my solar forum: Possible hybrid PV/hot water panel product? - Solar Electric Power Discussion Forum by Northern Arizona Wind & Sun (There are other threads as well, I chose this as being the closest to topic).

There are a myriad of technical/financial/efficiency issues at work here. At first glance it might seem like a great idea, but there are some limiting issue. For example, this might be a better solution for pool heating rather than domestic hot water. The reason being is that pool temps are much cooler (~90f?) than domestic hot water. DWH will run ~150F. So at 150F you are in a range of not adding much efficiency because you are not cooling the panels very much. Second, if you are running cold water (55f) behind a hot panel you have potential thermal shock issues with glass panels. Next, the infrastructure costs are not insignificant as are the parasitic energy cost of circulating coolant medium.

I think the conclusion amongst the solar community is that the potential Pv gains are likely to minimal, (remember that panel specs are based on 25C (77f)) so if you are running 150f water through them you aren't going to gain ef above spec. I think the consensus is that you money would be way better spent on DWH from flat plate or evacuated tube systems, and B Use water medium heat capture from A/C systems, allowing them to not only heat DWH, but to also run more efficiently. (A simple auto drain DWH system using potable water can be had for ~$2000. I have a simple home made system that cost me $300 and give me ~75% of our hot water needs year round in the grey Pacific NW. A Similar system in CA or AZ should provide ~100% pretty easily.)

Personally I think that this is a solution in search of a problem.

Icarus

 

I would have to see some estimates of the change in PV efficiency with temperature in order to determine what might be feasible. If it is only 10% or so improvement then that's not much of an energy budget to work with. If taking the panel temp down from 170 to 130 F was worth 30-50% then it would be another matter. Keep in mind that only a portion of the panel would be at the higher temp of the water outlet. The inlet end would be cooler.

Having a heat transfer fluid (glycol) makes the system more manageable, but the thermal transfer is reduced because of the extra heat exchange step. Of course, I would normally target the water heater at 120 F anyway (same as I do now with nat. gas.) Perhaps with thermal solar that is too low and more rise is needed to hold 120 F average...particularly for the morning shower.

Pool heating seems a tailor made application. Some sort of evaporative pool might also work, such as one that collected gutter downspout in a pond/cistern to use as an evaporative heatsink, so no net water consumption, and the inlet water temperature on each pass would still be blow that of a water heater. Someone might even use this heat source to run their composting toilet...

 

Shawn,

This is "plagerized" from a former design engineer at Outback Power Systems in a conversation we had on the subject several years ago.

"The relationships between power, voltage, current and PV cell temperature are “linear enoughâ€, in my view, for our applications. For example, a typical temperature coefficient of power might be expressed as “-0.5%/degree C (or K)â€. That’s a straight line with a negative slope.

Check the graphs in the linked Mitsubishi cut sheet; they’re pretty telling. See: Yahoo! - Not Found

The application key is to adjust cell operating temperature for ambient conditions. Specifically, cell operating temperature of a well-aligned PV module under full insolation can be ~34 C above ambient, depending on wind conditions. So, at 15 C (59 F) ambient, cell temp could well be 49 C (120 F), or 24 K (43 R) above the STC reference of 25 C (77 F). At -35 C (-31 F; brrrrr!), cell temperature could be ~-1 C (30 F), or 26 K (47 R) below STC ref. "

So without getting too engineering here, if you figure ~.05%/degree C will put you in the ball park. So lets say the panel temps are dropped 15C as result of "water heater cooling, that might be a ~7% gain under ideal conditions. Take into the energy cost of the water circ system and I bet it might near 5% (WAG on both subjects) So if on average, I would only yield 5% better harvest, plus some hot water, I think I would rather buy 5% more panel capacity. Now if that 5% were coupled with a significant % of hot water then perhaps there may be some merit.

Icarus

 

We don't have a water heater tank ... rather we have a flash heater system / tankless. I was thinking that if the flash heater's incoming (recirculated via sundrum or a similar system) water had been pre-heated even only 30 degrees f ... that would still be a big bonus ... simply because you'd use way less natural gas/propane to heat hot water up to 120ish. No? I was thinking one could simply make some kind of DIY system and stick it near the back side of the panels, since PV panels are typically built 6 or 8 inches above the roof material any ways.

 

We run a simple flat plate collector that circulates into a 50 gallon water heater (Un plugged) that serves as a preheat tank for a Tagaki flash demand gas water heater.

This simple home made system (cost me ~$300) produces enough hot water for two of us every day during the summer and about 25% in the winter in the Pacific NW gloom. I estimate that it produces ~75% of our hot water on an annual basis. The collector is ~2'X12'.

Pretty easy to do to save a bunch. As we suggest in the solar business Solar hot water should be the first thing you do, long before PV since it saves way more energy for way fewer dollars.

Icarus

 

Yea, if you got the space for both water heating solar & PV solar that'd be great. As mentiond previously, we don't have that luxury. We're off to MT in a couple days. When we get back 10 days later, I'm going to look into it. I'm feeling inspired!


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Hot water is much more forgiving than PV. Solar hot water will deal with shadows, partial shading, imperfect angles etc. An installation on a side wall is doable, as is one on a fence or a whole host of other places. In California, if you have enough sun to do justify Pv, you are really missing the boat by not doing solar hot water. A commercially available system can take up a pretty small foot print (even the system mentioned in this thread) and pay off quite quickly, especially with the tax credits it would qualify for. Pay offs in the 1-3 year range would be pretty easy to do. If you've got demand hot water, your halve way there already.

Icarus

 

icarus,

What sort of installed price would you consider reasonable for a commercial thermal solar water heating system? I ask because for me the payouts appear to be longer (still well less than PV of course). I've got my water heating cost cut back to about 170 ccF nat. gas a year for a family of four, and when I finally can get the master bath showerhead I'm waiting for I hope to cut that back by another 26 ccF/year. At that point daily tank losses for the nat. gas fired tank with R-6.7 blanket become a rather substantial component--as measured while I was away for 9 days. The interesting thing about this latter test is that it was in decent agreement with estimates I made of the head end losses in a fired tank. If I was salvaging a water heater for use as a solar collector tank I would use an electric water heater tank and add insulation to heads and walls. The gas fired tanks have some uninsulated/less insulated surfaces that make them less efficient.

Gas seems to average about $1/ccF here, although it is about half as much at present. Electric is now around $0.12/kwh and rising in prep for replacing the old powerplant down the road. So at this point even at $1/ccF and assuming I could get 75% replacement with solar, my savings would be limited to as little as $108/year. 75% replacement is no mean feat in my climate with 4 people showering in a given day. (50% would be more conservative.)

 

Shawn,

To be sure, I have no clue. My experience is limited to off grid PV and DIY experimental tinkerers stuff. My WAG would be, that somewhere in the neighbourhood of $3-4000 would by a pretty good DWH system.

Just for example, my small DIY home made flat plate system cost me ~$400. But that was with salvaged material. The water tank was a discarded 240 volt electric, the pipe was left over from a previous project. The pump was ~$150 and the controller was ~$150. The glass, steel, etc was all salvage.

This little system which is ~200' of 1/2 copper pipe in a continuous loop, under salvaged patio door glass, on top of 3/16" plate steel, all housed in a lumber box. This system with is ~30 sq ft, yields about 20,000 BTUs on a good summer day, measured by temperature rise in the tank. I figure that system gives me ~70% of my hot water needs on an annual basis. (Two people)

So for me, if 20k BTUs is the average use, that would translate to just about one quart of propane, at todays cost of ~$2/gallon, so it save me ~ $.50/day in the summer. My estimated usage might be ~90 gallons a year at that rate. So a 70 percent savings would be 63 gallons, or about $120/year, making a pay off of ~ 3 years.

Now if you started with a $4k system and included a demand water heater in the mix, pay off at that rate would be pretty slow.

The reality, is the numbers I hear, for good climates (remember, I live in the cloudy Pacific NW!) Cal, CO, FLA, TX even areas in the midwest etc the numbers that I hear are payoffs of about 3-5 years depending on your usage, as well as your cost of fuel.

Icarus