Feedback on proposals for San Diego: LG/Hanwha + Solar Edge

[SunEagle]

What you describe is not what I'm writing about. I'm referring to arrays stood off a roof by 6" or so with the concrete/clay tile under the array removed and replaced with composition shingles under that array, while the original tile remains on the rest of the roof.

Such an arrangement can allow water and whatever is carried with it to flow under the remaining tile below the array through the open slit created at the shingle to tile interface. Lack of drainage from accumulated junk carried with the water and little air circulation will keep things wet and promote mold growth and rot. Not only does my engineering judgment tell me that, but common sense sort of screams the same thing.

My experience of having been a peddler prior to an engineering career also makes me skeptical of some vendors motives and their eagerness to engage in such shenanigans. Seen similar stuff and the often common attitude from the inside. My apologies to all the ethical, honest vendors whose job is only made more difficult by the scumbags.

That roofing system is not a good thing. About the only reason I can think of for doing such a thing is that it is easier and cheaper for the vendor. That benefit however, probably does not transmit to the customer via lower prices and any likely damage may go unseen and unknown until the unnecessary damage is done. Even with a price break, I'm not sure how I'd go about putting a value on increased probability of a built in rot problem.

Too bad the design did not incorporate some type of heat exchange/cooling system between the panels and roof.

If the entire array was totally sealed around the edges to keep out moisture, yet had a way of also removing the heat from behind the panels (which maybe could be used to preheat a hot water system). It might work.

Unfortunately a system like that would probably cost a lot and be hard to maintain without a way to easily remove the panels to inspect the heat exchanger below.

[J.P.M.]

Too bad the design did not incorporate some type of heat exchange/cooling system between the panels and roof.

If the entire array was totally sealed around the edges to keep out moisture, yet had a way of also removing the heat from behind the panels (which maybe could be used to preheat a hot water system). It might work.

Unfortunately a system like that would probably cost a lot and be hard to maintain without a way to easily remove the panels to inspect the heat exchanger below.

Might work, but My guess is it wouldn't be cost effective or practical from a maint. standpoint. Might be a fun design project for eccentrics who know something of heat transfer and eng. design, but not much else.

[SunEagle]

Might work, but My guess is it wouldn't be cost effective or practical from a maint. standpoint. Might be a fun design project for eccentrics who know something of heat transfer and eng. design, but not much else.

I was thinking about food grade glycol as the heat transfer medium but I also remembered how glycol degrades certain pipe fittings after a while. Talk about a messy leak from that.

[jd31]

Let me ask a slightly different question then.

Would you pay a premium to have the solar company, with a roofing background, install a new roof membrane under the cement S tiles where the solar is to be installed? For me it would be about 450 sq ft of roof. The benefit, in my eyes, is that one day when I have to re-roof, they don't have to uninstall the solar. To be clear, this is not the flush mount option, it's a raised install above the S tiles.

[J.P.M.]

One of the so far not discussed issues on this thread is something that many believe to be a very good idea: Inspecting the roof around and under where a proposed array will be, and probably replacing the paper under a tile roof. On concrete or clay tile roofs, that has, in the past, often meant removing the tile from the affected area, replacing the paper on that portion of the roof, and replacing the existing tile while affixing the panel standoffs to the roof followed by, or mostly simultaneous with, the placement of the rest of the solar equipment.

That's one common and accepted way of doing it. For "S'" or "Mission style" clay tiles or some others, that can be a more difficult due to breakage or other issues and can take more labor. A somewhat cheaper way to get new paper on the roof is to use different roof material under the array and (temporarily) out of sight that's relatively cheap, and much less labor intensive - composition shingles. I kind of suspect labor cost is one of the big reasons why that method is becoming the darling of contractors. However, for reasons already discussed, my opinion is that as usually and commonly done and attempted, that is a very bad to do it that will only have the end result of creating more problems. Not only does it seem contrary to what I've seen on roofs all my adult life, it also seems to run contrary to common sense, or at least any common sense I have left.

The tile removal and repaper followed by reinstallation of the tile is a common procedure. FWIW, about half the solar installations in my HOA have done this. Happens all the time.

[J.P.M.]

I was thinking about food grade glycol as the heat transfer medium but I also remembered how glycol degrades certain pipe fittings after a while. Talk about a messy leak from that.

Treated as a heat exchanger application, which it really is, it's a real loser.

[SunEagle]

Treated as a heat exchanger application, which it really is, it's a real loser.

I never tried to determine the true cost associated with using glycol as a heat exchanger. So maybe in that area it is a loser.

I can tell you that is it used in many Food processing industries mostly in equipment like pastuerizers, filling machines and just about all equipment that transfers a food product that requires cooling without contamination.

It is also used in under floor heating systems to keep the ground beneath a food freezer from building up ice which will cause the floor to heave.

IMO I would say it transfers heat very well.

[J.P.M.]

I never tried to determine the true cost associated with using glycol as a heat exchanger. So maybe in that area it is a loser.

I can tell you that is it used in many Food processing industries mostly in equipment like pastuerizers, filling machines and just about all equipment that transfers a food product that requires cooling without contamination.

It is also used in under floor heating systems to keep the ground beneath a food freezer from building up ice which will cause the floor to heave.

IMO I would say it transfers heat very well.

To put it correctly, glygol or usually water/glycol mixtures are referred to as a heat transfer fluid or working fluid, not a heat exchanger. A heat exchanger is what the working fluid(s) go through, not the fluid itself.

There is (was ?) a chicken parts factory that had a line freezing nuggets using liquid N2. We did some refit work, took out the old line, replaced it with a bigger one. As part of the refit, took out a floor heater as you describe, replacing it with what was essentially a large slab of rigid foam insulation. Worked better and needed little maint. cost a lot less too.

I'm very familiar with glycol as a heat transfer fluid. Designed industrial systems for many power applications and others, including sanitary food and drug manufacturing equipment of the type you speak of for years.

What I was referring to in this thread was the application of removing heat from beneath a solar array, using piping, pumps and required support equipment and controls. What you'll probably get back with such a scheme ain't worth what you'll put in, in engineering, capital cost and maint. requirements.

As for glycol, or usually glycol/H2O mixtures' abilities as a heat transfer working fluid, while glycol/H2O has a lower freezing temp., and usually a somewhat higher vapor pressure (--> "boiling" temp.) than H2O by itself, what are called its transport properties - sp. gravity, sp. heat, viscosity, thermal conductivity and to some extent surface tension make it's ability to transfer heat inferior to plain old water. Sometimes glycol/H2O is required for the application, but H2O is the fluid of choice as a coolant wherever possible. Glycol H2O is used in lieu of H2O when required by that application, but not for its superior heat transfer characteristics.

[SunEagle]

To put it correctly, glygol or usually water/glycol mixtures are referred to as a heat transfer fluid or working fluid, not a heat exchanger. A heat exchanger is what the working fluid(s) go through, not the fluid itself.

There is (was ?) a chicken parts factory that had a line freezing nuggets using liquid N2. We did some refit work, took out the old line, replaced it with a bigger one. As part of the refit, took out a floor heater as you describe, replacing it with what was essentially a large slab of rigid foam insulation. Worked better and needed little maint. cost a lot less too.

I'm very familiar with glycol as a heat transfer fluid. Designed industrial systems for many power applications and others, including sanitary food and drug manufacturing equipment of the type you speak of for years.

What I was referring to in this thread was the application of removing heat from beneath a solar array, using piping, pumps and required support equipment and controls. What you'll probably get back with such a scheme ain't worth what you'll put in, in engineering, capital cost and maint. requirements.

As for glycol, or usually glycol/H2O mixtures' abilities as a heat transfer working fluid, while glycol/H2O has a lower freezing temp., and usually a somewhat higher vapor pressure (--> "boiling" temp.) than H2O by itself, what are called its transport properties - sp. gravity, sp. heat, viscosity, thermal conductivity and to some extent surface tension make it's ability to transfer heat inferior to plain old water. Sometimes glycol/H2O is required for the application, but H2O is the fluid of choice as a coolant wherever possible. Glycol H2O is used in lieu of H2O when required by that application, but not for its superior heat transfer characteristics.

I agree that the cost of using just about any type of fluid to transfer heat from a solar panel will not be cost effective. The savings by keeping the PV panel within heat specs would be minimal.

As you stated H2O is a better coolant but having Glycol mix allows you to use a lower temperatures in the fluid without the possibility of freezing the pipes. This allows you to get a faster (although not necessarily more economical) Delta T during the cooling cycle in a pasteurizer.

It all comes down to what is important to balance costs and production requirements. Glycol is ok for food processing but not a good fix for keeping solar pv cells happy.

[jd31]

Update on my situation.

I have a revised bid from one of the installers (roofing background) that puts me at $3.52 /watt installed before tax incentive. That includes a new roof (titanium membrane) where the solar will be installed, which is about 30% of my total roof. I have a cement S tile roof and under the solar they will replace with shingles.

So the net net is, after the tax incentive it's about 4% higher than the next bid without replacing the roof. Seems like a good idea?

[J.P.M.]

Update on my situation.

I have a revised bid from one of the installers (roofing background) that puts me at $3.52 /watt installed before tax incentive. That includes a new roof (titanium membrane) where the solar will be installed, which is about 30% of my total roof. I have a cement S tile roof and under the solar they will replace with shingles.

So the net net is, after the tax incentive it's about 4% higher than the next bid without replacing the roof. Seems like a good idea?

Since you ask:

Putting different roofing material under an array is, IMO, and for reasons detailed several times, a very bad idea.

[jd31]

Since you ask:

Putting different roofing material under an array is, IMO, and for reasons detailed several times, a very bad idea.

Sorry I should have clarified that I can, and likely will, have them install it over the existing tile roof. The shingles is an option that they prefer for aesthetics, which I told them is not at the top of my priority list.

My question is more about the benefit of redoing the roof membrane. For a 4% difference ($609 difference on a 7,150 watt system), it seems like a good option. Curious what other think in case I'm grossly missing something.

And another question about system size. My goal is for 100% offset. In general, do people aim for 100% offset or a higher percentage for overhead? 7,150 is over producing by 123% of my estimated need. I've only been in the home for 6 months so I'm devoid summer usage numbers. Using some projections, a 5,800 watt system would likely cover all my needs. I also have an old refrigerator I will be replacing and changing out windows. I know there's no right/wrong answer, but is there a general rule of thumb I should consider?

[J.P.M.]

Sorry I should have clarified that I can, and likely will, have them install it over the existing tile roof. The shingles is an option that they prefer for aesthetics, which I told them is not at the top of my priority list.

My question is more about the benefit of redoing the roof membrane. For a 4% difference ($609 difference on a 7,150 watt system), it seems like a good option. Curious what other think in case I'm grossly missing something.

And another question about system size. My goal is for 100% offset. In general, do people aim for 100% offset or a higher percentage for overhead? 7,150 is over producing by 123% of my estimated need. I've only been in the home for 6 months so I'm devoid summer usage numbers. Using some projections, a 5,800 watt system would likely cover all my needs. I also have an old refrigerator I will be replacing and changing out windows. I know there's no right/wrong answer, but is there a general rule of thumb I should consider?

1.) The up and coming cheap out method is to remove the tiles from the area where the array will be and put composition shingles in their place, creating the potential for a lot of problems. It saves a few $$ for the vendor and provides no benefit for the owner. What you don't want is a sump under an array or a way for water and the crap that gets carried with it to get under the tiles. A uniform roof surface with all penetrations properly flashed is a tried and true method of helping to ensure a leak tight roof.

2.) The smart money does the conservation first or have a good idea what the for-sure conservation measures will be before buying any solar. That way, the solar will be sized with the new and (hopefully) smaller load. That way, the array will not be oversized.

3.) The future, as it has always been, remains uncertain, but usually and often, it is not cost effective to offset 100% of an electric load with solar. Reason: Doing so usually replaces the cheapest electricity you buy. Solar, being a relatively expensive way to supply electricity, is best suited to replacing the most expensive portion of what you must buy. That's also why conservation is almost always cheaper (more cost effective) than solar.

4.) Therefore, under most current rate schemes and tariffs, offsetting more than 100% of a bill makes even less sense than a 100% offset which, again, may not be justifiable from a cost effective perspective.

[gregvet]

And another question about system size. My goal is for 100% offset. In general, do people aim for 100% offset or a higher percentage for overhead? 7,150 is over producing by 123% of my estimated need. I've only been in the home for 6 months so I'm devoid summer usage numbers. Using some projections, a 5,800 watt system would likely cover all my needs. I also have an old refrigerator I will be replacing and changing out windows. I know there's no right/wrong answer, but is there a general rule of thumb I should consider?

For my net metering, since there is always a minimum monthly charge regardless of whether I am making more than I use(kWh), it is a poor way to offset 100% of my electric bill. I would rather have a very small bill at the the true-up month rather than be paid wholesale or less for the excess energy I might have produced. It would be a long ROI time on the amount of extra expense of additional production capability to offset the monthly minimum charges.

[jd31]

Here's what I'm looking to pull the trigger on; hopefully sign an agreement this Friday:

- 6,325 watt system (109% of projected usage)
- LG 275 panels (qty 23) + SolarEdge 7600 DC Optimizer
- $/watt before tax incentive: $2.75*

* the actual cost is $22,074 or $3.48 /watt; however, that price includes a re-roof (separately quoted at over $4K from another roofing company) on the roof planes where the solar will be installed and other roofing work I need done. After removing those costs, my net cost for solar is $2.75/watt before tax incentive and $1.9/watt after.