For owners of solar panels, is their own roof the best location for those panels?

You know, I like this idea!

 

It is indeed an interesting idea, but it is not without it's problems.

In most current grid tie configurations, small scale PV solar is done (at best ) with net metering. That is that the utility agrees to buy all you can produce at retail by spinning your meter backwards. It also allows you to buy power from the utility to augment your needs, also at retail. With net metering at the end of the metering period (usually a year) the utility typically bills you the difference, but doesn't pay you for any surplus. In places where time of use metering is in use, it is often much easier to go into surplus (in terms of $$ if not kwhs) but most utilities won't pay you for the difference.

The reason that they don't pay for the difference is that buying your power at retail is a sort of loss leader for the utility. It is a good deal for them, because the (relatively) small amount of power that home sized grid tie systems produce allows them to load balance peak loads using PV.

The problem comes in with large scale PV installations. If a large number of Large industrial sized PV "farms" are on line, expecting to be paid retail, it throws off the economics. The utility makes money (and covers the overhead of providing the grid and (and keeping it up!) by producing power at one price, and selling it at a higher price. If they are forced to buy their power at retail, and sell it at retail the economics gets skewed.

That said, clearly going forward we are going to see increases in our cost of energy regardless. I suspect that it is possible (in todays tax credit/utility rebate environment) to think that if you could pool resources like you suggest, use the economy of scale to reduce the KWH installed cost, it might compete with retail grid tie, even at a wholesale price.

The one thing to beware of is the plethora of scams and near scams that continually pop up, suckering folks into something that seems like a great idea. What you are suggesting is a great idea, and the idea of having a well organized, well managed system has merit.

 

It occurred to me after thinking about this a bit more, this program already exists in some places. Puget Power allows you to buy green energy, and you pay a premium for it. (look at that as the "purchase cost of PV in the above example) Further down the bill, there is a "green energy" credit. (Consider this the reduction in your bill for your PV for example). The net/net is a more expensive power bill, but you are doing some small part to increase the availability/economic viability of RE. Not exactly what you were thinking, but it it comes pretty close.

 

If you put 'em on your roof, there's much less issue with loss. Put up solar farms that are hundred & hundreds of miles from the users & you diminish one of the panel's primary benefit ... reduction in loss.

.

 

Absolutely. Power generation that is closer to the consumer is far better than power generated at a distance. That is why Westinghouse (proponent of AC) won over Edison (proponent of DC). Today AC plays an even larger roll because nobody wants coal or nuclear power in their backyard, so it must be transmitted huge distances. Even with AC transmission lines there is still loss. I am hoping that one day power generation is done at or close to the consumer which would negate the need for AC altogether.

 

Thats true in one sense, and I am a proponent of local supply, but in other ways it is not true. For example, is it better to install a Pv system of say 2 kw in Seattle where it might generate ~1900 kwh/year, with it's performance net/net hampered by environmental conditions that you can't control (fog/rain/haze etc), or install the same 2kw system in Eastern Washington (Yakima)on a PV farm, generate 2500 kwh/year. (Pvwatts version 1 calcs)

Take that 2500 kwh, lose 5% in transmission (which would be a lot at HV!) and you would still be left with ~2375 kwh, ~25% better harvest net/net. (Not only that, there are system loses in the Seattle installation that I didn't calculate).

So from a geo-political point of view, putting PV resources in better solar locations gets ~25% better bang for the buck, which, in the grand scheme of things is not an insignificant difference. At present, grid tie Pv comes it before most subsidies ~twice what conventional power costs, so an efficiency increase of ~25% is huge!

 

Another advantage of the roof is that it is dead space that isn't being used. A PV farm on the other hand may end being placed in an area which might be better used for something else or just left alone.

 

Actually, long range transmission is more efficiently done with DC. High-voltage direct current - Wikipedia, the free encyclopedia

Yes, better siting can make a huge difference in how much power you can get out of a PV system. Just being able to angle the panels optimally without dealing with existing rooftops can be worth 10-20% alone.

Typical home installs are about 95% efficient in converting the DC power from the panels into AC power. Large scale installs might have inverters that are a few percent better, but it's not a big difference. The largest gains in efficiency with large installs come from economy of scale (along with typically better panel siting/angling as previously mentioned.

I don't quite follow - how are non-solar tax payers subsidizing solar tax payers? In the end - everyone is paying to increase our renewable energy production with it's associated benefits which is why there are tax credits for installing such systems.

 

Roof tops may be fairly handy locations for PV, but they are not necessarily an efficient use of that "dead space" . Not always well known, but PV solar is significantly more efficient the cooler the panel temps are. So roof tops tend to be hotter than a ground based system that has more free air around it. Roof tops, especially residential roof tops suffer quite often from less than ideal siting angles, both in azimuth as well as elevation. Give a choice between capturing energy into heat, or PV, heat wins hands down on a roof location. Here, extra heating is an advantage rather than a disadvantage. (Also by carrying off excess heat into water or other storage media can cut cooling costs, often significantly). It is also true that capturing heat is much more "forgiving" of less than ideal angles. (As well as being way more forgiving due to small shadowing, something that can kill a Pv arrays out put!).

From Drees:

"Typical home installs are about 95% efficient in converting the DC power from the panels into AC power. Large scale installs might have inverters that are a few percent better, but it's not a big difference. The largest gains in efficiency with large installs come from economy of scale (along with typically better panel siting/angling as previously mentioned."


While this is true, it is misleading. While inverters convert in the mid 90's AC/DC, system efficiencies (total) around ~75-77%. So in simple terms, ideally sited, a 1000 watt array, might deliver ~750 watts of peak power,,, on average. More when cooler, less when hot. In addition to inverter loses, there are fundamental wiring loses, Pv loses etc, so if you think you are going to get 1kw out of a 1 kw system, you will likely fall short.

 

This is, in fact, the whole point of the green tag (renewable energy certificate) market.

Most state utilities are under mandates to produce some fraction of power using "green" technology. It is inefficient to have (say) a high-latitude power company physically install solar. Instead, they buy the "green tags" from solar installed in a more favorable location.

So the big players (the utilities) do this as a matter of course. They don't actually transmit the power, they just trade in the difference between commodity fossil-fuel-generated electricity and green electricity.

Example: Say I'm the Vermont electric utility. Say I'm supposed to generate 10% of my electricity using green sources. I make a deal with a provider in Arizona, say: Arizona provider to install PV, but Vermont provider buys the green tags for it (renewable energy certificates). That PV now counts toward the VT green energy mandate.

As long as enough states have large enough green energy mandates, this pushes the system toward more green electricity generation. If not, not.

So, to continue the example, if the Arizona utility has to have 10% green power as well, it can't count that particular PV plant for satisfying its green mandate. The "greenness" of that electricity has already been purchased by Vermont. AZ ends up with (say) 20% PV physically attached to its grid -- 10% that counts toward its green mandated, 10% whose greenness was purchased by VT (say).

The upshot is that the VT purchase increases US consumption of PV, displacing fossil fuel generated electricity. As long as that was all you wanted from your green mandate -- if you weren't concerned about local smoke stack effects on the local area, but were just concerned with total GHG emissions -- then you've accomplished your task, at lower cost than it would have been if VT had physically installed PV in VT.

Hmmm. Maybe not the clearest thing I've ever written. You might get more from wikipedia:

[ame="http://en.wikipedia.org/wiki/Renewable_Energy_Certificates"]Renewable Energy Certificates - Wikipedia, the free encyclopedia[/ame]

Their "additionality" discussion is what I was alluding to, about having enough mandates in place so that the overall system is constrained. But also note how cheap the green tags are. They cite a median of $20/MWH (= 2 cents per KWH). And look at the variance. Clearly this is not yet a well-functioning mature market.

Anyway, my only point is that, in some sense, utilities already do what you suggest. Whether it actually accomplishes the goal of adding green power that would not otherwise be built, I'm not sure. At the scale of these purchases, it's plausible that it does.

Also might add that when you purchase retail carbon offsets, what you're typically buying is mostly electric utility green tags, being resold in consumer amounts. It's even less plausible that this so-called voluntary market actually adds to green energy production, but it's possible that it does.

 

Problem is that once you talk subsidies, you'd have to factor in ALL the subsidies to be oranges to oranges dead on. For example, trillions to to the military ... "protecting freedom" (how can you dare question that ). Put those trillions into hydro electric, solar, ev's, a killer public rail system, etc, instead of bombs, and you actually have stuff that supports the public, rather than drain off its wealth. Talk subsidies and you go down a slippery dark complex tunnel.

I posted this before, but it's worth repeating. The (up to) "20% PV efficiency loss due to high temps" conundrum has actually been defeated. There's a company that has created a wonderful product. A thin cooling jacket is mounted onto the back side of your black panels (what better color to absorbe heat?). The heated water is captured in a recirculating loop for your personal use, while the PV panels go up in efficiency that would otherwise drop, due to the heat being carried away. Of course in the middle of a 125 degree desert, you won't have enough folks around to benefit from the hot water so this is only a great fix for regular ol' suburbia. I'll try & find the name of the company again, and do an edit later.

.

 

Hill,

"I posted this before, but it's worth repeating. The (up to) "20% PV efficiency loss due to high temps" conundrum has actually been defeated. There's a company that has created a wonderful product. A thin cooling jacket is mounted onto the back side of your black panels (what better color to absorbe heat?). The heated water is captured in a recirculating loop for your personal use, while the PV panels go up in efficiency that would otherwise drop, due to the heat being carried away. Of course in the middle of a 125 degree desert, you won't have enough folks around to benefit from the hot water so this is only a great fix for regular ol' suburbia. I'll try & find the name of the company again, and do an edit later."

Hill, please do post the links when you find them. I know about this technology, but I know of no one who has used it and to what effect. While the idea is fairly clever, the implementation sound like it is a bit a of a technical nightmare. Mixing PV with hot water,, what happens when one or the other fails for example?

 

Here's their literature:

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

Here's their product as seen from the back side of different manufacturer's panels:

SunDrum Solar

Here's a couple commercial setups, not the residential/local I'm thinking of ... using recycled doodie water :

LGC: Resources: Energy:

And Isreal ... always on the leading edge of high tech stuff, is big on mirror/amplification and (almost necessarily requiring) liquid cooling ... beneficial due to the increased heat from the mirrors:

In Israel, solar power that won't need subsidies / The Christian Science Monitor - CSMonitor.com

Alternatively (and simpler) there are systems that take the PV electricity for running high ratio air compressors. That's another form of storing energy after the sun goes down.

 

It is a interesting/promising concept.

As a practical matter it would be interesting to see how the cost effect curve falls. For example, what does it cost to simply increase the PV output. While I am a great fan of new technology, my suspicion is that the improvement in PV performance is somewhat optimistic because to get useful water temperatures (~120f) you are getting PV temps higher than one would like. On the other hand, what would make a Pv run more efficiently? Circulating hot water that draws some heat away, but insulates the back of the panel from the air, or having more air circulation behind the panel.


All that said, it is dramatic proof ( to the nay sayers especially) that technology, when given a real chance can and does belly up to the bar to solve problems. This technology, concentrating PV systems, Concentrating steam systems, exotic salt solutions for parabolic systems etc, all push the technology forward. The nay sayers like to suggest that we are all going to have to "go back to the stone age" in order to have "solar power" make sense. The reality is that what we really need to do is improve our thinking, and use all our energy more efficiently.