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RegGuheert
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Re: How to increase the *fraction* of roofs covered?

Fri Feb 21, 2014 11:45 am

DNAinaGoodWay wrote:Here, excess production won't be paid for, but can be allocated to any user on the same utility. Whole solar farms exist that offset usage somewhere else. If you had that, maybe you could find a way to work out the payment details with your neighbors or whoever.
That's the kind of thing I was wondering about! That legislation seems to be pretty forward-thinking!
DNAinaGoodWay wrote:As for HI, the utilities could save cost by shutting down generators during the day.
Agreed. I suppose the fuel cost in HI must be a primary reason for their high rates, which makes PV seem extremely cheap.
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RegGuheert
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Re: How to increase the *fraction* of roofs covered?

Fri Feb 21, 2014 11:51 am

Smidge204 wrote:
RegGuheert wrote:In the discussion about the Ivanpah plant, Guy and I have been discussing the fraction of "impervious" surfaces you would need to cover with PV in order provide the amount of electricity used by the U.S. It's a big fraction, even assuming infinite, efficient storage: ~16%.
The Wiki states that the figure given (110,000 km^2) is "Urban." So reasonably this figure includes only areas within major city limits. In other words, you'd have to cover 16% of the cities in PV.

There are roughly 130.6 million residences in the US. ~67% of these are single family homes, with another 18% being between 2-20 units (condos or small apartment buildings).

The average size for single family houses is about 2,400 square feet. At 67% of 130.6 million, that's 210 billion square feet, or about 20 billion square meters.

20,000,000,000 sq.m. * 1 KW/sq.m. * 0.15 PV efficiency * 4 hours/day * 365 days/year = 4,380,000,000 MWh/year.

That's more than the 3,886,400,000 MWh/year quoted in the other thread. For single family homes only.

Granted, not every roof is suitable and the entire area of a given roof may be suitable. However, if you include structures other than single family homes - especially large commercial and industrial buildings, and parking lots - then that would be handily to make up the difference. You'd even have enough to displace a significant chunk of fossil fuel use through electrification of homes and vehicles, as well as synthetic fuels.
=Smidge=
Thanks! I hadn't noticed that it was only for the cities.

Note that the size of the the homes is floor area, not roof area, but perhaps the two numbers are comparable, since some houses will also have a roof over the garage. Also, there is a LOT of roof area on barns.

Our grid out here in the country cannot handle a huge load, so I doubt that we could feed much back to the cities, but there is a LOT of potential for generation out here.
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RegGuheert
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Re: How to increase the *fraction* of roofs covered?

Fri Feb 21, 2014 12:02 pm

Randy wrote:If your scenario was true and in the daytime each and every customer generated their electricity and did not rely on the grid at all during the daylight hours, that would (of course) result in a near zero flow on the grid during those hours. Existing large scale generating plants, however, would still need to be running at minimum output because as the sun starts to go down and solar output drops off, a "new" system peak is created from a utility generation perspective as people get home from work and fire up everything at home....In fact, my personal opinion is that this evening peak will just be a little bit less in the future than the current peak load we see in the afternoon.
Germany is in the situation now where their renewable production in the middle of clear, cool weekend days exceeds 50% of their load. As the load gets too low, the grid frequency grows. In the past, this was a real concern since ALL of the PV inverters were designed to drop off at 50.4 Hz. Hitting that limit would have resulted in the equivalent of a HUGE load being dropped onto the system nearly instantaneously. They now require new systems (or individual microinverters) to drop off over a range of grid frequencies ranging from 50.4 Hz to 50.6 Hz. Each inverter has its own frequency randomly programmed in. Systems beyond something like 25 kWh require remote control so that they can be curtailed. This all allows the system to respond more gracefully as the renewable fraction gets very high.
Randy wrote:And like you said about storage not be economical in large quantities for quite some time, you still need large scale generation in the evenings (in fact there will a large ramp up needed as the system shifts from daytime solar predominance to evening traditional generation), you'll still need relatively the same transmission system, distribution system, etc. in the evenings. I don't see a major cost savings in grid infrastructure by a "super buildout" of solar across the system. It can help during the daytime, of course, but in the evening and night time hours almost the same grid capability will be needed to serve customer load.
With EVs now growing in popularity, we now have the possibility to locate grid-support storage in the form of V2G and/or spend EV batteries along with PV generation. I think these two technologies can continue to support each other in a way that allows for more renewable generation while improving grid stability. It should be possible to eventually greatly eliminate any peaks that currently exist.
RegGuheert
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dgpcolorado
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Re: How to increase the *fraction* of roofs covered?

Fri Feb 21, 2014 2:02 pm

DNAinaGoodWay wrote:...As for HI, the utilities could save cost by shutting down generators during the day.
It's not that simple. The generators have to be running at some level to fill in the gaps when (frequent) clouds drop the solar output. The problem is exacerbated by the relative small size of the grid in Hawai'i; it isn't like one can rely on the massive grid on the mainland where excess power in one state can be moved to fill a production gap in another. Tiny grids are much harder to balance and even diesel generators take some time to spin up. What is needed is some sort of storage that is instantly accessible to smooth the peaks and troughs.
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Re: How to increase the *fraction* of roofs covered?

Fri Feb 21, 2014 6:05 pm

That is some of the rational from the utility companies in Hawaii. They really do not have the infrastructure to store all of the PV energy, (at least that is what they say). Therefore they are only allowing a 15% saturation level per circuit. I do not think they know what to do with all of the excess generation.
Almost 90% of all generation in Hawaii is from oil, hence the high price.
Cloud cover is a huge issue. There are days in the winter where I will produce 50kw or 10kw. That has to be hard at the generation end.
dgpcolorado wrote:
DNAinaGoodWay wrote:...As for HI, the utilities could save cost by shutting down generators during the day.
It's not that simple. The generators have to be running at some level to fill in the gaps when (frequent) clouds drop the solar output. The problem is exacerbated by the relative small size of the grid in Hawai'i; it isn't like one can rely on the massive grid on the mainland where excess power in one state can be moved to fill a production gap in another. Tiny grids are much harder to balance and even diesel generators take some time to spin up. What is needed is some sort of storage that is instantly accessible to smooth the peaks and troughs.
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Re: How to increase the *fraction* of roofs covered?

Sat Feb 22, 2014 11:03 am

Smidge204 wrote:
RegGuheert wrote:In the discussion about the Ivanpah plant, Guy and I have been discussing the fraction of "impervious" surfaces you would need to cover with PV in order provide the amount of electricity used by the U.S. It's a big fraction, even assuming infinite, efficient storage: ~16%.
The Wiki states that the figure given (110,000 km^2) is "Urban." So reasonably this figure includes only areas within major city limits. In other words, you'd have to cover 16% of the cities in PV.

There are roughly 130.6 million residences in the US. ~67% of these are single family homes, with another 18% being between 2-20 units (condos or small apartment buildings).

The average size for single family houses is about 2,400 square feet. At 67% of 130.6 million, that's 210 billion square feet, or about 20 billion square meters.

20,000,000,000 sq.m. * 1 KW/sq.m. * 0.15 PV efficiency * 4 hours/day * 365 days/year = 4,380,000,000 MWh/year.

That's more than the 3,886,400,000 MWh/year quoted in the other thread. For single family homes only.

Granted, not every roof is suitable and the entire area of a given roof may be suitable. However, if you include structures other than single family homes - especially large commercial and industrial buildings, and parking lots - then that would be handily to make up the difference. You'd even have enough to displace a significant chunk of fossil fuel use through electrification of homes and vehicles, as well as synthetic fuels.
=Smidge=
I don't know a single person who has a 2,400+ sq. ft. home on a single floor. Assuming that on average, single family homes are two floors, that's 1,200 of roof space. Assume again that only half is useable, certainly true if you're north-south oriented, less so if east-west. Also, what percentage of those homes are shaded? Certainly in the northeast, at least 20% of homes are too shaded for practical solar.

In summary, I suspect your estimate of usable roof space is over by a factor of 3 or 4.

I am also suspicious of the 16% figure. If I covered 100% of my roof with solar panels, I would basically break even between the house and the Leaf. I estimate that's probably more or less true on average for upstate NY. But that's just residential spaces covering their usage. What about commercial space? A typical office has hundreds of computers running 200-300W each, plus HVAC systems, industrial equipment, etc. I don't think the typical commercial building has enough roof space to break even.

I'd love to see the original calculations, though. I know upstate NY is hardly known for its sunshine, but according to NREL it's only better by a factor of 2 in the desert southwest.

http://www.nrel.gov/gis/images/eere_pv/ ... 012-01.jpg" onclick="window.open(this.href);return false;
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Re: How to increase the *fraction* of roofs covered?

Sat Feb 22, 2014 9:50 pm

We live in a single story 2400 sq. ft. home in San Jose, CA and there are quite a few others that are our exact same model. We also have a solar system covering our west facing roof.

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Re: How to increase the *fraction* of roofs covered?

Sun Feb 23, 2014 2:11 pm

Becky50 wrote:We live in a single story 2400 sq. ft. home in San Jose, CA and there are quite a few others that are our exact same model. We also have a solar system covering our west facing roof.
Interesting. Although I believe many of the towns in the bay area have a restriction on building height. I have relatives in Los Altos who recently expanded their house - they were forbidden to go up due to privacy concerns.

In other parts of the country, going up is preferable. In the northeast, it's very uncommon to have a single-story home, and those that do are hardly 2400 sq. ft.
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Re: How to increase the *fraction* of roofs covered?

Sun Feb 23, 2014 2:40 pm

GetOffYourGas wrote:I don't know a single person who has a 2,400+ sq. ft. home on a single floor. Assuming that on average, single family homes are two floors, that's 1,200 of roof space. Assume again that only half is useable, certainly true if you're north-south oriented, less so if east-west. Also, what percentage of those homes are shaded? Certainly in the northeast, at least 20% of homes are too shaded for practical solar.
I sort of whitewashed over this when I said that not every roof would be usable. The point of the exercise was to demonstrate that there is a lot of roof area on the US, and that we would not have to cover nearly a fifth of it to cover our electricity usage.

Also keep in mind that 2,400sq.ft. is the AVERAGE. You have a handful of very large homes offsetting a majority of smaller homes, and smaller homes tend to be single story. I don't feel the estimate is completely out of whack.

GetOffYourGas wrote:I'd love to see the original calculations, though.
There is a link to the other post/topic in the first post of this thread.

Both me and RegGuheert used, effectively, 4 kWh/day for our generation estimates. That number is such that it covers over 95% of the country even per your graphic. That potentially makes it a rather severe underestimate, but to really hash that out you'd need to include population distribution and blah blah blah.... it's just an estimate for discussion's sake, so assume a less-than-ideal case to be conservative.
=Smidge=

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Re: How to increase the *fraction* of roofs covered?

Sun Feb 23, 2014 4:52 pm

http://www.solarroadmap.com/producing-y ... ar-garden/" onclick="window.open(this.href);return false;

Here's a starting point for you. Now that I look into it, there are several states that are allowing this to happen, and more coming on board, so see if you can get it going where you are. Wave of the future baby!
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