What's the best way to describe the 'rating' of a solar power system?

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Permalink Reply by Paul on August 31, 2008 at 2:01pm

This came up Saturday night. -- some confusion still abounds.

A solar panel, a fuel cell, a diesel generator, a coal or nuclear plant, a geo-thermal site, a windmill, and a hydro dam arr all rated in units of power (Watts, and their metric multiplier prefixes, kW, MW, GW, etc...) which tell how much energy (Joules) they can deliver each second.

Energy is what we pay for and it comes in various sized blocks with **lots** of confusing names. From the tiny 'electron-volt" (1.60217646 × 10-19 joules) to the rather large & fanciful "cubic mile of oil" (159.942 × 10+18 joules)

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Permalink Reply by Todd McKissick on September 2, 2008 at 8:06am

Paul,

There's not only confusion about how to measure electricity. It goes a little deeper than that. Beyond the standard measurements for instantaneous power (kW - Kilowatt) and total energy (kWh - Kilowatt hour), there's a missing concept of when it's available.

This is referred to by different names depending on who's talking and what they're talking about. I think it's time to consolidate them into one or two measurements.

If you're discussing a traditional centralized large plant, we call this capacity and it refers to the amount of time the plant is capable of making power - usually around 90% of the time - and the downtime is mostly at your discretion.

If you're discussing wind, the capacity is referring to the % of time available again, but this time it's at mother nature's discretion for when you get it. If PV, this capacity time is 'generally' predictable on a longer time frame, but still beyond full control. If that PV or wind system incorporates batteries or other means of storage directly coordinated with it, it then becomes dispatchable as in the centralized plants. This should be valued at a higher level, since it lowers infrastructure costs as opposed to increasing them.

If a 'dispatchable' load (one that can be at least somewhat automated) can be coordinated to any opportunity source like PV or wind, and can match the power rating, that also makes the source fully dispatchable. An example would be controlling a 1 kW PV system and a 1 kW pool pump. From the grid's standpoint, it is better than being simply opportunity power.

IMHO, if we can begin discussing all fuel sources in terms of their power (kW), energy (kWh) and dispatchable capacity (on-call %), then we can get people to think more effectively on how they truly get us off fossil fuels and centralized utility monopolies. What does everyone think?

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Permalink Reply by Joseph Wojcicki on September 12, 2008 at 12:35pm

Good job Todd!
kW, kWh, J, eV (electron Volt) are properly written units.
Why there are so many errors in writing and as well as in understanding/interpreting in discussions?
1. Education systems. In elementary schools we have “Science” with stories instead professionally written lessons explaining step-by step PHYSICS and a little Chemistry. Biology and Geology should be separate units of teaching
2. First chapter in the school textbooks should start with explaining Unit Systems. SI = International (all the globe uses it in technical papers and contracts), US Customary, here in the USA – a little different to Imperial rarely used in UK. Remember: metric is not SI. US units for temperature are F ̊ and R ̊, in metric C ̊, and in SI - K
3. People are reading newspapers. Journalists/writers never took Physics courses or are ignoring notation and strict meaning of each unit. Most of their articles serve propaganda and they assume very low level of readers’ knowledge in physics. Editors/checkers (if any exist in the “team”) are even more focusing on words than proper writing of the units.
4. The tables of units and conversion are not in the Criminal Law Ruling. You may spend a night in jail after drinking an Irish green beer than for misleading children who wants to build Solar Powered Robot.
Good news: More grandpas and some grandmas are attending courses on the universities, some free for retirees so they can correct their grand children. God bless them!

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Permalink Reply by Paul on September 12, 2008 at 2:33pm

Hey Joseph,

I had a funny thought when you said: The tables of units and conversion are not in the Criminal Law Ruling. So I could tool down the highway past a sign that reads "SPEED LIMIT 55" and claim that I was traveling way less than 55 'meters per second' because they failed to put the units on the sign and there is no assumption in the law? [sorry]

BTW what is an Irish green beer, and what of jail?

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Permalink Reply by Joseph Wojcicki on September 13, 2008 at 9:39am

Associations in Solar Power Group @ PickensPlan
On 2008-09-13
Association # 1 to Hey
Paul, your “Hey “made me feel better. Are you from SE (SouthEast (? I did not check your Resume/CV/Bio so this is just my reckoning. Most Happy Yankees start with “Hi”.
Association # 2 to m/s (meters per second compared to miles per hour *3.6)
My Irish friend and a green lawyer commented:
“If you have ‘mps’ on your speedometer we have The Case!”
Our Irish beer is GREEN. May you say that Bud LIGTs are GREEN?
Association # 3 to wrong units’ capitalization.
My friend, kinda of a German teacher, takes -10 (unitless?) from the student’s test score when he found e.g. KW, kw, KWH etc in his/her answers. He explains this with an example: .ca is for Canada when CA is for California.
Association # 4 to “rated power”
Reading the rest of discussion I found many very nice explanations/examples of selecting the PV Panels’ Power (rated?). This is what should be always the final examples of practical solution for ANY Technical Question, right? I believe somebody uses letters CSP should give a brief explanation so everybody will understand what is about in his example.
Salute Paul! Association #5 : Pope Benedict declared this year to be St Paul’s Year!
Explain BTW, plz
Joe

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Permalink Reply by Dean Youngkeit on September 2, 2008 at 1:50pm

Give me a link to a graph comparing the above per acre, per unit life, and versatility parameters.

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Permalink Reply by Paul on September 2, 2008 at 2:49pm

Just by reading your profile, I'm certain that you know the differences!

Let's help clarify the terms for the less-involved members of the group :^)

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Permalink Reply by Chris Clement on September 2, 2008 at 3:07pm

$5 per watt is my standard for cost. More, you are paying too much. Less, you're ok. This is equipment cost for something that should last 2 decades. The cost per KWH will depend on how much you use it but will never be a "bargain" compared to the grid. What you get for your money is independence and a source of power that is always there for you.

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Permalink Reply by Paul on September 2, 2008 at 3:46pm

Hey Chris,

How about $3/Watt and 35-50 years, 8hrs per day design window???

The Light is Green!

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Permalink Reply by Gordon Myers on September 2, 2008 at 5:19pm

Good Afternoon Everyone.
I just received an electric bill saying I used 2800 kWh in a 32 day time period. How would that break down when figuring the size of solar systems. Since living in Arizona we get approx. 340 days of sunshine. I just don't want to get a system to small or one oversized. I have been told that a 3 kW, a 4 kW or a
5 kW system would work great.
Thanks far any help
Gordon




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Permalink Reply by Paul on September 2, 2008 at 6:34pm

Hi Gordon,

12.35 kW -- Let me show you the math:

340days of sun / 12months = 28.33 days of sun / month

8hr of sun / day of sun X 28.33 days of sun / month = 226.67 hr of sun / month

(2800 kWhr / month) / (226.67 hr of sun / month) = 12.35 kW

I ignored the 32 day period and called it a 'month' but that's a relatively small error. Now we have to differentiate between 'working great' and meeting your energy needs. We're ignoring a few things like incentives & inflation and O&M, but we're treating the systems as equally as possible; apples to apples.

The 5kW PV system will cost you 5,000W x 8.50/Watt = $42,500
The 12.35 kW CSP system would cost 12,350W X $3/W = $37,050

The CSP is $5,450 less than the PV, and provides 2.5 times as much energy.

Let's look at the 20 year "lifetime" energy for both systems:

first the total energy generated by both systems...
5kW x 226.67 hr of sun / month x 12 mo X 20 years = 272MWh
12.35kW x 226.67 hr of sun / month x 12 mo X 20 years = 672 MWh

Then the divide the total upfront cost by the energy to get the kWh rate:

$42,500 / 272 MWh = $156.25/MWh = $0.15625/kWh -- PV
$37,050 / 672 MWh = $56.13/MWh = $0.05613/kWh -- CSP

Look! CSP energy costs 1/3rd of PV over the 20 year life of the system. How does that compare to what you're paying today?

So even if the utility net-metered you at the 'discount' (or punishment) rate of $0.06/kWh, you still make a profit every moment the sun is shining!

The Light is Green!

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Permalink Reply by Gordon Myers on September 6, 2008 at 5:56pm

Thanks Paul

I went to the Green Summit EXPO here in Phoenix AZ this morning and I spoke with 3 different companies who sell and install solar systems. All three only sold PV systems and all three said I needed a 12.5 kW PV system for my home. at a cost of approx. $100,000.00 and that would only reduce my energy cost, not eliminate it all together.
I like your numbers much better then theirs. In fact one company said that the CSP system was junk and they wouldn't recommend it to anyone.
I find it very funny what some sales people will tell you to get a sale.

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