So, lets spec out and set up a system of LiFePo4 batteries and inverter

[lkruper]

Sunking:
In a properly designed Solar System with a 3-day reserve capacity forces to charge at least @ C/6 rate which is way faster than an EV will charge. C/2 is a very real possibility where Solar Insolation or Sun Hours drop to 3 hours or less. Example Seattle December/January drops to 1.2 Sun Hours. At that low of Solar Insolation you are looking at charging around C/2. Before anyone says "I do not use 3 day, I only use 2-day". That argument does not hold water because with less than 3 day autonomy means you have to charge at even higher rates approaching 1C or higher.

The terms Solar Insolation and Sun Hours are not interchangeable, Solar insolation is a ...

You have misinterpreted Sunking. The English word "or" also distinguishes between two different things. If Sunking had meant they were the same he could have said "Solar Insolation, which is, Sun Hours...." In the context that follows he discusses two different problems, one where there is not enough SUN in one day to charge a battery completely (Insolation) and the other where there is not enough TIME to charge batteries completely (Sun Hours).

In the case of low Sun Hours, even if there is adequate Solar Insolation, one would need to charge at a faster rate to completely charge the battery (ie C/6 which is faster than an EV will charge).

You should re-read Sunking's posts until you understand them completely and you will benefit.

[karrak]

Since tabular (usually) estimated hourly values, and thus day long integrated sums of GHI are commonly available, and since software is available to translate those GHI values to estimated P.O.A. values for the work of a few mouse clicks, the quantity and availability of that more quantifiable data, and it ease of use, makes the whole idea of "sun-hours" quite as unnecessary as it is anachronistic.

Thanks for your comments, very informative.

I am not sure what you mean by "P.O.A values"?

Do you know of any open source software that would evaluate a system design by running the GHI values through a simulator with the system specifications programmed into it? Don't need it for commercial use.

Simon

[J.P.M.]

Thanks for your comments, very informative.

I am not sure what you mean by "P.O.A values"?

Do you know of any open source software that would evaluate a system design by running the GHI values through a simulator with the system specifications programmed into it? Don't need it for commercial use.

Simon

You're welcome. Opinions vary on usefulness.

Since you ask:

P.O.A. == Plane Of Array. It is the value of the total solar radiation of all types - beam (the kind of irradiance that casts a shadow) and diffuse - the solar radiation received from the sky dome cloudy or clear as diffuse or reflected off clouds (and, BTW, all the stuff received from the sky on completely cloudy days), and/or beam and diffuse as reflected from the surroundings, either unintentionally or intentionally by the use of reflecting devices - all of that energy projected into the plane of the receiver (a solar array for example).

In the usual and accepted way of calculating P.O.A. irradiance, each of those beam components will have its own angle of incidence that can be determined geometrically, and each diffuse component, because diffuse does not have a strict origin, will have its own calculated "effective" angle of incidence". How to determine/estimate/define and use the diffuse angles of incidence is a fruitful subject for journals dealing with alternate energy.

The sum of all those projected incident irradiance components is the P.O.A. irradiance. The time integrated value of the irradiance is the insolation.

To your second question: NREL is your friend. There is a lot of stuff available for a few mouse clicks. Sounds like you are looking for models to estimates short or long term performance. Start with PVWatts. Read the help screens and FAQ's a couple of times before attempting a few runs. A relatively steep learning curve. After that, check out something called SAM - System Advisor Model.

Those two will probably get you the info you may be looking for. SAM is like PVWatts on steroids. It usually takes some prior knowledge of solar energy to get the most out of it, and/or to avoid shooting yourself in the foot.

A respectful suggestion meant in a friendly way, or as a rant depending on your outlook: Don't do anything with a piece of canned software that you can't do with a pencil, some paper and your brain. Without some understanding of what the canned stuff does, sooner or later, you'll get your goodies in the wringer. Models are a help, but not a substitute for actually knowing what you're doing.

I appreciate that most folks do not care to get as involved with the resource availability as I do. However, some familiarity is necessary as evidenced by how, for example, many folks are clueless as to why arrays facing away from the equator do not usually perform as well as those facing the equator, or why output usually falls off in the winter. Those folks are easy pickings for getting screwed and that is, IMO, a self inflicted condition.

For more details about the solar resource, see Duffie & Beckman : "Solar Engineering of Thermal Processes", ISBN 0-471-51056-4. That's also a decent undergrad level text dealing with solar energy in general.

As usual, take what you want of the above. Scrap the rest.

[Sunking]

To your second question: NREL is your friend.

Not a friend to Karrak. Only to USA citizens living in the USA. Karak lives upside down under. Our math and physics do not apply. Karrak makes up his own laws of physics.

[inetdog]

If the term "sun-hours" is interchangeable with any other terms, those other term might be "useless", or "misleading" at least in the context of determining or describing the availability of the solar resource for power generation purposes.

I've seen definitions of "sun-hours" such as "duration of time when the sun casts a visible shadow". Or, when an instrument called a Campbell-Stokes sunshine recorder "operates" - basically, burns an image on to treated paper. Both those, and others are lacking in necessary precision to be of any use for a lot of testing or design work. I note the referenced article has neither a power or energy term in the "sun-hours" description.

If linked to some (natural sunlight) irradiance quantity, the term "sun-hours" seems to have definitional problems several reasons besides units, dealing with angles of incidence, distribution of beam, diffuse and reflected irradiance, energy dist. as f(wavelength) and other things.

Although Sun Hours may be ill defined, the most common usage of that term on this forum can be described as:
Sun Hours value (in units of Hours, or Hours per Day depending on exactly how the term is used in a sentence) is the result of taking the total POA incident power per unit area integrated over a full day with the array in a fixed optimal orientation and dividing that by 1000w/m².

The result is a measure, in single conveniently sized number, of how much sun you can capture at a given location on a given day with an optimized fixed array.

There can be an argument that the "more correct" value would instead integrate the insolation in POA while keeping the array (or hypothetical measuring plane) oriented at all times toward the sun. I agree that there is ambiguity there.

Another way of deriving "Sun Hours" would be to take the PVWatts figure for a particular location and optimum fixed array orientation, back out the panel efficiency used to get incoming usable radiation and divide that intermediate result by 1000W/m².

It is absolutely correct that for scheduling loads and for looking at whether you can get a sufficient absorb time before the panel power drops too low you also need to look at how many hours of the day the sunlight falls on the panels, period. I would not use Sun Hours for that number, although a change in the first by changing season and or latitude will affect the second proportionally.

I guess we could indeed spend a lot of time arguing about this. Do you have a more useful metric or word for that metric to propose?

[Sunking]

Hardest thing for Engineers to learn is to KISS. Solar is so simple you do not even need a high school education. A fair number of installers and designers do not even have a high school diploma or even speak English.

[J.P.M.]

Inetdog: In respectful response:

And what would you have if/when you did that ? And, what use would that value have ? Since the description you give seems to imply that the trouble of getting P.O.A. irradiance (usually at least a minor PITA if done thoroughly) has been done and then summed over the period of interest, why is dividing that total by 1,000 W/m^2 be necessary or useful ? Seems to me as a result of the process of getting the integrated P.O.A irradiance, a lot of good, useful and powerful information has already been obtained. If some array has, say, 5,000 W/m^2 of P.O.A. irradiance over a day, having gone through all the trouble of calculating and/or measuring that quantity, why not just say that and avoid any possible confusion by introducing a time term (hours) ? I'd respectfully suggest that 5,000 Watts/m^2 is more useful, less confusing and equally as easy to write and communicate as "5 sun-hours".

I'm not sure what "sun hours" as you define it says about or determines optimization (assuming orientation) of a fixed array.

A simpler, perhaps more useful and commonly available number for estimating and design purposes might be something called the "Clearness Index". It's been around for a long time, easy to use and find either in tabular form for many locations, or easily calc'd for site and measured data purposes. It is simply the GHI at any location divided by the extraterrestrial irradiance received by a "horizontal" surface above the earth at the same orientation (I.E. parallel level ground or essentially normal to the gravity vector). It takes a lot into account, including the effects of location altitude above mean sea level, clouds, pollution, seasonal adjustments in incidence angles and a lot of other stuff. It can use any time period, from a second to a year or longer, depending on the need and application.

"There can be an argument that the "more correct" value would instead integrate the insolation in POA while keeping the array (or hypothetical measuring plane) oriented at all times toward the sun. I agree that there is ambiguity there."

And probably adds confusion.

"Another way of deriving "Sun Hours" would be to take the PVWatts figure for a particular location and optimum fixed array orientation, back out the panel efficiency used to get incoming usable radiation and divide that intermediate result by 1000W/m²."

Again, more work than necessary using data that is useful in and of itself. I'd do what is usually recognized as the standard - get the GHI, and translate it to P.O.A. - just like PVWatts (actually the TMY database) does, and use that for design or estimating purposes.

"It is absolutely correct that for scheduling loads and for looking at whether you can get a sufficient absorb time before the panel power drops too low you also need to look at how many hours of the day the sunlight falls on the panels, period. I would not use Sun Hours for that number, although a change in the first by changing season and or latitude will affect the second proportionally."

"I guess we could indeed spend a lot of time arguing about this. Do you have a more useful metric or word for that metric to propose?"

I, for one, don't consider this an argument. The short answer to your last sentence is yes. The Clearness Index. Universally used and understood. And much more useful for design. See Duffie & Beckman, Chap. 2, or the seminal papers by Liu and Jordon in "Solar Energy", vols, 3 and 7.

Respectful Regards,

[lkruper]

Inetdog: In respectful response:

And what would you have if/when you did that ? And, what use would that value have ? Since the description you give seems to imply that the trouble of getting P.O.A. irradiance (usually at least a minor PITA if done thoroughly) has been done and then summed over the period of interest, why is dividing that total by 1,000 W/m^2 be necessary or useful ? Seems to me as a result of the process of getting the integrated P.O.A irradiance, a lot of good, useful and powerful information has already been obtained. If some array has, say, 5,000 W/m^2 of P.O.A. irradiance over a day, having gone through all the trouble of calculating and/or measuring that quantity, why not just say that and avoid any possible confusion by introducing a time term (hours) ? I'd respectfully suggest that 5,000 Watts/m^2 is more useful, less confusing and equally as easy to write and communicate as "5 sun-hours".

I'm not sure what "sun hours" as you define it says about or determines optimization (assuming orientation) of a fixed array.

A simpler, perhaps more useful and commonly available number for estimating and design purposes might be something called the "Clearness Index". It's been around for a long time, easy to use and find either in tabular form for many locations, or easily calc'd for site and measured data purposes. It is simply the GHI at any location divided by the extraterrestrial irradiance received by a "horizontal" surface above the earth at the same orientation (I.E. parallel level ground or essentially normal to the gravity vector). It takes a lot into account, including the effects of location altitude above mean sea level, clouds, pollution, seasonal adjustments in incidence angles and a lot of other stuff. It can use any time period, from a second to a year or longer, depending on the need and application.

"There can be an argument that the "more correct" value would instead integrate the insolation in POA while keeping the array (or hypothetical measuring plane) oriented at all times toward the sun. I agree that there is ambiguity there."

And probably adds confusion.

"Another way of deriving "Sun Hours" would be to take the PVWatts figure for a particular location and optimum fixed array orientation, back out the panel efficiency used to get incoming usable radiation and divide that intermediate result by 1000W/m²."

Again, more work than necessary using data that is useful in and of itself. I'd do what is usually recognized as the standard - get the GHI, and translate it to P.O.A. - just like PVWatts (actually the TMY database) does, and use that for design or estimating purposes.

"It is absolutely correct that for scheduling loads and for looking at whether you can get a sufficient absorb time before the panel power drops too low you also need to look at how many hours of the day the sunlight falls on the panels, period. I would not use Sun Hours for that number, although a change in the first by changing season and or latitude will affect the second proportionally."

"I guess we could indeed spend a lot of time arguing about this. Do you have a more useful metric or word for that metric to propose?"

I, for one, don't consider this an argument. The short answer to your last sentence is yes. The Clearness Index. Universally used and understood. And much more useful for design. See Duffie & Beckman, Chap. 2, or the seminal papers by Liu and Jordon in "Solar Energy", vols, 3 and 7.

Respectful Regards,

I have never heard of the clearness index, which is not saying much because I have not heard of a lot of things. Could you show how one could use this to answer the question as to whether there is enough time to charge a battery with solar... say 2 hours of bulk + 2 hours of absorb for a particular location? Is there a web site with this data?

[Mike90250]

PV Watts manages to calculate sun hours somehow.

I think it includes the solar noon hour, plus some average of the less than noon hours, eventually giving 2.4 hours in some months, 5 in others.

It's not simple.. Solar noon counts for about 20 minutes, on a fixed array. Tilt, azimuth, AM fog, or afternoon thunder clouds is factored into their calculation.

[J.P.M.]

I have never heard of the clearness index, which is not saying much because I have not heard of a lot of things. Could you show how one could use this to answer the question as to whether there is enough time to charge a battery with solar... say 2 hours of bulk + 2 hours of absorb for a particular location? Is there a web site with this data?

I cannot. For one thing, I'm not sure what you're asking. But I'm not sure how sun-hours would be of any more use with or without information about the storage device or array information, including size. But that's not saying much either.

Maybe I'm missing something or my ignorance is causing the problem here. I don't claim much knowledge about off grid or battery storage systems, which is why I usually stay out of such discussions, but I'm pretty sure if I chose to become knowledgeable in such things, I'd either find the error of my ways with respect to my opinion of the efficacy of "sun-hours", or find that existing methods were as well or better suited to the purpose of which you speak. I suspect the latter, but I'd like to think I have an open enough mind and enough intellectual curiosity to learn stuff. Can you, or do you care to enlighten me ?

A Google search of "Clearness Index" + "Solar" will enlighten you in such concepts. As for a website, I'm sure they exist. I have lots of tabular values, but they're all in things called books. I guess my age is showing.

[J.P.M.]

PV Watts manages to calculate sun hours somehow.

I think it includes the solar noon hour, plus some average of the less than noon hours, eventually giving 2.4 hours in some months, 5 in others.

It's not simple.. Solar noon counts for about 20 minutes, on a fixed array. Tilt, azimuth, AM fog, or afternoon thunder clouds is factored into their calculation.

I respectfully differ. Where does PVWatts calculate something called "sun hours ? The closest it gets (if I understand some "definition" of "sun-hours") is the PVWatts output listing of kWh/m^2/day as resulting monthly average daily value of P.O.A. insolation for a TMY2, TMY3 or SolarAnywhere data set. That is, that #, (for the PVWattts monthly output) is the TMY GHI projected to P.O.A. monthly total insolation divided by the # of days in that month.

I've never seen a reference to solar noon hour or some averages as you write of in any context. Solar noon may account for something in a fixed or moving array, but I'm not sure what that means or what value it may have. The other things you write of - Tilt, azimuth, etc. all are important in estimating array output or explaining array performance, but that's a normal and common situation.

If you choose to call P.O.A insolation "sun hours", so be it, but that's what I'd call a local definition and not one that is used in the open literature or in general use. I've been around solar energy and R.E. for about 40 years, and while I've come across the use of the term "sun-hours", it's not in a context of anything usually much more than more than casual or as an example of a type of term to avoid due to its imprecision, ambiguity and lack of definition.

[lkruper]

I cannot. For one thing, I'm not sure what you're asking. But I'm not sure how sun-hours would be of any more use with or without information about the storage device or array information, including size. But that's not saying much either.

Maybe I'm missing something or my ignorance is causing the problem here. I don't claim much knowledge about off grid or battery storage systems, which is why I usually stay out of such discussions, but I'm pretty sure if I chose to become knowledgeable in such things, I'd either find the error of my ways with respect to my opinion of the efficacy of "sun-hours", or find that existing methods were as well or better suited to the purpose of which you speak. I suspect the latter, but I'd like to think I have an open enough mind and enough intellectual curiosity to learn stuff. Can you, or do you care to enlighten me ?

A Google search of "Clearness Index" + "Solar" will enlighten you in such concepts. As for a website, I'm sure they exist. I have lots of tabular values, but they're all in things called books. I guess my age is showing.

My question relates to my premise earlier that Solar Insolation is the sum total of irradiance in a 24 hour period and that Sun Hours is the window of time for that radiation. The former can inform our view as to the total possible energy storage while the latter will constrain the ability to charge a battery in that time frame because it could exceed its maximum charge rate, even if there is enough insolation in a 24 hour period.


I believe the discussion started because of a dispute over whether or not Solar Insolation was equivalent to Sun Hours. I have seen these two terms used identically, and in other instances have seen Sun hours refer to the actual hours the Sun shines in a particular season.

I thought that you had proposed that the Clearness index was equivalent to the literal number of hours in one day where a solar panel could produce sufficient insolation to charge a battery.

[Sunking]

JPM with all due respect, like all engineers, we are very susceptible to a disease called Paralysis by Analysis. It is a nasty disease we come in contact with in college. Professors and Instructors who have never done or built anything fill our little heads with Mush. A bunch of facts, theory, laws, and formulas we will never most likely ever use again. The cure is simple, KISS. This is a DIY site, no one knows or cares about all the little details. None of it is needed.

I learned solar from John Wiles of NMSU 15 years ago. You can look him up. He has forgotten more than you and I will ever know. One of the things he has forgotten is exactly what you aree trying to convey. It is all waste of precious time and money. I met John via NEC code panels in 1999. He gave me all the books and course material he teaches. After a year of going through the info and building a number of of-grid sites, I came up with a great little program. Man I was proud of it, and gave to John to see what he thought.

A month or two later during a code panel meeting we had lunch together and time to talk about the work I had done. I was so excited to get his feedback, I just knew I hit it out of the park. So when I asked John about it, he laughed and said: "You cannot see the forest for the trees stupid". He caught me off guard and I asked him to expand on that a bit.

• He asked: How long does it take me to collect all that data to input it and have the basic design. .
• I said: 3 to 6 hours depending on the scale.
• He laughed and said: It should take no more than 5 minutes to gather the data and do the design. If you are sharp requires no calculator or computer other than the protein computer or peice of meat between my ears.
• John then asked: Did you read read the Cliff Notes?
• I said: Yes
• He then asked: Why in the hell would I go to all that work when the Cliff Notes lined out a very simple design process.
• He then challenged me right on the spot and said: Pick any location on the map, look up Sun Hours on NREL and design a system that yields 10 Kwh of usable power in a day.
• He continued and said: do it his way, and my way, then get back to him.
  

So I did. His way is one of the Stickies. My way took roughly 3 hours, his way 5 minutes. End result was panel wattage was 200 watts difference. My way, the long method, was the lessor of the two and I thought I had proven him wrong. So I got back in touch with him with my answers. I had proven him wrong. I was so happy.

• He laughed and said: What was the cost difference and how much more money it put into my pocket?
• I said about $300 addeed cost to client, and netted me $15.
• He then really laughed hard and said: It took you 3 hours to make $15 stupid. Did I like working for $5/hour?

Ouch I was busted and finally seen the forest. For the next couple of years I did some jobs both ways. The pattern was obvious. My time wasting way either matched John's way, or slightly higher panel wattage. Since that time I learned I sleep a lot better using and extra 100 or 200 watts doing it John' way. Error on the side of caution and use worse case. You will sleep better at night.

So KISS it. It is a DIY site. Save the boring lectures and theory for a captive audience like a class room or conference with Engineering NERDS. No one here gives a damn or capable of understanding it.


. .

[J.P.M.]

JPM with all due respect, like all engineers, we are very susceptible to a disease called Paralysis by Analysis. It is a nasty disease we come in contact with in college. Professors and Instructors who have never done or built anything fill our little heads with Mush. A bunch of facts, theory, laws, and formulas we will never most likely ever use again. The cure is simple, KISS. This is a DIY site, no one knows or cares about all the little details. None of it is needed.

I learned solar from John Wiles of NMSU 15 years ago. You can look him up. He has forgotten more than you and I will ever know. One of the things he has forgotten is exactly what you aree trying to convey. It is all waste of precious time and money. I met John via NEC code panels in 1999. He gave me all the books and course material he teaches. After a year of going through the info and building a number of of-grid sites, I came up with a great little program. Man I was proud of it, and gave to John to see what he thought.

A month or two later during a code panel meeting we had lunch together and time to talk about the work I had done. I was so excited to get his feedback, I just knew I hit it out of the park. So when I asked John about it, he laughed and said: "You cannot see the forest for the trees stupid". He caught me off guard and I asked him to expand on that a bit.

• He asked: How long does it take me to collect all that data to input it and have the basic design. .
• I said: 3 to 6 hours depending on the scale.
• He laughed and said: It should take no more than 5 minutes to gather the data and do the design. If you are sharp requires no calculator or computer other than the protein computer or peice of meat between my ears.
• John then asked: Did you read read the Cliff Notes?
• I said: Yes
• He then asked: Why in the hell would I go to all that work when the Cliff Notes lined out a very simple design process.
• He then challenged me right on the spot and said: Pick any location on the map, look up Sun Hours on NREL and design a system that yields 10 Kwh of usable power in a day.
• He continued and said: do it his way, and my way, then get back to him.
  

So I did. His way is one of the Stickies. My way took roughly 3 hours, his way 5 minutes. End result was panel wattage was 200 watts difference. My way, the long method, was the lessor of the two and I thought I had proven him wrong. So I got back in touch with him with my answers. I had proven him wrong. I was so happy.

• He laughed and said: What was the cost difference and how much more money it put into my pocket?
• I said about $300 addeed cost to client, and netted me $15.
• He then really laughed hard and said: It took you 3 hours to make $15 stupid. Did I like working for $5/hour?

Ouch I was busted and finally seen the forest. For the next couple of years I did some jobs both ways. The pattern was obvious. My time wasting way either matched John's way, or slightly higher panel wattage. Since that time I learned I sleep a lot better using and extra 100 or 200 watts doing it John' way. Error on the side of caution and use worse case. You will sleep better at night.

So KISS it. It is a DIY site. Save the boring lectures and theory for a captive audience like a class room or conference with Engineering NERDS. No one here gives a damn or capable of understanding it.


. .

Derrick: I respect your credentials and experience, and your service to our country. I choose to not comment on most of what you write with respect to alt. energy mostly because we seem to travel in different circles.

In many ways, if you are preaching or lecturing to me, you are preaching to the choir with your recent post, including the idea, which I wholeheartedly endorse, that someone (including you and most everyone else on this planet) may have forgotten more than I will ever know.

As such, I'm the biggest KISS fan I know, if only for self protection. I've been practicing KISS for as long as I can remember. Say it every morning when I look in the mirror. However, I've observed that sometimes simple (minded) does not mean best. Sometimes it takes exposure to formal ideas to spot the inelegance in reducing things too far.

I believe I can, for many types of R.E. systems and applications do a 5 min. design using a method similar to similar to what you describe, and get as practically close to a good system as many hours of # crunching and design iterations would yield. I get that. Perhaps such as yourself, I've done and sealed enough designs, and corrected enough errors, both mine and others', to appreciate the elegance and safety in simplicity or avoiding overdesign.

I have written on more than a few occasions that formal education is not a substitute for real knowledge gained through experience. However, I do believe concepts learned by any means, sometimes in a classroom, can help folks work smarter, improving efficiency. Sometimes simplicity is synonymous with elegance. Sometime elegance takes insight. Sometimes insight takes knowledge gained through formal schooling, and as Edison suggested - sweat and persistence. I think it takes all of those and more.

Above all, simplicity does not mean simple minded.

My point with the use of the term "Sun-Hours", which point it seems I'm doing a poor job of explaining, is that it is imprecise, and as such can possibly lead to lead to confusion that's easily avoided in a simple way that's easy to implement.

The use of the term "daily GHI" or some such commonly recognized term with its readily available data and avoidance of the unnecessary confusion I believe to be inherent in the term "Sun-Hours" is at least as good or a better term and concept to use.

That's it, and that's all I'm suggesting.

Respectfully.

As usual, take what you want of the above. Scrap the rest.

[Sunking]

My point with the use of the term "Sun-Hours", which point it seems I'm doing a poor job of explaining, is that it is imprecise, and as such can possibly lead to lead to confusion that's easily avoided in a simple way that's easy to implement..

That was my point JPM. This is a DIY crowd and we have to dumb it down. Otherwise you just go round in circles trying to explain something they will never grasp. Sun Hours is universally accepted interchangeable with Kwh/m2. That is all they need to know.

Lighten up JPM I was not being critical. You should know me better than that by now. Don't take me so serious. I was just pointing out the crowd you are talking too need KISS, not theory.