SunPower SPR-E20-327-C-AC Power Testing

[J.P.M.]

Absolutely... gable and ridge vents should be adequate until we decide what to do with the aging water heater (replace in kind / solar / heat pump).

Well, made it through PV Dummies Chapter 7 (installing your solar system). Decent read / mostly about hiring a pro when you are out of your element, which is what we did, but considering doing the next one at little more hands on. Considering a small ground mounted system in the future at another location.

Also, re-did the previous PV Watts calculator, that I sent a few days ago) as I had forgotten to plug in age (four years). Came up 6,703 kWh annually (still more than the 5,600 kWh annually.

Also, we are starting a little smaller (less expensive) on the weather station, but it (Ambient Weather WS-2902C WiFi Smart Weather Station) does measure wind speed/direction, temperature, humidity, rainfall, UV and solar radiation. Data is collected by the manufacturer, and I can download five-minute intervals up to annually. You had mentioned previously that you know how to translate W/SQ Meter collected from a sensor that points straight up to "normal". Could you help me at least head in the right direction????? What is also interesting is that there is another one of these weather stations installed at the school just a block away and has temperature, wind and irradiance data collected every 5 minutes for at least the last year - not precise, but another reference point on daily irradiance levels in my neighborhood.

On to PV Dummies Chapter 8 (Digging into Landscape Projects). Hm... this one might be fun as I have been wanting to get our 60 + 90 watt pond pumps (running 24/7/365) off the grid for some time.

My comment about attic vents was inadequate in that I didn't convey the idea that an air source HPWH - or any air source heat pump for that matter - in a semi isolated space such as an attic will need careful consideration of the venting and how it's done with respect to both attic condition requirements and the supply air requirements for the heat pump. If ventilation is inadequate, the C.O.P. of the heat pump will drop as f(attic air temp.). The air will certainly be dry - maybe/likely with a lower dew point than the outdoor ambient - at least in non winter weather - but care in ventilation adequacy for the heat source for the pump is still needed.

On the PVWatts system loss factor: Many have found the 14 % default rate the model uses to be a little conservative for many applications with many users finding 10 % to be a better match with actual output over time for unshaded arrays.

Keeping in mind that the PVWatts model is for system design, and for many reasons not a predictor of output (although it's tempting to use it as such), I have enough data on my historical irradiance and actual array output to have been able to back into a system loss parameter for my system so that the PVWatts model output for running 31 day outputs over 8 years with few gaps in the data is reasonably within the model's stated tolerances for the same periods.

Using a system loss parameter of 12.8 % which includes a 3.5 % shading loss, the average running 31 day actual output/modeled output for the same periods = 0.989, high actual output/modeled output = 1.25, low actual output/modeled output = 0.651, std. dev. = 0.096, n = 2,989.

Those modeled numbers are quite close to that of two other models I use. One model, SAM from NREL, is probably best described as PVWatts on steroids. After you get a facile understanding of D.& B., you might want to take a look at SAM. Doing so can be an educational experience in itself. The other model is one I wrote several years before SAM made its appearance. It uses a lot of stuff in found in chap. 2 of D. & B. as well as other things found in the journals and some stuff of my own that enables estimates of things like backside irradiance on panels of any orientation and some stuff about view factors necessary for the calculation of that irradiance, and a few other things that deal with specific atmospheric constituents and how they affect irradiance on a wavelength basis. It will also use any input for the three irradiance components (beam, diffuse, reflected) from measured or modeled data for any clearness index from zero to 1.25. I put it on a spread sheet and it's ~ 100 Mb. It's not in a form that's presentable and really no more than a compilation of stuff that helps me explain some of what I've measured over the years.

As for translating GHI to POA, know that there are as many models to do that as there are people to attempt the chore. In spite of what many folks think, it's not an exact science, mostly due to the nature and the uncertainties associated with the atmosphere of the earth. My irradiance model starts with something called the HDKR model.

If I was giving advice on understanding irradiance models and how to translate GHI to POA irradiance, best start I'd suggest is at chap. 2 and then particularly 2.16 of D.& B. paying particular attention to the end of chapter bibliography and go from there.

What you may be looking for is similar to what it's been my experience to find many people want, including me: Simple answers to what looks to be a quest as clear and as simple as light. I wish it were so, but it turns out to be - depending on the level of confidence wanted in the data and results you get to be an inherently convoluted quest. Not conceptually difficult - if I can figure some of it out, anyone can - just don't expect accuracy with magic bullet one line solutions.

Going back to your original post and what you're looking for - an answer to why your array's output seems low, if you want a fast and reasonably accurate way to find POA irradiance and don't want the academic gyrations and all the B.S. & T. that goes with them, get a Davis Pro II, with an irradiance sensor and mount the sensor on the array in the plane of the array. As I mentioned previously and for several reasons mostly dealing with how the irradiance sensor sees/handles handles circumsolar and diffuse irradiance when in an off vertical orientation, it's not the best use of the instrument, but doing so will be faster than a perhaps unwanted trip through the details of solar irradiance and may be acceptable if a slight loss of accuracy is acceptable.

As for Ambient weather stuff, there are maybe 8 -10 WeatherUnderground sites near me (within maybe 8-10 miles or so). On clear days, the ones using Davis Pro II stations - 4 of them - seem to be in general agreement with one another and with my Davis on irradiance and other parameters. Others, including stuff from Ambient Weather - 3, I believe - even on clear days - seem to be all over the place. I think part of that may be that Davis owners may be more knowledgeable about what they're measuring.

Bottom line, if you want reliable data, get a Davis or if you just want reasonably accurate and precise irradiance data, get a pyranometer from an outfit called Kipp and Zonen or some others. There's good stuff out there to measure irradiance but you'll pay about as much or more for the pyranometer and ancillary equipment alone as you'll pay for the Davis and the accuracy will be about the same.

Reliable irradiance numbers are necessary for reasonable and consistent estimations of solar panel performance which you'll need as part of your task of figuring out where your array's performance problem(s) may be. This is not a plug, just what I've found over the years: You'll get what you pay for and the least expensive way to get reasonably accurate and precise as well as logged irradiance data at this time is with a Davis.

[ajonestx]

My comment about attic vents was inadequate in that I didn't convey the idea that an air source HPWH - or any air source heat pump for that matter - in a semi isolated space such as an attic will need careful consideration of the venting and how it's done with respect to both attic condition requirements and the supply air requirements for the heat pump. If ventilation is inadequate, the C.O.P. of the heat pump will drop as f(attic air temp.). The air will certainly be dry - maybe/likely with a lower dew point than the outdoor ambient - at least in non winter weather - but care in ventilation adequacy for the heat source for the pump is still needed.

On the PVWatts system loss factor: Many have found the 14 % default rate the model uses to be a little conservative for many applications with many users finding 10 % to be a better match with actual output over time for unshaded arrays.

Keeping in mind that the PVWatts model is for system design, and for many reasons not a predictor of output (although it's tempting to use it as such), I have enough data on my historical irradiance and actual array output to have been able to back into a system loss parameter for my system so that the PVWatts model output for running 31 day outputs over 8 years with few gaps in the data is reasonably within the model's stated tolerances for the same periods.

Using a system loss parameter of 12.8 % which includes a 3.5 % shading loss, the average running 31 day actual output/modeled output for the same periods = 0.989, high actual output/modeled output = 1.25, low actual output/modeled output = 0.651, std. dev. = 0.096, n = 2,989.

Those modeled numbers are quite close to that of two other models I use. One model, SAM from NREL, is probably best described as PVWatts on steroids. After you get a facile understanding of D.& B., you might want to take a look at SAM. Doing so can be an educational experience in itself. The other model is one I wrote several years before SAM made its appearance. It uses a lot of stuff in found in chap. 2 of D. & B. as well as other things found in the journals and some stuff of my own that enables estimates of things like backside irradiance on panels of any orientation and some stuff about view factors necessary for the calculation of that irradiance, and a few other things that deal with specific atmospheric constituents and how they affect irradiance on a wavelength basis. It will also use any input for the three irradiance components (beam, diffuse, reflected) from measured or modeled data for any clearness index from zero to 1.25. I put it on a spread sheet and it's ~ 100 Mb. It's not in a form that's presentable and really no more than a compilation of stuff that helps me explain some of what I've measured over the years.

As for translating GHI to POA, know that there are as many models to do that as there are people to attempt the chore. In spite of what many folks think, it's not an exact science, mostly due to the nature and the uncertainties associated with the atmosphere of the earth. My irradiance model starts with something called the HDKR model.

If I was giving advice on understanding irradiance models and how to translate GHI to POA irradiance, best start I'd suggest is at chap. 2 and then particularly 2.16 of D.& B. paying particular attention to the end of chapter bibliography and go from there.

What you may be looking for is similar to what it's been my experience to find many people want, including me: Simple answers to what looks to be a quest as clear and as simple as light. I wish it were so, but it turns out to be - depending on the level of confidence wanted in the data and results you get to be an inherently convoluted quest. Not conceptually difficult - if I can figure some of it out, anyone can - just don't expect accuracy with magic bullet one line solutions.

Going back to your original post and what you're looking for - an answer to why your array's output seems low, if you want a fast and reasonably accurate way to find POA irradiance and don't want the academic gyrations and all the B.S. & T. that goes with them, get a Davis Pro II, with an irradiance sensor and mount the sensor on the array in the plane of the array. As I mentioned previously and for several reasons mostly dealing with how the irradiance sensor sees/handles handles circumsolar and diffuse irradiance when in an off vertical orientation, it's not the best use of the instrument, but doing so will be faster than a perhaps unwanted trip through the details of solar irradiance and may be acceptable if a slight loss of accuracy is acceptable.

As for Ambient weather stuff, there are maybe 8 -10 WeatherUnderground sites near me (within maybe 8-10 miles or so). On clear days, the ones using Davis Pro II stations - 4 of them - seem to be in general agreement with one another and with my Davis on irradiance and other parameters. Others, including stuff from Ambient Weather - 3, I believe - even on clear days - seem to be all over the place. I think part of that may be that Davis owners may be more knowledgeable about what they're measuring.

Bottom line, if you want reliable data, get a Davis or if you just want reasonably accurate and precise irradiance data, get a pyranometer from an outfit called Kipp and Zonen or some others. There's good stuff out there to measure irradiance but you'll pay about as much or more for the pyranometer and ancillary equipment alone as you'll pay for the Davis and the accuracy will be about the same.

Reliable irradiance numbers are necessary for reasonable and consistent estimations of solar panel performance which you'll need as part of your task of figuring out where your array's performance problem(s) may be. This is not a plug, just what I've found over the years: You'll get what you pay for and the least expensive way to get reasonably accurate and precise as well as logged irradiance data at this time is with a Davis.

Understood – concerns about an air source HPWH ventilation – should we go down that route, what do you think about 2,100 CF of space in the attic? General recommendation from EPA is 1,000 CF minimum. There is also one 6 CF gable vent and 22 LF of ridge vent giving about 33 CF of ventilation there. In 77008, we have on average about 10 days below freezing. If a HPWH is considered, manufacturer specific data and installing contractor input will be evaluated further.

On the PVWatts system loss factor and your calculations (some of your words are a little Greek to me), are you suggesting that SAM and NREL models produce highs at 1.25% and lows at 65.1% of capacity?

On the PVWatts system loss factor. Admittedly, I made a trip to visit my ASHP and propane powered furnace to collect name plate data (among other things like installing PV for my trail camera) and left my D. &.B. book on the shelf. I will check Chapter 2 soon.

I started looking into the conversion of GHI to POA irradiance and got lost in the initial data that I found online – mostly what looked like rows and rows of calculations reminiscent of my days in Calculus class. I am hoping that chap. 2 and then particularly 2.16 of D.& B. will shed some light on this task. And no, I am not expecting accuracy with magic bullet one line solutions. I realize that things are plus or minus (especially dealing with mother nature) – right now, I am in the dark looking for the lamp switch that will shed some light on the topic. In the end, you are probably right about the Davis Pro II (slight loss of accuracy is acceptable). I mostly wanted to see what the task of converting GHI to POA entails before making a decision on Davis or other.

On the topic of ASHP vs. propane, I did find the heat pump (Lennox XP16-036-230-08). Efficiency is a little hard to nail down as you suggested: Online literature suggests a SEER of 16 and a 9.50 HSPF. I guess I am looking for the winter efficiency, so I divide 9.5 by 3.412 to get COP of 2.78. I called Lennox to get COP by temperature, but they were not willing to give me that data and told me to go through a dealer (who would have to call Lennox). Frustrating.

For the furnace (Lennox CX34-43-6F-1), the “Energy Star sticker” suggest an annual fuel utilization efficiency of 97.5. Propane conversation sticker suggested BTUH input of 88,000/31,000 and an output of 85,000/30,500. Running the numbers, it looks like I might be able to save about $300 to $400 (calculations attached) a year by getting the ASHP up and running – technician looked at it this week and said a pressure switch was missing which explained why the HP would not turn on during the winter.

OH, and finished Chapter 8, PV Dummies, Landscape projects – fun stuff. Gets you thinking more about awnings, shading, simple stuff. Never been a fan of solar powered walkway lights, but the writer did have some interesting ideas about solar powered water falls and the use of an upper water reservoir.

[J.P.M.]

Understood – concerns about an air source HPWH ventilation – should we go down that route, what do you think about 2,100 CF of space in the attic? General recommendation from EPA is 1,000 CF minimum. There is also one 6 CF gable vent and 22 LF of ridge vent giving about 33 CF of ventilation there. In 77008, we have on average about 10 days below freezing. If a HPWH is considered, manufacturer specific data and installing contractor input will be evaluated further.

On the PVWatts system loss factor and your calculations (some of your words are a little Greek to me), are you suggesting that SAM and NREL models produce highs at 1.25% and lows at 65.1% of capacity?

On the PVWatts system loss factor. Admittedly, I made a trip to visit my ASHP and propane powered furnace to collect name plate data (among other things like installing PV for my trail camera) and left my D. &.B. book on the shelf. I will check Chapter 2 soon.

I started looking into the conversion of GHI to POA irradiance and got lost in the initial data that I found online – mostly what looked like rows and rows of calculations reminiscent of my days in Calculus class. I am hoping that chap. 2 and then particularly 2.16 of D.& B. will shed some light on this task. And no, I am not expecting accuracy with magic bullet one line solutions. I realize that things are plus or minus (especially dealing with mother nature) – right now, I am in the dark looking for the lamp switch that will shed some light on the topic. In the end, you are probably right about the Davis Pro II (slight loss of accuracy is acceptable). I mostly wanted to see what the task of converting GHI to POA entails before making a decision on Davis or other.

On the topic of ASHP vs. propane, I did find the heat pump (Lennox XP16-036-230-08). Efficiency is a little hard to nail down as you suggested: Online literature suggests a SEER of 16 and a 9.50 HSPF. I guess I am looking for the winter efficiency, so I divide 9.5 by 3.412 to get COP of 2.78. I called Lennox to get COP by temperature, but they were not willing to give me that data and told me to go through a dealer (who would have to call Lennox). Frustrating.

For the furnace (Lennox CX34-43-6F-1), the “Energy Star sticker” suggest an annual fuel utilization efficiency of 97.5. Propane conversation sticker suggested BTUH input of 88,000/31,000 and an output of 85,000/30,500. Running the numbers, it looks like I might be able to save about $300 to $400 (calculations attached) a year by getting the ASHP up and running – technician looked at it this week and said a pressure switch was missing which explained why the HP would not turn on during the winter.

OH, and finished Chapter 8, PV Dummies, Landscape projects – fun stuff. Gets you thinking more about awnings, shading, simple stuff. Never been a fan of solar powered walkway lights, but the writer did have some interesting ideas about solar powered water falls and the use of an upper water reservoir.

Understood. I'm at 2d home just now. Will respond next day or so.

[ajonestx]

Understood. I'm at 2d home just now. Will respond next day or so.

When you get back (I am also away and will be back settled in a few days), I did also want to step back and better understand the terminology that we are using. I found an interesting source (below / on the internet, so it must be true ).

in the descriptions below, it would suggest that I should be looking for GTA (which is also POA, right)?

But... in some of the NREL data I downloaded from my area in 2020, there were 69 days at or above 1,000 W/SQM (GHI). In that same year, there were 37 (almost half) days at or above 1,000 W/SQM (DNI). I know (from experimentation and from basic general observations) that PV performs better when the sun is generally perpendicular to the POA, but this data suggests maybe not always as DHI plays a pretty large role in PV performance.

Question: if a particular weather station has the irradiation sensor pointing straight up, is it measuring GHI?

Question: speaking to "accuracy", If can get a less expensive weather station that has the irradiation sensor pointed straight up and bypass the effort to convert or try to get an external sensor mounted to the array at the correct angle, what kind of accuracy loss are we talking about?

Question: if I can identify a particular day and time with over 1,000 W/SQM (GHI) and my micro inverters are not generating power at the expected levels, this would identify the problem, right? Temperature loss and Wind plays into the equation too per your prior notes.

These questions are probably ahead of my education curve (as I am not yet complete with PV Dummies and D.& B.), but I am struggling with the basic terminology before equipment requirements can be established and making a purchase.

QUOTE FROM THE INTERNET BELOW:

There are various components of Solar Irradiance. These are explained as below:

DNI is acronym for Diffused Normal Irradiance which represents the amount of light that is coming perpendicular to surface. The surface here represents ground or something parallel to ground. This type of irradiance belongs to rays that come in a straight line from the direction of the sun at its current position in the sky. Solar collectors, panels maximize this DHI by means of tilting or rotating with angle of sun.

DHI is acronym for Diffused Horizontal Irradiance which represents solar radiation that does not arrive on a direct path from the sun, but has been scattered by clouds and particles in the atmosphere and comes equally from all directions.

GHI is acronym for Global Horizontal Irradiance which represents the total amount of shortwave radiation received from above by a surface which is horizontal (parallel) to the ground. GHI is the most important parameter for calculation of PV electricity yield. Both DHI & DNI are actually components of GHI and mathematically,

Global Horizontal Irradiance (GHI) = Direct Normal Irradiance (DNI)* cos(solar zenith angle) + Diffused Horizontal Irradiance (DHI)

GTI is acronym for Global Tilted Irradiance which represents irradiation that falls on a tilted surface. Unlike a horizontal surface which is parallel to ground, a tilted surface also receives small amount of ground-reflected radiation (REF). GTI is an approximate value for the energy yield calculation of fixed installed tilted PV panels.

Optimum angle refers to the angle at which Solar PV modules should be oriented in order to generate maximum electricity.

[J.P.M.]

When you get back (I am also away and will be back settled in a few days), I did also want to step back and better understand the terminology that we are using. I found an interesting source (below / on the internet, so it must be true ).

in the descriptions below, it would suggest that I should be looking for GTA (which is also POA, right)?

But... in some of the NREL data I downloaded from my area in 2020, there were 69 days at or above 1,000 W/SQM (GHI). In that same year, there were 37 (almost half) days at or above 1,000 W/SQM (DNI). I know (from experimentation and from basic general observations) that PV performs better when the sun is generally perpendicular to the POA, but this data suggests maybe not always as DHI plays a pretty large role in PV performance.

Question: if a particular weather station has the irradiation sensor pointing straight up, is it measuring GHI?

Question: speaking to "accuracy", If can get a less expensive weather station that has the irradiation sensor pointed straight up and bypass the effort to convert or try to get an external sensor mounted to the array at the correct angle, what kind of accuracy loss are we talking about?

Question: if I can identify a particular day and time with over 1,000 W/SQM (GHI) and my micro inverters are not generating power at the expected levels, this would identify the problem, right? Temperature loss and Wind plays into the equation too per your prior notes.

These questions are probably ahead of my education curve (as I am not yet complete with PV Dummies and D.& B.), but I am struggling with the basic terminology before equipment requirements can be established and making a purchase.

QUOTE FROM THE INTERNET BELOW:

There are various components of Solar Irradiance. These are explained as below:

DNI is acronym for Diffused Normal Irradiance which represents the amount of light that is coming perpendicular to surface. The surface here represents ground or something parallel to ground. This type of irradiance belongs to rays that come in a straight line from the direction of the sun at its current position in the sky. Solar collectors, panels maximize this DHI by means of tilting or rotating with angle of sun.

DHI is acronym for Diffused Horizontal Irradiance which represents solar radiation that does not arrive on a direct path from the sun, but has been scattered by clouds and particles in the atmosphere and comes equally from all directions.

GHI is acronym for Global Horizontal Irradiance which represents the total amount of shortwave radiation received from above by a surface which is horizontal (parallel) to the ground. GHI is the most important parameter for calculation of PV electricity yield. Both DHI & DNI are actually components of GHI and mathematically,

Global Horizontal Irradiance (GHI) = Direct Normal Irradiance (DNI)* cos(solar zenith angle) + Diffused Horizontal Irradiance (DHI)

GTI is acronym for Global Tilted Irradiance which represents irradiation that falls on a tilted surface. Unlike a horizontal surface which is parallel to ground, a tilted surface also receives small amount of ground-reflected radiation (REF). GTI is an approximate value for the energy yield calculation of fixed installed tilted PV panels.

Optimum angle refers to the angle at which Solar PV modules should be oriented in order to generate maximum electricity.

I agree you seem to be getting a bit ahead of yourself. Respectfully suggest you read chapters 1 & 2 of D.& B. before you get involved in junk on the net. The nomenclature D.& B. use is pretty universal and mostly, with a few mods over the years, came from 2 guys, Liu and Jordon, back in 1954 who did the first modern definitive analysis of flat plate solar thermal collectors.

A couple of items:

1.) GTA is an unknown acronym to me as is GTI. It looks like GTI is the same as the more commonly used P.O.A. (Plane of Array) irradiance.
2.) See D. & B. for nomenclature starting on P. 856 of the 4th ed.
3.) DNI is usually used for Direct Normal Irradiance, not Diffused Normal Irradiance. The conversion often used between horizontal diffused and POA diffused is treated in chap. 2 of D.& B. Diffuse irradiance is not necessarily and probably not evenly distributed over the sky dome or in the hemisphere over a tilted surface.
4.) GHI is all wavelength irradiance, not just shortwave irradiance.
5.) Strictly speaking GHI also includes albedo and a horizon brightening term.
6.) With respect to the 69 vs. 37 days of high irradiance you write of: 1,000 W/m^2 is a flux rate (instantaneous power or irradiance rate), not necessarily an indicator of a day with high day long insolation (that is, a high integrated daily energy total or insolation total). What NREL data did you download ? If was SAM or PVWatts, those models use the THY so also check/download something called the TMY (Typical Meteorological Year) and its manual. The TMY data will, in all likelihood, be different from any site's actual historical data for reasons made clear in the TMY manual.

As for your struggles, I too was once where you are (and probably more so). All my raggings about nomenclature may seem a bit or a lot like separating fly crap from pepper but there's a lot of detail involved in irradiance measurements and the precision in terminology make things a lot easier to talk about without getting confused. Take my word as reality for now on that.

To your questions:

Question 1: Yes, in a horizontal orientation, it's measuring global horizontal irradiance or GHI.

Question 2:

First off and by convention, since most all irradiance and insolation data is, unless stated otherwise, assumed to be referenced to the horizontal orientation, all the pyranometers are designed to be used in the horizontal orientation. See D & B. for a good discussion of irradiance and insolation measuring instruments. At the risk of using circular logic, any inaccuracies due to instrument orientation are, in the end, due to not being horizontal. That means, any inaccuracies in calculated using algorithms of off horizontal irradiances or insolation totals will lie with the algorithmic methods used - of which there are many. see D.& B. for several methods. FWIW, I use a modified HDKR method that seems to fit what my Davis produces pretty well and also gets me close to what my array model pukes out vs. what the array actually produces.

Once you get the reference convention straightened out and so speaking/writing of instrument uncertainty caused by using it in a non horizontal orientation, such accuracy loss will depend on the usual things such as sky conditions, solar zenith angle, collector orientation, albedo and other factors. Under clear skies with an equator facing slope of maybe 20 - 30 degrees or so, maybe an error of +/- 5 % of what the sensor says vs. what a corrected horizontal reading under the same conditions might give. Under completely cloudy skies, probably less error as the sky may be more uniform in terms of the direction of the diffuse irradiance. But a word of caution: That's only speaking of what the device is measuring or what it "sees", not the difference in irradiance due to the change in orientation alone. Some of the difference will be due to things like the "anisotropy" of the diffuse irradiance or horizon brightening terms which more sophisticated irradiance conversion algorithms account for in various ways. Then, there's the issue of albedo which, along with wind vector are the two items which have the most uncertainty of measurement and thus are the biggest problems of accurate array performance measurement at this time. Nor is that including any inherent instrument inaccuracy or variations.

If/As you get more familiar with the subject of what's often called "resource assessment" you'll come to appreciate the variation in methods and compromises in precision made necessary by the mostly unpredictable and unknown variations in the atmosphere as well as device operation conditions. Because it is so undefinable - and probably will remain so for some time - it's often fertile ground for mostly useless master's theses and other forms of white collar welfare. But that's off topic.

Question 3: Most likely not a smoking gun. For one thing, it's not likely to have a constant or quasi constant irradiance level of >1,000 W/m^2. For another, what's the "expected" level of output ?

FWIW, and not strictly part of your question, except as maybe one thing you can do or consider as an alternative answer to your question, what I've done (several hundred times) is use the Davis data (including GHI which I then convert to P.O.A irradiance) on very clear days at the time (within 1 minute) of minimum incidence angle on my array while measuring actual array input to and output from my string inverter to calculate instantaneous array and inverter efficiencies. Over time and using some statistical analysis, I'm pretty confident my S.P. 327's and my string inverter are performing per the data sheet that I'm working to.

More off topic and FWIW stuff: I've learned a lot from the experience as well as verifying (by comparison with about a dozen cooperating neighbors who have non S.P. arrays that while Sunpower stuff performs well, its performance is no better than other quality equipment which 8+ years ago cost about 20 - 25 % less than the Sunpower stuff I bought. So (and I knew tis walking in and not complaining), I paid a premium to see for myself and satisfy my curiosity but got to call B.S. on the S.P. claims of most efficient and can back it up (theirs is an AREA efficiency which is specious crap and IMO deceptive), which was my goal in getting the S.P. stuff in the first place - to see if their "most efficient" and other claims held up or were actually B.S. as I suspected. So, while their stuff is well made, in terms of production per $$ spent, S.P. stuff is, or at least was, overpriced and their advertising claims are B.S. Long, boring % less than Sunpower stuff story.

[ajonestx]

I agree you seem to be getting a bit ahead of yourself. Respectfully suggest you read chapters 1 & 2 of D.& B. before you get involved in junk on the net. The nomenclature D.& B. use is pretty universal and mostly, with a few mods over the years, came from 2 guys, Liu and Jordon, back in 1954 who did the first modern definitive analysis of flat plate solar thermal collectors.

A couple of items:

1.) GTA is an unknown acronym to me as is GTI. It looks like GTI is the same as the more commonly used P.O.A. (Plane of Array) irradiance.
2.) See D. & B. for nomenclature starting on P. 856 of the 4th ed.
3.) DNI is usually used for Direct Normal Irradiance, not Diffused Normal Irradiance. The conversion often used between horizontal diffused and POA diffused is treated in chap. 2 of D.& B. Diffuse irradiance is not necessarily and probably not evenly distributed over the sky dome or in the hemisphere over a tilted surface.
4.) GHI is all wavelength irradiance, not just shortwave irradiance.
5.) Strictly speaking GHI also includes albedo and a horizon brightening term.
6.) With respect to the 69 vs. 37 days of high irradiance you write of: 1,000 W/m^2 is a flux rate (instantaneous power or irradiance rate), not necessarily an indicator of a day with high day long insolation (that is, a high integrated daily energy total or insolation total). What NREL data did you download ? If was SAM or PVWatts, those models use the THY so also check/download something called the TMY (Typical Meteorological Year) and its manual. The TMY data will, in all likelihood, be different from any site's actual historical data for reasons made clear in the TMY manual.

As for your struggles, I too was once where you are (and probably more so). All my raggings about nomenclature may seem a bit or a lot like separating fly crap from pepper but there's a lot of detail involved in irradiance measurements and the precision in terminology make things a lot easier to talk about without getting confused. Take my word as reality for now on that.

To your questions:

Question 1: Yes, in a horizontal orientation, it's measuring global horizontal irradiance or GHI.

Question 2:

First off and by convention, since most all irradiance and insolation data is, unless stated otherwise, assumed to be referenced to the horizontal orientation, all the pyranometers are designed to be used in the horizontal orientation. See D & B. for a good discussion of irradiance and insolation measuring instruments. At the risk of using circular logic, any inaccuracies due to instrument orientation are, in the end, due to not being horizontal. That means, any inaccuracies in calculated using algorithms of off horizontal irradiances or insolation totals will lie with the algorithmic methods used - of which there are many. see D.& B. for several methods. FWIW, I use a modified HDKR method that seems to fit what my Davis produces pretty well and also gets me close to what my array model pukes out vs. what the array actually produces.

Once you get the reference convention straightened out and so speaking/writing of instrument uncertainty caused by using it in a non horizontal orientation, such accuracy loss will depend on the usual things such as sky conditions, solar zenith angle, collector orientation, albedo and other factors. Under clear skies with an equator facing slope of maybe 20 - 30 degrees or so, maybe an error of +/- 5 % of what the sensor says vs. what a corrected horizontal reading under the same conditions might give. Under completely cloudy skies, probably less error as the sky may be more uniform in terms of the direction of the diffuse irradiance. But a word of caution: That's only speaking of what the device is measuring or what it "sees", not the difference in irradiance due to the change in orientation alone. Some of the difference will be due to things like the "anisotropy" of the diffuse irradiance or horizon brightening terms which more sophisticated irradiance conversion algorithms account for in various ways. Then, there's the issue of albedo which, along with wind vector are the two items which have the most uncertainty of measurement and thus are the biggest problems of accurate array performance measurement at this time. Nor is that including any inherent instrument inaccuracy or variations.

If/As you get more familiar with the subject of what's often called "resource assessment" you'll come to appreciate the variation in methods and compromises in precision made necessary by the mostly unpredictable and unknown variations in the atmosphere as well as device operation conditions. Because it is so undefinable - and probably will remain so for some time - it's often fertile ground for mostly useless master's theses and other forms of white collar welfare. But that's off topic.

Question 3: Most likely not a smoking gun. For one thing, it's not likely to have a constant or quasi constant irradiance level of >1,000 W/m^2. For another, what's the "expected" level of output ?

FWIW, and not strictly part of your question, except as maybe one thing you can do or consider as an alternative answer to your question, what I've done (several hundred times) is use the Davis data (including GHI which I then convert to P.O.A irradiance) on very clear days at the time (within 1 minute) of minimum incidence angle on my array while measuring actual array input to and output from my string inverter to calculate instantaneous array and inverter efficiencies. Over time and using some statistical analysis, I'm pretty confident my S.P. 327's and my string inverter are performing per the data sheet that I'm working to.

More off topic and FWIW stuff: I've learned a lot from the experience as well as verifying (by comparison with about a dozen cooperating neighbors who have non S.P. arrays that while Sunpower stuff performs well, its performance is no better than other quality equipment which 8+ years ago cost about 20 - 25 % less than the Sunpower stuff I bought. So (and I knew tis walking in and not complaining), I paid a premium to see for myself and satisfy my curiosity but got to call B.S. on the S.P. claims of most efficient and can back it up (theirs is an AREA efficiency which is specious crap and IMO deceptive), which was my goal in getting the S.P. stuff in the first place - to see if their "most efficient" and other claims held up or were actually B.S. as I suspected. So, while their stuff is well made, in terms of production per $$ spent, S.P. stuff is, or at least was, overpriced and their advertising claims are B.S. Long, boring % less than Sunpower stuff story.

Ok. Back again after some travel to visit my ASHP and get that functional again above about 25 degrees. Still trying to get COP data for the ASHP so I can fine tune the balance point temperature sensor set point. My gut tells me it should be around 30 to 35 degrees F, but the HVAC folks say it should be at about 27.5F. Note they did not have COP data in hand to make this statement at the time. I may also have some direct sun on the sensor that is throwing off the measurements.

I did skim a little more of D.&B. chapter two - very in-depth that will take some time to absorb. Also, I appreciate the acronym descriptions and clarifications. Oh, and GTA was a typo - should have been GTI.

On the NREL - National Solar Radiation Database (NSRDB) data download - I tried to download PSM v3 TMY 2020 data, but it gave me a collection of values that included several other years of data (not just 2020). Next I downloaded PSM v3 2020 data and that was clean (just for 2020). I did the same for years 2017, 2018 and 2019 for comparison. Odd that 2021 is not available just yet - I guess it takes some time to make available.

Also understand that TMY = "Typical Meteorological Year" for use in evaluating general performance, but isn't actual historical performance a reasonable data point since SP is using that to claim that my panels are performing lower because there have been less sun hours for three years (since install)? As stated above, the TMY data that I downloaded appears to include random past years for each month. I just downloaded the 2020 TMY data again for my area just now and it included 2011 for January, 2020 for February, 2016 for January, 2014 for April, etc. Confused... If this is "typical", why present each month from a different year?

In your last note, you state that you, "use the Davis data (including GHI which I then convert to P.O.A irradiance) on very clear days at the time (within 1 minute) of minimum incidence angle on my array while measuring actual array input to and output from my string inverter to calculate instantaneous array and inverter efficiencies." This makes sense to me and probably why I am drawn to using the historical PSM yearly data as a reference point against prior power output levels and also to use the instantaneous GHI data against system (and module) level output to back check measured output.

Oh, and one additional interesting data point - just last week (after over a year of asking), SP provided a report that included maximum power output levels for each module for the first six months of 2021 and the first couple of months this year. It suggested that in March and April of 2021, each module hit at least 330 watts at some point and so far this year, each module (except two) has hit at least 300 watts.

While this last report from SP gives me some comfort, I still wish to study this further and set up something that allows me to backcheck what SP says. Right now, I have no tools to evaluate statements like, "system is performing as expected", nor do fully understand what is expected other than I think each module was tested and warranted to output 320 watts under ideal conditions. In your statement above where you evaluate your system under pretty darn close to ideal conditions, I would feel a lot more comfortable being able to accomplish the same. In the Building Commissioning (Cx) industry, they call this a Functional Performance Test (FPT).

On a related note, life's distractions has not allowed me to spend as much time as I had hoped over the last couple of weeks to read PV dummies, but I will continue with my studies!

[J.P.M.]

Ok. Back again after some travel to visit my ASHP and get that functional again above about 25 degrees. Still trying to get COP data for the ASHP so I can fine tune the balance point temperature sensor set point. My gut tells me it should be around 30 to 35 degrees F, but the HVAC folks say it should be at about 27.5F. Note they did not have COP data in hand to make this statement at the time. I may also have some direct sun on the sensor that is throwing off the measurements.

I did skim a little more of D.&B. chapter two - very in-depth that will take some time to absorb. Also, I appreciate the acronym descriptions and clarifications. Oh, and GTA was a typo - should have been GTI.

On the NREL - National Solar Radiation Database (NSRDB) data download - I tried to download PSM v3 TMY 2020 data, but it gave me a collection of values that included several other years of data (not just 2020). Next I downloaded PSM v3 2020 data and that was clean (just for 2020). I did the same for years 2017, 2018 and 2019 for comparison. Odd that 2021 is not available just yet - I guess it takes some time to make available.

Also understand that TMY = "Typical Meteorological Year" for use in evaluating general performance, but isn't actual historical performance a reasonable data point since SP is using that to claim that my panels are performing lower because there have been less sun hours for three years (since install)? As stated above, the TMY data that I downloaded appears to include random past years for each month. I just downloaded the 2020 TMY data again for my area just now and it included 2011 for January, 2020 for February, 2016 for January, 2014 for April, etc. Confused... If this is "typical", why present each month from a different year?

In your last note, you state that you, "use the Davis data (including GHI which I then convert to P.O.A irradiance) on very clear days at the time (within 1 minute) of minimum incidence angle on my array while measuring actual array input to and output from my string inverter to calculate instantaneous array and inverter efficiencies." This makes sense to me and probably why I am drawn to using the historical PSM yearly data as a reference point against prior power output levels and also to use the instantaneous GHI data against system (and module) level output to back check measured output.

Oh, and one additional interesting data point - just last week (after over a year of asking), SP provided a report that included maximum power output levels for each module for the first six months of 2021 and the first couple of months this year. It suggested that in March and April of 2021, each module hit at least 330 watts at some point and so far this year, each module (except two) has hit at least 300 watts.

While this last report from SP gives me some comfort, I still wish to study this further and set up something that allows me to backcheck what SP says. Right now, I have no tools to evaluate statements like, "system is performing as expected", nor do fully understand what is expected other than I think each module was tested and warranted to output 320 watts under ideal conditions. In your statement above where you evaluate your system under pretty darn close to ideal conditions, I would feel a lot more comfortable being able to accomplish the same. In the Building Commissioning (Cx) industry, they call this a Functional Performance Test (FPT).

On a related note, life's distractions has not allowed me to spend as much time as I had hoped over the last couple of weeks to read PV dummies, but I will continue with my studies!

I'm draining the swamp just now.

Meanwhile, check out: www.osti.gov/biblio/928611. It's the data manual for the TMY3 data set. then click the box that says "View Technical Report". That report, dated 2008 contains a pretty good and concise explanation of the what the NSRDB is, what it contains (as of 2008), how a TMY is put together, its origins and the logic behind the whole NSRDB including why most of the data for most all the stations - even today - is synthetic (modeled in one way or another). It's good background info for when you try to make sense of all the data that's available now and why a lot of it may not be helpful to what you're trying to do.

Also, when you get through or far enough along in your reading, Download SAM from NREL. It's a good tool once you understand what it does. It's also a good place (fast and easy) to get and use the NSRDB info you'll need without going through all the B.S. hoops NREL seems to have put in peoples' way these days. A word of respectful caution about SAM: Without the background knowledge you're acquiring now, using SAM is a bit like giving a loaded firearm to a 2 year old with the safety off.

[ajonestx]

I'm draining the swamp just now.

Meanwhile, check out: www.osti.gov/biblio/928611. It's the data manual for the TMY3 data set. then click the box that says "View Technical Report". That report, dated 2008 contains a pretty good and concise explanation of the what the NSRDB is, what it contains (as of 2008), how a TMY is put together, its origins and the logic behind the whole NSRDB including why most of the data for most all the stations - even today - is synthetic (modeled in one way or another). It's good background info for when you try to make sense of all the data that's available now and why a lot of it may not be helpful to what you're trying to do.

Also, when you get through or far enough along in your reading, Download SAM from NREL. It's a good tool once you understand what it does. It's also a good place (fast and easy) to get and use the NSRDB info you'll need without going through all the B.S. hoops NREL seems to have put in peoples' way these days. A word of respectful caution about SAM: Without the background knowledge you're acquiring now, using SAM is a bit like giving a loaded firearm to a 2 year old with the safety off.

Back again... spent a little time getting ready for another trip to visit my ASHP that needs a little TLC to get the balance point sensor set right - since Lennox will not share the COP (at different temperatures), HVAC tech also does not seem to be able (or may not have spent any time) locating COP data for my ASHP. Until then, I am just trying to raise the setting in increments of a couple of degrees until the ASHP stops turning on below freezing.

Also, spent some time this month getting the weather station installed on top of a 25-foot ladder (even grounded it). It is not the Davis, but I am going to try to work with this model for a bit. Don't beat me up too bad - I am sure that I will learn. In the meantime, I am learning a bit more about the weather for the purpose of PV performance (and also a little distracted using the weather data to help me with a parallel project learning how to conduct prescribed burns).

Uh, draining the swamp? Politically, I understand its meaning, but not in this context...

Also, got through PV Dummies Chapter 9 - Small projects with Big Results: kind of cheesy stuff, but admit to looking at solar powered exterior accent lights for the back deck just a few days ago, already installed radiant barrier, recently installed operable interior blinds (shutters) and will someday day re-evaluate installing storm windows over our 100 year old double hung single pane wood windows. Interesting section on Battery powered stuff - not sure if I had mentioned this in the past, but recently started experimenting with PV (about 20 watt) panels to keep a battery charged for a game / trail camera - quite the interesting exercise experimenting with shaded / wooded environments, battery sizing, estimated wattage output and estimating at how much energy the camera uses under varying conditions that change daily based on number of photos taken if the IR flash is used at night, cellular communication requirements and the biggest wildcard - some of the SLA batteries that I have been experimenting with just stop when the temperature drops below five degrees and comes back to life at around freezing. Solar light tubes are also on my list for the workshop - saw a model at an Amish grocery store that I like, but have been unable to source them just yet.

I did read the Technical Report for the TMY data - that explains a lot! As you implied, it appears to be a reasonable collection of different months from different years manipulated and tweaked some based on a predefined algorithm to produce reasonably prioritized data.

I also did watch the some of the video for what NREL SAM does - I will continue reading a bit before downloading the software, but it does appear to be a pretty nice source that will hopefully reconcile the vast amount of data available. I also prefer to understand my firearm before loading.

Back to PV Dummies, Chapter 10.

[J.P.M.]

Back again... spent a little time getting ready for another trip to visit my ASHP that needs a little TLC to get the balance point sensor set right - since Lennox will not share the COP (at different temperatures), HVAC tech also does not seem to be able (or may not have spent any time) locating COP data for my ASHP. Until then, I am just trying to raise the setting in increments of a couple of degrees until the ASHP stops turning on below freezing.

Also, spent some time this month getting the weather station installed on top of a 25-foot ladder (even grounded it). It is not the Davis, but I am going to try to work with this model for a bit. Don't beat me up too bad - I am sure that I will learn. In the meantime, I am learning a bit more about the weather for the purpose of PV performance (and also a little distracted using the weather data to help me with a parallel project learning how to conduct prescribed burns).

Uh, draining the swamp? Politically, I understand its meaning, but not in this context...

Also, got through PV Dummies Chapter 9 - Small projects with Big Results: kind of cheesy stuff, but admit to looking at solar powered exterior accent lights for the back deck just a few days ago, already installed radiant barrier, recently installed operable interior blinds (shutters) and will someday day re-evaluate installing storm windows over our 100 year old double hung single pane wood windows. Interesting section on Battery powered stuff - not sure if I had mentioned this in the past, but recently started experimenting with PV (about 20 watt) panels to keep a battery charged for a game / trail camera - quite the interesting exercise experimenting with shaded / wooded environments, battery sizing, estimated wattage output and estimating at how much energy the camera uses under varying conditions that change daily based on number of photos taken if the IR flash is used at night, cellular communication requirements and the biggest wildcard - some of the SLA batteries that I have been experimenting with just stop when the temperature drops below five degrees and comes back to life at around freezing. Solar light tubes are also on my list for the workshop - saw a model at an Amish grocery store that I like, but have been unable to source them just yet.

I did read the Technical Report for the TMY data - that explains a lot! As you implied, it appears to be a reasonable collection of different months from different years manipulated and tweaked some based on a predefined algorithm to produce reasonably prioritized data.

I also did watch the some of the video for what NREL SAM does - I will continue reading a bit before downloading the software, but it does appear to be a pretty nice source that will hopefully reconcile the vast amount of data available. I also prefer to understand my firearm before loading.

Back to PV Dummies, Chapter 10.

Draining the swamp use was in reference to being busy with projects as in "up to my ass in alligators and trying to drain the swamp".

On storm windows, given that what's there no is 100 or so years old, I'd have made storm windows one of my first considerations if not a more involved analysis to see if the extra incremental costs of entirely new windows could be justified over the cost of fitting old widows with storm windows. If nothing else, I'd have caulked and sealed (from the inside) what's there regardless of what else I did to upgrade the fenestration.

COP info as f(temp.) ought to be available in the literature, or at least an estimate should be possible from a careful perusal of the published data/spec sheets.
It's not only the freezing temp. of H2O but equally important are the ambient dew point temp. and/if/not the heat pump is variable speed/output.

Not a criticism, but based on my experience, I'd not place a lot of faith in the irradiance readings from non Davis weather stations. The Davis's in my area (including mine) seem to produce pretty consistent output that make sense with what I understand of irradiance. Not so with other products outputs, although that inconsistency may be a partial function of incorrect sensor placement, orientation and maintenance.

A couple of folks in my area put their stations on poles and discovered what a PITA is to get at the station for maintenance. Weather stations, including the Davis Pro II need more regular maintenance than folks think and for the most part such operations take more than one hand to accomplish. I learned from watching the mistakes of others on that point. My station's location is about 1m north of the centerline of my array with the wind cups about 15 cm higher in elevation than the highest array elevation, but the stations output is dedicated to measuring what's going on in the immediate vicinity of the array. Anyway, it's easy to get at (and also clean the irradiance sensor on clear days when I'm recording/measuring real time array output and input at the time of min. solar incidence angle on the array).

I haven't watched the SAM video yet but probably ought to as there's always something to learn.

Lot's of interesting stuff in the manual. One tidbit being that it reveals that most (~ 95% or so) of the TMY data is synthetic (algorithm generated) and - for the TMY 2 and TMY 3 data anyway, extrapolated from actual measurements from just 26 stations around the U.S. I suppose there's 2 (or more) ways to look at that: either the database is on shaky ground given its small base or, given that the results of models that use(d) the TMY data generally produce reliable results with intelligent use, the models are useful and reasonably fit for purpose.

[ajonestx]

Draining the swamp use was in reference to being busy with projects as in "up to my ass in alligators and trying to drain the swamp".

On storm windows, given that what's there no is 100 or so years old, I'd have made storm windows one of my first considerations if not a more involved analysis to see if the extra incremental costs of entirely new windows could be justified over the cost of fitting old widows with storm windows. If nothing else, I'd have caulked and sealed (from the inside) what's there regardless of what else I did to upgrade the fenestration.

COP info as f(temp.) ought to be available in the literature, or at least an estimate should be possible from a careful perusal of the published data/spec sheets.
It's not only the freezing temp. of H2O but equally important are the ambient dew point temp. and/if/not the heat pump is variable speed/output.

Not a criticism, but based on my experience, I'd not place a lot of faith in the irradiance readings from non Davis weather stations. The Davis's in my area (including mine) seem to produce pretty consistent output that make sense with what I understand of irradiance. Not so with other products outputs, although that inconsistency may be a partial function of incorrect sensor placement, orientation and maintenance.

A couple of folks in my area put their stations on poles and discovered what a PITA is to get at the station for maintenance. Weather stations, including the Davis Pro II need more regular maintenance than folks think and for the most part such operations take more than one hand to accomplish. I learned from watching the mistakes of others on that point. My station's location is about 1m north of the centerline of my array with the wind cups about 15 cm higher in elevation than the highest array elevation, but the stations output is dedicated to measuring what's going on in the immediate vicinity of the array. Anyway, it's easy to get at (and also clean the irradiance sensor on clear days when I'm recording/measuring real time array output and input at the time of min. solar incidence angle on the array).

I haven't watched the SAM video yet but probably ought to as there's always something to learn.

Lot's of interesting stuff in the manual. One tidbit being that it reveals that most (~ 95% or so) of the TMY data is synthetic (algorithm generated) and - for the TMY 2 and TMY 3 data anyway, extrapolated from actual measurements from just 26 stations around the U.S. I suppose there's 2 (or more) ways to look at that: either the database is on shaky ground given its small base or, given that the results of models that use(d) the TMY data generally produce reliable results with intelligent use, the models are useful and reasonably fit for purpose.

Ah... draining the swamp... understand some of those days.

As far as window upgrades go, there are definitely hot and cold days that would have made sealing them up the right choice, but for those days in between, we do love opening them up (at least two in every room in the house) and letting the breeze come through. We also spent a good bit of effort getting those darn cast iron counter-weights and cords replaced! We did get custom removable screens (on the outside) that block some direct light from the glazing and shutters on the inside, but have not yet pulled the trigger on some type of custom storm window yet. We also love our 100 year old "wavy" glass in our historic home - recent door replacements over the years always include modern insulated and weather-stripped units. Your strong recommendation to consider these upgrades above others is noted. Oh, but the floor insulation is getting replaced next week, so hope to continue sealing up the home over time. If there was an easy way to have two sets of exterior removable "screens"... one with glass or plexiglass to create a storm window and one with screen / breathable mesh that could be interchangeably placed depending on the season, but that sounds labor and storage intensive.

I really appreciate you pushing me again on the ASHP COP info... had searched for it previously, called Lennox (who refused to share it or did not know it existed) and called my HVAC installer (who seems to be too busy to look for it). Google finally found a chart (and two very hidden / small footnotes) that showed different types of matching unit combinations and low and behold there was a high temperature COP rating of 3.10 at 47°F db/43°F wb outdoor air temperature and 70°F db entering indoor coil air and a low temperature COP rating of 2.54 at 17°F db/15°F wb outdoor air temperature and 70°F db entering indoor coil air. Not bad even at 17 degrees

With this new data, your suggested formulas and the assumption that the COP range is linear between these two temperatures, I think that puts my potential savings to around $350 to $400 per year. Updated calculations attached. Considering the repairs needed to get the heat pump working in the first place, I should be able to get that money back in two to three years (unless we go geothermal which we are considering and that complicates our pay back scenario). Oh, and I just don't know enough about the engineering to be able to factor in dew point and variable speed complexities. I am just going to have to assume that these rough calculations are within lets say 10 to 20%???

On the topic of weather station manufacturers... you have been very clear that the Davis product line is well admired among many PV enthusiasts and I admit that we have chosen an inferior product. Your words are not received as criticism. We also considered maintenance with the placement of our weather station in our urban environment and put it at the top of a gable on a 60 cm pole about 1.5-m east of one end of our rooftop PV array and about 30-cm higher than the highest array elevation. I considered placing it closer to the array (on the roof), but the roof slope is too much for me without special felt boots and I was not inclined to create more roof penetrations. We also considered something much lower (and much further from the array), but wind turbulents and increased shade conditions were problematic. So far, my Ambient station appears to be giving me reasonable irradiance peak numbers (based on visual observations) in the 800's on cloudless days. Will have to evaluate over time and clean irradiance sensor with the regular cleaning of our array. Ladder access is marginally better than roof top pros / cons evaluated.

On the NREL data... Ha! "fit for purpose". I guess between varying seasonal conditions, clouds, rain, snow and the like that a reasonable TMY measurement and calculation used to evaluate possible future conditions over thousands of hours per year is a very good source that gets you pretty close to predicting the future. It's on my list to download the SAM software after a little more reading.

Well, PV Dummies Chapter 10 was a very good read. Mostly makes me think about an incredible untapped source of heat for so many. Amazing that we can only capture about 20% of the sun for PV and the rest is wasted. Water heating just makes so much sense on so many levels. I knew about direct vs. indirect systems, but did not know about passive vs. active, flat or plate or batch collectors, evacuated tube collectors, etc. The varying options and control scenarios are quite complex for such a simple concept. I can also understand with all the moving parts, either go VERY simple (if climate allows) or as you suggest, or maybe just go with ASHP water heating. In any case, it really makes me think this heat source could be better utilized to do more than just heat water. Hopefully Chapter 12 will give me some juicy details on the solar water heating controls.

At the risk of insulting some folks, I am not a fan of swimming pools - cleaned them as a youth / summer job. What a PITA! Skimming PV Dummies Chapter 11, sounds like it might be a waste of time, but then again, there is some engineering behind the concepts, so I'll take a look.

[J.P.M.]

Ah... draining the swamp... understand some of those days.

As far as window upgrades go, there are definitely hot and cold days that would have made sealing them up the right choice, but for those days in between, we do love opening them up (at least two in every room in the house) and letting the breeze come through. We also spent a good bit of effort getting those darn cast iron counter-weights and cords replaced! We did get custom removable screens (on the outside) that block some direct light from the glazing and shutters on the inside, but have not yet pulled the trigger on some type of custom storm window yet. We also love our 100 year old "wavy" glass in our historic home - recent door replacements over the years always include modern insulated and weather-stripped units. Your strong recommendation to consider these upgrades above others is noted. Oh, but the floor insulation is getting replaced next week, so hope to continue sealing up the home over time. If there was an easy way to have two sets of exterior removable "screens"... one with glass or plexiglass to create a storm window and one with screen / breathable mesh that could be interchangeably placed depending on the season, but that sounds labor and storage intensive.

Sealing is only permanent for fixed surfaces such as where molding and walls meet. Otherwise, it's either removeable or flexible to allow windows/doors to be operable. You can seal and still open the windows. Fenestration usually constitutes a significant portion of heat transfer in a home. For standard construction techniques, storm windows will usually knock that heat gain/loss in half or so. After changing thermostat setting and turning stuff off, sealing the exterior envelope of a dwelling is about the most effective, easiest and most cost effective way to lower a utility bill.
So, , and following the general precept of doing a mix of the most bang for the buck stuff and (probably) the cheapest of those measures first, seal up the house and get storm windows. My first house in Buffalo, NY, built in 1929, had with double hung wood windows that were original, sash weights and all. Aluminum storm windows were added at some point before I acquired the home. I used weather stripping to make them tight and replaced the sash weights with spring loaded clips on the moveable window frames to hold the windows open when needed/desired. Worked great. Because the sash weight spaces are, in effect, uninsulated wall spaces that constitute a significant portion of the exterior wall area of the dwelling. When I had the exterior walls insulated with blown in cellulose, I had the insulation vendor drill 1" holes in the exterior of the window frames and blow insulation into the sash weight spaces. I then sealed, caulked and finished the holes.
If economics/cost effectiveness of your efforts is a consideration, seal first. Depending on what restrictions you may have from HOA/preservation authorities, and what their mandates do to cost, I'd make sure storm windows are operable. One reason my A/C run time is so low here in CA is that I "sail" the house much as I have always done with all my homes- opening/closing windows and shades, fans and doing other measures as the diurnal temp. swings, solar angles and outside temps. allow minimizing HVAC run times for both heating and cooling.

I really appreciate you pushing me again on the ASHP COP info... had searched for it previously, called Lennox (who refused to share it or did not know it existed) and called my HVAC installer (who seems to be too busy to look for it). Google finally found a chart (and two very hidden / small footnotes) that showed different types of matching unit combinations and low and behold there was a high temperature COP rating of 3.10 at 47°F db/43°F wb outdoor air temperature and 70°F db entering indoor coil air and a low temperature COP rating of 2.54 at 17°F db/15°F wb outdoor air temperature and 70°F db entering indoor coil air. Not bad even at 17 degrees

One reason I never got a heat pump in buffalo was because with all the conservation measures I took with that 1st home, the balance temp. of the home was ~ 44 deg. F. That is, the heat loss was so low (as well a reduced thermostat setting) that aux. heat was only necessary when the outside temp. dropped to 44 F. Heat pumps at the time (late '70,s/early 80's) had more significant rolloff's of C.O.Ps as f(outside temp.). Also, I never needed A/C in Buffalo and the low(er) price of nat. gas vs. electricity to run a heat pump made it almost a no brainer.
Dirty little secret: Since Well, or superinsulated homes have lower balance temps., ASHP performance will drop simply due to the idea that ASHP's C.O.P. will drop as the building balance temp. drops.

You don't have a super or even well insulated home (yet), but if you really want good (but dated) information on retrofitting existing homes, some of the proceedings of a conference I took part in were published in book form (now out of print, but libraries may still have a copy) : "Home Remedies", ISBN # 0-9601884-0-1. Quite readable and full of common sense stuff you won't find elsewhere. FWIW, I'm in a group shot at the bottom of p11.



With this new data, your suggested formulas and the assumption that the COP range is linear between these two temperatures, I think that puts my potential savings to around $350 to $400 per year. Updated calculations attached. Considering the repairs needed to get the heat pump working in the first place, I should be able to get that money back in two to three years (unless we go geothermal which we are considering and that complicates our pay back scenario). Oh, and I just don't know enough about the engineering to be able to factor in dew point and variable speed complexities. I am just going to have to assume that these rough calculations are within lets say 10 to 20%???

Unless you decide to study Thermodynamics more than you already have, and given the vagaries and randomness of the of the environmental inputs, that 10-20 % is probably close enough.

On the topic of weather station manufacturers... you have been very clear that the Davis product line is well admired among many PV enthusiasts and I admit that we have chosen an inferior product. Your words are not received as criticism. We also considered maintenance with the placement of our weather station in our urban environment and put it at the top of a gable on a 60 cm pole about 1.5-m east of one end of our rooftop PV array and about 30-cm higher than the highest array elevation. I considered placing it closer to the array (on the roof), but the roof slope is too much for me without special felt boots and I was not inclined to create more roof penetrations. We also considered something much lower (and much further from the array), but wind turbulents and increased shade conditions were problematic. So far, my Ambient station appears to be giving me reasonable irradiance peak numbers (based on visual observations) in the 800's on cloudless days. Will have to evaluate over time and clean irradiance sensor with the regular cleaning of our array. Ladder access is marginally better than roof top pros / cons evaluated.

What you do for placement depends a lot on why you have the station. My Davis is very near my array because I want info about what's happening very near the array. One other nice thing about the Davis is that it has a fair amount of flexibility. I have a separate irradiance sensor and a fixture I can place under my array to measure and record irradiance in the plane of the array, both on top of and under the array. BTW, under clear skies and at time of maximum irradiance, the P.O.A. irradiance under the array averages about 3-5 % of the on top P.O.A irradiance - depending on lateral/vertical sensor location of the sensor under clear skies.


On the NREL data... Ha! "fit for purpose". I guess between varying seasonal conditions, clouds, rain, snow and the like that a reasonable TMY measurement and calculation used to evaluate possible future conditions over thousands of hours per year is a very good source that gets you pretty close to predicting the future. It's on my list to download the SAM software after a little more reading.

I'd think it's a reasonable source by virtue of having the algorithms and models that have used it been shown to produce reasonable agreement with systems it's been checked against.

Well, PV Dummies Chapter 10 was a very good read. Mostly makes me think about an incredible untapped source of heat for so many. Amazing that we can only capture about 20% of the sun for PV and the rest is wasted. Water heating just makes so much sense on so many levels. I knew about direct vs. indirect systems, but did not know about passive vs. active, flat or plate or batch collectors, evacuated tube collectors, etc. The varying options and control scenarios are quite complex for such a simple concept. I can also understand with all the moving parts, either go VERY simple (if climate allows) or as you suggest, or maybe just go with ASHP water heating. In any case, it really makes me think this heat source could be better utilized to do more than just heat water. Hopefully Chapter 12 will give me some juicy details on the solar water heating controls.

One reason PV cells' efficiency is relatively low (compared to, say solar flat plate thermal at <~> 40 % or so) can be at least partially explained by entropy. One reason why electricity is so versatile is because it's a very low entropy form of energy. But that versatility comes at a price. That price is paid in conversion efficiency. Solar thermal is a more efficient in/out conversion efficiency because it's converting low entropy sunlight to high(er) entropy heat. That's easier than making low(er) entropy (lower than hot water anyway) electricity, so the process efficiency is less - it takes more work to get more of what's called "availability" into the electricity. Beside, PV cells only "see" irradiance at frequencies < ~ 1.15 microns wavelength, or about 3/4 or so of the total energy of what hits the earth on a sunny day. Passive solar makes a lot of sense in sunny climates because it's easy to convert (degenerate ?) low entropy sunlight to high entropy thermal energy. Hell, it's so easy that it'll happen by itself. Conditions required to turn sunlight into electricity doesn't happen without some help.

Stuff to think about.

[bcroe]

Liking the version I heard long ago.
When you are up to your elbows in alligators, its hard to remember
that your objective was to drain the swamp.

[J.P.M.]

Liking the version I heard long ago.
When you are up to your elbows in alligators, its hard to remember
that your objective was to drain the swamp.

Not as hard to remember as it is hard to do.

[ajonestx]

Stuff to think about.

Your word "seal" caught me off guard as I had trained myself over many years to use the word "sealant" where most use the word, "caulk" - understand your lingo now. Ha! It never occurred to me to use a different type of sash hold open device like a spring loaded clips and insulate the counter weight cavity. Talking about this a little more got me to look for factory made storm windows (I had looked in the past to build something myself). Being from the south, I don't see these types of devices too often. Found something pretty reasonable at "affordablestormwindows.com". I also had it in my mind that storm windows had to be fixed. As you suggest above, they do come operable at a small premium and allows us to sail the house when the temperatures are right. We did a pretty good job of weatherstripping the old wood windows a few years ago, but this might be our next home improvement project (after insulating the floor this week).

Understand why a heat pump would not work at those lower heating efficiencies and also how you might not need to cool with it at all in Buffalo. We did run the HP to cool a little bit last summer up north – total cooling load on that meter in the summer was only about 180kWh over the hottest three months.

I plan to watch the furnace / heat pump frequency as to when it turns on and off over the spring and fall months - I have no idea what our balance temperature is. Now that the system appears to functioning, we can fine tune it. Natural Gas has always been the choice for fuel for us in the south, but with the home up north we only have propane (at over $2 a gallon) and electricity (at 8 cents per kWh). Given the special lower electrical rate (on it's own meter), we are inclined to use electricity to run the heat pump (when the COP / temperature makes sense).

I did find the "Home Remedies" book. Skimmed through it and found you on page 11! Is that you in the white shirt? Joking. Passing through the book, I found it interesting that for the folks who would attend the conference of this sort, or might pick up a copy of this book actually care about how the performance of their home affects their utility bills. This is in sharp contrast to main street media and marketing attempts to address what the majority of people are asking for – can I get a better deal on my electricity or gas price? It just does not occur to most people that you can pay less by using less electricity!

JPM Group Shot.JPG

Entropy and unavailable energy – your comment made me laugh, “Passive solar makes a lot of sense in sunny climates because it's easy to convert (degenerate ?) low entropy sunlight to high entropy thermal energy. Hell, it's so easy that it'll happen by itself.” I think that is the most amazing part of passive heating / cooling systems that most folks just do not get. I admit that it is easier to just adjust the thermostat than to move / adjust awnings or open and close windows.

One of these days, I will have to find an easy to use energy modeler (most are either over the top or just way too basic) to study some of these more passive options for heating and cooling. Until then, we are mostly stumbling along, sealing this, sealing that, insulating this, replacing that with more efficient equipment.

Well, PV Dummies Chapter 11 was a very good read. Personally, I am not a fan of swimming pools, but I guess if you have one, the idea of heating it with gas or electricity sounds awful! I like the simplicity of the systems described (as compared to the elaborate domestic heating systems), but understand how the domestic systems must be more complicated for health, safety and potential internals leaks. For some reason, this chapter made me think that heating water with solar energy could be used to heat a radiant floor system – some of my neighbors have wood burning boilers with their radiant floor systems. Seems like a pretty uncomplicated pair – solar heated water and a radiant floor system (with appropriate freeze protection when the sun is not out).

Off to PV Dummies Chapter 12.

[ajonestx]

J.P.M. - wanted to send you a note. got busy over the summer and well, still looking to learn, but slowing down a bit. Thank you for all of your wisdom sharing. I continue with my reading, but lost interest in specifically trying to get MI tested and confirming panel performance. Switching gears a bit and looking into installing geothermal heat pump (WSHP) this summer before installing new PV array to support. Hope all is well.