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

[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.]

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]

I believe I understand what you write.

I'd still be sure to provide adequate ventilation for the attic if only for the problem of moisture retention and condensation while taking care to keep all the critters out.

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.

[J.P.M.]

I did plug in my numbers into PV Watts (with the originally suggested shade loss of 16%) and the numbers were a little lower than what we were told back in 2016. The attached PV Watts report suggests 7,059 kWh annually vs. current actual average production of about 5,600 kWh annually. The 7,059 number appears to be the mean of the expected range from 6,829 to 7,249 (interesting… the top end is what we were told back in 2016).

I’ll hold off on D.& B. reading too much for now as that read appears to be pretty intense and I want to be able to focus on the content (and after PV Dummies as you suggested).

I had also forgotten about your prior recommendation to use the HPWH in lieu of Solar WH. I have started some initial research into this product line and (just thinking) since my WH is in the attic with my furnace, I might be able to use the HPWH to cool the attic space in lieu of an attic fan all together. I’ll have to evaluate the cost of the change from my 120V tankless water heater to the 240V HPWH (electrical upgrades being an additional cost element).

And you are not alone on the carbon footprint scene… I am not sure if I believe so strongly as you do in that particular direction, but (IMO) do believe we should continue looking for alternative energy sources to fossil fuels. Whether or not these fossil fuels are limited in supply or cause climate change (I’ll leave that argument up to the scientists), the pollutants they introduce into the air by burning them can’t be good for anyone. I also strongly agree that what you have accomplished over the last 50 years needs to be duplicated by others – if the arrogant carbon footprint people can “try” to quantify at a general level what you have done (and what others have NOT done), I will say they have done “some” good. I have personally watched the building codes evolve over the last several decades – these codes have raised the bar and quantified the types of efficiencies that you so dearly cherish. I might also say that with simple code changes, the average architect or engineer has an easier sell to his client (who is asking them to reduce cost) to provide more energy efficient design. From a political point of view, I also think that the building codes have evolved in part by the desire to reduce energy costs (and quantities), reduce energy dependance on foreign nations and also in part by pressure to reduce carbon emissions. Whether or not I like it, the carbon footprint tool did help me identify (maybe not accurately quantify), areas in my energy use portfolio that needed improvement.

I believe I understand what you write.

I'd still be sure to provide adequate ventilation for the attic if only for the problem of moisture retention and condensation while taking care to keep all the critters out.

[ajonestx]

Sounds like you're getting somewhere.

I'd still do my own PVWatts run using inputs that closely match your application and see how it compares to actual 365 day outputs.

If you want more detail about economic payback, and without getting too ahead or skipping around D.& B. too much (something I seriously don't recommend), see chap. 11, "Solar Process Economics". About as good a primer as I've seen on the subject. Less than 30 pages that's a straightforward discussion and treatment of engineering economics as it applies to solar energy systems of all types.

The dummies book used solar thermal as an example perhaps because it is a bit less complicated due to POCO rates and such, but be assured the economic concepts are the same whether the system being analyzed is a wood burning fireplace or a nuclear power plant or anything in between. By way of comparison, I did the design of some power boilers back in the day or some of the geothermal power plants near the Salton Sea and had to work with some folks on cost effective alternatives in the designs. The analyses were very similar to the way residential PV or other
systems including conventional HVAC systems can be analyzed for economic viability or for comparison of alternative equipment or investments.

On solar thermal, and as I wrote, if I was doing it today, and as much as I like solar thermal, I'd consider a PV system and a HPWH combination (or account for the HPWH load in the load of a residential PV system) in lieu of solar thermal for DHW, and particularly when compared to evac. tube collectors which, BTW are about as inappropriate a mismatch of temp. potential to requirements for DHW systems as it gets.

I never give much of a lick about specious crap like carbon footprint or trees saved or such useless and non-quantifiable brain pablum. IMO only, that's stuff that only masks the hard and real issues. The way I look at it, if I minimize how much energy I use to maintain my desired lifestyle and do so in the most cost effective ways possible by using life cycle costing methods and principles, I'll minimize the entropy increase to planet earth and the tree huggers and twits whose lifestyles and self-centered ignorant behavior are as much of a cause of the problem as the stuff they ignorantly rail against, can stick their self-righteous, pious, arrogant attitudes where the sun don't shine, and then get down where I've been for close to 50 years trying to make a real difference.

Skip the solar powered attic fan - the kind with their own dedicated PV. Instead, do the homework and analyze the heat load of your roof first by checking ASHRAE or other accepted methods. While sizing the fan, don't forget that all fan flowrates are given without pressure drops for air resistance and know that real fan volumetric flow rates for actual working conditions are much less which means a bigger fan. Then look at most PV slaved attic fans and you'll probably see that they are a joke in terms of getting close to doing an adequate job. You'll get a 3-4 year payback and an attic that's about as hot as before.

The dummies book is a decent start. The D.& B. tome is the real deal. Once you get into D.& B. stuff in the Dummies book will make a lot more sense.

I did plug in my numbers into PV Watts (with the originally suggested shade loss of 16%) and the numbers were a little lower than what we were told back in 2016. The attached PV Watts report suggests 7,059 kWh annually vs. current actual average production of about 5,600 kWh annually. The 7,059 number appears to be the mean of the expected range from 6,829 to 7,249 (interesting… the top end is what we were told back in 2016).

I’ll hold off on D.& B. reading too much for now as that read appears to be pretty intense and I want to be able to focus on the content (and after PV Dummies as you suggested).

I had also forgotten about your prior recommendation to use the HPWH in lieu of Solar WH. I have started some initial research into this product line and (just thinking) since my WH is in the attic with my furnace, I might be able to use the HPWH to cool the attic space in lieu of an attic fan all together. I’ll have to evaluate the cost of the change from my 120V tankless water heater to the 240V HPWH (electrical upgrades being an additional cost element).

And you are not alone on the carbon footprint scene… I am not sure if I believe so strongly as you do in that particular direction, but (IMO) do believe we should continue looking for alternative energy sources to fossil fuels. Whether or not these fossil fuels are limited in supply or cause climate change (I’ll leave that argument up to the scientists), the pollutants they introduce into the air by burning them can’t be good for anyone. I also strongly agree that what you have accomplished over the last 50 years needs to be duplicated by others – if the arrogant carbon footprint people can “try” to quantify at a general level what you have done (and what others have NOT done), I will say they have done “some” good. I have personally watched the building codes evolve over the last several decades – these codes have raised the bar and quantified the types of efficiencies that you so dearly cherish. I might also say that with simple code changes, the average architect or engineer has an easier sell to his client (who is asking them to reduce cost) to provide more energy efficient design. From a political point of view, I also think that the building codes have evolved in part by the desire to reduce energy costs (and quantities), reduce energy dependance on foreign nations and also in part by pressure to reduce carbon emissions. Whether or not I like it, the carbon footprint tool did help me identify (maybe not accurately quantify), areas in my energy use portfolio that needed improvement.

[J.P.M.]

J.P.M. etal. ok. PV Dummies Chapter 6 (Calculating Payback on Your Solar Investment) – ugh… this one hurt the most so far. I went back to the original estimates, and we were told that we could generate about 7,250 kWh per year and our payback was going to be about 12.9 years (I know I think said earlier that we were not in it for the money, but still nice to get some financial reward for our financial outlays). So far, we are averaging about 5,600 kWh per year and the current pay back is looking like about 15 years (assuming 4% increases in electrical cost in the future).

I also guess the author thought it was easier to use solar water heaters for examples and it really got me thinking about your suggestion to consider it (maybe the next time we replace the water heater). Not much real estate on the roof to put it with PV already hogging all the space though…

The great question in this chapter was, “How much would you pay to completely eliminate your carbon footprint altogether?” Timing was perfect as we just watched an update from Stanford this morning on global warming… crazy stuff just trying to stay below 1.5C. We thought we were doing pretty well until we went online to the EPA calculator and determined that our carbon footprint was annually about 50k CO2 emissions (about 15% higher than the US average).

This chapter also got me thinking about installing a solar powered attic vent (low-cost stuff with a 3- or 4-year payback). Well, on to chapter 7 (Installing Your Solar System).

Also, received my D&B Solar Engineering of Thermal Processes book (much deeper content to compliment PV Dummies). Will start skimming this one to evaluate content.

Sounds like you're getting somewhere.

I'd still do my own PVWatts run using inputs that closely match your application and see how it compares to actual 365 day outputs.

If you want more detail about economic payback, and without getting too ahead or skipping around D.& B. too much (something I seriously don't recommend), see chap. 11, "Solar Process Economics". About as good a primer as I've seen on the subject. Less than 30 pages that's a straightforward discussion and treatment of engineering economics as it applies to solar energy systems of all types.

The dummies book used solar thermal as an example perhaps because it is a bit less complicated due to POCO rates and such but be assured the economic concepts are the same whether the system being analyzed is a wood burning fireplace or a nuclear power plant or anything in between. By way of comparison, I did the design of some power boilers back in the day for some of the geothermal power plants near the Salton Sea and had to work with some folks on cost effective alternatives in the designs which was commonly done in the proposal stages of a (future) contract. The analyses were very similar to the way residential PV or other systems including conventional HVAC systems can be analyzed for economic viability or for comparison of alternative equipment or investments.

On solar thermal, and as I wrote, if I was doing it today, and as much as I like solar thermal, I'd consider a PV system and a HPWH combination (or account for the HPWH load in the load of a residential PV system) in lieu of solar thermal for DHW, and particularly when compared to evac. tube collectors which, BTW are about as inappropriate a mismatch of temp. potential to requirements for DHW systems as it gets.

I never give much of a lick about specious crap like carbon footprint or trees saved or such useless and non-quantifiable brain pablum. IMO only, that's stuff that only masks the hard and real issues. The way I look at it, if I minimize how much energy I use to maintain my desired lifestyle and do so in the most cost effective ways possible by using life cycle costing methods and principles, I'll minimize the entropy increase to planet earth and the tree huggers and twits whose lifestyles and self-centered ignorant behavior are as much of a cause of the problem as the stuff they ignorantly rail against, can stick their self-righteous, pious, arrogant attitudes where the sun don't shine, and then get down where I've been for close to 50 years trying to make a real difference.

Skip the solar powered attic fan - the kind with their own dedicated PV. Instead, do the homework and analyze the heat load of your roof first by checking ASHRAE or other accepted methods. While sizing the fan, don't forget that all fan flowrates are given without pressure drops for air resistance and know that real fan volumetric flow rates for actual working conditions are much less which means a bigger fan. Then look at most PV slaved attic fans and you'll probably see that they are a joke in terms of getting close to doing an adequate job. You'll get a 3-4 year payback and an attic that's about as hot as before.

The dummies book is a decent start. The D.& B. tome is the real deal. Once you get into D.& B. stuff in the Dummies book will make a lot more sense.

[ajonestx]

J.P.M. etal. ok. PV Dummies Chapter 6 (Calculating Payback on Your Solar Investment) – ugh… this one hurt the most so far. I went back to the original estimates, and we were told that we could generate about 7,250 kWh per year and our payback was going to be about 12.9 years (I know I think said earlier that we were not in it for the money, but still nice to get some financial reward for our financial outlays). So far, we are averaging about 5,600 kWh per year and the current pay back is looking like about 15 years (assuming 4% increases in electrical cost in the future).

I also guess the author thought it was easier to use solar water heaters for examples and it really got me thinking about your suggestion to consider it (maybe the next time we replace the water heater). Not much real estate on the roof to put it with PV already hogging all the space though…

The great question in this chapter was, “How much would you pay to completely eliminate your carbon footprint altogether?” Timing was perfect as we just watched an update from Stanford this morning on global warming… crazy stuff just trying to stay below 1.5C. We thought we were doing pretty well until we went online to the EPA calculator and determined that our carbon footprint was annually about 50k CO2 emissions (about 15% higher than the US average).

This chapter also got me thinking about installing a solar powered attic vent (low-cost stuff with a 3- or 4-year payback). Well, on to chapter 7 (Installing Your Solar System).

Also, received my D&B Solar Engineering of Thermal Processes book (much deeper content to compliment PV Dummies). Will start skimming this one to evaluate content.

[ajonestx]

Well, they have a min. ambient source design temp. But their operating C.O.P. above that ambient air temp. will depend on (among other things) the ambient air temp. with the operating C.O.P. decreasing toward 1.0 at the ambient air temp. decreases.

That's one reason why an air source heat pump providing heat to a very well insulated home when operated with a cool(er) indoor ambient winter temp. of, say 65 F or so may not achieve its expected operating C.O.P., especially if there is some significant internal heat generation going on in the dwelling that further lowers the dwelling's balance temp. (the temp. below which aux. heat is required to maintain a stated indoor ambient air temp.). Been there. Done that.

Because of conservation measures I'd taken, the winter balance temp. in my Buffalo home was ~ +42 F. That is, the dwelling did not need aux. heat until the outside ambient temp. dropped below 42 F. and stayed there or lower for several hours. A bonus was that the low building heat loss (including infiltration loss) made the building time constant about 20 - 23 hrs. depending mostly on wind velocity and (during the approx. 300 sunny hrs. over an average winter) the irradiance level during daylight hours. I decided against a heat pump for those reasons.

it will be interesting to see what I find when I get up to the house and get the HVAC technician to look at my set up. I found the controls manual for the Lennox Harrmony Zone Control System. It suggests that I might have a balance point temperature sensor. Once I get the actual heat pump and furnace data and talk to the HVAC technician, maybe I can figure out at what temperature the system switches from heat pump heating to auxiliary heating.

[ajonestx]

J.P.M. etal. ok. PV Dummies Chapter 5 (Evaluating Your Solar Potential) was a good refresher and got me thinking about a few things. I liked the simple diagrams that showed average sun hours - I knew there were areas that had more sun than others and it made me think back to a road trip last year along I-10 through Arizona. I can understand how so many square miles of PV would work so well out there in that barren desert!

Also, nice data points... 46 degrees of sun elevation between the summer and winter solstice, which got me looking into my current roof conditions. My roof (and PV) is 8:12 pitch or 34 degrees. At my latitude of 29.76, I should see optimal sun exposure (ignoring rain, shade, clouds, temperature, etc.) on (or about) March 9th and October 3rd when the sun is at 56 degrees above the horizon (or normal to my array).

Also interesting in that the book suggested that the PV surface area exposure to solar radiation is 90% or greater while the sun is within 25 degrees of normal to the panel (which suggests pretty good sun exposure during the months of Feb, Mar, Apr, Sep, Oct and Nov).

Some time back, I also attempted to find "solar data", but was unable to do so. For some reason (while looking for heat pump information), I was able to find it (I think since my search now included "TMY" it worked better).

• Interesting that in 2020, there were 37 documented days where DNI (direct normal irradiance in Houston) was 1,000 w/ sq. meter or greater for 37 days (most during the months of Nov, Dec, Jan, Feb and Mar).
• In 2019, there were 14 days at 1,000 or above (Dec, Jan and Feb).
• In 2018, there were 18 days at 1,000 or above (all in Jan except for a couple in Nov and Mar).
• In 2017, there were only 6 days at 1,000 or higher (all in January).

I would have expected to see a few at 1,000 w/ square meter during the summer months where sun radiation was passing through less atmosphere, but that just might be our climate and coincidence. Also interesting that in 2017, there were 7,760 half hour readings with DNI over zero. In 2020, there were 8,152 readings over zero. I was told at one point in time that part of my lower production issue was due to fewer sun hours over the last few years - not sure if this is the right way to measure this, but just by looking at local DNI data, there was an increase of about 5% (2017 to 2020).

Well, on to chapter 6 (calculating payback on your solar investment).

[J.P.M.]

With heat pumps, especially air source heat pumps, have a design temp, and anything below that will run on aux heat or emergency heating. Since that is essentially heat coils, that is when you chew up a lot of electricity.

Well, they have a min. ambient source design temp. But their operating C.O.P. above that ambient air temp. will depend on (among other things) the ambient air temp. with the operating C.O.P. decreasing toward 1.0 at the ambient air temp. decreases.

That's one reason why an air source heat pump providing heat to a very well insulated home when operated with a cool(er) indoor ambient winter temp. of, say 65 F or so may not achieve its expected operating C.O.P., especially if there is some significant internal heat generation going on in the dwelling that further lowers the dwelling's balance temp. (the temp. below which aux. heat is required to maintain a stated indoor ambient air temp.). Been there. Done that.

Because of conservation measures I'd taken, the winter balance temp. in my Buffalo home was ~ +42 F. That is, the dwelling did not need aux. heat until the outside ambient temp. dropped below 42 F. and stayed there or lower for several hours. A bonus was that the low building heat loss (including infiltration loss) made the building time constant about 20 - 23 hrs. depending mostly on wind velocity and (during the approx. 300 sunny hrs. over an average winter) the irradiance level during daylight hours. I decided against a heat pump for those reasons.

[heimdm]

With heat pumps, especially air source heat pumps, have a design temp, and anything below that will run on aux heat or emergency heating. Since that is essentially heat coils, that is when you chew up a lot of electricity.

[ajonestx]

Just keep in mind that the heat pump's C.O.P. is a pretty strong function of the ambient air temp. If you run simulations, it might be helpful to use TMY data from NREL for representative ambient air temps. as f(date, time of day)

Thanks for the suggestion . I built a little table to help me fill in the blanks based on NREL TMY data and summarized monthly averages as a generalization of expected savings. Once I get the REAL COP data, I can crunch the numbers to get a little closer. Having some difficulties building the formulas correctly as I have tied the heat pump numbers to the furnace numbers and when I test the formulas by changing the efficiency of the furnace to a "better" value, the savings does not improve as it would be expected. I must have plugged in something wrong, but will fix it eventually (probably after I get the equipment data next month).

[J.P.M.]

I'll be back up at the house in about three weeks, will collect manufacturer plate data (I think I have a manual too) for the furnace and the heat pump, then run some numbers at different temperatures.

Just keep in mind that the heat pump's C.O.P. is a pretty strong function of the ambient air temp. If you run simulations, it might be helpful to use TMY data from NREL for representative ambient air temps. as f(date, time of day)

[ajonestx]

Once you get familiar with all this, find your equipment's C.O.P.'s and combustion efficiencies and plug them into the above method using your current fuel and electricity costs for your specific comparison.

I appreciate the detailed response and the details for the calculations. I'll be back up at the house in about three weeks, will collect manufacturer plate data (I think I have a manual too) for the furnace and the heat pump, then run some numbers at different temperatures. On the surface, it looks like I would benefit from getting the heat pump back up and running. The numbers should confirm or deny.

[J.P.M.]

Since you brought heat pumps up, do you know of any tools that I can use to evaluate the cost difference between using propane (at $2.16 per gallon) for space heating vs. heat pump (at $0.08 per kWh)?

Several. But first you'll need to understand how heat pumps and their inverse (air conditioners) work, particularly with respect to something called the Coefficient of Performance. Also, you'll need to understand what is meant by the various terms related to furnace efficiency and be careful about how the terms are used, particularly by peddlers and those with money to make or egos to feed by muddying the waters with imprecise terms or just plain B.S.

1.) Confining the discussion to air source devices for now, the difference between heat pumps and air conditioners is that one is used to heat air and one is used to cool air. One perhaps oversimplified way to look at it is to say if you turned a window air conditioner around in the window and so reversed the direction of flow of the heat and cold coming off the device, you'd have a heat pump.

2.) The Coefficient of Performance of either a heat pump or an air conditioner is the ratio of useful heating (for a heat pump) or cooling (for an air conditioner) provided to the work (provided by electricity) by the device that's required to produce that output.

3.) Because you're in the U.S, it'll be useful to do the heat calcs in BTU (British Thermal Units) in the old imperial system, sometimes called the "customary units system. That means we'll need to know the work/heat equivalents and conversions between customary (Imperial or British) units and S.I. units which is the system that electricity units use. S.I. is the name for what was called the old (and similar but not identical "metric" system.

For this discussion, the heat (BTU) equivalent of a watt-hr. is: 1 Wh of electricity == 3.412 BTU, or, 1kWh of work energy == 3,412 BTU.of thermal energy.

4.) If 1 gal of propane is burned in a home heating device to heat air, the combustion will liberate (transfer) between 90,000 and about 83,000 BTU of heat to the air. The difference between the 90,000 and 83,000 has something to do with the heating equipment and whether or not it can wring the moisture (H2O vapor ) out of the products of combustion and use them as added heat value into the delivered heat total. But that's beyond the scope of this conversation. For now, I'll use the lower value unless what's called a "condensing" furnace or some such name is used.

5.) I'll assume the same ductwork and delivery system will be used for either system and again for simplicity of explanation, those duct losses are ignored for now. Just know that they can be substantial if not considered and minimized.

Now, on to roughly equivalent costs between heat delivered by a propane burning air heater and an air source heat pump.

Let's say that your non condensing propane burning air heating residential furnace has a combustion efficiency of 70 %. So, every gal of propane burned will create 83,000 BTU of heat that's transferred to the air going through the cold side of the unit's heat exchanger. However, for our gallon to kWh equivelance to stay true for this example, we need to take the furnace efficiency into account. We do this by dividing the 83,000 BTU/hr; by the furnace efficiency (==0.70) so, (83,000 BTU)/(0.70) = 118,571 BTU of propane needs to get burned to get 83,000 BTU of heat delivered at the furnace plenum (the furnace outlet).

That 118,571 BTU is the heat equivalent of (118,571 BTU)/(3,412 BTU/kWh) = 34.75 kWh after accounting for our 70 % furnace combustion efficiency.

But now, we've got to consider the C.O.P. of the heat pump. Without going into the Thermodynamics of why, know that a heat pump's C.O.P. goes down (that's bad) as the temp. of the heat source of the heat pump (if this case the outside air) drops. So to not muddy the water any more, assume the air source temp. is + 50 F and, at that temp, the heat pump will operate and a steady state C.O.P. of 3.0 before any parasitic losses for an air handler.

From our def. of C.O.P, from 2.) above, to produce 34.75 kWh equivalent of heat (That is = 118,571 BTU), our heat pump operating at a C.O.P. of 3.0 will need an input of : 34.75/3 = 11.58 kWh.

So, for this situation, and with all the qualifying conditions stated, from a fuel cost standpoint only, if 11.58 kWh of electricity gets billed for less than the cost of 1 gal. of propane, the heat pump is the more cost efficient fuel to heat a dwelling. Or, conversely, and for the same conditions and equipment, if 1 gal. of propane is burned and its cost is less than the cos of 11.58 kWh of electricity, from a fuel cost standpoint only, the propane is the cheaper fuel.

Once you get familiar with all this, find your equipment's C.O.P.'s and combustion efficiencies and plug them into the above method using your current fuel and electricity costs for your specific comparison.

Learn about heat pumps and why the C.O.P. varies with heat source temp., and don't be B.S.'d by crap like EER (which is no more than C.O.P. divided by 3.412 BTW), and especially something called SEER - one of the biggest cons the HVAC industry even pulled off on ignorant users by reinventing the calcs and nomenclature for the sole purpose of making things less clear.

I'm back to the old signoff of: Take what you want of the above. Scrap the rest.