Solar Panel Technology Comparisons

[chem geek]

There are three basic technologies used for solar heating of water. The least expensive is the unglazed flat black plastic mat like the ones sold by FAFCO shown here ([EDIT] now here). The glazed flat panel Gobi-style is about 3 times as expensive for the same area with an example from Heliodyne shown here. Evacuated tube panels are about twice as expensive per gross area as Gobi-style flat panels such as the Navitron shown here (or somewhat less efficient Silicon Solar panels here).

The maximum amount of sunlight in the summer in most locations with the panel pointed at the sun is around 1000 Watts per square meter. In the charts below I use 800 W/m² to represent a sunny day with either less than optimal panel placement or a less than perfectly transparently clear sky. A mostly cloudy (covering the sun) or overcast day varies in its brightness, but I use 300 W/m² as a reasonably bright overcast day. The efficiency data comes from this FAFCO PDF file, this Heliodyne PDF file and this Navitron PDF file.

[EDIT] As described here, if one uses the most efficient evacuated tube, then add 13% to the values in the evacuated tube columns. [END-EDIT]

....................... MOSTLY CLEAR AND SUNNY (800 W/m²) ............ CLOUDY OR OVERCAST (300 W/m²)
deltaT(ºC/ºF) .. flat black .. Gobi glazed .. evacuated tube ........ flat black .. Gobi glazed .. evacuated tube
. 0 / . 0 ................ 80% .......... 74% ............... 41% ........................ 80% .......... 74% ............... 41%
. 5 / . 9 ................ 68% .......... 71% ............... 40% ........................ 46% .......... 66% ............... 39%
10 / 18 ................ 55% .......... 68% ............... 40% ........................ 13% .......... 59% ............... 37%
15 / 27 ................ 43% .......... 65% ............... 39% ........................ N/A .......... 51% ............... 35%
20 / 36 ................ 30% .......... 63% ............... 38% ........................ N/A .......... 44% ............... 33%
25 / 45 ................ 18% .......... 60% ............... 37% ........................ N/A .......... 36% ............... 30%
30 / 54 .................. 5% .......... 57% ............... 36% ........................ N/A .......... 28% ............... 28%
40 / 72 ................ N/A .......... 51% ............... 35% ........................ N/A .......... 13% ............... 24%
50 / 90 ................ N/A .......... 46% ............... 33% ........................ N/A .......... N/A ............... 19%
60 / 108 .............. N/A .......... 40% ............... 31% ........................ N/A .......... N/A ............... 14%
70 / 126 .............. N/A .......... 34% ............... 29% ........................ N/A .......... N/A ................. 9%

If the Gobi glazed or evacuated tube panels use a heat exchanger to avoid having pool water through the panel, then the efficiency would be lower than shown by a factor. The chart above shows that the less expensive flat black plastic mat panel works well in sunny warm climates [EDIT] (with 2.4 MPH wind) [END-EDIT], but that the Gobi glazed flat panel performs better in cooler weather (or when there is more wind) and in overcast conditions. The evacuated tube is better for domestic hot water heating or for spas (especially in cold winter climates) because the temperature difference is greater and the volume of water is lower (so fewer panels are needed so the higher cost and lower base efficiency aren't as much of a problem).

Richard

 

[amjohn]

Why are "in-ground" unglazed panels and kits so much more expensive than "above ground". When I look at FAFCO and Sungrabber, they both make both kinds of systems. Other than price, I do not see a significant difference- any thoughts?
Also, some systems have a 1.5 inch header- claiming smaller tubes means slower flow which means better heat transfer, others 2" (FAFCO and Hi-deluxe) claim that "extra large 2” headers do not slow down your pool’s circulation and insure excellent solar heat transfer to your pool’s water. Smaller 1-1/2” headers can reduce your pool’s circulation and cause solar heating to be compromised".
Both of course claim theirs is superior. Any basis to one being better than the other?

 

[mas985]

I would assume that the first set also includes EPDM panels although performance does vary slightly.

Also, Florida Solar Energy Center has put together a pretty good resource for solar panels and has even estimated performance for many brands:

http://www.fsec.ucf.edu/en/industry/tes ... l_ratings/

They rate the performance in terms of BTU/sq-ft/day which can then easily be converted into daily temperature rise for a given pool size.

Most of the performance averages about 1000 BTU/sq-ft/day. I have measured my own system at over 270 BTU/sq-ft/hr (based upon a 4 degree delta rise from solar input to output, 65 GPM & 480 sq-ft panel). Most of the time, I only see about a 3 degree rise but occasionally I will get 4 degrees.

Also, from this reference:

http://www.builditsolar.com/References/SolRad/Lat40.htm

Incident solar energy on the hottest day of the year (when I see a 4 degree rise) for me is about 310 BTU/sq-ft/hr which means I am running close to 86% efficiency although my temperature measurements are not very accurate +- 0.5 degrees due to the readout of the controller so efficiency could be as low as 77%.

Other resources:

http://www.builditsolar.com/References/ ... rmance.htm
http://www.engineeringtoolbox.com/surfa ... _1213.html

 

[chem geek]

amjohn,

The FAFCO makes both in-ground (SunSaver, Revolution) and above-ground (Solar Bear Plus, Bear Economy so some sites call this SunSaver AGP). The main difference between the in-ground and above-ground panels is that the former are designed to self-drain while the latter are not so you can't use the above-ground versions in freezing conditions and you generally disable them (and manually blow them out and store them) for the winter. I don't know if the price difference is justified based on the differences in H/W to support such self-draining -- I suspect the in-ground has some premium associated with the overall higher price of in-ground pools.

It is absolutely not true that slower flow leads to better heat transfer. As can be seen from the "EFFICIENCY VS. FLOW" graph in this specification, you can see that faster flow is more efficient. The reason is that a faster flow results in a lower temperature and as you can see from the "MEASURED EFFICIENCY" graph (and my chart in the initial post in this thread), a lower temperature difference is more efficient. At higher temperatures, the panel is quite a bit hotter than the surrounding air so loses some of its absorbed sunlight as re-radiated or convected heat, especially with wind. An efficient panel is a cool panel. Remember that the amount of heat transferred is proportional to the product of the temperature rise and the flow rate. If you double the flow rate and have half of the temperature rise you still have the same amount of heat transferred (ignoring the efficiency difference). Remember that the turnover rate and flow rate are related -- if the flow rate is cut in half, then it takes twice as long to turnover the water so even though the temperature may be doubled it takes twice as long to have that temperature rise in the entire body of pool water -- a 4 degree rise every 2 hours is the same as a 2 degree rise every 1 hour. So, FAFCO is right in their claim and the others are wrong.

So the tradeoff in flow rate is that the pump electricity usage varies roughly as the cube of the flow rate so though higher flow rates make the panels more efficient (with diminishing returns as shown in the graph), the pump cost rises dramatically.

Richard

 

[chem geek]

Mark,

Thanks for the info and the links. The "Collector Efficiency" graph in this link you gave is similar to my first chart (for "sunny" conditions). A picture is worth a thousand words.

Richard

 

[amjohn]

Since "...a lower temperature difference is more efficient. At higher temperatures, the panel is quite a bit hotter than the surrounding air so loses some of its absorbed sunlight as re-radiated or convected heat, especially with wind...", then what about a situation where there is hot day time sun and winds? Would the larger tubes increase the heat loss due to winds averaging 10-20 mph, and would that exceed the advantage gained by increased flow rate of 2" headers?
Note: during mid summer we do not need to warm the pool- we will be using fountains to cool it off. I am looking at extending the season and having the water stay warmer during spring and fall cool spells. An example, right now, we are having 55-60 degrees at night with 85 degrees during the mostly sunny day, with a 10-20 mph wind during the daylight hours. Even with a solar cover, the pool loses a lot of heat during the night, and takes most of the day to warm back up. It is still cool to swim with the wind, and the "swim window" for the day is pretty narrow. My hope is that the solar heater would get the pool to a warmer temp earlier in the day and keep that temp into the evening.

 

[chem geek]

If the air is hotter than the panels (including having wind), it is still more efficient to have a faster flow rate. The reason is that the faster flow rate will keep the panels cooler than the air so the rate of convected heat transfer from the air to the water in the panels will be faster. The bottom line is that when you want to heat up water, you want it to remain as cool as possible so as to minimize the rate of its losing heat (or maximize the rate of its gaining heat).

The 1.5" vs. 2" tubes they are talking about are HEADER tubes and these are not the primary ones involved in the heat absorption from the sun. The header connects to many smaller tubes where the water flow is relatively speaking much slower in each tube but there are many such tubes (the flow is actually laminar rather than turbulent in such small tubes). The 1.5" vs. 2" has more to do with the additional head (pressure resistance to water flow) with the smaller 1.5" with the result being a lower overall flow rate assuming the pump RPM is the same. The header has a relatively small surface area so does not affect the heat gain or loss very much -- it's the far larger area of the small tubes between the headers that is most relevant for the heat gain/loss calculations. The reason for the small tubes is 1) to spread the water flow over a larger area and 2) to maximize the surface area of the water exposed to the hot side of the tube. An ideal panel would have a very thin "sheet" of water flow, but that's not practical to construct so the many small tubes approach is generally what is used and results in pretty good efficiency of 80% at typical flow rates (4 GPM per 4'x8-12' panel) vs. 90% efficiency for double that flow rate.

For your specific situation, the use of the much more expensive glazed Gobi-style flat panels would work much better because they won't lose efficiency very much from the wind nor from the lower amount of sunlight in spring and fall. Had I known about such panels as an option when we built our pool, I probably would have purchased them BUT they are around 3 times as expensive. It's a rather tough call to make even though I wouldn't have needed quite as many panels as I now have (my current panels solar turns off at 2 PM during the middle of summer so I have more than needed in order to have enough area to extend the season in spring and fall, but it's only partially effective at doing that).

I, too, am in a situation where the daytime gain and nighttime loss are about equal -- well, actually, I was in that situation in August and early September since right now I'm at a net loss due to cooler nights and days that are more overcast or cooler. Of course, even if you had better panels to have more of a temperature rise during the day, you'd still have a see-saw of highest temperatures in late afternoon and lowest temperatures in early morning. The only way to stabilize that is to have a more efficient solar cover or other insulating properties for your pool.

Richard

 

[launboy]

This may need to be in a seperate thread, but there is a lot of knowledge already in this one and I'd like to use some of it.

This is more of a question then an addtion, but what are the chances that 120 sq.ft of solar heat panels that get full sun from 9:30 a.m. until 5 p.m. will be able to heat a 400 gal. spa to 100-102*F?

Some more details... Panels powered by 1 HP pump that puts out 30 GPM with solar on. The equates to 10 GPM per panel with 3 panels plumbed in parallel. The panels are 2x20' flat black A.G type - 1.5 in. headers.

The spa holds aprox. 400 gal. including water in pipes. It has 2" hard foam insulation directly against all walls which are vinyl.(Homebuilt vinyl lined spa). The foam is R-7.8. it will have a cover, probably 2" foam with Reflectix on the underside(or maybe pool solar cover material) topped by plywood. Dimensions of the round spa are 6 ft. dia. 18" deep, 16" seat and an 18" deep footwell thats 40" in dia.

The total surface are of the spa(not counting the top) is 72 sq. ft.(is that right?). Actual water surface area is about 29 sq. ft. I'm hoping having a 4:1 ratio of solar S.A. to Spa S.A. will help me out, but I'm sure there is a point where the differencial between water temp. and air temp. is to great and the panels stop heating.
If it matters, the spa will have 8 jets total, 6 from a seperate spa pump and two powered by the pool pump(running solar).

We are having 75* daytime temps, full sun, and averaging wind less then 5 MPH, but at night it drops down to 50-55*F.

I'm just trying to figure out how much later into the fall we will be able to use this thing before only solar just isn't enough anymore. A guy on Youtube used his solar to supplement his stand-alone hot tub and said his system topped out at about 115* but I assume this was in the summer.

To sum it all up, what temp. and for how much longer(in Wisconsin) do you think I can reasonably heat my spa to using only solar panels, given the above info?

If this belongs in a new topic, feel free to move it there.

Thanks,
Adam

 

[mas985]

I have a 750 gallon attached spa on my pool and I can usually heat it up past 102 but only on very warm days. The panels must exceed that temp in order to do it.

The downside really is that I usually want the hot water spa at night or sun down. However, I found what I can do is heat the spa up during the day and some of the heat will carry over to night so I need less of the NG heater.

So as long as the panels exceed 102 deg, it should be no problem.

 

[chem geek]

The temperature difference between the 75F air temperature and the 102+ panel temperature is, say, around 30F. That's around a 40% panel efficiency with 2.4 MPH wind at 800 W/m² which is less than ideal peak summer sunlight. Even with ideal sunlight, the efficiency at low wind is around 45%. With your somewhat higher wind, you'll have somewhat lower efficiency (unfortunately, no data for that). At 30 GPM with 400 gallons that's a turnover time of around 13 minutes. You've got around 120 square feet of panel surface area which is 11.15 square meters. Southeast Wisconsin has peak solar radiation per day in June and July of 5.9 kWh/m². So the heating rate when the water is already near 102F is 40%*11.15*5.9 = 26.3 kWh = 22,641,000 calories. 400 gallons is 1514 liters so the temperature rise is 22,641,000/1514/1000 = 15ºC or 27ºF. However, the efficiency will be lower when the sun isn't overhead and the panel will lose heat when the sun is low (or absent).

So I would say that if you had your spa covered when not in use and you only ran the water through the panels during sunny days and decent hours of around 10 AM to 4 PM or so, then you'd probably have no problem heating the spa. Mark's (mas985) situation sounds like an uncovered spa so would lose heat fairly rapidly so wouldn't heat up as much during the day and would lose heat by the time he would get in later that evening which is what he described.

Richard

 

[launboy]

:bowdown: :super: ** Clap clap clap** Thanks you so very much! Knowin there is some science behind my theory is very reassuring. I've made some progress on the spa and have posted updates with picures in my thread (in the Build & Repair section).

Thanks again,
Adam

 

[salinda]

Richard,

I live close to you, around San Jose. My solar system is not ideal at all, which I have discussed on other threads. It was a bad design from the beginning.

I am now thinking ahead to a design for the future.

Anyhow, I have one very good (unshaded most of the summer, very hot) roof, mostly Southern exposure and a smaller excellent WSW exposure roof to work with. I think I want to abandon the eastern exposure roof for solar. It gets too much shade anyway. Unfortunately, I have a large pool. The WSW roof has some roof geometry issues that limit its usefulness for standard solar.

It looks like I could use a glazed system on a much smaller area to get the same output as a standard "mat" type of system. What I don't understand from you OP above is would you not use this system closed loop as designed? You would simply run the pool water through it and run it like standard solar?

 

[chem geek]

Salinda,

First of all, the glazed collectors are only more efficient than black flat plastic mats if the temperature difference between the air (during the day) and your desired water temperature is at least 15F or more or there is frequent wind. My black flat plastic mats have no problem heating the pool during the peak months of the summer from June through most of September for most days. It's only in trying to extend the season into late-April/May and late-Sep/October/early-November that they do not do well. They also don't do much on partly cloudy days when the glazed panels would be better, but only a little bit since there's only one-third as much sun energy on such days. Also, don't forget that I'm trying to keep the pool at 88F.

Unless you live in a windy area, the glazed panels should be considered only if you intend to keep your pool rather warm or want to extend the season as much as possible. Even with glazed panels, you will only be able to get by with less surface area if your main issue is extending the season and that you don't have any problem with heating during peak summer months. So, let's figure out what sort of heating you might expect.

Your pool is large at 35,000 gallons. That's a good news / bad news story. It takes more heat energy to move the temperature, but the pool also stores more heat so it can lose heat more slowly. This basically means that it takes longer to get your pool heated, but once it's heated it tends to stay warm longer (assuming your larger pool size is due to larger depth as well as surface area). If you have an area of solar panels equal to the surface area of your pool and if your pool has an average depth of 4.5 feet, then at 70% efficiency during peak noontime sun that's about 0.8F per hour of heating. I suspect that your pool volume is large not only due to larger surface area, but it's probably deeper than 6 feet at the deep end and may have an average depth of 5.5 feet instead. In that case, surface area of the panels equal to your pool's surface area would result in about 0.65F per hour and these are ideal numbers assuming the panels are pointed at the sun at noontime (roughly 1 PM during daylight savings time).

As for using the glazed panel with pool water vs. a closed-loop system with a heat exchanger, you lose efficiency if you go with closed-loop with a heat exchanger. There are only two reasons to go that route: 1) you have freezing temperatures and plan to operate the panel in such conditions (i.e. not drain it for the winter) or 2) your panels are not resistant to corrosion from pool chemicals and your water is more corrosive. In your case, you have an SWG which means your water is salty and conductive. It is true that most glazed panels use copper so it would be better to use a closed-loop system with a heat exchanger in your situation since the saltier conditions could corrode the copper more quickly. It's one thing if it's a gas heater with a copper heat exchanger goes out after 5 years or so, but replacing glazed solar panels is a much more expensive proposition so you'll want to protect them.

Before you get any system, you should figure out how your pool operates at different times of the year using a solar cover, which I presume you have. Figure out its peak temperature in the late afternoon and its lowest temperature in the morning. That will give you some idea of the rate of heat gain from sunlight and loss overnight from your pool and you should do these tests with the pool starting out at the temperature you desire (unfortunately, this isn't the time of year to be doing this). Your solar system will need to add heat at the rate necessary to make up for this difference of greater loss at night compared to gain during the day -- that is, it needs to increase the temperature of the pool at least at the rate it is lost over 24 hours. In your situation, if you go with glazed panels, then the main factor is going to be the 24-hour temperature drop. In San Jose, the daily solar insolation on a horizontal surface is 6.95 kWh/m²/d in June, 4.85 in September, 5.91 in April. A roof facing south and tilted at 20 degrees or so improves the peak in June to 6.95 * cos(37-23-20) / cos(37-23) = 7.15, to 5.81 in September and to 6.6 in April. If I use 6 kWh/m²/d and 60% net efficiency of glazed panels with heat exchanger and assume an average pool depth of 5.5 feet, then this is 5,162,524 calories heating 1.6764 cubic meters (1,676,400 ml) so that's 60% * 5,162,524 / 1,676,400 = 1.85C per day or 3.3F per day. So you can maintain a pool's temperature if the net heat loss overnight relative to gain from the sun on the pool during the day (with a solar cover) is around that same 3.3F. The pool will be cooler in the early morning and warmer in the late afternoon. Note that this all assumed an area of solar panels equal to your pool's surface area.

In short, glazed solar panels are not a panacea. You simply cannot extract more heat from the sun than it is delivering over an area and black flat plastic mat panels are very efficient unless the air is cool (relative to pool water temp) and windy or the sky is hazy or cloudy or the sun is lower in the sky (i.e. spring and fall).

Richard

 

[salinda]

Salinda,

In short, glazed solar panels are not a panacea. You simply cannot extract more heat from the sun than it is delivering over an area and black flat plastic mat panels are very efficient unless the air is cool (relative to pool water temp) and windy or the sky is hazy or cloudy or the sun is lower in the sky (i.e. spring and fall).

Richard

Thanks Richard! I haven't been using a solar cover other than the solar pill. The pool is large and has a spillover spa and we are also concerned about safety for our kids and pets. I think about it pretty hard every year, but haven't made the leap yet.

My deep end is 10 feet deep, so your assumptions about the volume are right. In the summer, the surface of the pool gets full sunlight most of the day. I would love to extend the pool season. I would also love temps in the high 80's for the full swim season. I used to have that regularly (even the 90's) without a cover but with the less-efficient pump. Of course, it cost me over $150 in wasted electricity per month to get there. Also, the past few summers seem cooler on average than in the past decade.

 

[chem geek]

Well if your shallow end is 3 feet and your deep end is 10 feet and the average is 6.5 feet, then that's even slower heating than I had been assuming with 5.5 feet as average depth. To get to 35,000 gallons, this would be 20' x 40' x 6'. Is your pool around 20' x 40' or equivalent in surface area?

As for pump energy costs, this is really a function of the head caused by having to go through a lot of solar panels, even in parallel since the total run length is still longer through the headers. The graphs shown in this spec for FAFCO SunSaver solar panels shows that each 4' x 12' panel has an effective area of 47.8 ft². You would need 17 such panels to have the same area as your pool (assuming it's 20' x 40'). The recommended flow rate for 80% efficiency at low wind and low temperature difference conditions is 4 GPM per panel so that's 68 GPM which is quite high. If you were plumbed with 2.5" or 3" pipe to/from the solar panels, then that would reduce the head loss. Another option is to operate at half the flow rate, or 34 GPM, for 2 GPM per panel which reduces efficiency to 70%.

What doesn't make sense is that it sounds like you got a slower pump to have a lower flow rate, but your solar efficiency went way down -- much more than would normally be expected from a drop of 80% to 70%. You had mentioned how the solar system may have been in bad shape for a while as you described here and that may have more to do with the problems than just the pump change. Either that or the "loop" system you have drops in efficiency a lot more with lower flow rates.

Richard

 

[salinda]

What doesn't make sense is that it sounds like you got a slower pump to have a lower flow rate, but your solar efficiency went way down -- much more than would normally be expected from a drop of 80% to 70%. You had mentioned how the solar system may have been in bad shape for a while as you described here and that may have more to do with the problems than just the pump change. Either that or the "loop" system you have drops in efficiency a lot more with lower flow rates.

Actually, most of the solar area I have are those coils--I believe that was a mistake. I think that they need a lot of flow and I have something like 12 of them, so combined they need A LOT. Also huge head losses are suffered here. I don't think that all of the coils are filling. In fact, I can feel some are stagnant in the few times I have gone up on the roof to figure things out. It is possible that I effectively have a much smaller system combined with the efficiency drop.

I really appreciate your help as a sounding board.

Oh, by the way, my pool is almost trapezoidal in shape, the shallow end being wider than the deep end because the wide steps angle out a little. Both long sides are long arcs with the radii going in similar directions (both curving to the right). In the middle of one of those curves is my spillover from my spa. My spa is also a little larger and a bit deeper than normal. I am not sure of the exact size because I didn't build it, but I have managed the chemistry over the past few years and the acid additions and subsequent ph drops correspond to anywhere from 32k-37k gallons. I should get out there and exactly measure it sometime, but I honestly haven't.

The bones of the pool are quite old (built in the '60's I think), but it was replastered a few years before I moved in and it is in such good shape that people usually assume that we built it recently.

 

[launboy]

Any chance you may be able to snap a couple of pics of the solar set up? This may help us better understand how the system is set up.

Thanks,
Adam

 

[Gooserider]

Interesting thread on the solar panels, Chemgeek, but I think we are looking at slightly different objectives... It appeared to me that you were talking about using solar ONLY for heating a pool, and nothing else. For that I tend to agree, the rubber mats are probably the best bet in terms of BTU's per dollar.

However I was thinking more in terms of doing solar for year round use, Domestic Hot Water (tap water) at all time, supplementing the heating system in the winter, and then using it for pool heating in the non-heating times just because the system is there, and one might as well put it to work...

For that, I think glazed panels are the best bet - they rival or beat evac tubes for overall performance at a much lower price, but still work well at an operating temperature of 100-120*F, which is good for DHW and radiant floor heating. Obviously for this type of application you would want to use anti-freeze and a heat exchanger, as drainback systems tend to demand excessive pump loads (at least for us with a 12/12 roof that goes to a peak about 35' above ground level) and run a seperate loop off the heating system to use it on the pool in the warmer weather.

I haven't really crunched hard numbers yet, but my early estimates are that this could help the pool quite a bit. Of course ANY heat at all will help considering that at present we don't have any sort of heat, not even a solar cover (can't use one as it isn't compatible with the Polaris...)

Gooserider

 

[chem geek]

Yup. I agree with your analysis. Just keep in mind that using anti-freeze and a heat exchanger reduces the efficiency. Nevertheless, if you're going to use the system for DHW or during the winter, then you have to use at least the glazed panels as you said. The evacuated tubes are better for really cold climates. According to this link, your lows aren't that much below freezing. The evacuated tube design only helps on cloudy days in your situation but the output is so low that it's hard to justify the higher price and the lower efficiency when it really counts when there is more sun or higher temps.

Richard

 

[PaulR]

at present we don't have any sort of heat, not even a solar cover (can't use one as it isn't compatible with the Polaris...)

I haven't seen any posts to suggest that, and the few people who responded in this post didn't think it would be a problem. I've ordered a cover for my own pool and I should be able to see for myself pretty soon.
--paulr