solar observations- incredible

[Davegvg]

I completely shut off my solar for the last 2 weeks to conserve eletricity while travelling and not using the pool.

I live a few miles from the beach and every night it cools down to the mid 50's regardless of the temps during the day and I returned to a pool sitting at a crisp 71 degrees.

We've had a heat wave in the last three days and I felt like swimming so I decided to see just exactly what I could get out of a 90 degree day so I RPM'ed up my epump to about 3100 RPM or 1300 watts just under the full 2 HP and let it rip as long as the solar tap called for heat.

I was able to get an ASTONISHING 15 degree rise out of the system in a single day.
The tap opened at 8:30 and closed around 4: 30 even though the panels were shaded starting about 4.

This computes to an astounding 2,502,000 btus delivered to the water in a single 8 hour day or just shy 313K btu per hour on average.
This is performance just a bit below what my 400K btu natural gas heater can achieve at the cost of around 5-6 dollars an hour.
I estimate the electricity cost was 1.56 cents to do this based on .15 cents a KW hour.

This is nothing short of incredible in my mind to achieve this level of performance for this tiny amount of money.




Uncle Dave

 

[chem geek]

12 4'x10' solar panels is around 45 square meters. Peak solar energy at ground level is around 1000 Watts per square meter with the sun directly overhead so around 45,000 Watts or over 150,000 BTU/hr. Solar panels are normally around 80% efficient though running at higher speeds as you did might get that closer to 90%.

So I suspect that some of your heating came from the sun directly hitting the pool since even with a white plaster surface around 60% of the sun's energy is absorbed by the water (with a dark pool surface, absorption can approach 90% -- see this thread for more details). Your pool surface area is probably around 50 square meters. If there was little wind and the air wasn't too dry, then there would be less heat loss from evaporation. Without a solar cover and with no solar heating except for the sun on the pool itself, it is not uncommon to have a 5 degree or higher gain during the day in a dark pool and a similar loss at night. So you might be getting around 90,000 BTU/hr from the sun hitting the pool directly (with a white surface; with a dark surface this would be over 130,000 BTU/hr).

Finally, with the air temperature being hotter than the pool water temperature, there is additional heat transfer through the panels and some to the pool surface itself. As shown in this thread, an 18ºF difference in air temperature when the air is cooler than the panel results in a change in panel efficiency from 80% to around 55% so conversely with the air temperature being that much higher, effective panel efficiency might go to 105% or in your case starting at 90% you might get to around 115% so nearly 175,000 BTU/hr at peak from the panels.

Even so, it's hard for me to see where you got over 300,000 BTU/hr on average over an 8 hour day. That is pretty remarkable. What color is your pool surface? What were the conditions that day -- was it a little windy, what was the humidity? What is your latitude (i.e. where do you live, roughly?) Was your pool covered and if so, was it a clear cover? If you had a cover to minimize heat loss from evaporation and there was some wind, then the heat transfer from the hot air to the panels would be more efficient than I calculated above. What brand/model solar panels are you using?

Richard

 

[Davegvg]

The sun does not hit the pool until 12, so from 8:30 till 12 the pool cannot be realizing solar gain.

Plaster is white in color. No solar or any cover. I live in west Los Angeles

I suspect the hot air temps (greater than 20 degrees hotter than the water) and relatively no wind (very rare for this micro climate) contributed as did the speed at which I was running the pump likely got me closer to the mediums most efficient rates- the water coming off the roof was usually 1 degree at most 2 hotter than the pool. Indicating I was getting almost everything out of the panels.

"Normally" I run the pump at 2800 RPM or about 900 watts with the solar tap open.

"Normally" I see 5-7 degrees of gain under regular daily cycling, but this day the numbers worked out to be simply astonishing.

Panels are 12 Aquatherm 4x10's pointed South an angled (dont know) only 1 story up.

I agree it seems way optimistic, but the numbers are the numbers- the SWG and the intake temps were within 1 degree of one another at start and finish so the temps were checked at 2 different places. I ran the numbers several times and posted this because I was just amazed at the difference from what I normally get, but the conditions in which this happened - I have never had before or had this setup to observe.

The florida solar center says this panel is good for 39800BTU a day so I get there is substantial difference from what the number should be vs what I saw.

I think what I observed was an absolute best case scenario, but the math is what the math is.
15 degrees of temp rise into a 20K body is 2,502,000 BTU's or 208K per panel.
(if one attributes the increase to just the panels which obviously cannot be the case)

Regardless- I was amazed. I think I had the "perfect storm" of solar pool heating that day.


Uncle Dave

 

[mas985]

Incredible, yes. Possible, no

LA is around 34 degrees latitude and from these tables, the incident energy on the pool for half a day would be about 1146 BTU/sq-ft. For the solar panels, assuming optimum 32 degree tilt, would be about 2308 BTU/sq-ft incident energy. Even if you assume all the heat goes into the water, the panels could only produce 1.1 M BTU and if the pool is about 550 sq-ft, the 100% gain would be 630k BTUs for a total ~1.7 M BTU. That would mean the efficiency of the Pool & Solar heat gain would have to be 147%. Even if you use the direct normal numbers you still can't get to 2.5 M BTUs.

Also, the most I have ever seen on my panels with a similar setup is a daily gain of ~10 degrees on a 105 degree day but it was still less than 100% efficiency.

Either the initial or final measurements must be in error. A 15 degree gain is simply not possible in 20k gallons.

 

[Davegvg]

The numbers are what they are-temps checked at 2 independent sources - and are not in error. Hence the post.

If thats the absolute maximum theoretical amount the solar could contribute the remaining gain had to come from "ambient".
(Im not yet convinced the panels temp weren't closer to 110 degrees with no wind sitting in the sun.)

It came from somewhere because I had a 15 degree rise in 8 hours.

I've had the system installed for 2 years and have never seen anything like what I saw yesterday.

Mark- I see you have a 2 speed pump. whats "high" speed on your pump?



Uncle Dave

 

[mas985]

I'm not sure what you mean by ambient heat gain but heat gain and loss comes from only four components; radiation, convection, conduction and evaporation. The panels and pool have a heat gain due to radiation but a heat loss due to radiation as well. There is a net gain due to radiation during the day but a net loss at night. Evaporation will nearly always be a net heat loss from the pool for both day and night. The pool and panels might gain something based upon air convection but it is not all that efficient of a heat transfer mechanism unless there is a strong wind. Even with the most optimistic assumptions, the pool should not gain more than about 8 degrees given the amount of sun hitting the pool during the day but that assumes the pool and panels absorbs everything that hits them and the convection gain is about as high as it can get. The bottom line is that you cannot have a heat gain that is more than what the sun can provide unless of course you had the heater running at the same time. If you still don't believe me, run the numbers yourself with this software.

Also, I am not sure I understand your question about the pump. All two speed pumps operate at either 1725 or 3450 RPM. I swapped out the impeller for a 1/2 HP Northstar impeller and the motor is a 1 HP uprated two speed off a Super Pump.

 

[Davegvg]

By "Ambient" I mean gain to the pool from the surrounding 20 degree or more hotter air. Not from the water in the panels
If you turn the solar off and the air is consistently 20 degrees hotter than the pool- the pool will start to heat up.

My poorly worded question about the pump was what HP is full rated speed.
I only saw 1/2 HP in your sig line and didn't want to assume all 2 speed pumps divide the speeds in order of 50%.
(Iv havent read the specs on all mfg'r 2 speed pump I look at the specs and see hundreds of pumps. )
So running solar you are using the pump at full 1hp and getting an uprated 1 HP of flow rate?


Ive read the efficiency documents supplied by you guys and even commented on them before as being excellent and definitively answering the question of flow rate vs performance ie its NOT more efficient to run the water slower -so I get the math.

I don't believe Im known on these forums for mendacity, but guys- how did I measure 15 degrees of rise from 2 different sources?

Ive been measuring a long time and not seen this level of rise - but I did yesterday.

Is it possible I have 2 separate defective sensors?



Uncle Dave

 

[Davegvg]

Im wondering if their isn't the possibility of a mini thermocline in the upper layer of water that was colder than the lower layers.
I would think It would mix it up quite quickly.



U.D.

 

[mas985]

By "Ambient" I mean gain to the pool from the surrounding 20 degree or more hotter air. Not from the water in the panels
If you turn the solar off and the air is consistently 20 degrees hotter than the pool- the pool will start to heat up.

Again there are only 4 ways to transfer heat as mentioned above so I think what you mean is convective heat gain as that is the only way air can transfer heat to water. Convective heat gain is proportional to the temperature difference between the heat and air but the air directly over the water. So if the air is not moving, then the air temperature over the water is close to the water temperature and a gradient forms which reduces the heat transfer. The only way for convection to be efficient is to have new air constantly replacing the old air. But then, when the winds are high, evaporation losses well exceeds anything gained by the convective heat gain by a margin of 5 to 1. If there is no wind, convective heat gain drops and evaporation losses decrease but the ratio of evaporation to convection is still about 5:1. However, the sum of both decreases by a factor of two so the net heat gain goes up without wind. The point is that without a solar cover, the sum of heat transfer for evaporation and convection is always a net heat loss.

I don't believe Im known on these forums for mendacity, but guys- how did I measure 15 degrees of rise from 2 different sources?

The most likely reason is measurement error. Exactly what were the sources and how was the measurement done?

 

[Davegvg]

By "Ambient" I mean gain to the pool from the surrounding 20 degree or more hotter air. Not from the water in the panels
If you turn the solar off and the air is consistently 20 degrees hotter than the pool- the pool will start to heat up.

Again there are only 4 ways to transfer heat as mentioned above so I think what you mean is convective heat gain as that is the only way air can transfer heat to water. Convective heat gain is proportional to the temperature difference between the heat and air but the air directly over the water. So if the air is not moving, then the air temperature over the water is close to the water temperature and a gradient forms which reduces the heat transfer. The only way for convection to be efficient is to have new air constantly replacing the old air. But then, when the winds are high, evaporation losses well exceeds anything gained by the convective heat gain by a margin of 5 to 1. If there is no wind, convective heat gain drops and evaporation losses decrease but the ratio of evaporation to convection is still about 5:1. However, the sum of both decreases by a factor of two so the net heat gain goes up without wind. The point is that without a solar cover, the sum of heat transfer for evaporation and convection is always a net heat loss.

I don't believe Im known on these forums for mendacity, but guys- how did I measure 15 degrees of rise from 2 different sources?

The most likely reason is measurement error. Exactly what were the sources and how was the measurement done?

The 2 sensors I used to determine rise were the the water temp sensor in the jandy aqualink, and the water temp sensor in the SWG.
I synced the jandys reading with a digital thermometer about a month ago.

I measured both at 8 am - 2 hours after the pump started and saw 71 on the jandy and 72 on the SWG.

At 430 PM when the solar tap shut off I had 86 on the jandy and 87 on the SWG. There was an almost linear rise of about 2 degrees an hour for 8 hours straight.

During the high RPM runningI often checked the Jandy vs the SWG.
The SWG would read between 2 & 3 degrees higher than the Jandy or 2 degrees higher than the inlet water reading.
At a tamer 2800 RPM i normally run - the temp rise in the SW cell is typically 4 degrees higher what the jandy reads is so based on the solar data printed here, I was pulling all but a degree or 2 off what the panels were absorbing.



Uncle Dave

 

[mas985]

At 2800 RPM and assuming decent plumbing, the JEP 2.0 should produce about 83 GPM with a filter pressure around 14 PSI. With a 4 degree rise, that would calculate to 2768 BTU/min and over 8 hours, and assuming the 4 degree rise it is constant which is unlikely, is 1.33 M BTU. I estimated about 1M BTU but my numbers include the roll off in solar irradiance which 1.33M BTU number does not. Because of evaporation and only 1/2 day in the sun, the pool's solar gain would be about 1/3 of the solar panels so the numbers are consistent with my estimates but no where near 2.5M BTU.

So something is not correct but I suspect the temperature senors may be non-linear in some way. Since they are both run off the same power supply, it should not be surprising that they read nearly the same the temperature and doesn't actually count as independent sources. Confirming with an in pool thermometer might help determine the accuracy of the sensors but even then you have to test it over a range of values to determine linearity.

 

[4JawChuck]

Sorry your free solar heat party got stomped on, nice to get something virtually for free for once isn't it!

I'll trade ya my 200K NG heater for your 400K NG heater, I could use it up here! :lol:

 

[chem geek]

Mark,

The reason I asked about whether there was a breeze was that there could be convection at the panels in that case. It's possible that some wind on the roof at panel level could have helped with the heating and maybe at pool level it was negligible so didn't increase evaporation. As I noted in my post, there is a conduction/convection increase with (almost) no wind that I roughly calculated going in the opposite direction to the FAFCO panel tables when the air is cooler than the water. It's just that this alone doesn't get close enough to what was seen, especially since the calculations I did were for peak direct overhead sun so peak noontime in summer (unlike your calculations that factor in sun angle during the day at the L.A. latitude).

I agree something just doesn't seem quite right about this, but whatever it was, HAPPY SWIMMING DAVE!

Richard

 

[Davegvg]

The jandy and the SWG have independent power supplies - and use different sensors, in different locations.

If you recompute the heat xfer based on the 3100 RPM I was turning rather than 2800 how much closer would I get?

I am curious as to just how hot the roof actually got given the system continued to run with the solar tap open for a half an hour past the time the panels were blocked at that high pump RPM. I normally see the system slow back down within minutes after panel blockage. That heat had to be coming from somewhere besides just the air temp.

4jawchuck- I dont feel "stomped"- regardless of weather ever see this performance repeated, I still think the solar is the single best investment I've made and in my case its the difference between a pool I use and one I look at.

Now Im really intrigued.....


Uncle Dave

 

[chem geek]

Unfortunately, FAFCO doesn't have their specs online anymore, but I found an earlier copy (on the Wayback Internet machine) that I've copied here where the graph on efficiency (with 2.4 MPH wind) had around 60% efficiency at 1 GPM, 70% efficiency at 2 GPM per panel which they called the minimum, 80% efficiency at 4 GPM per panel which they called recommended and extrapolating to 90% efficiency at 8 GPM per panel which they called maximum. 100% is defined as absorbing all of the sun's energy and converting it to heating the water when the air and water inlet temperatures are the same.

Basically, there is a loss in efficiency at slower flow rates due to the panel getting hotter as the water flows through it, leading to heat losses to the air at the same temperature as the inlet water. The chart in this post shows the drop in efficiency as the difference in air and water temperature gets greater (assuming that the air is colder than the water). When the air is warmer than the inlet water temperature, the opposite occurs where instead of heat loss there is heat gain and it should be at the same rate, but in the other direction. So based on the difference between 80% and 55% which is 25% for an 18ºF temp difference, and assuming you were starting out at 90% efficiency with your higher flow rates, I figured 90+25 = 115% overall panel efficiency which includes both sunlight absorption and air convection and air-to-panel conduction. I just realized that the table I used was for 800 Watts per square meter, not 1000, so that would just make the true number less than 115% relative to 1000 Watts per square meter.

The thing is that though this spec for the panels shows how efficiency varies with the temperature delta, there is no chart showing how all of this varies with different amounts of wind. Greater wind would reduce the slowdown in heat transfer associated with convection and be limited instead by conduction through the plastic. I just don't think that's enough.

What kind of solar panels do you have? Do you have any specs on them? Maybe they are made of a material that conducts well, such as metal (similar to a heat exchanger), but that would be rather unusual since it wouldn't work very well when the air temperature was cooler than the water temperature.

As for how hot the panels would have gotten, normally when the flow stops the pressure relief valve opens if the panels are self-draining which is usually the case for roof panels. The plastic would get very hot and based solely on their temperature efficiency loss chart, this implies a 0% efficiency with a temperature difference of around 57ºF so figure that perhaps the panels might get that much hotter so at 90ºF ambient that would be 147ºF (at 800 Watts per square meter; around 162ºF at 1000 Watts per square meter). This should give you an idea of why a dark roof gets very hot and why attic fans would be important to try and remove the heat back to the air to keep a house cooler (one could theoretically blow air across the panels to accomplish the same thing, but that's impractical).

 

[Brentr]

Davegvg, I too was very astonished as I initially got a 14-15 degree rise in one day back in March when I first installed our solar. I also have Aquatherm panels and love them. I also think it depends on the square footage ratio of panels to pool surface area. In my case we have a 1:1 ratio so I think we have an almost perfect system.

 

[Davegvg]

Chem Geek
Nothing special here just regular Aquatherm eco-sun model panels 4x10 on an ideal slanted dark shingle roof, I looked at all the specs on all the popular panels and these are within a % or 2 either way with every other black plastic panel on the market- they are really all about the same regardless of a particular manufacturers "arm waving".

Im pretty sure these are the ones - Aquatherm Industries Inc. Ecosun 16104-10
http://www.fsec.ucf.edu/en/certificatio ... poolap.htm


Brentr-
As I said before typically I see 5-7 degrees of rise in the system, and have for years. I monitor this thing like a hawk and constantly play with pump speeds, chlorinator settings, and measure rise times and chem levels- its kind of a hobby.

It's hard for me to question your performance because I saw about the same under as closely controlled monitoring as Im capable of performing with the equipment I've got. Ive got pretty close to a 1-1 ratio as well, just slightly under.

Thing is that day literally was the textbook ideal solar day -
No wind at all, super hot ambient air temps, blazing sun starting early in the day and as much RPM and pump performance as I though the panels could stand running through a 1 week fresh filter charge.
I also think I had a heat "microclimate" in my back yard- it has taller than average "fencing" contributing to still air.

It seems to me all the computations being thrown at the problem are correct, but maybe, just maybe, based on the actual temp rise we are missing something. Maybe one more additive, or a variable, or condition thats somehow kicking the actual delivered BTU under very rare situations. - maybe it is all my temp sensors - weird thing is they both return back to normal after being faulty, or maybe its a matter of linearity ....hmmmm...

In all likelihood I'm probably reading 1 degree high, on top of a water thermocline a few degrees cooler at the skimmer than the water below it - still all in all, a very impressive performance thats isn't likely to repeat itself for a long time-

Oh well .....Im enjoying the heat while it lasts before I have to use the heater in conjunction with the solar.

Thanks guys, great reading, and as usual awesome informative links.
We have the best site and guys in the business here on TFP.


Uncle Dave

 

[mas985]

I'm all for science projects and if willing we can probably figure out what is going on but it requires knowledge of a lot of variables. For example, you just stated that it was a still day with zero wind. That has a big effect and normally if the wind is zero on the ground it would be near zero at the panels as well as is typical on really hot days so convection would be minimized but so would evaporation but the later also depends upon dew point. Your 5-7 degree rise is more in line with what I would expect with your setup but it is really hard to envision conditions where it would go much above 10 degrees but I am willing to try.

If you have a date and weather station close to your location, we can figure out if the weather was unusual on that day.

Second, a few questions about your setup:

Filter pressure with and without solar at the RPM that was used during this day?

Do you divert all of the water through the solar?

Azimuth and elevation angles of your solar panels plus elevation? So far I have been assuming ideal pointing.

Pool surface area? I have been assuming 550 sq-ft

Panel surface area? I have been assuming 480 sq-ft


Note that I neglected to include the head loss of the solar panels so the flow rate I posted earlier is much too high. I get about 4 degree input to output delta at about 70 GPM with my panels which I think is close to where you are but I want to make sure with the filter pressures and RPMs.

 

[Davegvg]

Closest Wunderground station would be Cheviot Hills- I looked but cant seem to go back in time.

Temps were 90 in the shade (wall thermometer) and 106 in the sun according to the jandy ambient air sensor.

Filter pressure with solar on is 28LB @ 3100 RPM- ill have to run the same RPM with it off and tell you what that is.

Yes the three way valve diverts 100% through the solar on "on".

Azimuth and elevation angles of your solar panels plus elevation? So far I have been assuming ideal pointing.

Good question Ill need to measure, its likely close to ideal (based on three vendors from solar companies telling me so)

Pool surface area? I have been assuming 550 sq-ft - Close enough

Panel surface area? I have been assuming 480 sq-ft Close enough (I have black inlet and exit pipes as well as black panel connectors so that may add a foot or two.

Thanks

Uncle Dave

 

[mas985]

28 PSI? That is very high pressure and head loss. The JEP at 3100 RPM and 68' of head would produce only about 35 GPM. So the flow rate is much lower then what I had anticipated. Do you have smallish plumbing? Are you sure the filter gauge is working properly?

If you were getting 313k BTU per hour at 35 GPM, the temperature rise from panel input to output would need to be about 18 degrees delta and not 4. With only 4 degree delta and 35 GPM, panel output would only be 70k BTU/Hr. So something still doesn't make sense. Nothing seems to be consistent.

Also, Cheviot Hills is actually not a station. It is simply a marker for the middle of LA. Weather station markers have the temperature in the center. Once you pick a station, you can then look up the history. For example, here is a station output that you can set the date and get all of the data. Important parameters are max, min, ave, temperatures and average dew point temperature as well as wind speeds.