Solar Collectors vs Solar Panels

[Yinn]

I've got 10kW of grid tied solar electric powering the house. Roughly 30,000 BTUs of solar thermal vacuum tube collectors pre-heating my domestic hot water. I'm now looking to add heat to my pool and of course looking to do it via solar but came to a dead stop in my assumptions.

I was assuming to duplicate my existing system, utilize solar thermal vacuum tube collectors, run it through a heat exchanger and heat the pool that way. But looking through the boards here, many people are using solar panels driven directly off their pool pumps.

From an initial overhead, it does seem to be cheaper. A titanium heat exchanger alone is $400. Another $200 for a water pump, and then the cost of the panels themselves are more expensive ($800/ea). But the setup would be much more robust, resistant to bursting, more resistant to weather, and potentially alleviates stress on the pool pump itself.

That's about all I came up with, are there any other reasons? It's hard to find info on btu/sq ft on the pool specific solar panels, but that's another area that I can think of where space may be a concern.

Specific to me and what I'm looking to do:
We've been pretty happy with temps for our use during July and August. It takes about a month and a half to get up to temp on it's own, so we're looking to help bring it up to temp a bit faster - target is about one to two weeks. The intention is to buy commercial parts, but do a DIY install this year.

36,000 gallons
Pool is open 4-6 months
Avg. Startup Air Temp: ~65F High / 40F Low
Avg. Startup Water Temp: ~50F
Avg. Peak Air Temp: ~88F High / 65 Low
Avg. Peak Water Temp: ~80F
Avg. Closure Air Temp: ~70F High / 55F Low
Avg. Closure Water Temp: ~68F
Desired water temp: 80F

 

[RDspaguy]

You pretty well summed it up.
Do you have a surplus on your hot water system? You might add a bit to it and tie your pool in via heat exchanger. Keep costs down a bit and get the benefits of the heat exchanger. I saw alot of those in Boulder.

 

[Yinn]

You pretty well summed it up.
Do you have a surplus on your hot water system? You might add a bit to it and tie your pool in via heat exchanger. Keep costs down a bit and get the benefits of the heat exchanger. I saw alot of those in Boulder.

Right now I'm dumping any surplus into space heating via a buffer tank and water coil in the air handler, but it isn't much left.

That's actually what got me started thinking about heating the pool. I have a leftover taco controller, pumps, and zone valves from an old removed boiler. My initial thought was to purchase 5-6x vacuum collectors to throw on my pool shed that are estimated to produce 140k-168k btu/day. Run PEX to a heat exchanger to the pool and zone off a diverter to the buffer tank in the house. I figured in the months I'm not using it to heat the pool, I can use it to heat the house and save some space heating costs. The setup would run roughly $5,000 (excluding labor).

But it looks like something like the vortex system could be setup for roughly $1200 if I install it myself. That's a big enough difference that I may just consider throwing more solar electric up to cover any electrical heating costs in the winter. It wouldn't be as efficient but it seems more cost effective.

 

[RDspaguy]

Cost effective, yes. In my opinion, the heat exchanger will last longer and probably work better with fewer headaches. But that is a big $ to save a few headaches.

 

[Yinn]

Cost effective, yes. In my opinion, the heat exchanger will last longer and probably work better with fewer headaches. But that is a big $ to save a few headaches.

Today I was able to locate through a lot of 10 new flat plate collectors from a bankruptcy auction. They're not as efficient (~20k btu/day each) but they were $125/panel which is absurdly cheap for a glazed parabolic flat plate collector. At that price I had to jump on it.

I also picked up a 300k btu titanium heat exchanger ($450) all I need now is a mounting rack and a heat dump to avoid potential high stagnation temps on fringe days. I might run it for a week or two and if it does stagnate, install an outdoor water tank for a warm water outdoor shower.

 

[RDspaguy]

Good for you! You have the set-up I dream about. And the know-how to make it happen, from all appearances. Post some pics when you are done.

 

[mas985]

Hot water panels are designed for high temperatures and low flow rates while pool panels are designed for high flow rates at lower temperatures. If you compare the cost of each per sqft, the pool panels are likely to be cheaper and add more heat to the pool.

 

[Yinn]

Good for you! You have the set-up I dream about. And the know-how to make it happen, from all appearances. Post some pics when you are done.

I work slow especially since I'm still very much an amateur when it comes to piecing together solar thermal.

It will initially be a ground mount system. I have to build a shed, and the roof of the shed is where these panels will ultimately go. But that shed is probably a next year project. I wasn’t planning to do the solar first, but the deal on the panels pushed me into it.

Hot water panels are designed for high temperatures and low flow rates while pool panels are designed for high flow rates at lower temperatures. If you compare the cost of each per sqft, the pool panels are likely to be cheaper and add more heat to the pool.

I know this isn't a solar forums, but I'm going to geek out a bit anyway. I asked the question and then did a bunch of googling to answer my own questions. But in a nutshell, you're absolutely right on a general sense.

I didn't link this but I was able to find some government studies (example: https://www.milwaukee.gov/ImageLibrary/Groups/cityMilShines/Documents/CollectorGuide.pdf) and it shows that up to ~10F delta, an unglazed solar pool heater is actually more efficient than a flat plate or evacuated tube panel. But it declines rapidly and by a ~20F delta the hot water collectors becomes more efficient. The pool collectors are pretty much useless at a 30F delta. So depending on where one lives, the amount of sun obtained, this could change the economics quite a bit. For me, the ambient air temp on my extended seasons are going to be 20F lower than my desired pool temperature so I will need the higher deltas. My panels are designed for a max temp of 212F, much higher than pool collectors which probably goes in line with being able to achieve better performance with the higher deltas. But since it's an indirect heat exchanger system, I can turn the flow rates up on the solar collectors and run them as low as 95F which seems to be in line with what most pool collectors run and will be using a two stage pump to make sure I get the flow rates for extended/high delta months and peak/low delta maintenance months.

For comparison, I obtained a Vortex solar pool heater quote. They recommended 15 (!!!) 4x12 panels for my pool. At $250/each that's $3750 for panels alone to accommodate those extended months. I was shocked as that would have been 650,000 btus, but the drop performance drop off would explain why so many are needed. It also presented a problem because I don't have the recommended 13' high and 63' wide to lay these out. In comparison, I was able to buy 10 solar collectors at $125/each for $1250 that requires 8' high and 20' wide. My calculations are still a bit off so it's going to be a bit of trial an error. The ratings on the pool collector BTU are higher (~958btu/sq ft) than the hot water collectors (~833btu/sq ft) but I think the operating and temperature range is larger. Even on a fully cloudy day the hot water collectors will generate 250btu/sq ft)

Now I could be completely wrong here and actually need the full 650,000 btus; which would mean I need an additional 22 panels for an additional cost of $2750; which brings it pretty much in line with the cost of a pool solar heater in terms of cost and btu generation and size. But I'm impulsive and decided to give it a try....

I may sound confident, but I'm not. This is how I'm thinking right now, but I recognize I could be completely off base. I'd welcome any input and corrections. Thanks!

 

[mas985]

Yes, I agree that with colder ambient temperatures, glazed panels will be more efficient but you will lose efficiency during the summer months. I guess it just depends on your priorities.

 

[RDspaguy]

He said he already bought 10 at $125 each. Also has a heat exchanger, I think. Jumped on a deal. He already has a system for in home hot water, and is expanding and tying in to that.

 

[mas985]

But again, hot water panels are not the same as glazed pool panels. The later allow for higher flow rates. Hot water panels are very low flow rates and will not add much heat to the pool during the period of time the sun is shinning on the panels. Heat transfer is related to dT * GPM.

 

[Yinn]

Yes, I agree that with colder ambient temperatures, glazed panels will be more efficient but you will lose efficiency during the summer months. I guess it just depends on your priorities.

Indeed, but I don’t need much heating at peak. We’re pretty comfortable with the pool temps in July/Aug. it’s startup that’s taking a long time (1.5mo) that I’m looking to shorten.

But again, hot water panels are not the same as glazed pool panels. The later allow for higher flow rates. Hot water panels are very low flow rates and will not add much heat to the pool during the period of time the sun is shinning on the panels. Heat transfer is related to dT * GPM.

Can you clarify?

I’m going to be running the pool water through 2” heat exchanger at 80GPM. I picked a 300k BTU heat exchanger explicitly due to the desire for a 2” throughput to maintain flow and pressure. The expected pressure drop is about 6GPM so I should (this is all theoretical) still be outputting 74GPM to maintain circulation. So that is considered my pool loop side of the heat exchanger

My hot water panels are going to run on a separate dedicated loop. This will run at ~0.75-1.25GPM per panel for a maximum flow rate of 12.5GPM across all panels. Significantly less than the pool water, and varying based on what temp I’m trying to achieve.

Since I’m not going to be running my pool water directly through the panels like a pool system would, does the flow rate matter as much?

 

[mas985]

Flow rate can have an impact on efficiency. Halve the flow rate in the panel, and the water temperature change from inlet to outlet will double. Hotter water results in a larger temperature difference between the air and the water decreasing efficiency. Even though glazed panels are insulated, they still lose quite a bit of heat to the environment. So does the heat exchanger.

Have you gone through the heat transfer calculations to estimate how much heat will be added to the pool each day and how much of that heat will be lost by the pool? Even with a pool cover, much of the excess heat generated by the panels is lost each day when the weather is cooler.

 

[Yinn]

Flow rate can have an impact on efficiency. Halve the flow rate in the panel, and the water temperature change from inlet to outlet will double. Hotter water results in a larger temperature difference between the air and the water decreasing efficiency. Even though glazed panels are insulated, they still lose quite a bit of heat to the environment. So does the heat exchanger.

Have you gone through the heat transfer calculations to estimate how much heat will be added to the pool each day and how much of that heat will be lost by the pool? Even with a pool cover, much of the excess heat generated by the panels is lost each day when the weather is cooler.

I’m going to need a degree in thermodynamics..

I ran the btu numbers for what my pool needs, but it didn’t seem right. It was telling me I needed a heat source to produce ~450k BTU to heat and about 100k btu to maintain during peak (Rough numbers off the top of my head) But my pool currently does not ha e a heat source and does not lose temp during the day. Overnight losses haven’t been noticeable, by the time 10AM rolls around the temp is the same as 4PM the previous day.

I also used a heat transfer calculator I found online (Heat Calculator) for a liquid to liquid heat exchange. As an example, with the pool loop going at 74GPM with 50F incoming water temp at open. It looks like if I use a 7.5GPM flow on the solar loop at 120F; I can get a 2F/hr increase on the pool at a 74K btu/hr transfer. My current panels get 4.5hrs of full sun and an additional 4-5 hours of partial. On the lower end that would be a gain of 9F/day.

Does all that seem right to you?

 

[mas985]

But my pool currently does not ha e a heat source and does not lose temp during the day.

All pools lose heat during the day but they also gain heat during the day from the sun. So your net heat gain/loss may be 0.

I also used a heat transfer calculator I found online (Heat Calculator) for a liquid to liquid heat exchange.

They don't take into account heat loss through the exterior of the heat exchanger which could be significant if the temperature difference is large. Does the exchanger have insulation? Do they publish efficiency numbers based on water-air temperature differences?

This is the proper way to do heat transfer calculations, but much more complicated: Overall Heat Transfer Coefficient

As an example, with the pool loop going at 74GPM with 50F incoming water temp at open. It looks like if I use a 7.5GPM flow on the solar loop at 120F; I can get a 2F/hr increase on the pool at a 74K btu/hr transfer.

Heat gain is the highest when temperatures are the lowest. What you really need to look at is what it takes to maintain the 80F target, If the panels can add the heat that is lost everyday, then they can maintain pool temp. The issue is that the difference between the water and the air/environment is at it's maximum and heat loss is at it's greatest. So going from 50F to 60F is much easier than going from 75F to 80F.

A few questions:

What is size of the panel (LxW) and/or make/model?

What is the efficiency of the heat exchanger and/or make/model?

My current panels get 4.5hrs of full sun and an additional 4-5 hours of partial.

What does partial mean? Are the panels shaded at some point? What is the period of time where the panels get at least 50% of sunlight?

At what angle will the panels be installed (azimuth & elevation)?

Are the panels going to be installed on the roof or the ground? Wind exposure has a large impact on heat gain.

36,000 gallons
Pool is open 4-6 months
Avg. Startup Air Temp: ~65F High / 40F Low
Avg. Startup Water Temp: ~50F
Avg. Peak Air Temp: ~88F High / 65 Low
Avg. Peak Water Temp: ~80F
Avg. Closure Air Temp: ~70F High / 55F Low
Avg. Closure Water Temp: ~68F
Desired water temp: 80F

Does this mean you want 80F water temps at start up? Average air temp is around 50F so a 30F temp difference. About what month/day do you want 80F? It makes a difference to the available heat gain from the sun.

 

[Yinn]

What is size of the panel (LxW) and/or make/model?

Solargenix WS0503 (7x3.5)
http://www.domanibuilders.com/documents/Solar Energy/Solargenix Energy.pdf

What is the efficiency of the heat exchanger and/or make/model?

SP300Kti. 300k BTU. There's no efficiency numbers on it but the specs are just like this one on Amazon.

Amazon.com

www.amazon.com

What does partial mean? Are the panels shaded at some point? What is the period of time where the panels get at least 50% of sunlight?

Panels start receiving sun at 7am. Half of my panels are getting full sun (no shading) by 9am. All panels are receiving full sun by 11am until about 3:30pm where they will start getting shade. By 4:30pm half of them will be shaded and half will still be getting sun. By 5:30pm, most will be in shade.

At what angle will the panels be installed (azimuth & elevation)?
Are the panels going to be installed on the roof or the ground? Wind exposure has a large impact on heat gain.

When ground mounted (temporary) they will be 180S at ground level (110ft above sea lvl)
When roof mounted (~1 year from now) they will be ~160S and 10ft above ground (120ft above sea)
Ave windspeed in my area annually is ~13mph

Does this mean you want 80F water temps at start up? Average air temp is around 50F so a 30F temp difference. About what month/day do you want 80F? It makes a difference to the available heat gain from the sun.

No, I want by peak usage. For ease of calculation, I start the pool up on April 15. The water temp is 50F at startup. I'd like the water temp to be a minimum of 70F by May 1st and 80F by June 1st. If it helps, my pool gets full unshaded sun starting around 8:30am until about 4pm.

 

[mas985]

When ground mounted (temporary) they will be 180S at ground level (110ft above sea lvl)

By elevation, I meant the elevation angle that they will be pointed at. Are they pointed straight up or angled towards the sun?

Also, do you know the square footage of your pool? Rough estimate I got is about 1050 sqft for an average depth of 4.6'.

But just doing a back of envelope calculation. The manufacture claims a 20700 BTU/day output (95F panel temp) per panel with an incident solar radiation of 1600 BTU/day/sq-ft. This about 57% efficiency which is fairly low.

10 panels will produce 207000 BTU per day which will raise the water temp at most 0.7F neglecting heat exchanger efficiency. Not much of a heat gain to make an impact. Note that the panel area is only about 22% of the pool area. Normal coverage is about 50%-100%.

Note too that with just a solar cover, you can raise pool temps as much as 10F.

 

[Yinn]

By elevation, I meant the elevation angle that they will be pointed at. Are they pointed straight up or angled towards the sun?

They’re going to be mounted on a 12 pitch/ 45 degree angle.

Also, do you know the square footage of your pool? Rough estimate I got is about 1050 sqft for an average depth of 4.6'.

960 sq ft, 4.5 avg depth

But just doing a back of envelope calculation. The manufacture claims a 20700 BTU/day output (95F panel temp) per panel with an incident solar radiation of 1600 BTU/day/sq-ft. This about 57% efficiency which is fairly low.

Keep in mind that I’m going to be using this for domestic hot water and space heating in the winter when the pool is closed. Less efficient collectors are better at snow melting; which actually results in more total generation. What I’m lacking in peak power, I’ll be making up in run time - so to speak. This is important too because it will save me on electrical usage/running costs here.

10 panels will produce 207000 BTU per day which will raise the water temp at most 0.7F neglecting heat exchanger efficiency. Not much of a heat gain to make an impact. Note that the panel area is only about 22% of the pool area. Normal coverage is about 50%-100%.

Is that in addition? So right now it takes 45 days for the pool to heat up from 50 to about 70. Another 30 days to get to 80 or about .4F/day. That’s using my solar pool cover when the pool is not in use. In fairness, my pool is used everyday from around 10am to 4pm so the cover is only loss prevention at night.

So if I take the 0.4 currently sans heater plus your calc of 0.7, that would be 1.1F/day. In 15 days time I’d be able to get from 50 to 66.5 (goal was 70) and be able to hit 80 by mid May. Is that a correct assumption?

 

[mas985]

960 sq ft, 4.5 avg depth

960 * 4.5 = 4329 cut-ft = 32314 gallons. Your signature says 36000 gallons. Which is correct?

s that in addition? So right now it takes 45 days for the pool to heat up from 50 to about 70. Another 30 days to get to 80 or about .4F/day. That’s using my solar pool cover when the pool is not in use. In fairness, my pool is used everyday from around 10am to 4pm so the cover is only loss prevention at night.

No, heat gain is not cumulative because you have to contend with heat loss within the pool. As the pool temp rises, heat loss also increases (i.e. evaporation, radiation, convection & conduction) so each day a portion of that heat gain is lost and the higher the pool temp, the larger the loss. Especially without a solar cover. A pool is constantly losing heat and only the sun on the pool and the sun on the panels adds heat (Tair > Twater can also increase temps). At equilibrium, the net heat gain/loss is always 0 (i.e. total heat gain = total heat loss) at this point the remaining BTUs in the pool water determines the water temperature.

So for example, using your location, I assumed Newark, and a 6/1 date, here are some averages for your location:
High: 77F
Low: 59F
Ave: 68F
Ave Wind: 13 mph
Wind Break: None
Clouds: None

Assuming the above conditions are constant over a week. The resulting pool temps are the following:

Average daily pool temp w/o cover & w/o solar: 69F - baseline
Average daily pool temp w/o cover & with solar: 70F
Average daily pool temp with cover & w/o solar: 83F
Average daily pool temp with cover & with solar: 87F

As air temperatures rise, these numbers will also increase because heat loss is decreasing with increasing air temps.

 

[Yinn]

960 * 4.5 = 4329 cut-ft = 32314 gallons. Your signature says 36000 gallons. Which is correct?

My pool builder has 36,000 gallons listed on my build sheet, that's what I put in my signature. I used an online calculator at one point that gave me the 4.5ft avg depth at one point. It's a 25x40 ft rectangular pool. 7x20 of it is a tanning ledge that's 1ft deep. There's then 3 steps down that goes across the pool that brings it down to 3 feet and from there it gradually goes down to 9 feet. At the deep end, it's only 17ft wide as there's ledges on the sides of the pool.

No, heat gain is not cumulative because you have to contend with heat loss within the pool. As the pool temp rises, heat loss also increases (i.e. evaporation, radiation, convection & conduction) so each day a portion of that heat gain is lost and the higher the pool temp, the larger the loss. Especially without a solar cover. A pool is constantly losing heat and only the sun on the pool and the sun on the panels adds heat (Tair > Twater can also increase temps). At equilibrium, the net heat gain/loss is always 0 (i.e. total heat gain = total heat loss) at this point the remaining BTUs in the pool water determines the water temperature.

So for example, using your location, I assumed Newark, and a 6/1 date, here are some averages for your location:
High: 77F
Low: 59F
Ave: 68F
Ave Wind: 13 mph
Wind Break: None
Clouds: None

Assuming the above conditions are constant over a week. The resulting pool temps are the following:

Average daily pool temp w/o cover & w/o solar: 69F - baseline
Average daily pool temp w/o cover & with solar: 70F
Average daily pool temp with cover & w/o solar: 83F
Average daily pool temp with cover & with solar: 87F

As air temperatures rise, these numbers will also increase because heat loss is decreasing with increasing air temps.

I have a solar cover and it never reached 83F w/o solar. 80F was the highest it got and that was in late July. It would only be about 68-70 on June 1st. So are you saying then that even if I didn't go Solar and went with a heat pump heater - which maxes out at about 140k btu, that it wouldn't help at all?

I mean I already bought the parts so I can just keep it for space heating only; but its kind of discouraging to find out that I really have no options to get my pool heated. Even if I combined the 140k btu from a heat pump heater and the 200k btu from the solar panels it sounds like it would barely make a dent. Is that right?