Analyzing solar + heat pump performance/configuration

[mas985]

My wife says the solar guys stated that the heliocool panels were not as direction sensitive as most panels, and that we would want to save the good roof space for solar electric later.

The heat gain for any solar panel is governed more by the angle of incidence, area of capture, and the panel efficiency. Based on independent efficiency measurements (see the link in my sig), those panels are not much better than the average panel.

Anyway, assuming you have a few more days like yesterday, you should be able to maintain 75 degrees with just solar. But that assumes your cover prevents evaporation and some heat loss. Those covers look to be in the water and if mesh, then they probably don't hold the heat very well. So using just solar but without a cover would be around 69 degrees. BTW, about how much heat do you lose overnight? This can tell you how well your cover is holding in the heat.

 

[MattM]

Yesterday afternoon, at around 5pm when we turned off heat for the day - the water temp had just reached 72 degrees. We put on the cover and opened it up at 9am this morning before heating started and the water temp was 69 degrees. Most days, I think we lose 2 degrees at night..we might be at 2.5-3 degrees as the water gets warmer. Pool was uncovered all this morning while the dolphin vacuum ran - heat pump and solar turned on at 9:15am and around 1pm we reached 73 degrees - I expect we'll just barely touch 75 degrees around 5pm. I'm guessing if the cover had been on all morning, the temp might have gotten at least 1 degree higher.

 

[mas985]

Without a cover, I had estimated about 2.7 degrees of temperature loss over night so that is pretty close to what you are seeing. My guess is that your cover is not holding the heat much. What is the cover made of? Mesh, plastic, insulated, etc. Have you tried leaving the cover on during the day?

 

[Brentr]

MattM, I really think that you need a bubble cover to help keep down the heat loss both during the day and at night. I think that your safety cover does very little if any at all in keeping the heat in the pool. If you look at my data in my sig, you will see that I did not get near as much gain during the day with the bubble cover off and I lost quite a bit of heat during the night. I really think you should get one and experiment a bit. Hope this helps

 

[JamesW]

The automatic safety covers are solid vinyl and hold the heat in fairly well.

 

[MattM]

James is correct - it's vinyl and I do notice that it protects against heat loss due to wind quite well during the day - however, the standard temp loss at night is roughly 2 degrees but that is based on maintaining a water temp of ~65 the last few weeks with daily highs in the mid 60's and nightly lows between 40-45, occasionally have days in the upper 60's/lower 70's -- this week we got lucky and are seeing low's in the upper 40's and highs in the upper 70's. We swam in the pool for 1hr this afternoon with cover off and managed to touch 75 degrees water temp - but that was with heat pump + solar.

I think the only real option I have to improve anything would be changing the direction of the panels or adding up to 100 more sq ft to bring the total to 650 sqft. If the solar, on it's own, can maintain ~75 degrees from Feb-April, 84 from May - August, and 74 again from Sept-Nov we'll be happy enough.

Cover is on whenever we're not swimming, vacuuming, or performing other maintenance.

 

[mas985]

but that was with heat pump + solar

Given the water temperature, the panel temperature should be higher than that for most of your run time so I was wondering how much the heat pump is actually running?


Also, moving the panels from west to south should get you about 5 degrees more. But one interesting thing I noticed is that there is not much of a difference if you cover the pool during the day vs not covering it. The heat gain by the pool from the sun is offset by the heat loss without the cover.

[EDIT] Moving the panels to the south and using the cover 24/7, could get you as much as 10 degrees more. The higher the water temperature, the more heat that can be lost during the daytime when the cover is removed.

 

[MattM]

While both solar and heat pump are running in the middle of a warm day, what I'm noticing is that water leaving the solar panels and entering the heat pump is 3-5 degrees warmer than the pool water, and that when the heat pump is on - the temp is increased another 1-2 degrees within return pipes (so, heat pump seems to be contributing between 16-40% of heat being added to the pool depending on time of day, temperature, humidity, and how cloudy it is.)

 

[chem geek]

I have an electric automatic safety cover (open[/ur] and [url=https://www.troublefreepool.com/~richardfalk/pool/PoolClosed.jpg]Closed) and I estimate that it reduces the heat loss to around 50% compared with no cover, mostly by virtually eliminating evaporation, not so much by reducing conduction since the cover "feels" warm when the pool is heated. A bubble-type cover probably cuts the heat loss in half again so 75% compared to no cover.

When our pool is heated to 88ºF we lose around 2.5-3ºF overnight during summer nights that get to around 54ºF while the end of the season using gas heating has losses of around 4-5ºF when nights get down to around 40-45ºF. You can see our gas heating costs in the table in this post but note that our nights are colder than where you are and the pool is larger at 16,000 gallons. Also, solar is pretty marginal since the days aren't very warm (mid-60's) so solar efficiency is around 45% (see this post) if trying to keep the water very warm and the sun is lower with more cloudy days so less than half the solar energy compared to July (see the solar insolation chart). If you are only trying to heat the pool into the 70's, then that's not so hard -- it's getting it into the mid-to-high 80's that is tough except in summer.

 

[MattM]

Update: Brought out techs from solar company hot sun industries to analyze our setup and generate recommendations. They said the water GPM isn't that critical, solar heating efficiency is mostly important at the begining and end of season when one isn't getting that much sunlight and then panels don't get that hot and putting high amounts of water through the pipes can be counterproductive. I'm hoping to get back a proposal on what panels they can add or move later on, but apparently our roof isn't as big as I thought so there isn't a huge amount of free space left.

Also installed flow meter, digital, which is supposed to have 2.5% accuracy. Pool builder actually put in a a long straight pipe near the main equipment so that we would have as an accurate a reading as possible. Still, not sure if I should believe what it says.... states 38GPM even though RPM is at 2500 with solar on and we have relatively big pipes and lazy turns where possible. Current solar panel optimum is supposed to be 55gpm but if 65gpm/rpm, that would require pump running at 3575 rpm. ouch. Will monitor.

 

[ping]

I live about an hour north of you and have had similar weather as you and decided to turn on my solar a couple of days after you did. My panels are 90% square footage of my pool and are due west. I have a bubble cover on 24/7 for the most part. My water temp started at 63 degrees and as of last night the water is now 78 degrees. With todays weather I should hit at least 80 degrees tonight.

My pump starts at 11:00AM and shuts off at 4:00PM with the pump running on high speed. My solar starts running right away but usually shuts off around 3:00PM. I do notice if I run the pump on low speed the water temp coming out of the panels is at least 2 degrees higher than if I was running on high speed. I have not done enough tests to see if I get more heat generated by using high or low speed. Just thought I would let you know how my solar system is working in similar weather conditions.

 

[chem geek]

Lower speed will provide somewhat less heating. It comes out at a higher temperature, but less volume of water per time. The total heat would be the same except for the fact that with the panel at a higher temperature, it is losing more heat to the environment which makes it less efficient. For the FAFCO panels that I have, the efficiency drops from around 80% to 70% going from a flow rate of 4 GPM to 2 GPM (for one panel) then down to 60% at 1 GPM. Though flow rate does make a difference, the efficiency benefit is fairly small especially compared with the huge rise in energy costs required at higher flow rates since they rise faster than the GPM so even running different times to still get to one turnover, twice the GPM for half the time uses more than 50% more energy.

 

[carlscan26]

Matt,

I think you may be expecting too much from the solar this time of year...you can get that long swim season but you need to run the HP - a lot.

I'm at high 60s with just a solar cover. I live further inland than you but our temps are roughly the same - I have less costal layer than you do but this time of year it's not that different. I had my solar on during that hot spell and I was in the mid 70s at peak and then down as much as 6 degrees by morning. I do get a fair amount of wind though; my yard is more open than yours.

In the summer my solar barely ran with the cover on. We actually had to set the temp lower than we wanted because the passive heating with the cover would overshoot our desired temp by as much as 5-6 degrees. it got to 93 when I set the solar at 88! We actually took the cover off for July-mid Septemeber because even with the solar not running the pool was getting too hot. Even with the cover off the solar only ran for a few hours and then we weren't overshooting.

It depends on what you call swim-able water. 84 degrees is easy to do with solar alone late spring through early fall. I can do mid 70s right now but it's not worth it to me because I don't like to swim in it that cold when the ambient is mid 60s. 75+ should be doable year round if you are willing to run the HP and you may be able to get nearer to 84 with a second cover

I should clarify that I have almost 110% coverage. It's in a East-South-West setup and I have to limit the hours to 9:30-3:30 this time of year when sun is on all of the panels.

 

[ping]

I believe I have reached the peak water temp for my pool until we get more 80 degree days and warmer nights. My pool temp actually dropped one degree to 77 from the night before and yesterday was sunny and 75 degrees. Like carlscan26 said it is pushing it to get much more than this at this time of year.

 

[MattM]

Well, we were roughly 74 last weekend with heat pump + solar running together most of each day for a few days. We did some final construction last week which brough solar offline for a bit. Now, we're running pump from 8:30am - 6:30pm with heat enabled in solar preferred mode only from 9am-6pm which generally means that solar runs 7hrs and heat pump 2hrs with air temps between 62-72 during the day and in the 50's at night. Highest water temp today was ~70 degrees and at 7:30pm we're already back down to 69. I suspect water will reach 67 degrees by the time pump turns on tomorrow morning.

Hot sun industries has clarified that our panels are broken into three sections and one section is in such an angle/position and exposed to wind such that it is likely to be doing nothing to heat the water and may even be cooling (they are offering to prove this by putting temp and flow sensors on that section of panels). Furthermore, apparently all our panels were plumbed in serial rather than parallel. And, all our panels are especially sensitive to wind while our house is just a few miles from the coast and gets regular afternoon winds? No idea if all that is the case but that is what I've heard so far in their eval. On the other hand, it appears to have been connected to the roof properly with otherwise good plumbing/etc. Not sure yet how we'll respond but the hot2sun staff seem to be very professional and if they can collect data on our existing system to justify the conclusions, than we'll go with whatever changes they recommend which might include switching to different panels or at the very minimum plumb the existing panels to be in parallel and relocate/put in a valve to control whether water goes to the worst.

 

[carlscan26]

I misspoke when I said we hit high 60s with just the solar cover - it's low to mid 60s. Today we peaked at 66.

Wind can definitely be a factor but that should be true for any install. My understanding is that the thermometer placed on the roof should near the panels and located such that it is equally affected by wind and sun expososure that the panels will have.

Will they do that analysis for free? I'd still try to get some compensation or remediation from the solar guys or your pool builder if he subbed it to them.

 

[MattM]

Update -- according to hot sun industries, the roof plumbing is so bad that only 4 of the 15 panels will definitely be generating heat and that of all panels about 40% of them are on the direct north face and will generate essentially no heat even if plumbed correctly during early and late season when we need it the most.

Anyhow, here is a video of what our screenlogic interface is reporting for the solar heating (and heat pump too) - likely need to view full screen on reasonable sized monitor:
[youtube:kv4dlfh7]T15wyH66AfI[/youtube:kv4dlfh7]

 

[MattM]

diagrams below - first is how hot sun industries thinks the system is setup with plumbing and panels now with working panels marked with arrows and significant problems highlighted. Blue is used to mark incoming water and red returning water.

 

[MattM]

Initial thoughts for a redesign (keeping north panels as is because its not worth the effort to remove them from roof) but perhaps putting in a valve so they are only used mid summer - main focus is redoing the plumbing and adding some panels in the southwest corner to make up for lack of power from north:

 

[carlscan26]

Matt,

Do you have a program to run solar from 6:30pm - 7:30 pm or so? I'd turn that off as you won't get any heat that time of day. Also, I don't think you're running your HP enough to get any benefit from it - the data would appear to back that up too.

Depending on when you have sun, I'd run solar from 9:30ish to 3:30ish. At least that's the peak heat period for me give or take 30 mins, maybe a tad longer now that we're in Feb. I'd run HP before and after while ambient temps are still supportive of the HP.

Also, I'm not aware of any solar panels of this style that are not sensitive to wind; you have to go to closed cell designs to not have those issues; even the ones from hot2sun will be affected. That's why thermometer placement is important; you want it exposed to match the prevailing conditions at the panels.

Lastly, it appears most of the panels are over a crest in your roof vis-a-vis the piping from the pad - is that correct? If so, they also need to install a drain system on that far side so you can drain the panels should you be concerned about freezing etc. Preferably close enough to run a hose back to the pool to recover the water.