VRV on solar, adjustable?

[Dave95602]

Our variable speed pump needs to run a at minimum of 2300 rpm in order to keep the roof top vacuum release valve on the solar system from opening and allowing air into the line.

I would like to be able to balance flow rate and energy use (ideally lower) while using solar but I'm up against this min rpm. Are VRVs adjustable? Can they be closed off completely during the warm season keeping the system filled?

Thanks, Dave

 

[ajw22]

I don’t believe so. @mas985 @Dirk can comment.

You need your VRV working to allow the panels to drain when solar heat is off.

 

[Dave95602]

Thanks Allen,

Another benefit of keeping the solar system filled, would be more consistent levels in our trough. Our pool pours into a trough (no skimmers) so the pool level is always the same, but the trough depth changes as the solar system fills/drains.

 

[mas985]

I would like to be able to balance flow rate and energy use (ideally lower) while using solar but I'm up against this min rpm. Are VRVs adjustable? Can they be closed off completely during the warm season keeping the system filled?

No but if you move the VRV closer to the pump, pressure rises so it allows you to drop the RPM. Most installers put them on panel outlets but you can go with lower RPM when plumbed near the panel inlets.

Also, if you really want to lower RPM by a lot, then you can even plumb the VRV on the supply pipe about 6 feet above the pump.

 

[JamesW]

You can also restrict the return some to create some pressure on the VRV (About 1 psi).

 

[Dirk]

James and Mark describe ways to accomplish what you want, but that might not actually be the best for efficiency, in terms of both heating effectiveness and energy costs.

Solar panels have an optimal flow rate. The manufacturer of the panels should be able to guide you about that (my rate was spec'd in the installation manual). This optimal flow rate is what is required to heat the pool most energy efficiently, and so most cost effectively. Lowering the flow below the optimal rate will save you some energy cost, of course, and you'll lose heating efficiency, too. And it won't likely be a direct ratio. You will probably save more by running the panels at their optimum flow rate, and then just turning them off sooner.

This effect is just the physics of heating a pool at work, and unfortunately it's not obvious. Made more confusing because as you lower the flow rate (the RPMs) the water coming out of the returns will actually feel warmer. And this fools a lot of people into thinking that their pool will get warmer, too. But that's not how it works. You want the water out of the returns to be just very slightly warmer than the pool, but you want to pump as much water as you can afford.

It's counterintuitive. You'll get your pool warmer by moving a lot of just slightly warmer water rather than moving less hotter water.

Save on your electric bill by decreasing your runtime, not by lowering your flow rate.

Flow rate will vary from pool to pool for a given RPM, but for reference, I run 2210 RPM. So we're pretty close. I optimized my panels' flow rate using a flow meter (FlowVis). So I know my panels are getting the exact flow rate specified by my panels' manufacturer (40GPM). There are other methods to optimize flow without a flow meter, but I don't know them and didn't want to try and figure them out. I installed the FlowVis to eliminate the guesswork, because I wanted to eek out the most heat I could from my system.

Full disclosure, I don't have to think about energy efficiency, because I have PV solar panels on my roof, next to the solar heater panels. They paid for themselves in just a few years, and I now heat my pool for free. Point being: if you really want to get the warmest pool for the least cost, invest in PV solar.

 

[mas985]

Here is a typical efficiency chart for solar panels:

Solar Pool Heater: Best Flow Rate

Learn about solar panel flow rate vs. efficiency. The answer may surprise you! How much flow do solar panels need to heat a pool?

floridasolardesigngroup.com

Most manufactures use a similar chart in their documentation. I have also replicated this chart using my heat transfer spreadsheets so I believe they are fairly representative although they do represent specific conditions (e.g. inlet temperature and air temperature). There is also a similar chart in this wiki:

Solar Pool Heaters - Further Reading

www.troublefreepool.com

The target flow rate for these panels is 4 GPM/panel or around 0.1 GPM/sqft which is generally what is recommended by manufacturers. You will note that at 4 GPM, the efficiency is 80%. At HALF that flow rate, 2 GPM/panel, the efficiency is around 70% or about a 15% reduction in heating efficiency. So to get the same heat, you would need to run the pump 15% longer, if possible. However, the decrease in energy use is about 85% for a 50% reduction in RPM. So even adjusting for the longer run time, 1.15x, the savings in energy are still about 75% so well worth the effort. Especially if you are paying as much as I do in electricity ($0.40/kwh).

However, when panels are installed on the roof, in most cases, the flow rate needs to be well above 0.1 GPM/sqft in order to keep the VRV closed so more than likely, you can achieve a reduction in flow rate without sacrificing that much of a reduction in heat transfer.

 

[Dirk]

In case this jumped out at you...

FYI: Mark mentions a typical flow rate of "4 GPM/panel," where I stated 40 GPM. You multiply the manufacturer's flow rate recommendation by the number of panels. In my case, Heliocol called for a flow rate of 5 GPM/panel, times my eight panels, equals 40 GPM.

So I observed my FlowVis while I adjusted my VS pump's RPM to get my 2210 RPM. I monitor the FlowVis throughout swim season, and sometimes adjust the RPMs to account for the decrease in flow my system can experience as my filter clogs up.

I installed my VRV on the solar pipe leading to the roof, on the side of the house, not on the roof like some installers do. Mine is just under the eve. At the time, I didn't know to do that to help with the closing of the VRV, I put it under the eve to keep it out of the sun. I had read that they only need to be a ways above the pump, but they don't have to be all the way up on the roof, or attached directly to the array. I had also read that they are one of the parts of a system that can fail. I figured blaring sun all day could only speed up its demise. Plus, if I do have to swap mine out, I can do so from a 6' ladder, I don't need to crawl up on the roof, risking either my roof tiles or my neck.

Mine was threaded and screwed into a tee with a threaded port:




The only downside to this location is the sound it makes as it does its thing. Mine sounds a little like a croaking frog, and can be easily heard while we're in the yard (I can even hear it some in my house). Not bad, but not ideal.

It gulps air. I don't know if that would have been better or worse if it had been at the highest point of the system. Maybe @mas985 would know.

 

[mas985]

It would probably be quieter at the highest point. The issue is more than likely some of the water above the VRV (most gets sucked upward toward the return) traveling past the VRV while air is getting sucked in at the same time which is causing periodic closure of the VRV. This does not happen when plumbed at the highest point. But then you don't get the benefit of higher pressure and lower RPM either.

 

[Dave95602]

Thanks James, Dirk and Mark for the great info and giving me some options. Maintaining the optimum flow rate and decreasing the run time makes sense. As does backing off from that flow rate somewhat on the flatter part of the efficiency curve.

Sounds like moving the VRV down lower on the supply side is easy and worth a try. The vertical runs are on the back side of the detached garage so the noise will not be a problem, and we already have lots of frogs around this time of year…

We’ve got ~200 sqft of Helicol panels that want about 20 gpm flow. I’m pretty sure we can achieve that flow at lower rpm once the VRV is moved lower. I’ve got some other plumbing changes in the queue and will install a FlowVis to help dial in the flow rate.

We too have solar, and currently produce more than we consume (thank goodness for NEM 2.0), but we’re gradually moving more loads from gas to electricity and will eventually use up that margin. Already swapped out the hot water heater (Rheem heat pump) and HVAC (Mitsubishi heat pump) and, much to my wife’s dismay, I’ve been eyeing the gas clothes dryer and stove/oven…

 

[Dirk]

With your need being only 20GPM, I think you could definitely benefit from lowering the RPM after moving the VRV.

It's difficult to measure real-world results from this kind of optimizing. A simple test would be to run it one way for a week, and another way a second week. But the variables make it impossible to get fair data. It would be easy enough to determine the difference in electrical use, and then calculate the difference in cost. The problem is comparing the heating efficiency. I've never been able to figure out how to do that. No matter when you ran the experiment, you'd be hard pressed to come up with a pair of weeks with identical conditions: daytime air temps, night time air temps, cloud cover, etc. Even pool use could affect the data. There's no way to really know: did that tweak make my pool warmer that week? Or was it because the air temps were warmer? Or was it because there was less wind? Or because more people splashing around circulated the water more?

So I used the manufacturer's recommendation, fine tuned my flow rate as best I could, and now leave the rest to Mother Nature.

 

[mas985]

It's difficult to measure real-world results from this kind of optimizing. A simple test would be to run it one way for a week, and another way a second week. But the variables make it impossible to get fair data. It would be easy enough to determine the difference in electrical use, and then calculate the difference in cost. The problem is comparing the heating efficiency. I've never been able to figure out how to do that. No matter when you ran the experiment, you'd be hard pressed to come up with a pair of weeks with identical conditions: daytime air temps, night time air temps, cloud cover, etc. Even pool use could affect the data. There's no way to really know: did that tweak make my pool warmer that week? Or was it because the air temps were warmer? Or was it because there was less wind? Or because more people splashing around circulated the water more?

That is why using analytical models can give you both a fair comparison and fairly accurate results.

But as I posted above, you will always come out ahead with RPM reduction. The loss in heating efficiency is always more than compensated for by an increase in energy efficiency because the energy cost curve is much steeper than the heating efficiency curve so it will always be the case.

 

[Dave95602]

The loss in heating efficiency is always more than compensated for by an increase in energy efficiency because the energy cost curve is much steeper than the heating efficiency curve so it will always be the case.

True, but if Mark is producing more electricity than he is using, the cost curve is flat (or close to flat since NEM payback rates are just pennies per kWh).

 

[mas985]

PV definitely changes the economics so energy use is much less of a concern although I would not say the cost curve is flat. It still costs something to run at higher RPM even if it is less of a payback.

 

[Dirk]

But as I posted above, you will always come out ahead with RPM reduction. The loss in heating efficiency is always more than compensated for by an increase in energy efficiency because the energy cost curve is much steeper than the heating efficiency curve so it will always be the case.

Then I've been telling that incorrectly. I understand better now.

I have other considerations that I've been projecting onto others (that they may or may not have). I don't have an energy cost constraint, because of my PV, but I do have a runtime constraint and one other. I imagine others might as well. Mine has to do with available sun. I have eight hours max, so I want my pool to get as warm as possible in that time. I also want my pool to get as warm as it can as soon as it can, which extends my swimmable hours per day (swimmable as in my preferred comfort level).

Even with my new understanding of the energy cost curve, I would contend that only applies if cost is the primary, or only, consideration. If you cannot extend your runtime to compensate for the lower heating efficiency resulting from reducing RPMs, or if you want your pool to be warmer sooner than later, than I would think running the panels at optimum flow rate would be preferred.

There's another factor, too, though I'm not sure how much it would impact the overall cost of heating a pool. The sun on the water is doing a lot of the work. Maybe as much or more than the panels. My system usually shuts down before the end of my eight hour runtime, at which point the pump ramps down and I'm no longer paying for heating (just SWG). If the heater is working less efficiently, and so taking longer to heat the water, then what the sun adds to the equation will take longer, and so the pump will have to run longer. Does it follow that the sooner the pool can reach the desired temp, the sooner the pump can throttle back, and so the sooner the cost goes down? That might be a bit of a stretch, but I imagine there's something there.

I think where I got stuck is when I refer to optimum, I was more concerned about the max than the minimum. By that I mean: I think you can keep increasing heating efficiency by increasing flow (with more RPM). The more water you move, the warmer your pool will get. But there is a diminishing return: the curve flattens out as it gets to the top, such that increasing RPM become less and less advantageous, compounded by the inverse cost effectiveness of pumping water: the higher the RPMs the more it costs, but logarithmically, as in: 10% more RPMs is considerably more expensive than 10% in cost. Plus there is the physical limit of the plumbing and the solar panels, they can only take so much pressure.

I've always assumed that panel manufacturers find this happy medium for us. My 5GPM/panel gets me the warmest pool possible before the cost of doing a little better is not worth it.

I have it straighter in my head, Mark, than I can explain it, but please correct me if I've strayed.

 

[mas985]

I think you covered the key points and I agree with most of it.

There's another factor, too, though I'm not sure how much it would impact the overall cost of heating a pool. The sun on the water is doing a lot of the work. Maybe as much or more than the panels. My system usually shuts down before the end of my eight hour runtime, at which point the pump ramps down and I'm no longer paying for heating (just SWG).

Solar panels have an efficiency of around 80% depending on conditions and flow rate. The pool surface on the other hand is not nearly as efficient. Only about 65% of the incoming energy is captured in a white pool while the water also has to deal with evaporation heat loss so the overall heating efficiency is much lower than the panels. Even a black bottom pool would still have an efficiency less than that of the solar panels due to evaporation.

If the heater is working less efficiently, and so taking longer to heat the water, then what the sun adds to the equation will take longer, and so the pump will have to run longer. Does it follow that the sooner the pool can reach the desired temp, the sooner the pump can throttle back, and so the sooner the cost goes down? That might be a bit of a stretch, but I imagine there's something there.

The cost of the faster flow rate scenario is still much more than the cost of the lower flow rate with lower heating efficiency. This is worth repeating but a doubling in flow rate only adds about 15% more heat but it uses almost 8x the energy. So while you can turn off the pump sooner in the 2x flow rate scenario, it will only be 15% sooner and so it does not even come close to erasing the 8x in energy use increase. The relative economics between the two scenarios really doesn't change with the addition of the pool heat. Plus the pool itself will add heat with the pump on or off.

With regards to your point about adding as much heat as possible, I just wanted to put this into some context. I doubt you would even notice a 15% change in "added" heat by the panels. For typical panel size in most pools (~60%), the net heat gain is only about 3F per day so that would only reduce to about 2.6F which could be challenging to even notice.

Also, I want to point out that the efficiency curve that I showed earlier is for very specific test conditions where the sun is normal to the panels and the water temperature is very close to the air temperature. In reality, the efficiency of panels varies greatly dependent on the air to water temperature as shown in this chart:

Solar Collector Efficiency Calculator

Note that the 80% mark is very close to a 0F difference between the water and air temps. When you look at temperatures during the shoulder months of the season, the efficiency of the panels is far worse than 80%. But in the heat of the summer where the average air temperature during the panel run time is above the water temperatures, efficiencies can actually be above 100% because heat is being drawn from the air as well.

 

[Dirk]

I'm with ya on most of that. I may have to read it a few times to get the rest of the way there. And I think I can ignore some of it, because the cost of electricity is not really a factor for me. Though it is for the OP, so your info is sound.

With regards to your point about adding as much heat as possible, I just wanted to put this into some context. I doubt you would even notice a 15% change in "added" heat by the panels. For typical panel size in most pools (~60%), the net heat gain is only about 3F per day so that would only reduce to about 2.6F which could be challenging to even notice.

I think this is important. It's good to take a step back and see that in the grand scheme, it's not worth fretting too much about. I think my numbers are a little better, more like 5°. But I never really know how much the panels actually contribute. How much was the sun? Or the residual from the day before?

I justify the expense of the FlowVis because I use it several times each year, and can defer it's cost over the decades I'll use it. It makes me feel better knowing I have the flow rate where I want it, and that is a value (to me, anyway) that offsets its cost. But your point is taken, that even a rough guess at flow rate, or lowering it purposely to save on electricity, where one might not technically be at the optimal rate, is not going to translate all that much to what temp you can actually discern in the pool.

Your 3°F does confirm that my panels are doing OK. I know I'm getting at least that amount of increase. That was actually a source of disappointment at first. I thought I was going to swim a month early, and a month later. (I should have known the supplier was going to exaggerate!) And that I would be luxuriating in cozy warm water. The reality was much different. Maybe two weeks on each end of swim season. And even on the hottest days my pool rarely breaks 90°. In the fringe season, my pool is not cozy. It's more like: "OK, I'll get in to cool off from some Spring gardening, but I'm not gunna like it!"

If I had to do it over, I would. On any swim season day, my pool is more comfortable than it would be without solar. Not always comfortable, just more comfortable, if that makes sense. My panels increase the number of days I'm willing to get in. And increase the number of days that getting in is not a shock. And makes for more days where it is, indeed, very comfortable. So in that regard they are successful.

The best way to fully appreciate what solar can do for your pool is to keep your expectations low!

 

[mas985]

I think this is important. It's good to take a step back and see that in the grand scheme, it's not worth fretting too much about. I think my numbers are a little better, more like 5°. But I never really know how much the panels actually contribute. How much was the sun? Or the residual from the day before?

That's exactly why you need to look at this more from an analytical POV rather than relying on measurements. The number I quoted was for 60% area coverage, a 15 degree tilt, S pointing and about 80% efficiency and ONLY for the panels. You can't just go by how much the pool temperature rises during the day because there are multiple factors in that number.

You said you had 8 panels but what is the area of each? 5 GPM would imply 12x4 panels? What is the panel tilt and the direction they are pointed?

 

[Dave95602]

Thanks for all the help. With the VRV lowered about 5', now about 6' above the surface of the pool, I can run at 1800 rpm. 1600 rpm still causes air to enter. FlowVis is arriving Thursday, just in time for 3-4 days of rain, so it will be next week before it is installed. Looking forward to seeing what the rate is at 1800 rpm.

Another related question - have you felt the need to increase your priming rpm to get all the air out of the array? My pump has been priming at 2850 rpm for 5 minutes, and kicking it up to 3000 seems to pull some more air from the array.

 

[mas985]

Do you have a full controller? Some controllers like mine, ramp up the speed for 3 minutes to purge all of the air before settling down to the lower RPM. It is just a matter of configuration.