Analyzing solar + heat pump performance/configuration

[MattM]

So, our brand new pool with cover + solar + heat pump has been running for about 6 weeks now and we're located in San Diego with relatively mild weather and good sunlight all year long but slightly expensive electric....initial design assumes a mid Feb - late Nov swimming season...I've been monitoring how the panels and heat pump affect the water temp and we've maintained 65 degrees with solar only throughout most of January (unheated pools in the local area seem to be around 50 degrees).

Surprisingly, this week is relatively hot..and air temp may reach the 80's nearby...I've been running the heat pump + solar manually to get an early start on our season...water temp just passed 70 degrees and I might have it up to 74 degrees by end of tomorrow...at which point, I hope solar only should be able to maintain that temp until April/May when we get to a final desired water temp of 84. Heating would be setup as solar preferred in intellitouch and run for ~8hrs/day, with the heat pump going on when weather is cloudy. I suspect heating to have to rarely run during June-August.

I suspect we probably could use more solar panels...perhaps 100 sqft on the ideal sides of our roof facing the sun. we currently have a ratio of 1 sqft of solar for every sq ft of water surface, but the panels are facing in a non optimal direction and the pool gets both wind and shade. The current panels are 550 sq ft of heliocool which should have a peak output of ~400K btu. The heat pump is rated for 110K btu. Solar is on its own valve and pump speed, but the heat pump always has water passing through (~40-55 gpm).

Due to our electric rates, it would make sense to limit the amount of time the heat pump runs...and maximize solar, but perhaps run both near the start and end of each season. I'm not sure that screenlogic/intellitouch supports a way to run both at the same time or limit it to certain months.

I'm curious what steps others have taken when they analyze how well their pool heating is working, maximing heating during season startup, and especially dealing with the pecularities of joint solar/heat pump configuration or very long swimming seasons. Is there anything I can do to optimize this configuration?

 

[Brentr]

Sounds like you are doing everything correctly. I think the best think you can do is put a cover over it. I started up the solar in the last 2 days because it is very warm this past week in Jacksonville 70-75 degrees, and the pool temp went from 55 deg to 75 today. I also have a cover but I believe it is an 8mil thick cover which I bought from Lowes 3 years ago. I have not fired up the Heat Pump yet.

 

[MattM]

10-20 degree jumps in 1 day? With solar + heat pump running, the max daily rise I've seen yet is 6-7 degrees. Right now, with both on - we normally go up 4-5 degrees during the day and lose 2 degrees at night (air temp high 70-80, low 45-50). We have a safety cover on...there wasn't any choice in thickness for it. I suppose I could put another cover below the main one since the main one rolls 6 inches above the water but that sounds like overkill.

 

[mas985]

I can tell you what the theoretical heat gain/loss would be if you can tell me a few more things:

Time the sun hits/leaves the panels and pool.
The direction and tilt of the panels.

Also, either lat, lon of your location or a Wunderground weather station nearby. Weather in the San Diego area can vary by quite a bit depending on your exact location.

The model is not perfect but it can give you a rough idea.

 

[MattM]

solar doesn't turn on until around 9:15am and turns off around 4:45pm right now but that is with a 9 degree min start setting (4 degree diff turns off).
The panels seem to be set at no less than 30 degrees and no more than 45 degrees depending where they are on the roof.
The nearest minor weather station to our area is KCASANDI88 (Los Penasquitos canyon) and the major station a few miles further inland is miramar naval air (KNKX).
Panels are facing north east to north west (on purpose, solar electric panels will be installed later on south facing roof).

 

[bk406]

10-20 degree jumps in 1 day?

BrentR's pool is ~5800 gallons. Yours is 24,000.

 

[JamesW]

The heat pump is rated for 110K btu.

Note that the rated Btu is for a particular Ambient Air temperature and that lower Ambient Air temperatures will lower the output potential for the heat pump. (Chart)

Ambient Air Temp............Btu
........80 F...................108,000
........70 F.....................96,000
........60 F.....................84,000
........50 F.....................72,000

I suppose I could put another cover below the main one since the main one rolls 6 inches above the water but that sounds like overkill.

The safety cover should not be suspended above the water. It should be resting on the surface of the water.

 

[bk406]

The safety cover should not be suspended above the water. It should be resting on the surface of the water.

The solar cover should be resting on the water, the safety cover doesnt need too.

 

[mas985]

The solar cover should be resting on the water, the safety cover doesnt need too.

I agree, most safety covers are taught and suspended over the pool.

10-20 degree jumps in 1 day?

A 20 degrees has to be over several days because for a 5775 gallon pool, 20 degrees is 963,000 BTU. With only 192 sq-ft of panels, that is 5000 btu/sq-ft which is impossible for just the panels. The pool itself will heat some but even if the pool is the same size as the panels and is just as efficient, which it isn't, that is 2500 BTU/sq-ft which is more than what the sun produces this time of year. So I think that must be over several days.

30 degrees and no more than 45 degrees.
Panels are facing north east to north west

Are half the panels NE and half NW? Are the panels getting any sun this time of year other than maybe early morning/evening?

With that location, I would be surprised that you get much heat out them at all this time of year.

 

[JamesW]

[Edit]Note: I think the confusion is over safety covers like a Loop-Loc, vs. what the poster has, which is an automatic safety cover.[End edit]

WATER LEVEL: The ability of your cover to support any weight is completely dependent upon you maintaining the pool water at normal operating level. In many cases, the cover will not even support itself if run over a low or empty pool. Severe damage to the cover may occur and a great hazard may develop if your water level is not maintained properly (see winterizing instructions).

It is obvious that there is a limit to how much water, snow, and ice a pool cover can support. A cover owner must be aware that each gallon of water on the cover weighs eight (8) pounds and that a few inches of rain can quickly result in a ton of water that could cause damage to the cover if it is not supported by water underneath.
http://www.coverpools.com/files/docs/CP ... Manual.pdf

It is important to maintain the same water level all year. Pool covers do have a limit as to how much water, snow, and ice they can support. Without the support of water underneath the cover, tracks may pull down, and the cover may tear. Damage could also occur to the coping or may create a hazardous situation. A gallon of water equals (8) pounds, rain water accumulation can quickly result in a ton of water. Inspect your water level at least once a week. If you are intending to lower your water level when you close your pool for the winter, Aquamatic requires that you keep the cover rolled up in its recess or enclosure.

Your Hydramatic Cover is designed to float on the pool water surface and will only function properly when the pool water level is maintained within 1 to 2 inches of its normal operating level.

Remember, a cubic foot of water weighs approximately 62 lbs. Therefore, 1 inch of water on a 20 ft. by 40 ft. cover is equal to about two tons of weight or about 4000 lbs. However, if the pool cover is completely supported by pool water, the net effect of the weight is essentially negligible.

http://www.aquamatic.com/New_homeowners_manual.pdf

Water Level
Maintain the water level at the middle elevation of the skimmer opening. If the water drops below this level, the cover may operate under stress. Never cover the pool without the proper water level under the cover. The water supports all weight on the surface of the cover. If the water level is low and water builds op on the cover, severe damage may be done to the coping and/or cover system.

Although during the winter your pool cover requires less attention, water and/or snow will build up on the cover. If the water level in the pool is not properly maintained, serious damage to the cover can occur. Properly maintaining a pool’s water level prevents excess weight on the top surface of the cover that can cause damage to the cover system.

http://www.coverstar.com/cover-care

Since the INFINITY 4000™ is a “floating” safety cover, the ability to support the weight of heavy objects (i.e., several thousand pounds of snow) would be compromised by an inadequate water level in the pool. The cover has not been designed to accommodate situations where less than eighty-five percent of the cover’s under-surface is making direct contact with water.

It is the pool water itself that supports the tremendous weight of snow or other heavy objects. The cover simply provides a protective barrier between the water and any objects resting on the top of the cover. Failure to maintain an adequate water level when the cover is in use could result in extensive damage to the fabric or a separation of the track from the coping to which it is anchored.

More importantly, inadequate support created by an insufficient water level beneath the cover could compromise the cover’s ability to perform as a safety barrier. Therefore, be sure that your pool water is kept at its normal level, not just during the swimming season but during the “off season” as well.
http://pcsweb.poolcovers.com/Documents/PCS/60001.01.pdf

 

[JamesW]

Sounds like you are doing everything correctly. I think the best think you can do is put a cover over it. I started up the solar in the last 2 days because it is very warm this past week in Jacksonville 70-75 degrees, and the pool temp went from 55 deg to 75 today. I also have a cover but I believe it is an 8mil thick cover which I bought from Lowes 3 years ago. I have not fired up the Heat Pump yet.

At 300 Btu per square foot per hour (good direct sun), the solar would need 16.7 hours of sunlight to heat the pool by 20 F.
At 200 Btu per square foot per hour (indirect sun), the solar would need 25.1 hours of sunlight to heat the pool by 20 F.

The pool is about 225 square feet. If it also collects 300 Btu per hour per square foot, then the pool could gain 20 degrees in 7.7 hours.

 

[mas985]

Sorry, I now see we are talking about automatic covers. I thought it was one of these covers:
http://www.portcitypools.com/blog/bid/1 ... ed-To-Know

 

[MattM]

Well, I used the compass app on my phone to double check the directions the panels were facing....used something a little more robust this morning. Looks like most of the panels are facing west, and some of them north west. 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. but given the guys that did the install and post install support, I doubt I'll trust everything -- it does seem to work OK enough to get as much as 6 degrees/day with heat pump working and the panel temp peaks between 90-110 degrees and averages 80-100 from 10am-3pm. Keep in mind, the vacuum release valve was put on the return line about half way up the house north side so I'm considering having another solar company check the work and make any recommendations.

 

[Brentr]

Sounds like you are doing everything correctly. I think the best think you can do is put a cover over it. I started up the solar in the last 2 days because it is very warm this past week in Jacksonville 70-75 degrees, and the pool temp went from 55 deg to 75 today. I also have a cover but I believe it is an 8mil thick cover which I bought from Lowes 3 years ago. I have not fired up the Heat Pump yet.

At 300 Btu per square foot per hour (good direct sun), the solar would need 16.7 hours of sunlight to heat the pool by 20 F.
At 200 Btu per square foot per hour (indirect sun), the solar would need 25.1 hours of sunlight to heat the pool by 20 F.

The pool is about 225 square feet. If it also collects 300 Btu per hour per square foot, then the pool could gain 20 degrees in 7.7 hours.

Just to clarify Jamesw your are correct. When I built my pool I wanted to be able to heat it very efficiently. When I added my heat pump I was able to increase 1 degree every 27 mins. Remember my pool is 5800gals and I get one turn over in about 75 mins.
When I added the solar I was very surprised to get as much as a 10-15 degree rise in one day only using solar. I have 192 sqft of solar panels. Please see my solar install in the sig as I have this well documented.
I activated the solar for 2 days and the temp went from 55 degrees to 75 degrees. So that is about about 15 degrees increase per day. I am not home so I do not know how long the pump stayed on. We have been having very nice weather, yesterday the max temp was 81 degrees. I would assume that I can get more increase by putting on the heat pump.
Hope this helps. Thanks

 

[bk406]

Sorry, I now see we are talking about automatic covers. I thought it was one of these covers:
http://www.portcitypools.com/blog/bid/1 ... ed-To-Know

I thought we were talking about a loop loc type.


If you guys would dump all this solar and heat pump non sense and get a 400K BTU NG heater, you wouldnt have to worry with this.

Ok, that was partially toung in cheek, dont flame me! But seriously, gas heat is the best and fastest. and, NG is cheap as heck. Theres a reason why heat pumps arent that popular in high electic cost areas. NG beats >30 cents per kW hour most of the time.

 

[MattM]

automatic embedded safety cover -- track is right over tile dug into the gunite but not really visible due to coping above it, waterline is half way or slightly more up the tile and cover is at track level with perhaps the middle of the cover touching water in a few places - motor w/ electric lock opens/closes.

 

[MattM]

Well, I had the silly idea that wanting to swim at least every other day from February to November would be too expensive with gas and might be cheaper with big solar panels in our high sun location. The heat pump was just meant to be a backup so that water temp wouldn't plumet if we had a few cloudy or rainy weeks or if the solar needed a little more oomph, especially at the start or end of the season. Natural gas heaters are popular around here, that's definitely true...but my understanding was that they were primarily serving the market of owners that just wanted to swim once or twice/week, usually on the weekend, have a spa, and thus need super powerful heating for a few hours/week.

 

[JamesW]

automatic embedded safety cover -- track is right over tile dug into the gunite but not really visible due to coping above it, waterline is half way or slightly more up the tile and cover is at track level with perhaps the middle of the cover touching water in a few places - motor w/ electric lock opens/closes.

You can check with the manufacturer of your particular cover as to how much of the cover needs to be supported by the water. All manufacturers that I know of require at least 85 % of the cover be supported by the water.

You could use one or more pyranometer(s) and a data logger to record solar energy intensity and use that to help assess the efficiency and performance of your solar system.

http://www.apogeeinstruments.com/pyrano ... ndex1.html

http://www.instesre.org/construction/py ... ometer.htm

http://www.builditsolar.com/References/ ... rments.htm

Solar electric should help offset the cost to run the heat pump.

 

[bk406]

NG rates vary, but its bascially running about $1 per therm on average. For your pool, you could put 10 degrees per day in for about $5 a day. So for an entire month, if you lost 10 degrees overnight (doubtful in your climate with a solor cover), you could keep your pool heated for about $150 a month even in the coolest months you want to swim (i use cool since southern cali does not get cold compared to me) :-D

If you ran your heat pump enough to add 10 degrees with (100,000 BTU HP output), it would take 20 hours for that same 10 degrees. At 30 cents per kW hour, that around 6 bucks. A 400k gas heater will take 4-5 hours. Of course with solar woking as well, the numbers are going to be a little different. But, if i had solar, i would supplememt with gas, not a HP, especially in california.

 

[MattM]

Right, solar electric might alleviate the cost of the heat pump long term, and the heat pump should mostly be running just at start/end of season and during cloudy/rainy days. This assumes the solar heating works properly. Likewise, the calculations that favor natural gas are based on todays prices...it's unclear how it will all work out long term. Still, I know that I went against consensus here...I'm not a fan of gas and especially laying new gas piping all around the house to reach the pool equipment which is located by the house windows which have their own safety concerns w/ gas nearby. Perhaps I should have gone with a gas heater to supplement the solar, but it's too late now.

For the cover, I was told that 6-18 inches is ideal...and we're closer to 6.

Sounds like I just need someone to review how well the solar is working (it probably is doing OK given its location) but we might need to add another panel or two or move some panels to better spots just to ensure we meet our long term season goals. It doesn't seem like there is much else to look at for optimization.