Vacuum Tube Solar System

I recently built a pool and spa in one of the foggiest, coolest environments in California.  This pool, high on a bluff in Elk, is not heated by fossil fuels.  The only source of heat is an array of vacuum tube solar collectors.  I was at the site yesterday.  The pool was 84 and spa 103.  The solar system heats (3) 120 gallon storage tanks which distribute heat to titanium heat exchangers for the pool and spa.  The pool is the heat sink for the system.  A few weeks ago the owner reported that he pool was 95 degrees so he rolled back the auto cover and dumped 14 degrees overnight.  The pool has been in the mid '80s since.

The solar system also heats the residence, a 5000sf custom home with hydronic heating, in the winter.  When the home is up to temperature the excess heat is dumped into the pool and spa.  The residence is prioritized in the winter with the spa second and excess heat into the pool.  The priorities are set by thermostats.  When the tank reaches 110, the residence heat is turned on.  When it reaches 120, the spa heat is turned on and 130 the pool heat is activated.  It is extremely efficient, even in the fog.  The vacuum in the tubes is a perfect insulator.

There is a Munchkin 110k BTU back-up boiler but we don't anticipate having to use it.

The pool is a custom 2 sided zero edge design with one side a 105' radius.  This required an in-deck cover track on that side with a standard under-coping cover mount on the opposite side.  It covers the overflow troughs on the side and end for additional heat retention.  The spa is a pretty standard 6 jet spa, 6'-10" round.  The custom detail for the spa is the coping.  It was designed and manufactured with a 2" recess to accommodate an aluminum Be-Lite spa cover so that it is flush with the deck.

I got the typical plan view of a shape from the architect, nothing more.   The  owner wanted a vanishing edge pool.  I indicated to him the heat loss on a VE pool and suggested the zero edge design.  I did the design and specifications.  The owner already had a PV solar system and added a little more capacity to handle the pool.  Although the pool pump must stay on all daylight hours, the spa pump cycles with demand to save energy.

Another selling point for this type of solar heating system is that, since it heats the residence, it qualifies for federal and state solar tax credits which saved a substantial amount of the cost of the system.  The solar is utilized year round for optimum performance and savings.  We expect this residence, pool and spa to use no power or fossil fuels.

The solar contractor is Solar Hot Water Plus in Arcata, CA.  They work all over California and can consult, design and install: 3423660837?profile=original3423661017?profile=original3423660960?profile=original

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  • Was out at the site yesterday, 10/30 at 11am, prior to peak heating time.  Pool temp 82, spa temp 100, no other heating source.  I've done standard flatplate glazed systems, copper unglazed and standard plastic panels on pools of this size and have not come close to these temperatures in this climate.  We have had rain and much fog lately.

    • OK then.  Thanks for the real-world input.  I guess those specs for the glazed panels are unrealistic then even for pool-only situations.  They clearly indicate that they would not work well for hotter temps for domestic hot water (120+), but they should have worked for pools at 80-90.  On a cloudy day with 300 W/m^2 (30% of peak sunny day), the evacuated tube should do better than glazed panels for air/water temperature differences of 32F or higher so for an 88F pool that means the evacuated tubes would do better than glazed when air temps were below 56F.  Maybe with even cloudier/darker days and even cooler temps then that's making the evacuated tubes really shine.  Or perhaps the manufacturer's are lying about their specs and that the Florida Solar Energy Center wasn't testing cloudy conditions -- wouldn't be the first time.

      • Richard:  Thanks for the input.  I'll monitor the performance of the system.  I tried to get the owner to install the iAqualink so I could just log on and check things but he did not want to spend the extra money (which included automating the chlorine and pH systems since there are no real pool service guys in the area). 

        Where are you located?

        • I'm in the San Francisco Bay Area.

          • Thanks.  Yeah, most of my work is down that way:  Marin, Sonoma and Napa counties.  You most likely know about weather conditions on the Mendocino coast, about 3hrs north of you.

  • Take a look at Solar Panel Technology Comparisons. Though evaucated tube solar collectors are good for domestic hot water applications which have a lower water volume and a greater temperature differential, they are not usually as good or as cost effective as glazed panel collectors (e.g. Heliodyne) for heating swimming pools which are high volume and lower temperature differential and certainly not as cost effective as flat black mat (unglazed) panels, though these unglazed panels aren't good in cold windy climates.

    So for the cooler foggier area you describe, certainly glazed panels would have been a consideration and are roughly 3x the price of unglazed panels, but the evacuated tubes are roughly 2x the price of glazed panels but don't provide as much of a benefit.  In fact, in areas that get snow or ice, such evacuated tubes can be worse since they are "too good" so end up getting frost which blocks the sunlight.

    Now for heating the house as well as the pool, it's less clear whether glazed panels would work as well if you were heating the water hotter than would be done for the pool and spa (i.e. higher than 90-104°F).  If you were heating the water closer to hot water applications (120-140°F), then the evacuated tubes would be better than glazed panels in cooler environments.

    • Richard: I've been doing pool solar since the good old days of Jerry Brown's first administration in the late '70's.  Done them all.  This particular site is very foggy much of the time and rarely gets over 60 degrees there, even in the summer time.  It is an exceptionally challenging location for solar heating.  It was an expensive system ($60k) but he also got the federal tax credit (30%) and some with the state since it heats the residence and the pool/spa are secondary sinks.

      Additionally, the solar panel ratings are done in laboratory situations.  The solar sub sells both systems and has done side-by-side comparisons in Arcata, CA, another cool foggy location.  On sunny days the vac tubes are about 10% more efficient; on foggy days about 30%. 

      My home on the coast is hydronically heated and I have a VE/ZWL spa (most inefficient spa on the planet) and I am having them install a system for my home.


      • On sunny days, the evacuated tubes can't be more efficient than the glazed panels except for extreme temperature differentials because they have gaps between the tubes so on a per panel area basis they aren't able to capture as much sunlight.  So unless one is measuring efficiency in terms of collector (inside tube) area instead of gross area (how much space the panels/tubes actually take), they won't be more efficient on sunny days unless the temperature difference between the air and the water gets to 50°C/90°F.  It's on cloudy days where the difference in air/water temp only needs to be 20°C/36°F to make the evacuated tubes more efficient (which sounds like the situation you described, assuming the water is heated to be very hot).

        Are you measuring efficiency based on gross area (i.e. how much roof or rack space is actually taken)?  Also, what is the temperature of the water in the system?  Since you are also heating a spa it sounds like the temperature is quite high possibly near domestic hot water temperatures in which case I would agree that evacuated tubes are the best choice in that situation.  The efficiency measurements are consistent with real-world testing done by the Florida Solar Energy Center, but it is true that at a system level the efficiency of heat exchangers also is a factor, but should be independent of panel type assuming such an exchanger is used in both cases.

        At least for pools (not whole house heating), there have been complaints on forums for how the evacuated tubes weren't as efficient as claimed by installers, especially on sunny days.  For hot water heating in colder climates where the temperature differential is much higher, evacuated tubes are much better.
        • A great project Lee, Glad you guys have the space for all those panels, wow that's a lot but show it works. Richard, The problem as we have discussed before is when do you want to heat your pool, if it's in the summer during sunny spells then black rubber mats are fine and flat panels but if you want to extend the swim season by using the light end of the solar spectrum then evacuated tubes are the way to go. They are efficient but are frequently oversold, are the pools efficient to start with. was there any insulation on that pool Lee?

          What system do they use in the Antarctic and why?

          • I'm not comparing flat black mat which clearly couldn't cut the mustard in cooler windier environments.  I was comparing GLAZED panels, which are typically black-painted copper pipes connected with black fins enclosed in multi-paned glass vs. evacuated tubes that have gaps between them.  The evacuated tubes do best when the water temperature needs to be higher, as with domestic hot water heating (120-140F) but are generally worse when only needing to heat a swimming pool where the water temperature is lower (80-90F).  This particular installation sounds like it needs higher water temps for the whole house heating and Lee wrote "When the tank reaches 110, the residence heat is turned on.  When it reaches 120, the spa heat is turned on and 130 the pool heat is activated."  So for these sorts of temperatures, the evacuated tubes are more efficient in cold environments, but if ONLY the pool was being heated in the 80-90F range, then glazed panels would likely work just as well if not better.

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