Case Study

Subject: Problems with Fafco 4x10 polypropylene solar panels installed in 2010

Location: Cache Creek, BC, Canada

HISTORY

In 2010 the Cache Creek outdoor municipal pool was solar heated with 14) 4x10 Fafco solar panels under the Eco-Energy Program. This well intentioned subsidy was meant to kick start the commercial solar heating of swimming pools. Hot Sun discussed the design with one of the original engineers on the project and we were told the solar heater would be equipped with a pressure relief system that would isolate solar from pressure when solar was off and dump the excess water back to the surge tank. Upon inspection by Hot Sun in August of 2013 it appears that didn't happen. Instead failsafes were probably attempted using a building automation system.

Read Hot Sun's web sites covering mechanical considerations like pressure.

INSPECTION REPORT

Hot Sun's Ken Wright was asked to take a look at the system in August 2013. Reports from the owner were that piping was melting as temperatures had reached 180F on the roof.

It should be noted that Cache Creek can be very hot in the summer and on a windless day with solar panels stagnating full of water it is entirely believable that temperatures could reach 180F. At this temperature PVC pipe is quite soft and if not supported fully and not allowed to move freely with temperature could certainly sag and appear to be in danger of melting. This is primarily why we specify that solar heaters be set up to not operate under pressure. We want solar to always drain down when off not so much because we want to get the water out. Its because draining down reflects the fact the pressure is lower than the height of the roof. In this case what we may see is the effect of water in the system contributing to the overstress condition because of the fact the water allows that excess collector heat to transfer and migrate into vulnerable pvc pipes.

Observations

There are 8 Fafco 4x10 solar panels on each side of the roof and between panel 5 and 6 (left to right) there is a gap for a vent pipe and the headers are joined with pvc pipe over this span and the pipes are sagging. We added the straps to hoist the pipes back into position partway in the photos below.

The solar panels have expanded since installation. The roof connections are jammed against the edges of the solar panels as the collectors want to be wider than the mounting points allow. The collectors were probably installed properly in cold air temperature and we are now observing the system hot in the summer not operating. There could be a 150F temperature differtence and over that temperature difference 30 feet of polypropylene would want to be as much as 4" wider than when cold. The strap down scheme employed by all collectors of this type (rigid polyprpopylene) suffer this limitation and should be limited in bank size. The reason this isn't standard in the industry is Fafco has sold most of their systems in the US sunbelt over the last 40 years without these Canadian temperature extremes. Additionally solar systems usually operate when its sunny when they are pool temperature not solar stagnation temperature. We run into troubles in this industry when solar panels sit there in the Hot Sun doing nothing. That's when they get really hot.

There is no stagnation solar temperature sensor. Solar controls since the beginning of solar history use the temperature of a solar panel in the sun with no water going through it as the solar sensor. Compare this with the pool temperature and use that to decide if water flows to solar or not. Its a very simple and well established standard but this technology isn't mainstream engineering. The standards we keep talking about are the standards Hot Sun and others in the solar field have made up ourselves. These should be building code standards not just manufacturing standards but this isn't in the building code and engineers haven't been needed up until now. Now we have municipalities going with this technology and their procedures require that they use the services of engineering companies. They can't just hire small solar companies and pay them with taxpayer's money. Engineering firms provide that necessary layer of assurance that public moneys are being spent with proper engineering oversight. Its all very good in principle but when it comes to solar pool heating we have an unique situation. Its tricky stuff and engineering companies don't understand it and haven't learned the lessons of the past 40 years like some of the good solar companies have.

On this job it appears that solar is sensed based on the temperature of the water coming out of the solar panel. This is how heating systems are controlled. This is not how solar systems should be controlled.

The pressure at the plumbing location where solar ties in is about 14 psi. The public works official we met with on site thought that pressure was 20 psi. I would tend to trust his knowledge over a pressure gage. The solar panels are elevated about 8 feet above that point so if the tie in pressure was 14-20psi the pressure on the collectors with solar off would be about 10-16psi. We consider a solar stagnation pressure of 8 psi to be too high although we do have many Powerstrip systems operating long term at higher pressures but not in BC. In BC's deserts air temperatures can be 30 below zero in winter and in summer hotter than the Sahara desert. We can't get away with 8 or 10 psi in many locations in BC. You wouldn't think of BC as a worst case solar location like the San Fernando Valley in California but it certainly is. We need to be more diligent with solar design in some areas of BC than we do in main solar markets of the sunbelt and this is another case of solar design and control mistakes showing up 3 years later discrediting the entire movemnent the funding was intended to kickstart.

Rubber couplings between collectors were leaking when solar first started up each morning.

PVC pipes on the roof were bent between roof connections

There are reports of roof leaks and the roof will be replaced in upcoming budgets. What they do with solar is undecided.

Collectors are failing (springing leaks)

Conclusions and Assumptions

We have to make assumptions because we don't know why the original engineer didn't get his way on the design he wanted to implement. Our best guess is that the failsafing of the system was left to the controls company. Engineering companies may be inexperienced with unglazed pool heating with plastics at high temperature but at least they are mechancially astute. The engineer knew he needed to take pressure off the collectors when solar was off and we know this because Hot Sun had a discussion with him on that before the job was started. Controls companies don't know the first thing about what they are seeing when they start to monitor and commission a solar heater and once they are at that stage everyone else is already signed off on the job. The public works personnel on site told Hot Sun that at a certain temperature solar is locked out. He also told us the heat pump takes priority over solar. He has some access to the DDC system we believe. He's the guy trying to maintain and operate this thing without any guidance. The main problems they were having were resolved when the controls company allowed solar to operate when it got hot instead of the opposite. You don't prevent the high temperature and high pressure event from happening by shutting solar off. If anything you turn solar on to cool the collectors with pool water but obviously a better solution would be to alleviate the pressure in the first place.

High pressure creates grief at many levels and is generally considered to be the root of all evil. In this case that may be true but there is also the simple design flaw with strap down polypropylene collectors. Plastics need to expand and contract with temperature and over a single 4 foot collector the movement with temperature of 150F is about 3/8" so over 8 collectors its 3" and the spacing between the roof connection and the edge of the collector is no more than 1".

Rigid polypoeoplylene collectors should not be the standard building block of the solar industry. They only reason they are is that there is about $18 worth of material in a collector and it can be mass produced whereas a proper solar solution involves a much higher materials and labor cost. Often polypropylene collectors last a long time so there are always case studies contrary to this one and nobody publishes case studies like this because this is a sure fire way to reduce all interest in solar heating not just in the tried and proven untrue technologies of yesteryear.

When you have 10 psi in a solar heater that pressure goes away when the pool pump shuts off. That may happen nightly or it may happen only once a week for backwashing. The solar panels empty and fill up with air. Then when the pump turns back on you now have a compressed bubble of air in the solar system and then when solar tries to turn on the pump must work against a compressed air bubble that doesn't want to be forced down the return pipe to the pool. Its like trying to force a balloon full of air underwater. That could be the source of the large pressure pulse that causes the couplings to leak upon start up each morning. Its not a flaw with the rubber couplings Fafco uses. Heliocol would have you believe that their coupling solution is superior because it doesn't leak but under this kind of pressure the system just fails somewhere else. The proper remedy is to remove the pressure.

The sagging pipes are caused by a combination of the flawed collector design not allowing for enough movement with temperature and the fact the collectors stagnated under high heat. If we let the water out when solar was off we could get away with the pvc that was used. We sometimes look at using cpvc instead of pvc in that service

SOLUTIONS AND RECOMMENDATIONS

Job one with any unglazed plastic solar pool heater no matter what is to design the system so the collectors are not pressurized. How could this be done in this case? Maybe solar could be tied in downstream of new condensing boilers. We'd have to check the pressure there but we suspect its still high. Our recommendation would be along the lines of the intention of the original engineer on the job. To make his idea more failsafe we'd simply dump the solar heat into the surge tank. Its only something like 10% of the total system flow so its not going to matter that we're creating a recirculation loop taking water from downstream of the filter and returning it upstream. There are other methods we could employ. With any tricky red flagged solar system like this its important we monitor the system over time and watch how it behaves. Its important we commission by monitoring and looking at all the heaters and how they interact. The controls company can't do that. That's why we became a controls company. With our SWIM PC controls and monitoring technology we can be the ones commissioning our systems. Unglazed solar heating is popular because its economically viable but its loaded with technical trap doors and its really hard for the traditional engineering, controls and public works infrastructures to co-ordinate successfully with existing methodologies. It takes a long time and gradual acceptance curve as opposed to a short term blast of free money.

If Hot Sun is being asked what is wrong with this picture and how can it be remedied for the long term we have to say that Fafco solar panels should not have been used here. Powerstrips can be glued down sideways with headers at each end. There are no horizontal pipe runs subject to migratory heat. There are no roof penetrations except those holding headers and plumbing at the extreme ends. There is no movement with temperature of a rigid header manifold extending 30 feet side to side. What is wrong here is that BC made Powerstrips should always be used in BC applications especially when funding for the project is Canadian? It doesn't seem sensible to assist American companies in taking undeserved market share from Canadian manufacturers at the expense of Canadian taxpayers. Powerstrips are made in the US by Hot Sun Industries Inc and in Canada by Hot Sun Industries Ltd.

Powerstrips glued down sideways

Thirdly the controls need to be fixed. The system needs a solar roof sensor and a standard solar controller worth $350 or logic written into the DDC system that copies that basic algorithm. Of course benefit would be gained by optimizing the use of solar, heat pump and gas through ongoing monitoring. It is assumed that is happening already but apparently this has been one source of the trouble. The design should have been failsafed mechancially not by simply treating the automation system as the Godsend that it seldom lives up to being.

Powerstrips glued down sideways

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