We get asked from time to time what is the best way to install a thermocouple on a solar collector during installation. While certain manufacturers include a thermal well in their collector this both solves a problem and causes a problem. The same manufactures that supply thermal wells advocate only installing glycol based systems. The thermal well is installed in or near the top header on one side of the collector. Since the well is only on one side of the collector this can lead to extra line runs on the roof to cross from one side of the array where the solar fluid is exiting the collectors to the other side of the array where the thermal well is located. This is not ideal. Additionally, some manufacturers will install wells that are required to be immersed in the solar fluid. Not bad for a glycol based system that is always wet but on a drainback system this doesn’t work particularly well.
Several of the domestic manufactures have taken the approach of not supplying any wells with the installer then simply using a strap clamp to strap the sensor to the manifold. This has the advantage of allowing you to install the sensor on either side of the collector so no extra wire run. The disadvantage of this approach is that the sensor is then farther away from the collector thus making timely temperature detection more difficult.
One solution to this problem is to simply take advantage of the rubber grommet as the clamp for holding the sensor (see picture).

With this scenario you then are able to install the sensor on either side of the collector as well as get a close temperature indication of what is going on inside the collector.
Another solution is to cut a hole in the back of the collector and through the insulation and then affix the thermocouple to the back side of one of the fins. This has the advantage of getting the temperature reading in the middle of the collector but the disadvantage of voiding most manufacturers warranties.

With the cost of conventional energy ever increasing, consumers are becoming more interested in the use of solar energy. But which solar energy technology makes the most sense for the average consumer, solar heating or solar electric?

A solar heating system generally consists of solar collectors, a water storage tank, a pump and automatic controls. The estimated cost of a hot water solar heating system for a typical single family home is between $4,000 and $8,000, including installation. The system will replace approximately 80% of the energy used by a conventional hot water system, and represents a cost savings of $500 to $600 a year. The payback will average between 5 to 7 years after tax credits, meaning that after 5 to 7 years the total cost savings will have paid for the initial cash investment.

The components of a solar electric system are the photovoltaic solar collectors, an inverter, a charge controller and batteries. The projected cost of a grid-tied solar electric system for the same typical single family home is from $25,000 to $35,000 and will supply approximately 20-25% of the electricity usage of the home. The yearly savings would be between $500 and $600. The payback of the system is estimated at 15 to 25 years.

The initial costs of both of the solar hot water heating system and the solar electric system may be reduced by federal tax credits and state tax incentives. The cost savings per year and payback time for both systems will vary depending on the exact costs of the system components, actual current energy costs and future increases in energy costs.

Neither system is 100% efficient, since energy is lost due to conversion processes. Solar hot water heating systems are considered to be between 35% and 50% efficient, compared to solar electric systems which are 4% to 13% efficient.

In our opinion, because of the lesser initial expenditure, the quicker return of the investment and the higher rate of efficiency, the solar hot water heating system is the better, more cost effective choice.

When more pumping means less.

Problem:  A customer has a drainback system that turns on properly when the differential is achieved.  Once the system turns on the pump starts pumping and then shortly thereafter the flow can be heard dropping into the drainback tank.  Everything is working according to design.  A short while later (5 to 10 minutes) while the pump is still running the system ceases pumping over.  When the system originally started the site glass was at the top of the site glass.  After the water started falling back into the drainback tank the water level in the site glass was down about 6 inches from the full level.  After the system ceased pumping around the water level in the site glass was now 3 inches below the full level.  This situation repeated any time the system turned on.

The installer, thinking there was a problem with the pump, replaced the pump.  No change.  The installer then added another pump in series to address the problem.  The system operated identically except it “lost prime” faster than with a single pump.  What was the problem?

Answer:  The particular drainback tank that the installer was using had the return from the collectors coming straight into the top of the tank immediately above the line leaving the drainback tank going to the heat exchanger and then back to the collector.  After the pump started running the fluid coming from the collectors picked up air as it splashed in the drainback tank.  Enough of this air flowed out of the drainback tank and ultimately collected in the pump.  With a small amount of air in the pump body the pump was no longer able to generate enough lift to get the water past the highest point in the system and the prime was broken.  The solution to the problem was to reduce the flow out of the pump by partially closing the ball valve on the exit side of the pump.  By doing this the volume of flow going through the drainback tank was reduced.  This allowed enough of the air to come out of solution to prevent the pump from air locking.

I am a strong advocate of the work that the SRCC (solar rating and certification corporation) is doing in testing and certifying solar collectors but….  There are a number of factors that can have a significant impact on the certified performance that aren’t published in their certifications and can lead to significant differences in performance as well as longevity.  Below are a few things that have a significant impact on the performance of the solar collector that currently aren’t reported:

1.  Glass

1. Hardiness – as part of the certification process the solar collector glass has to undergo repeated thermal shocks to insure that it doesn’t fail.  It is commonly understood that in order to pass this test the solar collector glass must be tempered.  The tempering process is expensive for glass manufacturers and leads to significantly higher glass prices for the collector manufacturer.
2. Transparency – when the solar collector s are tested for certification they are tested as a whole and the transparency of the glass is not measured separately.  There are two steps that can be taken to increase the thermal transmission of the glass.  The first is going with low iron glass.  This is the most common grade of glass that is used in solar collectors.  The second step that can be taken is going with an anti-reflection coating on both sides of the glass.  This coating allows low angle light to pass through the glass rather than be reflected by it.  Unscrupulous manufacturers could substitute tempered, low-iron, anti-reflection glass for their certified collector (they are only required to produce 5 samples) and then use standard float glass to supply there everyday production.  This higher performing glass would give them a significantly higher performance number from the testing but then they could cut the cost of their glass by 75% by going with standard float glass for their production.

2. Absorber

1. Thickness – as a rule the thicker the absorber sheet the greater it’s ability to carry heat (less thermal resistance).  Unfortunately, a thicker sheet costs more to produce.  Currently, the published data does not specify at what absorber thickness the collector was tested so again an unscrupulous manufacturer could substitute a thinner material once they have received their solar collector certification with higher numbers.
2. Riser quantity – the shorter distance between the fluid channels (tubes) the less heat is lost.  The number and spacing of the tubes is not published.  It is possible (and has been reported to have happened) where a manufacturer increases the spacing between there tubes to save money on the solar collector and the contractor/homeowner are none the wiser since they are sold the solar collector based on the SRCC solar collector rating.
3. tube thickness – in order to pass the rigorous 160 psi standard required for open loop collectors manufacturers can reduce the wall thickness of their copper tube once the certification has been achieved.  This will save the manufacturer money but diminish the expected life of the collectors

Random variation

1. Every process has natural variation including the production of a solar collector.  Currently, the SRCC (or there certified testing labs) tests only 1 collector (from a batch of 5) to determine what the published data for that collector and manufacturer will be.  Even their testing procedures have variation in them (this has been much discussed in the industry).  With only taking a single sample you could infer that Michael Jordan either never missed a shot or never made one (both equally wrong).  Without several data points it is easy to see how seemingly identically constructed collectors have widely difference performance numbers.

So all I have shown is that unscrupulous companies can (and will) take advantage of the system but what can we do about it?

I would recommend that we convert the current system of 1 scheduled inspection every 10 years to one where the certified manufactures pay an annual fee for the testing that will be done that year.  The samples to be tested would then be pulled randomly from the manufacturers on the list based on surprise audits.  This would serve the purpose of leveling out the costs associated with the testing process (fees would be annual based on the number of models needing testing and therefore your chance of being selected in a given year), eliminating the complaints of back-logs in solar collector testing because you would have your certification by paying your fee until your solar collector is inspected,  and freeing the solar collector performance numbers from the influence of gaming that I discussed above.

10 things to look for when buying a solar water heater

1.  Certification – The Solar Rating and Certification Corporation (SRCC) reviews and certifies both solar collectors (OG-100) as well as the entire solar water heating system (OG-300).  This certification provides a minimum standard of quality for the industry as well as providing a basis for comparison between different products.  Don’t consider buying a solar water heater without both OG-100 and OG-300 certification.
2.  Collector frame materials – Solar hot water panels come primarily with either aluminum or steel frames and backs.  The aluminum products will come with either raw, painted or anodized frames.  The steel only come in painted versions.  Generally speaking if you are in an area away from the coast any of these styles will do as long as you can handle the aesthetics (generally painted or anodized will be preferred for appearance).  If you are within proximity of the coast the anodized surfaces will provide you the long term corrosion protection that the other materials won’t.
3.  Fluid path materials – a solar collector can come with aluminum, steel or copper tubing for the fluid to flow through .  The most common material in north america is copper although the other materials are available.  Be aware that the aluminum may suffer from galvanic corrosion as a result of the other copper, brass and bronze that are likely to be in the system and therfore prematurely degrade.  The solar collector (s) that have steel fluid paths are only appropriate for glycol systems.
4.  Warranty – while the SRCC certification mentioned above covers the minimums there are differences in both the length of the warranty as well as if they warrant labor.  It isn’t much good if a collector goes out after 6 months and then the contractor can’t get paid to replace the one on your roof.
5.  Installed base – It is a perfectly fair question to ask for references or what other solar heating project the installer may have done.  As a homeowner you need to be patient because depending on the area where you are located you might not have any choice of contractors.
6.  Absorber connection method – solar water heaters come with four main styles of producing the absorber

1. ultrasonic welding – this method is probably the most common method for attaching the absorber fin to the tubes but it leaves a line down the absorber that some homeowners find objectionable  The advantage is you can see the quality of the weld.
2. Soldering – this has diminished in popularity over the years since it is difficult to maintain the quality of the process although some companies still use it to great effect.
3. Mechanical bonding – the absorber is crimped around the tube that holds the water.  This can be an effective means to attach the tubes to the sheet but there is a risk of poor mechanical sealing and then the thermal performance of the collector is greatly diminished with the buyer being none the wiser.
4. Laser welding – this process is very capital intensive but provides an excellent seal between the absorber and the tubes without having the witness line associated with ultrasonic welding above.

7.  Pre-engineered – solar heating installations are technically complicated and require the greatest of care to insure that the system will function properly for years to come.  Either go with a pre-engineered system from a manufacture or stick with a very experienced installer.  Having an installer cut there teeth on your house isn’t the way to go with solar heating.
8.  Aesthetics – Congratulations you are leading the green revolution by going with a solar water heater.  Keep the positive vibes going by making sure the installation is good looking.  Nothing will turn off future solar customers more than an unattractive installation.
9.  Maintainability – murphy lives everywhere including in your solar heating installation.  Make sure the system is designed and installed so any of the components can be replaced should they fail while providing the minimum disruption to the system.  Insist that the components are isolated from the balance of the system to allow easy change out.  This also plays a part in the system design that you go with.  If you have easy access to internal heat exchange tanks then they become a reasonable option.  If you live in parts of the country where you local plumbing supply house doesn’t stock them then stick with an external heat exchange system so the tank can be replaced when it fails (and it will).
10.  Mounting hardware – How solar hot water panels tie into your roof is as important as the other components.  You don’t want you system to create problems with your roof.  The manufacturer should have had engineering done on the hardware to insure it can withstand any wind you might see in your area.  In addition, you should look to be sure that you only use materials on the roof that can stand the test of time, aluminum, and stainless.  Avoid using steel, galvanized steel or zinc plated hardware as part of the mounting system unless you live in the desert.

(the bottom of my tank)

Contractor leaves the job site satisfied that they have done a good job installing a solar hot water system only to be called back 2-4 weeks later to be told that the system doesn’t seem to be heating the customer’s tank.  The homeowner has been keeping a detailed log of the temperatures in the tank as well as on the roof and shows you a log that never shows the tank
sensor getting hotter than 105 degrees. They say the system has been running and they haven’t noticed anything else unusual but clearly the system isn’t working properly with the solar only getting the tank this hot.  What went wrong?

A modern differential control operates by measuring the temperature (via a temperature probe) in the bottom of the tank (T2) and the solar collector temperature at the outlet of the collector array (T1). Hot water tanks are constructed so they introduce any cold water into the bottom of the tank primarily via a dip tube that carries the cold water from the connection on
the top of the tank to the bottom of the tank. The Steca 0301U differential control (which represents greater than 50% of the control sales in the U.S.) only comes equipped with two sensors although it can take a third. In this case what the homeowner is seeing as the tank temperature is actually the temperature at the bottom of the tank (T2) where any cold water entering the tank is mixed with the solar heated water in the tank. The stratification in the tank allows the homeowner to have access to hot water (120 degrees +) at the top of the tank while only seeing “cold” water temperatures on the readout for the bottom of the tank.

solution: Supply the homeowner with a third temperature sensor that would then be mounted to the top of the tank showing what temperature the tank is actually delivering.