Article by Larry Weingarten & Zak Vetter
Way back in 1978, I (Larry) installed my first solar water heating system. I continued with solar thermal, installing new systems until tax credits left in 1986, then kept nearly all the local systems up and running for years after that. It became painfully obvious to me that simplicity is essential for the durability and longevity of any solar thermal system. Complex systems just die young. Back then, the holy grail of solar thermal was to come up with a system that would cost $1,000--which was never really done.
These days, you expect to pay $6,000 to $10,000 for a solar hot water system, installed. I have a friend, Martin Holladay, who wrote an article in March of 2012, titled “Solar Thermal is Dead.” He generated a lot of discussion with that article, including some dissent, so he wrote another article in December of 2014, “Solar Thermal is Really, Really Dead.” Martin looked at solar thermal prices and compared them to using photovoltaics (PV) and a heat-pump water heater to do the same job. After doing the math, PV and a heat pump appeared to beat solar thermal for water heating.
But, clearly, it depends on what your assumptions are! I’ll add that heat pumps are new enough that we don’t really know how long they will last. Also, good PV extracts about 20% of sunlight’s energy, while efficient solar thermal gathers around 80%, and even the “inefficient” system described in this article gets about 60%. These are reasons to continue to explore how to make solar thermal work.
Enter Zak Vetter. Zak asked me to help design and install a solar hot water system for him, but he had a set of goals I’d not heard before. He wanted:
Note that cost was not given as a limitation!
I had never worked with such a list. Many assumptions go into designing and building a traditional solar thermal system, and those got challenged by Zak’s goals. Here is a quick list of assumptions we typically work from:
Design rules also involve assumptions:
Clearly, Zak’s goals didn’t line up with the standard assumptions. But I’m glad he challenged convention, because ultimately we built a system that costs less and performs better than any solar thermal system I know of. The system cost right around $4,000 and provides for 95% of the yearly hot water need. A handy person could do the same job for around $3,000, if they built their own collectors.
Following is the thinking that got us there. Wanting efficient collectors forces us to build more complex, expensive systems due to overheating and freeze concerns. So, instead, we used really inefficient collectors! These are just coils of ¾” polyethylene tube, under an acrylic glazing.
There is no insulation in the collectors, so they cannot overheat and are unlikely to be damaged by freezing. The hottest we’ve measured in summer with no water flow is 170 F in the collectors, and they have frozen multiple times without problem. This type of collector has been under test in San Jose, California, for sixteen years with no trouble. Essentially, they are pool collectors, modified to produce domestic hot water, simply by adding glazing. They are commercially made by Gull Industries in San Jose. Here is what the coils look like installed on Zak’s roof.
The coils are 26 square feet each. Another benefit of using “inefficient” collectors is that we eliminated the need for copper pipe to and from them, by running PEX tubing. With traditional copper collectors that can stagnate in the summer sun at up to 400 degrees F, PEX tubing would melt pretty fast. But we were able to use poly pipe and PEX for nearly everything, simplifying the job even further. We purposely oversized the system, so it could coast through periods without sun and recover quickly when the sun returns.
The tank was another consideration. Normally, with any glass-lined tank (nearly all tank-type heaters in the US are glass-lined), you want to turn over the volume of the tank daily to prevent stagnation and odor problems. Turns out the anode that comes with all glass-lined tanks generates hydrogen gas, which some bacteria really like. We got around this by installing a 105-gallon Marathon tank, by Rheem. This is a non-metallic tank that needs no anode, so does not “age” the water. The benefit of this much storage is the ability to survive happily through sunless days. Here is what the tank looks like:
One other benefit of the Marathon tank is its insulation. It’s got three inches of foam, and the literature says it loses only five degrees F in 24 hours. Our data-logging suggests it’s more like six to eight degrees in our situation, but still, not bad. Insulation is something else we played with. Pipe insulation seldom comes really thick, yet keeping heat loss down increases the actual solar fraction and reduces the amount of back-up energy needed. So we decided to double up on the insulation where possible. Here are photos of how that worked:
Solar water heaters are normally designed as one- or two-tank systems. One tank is better, if you can make it work, as there is less equipment to lose heat from. These days, this can only be readily done with electric backup. So another thing we did was to disconnect the lower element in our single tank and use only the upper element for backup. This prevents the electric heat source from competing with the solar one. We wired it at 120 volts rather than 240, so there was no need to do anything more than just plug it in. It does take four times as long to heat at half the voltage, but Zak wanted a good test of the solar--so he hasn’t even plugged it in yet! The system was installed in November of 2014 and he has yet to use the back-up.
The system is managed simply with an off-the-shelf Goldline GL-30 solar controller. It measures the temperature at the solar collector and at the bottom of the tank. It compares the two and, when the collector is sufficiently hotter, turns on the pump. The control has adjustments for fine-tuning this set-point. Fortunately, we do not need the control for freeze or overheating protection.
The system was simple to install. If you look just at installation time, it took only six man-hours, which is very fast. In the good old days, a fast installation used to be three guys and one long day, or something like 24 person-hours. This system went in so quickly because:
Here are photos showing some of the time-saving hardware.
Performance so far has been good. We’ve data-logged at multiple points across the system in order to understand just how it’s working. Following is a graph of the Spring Equinox performance. You’ll see that the system produced water ranging from about 110F to a little over 140F.
The term “solar fraction” is used to indicate what percentage of one’s hot water is heated by the sun. Done right, determining the solar fraction would involve measuring total hot water use and subtracting the portion of water heating not provided by the sun.
We opted instead to simply notice when the solar-heated water was hot enough to shower with. If the stored water is around 105F or more, it’s good for showering. When we say the system is producing 95% of the hot water, it means Zak gets acceptable shower temps 95% of the time.
It’s a quick, non-mathematical way of understanding generally how the system is performing. If we took accurate measurements to determine solar fraction, it would probably be higher than 95%. But because we consider anything under 105 F inadequate, we’re presently not taking credit for water that isn’t quite hot enough, but is certainly well above groundwater temperature.
Following is a graph that shows the system at its worst. The vertical yellow bars represent periods of sunshine, and the vertical blue bars are night time. Between the 21st and 22nd you’ll even see rain! But note how just a few hours of winter sun on the 23rd gives the tank about a 20-degree boost.
Another two graphs show the differences between December and March. Note that, in these graphs, we measured outputs from each collector to see if all four were useful. It turns out that the first two collectors gathered more BTUs, but the following two collectors each bumped the temp up higher, so they really did help--particularly during the colder times of the year.
Where to go from here? Clearly there will be limitations on where this sort of system can be successfully installed. If these collectors are covered with snow, they might not function too well, so it could make sense to avoid areas that stay below freezing for extended periods of time.
Also, if tax credits are the main motivation, this system won’t do, as this collector/system isn’t yet SRCC-certified. Still, this system should cost less then most other systems, even without the benefit of tax credits.
There is a way to make the system cost even less, by making one's own collectors. It turns out you can buy enough of the right type of poly pipe to make a coil for $194, cutting collector cost by at least $300 each!
To wrap up, it’s clearly a good thing to bring fresh perspective to solar water heating. By intelligently questioning old ideas and by using newer materials and hardware, Zak pushed us to do better than I had believed possible!
Larry Weingarten was raised on the Monterey Peninsula of California and has been self-employed most of his working life. He got his general contractor's license in 1982. Larry has written articles on water heating and energy for various trade journals; has taught about these topics for PG&E, California State Parks, Affordable Comfort, and others; and has recently helped create DVDs on these and related topics. In 2006, he finished building an off-grid home; designed to be very efficient, comfortable, and inexpensive, it was the 13th home to meet the "1000 Home Challenge," a competition for creating superefficient homes. He likes cats.
Zak Vetter was also raised on the Monterey coast. He has been self-employed for over ten years, repairing and teaching about computers. Since 2008, Zak has been learning about the wide-ranging world of energy-efficiency while improving his own property. The solar water system in this article was inspired by a visit to Larry’s off-grid house, which demonstrated how much was possible with solar power.
Looking back over my working life of 50+ years, it seems clear that self sufficiency has always been the best way for me to be useful. Now, mix in a strong interest in water in its many forms and the wide world of animals and you'll know what's important to me.