Another Solar Myth Bites the Dust
Way back in 1978, I installed my first solar water-heating system. I continued with solar thermal, installing new systems until tax credits expired in 1986, and I 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 nobody ever really succeeded in doing. These days, you expect to pay $6,000–10,000 for a solar hot-water system, installed.
My friend Martin Holladay published an article in March 2012, entitled “Solar Thermal Is Dead.” He generated a lot of discussion with that article, including some dissent, so he published another article in December 2014, entitled “Solar Thermal Is Really, Really Dead.” Martin looked at solar-thermal prices and compared them to using 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 often the answer you get depends on your assumptions, and in designing and building this system we chose to challenge some of those commonly held assumptions. For one, heat pump water heaters are new enough that we don’t really know how long they will last. For another, there are great advantages to installing a system that, while not protected against freezing, will not be damaged by freezing. These are reasons to continue to explore how to make simple solar thermal work.
Enter Zak Vetter. Zak asked me to help design and install a solar hot-water system for his home near Carmel, California. It’s a roof-mounted system on a building that combines living space and shop. He had established a simple set of goals for the project:
- Greatly reduce or eliminate the need for off-site energy to provide all the hot water desired.
- Build a system that works well in less-than-ideal conditions. This means that even on a cloudy day, most (or even all) of the hot-water demand is met by the solar energy collected and stored in the system.
- Build a system that requires nearly zero maintenance.
I had never worked with such a demanding list. Many assumptions go into designing and building a traditional solar-thermal system, and these got challenged by Zak’s goals. Here are some of the assumptions we typically work from:
- Solar can provide, at best, 75% of your water heating.
- With freeze protection, solar is complex.
- Overheating is a big problem for solar.
Installing solar thermal is tricky.
Solar-thermal systems need yearly maintenance.
Design rules also involve assumptions:
- We want the most efficient collectors.
- Sizing a system for winter will cause overheating in summer.
- Parallel piping gathers the most Btu.
- Storage tanks should not be oversized since this will create stagnation problems.
- Freeze protection dictates system design.
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 95% of annual hot-water Zak’s family needs. Someone good with their hands 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 would have forced us to build a more complex, expensive system, to prevent freezing and overheating. So instead, we used really inefficient collectors! These are just coils of ¾-inch 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 top temperature we’ve measured in summer with no water flow is 170°F in the collectors, and they have frozen many times without a problem. This type of collector has been tested in San Jose, California, for 16 years and no troubles have surfaced. 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.
Each coil measures 26 square feet. Another benefit of using “inefficient” collectors is that we eliminated the need to run copper pipe to and from them, by running PEX tubing instead. With traditional copper collectors, which can stagnate in the summer sun at up to 400°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 United States 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 nonmetallic tank than needs no anode, so the water does not become aged, or contaminated, by slow turnover. The benefit of this much storage is that the system can continue to deliver hot water during sunless days.
One other benefit of the Marathon tank is its insulation. It has 3 inches of foam, and the literature says that it loses only 5°F in 24 hours. Our data logging suggests that it’s more like 6–8°F in our situation, but still, not bad. Insulation was 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 backup energy needed. So we decided to double up on the insulation wherever possible.
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 from which to lose heat. 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 4 times as long to heat at half the voltage, but Zak wanted a good test of the solar. The system was installed in November 2014, and he has yet to use the backup!
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 that protects against freezing or overheating.
The system was simple to install. If you look just at installation time, it took only six person-hours, which is very fast. In the good old days, a fast installation used to be three guys and one long day, or about 24 person-hours. This system went in so quickly for several reasons:
- We used PEX and polyethylene tubing.
- We assembled the exposed connections with Sharkbite push fittings.
- The collector manufacturer supplied us with a prebuilt control station.
- The collectors were installed on the roof using only one central bolt.
- We had easy access to the underside of the roof.
- The collectors are somewhat flexible and lightweight.
- The 105-gallon tank is lightweight and easy to move.
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.
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 simply to notice when the solar-heated water was hot enough to shower with. If the stored water is around 105°F, it’s good for showering. When we say the system is producing 95% of the hot water, it means that Zak gets acceptable shower temps 95% of the time. It’s a quick, nonmathematical 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 not taking credit for water that isn’t quite hot enough but is certainly well above groundwater temperature.
Figure 1 shows the system during the first days of spring, when the system is making an admirable contribution to the home’s hot-water supply. Figure 2 shows the system at its worst. The vertical yellow bars represent periods of sunshine, and the vertical blue bars represent nighttime. Between the 21st and the 22nd you’ll even see rain! But note how just a few hours of winter sun on the 23rd boosts the tank by almost 20°F.
The other two graphs, shown in Figure 3, illustrate the differences between December and March. In these graphs, we measured outputs from each collector to see if all four provided useful output. It turns out that the first two collectors gathered more Btu, but the second two collectors each bumped the temp up higher, so they really did help—particularly during the colder times of the year.
Read Solar Thermal is Dead, by Martin Holladay at Green Building Advisor.
The Relevance of This Design
Clearly there are 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 would make sense to avoid areas that stay below freezing for extended periods of time. But because there is no metal piping in this system, it can withstand occasional freezing. And if tax credits are the main motivation for installing solar hot water, this system won’t do, because it isn’t yet certified by the Solar Rating and Certification Corporation. Still, this system should cost less then most other systems, even without the benefit of tax credits.
It’s clearly a good thing to bring fresh perspectives to solar water heating. By intelligently questioning old ideas and by using newer materials and hardware—such as the Marathon tank, PEX piping, and polyethylene collectors—Zak pushed us to do better than I had believed possible.
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