Solar hot water: works well.
We’ve had our solar hot water system fully operational at our house for two days now, and so far it has been working extremely well. As expected, it has provided plenty of hot water for our family of three without the need for any additional auxiliary heating. So, not only do I get a hot shower in the morning, I get the pleasant sense of not totally screwing up the environment.
I had a hard time finding good information about solar hot water on the web - there’s a lot of marketing fluff from the manufacturers, and general theory, but not much data on what the systems are really like from the user’s perspective. I’ve learned a lot over the last couple of days from watching the installation and using the system, so I thought I’d share my observations. I’m no mechanical engineer, so a lot of what follows is just my unschooled analysis of what I’ve learned and heard about. Rely on my conjectures at your own risk!
The Collectors
The collectors are a pair of 4′x6′ panels that mount on the roof. The roof on our house tilts 20 degrees toward the south and is completely unshaded, so it’s really perfect for a solar installation. The only drawback is that it’s covered with terra cotta tiles, which require a little extra work when installing the panels. You have to install some curved brackets that fit under the tiles to support the panels, and you wind up breaking and repairing a few, so there’s some fiddling around. The panels themselves are slim (about 4″ thick) black aluminum frames, covered with anti-reflective glass (actually, shatterproof acrylic, I think). If you peer into them in bright sunlight, you can see an internal sheet of copper, tinted midnight blue, that captures the solar energy. Soldered to the copper sheet is a serpentine pipe that carries the heat exchange fluid.
This arrangement creates the mother of all greenhouse effects. The controller (more on that in a minute) reports the internal temperature of the panels. Yesterday, June 18 (nearly the solstice!), which was cool and partly overcast, the panels reached a maximum temperature of 225 degrees F. Palo Alto is at 37 degrees north latitude. That’s hot enough not only to fry an egg (or boil water), but also to cook a few slices of bacon to go with it. All that energy capture translated into heating up 80 gallons of water in our tank to 155 degrees F.
The Tank
It turns out that the humble tank, which you might naively think of as not that important, actually has a huge effect on the efficiency and performance of the system. There are two basic layouts: you can use a solar storage tank that gets warmed by the sun and feeds preheated water to your regular gas fired water heater, reducing the amount of work it needs to do. Or, as we did, you can use a single solar heated tank with an auxiliary 220V electric heating element. Mostly we chose the second option for space reasons, but it also turns out to be more energy efficient. The only drawback is that there’s a little less capacity (or so I hear). But an 80 gallon tank like we have is apparently more than enough at least for our three person family. The electric heating works fine - we had to use the tank in electric-only mode for a couple of days before the collectors got hooked up, and it was OK.
Why is the tank so important? And why is the single tank approach more efficient? Well, it turns out that there is a huge temperature gradient inside a hot water tank. The top will be at, say, 120 degrees, which is a normally usable temperature. Water at the bottom will be at 80 - 90 degrees. How you handle that temperature differential has a big impact on the efficiency of the system. All water heaters deliver hot water out of the top of tank, and pull in cold water into the bottom. But that’s not the whole story.
A regular gas water heater handles that gradient really badly. It heats the water at the bottom using a gas burner, which is the least efficient thing you can do (although I believe there may be new, energy efficient gas heaters that work differently). Effectively you’re putting all your energy into heating water that’s the coldest and least likely to be used. The electric element in the Schuco tank, on the other hand, is located at the top. That means that when auxiliary heating is needed, only a relatively small amount of water needs to be warmed to deliver usable temperatures. You don’t care if the water at the bottom is at 80 degrees, as long as what’s coming out the top is hot enough.
The solar tank also has a heat exchanger located near the bottom of the tank. A small pump circulates propylene glycol up through the solar panels, back down at high temperature into the top of the heat exchanger (near the midline of the tank), then down through the heat exchanger where it warms up the water, and out the bottom of the tank where it can be pumped back up into the panels again. The solar system is also heating water at the bottom of the tank, but that’s actually an advantage, at least when the heat is free and plentiful. You want to capture as much heat during the day as you can, so that it’s still hot the next morning for your shower. You can only get the water so hot before the tank gets damaged (about 170 degrees), so you want to eliminate the temperature gradient as much as you can to store the maximum amount of energy within the allowable operating range of the system. By the way, nothing disastrous happens at 170 degrees - its just that the lining of the tank doesn’t last as long. So the controller is set up to shut down the panels once the temperature reaches that high. It’s clear that you could, on a really hot sunny day, capture enough energy to boil the water in the tank, and that’s something that you really, really don’t want to do.
The other key thing about the tank is the insulation. You don’t get any heat at night, so it need to be able to store enough heat overnight for morning use. This works fine - water at 155 degrees at 6 pm was still at about 125 degrees at 8 am the next morning. This is also a place where the single tank solution is better, since you only have one tank bleeding heat.
The Controller
The controller monitors the temperature at the top and bottom of the tank and the temperature of the solar panels, and turns on and off the pump as appropriate. I mentioned the temperature limits: the controller has a “stagnation mode” that it uses if the tank gets too hot, where it drains the fluid out of the solar panels and into an expansion tank, stopping any further heat capture. The controller is quite nice - I haven’t read the manual yet, but it gives out lots of information about the temperatures it is observing and the activity of the system. Since I’ve become a hot water nerd, I find myself drifting out to the garage a few times a day to observe, with a quiet sense of satisfaction, the action of the sun heating up our water. I’m hoping I can get over it soon. The controller also turns the electric element on if needed to maintain the output temperature. It has logic (which I think is adjustable) to decide if it can wait a little bit–I believe the default is 30 minutes–if the panels are coming up to temperature and it could avoid using electricity. Here’s where you get to be extra-green: taking the risk of slightly cooler shower to save some electrons. I’m not sure I’m that green - I have the mode enabled, but we’ll see how it goes. Hey, we’re buying renewable power anyway (it’s an option in Palo Alto).
The Plumbing
It would all be pretty simple if it wasn’t for our recirculation pump. We’re using a wirelessly activated recirc pump, triggered by buttons at the faucets, to circulate hot water through a loop to all our faucets. You hit the button, wait a few seconds, and you have hot water almost instantly with very little waste. This gets a little tricky because it has to be plumbed together with an anti-scald valve. Remember, you could serve 170 degree water out of the tank, so you need a device that mixes in some cold water to moderate the temperature to something you could put on your skin without getting seriously burned. It’s a little tricky to plumb all this together. If I get inspired, I’ll write up a separate entry later about how this works. Here’s the problem: you have a closed loop connected to the water heater, and yet you need to inject cold water when you recirculate. How are you going to do that? You can’t compress water, can you? Solve that puzzle if you can! Hint - you can use cold water from the recirc loop OR cold water from the input line…
The pipes carrying the heat transfer fluid come together at the solar tank, where they tie into the controller. There’s an insulated box with three analog dial gauges, showing the temperature of the fluid leaving the tank (cool) and returning from the collectors (hot) and the pressure of the heat transfer fluid. It looks kind of cool.
Conclusion
The system is really beautiful. All the components seem to be of the highest quality, and so far it all seems to be working extremely well. We’ll see how it holds up over time, but since the pump is the only moving part, I imagine that it will work well for years. You know how you look at the way something is designed and manufactured and you can just tell that it’s good, well-thought-out equipment? That’s my impression, for sure. Anyway, based on my experience so far, I would highly recommend solar hot water to other people. I mean, it’s such a no-brainer: save as much carbon emissions each year as you would by not driving, get free hot water for ever, enjoy the same lifestyle, and all for a pretty small capital investment (for us, less than $5000 after incentives).
- John Seybold, Palo Alto
