A bad sunburn should be enough to teach a person respect for the closest star to our planet. Sitting on a black leather car seat on a sunny summer day should do likewise, especially if you happen to be wearing shorts. Even better, try leaning against the hood of that same car on that same day. And who hasn’t felt the scalding hot water coming out of a backyard garden hose that’s been baking in the sun all day?
The sun delivers an incredible amount of heat every second that it’s in the sky. When directly overhead, it delivers 1000 watts to each square meter. That’s the same amount of energy your iron consumes when you press your clothes.
In spite of all that, solar hot water systems are a tough sell. Why is that?
Before we get into it, let’s recap what a solar hot water system is. First, the purpose. It may simply heat up your pool. It may supply domestic hot water to a home or business – for showers, washing dishes, and doing laundry. It may supply hot water for heating a building, for example by means of a radiant floor. It may even (and this, I’ll confess, is an application that causes me to geek out vigorously, although I won’t spend any time on it today) provide building cooling. Yes, using heat to make something cool. Wild.
Solar thermal water heating systems have a number of key elements. The first and most visible is the collector. This may be just a series of tiny capillaries made of black plastic, arranged in a long panel that looks like a mat. It may instead be a flat glass plate over manifold copper pipes, easily mistaken for a skylight. Or, it may be a set of evacuated tubes arranged side-by-side. The collector typically sits on a rooftop, although it may be on a rack built directly on the ground.
The second element is the working fluid, which circulates through the collector to carry away the heat to where it is used. This may be simply water, as is the case with solar pool heaters. However, water has a very narrow band between its freezing point and its boiling point. If a wider band is required, a different working fluid is needed. For example, in a region with cold winters and hot summers, a good choice is a 50-50 mix of water and propylene glycol. This mixture will only freeze at around -26°C (-25°F) and boil at 144°C (291°F).
Up next is the heat exchanger. In pool heaters, there typically isn’t one – the sun-heated water mixes directly with the rest of the pool water. For systems with a non-water working fluid (like the glycol mentioned above), in the heat exchanger the working fluid flows past one side of a flat metal plate, and the water flows past the other side. The glycol heats the plate, and the plate heats the water.
Finally, there is storage. Again, with pool heaters this element is absent. For other systems, the hot water tank is not much different from a typical water heater tank, except there is no electric element or gas burner at the bottom. The water circulates through the heat exchanger and back into the storage tank until it reaches the desired temperature.
In summary, the working fluid is heated up in the collector, it gives up its heat to the water in the heat exchanger, and the heated water is stored until it is needed.
All that is simple enough. So why are these systems so rare?
First, the sun suffers from Rodney Dangerfield Syndrome – it don’t get no respect. People just don’t seem to believe that a heater that uses sunshine could work, or work well enough to ensure, say, a suitably hot shower. This despite all the lessons we repeatedly learn about sunburns, hot car seats, hot car hoods, and hot water in garden hoses. The sun is plenty hot. It’s photosphere is 5500°C (9900°F), after all. Perhaps we take it for granted because it is such a familiar, everyday thing.
I had a personal and novel experience with the reality of sun-heated H2O this week. I was on the roof of a new retirement complex, doing the finishing touches on an eight-panel solar hot water system. We put some water through the array to flush out any debris and check for leaks. The leaks were easy enough to find, as there was steam - yes, steam - shooting out of them. The panels were hot enough from the mid-April Canadian sunshine to flash water into steam. (We fixed the leaks and had the system up and running the next day.)
Second is the issue of timing. People think about when the sun is hottest – high noon – and compare that with when they most want hot water – for their shower first thing in the morning. What’s the good of hot water if it’s not hot when you need it? The answer is that the hot water stays hot for quite a bit longer than many would imagine. Anyone that’s used a double-walled, vacuum-insulated container like the ones made by Thermos will realize how well a properly-designed vessel can retain heat. One 100-gallon storage tank that my company installs is rated for a loss of 5°C (9°F) per 24 hours. To provide you with a hot (40°C, or 104°F) shower at 7AM, your system just needs to have heated the water to 42.5°C (109°F) when it calls it a day at 7PM the prior night.
That raises the third issue – reliability. The system may work just fine, but what if the sun doesn’t shine? A string of overcast days would make for a string of cold showers if your only source of hot water is sunshine. That is absolutely true, and that is why solar thermal hot water systems are rarely implemented without some form of backup, typically electrical.
The last issue, and the one that is the biggest barrier to wider adoption of solar thermal hot water systems, is economics. Four years ago, the price of natural gas was at an all-time high and people were looking very seriously at alternatives. Today, largely because hydraulic fracturing technology has offered up huge new reserves, the price of natural gas is one-seventh what it was. Solar thermal can only compete in areas without natural gas service, where electric water heaters are the only other option.
There’s only so much natural gas in the ground. Inevitably, the price will rise again. However, until that day comes, solar thermal hot water systems will be limited to applications where government subsidies are available, and locations where natural gas is not available.
But in those applications and locations, it is real, it works, and it is reliable.
And the fuel is and always will be free.
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