Solar Hot Water

(Solar hot water panel in Nova Scotia)

One of you asked about solar hot water heating, and another asked why we installed solar photovoltaic panels at our house instead of putting in solar hot water.

Normally, solar hot water costs much less to implement than solar power. In our case, we currently have a natural gas combi boiler and no big hot water tank. We would need to add a tank to our system. That made solar hot water a somewhat bigger project for us than it would be for someone who already has a hot water tank. We also don’t use much hot water in summer when we would get the most from a solar hot water system, and use more (for heating) in winter when the system wouldn’t produce well, so it wouldn’t reduce the energy we pull from infrastructure very much.

Our solar PV panels will be supplemented with a microwind turbine by the time the sun is too low for the panels to perform well. We use electricity throughout the year, so what we’re doing will reduce our pull from infrastructure more.

However, solar hot water is often a more productive and more cost effective choice than in our case. The friend who asked why we used our roof space for photovoltaic panels and not for water heating used to have solar hot water at a previous home. With more people in the house and therefore more hot water use, she knows at first hand that it was worthwhile.

That leaves the question of how hard it is to implement.

If you already have a hot water tank of some type, it shouldn’t be difficult. You may plumb it in to feed your existing tank so that your hot water is preheated by the sun and you only use energy from the grid to top up its temperature. Alternatively, you may set up a separate tank connected to the solar heating system and use that to feed your existing hot water tank. Which way you go about it depends on more variables than we should get into in a short overview. (Remember, as an example of such factors, that our household doesn’t have many people in it and our friend’s household has more.)

Then there is the question of how sophisticated the solar hot water system needs to be.

At the high end, you could buy a commercial system that circulates coolant through the solar panel and through a heat exchanger in the tank. The advantage of this approach is that like the coolant in a car’s radiator, the coolant can be chosen so that on a hot sunny day, the coolant will continue to be liquid at temperatures where plain water would turn to steam. You don’t want vapor in the tubes of the panel. Vapor doesn’t pump through in the same way and doesn’t have the same energy density characteristics. But with coolant, you generally do need a pump to keep it moving through the system, and you don’t want any leaks that contaminate the water in your tank with coolant. This means the sophisticated system has running costs, needs regular maintenance and has a couple of pieces of equipment (the pump and heat exchanger) vulnerable to breakdown that a simpler system might avoid.

A simpler system circulates the water itself through the solar panel. If it cools off too much in the tank before it gets used, it may go through the panel again to get reheated. This type of system may be active, with smaller tubes and a pump, or passive with larger tubes and relying on physics (the fact that hot water rises and cooler water sinks) to circulate the water. Obviously, the vertical relationship between panel and tank is essential for a passive system and doesn’t matter so much for an active system.

You can build the simpler type of system yourself if you want. You can even do it mostly with parts from a junkyard and some plumbing supplies from a hardware store.

DIY Solar Hot Water

Decades ago, when the magazine Mother Earth News was young, my aunt gave me a subscription. While I was working at the Johnson Space Center, the magazine published an article about how to build your own solar hot water heater out of a junked refrigerator.

First you had to remove everything from the back of it and lay it on its back. Whatever you found to use as a water tank nestled in the body of the refrigerator. You removed the structured inner lining of the door to expose the insulation there. Your heat collecting tubes nestled there, and you tucked them in with a glass panel cut to fit neatly under the door gasket (with holes for the inflow and outflow flex plumbing).

You propped open the refrigerator door facing the sun when you wanted to make solar hot water, and closed the refrigerator door at night to keep the heat in.

For a passive system, your tubing could be a grid of piping from the hardware store. The diameter needed to be relatively large so water would circulate fast enough to avoid getting steam in your system. For an active system, you could use narrower copper piping in a serpentine pattern (or sometimes even clean out and repurpose the heat exchanger you removed from the back of the refrigerator). The thinner tubing would allow you to pack more heat exchange piping into the door and get more efficient solar heat gain, but fluid would not naturally flow fast enough to avoid the steam issue. You needed a pump.

For fun, my boss at the lab decided to build one for himself. He used the active system design, but thought the tubing he used would be able to operate passively. When he told me about his choices, I remarked that I thought he would need a pump.

On a slightly grey day at 50 degrees Fahrenheit, he tested it. Steam came out.

He added a windshield wiper fluid pump from a junkyard and then it worked beautifully.

One More Thought

If you’re thinking of putting in your own system now, I have just one more detail to add to the ones you’ve glimpsed in these anecdotes. If you are on an urban water system, accustomed to having that standard pressure in your water lines, switching your hot water entirely to a gravity feed from your attic will probably not make you happy. In that situation, the cold water still has urban pressure but the hot water only has much lower pressure from the gravity feed. In that situation, mixer faucets don’t perform well. The powerful cold water overwhelms the wimpy hot water. Getting the temperature right for your shower becomes a frustrating battle.

Try to equalize the pressure for your hot and cold water, and you’ll be fine.