When we talk about solar power, it’s natural to first think about photovoltaic panels and turning sunlight into usable electricity for your home. However, electricity isn’t always the best type of energy to convert solar radiation into. Using solar electricity to cook or otherwise heat things is pretty inefficient. Any electrical heating puts a major strain on a solar power system, which is why using natural gas for cooking and general heating duties is a common solution.
The problem with that approach is that it still makes you dependent on external infrastructure; in this case, the gas company from whom you buy. You can only stockpile so much of it, which means it’s not really a renewable resource. Of course, this is not a problem for everyone, but if you want to be 100% solar you’ll need alternative technologies. A solar water heater is a major step towards being 100% solar. It uses solar energy to directly heat water for home use, rather than turning it into electricity first.
Even if you don’t have any other solar power sources, just changing your grid-electric water heater for a solar one can slash your utility bill significantly. Sounds good, right? That doesn’t mean you should just go out and buy the first thing labeled “solar water heater”. There are more decisions to make than you might think, and I’m going to walk you through the most important ones.
Before we look at specific designs, however, let’s go over the general operating principles that solar water heaters use.
How the Magic Happens
For a solar heater to provide you with hot water, it needs to get the energy of the sun into the water that you’ll use. Different heater designs have different ways of achieving this, but there are always three main components to every heater.
The first is also the most obvious. You need a solar collector of some sort – a device that absorbs solar radiation and releases it as heat. This heat moves to the next component – the transfer medium. This is the material that moves the heat from where it is collected to where it is needed. The last component is the one that actually stores your water. This is usually a geyser, tightly-insulated and large enough to store hot water for an entire day. After all, solar heaters don’t work at night.
Spoiled for Choice
There are five different main designs for solar water heaters. Each one of them has a different approach to turning sunlight into hot water. Each also has a unique set of pros and cons, which makes it essential to pick the right one for you.
There are two broad types of solar water heater. Direct systems circulate the water to the solar collector, where it is heated directly before passing to the storage tank. The main disadvantage of direct systems is that they are vulnerable to freezing temperatures. As you can imagine, frozen water in the pipe will wreak havoc on circulation.
Indirect systems don’t heat the water directly, but instead use something else, like special freeze-resistant fluids, to heat the water. The collector part of the solar water heater system works with either type of heater. The only difference is what the collector transfers the heat to. Before we look at the different overall heater designs, it’s good to know what types of collectors you can buy.
Solar Collector Technologies
The most visible part of a solar water heater system is the collector. These usually lie on the roof of the home. Because we want the circulation system to be efficient, it’s better to have the collector close to the water storage device. The longer the circuit, the more opportunity for heat to be lost. In general, there are two main collector designs, at least when it comes to mainstream equipment that you would buy off the shelf.
A flat plate collector is simple and relatively affordable. It consists of a flat metal plate, usually copper or aluminum. This plate is made black so that it absorbs as much light as possible. Metal pipes are welded directly to the plate. It’s through these channels that either water or some other medium circulates. That would seem like enough – however, the whole shebang is installed in a sealed box covered by a sheet of glass or sometimes plastic. This creates an air gap between the plate and the glass, turning the collector into a small greenhouse. All of this works to maximize the amount of heat captured and transferred to the fluid running through the collector.
The other type of collector is the heat pipe or evacuated tube collector. If you’re into modern computer hardware, you might have heard the term “heat pipe” before. A few years ago, heat pipes became very popular as a way to move heat away from hot processors to fansinks, which used airflow to transfer the heat to the outside environment.
In solar collectors, the heat pipes are sealed copper tubes. Inside these tubes is some purified water and a vacuum. Since the water is in a vacuum, its boiling point is very low – something like 30 degrees Celsius. At that temperature the water turns to steam, rising up the length of the pipe where it dumps the heat and turns back to a liquid again. It’s a closed system and is pretty effective at moving heat from one end of the pipe to the other. Water or another liquid medium circulates at the end where the heat is dumped and is taken to the water storage unit.
Evacuated tubes operate on a similar principle, but are designed differently. They are made from glass. The system consists of two tubes, one within the other. There’s a vacuum between these tubes, with the inner tube collecting the heat that gets trapped in the vacuum.
Direct Circulation Systems
This is a setup where water is directly circulated through the collector using a pump. Since direct systems like these are vulnerable to freezing, they usually have countermeasures in place, such as pumping hot water from the storage tank back up to unfreeze the external pipes. Another method is to flush the collector, but that interrupts its operation.
Since it uses a pump to circulate water, this system is also a little more power hungry. Thanks to the freezing issue you should only consider such a direct circulation system where the climate makes freezing rare.
This is the most cost-effective solution, with a relatively simple design. Also, because the water is heated directly there is less heat exchange loss. There are temperature sensors at the collector and in the storage vessel. The sensor is at the bottom of the tank, where the water is colder. When the water at the bottom of the tank is much colder than the water in the collector, the electric pump kicks in and pushes the cold water to the collector while bringing the warmer water in at the top.
This is a solar water heater design that’s perfect for parts of the world with lots of intense sunlight. These heaters are an example of passive solar water heaters. What this system does is circulate a special fluid that has antifreeze properties. This means there is no practical danger of the circulating fluid freezing up. The fluid goes through the collector, picking up heat, It is then circulated to a heat exchanger and the energy is transferred to the water itself.
Thermosiphon heaters are referred to as passive because there is no pump. The hotter the system gets, the more the fluid circulates. The tank is actually mounted higher than the collector, using the principle of convection to move things along.
Indirect systems and thermosiphon systems use the same closed-loop antifreeze principle, but in the case of indirect water heaters in general, an electric pump is still used. Despite needing to do an extra step of heat exchange, these water heaters manage efficiency figures of as much as 90%.
You can also get a variant of the general indirect system called a “drain-down” system. This setup uses gravity to drain the collector when the sun isn’t out, which prevents freezing.
Air Solar Water Heaters
Air heaters are another type of indirect water heater. The difference is that the exchange medium is plain old air. Obviously, there’s no worry that the air will freeze. If it was that cold, a hot shower is the least of your worries.
The main advantage of using a air-to-water exchange is that there’s almost no maintenance. These systems can’t leak and never freeze. The downside is that only about 50% of your solar heat actually makes it to the water. It’s a tradeoff worth considering if you need something that will be virtually maintenance free.
Hot, Wet, and Eco-friendly
While going full solar on the electric side is still pretty rare, solar water heaters are gaining popularity quickly. Unlike solar power, the efficiency numbers are high, so a small heater can supply a household with free hot water easily. On top of that, the reduction in grid power bills is a big pull.
I like to think of solar water heaters as the first step to going off-grid. It gets people used to the idea that the sun can provide us with cheap energy. It also shows that a little effort to convert your energy source can go a long way in the long run.