In this article we will talk about flat panel solar water heaters. The system consists of rectangular flat panel collectors. Above them is a 300 liter hot water tank that is connected to the collectors below. For this house two solar water heaters were installed together bringing the hot water capacity to about 600 liters.

1.   Working Principle

Now let's find out how the indirect system works. Another common term used for indirect systems is closed loop systems. To make it easier to understand we'll talk about two closed loops inside the tank. The first closed loop is the glycol and water mix circulating around the tank. Glycol is an antifreeze chemical as it prevents water inside the tank from freezing. This is especially important for areas that experience freezing winters. Once the glycol mixture is filled inside the tank it flows down to the flat panel collectors to be heated up by the sun's heat. At the top of the collectors are glazed glass sheets. They are very durable as they are designed to withstand tough weather conditions.

Below the glazed glass sheet we'll find copper tubes and a blackened heat absorbing plate. When the sun's heat passes through the glass sheet it gets absorbed by the heat absorbing plate. The plate is made out of aluminium as it's a good conductor of heat. It is painted black to ensure as much of the sun's heat is absorbed as possible. The copper tubes are in direct contact with the black aluminium plate to ensure maximum heat absorption. The glycol and water mixture pass through the copper tubes inside the collectors. The black plate transfers the heat it has absorbed from the sun to the copper tubes. The copper tubes then transfer that same heat to the glycol and water mixture flowing inside the tubes so there's a lot of heat exchange going on inside the collector.

The copper tubes run lengthwise across the rectangular collector providing a big surface area to heat the glycol mix because hot water is less dense than cold water. Hot water rises up the copper tubes and exits at the top of the collector. This is thanks to a phenomenon called convection.

From there the hot glycol and water mix flows back to the tank completing the first closed loop. However the hot glycol and water mixture inside the tank is not suitable for home use. It only circulates from the tank into the collectors and back to the tank. In other words none of this glycol and water mix is directed to your house, so the question is how do you get hot water inside your house?

The answer to that is with the second closed loop - the hot water to your house loop. The cold water enters the tank from an inlet pipe. From there it is directed into a flexible but strong stainless steel coil located inside the tank. Cold water circulates within this coil and leaves the tank from the same coil as hot water and through an outlet pipe.

So how does the cold water inside the stainless steel coil get heated up? The answer is through the glycol and water mix from the first loop. When this glycol mixture leaves the flat panel collectors and into the tank it's very hot. It's the heat from the glycol mix that heats up the stainless steel coil. When the coil gets hot it heats up the water flowing inside it which then leaves the tank as hot water to be used in your home.

2.   Benefits

Now let's look at some benefits of an indirect flat panel solar water heater.

●     One benefit is getting pressurised hot water to your house. It uses the same pump or tap pressure to distribute hot water around your home.

●     Another benefit is less scaling of water because of the flexible heat exchanging stainless steel coil. Any impurities inside the water gets washed away as there's always a constant flow of water

●     Compatibility with water pumps is another benefit of closed loop systems. This ensures a steady flow of water around the flat panel solar water heater.

3.   Accessories

●     To ensure constant pressure within the system a water tank is installed. It channels cold water from this tank to the solar water heating system on the roof . There's another small but important pump called a circulation pump. This provides sufficient pressure to hot water taps around your house. For example when you open the hot water tap on your shower you immediately get hot water. This pump removes the waiting time of getting hot water to your taps.

●     Another important and optional accessory is called a solar controller. It can digitally display the temperature of the water inside the tank at any given time of the day. This allows you to know how hot your water is. The controller also allows you to automatically trigger the electrical heater inside the tank when temperatures are too low from the sun.

●     The electrical immersion heater is also another optional accessory that is added to the tank. It boosts the temperature of the glycol mixture inside the tank when there isn't enough heat from the sun.

Some additional notes to consider: There are various capacities that you can choose ranging from 100 litres, 200 litres to 300 litres. The capacity that you want will be determined by your hot water needs, the number of people who are going to use the system on a daily basis and your budget.