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Controlling solar thermal systems

The controllers used in solar thermal systems come in many different shapes and sizes and offer a variety of different options. The most basic controllers just have a differential control – this activates the solar pump when the temperature at the sensor on the collector is at a higher temperature than the sensor on the storage tank.

However, there is a lot more a controller can do to help you to get more out of your solar system. Almost every controller we install is set up slightly differently, as each system is designed specifically for the site. The pipe length, for example, between the collector and tank will determine the most energy efficient setting for the switch on switch off temperature of the pump.

We install controllers with many additional functions as standard. The most important of these is pump speed control. This means the controller constantly measures the collector temperature and the bottom of the store temperature and varies the pump speed accordingly. The greater the difference between the collector and the storage vessel, the faster the flow through the system. This can significantly increase the annual yield.

In larger systems controllers can be used to direct the solar heat to the part of the tank (or thermal store) where it is needed most. When there is a heat requirement the solar energy will be directed to the top of the tank and when the top of the tank is satisfied it will be directed to the bottom of the tank.

Controllers also give you useful information about your system. The standard controllers display the current status of the system including the temperature of the collector and at the bottom and top of the store, so you can see how much hot water is available. It also displays the current pump speed percentage and a cumulative count of the pump operation hours.

Controllers can also be used for heat quantity measurement. This allows you to measure how much energy has been harvested by the system. The controller uses additional sensors to calculate how much energy has been inputted to the storage vessel and shows a cumulative count of how many Kwh of energy have been harvested.

There are many different varieties of controllers. It is important to choose one with additional functions to allow you to make the most of the energy available from the solar panels. Once your system has been installed your installer should take time to guide you through the system and show you how the controller works, what the different readings mean and the different functions available. You should also be left with a user guide for future reference.

About the author: Gabriel Wondrausch is founder and director of SunGift Solar, which installs solar thermal and other renewable energy systems in the South West of England. 

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