Introduction to Solar Thermal Technology

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In the previous two articles I talked about energy security and India’s energy security scenarios in 2047.

In this and the next few articles I will talk about solar thermal technology because the first thing that people in India think of when they think of solar is water heaters. I won’t blame them for that though since solar heaters have been around for a good three decades in India now; I actually know two gentlemen (personally) who installed their solar water heating systems in the late 1980s and the systems are still working! They needed some repairs during the last 25 years, but by and large, they have done their job very well and more than justified the investment I would say.

Solar Thermal collectors
As can be seen in the figure above, solar thermal collectors can be broadly classified in two categories: non-concentrating and concentrating. In the non-concentrating collectors, there are two main types of collectors: flat plate collectors (FPCs) and evacuated tube collectors (ETCs).

Solar Water Heater

The FPC that you see in the figure above is the most common type of solar water heater and the type that has been very popular in India for the last three decades. You can see them a lot on the roofs in cities like Pune because: firstly, there are many individual houses/bungalows in Pune, and since the houses/bungalows are owned individually, permission for installation is not an issue; secondly, there is a requirement for hot/warm water for 8-9 months in an year. These types of water heaters are also very popular in hotels and student hostels. Each unit generates 100 to 300 litres of hot water per day, with the maximum temperature being around the 60°C mark.

Solar Water Heater 1
FPCs work on the thermosyphon working principle. As the water gets heated, it rises up since hot water is lighter than cold water, and is replaced by cold water from the bottom. This works well for small residential heating systems, but for larger systems, forced-flow is required, as can be seen in the figure below.

Water Heater
In the forced-flow systems, a pump forces water through the system. When the temperature reaches a preset value, the pump is switched off. When hot water is used and is replaced by cold water, its temperature falls below the preset value, and the pump is switched on thus resuming the heating.


ETCs also work on the thermo-syphon working principle but differ from FPCs in their construction as can be seen in the figure above. ETCs consist of multiple tubes, each of which has concentric tubes with vacuum in between to eliminate conductive and convective losses. The other thing about ETCs is that the sun’s rays are always perpendicular to it because of their tubular construction, as can be seen in the figure below.


A quick comparison of FPCs and ETCs is given in the table below:

Have been around for almost 100 yearsHave been invented fairly recently.
Work well in locations with hot climate.Work well in cold climates as well.
Are not modular.Are modular.
Need 2-3 people for installation.Can be installed by one person.
Cost more for shipping since they occupy a lot of space.Cost less for shipping since they can be stored vertically.
Cost less than ETCs.Cost 20%-40% more than FPCs.

I hope you liked this brief introduction to solar thermal technology. In the next article I will talk about concentrated solar thermal (CST) technology.

Sustainably yours,

Prashant Karhade.
Writer, Publisher, Entrepreneur

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