How Solar Thermal Collectors Work

The world of solar thermal collectors can be very confusing. Terms like active and passive solar systems, low-, and high-temperature solar thermal collectors might seem daunting at first, but are in fact quite simple.

The goal of this article is to give you a brief overview over different types of solar thermal collectors, how these devices collect heat by absorbing sunlight, and from there, different ways we distribute and use this heat energy. Let’s start with the basics:


All solar thermal collectors such all essentially built around one functionality:

Converting sunlight into thermal energy (heat). Heat is an energy form that is both more useful and easier to store. However, the design of different types of solar thermal collectors can be very different.

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The Difference Between Passive and Active Solar

Solar thermal collectors can be both passive and active. Passive systems are by definition not reliant on external sources of energy to function, but rather use natural convection to transport heat, which takes place when there are different temperatures within a fluid.

In more practical terms, this means that as the sunlight is being collected and heat is absorbed, hot water rises. We now have a natural movement or flow of water.

Other common uses of passive solar systems include geyser or bubble pumps, space heating, solar cookers and solar chimneys. There are two types of passive solar thermal collectors on the market:

Active solar technologies, as you guessed, do use external sources of energy to function, most often electrical pumps and valves that deals with the heat transportation.

Solar thermal collectors are usually classified as low-, medium- or high-temperature. Low- and medium-temperature solar thermal collectors are mostly used in residential and commercial buildings, while high-temperature solar thermal collectors are needed to generate electricity on the utility-scale. Here are some typical examples of each category:


Low-Temperature Solar Thermal Collectors

The solar thermal collectors in the low-temperature category are usually based on very simple designs, many of which fits into the passive category. They are often found in swimming pools, or other places where the temperature-demand is quite low – at least compared to the temperature that is required to generate electricity. Most of them are based on flat panel designs.

Heating, cooling and ventilation (HVAC) is another excellent use of low-temperature solar thermal collectors. Read Building a Passive Solar Home – 5 Best Design Strategies to learn more about how we can design a building to take advantage of sunlight.


Medium-Temperature Solar Thermal Collectors

These solar thermal collectors are slightly more complex, but are able to supply a higher temperature. You often find medium-temperature solar thermal collectors being used to cover a building’s hot water demand (or parts of it). They can also be used to sustain a comfortable room temperature. As with low-temperature, medium-temperature solar thermal collectors are usually based on flat plate panels.

This is also the category where solar drying of fuels and food, solar cookers and solar distillation belongs.

Learn more about medium-temperature solar thermal collectors in the following article: (coming soon)

Flat plate solar thermal collectors on rooftop

High-Temperature Solar Thermal Collectors

High-temperature solar thermal collectors are of higher complexity and can usually be found in concentrated solar power (CSP) plants, due to the high temperature requirement of generating electricity by the use of steam turbines.

Concentrated solar power plants can be based on solar thermal collectors of many different designs. Learn more about the designs of high-temperature solar thermal collectors, CSP plants, and how we convert solar thermal energy into electricity in the following article (coming soon):
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