How do Solar Thermal Panels Work ?


The simplified answer

A selective coating absorber captures the photons from the electromagnetic spectrum (Sunlight) and gets hot. Heat trans fer fluid (anti-freeze) is pumped through the hot absorber therefore itself becoming hot and carrying the heat down to a hot water cylinder. The cold water in the cylinder is heated up to be used for baths and showers.

That is a very brief explanation, but some may prefer a more in depth answer to the question, “How do solar panels work?” Here is the more detailed response, and require a little physics to understand how heat energy moves about.

The Physics

To start off, heat is measured by temperature and only flows one way. You can’t make something hotter by placing it next to something that is colder! There are three ways in which heat energy can be trans- ferred :- Conduction: The heat energy moves through the material. A good thermal conductor such as metal allows heat energy to pass quickly and easily through it. For example place one end of an iron bar in a fire and the other end ( the bit you’re holding!) get hot quickly because metal is a good conductor of heat. The end of a wooden pole in the same situation can be held for a considerable period of time be- cause wood is a poor conductor of heat. All materials conduct heat in varying degrees and surprisingly enough water is a poor conductor of heat which allows hot water in a cylinder to stay hot even with cold water below it.

Convection; here the heat energy is carried by moving the material with its heat energy somewhere else. For example, a radiator in a room will heat the air next to it, that air then rises and carries its heat around the room warming the whole room. This is natural convection. Forced convection is where the material say for example the water in a boiler is physically moved by the pump to the radiator where the radiator then gets hot. Because of its very high heat capacity water is a very good medium for convecting heat from one place to another.

Radiation: this is the direct transfer of heat by the electromagnetic spectrum (Solar radiation). You can feel radiated heat coming off a fire.

In the case of a solar thermal panel we are trying to heat above the ambient temperature so conduction and convection will work against us by taking heat from the panel to the out- side world.

The only way you can get a higher temperature than ambient is by radiation therefore all solar collectors rely on Solar radiation!

All the energy from the Sun comes via radiation. In the vacuum of space there is no material to conduct and no material for convection. The sun (at 6000 C surface temperature) is hotter than the solar panel so the panel will get hot due to the solar radiation. (this is the only way to get a higher temperature than the surroundings so all solar panels need solar radiation to get hot!)

The Terminology

The effectiveness of the collector will depend on how good it is at absorbing radiation (Selective coating) and the heat loss to the ambient temperature (Insulation factor) . As the collector heats up, the amount of heat lost to ambient will depend on the difference in temperature between the panel and ambient.

The heat loss occurs by conduction, convection and radiation. The higher the difference (hotter the panel) the faster the heat is lost until the amount of heat lost is the same as the amount of heat gained by radiation. The solar panel will then stop getting any hotter and this temperature is known as the stagnation temperature. The more power- ful the incident radiation (measured in watts/M 2 ) the more energy the panel needs to loose so the higher the stagnation temperature becomes.

solar thermal efficiency curve

Any time there is daylight there is solar radiation but the amount of energy available will depend on the incident radiation 2 level (watts/m ). In bright sunshine there is about 1000 watts of available heat energy for every square meter, on cloudy days there is still solar radiation available but it will be at reduced 2 intensity (lower watts/M).

At stagnation the collector temperature is so high compared with ambient that any heat extracted will drop the temperature so effectively the efficiency is 0 ( 1000 watts/m2 in gives a very high temp but correspondingly high losses so available heat out is zero).

It therefore follows that the maximum efficiency occurs with the collector temperature being low compared with ambient ( 1000 W/m2 in gives low losses and around 930 watts of heat out hence the efficiency is 93%) If a solar panel is 93% efficient then it takes 93% of the available energy and turns it into heat, on a cloudy day it is still 93% efficient but the amount of avail- able heat is less hence the panel output is less.

Size matters! Because the solar radiation is uniform at so many watts/M then for any collector, if you want twice as much energy the collector needs to be twice as big!

Performance parameters

The term solar collector applies to a device that is specifi- cally designed harvest energy from solar radiation.

Everything will get hot when exposed to solar radiation (think of a garden hose in summer! The water can get quite warm but as soon as you try and move that heat to somewhere useful, a paddling pool for example, the water soon runs cold as the hose isn’t a particularly good solar panel!).

So the performance of a solar panel will depend on how good it is at “getting hot”.

We have already touched on the ability of a solar collector to deliver hot water depends on the Selective coating and Insulation factors.

The heat loss to ambient by conduction and convection can be limited by good insulation or stopped completely by a vacuum.

In a vacuum tube the heat loss can only be by re radiation, as a materials temperature rises then the radiant heat given off rises ( Black body radiation) the higher the temperature the shorter the wavelength of the radiation given off.

For example “red hot” is a lower temperature than “white hot” and the corresponding wavelengths of red light are longer than the rest of the colours that make up white light.

Longer wavelengths below red (infer-red) are where radiant heat is.

The ability of a material to absorb radiation depends on colour, black is very good at absorbing radiation but also good at re-radiating it (hold your hand over a material that is black and hot you can feel the radiant heat ). In the case of a solar collector we need a material that absorbs radiation effectively over the whole spectrum but doesn’t re-radiate it, hence the term selective coating.

This is why most solar collectors look dark blue/grey rather than matt black.

The stagnation temperature of a vacuum tube collector will always be higher than other collector types because the heat loss by conduction and convection is elim

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