Flatplate collectors

The most common collectors for solar domestic water heating systems in many countries today are flat-plate collectors. These mostly consist of three components:

• transparent cover

• collector housing

An absorber is inside the flat-plate collector housing. This absorber converts sunlight to heat and transfers it to water in tubes, which passes through the system. The collector housing is highly insulated on the back and sides to keep heat losses to a minimum; however, there are still some collector heat losses, which mainly depend on the temperature difference between the absorber and ambient air. These losses are subdivided into convection and radiation losses. Movements of the air cause the convection losses.

A pane of glass covers the collector and avoids most of the convection losses. Furthermore, it reduces heat radiation from the absorber to the environment in the same way as a greenhouse. However, the glass also reflects a small part of the sunlight that can no longer reach the absorber. Figures 3.6 and 3.7 show the mechanism and energy flow in flat-plate collectors.

The front glass cover reflects and absorbs a small part of the solar radiant power as shown in Figure 3.8; however, the majority of the solar radiation passes through the glass. The reflectance p, absorptance a and transmittance t

Figure 3.6 Processes in a Flat-plate Collector

can describe these processes. The sum of these three values must always be equal to one:

The absorption of the solar radiation heats up the pane of glass. If the glass is in thermal equilibrium, it must emit the absorbed radiation. Then, the emitted radiant power <&e is equal to the absorbed radiant power <&a, otherwise the glass would heat up indefinitely. Hence, the emittance £ is equal to the absorptance a:

On one hand, the front cover should let through the majority of the solar radiation. On the other hand, it should also keep back the heat radiation of the absorber and reduce the convection losses to the environment. Most collectors use a single glass layer made of thermally treated low-iron solar glass. This glass has a high transmittance (t ^ 1) and a good resistance to the influences of the environment. Front covers made of glass prevail against those made of plastic because the lifetime of plastic is limited due to a poorer resistance to ultraviolet radiation and the influences of weather.

Double glazing can result in further heat loss reductions but also reduces the transmitted solar radiant power and increases costs.

The corresponding radiant powers are:

Reflection at glass cover 8%

Absorption of glass cover 2%

Reflection at glass cover 8%

Absorption of glass cover 2%

Heat emission of absorber 6%

Absorber 60%

Was this article helpful?

0 0
Renewable Energy 101

Renewable Energy 101

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable. The usage of renewable energy sources is very important when considering the sustainability of the existing energy usage of the world. While there is currently an abundance of non-renewable energy sources, such as nuclear fuels, these energy sources are depleting. In addition to being a non-renewable supply, the non-renewable energy sources release emissions into the air, which has an adverse effect on the environment.

Get My Free Ebook


Post a comment