Difference Between Solar Panels

Difference Between Solar Panels? The starting point for the performance of any solar panel is the availability of energy from the sun. For Ireland the typical energy irradiated by the sun on a 30o inclined surface facing south is 1,074 kWh per sq metre per year. This is quite a sufficient resource and counters the notion that Ireland does not have the climate for solar panels. It must be noted that this 1074 kWh/m2/yr energy yield comes from a combination of direct, diffuse and reflected solar radiation.

Difference Between Solar Panels

The basis for energy collection by solar panels will be familiar to all, which is that a black surface will absorb the suns radiation more than other coloured surfaces – think of a white car and a black car next to each other under the same sun as a good example of the difference between solar panels. The temperature of the black car will be far greater than that of the white car. However a simple black surface will also loss or emit back this absorbed radiation, so for solar panels special selective surfaces are used to increase the efficiency of energy collection. These selective surfaces are balanced to have good solar absorbivity levels whilst minimising emissivity losses. Both flat plate panels and evacuated tube panels utilise these selective surfaces to maximise their efficiencies. So, apart from the cosmetics, where do the performance differences arise from these main types of panels? In general evacuated tube panels tend to be more efficient than flat plate panels and this difference in efficiency can be better understood when you look at the means by which heat is transferred.

Solar panels both gain and lose heat. The transfer of heat to the panels from the sun is almost entirely from radiation i.e. the solar radiation is absorbed by the selective surface of the panel which heats it. Once a solar panel has gained heat from the sun its efficiency is then determined by how much of this heat is then lost. The loss of heat from a flat plate solar panel has 3 transfer routes i.e. conduction, convection and radiation where as an evacuated tube has only 1 heat loss transfer route i.e. radiation. This is because the heat cannot be conducted or convected within a vacuum (similar to that of a flask). This basic principle means the heat loss from evacuated tubes is significantly lower than that of flat plate panels.

It should also be noted that flat plat panels tend to be very efficient in high sun and when receiving direct radiation whilst evacuated tubes are more even in their solar radiation absorption and absorb significantly more diffuse radiation (solar radiation broken up through cloud cover) than their flat plate counter parts. There are also differences between evacuated tube panels and again the above principles apply to maximising energy collection and retention. A common method of heat transfer to the water in an evacuated solar panel system is by means of a ‘heat pipe’. However whilst some evacuated tubes contain their ‘heat pipe’ within the tubes vacuum (like the Kingspan HP range and therefore minimising heat losses) some evacuated tubes for ease of manufacture mount their ‘heat pipe’ outside the vacuum area and rely on heat conduction from the glass selective surface to the ‘heat pipe’. This means a greater path to heat gain and introduces again some heat transfer losses of conduction and convection.