# What is the thermal performance of the DualSun Spring panel?

As with all thermal solar panels, the power of our DualSun panels varies according to the temperature of the water in the panel. The graph below shows the thermal power of our Spring panels (non-insulated and insulated versions) for different applications (each application has a different operating temperature).

At low temperatures (for coupling with water/water heat pumps and swimming pool heating), we recommend using the non-insulated version of our panels. For domestic hot water, we recommend using the insulated version of our panels.

For more details on the thermal behaviour of the DualSun SPRING panel, the following curves are available in the appendix of the data sheet - thermal behaviour:

• DeltaT as a function of the flow rate of the fluid in the panels
• Thermal power per m² depending on the temperature difference between the fluid circulating in the panels and the ambient air

Explanations on the thermal power graph above:

The thermal power (Pth) of a solar panel depends on:

1. the technical characteristics of the panel:

• a0 [%]: optical loss coefficient, which represents the percentage of the sun's radiative power that will actually be absorbed by the collector.
• a1 [W/m².°C]: heat loss coefficient, which represents the proportion of heat lost by convection and radiation to the outside.

2. the panel surface area (S) [m²].

3. solar irradiation (G) in standard test conditions (STC), i.e. G = 1000 W/m² with T°ext = 25°C and a wind u=1m/s

To improve the efficiency of a thermal solar panel, it is necessary to maximize a0 to absorb the maximum solar power.

As we can see from the curves above, an uninsulated panel is more efficient at operating temperatures below 30°C. In this case it is necessary to maximise a1, i.e. to improve the thermal exchanges towards the outside.

For operating temperatures above 30°C, as for domestic water heating, the insulated version is more efficient. The coefficient a1 must be minimised to transmit the maximum amount of solar energy to the water.

In order to achieve optimum overall efficiency (electrical and thermal), the DualSun team of engineers has calculated the conditions for setting up the systems according to the applications.

For more information on the thermal output of the DualSun Spring hybrid solar panel, please refer to its data sheet.

Why does the uninsulated Spring perform better at low temperatures?

The uninsulated Spring panel has a better optical performance a0 than the insulated Spring panel in standard test conditions (STC), for an ambient temperature of 25°C under 1000W/m2 of solar irradiation.

As can be seen from the curves above, the uninsulated Spring panel performs better than the insulated Spring panel to heat water up to 30°C.

This is due to two physical phenomena:

• The non-insulated version recovers more energy from its environment (air) when cold water (e.g. 10°C) flows through the panel. It also has slightly colder cells than the insulated version when it produces warm water (e.g. at 25°C). With cooler cells, it then loses less energy through radiation at the front towards the sky. Its performance is therefore better than that of the insulated version at low and medium temperatures, as long as the losses from the back side are not too great,
• The hotter the water circulating into the panel, the more energy the non-insulated version will lose through the back side. Its performance then decreases faster than the insulated version. With the insulated version, the losses on the back side are lower. After a while, the insulated version has a better performance.

Performance tests at different temperatures show that, under standard conditions, this crossover takes place at around 30°C.