How to read the thermal power curve on the DualSun Spring panel data sheet?

The thermal power curve provided in the DualSun Spring hybrid solar panel data sheet represents the "peak" thermal power depending on the application. 

Our results are displayed in accordance with the SOLAR KEYMARK label (based on ISO9806) which indicates the collector power results at 1,000 W/m² and 1 m/s.

These peak values, like those for photovoltaics, cannot be used directly to estimate annual performance: it is above all a benchmark for comparing different panels with each other. To estimate the annual production, it is necessary to use solar thermal software such as SOLO, PolySun, TRNSYS... with the certified coefficients of our collectors and to make a simulation on the hydraulic diagram for the given climate according to the real orientation and inclination of the project.

 

The formula for calculating the thermal power (Pth) is : 
Pth =  [ a0.G - a1.(Tm-Ta) ] x Surface 

 

The thermal power curve of the DualSun panels is plotted with the following values: 

Parameters

Notation

Value

Unit

Solar irradiation

1000

W/m²

Ambient temperature

Ta

25

°C

Fluid average temperature

Tm 

Abscissa variable that depends on the application

°C

 

The thermal coefficients (optical efficiency a0, losses a1) are those given in the data sheet of the DualSun Spring hybrid solar panels, they are given at a wind value of 1m/s. The surface area (m²) is also given in the data sheet.

 

For the water-water heat pump coupling application, the Spring collector is operating at negative dT (Tm<Ta): this is absolutely not impossible, and it is even really the case when the collector is coupled as the (cold) source of a heat pump! The fluid flowing through the collector is - even in winter - usually at a temperature below the ambient temperature. This means that the collector does not only collect solar radiation, but also convection energy from its surroundings. This means that the efficiency of the collector is higher than the optical efficiency of the solar radiation alone, which may seem surprising the first time you see it.

 

For the more mathematically minded, this is also what the formula says: 

If Tm < Ta (negative dT), then: Pth > a0.G

 

Note that the IEA experts of the Task 60 on hybrid solar energy (CEA INES, CETIAT, Fraunhofer ISE, TUV Rheinland, SPF...) also discuss this presentation scheme and could decide other reference values, in particular :

  • For the ambient temperature

Three temperatures have been suggested: 20°C (ISO9806 SRC standard), 25°C (STC cell temperature value) and 30°C (ISO9806 stagnation standard).

  • For the wind speed, which makes it possible to calculate the a0 and a1

Two wind speeds were suggested: 1m/s or 1.3m/s.

If other values were the subject of an international consensus, we would obviously make our data sheets consistent with them.

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