According to EN 12975-2, the following procedures can be applied for the determination of the effective collector capacity (the first three of these are alternative methods for the stationary collector test).

• Calculation from the physical data (with weighting factors for the components),

• determination from the response of the collector to a step-change of the fluid inlet temperature (Annex J.2),

• determination from the response of the collector to a step-change of the irradiance (Annex J.3),

• determination as a fit parameter of the quasi-dynamic collector test.

For details of the procedures see figure 2.

Unfortunately, the results of these procedures are not at all comparable. According to investigations at Fraunhofer ISE and at ISFH, the J.3 procedure leads to values that are
two to four and a half times as high as the calculated value. The most marked difference is found for vacuum-tube collectors of the dewar type ("Sydney” and similar collectors). According to ITW members, the results of the quasi-dynamic test agree quite well with those of the J.3 procedure. In a test at ISFH, the J.2 procedure gave a value that is even slightly below the calculated one.

These differences influence the thermal gains of the collector, which decrease with increasing capacity. For a dewar-type vacuum-tube collector, tested at ISFH, the calculated capacity was 9 kJ/m2K, while the result of the J.3 procedure was 40 kJ/m2K. The difference of calculated yearly collector yields1, caused by the difference of capacities, amounts to 9 %, which is quite remarkable.

Figure 1: Comparison of energetic balances in the 1-node and 2-node models.