In the following, the procedures mentioned above and their results for the effective capacity are analysed qualitatively. The arguments are based on fundamental physical considerations. In this analysis, the dynamic behaviour of the two nodes absorber (Tabs) and fluid (TF) is investigated. The effective collector capacity is nearly completely determined by the dynamics of these two nodes.

The amplitudes ATabs and ATF of changes of these temperatures in transients between two steady states are regarded. The thermal energy that is needed in the real collector to achieve the temperature steps ATabs and ATF corresponds with the effective capacity to be determined, which is, in the underlying one-node models, the capacity of the fluid node. Hence the larger ATabs in the test procedure, the higher will be the resulting effective capacity.

This becomes clear when regarding figure 1: In all the procedures of determination and in the underlying 1-node model, the energy AEcol needed to raise the fluid temperature by ATf is given by eq. (1). In a 2-node description (which is close to reality), this energy also includes the energy needed to heat up the absorber by the corresponding amplitude ATabs (see eq.(2)). As AEcol must be equal for both cases, eq. (3) follows. From eq. (3) the influence of ATabs on the effective capacity Ccol can be seen directly.

TRNSYS simulation of a standard SDHW system, collector area 5 m2.

SHAPE * MERGEFORMAT

In the following discussion, the fact is made use of that during steady state the difference Tabs — TF is proportional to the collector’s thermal performance.

The procedures, the courses of temperatures, the amplitudes ATabs and ATF and the resulting values of the thermal capacity are shown in figure 2.

In the calculation procedure (clause 6.1.6.2 of EN 12975-2) the weighting factors for fluid and absorber are equal, hence by definition the relation ATabs = ATF holds. The resulting effective capacity is relatively low.

The procedure according to Annex J.2 starts with a steady state where the inlet temperature Tin is equal to the ambient temperature Ta. In a step change, Tin is raised by 10 K. The irradiance G remains zero. So in the new steady state resulting after the temperature step, the heat flux is reversed, compared with normal collector operation: the collector loses heat, and consequently the fluid temperature is higher than the absorber temperature. Hence it follows for the amplitudes of the temperature steps that ATabs < ATF. The resulting effective capacity can be expected to be even lower than the one from the calculation procedure.

The procedure according to Annex J.3 starts with the same steady state as the J.2- procedure. Unlike above, the inlet temperature is kept constant here, and the irradiance is switched from zero to a high level. Here the thermal power of the collector increases from zero to a high value. Consequently, the same is true for the difference Tabs — TF. Hence for the amplitudes the relation ATabs >> ATF follows. A high effective thermal capacity is the result.

Collectors with a relatively low heat transfer coefficient habsF between absorber and fluid, as for example vacuum-tube collectors with a dewar construction, show particularly high values of Tabs-TF and, consequently, very high effective J.3-capacities. (When the irradiance is increased from 0 to 1000 W/m2, increases of ATabs « 30 K and ATF « 8 K can be expected for habsF « 30 W/m2K.)

In the quasi-dynamic collector test, the fluid inlet temperature is kept constant. The only dynamic effect that is taken into account are natural fluctuations of the irradiance. Hence it can be expected that the results for the effective capacity are similar to those of the procedure of Annex J.3.