Data plots:

A typical testing day is characterized for a certain number of heliostats focused over the receiver. This number is increased in case of mist that makes the solar direct radiation lower, but otherwise it is fixed. On the other hand, in one test day, the mass flow rate is decreased several times until the temperatures approach certain limits in each section. In a clear day, with an approximately constant solar radiation, the temperatures increase when the air flow rate decreases. From all the data recorded for the DAS, a plot is drawn for the most relevant parameters (see fig.2): direct solar radiation (in W/m2), overall mass flow rate (in kg/s, over the secondary axis in the chart) and air outlet temperatures (in °С) from the cups of interest (all of them adjacent and located at the central part of the receiver). Moreover, the average temperature of the 36 absorber cups is represented to have an idea of the temperature level in the whole receiver.

Selection of steady states:

To present the response of the volumetric solar absorbers under certain known parameters, it would advisable that the system was in a steady state. However, this is nearly impossible, since solar radiation is always slowly increasing until noon and decreasing from then. In addition, the clouds in the sky may cause transitories in the incoming radiation. The solution is to find quasy-steady states where the main input parameters of the receiver do not change very much. But also the response of the receiver has to be considered. Thus, the input and output parameters values must keep inside a certain range during a large period of time related to the response time. This time is calculated as the time that takes the receiver to react and to reach the 65% of the final value of the output variable after a quick change in one input variable, while the other input parameters remain constant. For this receiver, the response time is in the order of some minutes, between 3 and 5 depending on the temperature selected as output variable.

To identify the quasy-steady states, several criteria have been used, but in all of them the incoming radiation and mass flow rate, as input parameters, and receiver average temperature, as output parameters, must not change more than 4% or 5% in a previous period corresponding to 4 or 5 times the response time of the receiver. In other words, to consider that a measurement value is inside a steady period it is compared to the values taken in the last minutes (between 12 and 20, depending on the selected criterion). In addition, the change gradients of these parameter must be less than 1% (or 1.25%) in every former response time step.

As an example, all the experimental points for the 3 parameters of interest that comply with the all the requirements at the same time for the one of the strictest criteria are marked in the fig.2. This criterion compared every value of direct solar radiation and overall mass flow rate with their values during the last twenty minutes and they must not vary more than a 4% (and no more than 1% every 5 minute step). Meanwhile, the average absorbers temperature must not change more than a 5% (no more than 1.25 in every step).

After calculating the quasy-steady periods of a test, the mean value and the standard deviation of each variable are calculated from its values from the beginning to the end of the steady period. Thus, the number of experimental points is reduced from several thousands to some dozens in the best days, or just a few or even any steady data when direct radiation was unstable.