Environmental Engineering Reference
Figure 5.32 Total irradiance on collector surface and horizontal plane during July and energy yield
during regeneration hours only
From the 83 hours of required regeneration, the collector provided energy for 53
hours, that is during 64% of the time, and produced a total of 11 kWh m − 2 and a
month which is about the same amount that the waste heat delivered from the factory.
The other days were too cloudy to obtain the minimum temperature difference of 6 K.
The mean collector efficiency during the operating hours was 48%, which is a good
performance value (see Figure 5.32).
Looking in more detail at the control strategy and the timing of the regeneration
mode, the temporal sequences and phase shifts between regeneration operation and so-
lar thermal energy contribution can be clearly seen (Figure 5.33). During regeneration,
the temperature levels vary typically between 60 and 70 ◦ C. The two parallel-connected
air collector fields are capable of delivering this temperature level for nearly 7 hours
during a sunny day. As the air collector field only heats 55% of the total regeneration
air flow due to its small size, and the waste heat from the factory only delivers air tem-
peratures of 40 ◦ C, the complete regeneration volume flow of 11 000m 3 h − 1 reaches
maximum temperature levels of 60 ◦ C and auxiliary heating was always switched on.
This could be avoided by increasing the collector field size, which is not economi-
cal for the low number of regeneration hours. Also, the complete system simulation
showed that the auxiliary heating was not even necessary to reach the desired inlet
air temperatures and that the energy consumption of the auxiliary heating could be
The fixed high-temperature levels for regeneration also create a problem for the
early start-up of the desiccant unit, when solar thermal energy is not yet available
at sufficient temperature levels (see Figure 5.33). With air-based systems without
thermal storage this is difficult to avoid. It would be advisable to operate the desiccant
mode at low solar regeneration temperatures and admit lower dehumidification rates.