Active Thermal Cooling Technologies - Low Energy Cooling for Sustainable Buildings - page 140

These control problems can be avoided if the response time of the humidifiers is
taken into account and if the sorption wheel is not used in heat recovery mode for
compensating too low temperatures. Furthermore, the rotating heat exchanger should
only be switched on together with or 5 minutes after the exhaust air humidifier.
To determine the effective dehumidification potential of the sorption rotor, it can
be shown that temperature measurements combined with assumed enthalpy changes
(from laboratory experiments) are a much more reliable method than calculation from
relative humidity measurements. The measured dehumidification efficiency, which
is defined as the measured absolute dehumidification compared with the maximum
possible dehumidification down to the relative humidity of the regeneration air, is 80%
at a ratio of regeneration to process air of 66%.

Energy Analysis and COP

During the seven months of measurements, 34 710 kWh of cooling energy was
produced. For the calculation of all energy balances only sensible cooling energy
was calculated (ambient-inlet air temperature difference), as the enthalpy measure-
ments are too unreliable. At an average COP of 0.95 the heating energy required was
36 460 kWh. A main feature of the desiccant process is that heating energy is only
required for full desiccant operation and the humidification processes alone do not
require additional energy. If the heating energy is related only to those hours with full
desiccant operation, the COP is 0.5.
Full desiccant cooling operation is mainly required in the summer months, when
the humidification process alone is not sufficient for exterior air cooling. The average
COPs are highest when the process does not require heating energy and reach average
monthly values up to 4.8 (see Figure 5.29).
The low COP in June was to a large extent due to the malfunction of the exhaust air
humidifier, when the supply air temperatures could not be reached and the regeneration

7000 6.0

4.8

6000

5.0

5000

4.0

4000 3.1

3.0

3000

2.5 2.0 2.0

2000

1.0

1.0

0.4

1000

0.5

0 0.0

humidification only full regeneration COP

Figure 5.29 Monthly cooling energy and COP of the desiccant cooling unit in Althengstett