Active Thermal Cooling Technologies - Low Energy Cooling for Sustainable Buildings - page 149

Table 5.3 Summary of measured efficiencies of desiccant cooling units

Mataro Althengstett

Dehumidification efficiency 80 80

Humidification efficiency 86 85

Heat recovery efficiency 68 62

Control Strategy Modelling

The goal of the simulation was to determine the influence of the control strategy on
the system performance and to establish whether different sizing or orientation of
the collector field could improve the solar fraction and reduce the auxiliary energy
demand. The model can work with building cooling loads Q cool calculated using dy-
namic simulation tools or from measured loads obtained from the supplied cooling
energy (calculated using the supply air volume and the measured enthalpy differ-
ence between room exhaust and supply air). For each time step, the programmed
system controller calculates the required supply air temperature to cover the cool-
ing demand of the room for a given supply volume flow. To save electrical power,
the supply air volume flow is first set to 50% of its nominal value. If the resulting
supply air temperature is too low or if the cooling demand cannot be covered by
this reduced volume flow, the flow rates are gradually increased up to the nominal
flow:

Q cool
c p V supply

T required = T exhaust − (5.5)

The controller then switches on each of the components subsequently (first either the
exhaust air humidifier plus rotating heat exchanger for the Mataro control strategy or
the supply air humidifier for the Althengstett strategy) and calculates the supply air
temperature. If this temperature is still higher than the required temperature, the next
component is switched on, that is the next humidifier. Finally the sorption wheel starts
operating and if temperature levels are still not sufficient, auxiliary heating or cooling
takes place, depending on the system configuration.

Simulation Results

Using measured July data from the Althengstett system with a time resolution of
5 minutes, the model can be validated. Inputs to the model are the global horizontal
irradiance, ambient temperature and humidity. Room and supply air temperature and
the supply volume flowwere used for the cooling load calculation, which the controller
needs as an input.
The component switch-on times of the simulation model and measured results
in general correspond well (see Figure 5.40). The real component operating times