Sustainable Building Operation Using Simulation - Low Energy Cooling for Sustainable Buildings - page 198

practically unknown in the building energy community. The simulation model pro-
vides the environment to alternate full system simulations and real hardware testing
of the automation stations.
Finally, during the operation of the building, simulation models can be used for
online fault detection and diagnosis and even model-based control of energy plants
and building components. Simple control systems for radiators in test room configura-
tions have been analysed online using the building simulation tool ESP-r (Clarke et al .,
2002) and experiments have been carried out for model-based control of heat exchang-
ers and humidifiers in air-conditioning systems in car-painting processes (Uchihara
et al ., 2002). However, the simulation models have not yet been fully integrated into
commercial building management environments.
A strong argument for model-based controllers is the fact that conventional feed-
back controllers do not have much information on the system they are controlling
and nonlinearities in the plant characteristics can easily lead to non-ideal controller
actions. If the performance of a plant slowly deteriorates, the controller does not nec-
essarily recognize the system faults, as long as setpoints can still be reached. Salsbury
and Diamond (2001) investigated the advantages of using model-based feedforward
controllers simply to control the flaps and ventilators of an air handling unit. Not only
could the model be used to recommission the air handling unit (i.e. detect implemen-
tation faults), but also the controller improved accuracy by 10-43% (i.e. deviation
between setpoint and operating point). Models are provided for heating and cooling
coils and a mixing box.
Physical models are also very useful for controlling nonlinear systems, where the
different control loops are strongly coupled and single input, single output relations
do not give satisfactory results. Shah et al . (2004) developed physical models for
compression chillers to simulate pressure levels and vapour superheating as a function
of the expansion valve and evaporator cooling rates. The model-based control resulted
in better efficiency and capacity control of the cooling unit.
Both roles of simulation tools in the planning and operation phase of a sustainable
building will be shown in the following chapter. New developments in the modelling
of solar cooling systems will be presented first, which are necessary for the correct
dimensioning of a solar thermal plant. The analysis is followed by implementations
of online simulation systems for buildings and energy plants.
6.1 Simulation of Solar Cooling Systems
Solar or waste-heat-driven absorption cooling plants can provide summer comfort
conditions in buildings at low primary energy consumption. For the often used single
effect machines, the ratio of cold production to input heat (COP) is only in the range
of 0.5-0.8, while electrically driven compression chillers today work at COPs around
3.0 or higher. Solar fractions therefore need to be higher than about 50% to start saving