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A methodology is presented for a unified approach to building thermal control studies and building energy analysis. The same detailed building thermal models are used for energy calculations and for thermal models are used for energy calculations and for thermal control studies. Laplace transfer functions for the building are obtained by means of thermal network models that include both distributed parameter elements, such as thermal mass, and lumped elements, such as the room air thermal capacitance. For detailed models for which an analytical solution is not feasible, the s-domain transfer functions are obtained through a modified least squares polynomial fit to the discrete frequency responses. Laplace transfer functions are also used for HVAC system and control components. Transient thermal control studies are performed by means of a numerical Laplace transform inversion technique. Typical studies with the methodology led to the following results -(1) A separation of building thermal dynamics into short-term and long-term dynamics for convective loads begins at frequencies of about 35 cycles per day (period 41 minutes), with the room air thermal capacitance being important only for the short-term, high-frequency thermal dynamics. (2) Air temperature sensor time constants are shown to have a substantial effect on room temperature response to setpoint changes, such as an increase of 50% in the settling time (with P-I control) when the sensor time constant is increased from 30 s to 60 s.

KEYWORDS: predictive controls, ventilation, controls, temperature control, calculating, networks, rooms, models, thermal inertia, thermal capacity, sensors, heating, air conditioning.

Citation: Symposium, ASHRAE Transactions, vol. 96, pt. 2, St. Louis 1990