In the scientific literature, various weather models have been proposed and used for the simulation of solar thermal systems. The problem of the sensitivity of solar thermal systems to random fluctuations and persistent patterns in weather have been a source of concern and discussion for a long time. A general solution to this problem has proved to be elusive. This paper presents the results of weather sensitivity analyses performed on solar-assisted heat pump systems used in large buildings. The type of systems modeled usually incorporate relatively small collector areas (approximately equal to 5 to 15 percent of the building floor area). A detailed computer program was used for conducting hour-by-hour, zone-by-zone simulations of the buildings and the energy systems. Truncated Fourier series were used to represent weather data. It is concluded that various unusual weather sequences and perturbations do not affect the long-term energy consumption of the building so long as the average value of these variations is constant. Short-term energy consumption was found to be much more sensitive to these variations. It was also found that a uniform change in the average value of the weather functions, does, in fact, substantially influence building energy use.

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