Improved methods for the calorimetric determination of the solar factor in outdoor test cell facilities

Calorimetric methods for the performance assessment of building components have been largely applied in indoor laboratories and under steady-state conditions. Although effects of one or more outdoor weather parameters are sometimes mimicked by means of dynamic schedules, they never fully reproduce the complex interactions of the stochastic processes typical of real climate. The present work introduces improved measurement procedures to determine the solar factor under dynamic conditions, applicable to outdoor test cell experiments and which take into account the variation of internal energy in the control volume. An in-depth uncertainty analysis has been conducted in order to highlight the most relevant uncertainty sources and to suggest improvements to the measurement techniques. Based on an iterative application of the uncertainty analysis, we developed and optimised two new strategies to extract and measure the solar load entering through a test sample and a new design concept of test cell facility, which allows the configuration to be adapted according to various test objectives. In order to accurately analyse the storage and delay effects of the thermal capacities within the control volume of the calorimeter, lumped-parameter models of three alternative designs (the two proposed strategies and a reference, traditional one) have been developed and coded in Matlab. The simulation results suggest that, compared to a traditional solution, the two proposed solutions offer a higher measurement accuracy and measurement precision in the determination of the solar factor. In addition, the results indicate that rapidly variable solar irradiance levels are detrimental to the accuracy level of the solar factor measurement; therefore tests should be carried out under stable clear sky conditions.