An evaluation of the suitability of lumped-capacitance models in calculating energy needs and thermal behaviour of buildings

Simple, reliable building models have been receiving quite a bit of attention recently particularly with regard to diverse applications, such as building design for inexpert energy modellers, simulation of neighbourhood or city districts and model predictive control. The International Standard ISO 13790 and the German Guideline VDI 6007 use two different lumped-capacitance models (5R1C and 7R2C, respectively) based on deterministic, analytical procedures to identify their parameters. The current work investigates the suitability of these models in calculating peak loads and seasonal energy needs and their accuracy in estimating buildings’ dynamic behaviour. A room and an apartment were thus simulated using simplified models and with the benchmarked software TRNSYS. Four reference envelopes with different thermal insulation and heat capacities were examined in four climatic conditions. Each of the models was able to estimate quite precisely energy needs in both the heating and cooling modes, although the 7R2C model was slightly more accurate. The 5R1C model was, however, unable to follow the thermal response of the buildings during the cooling season, which in turn implied a systematic underestimation of the cooling peak load. The 7R2C model identified a significant reduction in the root mean squared error (RMSE) both in the indoor air temperature and in the heating/cooling loads with respect to the reference profiles. That model would seem then more suitable for the dynamic simulation of single thermal zones with hourly time steps in both heating and cooling modes.