Abstract
The technical standard EN ISO 13790 proposes a quasi-steady state method to estimate the energy performance of buildings both in the heating and in the cooling periods which is based on the concept of utilization factor. The standard also presents a procedure to improve the utilization factor definition through the dynamic simulation: the method prescribes the calculations of the losses, the gains and the actual energy need in three stages. The quasi steady state model considers operative temperature setpoint conditions, as a weighted average of the air temperature and the mean radiant temperature of the envelope delimiting the zone itself. Many of the simulation codes employed in refining the utilization factor definition are often used with a setpoint on the air temperature affecting the temperature-dependent fluxes, such as the thermal losses and distorting the comparison. On the other hand the quasi steady state model is also often used to estimate the dispersions and energy needs of buildings even when the indoor conditions are controlled by air temperature setpoint. In this work the influence of the use of the air setpoint temperature instead of the operative temperature in determining the losses has been assessed with the simulation code TRNSYS. A large number of cases has been developed basing on a factorial plan aimed to investigate the main variables related to the envelope of the building and its behavior: for example, variations in geometry (shape of the floor and dimensions of the glazings), wall insulation level, climatic conditions and the number of zones – a single zone or two equivalent coupled zones, have been considered. The setpoint strategy significantly affects the deviations between the simulated thermal losses and the EN ISO 13790 ones, with percentage deviations greater than 5% when an air temperature setpoint is used. In this context, considering a not insulated envelope, the percentage differences can grow to 20%. When an operative temperature setpoint is employed, both for insulated and not insulated walls, the deviations decrease under the 5%.