On the thermal buffering of naturally ventilated buildings through internal thermal mass

Abstract

In this paper we examine the role of thermal mass in buffering the interior temperature of a naturally ventilated building from the diurnal fluctuations in the environment. First, we show that the effective thermal mass which is in good thermal contact with the air is limited by the diffusion distance into the thermal mass over one diurnal temperature cycle. We also show that this effective thermal mass may be modelled as an isothermal mass. Temperature fluctuations in the effective thermal mass are attenuated and phase-shifted from those of the interior air, and therefore heat is exchanged with the interior air. The evolution of the interior air temperature is then controlled by the relative magnitudes of (i) the time for the heat exchange between the effective thermal mass and the air; (ii) the time for the natural ventilation to replace the air in the space with air from the environment; and (iii) the period of the diurnal oscillations of the environment. Through analysis and numerical solution of the governing equations, we characterize a number of different limiting cases. If the ventilation rate is very small, then the thermal mass buffers the interior air temperature from fluctuations in the environment, creating a near-isothermal interior. If the ventilation rate increases, so that there are many air changes over the course of a day, but if there is little heat exchange between the thermal mass and interior air, then the interior air temperature locks on to the environment temperature. If there is rapid thermal equilibration of the thermal mass and interior air, and a high ventilation rate, then both the thermal mass and the interior air temperatures lock on to the environment temperature. However, in many buildings, the more usual case is that in which the time for thermal equilibration is comparable to the period of diurnal fluctuations, and in which ventilation rates are moderate. In this case, the fluctuations of the temperature of the thermal mass lag those of the interior air, which in turn lag those of the environment. We consider the implications of these results for the use of thermal mass in naturally ventilated buildings.

The effect of gradual changes in wind speed or heat load on natural ventilation in a thermally massive building

“…There is also a distributed thermal mass, which exchanges heat with the interior air. Our model focuses on the convective heat transfer between the thermal mass and the air using a bulk model of the thermal mass (cf [4]). Convective heat transfer between the thermal mass and the interior air is typically described by a heat transfer coefficient which depends on a variety of factors, including surface orientation and air properties [1].…”

“…Convective heat transfer between the thermal mass and the interior air is typically described by a heat transfer coefficient which depends on a variety of factors, including surface orientation and air properties [1]. Here we consider an idealised problem, and assume the heat transfer coefficient has constant value f which is representative of the overall heat transfer rate, and typically around 2.5 W m À2 K À1 (cf [4]). The heat exchange between the thermal mass and the interior air is therefore given by…”

“…DTðtÞ (4) where Q 0 is the initial heat load. Similarly, we scale the wind forcing relative to the pressure difference associated with the buoyancydominated flow based on the initial heat load Q 0 :…”

“…Several theoretical models have examined the effect of thermal mass in attenuating the diurnal variations of the exterior temperature, within a ventilated enclosed space [11]. Approximate expressions have been derived for both the attenuation and the phase lag as a function of the ventilation rate, the rate of heat exchange between the thermal mass and the air, and the mass of material within the space [12,4]. In their analysis Yam et al assumed that there was a fixed ventilation rate, while Holford and Woods [4] modelled the interaction between buoyancy-dominated ventilation and thermal mass over a diurnal cycle.…”

“…Approximate expressions have been derived for both the attenuation and the phase lag as a function of the ventilation rate, the rate of heat exchange between the thermal mass and the air, and the mass of material within the space [12,4]. In their analysis Yam et al assumed that there was a fixed ventilation rate, while Holford and Woods [4] modelled the interaction between buoyancy-dominated ventilation and thermal mass over a diurnal cycle. In many buildings, this phase lag is of comparable timescale to the timescale of the diurnal forcing of either the heat load or the wind.…”