Adaptation of the Buildings for Efficient Passive and Free Cooling
Design of Building Envelopes
Besides the internal heat gains of occupancies, appliances, and heat losses from the building service systems, heat fluxes that enter the building interior through the building envelope can be the reason for overheating of the buildings and increased energy demand for cooling. In the planning phase of the buildings, several measures can be taken to decrease those heat fluxes (Figure 15.10) (Medved, 2014).
Transparent Building Envelope Structures
Transparent building structures enable that occupancies have visual contact with outdoor environment as well as the daylighting and the passive heating of the buildings, but they can also be the reason for increased energy demand for cooling. Because of that, transfer of the solar radiation into the building must be controlled. In principle, the following possibilities exist:
• By installing glassing that automatically changes the transmittance of the solar radiation according to the temperature of the surroundings, (known as thermochromic glassing) or the density of the light flux (photochromic glassing) or when exposing the thin layer of the tungsten oxide (WO,) inside the gap in the glassing to the hydrogen gas (gasochromic); nevertheless those techniques of so-called “smart windows” are very promising, but they are not applicable on the large scale yet.
FIGURE 15.10 The effectiveness of the passive and free cooling can be significantly increased by decreasing the heat fluxes that enters the building through transparent (left) and opaque building envelope structures (right).
FIGURE 15.11 Movable, high solar reflective outdoor shades installed 30 cm in front of the facade for buoyancy-driven convective cooling. Shades are computer controlled; however, manual intervention of the users must be enabled in any case.
- • By glass panes with very thin metal or oxides layer(s) that reflect a large part of the solar radiation; such glassing has low transmittance, although it cannot be adjusted to the needs of the occupancies (and the building).
- • As most efficient, by shading devices installed on the outer side of the window or glassed facade, such devices must be movable, high reflective for the solar radiation, and must be installed in the way that the air gap between shades and glassing is formed to enable buoyancy-driven convective cooling of shades (Figure 15.11).
Opaque Building Envelope Structures
The heat flux that passes sunlit opaque building structures can be decreased by painting the outer surface of those structures with “cold color.” These are paints have high reflectance of the visual part of the solar irradiation (wavelengths X between 0.38 and 0.76 pm), which causes this color to be bright white. Figure 15.12 shows heat flux that passes the flat massive roof structure with a darker surface and cold-paint coating.
FIGURE 15.12 (a) Painting the roof with “cool paint” (From LEXIS Coatings, 2019); transient thermal response of the concrete roof with 5 cm thick outside thermal insulation layer with thermal transmittance U 0.65 W/m2K in case of dark and “cool paint” coating; and (b) in the numerical simulations meteorological data from TRY for Ljubljana (Cfb) was used. (From Arkar, C. et ah, Earth Environ. Sci., 323, 2019.)
Case study: Transient heat flux q( (W/m2) on the interior surface of the flat concrete roof was determined over the period between June 1 to August 31 using typical meteorological year data for the City of Ljubljana (climate zone Cfb). The concrete layer is 20 cm thick, and the roof has a 5-cm thick thermal insulation layer on the outer side. No other layer was taken into account. Figure 15.12 shows qj of the roof with an average dark surface (with absorptivity of solar radiation as equal 0.7) and a roof painted with “cool paint” (as 0.18) for the first week of modeled period. The total heat gains determined by qf (integrated over the time when
Due to the energy, the environmental and the social benefits, green roofs and facades are becoming predominant solutions in urban planning and building envelope retrofitting, especially in the form of extensive green roofs because of low additional structural load, low maintenance, and low cost in comparison to intensive solutions. Extensive green roofs consist of (from top to bottom) a vegetation layer, a thin organic soil layer, a lightweight rock mineral wool growing media 2 to 5 cm thick, a drainage system, and a root membrane. Environmental effects are the consequence of the absorption of the greenhouse C02, particles and heavy metals in plants and the soil, as well as urban heat island mitigation. At least as important is the role of a green roof in retention and detention of the precipitation. When completely saturated, extensive green roofs store between 25 and 55 kg of water per m2 area. Social benefits can be seen from new urban areas intended for socializing of inhabitancies, urban food production in cooperatives, and even the settings up of beehives. Green roofs, however, also have a considerable influence on the thermal response of building structures due to the latent heat transfer by ET and freezing of the water in growing media (Arkar et al., 2018).
Case study: Transient heat flux (W/m2) at the interior surface of the building structure with an extensive green roof (Figure 15.13, left) was determined. The roof structure is built by the concrete (15 cm) and the outer thermal insulation (5 cm) layers. The simulation results are compared with the same massive structure without green roof and with dark outer surface (as 0.7; Ca) for the period between June 1 and August 31 using typical meteorological year data for the City of Ljubljana (climate zone DbF) (Arkar et al., 2019). It can be seen that during the presented week, the maximum daily heat flux toward the building interior is 5 to 6 times lower in case of green roof, compared to the building structure without vegetation (Figure 15.13, right). The total heat gains q/ of the structure with the green roof over the observed period are 1.67 kWh/т2, which is significantly lower compared to the structure without the green roof (q( equals 10.13 kWh/m2).
FIGURE 15.13 (a) Extensive green roofs in test stand at Laboratory for Sustainable
Technologies in Buildings; and (b) transient thermal response of the massive roof structure with green roof and the same roof structure without vegetation and with dark outer surface in the first week of July.
Building structures on the envelope of the buildings exchange long-wavelength radiation (IR radiation) with their surroundings and in case they are horizontal, mostly with the sky. The temperature of the sky is strongly dependent on the cover of the sky with clouds. When the sky is cloudy, the apparent sky temperature is only few К below the outdoor air temperature; meanwhile, in the case of a clear sky, the apparent sky temperature is tens of degrees lower than the outdoor air temperature. If the surface of the building structures has selective radiative properties, designed for radiative cooling, this means that such surfaces have thermal emissivity eIR in the range of the wavelengths X of the atmospheric window (8 to 12 pm) close to 1. The reason is that the atmosphere in a clear sky is almost totally transparent for those wavelengths, meaning, that building structures exchange the radiation with the space, which has the temperature of only few K. For all other wavelengths of the thermal radiation, including solar radiation and IR radiation outside of atmospheric window wavelengths, the surface of building structure must have a reflectance close to 1, to reflect all incoming radiation. In such a case, it is possible that the surfaces of building structures cool below the outdoor air temperatures, and it is possible that heat flux is directed outside of the building as it is shown in Figure 15.14. The figure shows the heat flux at the inner surface of the massive roof building structure with ideal radiative properties for radiative cooling (eIR = 1 for wavelengths 8 mm < X < 12 pm and pIR = 1 for wavelengths 8 pm > X > 12 pm) during the first week in July (TRY Ljubljana).
The thermal response of the building structures depends not only on the boundary conditions of the outer surface of the structure but on the capability of the construction to accumulate the heat during the periodic daily transient heat transfer as well. In engineering practice (EN ISO 13786:2017), the dynamic boundary conditions at the outside surface of the building structure are defined by variation (amplitude) of the air temperature 0e or the heat flux qe around average value 6e, qe and described by a sine function of time. The period of the process, which is in nature 24 hours, is replaced in calculation method by frequency w (rad), and because of that, the dynamic heat transfer in the building structure is determined by the heat transfer matrix Z of the
FIGURE 15.14 Heat flux at inner surface of the massive roof structure with ideal selective radiative properties for radiative cooling and at the same structure with common non- selective coating; term selective radiative properties indicate that one or more radiative properties (including absorptivity, reflectivity, transmittivity, and emissivity) are adjusted to the particular heat transfer problem.
building component. To evaluate the dynamic thermal response of the building structure, decrement factor f and time shift At are used. Decrement factor f is defined as the ratio of the modulus of the periodic thermal transmittance to the steady-state thermal transmittance U. In other words, f indicates how dynamic boundary conditions at the outer surface is reflected at the inner building structure. If the value of f is close to 1, the building structure will not suppress any change in the outdoor state; meanwhile, the building structures characterized by f equal 0 will act as perfect adiabatic structures. Therefore, the building built by structures with low' decrement factor (<0.3) will have better predispositions for efficient passive and free cooling due to lower heat flux qr Besides that, the maximum value of q, over the day will enter the interior of the building with a larger time shift At as w'ell (Figure 15.15).
FIGURE 15.15 (a) Heat flux at the outer and the inner surface of the massive; and (b) lightweight structure; both structures have equal thermal transmittance U 0.615 W/m2K, but obviously very different dynamic thermal properties. This can be noticed by decrement factor f equal 0.315 for massive and 0.998 for lightweight building structure.
Case study: Two building structures have equal (static) thermal transmittance, U, but are built as massive (including 15-cm-thick concrete layer) and lightweight structures built by thermal insulation layer only. The dynamic thermal response during real environment conditions was modeled, and results are shown in Figure 15.15. It can be seen that (1) despite both structures having equal thermal transmittance, heat flux that enters (and heat up) the building is significantly larger in lightweight building structures (there is no difference in and qe ~ qO and (2) there is almost no time delay (At) of heat flux in lightweight building structures. The theoretical decrement factor f for the massive structure is 0.315 and for lightweight structure it is 0.998.