Civil Engineering Reference
In-Depth Information
(at the melting temperature) change their state. In the process, they store latent heat while
remaining at a constant temperature. During winter nights, this energy can be released to
the rooms of the building if an appropriate thermal resistance is provided on the outer side
of the glass (e.g. a cavity with a glass pane). Summer radiation can be excluded by means
of shading systems or prismatic glass included in the glazing system.
As PCMs are translucent, their presence in the cavity still allows diffuse light to enter
the building.
In principle, translucent elements containing PCMs can also be adjustable, in order to
regulate the behaviour of the envelope according to season or time of the day.
Glass with variable solar transmittance
Performances required from glass may change drastically according to time of the day or
season. Active thin films on glazing are seen as a very attractive approach to solar con-
trol and daylight regulation in highly glazed buildings. While external shading devices
are usually the best approach to solar control, there are many reasons why their applica-
tion may be considered undesirable such as capital cost, aesthetics, maintenance, struc-
tural restriction and wind loading, especially for high-rise buildings. The use of highly
reflective glazing would be an alternative, but this can create glare issues for the exterior
surrounding.
A potential solution would be the use of glass with active layers that can modify
the light and energy transmission properties of the façade. Examples of this technology
include glass that change its properties when a voltage is applied (electrochromic glazing,
suspended particle devices), and glass that change its properties in response to an environ-
mental signal (thermochromic glazing, electrochromic glazing). Some companies expect
to provide systems with adjustable opacity (not just clear /reflective) in the near future.
Critical aspects of smart glass include installation costs, the use of electricity, durability,
as well as functional features such as the speed of control, possibilities for dimming, and
the degree of transparency of the glass.
3.1.1.4 Thermal Storage in the Building Fabric
The thermal capacity of a building (the amount of heat that can be stored in its ele-
ments) defines its dynamic thermal behaviour, that is, how quickly it heats up or
cools down when boundary conditions change.
While in traditional buildings, mass is used to reduce the inbound heat flow
through thermal lag and decrement, the levels of thermal insulation required by
European regulations shifts the importance on the surfaces that exchange heat with
the indoor environment. Thermal storage surfaces can absorb heat due to solar
radiation and internal gains in summer, thus reducing overheating of indoor air, or
absorb heat in winter, releasing it during the night or when the heating system is
switched off. Thermal mass is then able to stabilise indoor temperatures, reducing
peak loads on systems (Di Perna et al.
2011
).
These surfaces are also instrumental in defining user comfort, as operative tem-
perature is defined by both air and surface temperatures. Thus a warm surface will
increase comfort in winter, while a cool surface will be beneficial in summer pro-
vided heat can be removed during the night with ventilation.
The relationship among climate, insulation levels, thermal mass, dimensions of
glazed parts, and so on is specific to each building and the correct assessment of
