Civil Engineering Reference
In-Depth Information
comfort levels requires dynamic energy simulation tools case by case. Buildings
with thermal mass can save cooling energy in summer and often appear to be more
robust to misuse by the user (Dunster et al.
2008
).
Thermal storage can be realised either with traditional materials or with innova-
tive solutions such as phase change materials (PCM). It is also possible to artificially
activate thermal masses through the inclusion of radiant systems in slabs and walls.
Below are some innovations related to heat storage in buildings.
TABS—activation of thermal mass
Thermally activated building slabs (TABS) exploit thermal capacity of construction mate-
rials to create a heat sink in the floor slab. The use of a large exposed surface improves
thermal comfort (increase or decrease of mean radiant temperature).
In summer, the exposed mass of the floor slab can absorb excess heat from the rooms,
thus levelling out peak cooling loads. This heat can be removed via night flushing of the
rooms (natural or mechanical ventilation with cool outdoor air), or by circulating chilled
water in embedded pipes. For summer comfort, the exposed surface should be the ceiling
(higher yield). In winter, the exposed slab can soak up free heat from the sun and release it
at a later time. The slab can be kept warm with water circulating in the pipes.
While traditional radiant systems work in the superficial layers of building com-
ponents (floors, ceilings or walls), TABS are based on the activation of thick layers of
heavyweight materials. This is possible embedding plastic pipes in the structural concrete
slab, that shall remain exposed to the rooms to optimise heat exchange. Other solutions
use water pipes integrated in lost form shuttering to activate the concrete mass above. As
the thermal response of the system is slow, a management system that is able to predict
the required slab temperatures is required. This temperature is defined by internal gains,
weather and humidity.
TABS have a specific field of application in office buildings with discontinuous use.
This allows for night flushing of the building masses and/or off-peak charge and discharge
of stored heat. TABS are generally applied in new construction, as they need integration in
the structural components.
Building-integrated PCMs
A phase change material (PCM) is a substance with a high heat of fusion, where melting
and solidifying at a certain temperature is capable of storing and releasing large amounts
of energy. Heat is absorbed or released when the material changes from solid to liquid and
vice versa; thus, PCMs are classified as latent heat storage (LHS) units.
Initially, the solid-liquid PCMs behave like sensible heat storage (SHS) materials; their
temperature rises as they absorb heat. Unlike conventional SHS, however, when PCMs
reach the temperature at which they change phase (their melting temperature) they absorb
large amounts of heat at an almost constant temperature. The PCM continues to absorb
heat without a significant raise in temperature until all the material is transformed to the
liquid phase. When the ambient temperature around a liquid material falls, the PCM solid-
ifies, releasing its stored latent heat.
Within the human comfort range of 20-30 °C, some PCMs are very effective. They
store 5-14 times more heat per unit volume than conventional storage materials such as
water, masonry, or rock. Thus, they may be very effective as a heat storage element in build-
ings in substitution to heavier materials, such as concrete, which only absorb sensible heat.
PCMs can be organic substances, such as paraffin or fatty acids, inorganic (salt
hydrates) or a mix (eutectics). While PCMs have been used for a number of years in
healthcare, spacecraft and heating/cooling systems, their use as a building-integrated
component is relatively new and promising. It is possible to use either a layer of macro-
encapsulated PCMs as a building component, or add micro-encapsulated PCM powder
