2 Thermal mass
In cooperation with UCD
2.1 General principles about thermal mass
In historic buildings thermal mass is generally large, as they were built using heavyweight materials such as stone or brick. The most important features of thermal mass are that it moderates internal temperatures by averaging diurnal (day/night) extremes (stabilising internal temperatures) and it delays the time at which peak temperatures occur. Large thermal mass can store and later release large quantities of heat without a large temperature rise on the surface. Conversely, if the thermal mass is low, the room will be quickly overheated if there are heat gains (Figure 62).
Large thermal mass can be advantageous both in winter and in summer. In winter, peak demands for the heating system are lower and solar gains are utilised more efficiently. Solar gains heat up the structure during the day, which releases the absorbed heat with a delay in the evening: this is the basis for passive solar heating techniques. In summer, indoor climate is better as shown in Figure 62. Thermal mass combined with an effective shading and ventilation strategy might eliminate the need for mechanical cooling. However, large thermal mass can be disadvantageous in winter if the building is not in constant use or has intermittent heating [ZOL99]. In this case, heating up a large mass takes a long time.
Figure 62: Internal temperature profiles expected in buildings with high and low levels of thermal mass [TCC08]
Definition of thermal mass
The heat storing capacity or thermal mass of a material is commonly defined as the product of the mass and the specific heat capacity. The specific heat capacity of most building materials is around 0,8-1 kJ/kgK. Wood has a higher heat capacity of 1,6-3 kJ/kgK, but has a relatively low density. The volumetric heat capacity referring to 1 m3 volume describes the materials well. Dense structural elements, such as brick, concrete or stone usually have large volumetric heat capacity (Table 6).
Table 6: Volumetric heat capacity of different materials
|
|
Volumetric heat capacity, @ 20 °C (kJ/m3K) |
|
Air |
1.2 |
|
Insulation materials |
20 - 200 |
|
Wood |
1000 - 1600 |
|
Brick |
1300 - 1500 |
|
Rammed earth |
1400 - 1800 |
|
Concrete |
1800 - 2000 |
|
Glass |
1900 - 2200 |
|
Marble |
1400 - 2400 |
|
Water |
4180 |
The ability of thermal mass to charge and discharge effectively with a diurnal thermal cycle is mostly determined by its conductivity. Materials with high conductivity, e.g. metals heat up and cool down too quickly to work efficiently in a diurnal cycle. On the contrary, this process is too slow in low conductivity materials.
In the daily heat storage, only a limited thickness of the structure is active. According to EN ISO 13790, the inner 10 cm can be taken into account as the effective zone in a diurnal cycle. This can be used as a rule of thumb in common structures with the caveat that if there is an insulation layer in the first 10 cm, only the thickness up to the insulation layer should be considered. The heat capacity of a room or a building is then calculated by multiplying the heat capacity of each element by its total surface area in contact with the internal air.
2.2 How to preserve thermal mass
In historic buildings, the question is usually how to preserve the large original thermal mass during the retrofit. In order to perform effectively, thermal mass must be allowed to interact thermally with the interior. Thermal coupling by all forms of heat transfer is important [UCD06]:
§ Conduction: even if high heat capacity materials are applied in a room, low conductivity internal surfaces can “switch off” their effect. Carpets, interior insulation or suspended ceilings and raised floors isolate the heavy structure from the interior. Heavyweight structures should be exposed or conductive surface finishes, such as dense plaster, ceramic tiles or natural stone should be used as a thermal link. Slabs are the most convenient and most effective place to locate thermal mass as they receive direct solar radiation. Furniture, however, will obstruct the incoming radiation. Ceilings do not receive direct radiation, but will heat up through radiation and convection. They are less likely to be obstructed and hence can be especially effective in office buildings.
§ Convection can be improved by increasing the airflow over a mass surface or by introducing turbulence into the air steam.
§ Radiation is enhanced with dark, matt or textured surfaces, which absorb and re-radiate more energy than light, smooth, reflective surfaces.
In contrast to the importance of thermal coupling with the interior, thermal mass should be isolated from the influence of external air temperatures. The reaction of a building to thermal changes can be described with its time constant – the ratio of the effective thermal capacity and the heat loss coefficient of the building. The heat loss coefficient includes both transmission and ventilation losses. A higher time constant – and so a better utilisation of winter gains and a lower risk of overheating in the summer – can be reached either by increasing the thermal mass of the volume or by increasing the heat losses in the summer and decreasing in the winter. This means that exterior insulation will increase the effect of thermal mass, but if there is no insulation, the benefits might be lost towards the outside. Where a façade is highly glazed, windows with a low U-value should/can be applied to ensure enough heat losses in the summer. The air tightness of the building is also important.
2.3 How to create thermal mass
Innovative options, such as the application of ventilated hollowcore slabs, are not feasible in historic buildings, but are also not necessary. Areas where the risk of overheating in the summer is high, such as converted roof spaces, can be a critical issue. Here, it is advisable to install heavy insulating materials, for example wood fibreboards with a density over 100 kg/m3. Another possibility to create thermal mass is the use of phase change materials.
Phase change materials
Phase change materials (PCM) are special materials for increasing the heat storage capacity without adding extra weight to the structure. Here the phase change is used for energy storage. As the temperature increases, the material changes phase from solid to liquid and during the chemical process energy is absorbed. This way the room temperature will be lower. Later when the temperature decreases, the material changes phase from liquid to solid and dissipates the heat. This energy can be removed from the room through night-time cooling. Ventilation can be increased with a fan and the air blown directly on the panels. The temperature of the PCM itself remains constant during the reactions.
For building applications, the phase change should take place near the comfort temperatures, between 18 and 25°C. The phase change point depends on the type of material applied. If this temperature is too low, the heat storage capacity is exhausted too early, if it is high, starts too late and the influence is small.
Micro-encapsulated PCM (e.g. paraffin) can be mixed to interior plaster, wallboard panels or aerated cement blocks and applied in the building without any special measures, just like conventional materials (Figure 63). Encapsulation is important, as the PCM must not be in direct contact with other materials to avoid damages due to the “melting” process. 30 mm plaster coating with 30 % PCM has a heat storage capacity equivalent to 180 mm concrete [HEG06].
PCM can be coupled with active elements, for example alternative heat sinks to cool the fluid. PCM technology is still under development.
Figure 63: PCM plaster and actively chilled PCM system with capillary tubes [FRA08]
2.4 Control and ventilation strategies
The heating control strategy needs to be adapted to fully utilise the advantages of thermal mass. If the thermal mass is large, the thermal response will be slow. If night set-back is used, e.g. in office buildings, the longer time for heating up the building needs to be taken into account by the building management system. The cooling down of the building at night can be minimised by decreasing the heat losses – with an airtight and well-insulated building. The heating system has to be flexible and follow the internal conditions. If, for example, the mean radiant temperature is increased by incident solar radiation, the air temperature set point and the load on the heating system must be reduced accordingly.
An effective ventilation strategy adapted to the present thermal mass can eliminate or reduce the mechanical cooling load in the summer. Thermal mass absorbs a large portion of heat gains during the day (solar and internal heat – both significant in office buildings). These gains can be purged from the building by flushing the thermal mass with cool nighttime air by leaving windows open or by mechanically ventilating. Due to security reasons, hopper windows or vents can be applied. The optimal night air change rate depends on the function of the building, typically 2 to 5 air changes per hour are necessary. Increasing the air change rate will also increase the energy consumption of fans if mechanical ventilation is used.
2.5 References
[ZOL99] Zöld A. Energy conscious architecture (Energiatudatos építészet - in Hungarian). Műszaki Könyvkiadó, Budapest, 1999.
[TCC08] The Concrete Centre, UK, http://www.concretecentre.com/, 2008
[UCD06] UCD Energy Research Group. Thermal Mass & Sustainable Building, Irish Concrete Federation, 2006.
[HEG06] Hegger, Auch-Schwelk, Fuchs and Rosenkranz. Construction Materials Manual, Birkhaeuser, 2006.
[FRA08] Fraunhofer ISE, Germany, www.pcm-storage.info