Night ventilation of thermal mass
Provided by Nick Baker: Research Associate, The Martin Centre, University of Cambridge
What, Why, How, Extras
Fig 1: The principle of night ventilation; the mass of the building is cooled at night to provide a heat sink for internal gains during the day. (Click image to enlarge)
Night ventilation is the use of the cold night air to cool down the structure of a building so that it can absorb heat gains in the daytime This reduces the daytime temperature rise. It is usually applied to buildings that are not occupied at night, although an occupied building would probably be ventilated anyway.
Night ventilation can be driven by natural forces – i.e. stack or wind, but may use auxiliary fan power, either to provide sufficient airflow at times when the natural forces are weak, or to allow smaller ducts (causing greater resistance) to be used.
Typically daytime temperature depressions of at least 3oK can be expected and are worth achieving. In favourable conditions, it can be twice this. There are many variables affecting the actual figure and it is not possible to give a more accurate guideline here. If a preliminary reference to this guide suggests that night ventilation could be incorporated in a design proposal, it is strongly recommended that powerful simulation software be used to quantify design parameters.
Night ventilation is an overheating prevention strategy which uses little or no fossil energy, and together with other passive strategies such as natural ventilation and shading , can avoid the use of air-conditioning. This saves energy (and CO2 emissions), and once set-up would require lower maintenance than mechanical systems.
It may not, however, be without initial costs, since the requirement for ducts and controls may represent an additional cost.
The average diurnal temperature swing must be at least 5oK, and preferably greater than 7oK.
The building must have thermal mass that can be coupled to external air.
The same thermal mass must be able to be coupled to the occupied space.
A ventilation flow path of low resistance (i.e. large openings and large cross-section ducts) can be incorporated into the building so that it can be driven by stack effect and/or wind pressure. Or fans must be installed to enhance flow when needed.
Step 1: Thermal climate: Determine the typical thermal climate of the site – i.e. the diurnal temperature range (Tmax – Tmin; take particular notice of the months where overheating is most likely. Note that there might be local effects – urban environments have smaller temperature swings due to the heat retention of surrounding buildings. Test against the 5oK criterion. Note that for the smaller temperature swings, ventilation flows will have to be greater for the same cooling effect.
Step 2: Thermal mass:
Fig 2: The thermal mass must be coupled to the occupied space and have the maximum surface area possible. Thermal mass can be coupled remotely by mechanically driven air flow.
Thermal mass is any material of the building that can absorb heat. All material absorbs heat when its temperature increases; dense and conductive materials like concrete absorb much more than light materials like wood or insulation.
Determine the spatial disposition of the thermal mass in the proposed building, fig 2). Note that at night, the cool air must have direct contact with the thermal mass, and the larger the surface area, the better.
For the daytime cooling effect, the thermal mass that has been cooled must be coupled with the occupied interior where the thermal gains are anticipated. This can occur in two ways –
(1) by being the exposed room surfaces – e.g. walls, floor, ceiling. These cooled surfaces will absorb heat by both convection and radiation.
(2) coupled with the interior remotely by a convective link The second technique allows night ventilation to be effective even if all the room surfaces are lightweight. However, no direct radiative cooling of the occupants can take place.
Step 3: Airflow pathways: The principles have been covered under Stack Ventilation and Wind-induced Ventilation.
However two special considerations have to be made here. Firstly, the night ventilation flow path must direct the air to the surface to be cooled, not the occupants. This is relatively easy if a massive floor is to be cooled, since the cool night air will tend to sink onto the floor. Walls and ceiling (slabs) are less easy and careful consideration needs to be given to inlet and outlet points.
The second design consideration is the maintenance of security at night time, with large areas of opening.
Step 4: Thermal gains reduction: As in all passive techniques, it is always better to reduce demand than to struggle to meet a large demand. Night ventilation is much more likely to achieve a satisfactory daytime condition, in a building that is well shaded in the daytime, and has taken steps to minimise internal gains from equipment and lighting. This is a vital step.
Step 5: Controls: In principle, a small building could be controlled manually. In Mediterranean countries, traditional houses would be closely shaded and shuttered in the daytime, and opened up at night, as a matter of habit, without consciously thinking about night ventilation. This approach still could be appropriate in residential buildings, provided there are no conflicting factors such as noise.
For non-domestic buildings, some level of automatic control may be appropriate, and for the best efficiency, the control algorithm is not just - high rates at night and low in the daytime.
Consider two cases – first where the outdoor daytime temperature is lower than the maximum comfort temperature, and secondly, where the daytime temperature is higher than the target internal temperature. In the first case, the high rate of ventilation can take place 24 hr/day, since it will always lead to heat removal. In the second case, it is better to reduce ventilation in the daytime, to a minimum value sufficient to maintain air quality. High daytime rates will bring hot air into the occupied space, and increase the heat load on the structure.
In practice, an either/or situation rarely exists; the situation may change day to day, and even hour to hour within one day. A further complication is that at transitional seasons, vigorous night cooling could cool down the structure so much that heating could be required during the early hours of occupation. Complications such as this are best controlled by automatic intelligent systems, taking account of the key parameters such as internal and external temperature, and the temperature of the thermal mass itself. The control systems can be used to activate large flow-controlling elements such as windows and louvres, and optimise the performance of the system.
Step 6: Energy and fan power: Night ventilation can be driven by fans, and this carries the advantages of easier control, and in most cases, a reduction of opening and duct sizes due to the larger and consistent driving force of the fan. However, fans require significant amount of energy, and it is essential to carry out an estimate of the fan energy, in relation to the cooling affect achieved. It is helpful to define a Coefficient of Performance (COP), which is the ratio of the heat energy rejected, to the energy used by the fans. A conventional air-conditioning system will have a COP of 3 to 5, so a night ventilation system should aim for considerably better than this. These are advanced design considerations, and need powerful analysis tools to provide definitive answers
A hybrid system is one using both natural and mechanical driving forces, the fan only being used when natural forces are insufficient. This is a good strategy, enabling the building to operate predominantly passively, with a small occasional expenditure of energy for extreme conditions.
Fig 3: The temperature in a lightweight building (low thermal mass) rises above the overheating threshold, wheras that in the heavyweight building does not although the average daily temp is the same. In principle, night ventilation can reduce the average internal temperature further. (Click image to enlarge)
Obviously there is a close relation to normal daytime natural ventilation; indeed the two are inseparable. The main difference is the additional requirements outlined above – i.e. a sufficient temperature swing, thermal mass that can be coupled to both outside and inside, and openings that can provide sufficient air at night.
The incorporation of exposed thermal mass is a familiar recommendation in order to stabilise internal temperatures, and therefore reduce the daytime peak temperature. The principle is exactly the same as in night ventilation – the only difference is that the ventilation is being consciously controlled to selectively couple the mass to the air when it is below the 24 hr average. In principle, this could result in the average temperature of the building being below the average ambient temperature, although in practice, due to internal gains, this is difficult to achieve.
The problem of security has already been touched on. Where open windows are the ventilation inlets, this may cause problems when the building is unoccupied at night, especially on the ground floor. Two solutions are available – provide intruder proof grills or bars to the windows, although these may be considered unsightly. Or separate out the view, daylight and ventilation functions and have independent louvered air inlets. This is an example of how passive techniques such as this can incur extra costs.
There are two acoustic issues that may conflict. Firstly, any internal openings that allow air to circulate through the building, will conflict with good acoustic isolation, unless closeable during occupancy. Large openings (even when nominally closed) may also represent weak links to external traffic noise during the daytime.
Secondly, the exposure of thermal mass to the interiors, will inevitably lead to a more live acoustic in which noise control will be more difficult, and incur extra costs. This can partly be dealt with by using a convective link with the mass, together with free-standing acoustic absorbers.