On the natural ventilation of two independently heated spaces connected by a low-level opening

We study the buoyancy-driven natural ventilation of two spaces which are connected to one another by a low-level opening, and each of which is connected to the exterior through a high-level vent. Each space is heated uniformly by an independent source, which provides buoyancy driving the ventilation. Using laboratory experiments, we show that these conditions lead to each space becoming well mixed at steady state. In this regime, a net flow from one space to the other is driven by the buoyancy created in the downstream space. Although it is possible in theory for the flow to develop in either direction, our new experiments and theoretical model show that, in reality, if the vents of the two spaces are at the same height, then the actual flow regime will depend primarily on the relative strength of the heat loads. If the two heat loads are sufficiently different, only the flow from the weakly heated space to the strongly heated one is stable. If the two heat loads are comparable, both modes are stable, leading to multiple flow regimes. The problem is generalised to show that, if the heights of the vents are equal, then the flow regime will depend on the relative height of the vents, as well as the relative strength of the heat loads. There is a range of combinations of vent heights and heat loads that still allow multiple flow regimes. We identify the limits of each regime and outline principles for control.     

Using Display Energy Certificates to quantify public sector office energy consumption

The influence of internal and external characteristics on energy use in the public sector office stock in England and Wales is explored using a database of 2600 Display Energy Certificates (DECs) combined with other sources of disaggregated office information. The DEC office benchmarks are shown to match the median fossil thermal and electrical consumptions well. Analyses of heating, ventilation and air-conditioning (HVAC), size, occupancy density, building age, location and rateable value are considered. While newer offices are shown to have lower typical fossil-thermal consumption than older offices, this is counterbalanced by higher electrical consumption, resulting in higher typical CO₂ emissions. This has implications for the UK's emissions reduction targets for 2050, indicating that while building regulations that focus on thermal performance have been successful, a focus on electrical consumption (both regulated and unregulated) is key. The results are also compared with existing benchmarks for all UK offices, splitting the sample into four generic types, and compared with a similar smaller study of private offices. This indicates that public offices typically used less energy than the general benchmarks had previously predicted, particularly for prestige offices.     

The fluid mechanics of the natural ventilation of a narrow-cavity double-skin facade

This paper investigates the natural ventilation of a double-skin facade connected to a room in a multi-storey building. The room and the facade are connected to the exterior through vents at different levels. The room contains a horizontally distributed heat source analogous to occupants in an open-plan office or an underfloor heating system. The facade cavity contains a vertically distributed heat source analogous to a shading blind/louvers heated by solar radiation. These two sources of heat combine to provide buoyancy driving the ventilation. Two basic modes of facade operation are proposed and investigated. These two modes of operation should be alternated according to exterior climatic conditions. In colder seasons, the room draws air from the portion of the facade which extends one floor below the room, and solar radiation on the facade preheats supply air into the room. In warmer seasons, the room vents to the exterior through the portion of the facade which extends one floor above the room, and solar radiation on the facade enhances the ventilation and prevents overheating in the system. A quantitative model is developed to describe the fluid mechanics of the ventilation in these two modes of operation. The model is successfully tested with laboratory experiments. It shows how the height of the facade and the size of the openings can be adjusted to maximise the preheating of the room in colder seasons, and to prevent overheating in the room and the facade in warmer seasons. The model is used to explore the principles for design and control in different climatic and occupancy conditions.     

Stratification and oscillations produced by pre-cooling during transient natural ventilation

This paper investigates the transient natural ventilation of a warm room which vents to a cool exterior through a stack connected to the roof of the room, and into which air is drawn through a lower vent connected to a cooling unit. The temperature differences between the interior and exterior create positive buoyancy driving the ventilation. This results in fresh exterior air being drawn into the room through the cooler connected to the lower vent. After being cooled, this fresh air enters the room and displaces the existing warm air upwards and out through the stack. A sharp interface then develops between a lower layer of cooled invading air and an upper layer of warm original air. This interface ascends as the room continues to drain. However, the pre-cooled air is negatively buoyant, and so the flow rate and the rate of ascent of the interface gradually fall as the cold lower layer deepens. As a result, the temperature of the pre-cooled air progressively decreases towards that of the chiller, and the lower layer becomes stably stratified. Eventually, the ventilation ceases and the interface stops ascending when the positive buoyancy associated with warm air in the upper layer and stack and the negative buoyancy associated with cooled air in the lower layer are in balance. However, this equilibrium is unstable to downward motion. Colder, less buoyant air from the exterior in time displaces warm buoyant air in the stack, reducing the positive buoyancy of the upper layer and causing it to be outweighed by the negative buoyancy of the lower layer. A downward draining flow then commences. As the exterior air descends through the stack, it mixes with warm air in the upper layer, leading to the temperature of the upper layer evolving towards that of the ambient. As a result, the flow rate and the rate of descent of the interface gradually fall as the upper layer deepens and the colder lower layer drains from the room through the lower vent. Eventually, the flow ceases again when the positive buoyancy of the upper layer balances the negative buoyancy of the lower layer, but now the interface is arrested closer to the base of the room. This equilibrium is unstable to upward motion, and in time warm air from the upper layer displaces the exterior air in the stack. This causes the positive buoyancy to increase, and consequently the room drains upwards once more. In this way the pre-cooling makes the system oscillate between the upward and downward draining regimes, which ensue until the room is completely drained. The ventilation eventually stops altogether when the interface reaches the base of the room and the upper layer attains the temperature of the ambient. A simple model is developed to describe this transient oscillatory process and is compared with small-scale analogue experiments. This paper discusses the application of the model, and shows how pre-cooled draining may be employed appropriately to provide thermal comfort in an intermittently occupied room, when the exterior air is uncomfortably warm so that conventional flushing without pre-cooling may not be used effectively.     

Using Display Energy Certificates to quantify schools' energy consumption

The UK non-domestic sector accounts for 2 million buildings and 19% of national CO₂ emissions, representing a significant opportunity for emission reductions. However, substantial improvement of the stock requires a greater understanding of current energy performance characteristics. This paper explores energy consumption in English schools, using data from the Display Energy Certificates (DECs) database. DECs are a key step in understanding the non-domestic stock, incorporating national-scale statistical data, covering bottom-up details of the individual buildings. Significant variations in emissions between primary and secondary schools and academies exist, primarily caused by large differences in electricity consumption. Considering pupil numbers is shown to accentuate the differences, revealing a 47% rise in CO₂ emissions per pupil from primary to secondary schools, and a further increase between secondary schools and academies. The extent to which building characteristics, including location, heating, ventilation and air-conditioning (HVAC) and size, influence performance has also been evaluated. Location, HVAC and school density are shown to correlate with variations in energy use. Finally, a comparison of current school performance against past data reveals considerable reductions in fossil-thermal energy consumption over the last decade. However, this has been offset by a significant increase in electricity consumption, resulting in rising typical emissions across the school types.

Le secteur non domestique britannique représente 2 millions de bâtiments et 19% des émissions de CO₂ du pays, ce qui constitue une importante opportunité de réduction des émissions. Cependant, une amélioration substantielle du parc exige une plus grande compréhension des caractéristiques actuelles des performances énergétiques. Cet article examine la consommation d'énergie dans les écoles anglaises, en utilisant les données provenant de la base de données des Display Energy Certificates (DEC – Certificats de Performance Energétique à afficher). Les certificats DEC sont une étape clé pour mieux connaître le parc non domestique, intégrant des données statistiques à l'échelle nationale et couvrant en détail selon une approche ascendante les différents bâtiments. D'importantes variations des émissions entre les écoles primaires, les écoles secondaires et les académies existent, principalement dues à de grandes différences dans la consommation électrique. Il est démontré que la prise en compte du nombre d'élèves accentue les différences, révélant une hausse de 47% des émissions de CO₂ par élève entre les écoles primaires et secondaires, une hausse supplémentaire intervenant entre les écoles secondaires et les académies. A également été évalué le degré d'influence sur les performances des caractéristiques des bâtiments, au nombre desquelles l'emplacement, le chauffage, la ventilation mécanique et la climatisation (CVCA), ainsi que la taille. Il est démontré que l'emplacement, la CVCA et la densité des écoles sont en corrélation avec les variations de la consommation d'énergie. Enfin, une comparaison des performances actuelles des écoles par rapport aux données passées révèle des réductions considérables de la consommation d'énergie thermique fossile au cours de la dernière décennie. Cependant, ceci a été contrebalancé par une augmentation importante de la consommation d'électricité, qui s'est traduite par une hausse des émissions moyennes sur l'ensemble des types d'écoles.

indicateurs de référence, parc immobilier, émissions de CO₂, Certificats de Performance Energétique (DEC), consommation d'énergie, écoles     

On underfloor air-conditioning of a room containing a distributed heat source and a localised heat source

This paper studies air flow in an air-conditioned room containing a distributed heat source and a localised heat source, into which cool air is supplied at low momentum through openings at a low level, and from which old air is extracted from a high level. This situation may be analogous to an auditorium containing a distributed audience and a localised group of actors and lighting on a stage, into which cool air is distributed from underneath the seating, and from which old air is removed through a ceiling, for example. Using a combination of a theoretical model and laboratory experiments, the paper shows that, in such conditions, if the localised heating is sufficiently strong compared to the distributed heating, the room will become stratified into two layers at steady state. A layer of warm air lies atop a cooler layer that attains a temperature above that of the supply air. This temperature structure depends primarily on the supply air flow rate and the ratio of the distributed heating to the total heat flux. For a room with fixed heating, increasing the supply air flow rate raises the interface between the upper and lower layers, while cooling both layers. The temperature in the upper layer depends on the total heat flux, but the temperature in the lower layer depends on the flux of distributed heating. For a room with a fixed supply air flow rate and a fixed total heat flux, increasing the strength of distributed heating warms the lower layer, but does not affect the temperature in the upper layer. Such increase in the strength of distributed heating also raises the interface. To achieve sufficient ventilation and thermal comfort in an occupied lower zone while keeping any pocket of uncomfortably warm air well above it, cool air needs to be supplied within an appropriate range of flow rates, which depends on the ratio of the distributed heating to the total heat flux. The paper shows how to determine such appropriate ranges of flow rates for different heating ratios, using the model.