Trans RINA, Vol 155, Part C1, Intl J Marine Design, Jan - Jun 2013
Tchim=Average temperature inside the stack (K) Te= Temperature outside of the stack (K) ∆h=Difference in high from air entry to exhaust.(m)
Another proposed configuration of a naturally ventilated multihull is shown in Figure 8. Here the air is pulled in through the upper apex of the roof by a pressure differential cause by the movement of air through cross ventilation. Air is pulled through, due to the Venturi effect which is represented by the Bernoulli equation (Equation 2). This proposal makes use of cross winds, which are often a typical climatic characteristic of coastal environments, the effects of which are enhanced by the semi-rigid structure above the roof.
with only 6-9m2 of solar absorbers) and reduced room overheating by approximately 50%. This equates to a ratio of 0.24 - 0.36 m2 to house volume.
Ruiz et al[16] used trombe walls to induce natural ventilation in order to remove heat. A mathematical difference was created to determine the airflow rates of a trombe wall based in Mediterranean climates around Spain. This supported the work of Khedari [14], showing a proportional relationships between total solar irradiance and air flow rates. This is an important consideration when designing the exterior of a yacht as coastal are good locations for natural ventilation to take place.
The British Research Establishment Conservation
Support Unit (BRECSU) has established that in the UK fans consume at least two thirds of energy consumed for cooling in office buildings [17]. Whilst there is limited data to suggest that this is the case for other countries, there is a growing awareness that fans consume a large portion of energy used to cool buildings[18]. This makes natural ventilation a desirable alternative to cool the interior, reducing operation costs consumption.
and energy
Figure 8: Proposed catamaran natural ventilation system using Venturi roof to create differences in pressure to induce ventilation [2].
The multihull structure lends itself to passive cooling, not only because of its large surface area but also because of the largely empty air cavity in between its two hulls which provides a well shaded area for the intake of air pulled into the interior by the mechanisms mentioned above. Before the air from this cavity enters the interior it is firstly preconditioned by phase change materials.
These materials often composed of salt hydrates, which absorb heat energy as they change phase from solid to liquid. The temperatures at which phase change occurs can be tailored to operational need, but it provides the essential thermal mass that any passive designed structure requires. In the proposals shown in Figs 7&8 both thermal and humidity conditions of the incoming air will be treated at this point.
The Temperature difference is the primary driving factor for the natural stack effect. The temperatures within the area of operation of the catamaran [2] were predicted to vary from hot arid to hot humid conditions, which raises a number of concerns for naturally ventilated buildings and reduces the cooling potential of these technologies. However, the work of Khedari [14], determined the effectiveness of natural ventilation systems within hot humid areas with low diurnal ranges and warm winters. The research ascertained that despite climatic conditions, solar chimneys alone were able to reduce the room temperature near to that of ambient, and provided nearly 8-15 air changes of a room equating to 25m3 per hour
There are other benefits to passive designed structures over conventional HVAC systems. The first is that mechanical heating ventilating and air conditioning often account for a large fraction of the cost of construction [11]. The other benefit
is that of space saving, as the
innovative of natural ventilation system designs recover most of the volume as occupiable space unlike traditional HVAC system which consume up to 20% - 40% of the total volume of the building (for a typical commercial building – of ceiling height 3.6m) [11].
Overall passive design has the potential to provide benefits
manufacturer, through facilitating the demands of future environmental legislation. However,
both to the user, the environment and the with
weather
conditions of many locations being variable, natural ventilation techniques alone are unpredictable and unreliable. This challenge could be overcome through the use of a hybrid system design approach.
5. NATURAL LIGHTING
There are particular characteristic desirable for passively designed houses. One of which is an intrinsic feature of the morphology of a mono-hull yacht, the length to width ratio. It is possible to achieve 16 – 25% reduction in energy use through the application of natural lighting technologies with an elongated building. This is not possible in other compact buildings, over a length and width ration 1:1 [19].
Natural lighting is common in most passively designed houses. As illustrated in Figure 9 for a hot humid climate it can be used to significantly reduce the use of artificial lighting and in doing so increase the user psycho-
©2013: The Royal Institution of Naval Architects
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