WO2014064426A1

WO2014064426A1 - Ventilation system

Info

Publication number: WO2014064426A1
Application number: PCT/GB2013/052692
Authority: WO
Inventors: Thomas Lipinski
Original assignee: Ventive Limited
Priority date: 2012-10-23
Filing date: 2013-10-15
Publication date: 2014-05-01
Also published as: GB201218993D0; EP2917650A1; GB2507266A

Abstract

A ventilation system (1) for a building (2), the ventilation system (1) including at least an outflow duct (32) for directing air out of the building (2), the ventilation system (1) further comprising a ventilation promotion device (30), the ventilation promotion device (30) comprising a solar energy collector (36) adapted and arranged such that it can receive solar energy, a heat directing element (37) arranged to direct the thermal energy from the solar energy collector (36) to a convection promotion element (38), the convection promotion element (38) arranged to extend within the outflow duct (32), the convection promotion element (38) adapted to use the thermal energy received to promote air flow through the ventilation system (1).

Description

VENTILATION SYSTEM

This invention relates to a ventilation system. In particular, it relates to a passive ventilation system having a ventilation promotion device for aiding the movement of air through the ventilation system. It also relates to a ventilation promotion device for incorporation in a ventilation system to promote air flow through the ventilation system.

As buildings are becoming better insulated and more airtight, the need for adequate ventilation, to maintain a healthy indoor environment, is growing in importance. Ventilation systems with heat recovery are known. These systems comprise an outflow duct and an inflow duct and a heat exchange. A first electrically powered fan is used to move air from inside the building to the outside through the outflow duct, while a second electrically powered fan is used to bring fresh air inside through the inflow duct. The heat exchange comprises a region where the inflow duct and outflow duct are separated by a number of heat exchange plates or a thin membrane that allows heat transfer therethrough. Thus, the warm outgoing air is used to heat the cooler incoming air. These systems have been shown to be able to recover up to 90% of the energy of the outgoing air. These systems can therefore provide ventilation without a significant effect on heating requirement. However, if we take account of the electricity used by the fans, the overall saving in energy usage is very small and they are cumbersome to install. Passive ventilation systems are known that utilise the buoyancy of warmer air to drive the ventilation of buildings; the so called "passive stack effect". However, these systems have drawbacks as when the temperature of the air outside a building changes relative to the air inside the building the movement of air through the ventilation system can slow, stop and reverse depending on the relative conditions inside and outside the building. This makes passive ventilation systems ineffective in certain conditions.

According to a first aspect of the invention, we provide a ventilation system for a building, the ventilation system including at least an outflow duct for directing air out of the building, the ventilation system further comprising a ventilation promotion device, the ventilation promotion device comprising a solar energy collector adapted and arranged such that it can receive solar energy, a heat directing element arranged to direct the thermal energy from the solar energy collector to a convection promotion element, the convection promotion element arranged to extend within the outflow duct, the convection promotion element adapted to use the thermal energy received to promote air flow through the ventilation system.

This is advantageous as the ventilation system can operate passively, without fans or blowers, by focussing (and/or transferring) the sun's energy to a particular point in the ventilation system to promote air flow. The solar collector simply needs to be positioned such that it can receive sunlight. The heat directing element is used to direct the thermal energy from the solar energy collector to where it can be used to promote air flow by way of the convection promotion element. The convection promotion element can then impart that thermal energy to the air in the outflow duct or path such that it increases its buoyancy and thus draws further air through the outflow duct thereby improving the efficiency of the ventilation system. Thus, the heat directing element is particularly important in a passive ventilation system as it needs to direct or focus the thermal energy to a point in the system where it can passively aid ventilation. Preferably, the solar energy collector is located at least partly above the outflow duct. This is advantageous as the ventilation promotion device can be particularly space efficient and retrofitted to existing ventilation systems. Prior art systems that use solar energy to create an air flow collect the solar energy below the outflow duct. However, the heat directing element of the present invention leads to a flexible and efficient design as it is able to transfer the energy from a solar energy collector mounted above the device to the outflow duct. Thus, this is particularly advantageous when space is tight and the ventilation system opens onto a roof space. Preferably, the solar energy collector is mounted wholly above or above and around the outflow duct.

Preferably, the convection promotion element comprises an elongate structure arranged to be located within or form the structure of the outflow duct. Thus, air within the outflow duct has to pass the convection promotion element to leave the outflow duct. The release of thermal energy from the convection promotion element to air within the outflow duct heats the air (increasing its buoyancy) and thus promotes air flow through the duct. Preferably the wall of the outflow duct is insulated to reduce the flow of heat through the wall of the outflow duct. This is advantageous as the heat directed into the outflow duct by the heat directing element is used efficiently. Further, the insulation prevents the outflow duct from losing heat to, or being heated from the outside, which allows more control over the ventilation system to prevent over ventilation for example. Preferably the heat directing element is adapted and arranged to direct the thermal energy through the open end of the outflow duct.

The convection promotion element may include a heat transfer accelerator, which may comprise fins. This is advantageous as the fins will increase the surface area of the element so that it can impart more heat to the air in the outflow duct. The heat transfer accelerator may take other forms such as conductive coatings, textures, spikes, spokes or protrusions among others. The convection promotion element may, at least in part, comprise the internal surface of the outflow duct. Thus, the heat directing element may be arranged to direct the thermal energy from the solar collector to heat the internal wall of the outflow duct. The heat from the outflow duct's wall will be transferred to the air in the duct to promote air flow. In this case, the outflow duct may be insulated to reduce heat transfer through the duct wall. This will ensure that only the internal surface, where the heat directing element is focussing the thermal energy, will be heated such that it can encourage convection in the outflow duct, maintaining a steady, continuous and predictable ventilation rate. Preferably the convection promotion element includes a heat storage material for storing the thermal energy received from the heat directing element. The heat storage material may be a phase change material. The phase change material may be in granular form. This is advantageous as the convection promotion element can store energy to maintain air flow through the ventilation system even when sunlight is not being received by the solar collector, such as at night or when the sun is obscured by cloud.

The heat storage material may form part of or be impregnated into the internal wall of the outflow duct. Alternatively, if the convection promotion element is an elongate structure, the heat storage material is located within the elongate structure. This forms a heat storing central core. In one embodiment, the solar energy collector and at least a part of the heat directing element comprise a lens arranged to focus the sun's energy onto the convection promotion element. This is advantageous as the lens can be configured and positioned such that it focuses the sunlight on a particular point so that it can then be used by the convection promotion element. Preferably the lens is dome shaped. This is advantageous as the dome has been found to be effective at directing solar energy from a wide range of angles and elevations to compliment the sun's position in the sky over the day and seasons. Preferably, the heat directing element comprises two parts; a lens part and a thermally conductive part, the lens part is adapted to focus the sunlight onto the thermally conductive part and the thermally conductive part is arranged to conduct the heat to the convection promotion element. Alternatively the lens may comprise both the solar collector and the heat directing element, wherein the lens directs the thermal energy by focussing it onto the convection promotion element.

In an alternative embodiment, the solar energy collector is of an energy absorbent material and the heat directing element is of a thermally conductive material such that thermal energy received by the solar energy collector is conducted by the heat directing element such that it is directed to the convection promotion element. Preferably the solar energy collector is dome shaped. This is advantageous as the dome has been found to be effective at collecting solar energy from a wide range of angles and elevations to compliment the sun's position in the sky over the day and seasons. The dome may be arranged such that the concave face of the dome faces the convection promotion element. This is advantageous as the energy absorbent dome will also radiate the energy it collects. The dome's internal surface may be parabolic with the convection promotion element at a focal point of the parabolic dome. Preferably the heat directing element comprises a heat pipe. This is advantageous as a heat pipe can efficiently transfer the collected heat to the convection promotion element. The heat pipe may form the thermally conductive part in the above lens embodiment in which the lens focuses the sunlight onto an evaporating end of the heat pipe. In the energy absorbent dome embodiment, the evaporating end of the heat pipe may be mounted within the dome. Preferably the ventilation promotion device is mounted within a cowl of the ventilation system, wherein the cowl provides an exit to atmosphere for air flowing out of the outflow duct. Preferably the ventilation system also includes an inflow duct to channel fresh air into the ventilation system. Preferably the inflow duct is substantially annular and surrounds the outflow duct. Thus, the inflow duct is mounted around and concentrically with the outflow duct. Preferably the ventilation system is a heat recovery ventilation system and includes a heat exchange adapted to transfer heat between the air flowing in the outflow duct and inflow duct. Preferably the convection promotion element is mounted above the heat exchange. The invention is particularly advantageous in heat recovery ventilation systems because in such systems (in cold & moderate climates) the heat of the outgoing air is transferred to the incoming air, which will thus cool the outgoing air. Cooler outgoing air will not be as buoyant and thus the present invention is particularly useful for encouraging air flow in heat recovery systems where the outgoing air may have been cooled by heat recovery. In warmer climates, the air in the building is typically cooler than the air outside. In this situation, the invention is advantageous as it can be used to superheat the outgoing air above ambient temperature to encourage ventilation.

According to a second aspect of the invention we provide a ventilation promotion device for a ventilation system, the ventilation promotion device comprising a solar energy collector adapted and arranged such that it can receive solar energy, a heat directing element arranged to direct energy from the solar energy collector to a convection promotion element, the convection promotion element arranged to extend within an outflow duct of the ventilation system is adapted to use the thermal energy received to promote air flow in the ventilation system.

This is advantageous because the ventilation promotion device is able to harness the sun's energy and focus it to heat air within a specific part of a ventilation system to encourage buoyancy assisted airflow through the ventilation system. It will be appreciated that the optional features of the first aspect of the invention that relate to the ventilation promotion device apply equally to the second aspect of the invention. According to a third aspect of the invention we provide a cowl for a passive ventilation system, the cowl including an outflow duct for exhausting air from the ventilation system, the outflow duct including a heat storage device for regulating the temperature of the air flowing in the outflow duct.

This is advantageous as the relative buoyancy of the air in the outflow duct of a ventilation system affects the flow rate through the ventilation system. By regulating the temperature of the air in the outflow duct, the flow rate through the system can also be controlled. Thus, passive ventilation systems in particular can be further controlled by the heat storage device regulating the temperature. Storing heat in the outflow duct is also advantageous because if the cowl is used with a solar collector as described above, the stored heat can be used to maintain ventilation after sunset.

Preferably the heat storage device includes a phase change material. The use of a phase change material is advantageous as the temperature of the material during its phase transition is relatively constant as the energy is absorbed or released as latent heat. By selecting a phase change material having an appropriate phase change temperature, the flow rate through the cowl can be controlled such that it is kept relatively constant. In hotter climates, when the air in the outflow duct is relatively warm (as it has already passed the heat exchanger and gained near outside temperature) but marginally colder than ambient air, then it is heated up to temperature significantly above ambient by the convection promotion device and would therefore flow more quickly due to its buoyancy. The ventilation promotion device heats up the air in controlled and time/solar energy decoupled way (it keeps heating it up past sunset). When the air in the outflow duct is relatively cool (comparing to ambient) and would therefore not flow out at all, unless flowing down assisted by gravity or flow more slowly, the heat storage device can impart heat to the air. This will heat the air and encourage or quicken the flow rate. As can be appreciated, the transition temperature of the phase change material is selected to suit the climate and required flow rate through the ventilation system. There now follows by way of example only a detailed description of the present invention with reference to the accompanying drawings, in which; Figures 1 a to 1 c show diagrammatic views of passive ventilation systems incorporating an embodiment of the ventilation promotion device;

Figure 2 shows a sectional view of a known chimney cowl for a passive ventilation system;

Figure 3 shows a sectional view of a first embodiment of the ventilation promotion device; Figure 4 shows a sectional view of a second embodiment of the ventilation promotion device;

Figure 5 shows a sectional view of a third embodiment of the ventilation promotion device;

Figure 6 shows a sectional view of a fourth embodiment of the ventilation promotion device;

Figure 7 shows a sectional view of a fifth embodiment of the ventilation promotion device; and

Figure 8 shows a sectional view of a sixth embodiment of the ventilation promotion device. Figures 1 a to 1 c shows a ventilation system 1 installed within a building 2. Looking first and Figure 1 a, the ventilation system is a passive ventilation system and does not include fans or blowers to move the air through it. Instead, the ventilation system uses the passive stack effect. The ventilation system 1 is a heat recovery ventilation system which uses a heat exchange 3 to exchange heat between air leaving the ventilation system and air entering the ventilation system 1. The ventilation system 1 includes a chimney cowl 31 mounted at a high point of the building, the heat exchange 3, an air outlet vent 5 and an air inlet vent 6. The air outlet vent is connected to the heat exchange 3 by an outlet conduit 7. The heat exchange 3 is connected to the air inlet vent 6 by the chimney stack 8' rather than a dedicated conduit. The chimney cowl includes a ventilation promotion device 30. Figure 1 b is substantially similar to Figure 1 a except that the air inlet vent is connected to the heat exchange 3 by a conduit 8' rather than by a chimney stack. Figure 1 c shows the ventilation system 1 installed within a single room, or a large space, such as a school, office, hospital, factory or warehouse. In this embodiment, there are no conduits to extract air from/deliver air to specific points in the building. Instead, the inflow duct and outflow duct located in the heat exchange 3 and cowl 31 ' open into the room space. The inflow duct opens into the room substantially downwardly. The outflow duct extracts air from a substantially horizontal direction adjacent the ceiling. Circulation of air within the room space is shown by the arrows. Figure 2 shows a cross-section through a known chimney cowl. The chimney cowl includes an outflow duct 10 for allowing stale air to leave the ventilation system and an inflow duct 1 1 to allow fresh air into the ventilation system. The inflow duct 1 1 is annular and is mounted such that it surrounds and is concentric with the outflow duct 10. The cowl further includes a cap 13, which shields the outflow duct 12 from rain.

Figures 3 to 5 show embodiments of a ventilation promotion device 30 that is mounted within a roof-mounted ventilation cowl 31 , which may be a chimney cowl or other ventilation system/atmosphere flow path. Similar to the chimney cowl shown in Figure 2, the ventilation cowl 31 includes an outflow duct 32 and an annular inflow duct 33 mounted concentrically therewith. It will be appreciated that the inflow duct 33 does not have to be concentric with the outflow duct 32. The ventilation cowl 31 extends above the roofline 34 and is arranged to interface with a heat exchange 35, which may be below the roofline 34. The heat exchange 35 is cylindrical and configured to receive the inbound air through an annular flow path mounted in cooperation with the inflow duct. The outbound air is supplied to the outflow duct 32 through a centrally positioned, circular flow path mounted in cooperation with the outflow duct 32. In the embodiment of Figure 3, the ventilation promotion device 30 comprises a solar energy collector 36 adapted and arranged such that it can receive solar energy, a heat directing element 37 arranged to direct the thermal energy from the solar energy collector 36 to a convection promotion element 38. The convection promotion element 38 extends into the outflow duct 32.

The solar energy collector 36 comprises a solar energy collecting dome mounted above the outflow duct 32. The dome also covers the inflow duct 33. The dome is adapted to absorb solar energy. Thus, the dome is dark in colour and, in particular, black so that it absorbs heat from the sun. The material and/or coatings and/or colour of the dome are selected for low reflectivity, high conductivity and low emissivity (preferably using coatings). For example, the dome may be domed sheet metal covered with coatings or matt black aluminium or stainless steel. Optionally, the dome may be covered with glass to reduce the amount of heat radiating back out.

The heat directing element 37 comprises a tube that connects to the apex of the solar energy collecting dome 36. The tube is of metal and includes a plurality of holes. The heat directing tube is thus adapted to conduct heat from the dome 36 to the convection promotion device 38. It will be appreciated that the tube may not be perforated and, alternatively, could comprise a plurality of rods or any other construction that is able to conduct and direct heat to the convection promotion device 38.

The convection promotion element 38 is mounted substantially along the axis of the outflow duct 32, such that the outflow air is required to pass between the convection promotion element 38 and the internal walls of the outflow duct 32. The heat directing element 37 forms a tubular frame 40 of the convection promotion device 38 and extends therethrough to the heat exchange 35. The tubular frame 40 is filled with a heat storage material 41 . The heat storage material 41 is a granular phase change material such as encapsulated wax or salt hydrides, for example. The thermal energy from the heat directing element 37 is absorbed by the heat storage material 41. The heat storage material 41 is chosen to have a transition temperature between 50 and 70°C (depending on the geographical region/climate), and therefore the thermal energy is stored as latent heat in the heat storage material. The convection promotion element 38 also includes radiator fins 42. The radiator fins 42 extend radially outwardly from the tubular frame 40 and are aligned with the axis of the outflow duct 32. The radiator fins 42 are adapted to pass the thermal energy to the outgoing air in the outflow duct 32. It will be appreciated that the radiator fins 42 are optional.

The ventilation promotion device 30 or convection promotion device 38 may further include a heat retention cover (shown in the embodiment of Figure 7), which may comprise a coating, to limit the radiant/conductive loss of heat to atmosphere. This ensures that the heat received from the heat directing element 37 is used to heat air in the outflow duct 32 or stored by the heat storage material.

In use, sunlight incident on the solar energy collecting dome 36 will heat the dome. The colour, material and texture of the dome are selected to encourage absorption of heat from the sunlight. The heat directing element 37 conducts the thermal energy to the convection promotion device which stores part of the thermal energy in the heat storage material and also passes the thermal energy to the air in the outflow duct 32. The air in the outflow duct 32 will thus be heated (above ambient temperature) and will rise up through the outflow duct 32 and out of the ventilation system 1 . The air rising in the upper portion of the outflow duct 32 will also draw air through the remainder of the ventilation system 1. Thus, air in the conduit 7 and heat exchange 35 will be encouraged upwards to leave the ventilation system 1 , which will further draw air through the air outlet vent 5.

The use of the heat storage material acts to regulate the flow rate through the outflow duct. Over the transition temperature, the phase change heat storage material will continue to absorb energy but will remain at a relatively constant temperature. The selection of a phase change material with an appropriate transition temperature can act to regulate or limit the temperature of the air in the outflow duct 32 to an upper limit, while the phase change material is undergoing phase change. The limiting of the temperature of the air in the outflow duct 32 acts to regulate the buoyancy of the air and therefore the flow rate through the duct. This is advantageous to avoid over ventilating the building (while providing it with continuous ventilation). It is appreciated that once the phase change is complete, the temperature in the outflow duct will continue to rise. However, by appropriately selecting the quantity (or energy density) of phase change material and its transition temperature, an upper temperature "limit" can be achieved in practice.

When the sunlight is obscured or after the sun has set, the convection promotion element 38 can continue to operate using the thermal energy stored in the heat storage material. Thus, thermal energy will be transferred from the heat storage material 41 to the radiator fins 42 to continue the warming of air in the outflow duct 32. Figure 4 shows a second embodiment. The same reference numerals have been used for identical parts. In this embodiment, the convection promotion element 38 is identical to the first embodiment. The solar energy collector comprises a lens 46. In particular, the lens 46 comprises a domed Fresnel lens (other lens designs are envisaged including non-Fresnel lenses). The use of a Fresnel lens is advantageous as it can be made thinly compared to a conventional lens. Further, the ventilation promotion device 30 includes a transparent cover 47 to protect the lens. The cover 47 allows the sunlight to enter the lens 46 while protecting it from dirt and damage. The cover 47 is of toughened glass and may include additives to assist in keeping it clean. Such additives are known and will not be discussed herein.

The solar energy collector, being a lens, also directs the thermal energy of the sunlight. Thus, in this embodiment the heat directing element comprises the tube 37 as well as the lens 46. The lens 46 focuses the sunlight onto the top surface of the convection promotion element 38 and the tube 37. The tube may be coated or painted a dark colour, such as black, to aid in the absorption of the thermal energy from the focussed sunlight. The tube need not be perforated or in contact with the lens. Further, the convection promotion element 38 may include a focal point receiver (not shown) on its top face of a conductive material substantially at the focal point of the lens 46 to efficiently receive the focussed sunlight. The focal point receiver may include conduction legs that extend into the heat storage material 41 to efficiently receive and transmit the heat to the remainder of the convention promotion element 38. Alternatively, the focal point receiver may comprise an evaporator part of a heat pipe where the condenser part of the heat pipe is placed within the heat storage tube/section of the outflow duct. This is shown in the embodiment of Figure 6.

The operation of the embodiment shown in Figure 4 is substantially the same as that in Figure 3. However, sunlight is received through the optional cover 47 and is focussed by the lens 46 onto the perforated tube 37 and the top face 48 of the convection promotion element 38. The convection promotion element 38 operates in the same way as described in relation to the first embodiment. Figure 5 shows a third embodiment, which is a modification of the second embodiment. In this embodiment, the heat transfer element does not include a tube. Instead, the function of the heat transfer element is provided by the lens 46. Thus, the lens 46 acts as a solar collector and heat transfer element by directly focussing the light, and therefore the thermal energy, onto the convection promotion element 38. The lens 46 and its associated cover 47 are supported above the convection promotion element 38 by support rods 50 which, at their other ends, are connected to a mounting collar 51. The mounting collar 51 comprises the upper section of the tubular frame 40.

In a further embodiment shown in Figure 6, the heat directing element 37 comprises a heat pipe 60. The heat pipe, as is conventional, includes an evaporator part 61 and a condenser part 62. The embodiment of Figure 6 also includes a focal point receiver 63 mounted at the evaporator part 61 of the heat pipe and at a focal point of the lens 46. The focal point receiver will thus be heated by the sunlight focussed by the lens and will heat act to evaporate a working fluid in the heat pipe 60. The heat pipe 60 conveys the working fluid to the convection promotion device. The convection promotion device includes the condenser part 62 of the heat pipe to condense the working fluid and thus extract the thermal energy. This is advantageous as efficient transfer of heat can be achieved and, after sunset, less energy is lost by the convection promotion element to atmosphere as the conductive link (movement of working fluid) is broken. In this embodiment the support rods 50 are connected to the wall of the outflow duct. Further, in this embodiment, radiator fins 42 are not provided on the convection promotion device 38, which can warm the outgoing via heat transfer through its surface. Figure 7 shows a still further embodiment in which the solar energy collector 36 comprises a black dome as in the embodiment of Figure 3. The solar energy collector 36 includes a heat retention cover 70 to reduce the amount of re-radiation of collected solar energy. The heat retention cover 70 is of glass, although it may be of other materials or have coatings to aid absorption of heat but reduce radiation back to atmosphere. The heat directing element 37 comprises a heat pipe 60, as shown in the previous embodiment. The solar energy collector 36 is connected to the evaporator end 71 of the heat pipe 60.

Figure 8 shows a further embodiment. In this embodiment the heat directing element 37 comprises a tubular member that connects between the apex of the solar collector dome 36 and the convection promotion element 38. Although the above examples show a ventilation promotion device that extends into a chimney cowl, the ventilation promotion device could be arranged to be mounted in any other part of the ventilation system to promote convection therethrough. For example, it could be mounted within a heat recovery part or incoming/outgoing conduit among other places.

The Fresnel lens may have alternative constructions. The lens may have a smooth top and the Fresnel lens portion may be annular. Alternatively the lens may comprise prismatic concentrators or reflectors to maximise the efficiency of thermal collection/transfer.

The heat exchange 35 may not be located directly below the ventilation cowl 31. Alternatively, heat recovery could be performed remote from the ventilation cowl 31 , which may be advantageous in hotter climates. Alternatively, the heat recovery ventilation system may be adapted and arranged such that heat recovery is delayed by up to 12 hours, recovering the day's heat to use at night to pre-heat fresh air and the using the lower night temperatures to pre-cool air during the day. Thus, the ventilation system can provide not just fresh air but also control comfort passively.

In the above embodiments the inflow duct 33 is shown present in the ventilation cowl 31. However, the inflow duct 33 may be entirely separate from the ventilation cowl 31 and mounted somewhere else in the building. Further, the above embodiments disclose various types of solar energy collectors, heat directing elements, means to attach the solar energy collectors to the remainder of the device and several embodiments of the convection promotion element. It will be appreciated that these parts can be used in different combinations to those disclosed herein.

Claims

1. A ventilation system for a building, the ventilation system including at least an outflow duct for directing air out of the building, the ventilation system further comprising a ventilation promotion device, the ventilation promotion device comprising a solar energy collector adapted and arranged such that it can receive solar energy, a heat directing element arranged to direct the thermal energy from the solar energy collector to a convection promotion element, the convection promotion element arranged to extend within the outflow duct, the convection promotion element adapted to use the thermal energy received to promote air flow through the ventilation system, wherein the heat directing element is adapted and arranged to direct the thermal energy through the open end of the outflow duct.

2. A ventilation system according to claim 1 , in which the solar energy collector is located at least partly above the outflow duct.

3. A ventilation system according to claim 1 or claim 2, in which the convection promotion element comprises an elongate structure arranged to be located within or form the structure of the outflow duct.

4. A ventilation system according to any one preceding claim, in which the wall of the outflow duct is insulated to reduce the flow of heat through the wall of the outflow duct.

5. A ventilation system according to any one preceding claim, in which the convection promotion element includes a heat transfer accelerator to assist in transferring heat to the air in the outflow duct, which may comprise fins, conductive coatings, textures, spikes, spokes or protrusions among others.

6. A ventilation system according to any one preceding claim, in which the convection promotion element, at least in part, comprises the internal surface of the outflow duct.

7. A ventilation system according to any one preceding claim, in which the convection promotion element includes a heat storage material for storing the thermal energy received from the heat directing element.

8. A ventilation system according to claim 7, in which the heat storage material is a phase change material.

9. A ventilation system according to claim 7 or claim 8, in which the heat storage material forms part of or is impregnated into the internal wall of the outflow duct.

10. A ventilation system according to any one of claims 7 to 9, in which the heat storage material is located within the convection promotion element which comprises an elongate structure within the outflow duct.

1 1. A ventilation system according to any preceding claim, in which the solar energy collector and at least a part of the heat directing element comprise a lens arranged to focus the sun's energy onto the convection promotion element.

12. A ventilation system according to claim 1 1 , in which the heat directing element comprises two parts; a lens part and a thermally conductive part, the lens part is adapted to focus the sunlight onto the thermally conductive part and the thermally conductive part is arranged to conduct the heat to the convection promotion element.

13. A ventilation system according to claim 1 1 , in which the lens comprises both the solar collector and the heat directing element, wherein the lens directs the thermal energy by focussing it onto the convection promotion element.

14. A ventilation system according to any of claims 1 to 10, in which the solar energy collector is of an energy absorbent material and the heat directing element is of a thermally conductive material such that thermal energy received by the solar energy collector is conducted by the heat directing element such that it is directed to the convection promotion element.

15. A ventilation system according to any preceding claim, in which the solar energy collector is dome shaped.

16. A ventilation system according to any preceding claim, in which the heat directing element comprises a heat pipe.

17. A ventilation system according to claim 18 when dependent on claim 12, in which the heat pipe forms the thermally conductive part in which the lens focuses the sunlight onto an evaporating end of the heat pipe.

18. A ventilation system according to claim 16 when dependent on claim 14, in which an evaporating end of the heat pipe is mounted within the dome.

19. A ventilation system according to any preceding claim, in which the ventilation promotion device is mounted within a cowl of the ventilation system, wherein the cowl provides an exit to atmosphere for air flowing out of the outflow duct.

20. A ventilation system according to any preceding claim, in which the ventilation system also includes an inflow duct to channel fresh air into the ventilation system, which may be substantially annular and surrounds the outflow duct.

21 . A ventilation system according to any preceding claim, in which the ventilation system is a heat recovery ventilation system and includes a heat exchange adapted to transfer heat between the air flowing in the outflow duct and inflow duct.

22. A ventilation system according to claim 21 , in which the convection promotion element is mounted above the heat exchange.

23. A ventilation promotion device for a ventilation system, the ventilation promotion device comprising a solar energy collector adapted and arranged such that it can receive solar energy, a heat directing element arranged to direct energy from the solar energy collector to a convection promotion element, the convection promotion element arranged to extend within an outflow duct of the ventilation system is adapted to use the thermal energy received to promote air flow in the ventilation system.