GB2373849A

GB2373849A - Ventilation heat exchanger

Info

Publication number: GB2373849A
Application number: GB0107550A
Authority: GB
Inventors: Christopher John Martin
Original assignee: Individual
Current assignee: Individual
Priority date: 2001-03-26
Filing date: 2001-03-26
Publication date: 2002-10-02
Also published as: GB0107550D0

Abstract

A mixed-mode heat exchanger in which stale but warm exhaust air is drawn from a building by a fan 1 and heat reclaimed from this exhaust air is used to preheat fresh cold air drawn into the building by a fan 6. The front surface of the heat exchanger 9 absorbs shortwave radiation, and is covered by a transparent insulating layer 13. When solar radiation is incident on the device it passes through this transparent cover to be absorbed by the plate, from where energy is transferred to the incoming ventilation air stream. A controller may optionally be used to vary the speed of the fans and to implement additional modes of operation. In summer, when there is no requirement for mechanical ventilation water channels attached to the absorber may be used to transfer energy from incident solar radiation to a water supply as in a conventional solar water heater.

Description

MIXED MODE BUILDING VENTILATION AIR PREHEATER This invention relates to a way of preheating the ventilation air required in buildings using a combination of heat extracted from outgoing stale air and incident solar radiation.

The energy required to heat incoming ventilation air forms a significant part of the overall energy consumption of many buildings. As materials, techniques and regulations advance, buildings are constructed with progressively higher levels of fabric insulation. They may also be more tightly sealed, reducing unwanted ventilation. However, there is still a basic requirement for sufficient ventilation air to maintain acceptable indoor air quality levels. In very tightly sealed buildings this ventilation may have to be provided by mechanical means. Whilst this approach does not of itself save energy, it does result in more appropriately controlled ventilation rates. More importantly, it opens up the possibility of reclaiming heat from warm exhaust air and using it to pre-heat incoming cold air. Many systems exist to do this in both commercial and domestic buildings.

A solar collector of conventional design using air as its working fluid may alternatively be used to pre-heat ventilation air. However, this suffers from the disadvantage that it provides most of its gains in the summer, when mechanical ventilation may not be in use. Furthermore it provides no useful gain at night.

According to the present invention stale but warm exhaust air is drawn from the building and passes through a mixed-mode heat exchanger. Heat reclaimed from the exhaust air is used to pre-heat incoming cold air. The surface of the heat exchanger is coated in such a way as to make it absorb shortwave radiation, and covered by a transparent insulating layer, which may be made of glass and/or plastic, and may consist of single or multiple layers. When solar radiation is incident on the device it passes through this transparent cover and warms the absorber plate, from where energy is transferred to the incoming ventilation air stream.

In keeping with present practice in the fields of solar thermal collectors and photo-voltaic arrays the invention described here can be constructed as a stand-alone collector, which can be added to any building which is sufficiently airtight to benefit from it, or as a building-integrated system. The exact form of a building integrated system will depend on the detailing of the building into which it is incorporated. Furthermore the basic principles of operation of such systems are identical to those of a stand alone version. For this reason two examples of stand alone systems are described here by way of example with reference to the following drawings in which: Figure 1 shows a cross section through the device; Figure 2 shows a typical installation which preheats the ventilation air to a whole building, in this case mounted on a roof; and Figure 3 shows a smaller implementation of the device designed to preheat the ventilation air to a single room, in this case mounted on a wall beneath a window.

Referring to Figure 1 stale air is extracted from the building by the fan or fans 1. In some applications ductwork 2 may be used to allow the system to extract from a region not immediately adjacent to the system. The stale air is then exhausted through the mixed-mode heat exchanger, passing between the insulated back of the unit 3 and the heat transfer plate 4 and finally emerging from a vent 5. Simultaneously the fan or fans 6 draw air in through an inlet vent 7. Once again, ductwork 8 may optionally be used to distribute this incoming fresh air within the building. Before entering the building this air passes between the heat transfer plate 4 and the absorber plate 9. To maximise the collection of solar radiation the front surface of the absorber plate is coated with a finish which absorbs shortwave radiation. To reduce the loss of this energy from the front of the absorber plate that coating may also be chosen to have a low emmissivity to long wave radiation.

To enhance the transfer of energy to the incoming air stream the heat exchange plate may have turbulence producing devices on both sides, and the absorber plate may have them on its rear surface. These devices may be as simple as indentations in the metal plates as at 10, they may be separate items attached to the plates as at 11, they may be placed in the airstream, as at 12, or any combination thereof. The front surface of the absorber is covered by a transparent or translucent glass or plastic layer 13 which may be single or multi-layered.

As an additional refinement the fans 1 and 6 may be operated by an electronic controller 14 connected to sensors within the building 15, within the mixed mode heat exchanger itself 16, outside the building 17 or any combination thereof. This allows fan speed to be controlled in response to humidity or Carbon Dioxide levels inside the building. It also allows further modes of operation to be implemented : * under conditions of high solar radiation the incoming air may be heated to a temperature above that of the air being extracted from the building. In this situation switching fan 1 off increases the efficiency of the system, by avoiding the unwanted transfer of heat from the incoming air stream to the outgoing stale air, and at the same time reduces the electricity consumption of the device. The system then functions as a conventional solar collector to pre-heat ventilation air until radiation levels fall and the controller resumes mixed mode operation; * when the temperature outside the building exceeds a given level the system may be shut down altogether, on the assumption that ventilation can then be obtained by opening windows at no energy cost.

Most buildings do not have a requirement for preheated ventilation air during the summer. Water channels 18 may optionally be attached to or incorporated into the absorber plate of the device and used to produce hot water from incident solar energy during the summer months. In this mode the device acts as a conventional solar water heater.

Figure 2 shows a system which allows a whole building to be ventilated, using ductwork 2 to collect stale air and 8 to distribute the preheated fresh air. In this situation air will normally be extracted from damp or polluted areas such as kitchens and bathrooms 19, and returned to other living areas 20.

Figure 3 shows an implementation of the system intended to ventilate a single room, which can be conveniently wall mounted.

Claims

CLAIMS 1. A heat exchanger which preheats the ventilation air required by a building using a combination of heat extracted from outgoing stale air and incident solar radiation.
2. A heat exchanger as described in Claim 1 wherein the surface of the heat exchanger is coated in such a way as to make it absorb shortwave radiation, and covered by a transparent insulating layer, which may be made of glass and/or plastic, and may consist of single or multiple layers.
3. A heat exchanger as described in Claim 1 and Claim 2 in which to enhance heat transfer to the two air streams the heat exchange plate may have turbulence producing devices on both sides, and the absorber plate may have them on its rear surface. These devices may be a simple as indentations in the metal plates, they may be separate items attached to the plates, they may be placed in the airstream or any combination thereof.
4. A heat exchanger as described in Claim 1, Claim 2 and Claim 3 in which the fans drawing air through the device may be operated by an electronic controller connected to sensors within the building, within the mixed mode heat exchanger itself or outside the building. This allows fan speed to be controlled in response to humidity or Carbon Dioxide levels inside the building. It also allows further modes of operation to be implemented : under conditions of high solar radiation the incoming air may be heated to a temperature above that of the air being extracted from the building. In this situation switching fan 1 off increases the efficiency of the system, by avoiding the unwanted transfer of heat from the incoming air stream to the outgoing stale air, or when the temperature outside the building exceeds a given level the system may be shut down altogether.
5. A heat exchanger as described in Claim 1, Claim 2, Claim 3 and Claim 4 which may have water channels attached to or incorporated into the absorber plate which allow water to be circulated through the device and heated by incident solar radiation.
6. A heat exchanger substantially as described here with reference to Figures 1-3 of the accompanying drawing.