Abstract:
A heat collector comprises a transparent glazing exposed to the ambient. The transparent glazing is spaced from a back surface to define a plenum therewith. A plurality of perforations is defined through the transparent glazing for allowing outside air to flow through the transparent glazing into the plenum and substantially maintain the transparent glazing at the ambient temperature, thereby providing for higher thermal efficiency.

Description:
TECHNICAL FIELD 
       [0001]    The present application generally relates to a device suited for pre-heating fresh outside air by means of free energy, such as solar energy and/or heat recovery. 
       BACKGROUND ART 
       [0002]    Design of traditional glazed solar air heaters generally comprises a glass, polycarbonate or Lexan® transparent cover placed in front of a dark solar absorber. The front transparent cover is provided for minimizing heat losses from the top of the collector. Fresh outside air is traditionally admitted at on end of the collector between the front transparent cover and the solar absorber. The air passes through the collector along fins and absorbs heat from the solar absorber as it travels therealong. Warm or hot air is discharged at the opposite extremity of the collector. As air progresses inside the collector, its temperature rises above ambient. The higher the temperature in the collector is, the higher the heat loss towards the ambient becomes. Heat loss happens through the bottom, the edges and the top (where the glazing is) of the collector. Typically the edges and the bottom are insulated, so that heat loss mostly occurs through the top, that is by convection between the absorber and the glazing and then by conduction through the glazing. When the glazing becomes very warm, the collectors become less efficient. 
         [0003]    Various unglazed solar air heaters have also been designed over the years. Current transpired collector designs are such that the solar absorbing surface is located outside facing the sun, unprotected by means of a glazing. The perforated absorber is coupled to a fan which creates a negative pressure between the building (or the bottom of the collector) and the absorber. When the fan is in operation, the air is drawn through the absorber. The air passing through the perforations in the outer opaque absorber breaks the naturally occurring warm film of air on the outside facing side (the boundary layer) of the absorber. This method provides acceptable performances when the flow of air per unit area exceeds 6 cfm per square foot of collector. However, for unitary flow rates below 5 cfm per square foot, the amount of cool air leaching the perforated plate is insufficient to prevent the collector plate from heating up, thereby negatively affecting the overall thermal efficiency of the system. Efficiencies at the rate of 2 cfm per square foot drop to 30% or even less. 
       SUMMARY 
       [0004]    It is therefore an aim to address the above mentioned issues. 
         [0005]    Therefore, in accordance with a general aspect of the present application, there is provided a heat collector comprising a transparent glazing exposed to the ambient, the transparent glazing being spaced from a back surface to define a plenum therewith, a plurality of perforations defined through the transparent glazing for. allowing outside air to flow through the transparent glazing into the plenum, the perforations being distributed over a surface area of the transparent glazing, the plenum having an outlet, and air moving means to draw heated air from said plenum via said outlet. 
         [0006]    In accordance with a further general aspect, the back surface includes a solar radiation absorbing panel. 
         [0007]    In accordance with another general aspect, there is provided a device for heating air, the device comprising a perforated transparent surface allowing solar radiations to pass therethrough, a solar radiation absorbing surface located behind the perforated transparent surface for absorbing the solar radiations, and a gap of air defined between the perforated transparent surface and the radiation absorbing surface, the air flowing in the gap absorbing heat from the radiation absorbing surface while fresh ambient air flowing through the perforations of the perforated transparent surface providing for a minimal temperature delta through the transparent surface. 
         [0008]    In accordance with still another general aspect, there is provided a transparent and perforated surface exposed to the ambient. The perforated transparent surface is spaced from a back surface so as to define an air gap or plenum therebetween. Fresh outside air is drawn into the plenum through the perforated transparent surface. The back surface can, for instance, be provided in the form of a bottom of a solar collector, a building wall or roof, an outer surface of a greenhouse, a photovoltaic panel, the ground or any non-porous surface. Between the perforated transparent surface and the back surface, the gap of air is maintained under negative pressure due to mechanical or natural means. An outlet is provided for allowing the air flowing through the plenum to be drawn into a duct or a channel, for use as make-up, ventilation, process or combustion air to a device which consumes or needs thermal energy. 
         [0009]    The air in the plenum is heated either by incident solar radiation on the surface of the back panel, which acts as a solar absorber, and/or by heat escaping from the back surface. The device can therefore act as a solar air heater and/or as a heat recovery unit. When used as a solar air heater, the back surface can be of a dark color, so that incident solar radiation passing through the perforated transparent surface is absorbed by the back surface in the form of heat and not reflected back to outer space. However, if the back surface, for any aesthetic reason or other, must be of light color, the solar thermal efficiency remains higher than other conventional unglazed collector design. This is particularly true when the device is used as a heat recovery device, since the back surface can be of any color with no influence on efficiency (it can even be transparent like in the case of a greenhouse), but the lower the thermal resistance (insulation) of the back surface, the greater the heat recovery rate. The device can be simultaneously used for both functions of solar heating and heat recovery. 
         [0010]    If necessary, the preheated air leaving the device can have an auxiliary heating device located downstream (e.g. a gas-fired system) to bring its temperature to a given set point. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0011]      FIG. 1  is a schematic side view of a solar collector including a perforated transparent surface in accordance with an embodiment of the present invention; 
           [0012]      FIG. 2  is a schematic side view of another embodiment of a solar collector having a perforated transparent glazing; 
           [0013]      FIGS. 3 and 4  are schematic side views of ground-mount configurations of solar collectors having perforated transparent glazing in accordance with further embodiments of the present invention; 
           [0014]      FIG. 5  is a schematic side view of a wall mounted solar collector having a perforated transparent glazing; 
           [0015]      FIG. 6  is a schematic side view of a roof mounted solar collector having a perforated transparent glazing; 
           [0016]      FIG. 7  is a schematic view illustrating a perforated transparent glazing surrounding a greenhouse shell for pre-heating cold outside air before being drawn into the greenhouse by a ventilation system; and 
           [0017]      FIG. 8  is a graphic comparing the efficiency of perforated glazing collectors vs. unglazed perforated collectors as a function of the quantity of air flowing therethrough. 
       
    
    
       [0018]    The term “glazing” is herein intended to broadly refer to any transparent surface allowing the light to pass therethrough. 
       DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0019]      FIG. 1  shows a solar air heater  10  provided in the form of an elongated conduit-like enclosure mounted on a base and including a sun facing perforated transparent glazing  12  exposed to the ambient and placed in front of a back panel having an arcuate solar radiation absorber plate  14  applied over an insulation layer  15 . The back panel is generally provided in the form of a half-pipe wall covered with the perforated transparent glazing  12 . The absorber plate  14  can be of a dark color to maximize solar gain. The perforated glazing  12  can be provided in the form of a perforated polycarbonate or transparent UV-resistant plate. Other transparent polymers could be used as well. The glazing  12  can be rigid or flexible. The perforations can be distributed over the entire surface of the glazing or over only a selected surface area thereof. The density of perforations can be uniform or variable over the glazing surface. 
         [0020]    The perforated glazing  12  and the solar radiation absorber plate  14  define a plenum  16  therebetween. A fan or other suitable air moving means  17  is operatively connected to an outlet  18  provided at one end of the back panel to draw fresh outside air through the perforated glazing  12  into the plenum  16  before being directed to a ventilation system, such as a building ventilation system. The solar radiations passing through the perforated transparent glazing  12  are absorbed by the absorber plate  14 . The air in the plenum  16  picks up the heat absorbed by the absorber plate  14  before being drawn out of the plenum  16 . As air travels longitudinally along the plenum  16  between the absorber plate  14  and the perforated glazing  12 , additional fresh outside air is drawn through the perforated glazing  12 . In this way, the glazing  12  remains at a temperature substantially equal to the ambient temperature. Accordingly, the temperature differential between the incoming air and the ambient is equal to zero or close to zero, so that thermal efficiency remains at the highest possible value. Heat losses through the glazing cover are thus kept to a minimum. 
         [0021]      FIG. 2  shows a second embodiment in which like reference characters refer to like components. The solar air heater  10   a  shown in  FIG. 2  essentially differs from the solar air heater  10  shown in  FIG. 1  in that the solar air heater  10   a  has a planar configuration characterized by spaced-apart parallel transparent glazing and back panel. The back panel is provided in the form of a flat absorber plate  14   a  applied over a planar layer of insulation material  15   a.  The absorber plate  14   a  could be corrugated. Sidewalls or supports  19   a  are provided along the perimeter of the back panel and the perforated transparent glazing  12   a  in order to create a uniform air gap  16   a  therebetween. The perforated glazing  12   a  and the back panel are preferably co-extensive. The back panel  14   a  can be provided in the form of photovoltaic (PV) panels to provide the double function of air heating and cooling the PV panels, which produce more electricity when their surface is kept at cool temperatures. As shown in  FIGS. 1 and 2 , the perforated transparent glazing  12   a  is preferably supported at an inclination equal to the latitude of a given location, and facing the equator, depending on use. However, it is understood that the transparent glazing could be oriented and inclined otherwise. For instance,  FIG. 4  shows a horizontally oriented perforated transparent glazing, whereas  FIG. 5  shows a vertically oriented glazing. 
         [0022]    As shown in  FIGS. 3 and 4 , the solar air heater can be mounted directly on the ground, the ground surface forming the back panel of the device. In the embodiment of  FIG. 3 , wherein like reference characters refer to like components, the plenum  16   b  is formed by the perforated transparent glazing  12   b,  a building wall  20   b  and the ground G. The fresh outside air drawn in the plenum  16   b  is heated by the solar radiations absorbed by the ground G as well as by the heat escaping from the building through wall  20   b.  The fresh outside air flowing through the perforations defined in the transparent glazing  12   b  maintains the temperature delta across the glazing close to zero, thereby ensuring high thermal efficiency. The heated air is drawn out from the plenum  16   b  and circulated in the building B via the building ventilation system (not shown). As shown in  FIG. 4 , where like reference characters again refer to like components, the solar air heater can also be provided in the form of an enclosure having a perimeter wall  19   c,  a closed bottom end formed by the ground, and a top end covered by the perforated transparent glazing  12   c.  An outlet  18   c  connected to suitable air moving means is provided for withdrawing the heated air from the enclosure. 
         [0023]    As shown in  FIGS. 5 and 6 , the perforated transparent glazing  12   d  and  12   e  can be mounted in opposed facing relationship to a building wall  20   d  or the roof  22   e  of a building. In the embodiment of  FIG. 5 , the plenum  16   d  is formed between the outside surface of the building wall  20   d  and the adjacent vertically oriented perforated transparent glazing  12   d.  In the embodiment of  FIG. 6 , the plenum  16   e  is formed by the outside surface of the building roof  22   e  and the perforated transparent glazing  12   e.  In both embodiments, the heat escaping from the building envelope through the wall  20   d  or the roof  22   e  is recovered to heat the air in the plenum  16   d  and  16   e.  The roof  22   e  and the building wall  20   d  both act as solar radiation absorbers to further heat the ambient air drawn in the plenums  16   d  and  16   e.  The solar radiations pass through the perforated transparent glazing and are absorbed by the underlying building wall or roof surfaces and the air in the plenum absorbs the heat from the building wall or roof. As opposed to conventional solar walls or solar roofs wherein solar radiation are directly absorbed by dark panels covering the wall or roof of the buildings, the transparent glazing does not negatively alter the appearance (i.e. change the color of the building wall or roof) of the building. Unlike the prior art, the performance of the system is not influence or restricted by the color of perforated. panels installed on the building wall or roof. The perforated glazing  12   d  and  12   e  are transparent and thus they do not change the color of the building wall or roof. No compromise has to be done for aesthetic purposes. 
         [0024]      FIG. 7  shows a further potential application of the present invention. More particularly,  FIG. 7  illustrates a greenhouse B′ having a skeleton framework covered with a transparent skin  12   f  or membrane, as well know in the art. A perforated transparent glazing  12   f  is mounted to the greenhouse wall and roof to define a double-walled structure including an air gap  16   f  defined between the perforated transparent glazing  12   f  and the inner transparent skin  25 . In this embodiment, the perforated transparent glazing  12   f  acts as a second insulation layer for the greenhouse B′. The heat escaping from the greenhouse through the inner skin  25  is recovered in the air gap  16   f.  A fan or the like can be provided for drawing heated air from the air gap back into the greenhouse B′. The perforated transparent glazing  12   f  maintains the required transparency required for plant growth. 
         [0025]    As can be appreciated from the above embodiments, the device can be used in several applications including:
       Solar thermal air heaters   Solar fresh air preheater mounted on building walls or roofs   Hybrid solar air/water heating systems   Preheating of air-to-air and air-to water heat pumps   Transparent energy recovery device for greenhouses   Cooling of photovoltaic panels   Residential, low-cost solar preheater       
 
         [0033]    Also various apparatus can be provided downstream of the device for further processing the air. For instance, the device could be coupled to the following units:
       Gas-fired make-up air unit   Air-based heat pump (air-to-air or air-to-water)   Swimming pool heat pump   Combustion chamber   Heat recovery unit       
 
         [0039]    The above described transpired or perforated glazing offers numerous benefits. The incoming air is admitted throughout the glazing surface, either on a large proportion of its surface or over the entire surface. Accordingly, the glazing surface remains cold so that collector top heat loss is substantially prevented. Furthermore, the air temperature inside the collector remains relatively cold, lowering heat losses through the bottom and the edges. The proposed perforated transparent glazing design provides solar efficiencies at least as good as that provided by the perforated plate design at high flow rates. For lower flow rates, however, the solar efficiency remains high and by far exceeds that of opaque perforated collectors, and even exceeds that of glazed collectors, for less than half the cost. That can be readily appreciated from  FIG. 8 . More particularly, it can be seen that for flow rate between 2 and 6 cfm per square foot of perforated surface, the efficiency of a perforated glazing with a black backing surface is greatly superior to that a conventional black perforated sheet metal solar collector. The difference in performance is even more noticeable for light or white color solar collectors. The perforated glazing with a white color backing surface is up to 100% more efficient than a white perforated sheet metal collector. It can also be appreciated that the difference in performance between conventional unglazed perforated collectors and the above described perforated glazed designs is even more significant at low flow rates of, for instance, 3 or 4 cfm per square foot. 
         [0040]    It will be apparent to one skilled in the art that modifications may be made to the illustrated embodiments without departing from the spirit and scope of the invention as hereinafter defined in the claims.