PERFORATED TRANSPARENT GLAZING FOR HEAT RECOVERY AND SOLAR AIR HEATING
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.
The present application is a continuation-in-part of U.S. patent application Ser. No. 12/178,211 filed on Jul. 23, 2008 the content of which is incorporated herein by reference.
TECHNICAL FIELDThe 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 ARTDesign 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 one 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.
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.
SUMMARYIt is therefore an aim to address the above mentioned issues.
Therefore, in accordance with a general aspect of the present application, there is provided a method of improving the efficiency of a glazed solar collector comprising a glazed cover, a solar absorber disposed behind the glazed cover, and a plenum between the glazed cover and the solar absorber, the glazed cover forming an outer surface of the collector; the method comprising: providing multiple perforations through the glazed cover; and reducing heat looses to the environment through the glazed cover by minimizing a temperature delta across the glazed cover, including cooling the glazed cover by drawing outside air through the multiple perforations at a flow rate between about 2 to about 6 cfm per square foot of glazed surface.
In accordance with another general aspect, there is provided a glazed solar air collector comprising a perforated glazed cover transparent to solar radiation, the perforated glazed cover having opposed front and back faces, the front face of the perforated glazed cover forming an external surface of the collector and being directly exposed to the ambient, a solar radiation absorbing panel disposed behind the perforated glazed cover for absorbing solar radiation passing through the perforated glazed cover; a plenum defined between the back face of the perforated glazed cover and an opposed front face of the solar radiation absorbing panel, the perforated glazed cover having a plurality of perforations distributed over a surface area thereof and collectively forming a main outdoor air intake for admitting fresh outdoor air into the plenum, the distribution of perforations being selected to maintain a temperature delta across the perforated glazed cover close to zero, a secondary outdoor air intake provided at least one of a bottom and a side of the collector and disposed to direct an additional flow of outdoor air over predetermined surface areas of the solar radiation absorbing panel prone to overheating, and air moving means to draw heated air from said plenum via an outlet thereof.
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.
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.
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.
The term “glazing” is herein intended to broadly refer to any transparent surface allowing the light to pass therethrough.
DESCRIPTION OF THE PREFERRED EMBODIMENTSMore particularly,
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. All the air admitted or fed into the plenum 16 is fresh outdoor air drawn from the environment. As can be appreciated from
The solar rays passing through the glazed cover, i.e. 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 solar absorber 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. The perforated glazing 12 traps the heat within the plenum 16 until the heated air is drawn out of the heater via outlet 18. The influx of fresh outdoor air through the perforated transparent glazing 12 cools down the glazing 12 continuously, thereby preventing same from warming up. 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 which would otherwise occur with conventional uncooled glazed covers can thus be reduced to a minimum. The perforations in the glazed cover provide a simple and efficient cover cooling means. Integrating the cooling and air intake function in the glazed cover allows improving the efficiency of glazed ambient air heating solar collectors. Cooling the glazed cover by controlling parameters such as holes size, hole shape and distance between holes, as well as the geometry and the shape of the plenum allow to maximize heat recovery. For the heat to be removed over the surface of the perforated cover, the incoming air must efficiently “sweep” over the entire outer surface of the cover. In some applications, it is advantageous that the perforations in the glazed cover be as small as possible, i.e. the glazed cover should be as porous as possible. However, the diameter of the perforations may be limited by the manufacturing process of the glazed cover. For instance, for an injected molded glazed cover, it might be challenging to form the glazed cover with perforations having a diameter smaller than the 2 mm (0.08 inches). For 2 mm (0.08 inches) hole diameter and a nominal unitary flow rate of 4 cfm/sq.ft., or 75 m3/h per m2 of collector, for which optimum performance is wished for, applicant measured with thermography and small scale an effective “heat removal radius” of 1 cm (0.4 inches) around each hole, when side wind velocities are below 3 m/s. The hole spacing shall therefore be dimensioned so as to allow at least 100% of the collector surface covered by the heat removal surface.
From the table above, it can be seen that for an embodiment having 2 mm hole diameter perforations, a hole spacing of a maximum of 16 mm should be used to allow over 100% the collector surface to be covered by the heat removal surface and be incentive to winds below 3 m/s (3 m/s is the side wind velocity used to rate air collectors by the SRCC).
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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
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
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, sometimes for less than half the cost. That can be readily appreciated from
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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.
Claims
1. A method of improving the efficiency of a glazed solar collector comprising a glazed cover, a solar absorber disposed behind the glazed cover, and a plenum between the glazed cover and the solar absorber, the glazed cover forming an outer surface of the collector; the method comprising: providing multiple perforations through the glazed cover; and reducing heat looses to the environment through the glazed cover by minimizing a temperature delta across the glazed cover, including cooling the glazed cover by drawing outside air through the multiple perforations at a flow rate between about 2 to about 6 cfm per square foot of glazed surface.
2. The method of claim 1, further comprising cooling the solar absorber by providing a secondary flow of outside air over a front face of the solar absorber, including admitting outside air via at least one outdoor air intake provided at least one of a bottom and a side edge of the collector and, the air drawn through the perforations in the glazed cover and the air admitted through the outdoor air intake mixing together inside the plenum.
3. The method of claim 2, wherein the at least one outdoor air intake comprises a set of lateral air intakes provided in opposed side edges of the collector, and a set of bottom air intakes provided in the bottom edge of the collector, and wherein providing a secondary flow of outside air comprises admitting outside air through the bottom and the side edges of the collector.
4. The method of claim 1, further comprising increasing a linear velocity of the outside air flowing over the solar absorber by providing a secondary flow of outside air in the plane of the solar absorber, the secondary flow entraining the flow of outside air drawn through the perforations of the glazed cover.
5. The method of claim 1, comprising balancing the flow of outside air over the solar absorber by providing additional outdoor air intakes in areas of the solar absorber prone to overheating.
6. The method of claim 1, comprising establishing a temperature distribution profile over the surface of the solar absorber, identifying surface areas prone to overheating, and increasing air flow over said surface areas.
7. The method of claim 1, comprising balancing the flow of outside air over the solar absorber by gradually increasing a width of the plenum towards an outlet thereof.
8. The method of claim 1, comprising ensuring a minimum linear velocity of air over an entire surface area of the solar absorber by varying a distance between the glazed cover and the solar absorber along a length of the plenum, the distance being minimal at locations remote from an outlet of the plenum.
9. The method of claim 1, comprising increasing turbulences over an outwardly facing surface of the glazed cover by providing protuberances around an inlet end of the perforations, the protuberances projecting outwardly from the outwardly facing surface of the glazed cover.
10. A glazed solar air collector comprising a perforated glazed cover transparent to solar radiation, the perforated glazed cover having opposed front and back faces, the front face of the perforated glazed cover forming an external surface of the collector and being directly exposed to the ambient, a solar radiation absorbing panel disposed behind the perforated glazed cover for absorbing solar radiation passing through the perforated glazed cover; a plenum defined between the back face of the perforated glazed cover and an opposed front face of the solar radiation absorbing panel, the perforated glazed cover having a plurality of perforations distributed over a surface area thereof and collectively forming a main outdoor air intake for admitting fresh outdoor air into the plenum, the distribution of perforations being selected to maintain a temperature delta across the perforated glazed cover close to zero, a secondary outdoor air intake provided at least one of a bottom and a side of the collector and disposed to direct an additional flow of outdoor air over predetermined surface areas of the solar radiation absorbing panel prone to overheating, and air moving means to draw heated air from said plenum via an outlet thereof.
11. The glazed solar air collector defined in claim 10, wherein the perforated glazed cover comprises a plurality of perforated glazed panels assembled in a coplanar relationship.
12. The glazed solar air collector defined in claim 11, wherein each of said perforated glazed panels has top and bottom edges extending between opposed side edges, and wherein a tongue is provided along said top edge for engagement with a corresponding groove extending along the bottom edge of an adjacent panel.
13. The glazed solar air collector defined in claim 11, wherein thermal expansion clips project integrally outwardly from a perimeter of the panels to accommodate thermal expansion in a plane of the perforated glazed cover.
14. The glazed solar air collector defined in claim 13, wherein each of the thermal expansion clips comprises a resilient finger adapted to be deflected in a seat formed in the panel and to spring back to its original outwardly projecting position.
15. The glazed solar air collector defined in claim 13, wherein the thermal expansion clips comprise first and second vertical expansion clips projecting from a top edge of the panel and first and second pairs of horizontal expansion clips projecting from opposed side edges of the panel.
16. The glazed solar air collector defined in claim 11, wherein protuberances are formed around the perforations in the panels, the protuberances projecting outwardly from an outer surface of the panels.
17. The glazed solar air collector defined in claim 11, wherein each panel is provided in the form of an injection molded polycarbonate panel with built-in perforations, expansion clips and top and bottom tongue and groove adjoining edges.
18. The glazed solar air collector defined in claim 10, wherein the plenum is at least partly delimited by a building wall, the plenum being sealed from inside air contained within the building, the plenum being solely fed with outdoor air.
19. The glazed solar air collector defined in claim 10, wherein the perforations and the air moving means provide for a flow rate between about 2 to about 6 cfm per square foot of the collector.
Type: Application
Filed: Jun 20, 2012
Publication Date: Dec 13, 2012
Inventor: CHRISTIAN VACHON (Magog)
Application Number: 13/527,926
International Classification: F24J 2/22 (20060101); E04D 13/18 (20060101);