CROSS-REFERENCE TO RELATED APPLICATIONS This application is a continuation-in-part application of U.S. Patent Application Serial No. 08/432,224, filed May 1, 1995, the complete disclosure of which is herein incorporated by reference.

BACKGROUND OF THE INVENTION This invention relates to systems and methods for heating or cooling ventilated air. More particularly, the invention relates to systems and methods for heating or cooling fresh air entering a building with stale air exhausted from the building so that the incoming fresh air enters the building at or near the room temperature of the air in the building.

As a result of increasing energy costs, many buildings are being constructed to be more energy efficient. For example, many buildings, and particularly homes, are constructed to include double-pane windows, sealed doors, increased insulation, and the like. The use of such energy conservation measures are intended to help maintain a comfortable environment within the building while at the same time reducing energy costs incurred in heating or cooling the air within the building. With many buildings, and particularly with energy efficient "air tight" buildings, there is a need to exchange fresh air from the outside environment with the stale air within the building so that adequate living conditions within the building can be maintained. One concern when replacing the stale air with fresh air from the outside environment is the difference in temperatures between the stale air and the outside fresh air. For example, the outside air temperature can differ from the temperature of the air within the building

by as much as 70°F or more. When intaking the fresh air at such temperatures, the efficiency of the building's heating or cooling system is reduced since the incoming fresh air must be heated or cooled to the room temperature.

What is needed, therefore, are systems and methods for efficiently cooling or heating incoming fresh air that is exchanged for exhausted stale air. In one aspect, it would be desirable if the exhausted stale air were used to heat or cool the incoming fresh air.

SUMMARY OF THE INVENTION The invention provides an exemplary method for heating or cooling fresh air entering a building with stale air that is exhausted from the building. According to the method, the fresh air entering the building is directed through a plurality of elongate fresh air channels. Simultaneously, air exhausted from the building is directed through a plurality of elongate stale air channels. The stale air channels are aligned and alternate with the fresh air channels so that the fresh air entering the building is heated or cooled by the stale air exhausted from the building. In one particular aspect, the fresh air channels and the stale air channels are alternated every other channel. In another aspect, fans are employed to direct the fresh air and the stale air through the channels. Preferably, the fresh air is directed through the fresh air channels in a direction that is opposite the direction of the stale air flowing through the stale air channels. The air flow through the fresh air channels and the stale air channels can be either laminar or turbulent, and is preferably turbulent.

In another particular aspect, each of the fresh air channels and each of the stale air channels have a cross- sectional area m the range from about 0.05 m to 10 in , and more preferably from about 0.1 in to 5 in , and air is directed through the fresh air and the stale air channels at a rate in the range from 1 in per second to 1000 m per second, and more preferably from about 3 in ³ per second to 100 in per second. The temperature of the fresh air entering the

fresh air channels can either be hotter or cooler than the stale air in the building. In this way, the fresh air entering the building can either be cooled or heated until its temperature approaches or reaches the room temperature of the building.

The invention provides an air ventilation system for heating or cooling fresh air entering a building with stale air that is exhausted from the building. The system includes an enclosure defining an interior and includes a plurality of spaced apart walls that define a plurality of elongate channels extending along the interior of the enclosure. To enhance heat transfer between the walls, the walls are constructed of a thermally conductive material. A first fan is provided for moving fresh air through at least some of the channels and a second fan is provided for moving stale air through others of the channels in a direction opposite the flow of the fresh air. A means is provided for directing incoming fresh air into the fresh air channels, and a means is provided for directing incoming stale air into the stale air channels.

In one aspect of the system, an insulative material is provided to surround the enclosure. Preferably, the insulative material comprises a plastic material. In another aspect, the thermally conductive material used to construct the walls comprises aluminum.

The walls of the system preferably have a thickness in the range from 0.005 inch to 0.04 inch. In one aspect, the walls are planar and are parallel to each other. In an alternative aspect, the walls are rippled in geometry and are equally spaced apart along the length of the enclosure.

Preferably, the walls have about 5 to 25 ripples per every 10 inches, and the ripples have a depth in a range from about 0.1 inch to 1 inch, and preferably at about 0.25 inch. The walls are preferably spaced apart by a distance in the range from about 0.03 inch to 0.4 inch, and more preferably from about

0.03 to 0.2 inch, and the channels have a length in the range from about 5 inches to 100 inches. In a further aspect, each

of the channels preferably has a cross-sectional area in the range from 0.1 in ² to 5 in ² .

The invention provides a further embodiment of an air ventilation system for heating or cooling fresh air entering a building with stale air exhausted from the building. The system includes an enclosure having a fresh air intake port, a fresh air exhaust port, a stale air intake port, and a stale air exhaust port. A heat exchange unit is included within the enclosure, with the heat exchange unit having a plurality of spaced apart walls defining a plurality of alternating fresh air channels and stale air channels. A means is provided for directing fresh air from the fresh air intake port into the fresh air channels, and a means is provided for drawing the fresh air through the fresh air channels and out the fresh air exhaust port. The system further includes a means for directing stale air from the stale air intake port into the stale air channels, and a means for drawing the stale air through the stale air channels and out the stale air exhaust port. The invention further provides an exemplary heat exchange unit comprising a plurality of corrugated metal sheets, with the corrugations comprising alternating half circles. Each half circle has a radius, R, and the sheets are separated from each other such that the distance between midpoints of facing half circles is in the range from between R/2 and 2R. In this manner, the cross sectional area between the metal sheets will vary greatly along the length of the sheets, e.g. by a factor of about two to twenty, or greater. Since air speed between the metal sheets is generally . proportional to the cross sectional area between the sheets, the air speed between the sheets will greatly vary, e.g. the air will continuously increase and decrease in speed as it passes over the corrugations. Hence, when passing orthogonally over the corrugations, differences in both airflow speed and direction will be created to generate a turbulent air flow and to greatly increase the heat transfer efficiency. The heat transfer is so efficient that a short flow path may be employed to transfer the desired heat, which

in turn greatly reduces the amount of energy needed to force the air through the heat exchange unit. In this way, the overall efficiency of the system is increased.

In one preferable aspect, the radius, R, of each half circle is in the range from about 0.15 inch to about 1.0 inch, and more preferably from about 0.18 inch to about 0.3 inch. In another preferable aspect, the distance between midpoints of facing half circles is at or near R. In another aspect, each metal sheet includes two ends, with one end of each sheet being folded to produce an air intake or an air outlet. The non-folded end of each sheet is attached to the folded end of an adjacent sheet to form layers of channels with alternating air intakes and outlets.

In a further aspect, each sheet includes two sides extending between the two ends. A plurality of elongate strips are provided between adjacent metal sheets at each side to form a side wall between the channels. Each elongate strip preferably comprises an insert portion and a support portion. The insert portion is constructed of alternating half circle corrugations which mate with the corrugations in adjacent metal sheets. The support portion includes a pair of planar parallel surfaces which enable the elongate strips to be stacked upon each other. In this way, the elongate strips allow the metal sheets to be uniformly aligned and provide structural support. Preferably, the elongate strips are constructed of a polymer which serves to insulate the sides of the heat exchange unit.

The invention provides a further method for heating or cooling air by passing fresh air between some of a plurality of corrugated metal sheets so that the fresh air passes orthogonally over the corrugations . The corrugations comprise alternating half circles, with each corrugation having a radius, R. The sheets are separated from each other such that the distance between midpoints of facing half circles is in the range from between R/2 and 2R.

Simultaneously, stale air is passed between others of the plurality of corrugated metal sheets so that the stale air passes orthogonally over the corrugations.

In one preferable aspect, the fresh air passes through the channels in a direction opposite to the direction of stale air flow. In another aspect, the air flow between the channels is turbulent, with the turbulence being created in measurable part by variations in the cross sectional area between the metals sheets. Such variations in the cross sectional area in turn cause variations in air speed which creates the turbulent flow as the air passes orthogonally over the channels. In yet another aspect, the stale air may be either hotter or cooler than the fresh air when entering the channels. Preferably, the fresh air and the stale air are passed through alternating channels. In still a further aspect, the stale air is taken from a building and the fresh air is taken from an outside environment.

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 illustrates a schematic top view of a heat exchange unit according to the present invention.

Fig. 1A is a cutaway side view of the heat exchange unit of Fig. 1 taken along lines A-A.

Fig. 2 illustrates a schematic top view of an alternative embodiment of a heat exchange unit having rippled channel walls according to the present invention.

Fig. 2A is a cutaway side view of the heat exchange unit of Fig. 2 taken along lines A-A.

Fig. 3 is a schematic top view of an exemplary air ventilation system according to the principles of the present invention.

Fig. 4 is a top perspective view of an exemplary air ventilation system according to the present invention.

Fig. 4A is a top view of the air ventilation system of Fig. 4.

Fig. 5 is a bottom perspective view of a heat exchange unit of the air ventilation system of Fig. 4. Fig. 6 is an exploded perspective view of a particularly preferable embodiment of a heat exchange unit according to the present invention.

Fig. 7 is a side view of the heat exchange unit of Fig. 6.

Fig. 8 is a schematic view of corrugated metal sheets included in the heat exchange unit of Fig. 6. Fig. 9 is a schematic view of an alternative configuration of metal sheets which may be used in the heat exchange unit of Fig. 6.

DETAILED DESCRIPTION OF THE SPECIFIC EMBODIMENTS The invention provides systems and methods for heating or cooling fresh air entering a building with stale air that is exhausted from the building. In this way, the interior of the building is provided with needed ventilation while at the same time conserving energy by heating or cooling the incoming fresh air with the exhausted stale air.

The stale air exhausted from the building will preferably have a temperature that is substantially equivalent to the average room temperature of air within the building. Usually, the exhausted stale air will have a temperature in the range from about 55°F to 80°F, depending on a wide variety of factors, including the location of the stale air within the building, the time of day, the time of year, the use of the building, and the like. The invention provides for the heating or cooling of the incoming fresh air so that the temperature of the incoming fresh air is the same or closely approaches the temperature of the exhausted stale air by the time the incoming fresh air reaches the interior of the building.

The invention allows for such heating or cooling even when the temperature of the incoming fresh air varies greatly from the room temperature of the stale air. For example, the invention can be employed to effectively heat or cool the incoming fresh air to a temperature at or close to the temperature of the exhausted stale air even when the temperature difference between the incoming fresh air and the exhausted stale air is initially 50°F or greater. The systems and methods of the invention are extremely efficient so that such heating or cooling of the incoming fresh air can be

accomplished with little energy consumption, usually only requiring energy sufficient to rotate a conventional circulation or ventilation fan.

The invention provides for the heating or cooling of the incoming fresh air by passing the incoming fresh air and the exhausted stale air through adjacent channels and allowing heat to transfer between the walls of the channels. The channel walls are constructed of a thermally conductive material so that heat can efficiently be transferred between the fresh air and the stale air. Preferable thermally conductive materials include aluminum, copper, and the like, with aluminum being the most preferable because of its low cost and ease in forming into desired shapes.

The invention preferably includes a plurality of both fresh air and stale air channels, with the channels being aligned, i.e. generally parallel, with each other. The channels having the fresh air are alternated with the channels having the stale air, with the fresh air and stale air channels preferably being alternated every other channel. The channels are preferably arranged in a single layer, but can alternatively be stacked on top of each other to form multiple layers of both fresh air and stale air channels. In one exemplary aspect, the fresh air flows through the fresh air channels in a direction that is opposite the direction of the stale air channels. Such an arrangement is advantageous in facilitating the intake of fresh air from the outside of the building and the exhausting of stale air from the inside of the building. Alternatively, however, both the fresh air and the stale air may be directed through the channels in the same direction if desired. In a further aspect, air flow within the channels can either be laminar or turbulent, with turbulent being preferable as described in greater detail hereinafter.

Referring to Figs. 1 and 1A, one embodiment 10 of a heat exchange unit will be described. The heat exchange unit 10 includes an enclosure 12 having an interior 14 and two open ends 16, 18. The heat exchange unit 10 has a length L, a width W, and height H. Extending along the length L, and

between the two open ends 16, 18, are a plurality of spaced apart walls 20. The walls 20 and enclosure 12 define a plurality of fresh air channels 22 and stale air channels 24. Fresh air is directed through the fresh air channels 24 as indicated by arrows 26, while stale air is directed through the stale air channels 24 in the direction of arrows 28. The walls 20 are constructed of a thermally conductive material so that as fresh air passes through the channels 22, the fresh air is heated or cooled by the stale air passing through the channels 24. The enclosure 12 is preferably insulated with an insulative material so that heat transfer in the exchange unit 10 occurs only between the walls 20. The particular dimensions of the heat exchange unit 10 can vary greatly depending on the particular application, such as the size of the building, the temperature of the outside fresh air, the rate of air flow through the unit 10, the amount of heat exchange required between the fresh air and the stale air, and the like. The thickness of the walls will preferably be kept minimal to allow for maximum heat exchange between the walls 20. The rate of air flow through the channels 22 and 24 can vary greatly depending on a variety of factors such as the amount of ventilation needed in the building, the size of the channels, the temperature difference between the fresh air and the stale air, and the like. The fresh air and the stale air can be directed through the channels 22 and 24 at the same flow rate or at different flow rates.

The cross-sectional geometry of the channels 22 and 24 can vary greatly, but will preferably be orthogonal for ease of manufacture and installation. The walls 20 are generally parallel to each other to allow for a convenient and compact arrangement of the channels 22 and 2 .

Although the dimensions of the heat exchange unit 10 can vary widely, for most home applications, i.e. for homes having a square footage of about 2500 square feet, the heat exchange unit 10 will have a length L in the range from about 7 inches to 20 inches, a width W in the range from about 10 inches to 30 inches, and a height H in the range from about 4 inches to 10 inches. The walls 20 will preferably be spaced apart in

the range from about 0.03 inch to 0.2 inch, and the rate of flow through each of the channels 22 and 24 will be about 5 in ³ per second to 100 in ³ per second for temperature differentials of about 0°F to 120°F, more usually about 0°F to 60°F, between the fresh and the stale air at their respective intakes. For large scale commercial applications, these dimensions and flow rates can be increased.

Referring to Figs. 2 and 2A, a preferable embodiment 30 of a heat exchange unit will be described. The heat exchange unit 30 is essentially identical to the heat exchange unit 10 of Figs. 1 and 1A except that the heat exchange unit 30 includes a plurality of rippled walls 32 instead of the planar walls 20 of the heat exchange unit 10. For convenience of discussion, the heat exchange unit 30 will be described with the same reference numerals used to describe the heat exchange unit 10. The walls 32 are provided with a rippled geometry to ensure turbulent flow through the channels 22 and 24. By introducing turbulent flow, it has been found that the fresh air can more effectively be heated or cooled while passing through the fresh air channels 22. The ripples can be provided with a wide range of geometries and curvatures depending on the desired level of turbulent flow through the channels.

Referring to Fig. 3, an exemplary ventilation system 34 will be described. The ventilation system 34 includes an outer shell 36 covering a heat exchange unit 38. The heat exchange unit 38 can be essentially identical to the heat exchange unit 30 of Fig. 2. A fresh air intake 40 is provided for receiving fresh air into system 34. Conveniently,, a fresh air filter 42 can be provided to filter the incoming fresh air. A fan 44 is provided for drawing the fresh air through the fresh air channels of the heat exchange unit 38 where it is passed into the building through a fresh air outlet 46. The stale air channels in the heat exchange unit 38 are blocked off from the fresh air fan 44 so that only fresh air is drawn through the fresh air channels of the heat exchange unit 38.

A stale air intake 48 is provided for intaking stale air from the building where it is passed through the stale air channels of the heat exchange unit 38. The stale air is drawn through the heat exchange unit 38 by a stale air fan 50, and the stale air is exhausted to the atmosphere through a stale air outlet 52. The fresh air channels in the heat exchange unit 38 are sealed off from the stale air fan 50 so that only stale air is passed through the stale air channels of the heat exchange unit 38. The fresh air outlet 46 and the stale air intake 48 are in communication with the interior of a building, while the fresh air intake 40 and the stale air outlet 52 are in communication with the outside environment . In this manner, fresh air can be received into the system 34 from without the building through the fresh air intake. The fresh air is drawn through the fresh air channels of the heat exchange unit 38 at the same time that the stale air is drawn through the stale air channels of heat exchange unit 38. The fresh air and the stale air pass through the heat exchange unit 38 in opposite directions where the fresh air is either heated or cooled by the exhausted stale air as previously described. In this way, the fresh air entering the building is heated or cooled to a temperature reaching or closely approaching the room temperature of the building. One advantage of such a system is that the energy required to operate the system is minimal, usually requiring only enough energy to run the fans 44 and 50. Since such fans are normally included in the building's air ventilation system, the invention provides for the heating or cooling of the ventilated air using approximately the same amount of energy used in existing ventilation systems which do not heat or cool the incoming air.

One embodiment of an exemplary air ventilation system 54 is shown in Figs. 4 and 4A. The ventilation system 54 includes a housing 56 with sides 58 and 60 being shown removed for convenience of discussion. Fresh air from the environment is introduced into the system 54 through a fresh air intake 62. Travel of the fresh air through the system 54 is illustrated by the shaded arrows. The fresh air entering

the system 54 passes through a heat exchange unit 64 where it is heated or cooled by exhausted stale air. After passing through the heat exchange unit 64, the fresh air enters a common area 66 and is channeled into a pipe 68 where it is introduced into the interior of the building through a fresh air outlet 70. A fan 71 is provided between the pipe 68 and the common area 66 for drawing the fresh air through the system 54. As fresh air is passed through the system 54, stale air from the building is removed through a stale air intake 72. Travel of the stale air through the system 54 is illustrated by the unshaded arrows . The stale air proceeds through the heat exchange unit 64 where it exits into a second common area 74 and is channeled into a second pipe 76. The stale air is then exhausted from the building through a stale air exhaust outlet 78. The stale air is drawn through the ventilation system 54 by a second fan 79 between the second common area 74 and the second pipe 76.

Referring to Fig. 5, the heat exchange unit 64 will be described in greater detail. The heat exchange unit 64 includes an enclosure 80 having a base 81 and a top 83. The fresh air entering from the fresh air intake 62 (see Fig. 4) passes through the base 81 and into a plurality of fresh air channels 82. The stale air entering the stale air intake 72 (see Fig. 4) enters the heat exchange unit 64 through a plurality of stale air channels 84 at an opposite end of the base 81. The fresh air channels 82 and the stale air channels 84 are alternated every other channel and are separated by rippled walls 86. The walls 86 are constructed of a thermally conductive material and are preferably constructed of aluminum. The fresh air and the stale air pass through the heat exchange unit 64 in opposite directions with the fresh air being heated or cooled by the stale air depending upon the relative temperatures of the fresh air and the stale air as previously described. The stale air exits the stale air channels 84 at a right side 88 of the enclosure 80. At the right side 88, the fresh air channels 82 are closed so that fresh air entering the heat exchange unit 64 at the base 81 does not interfere with stale air being exhausted from the

right side 88 and into the common area 74 (see Fig. 4) . The fresh air exits the heat exchange unit 64 through a left side 90 of the enclosure 80. At the left side 90, the stale air channels 84 are closed so that the stale air entering the stale air channels 84 from the base 81 does not interfere with the fresh air being exhausted from the left side 90 and into the common area 66 (see Fig. 4) .

The median distance S^ between each ripple is preferably in the range from about 0.4 inch to 2 inch, thereby providing about 25 to 5 ripples per 10 inches. The median depth S ₂ of each ripple is preferably in the range from about 0.25 inch to 1 inch. As the fresh air and the stale air pass through the channels 82 and 84, respectively, a turbulent flow is created within the channels 82 and 84 thereby enhancing heat transfer through the walls 86.

The heat exchange unit 64 can be constructed to have a wide variety of dimensions depending upon the particular application. For purposes of illustration, the following example, which is in no way meant to be limiting, describes the effectiveness of the air ventilation system 54 in cooling incoming fresh air used to ventilate a building.

EXAMPLE In this example, the ventilation system 54 was employed to cool incoming fresh air with stale air exhausted from a building. The particular dimensions of the heat exchange unit 64 are as follows:

Top Length T 7 inches Width WD 9 inches

Height HT 3.25 inches

Base B 13.5 inches

Channel width C 1/16 inch

Distance between ripples S-^ 2/5 inch Ripple height S ₂ 1/4 inch

Channel wall thickness 3/500 inch

The incoming fresh air was at a temperature of 130°F while the temperature of the stale air at the stale air intake was 60°F. Both the fresh air and the stale air were circulated through the system 54 at about 50 cubic feet per minute. With such a configuration, the temperature of the fresh air entering into the building from the fresh air outlet 70 was 62°F.

Referring to Figs. 6 and 7 an alternative embodiment of a heat exchange unit 100 will be described. Heat exchange unit 100 may be employed to heat or cool fresh air entering a building with outgoing stale air. Heat exchange unit 100 may be included as part of an overall ventilation system with fans (not shown) for passing air through heat exchange unit 100 similar to the embodiments previously described in connection with Figs. 3-5.

Heat exchange unit 100 includes a plurality of corrugated metal sheets 102 which are preferably parallel with each other. The sheets 102 are arranged such that the corrugations of each sheet are aligned with the corrugations of an adjacent sheet. Each sheet 102 includes two ends 104, 106, and two sides 108, 110. Every other sheet 102 will have end 104 folded toward side 108 so that it may be attached to the end 104 of an adjacent sheet, thereby forming a stale air intake 112. Alternating sheets, i.e. sheets not having end 104 folded, will have end 106 folded toward side 110 so that it may be attached to a non-folded end 104 of an adjacent sheet, thereby forming a fresh air intake 114. Alternating between the stale air intakes 112 are fresh air outlets 116, and alternating between the fresh air intakes 114 are stale air outlets 118. In this manner, fresh air enters intakes 114 as indicated by arrow 120 and exits fresh air outlets 116 as indicated by arrow 122. At the same time, stale air enters intakes 112 as indicated by arrow 124 and exits stale air outlets 118 as indicated by arrow 126. In this way, the fresh and stale air pass through alternating channels 102 in opposite directions which are generally orthogonal to the corrugations. Construction of the heat exchange unit in this manner improves its manufacturability since the air intakes

and outlets may be formed using sheets 102, thereby requiring fewer parts.

Heat exchange unit 100 further includes a top plate 128 and a bottom plate 130 to enclose the two most extreme sheets. Plates 128 and 130 are preferably constructed of a polymer. To enclose sides 108, 110, a plurality of elongate strips 132 and 133 are provided. Strips 132 and 133 each include an insert portion 134 and a support portion 136. Strips 132 and 133 are essentially identical to each other except that strips 133 have a longer support portion 136 as described hereinafter. Strips 132 and 133 are alternated with each other as shown. Insert portion 134 is corrugated and is inserted between sheets 102 to fill the cross sectional area between the sheets. In this manner, strips 132 and 133 assist in keeping the sheets 102 equally spaced apart and provide structural support. Support portion 136 is configured so that strips 132 may be stacked upon each other in a uniform manner so that the sheets 102 may be secured together. For strips 133, the support portion 136 is longer and forms a side wall for the fresh air outlets 116 and the stale air outlets 118 as shown (which in turn prevents stale air from entering fresh air outlets 116 and fresh air from entering stale air outlets 118) . Bolts 138 (see Fig. 7) extend through plate 128, sheets 102, strips 132, and plate 130 to hold heat exchange unit 100 securely together. Elongate strips 132 are preferably constructed of a polymer to provide insulation to the sides of heat exchange unit 100.

Referring to Fig. 8, configuration of the corrugated metal sheets 102 will be described in greater detail. .The corrugations of each sheet are alternating half circles having a radius R. Preferably, the radius R is in the range from about 0.15 inch to about 1.0 inch, and more preferably from about 0.18 inch to about 0.3 inch. The distance A between the midpoints of facing half circles is in the range from about R/2 to 2R, with R being a preferred distance. Distance A is the longest distance between two adjacent sheets 102, while a distance B is the shortest distance, preferably being smaller by a factor of about two to twenty. As distance A approaches

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R/2, distance A relative to distance B will greatly increase to increase the variation in air flow speed along sheets 102. However, distance B will also approach zero making it more difficult to force air between the sheets. As distance A approaches 2R, distance B increases making it easier to force air between the sheets. However, distance A relative to distance B will decrease to reduce variations in air flow speed.

Hence, by varying distance A between R/2 and 2R, the cross sectional area between sheets 102 widely varies along the length of sheets 102 causing significant air flow speed variations along different portions of the air channel, e.g. air flows fastest at B and slowest at A (with air flows being generally proportional to the cross sectional area between the sheets) . Such variations in speed cause the air flow to become turbulent as the air passes orthogonally over the corrugations (with the corrugations also assisting in changing the direction of air flow) . Turbulent air flow which is produced in this manner greatly improves the amount of heat transfer between the sheets. When the radius is at or near R, a particularly good amount of heat transfer may be achieved. The amount of heat transfer between sheets 102 in the manner just described is so efficient that the air need only be passed over a small number of corrugations to obtain the desired amount of heat transfer. In this manner, the length of the air flow path may be reduced to in turn reduce the amount of energy needed to force the air between sheets 102. In this way, the overall system efficiency is greatly increased. As one example, by providing sheets having about 20 half circle corrugations which are separated at their midpoints by a radius R of 0.2 inch, overall heat transfer efficiencies of better than 95% have been achieved. This figure includes the energy needed to pass the air through the heat exchange unit. Referring to Fig. 9, an alternative arrangement of sheets 140 which may be used in the heat exchange unit 100 is shown. Sheets 140 have wave-like or alternating V-shaped corrugations over which air is passed in a direction

orthogonal to the corrugations. The distance between each sheet 140 is generally constant along any given point on the sheets 140. With this configuration, turbulence is caused by change in the direction of air flow as is passes over the corrugations.

Although the foregoing invention has been described in detail by way of illustration and example, for purposes of clarity of understanding, it will be obvious that certain changes and modifications may be practiced within the scope of the appended claims.