Solar Kiln Designs 1 -- Solar Heated, Lumber Dry Kiln Designs - Part 1

An in depth article by Gene Wengert and Luiz Carlos Oliveira

A discussion and a compilation of existing solar heated lumber dry kiln designs by Eugene M. Wengert and Luiz Carlos Oliveira
Department of Forest Products
Brooks Forest Products Center
Virginia Polytechnic Institute and State University
Blacksburg, Virginia 24061

Solar Energy Basics
Solar Dryer Design Concepts
Wood Drying Concepts
Solar Lumber Dry Kiln Designs

Solar Energy Basics

Solar energy is an abundant energy source throughout most of the world. But the cost of collecting it and the fact that it is a low intensity form of energy mean that solar energy may not be suitable for all energy-using applications. The following discussion points out a few of the theoretical and practical aspects of collecting and using solar energy as they relate to applications in lumber drying.

Solar Intensity
A flat, horizontal surface will receive an average of 1000 – 1200 BTU’s of solar energy per day in most locations within a broad band of 45 degrees latitude on either side of the equator. On sunny days the energy received will be higher than the average and on cloudy days will be less. Likewise, there are some locations that, because they are frequently cloudy, will receive less than this average; there are some exceptionally sunny locations that will receive more. The bulk of the energy appears to come from the solar disc. But as the solar energy passes through the atmosphere, it is dispersed so that significant amounts of energy come from other areas of the sky in addition to the area around the solar disc.

Collector Orientation
A surface will receive the most energy if it points directly toward the sun (i.e., is perpendicular to the sun), following the sun as it rises until it sets. As such equipment to accurately (usually within 5 degrees) track the sun is expensive A simpler arrangement is to adjust the flat collecting surface so that it faces due South in the Northern hemisphere (or vice versa for the Southern) and is slanted at an angle so that at solar noon the surface is pointed directly toward the sun. Such an arrangement would have several adjustments ranging from + to – 23 degrees (winter to summer) around the base angle. The base angle is measured from the horizontal and is equal to the latitude of the collector’s location. For example, a collector located at 18 degrees North would be tilted southward 41 degrees on December 21, 18 degrees southward on March 21 and September 21, and 5 degrees northward on June 21. Intermediate days would have corresponding intermediate angles.

Collectors that are not movable or adjustable can be set, for best year-round performance, at an angle (from the horizontal) equal to the collector’s latitude. An angle of (latitude +10) degrees (that is, steeper) is suggested for best winter performance; an angle of (latitude –10) degrees (that is, flatter) is suggested for best summer performance.

In all collectors, the area of the top of the collector which is perpendicular to the sun (i.e., the area of the shadow made) is the critical area. It is not the area of the absorber underneath the top of the collector.

Collector Design
The top of a collector will typically be covered with one or more layers of clear or nearly transparent material. This cover is called glazing. One layer is usually quite effective, but a second layer can substantially decrease heat losses while decreasing the transparency only slightly. Collector performance can be improved by 35% or so, depending on design, when a second layer is used. The benefit of a third layer of glazing is much less; when the cost is considered, the benefit is usually uneconomical for a third layer of glazing. Typical glazing materials include glass, rigid sheets of fiberglass reinforced polyester panels (corrugated or flat), and polymer plastic films such as Mylar, Tedlar, Kalwall, and others, that have been treated so as to avoid rapid deterioration due to ultraviolet light.

Under the glazing will be an absorber whose purpose is to absorb nearly all of the incident solar energy (i.e., minimal reflection and transmittance). Typically, the absorber is a wood or metal surface painted flat black. As the color is the primary factor in absorbency, it makes little difference if the absorber material is wood or metal. Once the energy is absorbed, it must be transferred as heat to the surrounding air. (For liquid collectors, the energy must be transferred to the circulating liquid.)

The space between the glazing and the absorber provides a chamber to circulate the air past the absorber and transfer this heat.

Because the absorber surface is hot, the surface will be emitting long wave (infrared) radiation. Glazing materials are chosen so as to be opaque to this radiation, thereby minimizing infrared energy losses through the glazing.

The absorbing surface can be of any convenient shape or orientation, so long as all the surfaces "seen" through the glazing are black. Recall that the orientation and area of the glazing (and not the absorber area) determine how much solar energy enters the dryer.

The surface of an absorber can easily exceed 100 degrees C (212 degrees F) in use. When air or liquid circulation is zero, temperatures may exceed 150 degrees C. Designs for collectors should include the possibility of thermal expansion of collector materials and should include possible thermal degradation effects (such as glue failure in plywood).

Solar Dryer Design Concepts

Basic Designs – Hot Air Collectors
Once the solar energy has been absorbed by the black absorbing surfaces and transferred to the air, this energy must be transferred to the lumber pile. At the same time, excessive amounts of the absorbed energy cannot be lost from the dryer before doing the primary task of evaporation. To accomplish these objectives, air is passed over the black absorbing surfaces and is then blown through the lumber pile To assist in this task, the lumber is stacked in layers, each layer being separated by spacers, called stickers, that facilitate the movement of air past the lumber surfaces and additionally help hold the lumber flat. Air flow should be fairly uniform throughout the dryer to facilitate even heating and even drying.

There are three basic hot air dryer designs – greenhouse, semi-greenhouse, and opaque walls with a separate collector. (Hot water solar systems would function inside the dryer the same as any hot water dryer; that is, solar heated water is the same as water heated by other means. Therefore, the hot water dryer design need not be discussed in this section.)

Energy is lost from a dryer in four ways: energy lost by conduction through the walls, roof and floor (including the glazing surfaces); energy lost by ventilation (exhausting warm, moist air from the dryer and bringing in the cooler, dryer outside air); energy lost (or used) to supply the heat of evaporation; and energy lost by transmission through the glazing, both visible and infrared energy. Usually the first three losses predominate.

In almost any climate, the dryer design should develop temperatures as high as possible. Higher temperatures result in faster drying due to faster water movement and lower relative humidities.

Greenhouse Designs. A greenhouse dryer typically is a frame structure with transparent or translucent glazing on the roof and 3 walls – east, west, and south. The glazing is usually plastic. The collector then is an integral part of the dryer. Because the thermal insulation properties of most glazing materials are poor, the heat losses by conduction through the walls are quite high. Likewise, there typically is a considerable loss of solar energy passing through the dryer without being incident on an absorber, unless special care is taken. To reduce conduction heat losses, greenhouse dryers will often use two layers of glazing. In sunny locations where solar energy is abundant, it may not be cost effective to save solar energy through improved designs. A solid north wall with a door facilitates loading and unloading. Drying may be slower than other designs and final moisture contents may be higher because of the generally lower average temperatures in the dryer.

Semi-Greenhouse Designs. The semi-greenhouse design usually has only the roof or the roof and south wall glazed; the other surfaces are opaque and insulated. This design reduces the conduction heat losses substantially, thereby resulting in higher dryer temperatures and faster drying (i.e., there is more energy available for evaporation). In addition, with this design there is very little opportunity for solar transmission losses. As before, two layers of glazing will reduce conduction losses through the collector. The semi-greenhouse dryer design is typically a wood frame structure with plywood or lumber sheathing. The collector is an integral part of the dryer. These designs will generally achieve lower final moisture contents than the greenhouse designs.

Opaque Wall Design. In this design, the lumber is placed in a solid, opaque walled and roofed chamber that is usually insulated, much like a standard lumber dry kiln. The solar collector is separate from the dryer, with hot air or hot water being ducted or piped into the kiln from the collector. The dryer can be well insulated, minimizing heat losses. This design lends itself to using supplemental heat, as collector losses at night or during cloudy weather have no effect if the connection between the dryer and collector is closed off. Final moisture contents can be extremely low with these designs.

Basic Design Considerations
Storage. In all designs there is the question of storage of energy for use at night or during cloudy weather. Although storage is technically feasible, it must be remembered that the glazing area admits only a finite amount of solar energy. There are two options: 1) this energy can be used immediately when it is received, thereby making the dryer very hot at midday but with very little temperature difference in the nighttime; or, 2) the energy can be stored and used throughout a 24-hour period, thereby keeping temperatures more uniform. From a wood technology standpoint, too much heat and too rapid drying can be detrimental for some species such as oak (Quercus spp.), but will not harm others such as pine (Pinus spp.) or poplar (Populus spp.) Most solar dryers are designed to provide as rapid drying as possible for the species being dried and with no energy storage capabilities.

If storage were to be beneficial, the collector size would likely have to be increased in order to supply extra energy that would or could not be used immediately. If the collector size were not increased, then the solar input per day, and therefore energy input to the lumber per day, would be the same both with and without storage and so there would be no benefit to storage in this instance. Likewise, without storage, dryer temperatures would tend to be higher, accompanied by lower humidities; this means more rapid drying. In fact, without storage, circulating fans would only have to run during daylight hours, thereby saving on electrical use.

Circulation. Air is circulated in the dryers to facilitate heat transfer from the absorber and to assure uniform drying. A typical air velocity through the lumber pile is 150 feet per minute. When this value is multiplied by the total sticker space openings, the result is the average cubic volume of air required. For example, with 16-foot long lumber stacked in 20 layers with ¾-inch (=3/48-foot) high stickers, the air flow is (16 x 20 x 3/48 x 150) 3000 cfm (cubic feet per minute). The fans are usually located in the hottest part of the collector to provide the best heat transfer; the risk of such a location is, however, that if the fans are shut-off on a sunny day, the excessive temperatures in the collector could cause damage to the fans. Due to the poor drying rates at higher humidities (and therefore potential inefficient use of electricity), fans would be run only when humidities are low. Low humidities would be typical during daylight hours; high humidities at night.

Ventilation. The drying rate in the dryer can be controlled directly by varying the relative humidity in the dryer. Low humidities result in faster drying and lower final moisture contents than high humidities. Humidity is controlled by venting – that is, controlled by exhausting some of the heated, moist air from inside the solar dryer and simultaneously bringing in cooler air from outside. When the cooler outside air is subsequently heated, its relative humidity is lowered, thereby assisting drying.

At the same time that the moist air is exhausted, there is also a loss of energy (that is, the exhaust air is hotter than the incoming air). This loss of energy and the benefit of venting must be considered together. Excessive venting will be wasteful and will result in cool dryer temperatures and therefore slow drying. On the other hand, inadequate venting may result in very high humidities which also will result in slow drying. In general then, venting should be sufficient to lower inside humidities, but not to substantially reduce inside temperatures. For many dryer designs, the vents are several times larger than needed and so would never be fully opened.

In practice, with wet lumber that is prone to checking and cracking, the vents are kept nearly fully closed for the first several days in order to keep humidities high and to keep drying from proceeding too rapidly. As the wood dries or for non-check-prone species, the vents are opened slightly to achieve moderate drying rates. For nearly dry wood, the vents are again closed most of the way in order to maximize heating and to develop low relative humidities needed to achieve low final moisture contents.

Glazing Materials. There are many commercial materials sold for glazing. Glass is one of the best materials from an energy efficiency standpoint, but severe weather (e.g., hail) or vandalism may make glass impractical. Plastic films with ultraviolet absorbers or stabilizers area easy to install, have a moderate cost, and can last for several years before turning cloudy or brittle. Translucent fiberglass reinforced polyester panels are often the lowest cost and most durable materials available. (One solar kiln near Va. Tech has used the same corrugated panels for 16 years!) The fiberglass panels do not transmit as much solar energy into the dryer as the films or as glass, but taking a few days longer to dry or making the collector a little larger may be a small price to pay for the increased service life and lower material cost. Using fiberglass glazing outside with a plastic film glazing inside as a second layer of glazing can provide good performance (that is, low heat loss and high solar gain) at a lost cost.

If storage is used, the losses through the collector during the nighttime, unless the collector is covered, could easily exceed the solar gain during the day. In Virginia this net loss by running 24-hours a day exists during the four cold winter months; in tropical locations, the net loss may never exist.

Insulated Walls. Heat is lost from the dryer by conduction through the walls and floor. Limiting these heat losses, even in warm climates, will result in better performance of a solar dryer, with more energy available for the main task of evaporation. In general the solar gain achieved from transparent walls is not large enough to offset the heat losses through these walls. Therefore, in most cases the walls should be insulated to reduce heat losses. The dryer construction should be tight enough on the inside so as to prevent the insulation from getting wet. A plastic sheet on the inside of the walls or coating the inside of the dryer with a vapor resistant coating such as aluminum paint is recommended. The use of preservative treated wood for the walls and floor would be a good practice to avoid insect and decay damage.

Collector Size
As discussed below, the amount of energy received by the dryer controls the amount of water that can be evaporated. Therefore, to control drying rates and avoid drying defects, the collector size can be specified. Consider the following examples.

For woods that are prone to cracking and splitting, a typical safe drying rate might be 3.5% moisture content (MC) loss per day. This is equivalent to an evaporation loss of 100 pounds of water per day per 1000 board feet of lumber. The energy required for this evaporation is (1000 Btu’s per pound x 100 pounds =) 100,000 Btu’s. Given that the average solar input is 1000 Btu’s per square foot of collector, the collector size required is 100 square feet per 1000 board feet of lumber. For species that can be dried faster, the collector to board foot ratio can be increased safely, while for more degrade prone species (or thicker pieces of moderate degrade prone species) the ratio can be smaller. The ratio required for a species (as calculated above) should not be exceeded in the design due to the risk of quality loss in drying. However, smaller ratios can be used with the only penalty being longer drying times.

Supplemental Heat
If supplemental heat is to be used, such as a dehumidifier or a wood stove, careful consideration should be given to the benefit of including solar energy at all because of the high heat losses likely to exist in the collector. Further supplemental heat will likely create drying stresses (casehardening) that will be sufficiently large at the end of drying so as to require a stress relief steaming or water spray treatment.