The Design, Construction, and Use of an Indirect, Through-Pass, … · 2011. 8. 25. · dryer...

62 Home Power #57 • February / March 1997

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Food scientists have found that by reducing themoisture content of food to between 10 and 20%,bacteria, yeast, mold and enzymes are all preventedfrom spoiling it. The flavor and most of the nutritionalvalue is preserved and concentrated. Vegetables, fruits,meat, fish and herbs can all be dried and can bepreserved for several years in many cases. They onlyhave 1/3 to 1/6 the bulk of raw, canned or frozen foodsand only weigh about 1/6 that of the fresh food product.They don’t require any special storage equipment andare easy to transport.

The solar dryer which will be described in this article iseasy to build with locally available tools and materials(for the most part) for about $150 and operates simplyby natural convection. It can dry a full load of fruit orvegetables (7–10 lbs) thinly sliced in two sunny to partlysunny days in our humid Appalachian climate or asmaller load in one good sunny day. Obviously theamount of sunshine and humidity will affectperformance, with better performance on clear, sunnyand less humid days. However, some drying does take

place on partly cloudy days and food can be dried inhumid climates. The dryer was developed atAppalachian State University in the Department ofTechnology’s Appropriate Technology Program. Overthe last 12 years we have designed, built, and testedquite a few dryers and this one has been our best. Itwas originally developed for the Honduras SolarEducation Project, which Appalachian Stateimplemented several years ago. The prototype for thatproject was constructed by Chuck Smith, a graduatestudent in the Technology Department. Amy Martin,another Appalachian student, constructed the modifiedand improved version depicted in this article. Solardryers are a good way to introduce students to solarthermal energy technology. They have the same basiccomponents as do all low temperature solar thermalenergy conversion systems. They can be easilyconstructed at the school for small sums of money andin a fairly short amount of time, and they work very well.While conceptually a simple technology, solar drying ismore complex than one might imagine and much stillneeds to be learned about it. Perfecting this technology

Drying is our oldestmethod of foodpreservation. For

several thousand yearspeople have beenpreserving dates, figs,apricots, grapes, herbs,potatoes, corn, milk, meat,and fish by drying. Untilcanning was developed atthe end of the 18th century,drying was virtually the onlymethod of foodpreservation. It is still themost widely used method.Drying is an excellent wayto preserve food and solarfood dryers are anappropriate foodpreservation technology fora sustainable world.

The Design, Construction, and Use of an

Indirect, Through-Pass, Solar Food DryerDennis Scanlin©1997 Dennis Scanlin

63Home Power #57 • February / March 1997

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has been one of our goals and while we are not thereyet, over the years we have come up with somedesigns that work pretty well. This article will describeguidelines for designing, constructing and using a solarfood dryer.

Factors affecting food dryingThere are three major factors affecting food drying:temperature, humidity and air flow. They are interactive.Increasing the vent area by opening vent covers willdecrease the temperature and increase the air flow,without having a great effect on the relative humidity ofthe entering air. In general more air flow is desired inthe early stages of drying to remove free water or wateraround the cells and on the surface. Reducing the ventarea by partially closing the vent covers will increasethe temperature and decrease the relative humidity ofthe entering air and the air flow. Thiswould be the preferred set up duringthe later stages of drying when thebound water needs to be driven outof the cells and to the surface.

TemperatureThere is quite a diversity of opinionon the ideal drying temperatures.Food begins cooking at 180˚F soone would want to stay under thistemperature. All opinions surveyedfall between 95° and 180˚F, with110°–140˚F being most common.Recommended temperatures varydepending on the food bring dried.Our experience thus far and theresearch of quite a few others leadsto the conclusion that in generalhigher temperatures (up to 180˚F)increase the speed of drying. Onestudy found that it took

approximately 5 times as long to dry food at 104˚F as itdid at 176˚F. Higher temperatures (135°–180˚F) alsodestroy bacteria, enzymes (158˚F), fungi, eggs andlarvae. Food will be pasteurized if it is exposed to 135˚Ffor 1 hour or 176˚F for 10–15 min. Most bacteria will bedestroyed at 165˚F and all will be prevented fromgrowing between 140°–165°. Between 60° and 140˚Fbacteria can grow and many will survive, althoughbacteria, yeasts and molds all require 13% or moremoisture content for growth which they won’t have inmost dried foods.

Some recommended drying temperatures are:

Fruits and Vegetables: (except beans and rice):100°–130°F (Wolf, 1981); 113°–140° (NTIS, 1982);temperatures over 65°C (149°F) can result in sugarcaramelization of many fruit products

Meat: 140°–150° F (Wolf, 1981)

Fish: no higher than 131°F (NTIS, 1982); 140°-150°(Wolf, 1981)

Herbs: 95°–105°F (Wolf, 1981)

Livestock Feed: 75°C (167°F) maximum temperature.(NTIS, 1982)

Rice, Grains, Seeds, Brewery Grains: 45°C (113°F)maximum temperature. (NTIS, 1982)

Temperatures Obtainable in our Appalachian DryerOur Appalachian dryer, with a reflector added, hasreached temperatures over 200˚F on a sunny 75˚F daywith all the vents closed. Preliminary experiences with a4' long reflector have demonstrated a 20˚F rise in the

Above: Yum...the apples are almost ready.

Below: Adjusting the vents and testing (tasting?) the progress.

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dryer temperature and a decrease in drying time. Byfully opening the vents the temperature can be broughtdown to within 10° or 20° higher than the ambienttemperature. The dryer can operate for most of the daybetween 120° and 155˚F by opening the exhaust vents1–2" (10–20 sq. in.). These are the temperatures at thebottom of the food drying area when the dryer has justbeen filled with food and a reflector is being used. Thetemperature drops significantly as it passes through themoist food. Chart 1 shows: the temperatures below thebottom tray of food, the temperatures above the top trayof food, and the ambient temperatures, right after a fullload of 25 sliced apples (about 8 lbs) had been placedin the dryer. The dryer on this day had a reflector on it.It was a clear sunny day with relative humiditiesbetween 62 and 93%. By the end of the day, apples onbottom 5 trays were dry, some apples on top 5 trayswere not. The temperatures were recorded with a PaceScientif ic Pocket Logger, model XR220, 1401McLaughlin Drive, P.O. Box 10069, Charlotte, NC28212, (704) 5683691

Chart 2 shows a dryer operating in the afternoon of itssecond day of drying a load of food. One can see howthe temperatures increase in the top of the dryer, as thefood in the top of the dryer dries. This test was notusing a reflector. By the end of this day all apples sliceswere bone dry, almost like crackers.

Possible temperature related problemsThere are a couple of potential problems associatedwith higher temperatures. One study reported slightlyhigher vitamin C loss in fruits dried at 167˚F than at131˚F. Greater vitamin loss has also been reported forthe direct style of food dryer which exposes the fooddirectly to the sun’s radiation (ASES, 1983). However,there are many other factors that affect vitamin loss andthe losses are different for different foods and differentvitamins. I need to explore this topic more fully.

Case hardening is another potential problem associatedwith drying at higher temperatures. If the temperature ofair is high and the relative humidity is low, there is the

possibility that surface moisture will be removed morerapidly than interior moisture can migrate to the surface.The surface can harden and retard the further loss ofmoisture. Solar dryers start off at low temperatures andhigh humidity and thus avoid this problem, I believe. Atleast I have not observed it.

Air flow and velocityThe second of three factors affecting food drying is airflow, which is the product of the air velocity and ventarea. The drying rate increases as the velocity andquantity of hot air flowing over the food increases.Natural convection air flow is proportional to vent area,dryer height (from air intake to air exhaust), andtemperature. However air flow is also inverselyproportional to the temperature in a solar dryer. As theair flow increases by opening an exhaust vent the dryertemperature will decrease. Ideally one would want bothhigh temperatures and air flow. This can be difficult toachieve in a solar dryer.

Air velocity in a natural convection collector is affectedby the distance between the air inlet and air exhaust,the temperature inside the dryer and the vent area. Thegreater the distance, temperature and vent area thegreater the velocity. It is often measured in feet perminute (FPM) or meters per second. With constanttemperatures, 230 FPM air velocity drys twice asrapidly as still air; at 460 FPM drying occurs three timesmore rapidly than in still air (Desrosier, 1963). Axtell &Bush (1991) suggest air velocities between 0.5 to 1.5meters per second which is about 100 to 300 FPM.Desrosier (1963) suggests even higher air velocitiesbetween 300 to 1000 FPM.

The quantity of air, measured in cubic feet per minute(CFM) or cubic meters per minute, is the product ofvelocity and area of the exhaust vent. Morris (1981)recommends 2–4 CFM per square foot of collector foran efficient performing natural convection solar airheater. If the air flows are too slow the collector will heat

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up and lose more heat to the air surrounding it. Anefficient solar thermal collector should not feel hot to thetouch. NTIS (1982) suggests 1/3 to 1/2 cubic metersper minute (11.5 to 17.5 CFM) per cubic meter of dryervolume as being a good flow rate for solar dryers.

Most designers of fossil fuel powered industrial fooddrying systems recommend considerably higher flows.Axtell & Bush (1991) of the Intermediate TechnologyDevelopment Group (ITDG) recommend between 0.3 to0.5 cubic meters per second or about 600 and 1000CFM. Desrosier (1963) recommends 250 CFM per SFof drying surface. For the dryer described in this articlewith 18 SF of drying surface that would equal a littleover 4,500 CFM.

Measured air velocities and flows in the Appalachian dryerOur solar dryers are only able to achieve air velocitiesbetween 50 and 130 FPM with natural convection. Lessthan most of the 100 to 1000 FPM rangerecommended. Air velocities were measured in thesolar collector’s air flow channel with a Kurtz 490 seriesmini-anemometer.

Our dryer also has less total air f low than isrecommended by most. During normal operation itallows 25–60 CFM. A tremendous difference from the600 to 4500 CFM recommended for industrial dryingsystems. It has around 9 SF of glazing and shouldallow, according to Morris, 18 to 36 CFM for efficientcollector performance. Our drying volume is about 3cubic feet (0.08 cubic meters) and would according toNTIS need between 1 to 1.5 CFM. Quite a bit less thanrecommended by Morris for efficient collectorperformance and also less than our dryer’s normaloperating performance.

Increasing air flows and air velocity seems to havepotential for increasing the performance of solar dryers.Unfortunately as the air flow increases the temperature

decreases in a solar dryer. Chart 3 depicts thetemperature decline when the vents are fully openedfrom a 1 1/2" opening and then almost fully closed. Wehave found temperature to be more significant than airflow in affecting the speed or rate of drying and so weusually reduce the air flows by partially closing theexhaust vents to increase the temperature. Byincreasing the power or performance of our solarcollector greater air flows will be possible whilemaintaining high temperatures.

Relative HumidityWhile not something one can do much about, therelative humidity is the third factor affecting food drying.The higher the humidity the longer the drying will take.More air will be required and the temperatures will needto be higher. Each 27˚F increase in temperaturedoubles the moisture holding capacity of the air(Desrosier,1963). In the Appalachian region where wehave tested our dryers we normally have a relativehumidity throughout the summer and early fall of 55 to100%. This moist air can’t hold as much moisture asless humid air could and as a result drying takes longerthan it might in a dryer climate. This humidity alsomakes higher temperatures desirable for our climate.

How to get the correct temperature and air flowThe temperature obtainable in the dryer will be affectedby several things: area of south facing glazing,insulation, air-tightness, area of vent opening, andambient temperature. The area of south facing glazingis an important design decision. The dryer pictured has9.2 SF of south facing glazing and approximately 3 CFof drying volume or 3 SF of glazing for every 1 CF ofdrying chamber. This is a good ratio. If one is interestedin drying speed, increasing the ratio of glazing SF percubic foot of dryer volume, adding more insulationand/or adding a reflector to the dryer would bedesirable. This will allow one to increase thetemperatures, air velocities and total air flow; anddecrease the drying time. The temperature rise in thedryer described can be as high as 125˚F above ambientwith a reflector and all vents closed. Normaltemperature rises without a reflector and with bothexhaust vents opened 1–3" (12–36 square inches)would be 50 to 70˚F. As mentioned previously, ourpreliminary testing indicates about a 20˚F increase intemperature by adding a reflector. The maximumtemperature observed was 204˚F. The higher Delta T’sand maximum temperatures will be reached withexhaust vent opening area reduced.

Designing for good air flows involves quite a fewconsiderations. The air flow channel should be properlysized. The depth of the channel should be 1/15 to1/20th the length of the collector. Making the air flow

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path as aerodynamic as possible is also desirable;especially for a natural convection collector. Althoughturbulence created by fins on the back of an absorberplate or corrugated metal has been shown to deliver asmuch as 40% more heat in active systems (Morris,1981). One should try to keep the intake and exhaustvents spread as evenly as possible along the width ofthe collector to allow easy air movement. The intake andexhaust area and profile should ideally be the same orlarger than the air flow channel. Air flow rates can beincreased, while keeping temperatures up between140˚F and 175˚F, by constructing a larger, more efficient,better insulated collector and/or adding a reflector to thecollector. Increasing the size and/or performance of thecollector can also increase air velocity by increasing thetemperature inside the dryer. A larger, more efficient orpowerful collector will allow one to more fully open upthe vents thus increasing the CFM or volume of airmoving through the dryer, while still keeping thetemperatures high in the dryer. The dryer described herehas 2 exhaust vents with a total of about 1.6 square feetof exhaust vent area. With the vents completely openthe maximum temperature attainable on a sunny 70˚Fday is only about 85˚F and so we normally decrease thevent area and CFM of air flow to increase thetemperature and decrease the drying time. The area ofexhaust vent during normal operation for several dryerswe have designed and constructed is 10 square inchesor less. This enables the dryer to achieve temperaturesover 130˚F and still allow air flow. It is desirable to haveadjustable vent covers so one can adjust for differentfoods and weather conditions. Ideally the temperature ina food dryer should be controllable. The air velocitycould also be improved by adding a fan, possibly PVpowered as has been discussed in a previous HParticle, or tall chimneys. Adding chimneys to a dryer andincreasing the distance between the air inlet andexhaust will increase the velocity and volume of airmoving through the dryer.

Collector designThe dryer uses a “Through Pass” collector configuration.Solar energy passes through a glazing material and isabsorbed by 5 layers of black aluminum windowscreening diagonally positioned in the air flow channel.The air around the absorber, the black screen, is heatedand rises into the drying chamber. A slight vacuum ornegative pressure is created by the rising air whichdraws in additional air through the inlet vent and thealuminum mesh absorber. This air is heated and theprocess continues (Illustration 1).

Through pass mesh type absorbers can outperformplate type absorbers by quite a bit if properly designedbecause the air must pass through the mesh resulting

in excellent heat transfer (Morris, 1981). At AppalachianState we have compared the various absorber plateconfigurations and have found the diagonally positionedmesh type absorbers to produce the highesttemperatures inside a box connected to the collector.Expanded wire lathe is recommended by some for themesh but needs to be painted and didn’t perform anybetter in our tests than the window screening. Usingstock black or dark gray aluminum window screeneliminates having to paint the absorber and is lessexpensive and time consuming than other options. Thebottom of the air flow channel can be painted black orsome dark color to absorb any solar energy that getsthrough the mesh or possibly painted a light orreflective color to reflect sunlight back on to absorbermesh. Morris (1981) recommends a dark color, whenwe experimented with this we found similarperformance with both strategies.

Another characteristic of our collector is it’s U-tubedesign. In addition to the air flow channel right belowthe glazing, there is a second air flow channel rightbelow the first one and separated by a 1/2" thick pieceof polyisocyanurate foam insulation board. This allowsair to circulate when the vents are closed to increasethe temperatures for pasteurization or to recycle air thathas not absorbed much moisture in the latter stages ofdrying (Illustration 2).

When the vents are open most air will be drawn up inthe top air channel and the bottom channel helps toreduce heat loss to the outside through the bottom ofthe dryer. The measured air flow velocity in this bottomchannel was about 15 FPM with the two exhaust vents

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open 1.5" each and went up to about 25 FPM when allvents were closed. This seems to support the recyclingtheory. I’m not sure this feature is necessary; but, itdoesn’t seem to hurt the performance and may behelpful some times. We need to look at this some more.

One significant decision, in addition to size, whichneeds to be made when designing an air heating solarcollector is what depth should the air flow channel be.The air flow channel depth for a through pass collectorshould be 1/20 the length of the collector (Morris,1981). The collector pictured is 60" long and has a 3"air deep air space (1/20 x 60") in both air flow channels.

Any kind of glazing will work for this design.Appalachian’s dryer has two layers of glazing; the outeris Sun-Lite HP, a fiberglass reinforced polyester (FRP),often referred to as Kalwall. It is available from SolarComponents Corporation for about $2.00/SF (121Valley Street, Manchester, NH,03103-6211, (603) 668-8186). The inner glazing is Teflon manufactured by theDuPont Company, (Barley Mill Plaza 30-2166, P.O. Box80030, Wilmington, DE 19880-0030, (302) 892-7835).There is a 3/4" air space between the two layers andthe glazings are caulked in place. The dryer should facedue south for best stationary performance. The altitudeangle of the glazing above horizontal should be thecompliment of the average noon altitude angle of thesun at your latitude for the months you expect to beusing the dryer or your latitude minus 10˚, if youprimarily intend to use it during the later part of thesummer and early part of fall. For our latitude here inBoone, NC of 36˚ that would be 26˚. The dryer picturedhas an angle of 36˚.

The sides and bottom of the collector and the sides,

door and top of the drying chamber are insulated with1/2" Celotex Tuff-R polyisocyanurate foam insulation. Itnormally is covered with an aluminum foil. I am going touse 3/4" in the next one constructed. Making sure youtightly construct the collector by making good tightfitting joints, especially the door, and using caulksand/or gasket material is also desirable. And finallyadding a reflector to the dryer and properly positioning it(about 20˚ above horizontal in early October to 40˚ inmid July at 36˚ N LAT) will improve the performance.

Materials Needed (approximate cost is $150, excludingstainless steel shelf screen)

One 4' x 8' 3/4" CDX exterior plywood for sides, ventcovers and door

One 4' x 8' 1/4" exterior plywood for bottom, roof andsouth wall of drying box

approx. 12 - 8' long 1x2 pine

Two 8' long PT 2x4 for dryer legs

Water resistant glue

Caulk or glazing tape

Eight 1/4" X 2 1/4" lag bolts and washers

24" wide by 30' long piece of black or dark grayaluminum window screen (.65/FT)

Ten 21" x 14.5" Stainless steel screen for dryingshelves ($6.62/SF) adds another $150 to cost or coulduse a vinyl or vinyl clad fiberglass screen for about.35/SF

24" X 12 ft. 0.040 Sun Lite HP plastic glazing($1.85/SF)

Two 3 1/4" strap hinges approx.

Fifty 1 1/2" galvanized deck screws

paint

Two 2" hook and eyes

One 4' x 8' 3/4" celotex foil faced polyisocyanurateinsulation board

Dryer Construction and DetailsThe dryer is primarily constructed of 3/4" exteriorplywood, 1/4" exterior plywood, 3/4" celotex insulationboard, dark aluminum screening, glazing, some 3/4"thick pine boards, and wood screws. The cutoutillustrations (Illustration 3 & 4) dimension the layout ofthe important plywood and insulation pieces.

I tried to improve on the design depicted in this articleby slightly increasing the glazed area (from about 9 to10 SF), the SF/CF ratio (from 3 to 3.5 SF/CF), thethickness of insulation used ( 1/2" to 3/4") and loweringthe collector altitude angle (from 36˚ to 26˚) to improvelate summer and early fall performance. I am also goingto develop a larger and more permanent adjustablereflector. Verify the measurements before blindly cutting

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everything out. I tried to be as accurate as I could;however, there may be some mistakes. The explodedisometric drawing (Illustration 5) and the multiview(Illustration 6) illustrate the basic construction.

Basically begin by laying out the dryer sides, the doorand the vent pieces on the 3/4" plywood. Cut these outwith a skill or jig saw. Cut the 1/4" plywood bottom outwith skill saw. Use the plywood side pieces to layout theinsulation board dryer side pieces and cut with a razorknife. Glue the insulation to the plywood sides and thenconnect the sides together by gluing and screwing ornailing the plywood bottom on and screwing the 22 1/2"long wooden struts made from 1x2 stock in place.Illustration 7 describes the location of the most criticalstruts. Cut out insulation where the struts join the sidepieces. Once the basic form is constructed everythingelse is applied as depicted in plans and photos.

Using the dryer1) The initial phase of drying is more dependent on airflow than temperature, so keep the bottom ventscompletely open and the top about 1/2 open or more.After 1 to 2 hours reduce the top exhaust vent openingto 1"–3", leaving the bottom vents completely open, and

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let the temperature rise. Keep the dryer under 180˚ F.Close all the vents at night to prevent rehydration of anyfood left in dryer. On cloudy days keep the bottom ventsclosed and the top vents almost closed to keeptemperatures as high as possible.

2) Keep everything as clean as possible; wash foodgently in cold water 3) Get fruit and/or vegetables indryer as quickly as possible after harvesting to preservevitamins

3) Remove blemished and woody areas of fruits andvegetables

4) Consider blanching most vegetables, by exposing tosteam for a few minutes and then dipping in ice water,to inactivate enzymes which can cause color, flavor andnutritional deterioration. Blanching helps preservecarotene, thiamine, and ascorbic acid. Blanching alsomakes cell membranes more permeable, whichpromotes more rapid drying and will kill potentiallyharmful micro-organisms. The blanched dried productwill often have a softer texture when rehydrated.Blanching apricots, peaches and pears imparts atranslucent appearance to the dehydrated product and

can also be used for fruits which will not havedetrimental color changes during drying: grapes, figs,plums and prunes. Don’t blanch onion, garlic,mushrooms, horseradish, herbs, or vegetables withcabbage like flavors

5) Consider sulfuring fruits. Sulfuring helps preserve thelight color of apples and apricots and also helpspreserve ascorbic acid (C), and beta-carotene (A), andhelps control microbiological and insect activity. It alsoprotects delicate flavors and increases the shelf life ofdried foods. Sulfuring involves burning elemental sulfurand exposing the fruit to the fumes for 1-5 hrs ordipping the fruit for 30 seconds in a 5–7% potassiummetabisulfate solution. When fruit has been adequatelysulfured the surface will be lustrous. Pretreatingtomatoes with potassium metabisulfate prior to dryinghas been reported to significantly improve the taste andaroma of sauce made from the dried tomatoes. Sulfurflowers are available at pharmacies or use pure sulfurfrom garden centers. Use 1 tbls/lb of fruit. Thiamine isdestroyed by sulfuring.

6) Slice food thin (1/8") for most rapid drying and cutuniformly.

7) Most vegetables should be dried until they feeldistinctly dry and brittle, around 10% MC.

8) If drying meat use lean meat, cut into very thin stripsand marinate before drying. Beef, turkey, chicken, andsalmon can all be dried.

9) If drying fish keep temperatures under 131˚F to avoidcooking it and consider salting 1–2 days before drying.Salting retards bacterial action and aids in the removalof water by osmosis.

10) The safe maximum percentages of water to leave inhome dried produce are: no more than 10% forvegetables and no more than 20% for fruits (Hertberg etal., 1975). Fruit can be considered dry when it isleathery, suede-like, or springy. No wetness should

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come out of a cut piece when squeezed. A few piecessqueezed together should fall apart and spring backwhen pressure is released. Vegetables should bebrittle, or tough to brittle almost crisp like crackers orpotato chips.

11) Put screen over the intake and exhaust vents tokeep insects out.

Tips for Storing Dried Foods1) Cool food to room temperature before packaging

2) Store dry fruits and vegetables in small, airtight,moisture, insect and rodent proof containers in dark,cool, dry and clean places. Glass jars, plastic bags, orplastic containers that can be sealed tightly are good.Store grains, roots, and legumes in places with good aircirculation (NTIS, 1982).

2) Dried meats and fish should be stored below 5°C(41°F) to avoid rancidity (NTIS. 1982).

3) Most fruits and vegetables will keep for 6 months ifstored at 70°F and 3-4 times that long at 52°F (Wolf,1981).

4) Meat and Fish can be stored dried for severalmonths in moisture proof, airtight containers. (Wolf,1981)

5) If drying herbs store in uncapped jars for 24 hrs, ifmoisture collects, herbs need additional drying

6) Refrigeration or freezing will extend life of dried food.

7) Carefully label the food.

Influence of dehydration on nutritional value of foodWhile all methods of food preservation result in adegradation of the food quality and drying is noexception, drying food does increase the concentrationsof proteins, fats and carbohydrates. Fresh peas are 7%protein and 17% carbohydrates; dried peas 25% proteinand 65% carbohydrates. Fresh beef is 20% protein anddried is 55%. There is; however, a loss of vitamins. Theextent of vitamin loss will be dependent upon thecaution exercised during the preparation of the food fordrying, the drying process selected, and storage ofdried food. In general indirect drying methods such asthe dryer described in this article retain more vitaminsthan sun drying or direct drying and also better thancanning. Ascorbic acid, and carotene can be damagedby oxidative processes. Thiamin is heat sensitive anddestroyed by sulfuring. The carotene content ofvegetables is decreased by as much as 80% if driedwithout enzyme inactivation by blanching or sulfuring.Thiamin will be reduced by 15% in blanched vegetablesand up to 75% in unblanched. In general more rapiddrying will retain more ascorbic acid than slow drying.Usually dried meat has slightly fewer vitamins than

fresh. Fruits and vegetables are generally rich sourcesof carbohydrates and drying, especially direct sundrying, can deteriorate carbohydrates. The addition ifsulfur dioxide is a means of controll ing thisdeterioration.

Influence of drying on Micro-organismsLiving organisms require moisture; so by reducing themoisture we are able to reduce the ability of molds,bacteria, and yeasts from growing. Bacteria and yeastsgenerally require moisture contents over 30%. Dryingfood lower than 30% is no problem in a solar dryer.Molds however can grow with as little as 12%. Moldsalso require air, so as long as dried food is stored in anairtight container molds should not be a problem. Also iffood was dried at over 140°F or if it was pasteurizedprior to and after drying all 4 of the problem causingagents will be destroyed. Salt can be also used tocontrol microbial activity if drying fish or meat. It is alsoimportant to start with clean food and utensils, andstore food away from dust, rodents, insects andhumidity.

Influence of drying on Enzyme activityEnzymes are produced when plant tissues aredamaged. Their production can lead to discoloration,loss of vitamins, and breakdown of tissues. Mostenzymes are inactivated at 158˚F. They also requiremoisture to be active and their activity decreases withdecreasing moisture. But dried food still has somemoisture so food deterioration due to enzymes can stillbe a problem. Browning of fruit for example and loss ofcarbohydrate content. One minute of moist heat at 212F will inactivate enzymes. This can be achieved byblanching. Sulfuring also deactivates enzymes.Surprisingly dry heat does not affect enzymes verymuch. Short exposures to a dry 400°F has little effect.Blanching times vary. In general 1–3 minutes for leafyvegetables, 2–8 for peas, beans, and corn and 3–6 forpotatoes, carrots, and similar vegetables.

AccessAuthor: Dennis Scanlin, 3137 George’s Gap Road,Vilas, NC 28692 • 704-297-5084Internet Email: [email protected]

Sun-Lite HP glazing is available from SolarComponents Corporation, 121 Valley Street,Manchester, NH 03103-6211 • 603-668-8186

Teflon glazing is manufactured by the DuPont, PO Box80030, Wilmington, DE 19880-0030 • 302-892-7835

Reference ListAmerican Solar Energy Society (1983). Progress in PassiveSolar Systems. Boulder, Colorado: American Solar EnergySociety, p. 682.

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Axtell, B.L. & Bush, A. (1991) Try dying it!: Case studies in thedissemination of tray drying technology. London, UK:Intermediate Technology Publications.

Desrosier, N.W. (1963). The technology of foodpreservation.Westport, Conn.: Avi Publishing.

Hertzberg, R., Vaughan, B., & Greene, J. (1976). Putting FoodBy. New York: Bantam.

International Labour Office (1986). Practical methods of foodpreservation.

Martinez, P.S. (1985 ). Production characteristics of a solarheated drying plant. Sunworld, 13(1,19), 19-21.

Morris, S. (1981). Retrofitting with Natural ConvectionCollectors. In T. Wilson (Ed.), Home Remedies: A Guidebookfor Residential Retrofit (pp. 152 - 161). Philadelphia, PA: Mid-Atlantic Solar Energy Association.

National Technical Information Service (NTIS). (1982).Improved Food Drying and Storage; a training manual.(reportno. A360.33). Washington, DC: U.S. Peace Corps.

Winter, Steven & Associates, Inc. (1983). The Passive SolarConstruction Handbook. Emmaus, Pa: Rodale Press.

Wolf, R. (1981). Solar Food Dryer Preserves Food for Year-Round Use; Using Solar Energy. Emmaus, PA: Rodale Press.

SNORKEL STOVE CO

camera ready b/w3.5 wide3.4 high


his articledescribes a series ofexperiments

conducted over the lastyear and a half withthree solar food dryers.The food dryers wereconstructed atAppalachian StateUniversity (ASU) usingplans published inHP57. The goal of thisresearch program wasto improve the designand to determine themost effective ways touse the dryer.

Dennis Scanlin,Marcus Renner,David Domermuth, &Heath Moody

Above, Photo 1: Three identical solar food dryers for testing against a control.

1 1/2 x 3/4 inchPine

1/4 inch PlywoodVent Covers

0.040 Sun-Lite HP Glazing

Screened Air Intake

3/4 inch Foil-FacedFoam Insulation

Drying Shelves

1/4 inch Plywood

1 1/2 x 3/4 inchPine

3/4 x 3/4 inchPine

1/4 inch Plywood

1 1/2 x 1/8 inchAluminum Bar Trim

3/4 inch PlywoodRoof

3–6 Layers of Lathor Screen

7 feet

6 feet

Figure 1: Cutaway View of the Appalachian Solar Food Dryer

©1999 Dennis Scanlin, Marcus Renner,David Domermuth, and Heath Moody


Solar Dehydration

These solar food dryers are basically wooden boxeswith vents at the top and bottom. Food is placed onscreened frames which slide into the boxes. A properlysized solar air heater with south-facing plastic glazingand a black metal absorber is connected to the bottomof the boxes. Air enters the bottom of the solar airheater and is heated by the black metal absorber. Thewarm air rises up past the food and out through thevents at the top (see Figure 1). While operating, thesedryers produce temperatures of 130–180° F (54–82°C), which is a desirable range for most food drying andfor pasteurization. With these dryers, it’s possible to dryfood in one day, even when it is partly cloudy, hazy, andvery humid. Inside, there are thirteen shelves that willhold 35 to 40 medium sized apples or peaches cut intothin slices.

The design changes we describe in this article haveimproved the performance, durability, and portability ofthe dryer, and reduced construction costs. This workcould also help in designing and constructing solar airheaters used for other purposes, such as home heatingor lumber drying. Most of our experiments wereconducted with empty dryers using temperature as themeasure of performance, though some of ourexperiments also involved the drying of peaches andapples. We have dried almost 100 pounds (45 kg) offruit in these dryers during the past year. Graduatestudents in the ASU Technology Departmentconstructed the dryers, and students taking a Solar

Access Doorto Drying Trays

Handles

Exhaust Vents

Collector Surface(glazing)

Air Intake

Reflective Surface(one test case)

Rear Side Front

Collector Chamber

Drying Chamber

Figure 2: Multiple Views of the Appalachian Solar Food Dryer

Above, Photo 2: Setting up the solar simulator.


Solar Dehydration

Energy Technology course modified them for individualexperiments.

MethodologyWe began by constructing three identical food dryers.Having three dryers allowed us to test two hypothesesat one time. For example, to examine three versus sixlayers of absorber mesh and single versus doubleglazing, Dryer One might have three layers of blackaluminum window screening as an absorber with singleglazing; Dryer Two, six layers of the same absorberscreen with single glazing; and Dryer Three, six layersof the same absorber screen with two layers of glazing.Once we set up an experiment, we collect data. Thislasts from several days to a couple of weeks until weare confident that the data is reliable. Then we trysomething different.

Using three food dryers also allows us to offer morestudents hands-on experiences with solar air heaters.Each semester, students take apart the dryers’ solarcollectors and rebuild them using different materials orstrategies. This classwork was supplemented withexperiments set up and completed by several graduatestudents.

Equipment for Data CollectionWe have two systems for measuring temperature. Thefirst system uses inexpensive indoor/outdoor digitalthermometers. One temperature sensor is placed insidethe dryer and the other one outside. Different locationsare used for the sensor inside the dryer. If food is beingdried, we normally place it under the bottom tray of foodand out of direct sunlight. This temperature data isrecorded on a data collection form every half hour orwhenever possible.

The other system uses a $600 data logger from PaceScientific to record temperature data. It is capable of

measuring temperature, relative humidity, AC current,voltage, light, and pressure. The logger does not have adisplay, but it’s possible to download the data to acomputer. The software that comes with the loggerallows us to see and graph the data. The data can alsobe exported to a spreadsheet for statistical analysis.

We measure air flows with a Kurz 490 series mini-anemometer. We weigh the food before placing it in thedryer, sometimes during the test, and at the end of eachday. We use an Ohaus portable electronic scale,purchased from Thomas Scientific for $111. Wemeasure humidity with a Micronta hygrometerpurchased from Radio Shack for about $20.

Solar SimulatorIn addition to outdoor testing with the actual fooddryers, we use a solar simulator (see Photo 2) built byDavid Domermuth, a faculty member in the TechnologyDepartment at ASU. With the simulator, we can domore rapid testing and replicate the tests performed onthe dryers, even on cloudy days. The simulator also letsus control variables such as ambient temperature,humidity, and wind effects. The unit can be alteredquickly because the glazing is not bolted on. Thesimulator was constructed for $108. It was built in the

Time

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Graph 1: Single vs. Double Glazing

Below, Photo 3: This dryer has both a vertical wallreflector and side reflectors.


Solar Dehydration

same way as the food dryer, but without the food dryingbox at the top.

The simulator uses three 500 watt halogen work lightsto simulate the sun. The inlet and outlet temperaturesare measured with digital thermometers. Thetemperature probes are shaded to give a true readingof the air temperature. We conducted the simulatortests inside a university building with an indoortemperature of 62–64° F (17–18° C). As we changedvariables, we noticed significant differences in outlet airtemperatures. The simulator did produce temperaturescomparable to those produced by the food dryers out inthe sun. However, we did not always achieve positivecorrelations with our food dryers’ outdoor performance.We may need to use different kinds of lights or alter ourprocedures somewhat.

ExperimentsWe have done at least twenty different tests over thelast year and a half. All were done outside with theactual food dryers and some were also repeated withthe solar simulator. The dryers were set up outside theTechnology Department’s building on the ASU campusin Boone, North Carolina. We collected some additionalinformation at one of the authors’ homes. Every testwas repeated to make sure we were getting consistentperformance. We tried to run the tests on sunny tomostly sunny days, but the weather did not alwayscooperate. The dips in many of the charts were causedby passing clouds.

Single vs. Double GlazingThe original design published in HP57 used two layersof glazing separated by a 3/4 inch (19 mm) air gap. Weused 24 inch (0.6 m) wide, 0.040 inch (1 mm) Sun-LiteHP fiberglass-reinforced polyester plastic for the outerlayer. For the inner layer, we used either another pieceof Sun-Lite, or Teflon glazing from Dupont. Sun-Liteglazing is available from the Solar ComponentsCorporation for about $2.40 per square foot ($25.83 perm2). These two layers cost over $50, or about one-thirdof the total dryer cost. We wanted to see if the secondlayer helped the performance significantly and justifiedthe added expense.

We set up two dryers with six layers of steel lathpainted flat black. One had single glazing and the otherhad two layers of glazing. The outer glazing was Sun-Lite HP on both dryers. The dryer with double glazingused Teflon as the inner glazing. The two dryers wereidentical except for the number of glazing layers. Thetests were run on nine different days between February17 and March 26, 1998. We opened the bottom ventcovers completely and the top vent covers to twoinches (51 mm). The ambient temperatures were cooland no food was being dried.

As Graph 1 shows, the double glazing did result inhigher dryer temperatures. This was on a sunny daywith clear blue skies and white puffy clouds, lowhumidity (30%), and light winds. The temperaturesthroughout most of the day were slightly higher withdouble glazing. However, the single glazed dryer workswell and routinely reached temperatures of 130–180° F(54–82° C). When this test was replicated with the solarsimulator, the double glazing also produced slightlyhigher temperatures.

Our conclusion is that double glazing is not necessaryfor effective drying. It does reduce some heat loss andincreases the dryer ’s temperature slightly, but itincreases the cost of the dryer significantly. Anotherproblem is that some condensation forms between thetwo layers of glazing, despite attempts to reduce it bycaulking the glazing in place. The condensationdetracts from the dryer’s appearance and may causemaintenance problems with the wood that separatesthe two layers of glazing.

ReflectorsOne possible way to improve the performance of thesedryers is to use reflectors. We tried several strategies:making the vertical south wall of the dryer box areflective surface, hinging a single reflector at thebottom of the dryer, and adding reflectors on each sideof the collector.

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Figure 3: Sun Anglesand Reflection with aVertical Reflector


Solar Dehydration

Vertical Wall ReflectorWe realized that the vertical southwall of the dryer box could bepainted a light color or coated withaluminum foil, a mirror, or reflectiveMylar (see Photo 3). A verticalsouth-facing wall reflector wouldreflect some additional energy intothe dryer’s collector, protect thewood from cracking, and preventdeterioration from UV radiation.Considering the fact that the angleof reflection equals the angle ofincidence, we were able to modelthe performance of this reflector,using a protractor and a chart of sunaltitude angles (see Figure 3). If thedryer is moved several timesthroughout the day to track the sun’sazimuth angle, then the reflectorconcentrates some additional solarenergy onto the dryer’s collectorduring most of the day.

Look at the temperatures recorded on Graph 2. A slightincrease in dryer temperature was recorded in the dryerhaving the south-facing reflective wall. The reflectedlight covers the collector most completely at mid-morning and afternoon. As the sun gets higher, the lightis reflected onto a smaller area of the collector.

Single ReflectorA single reflector was hinged to the bottom of thecollector (see Photo 4). This reflector was supportedwith a string and stick arrangement, similar to one usedby Solar Cookers International. With all reflectorsystems, the dryer has to be moved several timesthroughout the day if performance is to be maximized.This allows it to track the azimuth angle of the sun. Thealtitude angle of the reflector also needs to be adjustedduring the day from about 15° above horizontal in the

Reflective Surface

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35°Sun Angle

8:30 / 4:30 Sun

Figure 4: Single Reflector at Low Sun Angle

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Above, Photo 4: Setting the front reflector angle.

morning and evening to 45° abovehorizontal around noon (see Figures4 and 5). The reflector added10–20° F (2.4–4.8° C) to thetemperature of the dryer andremoved slightly more moisture fromthe food than a dryer without areflector.

Side Mounted ReflectorsA third strategy was to add reflectorsto both sides of the collector. Thiscaptures more solar energy than the


Solar Dehydration

reflectors would shade the collector in the morning andthe other in the afternoon.

We concluded that the vertical wall reflector and thesingle reflector mounted to the bottom of the collectorare the best ways to add reflectors, since tracking is notcrucial in these applications. However, these dryersroutinely attain temperatures of 130–180° F (54–82° C)without reflectors, which is hot enough for food dryingand for pasteurization. Based on our work so far,reflectors just don’t seem to be worth the trouble.

AbsorbersAll low temperature solar thermal collectors needsomething to absorb solar radiation and convert it toheat. The ideal absorber is made of a conductivematerial, such as copper or aluminum. It is usually thin,without a lot of mass, and painted a dark color, usuallyblack. The original dryer design called for five layers of

Reflective Surface

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Figure 5: Single Reflector at High Sun Angle

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Figure 6: Ideal Angle for Side-Mounted Reflectors

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Right, Photo 5:Side reflectors

folded ontoglazing for

transportation.other two strategies. We determined that the idealreflector angle would be 120° from the collector surface(see Figure 6). This assumes that the dryer is pointingtoward the sun’s azimuth orientation.

We performed an experiment to compare a dryer withtwo side reflectors and a vertical wall reflective surfacewith a dryer having no reflectors (see Photo 3). Bothdryers were moved throughout the test period to trackthe sun. The reflectors were mounted with hinges andcould be closed or removed when transporting the dryer(see Photo 5). Graph 3 shows the significant increasein temperatures attained by using these reflectors. Theproblem with this design was that if the dryer could nottrack the sun for one reason or another, one of the


Solar Dehydration

black aluminum window screening, which had proven towork well in other air heating collectors we hadconstructed. Other designs call for metal lath, metalplates such as black metal roofing, or aluminum orcopper flashing. We decided to try some differentmaterials and approaches to see if we could come upwith a better absorber.

Plate vs. ScreenFirst, we compared five layers of black aluminumwindow screen placed diagonally in the air flow channelto one piece of black corrugated steel roofing placed inthe middle of the channel (see Figure 7). We found thatthe mesh produced temperatures about 7° F (3.9° C)higher than the roofing in full sun. Other experimentshave shown that mesh type absorbers are superior toplate type absorbers. These differences might bereduced if we used a copper or aluminum plate insteadof the steel roofing.

Lath vs. ScreenNext, we compared three layers of pre-painted blackaluminum window screening to three layers ofgalvanized steel lath painted flat black. We found thatthe lath produced temperatures as much as 15° F (3.6°C) higher than the screen in our outdoor solar fooddryer tests. We got the same results when wecompared six layers of screen to six layers of lath (seeGraph 4). While we found that the lath produced slightlyhigher temperatures, it was harder to work with, neededto be painted, and cost slightly more than the screen.

When these tests were replicated with the solarsimulator, we had slightly better results with the screenthan with the lath in both the three and six layer tests.We were disappointed by the lack of positive correlationbetween our outdoor tests with the actual food dryersand our indoor tests with the solar simulator. But thereare many variables to control and quite a few peopleinvolved in setting things up and collecting data, so ourcontrol was not as tight as we would have liked. Despitethese problems, we are confident in concluding thatthere is not a great deal of difference in performancebetween lath and screen—both work effectively.

Layers of Absorber MeshWe then compared three layers of lath to six layers oflath, and three layers of screen to six layers of screen.Obviously the more screen used, the greater theexpense. The literature on solar air heatersrecommends between five and seven layers. Wearbitrarily picked three and six layers. In our outdoortests, we found that six layers of screen producedtemperatures 5–10° F (1.2–2.4° C) higher than threelayers. Likewise, when we repeated these experimentsoutdoors with lath, we found that six layersoutperformed both two and four layers (see Graph 5).

Steeply Angled Sections U-Tube Through Pass

Reverse Diagonal AbsorberNormal Diagonal Absorber

Dual Pass

Figure 7: Collector/AbsorberConfigurations

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Graph 4: Lath vs. Screen Absorber


Solar Dehydration

Tests performed in the solar simulator showed very littledifference between three and six layers. We used thesimulator to test one and two layers and no absorber.With no absorber, the temperature decline was over 60°F (33° C), dropping from 153 to 89° F (67 to 32° C) .The temperatures for one, two, three, and six layers oflath after one half-hour were 145, 155, 159, and 160° F(63, 68, 70, and 71° C). Based on our work, we feel thattwo or three layers of screen or lath are adequate foreffective performance, but adding a few more layers willproduce slightly higher temperatures.

Reflective Is EffectiveWhen constructing a solar air heater, you must decidewhat to do with the bottom of the air flow channel,below the absorbing material. In the next part of ourresearch, we placed aluminum flashing in the bottom ofthe air flow channels of two of the three dryers, on top

of the 3/4 inch (19 mm) foil-faced insulation (CelotexTuff-R, polyisocyanurate). The flashing in one of thedryers was painted flat black. The third dryer was leftwith just the reflective insulation board on the bottom ofthe air flow channel. This test was done with both theactual dryers and the solar simulator. In both cases, thehighest temperatures were attained with the reflectivefoil-faced insulation. The differences were substantial,with the reflective insulation showing readings as muchas 25° F (14° C) higher than the dryer with the blackaluminum flashing (see Graph 6).

Mesh InstallationThe original design called for the mesh to be insertedinto the collector diagonally from the bottom of the airflow channel to the top (see Figure 7). This seemed thebest from a construction point of view. In this test, threeconfigurations were compared: from bottom to top asoriginally designed, from top to bottom, and a series ofmore steeply angled pieces of mesh stretching from thetop to the bottom of the air f low channel. Thedifferences in temperatures attained were very small(see Graph 7), and we concluded that there was notmuch difference in performance.

U-Tube vs. Single PassAnother characteristic of the original design is the U-tube air flow channel. In addition to the air flow channelright below the glazing, there is a second air flowchannel right below the first one, separated by a pieceof insulation board (see Figure 7). We compared adryer with this U-tube design to a dryer with just astraight shot single channel and found no significancedifference in temperatures. We removed the insulationboard from our dryers and have completed all theexperiments detailed in this article without the U-tubesetup.

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Solar Dehydration

Active vs. PassiveWe experimented with several small, PV-powered fansto see if they would generate higher air flows andpossibly accelerate food dehydration. We tried threedifferent sizes: 0.08, 0.15, and 0.46 amps. We placedthe fans in the exhaust area of the dryer. Of the three,the 0.15 amp fan seemed to work the best. It increasedthe air flow from about 25 to 50 feet per minute (8 to 15meters per minute), but decreased temperaturessignificantly (see Graph 8). The larger fan did not fit inthe exhaust vent opening, and the smallest fan did notsignificantly increase the air flow.

Even with the fans in use, the drying performance didnot improve. In every trial, the passive dryer eithermatched or outperformed the active dryer. Eachmorning during a five-day experiment, we placedexactly the same weight of fruit in each dryer. We usedone to three pounds (0.4 to 1.4 kg) of apple or peachslices. Each afternoon between 2:30 and 5 PM, we

removed and weighed the fruit. On all five days, the fruitdried in the passive dryer weighed either the same orless than the fruit dried in the active dryer.

Vent OpeningThe dryers have vent covers at the top which can beadjusted to regulate the air flow and temperature. Thesmaller the opening, the higher the temperaturesattained. We wanted to know how much the ventsshould be opened for maximum drying effectiveness.We tried a variety of venting combinations while dryingfruit. For most of our experiments, we filled five toseven of the thirteen shelves with 1/8 inch (3 mm) fruitslices. We cut up, weighed, and placed an identicalquantity and quality of fruit in each of two dryers in themorning. Sometime between 2 and 6 PM, we removedthe fruit from the dryers and weighed it again. Wecompared openings of different measurements: a oneinch (25 mm) to a seven inch (178 mm), a 3/4 inch (19mm) to a five inch (127 mm), a three inch (76 mm) to asix inch (152 mm), a three inch (76 mm) to a nine inch(229 mm), and a three inch (76 mm) to a five inch (127mm). During these experiments, the bottom vents werecompletely open.

We found that higher temperatures were attained withsmaller vent openings, but that drying effectivenesswas not always maximized. The best performance wasobserved when the vents were opened between threeand six inches (76 and 152 mm), and temperaturespeaked at 135–180 °F (54–82° C) (see Graph 9). Withthe one inch (25 mm) and smaller openings and theseven inch (178 mm) and larger openings, less waterwas removed from the fruit. There was no difference inthe water removed when we compared three inches tofive inches (76 mm to 127 mm) and three inches to sixinches (76 mm to 152 mm).

Based on this work, we would recommend opening theleeward exhaust vent cover between three and sixinches (76 and 152 mm), or between ten and twentysquare inches (65 and 129 cm2) of total exhaust area.The exact size of the opening depends on the weatherconditions. With the vents opened between three andsix inches (76 and 152 mm), we have been able toremove as much as sixty ounces (1.75 l) of water in asingle day from a full load of fruit and completely dryabout three and one-half pounds (1.5 kg) of apple slicesto 12–15% of the fruit’s wet weight.

Construction ImprovementsAs we experimented with the dryers, we came up withsome design improvements to simplify the construction,reduce the cost, and increase the durabil ity orportability of the unit. To simplify the construction andeliminate warping problems caused by wet weather, wedecided to eliminate the intake vent covers during our

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Solar Dehydration

experiments. The vent covers at the top, if closed atnight, would prevent or reduce reverse thermosiphoningand rehydration of food left in the dryer.

The redesigned air intake now has aluminum screensecured to the plywood side pieces with wooden trim.We also redesigned the top exhaust vent cover toeliminate the warping problem caused by leaving thevent covers opened during wet weather. The newexhaust vent cover works very well (see Photo 6). Itspreads the exhaust air across the dryer’s width ratherthan concentrating it in the center. This should improveconvective flows and performance. However, the ventcover makes it more difficult to calculate the exhaustarea, and as a result, we mainly used the old design forour research this past year.

We added wheels and handles to the unit, as it is heavyand diff icult to move around. It ’s now easier tomaneuver, although it is still difficult to transport in asmall pickup truck. We purchased ten-inch (254 mm)lawnmower-style wheels for $6 each. The axle cost $2.With the wheels on the small legs at the bottom of thecollector, one person can move the dryer.

The original design specified thin plywood for the roof ofthe dryer. We replaced that with 3/4 inch (19 mm)plywood and covered the peak of the roof withaluminum flashing. We also used 1/2 inch (38 mm)wide by 1/8 inch (3 mm) thick aluminum bar stock andstainless steel screws to attach the glazing to thedryer’s collector. Each collector used fourteen feet,eight inches (4.5 m) of aluminum bar at a cost of $23.The 1/4 inch (6 mm) plywood strips used in the originaldesign were adequate and less expensive, but wouldhave required more maintenance.

Conclusions and RecommendationsThe dryer described in HP57 has worked well in ourtests. It produces temperatures of 130–180° F (54–82°C), and can dry up to 15 apples or peaches—about 31/2 pounds (1.6 kg) of 1/8 inch (3 mm) thick slices—inone sunny to partly sunny day. The best performance inour outdoor tests was attained with six layers ofexpanded steel lath painted black, although aluminumscreen works almost as well and is easier to work with.We also found that two or three layers of screen or lathwould produce temperatures almost as high as sixlayers. The surface behind the absorber mesh shouldbe reflective, and for best performance the exhaust ventcovers should be opened three to six inches (76–152mm). The cost of the dryer and the time to construct itcan be reduced by eliminating the U-tube air flowchannel divider, the second or inner layer of glazing,and the intake vent covers, and by reducing the numberof layers of screen or lath to two or three.

We made the unit more portable by adding wheels andhandles, and improved the durability by fastening thelegs with nuts and bolts, using aluminum bar to hold theglazing in place, and using 3/4 inch (19 mm) plywoodfor the roof. We would also like to take the insulationboard out of a dryer to see if it significantly impacts theperformance. This would further decrease the cost ofthe dryer. Soon, we hope to compare this design todirect solar dryers, which a Home Power reader hasrecently suggested can outperform our design. Thusfar, we have avoided direct dryers because of concernsabout vitamin loss in foods exposed to direct solarradiation.

We have tried to carefully explore all of the significantvariables affecting this dryer’s performance. We havebeen able to increase drying effectiveness with highertemperatures of approximately 30° F (16.6° C), whiledecreasing the cost by about $30. We havedemonstrated the best vent opening for dryingeffectiveness, and seen the impact that variables suchas double glazing, fans, reflectors, and absorber typehave on performance. We have also developed anddemonstrated a low cost solar simulator that can beused to test solar thermal collectors indoors.

AccessAuthors: Dennis Scanlin, Marcus Renner, DavidDomermuth, and Heath Moody, Department ofTechnology, Appalachian State University, Boone, NC28608 • 704-262-3111 • [email protected]

Above, Photo 6: The new vent design.


Solar Dehydration

Solar Cookers International (SCI), 1919 21st Street,Sacramento, CA 95814 • 916-455-4499Fax: 916-455-4498 • [email protected]

Sun-Lite HP glazing was purchased from SolarComponents Corporation, 121 Valley Street,Manchester, NH 03103-6211 • 603-668-8186Fax: 603-668-1783 • [email protected]

Scales, anemometers, and other data collectionequipment were purchased from Thomas Scientific,PO Box 99, Swedesboro, NJ 08085 • 800-345-2100609-467-2000 • Fax: [email protected] • www.thomassci.com

Data logger was purchased from Pace Scientific, Inc.,6407 Idlewild Rd., Suite 2.214, Charlotte, NC 28212704-568-3691 • Fax: [email protected] • www.pace-sci.com

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The Design, Construction, and Use of an Indirect, Through-Pass, … · 2011. 8. 25. · dryer...

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