diy - survival - how to build a solar food dehydrater

7/30/2019 DIY - Survival - How to Build a Solar Food Dehydrater

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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 dont 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 (710 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 ofTechnologys 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 that

project 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 still

needs 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. Until

canning 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 food

preservation technology fora sustainable world.

The Design, Construction, and Use of an

Indirect, Through-Pass, Solar Food DryerDennis Scanlin1997 Dennis Scanlin


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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 water

around 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 180F so

one would want to stay under thistemperature. All opinions surveyedfall between 95 and 180F, with110140F 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 180F)increase the speed of drying. Onestudy found that it took

approximately 5 times as long to dry food at 104F as itdid at 176F. Higher temperatures (135180F) alsodestroy bacteria, enzymes (158F), fungi, eggs andlarvae. Food will be pasteurized if it is exposed to 135Ffor 1 hour or 176F for 1015 min. Most bacteria will bedestroyed at 165F and all will be prevented fromgrowing between 140165. Between 60 and 140Fbacteria can grow and many will survive, althoughbacteria, yeasts and molds all require 13% or moremoisture content for growth which they wont have inmost dried foods.

Some recommended drying temperatures are:

Fruits and Vegetables: (except beans and rice):100130F (Wolf, 1981); 113140 (NTIS, 1982);temperatures over 65C (149F) can result in sugarcaramelization of many fruit products

Meat: 140150F (Wolf, 1981)

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

Herbs: 95105F (Wolf, 1981)

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

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

Temperatures Obtainable in our Appalachian DryerOur Appalachian dryer, with a reflector added, has

reached temperatures over 200F on a sunny 75F daywith all the vents closed. Preliminary experiences with a4' long reflector have demonstrated a 20F 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 day

between 120and 155F by opening the exhaust vents12" (1020 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 problems

There are a couple of potential problems associatedwith higher temperatures. One study reported slightlyhigher vitamin C loss in fruits dried at 167F than at131F. Greater vitamin loss has also been reported forthe direct style of food dryer which exposes the fooddirectly to the suns 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 of

air 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 f low 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 both

high 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 1.5

meters 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 24 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 flowsin the Appalachian dryerOur solar dryers are only able to achieve air velocities

between 50 and 130 FPM with natural convection. Lessthan most of the 100 to 1000 FPM rangerecommended. Air velocities were measured in thesolar collectors 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 2560 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 efficient

collector 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 eff icient collectorperformance and also less than our dryers 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 27F 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 cant 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 glazing

is 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 125F above ambientwith a reflector and all vents closed. Normal

temperature rises without a reflector and with bothexhaust vents opened 13" (1236 square inches)would be 50 to 70F. As mentioned previously, ourpreliminary testing indicates about a 20F increase intemperature by adding a reflector. The maximumtemperature observed was 204F. The higher Delta Tsand 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 between140F and 175F, 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 air

moving 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 70Fday is only about 85F 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 130F and still allow air flow. It is desirable to have

adjustable 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 didnt perform anybetter in our tests than the window screening. Usingstock black or dark gray aluminum window screen

eliminates 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 its U-tubedesign. In addition to the air flow channel right belowthe glazing, there is a second air flow channel right

below 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

Illustration 1


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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. Im not sure this feature is necessary; but, itdoesnt 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 collector

should 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.Appalachians 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 f t . 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 adjustable

reflector. Verify the measurements before blindly cutting

Illustration 2


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Illustration 3

Illustration 4


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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 door

and 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

Illustration 5

Illustration 6


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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. Dont 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 57% 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 and

aroma 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 131F to avoidcooking it and consider salting 12 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

Illustration 7


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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 5C

(41F) to avoid rancidity (NTIS. 1982).

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

drying, 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 controlling 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 140F or if it was pasteurizedprior to and after drying all 4 of the problem causingagents will be destroyed. Salt can be also used to

control 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 t issues aredamaged. Their production can lead to discoloration,loss of vitamins, and breakdown of tissues. Mostenzymes are inactivated at 158F. 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 still

be 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 400F has little effect.Blanching times vary. In general 13 minutes for leafyvegetables, 28 for peas, beans, and corn and 36 forpotatoes, carrots, and similar vegetables.

AccessAuthor: Dennis Scanlin, 3137 Georges 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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Homebrew

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 Food

By. 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


12/22


his articledescribes a series of

experiments

conducted over the last

year and a half with

three solar food dryers.

The food dryers were

constructed at

Appalachian State

University (ASU) using

plans published in

HP57. The goal of this

research program was

to improve the design

and to determine the

most effective ways to

use 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

36 Layers of Lathor Screen

7 feet

6feet

Figure 1: Cutaway View of the Appalachian Solar Food Dryer

1999 Dennis Scanlin, Marcus Renner,

David Domermuth, and Heath Moody


13/22


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 bottom

of 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 130180 F (5482C), which is a desirable range for most food drying andfor pasteurization. With these dryers, its 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.


14/22


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 its 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 Technology

Department 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

Single Glazing Ambient Double Glazing

DegreesF

20

40

60

80

100

120

140

160

180

8:00 9:00 10:00 11:00 Noon 13:00 14:00 15:00 16:00 17:00

Graph 1: Single vs. Double Glazing

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


15/22


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. The

temperature probes are shaded to give a true readingof the air temperature. We conducted the simulatortests inside a university building with an indoortemperature of 6264 F (1718 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.

Experiments

We 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 Departments 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 HP57used 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 cool

and 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, low

humidity (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 130180F(5482C). 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 dryers 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 dryers 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 side

of the collector.

8:00/16:00

Noon

8:00/

16:00

11:00/1

3:00

9:00

/15

:00

9:00/15:00

10:0

0/14:00

10:0

0/1

4:00

11

:00

/13

:00

Noon

Reflective Surface

Figure 3: Sun Anglesand Reflection with aVertical Reflector


16/22


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 dryers 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 t imesthroughout the day to track the sunsazimuth angle, then the reflectorconcentrates some additional solarenergy onto the dryers 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 Reflector

A 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

15 Reflector Angle

35Sun Angle

8:30 / 4:30 Sun

Figure 4: Single Reflector at Low Sun Angle

40

60

80

100

120

140

160

180

9:30 10:00 10:30 11:00 11:30 Noon

Time

DegreesF

With Reflector Without Reflector Ambient

Graph 2:Vertical Wall Reflector vs. No Reflector

Above, Photo 4: Setting the front reflector angle.

morning and evening to 45 abovehorizontal around noon (see Figures4 and 5). The reflector added1020 F (2 .44.8 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


17/22


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 130180F (5482C)without reflectors, which is hot enough for food dryingand for pasteurization. Based on our work so far,reflectors just dont seem to be worth the trouble.

Absorbers

All 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

45 Reflector Angle

80Sun Angle

Noon Sun

Figure 5: Single Reflector at High Sun Angle

60Reflector

Angle

Collector Surface

ReflectorSurface

ReflectorSurface

Figure 6: Ideal Angle for Side-Mounted Reflectors

20

40

60

80

100

120

140

160

180

8:00 9:00 10:00 11:00 Noon 13:00 14:00 15:00 16:00

Time

With Reflectors Without Reflectors Ambient

DegreesF

Graph 3: Vertical Wall & Side Reflectorsvs. No Reflector

Right, Photo 5:Side reflectors

folded ontoglazing for

transportation.other two strategies. We determined that the idealreflector angle would be 120from the collector surface(see Figure 6). This assumes that the dryer is pointingtoward the suns 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


18/22


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 in

the 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. Screen

Next, 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 15F (3.6C) 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 screenboth 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 510 F (1.22.4 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

40

60

80

100

120

140

160

180

9:00 10:00 11:00 Noon 13:00 14:00 15:00 16:00 17:00 18:00 19:00

Time

Lath Screen Ambient

DegreesF

Graph 4: Lath vs. Screen Absorber


19/22


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 60F (33C), dropping from 153 to 89F (67 to 32 C) .The temperatures for one, two, three, and six layers oflath after one half-hour were 145, 155, 159, and 160F(63, 68, 70, and 71C). 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-tube

setup.

20

40

60

80

100

120

140

10:00 11:00 Noon 12:30 13:00

Time

2 Layers

4 Layers

6 Layers

Ambient

DegreesF

11:3010:30

Graph 5:Two vs. Four vs. Six Layers of Absorber

20

40

60

80

100

120

140

160

5:00 7:00 9:00 11:00 13:00 15:00 17:00 19:00 21:00

Time

Bottom to Top Top to Bottom

Ambient Steeply Angled Sections

DegreesF

Graph 7: Absorber Installation Comparison

Black Flashing Aluminum Flashing

Ambient Foil-faced Tuff-R

DegreesF

20

40

60

80

100

120

140

160

11:45 12:15 12:45 13:15 13:45 14:15 14:45

Time

Graph 6: Collector Bottom Material Comparison


20/22


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 be

adjusted 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 one

inch (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 135180 F (5482 C) (see Graph 9). With

the 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 1215% of the fruits 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 durability orportability of the unit. To simplify the construction andeliminate warping problems caused by wet weather, we

decided to eliminate the intake vent covers during our

40

60

80

100

120

140

160

180

9:00 11:00 13:00 15:00 17:00 19:00 21:00

Time

PV-Powered Fan 3 Inch Vent Ambient

DegreesF

Graph 8: PV Exhaust Fan vs.Vent

40

60

80

100

120

140

160

180

200

7:00 9:00 11:00 13:00 15:00 17:00 19:00 21:00

Time

DegreesF

3 Inch Vent 6 Inch Vent Ambient

Graph 9:Three Inch vs. Six Inch Exhaust Vent


21/22


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 dryers 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 to

maneuver, 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 the

dryers 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 130180F (5482C), and can dry up to 15 apples or peachesabout 31/2 pounds (1.6 kg) of 1/8 inch (3 mm) thick slicesinone sunny to partly sunny day. The best performance inour outdoor tests was attained with six layers of

expanded 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 (76152mm). 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 the

performance. 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 dryers performance. We havebeen able to increase drying effectiveness with highertemperatures of approximately 30 F (16.6 C), whiledecreasing the cost by about $30. We have

demonstrated 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.


22/22

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: 704-568-0278

[email protected] www.pace-sci.com

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