related technologies

from: "peace corps/armenia",
date: mon, 26 aug 1996
subject: solar fruit dryer advice for armenia

dear solarcooking-l readers,

in search of advice on industrial-scale solar fruit drying

i am a peace corps volunteer working as an agri-business development
advisor in the republic of armenia
(former soviet armenia). over the past three years several
international organizations have sponsored pilot
projects to test the feasibility of solar drying fruits and
vegetables. i am in search of advice on how to
improve the economic efficiency of this effort thorough improved
solar dryer design.

armenia was an important producer of horticultural crops in the
soviet system. stone-fruit orchards in the
range of 50-100 hectares were not uncommon on the state-owned farms.
the orchards have since been
divided up into smaller plots of .5 to 5.0 hectares, privately held.
fruit marketing has become very
problematic because of energy supply and transportation disruptions
wrought by external conflict. it is
hoped that solar drying can offer some relief to this problem.

the dryers are, i believe, a common design with a wedge-shaped cross
section, about 2.5 meters tall at the
rear. i have seen similar designs in un publications and a book
published by the vita (volunteers in
technical assistance) organization. the wedge-shaped end panels and
the bottom half of the back wall are
made of local saw-cut building stones. these side panels and back
wall sit on a concrete foundation.
resting on the back wall are sheet-metal access doors through which
three-tiered racks hold the drying fruit
or vegetables. the entire face of the dryer is composed of 1.0 m. x
0.5m glass panels which rest upon
angle-iron studs.

the dryers built thus far in armenia are rather large, about 20 m in
length, and can hold about 500 kilograms
of wet fruit. assuming that marketing and other problems can be
worked-out related to dried fruits and
vegetables (especially solar-dried tomatoes), the cost of these
dryers still looks daunting. the 500 kg model
costs around 6,000 us dollars to build. at least two-thirds of the
cost owes to imported glass (25 us
dollars per 1x.5 m. panel) and angle-iron (used for framing).

can we do better?
as a casual observer of these fruit dryers, it seems to me that there
might be considerable room for
improvement. some questions which have occurred to me:

1. would the incorporation of insulating materials improve drying

2. are there more economical substitutes for glass?

3. might cost efficiency be improved through a mass-produced pre-
fabricated design?

4. reflectors?

5. might fan-forced air improve drying efficiency?

6. alternatives to building stones for end-panels and back walls?

where to start?
having thrown out a little information, i ask for advice on where to
start looking for more information?
where might the most up-to-date knowledge reside for the design and
manufacture of solar dryers for
industrial applications? any thoughts which you might have will be
most welcome.


chuck specht
peace corps volunteer/armenia


from: jim arwood,
date: tue, 27 aug 1996
subject: re: solar fruit dryer advice for armenia

i posed your question to a couple of engineers in my office and got the
following advice:

i doubt there is a commercial pre-fabbed unit available. seems like a
perfect fit for some sweat-equity effort on the part of the locals.
obviously, they can substitute some suitable uv resistant plastic for the
glass, and use wood for framing if it is abundant locally. i'd say the most
important thing is to have some ventilation so the evolving moisture can be
expelled to the atmosphere. too bad john yellot is not still with us!

jim arwood


date: tue, 27 aug 1996
subject: re: solar fruit dryer advice for armenia

as i mentioned in my e-mail to chuck yesterday, my wife's nephew spent a
month in uganda recently. i had asked him to check on solar cooking usage
there. as it turned out, he found that no one seemed to be using solar
cookers, but a number of people had solar food dryers. unfortunately, he did
not get any information on the design of those dryers. however, there must
be someone around who has spent time in uganda and can provide further
details on the techniques and equipment used there.

from: jeff jowett
subject: solar fruit dryer
date: monday, august 26, 1996

jim arwood passed along your request for information on solar fruit drying.
i do technology transfer here at the arizona department of commerce and i
have some solar knowledge. i have some thoughts regarding your problem and
i would be willing to look into this if you provide some more detailed
information. the following information would be very helpful:
1. photos, drawings, etc. of existing technology.
2. dimensions, materials (a little more info than what's in your email).
3. what kinds of fruits and how dry do they need to get? it is necessary
to know how much moisture, in an absolute amount, needs to be removed.
4. is there electrical power at the dryer sites? if so, at what voltage
and amps? is it reliable?
5. is there any climatological data available for the country/region(s)?
solar insolation data would be great, but general climate data would
suffice. this would include average daily ambient temperatures by month,
average daily high temperatures and average daily low temperatures. this
kind of information is needed to determine cost-effectiveness of glazing
types (like double- or single-glazing, for example), insulation thickness,
and thermal mass. relative humidity would be very helpful. this is
necessary to determine evaporation rates, air flow/recirculation rates and
other design criteria which are air/moisture ratio dependent. any climate
data and descriptions would be useful.
6. what is ratio of equipment cost to other costs, like freight, for the
$6,000, 500kg unit? my guess is that freight from the us to armenia is
exorbitant, meaning that using a similar cost, but lighter weight material
may save some $$$.

you can reach me at phone/fax: 602-280-1335 or email at

jeff jowett


from: jim arwood,
date: thu, 29 aug 1996
subject: re: solar dryer

subj: solar dehydrator
date: 96-08-27 09:03:54 edt
from: tait solar

dear chuck,

my friend, mr. jim arwood, at the az dept. of energy sent me a copy of your
message. in the mean time, i have just joined the solar oven email group.

jim contacted me because i work as a solar energy consultant and have
experience working with solar ovens and solar dehydrators. i have designed
and built a number of small food dryers and worked on a large scale dryer
for crop drying.

please email a cross-sectional drawing of your current dryer for reference

i cannot answer all of your questions without more information but will try.

1. insulation--- depends on air temperatures during the drying period. this
needs to be looked at carefully to justify the cost and headaches of
including insulation. off the top of my head, i would certainly consider
an insulated glazing alternative. there are numerous ones on the market.

2. yes. there are lots of alternatives to glass. though, keep in mind that
glass has the best properties, but i can certainly understand that it
is not the most cost effective choice.

3. absolutely. the only single units that should be built are prototypes
from that point, the design should incorporate a form of mass production.

4. yes, reflectors would help. although there are some down sides here.
material choice? wind damage? structure, etc.

5. yes, increasing the air flow can help and may be necessary to help
eliminate condensation. depending on the outdoor air temperature and the
glazing type, condensation can create unwanted problems. the length of the
food dryer is an important value. we found that a dryer too short in
length did not work effectively. at the same time, if the dryer is too long
then average effectiveness goes down because the air becomes saturated at
which point the drying process dries up. (i couldn't resist the pun.)

6. i cannot easily imagine why you would use building stones for the end
panels. the key is to have air move through to carry out the moister so
why the stone end panels? you do need to have some control of the air flow

the solar dehyrdator i worked on a number of years incorporated a slow
moving belt in which the "crop" was placed on one end and came out the other
end dried. i am not sure this could be done with tomatoes but i would
certainly consider it. maybe the drying effectiveness could be increased
with mirrors to the point of making it work.

there is certainly a lot to think about in designing a new commercial solar

let me know what you think about my comments.

david tait

tait solar co., inc.


from: barbara kerr,
date: mon, 2 sep 1996
subject: solar dryers for chuck specht

dr headley in the west indies has worked for years on
commercial solar food drying for high humidity, low economic
level uses in the carribean and south america. i met him
at a workshop put on by solar energy international in
carbondale, colorado. he seems knowledgable and might have
some advice that would help in your situation.

dr. oliver headley, phone: 1-809-425-1310 64, bridgetown, barabados, west indies,
alternate address:university of the west indies,dept. of
chemistry, cave hill campus, pob 64, bridgetown, barbados.

for ventilation, you might find a solar chimney useful. even
if, under your circumstances, it would have to be combined
with a fan, it would lower electricity costs. in the dryer i
have used for 11 years, a solar heated chimney draws from
the bottom of the racks of food in an enclosed cabinet.
solar heated air is fed in at the top and falls through the
food trays to the bottom, where the solar chimney pulls it
out. this exhausts the coolest, moist air which is exactly
what is needed.

solar water heaters

during my recent 6 weeks in guadalajara, mexico, i looked in vain for any solar water heaters. there were several companies which advertised in the yellow pages but, when i went to them, they were no longer in business.

this in a city which is about a mile high and which enjoys clear sun filled days much of the year. they also have flat roofs which would make the installations easy.

i mentioned this to a guy i met who is a real estate broker in california. he said that many solar water heaters were installed in his town, chico, during the 70s when the gov’t subsidised solar installations. he says that most real estate contracts he deals with now include the instructions to the sellers from the buyers to "remove all solar water heating panels and piping" as part of the sale contract.

dave johnson


dear well meaning friends,
this is contemprary urban myth, an unattributed falsehood, and self-defeating rumor mongoring. stop it!

houses with solar aspects are favored over those without. dhw, air heat, pv, sunspaces and greenhouses are all positive amenities.

ross donald


dave johnson asked about the bad press for solar water heaters. i do not know about mexico but opinions there often rest on experience in the states. there have been at least two events in the us that have set back this very promising technology.

in the 1940s, i understand that florida had quite a few solar water heaters which were of a design vulnerable to freezing. there was widespread damage during an unusually cold spell. simultaneously, gas water heaters were coming on the market at low cost and with low operating expense. and so solar water heating of the design in use at that time was replaced by the newest and most modern and most advertised "troublefree" gas water heaters.

in the 1970s with the event of the tax rebate, the quick fix artists jumped into the solar water heater field. various poorly designed and/or poorly built designs came on the market to reap and run. there was a lot of installation by people not really qualified in either solar or roof work. so many solar water heating units were installed that did not last and for which service could not be located when the roof or the unit began to leak..... and this led to real estate managers and homeowner headaches. in addition, some of these units were priced at $2000, $3000 or more...

i had three solar water heaters (i am persistent) that froze and broke. these were all designs that had water in small tubes. these were installed in tempe, arizona which rarely has freezing weather and yet one even broke when temps only reached 40 degrees. this was because the unit was a black body radiating into a clear black sky and so was colder than ambient.

on the otherhand, here in the high desert where nights routinely freeze in the winter, i installed a simple breadbox solar water heater in 1984. made from a stripped-down 40 gallon water tank placed inside an insulated, foil lined plywood box with double-glazing at a 45% slant, it was made for under $150. it is still in use 13 years later although the fiberglass glazing seriously needs to be replaced—a low cost and simple home repair. this solar water heater has a large mass which protects it from freezing. we have had temperatures as low as minus 27 degrees f for two nights in a row with no solar gain on the day between and yet there was no freeze damage—albeit no hot water either during that time.

i use this solar water heater as a stand-alone and so back it up with a pot on the wood fire if weather is that bad. many people use solar water heaters to preheat and backup with a gas or electric heater. raising the temperature of water from 80 f or 110 f. up to the usa customary 120 degrees f instead of from 40 f to 120 f results in significant energy savings particularly where a large volume of hot water is needed.

in addition to mass in single or double breadbox type solar water heaters, i am aware of two other freeze protection lines of development. (there may be others.) one is to fill the exposed small diameter heating tubes with something that will not freeze...oils or glycol, etc. and pump that heated substance through a heat exchanger in a water tank. and in the second line there are a variety of ways to have the water drain out of the small diameter tubes when the temperature drops. all three of these methods protect from freeze damage.

what can we learn from these fiascos and successes?

solar water heaters do save energy, can be inexpensive and durable. freeze protection is essential. durability and simple maintainance is very important. a final essential element is that must be someone to service commercial units over the years and to trouble-shoot if there are unexpected problems. for solar water heaters to take their place in the solar market,it may take special efforts to educate home owners, bank managers and real estate brokers that there are successful designs which will serve well and free up money for the home owner.

dave, can you tell us if any of this applies to what you know about the situation in mexico? anyone else know about other troubling aspects or other locations? please tell us what you know so we can have the broadest possible understanding.

michael bonke internet training
millrather str. 22
40591 duesseldorf
tel: 0211 97 79 023
fax: 0211 97 79 025


besuchen sie mich auf der cebit: halle 18, 1. og, stand b12 (global entrepreneurs network - gen)!

barbara kerr


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