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Title:

Design and fabrication of a solar air heater

tube with green house jacket for tobacco

drying

By Sithole Gerald T

(R068965N)

SUPERVISOR: Eng. Nazare

CO-SUPERVISOR: Mr. Musoni

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Background

Zimbabwe is facing serious electricity shortages as a result farmers are

not able to use more technologically advanced equipment such as theelectricity driers adding to that most of the resettled farmers dont

have electricity at all. The Government embarked on a rural

electrification programme which only benefited a few farmers and for

those lucky few load shedding hasnt spared them either. Farmers need

to dry their produce in a specified period to ensure quality control and

achieve higher capacities and this can only be achieved with a reliable

power source. An alternative power source can be the solution.

With problems associated with non-renewable energy ranging from

land degradation to pollution the focus has shifted to use of renewable

energy which is a cleaner alternative and is in abundance. Solar energy

has come into serious consideration over the years. Faster and effective

ways of drying agricultural products need to be devised which use

energy since it is cheaper and in abundance. One of the methods

devised is a solar air heater tube. The device constitutes of parabolic

trough which is the collector, having a pipe with three inner tubes

running along its length. The suns rays are then tracked and directed at

the pipe or absorber and the air in the inner tubes is then heated

through an oil bath surrounding the inner tubes which acts the heat

transfer medium. The temperature attained at the outlet is 50oCwhich

is not adequate to dry most agricultural products.

Taking tobacco for instance it needs temperatures as high as 700C for

curing .Using fossil fuels this temperature can be easily attained but

with issues of global warming and land degradation ways have to be

devised to make this solar type air heater tube reach temperatures

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above 700C. Drying produce in the sun has been used for centuries but

it also has its problems similar to the air heater tube, temperatures

required may not be attained and its difficult to regulate the

temperature, coupled to that it increases chances of produce spoilageand its labour intensive. With the current system this cannot be

achieved.

A drying system which reaches temperatures above 700C needs to be

put in place which is affordable yet does the job.

Problem statement

The solar air heater tube present (done by Leonard Phiri)does notreach the required temperature of 70

0C to cure tobacco

Hypothesis

Temperatures as high as 700C needed for tobacco curing can beattained if energy losses are prevented and if heat transfer is

maximized from the outer tube to the inner tube.

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Objectives

Main Objective

Design, fabricate and test an air heater tube with green housejacket

Specific objectives

Fabricate and test an absorber tube with a green house jacketaround it

incorporate a selective absorber coating on the absorber tube Create an impedance inside the inner tube

Methodology

Solution Options (maximizing heat transfer wall of inner tube to air

particles inside tube)

Option1

air is to be blown through the tube filled with small stonesOption 2

Smaller (12mm) solid tubes are to be fitted into 19mm inner andair is forced into the 3,5mm clearance.

Option 2 is to be chosen as it provides more heat transfer becauseof the turbulence of air created and small clearance which

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maximizes contact between air particles and inner wall of inner

tube

Maximizing solar radiation absorption and minimizing energy losses;

Using option 2 from above, a glass tube or similar material will beused to enclose the absorber and temperature is recorded.

Depending on availability a selective absorber coating is to beincorporated on the pipe which is either black chrome, black

nickel or TiNOX.

The air heater tube is fixed to a parabolic trough using bearingsand the set up is connected to a solar tracking device

Air is blown through the inner tubes and the temperature isrecorded

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Relevance of project to stake holders

This project will go in a long way to alleviate farmers who face different

challenges when drying their produce. Since it is solar based it willcome in handy to farmers who cannot access electricity. With the

objective of attaining the highest possible temperature and system

being able to regulate temperature farmers will be able to choose from

a wide range of temperatures depending on their produce which will

determine the appropriate temperature range. Some methods of

drying are not controllable such as sun drying as such may lead to crop

decay or over drying, with the air tube the problems would become athing of the past.

For academia it would address the problem of global warming and land

degradation since there would be an alternative fuel which is

environmental friendly thereby reducing the use of fossil fuels.

Embedding of the project

Work concerning renewable energy use as an alternative to non-

renewable energy sources had already begun. Part of this project was

done by Phiri Leonard who has the recommendations which this project

will be looking into. The work concerning the air tube didnt seek to

address minimizing energy loses which was one the recommendations.

At departmental level some students are partaking in this project on

how to build and couple many units of this heater tube and parabolictrough. Another student is looking at how to store the unused heat for

later use. Another former student devised a method on how to track

the sun using a mechanical solar tracking device.

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Work Plan

oct nov dec jan feb mar apr may

proposal

Design for maximizing

heat transfer

Design for reducing

energy losses

Fabrication

Testing

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Budget

Item Cost(USD)

Internet $50

Transport $20

Stationary $10

Materials $150

Accessories $50

Total $280

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References

I. Madamombe, October,2006, Solar power: cheap energy source forAfrica,

http://www.un.org/ecosocdev/geninfo/afrec/vol20no3/index.html/Sol

ar power cheap energy source for Africa.html. 22/11/10

Anonymous, November 2007 Solar Drying, Practical Action,

http://practicalactionconsulting.org/docs/technical_information_servic

e/Solar Dryings.html, 22/11/10

Anonymous, 2003 Solar Dryer, Gujarat Energy Development Agency,

http://www.geda.org.in/default.htm/Solar Dryer/Solar Energy.html,

18/11/10

Anonymous,Naturaland solar drying,FAO, http://www.fao.org/Grain

storagetechniques - Drying methods - Natural and solar drying.html

http://practicalactionconsulting.org/docs/technical_information_service/Solar%20Dryings.htmlhttp://practicalactionconsulting.org/docs/technical_information_service/Solar%20Dryings.htmlhttp://practicalactionconsulting.org/docs/technical_information_service/Solar%20Dryings.htmlhttp://www.geda.org.in/default.htm/Solar%20Dryer/Solar%20Energy.htmlhttp://www.geda.org.in/default.htm/Solar%20Dryer/Solar%20Energy.htmlhttp://www.geda.org.in/default.htm/Solar%20Dryer/Solar%20Energy.htmlhttp://practicalactionconsulting.org/docs/technical_information_service/Solar%20Dryings.htmlhttp://practicalactionconsulting.org/docs/technical_information_service/Solar%20Dryings.html