ABSTRACT
An active mix-mode solar dryer powered by wind generator and a passive solar dryer both integrated with and without C3H8O3 (glycerol) as thermal storage was designed, fabricated and evaluated with a pre-treated sliced potato. The dryer was designed to operate in a low sunshine belt of the southern Nigerian climate characterized by its low solar radiation intensity, hence the integration of thermal storage material. The study offered the total dryer performance analysis including the exergy, energy, and drying kinetics of the dried product. The main objective was to develop an active dryer system using only renewable energy (solar and wind energies). The results presented showed that the developed mix-mode solar dryer can save between 9- 16 hrs of drying time compared to ordinary sun drying method when drying 2 mm thick potato slices with the moisture content reduced from 64.2 % to 8.6 11.6 % wet basis. A shorter drying time of 4h hrs was recorded with solar dryer integrated with wind-powered axial fan and thermal storage while drying with passive solar dryer without thermal storage presented a longer drying time of 25 hrs. Dipping the potato in a salt solution for 30 seconds before drying quickens the drying rate of the potato compared to dipping it for 60, 90 seconds or blanching for 30 -90 seconds. Additionally, potato pre-treated with salt solution showed very attractive bright colour compared to blanch or none pre-treated potato. During the experimental periods, the mean radiation intensity ranged from 10 - 626 W/m2, the ambient temperature range was 24 oC - 50 oC and humidity of about 9 - 52 %. . Under these operating conditions, the average drying efficiency was 28.59% - 29.14 %. In terms of energy utilization for drying purpose, solar dryer, integrated with wind-powered axial fan only, showed higher drying efficiency of 80 % while drying with passive solar dryer with thermal storage presented lower energy efficiency. The total energy consumption for drying ranged between 435102.2 kJ and 498040.2 kJ while the specific energy consumption was 28.46 to 36.86 kWh/kg. The exergy efficiency ranged from 14.5 – 80.9 % during the sunshine hours.
TABLE
OF CONTENTS
Front Cover Text
i
Tittle
Page
ii
Declaration
iii
Certification
iv
Dedication
v
Acknowledgements
vi
Table
of Contents
vii
List
of Tables
viii
List
of Figures
ix
List
of Plates
x
Abstract
xii
CHAPTER 1: INTRODUCTION
1.1
Background of Studies
1
1.2
Statement of Problems
4
1.3 Objectives of the study 5
1.4 Justification of the study 5
1.5 scope of study 6
CHAPTER 2:
LITERATURE REVIEW
2.1
Drying
7
2.2
Crop Drying
8
2.3 Drying Mechanism 9
2.4 Methods of Drying
10
2.5 Solar Drying 12
2.5.1
Classifications of solar dryers
15
2.5.1a Natural
convection (Passive solar dryers) 16
2.5.1b Forced
convection (Active dryers)
17
2.5.2. Mode of heating or operation 17
2.5.2a.
Direct mode
17
2.5.2b
Indirect mode 18
2.5.2c Mixed mode solar dryers.
2.5.2d
Hybrid solar drying:
2.6 Evaluation of Solar Dryers 19
2.7
Previous Works on Solar Dryers 19
2.7.1 Direct Solar mode Dryers 19
2.7.2. Indirect
solar mode dryer
22
2.7.3
Mixed mode solar dryers
26
2.7.4 Solar dryers with thermal
storage
29
2.8 Thermal Energy Storage 30
2.9 Wind Power Energy 32
2.10 Wind Generators / Wind Turbines
32
2.10.1.
The principle of wind turbine
33
2.11 Energy and Exergy Studies 34
2.11.1 The exergy efficiency 36
2.12 Gaps Identified in the Review 36
CHAPTER 3: MATERIALS AND METHODS
3.1
Materials Used for Construction
38
3.2
Design Consideration 38
3.3
Design Calculation
39
3.3.1 Collector design 39
3.3.1a
Determination of useful heat for drying
39
3.4 Sizing of the Collector
40
3.4.1
Collector orientation and angle of tilt (β) of the solar collector 41
3.5
Air Flow Requirement
41
3.6
Design of the Wind Generator Rotor
42
3.7 Experimental procedure
43
3.7.1.
Sample preparation
43
3.7.2.
Dryer evaluation tests
44
3.8.
Performance evaluation
48
3.8.1
Drying kinetics
48
3.8.2.
Collector and dryer efficiencies
48
3.8.3.
Energy Analysis
48
3.8.4.
Exergy analysis of the drying process 49
CHAPTER 4: RESULTS AND DISCUSSION
4.1 Description of the Developed Solar Dryers
51
4.2. Dryer Performance
57
4.3 Drying Kinetics of the treated Potato Slice
60
4.4 Energy Utilization
66
4.5. Exergy Performance of the Solar Dryers
69
CHAPTER 5: CONCLUSION AND RECOMMENDATIONS
5.1 Conclusion
72
5.2 Recommendations
73
References
74
Appendices
LIST
OF TABLE
4.1:
Design parameters for solar dryer with wind air generator 52
4.2:
Performance parameters for solar dryer with and without axial fan, with and
without storage Material and axial
fan.
62
4.3:
Effective moisture diffusivity of potato for all treatment. 67
LIST
OF FIGURES
2.1:
Global horizontal irradiation for Nigeria. Source: (Chukwujindu 2017) 13
2.2:
Classification of solar dryers (Hii et al., 2012)
16
2.3:
Experimental setup of a cabinet type drier (Sodha et. al., 1985) 20
2.4: Natural
convection solar dryer modified (Olalusi and Bolaji 2005) 20
2.5: Natural convection cabinet type
solar dryer (El-Amin et al (2015) 21
2.6:
Schematic of solar dryer with natural convective heat flow (Singh et al. 2004) 22
2.7: Direct solar dryer
(Boulemtafes-Boukadoum and Benzaoui 2011) 23
2.8:
Mix mode solar dryer integrated with thermal storage material (Ndukwu et al.,
2017) 30
2.9: Different types of thermal storage of
solar energy (Baladin 1999) 31
2.10:
Working principle of a wind turbine with savonious rotor (Ali, 2013) 34
3.1:
Isometric view of the developed prototype solar dryer with wind
generator
powered axial fan
46
3.2: Isometric view of the developed prototype
solar dryer without wind generator
(All
dimensions is the same with figure 3.1) 47
4.1: Variation of Solar Radiation intensity with
time for the days of the test 57
4.2: Variation of dryer and ambient
temperature (sunshine hours) with time of the day
(a-SD
1, b- SD2, c-SD4, d-SD5) 58
4.3: Variation of dryer and ambient Relative
humidity (sunshine hours) with time of the day
(a-SD
1, b- SD 2, c-SD4, d- SD5) 60
4.4: Air speed
variation with drying time
61
4.5: Variation of moisture content
of different treated potato with drying time for the five drying conditions ( a-SD 1, b- SD2, c- SD3, d-SD4,
e- SD 5) 65
4.6: Variation of drying rate of
different treated potato with drying time for the five drying
conditions
( a-SD 1, b- SD2, c- SD3, d-SD4, e- SD 5)
66
4.7: Variation of the inlet (exin)
and outlet (exout) exergy of the solar dryers for sunshine
periods
with time 70
4.8: Variation of the exergy loss
profile of the solar dryers for sunshine periods with time 70
4.9: variation of the exergy
efficiency (exe ff) of the solar dryers for sunshine periods with time 71
LIST OF
PLATES
1: Solar dryer with baffles (Slama
and Combarnous 2011) 24
2:
indirect mode natural cabinet solar dryer for drying of fish (Onyinge et al,
2014) 25
3: Indirect mode
forced convention solar dryer (Bolaji, 2005) 25
4:
indirect mode solar dryer (Khama et al 2016) 26
5: Mixed
mode solar dryer (Dhanushkodi et al., 2014) 27
6: Mixed
mode greenhouse solar dryer (EL
Khadraoui et al., 2015) 28
7:
Developed prototype
solar dryer with wind generator 53
8:
Developed prototype solar dryer without wind generator 54
9: Dried
potato samples under different treatments (A- untreated potato slice,
B- Blanched potato slice and C- potato
treated with salt solution. 68
CHAPTER 1
INTRODUCTION
1.1 BACKGROUND OF STUDY
Drying
is a method of removing moisture from a product in order to reach the desired
moisture content. The purpose of drying can be to extend storage life,
enhancement of product quality, encourage ease of handling of the product,
further processing, and sanitation (Mujumdar, 2007). Drying involves the
application of heat to evaporate moisture after its separation from the food
products. Therefore it is a combined simultaneous heat and mass transfer
operation for which energy must be supplied (Mohanraj and Chandrasekar 2009)
Removing moisture from products
prevents growth and reproduction of microorganisms like yeasts, bacteria, and
molds which cause decay and also reduces many of the moisture-mediated deteriorative
reactions. It also causes reasonable reduction in weight and volume, minimizes
packing, storage, and transportation costs and enables storability of products
under ambient temperatures. This process is important for developing countries,
(Sharma, 2009)). In the developing
countries; the traditional method of drying is open sun drying, which causes
food contamination and nutritional deterioration. Open sun drying is define as
the spreading of commodity in the sun with no protection on a suitable surface,
hanging it on eaves of buildings and trees, drying on the stalk by standing in
stalks or bundles for cereals (Murthy, 2009). Exposing crops directly to
sunlight, or more precisely ultra-violet radiation, can greatly reduce the
nutritional content of the food products such as vitamins in the dried product.
According to Fagunwa et al., (2009) and Chimi et
al, (2008), open sun drying is mostly practiced by the local farmers. Also,
according to Murthy, (2009), more than 80% of food produced in the developing
countries is being produced by small farmers in and they dry their food product
by open sun drying; this is because solar insolation is free but the end
product could be degraded which results in a poor quality of the end product
(Jairaj et al., 2009). Therefore the
ultimate goal of any drying processes is to produce a dried product of high
quality at a minimal cost whilst maintaining high throughput (Esmaiili et al., 2007). Generally, drying is an
energy-intensive process and currently, the emphasis is on the use of
alternative renewable energy sources for the safety of the planet. Drying of
agricultural products using renewable energy such as solar energy is
environmentally friendly and has a less environmental impact (Alonge and
Adebayo 2012). Additionally, Rande and Forsan (2007) stated that of all the
available energy for drying, solar drying is the most economical because it
provides harmless and pollution-free energy with the lowest cost. Therefore
efforts have been made by researchers to develop solar drying systems using
locally available material. The challenge was that solar energy is not
adequately available throughout the year. Hence a solar dryer that will dry
most agricultural products throughout the year with good results is not obtainable.
This is a problem especially for those areas closer to the coastal region like
southern part of Nigeria with the lower solar insolation, limited sunshine
hours and the ambient air temperature frequently interrupted by cold winds from
the Atlantic (Ndukwu et al., 2017).
Therefore there is the need to address this problem. Equally, studies have
shown that thermal storage material like charcoal, coal rocks, bricks,
concrete, phase change materials such as desiccants and eutectics materials
with high latent heat storage has been used to cushion the contests posed by the uncertainty of weather that makes
solar dryer operation irregular. Currently material with high latent heat
storage is becoming very attractive in solar drying as thermal storage material
because of its high energy density.
Another
problem of solar dryer application in most African countries is the method of
hot moist air evacuation from the drying chamber. This is because; most of the dryers are
passive solar dryers which take a longer time to move the moist air out of the
dryer compared to active solar dryers (Ndukwu et al., 2018). This makes drying to take longer period which may
lead to rewetting of the product that will not only increase the drying period
but also may increase the microbial load after drying (Ndukwu et al., 2017). According to Sweelem et al., (2013), effective drying air
must be hot, dry and moving. Low adoption of active solar drying in most cases
is because of the need to make the system electricity free because most active
dryers require electricity to drive the fan. While solar drying has been proven
to be advantageous in drying agricultural products, allowing the drying to take
longer period may be injurious to the quality of the final product. One of the
proffered solutions for electricity-free air movement is the use of wind
generator or vortex wind machine (Sweelem et
al., 2013). A wind turbine generator produces air power when the ambient
wind speed is higher than the cut in speed (Sweelem et al., 2013). This air power can be transmitted to the suction fan
component of the solar dryer to make it active without electricity.
With
the closeness of the research location not far from the coastal region
characterized by low sunshine period, application of high latent heat storage
material in combination with utilization of renewable energy sources like wind
generator to power the fan to achieve a quicker drying process will interest
farmers in adopting solar dryers in this region.
Drying
is an energy consuming process, and efficient energy utilization is of utmost
importance (Akubulut and Dumus, 2010). Therefore, various models have been
developed to estimate the energy of the drying system based on the first law of
thermodynamics. But, the weakness of the first law of thermodynamics is that it
does not give much fact on losses or the quality of the energy moving through
the thermal boundary, consequently this may give a false sense about the
efficiency of an energy conversion device (Prommas et al., 2010). This is because it does not provide a measure of how
closely the performance of a system approaches reality (Midilli and Kucuk,
2003). Lately, there is increasing attention in the combination of the first
and second laws of thermodynamics, embodied in the idea of exergy to analyze
energy systems. Exergy concerns the estimation of the performance of energy conversion
devices and processes, by observing their performance at different points in a order
of energy conversion stages. With this information, proper efficiencies can be
estimated and the process steps having the major losses identified thus
providing a more realistic view of the process. Since these steps are not
adequately defined by the first law of thermodynamics which states that energy
is completely conserved; it is very difficult to approach reality with the
first law of thermodynamics. This makes the application of exergy analysis a
necessity. Therefore, exergy which is based on the second law of thermodynamics
is suitable for the assessment of the efficiency of drying systems.
Therefore
this research will test high latent heat storage material capable of capturing
and storing thermal energy which can be utilized during the off –sunshine
period to continue the drying process. The drying operation will adopt active
air circulation with wind generators to make the entire drying operation
electricity free with all energy resources renewable. In the end, energy and
exergy analysis will be carried out to determine the energy efficiency of the
system.
1.2 STATEMENT OF PROBLEM
The
challenge of solar drying is on how to capture and store the solar thermal
energy to be used during off-sun-shine hours. Another problem of solar dryer
application in most African countries is the method of hot wet air evacuation
from the drying chamber because; most of the dryers are passive solar dryers
due to lack of cheap means to power the fans that will help in that case. This
makes drying to take a longer period that may lead to rewetting of the product
which will not only increase the drying period but may also increase the
microbial load after drying. This problem is mostly faced by countries not far
from the coastal areas like southern Nigeria with lower solar insolation. This
is because of the frequent interruption of the sunshine hours by the sea or
ocean wind and cloud cover. Therefore
research has introduced thermal storage to see if they can solve this problem.
Some of these materials include rock, pebbles, charcoal, bricks, desiccants and
other materials with high energy storage densities. Research on this aspect is
still on-going and in most cases specific to a particular environment due to
the uncertainty of weather. Although some research has used photo-voltaic cell
and electrical powered fan to power the fan or blower to drive the air flow
stream through the solar collector and drying chamber this adds extensively to
the cost of the entire dryer. Therefore a frictionless self-powered wind
machine has been suggested. This study will, therefore, test some materials for
capturing and storing solar heat and also test the frictionless wind generator
mechanism for powering the axial fan for air circulation.
1.3 OBJECTIVES
The
main objective of this research is to develop a wind-powered active solar dryer
integrated with glycerin (
) as thermal storage for drying
pre-treated potato. However, the research specific objectives include to:
1. Design
and fabricate a mixed mode solar dryer integrated with glycerin as thermal
storage material and with and without
wind generator
2. Study
the process variables (drying rates, kinetics, diffusivity, etc.) of the dried
potato slices.
3. Carry
out energy and exergy analysis of the
dryer
4. Determine
the drying efficiency of the dryers.
1.4 JUSTIFICATION
Solar energy offers a
good alternative and environmentally friendly energy for drying of crops
considering the environmental challenges faced by the use of fossil fuel based
dryers. However, solar dryers are faced with the problem of crop rewetting when
the condition is unfavorable mostly during off sunshine hours or delay in the
evacuation of accumulated moist air in the drying chamber, this will increase
the drying time and sometimes the product quality is compromised. However,
active solar dryers with electricity powered blowers have been designed to
solve part of the problem above. The challenge of its operation in most African
countries is lack of electricity to power them; nevertheless, the wind is
available and free everywhere with some areas in Africa having high wind
density. Therefore designing solar dryers with the axial fan powered by the
wind and also integrating them with thermal storage material will help in
solving the above-mentioned problems.
1.5 SCOPE OF STUDY
The
scope of this research is limited to the development and performance evaluation
of a mixed mode solar dryer powered by a wind generator tested with and without
glycerin as thermal storage material.
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