ABSTRACT
A solar ovenis a device that converts solar energy into useful heat in a confined space (oven chamber) which can be utilised for cooking and baking purposes.The oven consists of plane reflectors to concentrate the solar radiation on the collector.The heat gain is maximum if the collector and reflectors are continuously adjusted such that the incidence angle of the reflected and direct radiations are minimised. In practise, it is always difficult to manually track the movement of the sun and the use of trackers can be very expensive. As such, an analytical model was developed to evaluate the optimum monthly collector and reflector tilts for maximum output hen employing the single axis tracking mode. The operation of the different components of the oven was modelled using TRNSYS, Microsoft Excel and EES programs alongside solar data for Zaria. Optimisation of the design was carried out based onweather conditions prevalent on the average day of the design month i.e. the month with the least solar radiation. The tilt angles of collector and reflectors required for the optimum collection of solar irradiation for each month were obtained from the simulation results of the oven model carried out for 12 months of the year. The optimum collector area and insulation thickness were also obtained through parametric studies by varying the aforementioned parameters until a stagnation temperature of 100áµ’C was obtained for the average day of the design month. The simulation results for the design with different collector areas and insulation thickness show that an area of 0.49m2 and thickness of 0.12m yields a stagnation temperature of 100áµ’C. However, the stagnation temperature achieved was insensitive to larger values of the design parameters.
TABLE OF CONTENTS
Title Page…………………………………………….……………………………………………….i
Declaration ……………………………………………………………………………………………..ii
Certification ………………………………………………………………………………………….iii
Acknowledgements …………………………………………………………………………………………………………iv
Abstract ……………………………………………………………………………………………….v
Table of Content …………………………………………………………………………………………………………….vi
List of Figures ………………………………………….…………………………………………….ix
List of Tables ………………………………………………………………………………..……..xvi
List of Appendices ………………………………………………………………………………….xvii
Nomenclature ………………………………………………..……………………………………..xviii
1.0 INTRODUCTION…………………………………………………………………………………………………….1
1.1 Background………………………………………………………………………………………………………………2
1.2 Statement of the Problem…………………………………………………………………………………………..3
1.3 The Present Work……………………………………………………………………………………………………..4
1.4 Aim and Objectives……………………………………………………………………………………………………5
1.5 Justification of the Work……………………………………………………………………………………………6
2.0 LITERATURE REVIEW………………………………………………………………………………………….7
2.1 Preamble………………………………………………………………………………………………………………….7
2.2 Solar radiation…………………………………………………………………………………………………………..7
2.3 Applications of solar radiation…………………………………………………………………………………..8
2.3.1 Solar power application…………………………………………………………………………………………….8
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2.3.2 Solar thermal applications………………………………………………………………………………………….9
2.3.3 Solar powered cooling systems…………………………………………………………………………………10
2.4 Solar cookers…………………………………………………………………………………………………………..11
2.4.1 Solar box oven……………………………………………………………………………………………………….11
2.4.2 Parabolic cookers……………………………………………………………………………………………………12
2.4.3 Panel cookers…………………………………………………………………………………………………………13
2.4.4 Heat accumulating solar cookers………………………………………………………………………………14
2.5 Historical background……………………………………………………………………………………………..15
2.6 Review of past works……………………………………………………………………………………………….16
2.7 Theoretical background…………………………………………………………………………………………..19
2.7.1 Incident radiation……………………………………………………………………………………………………19
2.7.2 Reflection of radiation…………………………………………………………………………………………….20
2.7.3 Transmission of radiation………………………………………………………………………………………..21
2.7.4 Absorption……………………………………………………………………………………………………………..21
2.8 Performance evaluation of solar oven……………………………………………………………………….22
2.8.1 Test protocol………………………………………………………………………………………………………….23
3.0 MATERIALS AND METHODS………………………………………………………………………………25
3.1 Description of Solar Oven System…………………………………………………………………………….25
3.2 Materials ………………………………………………………………………………………………………………..27
3.2.1 Reflectors………………………………………………………………………………………………………………27
3.2.2 Glazing………………………………………………………………………………………………………………….28
3.2.3 Insulation ………………………………………………………………………………………………………………28
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3.2.4 Casing ………………………………………………………………………………………………………………….29
3.3 SolarData……………………………………………………………………………………………………………….29
3.4 Design theory…………………………………………………………………………………………………………..30
3.4.1 Tracking mode……………………………………………………………………………………………………….31
3.4.2 Angle of incidence of reflected radiation……………………………………………………………………32
3.4.3 Exchange and shading factors…………………………………………………………………………………..33
3.4.4 Energy absorbed by collector……………………………………………………………………………………39
3.4.5 Heat balance equations……………………………………………………………………………………………40
3.5 Solar Oven Model and Optimisation ……………………………………………………………………….42
3.5.1 Thermal system modelling……………………………………………………………………………………….43
3.5.2 Determination of optimum collector slope…………………………………………………………………45
3.5.3 Determination of reflector tilt angle………………………………………………………………………….46
3.5.4 Determination of Design month………………………………………………………………………………..46
3.5.5 Determination of optimum collector area and insulation thickness………………………………..47
3.5.6 Temperature calculations…………………………………………………………………………………………48
3.6 Production of Oven Components……………………………………………………………………………..48
3.6.1 Insulated oven box………………………………………………………………………………………………….48
3.6.2 Absorber ……………………………………………………………………………………………………………….49
3.6.3 Level tray………………………………………………………………………………………………………………49
3.6.4 Glazing …………………………………………………………………………………………………………………49
3.6.5 Reflectors………………………………………………………………………………………………………………49
3.6.6 Door……………………………………………………………………………………………………………………..50
3.6.7 Support frame…………………………………………………………………………………………………………50
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3.7 Simulation of Solar Oven Chamber temperature………………………………………………………50
3.8 Experimental Setup……………………………………………………………………………..51
3.9 Test Procedure……………………..……………………………………………………………51
3.10 Error Analysis…………………………………………………………………………………52
4.0 RESULTS AND DISCUSSION……………………………………………………………..54
4.1 Optimisation of Design Parameters………………………..……………………………………544.1.1 Monthly optimum collector slope……………………………………………………………..54
4.1.2 Monthly optimum tilt angles of reflectors R1 and R2…………………………………………….56
4.1.3 Design month…………………………………………………………………………………..63
4.1.4 Optimum collector area and insulation thickness………………………………………………………..64
4.2 Experimental Results……………………………………………………………………………………………….67
4.2.1 Observations………………………………………………………………………………………………………….68
4.2.2 Error analysis…………………………………………………………………………………………………………77
4.2.3 System performance measurement……………………………………………………………………………77
4.2.4 Cost evaluation……………………………………………………………………………………………………….79
5.0 SUMMARY, CONCLUSIONS AND RECOMMENDATIONS…………………………..81
5.1 Summary………………………………………………………………………………………….81
5.2 Conclusions……………………………………………………………………………………..82
5.3 Recommendations……………………..………………………………………………………..83
REFERENCES…………………………………………………………………………………….84
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APPENDICES……………………………………………………………………………………..88
CHAPTER ONE
INTRODUCTION
1.1 Background
Energy is the focal point of all human activities; it is the basis of industrial civilization. Without energy, modern life would cease to exist. In the past, the demand for energy sources was minimal because it was primarily used for cooking and local production. But as time went on, population increase and technological advancement led to more demand for energy. The major sources of energy are the conventional sources, which include: fossil fuels, and nuclear fuels. Fossil fuels, which include petroleum, coal, and natural gas, provide most of the energy need of modern industrial society. Other uses are found in the transportation, residential heating, and electric-power generation. Nuclear fuels are used to generate electricity, but it is utilised mainly in the developed countries due to high level of supervision and maintenance required. The non- conventional (renewable) sources of energy include: hydroelectric power, solar energy, wind energy, biomass, ocean thermal energy, tidal energy, and geothermal energy, but the potential of these sources is still underutilised because they are much more expensive to harness than energy derived from fossil fuels. Hydroelectric power requires a large capital investment, so it is often uneconomical for a region where coal or oil is cheap. As such, they contribute a little percentage to the massive energy requirement of the world population. However, the fear of depletion of fossil fuels due to the fast rate of consumption has provoked further development of these alternative energy sources, such as solar energy.
Household energy need is one of the biggest issues in the daily lives of people around the world. The most important energy-consuming activities in most households are cooking, lighting
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and use of electrical appliances. Cooking accounts for a staggering 91 percent of household energy consumption, lighting uses up to 6 percent and the remaining 3 percent can be attributed to the use of basic electrical appliances such as televisions and pressing irons (Temilade, 2008). Cooking is an activity that must be carried out almost on a daily basis for the sustenance of life. An enormous amount of energy is thus expended regularly on cooking. Cooking may be classified in four major categories based on the required range of temperature, viz. baking (85-90°C), boiling (100 to 130°C), frying (200 to 250°C) and roasting (more than 300°C).
The primary household energy carriers are fuel wood, kerosene, electricity and liquefied petroleum gas (LPG). In Nigeria, Fuelwood is the most widely used, supplying over 80 percent of household energy, while less than 20 percent is supplied by the other sources and complemented by small quantities of coal and charcoal (Temilade, 2008). Fuelwood is often collected from the local environment in rural areas or purchased through markets in urban areas. Renewable energy alternatives include biogas, which is used for household heating, cooking and lighting, as well as agricultural and industrial activities.
Solar radiation presents an alternative energy source for a variety of applications. Solar radiation has been identified as the largest renewable resource on earth. The maximum intensity of solar radiation at the earth’s surface is about 1.2kW/m2, but it is encountered only near the equator on clear days at noon. Under these ideal conditions, the total energy received is from 6 – 8 kWh/m2 per day (Abdulrahimet al., 2011). Its intensity varies according to season, geographical location, and orientation of the collector. Solar energy is not available continuously because of the day/night cycle and cloud cover.
Solar cooking offers an effective method of utilising solar energy for meeting a considerable demand for cooking energy and hence, protecting the environment. Fortunately,
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Nigeria is among the twenty one countries with the highest potential for solar cooking (www.solarcooking.wikia.com). Nigeria lies within a high sunshine belt and thus, has an enormous solar energy potential. The mean annual average of total solar radiation varies from about 3.5 kWhm–2day-1 in the coastal latitudes to about 7 kWhm–2day-1 along the semi arid areas in the far North(Sambo, 2009). The country receives an average solar radiation at the level of about 19.8 MJm–2 day-1. Average sunshine hours are estimated at 6hrs per day. Solar radiation is fairly well distributed. The minimum average is about 3.55 kWhm–2day-1 in Katsina in January and 3.4 kWhm–2day-1 for Calabar in August and the maximum average is 8.0 kWhm–2day-1 for Nguru in May (Sambo, 2009).
1.2 Statement of the Problem
Cooking is a major necessity for people all over the world. The problem arises when cooking fuel is either scarce or highly expensive. Around the globe, hundreds of millions of people have limited access to cooking fuels (www.solarcookers.org). In situations where electricity and gas are not affordable, charcoal and fuel wood are the major substitutes and even charcoal can be very expensive. So fuel wood is the cheapest alternative left. Families either have to walk for hours to collect cooking wood, or spend the little money they have on fuel, leaving less money to buy food.
The problems related to the use of biomass as an energy source have been an issue of concern for more than three decades. The traditional stoves commonly used for burning biomass have long been found to be highly inefficient and to emit copious quantities of smoke due to the incomplete combustion of fuels. This inefficiency has also consequences on the environment, since intense collection of fuel wood has resulted into deforestation in highly populated areas. The
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use of such fuels has also adversely affected health. Fires release gasses into the air. This smoke, filled with particulates, is bad for the environment, but it is even worse for the people who are breathing that air. When people use open fires to cook indoors, they end up inhaling micro particles that can cause all sorts of health problems, including both lung and heart disease. Every year, the smoke from open fires and traditional stoves kills 1.5 million people (www.solarcookers.org).
Production and consumption of almost any type of energy have environmental impacts. Harvesting of fuel wood, in particular, contributes to deforestation, soil erosion, and desertification. In Nigeria, harvesting of fuel wood contributes to deforestation at a rate of about 400,000 hectares per year. If this trend continues the country’s forest resources could be completely depleted by 2020 (Oleg and Ralph, 1999).
For the rural and urban poor, connection to the electricity supply is often prohibitively expensive or unavailable, even though the price of electricity itself may be low enough to encourage a switch from other fuels. As at the year 2003, less than 45% of the Nigerian population had access to electric power (Suleiman, 2011).
1.3 The Present Work
The present work focuses on designing, simulating and constructing a solar oven that can augment conventional ovens in rural areas. The oven consists of plane reflectors to concentrate the solar radiation on the collector. The oven box is rotated about the vertical axis in order to track the movement of the sun (east to west). Oven inclination is also adjusted monthly to ensure optimum collection of solar radiation. Much emphasis was laid on the materials with suitable properties needed to give high temperatures required for most cooking operations as well as
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rigidity needed to keep the assembly in place in case of wind. Numerical simulation of the design model was carried out using transient system simulation programme (TRNSYS), Engineering equation solver (EES) and Microsoft excel to obtain optimum design parameters which would yield acceptable performance. The solar oven was then fabricated and finally subjected to experimental tests to validate the results obtained from the simulation.
1.4 Aim and Objectives
The aim of the present work is to design, simulate, construct and test an optimized solar box oven equipped with two plane reflectors that would be technically efficient, user friendly and cost effective in Zaria, Nigeria.
The specific objectives of the work are:
i. Select appropriate materials (bearing in mind cost, availability and durability) for the various parts of the solar oven.
ii. To study the influence of the following parameters on the solar oven’s performance:
 Collector slope
 Angular orientation of the reflectors
 Area of collector
 Insulator thickness
iii. Simulate its performance and validate it with experimental results from a test.
iv. Estimate the cost of a prototype of the system.
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1.5 Justification of the Work
i. The energy supply mix in Nigeria is presently dominated by oil and gas. In spite of a considerable solar resource in the country, it is still underdeveloped (Etiosaet al., 2008). Solarenergy contributes very little to Nigeria’s energy mix as it is currently at the early stage of development. Energy/electricity insufficiency in Nigeria can be alleviated through exploitation of the enormous renewable resources available to the country.
ii. It is normally estimated that areas of the world lying between latitudes 35°N and 35°S of the equator and which have at least 2000 hours of bright sunshine per year are ideal for the utilisation of the sun’s energy. Nigeria, located between latitudes 4°N and 14°N of the equator is very much within this area (Abdulrahimet al., 2011). A modest estimate of the solar energy potential in Nigeria with 5% device efficiency is put at 15×1014 kJ of useful energy annually. This translates to about 258.62 million barrels of oil equivalent or 4×105GWh of electricity production annually (Sambo, 2009).
iii. With the high cost of cooking fuels in Nigeria and the epileptic supply of electricity, majority of Nigerians are left with no option but to use fuel wood for cooking. The health and environmental hazards associated with this practice cannot be over emphasized. As such, solar cooking would provide an alternative to reduce total dependence on fuel wood.
iv. The use of solar energy for cooking will have a great impact in reducing electricity consumption. This energy saving could be used for other activities.
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