Solar Oven Final Report

University  of  Arizona   Tucson,  AZ,  85716

S.A.C.R.A.W Solar Oven Prepared for: Dr. Stanley Pau

 

 

 

Jack Speelman Rebecca Nelson Paola “Andy” Lopez Lorin Greenwood Stephanie Gilboy October 21st 2012

Figure 1: Shows the team members of Team S.A.C.R.A.W standing alongside the final solar oven on Solar Oven Testing Day.

 

 

 

Team S.A.C.R.A.W Solar Oven 2

 

Table of Contents

 

 

Cover 1 Table of Contents 2 Executive Summary 3 Introduction – Motivation/Background/Key Terms 4-5 – Criteria and Constraints 5-6 Main Body – Functional and Design Requirements 6-7 – Design Theory and System Model 7-10 – Design Description – Conceptual Design 11-13 – Design Description – Final Design 14-17 – Design Justification 17-18 – Evaluation of Results 18-19 – Test Procedure 19-20 Design Critique and Summary 20-22 Appendix – First Oven Spreadsheet Data and Design/Drawing 22-24 – Final Oven Spreadsheet Data and Design/Drawing 25-27 – References 28

 

 

 

Team S.A.C.R.A.W Solar Oven 3

Executive Summary

The objective of the solar oven project was to design, build, and test a productive solar oven that

could reach an interior temperature of 100˚C. This was obtained through converting solar energy,

also known as electromagnetic energy, into thermal energy. The first law of thermodynamics

was utilized by understanding that energy cannot be created or destroyed but could be

transformed and used to heat the interior of the oven.

 

Two solar ovens were constructed in order to fully maximize the temperature inside the oven

chamber. The first oven was used as a prototype and research tool in order to build an oven that

could reach the optimum temperature calculated. The initial oven was predicted at 170.26˚C but

only reached an interior temperature of 85.6˚C. This produced a preforming index number of

1.02 and a cost index of 6.05˚C/dollar. The improved oven had a predicted temperature of

176.58˚C using the ambient air temperature and solar density provided. The oven reached an

interior temperature of 99.6˚C. The performing index for the second oven was calculated to be

1.28 and a cost index of 4.34˚C/dollar.

 

 

 

Team S.A.C.R.A.W Solar Oven 4

Introduction

Motivation:

• Learn the concept of team work and how to work together with other people to

achieve a common goal

• Gain proficiency in Excel, Solid Works, and basic solar oven knowledge.

• Acquire knowledge of the transformation of solar energy to heat.

• Learn the basics of the design and construction process

Background:

The main goal of the solar oven project was to find out the best way to change solar

energy into thermal energy. To do this, teams needed to know the first law of thermodynamics.

The first law states that energy cannot be created or destroyed, but can be changed from one

form to another. In the Solar Oven Theory, the energy that is put into the oven should equal the

energy that comes out (Ein=Eout). Therefore, the solar energy in joules should equal the thermal

energy in joules. The oven does this by taking the energy from the sunlight and transferring it

into heat. Since the energy in equals the energy out it allows the equation stated above to be

true. Knowing that the power is equal allows Team S.A.C.R.A.W. to find the temperature of the

oven chamber.

Mathematics for the Solar Oven with Key Terms:

Predicted Temperature:

 

 

 

Team S.A.C.R.A.W Solar Oven 5

€

Tio = Tambient + IoAw ⋅ G⋅ τ

n ⋅ a Usb ⋅ Asb +Uw ⋅ Aw( )

 

Variables:

Tio =the temperature inside the cooking chamber

Tambient= the outdoor temperature on the day tested

G= the gain from the reflectors

Uw= the heat transfer coefficient of the window

Aw= the area of the window

Usb= the heat transfer coefficient of the sides and bottom of the cooking chamber

Asb= the total area of the sides and bottom of the cooking chamber

Constants:

a= absorption coefficient of the cavity walls

τ= the optical transmission coefficient of the cavity walls

Io= the incident solar power density

Performing Index:

 

Variables:

Tio =the temperature inside the cooking chamber

Tambient= the outdoor temperature on the day tested

Tpredicted= the predicted temperature of the oven

Cost= the amount of money and labor for the solar oven in dollars

(Source: “Solar Oven Basics” Engineering 102)

 

 

 

Team S.A.C.R.A.W Solar Oven 6

Criteria and Constraints:

• Cooking oven must be equal to 1000 cm3

o Length (L) and height (h) need to be a minimum of 5cm

• The cooking oven window needs to be a square (W=L)

• The oven must have access for a digital thermometer and have a rack to support a biscuit

• There must be two calculated Performing Index’s calculated PISTOD and PIcost

• The maximum M/L ratio is 3

• The minimum final oven temperature is 100ᵒ C

• Optimal final temperature is over 200ᵒ C

• Focusing lenses and parabolic designs are not allowed

(Source: Solar Oven Design Project and Report Guidelines)

Main Body

Functional and Design Requirements The overall purpose of constructing the solar oven is to convert the absorbed solar energy into

heat. Different requirements of the oven were given in order to fulfill the purpose of constructing

and designing the solar oven. Functional requirements of the oven are needed in order for the

oven to actual work. Functional Requirements for the project include:

• The oven chamber must reach a minimum temperature of at least 100 degrees Celsius.

This is the minimum temperature that is needed to cook the biscuit inside the solar oven

chamber

 

 

 

Team S.A.C.R.A.W Solar Oven 7

• Using exactly four reflectors, sunlight needs to deflect into the Mylar windows at a

certain angle in order to achieve maximum temperature.

• Insulation is needed to surround the chamber to avoid loosing any form of heat.

• The four reflectors given need to be placed at a certain angle to not let any of the sunlight

be steered away from the Mylar window.

• An object or stand supporting the oven may be used in order to have the window be

exactly perpendicular to the sunlight. This way, all the sunlight can directly enter the

window.

Design requirements are given to avoid any advantage towards any oven. These requirements are

given in order for every oven to be at the same advantage. Design Requirements for the project

include:

• The oven chamber dimension should equal exactly 1000 cm3.

• Access to the inside of the solar oven should be fairly simple. Different methods to access

the oven include: lifting the top, door-opening mechanism, the sliding tray mechanism,

and many other methods.

• A hole or small opening for the thermometer to have access to the oven chamber

• The ratio of the reflectors and the width of the widow (M/L) should have a ratio of three

or less.

• The reflectors should be flat and straight; not parabolic. Having parabolic curves could

result in an explosion in the solar oven.

Design Theory and System Model

To conquer the solar oven, Team S.A.C.R.A.W. had to learn more about the theory

behind the solar oven. The knowledge learned can be implemented into constructing the most

 

 

 

Team S.A.C.R.A.W Solar Oven 8

efficient solar oven. The different dimensions of the oven are variables that affect how much

solar energy is converted heat inside the solar oven chamber. By knowing how these variables

affect the temperature allowed Team S.A.C.R.A.W. to reach optimum temperature inside the

oven. The equation predicting the temperature inside the chamber is:

Figure 2: Shows the equation to predict the temperature inside the solar oven chamber

 

 

In the equation above, the variables that have the most effect on the predicted temperature (Tio)

are the variables that are related to the dimensions of the solar oven chamber (Asb and Aw). Asb

represents the total surface areas of the side and bottoms of the solar oven chamber. Aw

represents the surface area of the window that heat is transferred through. Since a given

constraint of the oven is that the volume has to equal 1000 cm3, increasing the Aw would result in

decreasing the Asb. Since Aw appears in both the denominator and numerator in the equation,

increasing the value would have no affect in the Tio , but decreasing the Asb would result in a

higher Tio since the variable only appears in the denominator.

In the equation, G represents the gain of heat that the solar oven reflectors acquire. The

following equation is used to calculate the value of G:

Figure 3: Shows the equation to solve for G, and the variables that affect the value

 

As shown in Figure 3, the number of reflectors (represented by r) affects the overall gain.

Without any reflectors, the gain would equal just one. A higher M/L ratio for the oven would

also increase the gain value. The maximum value of M/L that is allowed is three, and as shown

€

Tio = Tambient + IoAw⋅ G⋅ τ

n⋅ a Usb⋅ Asb +Uw⋅ Aw( )

 

 

 

Team S.A.C.R.A.W Solar Oven 9

in Figure 3, the highest M/L would be most ideal. However, as shown in the table below, having

a higher M/L ratio results in a smaller alpha value.

Table 1: Shows the M/L ratio and the alpha and omega angle associated with each ratio

M/L α Ω 3 16.31° 106.31° 2 21.47° 111.47° 1 30° 120°

 

The alpha angle in the higher M/L ratio is smaller than the smallest M/L ratio, which is one.

However, when multiplying the M/L ratio by the sin (α), the highest value still comes out for the

M/L value that equals three. The highest M/L value is preferred for constructing the oven

because it will allow for the largest reflector gain for the oven.

Two values in the solar oven predicted temperature equation depend on the weather:

temperature (Tambient) and the solar irradiance of the sun (Io) at that time of day. If the

temperature outside is low, the Tambient will also be low, which would result in a lower Tio. If the

sky is cloudy and hardly any sunlight is penetrating through, the solar density will be low, which

would also result in a lower Tio. The weather is a major component in a higher Tio. Because

Arizona relatively tends to have high temperatures and large amounts of sunlight, both values

should be high.

Two other given constraints in the solar oven predicted temperature equation are τ

(optical transmission coefficient of window) and a (absorption coefficient of the cavity walls).

Team S.A.C.R.A.W wanted as much power to be absorbed into the solar oven chamber as

possible, which would result in a higher Tio To achieve such results, both τ and a have to be as

close to one as possible. The optical coefficient of the window depends on the type of window

material used. An ideal window material used for higher τ would have to be highly transparent

 

 

 

Team S.A.C.R.A.W Solar Oven 10

across the visible and near-infrared spectrum. The absorption coefficient of the cavity walls (a)

depends on the color used. The color black absorbs the highest amount of sunlight in comparison

with others colors, and thus has a higher absorption coefficient.

In the solar oven chamber predicted temperature equation, having a low U (overall heat

transfer coefficient) would result in a higher predicted temperature. The value of U is inversely

related to the thermal resistance to the flow of heat energy (R). The equation below shows the

relationship between both values:

Figures 4: Shows the relationship between the heat transfer coefficient and the thermal resistance

 

The equation shows that having multiple materials with higher thickness (x) and higher thermal

conductivity (k) values result in a higher R-value, which results in a lower U value. Having more

insulation materials (such as cardboard, foam, newspaper) decreases the overall heat transfer and

increases the Tio.

One of the main goals of Team S.A.C.R.A.W for constructing the solar oven chamber is

to have the actual temperature acquired equal to the predicted temperature calculated. However,

the task is nearly impossible. Different sources of error can prohibit Team S.A.C.R.A.W from

achieving the desired goal. One of the main errors is not having the dimensions of the solar oven

match up with the ones calculated. For example, if Team S.A.C.R.A.W calculated the area of the

window to be 0.02 m2 and instead constructed a window with an area of 0.018 m2, the result

would be an actual value of Tio that differs from the calculated Tio. Another source of error

includes the deterioration of the materials due to the heat. The materials to construct the oven

must be careful considered. If the materials used are not able to withstand the heat and melt, the

solar oven’s durability will also decrease.

 

 

 

Team S.A.C.R.A.W Solar Oven 11

Design Description – Conceptual Design

Figure 5: Shows the outer view of the first oven Figure 6: Shows the inside view (the insulation) of the first solar oven constructed

 

constructed.

 

 

 

 

The first goal for Team S.A.C.R.A.W was to generate a solar oven that met the minimum requirements given by the instructor. Since every solar oven group had two official

trials to test out the ovens, Team S.A.C.R.A.W.s’ main focus was to achieve the optimum

temperature possible with the given constraints. The variables in our oven focused around the

dimensions of the oven chamber. The following table lists the variables that were adjusted for the

first solar oven.

Table 2: Shows the design variables and measurements for the first solar oven constructed

Design Variables Measurements

Aw 0.01 m2

0.92

a 0.9

r 0.7

40

Usb 0.642192854

Asb 0.05 m2

 

 

 

Team S.A.C.R.A.W Solar Oven 12

Io 691 W/m2

40 degrees

Tambient 29.4 degrees Celsius

G 5.205335351

 

The materials that were given by the instructor to construct the oven include:

cardboard, Mylar sheets, black paper, scissors, and a thermometer. The cardboard given was

used to construct the four reflectors, the solar oven chamber, and the outer box where the solar

oven chamber and insulation was kept. Black Duct tape was used to connect the solar oven

chamber to the lid of the outer box. To get to the oven chamber, the reflectors and Mylar sheets

where lifted from a hole centered in the top lid. Inside the chamber, there was a rack that was

intended to hold to biscuit in place. To position the angle of the oven in order for the oven

chamber to absorb optimum heat, backpacks and notebooks were used.

Two Mylar sheets were used and were on top of one another. They were placed

directly above the solar oven chamber and were connected to the reflectors. Having more Mylar

sheets minimized heat losses through the window, while still allowing solar energy to be

transmitted into the oven chamber.

Four reflectors were used and were covered with aluminum foil. Aluminum foil was

used because the material is low-cost and effective. Despite trying to have a smooth aluminum

foil covering the reflectors, Team S.A.C.R.A.W had to take into account the wrinkled sheets,

which affects the overall temperature inside the oven. The shape of the four reflectors was a

trapezoid, and to keep things simple, the height of each reflector was 30 cm. Having a trapezoid

shape for the reflectors allowed no gaps between the reflectors, a problem that would arise if

rectangular reflectors were used. The base of the reflectors was 10 cm wide, conforming to the

 

 

 

Team S.A.C.R.A.W Solar Oven 13

dimensions of the width of the mylar windows. In order to maintain a M/L ratio of 3 or less, the

area of the window had to be 10 cm by 10 cm. To keep with the constraint of having a solar oven

chamber with a volume of 1000 cm3, the solar oven ended up being a cube.

To maximize insulation, Team S.A.C.R.A.W built a huge outer oven to store all of

the insulation. The dimensions of the outer box were 0.62×0.62×0.1 all measured in meters.

However, the insulation material used – newspaper and printer paper – was all wrinkled up and

was not arranged in an organized matter (as shown in Figure 4). However, having high

insulation, allowed for a higher Uxb value, which increases the Tio value overall. Also, to not let

any heat escape the sides of the oven, the thermometer was placed inside the oven. Having that

particular setup prevents obstruction of window and loss of heat through mylar window.

To obtain the dimensions for the oven, Team S.A.C.R.A.W worked from the inside

out, starting with the dimension of the solar oven chamber. To keep things simple, the solar oven

chamber was made into a cube, and from those dimensions, the height of the reflectors was

determined. Having a maximum volume of 1000 cm3 and a M/L ratio of less than three put a

constraint on the oven since it prohibited from further increasing the height of the reflector.

 

 

 

 

Team S.A.C.R.A.W Solar Oven 14

Design Description – Final Design

Figure 7: Shows the construction of the new reflectors Figure 8: Shows the side view of the for the final oven final solar oven on Solar Oven Testing Day

 

 

 

 

 

 

The first solar oven testing allowed Team S.A.C.R.A.W to see what adjustments were

needed for the second oven. Needless to say, many adjustments were done. A new goal was

created – to reach the minimum temperature required, which was 100 degrees Celsius. Also, the

group strived to try to achieve a higher Performance index by reducing the total materials used,

thus reducing the total performing index. To decrease the cost without falling under the

minimum temperature required (100 degrees Celsius), Team S.A.C.R.A.W. took the following

steps: create a smaller insulation box, create smaller and more efficient reflectors, and improve

the design of cooking chamber in order to maximize the total volume. The following variables in

the table below show the values used to calculate the predicted temperature

Table 3: Shows the list of variables used to predict the temperature inside the oven chamber.

Design Variables Measurements

Aw 0.013225 m2

0.92

a 0.9

 

 

 

Team S.A.C.R.A.W Solar Oven 15

40

Usb 0.642192854

Asb 0.0219206522 m2

Io 619 W/m2

40 degrees

Tambient 29.4 degrees Celsius

G 5.205335351

 

Somehow, the first solar oven had dimensions that did not match up with the dimensions

that Team S.A.C.R.A.W. calculated. The group that dissected the oven noticed that the alpha

angle did not match up with the value that Team S.A.C.R.A.W had calculated. The angle value

that Team S.A.C.R.A.W that used based off of the M/L value, which was equal to three. The

group that dissected Team S.A.C.R.A.W.s’ oven measured out the angle to be 15.687 degrees.

The angle that Team S.A.C.R.A.W. calculated based off the M/L ratio was 16.31. Also, the M/L

ratio (that the group who dissected Team S.A.C.R.A.Ws’ oven measured out) was higher than

three. Knowing that these dimensions were inaccurate from the proposed dimensions given by

Team S.A.C.R.A.W, Team S.A.C.R.A.W focused more on accurately measuring out the

dimensions of the oven and making sure that all of the dimensions calculated for the oven

matched up to the actual oven.

Instead of constructing a cubic oven chamber, a rectangular-shaped oven was generated.

Team S.A.C.R.A.W decreased the height of the oven chamber to increase the area of the top and

bottom sides of the oven chamber. Since the width and length of the Mylar sheets are correlated

with the dimensions of the top lid of the chamber oven, the area of the window also increased by

making the oven chamber rectangular. The length of the square Mylar sheet for the new oven

 

 

 

Team S.A.C.R.A.W Solar Oven 16

was determined to be 11.5 cm, which also increased the height of the trapezoidal reflectors to

34.5 cm (to keep in with the M/L ratio of 3).

During the trial for the first solar oven, Team S.A.C.R.A.W noted that tape used did not

properly hold the oven in its place. A new brand of tape was bought, Gorilla duct-tape, which

withstood the heat and did not melt or peel off when exposed to the sun. Although it increased

the Performance Index cost since it was more expensive than the previous adhesive, this new

duct-tape improved structural rigidity and high-temperature performance. The duct-tape was

applied along the sides and corners of any joint component of the oven, which resulted in a

stronger structure and prohibited any air from escaping.

Instead of using wrinkled up newspaper and printer paper for insulation, Polyurethane

insulation foam was used. This foam did a better job of trapping heat and hardly left any room

for letting heat escape. The foam also had greater thickness than the newspaper and higher

thermal conductivity, thus increasing the resistance to escaping heat. However, it was discovered

afterwards that the foam expanded during testing. This increased the dimensions of the outer box

and may have possibly decreased the volume of the solar oven chamber.

The last improvement done on the final solar oven would be the dimensions of the

reflectors. Since the alpha angle was off in the first solar oven, a protractor was used to insure

that the angle matches to the one calculated by Team S.A.C.R.A.W. These new reflectors were

longer, were angled correctly to match the alpha angle calculated (16.31 degrees Celsius), and

had a higher reflectivity (aluminum foil was much smoother). Also to avoid loosing more heat,

the chambers’ sides were still connected with the reflectors when they were cut out. This way,

instead of loosing heat between the gaps of the reflectors and chamber, hardly any heat would be

lost. To have access to the insides of the chamber, the chamber and reflectors were lifted (since

 

 

 

Team S.A.C.R.A.W Solar Oven 17

they were connected). The Mylar sheets were placed from the top and were put at the bottom of

the reflectors. The cut out of the Mylar sheets had a slightly larger area than the window

dimensions, allowing the extra surface area on the Mylar sheets to connect the mylar window to

the top of the solar chamber (area where the solar chamber and reflectors meet).

Much more care and accuracy was placed into constructing the final oven in comparison

with the first oven. Also, to absorb more heat, the inside solar chamber was painted black. Black

is known to absorb the most amount of heat in comparison with other colors. Team

S.A.C.R.A.W also made the outer chamber more aesthetically pleasing by decorating it with red

paint.

Design Justification

To keep the first solar oven simple, Team S.A.C.R.A.W. made the solar oven chamber,

the basis for the construction of the oven, a cube. Team S.A.C.R.A.W.s’ main focus was on

simplicity and the performance of the oven, thus generating an oven with large reflectors and a

cubic solar oven chamber. With those factors in mind, Team S.A.C.R.A.W designed the oven

and constructed the oven based on those parameters. An example of a performance-orientated

component of the oven was the insulation. Team S.A.C.R.A.W decided to use as much insulation

as possible to achieve optimum temperature and a greater performance overall. Unfortunately,

having more insulation in the outer box chamber resulted in more material usage and more open

space for heat to escape (a factor that was not thought of until the day of solar oven testing). In

addition, the size of the reflectors was maximized in relations with the width of the window

(M/L = 3). Both actions were done without a thought about the performance index cost.

 

 

 

Team S.A.C.R.A.W Solar Oven 18

Once the oven was tested and dissected by another team, Team S.A.C.R.A.W noticed that

keeping the oven simple was not a method to achieve maximum performance. The overuse of

materials and lack of accuracy of the dimensions made the performance index relatively low. As

a result, the oven had the lowest temperature acquired out of all the other ovens in the class. For

the second oven, the team decided to replace many materials with more durable options and

made sure that the oven matched the dimensions that were calculated. Less cardboard was used

on the solar chamber and outer chamber, and also, different insulation was used. However, to

obtain a higher temperature, the size of the reflectors was increased. Increasing reflector size

resulted in a higher area of window and a lower surface area of the sides and bottom of the

chamber. The outer chamber box was decreased to reduce the insulation and the new insulation,

foam, did a better job of not letting heat escape than the newspaper and printer paper. The

performance index cost reduced with the usage of less material for the first oven. Also, the

overall performing index of the new solar oven was higher than the first solar oven.

 

Evaluation of Results

Due to the cloudy day and lack of sunlight, the final test did not yield results consistent

with the teams’ expectations and did not verify the final design process. The solar power density

given before the Solar Oven Throw down did not match up with the actual solar density value of

the weather. The result caused the predicted temperature calculated beforehand to be much

higher than the one actually calculated on that day (calculated to be around 109 degrees). The

temperature of the final oven was 99.6 degrees Celsius with a performing index of 1.28. The

predicted temperature of the oven on that day was calculated to be 176.58 degrees Celsius. The

large difference in predicted and actual temperature resulted in a lower performing index.

 

 

 

Team S.A.C.R.A.W Solar Oven 19

However, the main focus for the second design of the solar oven was to improve on the

performing index and durability of the oven. The first oven had a performing index of 1.02, a

predicted temperature of 170.26 degrees Celsius, and an actual temperature of 85.8 degrees

Celsius. No issues regarding the weather occurred on the first solar oven trial day – the day was

filled with sunlight and heat. Although the final oven did not reach the desired 100 degree

Celsius temperature, the final oven had a greater performance and better durability than the first

oven, which was the overall goal for Team S.A.C.R.A.W.

 

Test Procedure

Table 4: Shows the values measured in both ovens.

Value Measured First Oven Second Oven

Predicted Temperature (Tio) 170. 26 degrees Celsius 176.58 degrees Celsius

Actual Temperature Acquired 85.8 degrees Celsius 99.6 degrees Celsius

Performing Index 1.02 1,23

Performing Cost 6.05 degrees Celsius/dollar 4.34 degrees Celsius/dollar

 

As shown in Table 4, the measured values are much smaller than the predicted values. For the

first oven, Team S.A.C.R.A.W. identified that the result of disagreement between the predicted

and actual temperature value was because of the lack of accuracy done on the dimensions of the

oven. The angles of the reflectors were not measured accurately – the angles were just assumed

to be correct based off of the dimensions of the reflectors. Also, the insulation was not tightly

packed together; instead there were many openings for heat escape through. For the second oven,

more accuracy was placed on the dimensions of the oven. Also, a different material was used for

 

 

 

Team S.A.C.R.A.W Solar Oven 20

the insulation. However, the reason for the vast difference in degree of the predicted and actual

temperature was because of the weather. The weather on solar oven testing day was much cooler

than predicted. Also, not that much sunlight was penetrating through the clouds, resulting in a

lower solar irradiance value (I0)). The change in weather is out of Team S.A.C.R.A.Ws’ control,

thus no conclusion can be made on what improvements could have been done for the final oven

aside from testing it on a different day that includes more sunlight and less clouds.

 

Design Critique and Summary

Team S.A.C.R.A.W.’s objective was to build a solar oven that met the constraints and

performed at optimum temperature. The team needed to build the oven at a low cost but still able

to withstand the high temperatures made on the day of testing. Teamwork, collaboration, and

attention to detail were implemented to produce an oven that had the potential to reach 176.58°

C. During construction of the first and second oven, the team realized many areas of

improvement that would raise the temperature of their oven and produce a more efficient

product. For example, in the first oven constructed, a flimsy duct tape was used to hold together

the corners of the exterior of the oven. When the first oven was tested, the tape could not

withstand the high temperatures and the adhesives began to fall apart. Because of the error, gaps

began to form around the exterior oven, which allowed heat to escape. To prevent this from

happening again, the second oven was constructed with a heavy-duty black duct tape that was

able to withstand much higher temperatures than what could be reached with the solar oven. If

 

 

 

Team S.A.C.R.A.W Solar Oven 21

another oven was to be built, it should be constructed with the heat resistant tape to minimize the

heat loss. Another area that needed improvement from the first oven to the second was the angle of

the reflectors relative to the oven container. Precise measurements needed to be done in order to

have the optimal angle that would allow the most solar light to be reflected into the cooking

chamber. To fix this problem, S.A.C.R.A.W. used a more accurate protractor and ruler when

measuring the reflectors for the second oven. This way the angle was accurate and allowed the

most light to be reflected into the cooking chamber. If this oven was to be reconstructed it is

recommended that an emphasis is put on the measurement of the angles because this has a direct

impact on the amount of light reflected into the cooking chamber. The more light reflected into

the cooking chamber, the higher the temperature will reach.

The insulating material is a large component of how much heat will be retained and how

much will be lost through the walls of the oven. In the first oven, the team used crumpled up

newspaper and printer paper to insulate the cooking chamber. While the crumpled up paper took

up a lot of room, there was a lot of room for the air to move around between the pieces of paper.

The conductive heat loss was maximized because the molecules had a lot of air to move around

in and therefore they did not hold very much heat. For the second oven constructed, insulating

foam was used inside the oven. The foam sealed all of the edges of the oven to heat was not lost

through those and it also reduced the movement of air inside the chamber. The heat stayed

localized to the chamber instead of freely moving about the oven and escaping. This was

essential for the oven built by S.A.C.R.A.W. because on the day of testing it was very cloudy out

and the ambient air temperature and the incident solar density were not very high. This meant

that the oven only increased temperature when there was direct sunlight. The data oscillated

 

 

 

Team S.A.C.R.A.W Solar Oven 22

because every time the sun would go behind the clouds, the temperature would drop slowly and

when the sun would come back out, the temperature would shoot up. Because of the foam

insulation and the minimal amount of heat escaping the oven, S.A.C.R.A.W.’s oven was able to

maintain a temperature higher than the ambient air temperature when the sun was behind the

clouds so when it did come out and provide direct sunlight, the temperature was able to start at a

higher initial temperature and rise from there. If there had been direct sunlight on the day of

testing, the oven would be considered a viable cooking unit that could be used if desired to cook

food for consumption. Appendix

First Oven Spreadsheet Data and Drawing

Figure 9: Shows the drawing and dimensions for the first solar oven.

 

 

 

 

 

Team S.A.C.R.A.W Solar Oven 23

Table 5: Shows the value of each variable and their value. It also states the variables’ units and description

Variable Value Units Description Io 619 watts/m2 (solar power density)

τ 0.92 (transmissivity for single layer of mylar)

a 0.9 (absorptivity of oven chamber and contents)

r 0.7 (reflectivity of Al foil) Tambient 29.4 C ambient temperature L 0.1 m length of oven window

h 0.1 m height of oven chamber n 2 # of layers of mylar M 0.3 m length of reflectors Aw 0.01 m2 area of the window

Asb 0.05 m2 area of the sides and bottom

Vchamber 0.00099981 m3 volume of the oven chamber

M/L 3 ratio of reflector length to oven window length

Usb 0.642192854 heat transfer coefficient of the chamber

α 0.284602961 angle of the reflectors with respect to the Sun’s rays

G 5.205335351 gain from reflectors Table 7: Shows the dimensions of the oven

Table 6: Shows the materials used for insulation

Wall Element Thickness (m)

Thermal Conductivity (watts/m-C)

Inner cardboard wall x1 0.003 k1 0.064 Insulation (wadded newspaper) x2 0.18 k2 0.123 Outer cardboard wall x3 0.003 k3 0.064

 

Dimension Length (m) Length of window 0.1 Width of window 0.1 Length of oven 0.6096 Width of oven 0.6096 Height of oven 0.1524 Length of chamber 0.1 Width of chamber 0.1 Height of chamber 0.1 Reflector length 0.3

 

 

 

Team S.A.C.R.A.W Solar Oven 24

 

 

 

 

 

 

Tio Uw(single) Uw(double) Table Combo 1

10.1 4.9 66 225.319429 13.9 6.7 93 179.723146 18.7 9 121 145.571651 24.3 11.7 149 121.231541 31.6 15.2 177 101.530964 40.1 19.4 204 87.1142525

Value Measured First Oven Predicted Temperature (Tio)

170. 26 degrees Celsius

Actual Temperature Acquired

85.8 degrees Celsius

Performing Index 1.02 Cost Index 6.05 degrees

Celsius/dollar

Component Amount Cost ($)

Total Cost for Component ($)

Grand Cost ($)

Reflectors $0.31 $1.25 $1.25 Interior Chamber

$0.40 $0.40 $1.59

Exterior Chamber

$1.94 $1.94 $3.73

Mylar Sheets $0.25 $0.50 With Reflectors

Duct Tape $0.50 $0.50 $8.73 Paper $0.03 $0.03 With interior

chamber Newspaper $0.00 $0.00 $8.73 Grand Total $8.73

Table 8: Shows the heat transfer values for the windows

Table 9: Final Results for the First Oven

Table 10: Shows the breakdown of costs for the first oven

Figure 10: Shows the graph and equations of line used to predict the temperature of the fist oven

 

 

 

Team S.A.C.R.A.W Solar Oven 25

Second Oven Spreadsheet Data and Drawing

Figure 11: shows the drawing and dimensions for the final oven

 

 

 

 

Team S.A.C.R.A.W Solar Oven 26

Table 11: Shows the values for the variables used to predict the Tio for the second oven Variable Combo 1 Units Description Io 619 watts/m2 (solar power density)

τ 0.92 (transmissivity for single layer of mylar)

a 0.9 (absorptivity of oven chamber and contents)

r 0.7 (reflectivity of Al foil) Tambient 29.4 C ambient temperature L 0.115 M length of oven window

h 0.115 M height of oven chamber n 2 # of layers of mylar M 0.345 M length of reflectors Aw 0.013225 m2 area of the window

Asb 0.048001 m2 area of the sides and bottom

Vchamber 0.00099981 m3 volume of the oven chamber

M/L 3 ratio of reflector length to oven window length

Usb 0.642192854 heat transfer coefficient of the chamber

α 0.284602961 angle of the reflectors with respect to the Sun’s rays

G 5.205335351 gain from reflectors Table 12: Shows the values for the insulation used. Table 13: Shows the dimensions of the oven

 

Wall Element Thickness (m)

Thermal Conductivity (watts/m-C)

Inner cardboard wall x1 0.003 k1 0.064 Insulation foam x2 0.0925 k2 0.21 Outer cardboard x3 0.003 k3 0.064

Dimension Length (m) Length of Window 0.115 Width of Window 0.115 Length of oven 0.3 Width of oven 0.3 Height of oven 0.197 Length of Chamber

0.115

Width of Chamber 0.115 Height of Chamber 0.0756 Reflector Length 0.345

 

 

 

Team S.A.C.R.A.W Solar Oven 27

 

 

 

Table #16: Shows the breakdown of costs for the final oven

 

 

Tio Uw(single) Uw(double) Table Combo 1

10.1 4.9 66 358.947717 13.9 6.7 93 285.790364 18.7 9 121 229.865253 24.3 11.7 149 189.399928 31.6 15.2 177 156.270352 40.1 19.4 204 131.809152

Value Measured Second Oven Predicted Temperature (Tio)

176.58 degrees Celsius

Actual Temperature Acquired

99.6 degrees Celsius

Performing Index 1,23 Cost Index 4.34 degrees

Celsius/dollar

Component Amount Cost $

Total Cost for Component $

Grand Cost $

Reflectors $0.43 $1.72 $1.72 Interior Chamber

$0.05 $0.05 $1.77

Exterior Chamber

$0.71 $0.71 $2.48

Mylar Sheets $0.25 $0.50 With Reflectors Duct Tape $5.00 $5.00 $7.48 Foam $5.00 $7.50 $14.98 Grand Total $14.98

Figure 12: Shows the graph and equation of lines used to determine the predicted temperature for the final oven

Table 14: Shows the heat transfer values for the windows   Table 15: Shows the final results for the final oven.

 

 

 

Team S.A.C.R.A.W Solar Oven 28

References References from Solar Oven Handout

“Solar Oven Basics” University of Arizona. Engineering 102. 2012

Shawyer, Michael and Avilio F. Medina Pizzali. “FAO Fisheries Technical Paper 436: The use

of ice on small fishing vessels.” FAO Corporate Document Repository. Food and

Agriculture Organization of the United Nations. Rome. 2003. Web. 13 Oct. 2012.