1) A group of students designed and built a solar water heater for a class project with a $30 budget. They built a parabolic trough design out of coroplast and an emergency blanket. 2) Testing of the device found that it reached a temperature of 86°C, lower than the predicted 120°C due to wind and non-ideal conditions not accounted for in the model. 3) The students concluded that the experimental data was more reliable than the theoretical predictions, and that the project provided valuable experience with applying classroom concepts to real-world design constraints.

1. NORTHERN ARIZONA UNIVERSITY
Department of Mechanical Engineering
ME 495 Thermo Fluids Laboratory
Project
Solar Oven
Lab Instructor: Dr. Tim Becker
Submitted by
Group: 4
Nik Glassy
Mohammad Molani
Marissa Munson
December 4, 2014

2. Project objectives
Solar heaters are an inexpensive and environmentally friendly way to heat water or cook food.
They are generally easy to construct and have a variety of shapes,sizes and uses. The team was tasked
with designing, building and analyzing a solar water heater using a parabolic trough or a flat plate
collector. Using only a $30 budget, the heater was required to fit within 3 m3
and be transportable by a
single person without assistance. A final temperature of 65 o
C needed to be reached within 30 minutes for
the heater to be considered a success. Any moving parts were required to be entirely manual and the
heater was not allowed to have any additional heating elements.
Introduction
There are only a handful of viable solar water heater designs, especially on a limited budget. A
parabolic trough, parabolic dish, or flat plate collector are the only options that are feasible. The project
definition further restricted that to only be flat plate collectors and parabolic troughs. Of these two the
team chose to do a parabolic trough because a simple design was conceived, as can be seen in figure 1.
The design consists of two side panels made of coroplast cut in the shape of a parabola, 1m wide with a
focus at .2m from the bottom of the parabola. The trough is made of a single piece of coroplast glued onto
the side panels. The trough was then lined with a reflective emergency blanket. The collector was a pipe
painted matte black and located at the focus of the parabola. The glue used to attach the coroplast pieces
is common craft hot glue. The glue used to adhere the emergency blanket to the coroplast is an
aerosolized contact adhesive. Figure 2 shows the completed design.

3. Figure 1: CAD Model with Thermocouple Locations Notated
Figure 2: Completed Oven
Methods
Analysis ofthe oven
The experiment is assumed to be operating in still conditions, in other words the oven is tested
without having to account for convection due to wind. The absorption pipe is also considered to be a
black body, so it absorbs 100% of the energy that hits the surface. The analytical model is shown in figure
3. The basic mechanics of the system allow for a simpler analysis of the system because it can be assumed
that any small sliver of the cross section is the same as any other small sliver, in other words the oven can
be analyzed using a 2D analysis. There are 2 heat transfer mechanisms that prevent heat from being
applied to the pipe and therefore heating the water. The first is natural convection. Because the pipe is
painted black, it will collect a significant amount of heat. The air around the exposed pipe will heat the air
and cause it to rise. This is one area of thermal resistance. The other is the reflectivity of the emergency
blanket. The blanket is nearly impossible to apply perfectly smoothly to the coroplast, so it has wrinkles
resulting in scattering of some of the heat. The blanket is also not 100% reflective, causing it to absorb
some of the heat and transfer that to the underlying coroplast.
Figure 3: Resistance Network of Oven

4. The team chose to make the oven have a 1m^2 projected area into the sun to simplify the thermal
analysis. This allows the team to assume, from the project definition, that there is only 850W reaching the
oven. The part of the analysis that needed to be verified experimentally was the optical efficiency of the
emergency blanket used as the reflector. Initially the team made a conservative estimate that the blanket,
with all of its light scattering wrinkles, would have a 50% optical efficiency. That was corrected to be
80% after analysis.
Calculations to determine the final temperature on the pipe take into account the assumptions
stated above, the original 850 W/m^2 heat flux into the solar heater and the resistances shown in figure 3.
Assuming a 50% conservative estimate when calculating the reflectivity of the emergency blanket, the
total wattage being absorbed by the pipe is approximately 470 Watts. After taking into account the
resistances caused by energy absorbed by the reflector, the optical efficiency of the reflector and natural
convection around the tube, the final theoretical temperature of the pipe is approximately 120 o
C.
After taking measurements on the solar heater,it was clear that environmental factors should
have been taken into account for the theoretical analysis. These new factors were used to refine the
analytical model by including forced convection to account for wind cooling the pipe during testing. The
pipe itself should also not be considered a perfect black body. Not all the energy directed toward the pipe
is absorbed completely. The analytical model assumes 100% absorption by the pipe, however, a more
accurate value would be around 80% absorption.
Experimental design
Thermal measurements were taken in three places during testing: on the pipe at the focal point, on
the reflective surface,and on the back of the heater as a control temperature. The focal point measurement
was critical as this is where the most heat was being collected and where the water would be boiled at.
The temperature reading on the reflective material depicted how hot the rest of the heater would be
around the pipe. The reading on the back of the oven was a control point. The temperature readings from
this point were used to verify the rest of the measurement points.
Data was acquired using thermocouples and a LabVIEW VI system using a block diagram. The
thermocouples were attached to the measurement points described above. Signals were then taken from
the thermocouples and used as inputs for the block diagram. The data from the thermocouples was then
displayed on a waveform graph. The graph allowed for all three measurements to be seen simultaneously
and allowed the change in data and increase in temperature to be visualized. The data was also output to a
file for further analysis later.
The heat transfer model was updated based on the recorded data. The collected final temperature
was lower than the heat transfer model predicted. This was due to the assumption of a perfect black body
during solar absorption as well as environmental effects. Using this new data, the heat transfer model can
be updated to reflect more accurate temperatures that take into account forced convection and lower solar
absorption by the pipe.
Results
The system was evaluated on November 20, 2014. The data collected from that is shown in table
1 under Actual. Theoretical analysis was also done on the system to define a baseline with which to
compare the collected data. The difference in the two values is also shown in table 1. Tables 2 and 3

5. contain information on the uncertainty of the system. The largest error in the system is the error in the
accuracy of the thermocouple.
Table 1: Simulated vs. Actual Temperatures and % Error
Location Simulated deg C Actual deg C % Error
Control on Support 24 24 0%
Reflective Surface 53 42 26%
Collector Surface 120 86 28%
Table 2: Uncertainty for Individual Components
Component Uncertainty
Thermocouple 1.5 deg C
DAQ .02 deg C
Table 3: Total Uncertainty for the System
Total Uncertainty
1.50013 deg C
Discussion
Error in the data was most likely caused by environmental effects. The day testing was
completed, there was wind that would affect the temperature readings. Simulated temperature values were
calculated without taking into account environmental effects. Errors may have also been caused by the
heater not being perfectly lined up with the focal point. During testing, the heater needed to be adjusted
with the sun movement, as the shadow of the building was continually covering up part of the oven as
time went on. During some points, the heater may not have been fully aligned for short periods of time,
which could affect the temperature readings.
The most reliable temperature data is most likely the measured data acquired during testing.
While the simulated data is ideal, it is not guaranteed to reach the temperatures shown. The temperatures
acquired during testing with environmental effects and adjustment errors is a more reliable representation
of how well the heater performs.
Conclusions
In conclusion, building the solar water heater is very interesting where the team has gained a
great knowledge about how to apply what learned from the actual labs. Applying the knowledge gained
from the labs from calculations, and heat transfer theories helped the team build the project and how to
make it work. Most importantly, the team learned how to differentiate between the analytic data and the

6. experimental data and how to create a virtual instrument using LabView in order to collect the data for the
design. After analyzing the data and the results of this project, the team has build and understands a great
experience and knowledge of how the theories can work in reality and understand how to make it work.
The basis of building a low cost solar oven requires students to think critically about making the
most out of limited resources. This is incredibly important in industry because it is reality. There is only
so much that a company is willing to spend on a project and the engineer needs to be able to complete the
work at or under budget. The use of a data acquisition system for real life use is incredibly helpful.