NORTHERN ARIZONA UNIVERSITYDepartment 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

Project objectivesSolar 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 oC 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.

IntroductionThere 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.

Figure 1: CAD Model with Thermocouple Locations Notated

Figure 2: Completed OvenMethodsAnalysis of the 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

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 oC.

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.

ResultsThe 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 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

DiscussionError 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.

ConclusionsIn 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 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.