INTRODUCTION

People in developing countries spend 90% of their energy for cooking [1]. If 50% of the world’s fuel wood-using families switched to using solar ovens, around 346 million tons of fuel wood would be saved annually [2]. Chez Helios, the Engineers Without Frontiers (EWF) project team, is working with their partner organization, The STEVEN Foundation, to perform empirical tests of design parameters for solar ovens. The improvement and documentation of solar ovens can benefit people around the world. Solar ovens are not only environmentally friendly, but they are also pleasant to people’s health and wallets.

The EWF team members have built two identical solar box cookers based on the STEVEN Foundation’s plans. As depicted in Figure 1, these cookers consist of insulated boxes with glass covers and reflector panes to direct incoming light into the box. In the future, parameters to be tested may include interior oven coating, amount and type of heat retaining objects used, insulation, oven door position, and the reflective panel material, shape, and size. Performance measures include standardized cooking power and cost to build.

This end-of-semester report describes our project tasks in detail. We have provided a schedule for each of these, along with an overall project schedule for Chez Helios. Our results from our project are also summarized with a plan for transitioning and future recommendations. Each of our group members has provided their reflections from the semester, which give a personal insight into our project. At the end, technical documentation is provided to be easily followed by the next project team.

Figure 1 The two solar ovens made by the team set up for testing

PROJECT TASK DESCRIPTIONS

The Solar Oven team’s work for this semester consisted primarily of three main tasks. The first task was to design a solar oven based on the STEVEN Foundation’s plans, and to build two identical working models. The second task was to develop a method of testing the ovens based on the American Society of Agricultural Engineers (ASAE) standard. Finally, the third task was to perform the tests. The team broke up into two subgroups, a building team and a testing team, in order to accomplish these tasks. However, this distinction was largely for organizational purposes only; while team members focused more on the task to which they were assigned, every member worked on both tasks as necessary.

1. Building

The first task for this semester was designing and building a solar oven based on the STEVEN Foundation’s model. This was the larger and more time-consuming of the two tasks, so four team members were assigned to it: Cheryl, Kien-Man, Jen, and Jeff. During this semester, this team built two complete ovens and is in the process of building a third. We have also created blueprints for our design and developed a method of building the ovens that is more detailed and precise than the STEVEN Foundation’s original plans (See Appendices B and D).

Francis Vanek of the STEVEN Foundation provided the team with a solar oven that he had built and used for several years. We also had a design manual for the original STEVEN Foundation solar oven (Appendix D). The first major issue that we noticed immediately was that the design manual describes the oven design as of May 1996, and that Francis’s oven is a more updated model that does not correspond to the original design plans. We decided to attempt to recreate Francis’s oven as closely as possible, making small modifications to allow for the tools available in the Civil Infrastructure Lab and materials which we already had or could easily and cheaply obtain. Many of these changes were for our convenience in either building or testing, and most, we decided, would not significantly affect the performance of the ovens. In any case, our ability to test various oven parameters would not be affected by these minor design changes, as long as all of our ovens are built identically.

Major changes from the 1996 design manual are as follows:

§ For the outer wooden box (Appendix B.iii-B.iv), we used ¼-inch plywood for the sides and ¾-inch plywood for the base. The manual calls for using ¾-inch plywood for the entire box; however, Francis’s oven used ¼-inch on the sides. We decided to follow Francis’s oven, which is the STEVEN Foundation’s most recent model. This required other design changes due to the lack of wide sides, which were originally used to rest the reflector panels on. Furthermore, we added 90° angle brackets to the corners to regain structural stability that was lost in switching to thinner sides.

§ We built a frame for the glass cover out of 5/8 x 1½ inch wood, miter cut and slotted as shown in our plans (B.vii). The manual suggests this but gives no instructions for it. Francis’s oven uses a frame for the glass; however, the fact that his glass was broken from wear despite the frame suggested the need to design a better frame.

§ The wood frame at the top of the box was recessed by ¾ inch, rather than raised above the sides of the box as in the design manual. This modification was our own decision and not based on either the manual or Francis’s oven. This change allowed us to fit the door frame snuggly into the recessed part to prevent it from slipping. Francis’s oven solved this problem by cutting a recessed step into the frame; however, we did not have the tools to do this and so we had to opt for a simpler solution. The dimensions of the outer box were increased to allow for this change and keep the inside dimensions the same.

§ Due to availability, the metal liner was made of sheet metal thicker than 26-gauge (as used in the original plans as well as Francis’s oven). Furthermore, only the bottom was painted black while the sides were left shiny, as opposed to painting the entire inside black like that of Francis’s oven. We made this change because in other tests it had been shown to improve performance [3]. The testing team is considering testing this feature again.

§ For the reflector, eight panels (four rectangular panels along the sides and four trapezoidal panels in the corners, as shown in Appendix B.vi-B.vii) were used instead of the four indicated in the design manual. This was the major update that the STEVEN Foundation had made since writing the design manual, and is used in Francis’s oven. As these changes were undocumented, we were required to build these from scratch, including calculating the necessary geometry to allow the panels to fit snugly together. As a result, our reflective panels differ slightly from Francis’s.

§ An additional wooden frame to support the panels and hold them at a 30° pitch from vertical was designed and built. This added support was necessary because the eight-panel design was less rigid than the four-panel. Furthermore, the lack of wider box edges (created from the switch from ¾-inch to ¼-inch wide sides) meant that the panels would have to rest on something else. The panel frame rests upon nails in the glass door frame when the oven is tilted towards the sun. Testing experience revealed that the nails did not hold the reflector on the box as well as they should have at high angles of tilt, especially when the reflector panels were blown by the wind. In the future we will need to develop a more secure method of attaching the reflector to the box.

§ Tightly packed, rolled newspaper was used for insulation in our control ovens, as described in the building instructions (Appendix B.ii). The original plans and Francis’s oven both used fiberglass insulation; however, this insulation deforms due to the heat when the oven is used, as we saw on Francis’s oven. Therefore, we tried Styrofoam insulation; however, this also melts when the oven heats up. Thus, we decided upon newspaper, which we know to combust at 233°C, far hotter than our ovens will get. The testing team is considering trying other alternatives for insulation.

§ Nails, screws, rivets, and other connecting devices were used as available to us. Specifically, we used rivets instead of nails to affix the angle brackets to the reflective panels, and we used 1¼-inch screws instead of 2-inch nails to attach the frame to the box, as this would make it easier for us to disassemble the oven to make changes. These changes were made largely based on the materials available to us at the time, and they do not significantly affect oven performance.

The end result of the building team’s work for this semester is two complete ovens, which have been tested to perform similarly, and updated design plans with detailed notes so that future teams can build additional ovens identical to ours.

Figure 2 shows a detailed schedule for the building task. The time to complete this task was much greater than we had originally expected. Building the first oven involved a great deal of trial and error due to the fact we were constantly updating and modifying our design plans. Furthermore, we were hampered by our initial unfamiliarity with the Civil Infrastructure Laboratory, and finding time to work with the power tools (which required project supervisor Tim Bond’s presence, for safety reasons) was often difficult. Although we had hoped to build three ovens by the end of the semester and give the testing team enough time to run several tests, it took us about seven weeks to build our first oven and about two weeks to build our second. However, we now have a consistent design and familiarity with the building process, so future teams will be able to devote more of their time and efforts towards testing the ovens.

Figure 2 Schedule of building

2. Development of Testing Method

We based our test method off of the 2002 American Society of Agricultural Engineers (ASAE) X580 test standard for solar cookers (Appendix C). John and Dan implemented the test standard with slight modifications (Appendix B), following the schedule shown in Figure 3.

The STEVEN Foundation had limited involvement in this task. They shared their testing experiences, which were limited to measuring oven temperatures with a crude oven thermometer. We discussed their results with them, but indicated that we wished to work off the ASAE test standard because it allows us to compute the standardized cooking power of the oven, which is more valuable to know than simply how hot the air inside the oven gets, (which incidentally we are also measuring to be thorough). The STEVEN Foundation was pleased with our decision to use this rigorous test method.

Figure 3 Schedule of test procedure development

2.1. Test standard compliance

In order to develop our testing document, we took the ASAE standard and molded it to fit our needs. In two instances, we felt it was necessary to lift restrictions placed on testing conditions due to our climate. First, it called for testing at least 30 ten-minute intervals over a period of three days (i.e. at least 10 intervals per day). Given that the statistical likelihood of three consecutive partially sunny days in Ithaca, NY is minute, we decided to forgo the requirement that the tests be done on separate days. Secondly, we also excluded the requirement that the ambient temperature not fall below 20º C, again due to climate considerations. Waiving these requirements will have a minimal impact on the usefulness of our results, as tests at 15°C will not yield very different results than those at 20°C. In addition, since we will be doing all of our testing with a control oven, we can gain insight as to the effects of varying a parameter even if a day isn’t quite up to specs. Testing will take place on the blacktop parking lot behind Thurston Hall. The lot is well protected from the wind, yet still has good exposure to the sun.

We decided to comply with the standard’s requirement of 7.0 kg of water per square meter of incident light area. This is to make sure the oven is sufficiently loaded as to keep the water from boiling during the test. All ovens tested thus far have 1.00 m² of incident area and therefore require 7 kg ≈ 7.39 qt of water. The method devised calls for two 4-qt pots painted flat black in order to absorb maximum amounts of heat. However, we were unable to fit the 7.0 kg in our pots without the aid of a shelf inside the oven more complex than the crude one we currently employ. Thus the data in Figures 5, 6, and 7 (B.x-B.xi) was collected using 3.2 kg of water. We have every intention of using the full 7.0 kg in future tests. The testing done in section three was designed to compare the two ovens and prove that they performed the same, and thus as long as we had the same weight in each, using less water did not affect our data.

We have also decided that our control oven will not be tracked according to the sun’s movements. This will allow us to study the effect of such tracking and whether it makes a tangible difference in cooker performance.

2.2. Data collection

The data collection system is largely based on a pre-existing system owned and maintained by the Civil Infrastructure Lab. The data collection system includes a weather station which monitors wind speed, wind direction, and ambient temperature. We purchased a radiation pyranometer (required by the standard), which measures the insolation in W/m² at any given time. This is important because it allows us to characterize the power generated by the oven in relation to the energy given to it by the sun. We are then able to compare a test on a partly-sunny day with a test on a very sunny day. The power during each interval is adjusted for the amount of insolation, thus allowing a measurement of the oven’s power independent of the solar power given to it (Appendix C).

The apparatus also includes eight thermocouples per oven, which are used to monitor temperatures at crucial spots. In another deviation from the test standard, we chose to use seven more thermocouples per oven in order to gain additional information about the performance. The test standard requires only a thermocouple inside each water vessel and outside the oven for ambient conditions. Our method monitors the temperature on either side of the insulation, on either side of the glass cover, and of the air inside the oven. Our data is collected every 30 seconds, giving us maximum flexibility in choosing our ten-minute intervals. Every ten-minute interval in which environmental conditions stated in Appendices B and C hold is valid and included in analysis.

3. Testing of control ovens

Combining both previous tasks, we used our test method on the first oven we built, as shown in the schedule in Figure 4. This showed numerous improvements that could be made to the first oven, including the substitution of a different insulation material. During this test, our Styrofoam insulation melted onto the metal lining, yielding a big mess and unappetizing smell. When we discussed the insulation problem with him, Francis suggested that rock wool, straw, or newspapers be considered as alternative insulation materials. In addition, another layer of wood between the metal lining and the insulation could also do the trick, he said. For later tests, the control oven was insulated with crumpled newspaper. Other issues that still need to be addressed include the sealing of the metal liner with caulk (which we are hesitant to do until we find insulation we are happy with) and the installation of pins to hold the glass cover and reflectors in place. The data collection went very well, and the method was successfully executed. Ways of supporting and tracking the oven (moving it with the sun throughout the day) were examined and we ultimately found props for the cooker that would hold it at the proper angle and a shelf for the pots to keep them level. These are crude and a modification of these props and shelf would allow us to place the cooker at a more ideal angle with respect to the sun, increasing our power.

Upon the completion of the second oven, we tested both ovens simultaneously using our test method. As mentioned briefly earlier, we have not yet acquired the proper water vessels to allow the use of 7.0 kg per oven. Thus the water did boil and we had to throw out all the data intervals after the water had reached 95°C. However, there was meaningful data gained from this test, namely that the two ovens were nearly identical in performance (Figures 6 and 7, B.xi). The standardized cooking power at 50°C was 81.1 W for Oven 1 and 82.7 W for Oven 2, and their data plots (Figure 6) are nearly identical as well. This is exactly what we were hoping for because it will allow us to make a modification of Oven 2 and use Oven 1 as a control.

Figure 4 Timeline of other testing tasks

OVERALL PROJECT PLAN

Overall project schedule

Summary of semester outcomes

Our group successfully achieved the following:

Developed construction blueprints and instructions for building a solar box cooker (based on existing STEVEN Foundation model and plans).
Constructed two unmodified solar box cookers.
Developed a set of testing procedures that meets the ASAE X580 test standards.
Set up a testing station capable of testing two ovens simultaneously.

The work done in this semester will act as a solid foundation for future solar oven testing. Future teams can use and expand the testing station that we have set up to simultaneously test several modified ovens with a control oven. To speed up testing, future teams will want to use our construction blueprints to create 1-2 more standard solar ovens, and then modify them as needed.

With input from the STEVEN Foundation, our group has come up with several ideas for modifications to test. The modifications were chosen because they are feasible changes that could improve cooker performance, and very little data currently exists on how they affect performance:

Double glazing the oven door
Using another type of insulation
Painting the interior oven sides flat black
Placing a large thermal mass in the oven to improve heat retention
Using different reflective materials

Transition Plan

Summer solar oven testing will be a continuation of this semester’s goals, and ultimately a bridge to the fall semester. The group consisting of Cheryl Bauer and Brian Warshay, with Tim Bond supervising, will use the solar ovens built during the spring semester, along with the testing station, to measure different components of the oven itself and the environment surrounding it. During the summer, the team will test the effects of the above-mentioned modifications on oven performance. Other modifications that the group comes up with might be tested as well. Also, a third oven will be constructed identical to the two created during the spring semester, and it too will be modified and tested. We used SA-85, a hi-tech 3M product, for reflective material on the first two ovens. Unfortunately, this product is no longer made and we used our entire stock of it. An alternative reflective material will have to be used on the third oven. When testing variables other than reflective material, the material on all three ovens should be the same, so the summer group will likely also have to replace the SA-85 material on the first two ovens as well. VM2000 is a similar product made by 3M that could be substituted, but it is not UV-protected. VM2000 would have to be coated for UV protection in order to stand up to prolonged exposure to sunlight.

The main goal of the summer is to achieve the original project plan for this semester, including: testing different variable parameters to determine quantitatively the effect they have on oven performance and collecting results to place in an organized database that will be published. In addition, the design notebook from the spring, and a notebook created during the summer will be accessible to the solar oven group in the fall.

CONCLUSIONS AND RECOMMENDATIONS

Solar ovens have been the source of much unofficial research in the past ten to fifteen years; the solar box cooker is just one of many designs. Although several extensive manuals detail qualitatively the effects of varying certain parameters (reflective material, shape of collectors, etc.), there is little hard data available from official experiments conducted using the ASAE test standard for solar ovens [4, 5].

The primary objective of this project was to experimentally collect standardized, empirical data about solar ovens. This will augment the already available qualitative material and aid in further advancement in the field of solar ovens. Solar ovens are valuable for a variety of reasons, to a variety of populations. We did not intend to develop an oven to install in a particular location in the short term, rather we were concerned with developing quantitative data for the solar oven community in general, and specifically, our partner organization, the STEVEN Foundation. Installing these ovens somewhere would be a great future project.

This semester, we learned that working through the building of the first oven as a team step-by-step made the construction of the second oven much smoother once we effectively divided into separate teams. Our group effectively accomplished building two solar ovens that we are very proud of and an effective testing method with standards. While working these past four months, we kept accurate records and accounts of our every move making it easier for the next group to pick up where we left off. We have set solid groundwork for the ovens, and now the modification is ready to begin.

REFERENCES

1. Sperber, Bill. “Balancing the Scales.” The Solar Cooking Archive. 7 Apr. 1990. <http://solarcooking.org/balance.htm>

2. “The Untapped Market for Solar Cookers.” The Solar Cooking Archive. <http://solarcooking.org/market.htm> Feb. 2004.

3. “Solar Cooking Frequently Asked Questions.” The Solar Cooking Archive. <http://solarcooking.org/solarcooking-faq.htm> August 2001.

4. “The Bernard Solar Panel Cooker.” <http://p2.utep.edu/watts/manuals/bernard.pdf>

5. “The Solar Cooking Archive.” <http://solarcooking.org

Appendix: ASAE STANDARD X580

ASAE Standard: ASAE X580 (SE-414 Voting Draft revised by P. Funk on 04/03/2002)

Testing and Reporting Solar Cooker Performance

Developed by the Test Standards Committee at the Third World Conference on Solar Cooking (Coimbatore, Tamil Nadu, India, 9 January 1997); editorial revisions November 1998 and July 1999; revised March 2001 (Third Latin American Congress on Solar Cookers, La Ceiba, Atlintico, Honduras); edited and submitted for approval to ASAE Solar Energy Committee SE-414 (94^th Annual International Meeting, Sacramento, California, USA) 31 July 2001; revised and resubmitted 15 March 2002.

SECTION 1 — PURPOSE AND SCOPE

1.1 This Standard is intended to

1.1.1 Promote uniformity and consistency in the terms and units used to describe, test, rate and evaluate solar cookers, solar cooker components, and solar cooker operation.

1.1.2 Provide a common format for presentation and interpretation of test results to facilitate communication.

1.1.3 Provide a single measure of performance so consumers may compare different designs when selecting a solar cooker.

1.2 The scope of this Standard includes

1.2.1 All solar powered batch-process food and water heating devices (solar cookers). Devices designed to desiccate (dryers) are not covered.

1.2.2 Within the scope of this Standard a solar cooker shall be understood to include the cooking vessel(s) together with associated supporting, heat transfer and heat retention surfaces, heat storage and transfer media and associated pumps and controls, light transmitting and reflecting surfaces, and all associated adjustments, supports, and solar locating and tracking mechanisms as may be integral parts of a particular solar cooker.

SECTION 2 — NORMATIVE REFERENCES

IS 13429. 1992. Indian Standard- Solar Cooker- (3 Parts). Bureau of Indian Standards, New Delhi.

SECTION 3 — TERMINOLOGY

3.1 Absorber plate: Darkened surface converting light energy into thermal energy.

3.2 Angle, Azimuth: The angular displacement from south of the projection of beam radiation on the horizontal plane.

3.3 Angle, Zenith: The angle subtended by a vertical line to the zenith (point directly overhead) and a line directly to the sun.

3.4 Beam Radiation: Solar radiation received directly from the sun without atmospheric scattering.

3.5 Box-type cooker: A solar cooker with a well-insulated volume for the cooking vessel(s), typical designs having from zero to four plane mirrors.

3.6 Concentrating-type cooker: Any of various designs characterized by multiple plane or curved reflective surfaces. Many designs lack insulated walls but have large intercept areas to compensate for their comparatively greater heat loss.

3.7 Intercept area: The sum of the reflector and aperture areas projected onto the plane perpendicular to direct beam radiation (Figure 1). For convenience, use the average beam radiation zenith angle as calculated for the entire test period.

3.8 Load: The mass of water being heated by the solar cooker.

3.9 Test: All events and data comprising the measured solar heating of water in a device intended to cook food.

3.10 Tracking: Rotating the cooker in the horizontal plane to compensate for azimuth angle changes (box-type) or following the sun in two dimensions (concentrating-type).

SECTION 4 — GENERAL

4.1 This Standard specifies that test results be presented as cooking power, in Watts, normalized for ambient conditions, relative to the temperature difference between cooker contents and ambient air, both as a plot and as a regression equation for no less than 30 total observations over three different days.

4.2 This Standard specifies that cooking power be presented as a single number found from the above equation for a temperature difference of 50 C.

SECTION 5 — UNCONTROLLED (WEATHER) VARIABLES

5.1 Wind. Tests shall be conducted when wind is less than 1.0 ms^-1, measured at the elevation of the cooker being tested and within ten meters of it. Should wind exceed 2.5 ms^-1for more that ten minutes, discard that test data. If a wind shelter is required, 1) it shall be designed so as to not interfere with incoming total radiation and 2) The wind instrumentation shall be co-located with the cooker in the same wind shadow.

5.2 Ambient temperature. Tests should be conducted when ambient temperatures are between 20 and 35 C.

5.3 Water temperature. Test data shall be recorded while cooking vessel contents (water) is at temperatures between 5 C above ambient and 5 C below local boiling temperature.

5.4 Insolation. Available solar energy shall be measured in the plane perpendicular to direct beam radiation (the maximum reading) using a radiation pyranometer. Variation in measured insolation greater than 100 Wm^-2during a ten-minute interval, or readings below 450 Wm^-2or above 1100 Wm^-2during the test shall render the test invalid. For convenience, the pyranometer may be fixed on the cooker at the average beam radiation zenith angle as calculated for the entire test period.

5.5 Solar zenith and azimuth angle. Tests should be conducted between 10:00 and 14:00 solar time. Exceptions necessitated by solar variability or ambient temperature shall be specially noted.

SECTION 6 —CONTROLLED (COOKER) VARIABLES

6.1 Loading. Cookers shall have 7.0 kg potable water per square meter intercept area distributed evenly between the cooking vessels supplied with the cooker. If no cooking vessels are provided, inexpensive aluminum cooking vessels painted black shall be used.

6.2 Tracking. Azimuth angle tracking frequency should be appropriate to the cooker’s acceptance angle. Box-type cookers typically require adjustment every 15 to 30 minutes or when shadows appear on the absorber plate. Concentrating-type units may require more frequent adjustment to keep the solar image focused on the cooking vessel or absorber. With box-type cookers, zenith angle tracking may be unnecessary during a two hour test conducted at mid-day. Testing should be representative of local conditions, ie; how the typical consumer is expected to use the cooker.

6.3 Temperature sensing. Water and air temperature should be sensed with thermocouples. Each thermocouple junction shall be immersed in the water in the cooking vessel(s) and secured 10 mm above the bottom, at center. Thermocouple leads should pass through the cooking vessel lid inside a thermally nonconductive sleeve to protect the thermocouple wire from bending and temperature extremes. The sleeve should be secured with 100% silicone caulk to reduce water vapor loss.

6.4 Water mass. The mass of water should be determined with an electronic balance to the nearest gram using a pre-wetted container.

SECTION 7 — TEST PROTOCOL

7.1 Recording. The average water temperature (C) of all cooking vessels in one cooker shall be recorded at intervals not to exceed ten minutes, and should be in units of Celsius to the nearest one tenth of a degree. Solar insolation (Wm^-2) and ambient temperature (C) shall be recorded at least as frequently. Record and report the frequency of attended (manual) tracking, if any. Report azimuth angle(s) during the test. Report the test site latitude and the date(s) of testing.

7.2 Calculating cooking power. The change in water temperature for each ten-minute interval shall be multiplied by the mass and specific heat capacity of the water contained in the cooking vessel(s). This product shall be divided by the 600 seconds contained in a ten-minute interval, as:

P = (T_f- T_i)MC_v/600 [1]

where:

P = cooking power (W)

T_f = final water temperature

T_i = initial water temperature

M = water mass (kg)

C_v= heat capacity(4186 Jkg^-1K^-1)

7.3 Calculating interval averages. The average insolation, average ambient temperature, and average cooking vessel contents temperature shall be found for each interval.

7.4 Standardizing cooking power. Cooking power for each interval shall be corrected to a standard insolation of 700 Wm^-2by multiplying the interval observed cooking power by 700 Wm^-2and dividing by the interval average insolation recorded during the corresponding interval.

P_s = P_i(700/I_i) [2]

where:

P_s = standardized cooking power (W)

P_i = interval cooking power (W)

I_i = interval solar insolation (Wm^-2)

7.5 Temperature difference. Ambient temperature for each interval is to be subtracted from the average cooking vessel contents temperature for each corresponding interval.

T_d = T_w – T_a [3]

where:

T_d= temperature difference (C)

T_w = water temperature (C)

T_a = ambient air temperature (C)

7.6 Plotting. The standardized cooking power, P_s, (W) is to be plotted against the temperature difference, T_d, (C) for each time interval.

7.7 Regression. A linear regression of the plotted points shall be used to find the relationship between cooking power and temperature difference in terms of intercept (W) and slope (WC^-1). No fewer than 30 total observations from three different days shall be employed. The coefficient of determination (r²) or proportion of variation in cooking power that can be attributed to the relationship found by regression should be better than 0.75 or specially noted.

7.8 Single measure of performance. The value for standardized cooking power, P_s, (W) shall be computed for a temperature difference, T_d, of 50 C using the above determined relationship.

NOTE: for product labeling and sales literature an independent laboratory using a statistically adequate number of trials shall determine this number. While this value, like the fuel economy rating of an automobile, is not a guarantee of performance, it provides consumers with a useful tool for comparison and product selection.

7.9 Reporting. A plot of the relationship between standardized cooking power and temperature difference shall be presented with the equation, following the example in Figure 2. The report shall also state the standardized cooking power at a temperature difference of 50 C.

SECTION 8 — REFERENCES

Funk, P.A. 2000. Evaluating the international standard procedure for testing solar cookers and reporting performance. Solar Energy 68(1):1-7.

Mullick S.C., Kandpal T.C. and Saxena A.K. 1987. Thermal test procedure for box-type solar cookers. Solar Energy 39(4), 353-360.

Presented to

Chez Helios

The STEVEN Foundation

1. Building

OVERALL PROJECT PLAN

CONCLUSIONS AND RECOMMENDATIONS