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ASAE Standard: ASAE X580 (Draft prepared for Solar Energy Committee SE-414 by Paul Funk on 25/07/2001)

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 (following the Third Latin American Congress on Solar Cookers, La Ceiba, Atlintico, Honduras); edited and submitted for approval to Solar Energy Committee SE-414 (ASAE 94th Annual International Meeting, Sacramento, California, USA) 31 July 2001.


SECTION 1 — PURPOSE

1.1 This Standard is intended to 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.

1.2 This Standard is intended to provide a common format by which researchers can publish results.

1.3 This Standard is intended to provide a single measure of performance for consumers to use when selecting a solar cooker.

SECTION 2 — SCOPE

2.1 This Standard specifies test conditions, instrumentation, and procedures to assure uniformity and consistency of results.

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

SECTION 3 — COMPLIANCE DEFINITIONS

3.1 Compliance definitions. The accepted definitions of “shall,” “should” and “approved,” as included in this Standard, are:

3.1.1 “Shall” is intended to indicate requirements.

3.1.2 “Should” is intended to indicate recommendations, or that which is advised but not required.

3.1.3 “Approved” or “approval” refers to listing by a nationally recognized testing laboratory or agency.

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 observations.

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 m/s at the elevation of the cooker being tested. Should wind exceed 2.5 m/s for more that ten minutes, discard that test data. If a wind shelter is required, it should be designed so as to not interfere with incoming total radiation.

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

5.3 Water temperature. Test data shall only be recorded while cooking vessel contents (water) is at temperatures between 5 ºC above ambient and 5 ºC 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 W/m2 during a ten-minute interval, or readings below 450 W/m2 or above 1100 W/m2 during the test shall render the test invalid.

5.5 Solar altitude 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/m2 intercept area distributed evenly between the cooking vessels supplied with the cooker. If no cooking vessels are provided, inexpensive aluminum pots painted black shall be used.

6.1.1 Intercept area. Intercept area is defined as the sum of the reflector and aperture areas projected onto the plane perpendicular to direct beam radiation (Figure 1). As this quantity varies with latitude, date and time, the average beam radiation zenith angle should be calculated for the test period, and the cooker rotated in the horizontal plane (solar tracking) to compensate for azimuth angle changes.

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. Parabolic-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 anticipated consumer habits.

6.3 Temperature sensing. Water and air temperature should be sensed with thermocouples. Each thermocouple junction should be immersed in the water in the cooking vessel(s) and secured 10mm 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 centigram using a pre-wetted container.

SECTION 7 — TEST PROTOCOL

7.1 Recording. The average water temperature (ºC) 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 (W/m2) 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, M, (Kg) and specific heat capacity, Cv, (4186 J/kgK) of the water contained in the cooking vessel(s). This product shall be divided by the 600 seconds contained in a ten-minute interval, yielding the cooking power, P, in Watts.

P = (Tf - Ti)MCv/600 (1)

where Tf is the water temperature at the end of the 10 minute interval and Ti is the temperature at the beginning.

7.3 Calculating interval averages. The average insolation, average ambient temperature, and average pot 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 W/m2 by multiplying the observed cooking power, P, by 700 W/m2 and dividing by the average insolation, Iavg, recorded during the corresponding interval.

Ps = P(700/ Iavg) (2)

where Ps is the standardized cooking power.

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

Td = Tw – Ta (3)

where Td is the temperature difference, Tw is the cooking vessel contents (water) temperature and Ta is the ambient air temperature, all in degrees C.

7.6 Plotting. The standardized cooking power, Ps, (W) is to be plotted against the temperature difference, Td, (ºC) 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 (W/ºC). No fewer than 30 observations from at least three 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 75%, and shall be reported.

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

NOTE: for product labeling and sales literature an independent, approved 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 50ºC.

SECTION 8 — REFERENCES

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

Funk, P.A. The International Standard Procedure for Testing Solar Cookers and Reporting Performance. ASAE Paper No. 99-4017. 5 pp. 1999

Funk, P. A. International standards for testing solar cookers. World Solar Cooking and Food Processing; Strategies and Financing, Varese, Italy. 8 pp. 1999. (Proceedings)

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

Funk, P.A. Estándares Internacionales para la Prueba de Hornos Solares para Cocer. Tercer Congreso Latinoamericano y del Caribe de Cocinas Solares. (Proceedings. Accepted March 26, 2001)

Grupp M., Merkle T. and Owen-Jones M. (1994). Second International Solar Cooker Test. European Committee for Solar Cooking Research % Synopsis, Route d’Olmet, F-34700 Lodeve, France.

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.

Figure 1. Determining the intercept area where HR is the reflector height and HA is the aperture height in the plane perpendicular to beam radiation of zenith angle Z.

Figure 2. Example of adjusted cooking power plotted over temperature difference and the resulting regression line.