International Standards for Testing Solar Cookers
from ASAE Standard S580
Dr. Paul A. Funk
USDA Agricultural Research
Service
PO Box 578, Mesilla Park,
NM 88047 USA
(505) 526-6381, Fax (505)
525-1076, pfunk@nmsu.edu
Figures Referenced by this text.
More Complete Version of This Paper in PDF format.
1.
INTRODUCTION
Increased awareness of the growing worldwide need for alternative cooking fuels has resulted in an expansion of solar cooker research and development. This expansion, combined with globalization and growth in the solar cooking industry, makes the existence of standards critical. The objective of this paper is to present and justify the new solar cooker test standard. A new test standard was first proposed at the Third World Conference on Solar Cooking (Avinashilingam University, Coimbatore, India, 6-10 January, 1997). A committee of technical experts was formed at the conference to draft a standard. The first draft benefited from the diverse perspectives of people with experience in many different countries and circumstances. Correspondence over the following months refined it. The publication peer review process then brought the standard to its finished state. The standard specifies reporting the cooking power when water inside pots is 50șC hotter than the ambient air. This number, like the fuel economy rating of an automobile, is not a guarantee of performance: it is useful for comparison.
2.
JUSTIFICATION
With solar cooking moving into the mainstream of economic society, more players are becoming involved and transactions are becoming more complicated. The existence of a common language is essential to a host of parties:
·
Researchers
discussing the results of a promising experiment
·
Social
leaders evaluating the suitability of various cookers to their culture and
climate
·
Activists
picking do-it-yourself plans for a grass-roots dissemination strategy
·
Civil
servants qualifying manufactured cookers for government subsidy or tax credit
·
Volunteers
planning a solar cooking program for a Non-governmental organization
·
Industrialists
selecting a design for mass production
·
Merchants
deciding on models to retail to their customers
·
Consumers
choosing a unit appropriate to the needs of their family
All of
these individuals need a fair and repeatable means to make rational economic
decisions. A universal means is
required to make comparisons between technologies. An accurate means is necessary to be predictive of performance
once climate variables are taken into consideration. A satisfactory means will also be easy to apply in the various
situations people concerned with solar cooking might find themselves, be it a
well-equipped research laboratory in a large city or some remote rural
location.
Test
standards have existed for some time.
Unfortunately they have not been widely employed in papers discussing
solar cooker performance. The Indian
standard used for deciding on subsidy (Anonymous, 1992) is for one specific
design. It is based on the test
standard proposed by Mullick et al
(1987), a well reasoned one based on the laws of physics. Though more complicated to use and less
universal than the one being recommended, that standard inspired the new
one. The objectives of the new standard
are as follows:
1)
To
make testing as simple as possible, so it is practical to use,
2)
To
present cooker performance in widely recognized units, so it is easy to
understand,
3)
To
present information that will be predictive of performance at different
locations, so it is readily transferable,
4)
To
differentiate between solar cooker designs, so it is useful, and
5)
To
be repeatable, so it can be trusted.
The test
standard committee considered these objectives when the standard was
drafted. Through much discussion and
compromise the objectives were balanced against each other so that maximizing
one objective did not conflict with realizing another. Funk (1999) demonstrated that these criteria
are satisfied using the new standard.
3.
THE STANDARD
Recognizing the need for both a common format by which researchers can
share results and for a single measure of performance to facilitate consumers
selection of solar cookers, the test standard committee convened at Coimbatore
9 January 1997 agreed that:
The one
figure best representing thermal performance is effective cooking power, which
accounts for both different cooker sizes and heat gain rates. The unit of power with which most people are
familiar is the Watt. The influence
test conditions have on results can be minimized if uncontrolled variables are
held to certain ranges. Therefore, the
committee recommends the following test procedure and reporting format:
3.1. Uncontrolled (Weather) Variables
3.1.1. Wind. Conduct solar cooker tests when wind is less than
1.0 m/s at the elevation of the cooker being tested. If the wind is over 2.5 m/s for more that ten minutes, discard
the test data. Reason: Heat loss is
strongly influenced by wind velocity.
Wind velocities less than 1.0 m/s help to maintain a heat loss
coefficient close to the natural convection loss coefficient, yielding results
that are more consistent and repeatable.
If wind shelter is required, it must be constructed so it does not
interfere with incoming total radiation.
3.1.2.
Ambient Temperature. Conduct
solar cooker tests when ambient temperatures are between 20 and 35șC. Reason: Ambient temperature extremes
experienced in one location may be difficult to replicate at another
location. Cooking power is influenced
by temperature difference. A range of
15șC keeps variability moderate, yet permits testing in most locations for at
least half the year. Unavoidable
exceptions need to be noted.
3.1.3. Pot Contents
Temperature. Record data for water temperatures between
40 and 90șC. Reasons: Pot contents must
be above ambient for there to be heat losses.
The latent heat of vaporization severely depresses apparent cooking
power as water nears boiling. Avoiding
the upper limit reduces the probability of having anomalies in the data.
3.1.4. Insolation. Available solar energy is to 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 render the test invalid. Reason:
Maintaining moderate fluctuations in insolation levels reduces the variability
caused by thermal inertia effects.
Taking readings within 65% of the standard insolation level (which is
700 W/m2) reduces errors introduced by adjusting cooking power for
available insolation. It is expected
that most locations will meet these criteria.
If not, exceptions need to be specially noted.
3.1.5. Solar Altitude
and Azimuth Angle. The committee strongly
recommends that tests be conducted between 10:00 and 14:00 solar time. Reason: Solar zenith angle is somewhat
constant at midday, and the difference between insolation measured in the plane
of the cooker aperture and the plane perpendicular to direct beam radiation
will vary least. Exceptions
necessitated by solar variability (presence of clouds at midday during monsoon
season) or ambient temperature (midday is too hot) must be noted.
3.2.
Controlled (Cooker)
Variables
3.2.1. Loading. Cookers are
to have 7 kg water/m2 intercept area distributed evenly between the
pots supplied with the cooker. Intercept
area is defined as the sum of the reflector and aperture areas projected onto
the plane perpendicular to direct beam radiation
(Figure 1). The beam radiation zenith angle may be
averaged over the test period. Tracking
may compensate for the beam radiation azimuth angle. These two strategies should result in a constant intercept area,
facilitating load calculations.
Reasons: Water closely resembles food in density and specific heat, but
is more consistent. Intercepted
radiation is the best measure of available energy. Thermal performance is sensitive to loading rate. This particular value is close to the
various loading rates cited in previous publications.
3.2.2. Tracking. Azimuth angle tracking frequency must be
appropriate to the cookers 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 pot 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.
3.2.3. Temperature Sensing.
Thermocouples are recommended for their low cost, accuracy and rapid
response. Use pot(s) supplied with the
cooker. If unavailable, use inexpensive
aluminum pots most likely to be employed by the consumer. Thermocouple junctions should be immersed in
the water in the pot(s) and secured 10mm above the pot bottom, at the
center. Illustration of recommended thermocouple
mounting. Thermocouple leads
are to come through the pot lid (or wall above the water line) inside a
thermally nonconductive sleeve that will protect the thermocouple wire
from bending and from temperature extremes. Secure the sleeve with silicone caulk to reduce vapor
loss. Reasons: Proper thermocouple
placement can minimize errors that might be caused by thermal
stratification and sensor intrusion into the pot. The thermal storage capacity of
inexpensive aluminum cooking pots is insignificant compared to the thermal
storage capacity of the water contained by them.
3.3. Test Protocol
3.3.1. Recording. The average water temperature (șC) of all the pots in one cooker
is to be recorded every ten minutes, to one tenth of a degree if possible. The solar insolation (W/m2) and
ambient temperature are 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. Reason: Ten minutes is a long enough time
that the minor fluctuations in heat loss due to ambient temperature and wind
variability are expected to be negligible.
Ten minutes is a short enough time that the heat gain variability due to
gradual sun angle changes may be considered constant during the interval.
3.3.2. Calculating
Cooking Power. The change in water temperature for each ten
minute interval is to be multiplied by the mass and specific heat capacity
(4186 J/kgK) of the water contained in the pots. Dividing this product by the 600 seconds contained in a
ten-minute interval yields the cooking power in Watts. Reason: Solar cookers must heat food, and
sensible heat gain in a cooking pot is the best measure of a cooker's ability to
effectively heat food.
3.3.3. Calculating
Interval Averages. The average insolation,
average ambient temperature, and average pot contents temperature are to be
found for each interval.
3.3.4. Standardizing
Cooking Power. Cooking power for each interval is to be
corrected to a standard insolation of 700 W/m2 by multiplying the
observed cooking power by 700 W/m2 and dividing by the average
insolation recorded during the corresponding interval. Reason: To facilitate the comparison of
results from different locations and dates.
3.3.5. Temperature
Difference. Ambient temperature for each interval is to
be subtracted from the average pot contents temperature for each corresponding
interval. Reason: Heat loss increases
with the difference in temperature between the solar cooker interior and the
cooker's surroundings; pot contents temperature correlates to cooker interior
temperature.
3.3.6. Plotting. The standardized cooking power (W) is to be plotted against the
temperature difference (șC) for each time interval.
3.3.7. Regression. A linear regression of the plotted points is to be used to find
the relationship between cooking power and temperature difference in terms of
intercept (W) and slope (W/șC). At
least thirty observations are required.
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% or specially noted. Reasons: Statistical measures of goodness of
fit for the regression line require a fairly large sample, and systematic
errors are less likely to be repeated on different days. Excessive experimental error may invalidate
the test.
3.3.8. Single Measure
of Performance. The value for standardized
cooking power (W) is to be computed for a temperature difference of 50șC using
the above determined relationship.
Reason: One single number in common units familiar to most consumers
best facilitates the comparison of different devices. A temperature difference of 50șC strikes a balance between
overemphasis on the startup cooking power (where concentrating ovens are
strongest) and stagnation temperature (where box cookers tend to be superior)
and is just below that critical temperature where cooking begins to occur, the
temperature when a solar cooker succeeds or fails. NOTE: For product labeling and sales literature it is strongly
recommended that this number be calculated from a regression found by an
independent laboratory using a statistically adequate number of trials.
3.3.9. Reporting. Plot the relationship between standardized cooking power and
temperature difference, and present the equation. State the cooking power (standardized) at a temperature difference
of 50șC.
4.
APPLICATION
Four
solar cookers with the same sized cooking chamber and the same number of pots
were built with either good or poor insulation and either a large or a small
solar intercept area. The two levels of
intercept area were realized with one reflector (0.293 m2) or four
reflectors (0.966 m2). The
two levels of heat loss were realized either by having glazing (4.8 W/m2K)
or by leaving the cooking chamber unglazed (7.6 W/m2K). These cookers were tested according to the
standard. Figure 2 shows a plot of the
cooking power regression line for all four cookers. Each line is an average of four days of data, normalized for
insolation according to the standard.
The two cookers with large collector areas both enjoyed a high initial
cooking power (zero-intercept). The two
unglazed cookers both suffered a rapid decrease in performance as the
temperature difference increased. Their
high heat loss is reflected in the regression line as a steep negative
slope. The slope of the cooking power
regression line correlated to the heat loss coefficient independent of the
solar intercept area.
A solar
cooker was tested according to the standard in November of 1998, with automated
data recording (using an electronic datalogger) in Las Cruces, New Mexico. The results were compared with data taken
using the same cooker eighteen months earlier, in May of 1997, with hand held
instruments in Tucson, Arizona. Tucson
and Las Cruces are at the same latitude but different elevations (728 m and
1183 m, respectively). There also was a
big difference in solar altitude due to the different time of year of the two
tests. The cooker held five pots, each
filled with 424 grams of water. Water
mass was found volumetrically in a graduated cylinder calibrated with an
electronic balance. Other instruments
used in the first test were thermocouple wire (US$ 15) a thermocouple reader
(US$ 80) a radiation pyranometer (Li-Cor 200SA, US$ 180) and a volt meter (US$
100). Figure showing
tools used in the first test. The pyranometers current
signal
was passed through a 100 W resistor to get
voltage. The measured voltage was
converted into W/m2.
Insolation and temperature were recorded by hand in a notebook every ten
minutes. Although hand held instruments
did not provide as many observations as the electronic datalogger did, they
still gave enough data points to follow the test protocol and to fit a curve
with an R2 of 0.95.
Figure 3 illustrates results from the two
tests.
Performance decreased slightly with age.
4.3. Conclusion
The test standard can be followed independent of
infrastructure using hand-held instruments that cost less than US$ 400. The cooking power regression line clearly
distinguishes between cookers. Results
appear to be transferable and repeatable.
This test standard should serve the solar cooking movement well as it
continues growing.
Anonymous. (1992). Indian Standard- Solar Cooker- (3 Parts) IS
13429. Bureau of Indian Standards, New Delhi.
Funk P.A. (1999). Evaluating
the international standard procedure for testing solar cookers and reporting
results. 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.
