Comparison of Heating Efficiency by Microwave and
The typical methods of heating in chemistry
laboratories involve the use of electric hot plates, Bunsen burners and heating
ovens. Depending upon the application, microwave heating can be used to replace
these conventional methods. Microwaves works by directly coupling with the polar
molecules in a substance. This causes the particles to move and rotate which in
turn generates heat. Microwave ovens operate with electromagnetic non ionizing
radiation with frequencies between 300 GHz and 300 MHz. The corresponding
wavelengths span a range from 1 mm to 1 m, which places microwaves in-between
that of infrared radiation and radio waves. Most commercial microwave systems
however utilize irradiation with a frequency of 2450 MHz (wavelength λ = 0.122
m) in order to avoid interferences with telecommunication devices. The
corresponding electric fields oscillate 4.9 X 109 times per second and
consequently subject dipolar species and ionic particles (as well as holes and
electrons in semiconductors or metals) to perpetual reorientation cycles. This
strong agitation leads to a fast non-contact heating that is (approximately)
more or less uniform throughout the radiation chamber. Consequently, faster
heating rates are expected in microwaves.
In order to give more emphasis on the energy
aspect of these experiments, it is important to compute the actual power
consumption in both conventional and microwave heating methods. Every heating
system has in-built temperature sensors that turn on/off according to the
temperature control set in it. Therefore, the heating device consumes energy
only when the power is on. A simple calculation that uses the product of power
rating and time led to erroneous results. An inexpensive solution that could be
employed is a commercial power meters that can be connected online to the
heating device or the microwave. The one selected for this project is called
P3 Kill-A-Watt (P3 International Corporation, NY, USA) power meter, which is
P3 Kill-A-Watt meter is shown in Figure
1. It is a meter that is used for measuring the power consumption of house hold
appliances. This device records six parameters which including voltage, current,
wattage, frequency and energy consumption. The measurements are displayed on an
LCD screen. The power meter is connected to the main power supply unit while the
outlet of the appliance is connected to the power meter. The total power
consumed over a period of time is measured in units of Kilo Watt hour (KWH).
Both volts and amperes are measured using true RMS method.
diagram (left) and photograph (right) of a Kill-A-Watt power meter.
Electric hot plate
Kill-A-Watt power meter
1000 ml beaker
The efficiency of the two methods was compared
by using 1000 ml of water in two separate 1000 ml glass beakers and heated by
microwave and electric hotplate for different periods of time. The power rating
on the hot plate was 865 Watts (51.9 KJ/min) and that of microwave oven was 850
Watts (51 KJ/min). The rise in temperature as a function of time was recorded in
each case. The experimental setup is shown in Figure 2. In another set of
experiments the effect of volume was computed as follows. Hundred ml volume of
water was initially heated for a period of 3 minutes using hot plate. Different
volumes in the increments of 100ml were heated for 3mins and the final
temperature in each case was recorded. The experiment was repeated with
microwave heating, and the corresponding temperatures were recorded.
Experimental setup using the power meter.
Results and Discussion
In the first set of experiments, the variation
in temperature as a function of volume of water is presented in Figure 3 (a).
The temperature decreased significantly as the volume of water increased in both
convective and microwave heating. As the volume of water increased, it consumed
more energy to raise the temperature. We observed significant decrease in
temperature with convective heating, as heat transfer limitations are more
prominent with convective heating. For the same volume of water heated for same
duration of time, we could see a temperature difference between 20 - 40˚C, with
microwave heating being significantly more efficient. The energies consumed by
the two heating devices for heating same volume of water for the similar
duration of time are shown in Figure 3(b). The trends were similar, but the
absolute numbers were different for the two devices although their power ratings
were very close. The microwave consumed less energy but reached higher
temperature, which clearly demonstrated the higher efficiency of this approach.
(a) Plot of temperature as a funtion of volume at aconstant time duration of 3
mins. (b) Plot of energy consumed as a function of volume for a
heating time of 3mins.
Figure 4(a) shows the temperature of water as
a function of time at a constant volume of 1000 ml. During microwave heating,
the water temperature increased steadily and leveled off near the boiling point.
The rise in temperature was much more gradual with the hotplate. At high
temperatures, the vapor pressure of water was quite high, and some water was
lost via evaporation. The rate of increase in temperature with increasing time
was clearly higher for the microwave than for heating on the hotplate. Figure
4(b) shows temperature reached as a function of energy consumed by each device.
It is quite evident that for the same energy consumption, the microwave is a
significantly more energy efficient device.
(a) Plot of temperature as a function of time for heating 1000ml of water. (b)
Plot of temperature as a function of energy corresponding to Figure 4 (a).
Energy consumed by both heating devices as recorded by power meter:
Power Rating (KJ/min)
Actual Energy Consumed (KJ)
Difference in Calculated and Recorded Values:
Energy = Power x Time
Energy Consumed = 51.0 KJ/min x 3min
= 155.7 KJ
Hot Plate: Energy
Consumed= 51.9 KJ/min x 3 min
= 153.0 KJ
Therefore, the percentage of
error for microwave oven is,
and the percentage of error
for hot plate is,
For the same amount of energy
applied by each heating system, microwave heating reached higher temperatures.
As the volume of water increased,
more microwave energy was absorbed and overall efficiency increased, the
temperature reached by microwave was significantly higher than the convective
The power meter was important for
computing the energy consumption. Direct computation lead to 40-50% error.