Environmental Engineering Reference
In-Depth Information
Gas stove
Electric stove
Microwave
Water boiler
Energy demand in Wh/litre
Costs for 10,000 litres in
Electric kettle
0
50
100
150
200
250
300
Figure 1.1 Prices for Water Heating
a part of the energy in the incident solar radiant energy is converted to
electrical energy, i.e. the photovoltaic system converts non-usable energy to
high quality energy.
Technical systems perform energy conversions with varying efficiencies.
The following example should illustrate this.
The thermal energy , Q , which is needed to heat up one litre of water (mass
m = 1 kg) from the temperature
ϑ 1
= 15°C to
ϑ 2
= 98°C is calculated with
the heat capacity, c , of water c H 2 O
= 4187 kJ/(kg K) using:
Q = c m (
ϑ 2 -
ϑ 1 )
(1.2)
to Q = 348 kJ = 97 Wh.
A consumer magazine has compared different systems for boiling water. Figure
1.1 shows the results of different electrical appliances and compares them with
those from a gas stove. The graph seems to show that the gas stove has the
highest energy consumption while the energy costs are the lowest. The
explanation is not the low price of gas, but that the graph compares different
energy sources.
The electric stove uses electrical energy for water heating. Normally, this
type of energy does not exist in nature, except for lightning or in electric eels.
Power stations convert primary energy sources such as coal, gas or uranium
into useful electricity. Conventional power stations produce large amounts of
waste heat, which is emitted into the environment. They convert only a
fraction of the energy stored in coal, gas or uranium into electricity, and the
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