Energy & Exergy
Energy can not be consumed, energy can only
be converted from one energy form to another.
Energy is invariable!
For example, the chemical energy in the fuel is converted to heat energy and pressure in a combustion engine. These sources is transformed in turn into kinetic energy in the motor interior and further to the gearbox in the wheel axles and on the wheels. The car has thus kinetic energy, and we can transport us. On the road from the petrol to the wheels takes a lot of energy conversions, these are associated with losses in the form of eg friction. The petrol tank we used really pretty bad. The efficiency of a petrol engine is no better than about 30%. Additionally, we lose many of these additional percentage of energy transformations from the crankshaft to the wheels, see the picture.
Yet if we were to analyze the energy that we put into the engine, we could find back every Wh on the road out of the car. The largest amount of energy converted to heat in the exhaust gases in the cooling system and as friction in bearings etc. This means that the energy we entered comes out again, but in other forms than gasoline energy.
The energy form should be used for the correct task!
Modern electric motors have an efficiency of up to about 98%. Exergy of electricity can thus be set to 0.98 and 10,000 kWh of electricity then contains 9,800 kWh of exergy.
Oil burned to heat a house has an exergy of 0.84. This makes the following, 1 m3 of oil contains 10,000 kWh of energy and thus 8500 kWh exergy.
District heating with a temperature of 80 ° C which is converted to the room air of 20 ° C has exergy 0.17, providing to 10,000 kWh of district heating thus contains 1,700 kWh exergy.
We heat a house with 10,000 kWh, that means we consume:
9800 kWh exergy if we heat the house with direct electric,
8500 kWh exergy if we heat the house with oil or
1700 kWh exergy if we heat the house with district heating.
Exergy for district heating is of course in a previous energy conversion stage depends on how long the heat generated. Is the district heating a residue from elalstring that the CHP will exergy as low as 0.17.
The use of electricity for heating is exergy consumption!
Electricity with its high exergy should only be used for tasks electricity are good at. The electricity is most useful as lighting, to power electric motors or computers etc preferably in industry and other value-creating activities. Electricity should not be used to make 20 degrees in the house, this is because electricity has an exergy very close to zero. This also applies when heating with heat pumps of all kinds, beacuse the electricity is still the energy form in this process.
For the planet earth to survive and for our children and grandchildren to have a place to live, we have to use the right energy form for heating. Therefore you should choose a heating system for any kind of solid fuel heating, district heating or waste heat from eg industries.
No one can do everything, but everyone can do something!