Thermodynamic efficiencies

When considering the efficiency of energy use, it can be important to distinguish between efficiency as defined by the First Law of Thermodynamics and efficiency as defined by the Second Law. The second particularly applies when energy is used for heating.

A furnace used to heat a building may deliver say 80% of the energy released by full combustion of the fuel, the rest being lost through the pipes, flue, etc. That 80% is a First Law efficiency. An ideal thermodynamic device delivering 100 units of energy as heat to the inside of a building at a temperature of 20 °C from the outside at a temperature of 0 °C would require only seven units of energy. So the Second Law efficiency of the furnace is less than 6%.

Heat pumps (refrigerators or air conditioners working in reverse) are devices that make use of the Second Law and deliver more energy as heat than the electrical energy they use.15 Although typically their Second Law efficiencies are only about 30%, they are still able to deliver more heat energy than the primary energy required to generate the electricity they use. Because of their comparatively high capital and maintenance costs, however, heat pumps have not been widely used. An example of their substantial use is their contribution to district heating in the city of Uppsala in Sweden where 4 MW of electricity is employed to extract heat from the river and deliver 14 MW of heat energy.

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