Use of gas engines to generate electricity and waste heat for heating and cooling (called a cogeneration system) can provide similar increases in efficiency. In colder climates, heat from the earth, which is warmer than the air, could be pumped up to the desired room temperature, with similar efficiency improvements. In moderate climates this could improve efficiency of natural gas use by four times or more. ) Another way of describing it is as an air-conditioner in reverse that blows warm air into a building rather than out of it. (A heat pump uses electrical energy to pump up the energy present in outside air or soil to room temperature and transfer it into a building. The electricity could then be used to “pump” the heat from the cold air outside up to the desired room temperature. Measuring this system using the second law of thermodynamics allows us to see that the initial natural gas input could be used more efficiently and to greater benefit if its heat were not wasted.įor example, one could use natural gas as a hydrogen source for fuel cells to generate electricity at 60% efficiency (second law). Hence a typical natural gas home heating furnace has a high first law efficiency, often around 85 or 90 percent, but a low second law efficiency of only a few percent (depending on outside temperature). Thus, while most of the quantity of energy in the natural gas is transferred to the air that is used to heat the building, the capacity of the natural gas to do work has been almost entirely wasted. (This example could also apply to systems that provide hot water for heat.) A typical natural gas heating system degrades the heat of natural gas from possible temperatures over 1000 degrees Celsius to about 50 to 80 degrees Celsius. Let us consider the example of a natural gas heating system that provides warm air for heating a building. Energy at 20 degrees Celsius provides a comfortable living environment, but is essentially useless for producing mechanical work in everyday situations. Thus a kilogram of steam at 1,000 degrees Celsius will produce more mechanical energy than steam at 500 degrees Celsius other things (such as pressure) being equal. The temperature at which energy is available is a good measure of its quality - the higher the temperature of the energy, the more mechanical work we can theoretically get out of it. This maximum theoretical efficiency, called the Carnot efficiency, allows us to compare how well any particular real-world energy-using system is performing relative to the maximum theoretical performance. In 1824, a French physicist, Nicholas Léonard Sadi Carnot, described the most efficient (ideal) engine for converting heat to mechanical work. By contrast, the second law of thermodynamics allows us to know how well an energy system performs in terms of the quality of the energy. The first law of thermodynamics states that energy is conserved even when its form is changed, as for instance from mechanical energy to heat. Application of the second law of thermodynamics helps explain the various ways in which engines transform heat into mechanical work, as for instance in the gasoline engine of a car or in a steam turbine.Įfficiency measures based on the second law of thermodynamics take into account the quality of energy - unlike efficiencies based on the first law of thermodynamics which take into account only the amount of energy. The second law of thermodynamics states that you can move heat from a hotter place to a colder place without doing work, but that you need to work to move heat from a colder place to a hotter place.
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