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Exercising the benefits of exergy

Cutting energy bills and reducing emissions of pollutants are both high on the agenda and finding sustainable power sources and improving efficiency are usually the concepts we think of when trying to action those points. However, there is a problem. Heating buildings and water, cooking food, switching on the lights all require energy but they don’t all require the same “quality” of energy. Maintaining a room at a temperature the occupants find comfortable does not need such high-quality energy as does running electric lights.

I’ve touched on this elusive notion of the “quality of energy”, or exergy before. It is defined by scientists in terms of thermodynamics as being the amount of useful work a flow of energy can do. In practical terms one might consider the different uses of energy within a building. To maintain a room at 20 Celsius when it is much colder outside requires energy in the form of heat, but the supply need only be slightly higher than 20 Celsius, assuming the walls and roof are relatively well insulated and no one leaves windows and outside doors open. Looking at the thermodynamics though and we can see from the Carnot efficiency relation that the “quality”, the exergy, need only be 7% or so. There should be no need to operate a boiler with very high combustion temperatures and to have a heating system running at 70-80 Celsius if a building could be designed to maintain a comfortable temperature with a so-called low-exergy heating system, e.g. a ground source heat pump supplying a low-temperature floor heating system

Similarly, the production of the hot water in a building for washing, bathing and other activities usually requires it to be heated to about 55 Celsius. A “boiler” running at a much higher temperature than that is producing energy with far too high an exergy just to heat water. The exergy need only be 15% or so. In contrast, the efficient operation of electrical appliances and lighting requires the highest possible exergy of close to 100%. That said, incandescent lighting represents an incredible waste in that much of the energy is converted to heat rather than light and in motorised appliances heat is generated through friction and so wasted.

Unfortunately, burning fossil fuels does not give us a simple way to use energy at the optimal exergy level for different applications and so huge amounts of fuel are wasted raising temperatures above what are needed for a specific use. Dietrich Schmidt of the Department Energy Systems at the Fraunhofer Institute for Building Physics in Kassel, Germany, thinks he might have the answer. It should be possible to create so-called low-exergy, LowEx, buildings that exploit local heating systems and networks to fulfill the requirements for keeping rooms warm, for instance, without using a combustion process or boiling large tanks of water. Heat exchangers are usually utilizing low-quality energy in terms of the energy they supply, but if the heat is a few degrees above 20 Celsius, then that is adequate. No need to fill radiators with water at high temperatures or power up oil-filled heaters to even higher temperatures.

Schmidt has developed a model to allow architects and those running buildings a way to model the energy requirements and ensure that their buildings are “LowEx”. By adopting a “LowEx” approach it might be possible to increase the use of renewable energy sources for space heating purposes, cut fuel bills and lower emissions considerably.

Research Blogging IconSchmidt D. (2012). Benchmarking of low ‘exergy’ buildings, International Journal of Exergy, 11 (4) 473. DOI:

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