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Exergy and thermal experience

Posted March 9th, 2010 by Christopher Tweed

I have avoided using or engaging with the term “exergy” mainly because I’ve never quite understood it. Until now. I came across this post by John Michael Greer and it has helped me make connections between a few key ideas I have been thinking about recently.

Many discussions about renewable energy sources seek to demonstrate they are capable of replacing existing fossil fuel energy sources because they yield the same amount of energy over similar periods of time. In the extreme case it is argued that the amount of solar energy falling on the earth is numerically much greater than we currently need. This may well be so, but the crucial point is not the quantity of energy but the availability of that energy to “do work.” It is this variability in the quality of energy sources that undermines what is sometimes referred to as “the swap” in which current energy sources are simply replaced by renewables to support “business as usual.”

In the context of heated buildings, the ability of a source of energy to “do work” can be interpreted as delivering warmth to occupants. But as the post on exergy suggests, the concentration of heat is important and concentrated sources of warmth indoors are only available from fossil fuels. The erroneous assumption often made about warmth is that it doesn’t matter how it is delivered as long as it is capable of creating a comfortable environment. However, we know thermal comfort depends on the recent experience. If I return home on a cold day, what I want is not a uniform level of heating, which is increasingly the norm in new, highly insulated dwellings with small heating systems, but a high temperature heat source that will help me recover from the outside conditions quickly. There is an aesthetic pleasure to this which should not be underestimated.

My hunch is that if we examine thermally related behaviour in buildings we will find much of it is driven by the pursuit of thermal experience that often lies outside the conventional understanding of thermal comfort. It is important to take this on board when we consider the delivery of heat and warmth in dwellings because if we don’t, it is highly likely that the occupants will take steps to restore enjoyable thermal experience, regardless of the consequences for energy consumption or carbon dioxide emissions.


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