It is well known and stated to the point of being cliché that the sun provides us with more energy than we could possibly use, and that we should harvest this energy continuously, instead of relying on a limited supply of non-renewable sources.
From a Cradle to Cradle point of view, given that we have this abundance of energy, we describe issues related to energy as materials management challenges rather than energy shortages. Utilizing a cradle to grave approach with non-renewable resources depletes valuable materials. (Harmful) emissions are materials that need to be managed and energy systems which harvest current solar income should be built with appropriate, fully recyclable materials and should be designed in such a way that they are not harmful to people and the environment. When energy systems are healthy and beneficial, their efficiencies become less important as long as the systems and business cases are economically sound and competitive.
In Cradle to Cradle, energy is therefore approached from a quality perspective.
This is not to say efficiency is disregarded; it is not an end-goal but it can for instance be a tool to come to a tipping point. In this blog, I’d like to argue the concept of exergy as an even more valuable approach to energy challenges. Exergy is something surprisingly little people are acquainted with and it is often misunderstood or found to be confusing, so there is a challenge of some sort involved in explaining it concisely and understandably, and in keeping true to its technical definition while omitting the technicalities. Here’s a shot:
In a simplified sense, exergy is an indication of the usefulness of energy in a certain form. We all are aware of the fact that energy exists in many forms; electric, kinetic, thermal, chemical and many more. In all of these, the amount of energy in a body, or system, can be expressed in the same unit; the joule. This means that we can have an equivalent amount of energy in different forms, for instance in natural gas, electricity, or hot water. We are all also familiar with the law of conservation of energy. We know that when energy of a certain form is converted to energy of another form, no energy is lost, it just is converted to a different form of energy than the one(s) we desire. This is what we mean when talking about efficiencies.
So far so good.
Exergy is a measurement of what we can do with energy. It is in a sense the quality of energy. More technically; it is the amount of energy which can be converted to work. There is no law of conservation of exergy and indeed exergy can actually be lost, and is lost in practice.
The exergy of energy in a certain form can be expressed in joules, just like energy. The exergy factor of energy in a certain form can be expressed as the ratio to which this energy can be converted to work. To illustrate the difference between energy and exergy let us first look at an example of energy in two forms; natural gas and hot water.
Natural gas represents energy which has very high exergy content. Intuitively this should make sense as it is a very flexible source of energy. It can be used for different applications including propulsion, heating and the generation of electricity. Actually, nearly all of the energy in natural gas can be converted to work. Its exergy factor is almost 1. Hot water on the other hand, is much harder to convert to work, and the ability to do so depends on the temperature of the water. The exergy factor of water is lower than the exergy factor of natural gas, even if the amount of energy in both is the same. This is not due to efficiencies that might be improved, but fundamental to its nature.
Consider now the common situation of burning natural gas in order to heat a building. In this process, natural gas (which has high exergy content) is converted to a flame of about 1200 degrees Celsius, which is used to heat up water to about 70 degrees which is used to heat up the air in a building to around 20 degrees. In the end the amount of energy is maintained. However, the exergy of the water at 70 degrees centigrade in the boiler is extremely low. It has an exergy factor of only 0.146, which means that it gives that much opportunity for the extraction of work. In the conversion from gas to heat, more than 85% of its exergy was destroyed. Could we not have done much more with this gas? Or with the flame of 1200 degrees? And maybe more importantly: Should we be burning this valuable material which has so much potential, to warm up large volumes of air by a couple degrees?
Looking at exergy is refreshing as it provides us with a new point of view on energy systems. From this vantage point we can ask questions that may sound familiar such as: Are we doing things the right way, or are we perfecting the wrong things? It gives us an insight which, at least in some cases, can be complementary to Cradle to Cradle viewpoints. It leads us to the core of the discussion:
Why is a near 100% efficiency HRe boiler not the perfect solution?
What are appropriate technologies for various energy purposes?
These questions may be difficult to answer, but they are important nonetheless. There are more dimensions to energy systems than their energy efficiencies and these can be calculated and measured as well. Exergy can be used as a tool to help us move in the right direction and to improve and innovate our energy systems and usage.
Reading tip: Sabine Jansen’s Exergy guidebook for building professionals
Note: The figures represent 1; A boiler flame. 2; A heat pump, which is exergetically interesting as a means of providing heat of a low temperature. 3; Earth’s current solar income versus the global energy consumption. 4; A nuclear landmine, together with the title in analogy to the excessiveness of using energy with high exergy content for low exergy purposes.
Scientific Project Manager