The Law of Conservation of Energy does not seem to be a good indicator of the performance of energy systems in the built environment, as these systems achieve only a fraction of the theoretically possible performance. Focusing on exergy loss provides a much more accurate view of areas that could be improved. This is what Sabine Jansen proposes in her doctoral dissertation.
Her PhD research focuses on innovations from the past few decades, including heat pumps, waste-heat utilisation and cogeneration. These innovations have the potential to produce considerable energy savings. “Nevertheless, the performance of such systems is much lower than we would like and than it should be possible to achieve. Mapping out the exergy losses that occur will allow us to reduce the necessary input of high-grade energy”, argues Jansen. The use of ‘low-grade’ energy (e.g. ambient temperature) could actually be increased. In this way, the maximum yield of systems involving heat pumps could possibly be doubled, Jansen thinks.
Exergy is a concept that quantifies “the potential to produce labour” for various forms of energy. As compared to the usual energy analysis, exergy calculations offer the advantage of quantifying the ideal conversion from one form of energy to another. This allows comparison between the actual conversion and the ideal conversion. This is because, unlike energy, exergy can be destroyed. If you know how much exergy has been destroyed, you also know the ideal potential for improvement in a process or system.
In her study, Jansen enhances existing exergy analyses and provides insight into the exergy performance of current energy systems. She also suggests practical tips for approaching this in a better way. Reducing exergy loss can make it possible to achieve the same result with a greatly reduced input of high-grade energy. This therefore generates much higher performance. According to Jansen: “In theory, we could go down to 10% of our current energy usage by utilising minimal temperature differences, exergetically efficient generation (e.g. heat pump or cogeneration) and smart thermal storage."
She expects that the industry could benefit greatly from exergy analysis. For example, low-temperature heat pumps that work with small temperature differences could make better use of the quality of energy. Although there will always be some amount of loss, the current systems lose much more than is necessary. This is evident from a sample exergy analysis that Jansen included in her research. Her study also provides designers of the built environment with a tool for mapping out exergy losses, thereby reducing the amount of high-grade fossil energy need in buildings and systems. The current energy systems that are analysed in the dissertation have exergetic yields for heating that range from 3% percent (with a boiler) to 14% (with a heat pump). According to Jansen: “The theoretic ideal of 100% will never be achieved, but we should definitely advance to at least 25%. Exergy analysis shows that it is possible.”