Exergy and Power Plants

By Andy May

Key question: Can renewables ever replace fossil fuels and nuclear?

Understanding the value of renewables, vis-à-vis fossil fuels and nuclear power, requires that we consider that all energy is not equal in value. In fact, the quantity we call energy can be misleading and many experts prefer the quantity called “exergy,” which is defined in economics as (source Exergy Economics):

The maximum useful work which can be extracted from a system as it reversibly comes into equilibrium with its environment.”

Or it can be thought of as the measure of potential work embodied in a material or device. As Ayres, et al. (1998) argue exergy is a more natural choice as a measure of resource quantity than either mass or energy. Even today it seems BTU’s, a measure of heat of combustion, or MToe, million tonnes of oil equivalent, are commonly used and mislabeled energy (see the Exxon Outlook, 2017 or the BP Energy Outlook, 2017). In a previous post (here) I discussed EROI, or energy returned from energy invested. I complained in that post about the inconsistency and inaccuracy in current EROI and LCOE (Levelized cost of electricity) calculations. The problems mostly stemmed from comparing energy or electricity output from different sources (solar, wind, natural gas, coal, nuclear) as if all produced energy was equally valuable, which it isn’t. While comparing the heat of combustion or million tonnes of oil equivalent is clearly incorrect, Rud Istvan and Planning Engineer show that comparing the cost of producing megawatts of electricity, like the IEA and EIA do, is also incorrect, see here and here. Since exergy is a measure of useful work, it helps get around that problem. In a comment to that post, Captain Ike Kiefer posted a reference to Weißbach, et al. (2013) which has a much more valid EROI comparison (see figure 2) of conventional and renewable electricity sources in Germany. Since Germany is, in many ways, a testbed of renewable energy sources for the world; this is very helpful.