T₀ = 288.15 K · P₀ = 101.325 kPa
Energy is conserved. Usefulness is not.
The First Law says you can never lose energy. The Second Law says you will always lose the ability to use it. Exergy is what you lose.
Global · primary to useful
Nine tenths of the work never arrives.
Global primary to useful exergy efficiency has been estimated at 10 percent for 1990 and 11 percent for 2008. Broader country level accounting puts it near 20 percent. Under every method, most of the exergy humanity extracts is destroyed before it becomes light, motion, or heat at the temperature someone asked for. That is not a technology gap. It is a measurement gap.
Nakićenović, Gilli and Kurz 1996 · Cullen and Allwood 2010 · Country level PFU database 2023
dS ≥ 0
The only law that knows which way time runs.
Heat moves from hot to cold and never back on its own. Every real process generates entropy, and every unit of entropy generated destroys exergy in exact proportion to the temperature of the surroundings. Gouy and Stodola wrote it down more than a century ago. Nothing in engineering has repealed it.
Ẋdestroyed = T₀ · Ṡgen
η vs ψ
Ninety percent efficient. Ten percent useful.
A gas furnace delivers about 90 percent of the fuel's energy into the house, and destroys about 90 percent of its exergy doing it, because it takes a flame near 1,900 K and uses it to hold a room at 294 K. First Law efficiency measures how much energy arrived. Second Law efficiency measures how much of what the fuel could have done, it did. Only one of those numbers tells you to install a heat pump.
Cumulative exergy demand
Every object carries its own thermodynamic debt.
A tonne of primary aluminium, a wind turbine, a battery: each is the sum of every exergy destruction that occurred to bring it into existence, from ore body to gate. Cumulative exergy demand counts that in a single physical unit, with no political weighting and no allocation argument. It is the one lifecycle metric a physicist, an accountant and a regulator can all check.
The planetary budget
The largest energy resource on Earth is the one we are already destroying.
Practical improvement to the passive systems at the end of every energy chain, the buildings, vehicles and processes that finally consume the work, has been estimated to cut global energy demand by as much as 73 percent. No new resource. No new fuel. Just refusing to destroy exergy we have already paid for.
Cullen, Allwood and Borgstein 2011
Est. 2026 · British Columbia
A society for the Second Law.
Exerginity exists to advance the science, measurement and application of exergy: to establish energy quality, and not energy quantity, as the basis on which energy systems are designed, financed and judged. We publish, we convene, we fund, and we maintain the reference library.
Authoritative Sources & Standards
