1. Foundations of macro-scale exergy analysis
School of Earth & Environment
FACULTY OF ENVIRONMENT
Foundations of macro-scale exergy analysis
Paul Brockway
3rd International Exergy Economics Workshop
University of Sussex
14th July 2016

2. Foundations of macro-scale exergy analysis
Exergy: the basics
Macro-scale exergy analysis
Insights and implications
My role today is to cover the foundations of exergy analysis, such that we have enough knowledge to interact in the workshop.
By macro-scale I mean national or global level, since this is the scale of interest to us, e.g. for examining long run economic growth etc

3. What is exergy?
Exergy is known as “available energy” or “available work”
More formally exergy is defined as “the maximum amount of work that can be extracted from a system as it approaches equilibrium with its environment“
To illustrate how exergy accounts for its relative state (compared to outside environment) consider:
The LHS graphic – the water levels are the same (in equilibria), so no ‘work’ can be extracted
The RHS graphic – the difference in water levels means that work (via turbines) can be extracted.
So the types of dis-equilibrium (required for exergy to exist as available work) include potential, chemical, kinetic
Courtesy of Andre Serrennho

4. Exergy as a thermodynamic measure of energy quality
12V car battery versus the water molecules in this room
Hold the same energy
But the 12V battery has the energy in concentrated form
So we can see how exergy is a measure of (thermodynamic) energy quality. Let us keep that thought and see another example to reinforce this image
Both the car and the water molecules have the same energy (in thermal units),
but the car battery has chemical exergy difference relative to its surrounding environment. So we can extract work from the battery to power the car
The same is not true for the water molecules, they are in equilibrium with the rest of the building, so no work can be extracted
This is a key feature of exergy: its ability to measure thermodynamic energy quality
Courtesy of Science Europe

5. Exergy and thermodynamics
Exergy follows from application of two main thermodynamic laws:
First law (conservation of energy) and
Second law (not all energy can be converted into work)
Exergy is the convertible part of the energy; the remainder is anergy.
Energy = Exergy (usable energy)+ Anergy (useless energy),
Zoran Rant ( )
The inclusion of the second law is super-important. It thereby distinguishes the exergy (useful energy) from anergy (useless energy). Exergy (1956) and Anergy (1964) were both terms introduced by Zoran Rant.
Now recall the car battery – we can see now how it has lots of exergy, and very little anergy
This is in contrast to the water molecules: no exergy but lots of anergy.

6. Primary-to-final-to-useful exergy: light bulb
Useful exergy: “the heat or work usefully transferred by the device or system” (Carnahan et al, 1975)
Now I want to follow exergy through the energy conversion stages.
We start with primary to final stages of energy conversion. Energy economists are familiar with these two stages, and the IEA and others publish annual datasets for each country.
But the energy conversion goes further, to a useful stage – where in our light bulb example the useful energy is light.
The end stage is energy services, in this case illumination – this is what we are seeking.
Exergy can be estimated (in joules) at each stage (primary, final, useful). Its just (as we see here) the value of the usable energy (exergy) gets smaller each time, as we lose exergy via exergy destruction: second law (irreversible) losses.
The point about useful exergy stage is that it is the last point we can thermodynamically measure energy (as exergy) before it is lost and dissipates in exchange for energy services. This is why percebois (1979) preferred energy intensity (GJ/$) to be studied at the useful stage, since it “situates the analysis at the level of satisfied needs”
GEA Energy Primer, IIASA

7. Primary-to-final-to-useful exergy: car
Ok, now let us look at another example – a car
In this case we can see the primary stage (as oil), final stage (as diesel) and useful stage (as work done moving the forward)
The final energy service we seek is motion.
The motor and passive device losses leave useful exergy as ~ 20% of the energy content of the diesel, and still less of the starting oil
Useful exergy

8. Thermodynamic weaknesses of mainstream energy economics
Key macroeconomic energy questions remain:
How should we measure energy efficiency?
How large is the energy rebound effect?
How much energy will we need in the future?
Physical (GJ/tes), Monetary (GJ/$) Hybrid
Ok, so we have a better understanding of what exergy is, and the various stages: primary to final to useful
The next question is ok, so what? Why should we be interested? Well the point is that mainstream energy economics do not use second law based (exergy) efficiencies. Instead they use other metrics: physical, monetary, hybrid (e.g. ODYSSEE-MURE).
This means that the evidence base for key macro-economic questions (three shown here) can be built up from primary & final energy stages, and by considering energy efficiency via alterantive metrics. So including a national-scale efficiency metric based on thermodynamic energy quality may yield valuable insights.

9. Thermodynamic weaknesses of mainstream energy economics
This graphic illustrates what I mean. An existing mainstream energy economic modelling-to-policy chain exists. Energy efficiency and rebound can be included in such analyses, but they miss out second law approach. So they cannot be thermodynamically consistent, or at the very least, only give a view of the energy-to-economic interactions from one side.

10. Foundations of macro-scale exergy analysis
Exergy: the basics
Macro-scale exergy analysis
Insights and implications
So before we get to the really interesting bit (part 3) lets pass through and understand how to do national-scale exergy accounting. It gives important context.

11. National-level exergy accounting
Direct heat
Electricity
Transport
Manual Labour
A complete national-level time-series account of a single country in my (and others) experience takes a long time – maybe 6 months of work for one person.
First you have to collect primary energy data (mainly from IEA), then map through to useful exergy end use categories.
Direct heat (low/med/high) – follow primary to final to useful stages of energy conversion, estimate conversion efficiency including apply carnot temperature penalties
Electricity: find as much info as you can on electrical end uses: mech drive, light, heat/cooling. Allocate energy to those sectors, and attach time series exergy efficiencies. Calculate useful exergy for each end use
Transport – same as for the car, estimate useful exergy conversion efficiencies over time for each transport mode.
Manual labout
So we work through this in turn, for each sector over time for a given country.

12. National-level exergy accounting
To calculate useful work, we convert input exergy to end work done by end use, ie heat, mechanical drive, electricity applications and muscle work.
Take Mechanical drive: useful work is the residual energy moving the car forward after engine, drag, friction losses.
For heat, this is the 1st law energy flow transfer efficiency x 2nd law Carnot temperature penalty
So we work through this in turn, for each sector over time for a given country.

13. Foundations of macro-scale exergy analysis
Exergy: the basics
Macro-scale exergy analysis
Insights and implications

14. Insight #1: the observation of ‘efficiency dilution’ and how it constrains national-level efficiency
Practical limit to national-level exergy efficiency?
At a national-level, we see a divergence in trends between the US (flat) and the UK (rising). Both the UK and US exhibit ‘efficiency dilution’, except that the UK has stronger gains in device-level efficiencies which hide the dilution effect.
The observation of ‘efficiency dilution’ was a new finding for the UK and US. It means in these countries the gains of increasing device efficiencies (i.e. cars, boilers, lights) are being offset by using more of the less efficient processes. For example, it means using less high temperature heat and more low temperature heat (in UK). In the US it means they use a lot more air-conditioning (with very low exergy efficiency ~ 4%) than previously.
China has risen from agricultural to industrial super-power in 4 decades, and the efficiency gain (5 to 12%) reflects this.
Looking at the graph, it suggests there may be a practical limit on national-level exergy efficiency of 15-17%.
US-UK data from Brockway, P. et al (2014) Divergence of trends in US and UK aggregate exergy efficiencies , Environ. Sci. Technol. 48, pp.9874−9881

15. Insight #2: exergy analysis reveals the location
& magnitude of exergy losses
One of the key advantages (proposed in the literature) is exergy’s ability to pinpoint the location and magnitude of exergy losses. In turn, this means at an economy-level whether it is better to spend time/effort in the residential, commercial or industrial sector to reduce energy and (thus monetary) losses.

16. Insight #3: Primary energy vs useful exergy ‘decoupling’
Exergy also enables us to view energy decoupling from the other end of the energy conversion chain to normal.
Looking at this plot for the UK, we see primary energy decoupling from GDP.
However, at a useful exergy stage, there is less evidence of decoupling, since it increased by a factor of 1.5 since what supplied this gain (for the UK) was an increase of exergy efficiency, rather than primary energy. So the question emerges, if exergy efficiency gains are slowing (due to dilution), will we need more primary energy in the future?

17. Insight #4: Future energy demand may be
underestimated due to ‘efficiency dilution’
This slide starts to explore that suggestion. It shows China useful exergy increased 10-fold for , supplied by a 4-fold gain in primary energy and a 2.5-fold gain in exergy efficiency. if its useful exergy the economy needs (not primary energy), then continued gains in useful exergy will need to be supplied by increases in primary energy if efficiency gains slow – i.e. in the case of China its efficiency reaches an asymptotic limit of ~ 15%
2030

18. Insight #4: Future energy demand may be higher than we expect
We tested this assertion for China to 2030, projecting useful exergy and exergy efficiency to 2030, which then gave us estimates of future primary energy.
In our case, the results suggested primary energy by 2030 which was 20% than other published studies (IEA, EIA, BP). This suggests that existing methods (which work with primary or final energy) implicitly assume increases in exergy efficiency, which in reality will not occur to the extent supposed due to efficiency diltuion and overall stagnation.
Source: Brockway, P. et al (2015) Understanding China’s past and future energy demand: an exergy efficiency and decomposition analysis Applied Energy

19. Conclusions
Exergy analysis offers complementary insights to energy use and rebound versus traditional energy analysis
Energy efficiency and energy supply policies need to be account for:
Efficiency dilution
Energy rebound
Is it time to integrate exergy analysis into mainstream modelling-to-policy?
To conclude, the results suggest that the efficiency wedge in carbon reduction plans may not work as well as envisaged. So policies may need to increase renewables faster than currently planned.
Since exergy economics sits outside mainstream energy modelling, there is a chance to include it in mainstream models, to test its effect on results. This will help to make exergy analysis be COMPLEMENTARY to, not COMPETING with energy economics.
Exergy economics also needs to engage with policy makers, such at the recent Science Europe event on exergy aimed at European MEPs. This will help understanding in their community of its benefits, and perhaps start a ‘pull’ from policy makers to include exergy in energy-economic models.