Dr. Birol Kilkis
F-ASHRAE, Exceptional Service and Distinguished Service Awards ASHRAE
Science Encouragement Award, TUBİTAK, 1981
Country Manager, World Alliance for De-Centralized Energy
Professor Baskent University and Chair of Energy Engineering Graduate Program
Honored Member, America’s Registry of Outstanding Professionals
Executive Committee Member EU ESTTP
TTMD and Polar Teknoloji, Ankara Turkey



Decoupling sustainable development from CO2 emissions and global warming is the most urgent challenge of the World. While energy is required both in the forms of power and heat for sustainable growth and urbanization, CO2 emissions follow a parallel trend with sustainable growth. i.e. CO2 emissions continue to increase with sustainable development although 20+20+20 goals of EU are in place. These measures reduce the rate of increase in CO2 emissions but cannot de-couple the relationship. This paper claims that exergy rationality may achieve the desired decoupling by following new recommended metrics for Decarbonization goals of the EU and gives a practical example of district heating with solar PVT panels.

1. Introduction

‘We are paying for the quantity of energy but we are using only the quality (Exergy) of energy’

Prof.dr. Peter Novak[1]

The 20+20+20 goals of EU, namely 20% increase in energy savings, utilization of renewable energy sources, and efficiency, respectively each, may all reduce the CO2 emissions rate but are not sufficient to reverse (decouple) the ongoing parallel trend with sustainable growth. In Eq. 1 and Figure 1, these three goals are supplemented hereby with a fourth goal, namely exergy rationality, expressed by the term ψR. This is the rational exergy management model (REMM) efficiency, which watches the balance between the quality of energy among supply and demand points in the built environment. In Figure 1 HSDI is the Human (Sustainable) Development Index defined by UNDP and will replace HDI (Human Development Index) only if CO2 emissions rate falls below the 1990 level. Eq. 1 shows that CO2 emissions may be substantially reduced by increasing the exergy rationality, because there is a large window of opportunity between the current global ψR average of 0.20 and the practical bound that may exceed 0.70, without facing the dilemma of the diminishing returns, like the other goals of EU strategies face today.

It is clear that, if and only if, CO2 emissions are reduced by increasing ψR by reorganizing human activities by exergy rationality to such a level that natural sinking mechanisms assisted by artificial capture of CO2 emissions can take over and let CO2 emissions go below 1990 levels. Eq. 3, and Figure 2 show that exergy destructions must be minimum.



{Exergy destroyed upstream}



{Exergy destroyed downstream}



Figure 1. Phases of De-Coupling Between Emissions and Sustainable Development: Role of Exergy.

Here ε is the unit exergy (W/W) defined by the ideal Carnot cycle between two temperatures T1 and T2of a process.



Current EU strategies about electrification of heating and cooling with heat pumps do not make much sense for the Second-Law of Thermodynamics: a natural gas-fired thermal power plant burns natural gas at Tf = 2200 K and electricity is used by a heat pump with a COP of 3 for radiant floor heating at supply and return temperatures of 330 K and 320 K, respectively. Then, exergy-based COP, namely COPEX is less than one due to large exergy destructions (See Figure 2) and ψR, which is responsible for CO2 emissions is quite small. For an exergy-rational system (COPEXà1) COP must approach to 28.7, which is unpractical today. 283 K is the reference temperature, Tref.

{See Equation 2}






Figure 2. Exergy Flow Diagram (Eq. 2).

2. Justification of the Study

Today’s limited ability to cope with the real issues that affect sustainability, global warming, and energy security, originates from the fact that all energy projections, model studies, carbon emission predictions, and mitigation calculations, international protocols, and energy system designs are based only on the quantity of energy efficiency, leaving the root cause beyond touch. In fact, thermodynamic irreversibility that impacts harmful emissions with the end result of global warming lies within the new scope of rational exergy management, which deals with the quality of energy resources and the quality of energy required for different applications in the built environment. With current systems, which largely count on fossil fuels, there is an unbalance among them, that compounds energy spending and harmful emissions because the opportunities of fully utilizing the quality of energy resources are missed. A simple exergy input and output type of exergy balance, similar to the energy balance cannot provide the scale of the unbalances taking place today at large either. Figure 2 is the fundamental tool shown above to find the missing piece of the puzzle and establish the missing link. A key definition and a new methodology are necessary to bring all the energy supply, demand, and environmental issues and parameters on a common platform with a unified metric of rational allocation of energy resources in both quality and quantity. This is especially essential with the diversification, hybridization, and systems approach of the EU along with the increasing share of renewable energy resources, smart cities, and low-energy and low-exergy buildings (Sustainable and green buildings). All EU Directives, being issued so far are based however on the First-Law, which recognizes only the quantity of energy. In fact, every energy resource has a different quality, named exergy, which is the amount of useful work that can add value to the society and to the built environment at large, if utilized properly from a given amount of energy of a given type and quality. According to the Second-Law, it is important to use the right quality of energy at the right balance of quality demand at a given application, at the right time, and at the right order in addition to the right quantity (energy). Therefore, it is crucial to establish also a quality balance among different sources of energy having different exergy and different energy demand points at different exergy. This requires a new exergy allocation methodology and re-wiring of energy sources among all energy demand points. Even the forms and quantity of energy sources are balanced, with the demand points, remaining exergy imbalances cause exergy destructions, which are responsible for additional energy demand and additional – but avoidable – CO2 emissions, which are all invisible to the First-Law. Rational Energy Management Model [1] is the potentially key rescue mission to global warming and decoupling issues. Sustainable buildings of today are thermodynamically interconnected to the built environment, because buildings generate, share, and consume energy with all elements of the built environment, which also need to be sustainable. Such a level of interconnection along with the hybridized utilization of renewable energy sources, including reject heat of different types, temperatures, and exergy also makes it necessary to acknowledge and identify the existence of different sources and demands of exergy in different forms, temperatures, and locations. This further necessitates a holistic approach.

3. EU Literature

The only European initiative about the inclusion of exergy in EU energy analyses is published by Science Europe [2]. In appreciation and thanks to the motivation and insight into the subject matter by the Authors of this important publication, namely Dr. Paul Brockway, University of Leeds, Professor Jo Dewulf, University of Gent, Professor Signe Kjelstrup, Norwegian University of Science and Technology, Professor Susanne Siebentritt, University of Luxembourg, Professor Antonio Valero, CIRCE, Research Centre for Energy Resources and Consumption, University of Zaragoza, Dr. Caroline Whelan, Science Europe, their following statement is hereby repealed. In quote:

`Educators, researchers, policymakers, stakeholders, and citizens are urged to consider energy and natural resources on the basis of exergy, and in doing so understand that:

·         exergy measures energy and resource quality;

·         exergy-destruction foot-printing improves industrial efficiency;

·         exergy offers a common international energy-efficiency metric;

·         optimal use of our limited mineral resources can be achieved by the application of exergy rarity;

·         and exergy should be integrated into policy, law and everyday practice. `

4. Transformation of Non-Thermal Renewables

Mechanical energy, E of all types like the kinetic energy of flow, the potential energy of stored water of mass, m and specific heat, Cp may be mapped into the Carnot-cycle temperature field by a virtual source temperature, Tfby using the equivalency of energy and heat by Kilkis [2]. This relationship reduces to the following expressions for solar and wind energies, respectively.





ηw is the efficiency of the wind turbine. 1366 W/m² is the solar constant.

5. CO2 Emissions and Exergy Rationality

The compound CO2 emissions, including the effect of exergy destruction, is given in Equation 8.



This Equation, which is derived from the Rational Exergy Management Model (REMM), establishes the environment metric. If renewable energy sources are the primary energy source and power is also generated, then the first and the last terms drop.

The following expression, namely EDR is the Ratio of Emissions Difference, which must be close to one.



The term CO2 base, which is 0.63 kg CO2/kW-h is the standardized emission rate calculated with practical defaults for 0.5 kW-h thermal and 0.5 kW-h electrical loads per hour. c values are based on natural gas (0.2 kg CO2/kW-h, 0.85 is the typical boiler efficiency, and 0.35 is power generation and transmission efficiency. The current global average of ψR is taken to be 0.2.

6. Recommendations

Above review reveals that the Second-Law provides further insight and ability to further decarbonize EU and the globe in a more realistic and effective way, beyond the point where the First-Law stops. The key parameter, namely the ψR is a derivation from the Carnot Cycle in terms of supply and demand exergies. Thus, a simple transformation of all EU directives is possible by applying the factor ψR, which is shown in Table 1. For cold processes where the temperatures are below Tref, the term (Tref /Ts) in all equations are inverted.

Exergy-Based PER (Inverse of PEF)






{From fuel input at the plant to the point of use}






Exergy-Based PEF




Because the exergy of electric power and thermal power at different temperatures and enthalpy have a great difference, PEF and PEFX need to be broken down to thermal and electric power separately.

Exergy-Based PES (For co-generation)





Table 1. Sample Transfer Functions for EU Directives.

EU Terms

First-Law Definition



Performance coefficient



Multiply COP by ψR.

Primary Energy Ratio



(Inverse of PEF) Multiply PER by ψR.

Primary Energy Factor



Divide PEF by ψR.
Apply to electrical and thermal power separately.

Primary Energy Savings



Cogeneration applications. Eq. 14.

Tonne oil equivalent



Multiply Mtoe by ψRF.


Exergy-Based Mtoe




Here, ψR is indexed to the unit exergy of crude oil, εF, namely 0.881 W/W.

From the exergy point of view, it has been shown above in terms of PEXR that once the electrical power is generated- even from solar or wind- it must stay as electrical power instead of converting it to thermal power unless the average COP value of heat pumps in heating reach beyond a value of 8. This also shows that one needs a common base by converting exergy to cost or vice versa. In this respect, the cost of exergy destruction per unit supply exergy may be embedded into cost equations, like life cycle cost analysis optimizations. c is the unit cost.



7. Case Study

A solar PVT plant serves a 4DE district energy system. Power is partially used to drive the circulation pumps in the district. The rest of the generated power is distributed in the grid for electrical demand of different types and purposes, including mass transit. Hot water is distributed by a system of pipelines in the district. This heat may be converted to cold by individual absorption and or adsorption units on site of customers, on demand. Figure 3shows the basics of the system. The common mistake in the design and operation of such systems is the ignorance of the unit exergy difference between electrical and thermal powers. Among all ancillaries, which demand power circulation pumps need to be carefully optimized such that thermal exergy provided to the district must exceed electric power exergy demand of the pumps, ignoring other parasitic losses and ancillary demand.

Figure 3. District Energy System with Solar PVT Plant.




Here EXP is a function of the pipe diameter, D and the one-way distance of the loop between the PVT plant and the district, L. For the limiting case of Equation 15 and the given installed capacity, C of the PVT plant, providing heat to the district between 330 K and 320 K, feeding radiant panel heating and at the same time providing in a parallel piping heat at 340°C for DHW and to avoid Legionella risk in open systems, like showers and faucets. Thus, the overall exergy supply to the district takes place between 340°C and 320°C:



If for example, the power demand of the installed pump stations, Ps is 15% of the thermal capacity, C, COP of the district energy system between the plant supply and district demand points looks quite favourable:



But COPEX tells a different story:



1 W/W is the unit exergy of electricity. From Equation 5, ψR is 0.058 and the corresponding CO2 emissions responsibility (Although Solar Energy is the Primary Source) may be calculated from Equation 9. In order to improve the exergy performance of the system pipe diameter may be increased, which affects the Reynolds number but at the same time the flow speed in terms of the flow rate and the inner pipe cross-sectional area. A larger but optimal pipe diameter may be determined but Reynolds Number, Re must be above a certain limit for turbulent flow and the increase in the embodied energy, exergy, and cost corresponding to the pipe diameter increase must be balanced by the cost of electricity generation and operating exergy. The maximum L, namely Lmaxis related to exergy.




Therefore, the distance from the plant to the district is a function of the supply and pump demand exergy. Piping material is also important.

Electric power is in DC in order to avoid inverters and re-conversion to DC in some household equipment like TV screens, LED lighting, and computers.

8. Conclusions

The study presented in this paper shows the need to approach the Second-Law of Thermodynamics if EU wishes sincerely to pursue decarbonization further with all fairness to all stakeholders. Such a move will also become a role model for all other countries of the World. In quantified terms, the task is not of a paramount magnitude. Instead, Table 1 shows that a single key term, namely ψR shall transform all directives and rules in a simple fashion with a new mindset and perspective towards the exploitation, generation, transformation, and utilization of our limited energy resources for a truly sustainable future that we all envision.

We should use;

·         The right quality of energy,

·         At the right application,

·         At the right order of utilization,

·         At the right time and,

·         At the right location.

In conclusion, what EU needs is a strong willingness, motivation, mobilization of the stake-holders and mindset to accomplish such a monumental task with minimal effort with minimal transformation, simply by using a few key parameters. Such a move will especially be useful and effective in the decoupling process for new 4DE systems.

Furthermore, solar energy plants need to be optimally hybridized by other renewables, including district waste for biogas, with support from fossil fuels if necessary [3,4]. In order to improve COP and COPEX, Low-Exergy Buildings must be designed and installed along with high-efficiency pumps and electric motors need to be utilized [6].

9. Nomenclature


Pipe inside the cross-sectional area, m²


Unit emissions factor, kgCO2/kW-h


Unit cost of exergy destruction Euro/kW-h


Thermal plant capacity, kW or MW


Partial power generation efficiency


Partial heat generation efficiency


Carbon dioxide emission, kg CO2


Coefficient of Performance


Exergy-Based Coefficient of Performance


Electrical energy (load), kW-h


Ratio of carbon CO2 emissions differenceto the base emission, dimensionless


Exergy, kW or kW-h


Net solar insolation normal to PVT, /m²


Maximum district piping distance (one way),km or m


Megaton of oil equivalent


Exergy embodiedMegaton of oil equivalent


Primary energy factor


Exergy embodied primary energy factor


Primary energy ratio


Primary energy savings ratio


Exergy embodied primary energy savings ratio


Exergy-based primary energy ratio


Thermal energy (load), kW-h


Reference power generation efficiency


Reference heat generation efficiency


Temperature, K


Volumetric Flow, m²/h


One-way Circuit Length, km


Pressure, Pa


Power demand for pump stations, kW


Power demand for pump stations per kilometerof the pipe circuit in the district, kW


District Pipe Inner Diameter, m


Reynolds Number


Greek Symbols


Second-law efficiency, dimensionless


Power transmission and distributionefficiency


Rational exergy managementefficiency


Unit exergy, kW/kW


First-Law Efficiency


Second-Law Efficiency


Dynamic Viscosity, kg/m-s


Cost of exergy destuctio, Euro












Resource temperature, or Adiabatic FlameTemperature (Real or virtual), K


Crude oil


Thermal (Heat)

in, out

Inlet and outlet connections of a hydronic circuit

l, m

Local power plant, distant power plant,respectively

min, max

Minimum, maximum



sup, ret

Supply, Return



o, ref





Power transmission




Exergy, exergetic




Combined Heat and Power


Domestic hot water


Direct current


Fourth-Generation district energy system


European Union


Human Development Index


Human Sustainable Development Index


Megaton of oil equivalent (According to the First-Law)


Megaton of oil equivalent exergy (Accordingto the Second- Law)




Rational Exergy Management Model


United Nations Development Program

10. References

[1]    Kılkış, S. 2011. A Rational Exergy Management Model to Curb CO2 Emissions in the Exergy- Aware Built Environments of the Future, Doctoral Thesis, Division of Building Technology School of Architecture and the Built Environment, KTH Royal Institute of Technology, Stockholm, Sweden.

[2]    Kılkış, B. ve Kılkış, Şiir. 2015. Yenilenebilir Enerji Kaynakları ile Birleşik Isı ve Güç Üretimi (Combined Heat and Power Generation with Renewable Energy Resources) TTMD, Pub. No: 32, First Ed., ISBN: 978-975-6263-25-9, 371 pages, Doğa Pub. Co., İstanbul, 2015.

[3]    Kilkis, B., Kilkis, Siir, Kilkis, San. 2017. Optimum Hybridization of Wind Turbines, Heat Pumps, and Thermal Energy Storage Systems for Near Zero-Exergy Buildings (NZEXB) Using Rational Exergy Management Model, Paper No. 2, 12th IEA Heat Pump Conference, 15-18 May, Rotterdam 2017. Papers online, https://www.eiseverywhere.com/ehome/index.php?eventid=165152&tabid=558494 Also, abstracted in print, pp: 179-180.

[4]    Kılkış, B. and Kılkış, Şiir. 2017. New Exergy Metrics for Energy, Environment, and Economy Nexus and Optimum Design Model for Nearly-Zero Exergy Airport (NZEXAP) Systems, Energy (2017), doi: 10.1016/j.energy.2017.04.129.

[5]    Kılkış, Şiir. 2012. A net-zero building application and its role in exergy-aware local energy strategies for sustainability, Energy Conversion and Management 63: 208–217 · November 2012. doi:10.1016/j.enconman. 2012.02.029, Energy J.

[6]    Kilkis, B. and Kilkis, San. 2014. Energy and Exergy Based Comparison of Utilizing Waste Heat of a Cogeneration System for Comfort Cooling Using ORC Driven Chillers or Heat Pumps Versus Absorption/Adsorption Cycles, ASME ORC 2013, Conference Proceedings, 7-8 October, De Doelen, Rotterdam, the Netherlands.

[1] Honorary Member of IIR, 2003, Fellow and Life Member of ASHRAE 1999; Honorary Member of REHVA, SITHOK and SLOSE.

Dr. Birol KilkisPages 24 - 30

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