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Dr. Birol KilkisF-ASHRAE, Exceptional Service and
Distinguished Service Awards ASHRAEScience Encouragement Award,
TUBİTAK, 1981Country Manager, World Alliance for
De-Centralized EnergyProfessor Baskent University and Chair of
Energy Engineering Graduate ProgramHonored Member, America’s Registry of
Outstanding ProfessionalsExecutive Committee Member EU ESTTPTTMD 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.
‘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.
(1) |
{Exergy destroyed upstream} | (2) |
{Exergy destroyed downstream} | (3) |
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.
(4) |
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} |
(5) |
Figure 2.
Exergy Flow Diagram (Eq. 2).
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.
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. `
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.
(6) |
(7) |
ηw is the efficiency of the wind turbine. 1366 W/m²
is the solar constant.
The
compound CO2 emissions, including the effect of exergy destruction,
is given in Equation 8.
(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.
(9) |
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.
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.
(10) |
{From fuel input at the plant to the point of use} | (11) |
(12) |
(13) |
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.
(14) |
Table 1.
Sample Transfer Functions for EU Directives.
EU Terms | First-Law Definition | Second-Law | Comments |
Performance coefficient | COP | COPEX | Multiply COP by ψR. |
Primary Energy Ratio | PER | PEXR | (Inverse of PEF) Multiply PER by ψR. |
Primary Energy Factor | PEF | PEFX | Divide PEF
by ψR. |
Primary Energy Savings | PES | PESEX | Cogeneration applications. Eq. 14. |
Tonne oil equivalent | Mtoe | MtoEX | Multiply Mtoe by ψRF. |
(15) |
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.
(16) |
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.
(17) |
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.
(18) |
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.
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].
Ad | Pipe inside the
cross-sectional area, m² |
c | Unit emissions
factor, kgCO2/kW-h |
cEX | Unit cost of exergy
destruction Euro/kW-h |
C | Thermal plant
capacity, kW or MW |
CHPEη | Partial power generation
efficiency |
CHPHη | Partial heat generation
efficiency |
CO2 | Carbon dioxide
emission, kg CO2 |
COP | Coefficient of Performance |
COPEX | Exergy-Based Coefficient of
Performance |
E | Electrical energy
(load), kW-h |
EDR | Ratio of carbon CO2 emissions
differenceto the base emission, dimensionless |
Ex | Exergy, kW or kW-h |
In | Net solar insolation normal
to PVT, /m² |
Lmax | Maximum district piping
distance (one way),km or m |
Mtoe | Megaton of oil equivalent |
MtoEX | Exergy embodiedMegaton of oil equivalent |
PEF | Primary energy factor |
PEFX | Exergy embodied primary
energy factor |
PER | Primary energy ratio |
PES | Primary energy savings ratio |
PESEX | Exergy embodied primary
energy savings ratio |
PEXR | Exergy-based primary energy
ratio |
Q, QH | Thermal energy
(load), kW-h |
RefEη | Reference power generation
efficiency |
RefHη | Reference heat generation
efficiency |
T | Temperature, K |
V | Volumetric Flow, m²/h |
L | One-way Circuit Length, km |
P | Pressure, Pa |
Ps | Power demand for pump
stations, kW |
ΔPs | Power demand for pump
stations per kilometerof the pipe circuit in the district, kW |
D | District Pipe Inner
Diameter, m |
Re | Reynolds Number |
Greek Symbols
ηEX | Second-law efficiency,
dimensionless |
ηT | Power transmission and
distributionefficiency |
ψR | Rational exergy managementefficiency |
ε | Unit exergy, kW/kW |
ηI | First-Law Efficiency |
ηII | Second-Law Efficiency |
μ | Dynamic
Viscosity, kg/m-s |
ΔCEX | Cost of exergy destuctio,
Euro |
Subscripts
base | Base |
dem | Demand |
des | Destroyed |
E | Electric |
f | Resource temperature, or
Adiabatic FlameTemperature (Real or virtual), K |
F | Crude oil |
H | 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 |
ref | Reference |
sup, ret | Supply, Return |
s | solar |
o, ref | Reference |
p | Pump |
T | Power transmission |
w | Wind |
X, EX | Exergy, exergetic |
Acronyms
CHP | Combined Heat and Power |
DHW | Domestic hot water |
DC | Direct current |
4DE | Fourth-Generation district
energy system |
EU | European Union |
HDI | Human Development Index |
HSDI | Human Sustainable
Development Index |
Mtoe | Megaton of oil equivalent
(According to the First-Law) |
MtoEX | Megaton of oil equivalent
exergy (Accordingto the Second- Law) |
PVT | Photo-voltaic-thermal |
REMM | Rational Exergy Management
Model |
UNDP | United Nations Development
Program |
[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.
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