Evolution of Aggregated Exergy Efficiencies per end-use in the period 1960-2009

Miguel Palma∗

Environment and Energy Scientific Area, IN+, Instituto Superior Tecnico (IST),Universidade de Lisboa, Av. Rovisco Pais, 1049-001 Lisboa, Portugal

A final to useful exergy analysis of Portugal is done for the period 1960-2009. A revision of theexisting useful work accounting methods and works is made, and the existing approach for usefulwork studies is improved. Cooling is introduced as a new end-use category, conversion efficienciesfor heating processes are obtained for each energy carrier, and electricity shares per end-use areretrieved for each sector, therefore improving the accuracy of the useful work and aggregated secondlaw efficiencies. We show that cooling uses, which were not included before in useful work studies,are a very relevant end-use in Portugal (the aggregated efficiency difference with and withoutcooling is 3.4 percent in 2009) and that disaggregating the heating second law efficiencies for eachenergy carrier has a considerable effect on the aggregated efficiencies of the country (aggregatedefficiency difference with and without carrier-specific heating efficiencies is 1.3 percent in 2009).It is also shown that the heat process temperatures for the industrial sector, which are differentfrom country to country depending on the existing industry types, are very relevant for the finalaggregated efficiency. Stationary mechanical drive is the end-use category with the biggest share inthe useful work structure of the economy and it has been increasing continuously through all theperiod in study, reaching a 39 percent share in 2009. Transport has kept a relatively stable shareat 23 percent of the total useful work in 2009, while the heating categories have decreased from a53 percent share in 1960 to a 34 percent share in 2009.

Keywords: Energy, Exergy, Efficiency, Useful work, Energy use, Energy carriers.

I. INTRODUCTION

The energy consumption worldwide has been in-creasing at very high rates and, despite the currentinvestments in renewable sources of energy, fossil fuelconsumption still plays a big role in the economies ofboth developed and developing countries. Fossil fuelsare a scarce and not clean source of energy, and one ofthe topics of the current days is its depletion. This ismaking countries adjust their energy policy agendas,and, as referred by [1], there are three clear energypolicy themes that countries are concerned about:

• security of energy supply;

• environmental impact of energy in all its life cy-cle (production, transformation, use);

• liberalization trend in energy markets (notablyelectricity and gas).

These facets are leading towards a world that triesto use energy more efficiently, making better use ofthe natural resources available, and turning attentionto cleaner energies. Studies on the evolution of en-ergy usage are therefore important, in order to assessthe efficiency of energy consumption, and these aredone at the various levels of the energy flow, namelyprimary, final and useful energy. The primary to fi-nal conversion is mainly related to energy transforma-tions, comprising the energy sector. The final to use-ful conversion is related to the way energy is used toprovide an end-use, such as heat, movement or light.Studies at the level of useful energy are important tounderstand how well energy is being used at the end-use, and two different approaches can be taken, the

∗ miguelpalma@tecnico.ulisboa.pt

energy and the exergy one. The exergy approach ismore suited to evaluate the efficiency of utilization ofenergy because it assesses the quality of the energy be-ing used, evaluating the potential for producing work.

Useful exergy, or useful work analysis, has alreadybeen done by a variety of authors at the country level,in which different methods were applied. Differentpaths for these analysis exist in the literature, com-prising different approaches for presenting the results,choice of parameters to study, and time-spans. Recentexamples of useful work studies at the country levelare [2] and [3].

The main objectives of this study are related withthe improvement of the accounting of the energy datavalues to useful work studies by:

1. Creating an automatized program in R that canreceive general energy data and convert it intouseful information that can be used to analyzethe energy usage in a country;

2. Mapping of more detailed conversion efficienciesto be used in the calculation of useful work;

3. Analyzing the evolution of the useful work fig-ures in Portugal in the period 1960-2009;

4. Verifying the importance of disaggregating theconversion efficiencies by carrier;

5. Doing a sensibility analysis with a focus on im-portant parameters (temperature of heat pro-cesses and conversion efficiency of stationarymechanical drive processes).

The work was conducted using Portugal as a casestudy, as this is the country where the project wasperformed, and the one in which there is more infor-mation available and other projects with which it ispossible to compare the results (example: [4]).

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II. METHODOLOGY AND DATA

Recent studies conducted on energy efficiency of acountry generally analyze the energy system from anexergy approach, rather than an energy one. This isbecause exergy is a thermodynamic property that as-sesses potential for producing work, allowing for theaccounting of the energy resources used for produc-tive purposes within an economic system. It is thena much more adequate property to analyze economicevolution through the energy system analysis. It alsoevaluates the thermodynamic potential of the differ-ent end-use devices to the same reference, setting alimit for the second law efficiency of 100%, allowingfor the comparison between them. When using thefirst law efficiency this comparison is not possible, due,for example, to heat pumps or refrigeration systems,for which the Coefficient Of Performance (COP) ishigher than 100%. The chain of energy flow is gener-ally comprised of 3 different stages: primary, final anduseful level. These represent the energy transforma-tions from its origin, as an unchanged resource in thenature, to its final application. In this work we focuson the conversion from final to useful exergy, whichis the last step, corresponding to the energy actuallyused by the consumers, after losses and wastes havebeen accounted for.

To perform this study, energy consumption datais required. Final energy values are available fromnational statistic data-sets or international organiza-tions. To obtain useful work values and then aggre-gated efficiencies for the economy, a number of stepsand calculations is required, described below. Thedata base from which final energy data was retrievedwas the energy statistics from the International En-ergy Agency (IEA). The method followed is describedin the next sections.

A. Final energy to final exergy data

Energy from statistical data can be converted toexergy data using an exergy factor, which varies withthe energy carrier. In Table I, the exergy factors usedin the conversion are presented for the different energycarriers that are included in the IEA energy balances.

Table I: Considered exergy factors [5, 6].

Energy Carriers Exergy Factors

Coal and coal products 1.06

Oil products 1.06

Coke 1.05

Natural gas 1.04

Combustible renewables 1.11

Electricity 1.00

CHP and geothermal heat 0.60

B. Allocation of final exergy consumption toend-use categories

The data from IEA is organized in combinationsof economic sectors - energy carriers to a high levelof disaggregation, which makes it possible to allocateeach of these economic sectors (or flows of the econ-omy) to one end-use category. The usual end-use cate-gories are heat, mechanical drive (divided in transportand stationary mechanical drive), light and electricuses. These end-use categories were defined in previ-ous studies and they have been used by [2] in his work.In this study, a new category, cooling, is introduced,and its importance is discussed. Its introduction is re-lated with the fact that it is a service provided whichhas a significant share in final exergy figures of theeconomy. It is used in the residential and commercialsectors during summer for space cooling, and through-out the year for refrigeration of food. In the industrialsector it is used in the food industry for preservation ofgoods to be consumed, and in the chemical industriesfor preservation of chemical compounds, and we alsoconsider it in the transport sector for air conditioneruses.

Table II: End-use categories for useful work calculation [6].

Aggregated Disaggregated

HeatHigh Temperature Heat (500◦C)

Medium Temperature Heat (150◦C)

Low Temperature Heat (80◦C)

Stationary Oil - Mechanical drive

Mechanical Drive Coal - Mechanical drive

Transport

Steam locomotives

Diesel vehicles

Gasoline/LPG vehicles

Aviation

Navigation

Natural gas vehicles

Diesel-electric

Light Coal/Oil light

Electricity Treated separately

Cooling Treated separately

The allocation in Table II is made according to [6]and [7], although in these studies transport and sta-tionary mechanical drive were comprised in the samecategory (then called mechanical drive). A part of theenergy carriers, specially in the oil products, is usedfor non-energy purposes, which are not accounted forin this study since they are material flows and notenergy flows (asphalt, platics, etc).

While the allocation for end-uses of coal, com-bustible renewables, natural gas, oil products and heatis directly made according to the industry type, trans-port type, and use in residential, service and miscel-laneous sectors, for electricity this information is notreadily available. Electricity and cooling were treatedseparately from the other energy carriers, and theshares of utilization per end-use were obtained for eachsector from [8]. This data is for the United States of

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America (USA), but it is considered that in the pe-riod of focus of this study (1960-2009) the electric useswere similar among the developed countries.

C. Estimation of second law efficiencies

To convert the final exergy values obtained in theprevious steps to useful exergy values, second law ef-ficiencies are required. The major part of the secondlaw efficiencies depend on the first law efficiencies anda bibliographic study was made to gather them. [3]and [4] calculated the useful work values using secondlaw efficiencies that depended mainly on the end usecategory and not on the energy carrier. In this study,efficiencies are computed for the pairs energy carrier– end use category, in order to obtain more accurateresults. A brief explanation of the second law efficien-cies used is given, for each end-use category.

1. Heat

For the second law heating efficiencies it is necessaryto know the service and environment temperatures,as they are dependent on the temperature at whichthe heat is deployed, evaluating its thermodynamicpotential to produce work [9]. They are also relatedto the first law efficiency, η, by equation 1.

ε = η ·(

1 − T0

Ts

)(1)

In this work, first law efficiencies are retrieved byenergy carrier, instead of using generalized first law ef-ficiencies for all the heat processes from all the energycarriers. The carrier-specific first law efficiencies usedby [10] for the European Organization for EconomicCo-operation and Development (OECD) countries areused, divided between low and medium temperatureheat and high temperature heat. The energy carriersavailable for the disaggregation are coal, renewablefuels, oil, gas, electricity and heat. The first law effi-ciencies obtained were then normalized assuming theevolution of heat efficiencies used by [3] and [4].

The service temperatures defined for the heat pro-cesses are defined in Table II. Also, assuming thatspace heating uses occur just during the wintermonths, for the second-law efficiency of Low Temper-ature Heat (LTH) the temperature considered was thewinter temperature [4]. The annual average tempera-ture considered is 15.4◦C and the winter average tem-perature is 9.8◦C.

2. Transport

For gasoline engines, the theoretical maximum effi-ciency is defined based on the compression ratio andthe specific heat ratio (r), in equation 2.

ηtheoretical maximum = 1 −(

1

r

)γ−1

(2)

As done by other authors [3, 4], an evolution ofthe compression ratio of gasoline engines through thetime span pretended is used to calculate the theoret-ical maximum efficiency of these engines. The realefficiency is given by equation 3, in which the coeffi-cients αi denote losses which are described in TableIII (based on [2]):

ε = ηtheoretical maximum ·6∏i=1

αi (3)

The coefficient α5, which accounts for accessories(and Air Conditioner (AC)) losses, was included in thestudy of [2] and [3], although here a different approachis considered. Because AC is a service provided ratherthan an end-use, it accounts directly either for LTHuses or cooling uses.

Table III: αi coefficients for second law efficiencies of gasolineengines [2].

Coefficient Meaning Approximate value

1 Reduction due tostoichiometric devia-tions

0.75

2 Combustion andcylinder wall’s losses

0.75

3 Friction losses 0.85-0.90

4 Partial load 0.40-0.45

5 Accessories losses 0.90

6 Transmission Losses0.75 (automatic)

0.90 (manual)

It is considered that diesel engines are 25 percentmore efficient than gasoline engines, as these havebetter compression ratios and fuel-burning efficien-cies [9, 11]. Navigation engines run close to their fullpower, and generally at a more constant speed thanthe gasoline/diesel engines used in cars. That way,there are no losses for partial load, and accessoriesand transmissions losses are also not considered.

For transport uses given to ethanol the efficiencyused was 80 percent of the gasoline engine [12], as donein [13]. Kerosene is used mainly for aviation purposes,being its efficiency of utilization similar. Efficiencyfor aircraft engines (aviation gasoline) was taken from[11].

For steam locomotives information was taken from[14] and [15]. Natural gas vehicles efficiency data wastaken from [16] and [4].

3. Stationary Mechanical Drive

For the internal combustion engines used in Sta-tionary Mechanical Drive (SMD), the efficiencies fornavigation engines were used. This is based on theassumption that stationary engines work in a moreconstant regime as navigation engines do, as op-posed to car engines which mainly operate in accel-eration/deceleration. This usage leads to a more effi-cient use of navigation engines, as well as stationary

4

internal combustion engines [17]. Because gasoline en-gines work with lower efficiency than diesel engines,the gasoline SMD efficiencies were given a value 25percent lower than the diesel engines.

4. Light

Light efficiencies are defined differently from all theothers, due to the fact that they are not measured inenergy-units, but in lumens per watt – light emissionper energy input. Light emission is seen by the hu-mans in the visible spectrum, and the rate at whichthe emission happens is called the light flux, which hasunities of ’lumen’. The efficacy can then be measuredin terms of lumen-hours per kilo-watt [18]. [19] stud-ied lighting efficacies for the United Kingdom (UK) forthe last centuries, being this data used for Portugal aswell because lighting technologies did not differ con-siderably in developed countries. A first law efficiencycan be defined, as in equation 4.

η =output of light

input of energy(4)

which in the case of light turns out to have a unity oflm/W . The maximum value for the luminous efficacyis found to be 400 lm/W [3, 14], and this is the refer-ence value that is taken for the second law efficiency.The lighting efficiency is then given by equation 5.

η =η

400 lmW(5)

5. Electricity

Electricity efficiency data was taken from [8]. Theystudied the efficiency of electrical uses in the USA inthe period 1900-2010. Electricity end uses are dividedin 7 different subcategories, according to the utiliza-tion given: low and high temperature heat, stationarymechanical drive, transport, light, other electric usesand cooling. The other electric uses category includesall the uses that are not included in the other cate-gories, such as communication and electronic devicesand electrochemical uses in industry. Electricity al-located to transport is mainly used in railways. Ef-ficiency data for this category was obtained from thework of [3] and [2].

6. Cooling

A different approach from previous methods of cal-culating useful work for SMD is followed here, takinginto account that the real end-use for cooling devicesis the cooling of space or goods. In previous works,air conditioning and refrigeration uses were generallyincluded in the stationary mechanical drive category.This is based on the fact that the electricity is usedto power a mechanical movement, such as the rotor

in electric motors, or in the case of refrigeration andair conditioners, the compressor and fans, which canbe considered in the category mechanical drive. How-ever, for refrigeration and air conditioners, the com-pressor is not the end-use. This is an inconsistencyin the method used when addressing refrigerators andair conditioners as stationary mechanical drive andnot space heating/cooling.

A new category of end-use is created: cooling. Theshares of electricity allocated to cooling devices wereretrieved from [8], with estimates for cooling and heat-ing uses for air conditioners. Air conditioner uses incars for heating and cooling are also considered, withheating uses (during the winter season) going to theLTH uses and cooling uses (during the summer sea-son) going to the cooling category.

Conversion second law efficiencies for refrigerationand air conditioning end-uses were studied. The twoequipments are quite similar in the working principleand use the same components, and the difference relieson refrigerators being enclosed and insulated volumes[8]. Analyzing the cooling and heating devices from athermodynamic perspective, the coefficient of perfor-mance is the first law efficiency indicator.

In Table IV the first law efficiency for cooling andheating devices is shown. For cooling, Qin is the heatremoved from the cooled space, and Pin is the electric-ity required to power the chiller or compressor. Ex-tracting heat from an enclosed volume to maintain itstemperature at Tc, with an environment temperatureof T0, results in an ideal COP as expressed in TableIV. In heating the final use desired is to deploy heatat a temperature Th in an environment with a coldertemperature T0.

Typical values for the COP of electrically driven airconditioners and refrigerators are in the range 2 – 4[20].

To convert final exergy values into useful exergy val-ues, second law efficiencies are needed, which take theforms in Table IV.

Table IV: First and second law efficiencies for cooling and heatingdevices.

Cooling Heating

COP (real)Qin

Pin

Qout

Pin

COP (ideal)Tc

T0 − Tc

Th

Th − T0

2nd law COPreal(T0 − Tc

Tc) COPreal(

Th − T0

Th)

Typical values for environment temperature, andtemperature of hot and cold air deployed, with air con-ditioners in heating and cooling mode, respectively,are presented in Table V, for Portugal. Data is di-vided between heating and cooling uses, with the out-door temperature being the winter temperature forthe heating uses and the summer temperature for cool-ing uses, taken from [6] and [21]. The indoor airtemperatures were taken from the Portuguese legis-lation for the comfort temperatures for AC in build-ings (Regulamento das Caracterısticas de Comporta-mento Termico dos Edifıcios (RCCTE) in Decreto de

5

Lei n◦80/2006 de 4 de Abril, 2006).

Table V: Temperature parameters used for space heating andcooling efficiencies for Portugal.

Conditioningmode

Outdoor tem-perature (◦C)

Indoor temper-ature (◦C)

Heating 9.8 20

Cooling 28 25

For refrigerators, as done by [23], the efficiency iscalculated assuming a 1/3 load from the freezer boxat -15◦C and a 2/3 load from the cooler box at 4◦C.

D. Calculation of aggregated useful work values

After applying the mean efficiencies from final touseful, the total useful exergy is then aggregated indifferent categories according to useful work uses, asshown in Table VI.

Table VI: Aggregated end-use categories.

High Temperature Heat

Medium Temperature Heat

Low Temperature Heat

Stationary Mechanical Drive

Transport

Light

Other electric uses

Cooling

III. RESULTS AND DISCUSSION

Different types of analysis are presented, startingwith a comparison between an energy approach andan exergy approach. The effect of the changes in themethod for useful work accounting are shown, followedby an analysis of the exergy efficiency in Portugal.Finally, a sensibility analysis is performed.

A. Energy versus Exergy analysis

In figure 1, final to useful aggregated efficiencies arepresented for the first (up) and second (down) lawsand for all the sectors.

The aggregated end-use efficiency is much lower onthe exergy approach than on the energy one. For theyear 1970, for example, the energy efficiency was ap-proximately 39 percent, which means that for everyunit of energy used, approximately 1.5 units of en-ergy would be wasted. The exergy efficiency was ap-proximately 18 percent for the same year, 1970, whichmeans that for every unit of exergy used, 4 units ofexergy were wasted.

The transport sector depends mainly on the me-chanical drive category, for which the first law effi-ciency is the same as the second law efficiency. This

0%

10%

20%

30%

40%

50%

60%

70%

1960 1970 1980 1990 2000

Industry

Transport

Residential

Services

Miscellaneous

Total agg efficiency

0%

10%

20%

30%

40%

50%

60%

70%

1960 1970 1980 1990 2000

Industry

Transport

Residential

Services

Miscellaneous

Total agg efficiency

Figure 1: Final to useful efficiency for Portugal: first law (up) andsecond law efficiencies (down) per sector.

means that the efficiency for this sector won’t changea lot from the first to the second law. The residentialsector, with 41 percent energy efficiency in 2009, be-comes the least efficient sector together with the trans-port sector, showing a 19 percent exergy efficiency for2009.

B. Comparison with previous works

1. Introduction of the cooling category

The introduction of cooling as a useful work cate-gory results in the evolution on figure 7 of the finalexergy consumption in Portugal. Cooling has approx-imately a 7 percent share in 2009, being its share lowerin the useful work results because of its low conver-sion efficiency, as will be seen later in the chapter. Thecooling devices, including refrigerators and air condi-tioners have been increasing in recent years due toits generalization in cars and homes, with the func-tion of providing comfort. This is true for Portugalbut also for other developed countries, in which com-fort equipments such as Heating, Ventilation and AirConditioner (HVAC) systems are becoming commonin residential sites, shopping centers, offices and allkinds of indoor spaces.

It can be seen that the introduction of this cate-gory is useful, due to its high share in the final exergystructure. Its share has been growing at quite a con-stant rate which is a result of the evolution in comfortdevices in the developed countries.

The effect on the aggregated efficiency of the coun-try is shown in figure 2. The difference is of 3.4 percentin 2009, increasing as more recent years approach.

2. Heating processes - carrier disaggregation

The other main difference from the methodologypreviously used by [2] is the heating efficiencies. In

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12.0%

14.0%

16.0%

18.0%

20.0%

22.0%

24.0%

26.0%

1960 1970 1980 1990 2000

without cooling

with cooling

Figure 2: Effect of the introduction of cooling on the aggregatedefficiency.

this project, a disaggregation of the heating efficien-cies by energy carrier is made, to assess if this level ofdetail is relevant in a useful work study. The influenceof the usage of carrier-specific second law efficienciesis shown on figure 3.

12.0%

14.0%

16.0%

18.0%

20.0%

22.0%

24.0%

1960 1970 1980 1990 2000

Generalized heating efficiencies

Carrier-specific heating efficiencies

Figure 3: Aggregated exergy efficiency considering carrier-specificheat efficiencies and generalized heat efficiencies.

The difference between the two efficiencies is morethan 1 percent of the aggregated efficiency in someyears, being higher in the beginning of the study andin the period of the beginning of the 1990’s. The effi-ciency without considering carrier-specific efficienciesis an average efficiency considering all the carriers.When using this efficiency, which is closer to the onefor heat processes for oil, some carriers are assignedhigher efficiencies than the reality, which is the caseof combustible renewables and coal. What happens isthat the useful work values calculated for process heatfrom these carriers (coal and combustible renewables)are higher than the reality, bringing the aggregatedefficiency up in the first years of the study. When ap-proaching more recent years, mainly after 1997, theeffect is the opposite since natural gas, which has ahigher heating efficiency than oil, is being assigned alower efficiency than the real one, bringing the aggre-gated efficiency down, which is what happens from1997 onward.

3. Further disaggregation of electricity uses

Electricity uses were subject to a more detailedstudy, and the shares of end-use per sector were re-trieved, providing a more accurate result of the usefulwork distribution per end-use in each of these sec-tors. To compare with the shares that were being usedpreviously, the aggregated second law efficiencies areshown for the whole economy in figure 4.

The shares previously used allocated a lot of elec-tricity to lighting, which was found not to be realistic.

12.0%

14.0%

16.0%

18.0%

20.0%

22.0%

24.0%

1960 1970 1980 1990 2000

Undifferentiated electricity shares

Differentiated electricity shares

Figure 4: Aggregated efficiency considering new shares forelectricity.

Other electric devices also had a big share in all thesectors, and the new information found, from [8], con-tradicts this. As these two types of use, lighting andother electric devices, have a very low conversion ef-ficiency, they were bringing efficiency values down, asseen in the blue line on figure 4. The real efficiencycurve is higher, with values up to 2 percent higherthan the ones obtained before.

C. Evolution of the exergy efficiency in Portugal1960-2009

1. Final exergy per carrier

On figure 5 the final exergy consumption evolutionin Portugal for the period 1960-2009 is presented bycarrier. There was an increase from 120 PJ to 800 PJin final exergy consumption, which represents almosta 7 fold increase, over the period of 50 years. TheAverage Annual Growth Rate (AAGR) of final exergyconsumption was 4 percent.

Oil is clearly the energy carrier with the biggestshare through all the period, being the one that influ-ences the most the total final exergy consumption ofthe country. It continuously increases until the 1980’s,where it stabilizes for some years. There is also a de-creasing in oil consumption more recently due to theintroduction of natural gas in Portugal in 1997, whichhas been growing in importance. This substitutionof oil for natural gas is much more pronounced in theindustrial sector. In the transport sector, after contin-uous increase, oil consumption reaches a stable valueof approximately 260 PJ since 2002.

Coal (mainly present in industry) lost its impor-tance in the final exergy scene, as seen on figure 5,and it is also possible to see the growing importanceof electricity in the final exergy consumption, beinglikely that it will continue to increase.

The final exergy per sector is presented on figure6, in which it is shown that the transport sector hasbeen increasing while the industrial sector has beendecreasing in exergy consumption comparatively. Thetransport and industrial sectors evolved from a 21 and42 percent share, respectively, in 1960, to a 37 and 32percent share in 2009.

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0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

1960 1970 1980 1990 2000

Shar

e

Heat

Electricity

Oil and oil products

Natural gas

Combustible renewables

Coal

Figure 5: Final exergy consumption for Portugal per carrier, inrelative values.

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

1960 1970 1980 1990 2000

Exe

rgy

(sh

are

) Miscellaneous

Services

Residential

Transport

Industry

Figure 6: Final exergy consumption for Portugal per sector inrelative values.

2. Final exergy per end-use

When addressing final exergy values per end-use adifferent analysis can be made. Here, the focus is onthe final end-use, so it is not directly connected withthe natural resources utilization, but with the finalgoal intended for the energy. The final exergy perend-use is presented in figure 7, in which it is seenthat transportation uses have been increasing since1960 until 2009 comparatively, being the end-use withthe biggest portion of the final exergy structure, co-inciding with oil being the carrier with the biggestshare in the results per carrier. Figure 7 suggests thatthe industrial sector has been shrinking lately (alsoseen in figure 6), with its three major uses (SMD,Medium Temperature Heat (MTH) and High Tem-perature Heat (HTH)) decreasing in the share of totalfinal exergy.

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

1960 1970 1980 1990 2000

Shar

e

Cooling

OE

L

T

SMD

HTH

MTH

LTH

Figure 7: Final exergy consumption for Portugal per end-use inrelative values.

3. Second law efficiencies

The aggregated final to useful efficiency illustrateshow well a country is using its final exergy to provide

an end-use. The final to useful efficiency evolutionduring the period in study is presented in figure 8.

12%

13%

14%

15%

16%

17%

18%

19%

20%

21%

22%

1960 1965 1970 1975 1980 1985 1990 1995 2000 2005

Figure 8: Aggregated final to useful efficiency for Portugal.

The first period is characterized by a fast increasein the efficiency curve, which is mainly related tothe increasing of electric motors to provide station-ary mechanical drive in industries, in an electrifica-tion process that took place in Portugal. These de-vices have very high efficiencies, being responsible forthe improvement of the aggregated efficiency in thisperiod. Although this electrification process is contin-uous, happening also now-a-days, the increase in au-tomobile use, which has a lower conversion efficiency(10-12%), causes the efficiency curve to stabilize be-tween 20 percent and 21 percent in more recent years.The reduction in high temperature uses in industry isalso responsible for the stabilization of the aggregatedefficiency curve.

The second law efficiency results by end-use pre-sented here are for the country as a whole, in figure9. The efficiency of other electric devices has beensteeply decreasing since 1980 due to the decrease ofelectrochemical processes in industry.

25%

50%

75%

100%

0%

2%

4%

6%

8%

10%

12%

14%

16%

18%

20%

1960 1970 1980 1990 2000

Co

nve

rsio

n e

ffic

ien

cy

HTH

SMD

LTH

MTH

Transport

Light

OE

Cooling

Figure 9: Final to useful end-use efficiencies.

The efficiencies of heat processes have been increas-ing steadily over time, and this is a result of the in-creasing of the technological (first law) efficiencies, di-rectly related with the development of better heat en-gines. The LTH efficiency curve has a small decreasein 1988, a result of the sudden increase in combustiblerenewables consumption, recovering in the followingyears. As combustible renewables have lower efficien-cies than the other fuels, an increase in the consump-

8

tion of these will have as an effect the decrease of theoverall efficiency for a heating type of use.

Efficiency results by carrier are presented in figure10. These results were not presented in the usefulwork studies mentioned before, however they providean overview of the utilization of each carrier in theeconomy.

0%

10%

20%

30%

40%

50%

60%

1960 1970 1980 1990 2000

Co

nve

rsio

n e

ffic

ien

cy

Total coal

Total comb renewables

Total natural gas

Total oil and oil products

Total electricity

Figure 10: Final to useful efficiency per carrier.

The efficiency of utilization of the oil products hasbeen decreasing over time. This is related to the factthat the uses in industry are becoming less efficient.HTH uses from oil sources are decreasing and its us-age is being shifted to transport and lower heatinguses, due to the introduction of natural gas in indus-try, which is widely used for HTH uses. Coal, onthe other hand, has been increasing its final to use-ful efficiency. Being used for the residential sectorfor low temperatures, it is now used mainly for highand medium temperature processes in industry, whichbrought its efficiency higher.

In figure 11, the efficiencies per sector and the ag-gregated efficiency of the economy are presented.

0%

5%

10%

15%

20%

25%

30%

35%

1960 1970 1980 1990 2000

Industry

Transport

Residential

Services

Miscellaneous

Total agg efficiency

Figure 11: Final to useful efficiency per sector.

The industrial sector is, by far, the most efficientone, mainly due to the fact that electrical motorsfor stationary mechanical drive are more intensivelypresent in this sector, but also because its process heatefficiencies are generally at higher temperatures, be-ing also more efficient. In the service sector a decreasein efficiency can be seen in 1999, related with the in-troduction of natural gas, which started being usedfor low temperature heat applications (water heating),having a low efficiency. In the transport sector, effi-ciency is directly related with the transport end-useefficiency, since this is the main end-use present in thesector. It decreases with the increase in the use of thelowest efficiency devices of this sector - automobiles.The miscellaneous sector follows a constant patternwith a slow but constant increase in the more recentyears. This sector, comprising agriculture and fishing,

is mainly powered by oil and electricity, with the elec-tricity increasing in latest years, which results in theincrease of the efficiency.

D. Sensibility analysis

1. SMD

Even with the change in methodology described be-fore, with the introduction of cooling uses, SMD stillplays an important role in the useful work structureof the economy, coming mainly from electricity. Thesecond-law efficiencies used in the calculations wereconsidered as being for the best electric motors avail-able, based on the work of [8] for the USA. However,not only Portugal is not at the same technological levelas the USA, but also engines are not upgraded everyyear to new and more recent ones. To study the possi-ble error in the calculations, we apply a 10 year delaybased on the average life of SMD at homes (dishwash-ers washing machines, dryers).

14%

15%

16%

17%

18%

19%

20%

21%

22%

1960 1970 1980 1990 2000

10 year delay

no delay

Figure 12: Change in percentage in the aggregated efficiency withthe appliance of a 10 year lag in SMD efficiencies.

The results on the aggregated efficiency are not sig-nificant, as seen on figure 12, with a maximum changein efficiency of 0.2 percent.

2. Industry type influence on MTH and HTH uses

In order to study the influence of the industry struc-ture of Portugal in the heating process efficiencies thedifferent industries present in the country were studiedand their process heat temperatures are used for sec-ond law efficiency calculations. The method followedis very much similar to the one in [24], in which themost intensive industries were considered, and takenas representative of the entire industrial sector. ForPortugal, the final exergy for heat used in the indus-trial sector is mainly used in the industries shown inTable VII. The industry sector in Portugal is domi-nated by the non metallic minerals industry and thepulp and paper industry, followed by three smaller im-pact industries: textile and leather, food and tobacco,and chemical and petrochemical. Non metallic miner-als industry include the glass and cement industries,which are very significant in Portugal. The main typesof industries were studied, and their specific heat pro-cess temperatures were identified. These are presentedin Table VII.

9

Table VII: Most intensive industries and heating processtemperatures, Portugal.

Industry Ts HTH (◦C) Ts MTH (◦C)

Glass production 1450 160

Cement production 1450 160

Pulp and paper 500 160

Textile and leather 80 150

Food industry 300 150

Tobacco — 90

Chemical/petrochemical 500 150

Using the temperatures in Table VII to calculateweighted values of useful work for each industry, theaggregated efficiency of the country changes consider-ably. Both efficiencies, with and without the specificservice temperatures applied, are depicted in figure13.

12%

14%

16%

18%

20%

22%

24%

1960 1970 1980 1990 2000

Standard efficiencies

With detailed efficiencies per industry

Figure 13: Aggregated efficiency for Portugal, with and withoutthe specific service temperatures applied.

3. Ambient temperature effect on heating uses

Heating uses efficiencies are highly dependent onthe ambient temperature throughout the year, for thesecond law. The temperature used in this study is con-stant across the 50 years of the study, being the tem-perature for winter 9.8◦C (for LTH uses) and the an-nual average temperature 15.4◦C. A sensibility anal-ysis is here performed, using annual-specific tempera-tures, since 1960. The resulting aggregated efficiencyfor the whole economy is shown on figure 14.

14%

15%

16%

17%

18%

19%

20%

21%

22%

23%

1960 1970 1980 1990 2000

Effi

cie

ncy

Agg efficiency with Tamb variation

Agg efficiency without Tamb variation

Figure 14: Effect of varying ambient temperatures on aggregatedefficiency of the country, source: [21].

As seen, the change in the aggregated efficiency isnot relevant, being at maximum a difference of 0.3percent in 1963.

IV. CONCLUSIONS

To study the efficiency of utilization of energy inPortugal, an evaluation of the exergy flows was doneinstead of an analysis of the energy flows. This choicewas justified as energy analysis have disadvantages,with heat process efficiencies influencing a lot the ag-gregated efficiency of the countries and of the varioussectors. The industrial sector was the one that showedthe greatest difference between the first law efficiencyand the second law efficiency, due to the importance ofheat processes. The industrial sector energy efficiencywas 59 percent for 2009, while the exergy efficiency forthe same year was 32 percent. A difference can also beseen in the residential sector, which has low temper-ature heat applications. Its energy efficiency was 41percent, while its exergy efficiency was 19 percent for2009. The transport sector, on the other hand, hadenergy and exergy efficiencies of 13 and 15 percent,respectively. For this reason the first law efficiencycan be misleading when comparing different sectorsand consequently different types of uses in a system.The flow level chosen (useful exergy) is also a goodmeasurement of how productively a country is usingits available energy because it places the study closeto the end-uses, indicating the minimum amount ofwork necessary for a certain use.

Including cooling as an end-use category and ana-lyzing it with specific efficiencies allows for more de-tailed and accurate aggregated efficiency measures. Italso allows for the study of the evolution of this cate-gory alone, observing its growth in importance. Theaggregated efficiency of Portugal is highly influencedby the introduction of cooling uses, showing a 3.4 per-cent difference between the 2009 aggregated exergyefficiency with and without cooling.

The disaggregation of the energy carrier efficienciesshowed to be relevant because the setting of a gen-eral second law efficiency for all the carriers had theeffect of increasing the efficiency. If the same secondlaw efficiency is defined for all the carriers, the ag-gregated efficiency for the country will be inflated insome years, when the energy use for heating processesis more provided from coal or combustible renewables,as these tend to have lower conversion efficiencies. Onthe other hand, in years in which the natural gas ismore used to provide heat, the aggregated efficiencywill be deflated, as this carrier has higher conversionefficiencies, which happened in Portugal since 1997.

Electricity uses were subject to a more detailed al-location in this project, using sectoral shares of elec-tricity for each end-use. The residential, services andmiscellaneous sectors use electricity in different ways,and assuming the same shares of electricity end-use forall these sectors decreased the aggregated efficiency ofthe country.

Stationary mechanical drive is the category with thehighest share in the useful work structure of the coun-try, due to its very high conversion efficiency fromelectrical motors. Due to this, a sensibility analysiswas performed and a delay was applied consideringthat electric motors in the residential sector are notall from the latest technology available in the market.

10

The result shows that the influence of considering a10 year lag is not considerable, and the difference inthe aggregated efficiency of the country is not morethan 0.2 percent. However, the delay applied onlyhad its basis on the residential sector, due to the lackof information for the industrial sector.

The detailed study on the industrial heat processtemperatures revealed to be important on the aggre-gated efficiency of the economy. The difference on theaggregated efficiency is considerable, being on average1.3 percent per year in relation to an aggregated effi-ciency with standard temperatures for the heat pro-cesses in industry. This sector is a very significantsector in Portugal (accounting for more than 30 per-cent of the final exergy structure), and the definitionof these temperatures is relevant.

The study on the ambient temperatures variationshowed that the level of detail and accuracy obtainedfor the second law aggregated efficiency of the countrydoes not change considerably when using year-specificambient temperatures, being at maximum a differenceof 0.3 percent, mainly because of the change in lowheat process second law efficiencies, which are highlydependent on ambient temperatures.

The economy in Portugal is now-a-days dominatedby mechanical drive uses: stationary mechanical driveessentially in the industrial sector from electricitysources, and transport uses for the transport sector,

mainly in road vehicles. The exergy consumption hasbeen increasing almost continuously since 1960, goingfrom 120 PJ to 800 PJ in 2009. This represents a 7fold increase in final exergy consumption.

The useful work structure in Portugal had an in-crease from 20 PJ to 170 PJ in the period in study,which represented an 8.5 fold increase. The aggre-gated efficiency had a period of a steady increase,correspondent to the increase in electricity appliancesand in the usage of electricity for stationary mechan-ical drive in industry. The aggregated efficiency thenstabilized over the next years, slowed down mainly dueto the increase in transportation uses, typically witha lower conversion efficiency.

The low efficiencies of conversion in the transportsector suggest that research and development policiesshould be directed to the study and increase of thefinal to useful efficiency of transport uses, which area big part of the economy. The residential sector,with an increasingly share of the final exergy struc-ture, should also be a focus for improvement of final-to-useful efficiencies, namely in space heating/coolingdevices.

A more detailed analysis of the heat processes in theindustrial sector is desired for more accurate resultson the aggregated efficiency. It is left as future workthe tracking of conversion efficiencies for each ’energycarrier – industry type’ pair.

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