ene gies
Re iew
An O e iew o he Po uguese Ene gy Sec o and
Pe spec i es o Powe - o-Gas Implemen a ion
Ca los V. Miguel * , Adélio Mendes and Luís M. Madei a
LEPABE—Labo a o y o P ocess Enginee ing, En i onmen , Bio echnology and Ene gy, Facul y o Enginee ing,
Uni e si y o Po o, Rua D . Robe o F ias, 4200-465 Po o, Po ugal; [email p o ec ed] (A.M.);
[email p o ec ed] (L.M.M.)
*Co espondence: [email p o ec ed]; Tel.: +351-22-508-1519
Recei ed: 12 Oc obe 2018; Accep ed: 17 No embe 2018; Published: 23 No embe 2018


Abs ac :
Ene gy policies es ablished in 2005 ha e made Po ugal one o he op enewable powe
p oduce s in Eu ope, in ela i e e ms. Indeed, he coun y ene gy dependence dec eased since
2005, al hough emaining abo e EU-19 and EU-28 coun ies in 2015 (77.4% s. 62.4% s. 54.0%,
espec i ely). Da a collec ed om go e nmen al, s a is ical, and companies’ epo s and esea ch
a icles shows ha enewables and na u al gas assumed a g owing impo ance in he Po uguese
ene gy mix along ime, while oil ollowed an opposi e end. Recen ly, he coun y ema kably
achie ed a ull 70-h pe iod in which he mainland powe consumed elied exclusi ely on enewable
elec ici y and has se e al momen s whe e powe p oduc ion exceeds demand. Cu en ly, he main
op ion o s o ing hose su pluses elies on pumped hyd o s o age plan s o expo a ion, while o he
s o age al e na i es, like Powe - o-Gas (P G), a e no unde deep deba e, e en ually due o a lack
o in o ma ion and awa eness. Hence, his wo k aims o p o ide an o e iew o he Po uguese
ene gy sec o in he 2005–2015 decade, highligh ing he coun y’s e o owa ds enewable ene gy
deploymen ha , oge he wi h geog aphic ad an ages, upholds P G as a p omising al e na i e o
s o ing he coun y’s enewable elec ici y su pluses.
Keywo ds:
CO
2
cap u e and u iliza ion; ene gy dependence; powe - o-me hane; syn he ic na u al
gas; enewable powe ; ossil uels
1. In oduc ion
The Renewable Ene gy Roadmap 21 se les o 2020 and o he whole Eu opean Union a sha e o
ene gy om enewable sou ces o 20% [
1
]. Some coun ies, such as Po ugal, ha e al eady eached
o su passed such a a ge [
2
,
3
]; in ac , he cu en ene gy si ua ion in he coun y has signi ican ly
changed in he las decade, when enewable ene gy deploymen s a egies we e s ill unde deba e [
4
].
Po ugal was he ou h coun y o he Eu opean Union wi h a highe inco po a ion o enewable
elec ici y in 2015 (44.6%) a e Denma k (50.2%), Aus ia (62.6%), and Sweden (72.1%) [
3
]. The
Po uguese enewable annual elec ici y p oduc ion has inc eased almos ou old since 2005 and
eached 33.3 TWh in 2016, elying mos ly on hyd o (16.9 TWh) and wind (12.5 TWh) sou ces, oge he
ep esen ing 88% o he o al enewable powe p oduc ion [3].
In Po ugal, an annual su plus o enewable powe p oduc ion in he ange o 800–1200 GWh is
es ima ed o 2020 [
5
,
6
]. As enewable powe ele ance inc eases wi hin he ene gy sec o , de eloping
a way o e icien ly and economically s o e i s su pluses in pe iods o low demand becomes an u gen
p oblem o be ackled [
7
]. Among he sys ems a ailable o unde de elopmen o such a pu pose
(pumped hyd oelec ic s o age, comp essed ai ene gy s o age, elec ochemical and low ba e ies) [
8
],
powe - o-gas echnologies (P G) a e ecei ing inc eased a en ion, pa icula ly in Eu ope [
9
–
11
], and a
s o age po en ial o a leas 500 GWh has been o eseen in Po ugal [
6
]. One P G op ion could be o
Ene gies 2018,11, 3259; doi:10.3390/en11123259 www.mdpi.com/jou nal/ene gies
Ene gies 2018,11, 3259 2 o 20
use he su plus elec ici y o H2O elec olysis o ob ain H2(P H), bu i s s o age emains a challenge
and lacks a dedica ed in as uc u e o i s dis ibu ion [
2
]. Ano he way is o use ha “g een” H
2
and
blend i in na u al gas, bu only up o 10% wi hou majo e ec in he gas g id and end-use equipmen ,
o u he con e i o me hane (P M), also called subs i u e/syn he ic na u al gas (SNG), h ough
he Saba ie eac ion (Equa ion (1)) [
9
]. Me hane is a simple o s o e and anspo han pu e H
2
using he well-es ablished na u al gas in as uc u e and he e o e enabling he connec ion be ween
he powe and na u al gas g ids [12,13].
CO2+4 H2CH4+2 H2O∆H298 K=−165 kJ ·mol−1(1)
Syn he ic na u al gas can be la e econ e ed o elec ici y in pe iods o high demand o used as
eeds ock o uel. Thus, SNG can be seen as a secu e and e icien supply o enewable ene gy, while
simul aneously educing he dependence on (impo ed) ossil uels and suppo ing he ansi ion
owa ds a low-ca bon economy [14–16].
Baile a e al. [
10
] epo ed he exis ence o 43 P G p ojec s wo ldwide aking place in 11 coun ies,
wi h mos ini ia i es occu ing in Ge many (16 p ojec s), Denma k (7 p ojec s), and Swi ze land
(6 p ojec s) as a esul o s ong go e nmen al suppo . In he e iew by Qua on and Samsa li [
13
],
hese esul s we e upda ed, wi h Ge many s anding ou among o he coun ies wi h 45 p ojec s, ei he
inished, planned, ope a ing, o unde cons uc ion. The main d i e s owa ds P G in Ge many a e he
exis ence o geog aphic ad an ages o P G implemen a ion, like he a ailabili y o enough sui able
unde g ound gas s o age capaci y and a su icien gas ne wo k de elopmen o gas dis ibu ion [
11
,
17
],
as well as he coun y a ge s o inc ease i s powe gene a ion wi h o igin in enewable sou ces om
32% (in 2015) o 50% and 80% in 2030 and 2050, espec i ely [
18
]. In Po ugal, despi e being a
pionee ing coun y ega ding he adop ion and massi e di usion o wind powe pa ks ac oss i s
e i o y, he i s na ional esea ch p ojec in he coun y dedica ed o he opic was launched in
mid-2018 [
19
], dealing wi h he de elopmen o a cyclic so p ion- eac ion p ocess o simul aneous
CO2cap u e and con e sion o me hane o be coupled in P G applica ions (c . Figu e 1).
Ene gies 2018, 11, x FOR PEER REVIEW 2 o 20
P G op ion could be o use he su plus elec ici y o H
2
O elec olysis o ob ain H
2
(P H), bu i s
s o age emains a challenge and lacks a dedica ed in as uc u e o i s dis ibu ion [2]. Ano he way
is o use ha “g een” H
2
and blend i in na u al gas, bu only up o 10% wi hou majo e ec in he
gas g id and end-use equipmen , o u he con e i o me hane (P M), also called
subs i u e/syn he ic na u al gas (SNG), h ough he Saba ie eac ion (Equa ion (1)) [9]. Me hane is
a simple o s o e and anspo han pu e H
2
using he well-es ablished na u al gas in as uc u e
and he e o e enabling he connec ion be ween he powe and na u al gas g ids [12,13].
−
Δ−⋅
1
2 2 4 2 298 K
CO + 4 H CH + 2 H O = 165 kJ molH
(1)
Syn he ic na u al gas can be la e econ e ed o elec ici y in pe iods o high demand o used
as eeds ock o uel. Thus, SNG can be seen as a secu e and e icien supply o enewable ene gy,
while simul aneously educing he dependence on (impo ed) ossil uels and suppo ing he
ansi ion owa ds a low-ca bon economy [14–16].
Baile a e al. [10] epo ed he exis ence o 43 P G p ojec s wo ldwide aking place in 11 coun ies,
wi h mos ini ia i es occu ing in Ge many (16 p ojec s), Denma k (7 p ojec s), and Swi ze land (6
p ojec s) as a esul o s ong go e nmen al suppo . In he e iew by Qua on and Samsa li [13],
hese esul s we e upda ed, wi h Ge many s anding ou among o he coun ies wi h 45 p ojec s,
ei he inished, planned, ope a ing, o unde cons uc ion. The main d i e s owa ds P G in Ge many
a e he exis ence o geog aphic ad an ages o P G implemen a ion, like he a ailabili y o enough
sui able unde g ound gas s o age capaci y and a su icien gas ne wo k de elopmen o gas
dis ibu ion [11,17], as well as he coun y a ge s o inc ease i s powe gene a ion wi h o igin in
enewable sou ces om 32% (in 2015) o 50% and 80% in 2030 and 2050, espec i ely [18]. In Po ugal,
despi e being a pionee ing coun y ega ding he adop ion and massi e di usion o wind powe
pa ks ac oss i s e i o y, he i s na ional esea ch p ojec in he coun y dedica ed o he opic was
launched in mid-2018 [19], dealing wi h he de elopmen o a cyclic so p ion- eac ion p ocess o
simul aneous CO
2
cap u e and con e sion o me hane o be coupled in P G applica ions (c . Figu e 1).
Figu e 1. Powe - o-gas concep : sys em bounda ies wi h a cyclic so p ion- eac ion p ocess o CO
2
cap u e and con e sion/u iliza ion. Rep in ed om Chemical Enginee ing Jou nal, 322, C.V. Miguel,
M.A. So ia, A. Mendes, L.M. Madei a, A so p i e eac o o CO
2
cap u e and con e sion o enewable
me hane, 590–602, Copy igh (2017), wi h pe mission om Else ie .
The e a e ew s udies conce ning he assessmen o powe - o-gas implemen a ion po en ial in
Po ugal. The i s wo k was by Heymann and Bessa [6], who es ima ed he cos o P G p oduc s in
he coun y as a unc ion o he dis ance o wind powe pa ks and gas s o age acili ies. The le elized
cos o ene gy when conside ing SNG as a inal p oduc anged be ween 0.05–0.10 €/kWh. Recen ly,
Ca nei o e al. [20] p esen ed he oppo uni ies o la ge-scale ene gy s o age in geological o ma ions
in mainland Po ugal.
While P G demons a ion ac i i ies a e g owing as , pa icula ly in Eu ope, he cu en si ua ion
in Po ugal suppo s he indings by Ben o and Fon es [21] ha , ypically, Po ugal has an a e age
Figu e 1.
Powe - o-gas concep : sys em bounda ies wi h a cyclic so p ion- eac ion p ocess o CO
2
cap u e and con e sion/u iliza ion. Rep in ed om Chemical Enginee ing Jou nal, 322, C.V. Miguel,
M.A. So ia, A. Mendes, L.M. Madei a, A so p i e eac o o CO
2
cap u e and con e sion o enewable
me hane, 590–602, Copy igh (2017), wi h pe mission om Else ie .
The e a e ew s udies conce ning he assessmen o powe - o-gas implemen a ion po en ial in
Po ugal. The i s wo k was by Heymann and Bessa [
6
], who es ima ed he cos o P G p oduc s in
he coun y as a unc ion o he dis ance o wind powe pa ks and gas s o age acili ies. The le elized
cos o ene gy when conside ing SNG as a inal p oduc anged be ween 0.05–0.10
€
/kWh. Recen ly,
Ene gies 2018,11, 3259 3 o 20
Ca nei o e al. [
20
] p esen ed he oppo uni ies o la ge-scale ene gy s o age in geological o ma ions
in mainland Po ugal.
While P G demons a ion ac i i ies a e g owing as , pa icula ly in Eu ope, he cu en si ua ion
in Po ugal suppo s he indings by Ben o and Fon es [
21
] ha , ypically, Po ugal has an a e age
adop ion o ene gy- ela ed echnologies lag o one o wo decades ela i e o “co e” coun ies
(i.e., ene gy echnology de elope s/leade s, gene ally om he OECD-O ganiza ion o Economic
Co-ope a ion and De elopmen ) [
21
]. Hence, he p esen wo k aims o con ibu e o he cu en
s a e-o -a by p o iding a backg ound image o he Po uguese ene gy sec o (Sec ion 2), p esen ing
he main ac s and igu es, such as he coun y ene gy dependence e olu ion wi h ime (Sec ion 2.1), he
consump ion o ossil uels (Sec ion 2.2), and enewable powe p oduc ion (Sec ion 2.3). A e wa ds,
in Sec ion 3, equi emen s o powe - o-gas implemen a ion a e desc ibed, namely he a ailabili y o
enewable powe su pluses (Sec ion 3.1), ca bon dioxide sou ces o he me hana ion (Sec ion 3.2), and
access o he na u al gas g id o SNG s o age and dis ibu ion (Sec ion 3.3). In Sec ion 3.4, needs o
u u e esea ch a e iden i ied and, inally, in Sec ion 4, he main conclusions and he mos impo an
s eps ha all in e es ed pa ies should ake o aise awa eness ega ding deploymen o P G in Po ugal
a e p esen ed. Figu e 2shows a diag am p esen ing he app oach adop ed in his wo k.
Ene gies 2018, 11, x FOR PEER REVIEW 3 o 20
adop ion o ene gy- ela ed echnologies lag o one o wo decades ela i e o “co e” coun ies (i.e.,
ene gy echnology de elope s/leade s, gene ally om he OECD-O ganiza ion o Economic Co-
ope a ion and De elopmen ) [21]. Hence, he p esen wo k aims o con ibu e o he cu en s a e-o -
a by p o iding a backg ound image o he Po uguese ene gy sec o (Sec ion 2), p esen ing he main
ac s and igu es, such as he coun y ene gy dependence e olu ion wi h ime (Sec ion 2.1), he
consump ion o ossil uels (Sec ion 2.2), and enewable powe p oduc ion (Sec ion 2.3). A e wa ds,
in Sec ion 3, equi emen s o powe - o-gas implemen a ion a e desc ibed, namely he a ailabili y o
enewable powe su pluses (Sec ion 3.1), ca bon dioxide sou ces o he me hana ion (Sec ion 3.2),
and access o he na u al gas g id o SNG s o age and dis ibu ion (Sec ion 3.3). In Sec ion 3.4, needs
o u u e esea ch a e iden i ied and, inally, in Sec ion 4, he main conclusions and he mos
impo an s eps ha all in e es ed pa ies should ake o aise awa eness ega ding deploymen o
P G in Po ugal a e p esen ed. Figu e 2 shows a diag am p esen ing he app oach adop ed in his
wo k.
Figu e 2. Diag am p esen ing he s udy app oach adop ed in his wo k.
2. O e iew o he Po uguese Ene gy Sec o
2.1. Ene gy Dependence
The ene gy dependence (ED) is a pa ame e ha cha ac e izes he ex en o which an economy
elies upon impo s o mee i s ene gy needs. The indica o is calcula ed as ne impo s o p ima y
ene gy (i.e., (IMP) impo a ions minus expo a ions (EXP)) di ided by he sum o g oss inland ene gy
consump ion (GIC) plus in e na ional ma i ime bunke s (IMB) (c . Equa ion (2)) [22].
()
−
=×
+
% 100
IMP EXP
ED GIC IMB
(2)
The Po uguese ene gy dependence and he dependence o he Eu o-economic a ea (EU-19) and
Eu opean Union coun ies (EU-28) a e shown in Figu e 3 o compa ison.
Figu e 2. Diag am p esen ing he s udy app oach adop ed in his wo k.
2. O e iew o he Po uguese Ene gy Sec o
2.1. Ene gy Dependence
The ene gy dependence (ED) is a pa ame e ha cha ac e izes he ex en o which an economy
elies upon impo s o mee i s ene gy needs. The indica o is calcula ed as ne impo s o p ima y
ene gy (i.e., (IMP) impo a ions minus expo a ions (EXP)) di ided by he sum o g oss inland ene gy
consump ion (GIC) plus in e na ional ma i ime bunke s (IMB) (c . Equa ion (2)) [22].
ED(%)=IMP −EXP
GIC +IMB ×100 (2)
The Po uguese ene gy dependence and he dependence o he Eu o-economic a ea (EU-19) and
Eu opean Union coun ies (EU-28) a e shown in Figu e 3 o compa ison.
Ene gies 2018,11, 3259 4 o 20
Ene gies 2018, 11, x FOR PEER REVIEW 4 o 20
Figu e 3. Po uguese ene gy dependence (ED): (a) Along ecen yea s and (b) compa ison wi h EU-
19 coun ies in 2015. Da a aken om Eu os a online da abase [22].
Po ugal had he se en h highes ene gy dependence among he EU-19 and EU-28 coun ies in
2015 (c . Figu e 3b). None o he EU-19 coun ies had a nega i e ene gy dependence (c . Figu e 3b),
all depending on p ima y ene gy impo s o sa is y hei ene ge ic needs.
The no malized consump ion o p ima y ene gy (CPE) pe ype o sou ce in Po ugal is shown
in Figu e 4a, o he pe iod o 2000–2015. The coun y si ua ion is compa ed wi h hose om EU-19
and EU-28 g oup coun ies o he yea , 2015, in Figu e 4b.
The Na ional Ene gy S a egy, app o ed in 2005 by he Po uguese Go e nmen , se led s a egic
policies, such as he ene gy ma ke libe aliza ion, he p omo ion o ene gy om enewable sou ces,
and o echnologies wi h imp o ed e iciencies [23]. As a esul , he oil sha e ema kably declined
(i.e., 14.6%) in he ollowing decade, eplaced by na u al gas and ene gy om enewable sou ces,
whose alues inc eased by 4% and 9%, espec i ely, while he coal sha e p ac ically emained
cons an in he same pe iod (a ise o only 1.6%) (c . Figu e 4a). S ill, ossil uels ep esen ed 78% o
he consumed p ima y ene gy in 2015, a alue sligh ly abo e EU-19 (72%) and EU-28 (73%) g oup
coun ies, whose pa e ns a e nea ly iden ical (c . Figu e 4b). The emaining p ima y ene gy was
exclusi ely based on enewable sou ces (22%), making Po ugal he i h coun y wi h he highes
sha e o ene gy om enewables amongs he EU-28 coun ies [24]. The weigh o enewables
becomes mo e signi ican when conside ing p ima y ene gy consump ion exclusi ely o powe
p oduc ion pu poses. In ac , 45% o he elec ici y p oduced in 2015 was ob ained om enewable
sou ces [3]. Nuclea has almos he same weigh as he ene gy om enewable sou ces (ca. 13–15%)
in EU-19 and EU-28 g oups, al hough i is absen in some membe s, like Po ugal.
Figu e 4. No malized p ima y ene gy consump ion pe ype o sou ce in Po ugal: (a) F om 2000 o
2015 and (b) compa ison wi h EU g oups in 2015. Au ho s calcula ions based on da a aken om
Eu os a online da abase [22] and excluding he negligible con ibu ion o non- enewable was e
sou ces.
Yea
2000 2005 2010 2015
ED / %
40
50
60
70
80
90
100
Po ugal
EU-19
EU-28
ED / %
0 20406080100
Cyp us
Mal a
Luxembou g
I eland
Belgium
Li huania
Po ugal
I aly
Spain
G eece
Ge many
Aus ia
Slo akia
Ne he lands
La ia
Slo enia
Finland
F ance
Es onia
a) b)
Yea
2000 2005 2010 2015
CPEi / CPE o al x 100
0
20
40
60
80
100
a)
b)
EU-28 EU-19 Po ugal
CPEi / CPE o al x 100
0
20
40
60
80
100
Oil
Na u al gas
Coal
Renewables
Nuclea
Figu e 3.
Po uguese ene gy dependence (ED): (
a
) Along ecen yea s and (
b
) compa ison wi h EU-19
coun ies in 2015. Da a aken om Eu os a online da abase [22].
Po ugal had he se en h highes ene gy dependence among he EU-19 and EU-28 coun ies in
2015 (c . Figu e 3b). None o he EU-19 coun ies had a nega i e ene gy dependence (c . Figu e 3b), all
depending on p ima y ene gy impo s o sa is y hei ene ge ic needs.
The no malized consump ion o p ima y ene gy (CPE) pe ype o sou ce in Po ugal is shown in
Figu e 4a, o he pe iod o 2000–2015. The coun y si ua ion is compa ed wi h hose om EU-19 and
EU-28 g oup coun ies o he yea , 2015, in Figu e 4b.
The Na ional Ene gy S a egy, app o ed in 2005 by he Po uguese Go e nmen , se led s a egic
policies, such as he ene gy ma ke libe aliza ion, he p omo ion o ene gy om enewable sou ces,
and o echnologies wi h imp o ed e iciencies [
23
]. As a esul , he oil sha e ema kably declined (i.e.,
14.6%) in he ollowing decade, eplaced by na u al gas and ene gy om enewable sou ces, whose
alues inc eased by 4% and 9%, espec i ely, while he coal sha e p ac ically emained cons an in he
same pe iod (a ise o only 1.6%) (c . Figu e 4a). S ill, ossil uels ep esen ed 78% o he consumed
p ima y ene gy in 2015, a alue sligh ly abo e EU-19 (72%) and EU-28 (73%) g oup coun ies, whose
pa e ns a e nea ly iden ical (c . Figu e 4b). The emaining p ima y ene gy was exclusi ely based on
enewable sou ces (22%), making Po ugal he i h coun y wi h he highes sha e o ene gy om
enewables amongs he EU-28 coun ies [
24
]. The weigh o enewables becomes mo e signi ican
when conside ing p ima y ene gy consump ion exclusi ely o powe p oduc ion pu poses. In ac ,
45% o he elec ici y p oduced in 2015 was ob ained om enewable sou ces [
3
]. Nuclea has almos
he same weigh as he ene gy om enewable sou ces (ca. 13–15%) in EU-19 and EU-28 g oups,
al hough i is absen in some membe s, like Po ugal.
Ene gies 2018, 11, x FOR PEER REVIEW 4 o 20
Figu e 3. Po uguese ene gy dependence (ED): (a) Along ecen yea s and (b) compa ison wi h EU-
19 coun ies in 2015. Da a aken om Eu os a online da abase [22].
Po ugal had he se en h highes ene gy dependence among he EU-19 and EU-28 coun ies in
2015 (c . Figu e 3b). None o he EU-19 coun ies had a nega i e ene gy dependence (c . Figu e 3b),
all depending on p ima y ene gy impo s o sa is y hei ene ge ic needs.
The no malized consump ion o p ima y ene gy (CPE) pe ype o sou ce in Po ugal is shown
in Figu e 4a, o he pe iod o 2000–2015. The coun y si ua ion is compa ed wi h hose om EU-19
and EU-28 g oup coun ies o he yea , 2015, in Figu e 4b.
The Na ional Ene gy S a egy, app o ed in 2005 by he Po uguese Go e nmen , se led s a egic
policies, such as he ene gy ma ke libe aliza ion, he p omo ion o ene gy om enewable sou ces,
and o echnologies wi h imp o ed e iciencies [23]. As a esul , he oil sha e ema kably declined
(i.e., 14.6%) in he ollowing decade, eplaced by na u al gas and ene gy om enewable sou ces,
whose alues inc eased by 4% and 9%, espec i ely, while he coal sha e p ac ically emained
cons an in he same pe iod (a ise o only 1.6%) (c . Figu e 4a). S ill, ossil uels ep esen ed 78% o
he consumed p ima y ene gy in 2015, a alue sligh ly abo e EU-19 (72%) and EU-28 (73%) g oup
coun ies, whose pa e ns a e nea ly iden ical (c . Figu e 4b). The emaining p ima y ene gy was
exclusi ely based on enewable sou ces (22%), making Po ugal he i h coun y wi h he highes
sha e o ene gy om enewables amongs he EU-28 coun ies [24]. The weigh o enewables
becomes mo e signi ican when conside ing p ima y ene gy consump ion exclusi ely o powe
p oduc ion pu poses. In ac , 45% o he elec ici y p oduced in 2015 was ob ained om enewable
sou ces [3]. Nuclea has almos he same weigh as he ene gy om enewable sou ces (ca. 13–15%)
in EU-19 and EU-28 g oups, al hough i is absen in some membe s, like Po ugal.
Figu e 4. No malized p ima y ene gy consump ion pe ype o sou ce in Po ugal: (a) F om 2000 o
2015 and (b) compa ison wi h EU g oups in 2015. Au ho s calcula ions based on da a aken om
Eu os a online da abase [22] and excluding he negligible con ibu ion o non- enewable was e
sou ces.
Yea
2000 2005 2010 2015
ED / %
40
50
60
70
80
90
100
Po ugal
EU-19
EU-28
ED / %
0 20406080100
Cyp us
Mal a
Luxembou g
I eland
Belgium
Li huania
Po ugal
I aly
Spain
G eece
Ge many
Aus ia
Slo akia
Ne he lands
La ia
Slo enia
Finland
F ance
Es onia
a) b)
Yea
2000 2005 2010 2015
CPEi / CPE o al x 100
0
20
40
60
80
100
a)
b)
EU-28 EU-19 Po ugal
CPEi / CPE o al x 100
0
20
40
60
80
100
Oil
Na u al gas
Coal
Renewables
Nuclea
Figu e 4.
No malized p ima y ene gy consump ion pe ype o sou ce in Po ugal: (
a
) F om 2000 o
2015 and (
b
) compa ison wi h EU g oups in 2015. Au ho s calcula ions based on da a aken om
Eu os a online da abase [
22
] and excluding he negligible con ibu ion o non- enewable was e sou ces.
Ene gies 2018,11, 3259 5 o 20
The las 10 yea s up o he p esen ega ding ossil uels and ene gy om enewable sou ces
con ibu ions o he Po uguese ene gy sec o is p esen ed in Sec ions 2.2 and 2.3, espec i ely,
wi h an emphasis on enewable powe p oduc ion, as i is one o he main building blocks o
powe - o-gas echnologies.
2.2. Ene gy om Fossil Fuels
2.2.1. Oil
Up o now, Po ugal does no ha e indigenous oil ese es wi h economic iabili y, al hough
egula onsho e and o sho e explo a ion ac i i ies ha e been ca ied ou since 1940. The e o e, all
oil consumed by he coun y is impo ed. Table 1lis s he op i e supplie coun ies om 2014 o
2016. In he lis ed yea s, Po ugal impo ed oil om 13–15 coun ies and he op i e oil supplie s
we e esponsible o a ound 66–76% o he o al impo ed oil. Angola was he majo oil supplie wi h
a con ibu ion o ca. 25%. Di e si ica ion o oil supplie s along he yea s has con ibu ed o assu e
eliable and secu e access o ossil ene gy esou ces [25].
Table 1.
Top i e oil supplie s o Po ugal and hei co esponding sha e (based on da a aken om [
26
]).
Top-5 2014 2015 2016
1s Angola (26.1%) Angola (22.9%) Angola (24.9%)
2nd Saudi A abia (12.6%) Saudi A abia (14.2%) Russia (19.7%)
3 d Alge ia (9.9%) Kazakhs an (10.6%) Aze baijan (11.1%)
4 h Kazakhs an (9.7%) Alge ia (9.5%) Saudi A abia (10.8%)
5 h Aze baijan (9.2%) Aze baijan (9.0%) Kazakhs an (9.3%)
Imp. oil (106 on) 7.5 (ou o 11.17) 9.1 (ou o 13.73) 10.7 (ou o 14.09)
N . oil supplie s 14 15 13
Figu e 5shows he inal consump ion o oil by ac i i y sec o . The anspo a ion sec o is
esponsible o he la ges sha e (ca. 75–79%). Oil consump ion declined in all sec o s o he i e
yea pe iod, excep Ag icul u e/Fo es y, which emained p ac ically cons an . Wi hin he Po uguese
indus y sec o , he non-me allic mine als indus ies (e.g., cemen and glass) we e by a he ac i i ies
wi h highe oil consump ion (i.e., 50–60% o he oil consumed by he indus y sec o ).
Ene gies 2018, 11, x FOR PEER REVIEW 5 o 20
The las 10 yea s up o he p esen ega ding ossil uels and ene gy om enewable sou ces
con ibu ions o he Po uguese ene gy sec o is p esen ed in Sec ions 2.2 and 2.3, espec i ely, wi h
an emphasis on enewable powe p oduc ion, as i is one o he main building blocks o powe - o-
gas echnologies.
2.2. Ene gy om Fossil Fuels
2.2.1. Oil
Up o now, Po ugal does no ha e indigenous oil ese es wi h economic iabili y, al hough
egula onsho e and o sho e explo a ion ac i i ies ha e been ca ied ou since 1940. The e o e, all oil
consumed by he coun y is impo ed. Table 1 lis s he op i e supplie coun ies om 2014 o 2016.
In he lis ed yea s, Po ugal impo ed oil om 13–15 coun ies and he op i e oil supplie s we e
esponsible o a ound 66–76% o he o al impo ed oil. Angola was he majo oil supplie wi h a
con ibu ion o ca. 25%. Di e si ica ion o oil supplie s along he yea s has con ibu ed o assu e
eliable and secu e access o ossil ene gy esou ces [25].
Table 1. Top i e oil supplie s o Po ugal and hei co esponding sha e (based on da a aken om
[26]).
Top-5 2014 2015 2016
1
s
Angola (26.1%) Angola (22.9%) Angola (24.9%)
2
nd
Saudi A abia (12.6%) Saudi A abia (14.2%) Russia (19.7%)
3
d
Alge ia (9.9%) Kazakhs an (10.6%) Aze baijan (11.1%)
4
h
Kazakhs an (9.7%) Alge ia (9.5%) Saudi A abia (10.8%)
5
h
Aze baijan (9.2%) Aze baijan (9.0%) Kazakhs an (9.3%)
Imp. oil (10
6
on) 7.5 (ou o 11.17) 9.1 (ou o 13.73) 10.7 (ou o 14.09)
N . oil supplie s 14 15 13
Figu e 5 shows he inal consump ion o oil by ac i i y sec o . The anspo a ion sec o is
esponsible o he la ges sha e (ca. 75–79%). Oil consump ion declined in all sec o s o he i e yea
pe iod, excep Ag icul u e/Fo es y, which emained p ac ically cons an . Wi hin he Po uguese
indus y sec o , he non-me allic mine als indus ies (e.g., cemen and glass) we e by a he ac i i ies
wi h highe oil consump ion (i.e., 50–60% o he oil consumed by he indus y sec o ).
Figu e 5. E olu ion o oil p oduc s’ inal ene gy consump ion in Po ugal by ac i i y sec o om 2010
o 2015 (da a aken om [27]).
2.2.2. Coal
Figu e 5.
E olu ion o oil p oduc s’ inal ene gy consump ion in Po ugal by ac i i y sec o om 2010
o 2015 (da a aken om [27]).
2.2.2. Coal
A e na ional coal p oduc ion ceased in 1994, Po ugal dependence on impo ed coal o secu e
i s ene gy needs inc eased. Po ugal impo ed 4.5 millions o onnes o coal om Colombia (88.1%),

Ene gies 2018,11, 3259 6 o 20
he Uni ed S a es (6.6%), Sou h A ica (3.5%), and Uk aine (1.8%) in 2014 [
24
]. Impo ed coal is o
he bi uminous ype, being used essen ially o elec ici y gene a ion in wo coal- i ed powe plan s
loca ed in Sines (1250 MW) and Pego (620 MW). These plan s ac as a backup sys em, gua an eeing
ha powe demand is ul illed in pe iods o low enewable powe p oduc ion. Coal consump ion is
pa icula ly dependen on he hyd ological condi ions, namely when hyd opowe ou pu is lowe
du ing d ough pe iods. Coal is also consumed by end-use s om he indus y sec o , namely by
he i on and s eel indus ies, in chemical/pe ochemical plan s, and by he non-me allic mine als
indus ies (c . Figu e 6). Ne e heless, he amoun o coal used by hese end-use s is negligible when
compa ed o he quan i y used o elec ici y p oduc ion (e.g., 12 k oe s. 3246 k oe, espec i ely). S ill,
coal consump ion by he non-me allic mine als and chemical sec o s has dec eased conside ably om
2010 o 2015 (c . Figu e 6). The amoun o coal consumed in 2014 and 2015 by he non-me allic mine als,
i on and s eel, and chemical/pe ochemical sub-sec o s was simila (ca. 4 k oe/each) (Figu e 6).
Ene gies 2018, 11, x FOR PEER REVIEW 6 o 20
A e na ional coal p oduc ion ceased in 1994, Po ugal dependence on impo ed coal o secu e
i s ene gy needs inc eased. Po ugal impo ed 4.5 millions o onnes o coal om Colombia (88.1%),
he Uni ed S a es (6.6%), Sou h A ica (3.5%), and Uk aine (1.8%) in 2014 [24]. Impo ed coal is o he
bi uminous ype, being used essen ially o elec ici y gene a ion in wo coal- i ed powe plan s
loca ed in Sines (1250 MW) and Pego (620 MW). These plan s ac as a backup sys em, gua an eeing
ha powe demand is ul illed in pe iods o low enewable powe p oduc ion. Coal consump ion is
pa icula ly dependen on he hyd ological condi ions, namely when hyd opowe ou pu is lowe
du ing d ough pe iods. Coal is also consumed by end-use s om he indus y sec o , namely by he
i on and s eel indus ies, in chemical/pe ochemical plan s, and by he non-me allic mine als
indus ies (c . Figu e 6). Ne e heless, he amoun o coal used by hese end-use s is negligible when
compa ed o he quan i y used o elec ici y p oduc ion (e.g., 12 k oe s. 3246 k oe, espec i ely).
S ill, coal consump ion by he non-me allic mine als and chemical sec o s has dec eased conside ably
om 2010 o 2015 (c . Figu e 6). The amoun o coal consumed in 2014 and 2015 by he non-me allic
mine als, i on and s eel, and chemical/pe ochemical sub-sec o s was simila (ca. 4 k oe/each) (Figu e
6).
Figu e 6. Coal consump ion by he indus y sub-sec o s om 2010 o 2015 [27].
2.2.3. Na u al Gas
Po ugal has no na u al gas esou ces. Table 2 shows na u al gas impo o igins and
co esponding olumes o he yea s, 2014 and 2015.
Table 2. Na u al gas impo s (106 m3) in he yea s 2014 and 2015 [28].
Deli e y ype O igin 2014 2015
Pipeline (Na u al gas) 2736 3002
Alge ia 2196 2111
Spain 535 891
No speci ied 5 0
Ships (Lique ied na u al gas-
LNG) 1523 1776
Alge ia 102 210
Qa a 687 224
Nige ia 352 1166
No way 80 80
Spain 1 6 7
T inidad and Tobago 223 89
No speci ied 73 0
TOTAL 4259 4778
Yea
2010 2011 2012 2013 2014 2015
Final consump ion / k oe
0
10
20
30
40
Non-me allic mine als
I on & S eel
Chemical/Pe ochemical
Figu e 6. Coal consump ion by he indus y sub-sec o s om 2010 o 2015 [27].
2.2.3. Na u al Gas
Po ugal has no na u al gas esou ces. Table 2shows na u al gas impo o igins and co esponding
olumes o he yea s, 2014 and 2015.
Table 2. Na u al gas impo s (106m3) in he yea s 2014 and 2015 [28].
Deli e y Type O igin 2014 2015
Pipeline (Na u al gas) 2736 3002
Alge ia 2196 2111
Spain 535 891
No speci ied 5 0
Ships (Lique ied na u al gas-LNG) 1523 1776
Alge ia 102 210
Qa a 687 224
Nige ia 352 1166
No way 80 80
Spain 16 7
T inidad and
Tobago 223 89
No speci ied 73 0
TOTAL 4259 4778
1LNG impo ed using anke ucks.
Ene gies 2018,11, 3259 7 o 20
A ound 64% o he supplies we e ecei ed h ough a pipeline, while he emaining pa , lique ied,
was anspo ed o Po ugal in ships ha unload a he Sines e minal, on he sou he n pa o he
coun y. Only a negligible quan i y (6–7
×
10
6
m
3
) was impo ed using anke ucks exclusi ely om
Spain. The mos impo an supplie is Alge ia, wi h a sha e anging be ween 45–52%, while Qa a and
Nige ia we e he majo supplie s o lique ied na u al gas (LNG).
The inal ene gy consump ion o na u al gas by ac i i y sec o is shown in Figu e 7. The
indus y sec o accoun s o he la ges amoun o na u al gas consump ion (67–74%), ollowed by he
esiden ial (16–19%) and se ices (13–14%) sec o s. The use o na u al gas in he ag icul u e/ o es and
anspo a ion sec o s is negligible and bo h sec o s ep esen only ca. 1% o he o al consump ion.
Ene gy o anspo a ion pu poses is assu ed p edominan ly by oil p oduc s (as shown in Sec ion 2.2.1),
wi h na u al gas playing a negligible ole; o ins ance, he quan i y o oil and na u al gas consumed in
2015 o anspo a ion was 6245 k oe s. 13 k oe, espec i ely.
Ene gies 2018, 11, x FOR PEER REVIEW 7 o 20
1 LNG impo ed using anke ucks.
A ound 64% o he supplies we e ecei ed h ough a pipeline, while he emaining pa ,
lique ied, was anspo ed o Po ugal in ships ha unload a he Sines e minal, on he sou he n pa
o he coun y. Only a negligible quan i y (6–7 × 10
6
m
3
) was impo ed using anke ucks exclusi ely
om Spain. The mos impo an supplie is Alge ia, wi h a sha e anging be ween 45–52%, while
Qa a and Nige ia we e he majo supplie s o lique ied na u al gas (LNG).
The inal ene gy consump ion o na u al gas by ac i i y sec o is shown in Figu e 7. The indus y
sec o accoun s o he la ges amoun o na u al gas consump ion (67–74%), ollowed by he
esiden ial (16–19%) and se ices (13–14%) sec o s. The use o na u al gas in he ag icul u e/ o es
and anspo a ion sec o s is negligible and bo h sec o s ep esen only ca. 1% o he o al
consump ion. Ene gy o anspo a ion pu poses is assu ed p edominan ly by oil p oduc s (as
shown in Sec ion 2.2.1), wi h na u al gas playing a negligible ole; o ins ance, he quan i y o oil and
na u al gas consumed in 2015 o anspo a ion was 6245 k oe s. 13 k oe, espec i ely.
Figu e 7. Na u al gas o inal ene gy consump ion in Po ugal by ac i i y sec o om 2010 o 2015
(da a aken om [27]).
Table 3 lis s he na u al gas consump ion ac oss he indus y sec o s du ing 2005, 2014, and 2015.
Table 3. Na u al gas consump ion (k oe) by he Po uguese indus y (da a aken om [27]).
Indus y 2005 2014 2015 Δ (2015/2005)
Pape , Pulp, and P in 38.1 90.2 111.6 2.93
Cons uc ion 5.8 13.1 14.5 2.48
Chemical and Pe ochemical indus y 64.4 142.1 152.7 2.37
Food and Tobacco 66.5 124.6 147.2 2.21
Non- e ous me al indus y 7.6 12.9 16.0 2.09
Machine y 21.3 32.6 36.0 1.69
I on & s eel indus y 41.4 47.4 51.1 1.23
Tex ile and Lea he 128.6 131.4 131.9 1.03
Non-me allic Mine als (e.g., cemen ) 516.6 426.3 441.6 0.85
Wood and Wood P oduc s 9.7 9.0 7.8 0.81
Mining and Qua ying 6.3 5.1 4.6 0.74
T anspo Equipmen 28.8 14.9 16.6 0.58
Non-speci ied (Indus y) 20.9 7.2 5.8 0.28
To al 956.0 1056.8 1137.4 1.19
The alues lis ed in Table 3 show ha he non-me allic mine als sec o is he bigges consume
o na u al gas. Table 3 also highligh s he g owing ele ance o na u al gas o e ime. Indeed, since
2005, he annual consump ion inc eased in eigh ou o 13 indus ial ac i i ies (see ela i e a ia ion
Figu e 7.
Na u al gas o inal ene gy consump ion in Po ugal by ac i i y sec o om 2010 o 2015
(da a aken om [27]).
Table 3lis s he na u al gas consump ion ac oss he indus y sec o s du ing 2005, 2014, and 2015.
Table 3. Na u al gas consump ion (k oe) by he Po uguese indus y (da a aken om [27]).
Indus y 2005 2014 2015 ∆(2015/2005)
Pape , Pulp, and P in 38.1 90.2 111.6 2.93
Cons uc ion 5.8 13.1 14.5 2.48
Chemical and Pe ochemical indus y
64.4 142.1 152.7 2.37
Food and Tobacco 66.5 124.6 147.2 2.21
Non- e ous me al indus y 7.6 12.9 16.0 2.09
Machine y 21.3 32.6 36.0 1.69
I on & s eel indus y 41.4 47.4 51.1 1.23
Tex ile and Lea he 128.6 131.4 131.9 1.03
Non-me allic Mine als (e.g., cemen ) 516.6 426.3 441.6 0.85
Wood and Wood P oduc s 9.7 9.0 7.8 0.81
Mining and Qua ying 6.3 5.1 4.6 0.74
T anspo Equipmen 28.8 14.9 16.6 0.58
Non-speci ied (Indus y) 20.9 7.2 5.8 0.28
To al 956.0 1056.8 1137.4 1.19
The alues lis ed in Table 3show ha he non-me allic mine als sec o is he bigges consume o
na u al gas. Table 3also highligh s he g owing ele ance o na u al gas o e ime. Indeed, since 2005,
he annual consump ion inc eased in eigh ou o 13 indus ial ac i i ies (see ela i e a ia ion in he
las column). Amongs hem, he pape , pulp, and p in , he chemical and pe ochemical, and he ood
and obacco (2.21) indus ies s and ou gi en hei absolu e ene gy consump ion and ela i e a ia ion
Ene gies 2018,11, 3259 8 o 20
alues, mo e han duplica ing in all o hem in only 10 yea s. Globally, na u al gas consump ion
inc eased 19% in he 2005–2015 decade, which e lec s i s g owing impo ance o he Po uguese
indus y sec o .
2.3. Ene gy om Renewable Sou ces
The s a egic e o o eplace ossil uels by ene gy om enewable sou ces has made Po ugal
one o Eu ope’s leade s in his a ea [
24
]. Table 4lis s he amoun (in k oe) o ene gy om enewable
sou ces p oduced in Po ugal du ing he 2005–2015 decade.
Table 4.
Po uguese annual p oduc ion o ene gy om enewable sou ces (k oe) om 2005 o 2015 [
3
].
Renewable Ene gy Type 2005 2007 2009 2011 2013 2015 ∆(2015/2005)
Bio uels 0 162 226 330 274 321 -
Elec ici y 1599 1265 1456 1872 2369 1927 3.2
Biomass 22773 2891 3019 2571 2812 2781 1.0
O he enewables 320 23 36 61 74 82 4.1
To al 3392 4342 4737 4835 5530 5110 1.5
1
Includes he con ibu ion o hyd o, wind, pho o ol aic, and geo he mal powe ;
2
includes he con ibu ion o
biogas; 3includes sola ( o he mal pu poses) and (low en halpy) geo he mal sou ces.
Since 2006, Po ugal has p oduced biodiesel, which is inco po a ed almos comple ely in he
con en ional ossil diesel and only a small ac ion (ca. 1%) is di ec ly sold in he ma ke . Soybean
and, pa icula ly, colza oils a e he mos used aw ma e ials [
29
]. Mo e han hal o he ene gy om
enewable sou ces p oduced in Po ugal comes om biomass, al hough ha sha e dec eased om 82%
o 54%, when compa ing he alues o 2005 and 2015. The amoun o ene gy p oduced om biomass
emained nea ly cons an along he 2005–2015 decade, while he p oduc ion o elec ici y ipled,
eaching a sha e o 38% o he o al ene gy om enewable sou ces p oduced in 2015 (c . Table 4).
Elec ici y p oduc ion alues shown in Table 4include con ibu ions om hyd o, wind, pho o ol aic,
and geo he mal sou ces, and excludes con ibu ions om biomass in he moelec ic and co-gene a ion
plan s. In o ma ion ega ding he p esen ene gy p oduc ion s a us om bio uels and biomass can be
ound elsewhe e (e.g., [24]).
The in es men made on he di e en echnologies o powe p oduc ion om enewable sou ces
is highligh ed h ough he analysis o he ins alled capaci y (MW) alues lis ed in Table 5. The mos
es ablished enewable ene gy sou ces (RES) o elec ici y p oduc ion in Po ugal a e hyd o and wind,
bo h o aling o e 90% o he ins alled capaci y. Biomass is he hi d RES wi h a highe ins alled
capaci y, ollowed closely (in ecen yea s) by pho o ol aic, which ema kably inc eased om 3 MW o
451 MW in he 2005–2015 decade. Du ing his pe iod, he wind ene gy ins alled capaci y inc eased
almos 400% and was by a he ype o RES wi h he highes absolu e a ia ion (i.e., 3971 MW).
Table 5.
Renewable ene gy sou ces’ ins alled capaci y (MW) in Po ugal o elec ici y p oduc ion and
co esponding a ia ion in he 2005–2015 decade [3,30].
RES 2005 2007 2009 2011 2013 2015 ∆(2015/2005)
Geo he mal 18 29 29 29 29 29 1.6
Pho o ol aic 3 15 110 175 299 451 150.3
Biomass 429 449 518 712 718 726 1.7
Wind 1063 1699 3564 4378 4731 5034 4.7
Hyd o 4816 4853 4883 5330 5533 6053 1.3
To al 6329 7045 9104 10,624 11,310 12,293 1.9
Ene gies 2018,11, 3259 9 o 20
Among biomass, i should be men ioned he e olu ion o biogas p oduc ion, whose ins alled
capaci y inc eased om 8 MW (in 2005) o 85 MW (in 2015), while he capaci y o ene gy gene a ion
om u ban solid was es only inc eased 3 MW, eaching a o al capaci y o 89 MW in 2015.
The exploi a ion o he ins alled capaci y o powe p oduc ion om he di e en RES is p o ided
in Table 6, which shows ha , globally, he powe p oduc ion ipled in he 2005–2015 decade. In he
ollowing sec ions, he s a us o each RES o p oducing elec ici y is add essed.
Table 6.
Annual enewable powe p oduc ion (GWh) in Po ugal and co esponding a ia ion in he
2005–2015 decade [3,30].
RES 2005 2007 2009 2011 2013 2015 ∆(2015/2005)
Geo he mal 71 201 184 210 197 204 2.9
Pho o ol aic 3 24 160 282 479 799 266.3
Biomass 1651 1883 2086 2924 3052 3104 1.9
Wind 1773 4036 7577 9162 12,015 11,608 6.5
Hyd o 5118 10,449 9009 12,114 14,868 9800 1.9
To al 8616 16,593 19,016 24,692 30,610 25,514 3.0
2.3.1. Geo he mal
Among he RES, high empe a u e geo he mal esou ces a e con ined o he Azo es a chipelago
whe e his kind o ene gy plays an impo an ole. Two geo he mal powe plan s in ope a ion a
S. Miguel island, co esponding o a global ins alled capaci y o 23 MW, a e esponsible o he
p oduc ion o 42% o he consumed elec ici y (i.e., a ound 22% o he a chipelago o al demand). Plans
o inc ease he ins alled capaci y up o 28.5 MW un il 2019 ha e been epo ed [
31
]. The In e na ional
Ene gy Agency (IEA) epo ed ha enhanced geo he mal sys ems echnology, which uses he mal
ene gy om high- empe a u e ocks (d y ocks) loca ed a g ea dep hs, may be sui able o explo e
he po en ial geo he mal esou ces in he mainland and be es ed in he u u e [
24
]. S ill, Po ugal is
he i h coun y among IEA-29 membe s wi h he highes sha e o geo he mal ene gy used o powe
p oduc ion [24].
2.3.2. Pho o ol aic
Powe p oduc ion in pho o ol aic plan s was negligible in 2005 and eached 799 GWh in 2015,
being he RES wi h he highes ela i e a ia ion in he 2005–2015 decade (c . Table 6). Po ugal has
he bes yea ly sola i adiance in Eu ope a e Cyp us, pa icula ly in he Alen ejo egion, in he
sou he n pa o he e i o y, whe e he coun y has a cu en ins alled capaci y o 162 MW (ou
o a o al o 467 MW) [
3
,
32
]. The pho o ol aic plan loca ed in Mou a is he la ges in he coun y
comp ising an ins alled capaci y o 46 MW. I is expec ed ha sola ene gy will play an impo an ole
in decen alised powe p oduc ion, and a mini-gene a ion p og amme c ea ed in 2011 has a a ge o
ins all app oxima ely 250 MW o new capaci y by 2020 [
24
]. Be o e 2011, he lack o speci ic egula ions
o mini-gene a ion sys ems limi ed pho o ol aic di usion as he eed-in a i s se led in 2007 by he
Dec ee-Law No. 225/2007 ha e no been lis ed explici ly, being calcula ed mon hly o each sys em
based on a oided cos s, which leads o adminis a i e di icul ies as well as low anspa ency [33].
2.3.3. Biomass
The mos common biomass esou ces a ailable in Po ugal a e wood esidues, animal was e, and
municipal solid was e [
32
]. I was es ima ed ha he coun y’s o al biomass po en ial is 42.5 TWh/yea ,
wi h municipal solid was es as he main esou ce (17.0 TWh/yea ) [
32
]. I has been epo ed ha he
use o municipal solid was es, animal manu e, and was ewa e s a e s ill unde exploi ed [
32
]. In 2015,
586 GWh o powe was gene a ed om biogas and u ban solid was es (ca. 294 GWh each), oge he
ep esen ing 19% o he o al powe p oduced om biomass (i.e., 3.10 TWh). Powe p oduc ion om
Ene gies 2018,11, 3259 16 o 20
Ene gies 2018, 11, x FOR PEER REVIEW 16 o 20
Figu e 13. Po ugal na u al gas s o age and anspo a ion in as uc u e ( ep in ed om [36] wi h
pe mission om REN).
The na ional pipeline ne wo k has an ex ension o 1375 km wi h 202 pipeline s a ions [24]. Sines
e minal ecei es LNG om la ge essel ships wi h a capaci y om 45 × 10
3
up o 216 × 10
3
m
3
. These
ships unload in o h ee LNG s o age anks ha ing a combined capaci y o 390 × 10
3
m
3
, co esponding
o ca. 242 × 10
6
m
3
o na u al gas [24,36]. The e minal is equipped wi h i e apo ize s using sea wa e
as he mal luid o gasi y LNG, which is u he comp essed o 78 ba and injec ed in o he gas g id
[24]. The e minal acili ies also include a illing s a ion ha may load up o 4500 anke ucks a yea
o dis ibu e na u al gas o loca ions no co e ed by he pipeline ne wo k [24].
Ano he undamen al elemen o he na ional na u al gas g id is he combined unde g ound
s o age capaci y o 333 × 10
6
m
3
p o ided by six sal ca e ns loca ed in Ca iço. These ca e ns belong
o he Mon e Real sal s uc u e o he Lusi anian basin, s a egically placed in he middle o he main
high-p essu e pipeline (c . Figu e 13) [36,42]. The easons o i s cons uc ion we e: (1) he s o age o
s a egic ese es and (2) o balance supply and demand, namely due o seasonal and daily
luc ua ions, hus secu ing na u al gas supply [42].
Ca iço’s unde g ound s o age acili ies allow a gas injec ion and wi hd awal o 110 × 10
3
Nm
3
/h
and 300 × 10
3
Nm
3
/h, espec i ely. Be o e injec ion in o he g id, he gas is il e ed o emo e solid
and liquid pa icles, comp essed, and dehyd a ed in a e ical abso be ( he maximum inal gas
mois u e con en is 40 ppm ) [42].
The LNG e minal and Ca iço sal ca e ns p o ide a o al s o age capaci y o 575 × 10
6
m
3
.
Conside ing ha consump ion in 2016 was 4.6 × 10
9
m
3
[36], he exis ing combined capaci y can s ock
he equi alen o he amoun consumed by he coun y in 46 days. In he de elopmen plan o REN,
he ope a o o he gas ne wo k, he cons uc ion o 25 new ca e ns in Ca iço is o ecas ed o inc ease
he s o age capaci y up o 1.25 × 10
9
m
3
[43], al hough hese expansion plans we e epo ed o be
cu en ly unde e iew [24].
Addi ional unde g ound s o age capaci y in he Po uguese e i o y was es ima ed by Nunes
[43]. Se e al c i e ia we e adop ed o choose he bes loca ions. The c i e ia included ejec ing zones
Figu e 13.
Po ugal na u al gas s o age and anspo a ion in as uc u e ( ep in ed om [
36
] wi h
pe mission om REN).
The na ional pipeline ne wo k has an ex ension o 1375 km wi h 202 pipeline s a ions [
24
]. Sines
e minal ecei es LNG om la ge essel ships wi h a capaci y om 45
×
10
3
up o 216
×
10
3
m
3
. These
ships unload in o h ee LNG s o age anks ha ing a combined capaci y o 390
×
10
3
m
3
, co esponding
o ca. 242
×
10
6
m
3
o na u al gas [
24
,
36
]. The e minal is equipped wi h i e apo ize s using sea
wa e as he mal luid o gasi y LNG, which is u he comp essed o 78 ba and injec ed in o he gas
g id [24]. The e minal acili ies also include a illing s a ion ha may load up o 4500 anke ucks a
yea o dis ibu e na u al gas o loca ions no co e ed by he pipeline ne wo k [24].
Ano he undamen al elemen o he na ional na u al gas g id is he combined unde g ound
s o age capaci y o 333
×
10
6
m
3
p o ided by six sal ca e ns loca ed in Ca iço. These ca e ns belong
o he Mon e Real sal s uc u e o he Lusi anian basin, s a egically placed in he middle o he main
high-p essu e pipeline (c . Figu e 13) [
36
,
42
]. The easons o i s cons uc ion we e: (1) he s o age o
s a egic ese es and (2) o balance supply and demand, namely due o seasonal and daily luc ua ions,
hus secu ing na u al gas supply [42].
Ca iço’s unde g ound s o age acili ies allow a gas injec ion and wi hd awal o 110
×
10
3
Nm
3
/h
and 300
×
10
3
Nm
3
/h, espec i ely. Be o e injec ion in o he g id, he gas is il e ed o emo e solid and
liquid pa icles, comp essed, and dehyd a ed in a e ical abso be ( he maximum inal gas mois u e
con en is 40 ppm ) [42].
The LNG e minal and Ca iço sal ca e ns p o ide a o al s o age capaci y o 575
×
10
6
m
3
.
Conside ing ha consump ion in 2016 was 4.6
×
10
9
m
3
[
36
], he exis ing combined capaci y can s ock
he equi alen o he amoun consumed by he coun y in 46 days. In he de elopmen plan o REN,
he ope a o o he gas ne wo k, he cons uc ion o 25 new ca e ns in Ca iço is o ecas ed o inc ease
he s o age capaci y up o 1.25
×
10
9
m
3
[
43
], al hough hese expansion plans we e epo ed o be
cu en ly unde e iew [24].

Ene gies 2018,11, 3259 17 o 20
Addi ional unde g ound s o age capaci y in he Po uguese e i o y was es ima ed by Nunes [
43
].
Se e al c i e ia we e adop ed o choose he bes loca ions. The c i e ia included ejec ing zones ha
we e in a close dis ance o ai po s, oads, and houses, inside p o ec ed a eas, a away om he
sea and gas g id, o no in a plain ield. A e wa ds, h ee egions we e elec ed: Naza é, Caldas
da Rainha, and Peniche. The s udy conside ed a simila ca e n olume and dis ance among he
ca e ns, like he Ca iço acili ies, and an unde g ound s o age po en ial o 1
×
10
9
m
3
was es ima ed.
I he minimum dis ance o oads was limi ed o highways and ailways, he s o age po en ial
eached 1.65
×
10
9
m
3
[
43
]. Hence, a po en ial s o age capaci y o 3.14
×
10
9
m
3
is en isaged,
sa egua ding 249 days o consump ion (based on 2016 da a). Howe e , he p elimina y assessmen o
he unde g ound s o age po en ial made by Nunes [
43
] should be complemen ed wi h he necessa y
en i onmen al impac and economic s udies. Recen ly, Ca nei o e al. [
20
] sc eened p io i y si es o
ene gy s o age in geological o ma ions using a geog aphic in o ma ion sys em (GIS) and conside ed
spa ial, en i onmen al, and social cons ain s, as well as he p oximi y o a eas wi h wind o sola
ene gy po en ial, accessibili y o powe ansmission lines, and na u al gas ne wo ks. The au ho s
iden i ied si es ha could ac as ese oi s o unde g ound gas s o age (o hyd ogen o me hane)
(UGS), comp essed ai ene gy s o age (CAES), unde g ound pumped hyd o ene gy s o age (UPHES),
and unde g ound he mal ene gy s o age (UTES); hey concluded ha , o he Po uguese geological
con ex , he echnologies wi h bes applica ion po en ial seem o be CAES and UGS linked o P G.
Despi e he en isaged unde g ound s o age po en ial ye o be explo ed, pe spec i es o P G
deploymen in Po ugal would be e en mo e p omising wi h he cons uc ion o he hi d planned
connec ion wi h Spain in he na u al gas ne wo k and o p ojec ed connec ions linking he Ibe ian
Peninsula o F ance.
3.4. Resea ch Needs
The cu en challenges ega ding he echnologies in ol ed in P G p ocesses we e ex ensi ely
add essed in ecen publica ions (e.g., [
2
,
9
,
11
,
44
]), and o ha eason we e ou o he scope o his
wo k, al hough hei s udy has been he main ocus o p e ious au ho s (e.g., [
45
,
46
]) and u u e
esea ch ac i i ies [
19
]. Ins ead, he p esen wo k aimed o p o ide a pic u e o he ecen e olu ion
o he Po uguese ene gy sec o , highligh ing he emendous endea o and commi men o he
coun y o la ge-scale enewable ene gy deploymen and o aise awa eness abou wha seems
o be p omising condi ions o P G deploymen . Ne e heless, u u e esea ch s udies o o ecas
su plus powe in di e en ene gy case scena ios and he iden i ica ion and cha ac e iza ion o he mos
sui able CO
2
poin sou ces, besides echno-economic-en i onmen al assessmen s, will be c ucial o ind
p o i able business models and in eg a ed alue chains o P G deploymen in Po ugal. Among hem,
p ocess chains should be looked a , leading o oppo uni ies o O
2
(by-p oduc o H
2
O elec olysis)
alo iza ion, ecycling o H
2
O om CO
2
me hana ion, and ene gy in eg a ion o ackle cu en ba ie s
o comme cializa ion o P G sys ems.
4. Conclusions
The p esen analysis o he Po uguese ene gy sec o highligh s he coun y’s in ense dependence
on ossil uels o a o d i s ene ge ic needs, al hough, despi e his, i was he ou h EU-18 membe
wi h he highes inco po a ion o enewables in powe p oduc ion in 2015 (i.e., 44.6%), a alue ha
eached 57% in 2016 [
36
]. So a , he coun y’s op ions o manage he ene gy su pluses gene a ed
by elec ici y om enewable sou ces elies on pumped hyd o s o age plan s o powe expo a ion
o Spain. Hence, decen alized powe - o-me hane applica ions can be o s a egic ele ance o he
coun y, since powe p oduc ion om na u al gas will inc ease ollowing he decommissioning o he
Sines and Pego coal powe plan s by 2021. S o ing su plus enewable elec ici y as me hane would
also allow he di e si ica ion o na u al gas p o ision, minimizing he dependence and isk o sho age
supply om o eign coun ies, as i is ad ised by he Po uguese Di ec o a e-Gene al o Ene gy and
Ene gies 2018,11, 3259 18 o 20
Geology [
47
]. Addi ionally, a signi ican inc ease o na u al gas consump ion in he 2005–2015 decade
(ca. 19%) by se e al and impo an indus y sec o s was also shown.
Po ugal has impo an geog aphic ad an ages in a o o P M demons a ion p ojec s, such as
a well-de eloped na u al gas ne wo k nea wind pa ks and CO
2
sou ces, as well as a p omising
unde g ound s o age po en ial ye o be explo ed. Fo such a pu pose, he engagemen o all
s akeholde s (namely, academics, go e nmen al bodies, echnology and ene gy p o ide s, majo
CO
2
pollu ing companies, and na u al gas consume s) will be c ucial o es ablishing na ional and/o
egional esea ch and de elopmen oadmaps, whe e he ba ie s (e.g., echnical, legal, and egula o y),
challenges, and oppo uni ies o as P G deploymen should be iden i ied o coo dina ed ac ions.
Au ho Con ibu ions:
Concep ualiza ion, C.V.M.; W i ing—o iginal d a p epa a ion, C.V.M.; w i ing— e iew
and edi ing, A.M. and L.M.M.; supe ision, A.M. and L.M.M; Funding acquisi ion, A.M. and L.M.M.
Funding:
C.V. Miguel is g a e ul o he Po uguese Founda ion o Science and Technology (FCT) o his
PhD schola ship (SFRH/BD/110580/2015), inanced by na ional unds o he Minis y o Science, Technology
and Highe Educa ion and he Eu opean Social Fund (ESF) h ough he Human Capi al Ope a ional
P og amme (POCH). The au ho s acknowledge inancial suppo om p ojec s: (i) POCI-01-0145-FEDER-006939
(Labo a o y o P ocess Enginee ing, En i onmen , Bio echnology and Ene gy—UID/EQU/00511/2013) unded
by he Eu opean Regional De elopmen Fund (ERDF), h ough COMPETE2020—P og ama Ope acional
Compe i i idade e In e nacionalização (POCI) and by na ional unds, h ough FCT—Fundação pa a a Ciência e a
Tecnologia; (ii) NORTE-01-0145-FEDER-000005—LEPABE-2-ECO-INNOVATION, suppo ed by No h Po ugal
Regional Ope a ional P og amme (NORTE 2020), unde he Po ugal 2020 Pa ne ship Ag eemen , h ough he
Eu opean Regional De elopmen Fund (ERDF); (iii) POCI-01-0145-FEDER-030277— unded by he ERDF unds
h ough COMPETE2020—P og ama Ope acional Compe i i idade e In e nacionalização (POCI) and by na ional
unds (PIDDAC) h ough FCT/MCTES.
Con lic s o In e es : The au ho s decla e no con lic o in e es .
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