Applied Energy 122 (2014) 83–93

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Applied Energy

journal homepage: www.elsevier .com/locate /apenergy

Continuous power supply from a baseload renewable power plant

http://dx.doi.org/10.1016/j.apenergy.2014.02.0150306-2619/� 2014 Elsevier Ltd. All rights reserved.

⇑ Corresponding author. Tel.: +1 765 494 2257.E-mail address: agrawalr@purdue.edu (R. Agrawal).

Easa I. Al-musleh, Dharik S. Mallapragada, Rakesh Agrawal ⇑School of Chemical Engineering, Purdue University, West Lafayette, IN 47907, United States

h i g h l i g h t s

�We propose chemical-refrigeration cycles for energy storage.� Closed cycle between liquid CO2 and liquid carbon fuel.� Exergy metrics proposed to identify favorable candidate fuels.� Methane and methanol balance storage efficiency and volumes.

a r t i c l e i n f o

Article history:Received 8 August 2013Received in revised form 30 November 2013Accepted 6 February 2014Available online 28 February 2014

Keywords:StorageCarbonaceousMethaneMethanolCarbon dioxide

a b s t r a c t

A grand challenge for using intermittent renewable energy such as solar for baseload applications islarge-scale energy storage. Here, we propose an efficient means of implementing carbon recirculationcycles that enable dense energy storage. In these cycles, during the period of renewable energy availabil-ity, a suitable carbon molecule is synthesized from the stored liquid carbon dioxide and then stored in aliquid state. Subsequently, when renewable energy is unavailable, the carbon molecule is oxidized todeliver electricity and carbon dioxide is recovered and liquefied for storage. We introduce exergy basedmetrics to systematically identify candidate carbon molecules for the cycle. Such a search provides us thetrade-off between the exergy stored per carbon atom, exergy used to synthesize the molecule and theexergy stored per unit volume. While no carbon molecule simultaneously has the most favorable valuesfor all three metrics, favorable candidates identified include methane, methanol, propane, ethane anddimethyl ether. For cases where the molecule to be stored is gaseous under ambient conditions, we sug-gest synergistic integration between liquefaction and boilup of this gas and that of recirculating carbondioxide. This unique feature allows for minimizing the energy penalty associated with the recovery, puri-fication and liquefaction of carbon dioxide and storage of carbon molecules. Using process simulations weshow that these cycles have a potential to provide GWh of electricity corresponding to an overall energystorage efficiency of 55–58% at much reduced storage volumes compared to other options.

� 2014 Elsevier Ltd. All rights reserved.

1. Introduction

Energy storage at multiple time and energy scales remains anon-going challenge for transitioning from fossil fuels to intermit-tently available renewable energy sources as the dominantprimary energy supply. The impact of energy storage technologiesin enabling the use of renewable energy sources like solar, windetc. for different end uses is illustrated from the magnitude of en-ergy to be stored. For example, in the USA, on average, solar energyis available for only one-fifth of a twenty-four hour day [1]. Thismeans for an average 100 MW power supply from solar energy,one needs to store enough energy to supply �2 GWh of electricityfor a twenty-four hour cycle. This motivates the need to identify

methods for storing GWh levels of energy in a reasonable volume,which can also be subsequently delivered at a high efficiency. Here,we propose a GWh-level electrical energy storage system that isdense, energy efficient and makes use of carbon fuels and theirexisting infrastructure.

Among the known energy storage methods, current batteriesare known for their high storage efficiency (75–94%) [2]. However,their currently low energy densities (<2 GJ/m3) [2,3] and shortcycle life (e.g. �2500 for sodium–sulfur batteries) [3], make themimpractical for storing GWh levels of electricity. For example, com-mercially available sodium–sulfur batteries are typically installedto store electrical energy amounts at the MW h level, with a cur-rent world wide deployment of �270 MW [3,4]. Use of hydrogeneither as a cryogenic liquid or compressed gas results in low energystorage efficiencies typically near �20–30% [5] (Supporting Infor-mation, SI). Use of thermophysical materials like molten salts to

84 E.I. Al-musleh et al. / Applied Energy 122 (2014) 83–93

store thermal energy, which is subsequently transformed to elec-trical power via a steam Rankine cycle, is associated with low en-ergy density (<3 GJ/m3) and a storage efficiency that is constrainedby the cycle thermodynamics (<30%) [6–8]. For example, the Anda-sol solar power station in Spain, one of the largest plants usingmolten salt, has a storage capacity of �1.1 GWh of deliverable elec-tricity (or 150 MW for 7.5 h) [6]. On the other hand, compressed airand pumped hydroelectric storage, despite their relatively high en-ergy efficiencies and large scale energy storage capability(>3 GWh) [3], are constrained by the need for suitable geologicaland geographic locations respectively [9].

Carbon fuels (such as alkanes, alcohols, and ethers) offer anattractive storage solution owing to their high volumetric energydensity (e.g. gasoline is�32 GJ/m3), efficient conversion to electric-ity (50–70%) [10], and the well-established technology and infra-structure available for their utilization [11,12]. Candidate fuelmolecules suggested for energy storage applications include gas-eous methane [12–14], methanol [5,15], dimethyl ether [14,15],and diesel [16]. However, the long-term use of such fuels for en-ergy storage is contingent on our ability to synthesize them fromrenewable carbon and hydrogen sources. While hydrogen can begenerated from water, the use of atmospheric carbon dioxide orbiomass as possible renewable carbon source in such an open loopfashion is quite challenging [11,16,17]. For example, carbon diox-ide extraction from the atmosphere, or even from industrialexhausts, is an energy intensive process, which could substantiallyimpact the storage efficiency [12,18]. On the other hand, growingbiomass on agricultural land for energy use is generally con-strained by the available arable land as well as other environmen-tal issues [8,19]. Only the Sustainably Available (SA) biomasscomprising of crop residues and perennial grasses grown on mar-ginal lands are readily available for energy production [20,21].However, the specific availability of the limited SA biomass for en-ergy storage is uncertain mainly due to its anticipated competitiveuse for synthesizing chemicals as well as liquid fuels for transpor-tation [22]. Previous works have suggested closed loop storagecycles where the carbon dioxide formed during power generationis recirculated within the process [13,23,24]. This is consistentwith the cyclical nature of energy storage and warrants furtheranalysis to identify efficient and dense storage cycles.

The storage of a carbon fuel could by itself be a challenging task.Fuels having relatively high energy content per carbon atom suchas methane and ethane, exist as gases at ambient conditions. Thus,

Hydrogen + oxygen Water

Carbo Dioxid

Water

Hydrogen!

Ele

ctri

city

Electricity

Carbon dioxide

Carbon dioxide liquefaction and storage

Wre

Electricity

Heat

Fig. 1. Schematic of the proposed

they need to be either stored as liquids (close to ambient pressure)or as high-pressure gases (at ambient temperature or lower).Although liquefaction of these carbon fuels significantly reducesthe storage volume, it is associated with a relatively large refriger-ation energy penalty that could adversely impact the storage effi-ciency. High-pressure gas storage, on the other hand, isassociated with a lower energy penalty, but requires much largervolumes for storing the same quantity of energy compared to liq-uefaction. For example, consider the large-scale storage of naturalgas. A Liquefied Natural Gas (LNG) tank (�111 K and �1.1 bar)with a typical capacity of 100,000 m3 [25], is estimated to havean energy storage capability of �585 GWh in terms of Lower Heat-ing Value (LHV) (assuming NG is 100% methane). On the otherhand, the state of the art compressed natural gas VOLANDS

�stor-

age tank (comprising of bundles of cylinders operating at sub-ambient temperature of �243 K and �125 bar) is available for astorage capacity of up to �64,000 m3. This volume correspondsto a storage capability of �132 GWh on a LHV basis [26]. In theUSA, �4 million m3 of LNG, which is equivalent to �23 TW h(LHV), is stored for NG peak demand supply [27]. Thus, as evidentfrom the current NG storage practices, the volumetric and relatedpractical constraints of storing large quantities of compressed gasmake it less favorable compared to liquefied gas storage.

2. Efficient and dense cycles for energy storage

We propose the concept shown in Fig. 1, which can achieve effi-cient and dense energy storage using a closed carbon recirculationsystem. The cycle transforms carbon atoms back and forth betweenliquid carbon dioxide and liquid carbon fuel to enable the storageand then delivery of GWh levels of electrical energy. When renew-able energy becomes available, the stored liquid carbon dioxide isvaporized and reacted with hydrogen (provided by water dissocia-tion), to synthesize a carbon fuel. The synthesized carbon fuel isliquefied and stored. This section of the process is referred in thisarticle as ‘‘storage mode’’.

To meet the power demand in absence of the renewable energysource, the stored carbon fuel is vaporized and oxidized. The oxida-tion by-product, carbon dioxide, goes through Capture, Purifica-tion, and Liquefaction (CPL) processes prior to storage. This partof the process is referred as the ‘‘delivery mode’’. Although notessential, the water produced during the carbon fuel oxidation in

n e Carbon fuel + Water

Carbon fuel +OxygenCarbon dioxideCarbon

dioxide

Car

bon

fuel

Oxygen

Carbon fuel liquefaction and storage

+ Hydrogen

+ Water

Carbon fuel ater cycle

storage and delivery concept.

E.I. Al-musleh et al. / Applied Energy 122 (2014) 83–93 85

the delivery mode is also stored for reuse during the storage modeto minimize the net cycle water consumption. The storage efficiencyof the cycle is defined as the ratio of electricity recovered duringthe delivery mode to the electricity used during the storage mode.For the efficiency calculation, electricity used during the storagemode includes directly consumed electricity and any used heat ac-counted by converting it to exergy at the temperature of use. Usingexergy to account for the process heat input in the storage effi-ciency allows us to differentiate between heat used at differenttemperatures. Alternatively, if electrical heating is utilized (i.e.resistive heating), the process heat input may be converted to elec-tricity by including the energy losses during electricity to heatconversion.

Aside from the little make up carbon input (see SI), the pro-posed cycle eliminates the need for a carbon source. In addition,unlike the previously suggested approaches for energy storage[13,24], our approach is general and not restricted to any particularcarbon fuel, power generation or hydrogen production technolo-gies. Moreover, our proposal stores the carbon fuel and carbondioxide as liquids, which lends itself to synergistic integrationopportunities and energy efficiency benefits, as elaborated in Sec-tions 3.1 and 3.2.

Table 1Comparison of candidate carbon fuels for energy storage.

Carbon fuel EXC

(MJ/kmol C)a,b,dBoiling pointat 1 atm (K)a

EXH?C

(%)aEXV

(GJ/m3)a,c

GasesMethane 806 112 86 21Ethane 723 185 88 25Propane 692 231 89 26Dimethyl ether 685 249 97 20Ethene 657 169 93 26Propene 643 225 92 27Formaldehyde 523 255 112 14Carbon monoxide 239 81 – 7

Non carbon optionsHydrogen 234 21 – 9Ammonia 335 240 95 11

LiquidsMethanol 693 338 99 13Ethanol 654 351 93 19Iso-octane 652 399 89 27Diethyl ether 651 308 93 22Cetane 640 560 89 25Acetone 572 329 92 18Acetic Acid 433 391 93 10Formic Acid 270 373 116 3

a Numbers rounded to nearest decimal for presentation.b For non carbon fuels, EXC unit is MJ/kmol fuel.c Gases: at 1 atm and normal boiling point. Liquids: at 1 atm and 298 K.d Reference conditions for exergy calculations are 1 atm and 298 K.

3. Fuel selection for the cycle

The selection of a carbon fuel for the cycle in Fig. 1 impacts theoverall storage efficiency and storage volume. Here, we suggestfuel selection metrics to systematically compare different carbonfuel candidates for the cycle of Fig. 1 or any other energy storagestrategies using carbon fuels. Based on these metrics, it is possibleto identify favorable carbon fuel candidates, which can be furtherevaluated by conducting rigorous simulations or experimentation.The suggested metrics are: (1) EXC: carbon fuel exergy content permole of carbon, (2) EXH?C: exergy stored in the carbon fuel relativeto hydrogen exergy during the carbon fuel synthesis step, (3) EXV:carbon fuel exergy content per unit fuel volume under storage.

The exergy of a fuel refers to the maximum reversible work thatcan be generated from it (see Section A1). In general, EXC gives anindication of the moles of carbon atoms needed to store one unit(MJ) of exergy in the carbon fuel. Thus, choosing fuels with highervalue of EXC reduces the carbon demand for storing a unit of exer-gy. For the cycle of Fig. 1, increasing EXC reduces the carbon circu-lation between the two operation modes (i.e. storage and deliverymodes). This translates into reduced energy penalties of carbondioxide circulation (e.g. pressure drops, temperature differences,etc.) and carbon dioxide CPL. EXH?C indicates how much hydrogenexergy is wasted as heat of reaction during the carbon fuel synthe-sis step. The impact of this lost exergy can be minimized by recov-ering a portion of the heat of reaction as electrical power throughsteam generation. Alternatively, the heat of reaction may be uti-lized for heating process streams. However, such energy recoverymechanisms will only partially compensate for the hydrogen exer-gy that is lost due to increased hydrogen use. Therefore, choosing afuel with higher values of EXH?C may be beneficial and it couldminimize the exergy (electricity) requirement for hydrogen pro-duction during the storage mode of the cycle. The third metric,EXV, gives an indication of how much volume the fuel will occupyto store a unit amount of exergy. Fuels with higher values of EXV

will require lower storage volumes to meet a given energydemand.

Table 1 lists different carbon fuels (alkanes, alkenes, alcohols,ethers, carboxylic acids, ketones, and aldehydes) of which someare gases and other are liquids at ambient conditions. Although,carbon monoxide and ammonia are highly toxic and hydrogenand ammonia are not carbon fuels, they are listed in the table for

comparison. The initial conclusion that can be drawn from Table 1is that there is no fuel that is superior in all the three proposedmetrics. Gases, such as methane and ethane, are associated withhigher values of EXC when compared to liquids such as iso-octane,ethanol and methanol. However, the high exergy content of thesegases comes at the expense of lower values of EXH?C, particularlywhen compared against methanol. In general, carbon fuels that aregases at ambient conditions do not require energy intensive puri-fication from the water produced during the carbon fuel synthesisstep of the storage mode. However, this advantage is traded offwith the corresponding energy requirements for purification (fromthe unconverted hydrogen and carbon dioxide) and liquefaction,also during the storage mode. Energy input for gaseous fuel purifi-cation is also expected to substantially increase for cases with low-er carbon conversion per pass during the carbon fuel synthesisstep. Nevertheless, gases such as methane and ethane, when lique-fied, have comparable EXV as liquids such as iso-octane, as shownin Table 1. In case of carbon fuels with values of EXH?C greater than100% (formic acid and formaldehyde), additional work input is nec-essary for the carbon fuel synthesis reaction to proceed to comple-tion. In other words, the direct synthesis of formic acid andformaldehyde from carbon dioxide and hydrogen results in a posi-tive Gibbs free energy change which makes their synthesisdemanding [28]. Additionally, notice the much lower values ofEXC and EXV for these two candidates compared to other fuels inTable 1. Therefore, we have not considered formic acid or formal-dehyde as a feasible fuel candidate for the cycle of Fig. 1.

Among the different classes of carbon fuels considered inTable 1, methane has the highest value of EXC followed by ethane,methanol, propane, dimethyl ether and so on. Consequently, to thefirst approximation, if we assume that the EXC to electricity con-version efficiencies are similar for all the fuels, methane use inthe cycle will beneficially minimize the amount of carbon cycledto deliver a given amount of electrical power. Another interestingcandidate in Table 1 is methanol, associated with the highest valueof EXH?C among all fuels and the highest value of EXC compared toother liquids. Based on these observations, we have chosen todesign and simulate (using Aspen Plus

�) detailed processes

SOEC 1,223 K & 10 bar

Water

Hydrogen

Water recycle

Electricity

Heat

Liquid methane

CO2 CH4 + 2 H2O + 4 H2 673 K 20 bar

Refrigeration

Evaporation

Purification & liquefaction

Heat of reaction

Liquid carbon dioxide

Gaseous carbon dioxide

Electricity

Electricity

Land area for heat & electricity generation

Methane-rich gasH

ydro

gen-

rich

Liquid methane storage 101 K & 2 bar

Liquid carbon dioxide storage

227 K & 10 bar

Storage Mode

Gaseous methaneWater

recycle

Liquid methane

Refrigeration

Carbon dioxide CPL

EvaporationLiquid

Carbon dioxide

Carbon

dioxide-r

ich

gas

Hydrogen-rich

Electricity

SOFC 1,223 K & 10 bar

Heat Steam methane reforming 1,198 K & 10 bar

Hydrogen-rich

recycle

Liquid carbon dioxide storage

227 K & 10 bar

Liquid methane storage 101 K & 2 bar

Delivery Mode

re

cycle

Fig. 2. Simplified schematic of the proposed Liquid Methane-Cycle (LM-C). Detailed flowsheet is presented in Fig. 3.

1 Water needs to be almost completely removed to avoid solidification caused byexposure to subambient temperatures. Molecular sieve adsorbers are best suited forsuch high water removal rate.

86 E.I. Al-musleh et al. / Applied Energy 122 (2014) 83–93

applying the concept of Fig. 1 for methane (referred as LiquidMethane-Cycle or LM-C) and methanol (referred as Methanol-Cy-cle or Mo-C). It is also worth mentioning that there exist catalystsfor selectively synthesizing these fuels from carbon dioxide andhydrogen [29,30]. In addition, catalysts for methane synthesiscan achieve near equilibrium conversion (per pass), which is inexcess of 90% at 623 K [30]. In these examples we have used solarenergy (available for only one-fifth of a twenty-four hour) as anexample of intermittently available renewable energy. However,the concepts are valid for any intermittently available energysource. For example, wind energy harnessed as electrical energy(via a wind turbine) can be directly used in the cycle. Additionally,any heat requirement for the cycle can be met by indirect use ofwind-derived electricity (i.e. via resistive heating).

In addition to methane and methanol, there exist other optionsin Table 1, which warrant further evaluation, such as ethane, pro-pane and dimethyl ether. All these fuels have lower energy penaltyof liquefaction (as shown by their higher normal boiling points)and higher values of EXH?C compared to methane. Incidentally,dimethyl ether has second highest value of EXH?C in Table 1, whileethane and propane also have higher values of EXV than methane.These fuels will be the subjects of future investigations.

3.1. Liquid Methane-Cycle (LM-C)

Fig. 2 is a simplified schematic of the LM-C. During the energystorage mode, solar energy harnessed as heat and electricity isused for hydrogen production via high temperature steam

electrolysis using a Solid Oxide Electrolysis Cell (SOEC) [11]. Gas-eous hydrogen is then reacted with gaseous carbon dioxide accord-ing to the Sabatier reaction [30] to almost complete conversion,generating gaseous methane, water, and waste heat from the reac-tion at �623–673 K. The generated heat is recovered to evaporateand superheat the steam feed to the SOEC operating at 1223 K. Anyadditional heating needed to raise the temperature of the steam to1223 K is supplied by concentrating solar energy. After separatingco-produced water by condensation and molecular sieve dehydra-tion adsorber1, the gaseous methane stream is purified and liquefiedto be stored at a cryogenic temperature of �101 K and pressure of�2 bar. Typically, achieving this low temperature of liquefactionfor storage requires capital and energy intensive refrigeration sys-tems, and may suggest the use of high pressure gas storage instead.However, we propose integrating the carbon dioxide vaporizationstep into the methane liquefaction to reduce the need for low tem-perature refrigeration. In this case, liquid carbon dioxide precoolsthe methane to a temperature of �228 K during its evaporation step.The remaining cooling is carried out using Mixed Refrigerant (MR)refrigeration cycle [31], with the cycle compressors being drivenby solar electricity. The MR refrigeration cycle is known to be amongthe most efficient refrigeration systems [31]. It is worth emphasizingthat, due to the decreased load on the MR compressors, this synergy

E.I. Al-musleh et al. / Applied Energy 122 (2014) 83–93 87

between liquid carbon dioxide and gaseous methane directly trans-lates into capital cost savings.

During the delivery mode when solar energy is unavailable, thestored methane energy is extracted by vaporizing liquid methaneand oxidizing it via an integrated steam methane reformer–SolidOxide Fuel Cell (SOFC) unit to produce electricity [10,32]. The car-bon dioxide in the exhaust mixture leaving the SOFC goes throughthe CPL process and is stored for reuse during the storage mode. InCPL process, prior to subambient cooling, co-produced water is re-moved from SOFC exhaust via condensation and molecular sieveadsorber for recycle to the steam methane reforming section. Ordi-narily, carbon dioxide CPL is an energy intensive process, whichnegatively impacts the process energy efficiency. Similar to thestorage mode, we propose integrating the methane vaporizationstep with the carbon dioxide CPL step thereby eliminating the needfor a refrigeration cycle or other means of carbon capture. In thiscase, the energy penalty of liquefying methane during the storagemode (as measured by the electricity consumed by the MR com-pression) is partially recovered as cold refrigeration for carbondioxide CPL. The resulting carbon dioxide CPL process developedhas the potential to be used for other carbon capture processes.The proposed LM-C employing the synergistic liquefactionschemes described above is compared against a cycle using highpressure (205 bar) gaseous methane storage, referred as theGaseous Methane-Cycle (GM-C).

In addition to the synergistic vaporization and liquefactionsteps, the essential features of the proposed LM-C are the follow-ing. (1) The integration of the reforming process within the SOFC,

(a)

Fig. 3. Detailed flowsheet for the LM-C, (

a well-demonstrated technology [10], helps to recover an in-creased fraction of stored energy as electricity. Specifically, thereis no need to combust a portion of the methane feed to providethe reforming heat because the methane reforming process is car-ried out by soaking up the waste heat released during the opera-tion of the SOFC [10,32]. (2) The SOFC anode exhaust is rich incarbon dioxide and water (1.6 mol% hydrogen, 0.5 mol% carbonmonoxide, 17.9 mol% carbon dioxide and 80.0 mol% water), be-cause the fuel is electrochemically oxidized using oxygen ionstransported to the anode from the cathode side air feed. Hence, un-like conventional power plants, the exhaust is not diluted withnitrogen (or other diluents), thereby reducing the energy neededduring subsequent carbon dioxide CPL. (3) Traditionally, SOFCpower plants capturing carbon dioxide also consume unconvertedcarbon monoxide and hydrogen (generated during methanereforming) via enriched oxygen combustion downstream of theSOFC [33,34]. This generally requires expensive catalytic combus-tion [33] as well as capital and energy intensive air separation unit.In contrast, our system avoids combustion by utilizing liquid meth-ane refrigeration to separate carbon dioxide and recycle high pur-ity unconverted ‘‘fuel’’ (�66 mol% hydrogen, 23 mol% carbonmonoxide, and 11 mol% carbon dioxide) back to the SOFC unit,which further increases the SOFC power output. (4) The methanefuel fed to the SOFC, being free of sulfur and other corrosive mate-rials allows for increased heat recovery of the SOFC exhaust with-out severe metallurgy impact (arising from condensing corrosivecomponents). This enhanced heat recovery contributes to in-creased power output during the delivery mode. In addition, the

a) storage mode, (b) delivery mode.

88 E.I. Al-musleh et al. / Applied Energy 122 (2014) 83–93

capital and energy cost associated with fuel pre-treatment facili-ties are avoided. (5) During the storage mode, the operation ofthe SOFC unit may be reversed to operate it as SOEC for hydrogenproduction [11,13]. This not only has a potential to save additionalcapital needed for hydrogen production, but more importantly, po-tential daily thermal cycling of the fuel cell stack is avoided. Thiswill contribute to the smooth operation of the plant.

The detailed LM-C process, designed and simulated using AspenPlus

�, is shown in Fig. 3 with the basis of simulation presented in

Table A1. The main characteristics of the storage mode operationof the LM-C (i.e. Fig. 3a) are the following. (1) A Heat RecoverySteam Generator (HRSG) is utilized for power generation, in whichthe hydrogen steam mixture leaving the SOEC is used as the heatsource. The HRSG produces superheated streams of high pressure(120 bar, 733 K), medium pressure (30 bar, 593 K), and low pres-sure (5.7 bar, 503 K) steam, which are subsequently used for powergeneration via steam turbines shown in Fig. 3a. The system alsoequipped with a reheat mechanism in which the discharge of thehigh-pressure steam turbine, T-HP, (at �30 bar) is reheated to733 K prior to its further expansion in the medium pressure tur-bine, T-MP. (2) For improved carbon dioxide conversion to meth-ane, we model the use of a low temperature (623 K) and a hightemperature (673 K) reactor in series (with intermediate watercondensation). (3) Prior to recycling to the SOEC, water generatedduring the methane generation step is purified from the dissolvedgases, such as unconverted carbon dioxide, in a stripping columnusing steam. (4) The molecular sieve dehydration adsorber isregenerated during the delivery mode of the cycle using dry meth-ane (i.e. free of water) at 583 K (heated using the SOFC exhauststream, available at 1219 K).

Referring to the delivery mode operation of the LM-C (Fig. 3b),the dissolved gases in the water produced from the SOFC are recov-ered using a stripping column, similar to the stripping column used

(b)

Fig. 3 (cont

during the storage mode. Additionally, the molecular sieve dehy-dration adsorbers used in the carbon dioxide CPL are regeneratedduring the storage mode of the cycle in a similar fashion to theregeneration of the storage mode adsorber. In the carbon dioxideCPL, carbon dioxide recovery of �98.5% (per pass) at a purity of�99.8 mol% is achieved via cooling/condensation at multiple pres-sures of 42 bar (discharge pressure of molecular sieve adsorbers),21 bar (C-1 discharge), and 80 bar (C-2 discharge). The high pres-sure (i.e. 80 bar) liquid carbon dioxide is cascaded down to lowerpressures of 21 and 10 bar by first feeding the liquid from vapor/li-quid separator V/L-3 to V/L-2 and finally to V/L-1. The uncon-densed gases leaving V/L-3 is heated for expansion in expandersExp-1 and Exp-2.

3.2. Methanol-Cycle (Mo-C)

The Mo-C shares all the above characteristics and features of theLM-C, with the exception of the following. (1) A refrigeration cycleis required during the delivery mode for carbon dioxide CPL. Here,we have chosen to use an MR refrigeration cycle [31]. (2) Methanolis generated via single step carbon dioxide hydrogenation at 543 Kwith recycling of the unconverted reactants to ensure high conver-sion [29]. Similar to the LM-C, the heat of reaction is recovered toheat the water needed for the SOEC and any additional heating isprovided by solar energy. Fig. 4 shows the detailed process forthe storage mode of the Mo-C. The delivery mode operation ofthe Mo-C is similar to the delivery mode of the LM-C and ispresented in the SI.

Referring to Fig. 4, the stream exiting the methanol synthesisreactor is cooled (via heat exchange) to 316 K for methanol–watercondensation in V/L-1. The uncondensed gas stream (�73.5 mol%hydrogen, 3.7 mol% carbon monoxide, 22.2 mol% carbon dioxide0.5 mol% methanol, and 0.1 mol% water) produced from V/L-1 is

inued)

Fig. 4. Detailed flowsheet for the storage mode of the Mo-C. Delivery mode flowsheet is similar to Fig. 3b and is available in SI.

E.I. Al-musleh et al. / Applied Energy 122 (2014) 83–93 89

recycled to the reactor. The liquid stream from V/L-1 (essentially amethanol–water mixture) is further processed in a stripping col-umn followed by a distillation column. The stripping column puri-fies the methanol–water mixture from any dissolved gases, such asunconverted carbon dioxide. The purified methanol–water mixtureleaves the bottom of the column to be fed to the distillation col-umn for water–methanol separation.

While methanol does not require expensive refrigeration en-ergy for its liquefaction, it leaves the synthesis reactor as almost50 mol% methanol/water mixture according to the followingequation.

CO2 þ 3H2 ! CH3OHþH2O ð1Þ

As discussed below, the energy penalty from an energy inten-sive methanol–water separation process (e.g. a distillation column)has a substantial impact on the storage efficiency. Additionally,during the delivery mode, steam is needed for methanol reformingfor the SOFC. Therefore, instead of Mo-C, a cycle with �99.9% puremethanol storage, we propose a unique cycle for methanol, re-ferred to as Methanol Water-Cycle (MoW-C), where the metha-nol/water mixture is stored in a single tank. This eliminates theenergy penalty associated with methanol purification via a distilla-tion column (see SI for detailed flowsheet). However, if the cyclewere to use combined Brayton/Rankine cycle (or any power gener-ation method that directly oxidizes the fuel) during the deliverymode, then methanol would need to be dehydrated for combustionpurposes and its energy benefit will be unrealized.

4. Performance of the cycles

The performance of the LM-C, GM-C, Mo-C and MoW-C are de-rived from rigorous process simulation using ASPEN PLUS

�soft-

ware [45]. The simulation results are summarized in Table 2,with the cycles sized for an uninterrupted power output of

�140 MW. The simulation results predict base case storageefficiency values between �53% and 55% for the proposed cycles,with the LM-C having the highest efficiency of �55%.

For the same power output, the LM-C total electrical energy in-put during the storage mode is �2.3% higher than that required forthe MoW-C. This can be explained by the greater fraction of feedhydrogen exergy lost as heat of reaction (or lower value of EXH?C)during the generation of methane along with the additional com-pression energy required for methane liquefaction compared toMoW-C. For both LM-C and MoW-C, the heat of reaction from car-bon fuel synthesis is used to offset a portion of the solar heatrequirement (at 1223 K) for generating steam feed to the SOEC.However, the higher heat of reaction for methane results in theLM-C having �64% less external solar heat requirements than theMoW-C. Overall, using the LM-C rather than the MoW-C resultsin �60 MWh of exergy savings, which corresponds to 2.2% of theelectrical energy output during the delivery mode.

A main reason for the additional exergy input for MoW-C is the121 MW h of electricity consumption (due to lack of refrigerationsource) for carbon dioxide CPL during the delivery mode. Unlikethe LM-C where liquid methane is used as a refrigeration sourceduring its evaporation, methanol, available at 316 K, does not sup-ply any refrigeration energy when vaporized. Consequently, for thesame electrical power output, the MoW-C needs to store a greateramount of exergy as carbon fuel (methanol/water mixture) com-pared to the LM-C. This fact is further compounded due to the low-er value of EXC for methanol compared to methane, which resultsin methanol oxidation generating greater moles of carbon dioxidethan methane for delivering the same amount of power.

For the LM-C, if methane liquefaction and carbon dioxide CPLsteps are carried in a way such that refrigeration is not recoveredduring evaporation of either of the liquids, then �6% (or305 MW h) of the total electrical energy input to the cycle willbe consumed for methane liquefaction and carbon dioxide CPL.In this case, around 203 and 104 MW h of electricity are consumed

Table 2Simulation results for a uninterrupted �140 MW power supply.

Cycle LM-C GM-C Mo-C MoW-C

Storage mode Heat in (GWh) 0.14 at 1223 K 0.14 at 1223 K 0.38/0.53b at 1223/386 K 0.38 at 1223 KExergy of heat (GWh)a 0.1 0.1 0.38 0.27Electricity in (GWh) 4.85 4.89 4.71 4.74

Delivery mode Electricity out (GWh) 2.72 2.72 2.72 2.72

Overall cycle parameters Storage efficiencyc 54.9 54.4 53.3 54.3Carbon fuel volume (m3) 632 3086 1156 1679Liquid carbon dioxide volume (m3) 717 728 907 907Water volume (m3) 1340 1367 824 1232

a Exergy of heat = Exergy difference around the heat exchanger = DH�ToDS; where T0 = 298 K.b 0.53 GWh is used to purify the methanol from the coproduced water using a distillation column with a reboiler temperature of 386 K.c Storage efficiency is defined as: Electricity out during delivery mode/(Electricity in during storage mode + exergy of heat in during storage mode).

90 E.I. Al-musleh et al. / Applied Energy 122 (2014) 83–93

for methane liquefaction and carbon dioxide CPL, respectively. Theproposed refrigeration integration between the condensation andevaporation of the two components reduces this number to �4%,making the total electrical energy consumption for methane lique-faction and carbon dioxide CPL to be around 201 MW h. The result-ing energy savings compensates for the lower value of EXH?C

methane over methanol making LM-C, system wise, more efficientthan MoW-C. In addition, the LM-C using the synergistic liquefac-tion schemes has a storage efficiency that is comparable to that ofthe GM-C, where methane is cooled to 226 K (using vaporizing li-quid carbon dioxide) and then compressed to 205 bar for storage(Table 2). The GM-C consumes an additional �40 MW h of electric-ity during the storage mode compared to LM-C, due to the addi-tional methane circulation requirements to compensate for theenergy penalty of carbon dioxide CPL. In other words, part of thegenerated methane during the storage mode is oxidized in thedelivery mode to provide electricity needed (�103 MW h) for car-bon dioxide CPL process. Besides being slightly more efficient, amajor benefit for the LM-C is that for the same power output, theliquid methane volume is about one-fifth of the compressed meth-ane volume in the GM-C.

The proposed synergistic liquefaction and evaporation schemesare generally extendable to any gaseous carbon fuel used in the cy-cle of Fig. 1. Among carbon fuels candidates evaluated in Table 1,ethane, propane and dimethyl ether have their normal boilingpoint temperatures much closer to carbon dioxide liquefactiontemperature of �218 K at 5 bar� than that of methane. This is inaddition to the other favorable aspects of these fuels discussed ear-lier. The increased extent of overlapping between the boiling pointtemperatures of carbon fuels with carbon dioxide suggests thepotential for reducing external refrigeration for the carbon fuelliquefaction and/or carbon dioxide CPL. However, a direct compari-son of boiling points of different carbon fuels to predict externalrefrigeration needs may only be valid if the corresponding carbonfuel synthesis step has similar conversion and selectivity. To ourknowledge, this is currently not the case for ethane, propane, di-methyl ether and methane. For example, synthesizing dimethylether in a single step hydrogenation of carbon monoxide gives car-bon conversion per pass up to �60% with �95% selectivity [35].The energy penalty of separating the unconverted reactants andby-products (i.e. hydrogen, carbon dioxide, carbon monoxide, andmethanol) would consume part or all of the available refrigerationof the vaporizing carbon dioxide. Consequently, the remainingrefrigeration may not be sufficient to provide the refrigerationneeded for liquefying dimethyl ether. Detailed simulations of theseselect carbon fuels are needed to quantitatively compare themagainst the LM-C and MoW-C.

� Pressure and temperature lower than 218 K and 5 bar could result in dry iceformation which has the potential to disrupt the system operation.

Although, there is only slight difference in the storage efficiencybetween LM-C and MoW-C, there are significant differences in thestorage volume. Liquid methane requires �62% less volume thanmethanol/water mixture for the same power output from eithercycle. The LM-C also stores�21% less volume of liquid carbon diox-ide compared to the MoW-C. The lower total storage volume (car-bon dioxide and carbon fuel) of the LM-C is primarily aconsequence of the higher EXC and EXV (Table 1) of methane vs.50 mol% methanol/water mixture (EXV � 8.5 GJ/m3). This uniqueproperty of methane allows for delivering the required electricityusing fewer moles of carbon fuel (or carbon dioxide). Additionally,the proposed storage of methanol/water mixture in a single stor-age tank adds larger volume but reduces the energy consumptionfor the separation of water and increases the storage efficiency,as seen in Table 2. If methanol is to be purified, as in the case ofthe Mo-C, an additional �121 MW h of exergy input is requiredfor purification, which corresponds to 4.4% of the electrical energyoutput of the plant during the delivery mode. As seen in Fig. 5, theamount of water left in the stored methanol may be optimized toprovide the required trade-off between storage volume and the en-ergy used in the separation.

In case water produced during the delivery mode is also stored(�100% pure), then the corresponding volumes for all the four stor-age cycles are shown in Table 2. The water volume stored for GM-Cis �2% higher than LM-C, which is consistent with the trend in vol-ume of carbon dioxide stored. In case of methanol, the volume ofwater stored for MoW-C is �51% higher than Mo-C, due to thepresence of water in the carbon fuel stored during the storagemode. A brief comparison between the proposed cycles and thehydrogen storage options suggested in the literature (see Fig. 4

Fig. 5. Effect of: (1) methanol storage purity on the MoW-C storage efficiency andstorage volume and (2) Steam to Carbon Ratio (STC) on the storage efficiency of theLM-C.

Fig. 6. Storage efficiency and volumetric energy density of the LM-C (high-hi andlow-lo) vs. hydrogen (gas-GH2, liquid-LH2), batteries (Na–S, Li-ion), compressed airenergy storage (CAES), and pumped hydroelectric storage (hydro). Volumetricenergy density of LM-C does not consider volume of water stored. Details ofbatteries, compressed air, and pumped hydroelectric calculations are available inthe SI.

E.I. Al-musleh et al. / Applied Energy 122 (2014) 83–93 91

and SI), demonstrate the superiority of the proposed cycles (both interms of storage volumes and storage efficiency). As seen fromFig. 6, the developed cycles also compare favorably in terms ofstorage volume with batteries, represented by sodium–sulfur andlithium-ion batteries, compressed air storage, and pumped hydro-electric storage (see SI) [8,36–38].

Referring to Fig. 5, developments in steam methane reformingand SOFC anode catalysis to allow operation at lower steam to car-bon ratio (STC) could lead to enhancing the proposed LM-C storageefficiency. A minimum of one mole of steam per mole of methaneis required as per the reforming reaction stoichiometry. At thisstoichiometry ratio, the storage efficiency is identified to be almost58.5%. However, currently excess steam (which implies excesswater latent heat) is needed to avoid coke formation in the reform-ing and anode sections of the SOFC (arising from the presence ofmethane and carbon monoxide) [39,40]. On the other hand, thesteam requirement for methanol steam reforming is not as highas methane reforming (31). Thus, the improvement in storage effi-ciency that one may obtain by reducing the STC for the MoW-C/Mo-C will not be as significant as that for the LM-C case.

The storage efficiencies of the proposed cycles can also be esti-mated for the case of using wind-based electricity. In this case,heat needed by the cycles is supplied via resistive heating withelectricity-to-heat conversion efficiency of 98% [41]. When com-pared to the direct heat supply as in the solar energy case, the indi-rect supply of heat is expected to reduce the storage efficiency ofthe cycle. For the LM-C and GM-C, the storage efficiency whenusing wind is estimated to be 54.5% and 54%, respectively. Thesevalues are marginally lower than the values when using solar en-ergy, shown in Table 2. For the MoW-C, the shift from solar to windenergy results in a more pronounced storage efficiency declinefrom 54.3% to 53%. This is attributed to the higher heat demandof the MoW-C compared to the LM-C and GM-C. For the Mo-C,the increase heat demand arising from the use of a distillation col-umn (for methanol–water separation) results in lower storage effi-ciency of 48.2%.

Fig. A1. Representation of the reversible process used to calculate the fuel exergy.In case the fuel is hydrogen or ammonia, then the carbon dioxide stream iseliminated. If the fuel is carbon monoxide, then the water stream is eliminated.

5. Conclusions

The proposed fuel selection metrics and closed loop carbontransformation between liquid carbon dioxide and liquid carbonfuel provide an array of solutions for GWh level electrical energystorage in a renewable energy economy. Depending on the choiceof carbon fuel, we propose the following unique energy integrationschemes. If the carbon fuel is a gas, then the carbon fuel and carbondioxide vaporization and liquefaction steps are integrated to

reduce the energy penalty arising from using external refrigera-tion. If the carbon fuel is a liquid and its purification from wateris energy intensive, then storing a carbon fuel/water mixture inconjunction with fuel reforming followed by oxidation is the pre-ferred mode of operation. The fuel/water mixture compositioncan be adjusted to balance the storage volume and efficiency.The achievable balance between storage volume and storage effi-ciency for the cycle also depends on the choice of carbon fuel, asdiscussed via the proposed fuel selection metrics (EXC, EXH?C,

EXV). While there is no single fuel that is simultaneously mostfavorable among all the three metrics, we have identified somefavorable candidates, such as, methanol, methane, ethane, propaneand dimethyl ether. As demonstrated through detailed simula-tions, the use of methane and methanol in the proposed cycle offersome interesting features. For methane, these include the highestexergy content per mole carbon, dense energy storage as a liquid,and its refrigeration synergy with liquid carbon dioxide. In caseof methanol, it does not require refrigeration for liquefaction, min-imizes the fraction of hydrogen exergy lost as heat of reaction dur-ing its synthesis, and provides an opportunity for eliminating thepurification energy requirement by storing 50–50 mol% metha-nol/water mixture. All these features provide us with cycles withlow storage volume while maintaining reasonably high storageefficiency of 55–59%.

Acknowledgments

Research supported by Qatar University, by the Centre for DirectCatalytic Conversion of Biomass to Biofuels, an Energy FrontierResearch Centre funded by the U.S. Department of Energy (DOE),Office of Science, Basic Energy Sciences (BES), under Award #DE-SC0000997, and by the National Science Foundation SolarEconomy IGERT (0903670-DGE).

Appendix A.

A1. Fuel exergy calculations

The fuel exergy was calculated by applying the first and secondlaw of thermodynamics at reversible conditions for the processshown in Fig. A1. Ambient temperature (To) and pressure (Po) wereassumed to be 298 K and 1 atm, respectively.

Applying the first and second law of thermodynamics gives:

EXi ¼ �DG ¼ Gi þ GAir � GCarbon dioxide � GNitrogen � GWater ðA1Þ

where EXi is the Fuel i exergy per mole of fuel, Gi the Gibbs free en-ergy per mole of fuel i, GAir, GCarbon dioxide, GNitrogen, and GWater is theair, carbon dioxide, nitrogen, and water Gibbs free energy per moleof fuel i.

For a carbon fuel, exergy per mole of carbon (EXC) is calculatedby dividing EXi by the number of carbon atoms per mole of fuel.The fuel exergy per unit volume at storage conditions (i.e. temper-

Table A1Modeling and simulation basis.

Description Reference Value/description

Ambient conditions Basis 1.013 bar (1 atm)298 K 50% relativehumidity

Cooling water supply temperature [47] 305 KExpansion minimum pressure Assumption 1.1 barHeat exchangers maximum pressure [48] 100 barHeat exchangers maximum

temperature[48,49] 1073 K

Heating above maximum heatexchanger temperature is carriedout using solar concentrators withmaximum temperature

[50] 1273 K

Pressure drops in heat exchangers [47]

For boiling/condensation 0.10 barFor gases 0.21 barFor low viscosity liquid 0.34 bar

Heat exchangers minimumtemperature approach

[47]

T < 298 K 1 KT = 305–421 K 11.1 KT > 421 K 27.8 K

Heat exchangers heat leakage Assumption 0Storage tanks heat leakage Assumption 0

SOFC/SOEC(1)

Modeling and simulation approach [42,43]Maximum temperature approach [51] 270 KMaximum temperature [52] 1273 KDC to AC efficiency [53] 96%Maximum pressure [51] 10 barElectrical current-SOFC [42,43] Sufficient to provide

heat for the steamreformer

Electrical current-SOEC [54] At thermal neutralvoltage

Fuel reforming [10,32] Steam reformingintegrated inside thefuel cell

Oxygen concentration at SOEC anodedischarge

[55] 50% mole

Hydrogen concentration at SOECcathode inlet

[55,56] 10% mole

Phase separators pressure drop Assumption 0.21 barNumber of compression stages [47]Pout/Pin Stages<4 14–16 216–64 3

Compressor discharge maximumtemperature

Assumption 473 K

Gas turbines maximum inlettemperature

[57] 1573 K

Compressor isentropic efficiency [58] 80%Expanders isentropic efficiency [59] 85%Expander motor efficiency Assumption 97%Adsorber maximum pressure [60] 120 barAdsorber modeling approach [44]Methane dehydration molecular

sieve[58,61] 4A

Carbon dioxide dehydrationmolecular sieve

[58,61] 3A

Sabatier reaction modeling approach [30] EquilibriumMethanol reaction modeling

approach[29] Equilibrium

Cryogenic refrigeration for LM-Cstorage mode

[31] Mixed Refrigerant(MR)

Cryogenic refrigeration for Mo-C andMoW-C delivery mode

[31] Mixed Refrigerant(MR)

Storage mode sun availability Assumption 4.8 hThermodynamic modelingMixtures [46] Predictive Soave–

Redlich–KwongPure water [45] NBS/NRC steam tables

Table A1 (continued)

Description Reference Value/description

Heat Recovery Steam Generator(HRSG)

Water contamination Assumption Pure waterMaximum steam pressure [62] 128 barMaximum steam turbine

temperature[62] 791 K

Minimum steam turbine outletpressure

[63] 0.07 bar

Minimum steam turbine inletpressure

[64] 5 bar

Steam turbine discharge moisturecontent

[64] 10 to 15%

Number of steam pressure stages Assumption 3Number of reheats Assumption 1

Purge percentage AssumptionStorage mode 1%Delivery mode 1%

Minimum temperature for carbondioxide-rich streams

Avoidingfreezing

218 K

Minimum pressure for carbondioxide-rich streams

Avoidingfreezing

9 bar

Carbon dioxide liquid purity Basis >99 mol%

92 E.I. Al-musleh et al. / Applied Energy 122 (2014) 83–93

ature and pressure) is calculated by multiplying EXi by the fuelmolar density at storage conditions. The molar densities are ob-tained using the PSRK thermodynamic package available in AspenPlus

�. For fuels that are gases at 298 K and 1 atm, the molar densi-

ties are obtained at the gas normal boiling point of 1 atm. On theother hand, for liquids molar densities are all obtained at 298 Kand 1 atm. The exergy stored in the carbon fuel relative to thehydrogen exergy during the carbon fuel synthesis step, EXH?C, iscalculated as follows:

EXH!C ¼EXi

EXH2 � mðA2Þ

where m is the moles of hydrogen required for the synthesis of onemole of the carbon fuel.

Appendix A2. Modeling and simulation basis

Table A1 summarizes the modeling basis of the simulationresults presented here. For the SOFC, SOEC, and molecular sieveadsorbers, the model presented in [42–44] are adopted and imple-mented in Aspen Plus

�using Aspen calculator tool [45]. All other

unit operations involved in the proposed cycles are modeled andsimulated using the Aspen model library [45]. The PredictiveSoave–Redlich–Kwong equation of state [46] and NBS/NRC steamtables [45] are utilized for the thermodynamic propertycalculations.

For the SOFC/SOEC, the electrical power output/input arecalculated according to Eqs. (A3) and (A4), respectively. Here,_WSOFC

Reversible/ _WSOECReversible refer to the Gibbs free energy change of the

process at the specified operating conditions of the device. Theterm, _WSOFC

Losses/ _WSOECLosses, refers to the loss in electrical power (dissi-

pated as heat) owing to the irreversibilities during device opera-tion. These losses are modeled based on the experimental dataobtained for a planar type device [42,43]. For the SOFC, the fuelreforming and oxidation reactions are modeled using an equilib-rium approach [42,43]. Energy balance is applied around the SOFCand SOEC for residual heat calculations.

_WSOFCActual ¼ _WSOFC

Reversible � _WSOFCLosses ðA3Þ

_WSOECActual ¼ _WSOEC

Reversible þ _WSOECLosses ðA4Þ

E.I. Al-musleh et al. / Applied Energy 122 (2014) 83–93 93

where _W is power.In the SOEC, sweep air is fed to the anode side for oxygen dilu-

tion. High oxygen concentration tends to increase the degradationrate of anode material [55]. Therefore, air flow is adjusted to fix theoxygen discharge concentration at 50 mol% [55]. Similarly, to avoidcathode material degradation, part of the generated hydrogen isrecycled to the SOEC feed to maintain 10 mol% hydrogen concen-tration at the inlet of the cathode.

Appendix B. Supplementary material

Supplementary data associated with this article can be found, inthe online version, at http://dx.doi.org/10.1016/j.apenergy.2014.02.015.

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