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Renewable and Sustainable Energy Reviews

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

A review of liquefied natural gas refueling station designs

Amir Sharafian, Hoda Talebian, Paul Blomerus, Omar Herrera, Walter Mérida⁎

Clean Energy Research Centre, The University of British Columbia, 2360 East Mall, Vancouver, BC, Canada V6T 1Z3

A R T I C L E I N F O

Keywords:Liquefied natural gasMethane ventingBoil-off gas managementRefueling stationOnboard LNG tankFuel supply system

A B S T R A C T

The majority of operational liquefied natural gas (LNG) refueling stations in the world have no boil-off gas(BOG) management and rely on regular LNG delivery to condense the BOG. To reduce the pressure of LNGtanks onboard vehicles prior to filling, the BOG is vented to the atmosphere, is collapsed in the tank, or isreturned to the refueling station. In this study, different onboard LNG tank architectures are discussed, and thedesign strategies for LNG conditioning and BOG management technologies employed in LNG refueling stationsare analyzed. The critical analysis of different designs of LNG refueling stations indicates that 44% of designshave no BOG management, 28% of designs rely on liquid nitrogen condenser or a liquefier to condense the BOG,and 28% of designs compress the BOG to produce compressed natural gas. Our research shows that in Chinaand the U.S., where stations with BOG management are rare, the number of LNG refueling stations hasincreased by 32 and 3 times, respectively, between 2010 and 2015. This study highlights the fact that as heavyfuel oil and diesel are replaced by LNG, it is critical to pay proper attention to the design of the LNG supplychain and LNG refueling stations to minimize or eliminate BOG venting and reduce greenhouse gas emissions.

1. Introduction

Climate change is one of the main concerns of the 21st century [1],and eliminating the greenhouse gas (GHG) emissions from industrialand transportation processes is one of the most pressing challenges[2,3]. For many years, natural gas (NG) has been proposed as atransitional, low-carbon fuel [4]. More recently, renewable natural gas[5–10] has emerged as a potential link between existing distributioninfrastructure and renewable energy sources. The benefits associatedwith NG use have been reported by several authors focused oneconomic and market growth [4,9–19]. However, and despite thissignificant body of work, the overall benefits associated with NG useremain uncertain.

The announcements at the 21st Conference of Parties (COP) inParis indicate that reaching the 2 °C scenario targets would requireimmediate and significant changes over the next three decades (asopposed to changes occurring over centuries) [20]. The relative impactof methane (the main component in NG) compared to CO2 may have tobe revised to accommodate these more aggressive targets. Moreimportantly, the reduction in CO2 emissions from NG use must becompared to the impact of the corresponding methane emissions. Weillustrate the importance of these considerations by reviewing the state-of-the-art in liquefied natural gas (LNG) refueling stations. Withoutreliable data on the actual deployment technologies, most of the modelsand analyses comparing widespread NG use to the existing energy

options will remain incomplete.NG is composed of methane (83–99.7%), ethane, propane, butane,

and nitrogen [21], and has the lowest carbon-content compared topetroleum fuels, such as diesel and gasoline [22]. During combustion,NG emits less CO2 and lower levels of criteria pollutants than diesel.Fig. 1 shows that the replacement of diesel with NG can potentiallyreduce CO2 and NOx emissions up to 20% [23,24] and 90% [25,26],respectively, and SOx and particular matter emissions by almost 100%[24]. By regulation in Europe and North America [27,28], ultra-low-sulphur diesel (ULSD) was phased in for on-road vehicles between2006 and 2010. This regulation came into effect in North America foroff-road, rail, and inland waterway marine applications between 2007and 2014 [28].

NG is delivered in two forms to consumers who are not connectedto gas pipelines: compressed natural gas (CNG) and LNG. LNG is about600 times denser than gaseous NG at atmospheric pressure, and as aresult, LNG is the most efficient way of transporting NG across longdistances when pipelines are not available. The volumetric energydensity of LNG at −162 °C and 90 kPa is 22.2 MJ/L which is about 60%that of diesel and 2.45 times higher than that of CNG at 25 MPa(3,600 psig) [6]. This makes LNG an attractive fuel for heavy-dutytrucks [29–31], trains [22,32,33], and ships [25,34], where fuels withhigh energy densities are required.

LNG is a cryogenic liquid stored at temperatures as low as −162 °C.Heat transfer from the environment to the LNG causes the evaporation

http://dx.doi.org/10.1016/j.rser.2016.11.186Received 13 April 2016; Received in revised form 26 September 2016; Accepted 12 November 2016

⁎ Corresponding author.E-mail address: walter.merida@ubc.ca (W. Mérida).

Renewable and Sustainable Energy Reviews 69 (2017) 503–513

1364-0321/ © 2016 Elsevier Ltd. All rights reserved.

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of LNG, generation of boil-off gas (BOG), and consequently, an increasein pressure [35]. To maintain the LNG at low temperatures andpressures, LNG carriers release the BOG to atmosphere [36], re-liquefyit, or consume it in their engines [37]. In small LNG facilities, such asLNG refueling stations, the BOG gradually increases the pressure of thestorage system. By regularly delivering “unsaturated” LNG to theserefueling stations, the BOG is condensed and the storage tank pressurereduces before reaching its maximum allowable working pressure(MAWP) [38]. Unsaturated LNG refers to the LNG at a less than−143 °C and 0.34 MPa (35 psig) [39]. The MAWP of LNG storage tanksis set at 1.3 MPa (175 psig) [38]. In LNG refueling stations with lowfuel delivery rates, the BOG generation causes the pressure of LNGstorage tanks to rise and the chance of BOG release rate to theatmosphere increases [38].

CO2 and methane emissions account for 92% of global GHGemissions [2]. Methane is the main constituent of NG [21]. Recentstudies [30,40] showed that the well-to-wheels methane emissionsfrom NG value chain (including LNG) had up to 72 times more impacton climate change than CO2 in a 20-year period due to higher radiativeforcing of methane. Delgado and Muncrief [30] used the concept ofglobal warming potential (GWP) and the data available from theGreenhouse Gases, Regulated Emissions, and Energy Use inTransportation (GREET) model 2014 to compare well-to-wheelsGHG emissions of NG and diesel. With total emissions of 1.12% and1.19% for conventional NG and shale gas, respectively, their analysisindicated that switching from diesel to NG reduced the GHG emissions

by 4–5% over a 100-year period. However, in a 20-year time horizon,NG emissions corresponded to 19–24% increase in the GHG emissionscompared to diesel.

In 2015, Burnham et al. [41] analyzed and compared the methaneleakage in four links across the NG value chain (Table 1). They used theGREET model 2015 for their analysis.

Table 1 shows that, on average, 8.40 to 8.68 g methane/ m3 NG isemitted to the atmosphere across the NG value chain. This is equivalentto emissions of about 605 to 625 g CO2 equivalent/ m3 NG in a 20-yearhorizon [42]. Table 1 also indicates that the transmission and storagesector contributes to 33–35% of methane emissions, and the distribu-tion sector, which includes refueling stations, fueling process, andonboard LNG tanks, contributes to 28% of methane emissions. Thisshows that the transmission, storage, and distribution sectors are thelargest contributors to the methane emissions in the production anddistribution chain. As a result, preventing heat transfer to LNG andcontrolling BOG release will significantly reduce the GHG emissionsfrom these sectors.

A survey of the available literature shows that the BOG release ratefrom different designs of LNG refueling stations had not beenquantified accurately. Powars [38] reported that the average methaneventing from stations was about 1 vol% per delivery of unsaturatedLNG to the stations. Using a lumped-body model, Powars showed thata 15,000 gal capacity LNG station with a 1,000 gal LNG/day dispensingduring a 4-hr window remained under the MAWP of 1.3 MPa(175 psig), whereas the same capacity station with a 500 gal LNG/day dispensing during a 2-hr window reached the MAWP within 15days. In 2015, Hailer [44] measured the methane emissions from twoLNG refueling stations. Hailer reported that one of the operating LNGstations had a methane emissions of 0.1% to 1.5% of fuel dispensed tovehicles and the second station had a methane emissions of 0.9% to5.3%. Hailer also pointed out that the methane emissions from LNGrefueling stations were not necessarily due to the heat transfer to theLNG storage tanks. The BOG returned from vehicles to the station alsocaused a sudden pressure rise in the LNG storage tank and pressurerelief valves were activated.

Prior work has highlighted the importance of mitigating the releaseof methane along the supply chain [7]. However, there have beenlimited studies on the technological aspects of methane abatement inthe NG delivery chain. The main focus of this study is therefore on thetechnological aspects of LNG refueling stations and fuel supply systemsof LNG-fueled vehicles, and how these technologies contribute toreducing methane emissions from the natural gas supply chain. In this

0102030405060708090

100

CO2 NOx SOx Particularmatter

Air

pollu

tion

redu

ctio

n %

Fig. 1. Air pollution reduction% by combusting NG instead of diesel.

Table 1Methane emissions across different sectors.

Sector gmethane/ m3 NG (vol%) GREETModel 2015 [41] g CO2 equivalent/ m3 NG Global Warming Potential [42,43]

Conventional NG Shale gas Conventional NG Shale gas

20-year horizon 100-yearhorizon

20-year horizon 100-year horizon

Gas field 2.16 (0.30) 2.44 (0.34) 156 54 176 61Processing 0.92 (0.13) 0.92 (0.13) 66 23 66 23Transmission and storage 2.93 (0.41) 2.93 (0.41) 211 73 211 73Distribution (station pathway) 2.39 (0.34) 2.39 (0.34) 172 60 172 60Total emission 8.40 (1.18) 8.68 (1.22) 605 210 625 217

Nomenclature

BOG Boil-off gasCNG Compressed natural gasGHG Greenhous gasGWP Global warming potential

LCNG Liquefied-compressed natural gasLNG Liquefied natural gasLN2 Liquid nitrogenNG Natural gasMAWP Maximum allowable working pressureULSD Ultra-low-sulphur diesel

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study, we investigate different onboard LNG tank architectures, designsof LNG refueling stations, and mechanisms employed to manage andmitigate the BOG release rate. Also, we study current and future statusof LNG refueling stations in different countries.

2. LNG refueling station and onboard fuel tankfundamentals

LNG tanks on vehicles or vessels are required to supply NG to theengine at the appropriate temperature, pressure, and flow rate. Avariety of strategies can be employed. In the simplest embodiments, thefuel supply system relies on the pressure in the tank to providesufficient pressure to overcome the losses in the heat exchangers andpiping, and supply enough fuel to the engine. To improve fuel flowperformance, especially in the case of high-performance spark ignitionengines, these simple onboard tanks are filled with so-called warm orsaturated fuel [45]. Saturated LNG has been heated prior to filling, toincrease the saturation pressure, and therefore, supply the requiredfuel flow rate. Other, more sophisticated tank systems are capable ofaccepting so-called cold or unsaturated fuel which has not been pre-conditioned. LNG dispensing systems at stations are therefore requiredto provide unsaturated fuel, saturated fuel, and in some cases, supersaturated (super warm) fuel, depending on the fuel supply system ofthe destination vehicle.

Unsaturated LNG is dispensed at a less than −143 °C and 0.34 MPa(35 psig), and saturated LNG is dispensed at −125 to −131 °C, and 0.69to 0.93 MPa (85 to 120 psig) [39]. Unsaturated LNG has a higherdensity than saturated LNG, and as a result, more LNG can be storedonboard with longer LNG holding time. However, the unsaturated fuelin an onboard LNG tank has a low pressure, and auxiliary equipment inthe vehicle fuel supply system are required to increase the fuel pressurebefore it enters the engine.

It was shown that fueling a vehicle with unsaturated LNG comparedto saturated LNG increases the driving range up to 12% and the LNGholding time from 5 to 10 days [46], as shown in Fig. 2. According toSAE Standard J2343, the holding time of onboard LNG tanks in NorthAmerica is 5 days [47]. Holding time refers to the time an onboardLNG tank holds the LNG without venting [30].

Table 2 shows different vehicle fuel supply systems with capabilityof running on saturated and unsaturated LNG. As shown in Table 2,system (a) is the simplest vehicle fuel supply system that operates onlyby using saturated LNG. During fueling, LNG is sprayed from the top ofthe onboard LNG tank to condense the existing BOG at high tempera-ture. During operation, the LNG is transferred by the pressure gradientfrom the higher pressure tank to the vaporizer and engine [38,45].However, this fuel supply system may not be able to sustain full fuelflow at continuous high fuel demand.

Vehicle fuel supply system (b) shown in Table 2 is equipped with apressure-building circuit. The pressure-building circuit evaporates aportion of LNG to increase the BOG pressure in the vapor space andcreate a pressure gradient between the tank and the engine [45]. Usingthis fuel supply system, LNG does not require to be conditioned at therefueling station. However, this fuel supply system has two technicallimitations: (1) It is impractical to maintain the BOG at hightemperature separated from unsaturated LNG at low temperaturedue to vehicle or vessel vibrations, and (2) the pressure-building circuitrequires time to evaporate the LNG and increase the onboard LNG tankpressure if the tank pressure is initially low [45].

To resolve the latter issue associated with the vehicle fuel supplysystem with a pressure-building circuit, a compressor can be used toquickly increase the BOG pressure in the vapor space. The maintenanceand capital cost of the fuel supply system (c) with a compressor shouldbe considered. Besides, the mixing of the higher temperature BOG withthe remaining LNG in the onboard tank quickly increases the tem-perature of the entire mixture. To resolve this issue, the vehicle fuelsupply system can be equipped with a cryogenic pump (fuel supply

system (d) shown in Table 2). The pump is driven either electrically orhydraulically. The pump pushes the LNG from the tank to the vaporizerand engine under any saturation conditions. It should be noted thatcryogenic pumps are generally expensive and the durability of compo-nents can be problematic [45].

To prevent the pressure in onboard LNG tanks from exceeding theMAWP and BOG being released to the atmosphere, an economizer canbe added to the vehicle fuel supply system [45], as shown in Fig. 3. Thedashed box in Fig. 3 can be any of fuel supply systems (a)-(d) depictedin Table 2. The economizer permits a portion of BOG to be transferredto the engine thereby removing energy from the tank and reducing itspressure. This helps the pressure of the onboard LNG tank to remainbelow its MAWP and reset the LNG holding time of the fuel tank.However, drawing the BOG, which is mainly composed of methane, forsustained periods exclusively leaves the heavy hydrocarbons in theLNG tank [38,45]. This enriched fuel with low methane content maypresent problems for the engine when the economizer is disabled andthe system switches back to conventional LNG fuel supply mode.

Table 3 summarizes further advantages and limitations of fueling avehicle with saturated and unsaturated LNG.

To manage the BOG at high pressure in onboard LNG tanks, theBOG should be 1) condensed by the unsaturated LNG during fueling, 2)vented to the atmosphere prior to fueling (an undesirable action), or 3)transferred by a vapor return line to a station. In some tank designs,this vapor return line is routed through the fill receptacle, e.g., Refs[48–51]., and in other designs, it is provided with its own connection,e.g., Refs [52–56]. Therefore, the designs of LNG refueling stations anddispenser equipment must accommodate these different vapor returnarchitectures. When an onboard LNG tank is required to be filled froma station, the operating sequence can be:

1. No vapor back to station: Use the station pressure to overcome thetank pressure and condense the BOG (only possible if the tankpressure has sufficient margin below the relief valve pressure and thestation pump has sufficient discharge pressure available).

2. Vapor back to station: Use a vapor return line routed through the fillreceptacle or a separate vapor return line to reduce the tank pressureand then commence filling.

3. Vapor back to station and continue to vapor back during filloperation (only possible with a separate vent return line): Transferthe LNG from the refueling station to an onboard LNG tank and theBOG in the tank returns to the station by the vapor return line. Inthis method, the LNG pressure at the station does not need to be toohigh.

The BOG returned from onboard tanks to refueling stations mixeswith the BOG produced by the LNG station, and to prevent venting, itshould be condensed by an onsite LNG liquefier, liquid nitrogen (LN2)condenser, or be purged to a pipeline nearby the station with a

01234567891011

950

1000

1050

1100

1150

1200

1250

UnsaturatedLNG (3 bar)

Saturated LNG(10 bar)

Super saturatedLNG (15 bar)

LNG

hol

ding

tim

e (d

ay)

Driv

ing

rang

e (k

m)

Driving rangeHolding time

Fig. 2. Effects of unsaturated, saturated, and super saturated LNG on vehicle drivingrange and LNG holding time [24].

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compressor. These pieces of equipment add to the cost of the refuelingstation. In LNG refueling stations with sufficient LNG delivery tovehicles, each delivery of unsaturated LNG to the refueling stationserves the same purpose as a liquefier or LN2 condenser.

Liquefied-compressed natural gas (LCNG) refueling stations canalso be used to manage the BOG, e.g, Refs [51,53,55,57,58]. LCNGstations deliver CNG by pressurizing LNG. They are supplied with LNG

Table 2Different vehicle fuel supply systems and their capability of running on saturated and unsaturated LNG.

Vehicle fuel supply system Required fuel

Saturated (warm) LNG Unsaturated (cold) LNG

(a) Fuel supply system with a simple architecture[38,45]Yes No

(b) Fuel supply system with a pressure-building circuit[45]Yes Yes

(c) Fuel supply system with a compressorYes Yes

(d) Fuel supply system with a pump[38]Yes Yes

Fig. 3. Schematic of an economizer added to LNG fuel supply systems shown in Table 2.

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and can also deliver LNG to vehicles if equipped with a LNG dispenser.Combined LNG/LCNG refueling stations are capable to deliver CNG tolight- and medium-duty vehicles, and LNG to heavy-duty vehicles.Also, they have lower capital and operating costs than similarly sizedCNG refueling stations [59]. However, high-pressure reciprocatingLNG pumps used to generate the CNG require extended cooldowntime which generates significant quantities of BOG. This BOG mustthen be processed using a compressor and added to the CNG reservoir.The compressor can also compress the BOG generated in the LNGstorage tank and the BOG returned from vehicles to produce CNG.

To fuel vehicles with different fuel supply systems, LNG refuelingstations filled with unsaturated LNG should be able to deliver saturatedLNG by conditioning the unsaturated LNG. LNG conditioning is aprocess in which temperature and pressure of LNG increase to meet setvalues. Two methods of LNG conditioning at refueling stations are bulk

and on-the-fly conditioning. Fig. 4a shows a simplified schematic of aLNG refueling station with bulk conditioning. The refueling station iscomprised of a storage tank, sump tank, pump, heater, dispenser, andvapor return line. After filling the storage tank, the pump sends theLNG to the heater to rise its temperature and pressure. This processcontinues until the LNG pressure stored in the storage tank reaches aset point.

In contrast, a LNG refueling station with on-the-fly conditioningincreases the pressure and temperature of LNG simultaneously withthe fueling process, as shown in Fig. 4b. This method helps the storagetank to store more LNG with higher density for a longer time. However,the heater needs to be precisely designed to heat the LNG on-the-flywithin a short time without adding too much heat. Linde NorthAmerica Inc. designed and installed the first generation of LNGrefueling stations with on-the-fly conditioning in the U.S. in 2014[61]. Further information about advantages and limitations of LNGbulk and on-the-fly conditioning are summarized in Table 4.

Currently, several vendors are involved in the design and installa-tion of LNG and LCNG refueling stations. NorthStar, Inc. (CleanEnergy) [62], Chart Industries [63], CryoStar [64], ENN Canada[65], and Linde Group [66] are the main suppliers of LNG andLCNG stations with different technologies. Fig. 5 shows schematics ofLNG and LCNG refueling stations manufactured by Chart Industries. Itcan be seen in Fig. 5a that the LNG refueling station has a heater for

Table 3Advantages and limitations of fueling onboard LNG tanks with saturated andunsaturated LNG.

Saturated LNG fueled in onboard tanks with a simple architecture [45]

Advantages Limitations

• Fueling system is simple, reliable,durable, and less expensive.

• Fuel conditioning at station increasesstation cost and complexity.

• Lower fuel density and shorter drivingrange.

• Fuel starvation or choking at sustainedhigh loads.

Unsaturated LNG fueled in onboard tanks with a compressor orpressure-building circuit [45]

Advantages Limitations

• Does not need fuel conditioningat station.

• Compressor or pressure-building circuitis needed.

• Delays between refueling and pressurebuild are required to supply fuel toengine.

Unsaturated LNG fueled in onboard tanks with a pump [38]Advantages Limitations

• Does not need fuel conditioningat station.

• Denser low-pressure fuel andlonger driving range.

• Provides longer LNG holdingtime.

• Pump is needed.

Fig. 4. LNG conditioning at a refueling station: (a) bulk conditioning method [45] and (b) on-the-fly conditioning method [60].

Table 4Comparison of LNG bulk and on-the-fly conditioning [45].

LNG bulk conditioning

Advantages Limitations

• Straightforward, simple, androbust.

• Fully proven and is incommon use.

• Mass of fuel stored in the station is reduced.

• LNG holding time is reduced and risk ofventing is increased.

• Cannot deliver unsaturated LNG tovehicles.

LNG on-the-fly conditioningAdvantages Limitations

• More fuel is stored at station.

• Increases the LNG holdingtime.

• More complicated than bulk conditioning.

• Requires higher heat transfer rate tocondition LNG when it is transferred tovehicle.

• Requires good heat exchangers and precisecontrol systems.

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LNG bulk conditioning with no BOG management. This is the simplestand probably the most economical LNG refueling station design. Asshown in Fig. 5b, an LCNG refueling station is equipped with a highpressure pump to increase the LNG pressure, a vaporizer to convertLNG to CNG, and a CNG buffer storage tank to store the CNG forfueling vehicles.

A schematic of LNG refueling station with on-the-fly conditioningand an LN2 condenser manufactured by Linde North America Inc. isshown in Fig. 6. This station can manage the BOG if LN2 is regularlysupplied to the refueling station.

3. LNG refueling station design survey

In the following section, different designs of LNG refueling stations,which were obtained by analyzing published patents, are studied indetail (Table 5). These designs have been analyzed in terms oftechnologies employed in LNG refueling stations to condition LNG,and BOG management in onboard LNG tanks and refueling stations. Inorder to create the data set for this study, all published patentsdescribing a LNG station were included. However, patents that onlydescribed individual components of a LNG refueling station were notincluded in this analysis.

In order to compare the different refueling station designs, theyhave been classified based on the following criteria: delivering LNG orLNG and CNG, LNG conditioning at the refueling station, BOGmanagement in the onboard LNG tank, and BOG management at theLNG refueling station. The results of these classifications are shown inFig. 7.

Fig. 7a demonstrates that 72% of refueling station designs analyzeddeliver only LNG and 28% of those deliver both LNG and CNG. Interms of LNG conditioning, Fig. 7b shows that 83% of LNG refuelingstation designs analyzed can deliver conditioned LNG and 17% ofrefueling station designs deliver only unsaturated LNG. Of the LNGrefueling station designs with LNG conditioning capability, 33% ofthem perform bulk conditioning, 17% of them condition LNG in smallportions in a sump or secondary tank, and the remaining 33% ofstation designs condition LNG on-the-fly.

In terms of BOG management in an onboard LNG tank, as shown inFig. 7c, 28% of station designs support simultaneous vent to stationwhile filling, 17% of station designs support vent through the fill line inadvance of filling, and the remaining 55% of station designs have nocapacity to receive tank vapor and must rely on the collapse of vaporupon filling or vent to the atmosphere to manage the tank pressureduring filling.

Fig. 7d compares different station designs in terms of the BOGmanagement at refueling stations. It can be seen that 56% of LNGrefueling station designs have the capability for BOG management atthe station. Half of these station designs compress the BOG to produceCNG and the rest of them are equipped with a liquefier or an LN2

condenser to liquefy the BOG. 44% of the LNG refueling station designshave no BOG management. This means that the BOG generated overtime must be collapsed upon delivery of the next load of LNG or bereleased to the atmosphere.

Based on fundamental concepts discussed in this section, thefollowing criteria should be considered in the design of a LNG refuelingstation with minimum BOG emissions:

• Flexibility in fueling vehicles with different fuel supply systems.

• Minimize heat transfer to the LNG during dispenser cooldown.

• Increase the frequency of unsaturated LNG delivery to the station.

• Make a provision for the BOG processing when this is required.

4. LNG refueling stations around the globe

The vast majority of LNG refueling stations are distributed alongthe roads where heavy-duty vehicles travel. There are a limited numberof LNG refueling stations for inland waterway vessels and off-roadvehicles. Table 6 shows the existing and proposed LNG refuelingstations for on-road vehicles in different countries. China has thelargest number with 3,200 LNG refueling stations [74] followed by theU.S. with 122 LNG stations [75]. The total number of LNG stations inEurope is currently 46 and 6 more stations are either proposed or arecurrently under construction in Portugal, France, and Italy.

The rate of LNG deployment varies around the world. Fig. 8indicates that in China, the number of LNG refueling stations hasincreased from 100 in 2010 to 3200 in 2015 (a 32 fold increase) [74]. Itis expected that the number of LNG refueling stations in China will

Fig. 5. Schematic of (a) a LNG refueling station and (b) a LCNG refueling stationmanufactured by Chart Industries [63].

Fig. 6. Schematic of a LNG refueling station with on-the-fly conditioning and LN2

condenser manufactured by Linde North America Inc [61].

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Table 5Different designs of LNG refueling stations reported in the literature.

Ref. Focus of the design Components Technologyemployed in LNGrefueling station

Advantages Limitations

Cieslukowski [48] No loss single-line LNGrefueling station

• Storage tank

• Sump tank with aflexible roof

• Liquefier (LN2

tank)

• Saturation coils

• Conditioning in thesump tank

• Single line for vaporreturn and LNGfueling

• Delivering saturatedLNG

• It can deliver LNG atdifferent pressures andtemperatures

• It conditions LNG ondemand

• It can liquefy the BOG

• Single fueling line

• Complex design for the sump tankand fuel delivery system

• No pump to overcome the pressure ofthe remainder of BOG in the onboardLNG tank

• High dependency on LN2 for the BOGmanagement

Gustafson [67] No loss LNG refuelingstation

• Storage tank

• Centrifugal pump

• Eductor

• Saturation coils

• Flowmeter

• LNG bulkconditioning

• Vapor collapsemethod

• Delivering saturatedLNG

• It precools the pump andpipes by recirculating LNG

• It removes a portion of theBOG generated in the LNGstorage tank by the eductor

• Single fueling line

• The eductor cannot effectively removethe BOG at low LNG flow rates

• The pump and pipes are in directcontact with the environment leadingto high heat transfer to LNG over time

• No reliable BOG managementGoode [52] LNG refueling station and

its control system• Storage tank

• Centrifugal pump

• Saturation coils

• Flowmeter

• LNG bulkconditioning

• Vapor return method

• Delivering saturatedLNG

• It precools the pumps andpipes by circulating LNG

• It has a sophisticated control system

• The pump and pipes are exposed tothe environment causing extra heattransfer to LNG

• No BOG managementKalet and

Gustafson [68]LNG refueling station andconfiguration of vehicleonboard fuel tanks

• Storage tank

• Pump

• Vaporizer

• Pressure transducer

• Microprocessor

• Vehicle overflowtank

• LNG on-the-flyconditioning

• Vapor collapsemethod

• Delivering saturatedand unsaturatedLNG

• It delivers unsaturated LNGto collapse the BOG and thenvaporizes LNG to increasethe pressure of onboardLNG tanks

• Single line fueling

• It requires an overflow tank on vehiclewhich adds to the cost, complexity offueling system, and weight of vehicle

• It requires high power to vaporizeLNG within a short time to increasethe onboard LNG tank pressure

• No BOG managementPowars [69] LNG refueling station

driven by LN2

• Storage tank

• LN2 tank

• LN2 vaporizer

• No conditioning

• Vapor collapsemethod

• Deliveringunsaturated LNG

• BOG management

• Single line fueling• The LNG refueling station only

operates with LN2

• LN2 is directly in contact with LNGfor condensation of BOG andgenerating pressure

• No LNG recirculation in the LNGstorage tank which causes LNGstratification

Barclay [53] LNG and CNG refuelingstation with onsite LNGliquefier

• LNG liquefier

• Vaporizer

• Storage tank

• Expander

• Pump

• CNG buffer tanks

• LNG on-the-flyconditioning

• Vapor return method

• Delivering saturatedand unsaturatedLNG

• It converts NG to LNG onsite

• BOG management• It requires a NG supply pipeline with

enough pressure

• It has high construction costs due toonsite liquefier, two vaporizers, andan expander

• It liquefies the NG to produce LNGand then convert LNG to CNG. Thisprocess increases the energyconsumption of the station

Gustafson andKalet [57]

No loss LNG and CNGrefueling station

• Storage tank

• Two LNGconditioning tanks

• Vaporizer

• Saturation coils

• Compressor

• Flowmeter

• CNG buffer tanks

• LNG conditioned intwo conditioningtanks

• Vapor collapsemethod

• Delivering saturatedand unsaturatedLNG

• The compressor can removethe BOG from storage tankand reset the LNG holdingtime

• CNG is used to increase thepressure and temperature ofLNG in the conditioningtanks

• The CNG compressor is expensive. Asan alternative, a high pressure pumpcan be used.

• It requires a precise control system toadjust pressures in different parts ofthe station because there is no pumpin the design

• It has high construction costs due tocompressor, vaporizer, saturationcoils, and two conditioning tanks

Dehne [54] Zero-vent LNG refuelingstation

• Three storage tanks

• Pump

• Saturation coils

• LNG bulkconditioning

• Vapor return method

• Delivering saturatedand unsaturatedLNG

• It can deliver saturated andunsaturated LNG

• It requires 3 storage tanks of the samesize which add capital costs

• It stores significant amount ofconditioned LNG for a long time thatcontributes to BOG generation

• No BOG managementPreston et al. [70] Self-contained LNG

refueling station• Storage tank

• Sump tank

• Saturation coils

• Pump

• Flowmeter

• LNG bulkconditioning

• Vapor collapsemethod

• Delivering saturatedLNG

• It is a mobile system

• No precooling is required.The pump and flowmeterare submerged inside theLNG sump tank

• Single fueling line

• No BOG management

Forgash et al. [49]Kooy et al.[50]

Four different designs forLNG refueling station

• Storage tank

• Pump

• Saturation coils

• Compressor

• Flowmeter

• Sump tank(recommended)

• LNG bulkconditioning

• Single line for vaporreturn and LNGfueling

• Delivering saturatedand unsaturatedLNG

• It uses a compressor toremove the BOG from LNGstorage tank and purge it toLNG fueling line

• Using compressor adds to the stationcost and maintenance

• The BOG removed from the LNGonboard tank flows only by pressuregradient which may not be enough toremove the BOG properly

(continued on next page)

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Table 5 (continued)

Ref. Focus of the design Components Technologyemployed in LNGrefueling station

Advantages Limitations

Bonn and Gram[71]

LNG refueling station • Undergroundstorage tank

• Sump tank

• Pump

• Saturation coils

• LNG bulkconditioning

• Vapor collapsemethod

• Delivering saturatedLNG

• It is buried underground toreduce heat transfer to LNG

• No precooling is required.The pump and flowmeterare submerged inside theLNG sump tank

• It requires electricity for saturationcoils which increase their energyconsumption

• No BOG management

Emmer et al. [72] LNG refueling station • Storage tank

• Sump tank

• Positivedisplacement pump

• Flowmeter

• Venturi

• Saturation coils

• LNG on-the-flyconditioning

• Vapor collapsemethod

• Delivering acombination ofsaturated andunsaturated LNG

• It delivers unsaturated LNGto collapse the BOG and thenconditions LNG to increasethe pressure of onboard fueltanks

• Single line fueling

• The station needs information fromvehicles about their LNG onboardtank pressure, temperature, tank size,and fuel level which may not beprovided by all vehicles

• Microprocessor calculates the amountof LNG that should be conditioned foraddition to unsaturated LNG in theonboard tank which may not bealways accurate

• No BOG managementUrsan and Gram

[73]Pumping LNG and BOGsimultaneously in vehiclefueling system or LNGrefueling station

• Storage tank

• Reciprocatingpump

• Saturation coil

• Accumulator

• LNG on-the-flyconditioning

• Vapor collapsemethod

• Delivering saturatedand unsaturatedLNG

• Replacement of centrifugalpump with reciprocatingpump for removing the BOG

• It liquefies the BOG at highpressure

• Resets the storage tankholding time

• Single fueling line

• Difficulty in control the portion ofLNG and BOG removed from thestorage tank

Bingham et al.[55]

LNG and CNG refuelingstation

• Storage tank

• Pump

• Vaporizer

• CNG buffer tank

• two flowmeters

• two mixers

• LNG on-the-flyconditioning

• Vapor return method

• Delivering saturatedand unsaturatedLNG

• Its construction cost is lessthan conventional CNGstations

• It controls the LNGtemperature by adding CNG

• It adds LNG to CNG to reduce itstemperature which requires an extrapump to overcome the pressuredifference between LNG and CNG

• It cannot supply LNG and CNG at thesame time

Emmer et al. [51] LNG and CNG refuelingstation

• Storage tank

• LNG conditioningtank

• Reciprocatingpump

• Vaporizer

• Two CNGpressurizing tanks

• LNG level gauge

• Flowmeter

• LNG conditioned inthe conditioningtank

• Single line for vaporreturn and LNGfueling

• Delivering saturatedLNG

• It provides both LNG andCNG

• CNG is used to increase thepressure and temperature ofLNG in the conditioningtank

• It requires several LNG and CNGtanks which add to the cost

• It required a precise control system toadjust pressures in different parts ofthe station because there is no pumpin the design

Gram and Ursan[58]

LNG and CNG refuelingstation

• Storage tank

• Centrifugal pump

• Vaporizer

• Odorizer

• No conditioning

• Vapor collapsemethod

• Deliveringunsaturated LNG

• It provides both LNG andCNG

• Storage tank is buried toexperience less temperaturedifference with theenvironment

• It reduces construction costsof conventional CNGstations

• Single fueling line

• The net positive suction head of thepump should be accounted in thedesign

• Single pump for both LNG and CNGfueling

Lee and Heisch[60]

LNG refueling station • Storage tank

• Pump

• Saturation coil

• Conditioning vessel

• LNG on-the-flyconditioning

• Vapor collapsemethod

• Delivering saturatedand unsaturatedLNG

• It can provide LNG atdifferent temperature andpressure

• Single fueling line

• It may not be able to set thetemperature properly

• It is not a fast response system asLNG should be evaporated for heatingthe LNG delivered to vehicles

• It has a high BOG generation rate

• No BOG managementLee et al. [56] LNG refueling station with

LN2 condenser• Storage tank

• LN2 tank

• Condenser

• Flowmeter

• No conditioning

• Vapor return method

• Deliveringunsaturated LNG

• It uses LN2 for condensationof BOG returned from LNGonboard tanks

• The condensed LNG needs to enterfrom the bottom of the LNG storagetank by pressure gradient whichcannot not be sufficient to overcomethe LNG hydrostatic pressure in thestorage tank

• There is no pump in the design andpressure difference between thestorage tank and onboard tank maynot be sufficient to fully fill theonboard tank

• No BOG management in the storagetank

A. Sharafian et al. Renewable and Sustainable Energy Reviews 69 (2017) 503–513

510

reach 5000 by 2020. In the U.S., the number of LNG refueling stationgradually increased between 2010 and 2015 [79]., as shown in Fig. 8.The number of LNG refueling stations in the U.S. increased from 40 in2010 to 122 in 2015 (a 3 fold increase) and 54 more LNG refuelingstations are either proposed or are currently under construction.

Based on this analysis and the number of LNG refueling stationsdiscussed, it can be concluded that LNG usage in the transportationsector is increasing and will continue in the next decade. Equipmentdesign along the LNG distribution chain will play a major role. Newscenarios in BOG management of LNG refueling stations and vehicles’onboard LNG tanks should be implemented to get short- and long-termbenefits of switching from conventional petroleum fuels to LNG.

5. Conclusions

LNG can replace diesel in heavy-duty vehicles, ships, and trains.However, the unintended release of methane (the main constituent ofLNG) contributes more than CO2 to climate change in a 20-yearhorizon. Methane emission analysis across the supply chain indicatedthat transportation, storage, and distribution sectors are the largestcontributors to these fugitive emissions. LNG refueling stations andvehicles’ onboard LNG tanks are the main parts of the distributionsector. Our analysis showed that BOG management in LNG refuelingstations and onboard LNG tanks, flexibility of fueling vehicles withdifferent fuel supply systems, and minimizing heat transfer to the LNGbetween the storage tank and dispenser were parameters that neededto be considered in the design of LNG refueling stations. The majorityof LNG refueling stations available in the market had no BOGmanagement. Even patented designs included in our comprehensiveliterature review indicated that 44% of LNG refueling station designshad no BOG management. Our analysis of the LNG refueling stationmarket showed that the number of LNG refueling stations in Chinaincreased by 32 times between 2010 and 2015. In the U.S., the increasein number of LNG refueling stations was more gradual. The number of

(a)

(b)

(c)

72%

28%

0102030405060708090

100

LNG LNG + CNG

% o

f ref

uelin

g st

atio

ns

Fuel supplied by refueling station

33%

17%

33%

17%

0102030405060708090

100

Bulk conditioning Conditioning insump tank or

secondary tank

LNG on-the-flyconditioning

No conditioning

% o

f ref

uelin

g st

atio

ns

LNG conditioning status at refueling station

55%

28%

17%

0

10

20

30

40

50

60

70

80

90

100

Vapor collapse Simultaneous vent tostation while filling

Vent through the fillline in advance of filling

% o

f ref

uelin

g st

atio

ns

BOG status in onboard LNG tank in refueling process

(d)

56%

44%

0102030405060708090

100

BOG management No BOG management

% o

f ref

uelin

g st

atio

ns

BOG management status at refueling stationBOG managementNo BOG managementBOG management by CNG productionBOG management by liquefier (LN2)

28%

28%

Fig. 7. Comparison of different refueling station designs in terms of (a) delivering LNG or LNG and CNG, (b) conditioning LNG at refueling station, (c) managing the BOG in onboardLNG tank, and (d) managing the BOG at refueling station.

Table 6Number of existing and proposed LNG refueling stations by country.

Country Existing LNGStations

LNG Stations proposed orunder construction

Ref.

China 3200 1800 [74]USA 122 54 [75]Canada 12 – [6]Spain 12 – [76]Netherland 12 – [76]Australia 10 – [77]United Kingdom 9 – [76]Sweden 7 – [76]Belgium 2 – [76]Portugal 2 3 [76]France 1 2 [76]Italy 1 1 [76]Japan 1 – [78]

100

200 60

0

1844

2500

3200

5000

40 43

61

84

103

122

0

20

40

60

80

100

120

140

0

1000

2000

3000

4000

5000

6000

2010 2011 2012 2013 2014 2015 2020

No.

of L

NG

refu

elin

g st

atio

ns in

U.S

.anihC

nisnoitats

gnileuferG

NLfo.oN

Year

ChinaU.S.

Fig. 8. Number of LNG refueling stations in China [74] and the U.S [79].

A. Sharafian et al. Renewable and Sustainable Energy Reviews 69 (2017) 503–513

511

LNG refueling stations in the U.S. increased from 40 in 2010 to 122 in2015. This showed a considerable movement towards LNG in thetransportation sector and highlighted the importance of LNG refuelingstation and fuel supply chain designs to efficiently manage the BOG.

Acknowledgment

The authors gratefully acknowledge the financial support of theNatural Sciences and Engineering Research Council of Canada (Grantno. 11R24937) (NSERC) and the technical support of Westport PowerInc.

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