Vehicle refueling with liquid hydrogen thermal compression

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Vehicle refueling with liquid hydrogen thermal compression Guillaume Petitpas a,*, Salvador M. Aceves a, Nikunj Gupta b a b

Lawrence Livermore National Laboratory, 7000 East Avenue, Livermore, CA 94551, USA Shell Projects and Technology, India

article info

abstract

Article history:

We have modeled an approach for dispensing pressurized hydrogen to 350 and/or 700 bar

Received 25 January 2012

vehicle vessels. Instead of relying on compressors, this concept stores liquid hydrogen in

Received in revised form

cryogenic pressure vessels where pressurization occurs through heat transfer, reducing the

27 April 2012

station energy footprint from 12 kW h/kgH2 of energy from the US grid mix to 1.5e2 kW h/

Accepted 29 April 2012

kgH2 of heating. This thermal compression station presents capital cost and reliability

Available online 31 May 2012

advantages by avoiding the expense and maintenance of high-pressure hydrogen compressors, at the detriment of some evaporative losses. The total installed capital cost

Keywords:

for a 475 kg/day thermal compression hydrogen refueling station is estimated at about

Hydrogen refueling

$611,500, an almost 60% cost reduction over today’s refueling station cost. The cost for

Liquid hydrogen

700 bar dispensing is $5.23/kg H2 for a conventional station vs. $5.45/kg H2 for a thermal

Thermal compression

compression station. If there is a demand for 350 bar H2 in addition to 700 bar dispensing,

Cost-effective design

the cost of dispensing from a thermal compression station drops to $4.81/kg H2, which is similar to the cost of a conventional station that dispenses 350 bar H2 only. Thermal compression also offers capacity flexibility (wide range of pressure, temperature, and station demand) that makes it appealing for early market applications. Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

1.

Introduction

Hydrogen delivery and dispensing are key components of a hydrogen-based transportation system. Different delivery modes exist and their interaction with fueling station and vehicle storage technologies demand careful study in terms of cost and environmental performance to evaluate their feasibility and potential. Hydrogen can be delivered from the plant to the fueling station in different ways: (1) compressed gas (CH2) tube trailer, (2) cold compressed hydrogen gas [1], (3) liquid hydrogen (LH2) truck, (4) gaseous pipeline from the hydrogen production plant to the fueling station, and (5) pipeline to the edge of the urban area (city gate) and then liquid or gas truck to the station [2]. Table 1 shows the energy requirements for the

main options for dispensing H2 to 700 bar vehicles (room temperature). The last row is the option considered in this article and the corresponding station energy requirements will be further discussed. The LH2 path is frequently considered inefficient and expensive due to the high energy of liquefaction. However, transporting liquid H2 necessitates less energy than the station energy requirements, due to mechanical compression. As a result, recent analysis [4] has revealed that the total cost of delivering and dispensing LH2 is comparable to CH2 because (1) LH2 trucks have larger capacities, reducing capital and driver cost, (2) LH2 stores at the station in inexpensive Dewars, and (3) LH2 is easier to transfer into delivery trucks (see Table 2 for comparisons). These arguments are especially important in medium and large commercial fueling stations dispensing

* Corresponding author. Tel.: þ1 9254230348. E-mail addresses: [email protected] (G. Petitpas), [email protected] (S.M. Aceves). 0360-3199/$ e see front matter Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2012.04.137

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Table 1 e Energy requirements for delivering hydrogen to 700 bar vehicles for several alternatives based on estimates from [4] and from this study (last row), in kWh per kg of H2 delivered. The second row considers transporting H2 to the city gate then distributing it with 250 bar tube trailers. The table shows primary energy consumption assuming today’s USA electric grid, as well as electricity consumption in parentheses. Delivery mode

Liquefaction

Pipeline þ mechanical compression P-T250 þ mechanical compression Liquid trailer þ mechanical compression Liquid trailer þ thermal compression

24 (12 [3]) 24 (12 [3])

over 400 kg H2/day [5]. LH2 delivery also provides compatibility with vehicles storing cryogenic hydrogen, and CH2 can be obtained from LH2 through vaporization and subsequent compression. The advantages of LH2 distribution are illustrated in a recent California Air Resources Board (CARB) report [6]. This report predicts future deployment of H2 fueling stations for different sources of H2 (liquid delivery, gaseous delivery, or on-site steam methane reforming, SMR). CARB anticipates that, regardless of the rate of H2 vehicle introduction, most fueling stations will be supplied with LH2 (Fig. 1). Ease of distribution therefore favors LH2 delivery, even when all dispensed fuel is CH2. In typical hydrogen fueling stations (Fig. 2), LH2 is stored in a large Dewar, vaporized at low (near ambient) pressure, and then compressed. A cascade charging system (typically comprising three pressure vessels) helps downsize the compressors. A booster compressor enables vehicle refueling at 700 bar with a lower cost cascade rated for e.g. 500 bar. A refrigeration system is typically required when dispensing 700 bar CH2 to limit heating of the vehicle vessel during rapid filling operation. Today’s fueling stations (Fig. 2) waste LH2’s thermomechanical exergy through low-pressure evaporation. Thermomechanical exergy is the maximum theoretical (Carnot) work that can be obtained when bringing a substance into thermal and pressure equilibrium with the environment [7]. While negligible for gasoline and diesel fuels, LH2 thermomechanical exergy represents 10.4% of the lower heating value of hydrogen [8]. There is therefore potential to take

Transport 2.5 6 4 4

(0) (0) (0) (0)

Station 12 (6) 5.5 (2.75) 12 (6) 2 (0)

advantage of this exergy, e.g. to reduce capital cost and energy consumption of fueling station operations. A fueling station concept that takes advantage of LH2’s thermomechanical exergy is illustrated in Fig. 3. In this system, LH2 from the Dewar fills cryogenic pressure vessels in a cascade. LH2 inside the cryogenic vessel is then vaporized at constant volume by heat transfer through an in-tank heat exchanger, pressurizing considerably as the vessel heats up. When fully pressurized, dispensing starts. Cold hydrogen gas extracted from the cryogenic vessel is run through an external heat exchanger for further heating to the dispensing temperature (40  C) before flowing into the vehicle vessel. While not thermodynamically optimum (much exergy is still destroyed through irreversible LH2 heating), this thermal compression process greatly simplifies fueling station configuration by eliminating expensive and maintenance-prone mechanical compressors, in exchange for a cascade of cryogenic pressure vessels and, as later discussed, at the expense of potential H2 evaporative losses. Cryogenic pressure vessels e the key component of this fueling station concept e have been demonstrated for automotive use ([9e11]). Rated for 20e300 K operating temperature and potentially for very high-pressure (>700 bar), they hold promise for enabling constant volume pressurization without structural damage. Thermal insulation, a critical issue for automotive vessels, can be simplified for cascade vessels,

Table 2 e Comparison between two delivery modes for hydrogen: LH2 delivery truck vs. tube trailer compressed CH2 delivery truck. Liquid hydrogen Hydrogen delivered, kg [4] Equivalent number of filled H2 vehiclesa Capital cost per kg H2 capacity [4] Loading time per truck [2] (seconds/kg H2)

Tube trailer compressed hydrogen (250 bar)

4000 w870

550 w120

$160

$950

2 h (1.6 s/kg)

6 h (40 s/kg)

a Assuming the average “fill” is 75% of tank capacity, or approximately 4.6 kg per vehicle.

Fig. 1 e Station deployment for two fuel cell vehicle introduction scenarios (lower and upper bound) for different H2 delivery modes (liquid, gaseous or On-Site Steam Methane Reforming) [6].

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Fig. 2 e Schematic of a typical fueling station storing LH2 and dispensing CH2.

since dormancy requirements are diminished. Rapid pressurization of cryogenic vessels is made possible through intank heat exchangers [12]. In this paper we describe and model a thermal compression fueling station, estimating performance and cost and comparing it with conventional compressor-based fueling stations.

 

2.

Fueling station model 

Fig. 4 shows a circuit schematic of the thermal compression hydrogen fueling station concept [13] based on a dynamic cascade of insulated cryogenic high-pressure vessels containing an internal heat exchanger to accelerate pressurization during the warm up phase. The main model assumptions are:  Vehicles refueled at the station are equipped with hydrogen vessels storing 5.6 kg H2 (140 L internal volume for 700 bar or 233 L for 350 bar),  Vehicles are refueled when 1 kg H2 remains in the vessel [2], i.e. when pressurized to 90 bar for a 700 bar vessel or 53 bar for a 350 bar vessel,  The station uses a 1300 kg H2 Dewar [14] that operates at an essentially constant pressure of 3 bar. The Dewar depressurizes slightly as LH2 is extracted, and later repressurizes to





 



3 bar as residual H2 from the cryogenic vessel is recycled to the Dewar. While we have not optimized the Dewar pressure, 3 bar is a good compromise between LH2 density (65 g/L at 3 bar) and the need for driving potential (Dp) between Dewar and cryogenic vessels, Cryogenic pressure vessels in the cascade are rated for 860 bar and have 1 m3 internal volume, Heat transfer rate through cryogenic vessel insulation is negligible compared with heating rate from in-tank heat exchanger, Thermal equilibrium is assumed at all times between H2 and pressure vessel walls, The number of cryogenic vessels in the cascade depends on station demand. This number is determined later in the paper through transient station modeling, The cascade system is connected to a two-hose dispenser that can simultaneously refuel two vehicles. The hydrogen flow rate per hose is 1.67 kg/min [15], and the total cascade flow rate is 3.34 kg/min, The minimum dispensing temperature at the nozzle is 233 K (40  C) [15], The cascade dispenses from the least pressurized cryogenic vessel that (1) is at a higher pressure than the vehicle vessel, and (2) is not being thermally pressurized, Refueling from a cryogenic vessel ends when the pressure difference between the vessels (PcryogenicePvehicle) equals

Fig. 3 e Schematic of the proposed thermal compression fueling station concept.

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Fig. 4 e Circuit schematic of the thermal compression hydrogen fueling station.

1.4 bar [2]. At this point, refueling continues from the next higher pressure cryogenic vessel not being pressurized until the vehicle vessel is full,  We simulate a medium size hydrogen refueling station [5] dispensing 400 kg H2/day average, but we analyze the daily maximum (i.e. a summer Friday) at 475 kg H2/day [14],  Hourly station demand is given by the Chevron profile (Fig. 5). The peak demand is 35e37 kg H2/hour (w8 vehicles per hour or 4 per hose per hour) between 2 and 6 PM. Each of the cryogenic vessels in the cascade undergoes pressure and temperature cycles as they are filled with LH2,

Fig. 5 e Station demand profile for a medium size hydrogen refueling station dispensing 475 kg/day maximum on a summer Friday [2].

thermally pressurized, and emptied. One of these cycles is notionally shown in Fig. 6, where we consider that all vehicles refueling at the station are equipped with 700 bar vehicles. Temperature, pressure and density in the vessel at the end of each step are summarized in Table 3. Step 1 begins with a depressurized (1 bar) and relatively warm (74 K) cryogenic vessel (at time 0). The fill valve opens, allowing LH2 to flow from the Dewar into the cryogenic vessel. Flow continues until the cryogenic vessel fills with LH2 (at 65 g/ L, 24 K and 2.7 bar). Some LH2 evaporation occurs during filling due to the elevated initial temperature of the cryogenic pressure vessel. Unless mechanically compressed or refrigerated, low-pressure evaporated H2 cannot be used for refueling vehicles, and in the worst case it would have to be vented to the atmosphere or burned for rapid cryogenic vessel heating and pressurization, although in practice it could be used to e.g. generate electricity to run the fueling station and neighboring areas with a stationary fuel cell or internal combustion engine. When the vessel is filled to its maximum capacity (65 g/L at time t1), inlet and outlet valves are closed and the vessel is heated at constant volume, from 24 K to 2.7 bar to the rated pressure (860 bar at 160 K) at t2. The vessel is heated by running hydrogen from a previously heated vessel into the internal vessel heat exchanger on its way to refueling a vehicle (step 3) or recycling into the Dewar (step 4). Once the cryogenic vessel is at its rated pressure (at time t2), H2 is extracted to refuel vehicles via the dispenser (step 3). This step varies in duration as it depends on the relative pressures of the other cascade vessels and on the station hydrogen demand. The dispensing process ends (t3) when the cryogenic vessel pressure approaches (within 1.4 bar) the vehicle refueling pressure (90 bar for 700 bar vehicle vessels).

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Fig. 6 e Notional fill and empty cycle for a cryogenic pressure vessel in the cascade, for a station that dispenses exclusively to 700 bar vehicles. MOP is the cryogenic vessel’s Maximum Operating Pressure, and Pini is the minimum cryogenic vessel discharge pressure (equal to the pressure in the vehicle’s vessel when refueling starts plus 1.4 bar, i.e. 91.4 bar for 700 bar refueling).

Hydrogen extraction cools down the cryogenic vessel, ending the process at T ¼ 108 K and 20 g/L. At the end of step 3, the cryogenic vessel pressure is too low for refueling vehicles and some of the residual H2 is therefore recycled into the Dewar for future fueling operations and for pressurizing the Dewar back to 3 bar (step 4). In an effort to maximize the fraction of residual H2 recycled into the Dewar, residual H2 is first circulated through a cryogenic vessel in the cascade that is full of LH2 (at time t1 in the process). Cooling down recycled H2 reduces the rate of Dewar pressurization, permitting recycling of a larger fraction of residual H2. Extraction of recycled H2 during step 4 further cools down and depressurizes the cryogenic vessel. Recycling of the residual H2 ends when the Dewar pressurizes back to 3 bar. The remaining H2 in step 5 needs to be vented to the atmosphere, burned for vessel heating and pressurization, or used locally to produce electricity. Quantification of evaporative losses is important to the economic viability of this concept, and is discussed in the next section.

2.1.

Venting losses

As previously discussed, venting losses occur during steps 1 and 5 of the cryogenic pressure vessel cycle (Fig. 6). During

step 1, evaporation occurs due to the elevated vessel temperature. LH2 rapidly evaporates when refueling starts due to heat transfer from the vessel wall, requiring H2 venting to maintain a positive pressure gradient (and H2 flow) between the Dewar and the cryogenic vessel. From Fig. 7, a total of 6.7 kg H2 are vented from the system during the fill process, for a station that exclusively dispenses H2 to 700 bar vehicle vessels. The cryogenic pressure vessel starts step 5 with 9 g/L H2 (Fig. 8 and Table 3), and after venting to atmospheric pressure, the final density is 0.3 g/L (at the end of step 5). The amount of hydrogen vented in step 5 is therefore (9 g/Le0.3 g/L) *1 m3 ¼ 8.7 kg H2. Total losses are therefore 6.7 þ 8.7 ¼ 15.4 kg. The amount of hydrogen dispensed per cycle during step 3 is (65 g/Le20 g/L)*1 m3 ¼ 45 kg. The amount of hydrogen vented per kg H2 dispensed is thus 16.2/45 ¼ 34.2%. Evaporative losses can be reduced if the station dispenses H2 to 350 bar vehicles in addition to (or instead of) 700 bar vehicles. Dispensing to 350 bar vehicles increases the amount of H2 dispensed in step 3 (to 65-13.5 ¼ 51.5 kg H2), because vehicles drive into the station with a vessel at lower pressure (53 bar instead of 90 bar when 1 kg remains in vessel due to larger vessel size). The vessel is also colder at the end of step 3 (100 K, Table 3), reducing venting losses in steps 1 and 5 to 6.0

Table 3 e Temperature (T, K ), pressure (P, bar), and density (r, g/L) at the end of the 5 cycle steps (see Fig. 6) For cryogenic vessels dispensing H2 to 700 and 350 bar vessels. Vehicle pressure

700 bar 350 bar

End of step 1

End of step 2

End of step 3

End of step 4

End of step 5

T

p

r

T

p

r

T

p

r

T

p

r

T

p

r

24 24

2.7 2.7

65 65

160 160

860 860

65 65

108 100

91.4 54.4

20 13.5

92 77

33.7 8.8

9 2.7

74 74

1 1

0.3 0.3

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Fig. 7 e Pressure in both LH2 Dewar and cryogenic pressure vessel, along with temperature, stored mass, and vented mass in the cryogenic pressure vessel, as a function of mass of H2 to the cryogenic pressure vessel in the cascade during step 1, for a station that dispenses exclusively to 700 bar vehicles.

and 2.4 kg H2, for a total of 8.4 kg. Venting losses as a function of dispensed hydrogen therefore drop to 8.4/51.5 ¼ 16.3%. Please notice that 350 bar and 700 bar thermal compression stations in Table 3 are identical in construction: 350 bar stations can dispense 700 bar H2. However, demand for 350 bar H2 reduces venting losses, enabling reduction in H2 dispensed cost, as discussed later in the paper.

2.2.

Heat transfer analysis

Heat exchange control is a key aspect of thermal compression. External heat exchangers HX1 and HX2 and in-vessel heat exchangers (Fig. 4) have to be designed for rapid cryogenic vessel pressurization and for meeting the delivery

Fig. 8 e Pressure in liquid storage Dewar, and pressure and temperature in cryogenic pressure vessel, as a function of H2 mass in the vessel, during steps 4 and 5, for a station that dispenses exclusively to 700 bar vehicles.

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temperature requirement (40  C). These goals are best accomplished by using the hydrogen being dispensed to refueling vehicles as the thermal energy carrier. This process eliminates the need for circulation pumps, since the pressure inside the cryogenic vessel drives the process. Use of hydrogen also eliminates the risk of tube blockage due to freezing that might occur at the low operating temperature (25 K) if thermal energy carriers other than helium or hydrogen were used. Going back to Fig. 4, assume that cryogenic vessel 1 in the cascade is being pressurized and vessel 2 is dispensing hydrogen to a vehicle. Hydrogen is therefore extracted from vessel 2 and heated through HX1 before being circulated into vessel 1 in-tank heat exchanger. Vessel 1 therefore heats up and pressurizes while hydrogen from vessel 2 cools down. Reaching the desired 40  C dispensing temperature at the nozzle therefore demands H2 heating in HX2. Heat exchangers HX1 and HX2 enable effective warm up of the hydrogen leaving a cryogenic vessel and flowing into the dispenser. Heat exchanger dimensions are calculated assuming compact finned tube surface type CF-7.0-5/8J (Fig. 9 [16],), with hydrogen flowing inside the tubes and hot air at 500 K flowing outside the tubes. We assume that 500 K hot air is obtained by burning a fraction of the vented hydrogen with considerable excess air (at equivalence ratio 4 ¼ 0.1). While burning vented hydrogen fuel to heat a cryogenic fluid is far from thermodynamically optimum (on-site electricity generation would be much better), it is considered a worst-case operation that minimizes capital cost. Future work will analyze more exergetically efficient approaches for cryogenic hydrogen heating such as (1) use of solar thermal collectors; (2) use of waste heat, e.g., from a fuel cell or engine that consumes some of the hydrogen; or (3) installation of groundsource (buried) heat exchangers to heat H2 by heat exchange with the soil [17]. Heat exchanger volume and number and length of tubes were calculated using the ε-NTU method [18]. Table 4 shows the results of the heat exchanger analysis for HX1 and HX2. In order to provide enough heating to the cryogenic pressure vessel through the in-vessel heat exchanger (Fig. 4), a temperature of 350 K is needed at HX1 outlet. Hydrogen exiting a previously heated cryogenic vessel is between 100

Fig. 9 e Configuration of compact heat exchangers HX1 and HX2.

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Table 4 e Operating conditions and dimensions for heat exchangers HX1 and HX2.

Thot [K] Tin,H2 [K] Tout,H2 [K] qcold [kW] NTU ε Heating, MJ/kgH2 dispensed

HX1

HX2

500 100e160 350 233e171 1.44e1.07 0.625e0.559 4.18e3.07

500 200 233 31 0.12 0.11 0.56

and 160 K. As can be observed, HX2 needs considerably less energy since it only heats the hydrogen from 200 to 233 K (40  C). A total of 4.74 MJ (4.18 þ 0.56), or 1.3 kWh, is required for appropriately heating 1 kg H2. Considering that H2’s lower heating value is 120 MJ/kg, 4% of the dispensed H2 needs to be burned. This is less than vented hydrogen, and therefore vented H2 would suffice for providing the required heating. In-vessel heat exchangers enable rapid heating and pressurization of the cryogenic pressure vessels in the cascade. The cold hydrogen inside the vessel heats up by natural convection. The total required heating (including thermal mass of the tank) to dispense hydrogen from a cryogenic pressure vessel during a cycle is about 105 MJ. Heat transfer between flowing warm hydrogen and cold hydrogen inside the vessel is calculated with standard equations for fin efficiency, the SiedereTate correlation for forced convection in turbulent pipe flow, standard correlations for natural convection around vertical fin surfaces [18], and the ε-NTU method. We assume once again finned tube construction, with a 6-m long tube, 320 fins per meter, 1 cm inner tube diameter, and 2 cm outer fin diameter. Assuming that the warm hydrogen flows inside the inner heat exchanger at 350 K and 3.34 kg/s, the main model results are (1) total heating power is 100e70 kW at hydrogen temperatures 24e160 K inside the vessel, (2) hydrogen flowing inside the finned tube cools from 350 K to 200e270 K as it transfers heat to the vessel, and (3) the cryogenic vessel warms up in 20 min, assuming continuous 350 K hydrogen flow inside the heat exchanger (i.e. when the station has continuous refueling demand). Our calculations indicate that it takes about 40 min for a cryogenic pressure vessel to complete its cycle (steps 1 þ 2þ4 þ 5), not including step 3, which is a function of station demand. We, however, use 1 h for the station simulation as a conservative value that will give some excess capacity margin during peak station demand.

2.3.

Fig. 10 e Pressure in cryogenic vessels in the cascade (CV1 to CV8) and pressure in vehicle vessels being refueled (black lines) during the hours of peak station demand (1 PMe4 PM).

continuous station operation is an important parameter that affects cost. The station has a finite number of vessels that need to be promptly refueled and pressurized as they empty for continuous station operation. Refilling at 700 bar is most demanding in terms of number of vessels in the cascade. Fig. 10 shows pressure vs. time in 8 cryogenics vessels (CV1-CV8) and in refueling vehicles (black lines) between 1 PM and 4 PM, when 8 cars are filled per hour. Two vehicles are filled simultaneously. The analysis shows that a cascade with 12 cryogenic pressure vessels can meet the 475 kg H2/day demand, as 8 cars per hour can be filled for several hours beyond the 4 h peak station demand. A station that only

Station modeling

We have modeled the dynamic interaction between refueling 700 bar hydrogen vehicles and the thermal compression fueling station to validate the concept and calculate the required number of cryogenic vessels in the cascade for a 475 kg daily peak hydrogen demand. The minimum number of cascade cryogenic vessels that can support

Fig. 11 e Cost estimation for a cascade cryogenic pressure vessel rated for 860 bar and 1 m3 inner volume. Red sectors in the figure represent components that scale with the amount of hydrogen storage. Blue and gray sectors do not scale with H2 storage. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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Table 5 e Cost estimates for 400 kg/day thermal compression fueling station. Unit Price

Qty.

Cost

Source

$104,847 $16,460 $20,000 $50,000

1 12 2 1

$26,347

1

$104,847 $197,520 $40,000 $50,000 $100,000 $26,347 $518,714 $103,742 $611,457

H2A [2] LLNL estimate LLNL estimate Shell estimate H2A [2] H2A [2]

LH2 Dewar e 1300 kg Cryogenic vessels Burner/HX Valves, fittings, other Control and safety equipment Dispensers Total equipment Installation & integration Total installed capital cost

20%

dispenses H2 to 350 bar vehicles would require 8 cryogenic vessels.

2.4.

Cost analysis

A recent publication [19] details cost analysis of vehicular cryogenic pressure vessels when produced in large scale (500,000 units per year). The authors analyze a prototype vessel storing 10.4 kg LH2 and a smaller hypothetical vessel that stores 5.6 kg LH2. The publication lists the cost of every system component, including components that scale with amount of H2 stored (pressure vessel, vacuum insulation) and those that do not (valves, regulator, internal heat exchanger). Fixed costs (regulator, valves, etc.) amount to $940, and variable costs are $1146 for 5.6 kg H2 and $1818 for 10.4 kg LH2. From these values we can project the cost of large (cascade size) cryogenic pressure vessels with 1 m3 internal volume and 860 bar rating. The results are presented in Fig. 11. Higherpressure cascade vessels are estimated to cost $16,500, or $235/kg H2 (vs. $269/kg H2 for the 10.4 kg H2 prototype). Scaledependent components (carbon fiber, liner and fittings, multilayer vacuum insulation, and outer shell), account for 88% of the total cost, and the carbon fiber by itself represents 70% of the total cost. Table 5 shows the cost estimate for the 400 kg/day thermal compression fueling station. The total installed capital cost is calculated at $611,500. For comparison, today’s 400 kg/day, 700 bar compressor-based refueling stations are estimated at $1,500,000 [20]. Paster et al. [4] investigated the well-to-wheel costs of hydrogen for different delivery and dispensing modes. Among their results, they published hydrogen costs for the special case of a 15% H2 market penetration in Sacramento, CA, assuming steam methane reforming (SMR) hydrogen production and an average US grid electricity mix. Hydrogen costs are split into 4 different components: production, liquefaction, transport and station costs. These costs are reported in Table 6.

H2A [2]

Using these results, we can estimate that for a liquid delivery scenario to a traditional H2 refueling station (with mechanical compressors), the total hydrogen cost at the station would be $4.81/kg and $5.23/kg respectively for 350 bar and 700 bar refueling (Table 7). A thermal compression fueling station is less expensive and requires little electricity at the site but it is subjected to venting losses. Taking into account these factors and considering that production, liquefaction and transport costs are equal for all cases, we estimate that the cost of delivering H2 in a thermal compression station is $5.44/kg for a station that exclusively dispenses 700 bar H2. The cost drops to $4.81/ kg if there is a demand for 350 bar dispensing in addition to (or instead of) 700 bar dispensing. Thermal compression fueling stations present several advantages not included in cost estimates listed in Table 7: 1. A 350 bar mechanical compression station delivers only 350 bar H2, while the 350 bar thermal compression station dispenses both 350 and 700 bar H2, assuming only that there is enough demand for 350 bar H2 to depressurize cryogenic vessels to 54.4 bar (vs. 91.4 bar for a station that exclusively dispenses H2 to 700 bar vehicles). In the case of a “mixed fleet” (both 350 and 700 bar vehicles exist on the market), lowest cost refueling is obtained from the thermal compression station. 2. The results in Table 6 assume that there are no practical uses for all the vented hydrogen, except for heating hydrogen streams at heat exchangers. If practical, electricity generation at the station may contribute to reduce the cost of dispensed H2. 3. Maintenance and electricity consumption at the station are minimum: no mechanical components are required, aside from vessels and valves. 4. Thermal compression stations are intrinsically modular and offer capacity flexibility: the refueling station capacity can be increased as the demand expands by adding extra vessels in the cascade.

Table 6 e Cost of the different hydrogen operations from production to dispensing [4].

Cost [$/kgH2]

Production

Liquefaction

Transport (liquid truck)

Transport (pipeline-tube trailer)

Station (350 bar)

Station (700 bar)

Station (liquid)

1.32

1.61

0.6

1.7

1.28

1.7

1.3

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Table 7 e Hydrogen cost estimates for conventional vs. thermal compression stations. 350 bar thermal compression station can also dispense 700 bar H2, while 350 bar conventional (mechanical compression) stations dispense only 350 bar H2.

Conventional, liquid delivery þ compressors Thermal compression design

3.

350 bar refueling

700 bar refueling

$4.81/kg

$5.23/kg

$4.81/kg

$5.45/kg

dispensing from a thermal compression station drops to $4.81/kg H2, which is similar to the cost of a conventional station that dispenses 350 bar H2 only. It must be noted, however, that the conventional 350 bar station cannot dispense 700 bar H2, while the thermal compression station dispenses both 350 and 700 bar H2. All that is needed to reduce the cost is a demand for lower pressure H2. Estimated dispensing costs for the thermal compression station assume that evaporated hydrogen is vented to the atmosphere. Finding practical applicability for this hydrogen, e.g. for electricity generation in a fuel cell or internal combustion engine may contribute to reduce the cost of dispensed H2.

Conclusions

This paper reports modeling results for a thermal compression hydrogen fueling station. Instead of pumping hydrogen with expensive and maintenance-prone compressors, the station uses a series of cryogenic pressure vessels that can be fueled with high-density liquid hydrogen from a large instation Dewar, and then pressurized by constant volume heating to enable direct dispensing into vehicles. The concept station has been designed and modeled with a transient thermodynamic model. Vessels in the cascade are thermally insulated and include an internal heat exchanger for rapid pressurization. We model the daily maximum (a summer Friday) of a medium size refueling station (400 kg/day nominal, 475 kg/ day maximum) that can fill 350 bar or 700 bar room temperature vehicles vessels. The transient model indicates that 12 insulated high-pressure vessels operating in parallel are enough to supply the daily demand of 475 kg H2 (8 cars per hour during peak times). A conservative estimate yields $197,520 for the cryogenic high-pressure cascade system, considerably less expensive than today’s compressors and stainless steel cascades and without the need for frequent maintenance or electricity consumption. Some venting losses result from thermal compression station operation, and these increase the cost of dispensed hydrogen. Venting losses are estimated at 16.3% or 34.2% depending on whether there is demand for 350 bar H2 or only for 700 bar H2. Dispensing H2 to 350 bar vehicles enables more complete emptying of cryogenic pressure vessels in the cascade, since these vehicles are assumed to refuel at a lower pressure due to their larger vessel size (350 bar vehicles retain 1 kg H2 at 53 bar while 700 bar vehicles retain 1 kg H2 at 90 bar). The total installed capital cost for a 475 kg/day thermal compression hydrogen refueling station is estimated at about $611,500, an almost 60% cost reduction over today’s refueling station cost. Thermal compression stations also avoid expenses due to maintenance of high-pressure hydrogen compressors. In addition to cost benefits, thermal compression stations offer capacity flexibility: the refueling station capacity can be increased as the market expands by adding extra vessels in the cascade. Hydrogen dispensing costs are quite comparable if H2A economic parameters are used. The cost for 700 bar dispensing is $5.23/kg H2 for a conventional station vs. $5.45/ kg H2 for a thermal compression station. If there is a demand for 350 bar H2 in addition to 700 bar dispensing, the cost of

Acknowledgments This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.

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