Evaluation of a thermally-driven metal-hydride-based hydrogen compressor

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Evaluation of a thermally-driven metal-hydride-based hydrogen compressor Nelson A. Kelly a,*, Robert Girdwood a,b a

Chemical Sciences and Materials Systems Laboratory, General Motors R&D Center, 480-106-224, 30500 Mound Road, Warren, MI 48090-9055, USA b Robert Girdwood Aerotek Inc., 901 Tower Drive, Suite 200 Troy, MI 48098, USA

article info

abstract

Article history:

Currently, the hydrogen storage method used aboard fuel cell electric vehicles utilizes pres-

Received 2 January 2012

sures up to 70 MPa. Attaining such high pressures requires mechanical gas compression or

Received in revised form

hydrogen liquefaction followed by heating to form a high-pressure gas, and these processes

22 March 2012

add to the cost and reduce the energy efficiency of a hydrogen fueling system. In previous work

Accepted 14 April 2012

we have evaluated the use of high-pressure electrolysis, in which hydrogen is generated from

Available online 14 May 2012

water and the electrolyzer boosts the hydrogen pressure to values from 13 to 45 MPa. While electrolytic compression is a novel and energy efficient method to produce high-pressure

Keywords:

hydrogen, it has several limitations at present and will require more development work.

Hydrogen compressor

Another concept is to use hydrogen absorbing alloys that form metal hydrides, in combination

High-pressure hydrogen production

with a heat engine (hot and cold reservoirs), to drive a cyclic process in which hydrogen gas is

Fuel-cell electric vehicle

absorbed and desorbed to compress hydrogen. Furthermore, by using a thermally-driven

Compressor efficiency

compressor, the hot and cold reservoirs can be obtained using renewable energy such as sunlight for heating together with ambient air or water for cooling. In this work we evaluated the thermodynamics and kinetics of a prototype metal hydride hydrogen compressor (MHHC) built for us by a research group in China. The compressor utilized a hydrogen input pressure of approximately 14 MPa, and, operating between an initial temperature of approximately 300 K and a final temperature of 400 K, a pressure of approximately 41 MPa was attained. In a series of experiments with those conditions the average compression ratio for a single-stage compression was approximately three. In the initial compression cycles, up to 300 g of hydrogen was compressed for each 100 K temperature cycle. The enthalpy of the metallicalloy-hydriding reaction was found to be approximately 20.5 kJ per mole of H2, determined by measuring the pressure composition isotherm at three temperatures and using a Van’t Hoff plot. The thermodynamic efficiency of the compressor, as measured by the value of the compression work performed divided by the heat energy added and removed in one complete cycle, was determined via first and second law analyses. The Carnot efficiency was approximately 25%, the first law efficiency was approximately 3e5%, and the second law efficiency was approximately 12e20%, depending on the idealized compression cycle used to assign a value to the compression work, as well as other assumptions. These efficiencies compare favorably with values reported for other thermally-driven compressors. Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

* Corresponding author. Tel.: þ1 586 986 1623; fax: þ1 586 986 1910. E-mail address: [email protected] (N.A. Kelly). 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.2012.04.088

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1.

Introduction

Several major automobile manufacturers, including General Motors, have announced plans for beginning sales of fuel cell electric vehicles (FCEV) powered by hydrogen beginning around 2015 or shortly thereafter [1e5]. This follows extensive real-world testing in demonstration fleets, including “Project Driveway” in which over 100 Chevrolet FCEV have been driven nearly two million miles [1,6]. This initiative toward FCEV is the centerpiece of a major transformation for the automobile, referred to as a “new DNA”, in which the transportation sector is moved from the present mechanical system powered by petroleum to a system energized by hydrogen and powered by electric motors [1]. FCEV have no tailpipe emissions other than water vapor, and hydrogen can be generated from a variety of sources including renewable sources, thus creating a potentially sustainable system. The fuel source and storage system for the first production FCEV for sale will most likely be similar to the latest demonstration fleets. That is, they will be powered by hydrogen generated predominantly from natural gas and the hydrogen will be stored in high-pressure tanks aboard the vehicle. One concern facing the introduction of FCEV is the lack of a hydrogen fueling infrastructure. It is possible that small distributed hydrogen stations, including home fueling [7e10] could help facilitate the introduction of FCEV. One possibility for such distributed hydrogen systems involves the electrolysis of water. Water electrolysis can also be a keystone for large, renewable hydrogen generation sites [11e13]. A major question then becomes, “how can hydrogen be pressurized for storage, including storage aboard an FCEV”? We have studied the production of high-pressure hydrogen using solar energy and a high-pressure electrolyzer because this is one environmentally-attractive way to generate hydrogen for FCEV in the longer term [7e9]. We initially produced hydrogen at over 41.4 MPa (6000 psi) using water electrolysis powered by solar photovoltaic (PV) electricity [7]. Our goal was to extend that pressure to 70 MPa (10000 psi) so that we could fuel GMs latest generation of vehicles that use high-pressure 70 MPa tanks to store sufficient hydrogen to have a range similar to a conventional vehicle. However, we found that our initial system had limitations in that its requirement of a pressure-balanced high-pressure oxygen and hydrogen system was more complicated than we had anticipated [8]. At such high pressures, hydrogen permeated through the electrolysis cell membrane and into the oxygen side of the system where it exothermically combined with the oxygen and caused failure of the elastomeric (non-conducting) hoses exiting the cell. Therefore, we had to reduce the operating pressure of the electrolyzer to approximately 13.8 MPa (2000 psi). Although we still believe that high-pressure electrolysis will eventually be used to directly generate 70 MPa (approximately 10000 psi) hydrogen, it will require further research and engineering to achieve this goal. Therefore, we sought a compressor to boost the pressure of 13.8 MPa electrolytic systems (current technology) back to over 41.4 MPa. One

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attractive possibility was a thermally-driven, hydride-based compressor. Thus, the same type of research that is being conducted to store hydrogen aboard an FCEV could also be used to compress the hydrogen. Also, in keeping with our theme of using renewable energy to drive the generation of hydrogen, such a compressor could be run as a “heat engine” using waste heat, perhaps even waste heat from the solar system where solar heating of PV modules actually degrades the PV output versus cooler modules. Waste heat from the electrolyzer could also be recovered to drive a thermallybased compressor. Due to research collaborations on hydrogen storage with Zhejiang University in Hangzhou China, we decided to use a hydride-based compressor designed and built to our specifications (amount of hydrogen compressed and compression ratio) by Professor Xinhua Wang and co-workers [14e19]. They have studied the use of modified misch metals in order to tune the thermodynamic and kinetic response of the compressor. Their work follows the general idea of combining metals that form stable hydrides along with metals that form unstable hydrides to form alloys that dissociatively absorb hydrogen. By utilizing such combinations, Wang and coworkers have been able to optimize the compressor characteristics with respect to the energetics, the reversibility, and the kinetics of the process. We will hereafter describe our testing of a prototype system built in China and shipped to us for evaluation.

2.

Experimental

2.1. General description of the metal hydride hydrogen compressor (MHHC) system 2.1.1.

System design

The system was designed to our specifications and assembled in China by Professor Xinhua Wang at Zhejiang University. The design was based on previous hydride-based compressors built by Wang and co-workers [15,16,18]. We specified the compression ratio (about three), the design initial and final pressures, and the goal of compressing about 125 g of hydrogen per hour. These specifications were based on the output of our Avalence high-pressure electrolyzer [7] and our desire to boost the 13.8 MPa (2000 psi) pressure output from that system to 41.4 MPa (6000 psi). One of the benefits of MHHC is to easily scale its parameters to fit the user’s requirements. Wang and co-workers have conducted an extensive research and development program to tune and optimize the hydrogen compression alloy utilized in thermally-based hydrogen compressors, as described elsewhere [14e19]. The system sub-units were shipped to us and assembled for testing with minor modifications to the electric drive motors and some non-standard fittings. The system takes hydrogen at about 13.8 MPa (2000 psi) and compresses it to 41.4 MPa (6000 psi) using electrical energy to maintain hot and cold reservoirs to drive the hydriding and de-hydriding reactions of the hydrogen compression alloy. Fig. 1a and b show pictures of the MHHC components set up in the garage in Building 42B at the GM Milford Proving Ground

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Fig. 1 e Pictures showing two views of the MHHC components: a) from left to right; gas control, hydride bed and oil system, gas dryer, and water chiller, and b) components as used in the study, from left to right; water chiller, hydride beds, gas control unit, and high-pressure storage tanks (on floor). The hydrogen source tanks (green) are also shown in the lower picture at the far right. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

(MPG). The system consisted of: 1) two hydride beds, consisting of metal cylinders containing a misch metal alloy, together with heaters for raising the temperature in the hydride beds, 2) hot and cold oil tanks equipped with pumps, tubing, and valves for heating and cooling the hydride beds using flowing oil, 3) a gas control panel for directing hydrogen gas flows and controlling and measuring the system pressure and temperature, 4) a water chiller for cooling the oil in the “cold” oil tank, 5) high-pressure hydrogen storage tanks, and 6) a water removal system for drying the hydrogen before it entered the hydride beds. The water removal system was not needed and will not be discussed further, because we used dry hydrogen supplied from commercial gas bottles (a six pack of cylinders with pressures of about 16 MPa e located on the right in Fig. 1b) in our study. Next we will describe the overall compressor operation, followed by a description of the individual system components. Fig. 2a shows a close-up of the two identical MHHCs located on top of a cart with the oil circulation system below the compressors. Fig. 2b shows a view of the compressors length-wise, with the gas entrance and exit fittings on the front, and oil entrance and exit fittings on the top. Fig. 2c

shows a close up of the cylindrical metal tubes containing the hydrogen compression alloy prior to their insertion into rectangular metal boxes containing the oil thermal fluid. The compressor metal-containment boxes are similar in size, shape, and construction to the metal boxes used in the two high-pressure hydrogen storage tanks located on the floor to the lower right in Fig. 1b. However the compressor boxes were covered with insulating material for safety and to prevent heat exchange with the room. The cylindrical tubes containing the hydrogen compression alloy were made of type 310 stainless steel (density ¼ 0.5 kg/L) and were built to withstand the high pressures reached in the compressor. The cylindrical compression tubes, as well as the box containing them, were custom built. The compressor could be heated in two ways: 1) using internal resistance heaters immersed in an oil heat transfer agent (Therminol 55) inside the compressor box, and 2) using resistance heaters in an oil reservoir containing the Therminol 55 heat transfer agent. A pump circulated hot oil through the compressor when the hot oil reservoir was used to heat the oil. The hot oil tank and each compressor contained a thermocouple to measure the temperature. The temperature was controlled by a proportional temperature controller that regulated the output current to the heaters to attain the desired set point typed in by the operator. A cold oil reservoir used a chiller immersed in the oil reservoir to cool the oil. This proved to be inadequate for the purposes of using the compressor in an efficient cyclic process, as it lacked sufficient cooling capacity to quickly cool a hot compressor, but it was adequate for our evaluation in which rapid cycling was not a concern. The flow of oil through the compressors was controlled by valves that were manually set by the operator.

2.1.2.

Operation of the hydride compressor

The compressor was designed to compress hydrogen by absorbing hydrogen at low temperature (room temperature to about 303 K) and pressure (13.8 MPa) and releasing the hydrogen at high temperature (approximately 403 K) and pressure (41.4 MPa). Using two identical compressors containing the hydrogen compression alloy, the system could be manually operated such that one compressor was being charged with hydrogen at low pressure while the other was desorbing hydrogen at high pressure. A cold oil reservoir, maintained at approximately 303 K was used to cool the compressor and a hot oil reservoir maintained at approximately 403 K was used to heat the compressor. Alternatively, an internal heater in the compressor could be used for compressor heating without circulating the hot oil. Thus, the compressor could be used in a continuous process to compress hydrogen from our solar electrolysis system [8] from 13.8 MPa to 41.4 MPa by continuously charging and discharging the two hydride-based compressors. However, we did not use the system in that manner because we had discontinued the operation of the solar electrolysis system by the time the compressor was operational. Rather, we evaluated the compressor as a stand-alone unit, in order to determine its operational characteristics, and in particular, its efficiency. More detail on the individual system components will now be given.

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Fig. 2 e Pictures showing the MHHC and key parts: a) compressors and oil circulations systems, b) two compressors and insulation covering the rectangular box containing the high-pressure metal tubes, and c) the high-pressure cylindrical metal tubes containing the metal hydride powder and Al heat transfer agent prior to insertion into the rectangular metal box.

2.2.

Details on system components

2.2.1.

Hydride compressors and oil reservoirs

The hydride compressors were built in China based on the design of Wang and co-workers [15,16,18]. They consist of cylindrical stainless steel tubes filled with a powdered misch metal and contained within a rectangular box filled with oil. There are 16 cylinders arranged in a 4  4 matrix, as shown in Fig. 2c. The cylinders had an outer diameter of 38 mm, an inner diameter of 26 mm (wall thickness of 6 mm) and a length of approximately 900 mm. An electrically-powered, resistance heater is also enclosed within the box and it was used to heat the oil (heat transfer fluid) and thus the stainless steel cylinders containing the hydrogen compression alloy. Hoses were connected to the metal box to circulate either cold or hot oil through the box so that the tubes containing the special hydrogen absorbing alloy were cooled or heated, respectively. In a compression cycle, the compressor is charged with 13.8 MPa hydrogen when room temperature oil was circulated through the interior of the metal box, and then the heated oil was circulated through the box to raise the hydrogen temperature to 403 K and the pressure to 41.4 MPa. Therminol 55 oil was used as the heat transfer fluid. The properties of this oil are listed in Table 1. Table 2 lists the dimensions and capacities of the two MHHC. The 16 metal cylinders were connected to the gas control panel to allow charging with 13.8 MPa hydrogen or

discharging 41.4 MPa hydrogen. The volume of the 16 metal cylinders is approximately 4.1 L, and they contained about 27 kg of the misch metal alloy [21]. By measuring the change in the height of the oil in the cold and hot oil tanks, the volume of the tanks (from their physical dimensions), and the change in oil height in the tanks when they filled the volume of the compressor box (external to the cylindrical tubes), we determined the total internal volume in the compressors. Some of nominal values in Tables 2 and 3 were provided by Prof. Wang [21]. The total gas free volume of the compressor was determined using volume expansion of helium in a de-hydrided compressor to a known volume. Table 3 lists the dimensions and capacities of the cold and hot oil circulation systems that were used to cool and heat the compressors. The hot and cold oil systems have separate

Table 1 e Properties of the oil used for control of the compressor temperature [20]. Oil properties Type Density at 300 K, g/ml Density at 400 K Cv at 300 K, kJ/(kg K) Cv at 400 K, kJ/(kg K)

Therminol 55 0.867 0.800 1.93 2.29

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Table 2 e Dimensions, mass, and nominal capacities of the MHHC based on our measurements and reference [21]. Height, m Width, m Length, m Box volume (excluding cylinders), L Cylinder exterior volume, L Oil volume, L Metal cylinder total internal volume, L Metal cylinder total free gas volume, L Metal cylinder mass (stainless steel) kg Stainless steel heat capacity, kJ/(kg K) Hydrogen absorbing alloy mass, kg Cv of hydride material, kJ/(kg K)

0.178 0.1905 1.080 36.6 18.1 17 7.6 4.1 68 0.50 27 0.50

reservoirs and pumps that circulate the cold or hot oil. The hot reservoir has an electrically-driven (resistance) heating element. The temperature was controlled by a Yudian temperature-control system in which the temperature is measured with a type E thermocouple, and depending on the temperature measurement and the digital set point, the system supplied current to the heater. Each compressor had a similar resistance type heater and temperature controller. The cold oil system had a coolant coil that circulated chilled coolant (water and antifreeze) through coils to cool the oil in the reservoir. The thermocouple in each compressor was calibrated by comparison to traceable temperature gauges that were in turn calibrated in an ice bath and in boiling water at a known atmospheric pressure.

2.2.2.

Hydrogen compression alloy

The hydrogen storage alloy composition has the composition (MmCa)Ni4.95Al0.05 where Mm as a cerium-rich misch metal composed of 48.6% Ce, 23.8% La, 16.8% Nd, 8.1% Pr, and 2.7% other metals, on a mass percentage basis [17]. Wang and coworkers [16,17] discuss testing a range of such alloys for a compressor in order to optimize the properties of the alloy with respect to the thermodynamics, kinetics, and durability for use in a compressor. The relationship between the compressor characteristics and the alloy pressure plateau, kinetics, reaction enthalpy (DH), and percentage hydrogen storage for the best alloy is discussed elsewhere by Wang and co-workers [15,16,18]. In addition to the alloy, the 16 cylindrical tubes contained aluminum for better heat transfer. The aluminum was inert with respect to interaction with

Table 3 e Dimensions and nominal capacities of the cold and hot oil tanks.

Height, m Width, m Length, m Volume, L Oil height, m Oil volume, L

Cold oil tank

Hot oil tank

0.495 0.216 0.445 47.5 0.419 40.2

0.495 0.279 0.432 59.8 0.330 39.8

hydrogen [19] and its amount was small relative to that of the hydride material [21].

2.2.3.

Gas control panel

The gas control panel was used to direct the gas flows between the hydrogen source cylinders (regulated to 13.8 MPa), the hydride compressors, the high-pressure storage tanks, a calibrated volume, and a gas venting system on the roof of the building. Ball valves and manifolds made of 1/8” stainless steel tubing and stainless steel Swagelock fittings were used to control the gas flows. The gas control panel contained the gauges for measuring pressure in the system. Four gauges for measuring the pressure in each compressor and in each of the high-pressure tanks were mounted on the gas control panel. The larger and more accurate gauges were used to measure the pressure in the compressors. We added a very accurate digital gauge to the gas control panel to more precisely and easily measure the compressor pressure. It could be attached to either compressor by simply switching the fittings from the gauge. The gas control panel also contained the Yudian temperature controllers used to measure, and control the compressor and hot oil tank temperatures. In addition, an electrical power meter was added to the gas control panel to measure the electrical energy consumed by the heaters in the compressors and the hot oil tank.

2.2.4.

Cold oil chiller

A chiller (KYKY Technology Development Ltd, model LS45A) containing a water/antifreeze coolant was used to cool the oil in the “cold” reservoir. In practice, this was only used to cool down the system to prepare for another experiment, because we were not using the system as a process compressor, as it was originally designed. In continuous cyclic operation, the cold and hot oil, at 303 K and 403 K, respectively, would alternatively be circulated through each compressor to continuously compress hydrogen at a pressure of 13.8 MPa to a pressure of 41.4 MPa, and the 41.4 MPa hydrogen would then be stored in one of the high-pressure tanks. However, we only used the chiller to speed up cooling so we could perform another test.

2.2.5.

High-pressure tanks

The two high-pressure tanks were designed to receive and store the high-pressure hydrogen from the compressors. Their design and construction was very similar to the boxes and cylindrical tubes used in the hydride compressors, i.e., 16 cylindrical metal tubes contained in a rectangular box. The tanks were connected to the gas control panel with 1/8” stainless steel tubing and Swagelock fittings. The volume of the tanks was 12.4 L as determined by filling them with hydrogen at a known pressure, expanding each one to a known volume, and measuring the final pressure. In addition we utilized a gas cylinder with a volume of 42.2 L to store hydrogen compressed by the MHHC. This known volume was used to determine the mass of hydrogen compressed in a cycle of the compressor using the BeattieBridgeman equation of state for hydrogen [22] to calculate hydrogen mass from the measured pressure and cylinder volume.

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2.2.6.

Source hydrogen

Airgas (Radnor, PA) UHP hydrogen (999.99% pure) was used to supply the “low-pressure” hydrogen for the compressor system. Both low pressure tanks (approximately 15.9 MPa) and high-pressure (approximately 41.4 MPa) tanks, regulated down to a lower pressure of 13.8 MPa, were used as source gas to supply the compressor.

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However, for the pure resistive load of the compressor heaters, the reactive power was zero, and all of the power was real, so that the power in watts was the product of volts times amperes. The accuracy of this system for electrical energy measurement (kWh) was stated to be 0.5% by the manufacturer.

2.3.4. Measurement of the density and pressure-composition of the hydrogen pressure alloy 2.3. Temperature, pressure, and electrical measurements 2.3.1.

Temperature measurement and control

Temperatures were measured using type E thermocouples that were connected to Yudian AI-808 temperature controllers made in China. There were three main thermocouples and three Yudian controllers to measure and control the temperatures in each compressor and in the hot oil tank. Depending on the temperature set point on the Yudian temperature controller and the measured oil-bath temperature, the Yudian system sent electrical power to a resistance heater to attain the temperature set point. As the set point was approached, the Yudian temperature controller reduced the power output to avoid over shooting the set point. All three type E thermocouples were removed from the compressors and hot oil tank and placed in a thermally insulated container containing an ice water slush along with two reference thermometers (type K thermocouple and VWR thermistor) for calibration. The reference thermometers were calibrated in an ice slush and boiling water prior to calibration of the Yudian thermocouples. Then, all five temperature probes were put in the hot oil tank and compared at temperatures up to approximately 400 K, so that the accuracy of the temperature measurements was assured over the range of interest to our study.

2.3.2.

Pressure measurement

Initially, the compressor pressure was measured using Bourdon-tube analog gauges with a pressure range from 0 to 60 MPa that were made in China (HongQi, MC gauge) and mounted on the gas control panel. There was one gauge for each compressor. For convenience, as well as to improve the accuracy of the pressure measurements, a NIST traceable General Electric Druck DPI 104-IS digital pressure gauge was added to the system. The digital gauge had a full scale reading of 70 MPa with a resolution of 0.0001 MPa and an accuracy of approximately 0.1 MPa. Both of the analog gauges were compared to the digital gauge, and their readings were verified. Most tests subsequently used the digital gauge for all pressure measurements. Based on NIST-traceable calibration over the range from 0 to 70 MPa, the pressure measurements were accurate to 0.035 MPa.

2.3.3.

Electrical energy measurement

The electrical energy consumed by the heaters in the compressors was measured with an ION model 7330 PowerLogic meter (Schneider Electric). The voltage and current measurements were compared in a spot check to a calibrated Fluke multi-meter for verification of the ION meter readings. The ION meter measured both real and reactive power.

A sample of the metal hydride material in the compressor was used to determine the density of the alloy. A coarse sample of the alloy was first ball milled to a fine powder. The density of the powder was measured using a Micrometrics (Norcross, GA) AccuPyc 1340 gas displacement pycnometry system. A sample of the ball-milled alloy was placed in an Advanced Materials Corporation (Pittsburg, PA) pressure-composition isotherm measurement system. The sample was first degassed at high vacuum. Then pressure composition isotherms were obtained at temperatures of 309.3, 312.0, and 328.0 K in order to obtain the enthalpy of the alloy hydriding reaction, as will be explained later.

3.

Results and discussion

3.1. Theoretical analysis of the operation of the thermally-driven MHHC 3.1.1.

Metal hydride-based compression

The MHHC operates by using heat to do the work of compression. Ideally, the heat could come from some renewable process, such as solar or geothermal energy, or even waste heat, but for the purposes of testing we used grid electricity and resistance heaters to supply the heat. This allowed us to carefully control the temperature and pressure and measure the compressor response to temperature changes, as well as to determine its thermodynamic efficiency. The compressor operates as follows. The steel tubes in the compressor contain a specially formulated porous metal alloy, M, that can absorb (called chemisorption) hydrogen gas in an exothermic reaction to form a metal hydride. The reaction can be written: M þ n H2 4MH2n þ heat

(1)

The reaction is reversible, and depending on the temperature, the equilibrium can be to the left (M and H2) or to the right (MH2n). The reverse reaction is endothermic, so heat (energy) must be added, and we did that in our study with electrical resistance heating of the oil surrounding the compressor high-pressure cylinders. At low temperature, the fully hydrogen-saturated hydride is favored (the products on right hand side of the chemical equation), while at high temperature the metal and hydrogen gas are favored (the reactants on the left-hand side of the equation). The heat added is equal to the heat of reaction plus heat to heat up the metal tubes and the oil (thermal fluid) in the compressor. The absorption mechanism for compression in a metal hydride compressor has been thoroughly reviewed [23,24] and

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we will briefly summarize it. First, at a constant low temperature (Tl), gaseous hydrogen dissolves into the metal matrix as atoms (the so-called a-phase) and forms a solid solution. As the hydrogen pressure increases, the concentration of Hatoms in the metallic matrix increases, and a new and separate phase (the so-called b-phase) begins to form. The lattice structure is that of a metal with the hydrogen atoms at interstitial sites within the lattice. Equilibrium is reached between the a and b phases at the low temperature and heat is released during the formation of the hydride. The system is then closed off, the temperature is increased to the high value (Th), and the high pressure is attained. The a phase is once again formed as hydrogen is released from the hydride at high temperature. The pressure of hydrogen gas in equilibrium with a hydride-forming alloy varies with the system temperature according to the Van’t Hoff equation: lnðPd Þ ¼ DH=ðR  TÞ  DS=R

(2)

where DH is the standard enthalpy change and DS is the standard entropy change of the reaction shown in Eq. (1), Pd is the absorption or desorption plateau pressure at temperature T, and R is the gas constant. When the hydride absorbs hydrogen at a low temperature Tl and low pressure Pl, and desorbs hydrogen at a high temperature Th, then hydrogen with higher pressure, Ph, is obtained. The operating principle of a metal hydride hydrogen compressor (MHHC) is based on the property of the metal hydride system expressed in the Van’t Hoff equation. Compared with mechanically driven compressors, MHHCs have many merits such as no moving parts in the compressor, no vibration, no noise, and, because the hydrogen storage alloy

has the characteristic of selective absorption of hydrogen, an MHHC is a combination of hydrogen compressor and a hydrogen purifier [17]. The compressor energetics can be understood with reference to Fig. 3 showing the four basic steps in a compression cycle. The starting point, labeled state#1, is a compressor with a temperature of 303 K and charged with hydrogen at a pressure of 14 MPa. The compressor is heated to achieve a temperature of 403 K, and the pressure in the compressor rises to 41 MPa; this is state#2. This high-pressure gas is expanded to a storage tank, where it increases the pressure in the tank to a value greater than 14 MPa (the storage tank would be charged with 14 MPa hydrogen prior to receiving higher pressure hydrogen from the compressor); this is state#3. The pressure in the compressor is at X MPa, where 14 MPa < X < 41 MPa. Next the compressor is cooled to 303 K, and the pressure drops to a low value Y, where Y < 14 MPa; this is state#4. Finally, hydrogen at 14 MPa is added to the compressor, and the compressor is continuously cooled to remain at 303 K as heat is generated by the exothermic absorption of hydrogen into the metal to form the metal hydride. The compressor returns to state#1 and is ready to begin another compression cycle. The compression ratio, Rp, of the compressor is: Rp ¼ Ph =Pl

(3)

The cycle shown in Fig. 3 can be repeated over and over to continuously compress hydrogen from 14 MPA to 41 MPa. Our MHHC was designed with two independent compressor systems in order to double its capacity if it were used in a continuous process to compress hydrogen, as discussed earlier.

Fig. 3 e Simplified schematic representation of the distinct states in the cyclic operation of the MHHC.

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3.2. Pressure-composition curves of the alloy used in the MHHC and the enthalpy change for the hydrogen absorption reaction A ball-milled sample of the alloy was used to determine the pressure-composition isotherm for the alloy used in the compressor.First, the density of the alloy was measured with a pycnometry system. A density of 8.22  0.01 g/milliliter was obtained. Next the sample was degassed at high vacuum and room temperature, and pressure-composition isotherms were determined at temperatures of 309.3, 312.0, and 328.0 K using an Advanced Materials Corporation (Pittsburg, PA) pressurecomposition isotherm measurement system. Fig. 4a shows an example of a pressure-composition isotherm at 309.3 K. Notice the pressure plateau occurring between about 0.3 and 1.1H2 percent by mass or weight. A pressure plateau was not observed in an experiment conducted at 348.8 K. This indicates that this temperature was above the critical temperature where both the a and b phases coexist in equilibrium [23]. A Van’t Hoff plot (natural logarithm of the plateau pressure versus the reciprocal of the isothermal temperature in K) for the three pressure-composition isotherm experiments is shown in Fig. 4b. The slope in the Van’t Hoff plot (2462 K) multiplied by the gas constant R (8.314 J/mole K) is equal the enthalpy change of the chemical reaction describing the adsorption of hydrogen by the metal hydride material, Eq. (1).

a

b

Fig. 4 e Plot of: a) pressure-composition isotherm for hydrogen absorption at 309.3 K, and b) Van’t Hoff plot of the plateau pressure and temperature from three pressure composition isotherms. The midpoint of the pressurecomposition isotherm, shown by the dotted vertical and horizontal lines in the upper figure, was used as the plateau pressure in the lower figure.

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This gives an enthalpy change, DH, of 20.5 kJ/mol. This is in good agreement with an estimate of the reaction enthalpy change for the metal hydride material chosen in the design of the compressor (approximately 24.5 kJ/mol) by Wang and coworkers [22]. Based on the H2 mass percent measured at the pressure plateau for the test at 309.3 K (0.75%) and the mass of hydride material in the compressor (Table 2, 27 kg) we calculate that approximately 200 g of H2 is absorbed at equilibrium for the MHHC at that temperature. This is in reasonable agreement with the estimate in section 3.3 at a slightly lower temperature (303 K) and higher pressure (14.8 MPa), where we would expect that more H2 would be absorbed. An important phenomenon behind the operation of our MHHC is the very large difference between the volume of the absorbed (bound) and desorbed hydrogen gas to achieve a large compression ratio for the smallest temperature change. For example the change in volume for hydrogen bound in the alloy, with a density of approximately 8 g/ml, changing to a gas at 13.8 MPa, with a density of approximately 0.01 g/ml at room temperature, is about a factor of 800.

3.3.

Experimental evaluation of the MHHC operation

3.3.1.

Temperature and pressure rise tests

A series of tests were performed to determine the compression ratio of the compressor. The first series of tests had an initial pressure of approximately 13.7 MPa and an initial temperature of approximately 300 K. The temperature of the compressor was increased to approximately 410 K using either the heater in the compressor to just heat the oil, or alternatively, using hot oil circulated through the compressor using the hot oil reservoir. The pressure increased to approximately 41.6 MPa due to the heating. Fig. 5a shows the results from a typical experiment. For this test, circulating hot oil was used, resulting in a more uniform heating of the compressor than using the compressor internal heater. The hot oil system had much more mixing and turbulence than the more stagnant compressor internal heating system. The initial temperature and pressure were 296.7 K and 13.28 MPa, respectively. Over a time period of 410 min, the temperature was increased to 401.4 K in 10 K increments. Each 10-min temperature was achieved by dialing in the temperature on the Yudian heater control and waiting until a stable temperature and pressure was achieved before moving to the next temperature-pressure point. Depending on whether the compressor internal or the hot-oil circulation system was used, it took roughly 5e15 min to reach the temperature set point. A lesser time was taken using the internal heater option to heat the compressor because that method less oil was heated (17 L vs. 36 L) and there was no heat loss in the lines, circulating pump, or sides of the metal oil reservoir. A rest period of over a half hour at each temperature setting resulted in a stable pressure reading, indicating that the compressor had reached an equilibrium state, both with respect to the heating system and the chemistry occurring in the metal hydride/hydrogen system. For the example shown in Fig. 5a a final pressure of 39.00 MPa at 401.4 K was attained, yielding a compression ratio for this experiment of 2.94. Fig. 5b shows a plot of the data that in a Van’t Hoff format, i.e., ln(P) versus the inverse of the

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a

b

Fig. 5 e Data from one temperature-pressure rise test in which a compressor charged with 13.8 MPa H2 at low temperature (297 K) was heated to 401 K: a) plot of T and P versus time, and b) Van’t Hoff type plot of the data.

Van’t Hoff type plots for the data from both compressors over a time period of several months. The slope started out with a value less than 1400, but after two or three tests it attained a relatively constant value between 1240 and 1280, indicating some conditioning of the metal hydride was occurring. All of the subsequent tests discussed in this report utilized a “conditioned” compressor. The average compression ratio for the tests in Table 4 was approximately three. A second series of seven experiments was conducted with a lower initial pressure of about 8.4 MPa but with similar initial and final temperatures to those of the higher initial-pressure experiments in Table 4. The initial conditions for each experiment and the results are listed in Table 5. The final pressure in these tests was approximately 33.1 MPa and the compression ratio was 4.0. It is interesting that the change in initial pressure also yielded a significant change in the slope of Van’t Hoff plots of the data. Comparing the higher initial pressure tests with the lower initial pressure tests the slope went from about 1260 K to 1570 K. If these plots could be used like a “true” Van’ Hoff plot to determine the DH of the reaction (see Eq. (2)), then the slope should have been independent of the initial pressure. The change in slope for the two different initial pressures indicates that the slopes of the Van’t Hoff plots in Tables 4and 5 cannot be used to derive DH for the reaction in Eq. (1). Also, the DH corresponding to a slope of 1260 (mean value in Table 4) is 10.5 kJ/mol; this is too low by a factor of two compared to the DH for hydrogen absorption in an alloys reported in the literature of 20e30 kJ/mol [15,17].

3.3.2. absolute temperature. The data appears to fit the Van’t Hoff format with a resulting slope of 1251 K. Table 4 shows the results of a total of 15 similar experiments. The data in Fig. 5a and b is from test 6 with compressor 2 in Table 4. Fig. 6 shows a temporal plot of the slopes for the

Compression due to gas heating

Because some of the gas compression is simply due to gas heating, we calculated how much the pressure would be increased by increasing the gas temperature from 303 K to 403 K. The NIST web site [25] was used to calculate the change in pressure for an isochoric process with a starting temperature of 303 K and an ending temperature of 403 K for a starting

Table 4 e Compressor pressure rise tests, initial pressure, Pi [ 13.7 ± 1.2 MPa. Compressor 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 Mean of 10 testsb

Test

Tl, K

Th, K

Pl, MPa

Ph, MPa

1 2 3 4 5 6 7 1 2 3 4 5 6 7 16

313.8 293.8 299.4 296.3 294.5 313.4 293.6 296.6 297.0 304.7 302.1 293.8 296.7 299.8 302.4 300.3.1  5.8

422.2 423.2 413.2 413.2 413.2 400.7 391.4 413.2 412.7 412.9 413.2 413.2 401.4 413.6 388.9 408.3  8.5

12.50 11.60 13.50 13.00 12.60 16.50 12.85 10.20 11.80 13.50 13.40 12.50 13.28 13.83 14.76 13.7  1.2

41.50 44.60 41.40 40.60 40.70 40.80 36.36 40.70 41.40 41.60 41.40 42.70 39.00 41.66 37.82 40.8  1.4

Comp. ratio, Ph/Pl 3.32 3.84 3.07 3.12 3.23 2.47 2.83 3.99 3.51 3.08 3.09 3.42 2.94 3.01 2.56 3.0  0.3

Slopea, K 1432 1328 1228 1217 1222 1291 1219 1518 1370 1357 1278 1295 1251 1214 1259 1261  45

a The slope is from a linear regression of ln(P) versus 1/T. b From the 15 tests, the first two tests for compressor #1 and the first three tests for compressor #2 were excluded because the slope was changing during this initial “break in” period. The  is the standard deviation computed from the 10 tests.

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Fig. 6 e Temporal trend in the slope of Van’t Hoff type plots, ln(P) versus the inverse of the absolute pressure of the compressor, for the pressure-temperature rise tests with an initial pressure of approximately 13.7 MPa (data from Table 4).

pressure of 13.8 MPa. The result was that heating the gas at constant volume increased the pressure from 13.8 MPa to 18.4 MPa (about a 33% increase). The compression ratio calculated based on Gay-Lussac’s law, with Tl ¼ 303 K and Th ¼ 403 K gave the same compression ratio Th/Tl ¼ 1.33, as the NIST calculator. For the MHHC the compression ratio was about 2.85 for a Tl of 303 K and a Th of 403 K. There is over a factor of two improvement in using the hydride compression versus simple gas heating compression. Thus, most of the pressure increase to reach a pressure of 41.4 MPa is due to the de-hydriding reaction.

3.3.3.

Mass of hydrogen absorbed by the MHHC at Tl

We determined how much hydrogen the compressor absorbed at the lowest temperatures used in our study (room temperature of about 296 to about 303 K). This was accomplished by adding hydrogen at a known pressure to an isothermal compressor using a precise regulator connected to a 41.4 MPa H2 source tank of known volume. Prior to the test, the hydrogen in the compressor was removed by heating the compressor to 423 K, venting the hydrogen to a pressure of about 0.1 MPa (approximately atmospheric pressure), cooling the compressor overnight to room temperature, heating it to

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423 K a second time, and venting it to about 0.1 MPa at 423 K. When the compressor cooled to room temperature, the pressure was less than 0.1 MPa each time. Hydrogen was then added in 1 MPa pressure steps from 1 MPa to 10 MPa, and the pressure remaining in the known-volume cylinder at equilibrium was measured. From the pressure change for the 1 MPa pressure increment, measured after the compressor had reached equilibrium at the given temperature, the Beattie-Bridgeman equation of state [22] was used to compute the mass of the hydrogen for the measured pressure change in the known-volume cylinder. The results of the two tests are shown in Fig. 7a and b. Hydrogen was absorbed in a monotonically increasing amount with pressure up to about 6 MPa, after which there was very little increase in the amount of hydrogen absorbed. The mass of hydrogen absorbed was about 370 g. This can be considered the mass of hydrogen that the compressor can absorb. Based on the amount of hydride in the compressor (27 kg), the hydrogen weight percent of the system at this point is about 1.4%. The pressure plateau at about 6 MPa is an indication that the a to b phase transition is occurring continuously at that pressure for a compressor temperature between 293 K and 303 K. This is consistent with the earlier pressure plateau determined earlier using an apparatus designed to make isothermal pressure-composition measurements in section 3.2.

3.3.4. Mass of hydrogen compressed by the MHHC working between Tl and Th We determined the mass of hydrogen compressed in cyclic operation of the MHHC in charging it at low temperatures and pressures, Tl and Pl, respectively, heating it to reach Ph and Th, and then discharging it to a storage tank. The BeattieBridgeman equation of state, together with the known volumes of the charging cylinders and storage tank, were used to calculate the mass of hydrogen from the pressure measurements. Table 6 shows the results of a series of seven such tests that were used to increase the pressure in the storage tank to nearly 39 MPa. In each test the compressor was charged with hydrogen at about 14 MPa at a low temperature, Tl, of about 296 K and was discharged at a high temperature, Th, of about 403 K, followed by a second discharge at an even higher temperature of 422 K. In the first such test, referred to as cycle 1, 354 g of hydrogen was absorbed by the compressor at a temperature of 292.8 K and a pressure of 13.17 MPa. At a temperature of 398.4 K, 321 g of H2 was desorbed into the

Table 5 e Compressor pressure rise tests, initial pressure [ Pi [ 8.4 ± 0.8 MPa. Compressor 2 2 2 2 2 2 2 Mean of 7 tests

Test

Tl, K

Th, K

Pl, MPa

Ph, MPa

Comp. ratio, Ph/Pl

Slopea, K

8 9 10 12 13 14 15

302.2 294.9 294.4 306.3 296.9 297.4 294.1 298.0  4.6

406.9 396.7 397.8 398.0 407.5 403.3 413.3 403.3  6.2

9.57 8.78 7.77 8.84 8.50 7.70 7.29 8.4  0.8

37.64 31.84 31.63 30.53 36.24 31.13 33.10 33.1  2.7

3.93 3.63 4.07 3.45 4.26 4.04 4.54 4.0  0.4

1563 1506 1583 1623 1558 1564 1573 1567  35

a The slope is from a linear regression of ln(P) versus 1/T.

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Weight percent of H2 in hydride

H2 added to compressor, grams

a

Hydrogen gas pressure, MPa

Weigh percent of H2i in hydride

H2 added to compressor, grams

b

Hydrogen pressure, MPa

Fig. 7 e Results of tests to determine the mass of hydrogen absorbed into the metal hydride beds as a function of the hydrogen pressure at: a) 303 K, and b) 293e296 K.

storage cylinder, while at 419.5 K (cycle 1a) even more hydrogen was released, so that ultimately 347 g of hydrogen was compressed in cycle 1. The pressure in the storage cylinder increased from 0.1 MPa to 11.04 MPa as the result of cycle 1. In cycle 2 the compressor was again charged at low temperature and pressure, shown in Table 6, such that 347 g of H2 was added to the compressor. The compressor was heated

to 410 K to attain an internal compressor pressure of 43.0 MPa, and this was then expanded to the storage cylinder that was at a pressure of 11.04 MPa. This added 237 g of hydrogen to the storage cylinder. Further heating the compressor to 419.3 K (cycle 2a) resulted in a total mass of hydrogen of 294 g transferred to the storage cylinder out of 347 g initially added to the compressor. This is 85% of the H2 stored in the compressor at the low temperature, Tl. The pressure in the storage cylinder after two compression cycles was 20.9 MPa. In cycle 3 in Table 6, 239 g of hydrogen was added in the compressor charging. Heating the compressor to 408 K led to the release of in 83 g of hydrogen into the storage cylinder. Increasing the temperature further to 422 K (cycle 3a) increased the hydrogen transfer to 153 g, and a final pressure in the storage cylinder of 27.26 MPa. In cycles 4, 5, 6, and 7, successively lesser amounts of hydrogen were compressed by cycling the compressor as shown in Table 6. This was to be expected, and would occur with a mechanical compressor, as the receiving or storage cylinder pressure approached the compressed gas pressure released by the compressor. Although the compressor was able to reach a final pressure of nearly 39 MPa in seven cycles, in a practical thermal compressor it would be advantageous to have a second stage that would utilize gas at the pressure at the end of the third cycle (27.3 MPa) as the first cycle of a second compression stage at low temperature.

3.3.5.

Compressor kinetics

In order to determine how fast the compressor reached the equilibrium pressure after a sudden temperature increase, an experiment was performed in which the compressor temperature was rapidly increased and the pressure was recorded once per minute to follow its response to the temperature change. Fig. 8 shows the temperature and pressure for compressor #2 over the test period of interest. The initial temperature and pressure in compressor #2 were 306 K and 14.6 MPa, and the compressor was allowed to equilibrate at this temperature for about 15 min. Meanwhile, the

Table 6 e Absorption and desorption cycling of the compressor to build up pressure in the storage cylinder. Charge compressor

Discharge compressor

Storage tank P

Variable

Tl

Pl

H2 mass

Rp

Th

Ph

H2 mass

P, initial

P, final

Units

K

MPa

grams

Ph/Pl

K

MPa

grams

MPa

MPa

292.8

13.17

354

3.1

40.64

13.02

347

3.3

298.2

14.68

239

2.8

295.8

15.36

153

2.7

294.6

15.09

79

2.8

294.5

14.83

56

2.8

296.1

14.48

59

2.8

321 347 237 294 83 156 55 86 41 70 25 52 27 53

0.10

296.9

398.4 419.5 410.2 419.3 408.4 422.0 403.6 419.9 397.5 423.1 396.7 423.3 407.8 427.7

10.09 11.04 19.00 20.90 24.22 27.26 29.64 31.02 32.90 34.25 35.47 36.74 37.61 38.82

Cycle 1 1a 2 2a 3 3a 4 4a 5 5a 6 6a 7 7a

42.99 41.00 41.00 41.83 41.10 41.2

11.04 20.90 27.26 31.02 34.25 36.74

Equilibrium State #2

Equilibrium State#1

T=363 K P=27.7 MPa

T=306 K P=14.6 MPa

Compressor pressure, MPa

Compressor temperature, K

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Run time, minutes Fig. 8 e Results of a kinetic experiment conducted to determine the response of the compressor pressure to a sudden change in temperature.

temperatures in the hot oil tank and compressor #1 were increased to 403 K to generate the maximum possible amount of hot oil possible with our system; both the heater in compressor #1 and the heater in the hot oil tank (and both Yudian T-controllers) were used for this heating. The heated “cold” oil was drained from compressor #2 into the cold oil tank. Then, about 15 min into the test, the hot oil in the reservoir was circulated through compressor #2. The heater in the hot oil tank was turned off at this point, so no further electrical energy was added to the system. The temperature in compressor #2 rapidly increased to 374 K, and then slowly fell as the metal parts in the compressor absorbed heat from the hot oil. The pressure increased from 14.6 to 21.5 MPa in about 7 min in response to the temperature increase. At that point the hot oil pump was turned off, and the hot oil that was prepared and saved in compressor #1 (403 K oil) was drained into the hot oil tank. At about 23 min into the test, the hot oil was circulated through compressor #2, resulting in a rapid 21 K increase in the temperature (from 343 K to 364 K). The pressure responded over a period of about 7 min, increasing from 21.5 MPa to 25.9 MPa. At this point it was clear that the equilibrium temperature attained in compressor #2 by adding the hot oil was about 363 K, so the temperature controller for the hot oil bath was set at this temperature to maintain this condition at a time of about 30 min into the test. At this constant temperature, the pressure in the compressor continued to slowly rise to a value of 27.7 MPa at the end of the test period shown in Fig. 8. The temperature change for a 15-min time period from 15 min to 30 min in Fig. 8 was 57 K (306 Ke363 K), and the pressure increase was 11.2 MPa (14.7 MPae25.9 MPa). The pressure continued to increase slightly up to the 50-min mark. However, the pressure reached approximately 92% of 50-min equilibrium value at the 30 min point of the test. Thus, from a practical standpoint, the compressor has fast kinetics, and the equilibrium chemical states are attained just slightly slower than equilibrium temperature states (the temperature changes for the metal tubes and metallic hydride in the compressor). Based on the results of this test, we conclude

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that it takes about 15 min to get the compressor metal tubes, and the metallic hydride within the tubes, heated from 303 K to 363 K. For the 10 K temperature increments in the tests in section 3.3.1 (shown, for example, in Fig. 5), the compressor heaters limited the rate at which the compressor temperature could be increased, and the measured pressures for each for the temperature plateaus attained for each 10 K temperature increment, were equilibrium values.

3.3.6.

Compressor energetics

From Fig. 3, we can see that there are three places where energy must be added or removed; 1) heat Q1 is added in going from state#1 to state#2, 2) heat Q2 is removed in going from state#3 to state#4, and 3) heat Q3 is removed in going from state#4 to state#1. The total heat energy we must account for in operating the compressor over one complete cycle, Qtot, is the sum of the absolute values of Q1, Q2, and Q3. Qtot ¼ jQ1 j þ jQ2 j þ jQ3 j

(4)

First, we will consider the heat Q1 that must be added in going from state#1 to state#2. This heat does two things: 1) it heats the compressor, including the stainless steel cylinders and the hydride material, by 100 K, and 2) it provides the energy to de-hydride the alloy, driving the reaction shown in Eq. (1) to the right (M þ nH2). Note: the oil in the compressor is simply a heat transfer medium, and in cyclic operation the oil would initially be cooled to Tl in the cold oil tank and initially heated to Th in the hot oil tank, so we will not consider this energy in our analysis, and will focus on the energy needed to heat the metal tubes and hydride material as contributors to Q1. Heating the compressor requires sensible heat, Qs, that causes a temperature change, as well as latent heat, Ql, that drives the de-hydriding reaction: Q 1 ¼ Qs þ Q l

(5)

The sensible heat can be estimated from the mass of the materials in the compressor, their heat capacities, and the temperature change (100 K). Qs ¼

X

ðmi  Ci Þ  DT

(6)

where mi is the mass of component i, Ci is the heat capacity of component i, the sum is over the components that are heated, and DT is the temperature change of the system. From Table 2, the total mass of stainless steel in the 16 compressor cylinders containing the alloy and the metal jacket or box containing the cylinders is 66 kg, and the mass of hydride material is 27 kg. The amount of Al added to the powdered alloy for heat transfer was small [21], and was neglected in this calculation. The heat capacities of the metal in cylinders and alloy are both approximately 0.5 kJ/(kg K) [18,21]. Based on these quantities, the heat energy needed to increase the compressor temperature by 100 K, Qs, is 4.65 MJ or 1.29 kWh. The latent heat, Ql, needed to de-hydride the MH2n alloy shown in Eq. (1) is equal to the heat of the reaction, DH times the number of moles of hydrogen that is desorbed, nH2 Ql ¼ DH  nH2

(7)

From the experiments on the pure alloy discussed in section 3.2.1., DH is 20.5 kJ/mol, and from data for the first

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compression cycle in Table 6 and discussed in section 3.3.3., the mass of hydrogen desorbed at 398 K is approximately 320 g or 159 mol. The latent heat needed, Ql, is thus 3.25 MJ or 0.90 kWh. In addition, a small amount of energy, about 0.09 kWh, is needed to heat the 320 g of hydrogen gas, desorbed at high pressure, by 100 K. Thus, the total heat needed to move the compressor from state#1 to state#2 is approximately 2.29 kWh. The heat Q2 that is removed in going from state#3 to state#4 is equal to Qs (although it is opposite in sign). If we used the chiller to remove this heat, we would need to account for the efficiency of the chiller. However, this was not measured because we did not optimize this process. An efficient cooling system that could remove Qs ¼ 2.29 kWh of heat would be needed. The heat Q3 that is removed in the hydriding process, state#4 to state#1, is equal to Ql (but opposite in sign to the latent heat added in going from state#1 to state#2). Notice that the heat removed in going from state#3 to state#1 is equal to (but opposite in sign) the heat added in going from state#1 to state#2. The heat could be removed by circulating the thermal fluid through a radiator that is air cooled, rather than using a chiller. For simplicity, and to provide an estimate of the compressor energy usage, we will assume that the energy needed to cool the compressor is equal to the energy needed to heat it, 2.29 kWh. Thus, Qtot is equal to twice Q1, and 4.58 kWh is a rough estimate of the energy needed to operate the compressor over one cycle in the initial portion of the compression process shown in Table 6. Slightly lesser amounts of energy are used in the later cycles where lesser amounts of hydrogen are compressed (so Ql is reduced), but we still must add the sensible heat to heat up the compressor tubes and alloy (Qs stays the same).

3.3.6.1. Compressor electrical energy usage. The electrical energy needed for heating the compressor by approximately 100 K (from 303 K to 403 K) was measured using the ION model 7300 PowerLogic meter. The internal compressor heater was used for this test, so that only the compressor (metal tubes, hydride, oil, and metal case) were heated. Three types of tests were conducted: 1) a test with no hydrogen present, 2) a test with the normal amount of hydrogen present (approximately 14 MPa) in which the hydrogen was compressed, and 3) a test in which the normal amount of hydrogen was present and the hydrogen was allowed to flow into the storage cylinder during the latter portion of the test. In the first type of test only the oil, metal tubes, and alloy were heated. In the second test, some of the energy was used to drive the hydrogen out of the alloy, but the hydrogen pressure increased, so that the amount of hydrogen desorbing was minimal. In the third type of test the maximum amount of hydrogen desorbed, because the storage cylinder pressure was only about 1 bar Fig. 9 shows a comparison of the electrical energy added versus the hydrogen pressure for the three types of tests. The difference between the test with no hydrogen present and the other two tests represents the latent heat of de-hydriding the metal alloy (Eq. (1)) under the conditions of the tests. At a temperature of 400 K the difference between the two tests in Fig. 9 is approximately 0.3 kWh. Since the system was closed in the test with hydrogen present, only a fraction (in this case about

4.0

Electrical energy added, kWh

10910

3.5 3.0 2.5

no H2 H2 present H2 present, expand

2.0 1.5 1.0 0.5 0.0 290 300 310 320 330 340 350 360 370 380 390 400 410 420

Compressor temperature, K Fig. 9 e Electrical energy needed to heat the compressor from room temperature to approximately 420 K at three different conditions: 1) no H2 present, 2) H2 present and pressure increased in a closed system, and 3) H2 present and pressure increased with H2 expanded to a storage tank during the heating.

a third) of the hydrogen that would have been emitted if the hydrogen was allowed to fill the storage tank was released by the alloy. Thus, only a fraction of the latent heat energy used in cycle 1 of Table 6 was added in the hydrogen-containing test. Notice in the test in which the compressed hydrogen was expanded from the compressor to the storage cylinder at a temperature of approximately 375 K, the temperature decreased even as electrical energy was added to the compressor. This difference between the “no H2 present” and “H2 present, expand” tests represents the heat of de-hydriding about 300 g of H2. This difference is about 0.7 kWh, in rough agreement with the 0.9 kWh calculated from the DH determined earlier and the number of moles of hydrogen released by the hydride. Five repeat tests of the second of experiment type described above (normal amount of hydrogen present with a closed system) were conducted over a period of several months during the study. It took 2.73  0.05 kWh of electrical energy to accomplish an approximately 100 K temperature change when hydrogen was present. This shows that the results were very consistent from test to test. Slightly more electrical energy, 3.1 kWh was used for the experiment where the compressor was rested at 10 K intervals in the temperature-pressure rise tests described earlier in section 3.3.1 (Table 4 and Fig. 5). More electrical energy consumption would be expected in those equilibrium tests, versus a quick ramp-up to the final temperature, since some energy was used to maintain the compressor at a constant temperature during the rest periods to assure that equilibrium was reached.

3.3.6.2. Comparison of measured electrical energy consumed with predicted value. The electrical energy consumption measured in the above tests can be compared to the energy

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calculated using the heat capacities of the components in the compressor (including the 17 L of oil) and that of the dehydriding reaction, as discussed in section 3.3.6. The volume of oil that was heated in the compressor in this test was 17 L. Based on the density and heat capacity of the oil, averaged over the range of the temperature change (Table 1), 14.2 kg of oil with a heat capacity of 2.11 kJ/(kg K) was heated by 100 K, requiring 0.83 kWh of energy. Adding this to the energy to heat the compressor and de-hydride the alloy, calculated in section 3.3.6 (2.29 kWh), gives a total energy of 3.12 kWh. This is in rough agreement with the measured value of 2.73 kWh, and very close to the 3.1 kWh in the equilibrium test, in which the compressor was rested for 10 min at each temperature in the 100 K temperature rise. The compressor internal heater temperature is measured near the compressor cylinders, and the temperature of the metal jacket around the cylinders probably did not reach equilibrium with the internal compressor temperature in the three tests in which the temperature was increased at the maximum rate possible with the internal compressor heater. This, we believe, resulted in an overestimate of the energy added in the calculation (the actual compressor temperature was less than that measured by the thermocouple).

3.3.7.

Compressor efficiency

Several research groups have used different approaches, containing different assumptions, to calculate the efficiency of thermal compressors. As long as the assumptions are outlined, each approach can be justified. In essence, the MHHC is a heat engine that converts heat into work. The way we powered the compressor, i.e., with electrical energy for heating and cooling, would not be used in operation, because it takes high-grade energy (electricity) and converts it into lowgrade energy (low temperature heat sources and sinks). In practice, the heat source and sink would utilize renewable energy such as solar thermal or geothermal, and the heat sink would utilize the atmosphere including the air, earth, and ground water. For such a renewable heat source, the efficiency is not so important. The electrical heating provided a convenient method for precisely controlling and evaluating the compressor operation. There are three measures of efficiency that are important in evaluating the thermodynamic efficiency of the MHHC. They are: 1) the Carnot efficiency, 2) the first law efficiency, and 3) the second law efficiency. The efficiency of heat engines faces an efficiency limitation known as the Carnot efficiency. The Carnot efficiency is the theoretical maximum efficiency attainable by a heat engine operating between a cold reservoir at temperature Tl and a hot reservoir at temperature Th [18,23,25e33]. The Carnot efficiency is defined as: hc ¼ 100%  ðTh  Tl Þ=Th

(8)

For our MHHC, Tl ¼ 303 K and Th ¼ 403 K, so the Carnot efficiency is: hc ¼ 100%  ð403  303Þ=403 ¼ 24:8%

(9)

The first-law efficiency, h1, is the thermal efficiency in terms of the work done, W, for the amount of heat added, Q, as shown in Eq. (10):

h1 ¼ 100%  W=Q

10911

(10)

In our system, Q is the amount of electrical energy needed to heat and cool the compressor by 100 K through one complete cycle to achieve the hydrogen compression. Based on the discussion in the section 3.3.6, Q is 4.58 kWh. We will describe the evaluation of W for different assumptions about the value of the compression work below. The second-law efficiency, h2, is the ratio of the first-law efficiency to the Carnot efficiency: h2 ¼ 100%  h1 =hc

(11)

The second law efficiency is a measure of how well the MHHC efficiency approaches the ideal Carnot heat engine efficiency of 24.8%. In order to compare the MHHC compressor to traditional mechanical piston-type compressors, we computed the energy (work) required by mechanical compressors to compress hydrogen at 14 MPae41 MPa (the range of pressures for our MHHC) for different compression schemes describing the mechanical compression process. The first two cases are limiting cases, and the third is intermediate between the two limiting cases. They are: 1) a reversible isothermal compression (the most efficient possible), 2) a reversible adiabatic compression, and 3) a polytropic compression. The work required for an isothermal compression from a starting pressure Pl to a final pressure Ph is: Wisothermal ¼ Pl  Vl  lnðPh =Pl Þ

(12)

where Vl is the specific volume of hydrogen at a pressure Pl. The isothermal compression work is the minimum amount of work that is needed to achieve a gas compression, and it can only be achieved in a system that changes reversibly (slowly, such that the pressure changes gradually, and all of the heat of compression is removed). This is never achieved in the real world. Another type of idealized mechanical compression is an adiabatic compression, and if it is done reversibly, so the process is isentropic, the work can be calculated as: h i Wadiabatic ¼ g=ðg  1Þ  Pl  Vl  ðPh =Pl Þðg1Þ=g 1

(13)

Based on values in the NIST data base [24] at the lowest temperature and pressure for the typical experiment in Table 4, g ¼ 1.419, while at the highest temperature and pressure g ¼ 1.399. So, g ¼ 1.41 was chosen as a best value for the calculation of the adiabatic compressor efficiency over the conditions we tested. Also, from the NIST site [24], Vl at a pressure of Pl (14 MPa) and a temperature of Tl (303 K) is 0.0981 m3/kg. A polytropic compression is often used to calculate the real-world compression work for a mechanical compressor and it represents a process intermediate between the isothermal and adiabatic cases. The equation describing a polytropic compression has a form similar to the adiabatic case, with a polytropic coefficient, np, that is less than g h i   Wpolytropic ¼ np = np  1  Pl  Vl  ðPh =Pl Þðnp 1Þ=np 1

(14)

The compression work was evaluated for the three cases discussed above, as well as one typical real-world case; an

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adiabatic compression with a mechanical compressor efficiency of 70% (Eq. (13) with an added term, 0.7, in the denominator that accounts for irreversibility in the process). The results of these four scenarios are given in Table 7. In summary, the value of the compression work achieved by a mechanical compressor ranges from 0.413 to 0.695 kWh depending on the theoretical compression cycle used to compute the value of the work done. The next step is to use the estimate of the heat energy added to and removed from the compressor in one cycle, along with the Carnot efficiency, to determine the first and second law efficiencies for the four scenarios used to assign a value to the compressor work. The heat needed to drive the compressor through one cycle, compressing 320 g of hydrogen, was estimated in section 3.3.6 as 4.58 kWh. The calculated Carnot efficiency is 24.8%. Table 7 shows the calculated first and second law efficiencies for the MHHC based on each of the four theoretical scenarios used to evaluate the value of the compression work. Depending on which of the four compression scenarios is used as the basis for assigning a value to the compression work, the first law compressor efficiency varied from 2.9 to 4.9% and the second law efficiency varied from 11.6 to 19.6%. Our calculations correspond to the efficiency for the initial cycles in Table 6 where the greatest amount of hydrogen is compressed, i.e., approximately 320 g. The compressor efficiency decreases for later cycles where the storage pressure increases, and lesser amounts of hydrogen are compressed with each cycle, but similar amounts of energy must be used to heat and cool the compressor (Qs). A multi-stage compressor would help to alleviate this loss in efficiency. Different researchers have used different standards to compute the efficiency thermally-based compressors. For example, Wang et al. compare MHHC to an isothermal compression but do not consider the sensible heat, Qs, needed to heat the compressor containment vessels for each compressor cycle in their evaluation of compressor efficiency

Table 7 e Theoretical compressor work calculationsa for a compression from Pl [ 13.8 MPa to Ph [ 41.4 MPa and the first and second law efficienciesb corresponding to that work. Type of work Isothermal Polytropic Adiabatic Adiabatic, non-reversible

Energy, kWh

h1, %

h2, %

0.132 0.150 0.156 0.222

2.9 3.3 3.4 4.9

11.6 13.2 13.7 19.6

a The work energy was calculated from Eq. (12) to Eq. (14) with the specific volume for hydrogen, V0 ¼ 0.09812 m3 kg1 at 303 K and 13.8 MPa [24], the polytropic coefficient np ¼ 1.3, the heat capacity ratio g ¼ 1.41. For the non-reversible adiabatic case, a compressor efficiency of 0.7 was used, i.e., it takes 1.43 times the reversible adiabatic work for a real compressor that has friction. The work energy was calculated for the compression of a hydrogen mass of 320 grams. b The first law efficiency, h1, was calculated from Eq. (10) with using the work in column 2 and a heat, Q, of 2.8 kWh. The second law efficiency, h2, was calculated from Eq. (11) using a Carnot efficiency for Tl ¼ 303 K and Th ¼ 403 K of 24.8%.

[18]. Some research groups use the adiabatic cycle for computing the value of the compressor work [27,29,33,35] and consider the heating of the inert parts of the compressor, but not the cooling that must be performed to complete a compression cycle and return the compressor to its initial state. In general, our measured first law compressor efficiencies of about 3e5% for the reversible and non-reversible adiabatic case are similar to values reported for other thermal compressors [27,29,35].

3.4. MHHC compression as an enabler for storing highpressure hydrogen 3.4.1.

Summary of recent work on MHHC

We have already discussed how our compressor design was based on the work of Wang and co-workers [15e19] who extensively studied using the absorption of hydrogen by special alloys to achieve hydrogen compression. The properties they considered included: the pressure-plateau slope, hysteresis in cyclic operation, storage capacity, activation, alloy volume changes, decrepitation, system energetics, system reaction kinetics, thermal conductivity, impurity resistance, cyclic stability, safety, and cost. Wang and coworkers have optimized the compression characteristics AB5-type alloys using Ce-rich misch metal (Mm), where the Mm had a fixed ratio of La:Ce:Pr:Nd, and adding in La, Ca, Ni, and Al in various quantities [17]. The alloys considered were Mm0.8-xCa0.2LaxNi5 (x ¼ 0e0.7) with some substitution of the Ni by Al to improve the hysteresis properties. Following an extensive set of measurements of important alloy properties, alloys with the general composition Mm0.7Ca0.2La0.1(Ni4.95Al0.05) were selected to build compressors [15,16]. By adding a second stage with an optimized AB2-type alloy, a two-stage compressor that operated between temperatures of 298 K and 423 K were built [15]. This two-stage MHHC could pressurize approximately 170 g of hydrogen per hour from a starting pressure of 5 MPa to a final pressure of 70 MPa with a 1-h cycle time [15]. Wang and co-workers calculated the efficiency of their MHHC based on comparison to an ideal isothermal compression, and only considered the heat necessary to heat the hydrogen compression alloy plus that released by the hydride when it is formed, but neglected the heat needed to heat and cool the thermal fluid and the metal tubes containing the hydrogen compression alloys. The reported value was 18.6% [18]. Several other groups have studied MHHC. Hopkins and Kim [33] studied the compression characteristics of a two-stage thermal compressor using LaNi5 and Ca0.6MmNi5 as the working metal hydride to reach a compression ratio of 12 working between the temperatures of 283 K and 363 K (cold and hot water). Kim and co-workers [35] used a system containing LaNi5 and LaNi4.75Al0.25 together with a thermal system operating between a low temperature of 293 K and a high temperature of 363 K to achieve a compression ratio of up to 6 and an isentropic efficiency approaching 6%. The pressure rise achieved in their system went from in initial pressure of 0.7 MPa to a final pressure of 4 MPa yielding a compression ratio of about 6. Muthkumar and co-workers have studied compressors using misch metals plus nickel and either aluminum or iron [27e30]. They describe a small MHHC (4 g

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capacity) based on an MmNi4.6Al0.4 alloy that uses hot water to compress hydrogen from 0.5 MPa to 4.4 MPa with a maximum efficiency of 7.3% [29], and further discuss the effect of different operating parameters on the compressor performance [27]. Singh and co-workers [34] tested a host of materials with the general formula (La Mm)Ni5-yFey to obtain pressure-composition isotherms that were used to derive DH and DS using Van’t Hoff plots. They found that they could vary DH from 24.5 kJ/mol to 6.9 kJ/mol. Laurencelle and coworkers [36e38] used a system with three AB5-type hydrides to build a three-stage compressor operating between temperatures of 293 K and 350 K to achieve a compression ratio of 20. They comment that the compressor could be driven by waste heat form the alkaline electrolyzer that produced the hydrogen to be compressed; the goal for the compressor was to take hydrogen at near atmospheric pressure and compress it to approximately 20 MPa. Talaganis and co-workers [39] built a two-stage compressor based on MmNi5 to reach a compression ratio of 3.65. Each absorption/desorption stage operated between a low temperature of 293 K to a high temperature of 363 K. In summary, most of the recent work on MHHC has used systems that are generally similar, but with tuning of certain parameters, to achieve specific goals.

3.4.2.

Outlook on MHHC

Ultimately, it is desirable to use photovoltaic-driven electrolysis to both generate hydrogen from water and pressurize the hydrogen for storage and dispensing to FCEV. In that way the hydrogen is renewably generated, the generation and compression are achieved without any mechanical compressor, and the overall system efficiency is greatest (Appendix A). Electrolytic compression approximates and ideal isothermal compression and MHHC is only about 3% as efficient as that process (Table 7). However, electrolytic compression is not yet a proven technology, especially at pressures of 70 MPa [8]. Mechanical compressors are also much more efficient than MHHC; from Table 7, MHHC is only about 5% as efficient as a mechanical compressor operating over an adiabatic, nonreversible cycle. However, mechanical compressors have moving parts that wear out and can contaminate the hydrogen they compress, while MHHC actually purifies the hydrogen compressed. Although the efficiency values for MHHC are much lower than those for both electrolytic and mechanical compression an important feature is that they have the potential to operate using solar thermal energy and low-grade waste heat. In particular, solar thermal energy is the world’s greatest energy resource, and many very large solar thermal projects are planned around the world to convert solar energy to heat, and then operate a heat engine to make electricity [40e45]. In general such projects use concentrated solar power (CSP) to create a molten salt at high temperature (>1000 K), and then the molten salt stores the solar energy to run a steam turbine and generate electricity. However, there are a range of solar thermal technologies with temperatures going as low as 500 K [43]. Solar thermal generation is in general less costly than solar photovoltaic electricity generation, and it is a more mature technology [43]. Using solar thermal energy to generate electricity is especially attractive in the desert regions of the world where direct solar energy, that can be

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focused and concentrated to achieve high temperatures, is available. Very large scale solar thermal generation systems using CSP and costing over one billion dollars are planned over the next decade using government owned lands in six states in the Southwestern U. S [42]. Even lower-grade solar thermal could be used for hydrogen compression in an MHHC, since that heat engine can operate in a much lower temperature regime than the steam turbines generally used in a solar thermal electric generating system. This would reduce the importance of efficiency, and not utilize high-grade energy, like electricity, to do the work that low-grade heat, form a renewable and pollution-free source could do. Compressor efficiency is also of less importance if we utilize waste heat to drive the compression process, so this feature is an advantage over electrolytic or mechanical compressor that do not operate as heat engines. Even lowergrade heat from a solar thermal water splitting system could be used on an MHHC (also a heat engine process). Another desirable feature of MHHC is that it can be scaled easily to any size [15,19]. Thus, a large MHHC compression system could be added to a solar-thermally driven electric power plant that produces hydrogen for fueling FCEV, or a much smaller MHHC system could utilize a small solar heating system, with hot and cold water reservoirs, and multiple compression stages for a home hydrogen FCEV fueling system. Very recently, Wang and co-workers [46] described an MHHC with an output pressure of 70 MPa e the present high-pressure standard for FCEV fueling. Until electrolytic compression becomes technically feasible, MHHC is a viable method to compress hydrogen due to its many attractive properties.

4.

Summary

We tested a metal-hydride based hydrogen compressor as a means of boosting the pressure from our 14 MPa electrolyzer to over 41 MPa. Although this was not a high enough pressure for FCEV fueling, it allowed a convenient platform for evaluating the general concept of using the metal hydrogen absorption reaction as a compressor. This compressor was very different from piston or diaphragm based mechanical compressors in that it did not have moving parts (other than valves and pumps for circulating the heating and cooling fluids). Also, the electrical heating and cooling were used for evaluation purposes only. To be useful in promoting a hydrogen economy, the compressor would need to use solar thermal, low-grade waste, or geothermal heat. The compressor worked well and provided the factor of three compression that was desired. Early compression cycles compressed over 300 g of hydrogen. Automation of the process and use of waste heat is needed to make it a practical alternative to mechanical or electrolytic compression. A heat engine utilizing a small temperature difference between the cold and hot reservoirs as its power source is inherently inefficient. Although electrolytically-produced highpressure hydrogen is still the preferred pathway to pressurize hydrogen, a thermal compressor is a desirable method to compress hydrogen in the absence of such an electrolyzer. A means of storing hydrogen at high gravimetric and volumetric efficiency would make energy efficient compression moot, and is still needed in the long run.

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5.

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Conclusions

1. A single-stage metal hydride hydrogen compressor (MHHC) based on the absorption of hydrogen in an alloy at low temperature and pressure and the thermally-driven desorption at high temperature was able to boost the initial hydrogen pressure by a factor of three. The initial pressure and temperature were approximately 13.7 MPa and room temperature, respectively. A temperature increase of about 110 K led to a pressure increase to 40.8 MPa. In principal the hydride compressor operates in a nearly reversible manner, slowly increasing the pressure, with negligible friction losses that would be present in piston or diaphragm compressors that utilize continuous cycles of high and low pressure for each small amount of gas compressed. The MHHC lends itself to quiet operation on low-grade thermal energy. 2. The Carnot efficiency of the compressor was approximately 25%. The Carnot efficiency is the maximum efficiency of a heat engine. The first law efficiency was approximately 3e5% and the second law efficiency was approximately 12e20%, depending on the idealized type of compression used to assign a value to the compression work performed by the compressor. Although the first and second law efficiencies compared favorably with previous MHHC, there is an opportunity for increasing efficiency through reducing the energy needed to heat the pressure containing cells by reducing their mass, and even for making efficiency a less important parameter by utilizing renewably generated heat or waste heat from some other process, such as a solar PV power system, or an electrolyzer for generating hydrogen. 3. The thermal compression system displayed fast kinetics, and in a process compressor, it could be run very quickly with a large hot and cold reservoir to rapidly change the temperature and increase the gas throughput. 4. Hydride compression is a technically viable alternative to mechanical compression. Although electrolytic compression may eventually be the simplest and most efficient method, hydride compression is a good alternative in case high-pressure electrolytic compression remains elusive. 5. Improvements involving automation should be made to the MHHC to make it practical for continuously compressing hydrogen. For example, automated valves and a programmable electronic system to control the valve opening and closing would eliminate the need for the constant attention of an operator. Also, as better materials are found for on-board hydrogen storage, they may lend themselves to compressors, because the binding energy of approximately 20 kJ/mol between hydrogen and the hydrogen absorbing alloy in our compressor is within the range of enthalpies that is useful for on-board hydrogen storage systems.

Acknowledgments We wish to acknowledge the Professor Xinhua Wang of Zhejiang University (Hangzhou, China) for building and

commissioning the MHHC. Valuable help in installing and maintaining the MHHC was provided by Carlos Franca and his team at the Milford proving ground including Vance McCabe and Jerry Kowalski. We are grateful to Mei Cai of GM R&D who helped with the compressor commissioning, and Qiangfeng Xiao who performed the hydride enthalpy measurements. We also thank Brian Prokuda of Keweenaw Power Systems, Inc. who designed and installed the electrical energy measurement system on the compressor.

Appendix A. Energetics of electrolytic compression of hydrogen The energy needed to split water to make hydrogen and oxygen in an electrolyzer can be calculated using the Nernst equation [44]:   Electrolysis voltage ¼ E0  ðR  TÞ=ð2  FÞ  ln ½H2    ½O2 0:5 =½H2 O

(A1)

In Eq. (A1), T is temperature in degrees K, R is the gas constant (8.314 V coulombs/K mole), and F is the Faraday constant (96,484 coulombs/mole of electrons), and the H2 and O2 concentrations are equal to their pressures. If [H2] ¼ [O2] ¼ 0.1 MPa and the temperature is 298 K, then the electrolysis voltage is 1.23 V. The quantity ((R x T)/(2  F)) is equal to 0.0128 V at 298 K, and the concentration of water is relatively independent of gas pressure and is taken as one since it is in its standard state. Note, the sign on Eo is negative since 1.23 V is needed to split water; if the system was consuming H2 and O2 to produce electricity in a fuel cell, the sign would be positive and the reaction would be spontaneous. The Nernst equation can also be used to determine how much extra energy is needed at higher hydrogen and oxygen pressures. For example, if both [H2] and [O2] are increased to 10 atm, then the term to the right of V0 becomes 0.044 V and the voltage needed to split water is 1.273 V. This is a relatively small increase in overvoltage, 3.6% at the higher pressure, due to the logarithmic pressure dependence in Eq. (A1), and this provides an impetus for using a high-pressure electrolyzer to produce high-pressure hydrogen, versus a low pressure electrolyzer followed by a compressor. For higher pressures, one cannot simply use pressure in the Nernst equation, and the activity, called the fugacity for gases, needs to be used. The fugacity is equal to the pressure at low pressures, but, it is higher than the pressure at high pressures. The increase in the voltage for water electrolysis as a function of pressure, based on Eq. (A1) is shown in Fig. A1 for pressures up to 70 MPa. Both pressure and fugacity were used in the Nernst equation to compute the curves shown in the figure. Using the pressure for H2 and O2 (as for an ideal gas) caused a slightly lower overvoltage for water splitting at high pressure. Thus, the effect of gas non-ideality is minor, due to the logarithmic dependence of voltage on pressure or fugacity in the Nernst equation. The electrolytic water splitting and compression of hydrogen and oxygen to a pressure of 70 MPa takes about 10.6% of more energy than water electrolysis under standard conditions based on the 0.13 V increase

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calculated using the Nernst equation (100 %  0.13 V/ 1.23 V ¼ 10.6%). This is identical to the energy for a reversible, isothermal compression. As discussed elsewhere, kinetic effects in water electrolysis are very important, and they increase the water electrolysis voltage to values of from 1.8 to 2.0 V in commercial electrolyzers [7,10]. Moreover, the kinetic overvoltage of the electrolysis may decrease due to bubble compression, so electrolytic compression may be “free” on an energy basis, i.e., the thermodynamic voltage and energy increase at higher pressures is offset by a kinetic voltage and energy reduction at high pressures. However, the difficulties in making electrolyzers robust enough to contain high pressures, as well as the need for maintaining high-pressure oxygen in some designs, has hindered the implementation of high-pressure electrolysis. 0.140

In crease in voltage above 0.1 MPa value

H 2 + O2 voltage, fugacity based 0.120

H 2 + O2voltage, pressure based

0.100

0.080

H 2 voltage, pressure based 0.060

0.040

O2 voltage, pressure based 0.020

0.000 0

10

20

30

40

50

60

70

H2 and/or O2 pressure, MPa

Fig. A1 e Increase in the thermodynamic voltage for water electrolysis, i.e., the hydrogen evolution reaction and oxygen evolution reaction, as a function of the hydrogen and oxygen gas pressure or fugacity, based on the Nernst equation.

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Nomenclature a-phase: Metallic phase b-phase: Metal hydride phase Cv: Heat capacity at constant volume, kJ kg1 K1 Ci: Heat capacity of species i, kJ kg1 K1 E0: Standard water electrolysis voltage, 1.23 V (Gibbs free energy) F: Faraday constant, 96485 coulombs per equivalent g: Heat capacity ratio, 1.41 in this work for hydrogen MHHC: Metal hydride hydrogen compressor h1: First law efficiency, % h2: Second law efficiency, % hc: Carnot efficiency, % np: Polytropic coefficient, 1.3 in this work J: Joules mi: Mass of species i, kg M: Metal hydrogen compression alloy MH: Hydride form of the metal hydride compression alloy Mm: Misch metal (alloy of rare-earth elements) P: Pressure, MPa Pd: Plateau pressure, MPa Ph: Compressor high pressure at Th, MPa, typically 41.7 MPa Pl: Compressor low pressure at Tl, MPa, typically 14.7 MPa Q: Heat, J Ql: Latent heat of a chemical reaction or phase change, J Qs: Sensible heat, (mass x heat capacity x temperature change), J R: Gas constant, 8.314 J K1 mol1 Rp: Compression ratio, Ph/Pl T: Temperature in K Th: Compressor high temperature, K, typically 403 K Tl: Compressor low temperature, K, typically 303 K DH: Change in enthalpy in a reaction, kJ mol1 DS: Change in entropy in a reaction, J K1 mol1 DT: Change in temperature, K W: Compression work, J