Understanding the Application Envelope for Metal Hydride Compressors (Techno-Economic Considerations)

Abstract

Currently, H₂ compression is one of the highest-cost items, both in terms of capital and operating costs, at H₂ refuelling stations. Metal hydride (MH) compressors are an alternative H₂ compression technology, which uses heat rather than electricity to provide the driving force for compression. Where waste heat is available, these compressors have the potential to be lower in cost than current mechanical alternatives. While the development of metal hydride compressors has been underway for the last 40–50 years, only a few have made it through to demonstration at industrial sites. To better understand where these compressors see best potential, we have completed a high-level assessment of the levelised costs associated with MH compression. We explore the impact of cost assumptions (capital and operating cost items) on the overall cost of MH compression over an assumed 10-year life. Results indicate that MH compressors have similar capital costs to currently available mechanical compressors but have a significant advantage in operating costs where waste or solar heat is available. This analysis highlights that it is the cost of energy that has the greatest impact on the cost competitiveness of the metal hydride compressor.

1. Introduction

Despite the recent wave of project delays and cancellations, low-emission H₂ is still expected to play an important part of global ambitions to limit global warming. In its 2025 global H₂ review, the International Energy Agency noted that final investment decisions for low-emission H₂ projects are at over 200, a significant increase from the previous review completed in 2020 [1]. This reflects the continued importance (though perhaps slower than previously expected) of H₂ for achieving climate goals, energy security and industrial decarbonisation [1]. As efforts to decrease carbon emissions accelerate, H₂ usage is anticipated to increase [2,3]. One potential early adopter of H₂ is the automotive industry, particularly for long-haul and heavy-duty transport, where fast-refuelling times and increased range give fuel cell electric vehicles an advantage over battery electric [4]. There are still challenges however with the widespread use of H₂ for transport, including costs that still need to be reduced to encourage greater uptake. Due to its low density, compression and/or liquefaction of hydrogen is required for it to be stored at a reasonable volume. Its low density also makes compression of hydrogen more energy-intensive compared to other gases. The high diffusivity and fluidity of hydrogen create additional difficulties in the development of mechanical facilities for its compression [5]. Current fuel cell electric vehicles (FCEVs) utilise H₂ gas compressed to 200–350 bar, with efforts underway to increase this to 700 bar, requiring continuous pressure in refuelling stations above 900 bar. A report by the National Renewable Energy Laboratory (NREL) [6] evaluated the costs of compressing, storing and distributing H₂ at a theoretical H₂ refuelling station. This study highlighted that approximately two-thirds of the cost of delivering H₂ at refuelling stations was due to the cost of compression, regardless of the H₂ delivery method used.

Metal hydrides (MHs) have been extensively evaluated for H₂ storage applications [3] but can also be used for compression. MH H₂ compression utilises a reversible heat-driven interaction of a hydride-forming metal alloy or intermetallic compound with hydrogen gas [7]. Metal hydrides can absorb large amounts of hydrogen at a constant pressure, i.e., the pressure does not increase with the amount of hydrogen absorbed [8]. The formed hydrides can then, under certain temperatures and pressures, desorb the stored hydrogen [9]. At low temperatures, H₂ will be absorbed at lower pressures. If the metal hydride is then isolated and heated, it will desorb the H₂ at a higher pressure, achieving compression (see Figure 1). Metal hydride-based compressors have the advantages of simplicity in design and operation, absence of moving parts, compactness, safety and reliability, and the possibility to consume waste industrial heat instead of electricity [7]. Rusanov et al. (2020) [5] showed that when you take the full energy conversion into account, including heat to electrical power at the power station, then the MH compressor is more efficient than mechanical compression.

Metal hydride (MH) hydrogen compressors offer several potential advantages over conventional mechanical compressors. These include lower maintenance needs due to the absence of moving parts, the ability to use waste or solar heat instead of electricity for compression, and the delivery of high-purity hydrogen as a result of the inherent chemical nature of the process. A good overview of MH compression and its operating principles is provided in Lototskyy et al. (2014) [7]. Despite being developed over many years, and several working prototypes of MH hydrogen compressors (MHHCs) being built (examples include [10,11,12,13,14,15]), they still lack significant market uptake (currently, MH compressors can be purchased from Hystorsys [16]).

Despite their well-known low thermodynamic efficiency and lack of an existing market, Lototskyy and Linkov (2022) [15] suggest that MHHCs offer promising solutions for niche applications. They highlight the modular design and scalability of the technology, noting that downscaling in particular offers compact and effective solutions/options for handling high-pressure H₂ in laboratories. Applications subject to weight and space constraints are a further potential niche for MHHCs (e.g., the spacecraft compressor developed by Jet Propulsion Laboratory [17]).

The advantages of MHHCs over conventional mechanical compressors follow from their utilisation of available heat in place of electricity: greatly reduced electricity cost, reduced maintenance and near-silent operation. In contrast, a NREL report found that for a 700-bar H₂ refuelling station, the mechanical hydrogen compressor contributed 58% of the capital costs and about 65% of the operating costs [6]. It was the second-largest contributor to maintenance hours (24%) and frequency of incidents (18%) [15,18].

For MHHCs, the two main cost items are the capital costs of the MH and the pressure vessels. Corgnale and Sulic (2018) [19] suggested that the MH constitutes 40–85% of the installed cost. Their sensitivity analysis predicted that MHHC costs could be dramatically decreased by reducing the MH cost, reducing the cycle time, and increasing the MH density inside the vessel (i.e., less dead space). According to Johnston et al. (2022) [12] and Lototskyy et al. (2020) [11], however, the costs of pressure vessels were higher than estimated by Corgnale and Sulic, resulting in the MH constituting 20–25% of the MH compressor cost. As throughput increases, the volume of the pressure vessel in each stage of the MHHC increases, leading to rapidly escalating costs. This approach will eventually become unrealistic, necessitating modular designs replicating vessels at an optimum scale. This in turn will benefit the economic scalability of MHHCs.

Stamatakis et al. (2018) [20] completed a preliminary market evaluation for MHHCs and identified three significant niche markets: (1) chemical industry, by utilisation of waste industrial and/or available renewable heat; (2) hydrogen filling stations for vehicles; and (3) renewable energy systems and H₂ autonomous power systems for remote communities. Hydrogen compressors currently used at refuelling stations are generally either diaphragm or reciprocating compressors. Poor reliability continues to plague forecourt hydrogen compressors because current designs assume prolonged operation at peak pressure. This operating regime is not representative of the conditions to which forecourt hydrogen compressors are exposed [20].

Direct costs (excluding labour) are available for the refuelling system operated at the Impala Platinum Refineries in South Africa [11]. This refuelling station incorporated a one-stage MH compressor and compressed 13 Nm³/h H₂ (approx. 1.1 kg/h) from 50 to 200 bar using 140 °C steam. The total cost of the refuelling station is given as 1.94 million Rand (approx. $140,000 USD in 2015). The main cost components were the MH compressor, H₂ dispensing and assembly costs. For the MH compression system (475,000 Rand, 25% of total costs), about 25% of the cost was due to the metal alloy (118,750 Rand), with the remaining 75% being the cost of the pressure vessels (356,250 Rand). Operating costs for the refuelling station were estimated to be 100,000 Rand (approx. $7000 USD in 2017). The majority of these costs related to upgrading the prototype compressor to eliminate defects identified through prolonged operation (e.g., blocking of filters). They concluded that the capital and operating costs of this refuelling station were significantly lower than for standard refuelling stations available on the market. The cost benefits were due to (i) a lower H₂ dispensing pressure (185 bar), which enabled the use of standard gas service components; (ii) slow pressure ramping, which prevents overheating of the supplied H₂ and thus eliminates the requirement for deep cooling; and (iii) the replacement of a mechanical H₂ compressor with an MH compressor.

Several cost studies have been completed for theoretical and demonstration-scale metal hydride compressors. The results from some of these studies are summarised in

Table 1. Summary of suggested costs for metal hydride (MH) and mechanical (MC) compressors.

Scale and Cost	Capital Cost	Operating and Maintenance Cost	Overall Cost	Economic Assumptions	References and Comments
For forklift 1.92 kg H2 30–200 bar over 7 h (0.3 kg H2/h)	MH—€16,000 MC—€71,000		MH—€6/kgH2 MC—€16/kgH2	Costs provided per unit of compressor. Cost of electricity based on Italian mix	[21] Type of mechanical compressor not provided. Compressor costs from web search. Waste heat assumed free. Majority of capital cost for MH compressor from alloy
7 to 250 bar 56 Nm3/h (5 kg/h)	MH—€130,000 MC—€145,000	Annual power cost MH—€100, MC—€4000 Annual maintenance MH—€1000, MC—€4000		Power €0.10/kWh, waste heat €0.0/kWh. MC annual re-build, MH compressor valve replacement every other year.	[20] Type of mechanical compressor not provided. Weight of MH compressor about 1/3 that of mechanical compressor.
100–875 bar 1 and 100 kg H2/h			Installed cost $46,000–$73,000 for 1 kg/h H2 $2,400,000–$5,200,000 for 100 kg/h H2 Cost range due to different costs of MH materials	Installed cost estimated by applying installation factor to free on-board component cost. Free on-board cost = cost of metal hydride + cost of tube heat exchanger.	[19] Costs in USD 2017 Second stage in hybrid system with electrochemical compressor (EHC) for LP stage. Minichannel reactor design with TH = 150 °C. Desorption heat assumed supplied from EHC. 10 min ABS/DES time with no degradation for 35,000 cycles. Conductivity of 8 W/mK (10 wt% ENG) and volume expansion of 15% assumed. (US DOE target uninstalled cost for 100 kg/h H2 is $275,000 USD2020) 1 compression stage, 2 parallel tanks per stage for continuous output. Used factor to increase from FOB to installed cost, where FOB cost includes MH and cost of heat exchanger. Costs found to be very sensitive to cycle time. Costs found to be comparable to current mechanical compressors. Lowering MH cost and increasing bulk density suggested as methods for reducing system cost.
100–875 bar 100 kg H2/h	4000 kg for each MH (HP and LP). MH costs $136,000–$344,000 ENG cost $36,000–$55,000 Vessel cost $868,300 (LP), $1,129,100 (HP)		Total cost compressor ~$2M	Initial cost estimates based on scaling calculations from prototype system. Costs for MH, heat transfer additives and pressure vessels taken from received quotes.	[12] Scaled up from prototype achieving 33.6 gH2/h. Assumed 2 beds per stage, 2 stages of compression, 12 min half cycles, 1 wt% utilisation of MH. Cost of alloy from commercial vendors ranged from $17 to $43 per kg. Scaling calculations used to reduce number of vessels to 10 vessels per bed. Quote from manufacture at $43,000/$56,000 (LP/HP) per vessel. Manufacture of Type III composite vessel suggested vessel cost of ~$10,000 per vessel possible, dropping HP vessel cost to $200,000 (5 x cost reduction compared to Nitronic 50 vessel)

Note: MH—metal hydride compressor; MC—mechanical compressor; EHC—electrochemical hydrogen compressor; ENG—expanded natural graphite; LP—low-pressure compression stage; HP—high-pressure compression stage; ABS/DES time—absorption/desorption.

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When a particular use case is identified, it is possible to design and cost a potential compressor, facilitating a detailed comparison of functionally equivalent MH and mechanical compressors. Such a comparison would include a price for waste heat supplied to the MH compressor, which would depend on site-specific requirements. The waste heat available (particularly temperature) would also then be used to determine specific alloys for each stage of the compressor. What is less clear are the scenarios where MH compressors deliver a greater economic advantage over mechanical counterparts (i.e., where the site is not yet known). To shed light on this, the authors have obtained commercial quotes and identified costs for mechanical compressors available in the literature. High-level designs of theoretical MH compressors able to achieve the same compression ratio and throughput as the quoted mechanical compressors were then completed. This necessarily requires some assumptions for the design of the MH compressor. The impact of these assumptions is then explored through a sensitivity analysis. Through this comparison, we hope to identify the application range where MH compressors see greatest advantage. The calculations reported here are indicative only; the intent is not to attempt an accurate analysis of the cost of a specific design, but to make a high-level comparison of costs, to achieve an understanding of how the major cost components impact the overall feasibility of different MHHCs deployed in different scenarios. It should also be noted that we have not at this stage included any cost reductions due to learning that would be expected should a market for MHHCs develop (as is currently the case for mechanical compressors). Thus, the costs provided here for MHHCs are conservative in nature, and additional cost reductions could be expected with scale and market development.

2. Methods: Levelised Cost of Hydrogen Compression

Detail on assumptions and methods used to calculate the cost of the theoretical metal hydride compressor can be found in Appendix A.

As we are completing a high-level comparison, we are only considering basic operating costs, including the cost of electricity. At this stage, we assume that labour required for installing the MH and mechanical compressors is comparable and hence excluded from consideration at this stage. While it has been suggested that maintenance requirements of MH compressors should be lower than that of mechanical compressors, at this stage, we assume they are comparable and again remove them from consideration.

When calculating the levelised cost of hydrogen compression, we assume a lifetime of 10 years for both the mechanical and MH compressors, a discount rate of 9%, and a capacity factor of 0.85. We assume that electricity is at a cost of $100/MWh_e, and in the first instance, that waste heat is available at no cost.

The levelised cost of hydrogen compression ($/kgH₂) is then calculated via Equation (1). CAPEX is treated as a year-0 lump-sum cost while OPEX is discounted annually over the project lifetime. The resulting levelised cost is then normalised on a $/kg-H₂ basis.

$$Levelisedcostofcompression={\displaystyle \frac{CAPEX+\mathsf{\Sigma }\frac{annualOPEX}{(1+r{)}^n}}{H2\times CF}}$$

(1)

where

• CAPEX—purchased cost of the equipment ($);
• Annual OPEX—operating cost over a year ($);
• H2—annual H₂ throughput (kg/year);
• CF—capacity factor (0.85);
• r—discount rate (9%);
• n—lifetime of asset (10 years).

In the net present value calculations, CAPEX is assumed to be spent in the first year. OPEX is then spent over the subsequent 9 years of the asset life. Electricity costs are assumed to be constant over the asset life. All cost estimates represent uninstalled equipment costs for both the MH and mechanical compressors. We have not at this stage included a generic installation multiplier, as installation and site integration costs are expected to scale in a similar manner for both technologies.

Example

We received a quote for a compressor to compress H₂ from 1 to 16 bar_a at a rate of 87 Nm³/h H₂ throughput (7.22 kg/h). The cost of this compressor was approximately $196,000 AUD (2023; unless stated otherwise, costs are in AUD 2023).

Increasing the pressure from 1 to 16 bar can be accomplished by one stage in an MH compressor. Alloys that can achieve this include LaNi_4.65Mn_0.35, LaCu₅, MmNi_4.2Al_0.8, La_0.7Ce_0.2Co_0.5, and LaNi_4.25Co_0.5Sn_0.25 [22]. These alloys all have equilibrium pressures below 1 bar at 25 °C and above 16 bar at 150 °C. We assume that an AB5 alloy (e.g., properties comparable to LaNi₅) is used in the single stage of the compressor, with the time required for each half cycle (e.g., absorption or desorption) being 20 min. This is comparable to the half cycle time experienced in the MH compressor used in the refuelling station at the Impala refinery [11]. We assumed two parallel sections per stage, so that as one is absorbing, the other is desorbing to give nearly constant flow from the compressor. This means that 2.41 kg of H₂ needs to be moved during each absorption/desorption step to meet the target flow rate of 7.22 kg/h. For LaNi₅, the maximum H₂ uptake is typically around 1.4 wt%. The lower end of the pressure plateau is typically below 0.2 wt%. Thus, we have assumed a cyclic capacity of 1 wt%. This means that 241 kg of alloy is required for each absorption/desorption step. To improve performance, thermal conductivity enhancers are often added to MH vessels. Examples include expanded natural graphite (ENG) typically added at 10–15 wt% of the alloy. We assume that 15 wt% ENG is added to each vessel (36.1 kg) as this is the fraction used in the prototype compressor operated by Johnson et al. (2022) [12]. This brings the total mass in each vessel to 276.8 kg. As MHs absorb H₂, they expand, and as a result, dead space is required in each vessel to accommodate this expansion. For LaNi₅, we assume that the non-hydrogenated density is 3000 kg/m³ to accommodate this expansion (based on experience with LaNi₅ in our laboratory (unpublished)). This means that the volume required to house the metal hydride is 0.092 m³ per vessel. If we assume a vessel diameter of 0.25 m, and 10% of the cross-sectional area is lost to heat transfer tubing, a vessel 2.09 m long is required for each absorption/desorption step. We have assumed that so long as the length of the pressure vessel is below 8 m (see

Table A1. Cost of pressure vessels [6].

Type of Storage Vessel	Cost
ASTM SA 372 Grade J class 70 vessel 0.4 m OD, 7.3 m long 430 bar	$18,000 USD 2007 (approx. $38,400 AUD 2023)
Carbon wrapped ASME type 1 cylinder 0.6 m OD, 7.3 m long 193 bar	$20,700 USD 2007 (approx. $44,000 AUD 2023)
Type 2 cylinder 0.46 m ID, 2.4/4.4/8.8 m lengths 900 bar	$36,000 USD 2007 (approx. $76,900 AUD 2023)
Type 4 cylinder 0.6 m OD, 2 m length 900 bar	$24,000 USD 2007 (approx. $51,000 AUD 2023)

), the pressure vessel can be manufactured, and additional pressure vessels are not required. We have not scaled the cost for length as the change in cost is expected to be minimal. Thus, in each 20 min cycle, we have one vessel absorbing, and one vessel desorbing, with each vessel containing 276.8 kg alloy and ENG. The total mass of alloy for the whole compressor is 481.4 kg, and the total mass of ENG required is 72.2 kg. Using the quote we received for LaNi₅ ($100/kg, see Appendix A2.2) as the estimate for the cost of the alloy, the cost of the alloy for the compressor is $48,140. Using the quote from Johnson et al. (2022) [12] for the cost of the ENG ($70/kg), the cost of ENG is $5055. At $30,000 each, the cost of the two pressure vessels required comes to $60,000. Thus, the approximate total capital cost of the compressor is $113,000 (AUD 2023). Adding in the cost of the heat transfer pumps and air cooler (see Appendix A) brings the total uninstalled cost to $119,070. This is lower than the uninstalled cost of the mechanical compressor.

To provide a more thorough understanding of where the MH compressor could see an advantage, we have looked through the literature for additional quoted costs of current commercially available mechanical compressors. These are provided in

Table 2. Summary of uninstalled capital costs identified for mechanical compressors.

Title 1	Type of Compressor and Throughput	Cost (AUD 2023)
Own quote 1	Compression from 1 to 16 bar. Throughput 49–87 Nm3/h (nominally 7.2 kg/h)	$196,000
Own quote 2	Compression from 10 to 220 bar 11 kg H2/h, two-stage compressor Power requirement 75 kWe	$400,000
Parks et al. (2014) [6] Vendor B	Two-stage diaphragm compressor (20–350 bar), then dry-running piston compressor (350–950 bar). 33 kg H2/h	$980,000
Parks et al. (2014) [6] Vendor A	Two-stage diaphragm compressor (20–120–350 bar). 35 kg H2/h	$803,000
Parks et al. (2014) [6] Vendor A2	Single-stage two-head compressor (450–900 bar). 56 kg H2/h	$908,000

(taken from Parks et al. (2014) [6]), in addition to a second compressor quoted for use in Australia. Following the methodology outlined above, we have sized a theoretical MH compressor able to achieve the same throughput and compression ratio. The high-level details of the MH compressor to match each of these cases are provided in

Table 3. Cost information for mechanical and theoretical MH compressors. For further detail, see Appendix A.

Mechanical Compressor	Quote 1	Quote 2	Vendor B	Vendor A	Vendor A2
Compression range (bar)	1–16	10–220	20–950	20–350	450–900
H2 throughput (kg/h)	7.221	11	33	35	56
Capital cost (AUD 2023)	196,000	400,000	980,000	803,000	908,000
Efficiency (kWh/kgH2)	3	6.8	5	3	5
MH compressor detail
Mass H2 that needs to be moved per 20 min cycle (kg)	2.407	3.67	11	11.67	18.67
Mass alloy per 20 min cycle (kg)	240.7	366.7	1100	1166.7	1866.7
Number of stages	1	2	3	2	2
Number of vessels per stage	2	2	2	4	4
Total number of vessels	2	4	6	8	8
Mass alloy per vessel (kg)	240.7	366.7	1100	583.3	933.3
Mass ENG per vessel (kg)	36.1	55	165	87.5	140
Total mass per vessel (kg)	278.8	421.67	1265	670.8	1073.3
Volume alloy + ENG (m3)	0.092	0.141	0.422	0.224	0.358
Number of heat transfer tubes	10	15	25	25	35
Vessel ID (m)	0.25	0.25	0.3	0.3	0.35
Vessel length (m)	2.09	3.17	6.71	3.56	4.2
Cost of alloy stage 1 ($)	48,100	73,333	220,000	233,333	373,333
Cost of alloy stage 2 ($)	-	36,667	110,000	116,667	186,667
Cost of alloy stage 3 ($)	-	-	110,000	-	-
Total cost of ENG ($)	5055	15,400	69,300	49,000	78,400
Cost of vessel stage 1 ($)	60,000	60,000	60,000	120,000	280,000
Cost of vessel stage 2 ($)	-	60,000	140,000	280,000	280,000
Cost pf vessel stage 3 ($)	-	-	140,000	-	-
Cost of air cooler ($)	3053	9534	41,975	30,730	49,303
Cost of pumps ($)	2822	5735	14,001	11,612	15,420
Total cost of compressor ($ AUD)	119,070	260,687	905,276	841,342	1,263,123

.

3. Results

Noting the back-of-the-envelope nature of these calculations, the capital costs of the metal hydride and mechanical compressors are fairly comparable. To provide greater insight into the overall costs of the compressors, the levelised cost over an assumed lifespan of 10 years is provided in Figure 2. In this first comparison, we have assumed that waste heat for the MH compressor can be sourced at no cost.

Figure 2 highlights the large impact of the operating cost (mainly electricity) on the overall cost of mechanical compressors. For MH compressors, the capital cost dominates the overall cost (see Appendix A for the cost distribution of the MH compressor). The low operating cost of the metal hydride compressors stems largely from the assumption that waste heat is provided free to the compressor. Figure 3 explores the impact of this assumption with a cost of $10/MW_th applied to the operating cost of the MH compressor.

Figure 3 highlights that even with a cost of $10/MW_th applied to the metal hydride compressors, they are still economical compared to mechanical compressors. However, the cost for Vendor A is now within the error of this analysis, and in this instance, there is no clear cost advantage of the metal hydride compressor. Clearly, a rigorous design and cost analysis would need to be completed before settling on a particular compressor for a particular use case. However, Figure 3 highlights that even if there is some cost applied to the waste heat, MH compressors could still potentially see an advantage. In Appendix A, we explore the impact of supplying part of the thermal energy requirement for the metal hydride compressors via electricity.

3.1. Sensitivity Analysis

In this section, the impact of the assumptions made when sizing and costing the metal hydride compressors is explored. Here, we selected the comparison to Vendor A, and we evaluated the impact of electricity, pressure vessel and alloy costs, and efficiency assumed for the mechanical compressor. These results are provided in Figure 4.

From Figure 4a, it can be seen that where waste heat is free, the electricity cost would need to drop to $8/MWh_e for the cost of a mechanical compressor to be comparable to that of the metal hydride compressor. From Figure 4b, it can be seen that even if the pressure vessel costs were 50% higher than estimated (at $45k for the low-pressure vessel and $105k for the high-pressure vessel), the levelised cost of the metal hydride compressor is still competitive compared to the mechanical compressor. From Figure 4c, it can be seen that the costs of the alloys would need to double from those considered here (to $200/kg for AB5, $100/kg for AB2 and $150/kg for ENG) for the cost of the metal hydride compressor to match that of the mechanical compressor. Finally, from Figure 4d, we can see that the efficiency of the mechanical compressor would need to improve to 1 kW_e/kgH₂ to have similar costs to the metal hydride compressor over the assumed 10 yr lifespan.

3.2. Limitations of Current Economic Assessment and Recommendations for Future Work

This paper is intended to provide a high-level comparison of the costs of metal hydride and mechanical compressors so that the areas where MH compressors potentially see an economic advantage could be identified. As noted in the introduction, as a specific site or use case has not yet been identified, it is not possible to select specific alloys for each stage of the compressor, nor incorporate site-specific integration costs at this stage. In addition, we have not yet explored the impact of metal hydride deactivation, which can only be predicted once specific metal hydrides have been selected. In addition, for this first-pass comparison, we have excluded maintenance and labour costs. Due to their absence of moving parts, MH compressors are anticipated to have lower maintenance requirements than mechanical compressors [7]. Inclusion of maintenance costs in the cost comparison is anticipated to further enhance the cost competitiveness of MH compressors.

It is recommended that before a final investment decision is made, such costs need to be determined. At that point, a more detailed techno-economic comparison can be completed, providing a more robust comparison of the two technologies for the particular scenario of interest.

4. Summary and Conclusions

This paper considers a back-of-the-envelope cost comparison between metal hydride and mechanical compression. The aim was not to provide a detailed comparison between the two technologies. Rather, the intent was to explore the scenarios where metal hydride compressors are likely to see the greatest cost advantage.

Five compressor specifications corresponding to received quotations for the commercial supply of mechanical compressors were adopted, and each was matched by a metal hydride compressor, based on cost assumptions for hydrogen storage alloys, pressure vessels, etc.

With the exception of the “Vendor 2” scenario (450–900 bar compression, 56 kg/h throughput), the predicted capital cost of the metal hydride compressor was similar or significantly lower than the cost of the equivalent mechanical compressor. While the absolute numbers should not be taken too literally, the trend towards a lower relative capital cost for smaller metal hydride compressors appears to be significant.

Adding operating costs changes the picture considerably in favour of metal hydride compression. Where waste heat can be supplied free, or at a small cost, metal hydride compressors are expected to achieve significantly lower levelised costs of H₂ compression compared to mechanical alternatives. The sensitivity analysis highlights that the electricity price would have to drop to $8/MWe (AUD) for the mechanical compressor to be cost-competitive when heat can be sourced for MH compression at no cost.

The sensitivity analysis shows that while capital costs dominate for the metal hydride compressor, these would need to double from those estimated here (alloy costs increased to $100–$200/kg (AB2/AB5) and pressure vessels costs $90,000–$150,000 (low/high pressure)) for the mechanical compressor to be cost-competitive (where waste heat can be sourced at no cost).

This analysis highlights that the cost of electricity for mechanical compression has the greatest impact on cost-competitiveness. Metal hydride-based compressors see the greatest cost advantage where electricity costs are high, and where waste or solar heat can be provided at low or no cost.