Fulfilling the Promise of Green Hydrogen
Electrolysis powered by renewable energy to produce green hydrogen is one of the fastest growing and most promising decarbonization solutions available today to help industry meet lower emissions targets. In 2019 there was less than 100 MWe of electrolyzers (equipment that uses electric current to break apart water molecules into hydrogen and oxygen gas) installed globally, but today there is close to 1 GWe of operating electrolysis capacity, half of which was added just last year. The growth is set to continue: about 11 GWe of capacity is at an advanced stage of planning for deployment in the next 2 - 4 years, and a huge 480 GWe of capacity has been announced, heralding unprecedented growth in the green hydrogen market over the coming decade.
Most of the hydrogen produced today is used in large-scale chemical and refining plants, usually producing more than 10 tph of hydrogen. This is equivalent to 400 MWth of electrolysis for each large-scale hydrogen consumer. Most industrial applications that use hydrogen require mechanical compression to raise the pressure from the 20 - 30 bar typically provided by the incumbent steam methane reforming process (SMR or ‘grey’ hydrogen), to anywhere between 50 and 200 bar depending on the end-use application requirements. As a consequence, the hydrogen compressor is a critical piece of equipment in the hydrogen value chain. Compression technology has been used and optimized for decades. Most hydrogen compressors operating today are positive displacement machines, primarily reciprocating, which enables high efficiency and high-pressure ratio per stage on low-molecular-weight gases like hydrogen. There are thousands of hydrogen reciprocating compressors operating efficiently and reliably at refineries, petrochemical and chemical facilities (ammonia and methanol), spanning a wide range of power capacities from as low as 100 hp to as high as 20 000 hp (equivalent to approximately 0.1 - 15 MW of energy).
While hydrogen compressors have been around for some time, a new and important requirement has been added more recently. Unlike grey hydrogen – which is produced on a continuous basis – green hydrogen production can be intermittent due to the inherent variability of renewable energy inputs like wind and solar. Consequently, there is a mismatch between the operating requirements of the compressor and the variable nature of the energy input. This article will discuss the main requirements and design considerations for hydrogen compressors paired with electrolyzers for industrial-scale intermittent hydrogen production and use. It will also show how companies like Electric Hydrogen (electrolysis OEM) and NEUMAN & ESSER (compression OEM) think about the various tradeoffs at play to provide a solution to customers that is fit-for-purpose to produce zero emission green hydrogen at large scale to decarbonize industrial applications.
Electrolysis to produce green hydrogen has been historically more expensive than traditional hydrogen production solutions like SMR. Electrolysis OEMs, such as Electric Hydrogen, are combining scale and innovation in the electrolyzer stack and plant design to drive costs down and make green hydrogen at competitive economics. It is important that electrolysis OEM and compression OEMs work together to optimize the entire electrolysis system and reach the lowest-cost solution. It is also important to put these investment costs in perspective: all-inclusive electrolysis plants (inclusive of power conversion, and all elements of balance of plant but not additional compression beyond the electrochemical one) range between US$850/kWe,in and US$1700/kWe,in. Compression systems for these plants add between US$50/kWe,in and US$100/kWe,in depending on the desired flow rate, the suction pressure provided by the electrolyzer, and the discharge pressure required by the end user. These costs need to come down in order to make green hydrogen a viable alternative to grey hydrogen.
Reliable equipment is needed to minimize downtime and maximize the utilization of the equipment upstream and downstream of both the electrolysis plant and the compressor. Opportunity costs or penalties from not producing the contracted hydrogen gas can be burdensome and justify investments in system redundancy and reliability to avoid downtime. For example, to improve the compressor’s reliability, the API 618 standard for reciprocating compressors is commonly adopted to extend the mean time between failures (MTBF). Such a standard does not yet exist for electrolyzers and would be beneficial. Redundancy is a common strategy to guarantee uninterrupted plant operations – typically, a spare compression unit is installed on-site, either sharing the flow with another compressor unit, or as a standby compressor that only operates when the main unit is down.
This is the new requirement created by the inherent intermittency of renewable energy such as wind and solar. Electrolysis systems have been developed to provide highly dynamic operations: advanced proton exchange membrane (PEM) systems provide fast response time. They are able to ramp hydrogen production up and down at about 1 MWe/s, which is sufficient to load following solar photovoltaic (PV) and wind power sources. PEM systems can also be turned down to low operating limits – usually 10 - 15% of system nameplate capacity, without any issues. Partial operations for electrolyzers come with an efficiency advantage as the conversion of kWh of energy to kg of hydrogen gas is better at a reduced system load. It is typical to observe PEM systems conversion efficiency at 72% at full load and as high as 78 - 80% closer to the minimum turndown (the remaining energy is displaced as heat). As a consequence of these dynamic operations, there is a need for compressors that can maintain smooth dynamic operations down to the electrolyzer minimum turndown limit. This removes the need for an expensive buffer storage tank, which is commonly required if the produced hydrogen is at lower pressure (e.g. atmospheric alkaline electrolysis). To do this, a combination of a stepless capacity control system for reciprocating compressors as well as the number of compressor units can be used to offer a fully flexible solution that can be paired with any electrolysis system, independently of how dynamic its operations are. These two important design methods and their implications on the main compression system requirements are discussed in more detail in the following section.
Several critical design features need to be considered when looking at integrating a compression system with green hydrogen production plants. The variables with the largest impact on project performance and economics are:
The compression arrangement is defined as the number of compressors necessary to meet the total required hydrogen gas flow rate. Common arrangements are 1 x 100% (no spare), 2 x 100% (spare), 2 x 50% (no spare), or 3 x 50% (spare). There is a tradeoff between the required redundancy at the site to guarantee a higher uptime and the overall budget available on the project to procure and install the compressors. A larger quantity of smaller units will not take full advantage of procurement economies of scale, but large reciprocating compressors might be more difficult to source and construct and typically require large concrete block foundations, which increases installation costs.
The number of compressors installed on a hydrogen gas process flow defines the compression redundancy available at the plant. A 2 x 100% arrangement, which includes a spare unit, provides full redundancy, almost guaranteeing uninterrupted plant operation. A 2 x 50% arrangement with smaller individual units provides reduced total installed cost as no spare capacity is included but does not permit total capacity availability when one unit is down.
Due to their positive displacement principle, reciprocating compressors can perform at partial load operation while keeping efficiency constant. This is one of the most significant advantages of this compressor technology and is critical for green hydrogen production plants.
A lack of the right capacity control method, with simply relying on step unloading with discrete operation points (100%, 75%, 50%, and 0% operating loads), will inevitably lead to unnecessary gas recycling, which will increase overall power consumption.
A comprehensive capacity control suite is available to operators today, including a range of solutions going from simple pneumatic actuated valve unloaders to complex stepless capacity control systems using hydraulic actuators. Stepless capacity control allows seamless continuous operations but is usually an add-on and increases CAPEX. The API 688 standard provides considerations on stepless capacity control systems for reciprocating compressors.
A machine monitoring system (MMS) is fundamental to ensuring long operational uptime. The main objective of an MMS is to provide critical, real-time information about the compressor condition to allow operators to identify potential malfunction or underperformance of main components, therefore planning preventive actions and avoiding unexpected shutdowns.
Modern MMS offers a wide range of functionalities and can be designed to meet any specific project requirement. Basic functions include frame vibration, piston rod drop systems and process gas pressure and temperature monitoring.
Advanced monitoring functions include cylinder dynamic pressure, crosshead vibration, and valve temperature. MMS requires upfront engineering efforts and additional acquisition costs, but the payback can be very short if the calculation includes a potential loss of production due to an unexpected shutdown.
Many applications for low-carbon hydrogen have stringent oil contamination limits. This can be achieved by eliminating cylinder lubrication during compression or through an oil removal system after lubricated compression. Non-lubricated cylinders have additional design considerations for piston rings and guide rings design that provide adequate resistance to friction while guaranteeing an acceptable wear rate without the oil film protection. Alternatively, operators looking to take advantage of the longer uptime of lubricated compressors, or operators with applications with a final discharge pressure above 200 bar, can choose to install high-efficiency oil removal systems downstream of the compressor. These systems are available today, have demonstrated field-proven experience, and can handle large volumes of flow and high pressures while removing oil on the hydrogen stream up to 1 part per billion (ppb).
Electrolysis and compression OEMs are working together to ensure that green hydrogen projects have access not only to the latest technologies at the lowest possible cost, but also to ensure that all the other project operations requirements are met. Installed costs can be minimized, but that alone will not lead to the lowest overall total cost of ownership when considering downtime as well as the need to operate flexibly and accommodate the relatively low partial load that electrolyzers (especially PEM) can handle today. Companies such as Electric Hydrogen and NEUMAN & ESSER are working together to provide hydrogen project developers and EPC companies with solutions that can be seamlessly integrated and provide the best trade-off, satisfying all the requirements of a green hydrogen project: a sweet spot to optimize cost-uptime-operational flexibility.
A significant amount of green hydrogen will be necessary to meet decarbonization targets set by different countries and companies. Today we are seeing rapid growth in the electrolysis market, and most, if not all, green hydrogen production applications require hydrogen compression. Different players across the green hydrogen value chain need to work together to identify opportunities to not only reduce initial investment costs and increase efficiency, but to also ensure maximum overall plant uptime and flexible operations. This article described some of the collaboration efforts between an electrolyzer OEM and a compressor OEM in order to unlock the overall lowest cost compressed green hydrogen, therefore benefitting project developers and final hydrogen end users.
