DOE has prepared a Multi-Year Research, Development, and Demonstration Plan to provide hydrogen as a viable fuel for transportation after 2020, in order to reduce the consumption of limited fossil fuels in the transportation industry. Hydrogen fuel can be derived from a variety of renewable energy sources and has a very high BTU energy content per kg, equivalent to the BTU content in a gallon of gasoline.
The least expensive delivery option for hydrogen involves pipeline transport of the hydrogen from the production sites to the population centers where the vehicles will see the highest demand and thus have the greatest impact on reducing the US dependency on fossil fuel. The cost to deliver the hydrogen resource to the refueling stations will add to the ultimate cost per kg or per gallon equivalent that needs to be charged for the hydrogen fuel. In order to keep the cost per gallon (equivalent) of hydrogen fuel as low as possible to the consumer, it is necessary to reduce the cost of its transmission to the refueling station. This can be done by reducing the cost of the compressors, as well as all other refueling station components, while also increasing their energy efficiency. DOE has set a target of $1/GGE (Gallon of Gasoline Equivalent).
To meet these goals, the DOE has commissioned Concepts NREC (CN) with the project entitled: “The Development of a Centrifugal Hydrogen Pipeline Gas Compressor”. CN’s engineering project plan consists of three distinct phases, as detailed in Figure 1. Phase I of this project has been completed and is described in this article.
The engineering approach used by CN to accomplish these goals is to utilize state-of-the-art aerodynamic/structural analyses to develop a high-performance centrifugal compressor system for pipeline service. The centrifugal-type compressor is able to provide high pressure ratios under acceptable material stresses for relatively high capacities (flow rates that are not usually higher than what a piston compressor can provide). CN’s technical approach also includes the decision to utilize commercially available, and thus proven, bearings and seal technology to reduce developmental risk and increase system reliability at a competitive cost. CN has researched the use of a material that is compatible with hydrogen and that can enable the highest possible impeller tip speeds, which results in the reduction of the number of required stages while meeting DOE’s goals for overall pressure ratio and efficiency.
In order to minimize the development time and ensure industrial acceptance of the design for the new pipeline compressor system, CN has assembled a project team to assist in the advanced engineering of the compressor while also preparing an implementation plan that can provide for near-term industrial pipeline applications. The team consists of Praxair (to provide industrial user experience and “hands-on” experience), Texas A&M University (to provide materials science expertise and testing), and HyGen Industries (to provide experience in hydrogen fueling infrastructure).
The engineering challenge to implement this technical approach is to design a compressor stage that can achieve the highest acceptable pressure ratio and thermodynamic efficiency per stage, but also by using as few stages as possible and the smallest diameter. For centrifugal-type compressors, the pressure ratio is proportional to rpm2 and radius2. Thus, even a small increase in tip speed or impeller radius results in significant increases in pressure. The aerodynamic design challenge in reducing the number of stages is to maximize the tip speed of the centrifugal compressor impeller, while also staying within acceptable design safety levels of the strength limitations of material and utilizing advanced diffuser systems to maximize recovery of dynamic head into static pressure. However, material stresses also increase proportional to the square of the rotor speed (rpm2) and radius (radius2) and also by material density. Ultimately, the major constraint is imposed by the limitations of the maximum stress capability of the impeller material. This constraint is further aggravated by the need for the material selection to consider the effects of hydrogen embrittlement on the strength of the material.