The envisaged system is deliberately simple.  Some suggestions for development of the model, and possible changes to the system itself, are outlined below.

Suggested Project Developments

Possible improvements to the model

  •  More accurate modelling of the compression, to allow for likely rise in temperature of the hydrogen while it is being compressed. 
  • Inclusion of a term to allow for some expected losses of hydrogen (though it is expected these would be small).
  • Inclusion of a heat exchanger at the compressor, which will be necessary to minimise heating of the gas during compression. Such equipment will consume energy, and require capital and running costs, and the space it would occupy may be significant.   Some compressors may include such a system.

Possible project variations

  •  Heat recovery
        The fuel cell will inevitably generate heat, in quantities broadly similar to its electrical output of 3MW. The current simple system does not have any heat recovery incorporated into it. Clearly, there is an opportunity to capture and use this significant quantity of heat.  
        There may be practical challenges finding a suitable use for heat which would be generated for short periods, and unpredictably, as the time of operation depends on the needs of National Grid. Furthermore, as it is envisaged the plant would run at near ambient temperature (realistically, around 50oC is more likely) it is likely the temperature of the cooling water (or other cooling medium) would not be high.  
        However, the modular small-scale approach to the plant construction allows flexibility in its location: site selection criteria could include the presence of a suitable user for occasional heat.

  • High temperature electrolysers and fuel cells
        Fuel cells and electrolysers operating at up to 1000°C were out of scope for this project. They are still very much at R&D stage, but offer potential advantages, including lower losses, and the greater opportunity to collect and use heat inevitably generated in the fuel cell, which would improve overall efficiency  [1-3].

  • Different storage pressure or mode for the hydrogen
        If space permits, one could consider not compressing the hydrogen at all, or compressing it to a pressure lower than 200 bar, which would reduce capital costs, maintenance costs, and also consumption of electricity required for compression. Considering the fuel cell alone is likely to be considerably larger than the storage tank (using equipment that is available to date, based on information about a Hydrogenics fuel cell plant [4], described in Project/Intro section) one could consider such high compression is not necessary, and that a lower storage pressure may be more appropriate.  
        Selecting an electrolyser which operates at higher pressure would also reduce the necessary compression work [5]. Alternatively, if space is at a premium, hydrogen can also be stored in smaller volumes, by compression to higher pressures. Hydrogen can also be stored as a cryogenic liquid, or solid in metal hydride form [6]. While these approaches are will entail additional costs, and the latter use technologies are immature at present, in some circumstances these approaches may be advantageous.

  • Introduction of an expander into the system
        The current system does not capture any of the energy released when the compressed hydrogen is expanded to ambient pressure.  It is possible to incorporate an expander (a turbine) through which the hydrogen may be run between the storage tank and the fuel cell. The expander would generate electricity: less than that used to compress the hydrogen, due to inefficiencies in both compressor and expander, but potentially still useful.  Such an arrangement would contribute to improved round-trip efficiency. 
        Naturally, the additional equipment would require additional capital investment, and would also increase operation and maintenance costs. The amended system would need to be modelled to explore whether such an addition would improve financial performance. 

  • Collection of oxygen
        The proposed system has the oxygen generated at the electrolyser vented to air. There is potential to collect this gas, again, most likely to be under pressure. It could either be used by the plant itself, to supply the fuel cell, or it could be sold as a further revenue stream. Again, the model would need to incorporate projected capital and operational costs for equipment to collect, compress and store the oxygen, and the revenue that could be generated by the use or sale of the oxygen, to assess the effect this addition would have on financial performance. 
[1]            US Department of Energy, Energy Efficiency & Renewable Energy, and Fuel Cell Technologies Programme. (2010). Fuel Cells.
Available: https://www1.eere.energy.gov/hydrogenandfuelcells/pdfs/fct_h2_fuelcell_factsheet.pdf
[2]            C. Spiegel, Designing and building fuel cells. New York: McGraw-Hill, 2007.
[3]            P. Millet, Fundamentals of water electrolysis in "Hydrogen production by electrolysis" ed. Godula-Jopek, A.: Weinheim, Germany : Wiley-VCH, 2015.
[4]            Hydrogenics. (2013). Fuel cell megawatt power generation platform.
Available: http://www.hydrogenics.com/hydrogen-products-solutions/fuel-cell-power-systems/stationary-stand-by-power/fuel-cell-megawatt-power-generation-platform
[5]            O. Ulleberg, T. Nakken, and A. Ete, "The wind/hydrogen demonstration system at Utsira in Norway: Evaluation of system performance using operational data and updated hydrogen energy system modeling tools," International Journal of Hydrogen Energy, vol. 35, pp. 1841-1852, 2010.
[6]            A. Godula-Jopek, Hydrogen storage options including constraints and challenges.  In: "Hydrogen production by electrolysis", ed. A. Godula-Jopek. Weinheim, Germany: Wiley - VCH, 2014.