The feasibility of using a sustainable and clean energy source to replace the battery for an underground rail station's emergency power supply is presented in this study.
The fuel cell is truly the world’s forthcoming sustainable and clean energy source. One reason is the fuel cell's power-generation efficiency. Fuel cells offer the ability to convert chemical energy directly into electrical energy with a very low environmental impact. Fuel-cell technology shows great promise and is already used in various systems globally. For example, in the Middle East, fuel cells are being considered for use in rolling stock; in Hong Kong, fuel cell technology is used in a telecommunications data
WSP | Parsons Brinckerhoff conducted an evaluation
This article presents the various types of fuel cell technologies, the two technologies chosen as most applicable for this study and the results of the evaluation which considers: application, heat dissipation, space requirements, life-cycle cost, and safety and maintenance considerations.
Fuel Cell System
A fuel cell is an electrochemical device used to create electricity through a reaction between a fuel (such as hydrogen) and an oxidant (such as oxygen) in the presence of an electrolyte. In addition to producing electricity, the reaction generates by-products, typically water and heat, thereby making fuel cells an environmentally friendly energy source.
Fuel cell plants are now being installed in some countries for continuous power generation and emergency backup power, replacing diesel generators and uninterruptible power supply (UPS) systems. A fuel cell produces direct current (DC) power, not alternating current (AC) power. Therefore an alternating current (AC) fuel cell system requires an inverter and sometimes a transformer, depending on the output voltage and output power required.
Types of Fuel Cell Technologies
Fuel cells are classified primarily by the kind of electrolyte they employ. This classification determines the kind of chemical reaction taking place in the cell, the kind of catalyst required, the operating temperature range, the fuel required, and other factors which can affect the fuel cell performance. These
(Source: U.S. Department of Energy, Office of Energy Efficiency & Renewable Energy; http://energy.gov/eere/fuelcells/comparison-fuel-cell-technologies )
Proposed Fuel Cell Technology for an Underground Railway Station Application
With the consideration of fuel cell capacity, the operating conditions in the rail station environment, the operating temperatures of the different fuel cell types, availability in
Polymer Electrolyte Membrane Fuel Cells (PEMFC)
For the PEMFC installation, suppliers proposed storing the hydrogen in high-pressure gas cylinders in the railway station. However, as hydrogen is flammable and can be explosive under specific conditions, it is preferred not to have large amounts stored in an underground station. Besides cylinders, hydrogen gas can be stored in a metal-based catalyst in solid state form which is non-flammable. The hydrogen will be converted back by the catalyst when the fuel cell is in operation. A PEMFC system is subject to approval by the local fire services department for an underground installation.
Reformed Methanol Fuel Cells (RMFC)
For RMFC systems, methanol is used as the fuel because it is naturally hydrogen-dense and can be ‘steam reformed’ into hydrogen gas for the fuel cell. However, the flash point of diluted methanol is 19˚С (66˚F) and the boiling point of diluted methanol is 73˚С (163˚F). In Singapore, it is classified as category 2 of dangerous goods.
Comparison of Space Requirements for Emergency Power Supply (EPS) Systems with Batteries, PEMFC and RMFC
Table 2 presents an overall comparison of systems which considers: application, heat dissipation, space requirements, life-cycle cost, and safety and maintenance considerations.
Recommendations
With the currently available fuel cell technologies, we have reservations about recommending the replacement of an emergency power supply (EPS) system battery in an underground railway station with a fuel cell due to the following reasons:
- The storage of hydrogen gas for PEMFC with a metal-based catalyst and a diluted methanol fuel cell is relatively safe. However, it is a new technology with high initial cost and maintenance cost.
- The diluted methanol fuel cell for RMFC has been used for outdoor fuel cell systems with
small capacity (around 5kW to 15kW). - Although the lifetime of fuel cells is quite long, both types of fuel cell systems require an additional fuel supply to generate the hydrogen gas for the fuel cell. Based on the supplier information, the life cycle cost is extremely high compared to EPS with batteries.
- Current fuel cell technology creates large amounts of heat when generating electricity. A higher rating of
fuel cell is required to support ventilation during fuel cell operation for heat extraction and oxygen supply.
Based on the 80kVA capacity, the space requirement of a fuel cell system and its associated ventilation system will be more than an EPS with batteries.
Conclusion
It is suggested that the continued development of fuel cell technologies will enable the battery for the EPS to be replaced with a fuel cell when the following advances are realized:
- an increase in fuel cell efficiency to significantly reduce the heat removal requirement;
- better storage methods for hydrogen gas;
- lower initial cost, operation cost, and maintenance cost of fuel cell technology so it is commercially attractive; and
- approval by the local fire services department on the use of such technology in an underground area.
Although this study concludes that fuel cell technology is not recommended in this case, this technology is truly the world’s forthcoming sustainable and clean energy source. This technology and the lessons learned from this study may be useful when applied to other projects.
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