Fuel Cell Technology

This section provides an introduction to fuel cell technology. Fuel cells have recently been identified as a key technological option for improving both building energy efficiency and reducing emissions en-route to a zero carbon built environment (Hawkes et al. 2009; Kazempoor et al. 2011). Fuel cells are not heat engines, and thus their efficiencies are not limited by the Carnot efficiency. By combining hydrogen and oxygen in electrochemical reactions as shown in Fig. 2.1, fuel cells have the potential to produce highly efficient electrical power with little or no emission of environmentally damaging pollutants such as CO2.

The exothermic nature of the electrochemical reactions makes fuel cells ideal candidates for CHP and tri-generation system applications. Invented in 1839, fuel cell technology is by no means new, however, in the past, fuel cells have struggled to flourish particularly in terms of commercialisation and market application. In the past fuel cells have been used in bespoke projects. NASA used fuel cells in some of their early space shuttles to provide electrical power, heat and water. The reasons for this stalled start are mainly cost but also technological reliability, lack of interest from investors and supporting infrastructure (Gencoglu and Ural 2009). Fuel cells are often categorised by the type of electrolyte. This is determined by the type and purity of the fuel and oxidant used and the operating temperature. There are currently six types of established fuel cells on the market (O’Hayre et al. 2006).

• • Proton Exchange Membrane Fuel Cell (PEMFC)
• • Alkaline Fuel Cell (AFC)

Fig. 2.1 Fuel cell operating concept (ElianEnergy 2011)

• • Direct Methanol Fuel Cell (DMFC)
• • Phosphoric Acid Fuel Cell (PAFC)
• • Molten Carbonate Fuel Cell (MCFC)
• • Solid Oxide Fuel Cell (SOFC)

The first three fuel cells are classified as low temperature (80-250 °C), whilst the remaining three are medium to high temperature (250-1000 °C). The operating temperature is often a significant factor when determining which type of fuel cell should be used in a particular application. This is due to a number of factors including; heat usability, start-up time and ability to vary output.

Owing to the variety of types of fuel cells and their modularity, fuel cells have the ability to cover a range of building applications from a single family home to an entire hospital (Kazempoor et al. 2011). Fuel cells are now recognised, across a variety of markets, most significantly the stationary, as a superior technological option compared to conventional combustion based generators (FCT 2013). As a result, the stationary sector is currently the largest user of fuel cell technology, showing year on year growth, demonstrated in Fig. 2.2. In 2012 alone, 125 MW of fuel cells for stationary applications were shipped, a 53 % increase on 2011 figures, representing the rapid expansion of the sector. Furthermore, Ceramic Fuel Cells Ltd. (CFCL) reported that the domestic housing SOFC market is around 17,000 kWe installed per annum, a large market potential. E.ON believes most UK homes are technically suitable for fuel cell CHP, equal to a potential total installed capacity of 24 GWe (Harrison 2012). Fuel cells are an attractive option for building applications because of their high electrical efficiency (even at part load), low emissions, near silent operation, flexibility of fuel use and useful heat output.

Fig. 2.2 Fuel cell use by application 2009-2013 (FCT 2013)

Table 2.1 Summary of PEMFC and SOFC characteristics (Minh 2004; Wu and Wang 2006; Gencoglu and Ural 2009; Hawkes et al. 2009)

Of the six fuel cell variants mentioned, the low temperature PEMFC and the high temperature SOFC demonstrate the greatest promise for early market application, attracting the most attention and investment in building application projects (Sammes and Boersma 2000; Gencoglu and Ural 2009; Kazempoor et al. 2011; Ellamla et al. 2015). PEMFC and SOFC characteristics are summarised in Table 2.1.

Currently it is estimated SOFC developments are around 5 years behind PEMFC, however many commercial developers believe the future of fuel cell technology in the built environment lies with SOFC systems. This is due to lower capital cost as they do not need to use expensive platinum catalysts like PEMFCs. Furthermore, as highlighted in Fig. 2.3, SOFC based systems operating on natural gas offer significant advantages in terms of system complexity and efficiency when compared to PEMFC based systems. This is on account of SOFCs ability to run directly on natural gas, with fuel reformation occurring directly on the anode (Hawkes et al. 2005; Boyd 2008). In comparison, PEMFC systems require large, complex and expensive external reforming systems to produce very pure hydrogen, otherwise accelerated stack degradation will occur.

Furthermore, SOFCs are often cited as a more attractive fuel cell option for tri-generation system applications because of their high quality exhaust heat (Brouwer 2010). Extensive literature searches have highlighted that extensive work has been carried out on domestic scale fuel cell CHP systems (Beaussoleil- Morrison 2008; Beausoleil-Morrison and Lombardi 2009; Farhad et al. 2010; Choudhury et al. 2013). However, only a small amount of work on domestic scale

Fig. 2.3 Fuel processing reactions (methane to hydrogen) and their influence on the complexity and efficiency of fuel cell systems (Steele 1999)

SOFC tri-generation system has been found, and this has been predominately simulation based work (Pilatowsky et al. 2007, 2011).

This section has provided an introduction to fuel cell technology and its application in the built environment. Next, Sect. 2.3 presents a review of liquid desiccant air conditioning.