Tri-generation System Sub Component Analysis
In this section, the SOFC CHP and liquid desiccant air conditioning system sub components are described and their operational performance detailed through parametric analysis. Using the parametric analyses, the sub-component system sizes and operation are optimised to facilitate tri-generation integration. Next, Sect. 4.2.1 presents the SOFC CHP system sub-component description, modelling and parametric analysis.
Solid Oxide Fuel Cell CHP System
The aim of this section is to develop a SOFC CHP model so that the complete trigeneration system may be simulated in Sect. 4.3. The requirements of the SOFC CHP model are electrical and thermal output, electrical and CHP efficiency and heat to power ratio. The main independent variables considered in the SOFC modelling are: current density and fuel utilisation factor. These variables have been selected as they are influential on the performance of SOFC CHP and tri-generation systems (Al-Sulaiman et al. 2010).
A great advantage of SOFC technology is the potential for internally reforming a hydrocarbon fuel such as methane to hydrogen, thus removing the need for the expensive and complicated external reforming equipment seen in PEMFC systems. SOFCs also offer higher electrical efficiencies, long-term stability and relatively low costs compared to other fuel cell variants. However, disadvantages include high operating temperatures required for electrical and ionic conduction, meaning long start up times, issues with mechanical and chemical compatibility and little provision for on/off cycling. As a result, a large amount of research has focussed on lowering the operating temperatures of SOFCs to 500 °C and below (Tuyen and Fujita 2012; Fan et al. 2013). Furthermore, the maximum operating temperature of standard metals such as stainless steel is below 650 °C, therefore leading to much lower cost materials and components.
Unlike other fuel cells, SOFCs can have multiple cell geometries. There are three SOFC geometries: planar, co-planar and micro-tubular. In the most common planar type design, as shown in Fig. 4.2, an electrolyte is sandwiched between an anode and cathode to form flat cells, which are assembled into stacks. The fuel (hydrogen or short chain hydrocarbons) and oxidant (oxygen or air) flow though the unit via channels built in the anode and cathode respectively. A number of these cells are connected in series to form a stack to produce a useful electrical output. Conventionally, the anode, cathode and electrolyte materials used in SOFCs are: Ni-YSZ (Nickel-Yttria-stabalised zirconia), LSM (Lanthanum strontium manganite) and YSZ (Yttria-stabalised zirconia) respectively; however other material options do exist.
The YSZ electrolyte materials used in SOFCs do not become electrically and ionically active until they reach very high temperature and as a consequence the stacks have to run at temperatures ranging from 500 to 1000 °C. Air is supplied to the cathode, where reduction of oxygen into oxygen ions occurs: O2 + 4e- ^ 2O2- The oxygen ions then diffuse through the solid oxide electrolyte to the anode where they electrochemically oxidise the hydrogen fuel to produce water and electrons: 2H2 + 2O2- ^ H2O + 4e-. An external circuit allows
Fig. 4.2 Planar type SOFC construction and operation (Tuyen and Fujita 2012)
the flow of electrons from the anode to the cathode and thus current generation. Because there is a potential difference generated across the cell, electrical power is produced. The cycle then repeats as the released electrons re-enter the cathode. The overall reaction occurring in a SOFC is 2H2 + O2 ^ 2H2O. When using methane as a fuel, the CH4 reacts with O2 via internal reforming, producing H2O and some CO2, as shown in Eq. 4.2. This is discussed in more detail below.
Due to its ready availability, natural gas is the most commonly used fuel in SOFC building applications. Because the typical composition of natural gas is 70-90 % methane, the modelling work assumes a direct methane fuel input to the SOFC from the domestic gas stream, an assumption adopted by previous researches in numerical studies (Blum et al. 2011). In reality the natural gas would need to be de-sulphurised with a small amount of pre-reforming before being supplied to the SOFC. When operating with methane, hydrogen is produced through internal steam reforming on the fuel cell anode. In this work, a planar type Ni-YSZ anode supported SOFC with LSM cathodes and thin YSZ electrolytes, operating with methane at a temperature of 800 °C has been modelled. A description of the SOFC modelling procedure is provided in Sect. 18.104.22.168.