*3.3. Fuel Cell Stack and Its Cooling System Design*

The power unit of the CHP in the present research consists of a prototype of hightemperature PEM fuel cell and its novel cooling system that were designed and developed specifically for this project. The 40-cells high-temperature PEM fuel cell stack (HT-PEMFC) is formed by 41 JP-945 graphite bipolar plates 280 mm high × 195 mm wide × 5 mm thick manufactured by Mersen, as well as two stainless steel end plates where all the connectors for the reactant gases, H2 and O2/air, are placed. The flowfield geometry in both anode and cathode sides consisted of straight parallel channels with a land-to-channel ratio of 1, as recommended by the MEA manufacturer. The anode side was formed by 47 channels 1 mm wide, 1.5 mm deep, and a total length of 210 mm, while the cathode side is formed by 87 channels with a width of 1 mm and a depth of 2 mm, and a total length of 120 mm. With this design, pressure losses were minimized to 5.87 Pa in the anode and 2.6 Pa in the cathode, ensuring both the homogenous distribution of the reactant gases over the electrodes and the correct water management. Commercial high-temperature membrane-electrode assemblies (MEAs) G1018 Dapozol-110, manufactured by Danish Power System (DPS) with a rectangular active area of 163.5 cm2, were used [38]. The MEAs are formed by phosphoric acid doped PBI polymeric membranes, with a nominal thickness of 650 ± 50 μm, gas diffusion layers of non-woven carbon paper and a platinum load of 1.5 mg cm−<sup>2</sup> in both electrodes. The nominal thickness of the electrodes is 250 μm, including the GDL, the microporous layer and the catalyst layer. To obtain the best results, a minimum compression rate of 13% is advised, as well as a recommended working temperature ranging from 150 ◦C to 180 ◦C. Figure 6a shows the manufactured prototype developed by the PEMFC research team from LIFTEC-CSIC in Zaragoza (Spain), which has an ample expertise in this field [39,40]. Figure 6b shows the electrical and thermal performance of the HT-PEMFC stack. The vertical axes represent the voltage (left axis and red curve) and the power (right axis and green curves), and the horizontal values are the current produced by the electrochemical device. Solid green line corresponds to the electrical power, and the dashed green line is the estimated thermal power.

**Figure 6.** Fuel cell system developed for the MICAPEM project. (**a**) Fuel cell stack designed and manufactured by LIFTEC-CSIC laboratory. (**b**) Performance curves obtained in the test bench. (**c**) High-temperature PEM fuel cell cooling system.

> The fuel cell rated operating point is set in 1.6 kW and 1.9 kW for electrical and thermal power respectively, which ensures a long lifetime of the MEAs [41]. Despite this, the fuel cell is capable to achieve a maximum electrical and thermal power of 2.5 kW and 4.8 kW, respectively with excellent performance. Both operating points are considered in the primary energy analysis.

> High-temperature PEM fuel cells work in a temperature range from 120 ◦C to 180 ◦C that has to be controlled to avoid fast degradation. Figure 6c shows the manufactured novel cooling system specially designed to preheat the stack before starting, and to maintain the required temperature during the HT-PEMFC operation. The novelty of the system is the use of an isothermal oil bath with a dielectric oil that helps not only to keep the fuel cell temperature in the suitable range, but also as an energy buffer system extracting heat from the oil. Preliminary results show that for such system the heat extraction efficiency is higher than the value selected for the algorithm (85%), but no re-calculations have been performed with this higher efficiency.
