Question 13.2: A PEM fuel cell-based residential combined heat and power (C......
A PEM fuel cell-based residential combined heat and power (CHP) plant (refer to Figure 13.8a) supplies electric power and useful heat (hot water) to a house. The CHP plant operates under the following conditions:
• Power demand is P_{el} = 0.5 kW, load duration \tau_{el} = 20 h/day
• Electrical efficiency of the PEM fuel cells is \eta_{el} = 0.45
• Heat to power ratio (HPR) is 0.6 (on the daily basis)
• Overall efficiency \eta_{CG} or EUF of the cogeneration plant is 0.62
• Fuel is hydrogen with an LHV of 120 MJ/kg
• Water is heated by fuel cell exhaust heat from t_{c} of 20°C to t_{h} of 55°C
• Isobaric specific heat of water is c_{p} = 4.19 kJ/kgK
Calculate (i) the daily power generation, (ii) daily useful heat production rate, (iii) daily hot water production rate, and (iv) daily fuel requirements of CHP plant.
1. Daily power generation of the CHP plant
E_{\mathrm{el}}=\tau_{\mathrm{el}}\times P_{\mathrm{el}}=18\times0.5=9\,\mathrm{kWh}/\mathrm{day}=32.4\,\mathrm{MJ/day}2. Daily useful heat production rate
Q_{u}= \mathrm{HPR}\times E_{\mathrm{el}}=0.6\times9=5.4~\mathrm{kWh/day}=19.44~{\mathrm{MJ}}/\mathrm{day}3. Daily hot water production rate
m_{w}=Q_{u}/c_{\mathrm{p}}\;(t_{\mathrm{h}}-t_{\mathrm{c}})=19.44\times10^{3}\,\mathrm{kJ}/\mathrm{day/}[4.19\,\mathrm{kJ/kgK}\times(55-20)]\,K= 176.75 kg/day
4. Daily fuel energy requirements of the CHP plant
Q_\mathrm{f}=(E_{\mathrm{el}}+Q_{\mathrm{u}})/\mathrm{EUF}=(32.4+19.44)/0.62=83.6~\mathrm{MJ/day}5. Daily fuel (hydrogen) requirements of the CHP plant
m_\mathrm{f}=Q_\mathrm{f}/\mathrm{LHV}=83.6/120\approx0.7\mathrm{~kg/day}