Combined Micro-Systems - Design for Micro-Combined Cooling, Heating and Power Systems - page 71

Heat and
water

Clean
Exhaust

Useful heat

Natural
Gas

Fuel
Reformer

Hydrogen
Rich Fuel

Power
Section

Energy
Convertion

DC Power

AC Power

Air

Fig. 10 Conversion system with a fuel cell [ 13 ]

to-heat ratio, suited for
modern low-energy buildings. Fuel cells provide a higher proportion of electricity
than other CHP technologies. The fuel cell systems most widely commercially
deployed are PAFC and PEMFC, with 5,000 PEMFC fuel cells having been
installed in Japan [ 14 ] in 2009 alone. Most operational experience is therefore
limited to these fuel cells, although there are increasing numbers of SOFC and
MCFC systems installed. Combined heat and power systems of building scale is
1

Fuel cells offer the advantage of nearly 1-to-1 power —

10 kWe. Traditional CHP systems are mature and a useful transitional technol-
ogy, while micro-CHP, biomass CHP and even fuel cell systems (using CO 2 -free
hydrogen) may emerge as abatement option. The advantages of fuel cell CHP over
traditional mechanical CHP are the greater degree of modulation and the heat-to
power ratio is typically much lower. In general, fuel cells have high electric effi-

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ciency, under variable charge, and reduced emissions. However, energy generation
seems to represent another promising market that fuel cells could conquer. Actually,
with the exception of PAFC cells, fuel cells are not completely viable for sale yet.
World wide, there is a total capacity of over 40 MW, PAFC fuel cells. A detailed
comparison of the characteristics of these fuel cells is given in Table 5 .

Table 5 Performance characteristics for representative commercially available and developmental
natural gas fuel cell based cogeneration systems [ 15 ]
Fuel cell type PEM PEM PAFC SOFC MCFC

Nominal electricity capacity (kW) 10 200 200 100 250

Electrical efficiency (%) HHV 30 35 36 45 43

Fuel input (MMBtu/hr) 0.1 2.1 1.9 1.0 2.4

Operating temperature [ ° C] 70 70 200 950 650

Cogeneration characteristics
Heat output (MMBtu /hr) 0,42 0,76 0,85 0,34 1,9

Heat output (kW equivalent) 11,7 211 217 100 556

Total overall efficiency (%) HHV 65 72 81 77 65

Power/heat ratio 0.85 0.95 0.81 1.25 1.95

Effective electrical ef ciency (%) 55 65.0 81 65.6 56.5