Combined Micro-Systems - Design for Micro-Combined Cooling, Heating and Power Systems - page 70

Table 4 Main characteristics
of the CHP systems with
Stirling engine

Power range (kW e ) 0.003 - 100

Power to heat ratio 0.33

Electrical ef ciency (%) 10 - 25

Thermal ef ciency (%) 40 - 80

Total efficiency (%) 70 - 90

Fuel type All fuels

1.5 Fuel Cell

Fuel cells use an electrochemical process that release the energy stored in natural
gas or hydrogen fuel to create electricity. Heat is a by-product. Fuel cells that
include a fuel reformer use hydrogen from any hydrocarbon fuel. Fuel cells
transform the electrochemical energy into electricity and heat, through combining
hydrogen with oxygen in the presence of a catalyst (Fig. 9 ).
Through a catalytic reaction, the hydrogen at the anode produces ions and elec-
trons. Ions may pass through the electrolyte and reach the cathode through the external
electric circuit. The reaction at the cathode leads to heat and water generation.
Besides hydrogen, other gases (like pit gas) may be used, especially for high
power cells. The scheme for this case is shown in Fig. 10 . The fuel

-

oxygen mix
may be achieved outside or inside the cell, depending on the work temperature of
the fuel cell.
Fuel cells are similar to electric batteries and have the capacity to produce
continuous current through an electro-chemical process. While an electric battery
produces energy during a time span which is limited by the stored chemical energy,
fuel cells may operate inde

nitely.
Five main types of cells are known today, being classi ed according to the type

of the electrode used:

alkaline (AFC)

￿

with phosphoric acid (PAFC)

￿

with melted carbonate (MCFC)

￿

with solid oxide (SOFC)

￿

with a proton exchange membrane fuel cell (PEMFC).

￿

Anode: 2H 2

+

+ 4e -

4H

Electrolyte: Load

O 2 + 4e - + 4H + Cathode: 2H 2 O

Fig. 9 Electro-chemical conversion in a fuel cell [ 12 ]