3.
IMPLICATIONS
3.1
Energy Efficiency Implications
In a fuel
cell system, electricity and heat are produced by combining
fuel and air. In this way hydrogen can be converted into
electricity with a much higher efficiency than fossil
fuels burnt in a thermal power plant. The efficiency of
electricity production in a thermal power plant is approximately
45 %, while fuel cells in practice have an efficiency
of 40 to 60 %.
The efficiency
of power plants operating on a thermodynamic cycle has
an upper limit, which is the Carnot efficiency. For example,
this limit for a Rankine cycle operating with maximum
steam temperature of 540C at an environment with a temperature
of 25C is 63.3%. Fuel cells convert the chemical energy
of the fuel to electricity directly, with no intervention
of a power cycle. Consequently, the Carnot efficiency
does not apply to fuel cells, which offer high efficiencies
independent of their size. The efficiency of fuel cells
could theoretically reach 100%.
In practice,
several losses in the various components of a fuel cell
system, which consists of the fuel reformer, the cell
stack, the inverter and the auxiliary equipment, result
in efficiencies much lower than 100%. Thus, the electric
efficiency of phosphoric acid fuel cell units, which are
commercially available, is in the range of 37-45%, and
it depends on the quality of fuel and the operating temperature.
At 50% load, the efficiency is equal to and sometimes
higher than the efficiency at full load. The total efficiency
of a cogeneration system reaches 85-90%, while the power
to heat ratio is in the range 0.8-1.0.
As the technology
develops further, in particular for the molten carbonate
and solid oxide fuel cells, electric efficiencies higher
than 50% are expected. Integrated with gas- and steam-turbine
combined cycles, systems based on molten carbonate fuel
cells are expected to have electric efficiency of 55-60%,
while for systems based on solid oxide fuel cells the
expected electric efficiency is 60-65%.
Technical characteristics of cogeneration systems
|
System
|
Electric
power |
Annual
average availability |
Electric
efficiency % |
Total
efficiency |
Power
to heat ratio |
|
MW
|
% |
Load 100% |
Load 50% |
% |
__ |
| Steam turbine |
0.5-100* |
90-95 |
14-35 |
12-28 |
60-85 |
0.1-0.5 |
| Open
cycle gas turbine |
0.1-100 |
90-95 |
25-40 |
18-30 |
60-80 |
0.5-0.8 |
| Closed
cycle gas turbine |
0.5-100 |
90-95 |
30-35 |
30-35 |
60-80 |
0.5-0.8 |
| Joule-Rankine
combined cycle |
4-100* |
77-85 |
35-45 |
25-35 |
70-88 |
0.6-2.0 |
| Diesel
engine |
0.07-50 |
80-90 |
35-45 |
32-40 |
60-85 |
0.8-2.4 |
| Reciprocating
internal combustion engine package |
0.015-2 |
80-85 |
27-40 |
25-35 |
60-80 |
0.5-0.7 |
|
Fuel cells |
0.04-50 |
90-92 |
37-45 |
37-45 |
85-90 |
0.8-1.0 |
| Stirling engines |
0.003-1.5 |
85-90 (expected) |
35-50 |
34-49 |
60-80 |
1.2-1.7 |
| * The value 100 MW is a usual upper limit for industrial applications.
Systems of this type can have higher capacities
too. |
Geothermal Energy and Other Distinctive Energy
Sources