Comparison of Fuel Cell Systems [12].
Parameters |
Fuel cell type |
|||||
PEMFC |
AFC |
PAFC |
MCFC |
SOFC |
DMFC |
|
Electrolyte |
Solid polymer membrane |
Liquid solution of KOH |
Phosphoric acid (H3PO4) |
Lithium and potassium carbonate (LiAlO2) |
Stabilized solid oxide electrolyte (Y2O3, ZrO2) |
Solid polymer membrane |
Operating temperature(°C) |
50-100 |
50-200 |
200 |
650 |
800-1000 |
60-200 |
Anode reaction |
H2→2H++2e- |
H2→2(OH-)→2H2O+2e- |
H2→2H++2e- |
|
|
|
Cathode reaction |
|
|
|
|
|
|
Charge carrier |
H+ |
OH- |
H+ |
|
O- |
H+ |
Fuel |
Pure H2 |
Pure H2 |
Pure H2 |
H2, CO, CH4, other |
H2, CO, CH4, other |
CH3OH |
Oxidant |
O2 in air |
O2 in air |
O2 in air |
O2 in air |
O2 in air |
O2 in air |
Efficiency (%) |
40-50 |
50 |
40 |
>50 |
>50 |
40 |
Cogeneration |
- |
- |
Yes |
Yes |
Yes |
No |
Reformer is required |
Yes |
Yes |
Yes |
- |
- |
- |
Cell voltage |
1.1 |
1.0 |
1.1 |
0.7–1.0 |
0.8–1.0 |
0.2–0.4 |
Power density (kW/m3) |
3.8–6.5 |
1 |
0.8–1.9 |
1.5–2.6 |
0.1–1.5 |
0.6 |
Installation cost (US $/kW) |
<1500 |
1800 |
2100 |
2000–3000 |
3000 |
- |
Capacity |
30 W, 1 kW, 2 kW, 5 kW, 7 kW, 250 kW |
10–100 kW |
100 kW, 200 kW, 1.3MW |
155 kW, 200 kW, 250 kW, 1 MW, 2 MW |
1 kW, 25 kW, 5 kW, 100 kW, 250 kW, 1.7 MW |
1W to 1 kW, 100 kW to 1 MW |
Applications |
Residential; UPS; emergency services such as hospitals and banking; industry; transportation; commercial |
Transportation; space shuttles; portable power |
Transportation; commercial cogeneration; portable power |
Transportations (e.g. marine-ships; naval vessels; rail); industries; utility power plants |
Residential; utility power plants; commercial cogeneration; portable power |
It is used to replace batteries in mobiles; computers and other portable devices |
Advantages |
High power density; quick start up; solid non-corrosive electrolyte |
High power density; quick start up |
Produce high grade waste heat; stable electrolyte characteristics |
High efficiency; no metal catalysts needed |
Solid electrolyte; high efficiency; generate high grade waste heat |
Reduced cost due to absence of fuel reformer |
Disadvantages |
Expensive platinum catalyst; sensitive to fuel impurities (CO, H2S) |
Expensive platinum catalyst; sensitive to fuel impurities (CO, CO2, CH4, H2S) |
Corrosive liquid electrolyte; sensitive to fuel impurities (CO, H2S) |
High cost; corrosive liquid electrolyte; slow start up; intolerance to sulphur |
High cost; slow start up; intolerance to sulphur |
Lower efficiency and power density |
F. combined cycle cogeneration systems
Some thermodynamic cycles work between high temperature levels and others work between moderate temperature levels. In order to obtain a higher efficiency, a high-temperature topping cycle is combined with a medium- or low-temperature bottoming cycle. The rejected heat from the topping cycle is recovered in the bottoming cycle to produce mechanical/electrical energy. The most common combined cycle systems are the combined brayton – Rankine cycle based systems (Fig. 18). The advantages of this cycle could be observed in Fig. 19. The gas turbine plant operates between 300K and 1700K and rejects heat at 800K. The steam turbine plant in its turn operates between 300k and 750K and rejects heat at 300K. By combining the cycles, a large part of rejected heat in topping cycle is used in the bottoming cycle. The disadvantage of gas topping cycle consisting in high exhaust temperature becomes advantage for the steam bottoming cycle. We can notice in the T-s diagram that the combined cycle covers a larger area, resulting in a higher efficiency.
Figure 18. Brayton - Rankine combined cycle cogeneration system.
The overall efficiency of the Brayton - Rankine combined cycle cogeneration system is:
(1)
where:
are the electrical power generate by gas turbine and steam turbine,
respectively;
is the heat flow generated by steam system;
are the heat flow produced by fuel combustion in gas turbine and heat
recovery steam generator respectively.
Figure 19. T-s diagram of combined cycle (Brayton cycle with Rankine cycle).
In order to find what the cycles are suitable for combination they should be ranked according to their operating temperature range (Fig. 20) [14]. The Rankine and Stirling cycles are suitable both for topping and bottoming cycle. The Brayton, Otto and Diesel cycles and also the high temperature fuel cells can be better used as topping cycles. The Kalina cycle (a modified Rankine cycle operating with ammonia-water mixture), organic Rankine cycles and low temperature fuel cells can be used only as bottoming cycles.
Figure 20. Thermodynamic cycles arranged according to their temperature range of operation [14].