5.4.1 Hybrid FC–Gas Turbine Systems
Examples of hybrid FC power generation cycles include the combined high-­
temperature FCs and gas turbines, reciprocating engines, or another FC. These
represent the hybrid power plants of the future. The conceptual systems have the
potential to achieve efficiencies greater than 70%. The hybrid FC/turbine (FC/T)
power plant will combine a high-temperature, conventional MCFC or a SOFC with
a low-pressure-ratio gas turbine, air compressor, combustor, and in some cases, a
metallic heat exchanger [173]. The synergistic effects of the hybrid FC/turbine technology will also provide the benefits of reduced greenhouse gas emissions. Nitrous
(NOx) emissions will be an order of magnitude below those of non-FC power plants,
and carbon monoxide emissions will be less than 2 parts per million (ppm) [174].
There will also be a substantial reduction in the amount of carbon dioxide produced
as compared to conventional gas power plants.
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FCs are inherently attractive for distributed power applications, but their use is
limited by the need to package them in a system balance of plant (BOP) that allows
them to function effectively. All FCs, especially high-temperature FCs, require spent
fuel utilization/waste heat recovery subsystems. Low-temperature FCs also require
fuel reforming subsystems. A common approach for providing these BOP functions
has been to integrate the FC with another generating technology. There can be many
different cycle configurations for the hybrid FC/turbine plant. In the direct mode,
the FC serves as the combustor for the gas turbine while the gas turbine is the BOP
for the FC, with some generation. In the indirect mode, the FC uses the gas turbine
exhaust as air supply while the gas turbine is the BOP. In indirect systems, hightemperature heat exchangers are used [175].
The combination of the FC and turbine operates by using the rejected thermal energy
and residual fuel from a FC to drive the gas turbine. The FC exhaust gases are mixed
and burned, raising the turbine inlet temperature while replacing the conventional combustor of the gas turbine. Use of a recuperator, a metallic gas-to-gas heat exchanger,
transfers heat from the gas turbine exhaust to the fuel and air used in the FC. Figure 5.11
illustrates an example of a proposed FC/turbine system. One item to note in Figure 5.11
is that the combustor shown prior to the turbine is for start-up purposes only.
Countries around the world are developing interest in the high-efficiency hybrid
cycles. A 320-kW hybrid (SOFC and gas turbine) plant entered service in Germany
in 2001, operated by a consortium under the leadership of RWE Energie AG. This
was followed in 2002 by the first 1-MW plant, which was operated by Energie BadenWurttemberg AG (EnBW), Electricite de France (EDF), Gaz de France, and Austria’s
TIWAG [174]. The hybrid power systems based on high-temperature FCs and gas turbines have been extensively analyzed and studied by various universities, industries,
FIGURE 5.11 Diagram of a proposed Siemens–Westinghouse hybrid system. (Taken from
DOE Project Fact Sheet—FC/ATS Hybrid Systems [173].)
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and the US Department of Energy for stationary power generation applications [178–
187]. Successful development and commercialization of FC/turbine hybrid power
generation will allow the following: (1) extremely high efficiency compared to other
fossil fuel systems; (2) ultralow emissions without additional cleanup; (3) siting flexibility with environmentally friendly energy systems; and (4) fuel flexibility.
The SOFC–micro-turbine hybrid systems are also being investigated for use as APUs
in commercial airplanes to provide the power to all the electrical loads [188], and in
railroad vehicles to provide the power to all the accessory loads. Combination of a hightemperature FC with a turbine/micro-turbine has several important ramifications to the
energy and transportation industry. The SOFC systems are being developed in the range
of 5 kW for automotive applications to several megawatts for power generation applications. The micro-turbines are being developed from 30 kW to 30 MW, and the gas
turbines power is in the range of 100–1,000 MW. The study of Rajashekara [189,190]
analyzes 500-kW SOFC–gas turbine, which can be used as an APU in cruise ships,
airplanes, and trains. The same system could be used for distributed power generation
applications. If more power is required, more of these systems could be paralleled or
the individual systems can be distributed to meet the power demands of the local loads.
An SOFC/gas turbine hybrid system of 500-kW power is depicted in Figure 5.12.
The fuel is first reformed to obtain the hydrogen-rich reformate and fed to the anode of
the SOFC. The ambient air is drawn using a compressor and pressurized to about 300–
400 kPa (3–4 atm). The compressed air is heated using the exhaust of the gas turbine
with a heat exchanger and fed to the cathode. The cathode exhaust from the SOFC and
the unused fuel from the anode are burned in a combustor to increase the temperature
of the exhaust to about 1,000°C to meet the requirements of the turbine. The heat and
the pressure difference drives the downstream turbine to generate more power without
using additional fuel. The turbine exhaust after heating the compressor exit air is also
used for heating the fuel that is going into the reformer. The turbine drives the generator
FIGURE 5.12 SOFC–gas turbine hybrid system [189,190].
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and produces a three-phase AC output. This AC power is first converted to the DC
power and then combined with the DC output from the FC using the power-conditioning system. This DC is converted to the AC output before feeding to the utility.
The gas turbine and the SOFC are tightly coupled in the system [191]. Also, operating with elevated pressure will yield increased power and efficiency for a given
cycle. The use of a pressurized SOFC will also lead to optimum integration with the
gas turbine. The gas turbine supplies heated compressed air to the SOFC. During
normal operating conditions, no additional air or fuel is needed to the gas turbine
unit. The reforming process could be endothermic, autothermal, or partial oxidation process. The fuel and air utilization could be varied to give the best system
performance within the constraints of stack cooling and heat exchanger metrics. For
stationary power generation applications, generally, natural gas is used as the fuel.
For trains and ships, diesel fuel is used, and for airplanes, jet fuel is used. The system
can use other types of fuels also. The reformer has to be capable of converting these
fuels to hydrogen-rich fuel for the FC.
In the study by Rajashekara [189,190], the system was modeled using literature
equations [192–194]. The selection was constrained by the operating pressures of
the turbine and the compressor. For 500-kW power, the analysis showed that combining the turbine with the SOFC will result in higher efficiencies of the order of
about 68%. If a larger stack size of 500,000 cm is used, the stack power density
would be 0.89 W/cm and the combined efficiency would be 71.5% for a 500-kW
hybrid power system. The DC output of the SOFC and the AC output of the turbine–generator system can be combined using several different power-conversion
configurations. In Figure 5.13, the fuel-cell voltage is converted to a focusing of a
boost converter and an inverter, and then combined with the three-phase AC output
from the generator at the secondary of the three-phase transformers. Similar powerconversion techniques or variations of these are being used by different fuel-cell
system manufacturers [195].
FIGURE 5.13 Electrical system diagram of the hybrid SOFC/turbine systems [189,190].