Electricity generated from gas-fired power plants accounts for approximately 21% of the world’s total electricity generation. This electrical energy totals to about 1168 GWe. Europe has a total generation capacity of 804 GWe. 22% of this capacity is generated from gas power plants . Generally, gas turbines come in two configurations namely: Open-cycle Gas Turbine, OCGT and Closed-Cycle Gas Turbine, CCGT. Gas turbines are considered as the opponents of Renewable Energy Sources, RES, and they are certainly not environment friendly. In this article we will provide a reference to the Gas Turbines and their techno-economic characteristics so that we will have some basic knowledge of this technology.
OCGT configuration is more basic when compared to the CCGT configuration. An OCGT configuration is composed of a rotating compressor that is joined to a turbine at its downstream. Between the compressor and the turbine, there is a combustion chamber. During operation, air is drawn into the compressor from the atmosphere through a filter. This air is then compressed in the compressor to a higher pressure. The compressed air is then forced into the combustion chamber where it is mixed with the gas and ignited. The combusted mixture is at a very high temperature and possesses high energy. The compressed high temperature air-gas mixture is then released into the turbine through a nozzle. The combusted mixture expands in the turbine and releases the energy. This energy is converted into mechanical energy by rotating the shaft that is coupled to the generator. This rotation provides power to both the compressor and the generator. The generator converts the mechanical energy into electrical energy. The exhaust gases are then released to the atmosphere. This is a great loss since the exhaust gases still contain high energy which can be used to generate extra power.
Over two thirds of the total power generated from the gas turbine drives the compressor while the other third drives the generator to generate electricity. OCGT plants are relatively less efficient, with efficiency values of 35 to 42%.
CCGT are more efficient than OCGT. For the last two decades, CCGT have been preferred to the OCGT when building a new gas power plant. CCGT plants have a compressor and a gas turbine just like the open-cycle gas turbines. However, the CCGT do not discharge the exhaust gases to the atmosphere. The exhaust gases are directed to a heat recovery steam generator where it powers a steam generator to generate more electricity. From the heat recovery steam generator, the exhaust gases are then released into the atmosphere at a relatively lower temperature of about 900. CCGT plants usually have a single gas turbine and one gas turbine. However, large gas-fired power plants have multiple gas turbines. The total power generated by a CCGT power plant is therefore the cumulative power from the gas turbine and the steam turbine. Large CCGT power plants may have more than one gas turbine.
Modern CCGT plants have high electric efficiencies ranging from 52% to 60% when at full load. Intensive research in CCGT is ongoing and is aimed towards raising the efficiency values by increasing the inlet temperatures. The research also aims to reduce the investment costs as well as the emissions. This is done in order to comply with the regulations that have been adopted globally to minimize environmental pollution, which contributes to global warming.
A CCGT plant that has 1700°C gas-turbine can achieve between 62 to 65% electrical efficiency. The CCGT efficiency values of 52 to 60% are expected to increase to over 64% by year 2020. OCGT efficiency will also increase from the current values of 35 to 42% to over 45% by year 2020. CCGT is an established technology and is among the major options for intermediate-loads and base-loads electricity generation. CCGT plants have modular flexibility and are highly adaptable to the changes in both power demand and grid requirements. The relatively high efficiency of CCGT plants and the usage of natural gas as fuel reduce their emissions. For instance, a CCGT emits 50% less carbon dioxide and approximately nine times less oxides of nitrogen per kWh than the current coal power plants. In addition, Selective Catalytic Reduction techniques can be applied to minimize NOx emissions.
The high material and equipment cost, particularly rise in steel prices has greatly increased the investment cost of a CCGT. For instance, an 870 MWe CCGT plant will require €700 million to install. This is equal to €800/ kWe. CCGT is an established technology and decrease in the investment costs of this technology due to technological learning, is expected to be very minimal. For example, the current start up cost of €800/ kWe is expected to decrease to €827/ kWe in year 2020 and €655/ kWe by year 2030. The investment cost of OCGT plants is also expected to have slight decrease from €655/kWe to €618/kWe in 2020 and €581/kWe in year 2030. The annual operation and maintenance cost of both the CCGT and the OCGT plants is about 4% of the investment cost. This means that, the annual CCGT operations and maintenance costs will decrease from €32/kWe to €24kWe in 2030. On the other hand, the operation and maintenance costs of the OCGT power plants will decrease from the current €26/kWe to €24/kWe in the year 2030. When compared to OCGT, the CCGT plants have overall generation costs of about €52/MWh. On the other hand, the OCGT have an estimated operations cost of about €155/MWh with the fuel cost about €42/MWh. The fuel cost for an OCGT plant is usually 50% more than that of a CCGT plant. Therefore, the efficiency of a CCGT is approximately 67% higher than the OCGT.
CCGT technology offers great competition to all power generation technologies and has significantly increased its share in the electricity market. The main disadvantage of this technology is the environmental pollution that causes by emitting significant quantities of carbon dioxide (CO2) and nitrogen oxide (NOx). This is contrary to the RESs which respect the environment and provide an alternative which needs be explored more by all us. RESs are the main focus of our efforts and we believe that the future is in Renewable Energy Sources.