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Subsections


THERMODYNAMIC POWER CYCLES

Power Cycles

There are four power cycles that are generally used in the generation of electricity; the Rankine cycle, the Brayton cycle, the Otto cycle, and the Diesel cycle. The Rankine cycle is used in most base load power plants throughout the United States. The Brayton cycle is the power cycle used for gas turbines, often used in peaking applications. Finally, the Otto and Diesel cycles drive reciprocating engine generators, generally used in smaller power generating applications.

In order to properly discuss the cycles, some discussion of the terminology used is required. First, the term efficiency must be discussed. Every cogeneration cycle contains a number of components, each with a related efficiency. The fuel-to-electrical efficiency encompasses both the prime mover and the electrical generator (including whatever gearing is necessary between the prime mover and the generator). Generally speaking, reciprocating engines have the highest fuel-to-electrical efficiency, with gas turbines second, and steam turbines last. The fuel-to-electrical efficiency of a particular system is also referred to as the heat rate. The heat rate is the number of British Thermal Units (BTU) required to produce one kilowatt-hour of electricity. For a 100% fuel-to-electrical efficiency, this number is 3413, but the second law of thermodynamics limitations and irreversibilities in the cycle increase this number to roughly 10,000 or higher. Figure 2 shows the heat rate of some typical gas turbines. Note that in general, gas turbines become more efficient as they increase in size, becoming asymptotic at a value between 8,000 and 10,000 BTU.8

Figure 2 - Heat Rate as a function of Gas Turbine Size.

The choice of a cycle for a cogeneration unit is not as straightforward as it might first appear. Initial intuition might lead to the cycle with the highest fuel-to-electrical efficiency. It is important to remember that not just electricity is being generated from these cycles. In cogeneration applications, the thermal energy is as important as the electricity. Many industrial applications require vast amounts of steam, and a cycle with a very high electrical efficiency may be incapable of providing the required quality of thermal energy.

Capital costs must also be considered. This is most evident in a retrofit application. If some of the equipment required to operate a given cycle is already available, a given cycle may become the better economic choice, even though it is less efficient. Remember also that other issues such as fuel availability, ease of installation and maintenance, and local electric rates all play a part in the overall choice of a system.

Rankine Cycle

All large steam based power plants operate on the Rankine cycle. Basically, the Rankine cycle utilizes a steam boiler to produce high pressure, high temperature steam. The steam, leaving the boiler at temperatures and pressures as high as 1000 F and 4500 psig, is routed through a steam turbine where it is expanded to produce shaft work that drives an electric generator. In order to increase the efficiency of the overall process, the expansion of the steam is generally performed in stages. After passing through a high pressure turbine stage, the steam is returned to the steam generator to be reheated. After the final expansion stage, the steam is routed to a condenser, where it is returned fully to liquid form and pumped back to the steam generator. By using this method of power production, electric facilities are able to approach 40% efficiency.

Steam Turbine Types

The steam turbines used in the Rankine cycle can be of many types readily available. The most basic type is the backpressure turbine. The backpressure turbine produces electricity through the expansion of high pressure steam to atmospheric pressure and higher. The exiting steam can be directed to an industrial process (in a cogeneration system), or sent to a condenser.

A condensing turbine operates similarly to the backpressure turbine, but the low pressure side of the turbine is below atmospheric pressure. This results in a greater fuel-to-electrical efficiency for the cycle, but the rejected steam is of much lower thermodynamic value, thereby decreasing its value for thermal recovery. Electrical plants, whose sole purpose is the generation of electricity, use condensing turbines.

The extraction turbine allows for the removal of steam from the turbine during expansion. The turbine can be designed to provide a wide array of extraction pressures and flows. This allows for a great deal of flexibility when trying to satisfy both a thermal and electrical load. Extraction turbines are frequently used in cogeneration applications, though the efficiency of the extraction turbine is lower than other turbine types.

Cogeneration and The Rankine Cycle

The classic Rankine cycle is not generally used for heat recovery. The large base load power plants which operate on the Rankine cycle do not recover the heat from either the exhaust or the condenser. To be technically classified as a cogeneration cycle, the Rankine cycle must provide some heat recovery. This heat recovery can be taken from the combustion exhaust (if it is suitably clean), from the condenser, or, if an extraction turbine is used, directly from the steam turbine itself.

One attractive feature of a Rankine cycle system is its capability of operating on many types of fuel. Coal, chemical wastes, heavy and light oils, biomass, and even natural gas are used to fuel Rankine cycle plants. Furthermore, steam turbines are readily available in almost any size. These features combine to give the Rankine cycle unmatched flexibility to meet specific load conditions. Rankine cycle plants have a long life span, and all parts of the system are reliable and require relatively little maintenance.9

New Rankine based cogeneration systems can be quite expensive. The heat exchangers (boilers and condensers) are very expensive in terms of both equipment cost and installation cost. Usually, Rankine cycle cogeneration systems are economically attractive if the boilers and condensers are already in place, and a steam turbine is all that is required to cogenerate electricity.

Brayton Cycle

The Brayton cycle is the gas turbine power cycle. It is an open cycle where ambient air is compressed to a high temperature and pressure before it is fed to the combustion chamber. In the combustion chamber, the air-fuel mixture is ignited, dramatically increasing the temperature of the mixture. These hot gases are then expanded in a turbine, which can be coupled to an electric generator or some other load. In a turbine cogeneration system, the exhaust gases are collected and passed either to process (the relatively clean exhaust is suitable for high temperature drying), or to an HRSG. Because the exhaust gases leave the gas turbine at such high temperatures (~1000F), they have good thermal quality and are capable of producing large amounts of high pressure steam. Also, since so much excess air is used during the combustion process, the exhaust contains enough oxygen to sustain further gas firing, resulting in more usable heat. Supplementary fired HRSG units are capable of achieving total fuel utilization efficiencies of 90+%.10

Brayton cycle cogeneration systems have low capital costs, as well as low maintenance costs. Turbine systems generally last for twenty years if properly maintained, so the life span of the system is excellent. With turbines available anywhere from 3 to 250 MW, almost any size system may be built. Figure 3 shows the trend of cost vs. size for gas turbine generator sets. Installation costs and HRSG costs are not included. The cost for a HRSG capable of fully utilizing the thermal energy from a gas turbine adds about 15% to the cost of the gas turbine. Installation costs (includes engineering and construction) vary considerably from location to location, but typically add between 30-50% to the cost of the turbine.

Figure 3 - Cost per kW for Gas Turbines as a function of kW Rating.

Due to the high parasitic power requirements of the compressor and gearbox losses, the gas turbine cycle generally has a sharply decreasing efficiency at less than full loads. Typical full load efficiencies for a simple gas turbine hover around 30%. Gas turbines can operate on almost any type of gaseous fuel, but only lighter fuel oils (No. 2) can be used as a substitute. Also, when operating under sharply changing loads, maintenance costs are increased and life span is greatly decreased.

Regeneration

Regeneration is a method used to increase the fuel-to-electrical efficiency of the Brayton cycle. By using the exhaust gases leaving the turbine to preheat the incoming ambient air entering the combustion process, the fuel-to-electrical efficiency can be increased. The effectiveness of the heat exchanger between the exhaust and combustion air determines the exact increase in fuel-to-electrical efficiency of the regenerator-gas turbine system, but the increase is substantial. In a cogeneration system, this may not be desirable, since both the thermal energy and electrical energy are used in a cogeneration process. Therefore, regeneration should only be considered if the electrical load exceeds the thermal load by a considerable margin.

Otto and Diesel Cycles

The Otto and Diesel cycles are used to power reciprocating engine generators. They are treated as a closed, 4 stroke cycles for the purpose of analysis. The two cycles are similar in theory, with one major difference; the Otto cycle is a spark-ignition cycle, whereas the Diesel cycle is a compression ignition cycle. In the Otto cycle, the piston begins in the top of the cylinder. As the piston moves down into the cylinder, the air-fuel mixture is fed into the combustion chamber. The piston then rises to the top of the cylinder, compressing the mixture. A spark is ignited inside the cylinder, burning the air-fuel mixture and forcing the piston to the bottom of the cylinder. As the piston is forced down, it turns a crankshaft, generating work. The piston returns to the top position, forcing out the exhaust and completing the cycle.

The Diesel cycle operates similarly. The main difference is that only air is fed into the combustion chamber during the first process. The fuel is injected after the air is compressed to near top-dead-center (TDC). At this point, the air inside the cylinder is hot enough to cause the fuel to burn without a spark. The remaining two processes are identical to the Otto cycle.

In the Otto cycle, the air-fuel mixture does not get hot enough to burn before the spark is ignited. The compression of the air-fuel (or air) is the physical process which controls this. Compression ratios for the Diesel cycle are higher than that for the Otto cycle, causing the compressed temperature inside a Diesel cycle cylinder to be higher than the Otto cycle cylinder. In fact, the auto-ignition temperature of the air-fuel mixture limits the compression ratio of the Otto cycle. If this temperature is exceeded, spontaneous combustion (known as knocking) takes place at the wrong time in the cycle, robbing the engine of power and efficiency.

Diesel and Otto cycles are used in smaller applications, ranging from 10's of kW to 2-3 MW. Boosting the output of reciprocating engines is possible through either turbocharging or supercharging the engine. Both processes increase the input pressure of air-fuel fed to the motor, allowing it to produce more power. This lowers the cost on a kW basis, but it also decreases the efficiency and the life span of the engine.

Reciprocating engines are a popular choice for smaller cogeneration applications. The cycles have a higher fuel-to-electrical efficiency than the simple Brayton and Rankine cycles. The reciprocating engine cycle responds better to changing loads, which ideally suits it to smaller applications. Also, capital costs for a reciprocating engine generator are relatively low.

Reciprocating engine driven generators generally require more maintenance than other types of prime movers discussed here. Also, the overall life span of reciprocating systems is not as long as the other systems. The recoverable heat from a reciprocating generator is lower than other systems because of both the higher fuel-to-electrical efficiency and the heat lost in the coolant system. Figure 4 shows a breakdown of the energy output from a reciprocating engine. Coolant heat losses constitute roughly 30% of the heat input. The coolant heat is generally of low quality and unless special circumstances exist, cannot be utilized.
 

Figure 4 - Energy Balance for Naturally Aspirated Reciprocating Engine.11