POWER PRODUCTION ENERGY EFFICIENCY PRACTICES
3.2 INCREASING THE THERMAL EFFICIENCY AND CAPACITY OF PLANT
Table 3.2: Resistance of standard pipefittings measured as equivalent pipe length (meters)
Pipe size Standard elbow Standard bend Tee (flow Gate valve Globe valve
(mm) (90°) (90°) through branch) (open) (open)
50 1.5 0.6 3.0 0.7 17
65 2.0 0.8 4.0 0.85 22
80 2.4 1.0 4.8 1.0 27
100 3.0 1.2 6.0 1.3 34
125 3.75 1.5 7.5 1.6 43
150 4.5 1.8 9.0 2.0 51
200 6.0 2.4 12.0 2.6 68
Source -ERI, 2000d: 12
The effective way to reduce the energy consumption in steam utilisation is to determine the correct pressure of the process and using the right size of pipe and insulation, steam traps and air vents.
Lighting
Lighting is very important in power stations for a better working environment. Depending on the types of lamps and electrical fittings used, the amount of energy consumed and maintenance costs can be significant. Upgrading the lighting system with advanced technologies (high intensity discharge lamps, electronic ballasts and compact fluorescent lamps and metal halide fixtures) and controls (photo cells and occupancy sensors), considerable energy consumption can be reduced.
defers considerably and this will affect the ability to maintain reliable integrated operation. Failure of a component, or its failure to meet performance specification, results in the inability of the plant to perform efficiently or to generate at designed output.
There are many circumstances under which Power systems may fail to meet their performance and reliability expectations. For instance, imperfect information regarding conditions of service can result in exposure to stresses higher than the designed stresses; unexpected trace materials in the fuel supply can result in higher corrosion; improper operation or failure of control components may also shorten components lives. Air in-leakage affects the steam turbine output by increasing the backpressure. It also affects the performance of the condenser due to increased dissolved oxygen (Oz) and carbon dioxide (C02) that will enhance corrosion. An increase of 1 in-Hg backpressure can reduce the turbine capacity by 2% and the corresponding heat rate increase is severe. For every 0.1 in-Hg increase in turbine backpressure the corresponding heat rate increase is approximately 16 Btu/kWh (www.platts.com/engineering/issues/Power/0201/0201pwr_fom.shtml).
Therefore maintenance is very necessary to achieve the reliable and safe operation of any generating unit. Inadequate maintenance accelerates depreciation and leads to excessive damage at an exceedingly high cost in both repairs and down time. Such loses can be prevented. Generally, maintenance of electric generating units falls into three categories. These are proactive, reactive and predictive (Sullivan GP et al, 2002:5.1-5.4).
Proactive maintenance is planned maintenance and it is an action performed based on fixed calendar schedules or hours of service (regardless of the condition of equipment) that spot, prevent, or moderate degradation of component or system with the aim of extending its useful life. Actions like regularly changing of lubricants, replacements of seals, major overhauls that are done according to manufacturers' recommendations o~ utility' experience can extend equipment life and increase reliability.
Reactive maintenance is an unplanned maintenance. This is carried out when components or systems fail or experience performance deterioration. This involves the replacement of an identical part or an
improved design or material. This type of maintenance can also be initiated by discovery of situations that lead to component failure if not corrected.
Predictive maintenance is "measurements that detect the onset of a degradation mechanism, thereby allowing casual faults to be eliminated or controlled prior to any significant deterioration in the component physical state" (Sullivan GP et al, 2002:5.3). It is used to define needed maintenance task based on equipment condition. Predictive capability (monitoring equipment) allows threatening conditions to be exposed and alleviate prior to failure. This can avoid the cost of lost generation, wear and tear on equipment that occur during the shutdowns and start-ups and safety risks associated with failure.
Utilising combined cycles
The relative low efficiency of gas turbine can be due to either low temperature input or high temperature in the exhaust gases. The heat in the exhaust gases may be used to raise the efficiency by regeneration - heating the compressed air before the combustion process. Using the appropriate plant, the heat may be used more effectively to raise steam by feeding the engine exhaust gases into a boiler, to drive a steam turbine and generate more power. Steam-turbine-gas-turbine combined cycle offers an effective method of increasing the heat rate and power output of steam turbines. With such a combination, the heat rate and power output improvement can range between 1.8 -6.5% and 5.8 - 21% respectively (Bennett K F, 002).
Gas-turbine upgrade
The use of gas turbines in hot climates has a major disadvantage. Its output decreases significantly as the ambient inlet air temperature increases. The lower density of the warm air reduces the mass flow through the gas turbine. According to Hicks (1998), the three most common methods to handle such problem and increase the output include:
1. Injecting water or steam into the combustor. This can significantly increase power output, but the overall combined cycle efficiency will be affected. For steam injection, steam can be extracted either from the high-pressure turbine or HRSG section.
2. Precooling using evaporative coolers, mechanical chillers or absorption coolers. This involves spraying water into the inlet air stream to cool the air near the ambient wet-bulb temperature. It is a technique that boosts the gas turbine output by increasing the density and mass flow of the air entering the unit. The overall output can be increased by about 4%-5%, and the incremental cost can be about $200/kw.
3. Supplementary firing of the HRSG. This method lowers the cycle efficiency and has a higher emissions and costs for larger plants.
The choice of any option has to be balanced between capital investment and overall performance of the system as the addition and operation of other components will be affected.
Phillips and Levine (2002) show that supercharging and coating of gas turbine internal components and compressor blades can upgrade gas turbine performance. Supercharging of a gas turbine requires the addition of a fan to boost the pressure of the air entering the inlet compressor, its impact is more significant if it is coupled with inlet precooling downstream of the fan. Supercharging and precooling could increase output by 24% while improving the heat rate by 4%. Applying thermal barrier coating provides an insulating barrier between the hot combustion gases and the metal parts. This will allow increased firing temperature and longer component life. Coating of gas turbine compressor blades provides smoother, aerodynamic surfaces that increase compressor efficiency since smoother surfaces tend to resist fouling.