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New coal-fired plant construction today costs in the neighborhood of $2 million to $3 million per megawatt (MW). This new facility must then be staffed and operated with new resources and additional fuel consumption.

An alternative to new plant construction is to increase plant efficiency, and optimize existing plant output capability. For plants which are at least 20 years old, parasitic (i.e. on-site) power consumption can run as high as 8-15 percent of gross MVA generation. New, more efficient auxiliary system technologies can markedly reduce parasitic power consumption, and do so at investment costs substantially below the two-dollars-per-watt required to add new capacity.

This article briefly presents a survey of candidate areas where utilities can reduce large parasitic and systemic power losses, while adding substantially to the capacity of their existing fossil-fired generation fleet. This can buy utilities substantial time while the future direction of generation technology clarifies itself in the technical, political and economic arenas.

The True Value of Implementing Efficiency Techniques

The typical coal-fired utility power plant, as mentioned above, has internal components which consume anywhere from 8 to 15 percent of gross MVA generation. The revenue of the plant relies almost entirely on the ability to deliver net MW to the grid, and without financial compensation for reactive power delivery margins required by system dispatch. The internal consumption consists mainly of motor loads to drive high-powered fans and pumps for combustion & draft air supply, feedwater, condensate, and cooling water (or cooling air). But many other losses, large and small, add up as well, robbing the plant of available efficiency and capacity.

As utilities have cut back on staff as a response to "competition," among the first positions eliminated were the performance engineers, who tracked the thermal and electrical performance of fleets, unit by unit. Their efforts typically resulted in keeping facilities thermally "tuned up," and kept efficiencies up by 0.1 to 1.5 percent. Fuel cost variations and power price variations literally swamped out the value of any such small efficiency improvements.

Traditional financial analysis done for plant efficiency improvement design or retrofit projects does not include the true value of incremental improvements made available through judicious use of high efficiency techniques. These techniques may or may not add to the capital cost of the project (such as in the installation of a new or replacement motor), or may add some to the capital cost of the project, but with worthwhile returns from an operating cost and/or revenue enhancement standpoint. For example, utility transformer purchasers have some standard rules for up-front valuation of large plant transformers, often up to $3k per kilowatt of reduced equipment losses. This is a very clearly understandable way of valuing higher efficiency.

The True Breadth of Available Efficiency Improvements

Efficiency improvement programs can be implemented via the use of several available technologies, many available as "off the shelf" from plant equipment suppliers. These programs include:

• Replacement or repair of step-up or unit transformers which are damaged, or of high age, and therefore of lower than best-possible efficiency. The step-up transformer "touches" every bit of power emerging from the plant.
• Replacement and resizing of motors for best efficiency (and best efficiency point of operation) for major air, fuel and fluid handling systems. This is far better than rewinding a motor, as that typically reduces its efficiency by several percentage points. Gains here can easily exceed 10 percent of the power consumed by the motor.
• Use of variable speed drives rather than mechanical throttling mechanisms for systems where there is either wide variation in load, or there is a case of having an oversized motor already in place.
• Novel power factor response mechanisms which can allow the plant to run with a power factor closer to unity than has been traditionally been demanded (improving from 0.86 ? 0.95, for example).
• Improving feedwater heater chain efficiency, especially with the use of new highly efficient compact heat exchangers, versus older shell & tube designs. Reducing extraction steam to heat feedwater improves thermal efficiency. Maintaining generator cooling system hydrogen purity to well above the minimum 95 percent, thereby reducing friction losses by hundreds of kilowatts.
• The final stages of LP turbines can benefit from state-of-the-art blade designs, netting up to 10 percent more shaft power contribution from the LP turbine (a 5% overall power gain) without consuming any additional fuel.
• Greenfield site development can be optimized for efficiency, improved maximum output, and lowest cost-of-operation, if these are specific targets of the site development team. This is best achieved by avoiding copycat designs of existing plants.
• Examination of other parasitic plant operations and losses such as excessive compressed air consumption and/or leakage, steam trap management and maintenance, etc.

Large Motor Efficiency Improvements

A typical 600MW coal-fired power plant will have 10-15 large motors (5-25MW), and a somewhat larger complement of medium sized motors (0.1 - 5MW), perhaps 15 to 25 units. In the design history of these motors as part of the plant, the original engineering spec for the motor power and torque ratings were probably increased to accommodate pump/fan performance margins, delivery performance guarantees, and so forth. So an application originally requiring a 2MW motor operating at or near its best efficiency point (BEP) may have been "spec'd up" to a 3MW or more, and will consequently operate well away from its BEP, and can dissipate 10% or more extra power than necessary to do its originally intended job. Over a 40 year life of baseload operation, this precise situation would waste 56GWhr for just one such motor. Multiply that times the cost to produce those GWhr, or even worse, the lost opportunity to sell them, and it becomes obvious that the cost of the motor itself is dwarfed by the cost of the energy it wastes.

Variable Frequency Drives for Fans and Pumps

The actual operating points for air and fluid pumping systems can be well below their actual design limits, due to performance margins adding up during the design and procurement processes used when the plant was designed and constructed. Conversely, post-construction design changes, such as back end flue gas scrubbing, may cause the originally spec'd equipment to now be a bit lacking in the necessary performance. In either case, a variable speed drive can solve a number of operability and controllability issues which arise in these scenarios, and do so in an energy efficient manner.

A significant additional benefit from new VFD's is that they reduce downtime compared with first and second generation drives that still exist in many plants. A single avoided plant outage can often pay for a drive retrofit.

Capacity Increases via Power Factor Improvements

Unit power factors are typically set by dispatch at 0.80 to 0.90, depending on network load conditions. These represent capacity losses, as real MW power output is sacrificed to create MVAR support. A tighter range of 0.88 to 0.93 would be an improvement over the current range. This allows generating stations to shift MVAR (unsold) production to MW (sold), improving their net efficiency and revenue. Techniques and equipment exist to make this possible. Within the power station, auxiliary motors can be driven with variable speed drives that include front-end power electronics which will improve the Power Factor found on the busses. This in turn makes the station auxiliaries look like less of an inductive load, and reduces the need for additional power factor compensation from the generator.

Other Parasitic and Systematic Losses

The above survey of technology addresses the main efficiency losses from an electrical system standpoint. There are other plant operational aspects which can be addressed from an efficiency standpoint.

In the thermal cycle portion of the plant, turbine improvement programs have already demonstrated 5-8 percent increases in capacity without increasing fuel consumption. Coordinated boiler-turbine control with advanced control techniques, allows the thermal system to operate more closely to design parameters without compromising safety, due to dramatic reductions in thermal process variability. This improves both capacity and efficiency, often exceeding 1 percent in each case.

For the typical large hydrogen-cooled generator, the purity of the hydrogen is nominally maintained at 95 percent for safety purposes. An improvement in purity to 98-99 percent will improve generator efficiency more than 0.1 percent. This simple, automated improvement can pay for itself in a matter of a few months on a large base-loaded plant.

Valves and steam traps which are not well maintained and leak can add dozens of Btu to plant heat rates. The same is true of alternate feedwater heater drain leaks. Regular monitoring of valves, traps and drains, with maintenance performed as needed to ensure proper operation, is the standard approach to prevent these efficiency losses.

Finally, the compressed air needs of a plant, for instrumentation and other subsystems, may be consuming far more air and power than necessary. An air consumption audit and analysis can reveal how much air is being wasted, and how much power is being added to the parasitic load unnecessarily.

IV. Designing & Building New Generation Projects

Greenfield sites offer among the best opportunities to use intelligent "whole systems" design approaches, which specify efficiency as a primary design objective, along with long term reliability and minimized cost-of-operation and life-cycle costs. The typical mistake made during new plant auxiliary system design is to go for the lowest capital cost, without (much) regard paid to efficiency, and operating / life-cycle costs. Small incremental increases in design study effort, and careful equipment sizing & selection, can reap huge operational benefits. The onus for getting the best results should be placed on the plant designer.

One way to insure the most efficient plant design wins is to set specific efficiency targets for auxiliary systems, along with cost-benefit analysis, and enforce these as a metric during the bidding and design execution processes, with an expressed de-emphasis of up-front capital costs. When plant operation begins over the full expected load range, auxiliary system efficiency testing should confirm the design expectations, and the Engineering Procurement and Construction firm (EPC) should be held contractually accountable for meeting those efficiency expectations, just as they are held accountable for the plant meeting its larger expectations of schedule, output capacity, gross and net heat rates, etc.

Conclusion

As world energy demand continues to drive power generation projects into ever expanding geographic regions and new areas of technology, the need to manage performance becomes increasingly more important. If a generating station is audited from head-to-toe for potential efficiency improvements, and an action list is prepared for efficiency improvement projects during the span of a few major outages, there are a number of potential "low-hanging fruit" which are ripe for the picking at the appropriate time. A comprehensive plan for efficiency improvements can typically yield multiple-percent output improvements, at costs which are far lower than building new units. This helps the generation planning program for the fleet, and can reduce or eliminate significant amounts of capital spending on new plant construction, and all the issues and costs associated with bringing a new fossil-fired plant into the operating picture.