How One Plant Flourished After Installing a Direct Drive System
by Jeffrey Alascio
December 13, 2017

Dry cooling at power generation sites has become a popular choice due to environmental factors, geographical site planning and water scarcity on-site. However, environmental and reliability issues are still prevalent due to integral components of the overall cooling system. Also, these large air-cooled condenser structures consume considerable operating and parasitic load costs.

The issues can be minimized by a direct drive motor and adjustable frequency drive (AFD).

1. Reliability

Fewer moving parts mean the chance of a premature breakdown is lessened. Power generation facility operators’ main challenges with air-cooled condenser (ACC) units boil down to two things: reliability and efficiency. More specifically, the fan drive system components revolved around the cost and frequency of downtime. The time required to repair can range from 10 hours to several days, and if unrepairable, days to months for replacements.

Operating conditions fall within a wide spectrum of environmental conditions from site to site. These must be reviewed when selecting drive components to ensure equipment performance.

Case study impact: A power generation plant in the western part of the United States said it would need to rebuild or replace all gearboxes within 10 years of installation due to operating conditions. Additionally, the winter months require the fan to continuously operate at 15 percent of full load to ensure proper oil flow to the gearbox.

2. Efficiency

ACC units are among the most significant parasitic loads in a power generation plant. The past power transmission solution of gearbox and high-speed motor creates mechanical inefficiencies that are eliminated by the direct drive permanent magnet motor and AFD solution. A direct drive, highly efficient motor and AFD performance combined can significantly reduce operating energy—but more importantly reduce the parasitic load. The use of a permanent magnet alternating current (AC) motor has a higher operating efficiency as compared to an induction motor because it does not require the energy to create the magnetizing current.

Case study impact: Combining a highly efficient motor that can provide the performance at slow speeds and reduce the parasitic load is an attraction, especially when plant operators multiply the efficiency gains over 45 cells. The goal is to improve operating efficiency by 10 percent. In this specific case, efficiency gains were 8.5 to 9 percent.

3. Environmental

A direct drive solution eliminates the potential for oil leakage and thousands of dollars in cleanup as well as frequent inspections to ensure environmental compliance. Oil leaking from gearboxes presents a significant cost due to maintenance, contaminating surrounding equipment and environmental impacts. Direct drive permanent magnet motors and drives virtually eliminate these concerns.

Case study impact: Eliminating a harmful substance from the site made the direct drive option appealing due to the environmental concerns of usage, maintenance and storage.

4. Maintenance

Maintenance is a significant cost to any operation. Major maintenance costs within ACC units are time and frequency of oil changes for gearboxes, motor lubrication schedules (based on load operational time frames) and cleaning procedures. Eliminating oil changes and using a direct drive motor operating at slower speeds reduces greasing frequency and simplifies equipment cleaning processes.

Case study impact: After reviewing operational conditions of the direct drive motor solution, bearing lubrication schedules are set to 12-month intervals. By converting the fan drive system to direct drive technology, the plant estimates they can reduce labor costs by 75 percent per unit.

5. Noise

Noise can cost millions to alleviate after start-up if ignored during site planning. The massive ACC units are erected between 45 and 75 feet in the air with multiple 36-foot diameter fans operating at 100 percent speed. Noise also must be within safety limits of the Occupational Safety and Health Administration (OSHA). One way to reduce noise pollution is to specify and install direct drive motors. This allows slower, or optimized, fan speeds across an entire system, effectively reducing sound levels to a minimum.

Case study impact: Sound data was recorded before and after the new direct drive system was installed. Measurements were taken outside the ACC on an interior walkway with the motor/gearbox registering 82.5 decibels (dBA). After installation commissioning, sound level tests showed the direct drive solution registering at 81.5 dBA. Although one decibel is minimal, multiplying that over a 45 module structure can have significant noise reductions.