Identifying the Best Option for Closed Loop Cooling

By Todd Jastremski, Mike Dupuis and Todd Briggeman

The hydro facilities along the Menominee River in Wisconsin have experienced significant issues with zebra mussels. Populations of this invasive species are migrating upstream into the river system. The mussels are an issue for hydropower facilities because of their rapid growth and ability to attach to steel and concrete structures. The mussels will grow one on top of another, creating layer after layer of organic buildup that is very difficult to remove.

In cooling systems, these mussels can invade the small-diameter cooling water piping and form an interior lining of organic material. This coating reduces thermal transfer efficiency of the cooling system and blocks the flow passage for the cooling water, which can reduce water flow through the system. The combination of reduced efficiency and reduced flow results in a loss of heat dissipation, and bearing oil temperature increases. This also results in maintenance challenges in cleaning the generator cooling system and the potential for outages.

A Berg Chiller was tested at the 5-MW Almonte Generating Station as one alternative to provide turbine and generator cooling.
A Berg Chiller was tested at the 5-MW Almonte Generating Station as one alternative to provide turbine and generator cooling.

We Energies is constructing a new powerhouse at its Twin Falls Dam site to replace an aging powerhouse and provide additional spillway capacity. While mussels are not currently a problem at this powerhouse, We Energies is concerned they will become an issue in the near future. Mussels have been found at facilities about 50 miles downstream of the Twin Falls Powerhouse.

Of particular concern is the damage and blockage these species cause to cooling water piping. Generator cooling for hydropower facilities is traditionally provided by a once-through system that passes river water through the generator bearing heat exchangers to remove heat. However, with this arrangement, the presence of mussels could mean blockage and reduced functionality of the heat exchangers and need to clean or even replace the heat exchangers due to mussel buildup and scaling. Therefore, for the new Twin Falls Powerhouse, We Energies desired a different method of cooling for the generator bearings.

Cooling water alternatives

In evaluating and selecting alternatives, the primary goal was to achieve a closed loop cooling system that would not use river water. The cooling requirements for the project were dictated by the generator bearing requirements. The two 4.5-MW turbines being installed at Twin Falls are vertical Kaplan units in a saxo configuration and are being supplied by Canadian Hydro Components (CHC). The vertical generators are air-cooled but require additional external cooling for the upper combined guide and thrust bearing. The basic requirements for cooling are: generator bearing heat rejection of 17 kW at full load output, upper bearing flow rate of 40 gallons per minute, and maximum generator bearing water inlet temperature of 86 degrees Fahrenheit.

To achieve these goals, three cooling methods were evaluated.

Closed loop oil-to-air cooler

We Energies has used the closed loop oil-to-air cooler as a retrofit for cooling at some of the downstream facilities that are experiencing mussel issues. The system involves pulling oil from the bearing oil bath and circulating it through a forced air oil-to-air heat exchanger (radiator with fan) for cooling. With this method, the water-carrying cooling coils are removed from the generator oil bath.

In this configuration, the oil is recirculated continuously when the turbine and generator are operating. The fan on the radiator cooler is cycled based on the exiting oil temperature from the radiator.

Air-to-water cooling fans were installed in the lower level of the Almonte Generating Station for primary cooling.
Air-to-water cooling fans were installed in the lower level of the Almonte Generating Station for primary cooling.

The oil-to-air system is simple, with minimal moving components. The recirculation pump and cooler fan are the moving equipment and there are filters that require routine maintenance to keep the oil clean. It has proven effective in We Energies facilities largely because of the generally cool powerhouse ambient temperatures (especially during winter months) in northern Wisconsin and the upper peninsula of Michigan.

The disadvantages of the system are in the backup cooling capability and potential failures. This system relies solely on the differential temperature between the ambient air in the powerhouse and the bearing oil for cooling. On a hot day, if sufficient heat is not removed from the powerhouse, the ability of the cooler to maintain the generator oil temperature may be compromised. There is no backup cooling source. However, this has not been an issue for these facilities and the system has been in operation for five years.

Failure of the oil-to-air cooler would occur if there was a rupture in the oil piping or if the radiator should leak. A leak could result in emptying of the oil bath. There are oil level switches installed in the bearing oil bath and temperature monitoring on the bearing pads to help ensure a leak would be realized and the generator could be stopped before damage occurs.

Chiller with raw water backup cooling

For the new Twins Falls generators, the team was not comfortable with the disadvantages presented by the oil-to-air cooling system. Particular concerns were the lack of redundancy in a warm powerhouse and potential to empty the oil from the bearing oil bath. Therefore, a solution that used a chiller with a river water backup cooling heat exchanger was considered and tested.

For this particular application, CHC developed a system for using a chiller to cool the water circulated in a closed-loop system for the generator bearings. The system includes a single chiller sized to provide cooling for both generators. To provide back up to the chiller, a heat exchanger would be installed that would use river water on a temporary backup basis only to cool the closed-loop water system. The raw water heat exchange would only be used in event of a chiller outage.

To prove out the system, CHC worked with the nearby the 5-MW Almonte Generating Station, owned and operated by Mississippi River Power Corp. The equipment was installed into the turbine floor and operated for six months. One difference between the test system and the proposed system for installation at Twin Falls was that the test system had another source of water from the local potable water distribution system that could provide clean “town water supply” as a backup cooling source, in addition to the river water.

Cooling coils were installed on the turbine intake penstock piping to provide secondary cooling capacity.
Cooling coils were installed on the turbine intake penstock piping to provide secondary cooling capacity.

The primary component of this system is the Berg Chiller. To provide sufficient cooling capacity for both units, a 15-ton unit was proposed. The chiller runs chilled coolant through a set of heat exchangers to cool water that is being recirculated to the generator bearing. The system was controlled by a programmable logic controller (PLC) with a full instrumentation system installed for data acquisition and performance monitoring.

The system was somewhat complicated by the fact that a single chiller would provide cooling for both generators. This required cross connecting piping and solenoid control valves to isolate flow from the generator that is not in operation. The generator capacity at the test facility was slightly lower than the capacity for the new Twin Falls units. However, due to higher thrust loading and a different bearing design, the heat load was actually slightly higher. This provided a good comparison test for the chiller system.

After six months of development, operation, and testing, several observations were made in terms of operation, configuration, and reliability:

Capacity: The chiller system as specified was slightly oversized because the heat generated from the bearings was not as much as expected.

Regulation: Temperature regulation was adequate. Chiller systems are designed with on/off binary modes of operation, so a variable-frequency drive pump controlled by the PLC was initially put in series to help regulate bearing temperature.

Power consumption: Rated consumption of the chiller was 9.5 kW.

Restart timing: If the chiller unit was interrupted – due to a line fault, sensor fault or by the operator – a five-hour wait time was required to allow the compressor crankcase heater to evaporate the refrigerant in the crankcase oil. Although this time could be reduced by a few hours through revising the protection settings, the interrupt time was not compatible with the continuous operation We Energies desired.

Outages: In the six-month period of operation, there were four outages. Two were in the last two months, relating to indications of compressor failure. It is unclear whether this is a spurious fault, as the team was not able to trace the point of failure.

Servicing: Outages require trained HVAC personnel to investigate. The chiller system includes a compressor, refrigerant and condensing fan unit, all of which require a skilled technician to perform troubleshooting and repairs.

Reliability: It is believed the chiller’s problems can be sorted over time to reduce outage rates. However, the complexity of this system does indicate that a higher outage rate over an oil-to-air cooling system is anticipated.

Back‐up mode of operation: There is no effective back‐up mode for this system other than running it open loop and using river water to cool the bearings. Adding some form of passive cooling was considered, but this would add another layer of cost and complexity.

Side benefits: The Almonte station turbine floor is below tailwater level and is typically very cool. As a result, the plant operations staff had to use temporary heaters when working the lower level for extended periods of time, especially during winter months. The heat released by the chiller into the lower level during operation provided warmth that the staff greatly appreciated.

The chiller was effective at removing heat from the generator bearings, and the system did provide more than adequate cooling capacity. However, the disadvantages – particularly related to the maintenance and potential down time of the chiller system – were not acceptable for long-term reliability in a continuously operating powerhouse. The redevelopment team rejected this configuration.

Closed loop water-to-air cooler with intake cooling coil backup

The final approach considered took lessons learned from the previous two approaches to create a hybrid alternative. This alternative used a closed-loop water circulation system similar to the chiller system to prevent the issues associated with potential oil contamination and oil spill posed by the air-to-oil cooler. However, instead of using a chiller, this approach uses a radiator and fan similar to the approach used for the oil-to-air coolers already in service. This simplified the equipment and reduced the outage concerns caused by the chiller. Finally, to provide backup cooling to the radiator, CHC proposed and developed a cooling jacket that could be installed on the turbine inlet piping and use the cool steel intake piping as a secondary heat sink when ambient temperature in the powerhouse is too high for effective cooling with the radiator. Each turbine’s unit control PLC regulates the generator oil bearing temperature through the cooling system to maintain a constant oil viscosity.

This approach relies on the water-to-air cooling in the radiator as the primary cooling source. As the ambient temperature and cooling water inlet temperature to the bearings increase, flow is directed to the cooling coils for additional cooling capacity. To simplify the system, the cooling loop for each generator is independent, with separate radiators, recirculating pumps and controls.

To test this alternative, CHC replaced the chiller at the Almonte station with the radiator and fan system. Due to the low ambient temperatures on the turbine floor, the radiator was able to provide sufficient cooling during most of the year. During times of high ambient temperature in the powerhouse, the backup heat exchanger using city water provided supplemental cooling similar to the cooling that will be provided by the cooling coils installed on the intake.

This system had the benefits of the chiller system without the outage concerns and technician-specific training required for maintenance. It can provide cooling using the backup coils even during high ambient temperatures in the powerhouse. It is also mechanically simple, using only a pump, fan and motor-controlled valve for operation. And it addressed some of the shortcomings of the oil-to-air cooling system by recirculating cooling water rather than critical lubricating oil while providing a passive backup cooling alternative. All of this would occur while maintaining a closed-loop reduced maintenance system that would not be susceptible to zebra mussels.

Conclusion

While the chiller system provides an effective method for cooling generator bearings, its complexity, maintenance requirements and robustness did not fully meet the requirements for continuous hydroelectric equipment operations. Conventional oil-to-air cooling systems are more reliable and simple yet have limitations that would need to be addressed for an application at Twin Falls, particularly low efficiency at higher ambient temperatures and having the generator oil flow in an external circuit. The water-to-air cooler with backup cooling coils provides the right mix of simplicity of operation and maintenance with a reliable and flexible cooling system.

The proposed system was installed in the new Twin Falls powerhouse in the winter of 2016 and is the process of being commissioned. The system will be fully operational by the end of June.

Todd Jastremski, P.E., is manager of the hydroelectric operations division for We Energies. Mike Dupuis is president of Canadian Hydro Components. Todd Briggeman, P.E., is a hydropower mechanical engineer in the hydropower and hydraulic structures department of Black & Veatch.

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