Hybrid cooling tower solves environmental concerns

A 30 MW plant in a Japanese national forest faces stringent regulations for plume abatement

By George A. Kast

Psychrometric Systems Inc.

Officials of the Iwate prefecture in northern Japan, the Japanese national forest service, Tohoku Electric, Toshiba Engineering and Construction Co. and Psychrometric Systems Inc. (PSI) all have reason to celebrate. The groups worked together to design and construct a new geothermal power plant in the middle of a national forest. The project involves new technology in cooling tower design that increases the utility`s environmental sensitivity to the surrounding area. The new hybrid cooling tower is operating above expectations.

In Japan, environmental impact is always a top priority in the design and construction of any power generation site.

The designers of the Kakkonda Geothermal Power Plant No. 2 took those concerns to new levels due to the plant`s location in the middle of a national park. Because the park is heavily wooded, the companies were forced to consider the plant`s impact on vegetation, as well as on people who use the park for hiking and other outdoor recreation.

The main concerns were the cooling tower, as well as the plume condensation and its added constituents. To obtain operating permits, Tohoku Electric and Toshiba had to determine the effects of the operation cooling tower on the local habitat. The primary considerations were plume condensation in the forest, noise emissions from the tower and the dispersion of H2S and CO2 gases which are byproducts from the geothermal wells discharged in the cooling tower air stream. PSI, of the US, was chosen to supply and oversee the cooling tower installation.

Plume abatement

In a normal wet evaporative cooling tower, most of the heat is rejected by evaporation of a small portion of the cooling water into the exiting air stream. The saturated air in the exit stream can cause a visible plume under cool, humid and ambient conditions. During freezing ambient conditions as the plume rises from the cooling tower stack, it condenses and causes ice formation.

The buildup of these frozen droplets on deciduous trees adds weight, can create additional accumulation of snow and eventually may cause breakage of tree limbs. Therefore, the cooling tower design had to minimize the plume and its condensation effect on the national forest surrounding the Kakkonda plant.

Plume abatement coils were placed in the plenum area of the tower to introduce heated ambient air that is then mixed with the warm, saturated, wet-exit air stream, thus lowering the moisture content of the combined exit air stream and reducing the tendency of plume formation and condensation (Figure 1).

The fin tube coils use the warm cooling water, which enters at a design temperature of 41.4 C, to heat the ambient air introduced into the exit air stream. Air flow is modulated by dampers to provide the necessary cooling during seasonal operation. This hybrid (plume-abated) cooling tower is designed to accomplish 100 percent of its heat rejection by wet cooling during summer conditions. The tower then achieves 20 percent of its cooling through dry cooling and 80 percent by wet cooling during winter operation.

A proper mixture of air between the dry and wet sections must be maintained during plume abatement operation to accomplish the reduction in air moisture content prior to exit. The 80/20 percent mixture of wet and dry cooling was determined by PSI through extensive plume modeling (Figure 2).

By simulating the ambient conditions and makeup of the plume itself, a model was developed to determine the condensation levels and characteristic of the plume after discharge. Special consideration was given to local topography and wind directions. Because the Kakkonda geothermal site is situated in a valley surrounded by hillsides of vegetation, particular attention to plume attachment to the sloping hillside was incorporated into the design.

A typical cooling tower configuration is rectangular in shape with fans situated in a row. This is commonly referred to as an in-line tower configuration. At Kakkonda however, the exhaustive modeling exercise called for a tower four-cell (four fans) tower design where the fans are grouped together in a cluster. Modeling of this configuration determined that the plume would be more concentrated and rise higher into the atmosphere before dissipating.

Cooling tower design

To aid in the higher plume rise, PSI incorporated 4.27 meter-tall fiberglass fan stacks, increasing the discharge height and providing a longer mixing time for the wet and dry combined air stream. To accomplish the required airflow, large diameter (10 m) fans were used. The larger diameter fans with the wide core blade width also helped reduce the overall noise emission from the tower–a permitting requirement for the project.

The circulating cooling water is used to condense the steam for the 30 MW plant. Condensate from the cooling towers can have high levels of H2S, which is very corrosive to most metallurgy. Therefore, 316L stainless steel hardware was used. The fin tube coils use the circulating water, which is partially geothermal condensate, and are constructed of 316L stainless steel.

The efficiency of a fin tube coil is highly dependent on the cleanliness of the inside tube wall and the cooling fins. Therefore, an access walkway around the perimeter of the tower at the coil level has been provided for easy cleaning access.

Special consideration was also given to choosing a suitable fill material in the wet section of the tower. Under normal water quality conditions, cross-corrugated film, fill heat transfer media made from PVC is used. The typical flute opening is 190 mm and consists of several film packs stacked on top of one another. Due to the potential fouling or plugging by the condensate, non-fouling types of fills were used.

The top layers of fill consist of a polypropylene splash grid which has openings of approximately 101.6 mm. This is used to break up the water which then flows through a straight-fluted film fill with large flute openings. The combination of the two fills proved to be the most effective way of accomplishing the heat rejection duty and reducing the fouling effect of geothermal condensate, while minimizing the overall cooling tower size.

When designing a wet/dry tower in a series configuration, special attention must be paid to the design side of the piping for maximum flexibility of the water flow. In this case, several valves were installed to allow shutoff of each cell independently. The wet and dry sections of each cell can also be operated independently through several valves and bypasses. By not operating the dry section in the summer, several feet of pumping head were saved, reducing overall parasitic loads.

Another common consideration with a cooling tower is the drift loss. This is the entrained water carried out of the cooling tower through the fan stack in the air stream. This is a normal permitting concern but was especially important at Kakkonda because of the environmental issues and the possible attribution of the drift to the condensation problem. Low loss drift eliminators (not to exceed 0.005 percent of circulating water), with a very low pressure drop, were used so as not to drive up the horsepower requirements and associated additional noise emissions.

Environmental model

Since the opening of the plant in February 1996, the results are better than anticipated. Plume condensation measurement in the forest shows less moisture than predicted; the large fans are indeed cutting noise emissions; and the concentrations of H2S and CO2 are far below required levels.

The first Kakkonda plant yielded 50 MW with two cooling towers that emitted a significant plume. These towers were not plume-abated. Today the new plant delivers 30 MW with only one plume-abated tower.

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Kakkonda Geothermal Power Plant No. 2.