By Drew Robb
Is evaporative cooling of gas turbines only suited for hot dry areas of California, Nevada and Arizona and not in humid areas of Florida or the colder climates of the North? The answer is no. According to a recent analysis of the American Society of Heating, Refrigeration and Air Conditioning Engineers (ASHRAE) weather data, contrary to commonly held beliefs, evaporative cooling operates well in most areas of country. The cold temperatures of Seattle and Green Bay and the high humidity of Houston and Tampa are not obstacles for gas turbine evaporative inlet air cooling.
Cooling potential is measured in cooling degree hours (CDH). To understand this concept, it is first necessary to define a few terms in psychrometrics, the study of the properties of air-water mixtures.
- Dry bulb: The ambient temperature taken with a thermometer with a dry sensing bulb. (The temperature shown on an everyday thermometer).
- Wet bulb: The temperature of air measured by a thermometer with its sensing bulb covered with a wet wick. (This is the lowest air temperature that can be reached by evaporative cooling).
- Wet bulb depression: The difference between the dry bulb and wet bulb temperatures (For example, with a dry bulb of 100 F and a wet bulb of 70 F, the wet bulb depression is 30 F).
To arrive at the cooling degree hour figures for various U.S. locations, researchers at Mee Industries in California used ASHRAE weather data. Essentially, ASHRAE weather data is a historical collection of dry bulb and wet bulb temperatures recorded every hour for major U.S. cities.
On a given day, there may be 8 hours with a 10 F wet bulb depression and 8 hours with a 15 F wet bulb depression. Therefore, (8 x 10) + (8 x 15) = 200 CDH are possible for that particular day. By working out the number of cooling degree hours for each day of the month and each month of the year, the monthly and annual totals were calculated for 228 U.S. cities.
Figure 1 shows the amount of cooling potential in each state. While this serves to give an overview of the amount of power augmentation that is potentially available throughout the country, local variations may exist. Though most states operate within a 10 F temperature band, a few vary significantly. In California, for example, San Francisco has around 17,000 CDH, nearby Sunnyvale has 25,000 CDH and Fresno (about 100 miles distant) has 64,000 CDH. Cooling degree hours indicate how extra power can be achieved from a gas turbine through evaporative cooling of the intake air. A gas turbine’s output changes with ambient temperatures and for every 1 F drop in temperature there is a gain in power output of approximately 0.4 percent. Thus, a 100 MW turbine with 200 cooling degree hours available for a given day means 80 MWh (100 x 200 x 0.4%) extra energy available from evaporative cooling that day.
California, Arizona, Texas, New Mexico and Nevada benefit from evaporative cooling because they have the most cooling degree hours. However, there are a couple of unexpected candidates in the “best” list, Kansas and Oklahoma. Each of these states experience more than 40,000 CDH per year. Another surprise is the number of Northern and Eastern states that scored well. New Jersey, Maryland, Virginia, and the Carolinas show up in the 30,000 to 39,999 CDH range, as do Nebraska and Idaho.
Indeck Idaho Management, an IPP headquartered in Rupert, Idaho, used a media-type evaporative cooling system at one of its smaller plants, a base loaded Solar Mars 100 turbine ISO rated at 10 MW. However, the turbine’s capacity degraded noticeably between May and September. To rectify the problem the plant installed a Mee Fog inlet air cooling system. In operation, the fogger increased the gas turbine’s capacity by approximately 700 kW. On hot dry days, the plant operator reports 43 F of cooling using the old evaporative cooler and augmented with fog inlet air cooling.
Most of the North and East fall between 20,000 and 39,999 CDH. Only Montana, Minnesota, Northern Michigan and Maine are in the 10,000 to 19,999 bracket. Minneapolis has 24,000 CDH. Plants in this CDH range, however, report that evaporative cooling is still a worthwhile proposition. The state with the least number of cooling degree hours is Alaska. Nome’s 260 CDH per year make inlet air cooling uneconomical. However, Fairbanks, with 9399 CDH, is a viable location for evaporative cooling.
At TransAlta Energy Cogeneration in Mississauga, Ontario a fog system was installed to augment the existing 800 ton absorption chiller. According to the plant manager, its two GE LM 6000 aeroderivative, base loaded turbines (rated at 42 MW) required about 1600 tons of chilling per machine to achieve adequate inlet air cooling during the summer months. The addition of fog intake cooling provided an additional 2 to 3 MW from each gas turbine.
In regards to the more humid regions, there appears to be no foundation for the popular idea that the humid Gulf Coast/Southeast is a poor location for evaporative cooling. Houston and Corpus Christi, both with 43,000 CDH; New Orleans, 37,000 CDH; Baton Rouge, 40,000 CDH; Mobile, 38,000 CDH; Tampa, 47,000 CDH; and Miami, 53,000 CDH all have the potential for evaporative inlet air cooling. Because of this, power producers such as Florida Power Corp, Florida Power and Light and UtiliCorp are all investing heavily in fog technology to increase their peak load capacity.
“Although Florida might have 90 percent humidity in the early morning when the temperature is 60 F the humidity usually drops to around 40 percent as the temperature increases to 100 F,” says Dr. Eli Chaker, Senior Research Scientist, Mee Industries.
Rule of Thumb
How do you determine if a site has enough cooling degree hours to make it economically feasible to install a fog inlet air cooling system? A useful rule of thumb is to work out the CDH using ASHRAE weather data or the information in Figure 1. Using this information, the increase in MWs for a specific turbine can be calculated. In general, the fogging system is sized for the month with the greatest number of cooling degree hours.
During July in a Northern state, there may be 20 F of cooling possible for 150 hours (3000 CDH). This would result in an 8 percent power increase on a gas turbine. Fargo, N.D, achieves this level of cooling, or greater, for five months of the year. In August alone, Fargo exceeds 6000 CDH. On a 100 MW turbine this means an increase of 8 MW.
Typically, the installed cost of a fogging cooling system is $30,000 per MW increase in capacity. Using this figure, a fogging system would cost $240,000 to increase a gas turbine’s output 8 MW.
Sizing the System
Assuming a particular site has 23,000 CDH available per year and has a 100 MW gas turbine installed, the total amount of energy available from cooling for the year would be 9,200 MWh (23,000 x 0.4 percent x 100 MW = 9,200 MWh). With electrical power selling for around $40/MWh this extra power would be worth $368,000 giving a payback of less than 1 year.
Table 1 shows that a fogging system may be economical down to about 5000 cooling degree hours per year. However, below 5000 CDH it may have too long a payoff. This assumes the purchased price of electricity is $40/MWh.
Since the fog cooling system allows a power plant to increase its capacity, a better way to look at the value of the fogging system is by the Return on Investment (ROI) over a 5 or 10 year period. ROI requires a rather complex equation so is not covered here, but ROI’s in the 50 percent plus bracket are quite possible with fog cooling.
Clearlake cogeneration plant in Pasadena, Texas, with three 110 MW Westinghouse 501-D5 gas turbines, has a ROI of approximately 30 percent per annum. Since installing fogging systems, Clearlake has increased the plant’s output by 2 MW.
For most of the country, there are more than enough cooling degree hours to make evaporative cooling a viable option. Even in colder cities like Seattle, where CDH is zero for 5 months of the year and under 400 CDH for another two months, the summer produces 15,000 CDH, making it a good location to install fogging systems.
For most of the South, the Southwest, and California, there are often enough CDH to achieve year round power gains from a fog system. To avoid any possibility of icing, fog systems are usually not used at temperatures below 50 F, but there are still a number of cooling degree days that are possible between 50 F and 60 F, says Chaker. The amount of additional power that can be obtained at low temperatures makes fogging economical in certain cases.
Drew Robb is a Los Angeles based writer specializing in engineering and technology. Mr. Robb can be contacted at drewrobb @mediaone.net