If you’re outside working on a hot summer day, it’s inevitable that moisture will appear on your brow. A dry breeze evaporates the perspiration and cools down your body. This is nature’s way of keeping you productive because no one works efficiently when they are too hot. Nature’s cooling technique is effective — as far as it goes. But clearly, you’ll cool down more in an air conditioned room than by relying on outdoor breezes. This is especially true when it is humid, since damp air cannot absorb as much moisture as dry air.
So AC is more effective and predictable than nature to remove the sweat off your brow on a hot August day.
How does this apply to power generation? Like us, the gas turbines used in power plants operate less efficiently when the air is too hot.
And interestingly, the two most common technologies for cooling power plants — evaporative cooling and mechanical chilling – mirror the way we cool down our bodies. They even share similar pros and cons.
Here is a more detailed description of the two cooling technologies, as they are used in power production.
In evaporative cooling, shower like nozzles create a curtain of water, and an air stream is sent through the water curtain to cool the air. The cooled air then enters a compressor where it gets compressed. It then enters a chamber where it combines with fuel to combust and activate the power plant’s turbine in order to produce power.
Evaporative cooling happens with varying degrees of efficiency. Much depends on the humidity of the place. A power plant facility in Oregon is likely to be less successful at evaporation than one in Arizona. A dry day anywhere lends itself to more efficient evaporative air cooling than a humid day.
In contrast, mechanical chilling uses an AC-like system to chill the air before it enters the combustion turbine. The process is akin to refrigeration that funnels vapor compression. The power plant operator controls the process and the temperature. Whether it is a dry or a humid day will not matter; the combustion turbine is fed chilled air that is a consistent and optimal temperature.
So what is the practical value of one approach over the other? Which makes sense when?
There are several factors to consider when making a decision on which air cooling technology to install in a power production facility. These include:
- The expense or scarcity of water
- The value of predictable power plant output
- Immediate capital cost versus long-term economics
Growing concern exists about use of water in energy production. This is particularly true in high-growth desert regions in the Middle East or places like California that suffer from a severe drought while demanding large amounts of electricity. Cooling through evaporation necessitates large volumes of water. On the other hand, mechanical chilling can be done in a closed cycle that runs dry or in an open cycle that uses water.
The efficiency of a power plant that uses evaporative cooling shifts with the weather. The plant may produce fewer megawatts on humid days when it is unable to properly cool the air entering the combustion turbine. Like us, the plant using this system just doesn’t work as efficiently when it is hot and humid.
In contrast, humidity has little influence on the generating facility that uses mechanical chilling. Output is consistent regardless of the weather. The operator can adjust the thermostat as needed, and generate much more power.
Just as nature’s breezes are less expensive than central air conditioning, evaporative cooling costs less to install than does mechanical chillers.
But, of course, capital costs are only one input into power plant economics.
Another big consideration is the power plant’s ability to produce megawatts when market prices are high. Peak electricity pricing typically occurs on the hottest, most humid summer days, as demand rises because homes and office buildings are cranking up their air conditioners. This is often the time when power plants are also most likely to fail, which limits supply, further escalating prices.
Unfortunately, these peak summer days when electricity prices are high also are when plants that rely on evaporative cooling generate fewer megawatts. So while the plant owners may have saved money on installation costs, they also lose the opportunity to fully capture peak profits.
So in considering evaporative cooling versus mechanical chilling, power plant owners have much to consider. How humid or dry is the local climate? Is water a precious resource? And finally, does the system’s sticker price reveal its true costs when power market prices are taken into account?
In power markets, where prices can shift dramatically within minutes, control and predictability have value. We can control the thermostat; we cannot control the weather.