Your light bulb is powered by water

This article was first published in Albuquerque's Local IQ MagazineArchives are available at Issuu

First published in LocalIQ 15 Nov 2014

There’s a debate going on now, from house to house in Albuquerque, about evaporative cooling versus refrigerated air. As the Duke City grows and develops, it gets hotter (cities trap heat) and the humidity rises (as we water our lawns). Evaporative cooling works less well in higher humidity, and one of the arguments for refrigerated air sounds good: evaporative coolers use water, refrigerated air doesn’t use water. We live in the desert, water is scarce, therefore refrigerated air is better.

Whoa! Let’s look at this argument a little more closely, as a scientist might. By the end of the next paragraph you will have learned something just as cool as Einstein’s theory of relativity. It’s a rule of thermodynamics, called the Carnot Cycle, and it was published in 1824 in Reflections on the Motive Power of Fire.

The Carnot Cycle describes the loop a perfect heat engine goes through while creating energy. It traces out a perfect rectangle when plotted against temperature. Think of it like this: We start in the upper left hand corner, moving right, by letting something hot do work — let’s say this something pushes a piston. Then we drop down the right side of the rectangle, cooling to a lower temperature. We draw the bottom of the rectangle, right to left, allowing the piston to return to it’s original position, then we complete the rectangle by increasing the temperature to start over at the beginning. The bigger you make the square, the more energy you get out of the cycle. So to get the biggest bang for your buck, you want the low temperature to be as far away from the high temperature as possible.

Got it? Like the speed of light, the Carnot Cycle is nature’s speed limit. No matter how a power plant is designed, it can never produce more energy than the Carnot Cycle, which in turn is limited by temperature. A coal or natural gas plant creates high temperatures by burning fossil fuels. After using “hot” to move the piston, these plants are left with “warm.” What they want is “cold.” How to get the maximum temperature difference? The universal answer has been to use water for cooling.
How much water is your electricity using? Natural gas uses 180 gallons per megawatt hour (a megawatt hour is equivalent to the amount of electricity used by about 330 homes during one hour). Coal takes 200 gallons.

In addition to coal and gas, nuclear and biofuels also use water for cooling. Nuclear uses 560 gallons of water per megawatt hour. Solar thermal “power towers” are heat engines and are also using water: 835 gallons per megawatt hour.

How can you make electricity without using up water? You could change to energy processes which don’t need cooling. I can think of three: hydropower, wind and solar photovoltaics.

Hydropower wastes a shockingly large amount of water in a completely different way — backing up the water behind the dam creates a lake with a large surface area, which evaporates much more water than the running river ever did. At Elephant Butte Reservoir near Truth and Consequences NM, for example, water loss due to evaporation can total as much as 250,000 acre-feet per year (a family in the Southwest typically uses one acre-foot of water per year).

Wind and solar, on the other hand, are an efficient way to generate electricity without using water. Perhaps with solar you might hose the dust off your solar panels once a year.

So back to your summer cooling needs. Is refrigerated air better for the environment than evaporative cooling? Unfortunately, you can’t win by choosing between these two options — the solution is outside the box.

Do you remember debating “paper or plastic?” The answer was remembering to bring your reusable shopping bags. If you value and want to conserve water in Albuquerque, the most important thing you can do is make your house as energy efficient as possible.

Remember, unless you’re using wind or solar power, every light bulb you turn on is powered by water.





Articles and cartoons on Teaspoon Energy by Kristy Dyer are licensed under a Creative Commons Attribution-NoDerivatives 4.0 International License You can reprint this as-is for free.