The Curious Connections Between Water and Energy
Quickly — name three ways in which water is intimately tied to energy. Ready? What answers did you come up with? If the word “energy” made you think of commercial energy production, then your first answer was probably hydroelectric dams. You might also have thought about the use of water to cool nuclear power plants, or the use of large amounts of water in hydraulic fracking and coal mining. If you are mechanically oriented, then you might have thought of steam engines or hydraulic systems. If you are a botanist or a biochemist, then perhaps you thought about the crucial role that water plays in photosynthesis — the mechanism by which green plants capture the energy of sunlight. But if the word “energy” made you think of the human body, then perhaps you thought about how your skin releases sweat when your body gets overheated, cooling your body by drawing away excess heat energy. The upshot is that there are many, many connections between water and energy — and any list of just three examples would barely scratch the surface.
Let’s start with the example of perspiration. Why does sweating cool the body? The sweat that we produce emerges from the skin at the same temperature as the skin, so it’s not quite the same as getting doused with a shower of cold water. You probably know the answer already — that sweat cools us by evaporating. But this answer just kicks the can down the road. Now we have to ask: How does the evaporation of water from our skin make us cooler?
The answer is that it takes a lot of energy to convert water from a liquid state to a gaseous state. As the sweat on your skin vaporizes into the air, it steals heat energy as it disappears. This process actually lowers the temperature of your skin — which then lowers the temperature of the flesh and blood directly beneath the skin. The cooled blood then circulates to other parts of your body, thereby cooling these places too. Meanwhile, additional hot blood replaces the cool blood near the skin, allowing the cooling of your blood to continue.
When I first heard this explanation many years ago, I was quite skeptical — the story did not make sense to me. If you put cold water on your skin, then it will steal heat energy by conducting the heat away, due to the difference in temperature between the skin and the water. But sweat is warm when it emerges from your skin — which, from my point of view, ruled out the possibility of heat conduction. Therefore what drives the heat energy to move from your skin into the evaporating water?
The answer is that the evaporation of water from your skin lowers the temperature of the water that is left behind. Consequently, the sweat on your skin soon becomes colder than the skin — which allows heat energy to be conducted away from your skin into the remaining water. But once again, this answer just kicks the can down the road. What makes the water evaporate, and how does evaporation reduce the temperature of the remaining water?
To answer this question, it is helpful to think about the individual molecules of water. All molecules, including water molecules, tend to move around. The amount of motion is tied to the temperature of the matter of which the molecule is a part. In fact, heat can be described as the movement of molecules within a substance, and temperature as a measure of the average amount of motion among those molecules. The word “average” is key here. At any given moment, some of the molecules are moving faster than the average, and some are moving slower. In a bead of water on your skin, the fastest-moving water molecules can fly off into the air — which is the definition of evaporation. Because only the fastest molecules escape, the remaining water molecules now have a lower average amount of motion — which means that the water temperature has been lowered. As bizarre as this explanation may sound, this is how a bead of water or sweat on your skin becomes colder than the skin.
Now consider a situation that has some superficial similarities. It is a hot day in summer, and you pull a canned drink out of the refrigerator and put it on the table. You walk off, intending to come right back, but ten minutes pass before you actually return. You now see that the can is covered in beads of water, and that quite a bit of water has run off the can onto the table. Did all of this water condensation help to keep the can cold, in the same way that water on your skin cools you off? Or did the condensation on the can have the opposite effect?
Think about the details of what actually happened. The water that condensed on the can came out of the air, transitioning from water vapor to a liquid. This is exactly the opposite of evaporation, where the water changes from liquid to vapor. Instead of drawing away heat energy, as evaporating water does, the water condensing on the can gives up heat energy, thereby adding heat to the canned beverage.
Now consider these two scenarios — a sweating human body versus a “sweating” canned beverage — in two different climates. One locale is in the desert, with very low humidity. The other locale is lush and green, and is therefore quite humid. On this particular day, it is sunny and 86° F (30° C) in both locations. If you were to go for a vigorous hike in either location, then you would certainly sweat a lot, but in the desert the perspiration would evaporate quickly, keeping you relatively cool. In the humid location, the rate of evaporation would be much slower, preventing your body from cooling as much, and therefore you would suffer greater effects from the heat. Now suppose that you put a cold canned beverage on a shady table in both climates, and then returned to drink the beverage ten minutes later. In the humid climate, the can would be covered with heavy condensation, while the can in the desert would have little or no condensation. In both places the canned beverage would have experienced some warming due to factors other than condensation, but the condensation in the humid climate would have greatly speeded up the warming process. Therefore, as odd as it may seem, the desert is the better location for keeping your beverage cold.
In our examples so far, we have seen that it takes a lot of energy to convert liquid water to a gas, but that the energy is recovered when gaseous water changes back into a liquid. Imagine now that you have placed a pot of very cold water on the stove top. You turn up the burner to its highest setting, and you place a kitchen thermometer into the water. Over the course of many minutes, you see the temperature of the water increasing at a fairly steady rate. Eventually the water reaches 212° F (100° C), and it visibly begins to boil. You leave the burner at the same high setting until nearly all of the water has evaporated. At the end of the experiment, you note that it took much longer to boil off the water than it took to heat the water to a boil. You also note that the temperature of the boiling water never exceeded the boiling point.
This means that for the first part of your experiment, most of the energy from the burner went into heating the water. But after reaching the boiling point, the water could no longer be heated. Instead, all of the energy went into evaporating the water. The fact that it took longer to boil off the water than to heat the water is another sign that a great deal of energy is needed to force the transition of water from a liquid to a gas. We can quantify this. It takes exactly 100 kilocalories of energy to heat one liter of water from the freezing point all the way to the boiling point. But it takes 226 kilocalories of energy to convert a liter of boiling water into steam.
As a side note, the very definition of a calorie is another connection between water and energy. A calorie is defined as the amount of heat energy required to raise the temperature of one gram of water by one degree Celsius. But in popular terminology, a calorie is a measure of the stored chemical energy in our food. Oddly, a food calorie is exactly equal to 1000 calories — a kilocalorie. In other words, a food calorie is the amount of energy required to raise one kilogram of water (2.2 pounds) by one degree Celsius (1.8° F). By the way, one liter of water (about a quart) happens to weigh exactly one kilogram, under standard conditions.
Now let’s turn our attention to water at a much bigger scale. Imagine a huge hydroelectric dam, such as the Grand Coulee Dam on the Columbia River in the western United States. (If you want to think even bigger, then imagine the Three Gorges Dam in China, or the Itaipu Dam on the border between Brazil and Paraguay.) Dams such as these can produce enormous amounts of electricity. But compared to our earlier examples, this is a different kind of relationship between water and energy. The electricity is generated by releasing water from a dam, allowing the water to flow down through an inclined shaft, striking and turning a set of turbine blades along the way. There is no need for the water to change state. Instead, the process is driven by the weight of the water, and by the fact that the water can be released to fall or flow to a lower location, powered by gravity.
The energy of water that is falling or flowing downhill can be used in other ways besides generating electricity. A couple of centuries ago it was common to build mills along rivers and streams, driven by water wheels. These mills could be used to grind grain or to drive factory equipment. In other words, the potential energy of the water was converted into mechanical energy, not electrical energy. This process can be compared to an old grandfather clock driven by a weight suspended inside the cabinet. Someone needs to lift the weight periodically — putting it back to its highest position — so that the clock continues to have the energy to keep ticking. Gravity pulling on the weight is what drives the entire clock mechanism.
Just as a grandfather clock needs a person to lift the weight back up from time to time, a hydroelectric dam needs to have its water replenished — which requires that the water be lifted back into the sky so that it can fall again as rain. The energy to lift the water comes from the sun, which drives the evaporation of the water, as well as driving the air circulation that lifts the evaporated water thousands of feet above the ground. It takes a huge amount of energy to lift all that water — and therefore the water in the atmosphere represents a massive amount of potential energy. This energy dissipates as the water falls back to the ground and then flows down to the ocean. However, some of that potential energy is intercepted by the hydroelectric dam, allowing us to generate electricity.
An interesting thing to consider here is that the water changes state twice during this process — from a liquid to a gaseous vapor, and then back to a liquid. A great deal of heat energy is sopped up by the water when it evaporates from the earth’s surface. Later, when that water vapor condenses to form water droplets in a cloud high above the earth, all of that heat energy is released into the atmosphere. The air containing the water vapor could have traveled a long distance before converting its moisture into droplets. This movement of heat energy through evaporation and condensation is an important factor in how the weather systems of the world behave.
Let’s clarify a key point that might have slipped by unnoticed. The water that evaporates from the surface of the earth captures the energy of sunlight by two distinct mechanisms: 1) by changing state from liquid to vapor, and 2) by getting lifted high above the surface of the earth. Therefore the water also gives up its stored energy by two distinct mechanisms: 1) by condensing into water droplets high in the sky, and 2) by falling back to earth and flowing down to the elevation from which it evaporated.
So far we have looked at cases where the phase change from liquid to gas (or the reverse) plays an important role in the transfer of heat energy, and we have looked at cases where the energy of falling water is used to generate electricity or power a mill. If we were to look at other connections between water and energy, would they fall into these same two patterns, or would we see additional patterns?
Consider the case of a steam engine. As the name implies, such an engine depends upon steam — or more precisely, a phase conversion from liquid water to steam. The energy for the engine comes from burning some type of fuel, such as coal. The water and the steam are housed in a closed system, and as the boiler containing the water is heated by the burning fuel, some of the water turns to steam. Water expands greatly as it becomes steam, building up high levels of pressure inside the system. It is this pressure that can be put to work, driving the wheels of a locomotive or the propeller of a boat — or any of several other mechanical tasks. Thus a steam engine does indeed depend on a phase transition of water — but not to move heat from one place to another. Instead, it relies on the expansion of water during this change of state to generate mechanical energy.
Now consider a hydraulic system. Water or some other liquid is enclosed in a system consisting of pipes or tubes. Pressure is applied to the water at one place in the system, and this pressure is immediately distributed by the water to all parts of the system. This is analogous to the brakes in an older car, where the pressure of your foot on the brake pedal is distributed by the brake fluid lines to all four brakes. (Of course, these days everyone has power brakes, which means that the force applied by your foot is augmented by additional machinery.) In some ways a hydraulic system is like a steam engine, because in both cases an increase in pressure within a closed system does the work. However, in a hydraulic system, the pressure does not come from the expansion of water into steam, but by the application of an external force to the fluid in the system. In other words, a hydraulic system is not an engine, but a means of distributing a force, allowing the work to occur in some place other than where the force is applied.
Imagine a water distribution system that serves a town or small city — specifically a community that stores its treated water in water towers. It requires a lot of energy to lift the treated water up into the towers, but after it has been lifted, no additional energy is required to distribute the water. The weight of the water in the tower puts the entire distribution system under hydraulic pressure. Anybody anywhere in the town can open a water faucet, and the pressure will drive the water out of the open tap. Theoretically you could fit any one of these taps with a tiny turbine, thereby converting some of the potential energy into mechanical or even electrical energy.
There are yet other ways in which water is connected to energy. As you probably know, snow tends to appear blindingly white in bright sunlight. If you go skiing on a sunny day, then you should wear sunglasses to prevent snow blindness. This high degree of reflectivity means that much of the sunlight that falls on snow or ice bounces back into space. But if the snow or ice melts, exposing what lies beneath — bare rock, soil, plant material, or liquid water — then the exposed surface will instead absorb much of the sunlight that falls on it. Therefore the presence or absence of a snow cover has a great effect on whether a region will absorb sunlight and warm up, or instead reflect sunlight and stay cold. Either way, there can be a feedback loop, where the lack of snow cover causes additional warming which causes the loss of more snow cover — or where an unusually large extension of snow cover causes unusual cooling that results in further expansion of the snow cover.
There are other ways in which water affects the energy budget of the earth — that is, the balance between incoming solar radiation and the outgoing radiation returning to space. Clouds, like a cover of snow, are a bright white when seen from above — and therefore clouds also reflect a lot of solar energy back into space. The common attribute between cloud droplets and snow is the huge amount of surface area compared to the volume, thereby greatly increasing its reflectivity. In contrast, water vapor in the air — which has no surface to reflect light — is perfectly transparent to visible light. On a clear day you can easily see an aircraft several miles above your head, despite the water vapor in the air.
On the other hand, water vapor in the air is not transparent to all wavelengths of infrared radiation (IR). A high percentage of the energy that the earth sends back into space is in the IR band, and some of this energy is intercepted and absorbed by water vapor before it can escape. This is an important factor in the balance between incoming and outgoing radiation. This is why a cloud cover at night will prevent a region from cooling very much during the night, compared to a clear night. Water has still another big effect on this balance. Much of the carbon dioxide — a potent greenhouse gas — that enters the atmosphere eventually dissolves in the ocean. There is a constant balance between the CO2 in the atmosphere and the CO2 in the oceans. If you increase or decrease the CO2 in either place, then the equilibrium is affected, and CO2 will move from one to the other to restore the balance. This has several effects, including an effect on the energy budget of the earth. (For more details, see Are Greenhouse Gases a Myth?)
And now for one final example — yet another process in which water and energy are connected. As you know, green plants are able to capture the energy of sunlight through a process called photosynthesis. This process requires water, carbon dioxide, and sunlight as inputs. Chlorophyll in the plants uses the energy of sunlight to convert the water and carbon dioxide into molecules of sugar. (Oxygen is released as a by-product.) In other words, the plant converts light energy into chemical energy, in the form of high energy bonds within the sugar molecule (carbon-to-carbon and carbon-to-hydrogen bonds). This stored energy can be recovered at any time by converting the sugar molecules (along with some oxygen) back into water and carbon dioxide. However, this reverse process does not release light. Instead, it makes the energy available for the various metabolic processes that a plant must perform in order to survive. If an animal eats the plant, then it can make use of the stored chemical energy to drive its own metabolic processes. Therefore the living cells of an animal consume oxygen to release the energy, and these cells give off water and carbon dioxide as by-products.
We have now examined a wide variety of ways in which water is deeply connected to energy. And yet there are many other examples that we could have examined. Most of these unmentioned examples rely on the same principles discussed above, but a few rely on yet other principles. Can you think of additional examples of the connections between water and energy?