Energy Use—chapter 8:
Keeping comfortable, or simply staying alive, requires a temperature that’s not too hot and not too cold. As warm-blooded creatures, we have a built-in system for heat control. As intelligent humans, we augment our built-in systems with technologies—clothing, housing, and fire—that enable us to live in places where we could not otherwise survive. We have also learned to use heat in new ways—to make ceramics, to refine metals, to power automobiles and factories, and to fire rockets into space.
To conserve energy, we need to be able to control the flow of heat.
The science of heat energy has its own name—thermodynamics. To keep the heat we use in the places we want it, and out of the places we do not want it, we must understand the three processes of heat flow: conduction, convection, and radiation.
Heat Flow By Conduction
Your most vivid experiences of heat moving has probably been when you directly touch something. Climb into a car that has been sitting in the hot sun. The upholstery is toasty hot from hours of heating by the greenhouse effect. Ouch! How about the way your mouth feels after you bite off a big chunk of popsicle? In both cases your sensations are an immediate result of heat moving by conduction.
Conduction is movement of heat that occurs when two objects are in direct contact with each other.
One principle of thermodynamics is that heat flow by conduction goes from the hotter object to the cooler one. When you bite a popsicle, you are sensing the flow of heat from the inside of your mouth into the chunk of popsicle. When you sit on a hot car seat, your skin is sensing the flow of heat from the car seat into your body.
Put your hand on a page of a book, then the floor, a window, a rug, something metal, and something plastic. Do they all feel the same temperature? They may be the same temperature, even if they do not feel the same temperature. How can this be? The reason is in how well they conduct heat. Metal conducts heat very well so when you touch it at room temperature, heat flows quickly out of your body—you feel cold. Something made of wood, also at room temperature, feels warmer because heat flows very slowly out of your body into the wood, which is a poor conductor of heat.
Insulation--Preventing Heat Loss By Conduction
In the walls or ceiling of a house or apartment, insulation helps minimize house heating costs and energy waste by keeping heat from escaping. In 2013, 40% of total U.S. energy consumption was consumed in residential and commercial buildings, or about 40 quadrillion British thermal units. [from U.S. Energy Information Administration. The British thermal unit (BTU or Btu) is the amount of energy (heat) needed to cool or heat one pound of water by one degree Fahrenheit.] Heating and cooling are generally the biggest energy uses in buildings.
Insulation conducts heat poorly. The measure of a material’s ability to resist the flow of heat is called its R-value. A standard single pane window has an R-value much less than one. A lot of heat escapes through a single pane window. In cold climates, it is wise for home owners to insulate their ceilings to at least R-30.
R-value is the difference in temperature on either side of a material, divided by the rate of heat flow in Btus per hour per square foot.
For example, a window with an area of one square foot, that is one degree Fahrenheit warmer on one side, lets heat flow through at a rate of 1 Btu per hour. It would be R-2 if it allows only one-half a Btu to flow through per hour; and R-30 if just one thirtieth of a Btu flows through per hour.
A home builder in a cold climate is always interested in the best insulation. Goose down—tiny feathers used in expensive pillows and sleeping bags—has a very high R-value. But insulating a house with goose down would not be very practical. For a building, the choice of insulation depends not only on R-value but also on cost, strength, durability, ease of handling, and fire safety. Materials used for insulation include fiberglass, foam boards, sheet rock, double pane windows, and wood. Some insulation is even made from cattle hair or sugar cane fiber.
Air conducts heat very poorly. Yet, when a house is built in a cold climate the air spaces in the walls and the ceilings are often filled with insulating materials. Why isn’t the air alone enough to do the job? The answer is that air does not sit still, and when air flows, its energy flows with it.
The flow of heat by way of a moving fluid is called convection.
As air warms up, it expands and becomes less dense than the surrounding cool air. The warm (less dense) air is pushed upwards by the cold (denser) air. That principle is what makes a hot air balloon float upwards. A heater in a room warms the air just above it. The air rises to the ceiling, cools and drops to the floor, where it is heated again. That is a convection current.
Convection currents do not always distribute heat where it is needed. A layer of hot air near the ceiling does not give much comfort to the people below. A ceiling fan can send the warm air back down. Although a fan uses electricity, it may save on the overall cost of energy bills.
A fire in the fireplace might be a good way to warm the place up, right? Wrong! Convection strikes again, in a big way. Convection does not just carry smoke up the chimney; it carries warm air up the chimney too! An airtight wood stove works better by limiting convection.
Radiant Energy--Heat Flow at the Speed of Light
What material would stop heat flow best of all? The best heat barrier is nothing—a vacuum—empty space. But even a vacuum cannot stop heat completely. If it could, we would never be able to feel the warmth of the Sun across the emptiness of space. Some heat energy passes between objects that are not touching, even if there is empty space between them. That process of heat flow is called radiation; and the kind of energy that travels by radiation is called radiant energy (or electromagnetic energy).
Heat is always being converted into radiant energy. You can see (and feel) this transformation every time you use a incandescent light bulb. Inside the light bulb is a tungsten metal filament—the long narrow wire—which heats up to more than 1832°F as electricity flows through it. Some of the heat changes into brilliant white light—a visible form of radiant energy.
Look around you now. There is probably nothing near you that is as hot as the inside of a light bulb, but everything around you has some heat in it. Everything around you is giving off some of that heat as radiant energy. Only the very hot things such as light bulbs give off light you can see. Cooler objects give off infrared energy that you cannot see.
Objects lose heat when they give off radiation, and they heat up when they absorb radiation.
Which gets hotter in the sunshine; a white concrete sidewalk, or a black asphalt road?
You certainly know the answer if you have ever stepped off the sidewalk to cross the street in bare feet on a very sunny afternoon. Dark surfaces absorb visible light better than light surfaces. The absorbed energy becomes heat. Most objects, no matter how dark or light they look, absorb infrared radiation, so heat lamps are made to emit infrared energy.The Electromagnetic Spectrum
Traveling waves of electric and magnetic fields are known as electromagnetic radiation. Like water waves which can be long rolling swells or short choppy waves, electromagnetic waves can be long or short. The length of the wave—called the wavelength—determines how it behaves.
Visible light has wavelengths between 400 and 700 nanometers. (A nanometer is one billionth of a meter.) The exact wavelength determines the color of the light.
Visible light is partly absorbed and partly reflected by most materials. We see things by the light they reflect. Objects are warmed by the light they absorb. Infrared radiation, which has longer wavelengths than visible light, is easily absorbed by most materials and will heat them up. Ultraviolet light (UV) has shorter wavelengths than visible light. UV light can heat an object, but it can also cause chemical changes and damage to materials including skin.
Near one end of the electromagnetic spectrum is radiation with the longest wavelengths (microwaves and radio waves), while at the other end is radiation with the shortest wavelengths (X-rays and gamma rays).
The Sun radiates the entire spectrum. Most of the energy in sunlight that reaches Earth is in the form of visible light and infrared, but there is a lot of ultraviolet radiation as well. The warm feeling you get from sunshine on a cool day is from infrared radiation. Ultraviolet radiation from the Sun causes burns and skin cancer. Fortunately most of the ultraviolet radiation never reaches the surface because it is blocked by ozone gas in the upper atmosphere.
Passive And Active Solar House Heating
If your home is heated by burning natural gas, the heat is probably spread around to the living spaces in the house by blowing hot air through ducts or pumping hot water to radiators. In solar heated buildings, heat can come into the living spaces directly through the windows, or accumulated in solar collectors using either hot air or hot water for collecting heat. The heat is stored in rock bins (with hot air solar collectors) or water tanks (with solar water heating panels). In passive solar houses, the collected heat is distributed through the house by convection. If fans or pumps are used to circulate the heat through the living spaces, then the building design is active solar.
A refrigerator is a double-walled insulated box with a system of pipes between the walls that have a “working fluid” flowing through them. When the fluid runs through the pipes inside the insulated box, it changes from a liquid to a gas as it absorbs heat from inside the box. The fluid is then compressed and pumped outside the insulated box to condenser coils where it changes back to a liquid by releasing heat it absorbed from inside the box. The condenser radiates heat to the surrounding air outside the refrigerator.
Building A Better Refrigerator
In 1993, a consortium of 24 utility companies sponsored a contest with a $30 million prize for the best new refrigerator on the market. Entrants had to build a product 50% more energy efficient than current refrigerators. They also had to have a plan to market their product.
Another requirement for the winning refrigerator was that it could use no chlorofluorocarbons—CFCs. For decades the working fluid in most refrigerators has been freon, a CFC fluid. Freon released from air conditioners and refrigerators is a major contributor to the destruction of ozone in the upper atmosphere that protects us from the lethal ultraviolet rays of the Sun. It has become urgent that freon in old refrigerators be recycled rather than released to the air and that alternatives to freon be used as working refrigerator fluids. Whirlpool won that contest but you can find energy efficient refrigerators among all the major brands.