For the InstructorThese student materials complement the Renewable Energy and Environmental Sustainability Instructor Materials. If you would like your students to have access to the student materials, we suggest you either point them at the Student Version which omits the framing pages with information designed for faculty (and this box). Or you can download these pages in several formats that you can include in your course website or local Learning Managment System. Learn more about using, modifying, and sharing InTeGrate teaching materials.
Student Reading: Thermal Energy from Light
What will happen if we continue to rely on fossil fuels for the next 1,000 years? Instead we must find alternative forms of clean energy. Solar energy is free, clean, and will be around to use as long as Earth is habitable.
There are two ways to use sunlight to make useful forms of energy. One is to use photovoltaic panels to make electricity. The other, simpler approach is to convert sunlight to heat for such things as warming a building, making hot water, cooking, or producing "steam" that can power an electrical generator.
Learning Goals: Students will be able to:
- Articulate the relationship between the fundamentals of nuclear fusion and sunlight.
- Use hands-on activities to understand that sunlight is comprised of different wavelengths as represented by colors.
- Recount the historical development of solar heating and solar cooking.
- Create an annotated diagram of a solar-powered hot water system for household use.
- Use data they collect from experimentation to discover the relationship between energy uptake and color for solar collectors.
- Explain the greenhouse effect, including the role of short and long wavelength radiation, and relate this to data they collect from experimentation.
What is heat?
All substances above absolute zero (-273.15o C) are able to transfer heat to a colder object. Temperature is a measure of molecular motion of vibration. The warmer the substance, the faster its molecules are moving or vibrating. Heat is the amount of thermal energy transferred between objects. Physicists would not say that an object contains a certain amount of heat, but they would say that it could transfer a certain amount of thermal energy to another object, and they would call that heat. The units of heat include the: Joule, Watt, and calorie. Joule = (kg x 1m2)/s2 = W x S. kg is a kilogram, m is for a meter of distance, s is time in seconds. W is for a Watt.
We often like to think of a specific amount of energy used in terms of kilowatt hours (kWh), as that is how we buy electricity from the power company. Since the joule is a Watt-second, the kWh is thus 1000 x 3600 seconds = 3.6 MJ (megajoules). A Joule is the amount of energy released by a 100 g apple that falls a distance of 1 m. A kWh is the amount of electricity used by ten 100-watt incandescent light bulbs for an hour.
Another measure of heat is the calorie. It is the amount of heat needed to raise one g of water (= 1 ml, or 1 cubic centimeter of water) 1oC. 1 calorie = 4.184 Joules.
We can think of heat by the kind of work it does when it is transferred from one object to another. Sensible heat is that which causes the temperature of an object to increase. But does adding heat always cause the temperature to increase? No! Adding heat to an ice cube may cause some of it to melt, but the water changed from solid to liquid may still be at the same temperature of 0o C. When the temperature remains constant, but the added or removed heat causes a change in state, this is called latent heat. Recall that a change of state occurs when substances change forms between the conditions of being a solid, liquid, or gas. Generally it takes a lot more heat transfer to do a change of state than it does to simply raise the temperature of an object. For example, it takes 1 calorie to raise the temperature of 1 gram of liquid water by 1oC. Once at a temperature of 0o C, 79.5 calories must be removed to change the liquid water to 1 g of ice. For a gram of liquid water that is at 100oC, it takes another 539 calories to change it into water vapor. As you will see below, by using change of state rather than just change of temperature, much more energy can be stored or released from a system designed to do useful work.
How is heat transferred between objects?
Heat always flows spontaneously from an object of higher temperature to an object of lower temperature. When the objects are touching, the heat is flowing by conduction. If you place your hand on a hot cup of coffee, heat from the coffee will flow to your hand. If instead you place your hand near the hot cup of coffee, say 2 cm away, you will still feel your hand getting warmer. The infrared rays of heat energy are flowing away from the cup, and you are feeling them on the skin of your hand. When energy flows through space like this, it is called radiation. That is exactly how the energy travels from the sun to Earth, by solar radiation. Heat can also move from one place to another by being carried in a moving fluid (liquid or gas). This is called convection. Passive convection occurs when a warm object transfers heat to a fluid, and as a result the fluid becomes less dense, and floats up. The air above the hot cup of coffee is warmed by conduction and radiation. Being warmer, it becomes less dense and floats up, being replaced by cooler air that slides in to take its place. On a much larger scale this is what happens in the atmosphere and oceans, and it is how heat is transported around Earth. Active convection occurs when a force is applied to move the fluid that is carrying the heat. We use electric-powered fans to circulate heated air in our houses. We use pumps to circulate heated water/antifreeze solution to cool automobile engines.
The rate of heat flow between objects is proportional to the difference in temperature between them. When there is a big difference, heat flows fast. If the temperature difference is small, the flow of heat is reduced. Consider a hot pot taken off the stove and placed on a tile countertop. At first it cools quickly. As it cools, the difference in temperature between the pot and the room air becomes less, and so it takes a long time for the pot to lose enough heat to match room temperature. Two objects at the same temperature are said to be in equilibrium. At that point any heat gained by the pot from the air is equal to the same amount of heat lost by the pot to the air.
The sun is our star. It is a massive ball of dense gases, mostly hydrogen (91.2%) and helium (8.7%). The vast gravity of the sun packs together the gases in the core so tightly that it causes 4 hydrogen atoms (1 proton and 1 electron) to fuse together to produce 1 helium atom (2 protons, 2 neutrons and 2 electrons) and energy. The energy released comes from a loss of a small amount of mass during the fusion process. Recall that Einstein showed that mass could be turned into energy and vice versa with his equation: E = mC2. This nuclear fusion results in the release of a large amount of energy. The released energy includes heat (infrared radiation), visible light, ultraviolet light, and various high energy rays and particles. See an animation of fusion in the sun.
Energy travels in waves. The distance between one wave top to another is called the wavelength. The wavelength determines the kind of energy. Heat (infrared) has a longer wavelength than visible light. What is the wavelength in nanometers (nm) for infrared? ____ What is it for blue light? ___ Use the figure of the electromagnetic spectrum to answer.
Of the sunlight that reaches Earth's surface, 54% is already heat (infrared), 45% is visible light, and about 1% at shorter wavelengths (ultraviolet). When sunlight hits an object, it can be reflected or absorbed. If it is reflected it bounces off at the same wavelength. But if it is absorbed, the short wavelength energy is changed to long wavelength (heat).
The Greenhouse Effect
The infrared energy coming from the sun is not enough to keep Earth as warm as it is. Energy from visible light and ultraviolet light has to play its part, too. Of all the solar energy reaching the atmosphere, about 29% is reflected back to space. Much of what is reflected back to space is visible light, which is why Earth appears as a glowing blue and white globe when photographed from some distance away. About 23% of the initial solar energy is absorbed by gases and particles in the atmosphere, and the remaining 48% is absorbed by the land, ocean, plants, and essentially any object at Earth's surface. When visible light is absorbed by an object, the object converts the short wavelength light into long wavelength heat. This causes the object to get warmer. But this is only part of the story as to why Earth is warm. Something has to keep that heat from quickly radiating back out to space.
The atmosphere is comprised of 78% N2 (nitrogen gas) and 21% O2. The remaining 1% consists of various gases including the greenhouse gases listed above. The spaces between N2 and O2 molecules in the atmosphere are large enough to let both long wavelength and short wavelength radiation pass through. Although CO2 accounts for only a tiny fraction (0.04%) of the atmosphere, it is a potent block to long wave radiation. The longer wavelengths essentially cannot fit through the distance between CO2 molecules. The other greenhouse gases have the same effect.
As mentioned at the beginning of this unit, burning of fossil fuels since the start of the Industrial Revolution has significantly increased the level of CO2 in the atmosphere. In 1800, near the start of the Industrial Revolution, the CO2 concentration in the atmosphere was about 250 parts per million (ppm). In May of 2013, CO2 concentrations exceeded 400 ppm, the highest level in the past 3 million years. Accordingly, Earth's average temperature increased 1oC over the last century. This is the reason for the ongoing global climate change.
Putting the greenhouse effect to work
In 1776, Swiss-French scientist Horace Benedict de Saussure built the first solar collector to make use of the greenhouse effect. He noted that the closed carriages of the day that had glass windows would get warm even on cold days — just like your experience with automobiles. He built boxes covered with layers of glass that had black cork in them to absorb the light. He recorded temperatures above 100oC. URL for image of image of de Saussure's hot box.
Interestingly, de Saussure put his "hot box" to work as a scientific instrument. He was interested in discovering why it is generally colder at higher latitudes. He brought the box to the top of a mountain to measure the maximum temperature produced, and repeated the procedure the next day on the low altitude plain. The box reached the same temperature at both locations, despite the outside air of the plain being 43oF warmer than on top of the mountain. From this he concluded that the thicker blanket of air overlying the plain provided more insulation than the thinner atmosphere of the mountaintop.
The hot box concept was put to practical use by astronomer Sir John Herschel who was on an expedition in South Africa in the 1830s. He built a solar hot box to cook meals.
Solar cookers require no fuel. There are two advantages to this. Fuel is often scarce in poor countries. Kerosene is expensive and firewood, charcoal, dried manure, etc. may be in short supply. Second, cooking fuels often burn in a very dirty way, causing much soot and smoke. This creates real health problems, particularly for women and children in countries with traditions of cooking in houses that have poor ventilation. The Global Alliance for Clean Cook Stoves is a United Nations and private partnership to address this problem. They state, "The World Health Organization (WHO) estimates that exposure to smoke from the simple act of cooking is the fifth worst risk factor for disease in developing countries, and causes almost two million premature deaths per year — exceeding deaths attributable to malaria or tuberculosis. In addition, tens of millions more fall sick with illnesses that could readily be prevented with increased adoption of clean and efficient cooking solutions."
Traditional solid fuels also cause severe environmental damage. Forests are often destroyed by the removal of wood for cooking. And the polluting smoke and soot that are indoor health hazards also enter the atmosphere to cause general air pollution. Considering that about 3 billion people, or three out of every seven people on Earth, eat meals prepared on dirty open cookstoves, the pollution adds up quickly. The developing nations with the worst poverty tend to be found in sunny subtropical climates that fit well with solar cooking.
Solar cooking also has its place in wealthier nations. Why heat up the kitchen baking in a traditional gas or electric oven on a hot summer day when a solar cooker will do the job outdoors in an hour or less?
Solar laundry drying
Once a common feature of the human landscape, laundry lines are now a rare sight in the United States. Many communities have banned outdoor drying of laundry. The argument is that hanging clothes is an eyesore that lowers property values — it makes the community look "poor." This is an example of how bias against poor people hurts the environment and the pocketbooks of the middle class who aspire to appear affluent. Local organizations around the United States work to overturn prohibitions on outdoor laundry drying. One national organization is called Project Laundry List.
A combination of new finds of cheap natural gas, aggressive marketing by electric utility companies, and improved designs of gas and electric heaters all but killed the solar hot water industry in the United States. However, the technology was embraced in post-World War II Japan. Energy was in short supply, and the country was poor from wartime devastation. So cheap solar hot water was a natural choice for Japan. Today, more than 10 million households in Japan heat their water with the sun.
The 1974 "energy crisis" (resulting from a war in the Middle East) renewed interest in all things solar in the United States. Solar hot water systems reappeared on the market in the late 1970s. They all consisted of two basic parts, a collector panel and a storage tank. The collector panel contained a system of small black pipes on a black background, and was covered with glass. "Drain back" systems would fill the collector panels when sensors indicated that they were warming in the morning sun. A pump would then circulate the heated water to an insulated storage tank. At night the pumps would turn off and allow all of the water to drain out of the panel — an important feature in places where temperatures dropped below the freezing point of water at night. When water freezes, it expands and will break the pipes. The drain back systems worked well in places free of frost, but at higher latitudes many failed due to incomplete drainage at night.
Sometimes a series of cloudy days will exhaust the supply of stored hot water. A backup electric resistance heater coil built into the tank will ensure a supply of hot water until the sunshine returns.
Safety considerations — All tank-type water heaters have three safety considerations. First, as the water is heated, it expands, and the resulting pressure could cause the tank to explode. A pressure relief valve at the top of the tanks protects against this. Second, the hot water can also cause scalding burns to the user, so temperatures must be set low enough to prevent this. Finally, a tank with too low of a temperature can encourage the growth of pathogenic bacteria like the one that causes Legionnaires' disease. So it is best if tanks are kept at a minimum of 60o C (140o F), but the water should be distributed at 50o C (122o F).
Tankless (on demand) hot water heater
Solar energy can be used to heat buildings. Ancient architects understood how building and positioning structures could take advantage of solar resources. Such passive designs are covered in a different unit. Here we will focus mostly on active designs for space heating.
Simple thermal air siphons also can be added to existing windows. These devices also combine the greenhouse effect with natural passive convection. The collector can be set to have a more effective angle for collecting sunlight. Including a small solar powered fan, like those used to cool desktop computers, will make the unit more efficient. Care must be taken in insulation to seal around the gaps created in the double hung windows. Otherwise any heat gain will be lost by penetration of cold air through the leaks.
Space heating with solar hot water systems
Radiant floor heating works best with most solar hot water systems because such systems produce fluid temperatures in the winter of only 140–160o F. Recall that the rate of heat flow between objects is proportional to the difference in temperature. To transfer enough heat into the building, the radiating surface must be large, as is the case with radiant floor systems. Baseboard or old-fashioned cast iron radiators would not supply enough area for radiation at these temperatures. They are designed to work at the higher temperatures achieved by using natural gas or heating oil as the source of energy.
Evacuated tube collectors consist of an inner glass tube within an outer glass tube. They are separated by an evacuated space. The term evacuated means that all the air was pumped out. This greatly reduces the rate of heat loss by conduction and convection, as there is no air to conduct or carry the heat between the inner and outer tube. So sunlight passes through the two layers of glass and is changed to long-wave radiation when it is absorbed by a dark collector, and that heat is then trapped in the collector. Evacuated tubes are more efficient than traditional greenhouse-type, flat-plate collectors because of the insulation provided by the vacuum between the layers of glass.
The glycol-water fluid can be directly circulated through the evacuated collector pipes and connected directly to the rest of the system. But a more efficient approach is to include a heat pipe in the system.
Combining heat pipes into evacuated tube designs makes for a very efficient solar thermal collector. However, the evacuated tubes are fragile compared to flat-plate collectors and will potentially require more maintenance over time.
Solar thermal energy for air conditioning
Propane-fired absorptive refrigerators are used in recreational vehicles and cabins. Waste heat from power plants and industrial processes powers large air conditioning units using this mode. Solar power can be used to provide the heat source. One such purely solar-powered system is in South Africa, featured in a story at: solar powered air conditioner.
Thermal-based absorptive air conditioning requires a lot of heat. Another approach is to combine the advantages of absorptive and compression based systems. Such hybrid systems employ a solar collector to superheat the refrigerant, requiring less work by the compressor hybrid solar air conditioner.
Solar energy for making steam and clean water
Researchers at MIT recently invented a black carbon fiber solar sponge that floats on top of water. It captures sunlight and converts it to heat that vaporizes the water. Since the heat is concentrated on the wet sponge, it is not dissipated to the volume of water below, making for a very efficient process. View this Youtube video about the MIT black solar sponge. MIT is also working on reverse osmosis technology to design portable solar desalination plants for use near saltwater.
Collecting your thoughts: Systems thinking and reflection
You have just learned about the history of using energy from the sun to create heat and do work. Take a few moments to consider how this all fits together. What is regulating the temperature of the planet? How are humans influencing this? Think of Earth and its atmosphere as a system. Positive feedback loops destabilize systems and negative feedback loops bring stability. Global warming is melting the sea ice of the Arctic Ocean, so that in the summer sunlight will fall on blue water rather than white ice. From what you learned, what will this do to the rate of heat uptake? The melted ice also makes it easier to drill for oil in the Arctic Ocean. Will that be a negative or positive feedback mechanism for the warming of the planet?
When you do the hands-on experiments, you will notice that the solar collectors you test and the solar oven you use will get to a certain temperature and not get any warmer. Think of these devices as systems. Why does the temperature stabilize? What is happening with the additional energy being turned into heat? Could you modify the oven or solar collector in some way so it will reach a higher temperature?
What would be the pros and cons of using solar energy to heat water or power air conditioning on your college campus?