Student Reading: Thermal Energy from Light

Learning Goals: Students will be able to:

1. Articulate the relationship between the fundamentals of nuclear fusion and sunlight.
2. Use hands-on activities to understand that sunlight is comprised of different wavelengths as represented by colors.
3. Recount the historical development of solar heating and solar cooking.
4. Create an annotated diagram of a solar-powered hot water system for household use.
5. Use data they collect from experimentation to discover the relationship between energy uptake and color for solar collectors.
6. 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 (-273.15^o 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) 1^oC. 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 0^o 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 1^oC. Once at a temperature of 0^o 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 100^oC, 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.

What is sunlight and how is it produced?

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.

Converting Sunlight to Heat

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.

Einstein showed that while light travels in waves, it also is made of particles, called photons. The energy associated with a photon is determined by its wavelength. Shorter wavelength waves of photons have more energy per photon than longer wavelength photons.

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

One reason that Earth can support life is that it is very warm, considering its distance from the sun. The average temperature at the surface for the entire earth is 15^o C. The moon is the same average distance from the sun as is Earth. Yet the moon has a much lower average surface temperature of -35^o C. (Recall that water freezes at 0^o C, room temperature is 22^o C and water boils at 100^o C.) So why is Earth so much warmer than the moon? Gravity! The mass of the moon is only 1.2% of the mass of Earth. Since gravitational force is proportional to mass, the moon has much less gravity than Earth and cannot hold on to a gaseous atmosphere. It is Earth's atmosphere that keeps it warm.

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.

What keeps the heat from radiating away so quickly? It is greenhouse gases. The most important of these are carbon dioxide, methane (CH₄), water vapor and nitrous oxide (N₂O). These are called greenhouse gases because they let the short wavelength visible light pass through the atmosphere, but block much of the long wavelength (heat) infrared energy from escaping. These gases provide the same function as does the glass in the roof and walls of a greenhouse used to grow plants in cold climates. Sunlight passes through the glass and it is absorbed by the plants and other objects in the greenhouse. When absorbed, the short wavelength sunlight is converted to long wavelength heat. The glass walls and roof keep much of the heat from escaping. Anybody opening a closed automobile that has been sitting in sunshine on a clear cold day has experienced the greenhouse effect firsthand.

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 1^oC 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 100^oC. 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 43^oF 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 There are two different approaches to using solar energy to cook food. Solar concentrators use either mirrors or lenses to collect sunlight from a larger area and focus it on a smaller area where the cooking takes place. Greenhouse-style solar ovens use the system invented by de Saussure discussed above. These have glass doors that allow sunlight in, but seal tightly to minimize the escape of heat. The interior of the cookers is black to maximize the absorption of light. Some of the greenhouse-type cookers also have attached reflective mirrors to help concentrate the light.

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."

A kitchen can be blackened by soot from an indoor stove that has no chimney. This photo was taken in Bali, Indonesia in 2013 and is typical of rural kitchens in that country. Imagine the effects the smoke has on the health of cooks. Photo by B. Cuker.

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

Prior to about 1965, most people in the United States dried their laundry by hanging it on lines. Almost every dwelling had laundry lines. In rural and suburban areas they were typical features of side yards or backyards. In cities the lines were often run between adjacent apartment buildings. A pulley system allowed one to work out of a window to add and remove items held on with clothespins. In colder and wetter climates, basements and back porches provided good drying spots, as well as indoor racks in kitchens or laundry rooms. Electric and gas clothes dryers were first introduced in the 1940s, and by 1950 they were in 10% of households. Now most people in the United States use electricity or gas to dry laundry. About 75% of households have these appliances, and people also use dryers at Laundromats. Typically between 6 and 11% of the annual household energy budget goes to drying laundry.

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.

Solar hot water systems Making hot water consumes about 18% of the energy used by a typical household in the United States. Usually electricity or gas is used to heat water in tanks. But it was not always this way. Until the early 20th century, hot water on demand (from a faucet) was a rare luxury. In 1900 households were still transitioning to indoor plumbing in many parts of the United States. To make hot water for washing and bathing, most people had to heat large pots on stoves. Some stand-alone hot water heaters were available, but had to be lit by hand for each use, and carefully monitored so as not to explode.

Prior to the modern electric or gas hot water heater, a company from sunny California sold the first commercial solar hot water system in 1891, called the "Climax." (Edmund Ruud invented the first automatic electric hot water heater eight years later in 1899). The solar-powered Climax consisted of a set of black tanks in a glass-covered box placed on the roof. Thousands were sold, but they tended to cool off quickly at night. In 1909, William J. Bailey brought to market an improved design that separated the collection of solar energy to a glass-covered box of small tubes. This allowed the storage tank to be insulated and preserve its heat throughout the night. Bailey's design quickly replaced the Climax and was standard on many houses built in Florida in the 1920s. Bailey's design is the basis for modern systems.

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.

The more common installation today uses a pressurized mix of glycol and water (like antifreeze used in automobile engines) to transfer the heat between the collector and storage tank. A heat exchanger transfers the thermal energy to the tank. This system requires a pump to circulate the heated antifreeze mixture between the solar collector and the tank. Once it arrives at the tank, the hot antifreeze passes through a system of small pipes on either on the side of the steel tank, or running through it. This is the heat exchanger that transfers thermal energy from the antifreeze to the copper pipe and from the pipe to the water in the storage tank. There are thermal sensors located in the tank and at the solar collector. A small computer in the controller will turn on the pump when temperatures in the collector exceed the temperature in the tank by about 8^oC. When controller temperature drops due to cloud cover or approaching night, the controller stops the circulating pump. Temperatures may reach 300^o F at the collector. Typically traditional hot water heaters are kept between 120^o and 140^oF, but solar-powered tanks are set to go as high as 170^o F to maximize capacity for periods when the sun is not shining.

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 60^o C (140^o F), but the water should be distributed at 50^o C (122^o F).

Tankless (on demand) hot water heater

Solar hot water systems get all of their energy from the sun, except that small amount used to power the circulating pump and run the small computer and sensor system. Solar hot water systems are therefore the most sustainable choice to make. However, they are expensive to install, costing usually about four or five times as much as traditional electric or natural gas systems. An alternative approach is to install a tankless or on-demand system heated by electricity or natural gas. Traditional tank systems lose much of the energy from the storage tank by conduction, convection, and radiation. The tankless systems only turn on when the hot water faucet is opened. This saves on the loss of heat from a storage tank. Homes that uses less than 41 gallons of water a day may save 23–50% of the energy used by a traditional tank system. The efficiency improvement drops down to 8–14% for homes that use about 80 gallons a day.

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.

In 1965 French engineer Felix Trombe used an 1881 design of Edward Morse to create a thermal–siphon device to heat homes. This appliance combines the greenhouse effect, convection, and heat storage by a solid. A concrete or stone wall is built right next to an existing sun-facing wall. Glazing of glass or clear plastic is placed over the wall with an air gap of a few centimeters. This is the familiar greenhouse concept. Holes are placed in the concrete wall at the top and the bottom. These holes are connected to short lengths of pipe that extend to the inside of the building. When sunlight heats the wall, it causes the air to expand and float up to the top, where the warm air then exits into the building. This warm air is replaced by cool air from the building drawn in to the bottom of the wall through the lower set of pipes.

The concrete or stone wall serves as a heat reservoir. Even after the sun has set, the masonry will continue to radiate and conduct heat to the air in the cavity and maintain the convection cell. Electric blowers can be installed in Trombe wall systems to improve the rate at which they deliver heated air to the building. The blowers are wired to through small "snap switches" that turn on the motor at 120^o F and turn it back off when the temperature drops to 90^o F. Trombe walls can be integrated into the design of new buildings, or added to the sun-facing wall of existing structures. It is important the Trombe wall provides heat to the building in the winter and not be an additional heat load in the summer. Design and nature help solve this problem. During the winter the sun is low in the sky, even at noon. This provides sunlight at a fairly small angle to the wall. In summer the sun is high in the sky, and its rays intercept the heat wall at a large angle. It requires only a short overhang or awning to shade the Trombe wall from the high summer sun.

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

The active solar hot water systems discussed above form the basis for another way to provide space heating. Essentially a solar hot water system is sized to meet much or most of the space-heating needs for the building. This means many more collector panels, and increased tank storage capacity. Such systems work best when used with radiant floor heating (hydronic). Radiant floor heating uses small copper or plastic tubing to pass the heated fluid (usually glycol solution) under the flooring material. The hot tubing heats the floor from beneath, and the flooring in turn radiates the heat into the space above. This works best for wood or tile floors, as carpeting insulates the floor. For an existing structure that is not on a slab (there is a basement or crawl space), this requires stapling the tubing to the bottom side of the floor and adding insulation under the tubing.

Radiant floor heating works best with most solar hot water systems because such systems produce fluid temperatures in the winter of only 140–160^o 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.

However, it is possible to achieve higher temperatures with solar hot water systems using a different type of collector. Evacuated tube collectors coupled with heat pipes are more efficient than traditional flat plate greenhouse-type collectors.

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.

Heat pipes were invented by George M. Grover in 1962. They are in wide use today, and if you are reading this on a laptop computer, chances are that a heat pipe is helping to cool the electronics in your device. Heat pipes combine the principles of thermal conduction, convection, and change of state to maximize heat transfer. They are often made of copper tubing that has been sealed and partially evacuated. A liquid such as water, sodium solution, alcohol, or ammonia is also sealed into the tube. Because the tube is partially evacuated, the liquid can easily evaporate and become a gas. In practice one end is placed near the source of heat and the other is placed where the heat is desired. At the hot end, the liquid evaporates. As you will recall, that change of state requires lots of heat. The gas then travels by convection to the cooler end, and when it gets there it re-condenses, thus liberating much heat. The liquid flows back to the hot end to continue the cycle.

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

It may seem strange, but heat can be used to make things cooler. To understand this, let us review the basics of how refrigerators and air conditioners work. A mechanical refrigerator is a device used to extract large quantities of heat from one area and dissipate it in the surrounding environment. It is based upon compression and expansion. Refrigerators cycle substances called refrigerants between the liquid and gaseous phases. As you will recall, it requires a lot of latent heat to vaporize a liquid. And that same latent heat is liberated when the gas re-condenses to a liquid. Mechanical refrigerators use motors to turn pumps that pressurize the refrigerant, turning it into a liquid. The pressurized liquid is then allowed to expand and return to the gaseous state. When it does, it takes heat from the environment. For a window air conditioner, the heat produced in the compression phase is exchanged with the outside air. The heat taken up during the expansion phase comes from air in the building.

The second kind of refrigeration is called absorptive. Like the compression refrigeration, the absorptive unit also uses a refrigerant. However, it runs on thermal energy rather than mechanical energy. The cycle starts when the refrigerant evaporates (goes from liquid to gas) under low pressure. This evaporation removes heat from the desired area, like the inside of a refrigerator. The now heat-bearing gaseous refrigerant is then absorbed (dissolved) into another substance that is in the liquid phase. The combination of refrigerant and absorbent then travels to a place where it is heated. The heat causes the refrigerant to boil out of the absorbent, returning to a free gas. The refrigerant then passes through a heat exchanger to shed thermal energy and re-condenses to a liquid. It is now ready to restart the cooling cycle.

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

Solar energy can be concentrated to provide sufficient heat to vaporize water. This is useful for purifying water, as the vapor leaves behind any dissolved or suspended impurities, such as salts and metals. The vapor is captured, cooled, and condensed to provide clean water. View this YouTube video on a simple solar still for providing 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?