Conversion of Solar Energy into Electrical and Thermal Energy Perry LI and Moe MOMAYEZ NSF GK 12 Project Faculty Advisors for Solar Energy College of Engineering, University of Arizona Introduction Energy is essential to our lives on the Earth. It makes cars go and makes machines run. When you feel warm, you are feeling heat energy. When you see things, it is because of the energy called light. Today, the world gets more than 80 percent of its energy from fossil fuels (coal, oil, and natural gas). In the United States, this figure is even higher. More than 85 percent of energy in the United States comes from these sources. We cook our food and heat/cool our homes using fossil fuels. Burning fossil fuels, however, causes problems. Current technology does not allow complete and clean burning of coal and oil, which releases potentially harmful substances into the air. These substances have been shown to cause pollution and contribute to the climate change. Many scientists believe that unless we decrease our use of all three fuels, great damage could be done to Earth. In addition, fossil fuels reserves are limited. In time, we will need to replace them with other sources of energy. It is likely that in the future, we will still use coal, oil, and natural gas, but we will need to rely less on these fuels and more on renewable sources of energy, such as solar energy. Coal Figure 1: Fossil Fuels Figure 2: Renewable Energy
The Sun is the biggest source of energy in our lives. The Sun gives off enormous amounts of heat and light in all directions. Solar energy drives the winds. It also powers the currents in the ocean (the movement of ocean waters). In fact, solar energy makes life on Earth possible: sunlight provides the energy the plants need to convert carbon dioxide into organic compounds and releasing oxygen as a waste product in a process called photosynthesis. Many life species on Earth depend on photosynthesis directly as a source of energy or indirectly as a source of food. It is estimated that the amount of energy stored by photosynthesis is about 100 terawatts, more than six times the amount energy used by all the countries combined (in 2008 the world used 16 terawatts of energy). Sunlight is also used to produce electricity in two basic ways. One uses solar cells, which convert sunlight directly into electricity. This approach is called photovoltaic. (Photo means light, and voltaic refers to electricity.) Solar cells are often called photovoltaic cells, or PV cells, for short. The other approach to making electricity is called thermal. (Thermal comes from a Greek word meaning heat ). Solar thermal power systems use the Sun s heat to run a machine that generates electricity. Objective This short course and associated projects will focus on solar energy, how solar energy is converted into electrical energy and thermal energy, as well as how thermal energy is converted into electrical energy, which is the most widely used form of energy in our lives. To convert sunlight directly into electricity, solar or PV cells made of materials known as semiconductors are used. How semiconductors convert light into electricity will be discussed. When solar thermal energy is converted into electrical energy, water is typically used as a working substance in a solar thermal power plant. Why water is needed and how much is needed will also be discussed in this course. Physics of Solar Radiation Essence of Solar Radiation According to the laws of physics, any object whose temperature is above zero degree Kelvin will emit electromagnetic radiation, which is one of the five fundamental forms of energy. The higher the temperature of the object, the higher the energy it emits. The Sun has a huge mass and the fusion reaction in it creates tremendous amount of energy with measured surface temperatures of about 6000 ºK. Because of its high surface temperature, the Sun emits electromagnetic radiation into the universe. As a planet close to the sun, the Earth receives vast amounts of sun s radiation. The Sun s energy reaches the earth in the form of electromagnetic radiation. In other words, the Sun s light or radiation is essentially a waveform that carries energy. The visible light at different wavelengths has different colors and therefore carries different amounts of energy. In addition to the visible light, solar radiation also contains ultraviolet and infrared radiation. The energy distribution of solar radiation in the entire spectrum of wavelengths is shown in the following figure. 2
Figure 3: Energy Carried by Radiation at Different Wavelength Direct Conversion of Solar Energy into Electrical Energy Sunlight is energy carried by phonons which behave both as a waveform and particles (with no mass). When the energy of phonons is trapped by special types of materials such as silicon (with impurities) used in solar panel, it will excite electrons in the material and set them into motion, creating an electric current. There are different materials that can generate electric currents when hit by the sunlight. Properties and features of power generation using solar panels will be discussed separately. Conversion of Solar Energy into Thermal Energy When sunlight hits the surface of regular materials, phonons will excite the molecules at the surface and will force them to vibrate at a high rate causing the surface temperature of the material to rise. This is the process by which solar radiation is converted into heat. The absorption property of a material is an important factor that controls the amount of energy that the material can trap. When considering the visible light absorption, color is an important factor. We know that black surfaces absorb the maximum amount of visible light. If an object absorbs radiation a hundred percent, then it is called a black body, which is a specific term used in radiation heat transfer. One can make a physical black body by making a cavity in an object. If light is shone into the cavity, most of the energy is reflected around and absorbed by the surface material inside the cavity. 3
Figure 4: Examples of black body based on physically structured cavities Conversion of Solar Thermal Energy into Electrical Energy Solar thermal energy can be directly used for heating, drying and other processes requiring heat. Solar thermal energy can also be converted into electrical power in a thermal power plant, in the same manner as how heat from fuel combustion is converted into electrical power in a regular coal fired power plant (heat from the fuel combustion is used to generate steam and steam is used to propel and rotate turbines connected to generators). Figure 5: Concentrating Sunlight to Generate High Temperature Heat 4
Issues related to the use of thermal energy to generate electricity in a thermal power plant will be discussed in this course. In general, a thermal power plant needs high temperatures and cooling devices (also called heat sinks; ask why and find the answer). In general, the higher the temperature at which the power plant operates relative to the ambient temperature, the more efficient the rate of energy conversion becomes. Water is usually used in cooling devices which creates a challenge with respect to water availability in hot, arid and semi arid environments. Role of Water in Converting Solar Thermal into Electrical Energy Learn principles of thermal power generation with and without water cooling. Learn basic principles of internal combustion engines such as Rankine cycle and Stirling engines (to be provided by Energy and Fuel Cell Laboratory.) Figure 6: Typical Solar Thermal Power Plant 5
Projects Solar Energy Collection and Storage 1. Survey of daily hours of sunlight Active learning: Google information and in class discussion 2. Test power (Current and Voltage) generation and calibration of solar panels Measure and plot I V curves to determine the power output and efficiency (see page 7). 3. Conduct experiments to determine the effects of environmental elements (dust and moisture) on the power generation of PV modules. 4. Use tracking systems to maximize power production. 5. Solar stove (from Energy and Fuel Cell Laboratory) to study principle of using optical devices (parabolic troughs, or lenses) to concentrate sunlight. Scope: Material and tools needed: Issues to investigate: Prototyping and fabrication: Through this project, students will learn how optical devices work to concentrate light to a focus point and how the heat is transferred and used for heating, drying, and power generation. Parabolic dish reflector; Fresnel lenses, heat receiving plate, bearing, shaft, a sundial, supplies such like bolts, nuts, metal pieces for holding and fastening. How sunlight is focused and used safely? How sunlight is tracked and focused all the time in the day? How heat is carried away? What temperatures are needed to cook food? Build a prototype solar heat collector and test temperature variation in the day. 6. Make holes and cavities and black the surfaces of an object and to measure the temperatures of the object and compare with other objects that are not black and have no cavities. 7. Thermal energy storage The significance of solar thermal energy storage for extended power generation and flexibility. 8. Other energy storage technologies Battery Compressed air Hydrogen Pumped water 6
Power and Efficiency of Solar Panels Learning Outcome After measuring current and voltage, students are able to determine the true power output and efficiency of a solar panel. Lesson Overview In this lesson students determine output, maximum power point, and efficiency of solar panels. They also: measure a solar panel output as a function of the electrical load (resistance) connected to it plot the power output and create a performance graph for the panel use a solar panel to process energy and information into a more useful form plot solar panel output current versus radiation to obtain a calibration curve for their panel measure the efficiency of the panel Materials Solar panel Assortment of power resistors or an adjustable power resistor capable of handling the power Digital multimeter or data logger / DAS capable of handling the voltage and current Digital Solar Meter (Pyranometer) Teaching Demonstrate how to connect the multimeter to the solar panel leads and how each power resistor is connected to the panel and the voltage is measured with the meter, as shown in the drawing on the next page. Show the students how the meter is also used to measure the open circuit voltage and the short circuit current. The table on the next page shows the data from a set of measurement on 3 solar panels connected in series. In this example power resistors of 100, 50, 25 and 3 ohms were used. The current through each resistor is calculated by dividing the measured voltage by the resistance. The power is calculated by multiplying the voltage times the current. Tell the students to note that no power is generated when the open circuit voltage and short circuit current is measured. A better view of the data is obtained by plotting a graph of the voltage versus current. The output of the solar panel on the next page shows a characteristic behavior that is common to all solar panels. The maximum power is generated at the operating point which forms the 'knee' in the curve. In this case, this is at approximately 20 watts, corresponding to a load resistance of 25 ohms. The maximum power point is where the product of current and voltage is a maximum. Expressed graphically, the maximum power point it where the largest area rectangle can be formed beneath the curve. The efficiency of the solar panel is expressed as the ratio of maximum output to the input power as measured by the Pyranometer. The output and input power values may be expressed in watts (W), watts per meter (W/m), watts hour (Wh) or watts hour per meter (Wh/m). 7
Resistance (ohms) Voltage (V) Current (A) Power (W) W=V x A A=V/resistance Example: Resistance (Ohms) Voltage (V) Current (A) Power (W) Open Circuit 30.9 0.00 0.00 100 28.8 0.29 8.30 50 27.0 0.54 14.58 25 22.5 0.90 20.25 3 3.1 1.03 3.20 Short Circuit 0 1.10 0.00