Introduction Welcome to the Engineering Energy Efficiency Project! Your design challenge in this project is to build a model house that uses very little energy to keep warm and takes advantage of solar radiation to cut down its energy use even more. Through repeated testing and modifications, you can make your model better and better. In your final report, you will present how well your house performs and what you have learned about energy-efficient design. Although what you build will just be a model made of paper, clear plastic, and cardboard, and heated by a light bulb, the science and engineering principles are the same as in a real house. The tests of your model would work on a real building. So this project is about a real-world situation and a real-world problem. A substantial portion of home energy use in the United States is devoted to heating and cooling. The value of improving the energy efficiency of buildings is enormous. Outdated or negligent design and building practices waste vast quantities of fossil fuel that contribute carbon dioxide to the atmosphere. This is unsustainable in the long run. It is also quite unnecessary. We can construct and renovate buildings that are much better! Your generation has the task of making energy efficiency something that everyone knows and cares about. Some of you will also be the engineers who participate in that transformation. This is a Concord Consortium research project, but it's also a chance for you to apply your creative energies to an exciting and challenging task, work with your hands, build and test real structures, and have a good time. We look forward to seeing your designs, which we know will be diverse, beautiful, and energy efficient! Ed Hazzard Charles Xie
Table of Contents Build and Test a Standard House 1 Building Instructions 3 Heater Test #1: Warm Up the House 6 Heater Test #2: Keep the House Warm 15 Solar Heating Test 23 Heat Transfer 31 Heat and temperature 31 Conduction 47 Experiment 1: conductivity of different materials Experiment 2: multiple layers Convection 57 Experiment 1: natural convection in a cup Experiment 2: wind chill Experiment 3: infiltration Radiation 75 Experiment 1: infrared radiation detection Experiment 2: Infrared radiation transmission Energy from the Sun 87 Experiment 1: Represent the sun s path through the sky Experiment 2: Solar heating
Design and Build Your Own House 97 Building Instructions 102 Heating Test 103 Solar Heating Test 108 Modify Your House 115 Building Instructions 119 Heating Test 120 Solar Heating Test 125 Summer Cooling 133 Cooling Test #1 134 Modifications for Cooling 138 Cooling Test #2 141 Final Report 147
Introduction The overall goal of this engineering project is to construct a house that is energy efficient to keep warm or cool, has a steady inside temperature, and can be heated by the sun. You will be working with a model rather than a full-sized house, but the principles are the same. By the end of this project, you will understand the heat transfer basics and design issues that you would need to tackle a real energy-efficient house Before you start on your own house design, you will build and test a pre-designed standard house to familiarize yourselves with the materials, building methods, and measurements you will use to evaluate your design. This is called a standard house because everyone will start by building the same one. Also, it will be the standard against which you can compare the performance of your own design later in this project. The standard house meets the same criteria that you will follow when you build your own design. Your teacher will provide a cutout, which you can trace onto card stock, cut out, fold, and tape together. You will also make a base for the house out of foamcore and windows out of clear acetate. This project uses a standard procedure for measuring the thermal performance of a house. For the house to lose heat, there must be a temperature difference. The interior of the house must be warmer than the outside. Since you can t cool down your classroom to 0 C, you will warm up your house to 15 C above room temperature. This is done with a light bulb heater inside the standard house. As with a real house, what matters is how long the furnace must be on to keep the house warm. The more it s on, the more energy is used per day and the greater your heating bill. To imitate this situation, you will record what percentage of time the light bulb heater must be on to keep the house at 15 above room temperature. Finally, you will perform the same test, but with a bright light shining on the house, imitating sunshine. You can then tell how much your energy bill is reduced by solar heating. Note that your data will consist of a description of the experiment, a data graph, times entered into a table, calculations, and notes. All of this material must be placed in this workbook when you finish the experiment. Build and Test a Standard House Build a model house and measure how much energy is needed to keep it warm. Standard House 1
Tools & materials Two temperature sensors Computer with data logging software (Vernier Logger Lite) Logger Lite for plotting temp erature Full-scale cutout of standard house and base for tracing (two per class) Metal ruler (cm) Scissors Safety utility cutter Pencils Card stock Cardboard surface to cut on Acetate sheets for windows Masking tape and/or clear tape Two temperature sensors One 40 W light bulb heater in a socket with an inline switch, covered with foil One 300 W light bulb sun in a gooseneck fixture Power strip Standard House Description The standard house has a floor area of 400 cm 2. The window is on the south side, and its area is 80 cm 2. There is enough room inside for the light bulb (15 cm high) and its base. There is a 12 cm diameter hole in the floor for the light bulb heater. Materials for the initial design are limited to cardstock, clear acetate, and masking tape, except for the foamcore base. The house sits on a foamcore base, larger than the house. The base is labeled with the directions north, south, east, and west for testing purposes, so that you can picture the house with a real orientation with respect to the sun. Initially, two sensors are inserted through the walls of the house, one high and one low, in specific positions. * Note that this workbook is written for the latitude and climate of the northern United States. Other climates may have quite different design issues, and the sun s path changes in other latitudes. 2 Standard House Building Instructions
Building Instructions 1. Trace the two pieces of the standard house on a piece of 20x30 inch cardstock, using the full-scale template provided by the teacher. Note how they must be arranged to fit on one sheet. Be sure to mark the locations for the two sensors as shown on the template. 2. 3. 4. 5. 6. Cut out the two pieces, using scissors. Use a sharp pencil to make holes for temperature sensors in premarked spots. The holes are 5 cm and 15 cm above the floor. Cut out the window and tape a piece of acetate over it on the side of the cardboard that will be inside the house. Cut a circle out of the base, as in the template, so that the heater light can fit in. The circle happens to be the same size as a CD. Fold the cardboard along the dashed lines. Use the edge of a table to make straight folds. Building Instructions Standard House 3
7. Tape the edges together. 8. Cut out the foamcore base using a utility knife, cutting on a cardboard surface. 9. Label the base with directions: North, South, East, West. 10. Place the bulb with foil on the foamcore base. 11. Feed the power cord of the bulb through one corner of the house, as in the picture below. Then tape the joints closed around it. 4 Standard House Building Instructions
12. Place your house and bulb on the foamcore base so that the window faces south and the bulb fits through the hole in the base of the house. 13. Write your team members names on the house so that you can see them if you take a photo of the house. 14. Your house will look similar to the house pictured below. Note the power cord coming out of one corner of the house. Building Instructions Standard House 5
Heater Test #1 WARM UP THE HOUSE How much energy does it take to heat up a house? Introduction Your goal is to measure how much energy it takes to warm up your house to 15 C greater than the air around it. To do this, you will: Raise the house to the target temperature using the light bulb heater. Calculate the energy required to warm up the house. Tools & materials Standard house Two temperature sensors Computer with data logging software (Vernier Logger Lite) Logger Lite for plotting temperature One 40 W light bulb heater in a socket with an inline switch, covered with foil Power strip Extension cord, if needed As you perform the following steps you will look at the graph generated by Vernier Logger Lite, which will record the time and temperature automatically and represent them graphically. Note that you can annotate the graph, that is, write notes on it. Always save your data graph. Also print the graph and place it in your workbook. Predictions There are two temperature sensors in the house, one higher and one lower. Do you think they will show different temperatures? Why? Note the American students: You will use the Celsius scale for these measurements, so here s a quick exercise to remind you about Celsius vs. Fahrenheit. Fill in Table 1. C = 5/9(F 32) or F = (9/5)C + 32 6 Standard House Heater Test #1
Table 1: Celsius vs. Fahrenheit Temperature in C Temperature in F Water freezes Water boiles Room temperature 20 A hot day 100 For example, suppose the room temperature is 23 C. The target temperature for the warmed-up house will be 15 C higher. What will these temperatures be as measured on the Fahrenheit scale? Fill in Table 2. Room temperature 23 Table 2: Experimental Conditions Temperature in C Temperature in F Target house temperature 23 + 15 = 38 Outdoor temperature if it were 15 C below room temperature 23-15 = 8 The last calculation is to show that our experimental conditions are have the same temperature difference as a house kept at 23 C when the outdoor temperature is 8 C. It s a cold day, but not freezing. Heater Test #1 Standard House 7
SET UP THE SENSORS Procedure 1. 2. 3. 4. 5. 6. Connect the two temperature sensors to your computer. Open the Logger Lite file that goes with this experiment: std house warm up.gmbl Touch the sensors to make sure they work. (You should see the graph go up.) Measure the room temperature and record it in Table 3 below. We will assume it stays reasonably constant throughout the experiment. Experiment with changing the range of the Y-axis, which is done by dragging along the Y-axis. You can expand it to see very small changes! Calculate your target temperature: 15 C above room temperature. 8 Standard House Heater Test #1
COLLECT DATA Procedure & data collection 1. Insert temperature sensors in the holes you made in the wall of the standard house. Each sensor must be pushed through the wall so that it is 3 cm from the wall. Place tape over the hole to keep the sensor in place. Heater Test #1 Standard House 9
2. Start collecting data. 3. Wait 10 s. 4. Switch the light bulb heater on. 5. When the upper sensor reaches the target temperature, switch the bulb off. Record the time in Table 3 below. 6. The graph produced by Logger Lite should look similar to the graph below, although yours will include the lower sensor as well. It may be helpful to annotate your graph with similar labels. 7. Save the Logger Lite file. 8. Print your Logger Lite graph and make a copy for each team member to put in their workbook. 10 Standard House Heater Test #1
Table 3: Warming Up the House Data file name: Room temperature: Target temperature (upper sensor): C C Heater is turned ON at s Heater is turned OFF at s Total time heat is ON: s Energy requirement to warm up the house: Q = 40 watts * (total time on): W Comments How to calculate the energy requirement If we symbolize the amount of heat energy by Q, the power by P and time by t the relationship between heat, power, and time can be represented as: Q = Pt Since the power of the light bulb is 40 Watts (40 Joules/sec), the time it is on can be multiplied by 40 to give the total energy used to heat up the house. Q (joules) = 40 (watts) * time (seconds) Use this calculation to complete Table 3. Heater Test #1 Standard House 11
Analysis Refer to your data and graph files to answer the following questions. How do the temperatures recorded by the upper and the lower sensors compare? Why do you think the temperatures recorded by the upper and lower sensors show this relationship? Describe the shape of the graph as the house warmed up. Why does it have that shape? Specifically, why is it steeper at the beginning and less steep as the temperature rises? Remember that the power input of heat is constant throughout the experiment. Compare the energy requirement of your standard house to other teams. What s the range? If they are different, what do you think would explain the differences? 12 Standard House Heater Test #1
Connection to buildings Background This experiment is a little unrealistic. It s as if the building started out at the outdoor temperature and had to be brought up to a standard indoor temperature. Usually buildings are about the right temperature already, and the heater keeps them there. It takes lot of energy to heat up a whole building. Why do you think this is true? Heater Test #1 Standard House 13
Summary What characteristics of the house do you think affected the amount of energy necessary to heat it up? Think about size, building materials, window area, etc. What could you do to the house or the heater to warm the house up faster? 14 Standard House Heater Test #1
Heater Test #2 KEEP THE HOUSE WARM Introduction Your goal is to measure how much power it takes to keep your house 15 C warmer than the air around it. To do this, you will: Turn the heater on and off so that the temperature stays within 1 C of the target temperature. What is the power requirement to keep a house warm on a cold day? Record the times when the heater is turned on and off. Calculate what percentage of time the heater has to be on to keep the house warm. Multiply that percentage by the heater power to get the actual power supplied to the house. Heater Test #2 Standard House 15
Collect data 1. 2. 3. 4. 5. Now use just one sensor the upper one. Open the Logger Lite file that goes with this experiment: std house keep warm.gmbl Assume that room temperature has not changed. Calculate the target temperature (room temperature + 15) and enter it in Table 4 below. Turn the heater on. Start collecting data when the sensor is a few degrees below the target temperature. 6. Refer to the above sample graph, which should look roughly like yours. When the sensor reaches one degree above the target temperature, switch the heater OFF and record the time in Table 4 below (A). Note that the data table in Logger Lite makes it easy to note the current time while data is being collected. Note: the temperature may continue to rise for a time. That s OK. 7. When the sensor drops to one degree below the target temperature, switch the heater ON and record the time in Table 4 below (B). Note: the temperature may continue to fall for a time. That s OK. 16 Standard House Heater Test #2
8. 9. When the sensor again reaches one degree above the target temperature, switch the heater OFF and record the time in Table 4 below (C). Stop collecting data. 10. Click the scale icon to fit the graph to your data. 11. Save the Logger Lite file. 12. Print your Logger Lite graph and make a copy for each team member to put in their workbook. 13. Calculate the power requirement to keep the house warm by filling out the rest of Table 4. Data file name: Table 4: Keeping Warm Room temperature: Target temperature: C C Upper limit (target temperature + 1): C Lower limit (target temperature - 1): C Event Time (from data table) A. Turn heater OFF at upper limit (point A) B. Turn heater ON at lower limit (point B) C. Turn heater OFF at upper limit (point C) D. Total cycle time (C - A) E. Total time ON (C - B) F. proportion of time the heater is on (C - B) / (C - A) G. Power requirement 40W * F Heater Test #2 Standard House 17
How to calculate the power requirement You used the energy provided by the heater to heat up your house and maintain it at your target temperature. The bulb you used as a heater has a power of 40W. This means that it releases 40 joules of energy per second. But since it wasn t on all the time, the house used less than 40 W to stay warm. The power rating of your house is: Power rating = 40 W * time on / total time Note that the total time should be a full cycle, from OFF to ON to OFF again. The steps of the calculation are set out in Table 4 above. 18 Standard House Heater Test #2
Analysis Refer to your data and graph files to answer the following questions. In your own words, explain the difference between energy and power. How does the house lose heat? Where does it lose the most? Take an educated guess. You will learn more about this later in the project. In the vertical cross-section below, show how you think heat circulates and escapes from the standard house. Heater Test #2 Standard House 19
Connection to buildings Background The light bulb in this test house is like the furnace or boiler in your house. It has a fixed output and is on part of the time. Heating units are sized so that they would be on all of the time only on the coldest days, when there would be the greatest temperature differences between inside and outside, and hence the greatest rate of heat loss. If you improved the energy efficiency of your house, the heater would be on less time and use less total energy over the year. Your house has a thermostat, which does exactly what you did by hand in the experiment: it turns the heater on when the house temperature is below the set temperature, and off when the temperature rises above the set temperature. If you graphed the temperature in your house, it would be a wavy line like the graph in this experiment. In a real house, the yearly energy requirement would be calculated by looking at the energy bill (for example, 400 gallons of oil multiplied by 130,000 BTU/gallon = 52 million BTU = 15,200 kwh). Note: in the USA, both kilowatt-hours (kwh) and British thermal units (BTU) are in common use as heating energy units. If you want to interpret your energy bill and compare electrical energy to oil or gas energy, you will need to convert from one to the other. 1 kwh = 3412 BTU 20 Standard House Heater Test #2
Explore Optional exercise: Look up your actual heating energy use. Convert the results to kwh per square meter. If your boiler heats domestic hot water as well as the house, subtract the monthly summer use from each winter month to obtain just the heating energy. Make use of the following approximate conversion factors: 1 m 2 = 10 ft 2 1 kwh = 3400 BTU For oil, 1 gallon = 120,000 BTU (allowing for a boiler effiency of 85%) For natural gas, 1 Therm = 100,000 BTU What is the energy use per square meter per year for your house, in kilowatt-hours? Heater Test #2 Standard House 21
Summary How do you think you could reduce the energy needed to maintain the house temperature in this model? Explain what you would do and how it would help. Later on in the project you will have a chance to build and test your ideas. 22 Standard House Heater Test #2
Solar Heating Test Introduction During the last session you built your house and heated it using a light bulb heater. That situation mirrors the nighttime when there is no sunlight. Now you will add a very bright light bulb (300 W) outside as the sun. Its position will be roughly that of the sun at noon in winter in the northern United States (40 N). You will test your house using a single temperature sensor the one at the top of the house to represent the house temperature. How can energy provided by the sun reduce the house heating requirement? You will turn the heater on and off, but leave the sun on all the time. This will simulate a sunny day. Prediction How much do you think the sun shining will affect the power needed to keep the house warm? Record your prediction: Current power (watts): Predicted power with the sun shining (watts): Solar Heating Test Standard House 23
SET UP THE SENSOR Tools & materials Procedure Standard house One temperature sensor Computer One 40 W light bulb heater One 300 W light bulb in a gooseneck desk lamp (the sun ) Power strip with switch Template for measuring sun s angle Extension cord, if needed 1. 2. 3. 4. 5. 6. Remove the lower temperature sensor, leaving the upper one in place. Connect the temperature sensor to your computer. Open the Logger Lite file that goes with this experiment: std house solar heating.gmbl Open Logger Lite. It will display the temperature. Record the room temperature in Table 5 below. Calculate the target temperature (room temperature + 15) and enter it in Table 5 below. Set up the gooseneck lamp with a 300 W bulb in it, due south of the building. The tip of the bulb should be 20 cm from the house window and aimed downward at about a 35 angle, as if it were noon in winter. Use the template provided by the teacher to position the sun. 24 Standard House Solar Heating Test
COLLECT DATA Procedure 1. Switch the light bulb heater and the 300 W sun on. NOTE: this bulb is very hot. Be careful not to touch it, and wait until it cools down to move or store it. Turn it off except while doing the experiment. 2. 3. 4. 5. 6. 7. 8. 9. Start collecting data when the sensor is a few degrees below the target temperature. When the upper sensor reaches one degree above the target temperature, switch the heater OFF and record the time in Table 5 below (A). Leave the sun on. When the upper sensor reaches one degree below the target temperature, turn the heater ON. Record the time in Table 5 below (B). When the sensor again reaches one degree above the target temperature, switch the heater OFF and record the time in Table 5 below (C). Stop collecting data. Click the scale icon to fit the graph to your data. Save the Logger Lite file. Print your Logger Lite graph and make a copy for each team member to put in their workbook. 10. Calculate the power requirement to keep the house warm by filling out the rest of Table 5. Solar Heating Test Standard House 25
Table 5: Solar Heating Data file name: Note: Sun is ON for the whole experiment. Room temperature: Target temperature: C C Upper limit (target temperature + 1): C Lower limit (target temperature - 1): C Event Time (from data table) A. Turn heater OFF at upper limit B. Turn heater ON at lower limit C. Turn heater OFF at upper limit D. Total cycle time (C - A) E. Total time ON (C - B) F. proportion of time the heater is on (C - B) / (C - A) G. Power requirement 40 W * F H. Power requirement without the sun (from previous experiment) 26 Standard House Solar Heating Test
Analysis How does the power necessary to maintain the temperature at noon (sun shining) compare to the power necessary to maintain the temperature during nighttime (no sun)? What would you do to increase the solar energy gain in your standard house? Later on in the project you will have a chance to build and test your ideas. Solar Heating Test Standard House 27
Connection to buildings Explore Think about a real house, such as your own. How could you reduce the energy necessary to heat your house by using sunshine? Here s a challenging question. What would you do to increase heat gain during sunny periods, but minimize heat loss at night? 28 Heat Transfer Solar Heating Test
Summary Write a paragraph to summarize today s experiments, results and interpretation/explanation. Solar Heating Test Standard House 29
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Heat Transfer Heat and temperature As warm-blooded animals, we all care about heat and temperature! Our survival, not to mention comfort, depends on keeping our bodies at a constant temperature, despite huge changes in the environment. The focus here is on buildings, but the same principles apply to our bodies. Every day, we experience conduction (heat transfer through clothes), convection (moving air or water), and radiation (especially sunshine), which are the basic ways that heat is transferred. What is heat? How is it measured? In buildings, temperature is a key part of comfort. The more efficiently it can be kept at the proper level, the better, since a large part of the nation s energy budget is devoted to the heating and cooling of buildings. Note to American students: in the construction industry in the USA, none of the units are metric. This can be very confusing. This project will use metric units and mostly avoid calculations, focusing instead on the comparative values of different materials. Introduction Energy is a special quality in science and engineering. It has many forms thermal, kinetic, potential, chemical, electrical, nuclear, and radiation. It can change form, but the total amount of it is constant. In other words, energy is not created or destroyed; it just changes form. This principle, called the Conservation of Energy, is central to understanding heat flow. Think about Earth. Do you think the total energy on Earth is constant? Why or why not? Thermal energy is the total kinetic energy of the molecules of a substance. It is the energy needed to raise the temperature of the substance from absolute zero, which is -273 degrees Celsius or 0 degrees Kelvin to its actual temperature. It is measured in Joules, kilojoules, or other units of energy. Heat (Q) is the thermal energy that can be transferred between two sys- Heat and Temperature Heat Transfer 31
tems by virtue of a temperature difference. It is usually much smaller than the total thermal energy because normal temperature differences are small. Temperature measures the average kinetic energy of the molecules of a substance. Kinetic energy includes all of their motion: vibration, translation, and rotation. Molecules are always moving except at absolute zero, which is defined as the temperature at which all motion stops. Here s a diagram to help picture what heat, temperature and thermal energy mean. Imagine that you have two blocks of a material and you are able to heat or cool between 0 C and 100 C. The two boxes represent the total thermal energy in the two blocks. One is larger than the other, so it has more thermal energy. This is represented by the width of the box. The temperature, shown both in degrees Kelvin and degrees Celsius, is represented by the height of the box. The upper part of each box, between 0 C and 100 C, is the heat the portion of the thermal energy that can be transferred. 32 Heat Transfer Heat and Temperature
Draw a diagram to show that the heat needed to warm 50 g of wood 20 C is the same as the heat needed to warm 100 g of wood 10 C. When you turn on your house heating system, what is transferred to the house heat or temperature? Put another way, what are you paying for when you buy oil, gas, or electricity for heating heat or temperature? When the temperature increases, what can you say about the movement of the air molecules in your house? Heat and Temperature Heat Transfer 33
Heat transfer and thermal equilibrium Heat flows from a hotter to a colder body until the two are in equilibrium at the same temperature. The total amount of heat remains the same, unless heat is lost or gained from the system. Here s a simple example. Two cups of equal amounts of water, one at 50 C and one at 10 C, are mixed together. What is the resulting temperature? Why? Write down the principles you used to determine the result. Here s another simple example. Two identical blocks of aluminum, one at 50 C and one at 10 C, are placed in contact and surrounded by very good insulation. What will happen to the temperature of each block? Will they ever be the same? What will the final temperatures be? Why? Write down the principles you used to determine the result. Heat can be transferred by conduction, convection, and radiation, which will be explored in the experiments in this section. 34 Heat Transfer Heat and Temperature
Heat storage The heat stored in a material, called its thermal heat capacity, is Q = c p m T Q = heat (KJ) c p = specific heat capacity (kj/kg K) m = mass (kg) T = change in temperature of the material Expressed in words, this equation says that the thermal energy stored in a material depends on its heat capacity per unit mass (different for different materials), its mass (how much of it there is), and the change in temperature of the object. Note the units for cp (kj/kg K). Specific heat capacity is the kilojoules of energy that it takes to raise one kilogram of a material one degree Kelvin (which is the same as one degree Celsius). Note that heat capacity is the total heat per degree of temperature change stored in an object, whereas specific heat capacity is the heat stored per unit mass. Different materials can store different amounts of heat because they have different specific heat capacities. For example, for a given change in temperature, the same amount of heat is stored in a roomful of air, a cubic foot of bricks, or a gallon of water. Heat and Temperature Heat Transfer 35
Air doesn t hold much heat, and most heat storage in buildings is in the solid materials plaster walls, concrete floors, etc. Very little of it is in the air, which is quick to heat up, and quick to cool down. Water has a very high heat capacity, that is, it takes a lot of energy to change the temperature of water a small amount, compared to many other materials. This is very significant in both natural and man-made systems. For example, much more heat is stored in the world s oceans than in its atmosphere, which is important when thinking about climate change. As another example, a much smaller volume of water is needed than air to transport heat from one place to another say from the furnace to the rooms of a house. 36 Heat Transfer Heater and Temperature
Power and energy Power (P) is how fast the heat flows or changes. It is measured in watts, which is the same as joules/second. P = Q /t watts = joules / seconds We can also say that the amount of heat is the power multiplied by the time. Q = Pt joules = watts * seconds For example, a 40 W light bulb that is on for one minute produces 2400 Joules of energy. 2400 J = (40 J / s) (60 s) Here s a typical calculation that has practical importance. Compare a 100 W incandescent bulb with a 20 W fluorescent bulb that produces the same amount of visible light. How much electrical energy, in kilowatt-hours, does each bulb use in one year? (Remember that watts = joules/second.) Write down how you convert from Joules per second to kilowatt-hours per year. If electricity costs $.15 per kwh, what is the cost saving by switching to a fluorescent bulb? Heat and Temperature Heat Transfer 37
Analysis Explain the difference between energy and power. Give an example using thermal energy. Do you think you would need a greater volume of water or air at the same temperature to heat up your house? Why? When and where is it useful to store heat? The shortest day of the year is December 20, but the coldest time of the winter is the end of January and into February, about six weeks later. Similarly, the longest day of the year is June 21, but the hottest time of the summer is the end of July and into August. Why? Explain this in terms of the heat capacity of the earth. 38 Heat Transfer Heat and Temperature
Connection to buildings Explore How would a building with a high heat capacity behave differently from a building with a low heat capacity? Which building might be more comfortable, and why? Heat and Temperature Heat Transfer 39
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Heat Transfer Conduction Introduction Conduction is the transfer of heat through solid materials. Thermal conductivity is the measure of how fast a material conducts heat. The opposite of conductivity is resistivity, or insulating value. Metals, like aluminum or iron, conduct very well, that is, they are good conductors and poor insulators. Materials with air trapped in them, like wool, bedding, or Styrofoam, conduct very slowly; they are good insulators. Most solid materials, like wood, plastic, or stone, are somewhere in between. How does a house lose heat through its walls and roof? Factors that affect heat conduction The rate of heat transfer by conduction depends on the conductivity, the thickness, and the area of the material. It is also directly proportional to the temperature difference across the material. Mathematically, it looks like this: Q / t = ka( T / x) Where: ( Q / t) is the rate of heat conduction (kj / s) T is the temperature difference across the material x is the thickness of the layer (m) A is the area of the material (m2) k is the thermal conductivity of the material per unit thickness (kj/m/s/ C) Conduction Heat Transfer 41
Your goal in these experiments is to measure and compare the thermal conductivity of different materials that you might use in the walls of your model house. You can do this by heating one side of a material and measuring the temperature difference between the hot side and the cold side. Your source of heat will be a cup of hot water. The situation is just like measuring the inside and outside temperatures of a house wall on a cold day. The assumption in this experiment is that the temperature difference is roughly proportional to the resistivity of the material. This is not strictly accurate, but it does provide a way to compare different materials. Note that since the different materials are different thicknesses, this experiment measures the thermal conductivity of the whole material, rather than its conductivity per unit thickness. 42 Heat Transfer Conduction
Experiment 1 CONDUCTIVITY OF DIFFERENT MATERIALS Procedure & data collection 1. Pick a test material from the available collection of sample squares. 2. Attach the two temperature sensors to the computer. 3. Open the Logger Lite file that goes with this experiment: conduction exper 1.gmbl 4. Fill a foam cup with very hot water and bring it to your work station. 5. Measure the room temperature and the hot water temperature by putting one of the sensors first in air and then in the water in the cup. Record them in Table 1 below. 6. Tape a temperature sensor to each side of a piece of material. The tape should cover the sensor and hold it tightly to the surface. Tools & materials Two temperature sensors Computer Pencils Metal ruler (cm) Scissors Safety utility cutter Hot tap water Styrofoam cups Squares of different rigid materials (aluminum, cardstock, cardboard, foamcore) large enough to cover the cup Masking tape and/or clear tape 7. Place the material on top of the cup and hold it firmly in place, touching only the edges. 8. Observe the temperature graphs. After they stop changing very quickly (about three minutes), stop data collection and scale the graph. Conduction Heat Transfer 43
9. Write down the steady state temperatures in Table 1 below. 10. Pick another material and repeat steps 6-9. Record all the data as different runs on the same Logger Lite file. Be sure to label the material of each run. Here s an example of a labeled graph. The thicker lines are the current experiment, and the thinner lines are a previous run. 11. Save the Logger Lite file. 12. Print the data graph and include it in each student workbook. 44 Heat Transfer Conduction
Table 1: Conductivity of materials Material Water temp Air temp Inside surface temp Initial conditions Aluminum Outside surface temp Difference across material Cardstock Foamcore Notes: Conduction Heat Transfer 45
Analysis Why do you think there is a difference between the inside and outside temperatures of all the materials? List the materials in order from greatest to least temperature difference. Based on these measurements, which material do you think is the best conductor? Which material do you think is the best insulator? Describe an everyday situation where you have directly experienced the difference in conductivity between wood and metal. With different materials, you probably note differences in the inside temperatures, even though they are all right above the hot water. Why do you think that is? 46 Heat Transfer Conduction
Temperatures inside walls Most house walls consist of several layers. The total insulating value is the sum of all of them. In addition, there is some insulating value of the still air right next to the surface, both on the inside and the outside of the wall. Note: only air that is not moving provides some insulating value. If the wind is blowing, the insulating layer of air is stripped away. Here is a diagram of a typical wall and the temperature change across the materials. The temperature drop is directly proportional to the insulating value of each portion of the wall. That is, the largest difference in temperature will be across the most insulating part of the wall. Conduction Heat Transfer 47
Experiment 2 MULTIPLE LAYERS Procedure & data collection 1. 2. 3. 4. 5. 6. 7. 8. 9. Open the Logger Lite file that goes with this experiment: conduction exper 2.gmbl Pick one of your materials and try to increase its insulating value by adding layers of the same material. Different teams should work with different materials so they can be compared. Start with a new cup of hot water, since it may have cooled down quite a bit. Repeat the previous experiment with your extra insulation. Try one layer, then two, and then four. Record all the data as different runs on the same Logger Lite file. Be sure to label the material and number of layers of each run. Observe the temperature graphs. After they stop changing very quickly (about three minutes), stop data collection and scale the graph. Write down the steady state temperatures in Table 2 below. Save the Logger Lite file. 10. Print the data graph and include it in each student workbook. Table 2: Conductivity of layers Material Used: Temperatures Material Water Air Inside surface One layers Outside surface Difference across material Data file name Two layers Four layers 48 Heat Transfer Conduction
Analysis How did the temperature difference across the material change when you added more layers? How did the inside temperature change when you added more layers? Would you prefer a thicker or a thinner wall for your model house? Conduction Heat Transfer 49
Conductivity and heat capacity combined It s easy to confuse storage capacity with insulating value. The diagram below is an analogy. Imagine a barrel with an inlet and an outlet near the top. The larger the inlet and outlet, the greater the conductivity. The larger the barrel, the greater the storage capacity. Insulating materials generally have low heat capacity (because they are mostly air) and low conductivity. It takes very little heat to warm them up, but getting the heat into them is slow. So the heat flow looks like this: Masonry materials generally have high heat capacity and fairly high conductivity. It may take a lot of heat to warm them up, but once they are full, the heat will flow through them quickly. The heat flow looks like this: 50 Heat Transfer Conduction
Metals have a low heat capacity and very high conductivity. They are easy to heat up, and heat flows through them easily. The heat flow looks like this: Which is more valuable for reducing heat loss in a wall low conductivity, or high heat capacity? Conduction Heat Transfer 51
Connection to buildings Background In the building trades, the rate of heat loss is called conductivity (U). The most common measure of conductivity is its inverse: resistance to heat flow, called R or R-value. R (thermal resistivity) = 1 / U (thermal conductivity The greater the value of R, the more slowly heat is lost. Doubling R-value means the rate of heat loss is cut in half. The building trades don t use metric units. For instance, heat flow is measured in British Thermal Units (BTU) per hour, instead of kilojoules per second. Temperatures are in Fahrenheit rather than Celsius. Thickness is in inches, and area is in feet instead of meters. To do real calculations on a building, you must get used to doing lots of conversions of units! This project will focus on the relative behavior of different materials, rather than exact calculations. R can be given per inch of material or for the whole assembly. For example, many common insulating materials have an R-value of 3 to 5 per inch, in standard American units. Fiberglass in a 5 ½ wood frame wall adds up to about R-20. Insulation in ceilings and roofs, where there s more room for insulation, is commonly R-30 to R-40. Windows typically have the lowest R-value in the building envelope: R-1 for single glazed, R-2 for double glazed, and R-3 or 4 for triple or specially treated glazing. So the typical wall is five to ten times as insulating as the typical window. But there is five to ten times as much wall area as window area, so the two elements contribute equally to the total heat loss, roughly speaking. 52 Heat Transfer Conduction
Note that the true insulating value of a wall or ceiling depends very much on the quality of workmanship. Gaps and voids can radically reduce the nominal R-value. Material Approximate R-value in US units 2x4 wall with insulation 12 2x6 wall with insulation 20 12 of attic insulation 40 12 masonry or concrete foundation wall 2 Single sheet of glass 1 Insulated glass 2 High-performance insulated glass 3 Insulated door 5 Masonry is surprising. It has a high thermal heat capacity, but its R-value is low. That is, it stores a lot of heat, but it also conducts heat well. An 8 masonry or concrete wall has the same R-value as a double-glazed window! Conduction Heat Transfer 53
Connection to buildings Explore Find out the materials your house is made of. What materials are used to provide insulation in the walls and ceilings? For example, are they insulated with fiberglass, cellulose (ground-up newspaper), rigid foam insulation, etc.? What parts of your house lose the most heat by conduction? Describe the advantages of a well-insulated house. 54 Heat Transfer Conduction
Summary Think about a house you d like to design. What kind of materials would you use for the walls and why? Include R-value, thickness, layers, and heat capacity. Would you want to have air layers between layers of other materials? Why? Conduction Heat Transfer 55
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Heat Transfer Convection Introduction Convection is defined as the circulation of fluids (liquids or gases), either natural or forced. Natural convection is caused by density differences. Hot air rises because it is less dense than cold air, so air will rise above a heater and sink near a cold window. Forced convection refers to fluids being pushed around by outside forces. A fan or a pump are forms of forced convection. Natural convection Hot air rises, because it s less dense than cold air. Warm air in a room quickly rises upward, and cold air sinks downward, even if the temperature differences are quite small. How do fluids and gases carry heat from one place to another? Can air carry heat into and out of a house? Convection Heat Transfer 57
Experiment 1 NATURAL CONVECTION IN A CUP Tools & materials Procedure & data collection Two temperature sensors Computer Scissors Tape 1. Cut out two pieces of cardstock slightly larger than the tops of the two cups. 2. Tape the temperature sensors to the undersides and fold over the corners to fit on the cups. Two plastic or Styrofoam cups Two pieces of cardstock to cover the cups Shallow bowl of hot water Loose insulation such as crumpled paper, foam packing beads, fiberglass, or cellulose 3. Fill one cup with loose insulation. Leave the other cup empty. 4. Place the cups in a shallow pan. 58 Heat Transfer Convection
5. Place the cards on top with the temperature sensors on the lower side. 6. Connect the temperature sensors. 7. Open the Logger Lite file that goes with this experiment: convection exper 1.gmbl 8. Start data collection. Wait for a minute or so until the sensors settles at roughly the same temperature. 7. Add a small amount of hot water to the pan. If you add too much, the cups will start floating. 8. Note the changes in temperature of the two sensors. 9. Save your Logger Lite file. Convection Heat Transfer 59
Analysis Which temperature changed the most, the empty cup or the filled cup? About how long did it take for there to be a noticeable difference? In both cases, the cup space was mostly air, because these insulations (foam beads, fiberglass, cellulose) are mostly air. How do you explain the difference? What would it be like if there was no natural convection, that is, if air didn t move around when heated or cooled? Would you be comfortable? 60 Heat Transfer Convection
Forced convection Wind chill describes the cooling effect of moving air across a warm surface, such as our skin. But wind chill can also affect a building. The cause of wind chill is simple, and it depends on the difference between conduction and convection. Air is a very good insulator, if it doesn t move. Most good insulators wool, foam, fiberglass trap air in tiny pockets so that it can t circulate. Heat conducts very slowly across each little air pocket. On the other hand, air moves very easily in larger spaces, driven by even the slightest temperature differences. When it moves, warm air carries heat from one place to another. Water can also carry heat from one place to another by being pumped through pipes, that is, by forced convection. The great advantage of water is its enormous specific heat capacity. Large amounts of heat can be transported from the boiler to all corners of the building. It is then transferred to the air in various ways. Picture a hot surface (such as your skin) with cold air above it. Right next to the surface is a thin layer of still air that provides some insulating value because it is not moving. Imagine happens when you turn on a fan. Your skin cools off because the still air layer is stripped away, and the skin surface directly exposed to the cold air. Convection Heat Transfer 61
Experiment 2 will measure the effect of moving air on surface temperatures. Experiment 3 will test the effect of openings in a house that let air go in and/or out. Recall the upper and lower temperatures in your first standard house experiment. Was there evidence of convection inside the house? 62 Heat Transfer Convection
Experiment 2 WIND CHILL Procedure & data collection Tools & materials In this experiment you will measure this effect of moving air on surface temperature. One temperature sensor Computer Standard house with heater 1. Tape the temperature sensor to a piece of cardstock and tape the card down over a Styrofoam cup of hot water so it won t blow away. Metal ruler (cm) Scissors Safety utility cutter Fan (optional) Clear tape Styrofoam cup filled with hot water A piece of cardstock to cover the cup 2. Open the Logger Lite file that goes with this experiment: wind chill.gmbl 3. Start data collection. Wait for two minutes or so until the sensor settles at a steady temperature. 4. Turn the fan on while continuing to record temperature. If you don t have a fan, use a piece of cardstock to fan air across the sensor. Don t blow your breath is not at room temperature! 5. Wait until the temperature is stable again and turn the fan off. 6. Wait until the temperature is stable again. 7. Enter the temperature data in Table 3 below. 8. Save your Logger Lite file. Convection Heat Transfer 63
Table 3: Wind chill Measurement Temperature Fan off Fan onn Fan off Average difference of fan on vs fan off 64 Heat Transfer Convection
Analysis Explain why the fan changes the temperature. Think of other examples of the wind chill effect and how it is minimized. Convection Heat Transfer 65
Infiltration Infiltration refers to outside air leaking into a house. This implies that inside air is also leaking out (exfiltration), so infiltration is loosely used to describe the exchange of air between inside and outside. If the inside air is warm and the outside air is cold, lots of heat can be lost, and the house will be drafty and uncomfortable. Infiltration can be driven by two forces: a) the stack effect or the chimney effect, where rising hot air pushes outward at the top of a building and cold air is drawn inward at the bottom; b) wind, which creates greater pressure on one side of a building than the other. 66 Heat Transfer Convection
Experiment 3 INFILTRATION Procedure & data collection This experiment will test the effect of openings in a house that let air go in and out. You will cut slots in the wall of your house and measure the power requirement for two conditions: A. House with the lower and upper slots open wide B. House with the lower and upper slots taped shut 1. Cut slots in the side of your standard house, one high and one low, in the wall opposite the one with the sensor. Each slot should be about 1 cm high and 8 cm long. Leave the lower edge attached like a hinge. Fold the paper out so that each hole is wide open. Don t worry, you can close these openings with tape after you are done with this experiment. Convection Heat Transfer 67
2. Install the sensor through the side of the house opposite the slots, in the upper position. 3. Open the Logger Lite file that goes with this experiment: house infiltration.gmbl 4. Measure the room temperature and add 15 C. This is the target temperature. 5. Turn the heater on. 6. Start collecting data when the sensor is a few degrees below the target temperature. 7. When the sensor reaches one degree above the target temperature, switch the heater OFF and record the time in Table 4 below (A). 8. When the sensor drops to one degree below the target temperature, switch the heater ON and record the time in Table 4 below (B). 9. When the sensor reaches one degree above the target temperature, switch the heater OFF and record the time in Table 4 below (C). 10. Stop collecting data and save the Logger Lite file. 11. Calculate the power requirement to keep the house warm by filling out the rest of Table 4. 68 Heat Transfer Convection