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Build a Passive Solar Design Laboratory

This easy to make but effective project introduces students to all the basics of passive solar design, and allows extensive experimentation with these design principles. The information learned from this experiment can also be used to establish an effective choice of materials, such as identifying the most effective absorbing materials, for building more ambitious solar energy devices.

The experiments may be performed outside, but because the experiments each take a fair amount of time (15 to 30 minutes for the shorter experiments, up to several hours for the longer experiments), they will be difficult to carry out if the temperature and/or the direct sunlight is changing a lot. Sunlight coming through a window may be easier to use, and use of a lamp will be easiest. You might want to run experiments inside first using the lamp, and then try to repeat them outside.

Background

The phrase "passive solar" was coined by Benjamin "Buck" Rogers in the 1970s. Buck is a solar scientist and pioneer who lives and works in New Mexico, and stresses the importance of basing passive solar design on sound scientific principles. Passive solar design itself has been employed by many peoples of the World for thousands of years, including the Greeks, Romans, Chinese, and the Native American Peoples of the American Southwest. Nowadays, passive solar design is a sophisticated art practiced by many architects, builders, and home owners. 

The Three Components of Passive Solar Design

There are three essential components to passive solar design:

• Solar Gain - Arranging for sunlight to enter a building to provide energy, and using highly absorptive surfaces to absorb the solar energy once it enters. Also, one needs to keep sunlight out during the summer. 
• Insulation - Retaining the energy provided by solar gain by trapping air inside, and by minimizing conduction, convection, and radiation through the walls, doors, and windows of the building. 
• Thermal Mass - The rate at which a house cools and heats up can be slowed down by placing objects in the house which store a large amount of thermal energy, for example, thick adobe walls, or even barrels of water. The thermal mass must be placed inside the insulation surrounding the house.  This applies to the floor as well! Proper addition of thermal mass will help greatly to keep the temperature of the building more nearly constant.

After describing these ingredients, ask the students the following questions: 

• Which ingredients are essential for every home to keep heating and cooling costs low?  Answer: Insulation is deemed important by just about everybody, including those who don't prefer to consider solar heating or the thermal benefits of thermal mass.
• How can thermal mass still be useful, even without solar gain? Answer: Thermal mass is always helpful in keeping the house at a constant temperature, and to stay warm for a while if the heating system fails. It can also be useful if the heat source is intermittent, for example, if you light a fire in the evening, and want the house to still be warm in the morning. 
• Which of the three ingredients are important in a very hot climate? Answer: both thermal mass and insulation are important for hot climates too, for keeping the house cool. In this case, insulation helps to keep energy outside. 

What you will need: 

Note: There are many possible variations on the materials that may be used, and we encourage you to experiment. The list we give below is just a particular set of choices, which will hopefully be easy to mimic. In the data given below, we used precisely the materials listed below. The numbers you generate are likely to be a bit different, and you should use our numbers just to a get a rough feel for what to expect, not as a target. 

Note on safety: A few things to definitely avoid are the following: plexiglas (it can warp when heated), heat lamps (too much energy, and they have too much energy in the infra-red range of the electromagnetic spectrum), glass with raw, unsanded edges (if you can't have the edges sanded, at least tape them), and of course, avoid anything that can be explosive or toxic when exposed to heat.

• A styrofoam picnic cooler (size should be roughly 1 ft x 1.5 ft ).
• A cardboard box of nearly the same dimensions as the cooler.
• Two pieces of clear, ordinary, window glass, 1/8 to 1/4 inch thick, with edges sanded or at least taped, that completely covers the opening of the picnic cooler and the cardboard box.
• A roll of self-sticking foam insulating tape, long enough to seal the perimeter of the cooler three and half times.  Thickness is not critical (1/4 inch is fine).
• A glass thermometer, and a second one if your going to run experiments outdoors.  The thermometer(s) should preferably be mounted in plastic brackets (to avoid danger of breakage and injury). You might want to spring for a pair of mounted thermometers, one of which has a remote sensor bulb for measuring the temperature outdoors. This is the type illustrated in the diagram above. 
• A 120 watt spot light (or thereabouts, not to exceed 150 watts)
• One light socket with a mounting clamp of adequate rated power to  handle the wattage of the spot light. 
• Materials for a variety of different light absorbers/reflectors:
  • a sheet of foil
  • a packet of construction paper
  • some high quality ink jet paper
  • a can of flat black spray paint (check to make sure that the paint contains "carbon black", which is actually carbon soot, the same stuff india ink is make of. This is usually printed in extremely fine print somewhere on the can).
• Materials for providing thermal mass: 
  • A tray containing several pounds of gravel
  • Some large rocks with together weigh the same as the gravel 
• A digital clock with a seconds readout - you may want to use a cooking timer as well.
• Another small cardboard box to provide some extra cardboard.
• A pen, sheets of notebook paper, a ruler, and masking tape.

Preparation for Experiments

1. You will need a way to mount the reflectors/absorbers in the cooler, and in the cardboard box. These should be mounted parallel to the glass plates (which will lie on and cover the top of the boxes), roughly three inches from the glass cover. Note that this will leave room below them for any thermal masses that may be included. Many styrofoam coolers come with a lip around the inside of the box. We suggest making a two "shelves" out of cardboard to rest on this lip, one at each end of the boxes, each about four inches in width (don't block more than half of the air flow with the shelves). Use some wooden pegs (say, made from old pencils) to hold the shelves if a lip is not already present (don't make holes all the way through the cooler sides - if you accidently do, plug them with crumpled paper and seal them with tape). Do the same thing with tape for the cardboard box.
2. Seal the seams in the cardboard box with tape to prevent unintentional air loss.
3. You need to place the thermometer in the cooler, not too close to the top, such that direct light will not fall on it but that you can still read it easy. We suggest folding a sheet of paper over in such a way to make a small stand to hold the thermometer that will simultaneously shade it from the light while tilting it towards one end of the cooler so that it can be easily read. Place the stand on one of the shelves in the cooler, and place the thermometer on the stand. (Optionally, if you bought the thermometer with the sensor bulb, mount the sensor bulb about 2/3 of the way up on the coolers side with tape, and run the wire outside. Do not make a hole in the box side!). 
4. Adhere insulating tape along the perimeter of the top edge of the picnic cooler with insulating tape, such that the cooler will be sealed when you place the glass on top it. Do the same for the cardboard box.
5. Likewise, attach more insulating tape around the edge of one of the glass sheets, such that if the glass plates are put together there will be a sealed airspace between them.
6. Mount the spotlight firmly over the center of the picnic cooler, about 4 inches from the cooler top. Place the glass plate on the cooler, (just the one without insulating tape on its perimeter for the timing being), a measure the distance of the lamp from the glass. Write this distance down, because you want to make sure that it stays the same for all the experiments.

Experiments

A note on experimental procedure: When making comparisons between different situations, for example, when comparing the absorption power of differently colored materials in Experiment #1, always keep as many variables, other than the one you are explicitly changing, as constant as possible. For example, make sure the thermal mass, the absorber area, and the position of all the absorbers, is at least approximately the same across trials. Otherwise, subtle effects may dramatically bias the results. For instance, you might assume that the styrofoam picnic cooler is a good reflector because it is white. However, it turns out that because of the depth of the box, and the fact that light is strongly scattered by the (optically) rough surface of the box, the picnic coolers are usually very good absorbers. 

Experiment #1: Measuring solar gain to determine the best aborbers

Purpose: To discover the principle of solar gain - that you can heat up a  house while the sun is shining if you get light inside and absorb it! 

Procedure: Place a series of different types of absorbers and reflectors inside the cooler, one at a time, and measure how fast and how high the temperature gets for each type. Try (at least!) the following absorbers/reflectors:

1. Different colors of construction paper: black, white, green, etc.
2. A black sheet of construction paper spray painted flat black.
3. A sheet of foil (a reflector).

Make sure all the absorber/reflector are the same size, and placed in the cooler in the same way. Measure the temperature every five minutes to the nearest degree, and record each measurement along with the time the measurement was taken. Assume that the maximum temperature has been reached when the number you read hasn't changed for two measurements in a row. After the maximum temperature is reached, turn off the spotlight. For at least one of the measurements, say, for the spray painted black absorber,  continue to take temperature measurements to capture the decrease in temperature as well. This additional data can be used later in Experiment # 3 below.

Now plot the data you've measured, by plotting the differences in temperature from room temperature versus the time since the beginning of the experiment. To do this, first subtract the first temperature measured (the room temperature) from all the other temperatures measured ( to get the differences in temperature), and subtract the first time recorded from all the other times (to get the changes in time). For example, the raw data we obtained for white construction paper was:

When the first temperature is subtracted from the rest of the temperatures, and similarly for the times, we obtained

You can see from the data that the temperature changes at a rate of roughly 1 degree per minute. 

When these data are plotted we obtain:

Discuss the results.

What you should find: You will find that different absorbers are associated with very different maximum temperatures, and thus have very different absorption abilities.  Specifically, you should find that truly dark absorbers work much better than lighter colored ones, for example, the black painted absorber should do much better than any color of the construction paper, including the black paper! All the absorbers should do much better than the foil (which just reflects most of the light back out). The temperatures we measured (using a 120 watt spotlight four inches away), were

As demonstrated by this data, you should also find that just because something is colored black, i.e. the black colored construction paper, it may still not be a very good absorber, and may even be worse than something which appears to be lighter colored, i.e. the green construction paper. 

Ask the students the following questions: 

• Why is it true that some apparently black things don't absorb well?. Answer: To be a good absorber, something needs to have a sufficient number of atoms in its surface which absorb well, and these atoms have to absorb over a sufficiently wide range of frequencies, as well. This is especially true if you are using an incandescent spotlight, which has most of its energy at wavelengths outside the visible range (in the infrared and longer). These requirements are stronger than what is required for your eye to perceive something as being colored black.

• Why don't the contents of the cooler just keep getting hotter and hotter until the cooler melts or catches on fire. Answer: As they get hotter, the contents lose energy at faster and faster rates to the outside world by conduction and radiation. But the rate of solar gain remains the same, regardless of the content's temperatures. So, eventually, the rate of loss equals the rate of solar gain, and the temperature stabilizes.

Experiment #2: Determining the best insulation

Purpose: To discover the necessity of having good insulation to achieve effective heating, including the need to trap air.

Procedure: Repeat Experiment # 2 with the cardboard box instead of the cooler, but this time just for black spray painted construction paper, and also just measure the maximum temperature. Now try to lower the temperature by punching a small hole in the box with a pencil, and wait for at least ten minutes before measuring the temperature. If the temperature doesn't change much, trying punching some more holes, one by one, even ten minutes or so, recording the temperature every few minutes. 

What you should find: You should find that the cardboard box does not achieve as high a temperature as the styrofoam cooler, so that cardboard is not as good an insulator as styrofoam. You should also find that even small air holes dramatically lower the maximum temperature.

Ask the students the following questions: 

• Why does the better insulated box get to a higher temperature? Answer: At a given temperature, the insulated house loses energy at a slower rate than the uninsulated box for a given temperature. But the rate of energy input of the two houses is always the same. Therefore, it follows that the better insulated box has to be at a higher temperature before its rate of loss equals the rate of gain.

Experiment #3: Determine the effect of thermal mass

Purpose: To discover that thermal mass slows down how fast the temperature can change, but does not effect the maximum temperature achievable.

Procedure

1. Now place the tray of gravel (the thermal mass) in the cooler, making sure the thermal mass are completely shaded by the absorber (you don't want to add or subtract to the absorbing power).  
2. Plot the temperatures versus time, in the same manner as was done in Experiment #1.
3. Compare the results to the results of Experiment #1 for the same absorber, and discuss the comparison.

What you should find: With significant thermal mass, the cooler should show a marked tendency to warm and cool more slowly. 

Why is this the case? Answer: With the thermal mass present, it takes much longer and much more heat to heat up the inside of the cooler because much of the heat is absorbed by the thermal mass.  

Experiment #4: Determine the role of surface area on thermal mass aborbtion

Repeat experiment #3 with the large rocks substituted for the gravel.

What you should find: You should find that the cooler should warm at a rate faster than with the gravel, but slower than with no extra thermal mass.

Why is this the case? Answer: Even though the rocks weigh the same as the gravel, the surface area of the rocks is much less than that of the gravel, so it takes much more time for the energy to absorbed by the rocks than the gravel. 

Experiment #4: Determining the effect of thermal mass on variations in temperature.

Purpose: To demonstrate that thermal mass lessens daily variations in temperature in a solar home.

Procedure

Without significant thermal mass, record the temperature every five minutes while turning the light on and off for successive intervals of fifteen minutes. Do this until you have a good idea of the what the maximum and minimum temperatures are. Now repeat this with significant thermal mass.

What you should find: You should find that there is much less difference between the maximum and minimum temperatures of the cooler when it has significant thermal mass inside.

What you should learn: Because the Sun doesn't shine all the time, thermal mass is necessary to help keep the temperature of a passive solar house approximately constant. 

Ask the students what the effects of increasing or decreasing thermal mass would be. Answer: the more thermal mass, the slower the house changes temperature.

Ask the students how one can add thermal mass to a real house. Answer: Using walls make of heavy material, such as stone, brick, or adobe, or by adding something with lots of mass, such as some big barrels of water, are good ways of providing thermal mass. Some people once built a solar grocery store in Wisconsin, and found afterwards that the canned and bottled goods on the shelves alone provided all the thermal mass the building needed! Point out that its crucial to have enough surface area for the thermal mass to function effectively. In fact, the area of the thermal mass should be about six times as large as the south facing windows of the house - alot of area! 

Ask the students if a house's insulation should be placed inside or outside the thermal mass. Answer: The thermal mass must be inside the insulation. This includes the floor! Many houses do not have a thermal barrier in the floor. This could be important in the house's thermal dynamics, especially in colder climates.

Ask the students what thermal mass there might be in their own homes.

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