Designing solar hot air collectors

Engineering design is a lot of fun. The variety of engineering systems students can realistically design and build in classrooms is, however, limited by the constraints of time, resources, and student preparedness.

Currently, construction toys and computer programming are perhaps the most frequently adopted student projects for learning engineering design. These applications cover a number of domains such as robotics and software engineering. 

In our Engineering Energy Efficiency project, we have been working on adding a new option of engineering project that students and teachers can choose to learn and teach engineering.

This Green Building Kit we are developing needs only paper, cardstock, foam board, among other typical office supplies and widely available sensors. Yet, it will allow students to design, build, and test energy-efficient model houses with considerable green features.


An example I am working on is a hot air collector (HAC, also known as the Trombe wall). This is actually very easy to construct (hence a popular DIY project for those who are "green"-minded and handy). It is not difficult for students to add an HAC unit to the sun-facing wall of a model house.

In order for students to have fun with this design challenge, we need to show them that there are a variety of things that they can learn, emulate, test, and invent.

HAC units are usually installed to the part of the sun-facing wall that is not occupied by windows. Windows are necessary to a house because they let light in, but they generally lose more heat than an insulated wall. An insulated wall keeps the heat inside the house, but it does not do anything to collect the heat from the sun and give it to the house. The idea of hot air collector is to use the surface area of the wall that is exposed to the sun to collect some solar energy for warming up the house.

If you think about this engineering design task, it is really a problem about the optimal use of the sun-facing wall surface. So where should we put windows and HAC units and what is the best way of using them? The above images show a variety of designs. Click each image to enlarge it and see the details of each design.

The fourth design combines the benefits of windows and HAC units. It is basically a large HAC unit with the middle part replaced by a window. On the one hand, sunlight still can shine into the house through the two layers of glazing (we automatically have a double-pane window). On the other hand, as the HAC unit is tall, the convective heat exchange between the HAC unit and the room will be more significant. I haven't seen an HAC design like this, so this is my little "invention." Well, I am pretty sure some guy has thought of this before and there is probably a pending patent for this, but never mind about this, I am just demonstrating how an engineering design process in the classroom could be made more inventive.

Our next step is to make it possible for students to add these green architectural elements (HAC is just one of them) in one of our flagship products: Energy3D. Energy3D already has a powerful heliodon for solar design. 

The Physics Teacher Magazine features IR article

The Physics Teacher Magazine published by the American Association of Physics Teachers (AAPT) selected our article "Infrared Imaging for Inquiry-Based Learning" as a featured article on the September 2011 issue. A featured article is made free to the public. Each issue chooses three featured articles.

In this paper, we described a series of IR experiments that can be readily used to teach the basic concepts of heat transfer and their applications to engineering.

Students enjoyed Energy3D in Engineering Energy Efficiency Summer School 2011

Click to enlarge

This week 11 students of different ages (10-17) participated in our three-day summer school for the Engineering Energy Efficiency project. They were charged with using Energy3D to design their own model houses on a computer first and then construct them using inexpensive materials.

Although Energy3D is still in its alpha phase, it seemed to work remarkably well for these students who used the Mac computers we provided (thanks to Dr. Saeid Nourian, the lead developer of the software). Despite of some glitches, the students easily designed their own computer models. Creating the roof, the hardest part using other programs such as SketchUp, has been greatly simplified in Energy3D.

Interestingly, of the five groups, none used the template houses we provided to help them get started, indicating the fact that the students actually preferred designing their very own houses from scratch.

Strange thermal conductivity of leaves?

One way to tell if a plant is a plastic fake or not is to touch a leaf. If it feels cool, the plant is a real one. Have you ever wondered why a leaf feels cool? (A leaf of an indoor plant always rests at about the room temperature, plastic or real. It is not really cooler before you touch it. You can confirm this by measuring its temperature using a sensitive temperature sensor.)

We know metals feel cold because they conduct heat fast. Within a given amount of time, our fingers lose more thermal energy to a piece of metal than to a piece of wood.

Do leaves also conduct heat fast? On the contrary.

Let's put a fresh leaf on top of a piece of dry paper. The first set of IR images in this post shows what happened after I used two fingers to touch the leaf (on the left) and the paper to warm them up. The result tells that the leaf actually conducted heat more slowly than the paper, which has much lower thermal conductivity than metals.

Source: Wikipedia.

Now, we have a problem. We know leaves feel cooler than paper. But leaves conduct heat more slowly than paper! Our sense of touch honestly tells us that our fingers lose more thermal energy to leaves than to paper. So where does the thermal energy go on a leaf, if it doesn't diffuse to other parts?

My theory is that the thermal energy goes to heat up the water in the spongy layer of the leaf. The spongy layer lies beneath the palisade layer--the waxy surface layer of the leaf. Its cells are irregular in shape and loosely packed--hence the name "the spongy layer." During transpiration, the spongy layer is full of water in the spaces before they exit stoma. The specific heat of water is considerably high--4.18 J/(g*K) and the spongy layer is filled with water.

My theory is backed by the fact that a dry leaf conducts heat as fast as paper (IR images not shown here). This should not surprise you as paper is made of dehydrated wood fibers. 

Now, the question is why the water in the spongy layer doesn't dissipate thermal energy quickly as water in a cup does (I confirmed the energy dissipation in water by IR imaging, which is not shown here). The thermal conductivity of liquid water is about 0.58 W/(m*K), compared with 0.024 W/(m*K) for air, 0.016 W/(m*K) for water vapor, and 0.05 W/(m*K) for paper. Somehow, the water trapped in the spongy layer cannot conduct heat like free water does.

Let's get get a wet (20% of full water absorption capacity) sponge (left) and a dry one (right) and look at their thermal conductivities under an IR camera. Again, I used my fingers to leave a heat mark on each. The second set of IR images shows a surprising result: the wet sponge appeared to conduct heat more slowly than the dry one!

Does this thermal conductivity protect plants' leaves? Have you wondered why some plants are anti-freezing and some are not? Leaves may have very complicated thermal regulation that we don't quite understand.

Updates on 8/14/2012: 

See these YouTube videos of IR imaging:

Fresh leaf vs. dry leaf: http://www.youtube.com/watch?v=5I2eAU6AZ3Y
Wet sponge vs. dry sponge: http://www.youtube.com/watch?v=2LGfriM3O0Y