Tag Archives: Stack effect

Fireplaces at odd with energy efficiency? An Energy2D simulation

In the winter, a fireplace is the coziest place in the house when we need some thermal comfort. It is probably something hard to remove from our living standards and our culture (it is supposed to be the only way Santa comes into your house). But is the concept of fireplace -- an ancient way of warming up a house -- really a good idea today when the entire house is heated by a modern distributed heating system? In terms of energy efficiency, the advice from science is that it probably isn't.

Figure 1. A fire is lit in the fireplace.
When the wood burns, a fireplace creates an updraft force that draws the warm air from the house to the outside through the chimney. This creates a "negative pressure" that draws the cold air from the outside into the house through small cracks in the building envelope. This is called the stack effect. So while you are getting radiation heat from the fireplace, you are also losing heat in the house at a faster rate through convection. As a result, your furnace has to work harder to keep other parts of your house warm.

Figure 2. No fire.
Our Energy2D tool can be used to investigate this because it can simulate both the stack effect and thermostats. Let's just create a house heated by a heating board on the floor as shown in the figures in this article. The heating board is controlled by a thermostat whose temperature sensor is positioned in the middle of the house. A few cracks were purposely created in the wall on the right side to let the cold air from the outside in. Their sizes were exaggerated in this simulation.

Figure 1 shows the duty cycles of the heating board within two hours when the house was heated from 0 °C to 20 °C with a fire lit in the fireplace. A heating run is a segment of the temperature curve in which the temperature increases, indicating the house is being heated. In our simulation, the duration of a heating run is approximately the same under different conditions. The difference is in the durations of the cooling runs. A more drafty house tends to have shorter cooling runs as it loses energy more quickly. Let's just count those heating runs. Figure 1 shows that 15 heating runs were recorded in this case.

Figure 2 shows the case when there was no fire in the fireplace and the fireplace door was closed. 13 heating runs were recorded in this case.

What does this result mean? This means that, in order to keep the house at 20 °C, you actually need to spend a bit more on your energy bill when the fireplace is burning. This is kind of counter-intuitive, but it may be true, especially when you have a large drafty house.

Figure 3. In a house without cracks...
How do we know that the increased energy loss is due to the cracks? Easy. We can just nudge the window and the wall on the right to close the gaps. Now we have a tight house. Re-run the simulation shows that  only 11 heating runs were recorded (Figure 3). In this case, you can see in Figure 3 that the cooling runs lasted longer, indicating that the rate of heat loss decreased.

Note that this Energy2D simulation is only an approximation. It does not consider the radiation heat gain from the fireplace. And it assumes that the fire would burn irrespective of air supply. But still, it illustrates the point.

This example demonstrates how useful Energy2D may be for all precollege students. In creating this simulation, all I did is to drag and drop, change some parameters, run the simulation, and then count the heating runs. As simple as that, this tool could be a game changer in science and engineering education in high schools or even middle schools. It really creates an abundance of learning opportunities for students to experiment with concepts and designs that would otherwise be inaccessible. Similar experiences are currently only possible at college level with expensive professional software that typically cost hundreds or even thousands of dollars for just a single license. Yet, according to some of our users, our Energy2D rivals those expensive tools to some extent (I would never claim that myself, though).

Engineers use Energy2D to simulate rocket mass heaters

Link to simulation
A rocket mass heater is an innovative and highly efficient space heating system, which is popular among natural building DIYers since its invention in 1970s. A number of engineers who are interested in rocket stove design have used our Energy2D software to visualize the thermal physics involved.
Link to simulation

Martin Karl Waldenburg from Germany has designed a series of simplified rocket stove simulations. With his permission, we have published his simulations on our Energy2D website. This blog post provides links to three of his simulations. Another one was created by Pinhead of the Rocket Stove Forum (who also gave us permission to publish his simulation).

Link to simulation
Link to simulation
Since Energy2D hasn't supported chemical reactions yet, in all these simulations, burning is simulated using a heater with a fan to approximate the driving pressure due to combustion.

We will continue to work on Energy2D's computational engine and improve its graphical user interface. Currently, we are plowing through the math needed to model thermal radiation, chemical reactions, and phase changes. Once these features are added, we hope more people will find it useful, educational, and entertaining.

Who says kids cannot invent?

I was recently involved in a few pilot field tests in which high school students were challenged to build an energy efficient scale model house. We observed something amazing. Initially, I was worried that students may end up building houses that are so similar to each other that the entire research will be invalidated. But that did not happen.

In one field test, a group of students created a pyramid and discovered an effect that I would call "a heat funnel." The images to the right show the pyramid heated by a 40W light bulb on the floor inside and an infrared signature showing the equilibrium temperature distribution. The students observed that the temperature at the tip of the pyramid reached nearly 150°C--enough to boil water! This amazing heating effect is due to the fact that hot air rises to the top in a way similar to how water flows down in a funnel. Just like the bottleneck of the funnel records the highest speed of water flow, the top of the heat funnel records the highest temperature of heat flow. The water funnel is usually explained using the conservation of mass, whereas the heat funnel can be explained using the conservation of energy. The density of thermal energy must increase when the heat conduit narrows in order for energy to conserve. Therefore, the temperature at the tip can be very high because its cross section is very small.

Although they did not expect the temperature at the tip to be so high, the students were fully aware of the convection effect, because they cut some slits at the bottom of the pyramid to let fresh air in in order to keep the air flow through it (you can see a slit from the photo on the left). This is the stack effect that drives a chimney. At the top of the pyramid, the hot air just exits through the tip, which naturally has small passages for the air because it was not perfectly sealed. Had students had a sensitive air speed meter, they would have observed a small but appreciable jet stream coming out from the tip (would they?), just like steam from the vent of a cooking pot.

In another field test, a group of students created a sliding roof that can provide overhang shading in summer and increase roof insulation in winter (see the images to the right).

I must confess that, as a physicist, I have never heard of or thought of the heat funnel effect until I saw it in the classroom. Pondering about this effect, I realized that it might be non-trivial and could have some engineering implications. For example, might this effect be used to build some kind of solar updraft pyramid for generating electricity? I have heard that in the US there are huge solar power plants that utilize the optical focus effect to create high temperature to boil water, which in turn creates steam to push an electrical generator. How about a heat funnel generator that will work sunny or cloudy?

The sliding roof invention is impressive in that the students figured out an engineering solution that solves two problems: winter insulation and summer shading. The students also had an idea of putting solar panels on the sliding roof and the base roof. This smart design, which increases the solar reception area, will turn the unwanted solar heat into electricity instead of reflecting it off. This is not just a single solution that solves one problem. This is a stone that kills three birds. Isn't this exactly what we strive to teach in our engineering classes?

These inventions of students should convince you that students are not just learners. If we give them creative tools and interesting projects, they can be inventors as well. Sometimes, their inventions will surprise even seasoned scientists and engineers. Science and engineering education should make more opportunities for these young inventors to rise to the top.