Posts Tagged ‘Green building’

Simulating cool roofs with Energy3D

August 20th, 2014 by Charles Xie
Fig. 1: Solar absorption of colors.
Cool roofs represent a simple solution that can save significant air-conditioning cost and help mitigate the urban heat island effect, especially in hot climates. Nobel Prize winner and former Secretary of Energy Steven Chu is a strong advocate of cool roofs. It was estimated that painting all the roofs and pavements around the world with reflective coatings would be "equivalent to getting 300 millions cars off the road!"

With Version 4.0 of Energy3D (BTW, this version supports 200+ worldwide locations -- with 150+ in the US), you can model cool roofs and evaluate how much energy you can save by switching from a dark-colored roof to a light-colored one. All you need to do is to set the colors of your roofs and other building blocks. Energy3D will automatically assign an albedo value to each building block according to the lightness of its color.

Figure 1 shows five rectangles in different gray colors (upper) and their thermal view (lower). In this thermal view, blue represents low energy absorption, red represents high energy absorption, and the colors in-between represents the energy absorption at the level in-between.

Now let's compare the thermal views of a black roof and a white roof of a cape code house, as shown in Figure 2. To produce Figure 2, the date was set to July 1st, the hottest time of the year in northern hemisphere, and the location was set to Boston.

Fig. 2: Compare dark and white roofs.
How much energy can we save if we switch from a perfectly black roof (100% absorption) to a perfectly white roof (0% absorption)? We can run the Annual Energy Analysis Tool of Energy3D to figure this out in a matter of seconds. The results are shown in Figure 3. Overall, the total yearly energy cost is cut from 6876 kWh to 6217 kWh for this small cape code house, about 10% of saving.

Figure 3 shows that the majority of savings comes from the reduction of AC cost. The reason that the color has no effect on heating in the winter is because the passive solar heat gains through the windows in this well-insulated house is enough to keep it warm during the sunshine hours. So the additional heat absorbed by the black roof in the same period doesn't offset the heating cost (it took me quite a while to figure out that this was not a bug in our code but actually the case in the simulation).

Fig. 3: Compare heating and AC costs (blue is white roof).
Of course, this result depends on other factors such as the U-value and thermal mass of the roof. In general, the better the roof is insulated, the less its color impacts the energy cost. With Energy3D, students can easily explore these design variables.

This new feature, along with others such as the heat flux visualization that we have introduced earlier, represents the increased capacity of Energy3D for performing function design using scientific simulations.

Here is a video that shows the heating effect on roofs of different colors.

Visualization of heat flux in Energy3D using vector fields

August 14th, 2014 by Charles Xie
Fig. 1: Winter in Boston
One of the strengths of our Energy3D CAD software is its 3D visualizations of energy transfer. These visualizations not only allow students to see science concepts in action in engineering design, but also provide informative feedback for students to make their design choices based on scientific analyses of their design artifacts.

Fig. 2: Summer in Boston
A new feature has been added to Energy3D to visualize heat transfer across the building envelope using arrays of arrows. Each arrow represents the heat flux at a point on the surface of the building envelope. Its direction represents the direction of the heat flux and its length represents the magnitude of the heat flux, calculated by using Fourier's Law of Heat Conduction. Quantitatively, the length is proportional to the difference between the temperatures inside and outside the building, as well as the U-value of the material.

Fig. 3: Winter in Miami
The figures in this post show the heat flux visualizations of the same house in the winter and summer in Boston and Miami, respectively. Like the solar radiation heat map shown in the figures, the heat flux is the daily average. The U-value of the windows is greater than those of the walls and roof. Hence, you can see that the heat flux vectors in the winter sticking out of the windows are much longer than those sticking out of the walls or roof. In the summer, the heat flux vectors point into the house but they are much shorter, agreeing with the fact that Boston's summer is not very hot.

Fig. 4: Summer in Miami
Now move the same house to Miami. You can see that even in the winter, the daily average heat flux points inside the house, agreeing with the fact that Miami doesn't really have a winter. In the summer, however, the heat flux into the house becomes significantly large.

These visualizations give students clear ideas about where a house loses or gains energy the most. They can then adjust the insulation values of those weak points and run simulations to check if they have been fixed or not. Compared with just giving students some formulas or numbers to figure out what they actually mean to science and engineering practices, experiential learning like this should help students develop a true understanding of thermal conduction and insulation in the context of building science and technology.

Here is a YouTube video of the heat flux view.

Green building design with Energy3D: How big should south-facing windows be?

April 17th, 2014 by Charles Xie
Many people know that south-facing windows can help to heat a house in the winter because they let a lot of sunlight in. Exactly how much of the south-facing wall should we allocate to windows? What are the downsides? How can we avoid them? Our Energy3D software allows students to explore the problems and find the solutions.

Figure 1


Suppose we have a simple house like the one shown in Figure 1 and we are in the Boston area. Energy3D supports students to try a design choice, run a simulation, collect the data, analyze the result, and evaluate the solution -- all in real time as is shown in the video in this post. Energy3D's powerful simulation and analysis tools provide instantaneous feedback to students so that their design processes can be guided and informed by the scientific and engineering principles built in the software. Let's use the investigation of south-facing windows described above as an example.


Figure 2 (Excel graph)
Suppose a student follows the design trajectory as shown in Figure 1. A challenge is to keep the yearly energy cost needed to maintain the temperature of the house at 20℃ to be as low as possible. The student begins with adding a small window to the south side of the house. By running the seasonal energy analysis tool in Energy3D, she immediately discovers that, by adding a small window, she can cut the energy cost a bit. Then she enlarges the window and finds that more energy can be saved. So she goes on to increase the size of the window. However, she finds that, at some point, larger windows on the south side start to cost more energy. After she adds two large windows, the energy cost increases over 15%, compared with the case of no window at all. Figure 2 shows the energy cost, broken down to heater and AC, as a function of the window area. That doesn't quite make sense to her. So she has to stop and think about why.
Figure 3 (Energy3D graph)


The trend in Figure 2 suggests that, with the enlargement of windows on the south side, the cooling cost continues to rise while the heating cost levels off. A monthly breakdown in Figure 3 reveals this trend more clearly. As shown by the golden dashed line in Figure 2, the solar heating through the windows increases rapidly when their total area gets enlarged.

Figure 4 (Energy3D graph)
Figure 5 (Energy3D graph)
If she wants to keep the large window area in the south side (for natural lighting and sanity of the occupants!), she has to reduce the solar heating effect through the windows in the summer. One way to do this is to plant tall deciduous trees in front of the windows as shown in Figure 1. The trees provide shading for the windows in the summer but let sunlight shine through to the windows in the winter (in Energy3D, deciduous trees have leaves from May 1st to November 30th). Figure 4 shows the effect of the two deciduous trees on the solar gain through the two south-facing windows. From the graph, she can see that the trees cut down the solar heating in the summer. As a result, the AC cost is reduced, as shown in Figure 5, whereas the heating cost is almost unchanged.

She concludes that, with the trees planted to the south of the house, the net energy cost over a year can be decreased to lower than the case of no window at all, providing an acceptable solution that takes care of view, lighting, and landscaping.

The Energy3D graphs in this blog post show that students can keep the results of previous runs (the curve of each run is labeled by a number) and superimpose new data on top of them. As the data view can get quite complex, Energy3D provides options to turn on/off data types and runs. The embedded video shows how those features work for visualizing and analyzing the simulation results.

PS: Some readers may notice that our calculations predict higher AC cost in September than in August or July. This is because when those calculations were done, the house had no window on the east or west side. Adding windows to those sides, the AC cost will peak around July or August -- even when the trees are not present.

Building performance analyses in Energy3D

April 6th, 2014 by Charles Xie
Energy3D (Tree image credit: SketchUp Warehouse and Ethan McElroy)
A zero-energy building is a building with zero net energy consumption over a year. In other words, the total amount of energy used by the building on an annual basis is equal to or even less than the amount of renewable energy it produces through solar panels or wind turbines. A building that produces more renewable energy than it consumes over the course of a year is sometimes also called an energy-plus building. Highly energy-efficient buildings hold a crucial key to a sustainable future.


One of the goals of our Energy3D software is to provide a powerful software environment that students can use to learn about how to build a sustainable world (or understand what it takes to build such a world). Energy3D is unique because it is based on computational building physics, done in real time to produce interesting heat map visualization resembling infrared thermography. The connections to basic science concepts such as heat and temperature make the tool widely applicable in schools. Furthermore, at a time when teachers are required by the new science standards to teach basic engineering concepts and skills in classrooms, this tool may be even more relevant and useful. The easy-to-use user interface enables students to rapidly sketch up buildings of various shapes, creating a deep design space that provides many opportunities of exploration, inquiry, and learning.


In the latest version of Energy3D (Version 3.0), students can compute the energy gains, losses, and usages of a building over the course of a year. These data can be used to analyze the energy performance of the building under design. These results can help students decide their next steps in a complex design project. Without these simulation data to rationalize design choices, students' design processes would be speculative or random.

A complex engineering design project usually has many elements and variables. Supporting students to investigate each individual element or variable is key to helping them develop an understanding of the related concept. Situating this investigation in a design project enables students to explore the role of each concept on system performance. With the analytic tools in Energy3D, students can pick an individual building component such as a window or a solar panel and then analyze its energy performance. This kind of analysis can help students determine, for example, where a solar panel should be installed and which direction it should face. The video in this post shows how these analytic tools in Energy3D work.

Spring is here, let there be trees!

March 28th, 2014 by Charles Xie
Trees in Energy3D.
Trees around a house not only add natural beauty but also increase energy efficiency. Deciduous trees to the south of a house let sunlight shine into the house through south-facing windows in the winter while blocking sunlight in the summer, thus providing a simple but effective solution that attains both passive heating and passive cooling using the trees' shedding cycles. Trees to the west and east of a house can also create significant shading to help keep the house cool in the summer. All together, a well-planed landscape can reduce the temperature of a house in a hot day by up to 20°C.

The tree to the south side shades the house in the summer.
With the latest version of Energy3D, students can add trees in designs. As shown in the second image in this blog post, the Solar Irradiation Simulator in Energy3D can visualize how trees shade the house and provide passive cooling in the summer.

The Solar Irradiation Simulator also provides numeric results to help students make design decisions. The calculated data show that the tree to the south of the house is able to reduce the sunlight shined through the window on the first floor that is closest to it by almost 90%. Students can do this easily by adding and removing the tree, re-run the simulation, and then compare the numbers. They will be able to add trees of different heights and types (deciduous or evergreen). There will be a lot of design variables that students can choose and test.

A design challenge is to combine windows, solar panels, and trees to reduce the yearly cost of a building to nearly zero or even negative (meaning that the owner of the house actually makes money by giving unused energy produced by the solar panels to the utility company). This is no longer just a possibility -- it has been a reality, even in a northern state like Massachusetts!

SimBuilding funded by the National Science Foundation

July 28th, 2013 by Charles Xie
A thermal bridge simulation in SimBuilding
Building science is, to a large extent, a “black box” to many students, as it involves many invisible physical processes such as thermal radiation, heat transfer, air flow, and moisture transport that are hard to imagine. But students must learn how these processes occur and interact within a building in order to understand how design, construction, operation, and maintenance affect them and, therefore, the wellbeing of the entire building. These processes form a “science envelope” that is much more difficult to understand than the shape of the building envelope alone. With 3D graphics that can visualize these invisible processes in a virtual building, simulation games provide a promising key to open the black box. They offer a highly interactive learning environment in which STEM content and pedagogy can be embedded in the gameplay, game scores can be aligned to educational objectives to provide formative assessments, and students can be enticed to devote more time and explore more ramifications than didactic instruction. A significant advantage is that students can freely experiment with a virtual building to learn a concept before exploring it in a real building with all the consequences and costs that may entail.

A new grant ($900K) from the National Science Foundation will allow us to develop a simulation game engine called SimBuilding based on computational building simulation. The application of advanced building simulation technologies to developing training simulation games will be an original contribution of this project. Although building simulation has become an important tool in the industry and can be very helpful in understanding how a building works, it has never been used to build simulation games before. SimBuilding will unveil this untapped instructional power. Furthermore, this game engine will be written in JavaScript and WebGL, allowing it to run on most computing devices.

Amanda Evans, Director of Center of Excellence for Green Building and Energy Efficiency at Santa Fe Community College in New Mexico, will be our collaborator on this grant.

Significant gender differences found (confirmed?) in CAD research

March 13th, 2013 by Charles Xie
A student design
In a pilot study conducted in December 2012, high school students in an engineering class used our Energy3D CAD tool to do an urban solar design project -- they must consider the sun path in four seasons and the existing buildings in the neighborhood as the design constraints to optimize solar penetration to the new buildings and minimize obstruction of sunlight to the existing buildings.

Energy3D can log any student actions and intermediate steps, which provide extremely detailed information about student design processes. With such a high-resolution lens, we could characterize student patterns and analyze how they solve the design challenge closely. For example, the CAD log allows us to reconstruct the entire design process of each student and show it in an unprecedentedly fine-grained timeline graph. A timeline graph may show how students went through different iterative steps while shaping their designs. For instance, did they consider the interactions among the buildings they designed? Did they go back to revise a previously erected building that may be affected by a newly added one? The timeline data we have collected show that the students' designs demonstrated more iterative features as they moved on to explore and design alternatives following the initial attempts (perhaps encouraged by the gained familiarity with and confidence in the CAD tool).

A design timeline (click to enlarge)
Our analyses also suggest that there appears to be a significant gender difference in both design products and processes. The main differences are: 1) The boys tended to push the limit of the software and produced unconventional designs that looked "cool" but did not necessarily meet the design specifications; and 2) The girls spent more time carefully revising their designs than building new structures. While these findings may not be surprising to some seasoned educators, the significance is that this may be the first time this kind of gender difference was revealed or confirmed by empirical data from CAD logs. Using CAD logs may provide a fairer basis of assessing student performance based on the entire learning process rather than just looking at their final products or self reports.

Summary of the results
The implication of this study is that if we can identify patterns in student design learning and understand their cognitive meanings, we could devise a software system that can provide real-time feedback to help students learn in the future. For example, could the software prompt students to consider the design criteria more when it detects that students are ignoring them? Could the software stimulate students to think out of the box more when it detects that students are underexploring the design space?

For more information about this research project, visit: http://energy.concord.org/research.html.

Using Energy2D to simulate Trombe walls

February 26th, 2013 by Charles Xie

A Trombe wall is a sun-facing wall separated from the outdoors by glass and an air space. It consists a solar absorber (such as a dark surface) and two vents for air in the house to circulate through the space and carry the solar heat to warm the house up. In a way, a Trombe wall is like a machine that uses air as a convey belt of thermal energy harvested from the sun. Trombe walls are very simple and easy to make and are sometimes used in passive solar green buildings.


Hiding sophisticated power of computational fluid dynamics behind a simple graphical user interface, our Energy2D software can easily simulate how a Trombe wall works. The two images in this blog post show screenshots of a Trombe wall simulation and its closeup version. You can play the simulation on this page and download the models there. If you open the models using Energy2D, you should be able to see how easy it is to tweak the models and create realistic heat flow simulations.

Solar chimneys operate based on similar principles. Energy2D should be able to simulate solar chimneys as well. Perhaps this would be a good challenge to you. (I will post a solar chimney simulation later if I figure out how to do it.)

Designing solar hot air collectors

September 1st, 2011 by Charles Xie
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.