Designing an Energy-Plus House
By Charles Xie
Are you looking for high school engineering design projects that meet the requirements of the Next Generation Science Standards (NGSS)? Do you need free, high-quality software and curriculum that engage students in solving complex real-world problems like scientists and engineers and yet can be easily implemented? Do you want students to be more technically prepared to tackle energy and environmental issues in the future? If you answered “yes” to these questions, this tutorial is for you.
First, let's briefly introduce the Aladdin software. Aladdin is an integrated CAD+CAE program for designing buildings that take advantage of renewable energy sources such as solar energy to reduce fossil fuel use (and other engineering systems that have sustainability goals). Based on the weather data of 750 worldwide locations, Aladdin allows students around the world to design sustainable engineering solutions for their climates. It aims to engage students in science and engineering practices as required by NGSS. The streamlined capabilities of design, simulation, analysis, and visualization within Aladdin enables students to test and evaluate multiple design ideas through rapid cycles of virtual experimentation. With Aladdin, you can challenge students to design an energy-efficient house that, over the course of a year, produces more renewable energy than the energy required for heating and cooling it — this type of house is called an energy-plus building. In addition to this goal, students must also meet a set of other design criteria and constraints. For example, the house should adopt an architectural style specified by the client, the size cannot be too big or too small, and the cost must not exceed the construction budget.
The science of building energy
Figure 1. A 3D model of a 246m² Colonial style house created in Aladdin as a starting point (download).
Aladdin allows students to quickly sketch up realistic-looking houses using a basic set of design elements, including walls, roofs, windows, trees, solar panels, and so on (Figure 1). Students can adjust the properties of each element such as size, location, orientation, R-value, solar heat gain coefficient, heat capacity, color, and more — this is where they make meaningful connections to basic science concepts that dictate the energy exchange between the house and the environment.
Whenever students want to evaluate the effect of a change on the energy consumption and generation of the house under design, they can run the built-in physics-based simulator, which produces a visualization shown in Figure 2. The color map represents the distribution of the intensity of solar radiation that reaches the exterior surface of the house. The arrows represent the distribution of the intensity of heat flux flowing into or out of the house. As the visualization shows, windows and doors tend to lose more thermal energy in the winter than walls because they are less insulative; a surface that faces the Sun tends to receive more solar radiation than a surface that does not.
Figure 2. Visualization of the effects of solar radiation and heat transfer on the house (November 1st, Boston, MA).
Engineering analysis for making design decisions
Science concepts give us qualitative understanding, but engineering design demands quantitative results for decision making. To this end, students can plot a line graph that itemizes and summarizes daily or annual energy use. Figure 3 shows the obvious seasonal trend of the energy consumption — heating dominates in the winter, cooling dominates in the summer, and both are needed in the spring and fall (but the total energy curve peaks in the winter and summer):
Figure 3. The annual energy consumption for heating and cooling under the default condition (no passive or active solar solutions).
In this article, we first explore two strategies to achieve the goal of achieving energy surplus for the house under design: active and passive solar. Photovoltaic solar panels are typically considered as active solar systems as they rely on extrinsic devices to convert sunlight into electricity. Passive solar buildings, on the other hands, use intrinsic elements such as windows, walls, and floors to collect solar energy in the winter and reject solar energy in the summer.
Active solar design: Add solar panels
An energy-plus house often relies on active solar energy systems to generate power locally. Putting solar panels on the roof is a common solution. Since our goal is to produce as much energy as possible, let's opt for one of the best solar panels in the residential market (as of 2020) — the SPR-X22-370 model manufactured by SunPower, which boasts a solar cell efficiency of 22.7% and a power output of 370W. The model is also supported in Aladdin (so there is no need to gather the product specs and input them into the software manually). We should also try to fit as many solar panels as possible in the south-facing side of the roof. It turns out that we can install a 9×4 array of 36 panels if we choose the landscape orientation (Figure 4).
Figure 4. 36 SPR-X22-370 solar panels are added to the sun-facing side of the roof.
If we run the annual analysis again, we can see that the solar panels generate a lot of energy, almost enough to match the total heating and cooling energy throughout the year. Note that this does not mean that the house can go off the grid. As Figure 5 shows, the daily energy generated by the solar panels is not enough to heat the house in the winter, although it is more than enough to cool the house in the summer (this seasonal imbalance is a typical problem of solar energy). In Figure 5, we switch to an area plot to look at the result in a different way. The purple area above the zero axis is the additional energy needed to maintain the temperature inside the house and the purple area below the zero axis is the extra energy that can be sold to the grid through a net-metering program.
Figure 5. The annual energy generation from solar panels offsets the net consumption significantly.
Passive solar design: Change the solar heat gains of windows
One way to avoid excessive solar heating in the summer is to choose windows with a lower solar heat gain coefficient (SHGC). This way, we do not have to compromise the windowy look of the house. A window with a lower SHGC value allows less sunlight to pass through to heat up the house. Given the fact that the day is much shorter in the winter than in the summer, such a change may result in a net positive change for the house, bringing us closer to the goal. Figure 6 shows the improvement of halving the SHGC values for all the west and east-facing windows. Our house now consumes net energy of about 835 kWh per year, less than the average electricity consumption per month of an American home (877kWh).
Figure 6. Applying a lower solar heat gain coefficient for the west and east-facing windows reduces the net energy consumption.
You may be wondering why we don't reduce the SHGC values for the south and north-facing windows. It turns out that doing so does not result in a net improvement as the south-facing windows are very helpful in the winter. As for the north-facing windows, they do not receive a lot of solar radiation to begin with anyways. You can confirm these findings on your own.
Non-solar solution: Improve insulation
To move towards the goal, we can install more solar panels on the north-facing side of the roof. But considering the much lower productivity on that side of the roof and the high expenses of solar panels and their installation, it will not be a cost-effective choice. Another way to reduce the energy consumption is to improve insulation. For example, if we decrease the U-value of all the windows by 20% (meaning that we have to use better windows), we can accomplish the goal (Figure 7)!
Figure 7. Applying a lower U-value for all the windows achieves the energy surplus.
The energy-plus house design challenge meets the NGSS engineering standards in several ways: 1) it is a direct response to HS-ETS1-4 that requires students to use a computer simulation to model and solve real-world problems, 2) it promotes systems thinking as students explore how individual elements interact to affect the overall performance of a house, and 3) it creates many opportunities for learning about trade-offs and optimizations as the CAD-CAE fusion greatly accelerates the feedback loop necessary for iterations.
Although the engineering projects based on Aladdin are limited to virtual design, they have distinct advantages: 1) students should have the opportunity to learn CAD/CAE technologies as nearly every engineer today uses them, 2) software can simulate situations that are not possible to create in a school lab (e.g., waiting for a year to determine the annual energy use of a real house), and 3) the cost of implementing these projects is minimal—you only need computers that can run the free Aladdin software.