Saturday, January 14, 2017

Importing and analyzing models created by other CAD software in Energy3D

Fig. 1: Solarize a COLLADA model in Energy3D
Fig.2: A house imported from SketchUp's 3D Warehouse
Energy3D is a relatively simple CAD tool that specializes in building simulation and solar simulation. Its current support for architectural design is fine, but it has limitations. It is never our intent to reinvent the wheel and come up with yet another CAD tool for architecture design. Our primary interest is in physics modeling, artificial intelligence, and computational design. Many users have asked if we can import models created in other CAD software such as SketchUp and then analyze them in Energy3D.

Fig. 3: A house imported from SketchUp's 3D Warehouse
I started this work yesterday and completed the first step today. Energy3D can now import any COLLADA models (*.dae files) on top of a foundation. The first step was the inclusion of the mesh polygons in the calculation of solar radiation. The polygons should be able to cast shadow on any object existing in an Energy3D model. This means that, if you have a 3D model of a neighboring building to the target building, you can import it into Energy3D so that it can be taken into consideration when you design solar solutions for your target. Once you import a structure, you can always translate and rotate it in any way you want by dragging its foundation, like any existing class of object in Energy3D.

Fig. 4: A house at night in Energy3D
Due to some math difficulties, I haven't figured out how to generate a solar radiation heat map overlaid onto the external surfaces of an imported structure that are exposed to the sun. This is going to be a compute-intensive task, I think. But there is a shortcut -- we can add Energy3D's solar panels to the roof of an imported building (Figure 1). In this way, we only have to calculate for these solar panels and all the analytic capabilities of Energy3D apply to them. And we can get pretty good results pretty quickly.

Fig. 5: A 3D tree imported from SketchUp's 3D Warehouse
Figures 2-4 show more examples of how houses designed with SketchUp look like in Energy3D after they are imported. This interoperability makes it possible for architects to export their work to Energy3D to take advantage of its capabilities of energy performance analysis.

Being able to import any structure into Energy3D also allows us to use more accurate models for landscapes. For instance, we can use a real 3D tree model that has detailed leaves and limbs, instead of a rough approximation (Figure 5). Of course, using a more realistic 3D model of a tree that has tens of thousands of polygons slows down the graphic rendering and simulation analysis. But if you can afford to wait for the simulation to complete, Energy3D will eventually get the results for you.

Friday, January 6, 2017

Why is Israel building the world's tallest solar tower?

Fig. 1: Something tall in Negev desert (Credit: Inhabitat)
The Ashalim solar project (Figure 1) in the Negev desert of Israel will reportedly power 130,000 homes when it is completed in 2018. This large-scale project boasts the world’s tallest solar tower -- at 250 meters (820 feet), it is regarded by many as a symbol of Israel’s ambition in renewable energy.

Solar thermal power and photovoltaic solar power are two main methods of generating electricity from the sun that are somewhat complementary to each other. Solar tower technology is an implementation of solar thermal power that uses thousands of mirrors to focus sunlight on the top of a tower, producing intense heat that vaporizes water to spin a turbine and generate electricity. The physics principle is the same as a solar cooker that you have probably made back in high school.

Why does the Ashalim solar tower have to be so tall?

Surrounding the tower are approximately 50,000 mirrors that all reflect sun beams to the top of the tower. For this many mirrors to "see" the tower, it has to be tall. This is easy to understand with the following metaphor: If you are speaking to a large, packed crowd in a square, you had better stand high so that the whole audience can see you. If there are children in the audience, you want to stand even higher so that they can see you as well. The adults in this analogy represent the upper parts of mirrors whereas the children the lower parts. If the lower parts cannot reflect sunlight to the tower, the efficiency of the mirrors will be halved.

Fig. 2: Visualizing the effect of tower height
An alternative solution for the children in the crowd to see the speaker is to have everyone stay further away from the speaker (assuming that they can hear well) -- this is just simple trigonometry. Larger distances among people, however, mean that the square with a fixed area can accommodate less people. In the case of the solar power tower, this means that the use of the land will not be efficient. And land, even in a desert, is precious in countries like Israel. This is why engineers chose to increase the height of tower and ended up constructing the costly tall tower as a trade-off for expensive land.

Fig. 3: Daily output graphs of towers of different heights
But how tall is tall enough?

Fig. 4: Energy output vs. tower height
This depends on a lot of things such as the mirror size and field layout. The analysis is complicated and reflects the nature of engineering. With our Energy3D software, however, complicated analyses such as this are made so easy that even high school students can do. Not only does Energy3D provide easy-to-use 3D graphical interfaces never seen in the design of concentrated solar power, but it also provides stunning "eye candy" visualizations that clearly spell out the science and engineering principles in design time. To illustrate my points, I set up a solar power tower, copied and pasted to create an array of mirrors, linked the heliostats with the tower, and copied and pasted again to create another tower and another array of mirrors with identical properties. None of these tasks require complicated scripts or things like that; all they take are just some mouse clicks and typing. Then, I made the height of the second tower twice as tall as the first one and run a simulation. A few seconds later, Energy3D showed me a nice visualization (Figure 2). With only a few more mouse clicks, I generated a graph that compares the daily outputs of towers of different heights (Figure 3) and collected a series of data that shows the relationship between the energy output and the tower height (Figure 4). The graph suggests that the gain from raising the tower slows down after certain height. Engineers will have to decide where to stop by considering other factors, such as cost, stability, etc.

Note that, the results of the solar power tower simulations in the current version of Energy3D, unlike their photovoltaic counterparts, can only be taken qualitatively. We are yet to build a heat transfer model that simulates the thermal storage and discharge accurately. This task is scheduled to be completed in the first half of this year. By that time, you will have a reliable prediction software tool for designing concentrated solar power plants.

Thursday, January 5, 2017

Designing on lot maps in Energy3D

Energy3D allows users to import an Earth View image from Google Maps and then design 3D structures on top of it. The image provides the reference frame, boundary lines, and other visual aids for getting the geometry right. What if there is no Google Map image, or the Google Map image is outdated, or you simply want to draw on a different substrate other than a Google Map image?

Bob Loy, a teacher at Creekside Middle School in Carmel, Indiana, has such a situation. His school is working with a builder to engage young students to design new constructions in their areas. His goal is for them to design houses that fit on assigned lots planned by the builder and then make them as energy-efficient as possible by applying all sorts of solutions, including insulation, passive solar strategies, and solar panel technologies.

Upon his request, I have added a new feature to Energy3D (V6.2.7) to enable users to import an image from a file to serve as the ground for designing a building, a solar farm, or anything made possible by Energy3D.

Since users can import any image that represents any size in the real world, it is their responsibilities to make sure that the dimension and orientation of the image that appears as the ground in an Energy3D model is accurate. Setting the correct dimension can be done by rescaling the image after it has been imported. There are some other requirements of such images, though. For instance, they have to be a square image (its width and height must be the same) with a reasonably high resolution (otherwise they will appear to be too blurry to look once they are transformed into 3D textures). Users must know the scale of such an image, i.e., the exact length in the real world that a unit length in the image represents. Once the image is inside Energy3D, users should measure its width or height within Energy3D and then rescale the image to make sure that the measurement matches the value in the real world. Currently one can use a foundation object in Energy3D as a ruler, but a real ruler should and will be added in a future version to measure any distance in a more intuitive manner.

Saturday, December 31, 2016

Modeling the six MW solar farm at the Palmer Metropolitan Airfield in Massachusetts

Fig. 1 Aerial view of PMA (courtesy of Borrego Solar)
Fig. 2 The polygon tool for drawing land parcels
The Palmer Metropolitan Airfield (PMA) solar farm (Figure 1) is the first and, at 6 MW, the largest Massachusetts Department of Energy Resources qualified brownfield project under the SREC II solar energy incentive program. The solar farm consists of 20,997 solar panels of three different types (5,161 Suniva, 13,851 Yingli, and 1,985 Canadian Solar), connected by 74 SMA string inverters. It is expected to produce an estimate of 8.5 GWh annually, enough to power 1,000 homes and offset 4,000 tons of carbon dioxide every year -- according to this news source. The PMA solar farm was engineered by our partner, Borrego Solar, the third largest company in the commercial solar market in the US.

Fig. 3 The Automatic Layout Wizard for solar rack arrays
The PMA solar farm is the first test of Energy3D's capacity of seriously designing utility-scale (greater than 1 or 5 MW, depending on your point of view) photovoltaic solar power plants. This design capacity was enabled by three critical new features that were added only recently to Energy3D (V6.2.2): 1) A tool to draw polygons that represent parcels of land for solar farms; 2) a tool to automatically generate solar panel and rack array layouts within selected parcels of land; and 3) accelerated graphical user interface and numerical simulation to handle 10,000+ solar panels (which I have blogged earlier this month).

Fig. 4 The result of the Automatic Layout Wizard
Since Energy3D can import an Earth view image from Google Maps, you can directly draw polygons on top of the image to trace the parcel of land for designing your solar farm (Figure 2). Note that if you have multiple parcels of land that are separate from one another, you may have to use multiple foundations in Energy3D as each foundation is allowed to have one and only one polygon for the time being.

Fig. 5 Heat map representations of output in four seasons.
Fig. 6 Annual yield vs. tilt angle
As soon as you are all set with your land plans, you can use the Automatic Layout Wizard of Energy3D to add solar panel rack arrays (Figure 3). This wizard will automatically generate the array layout within the selected land parcel and assign properties to the solar panels based on the parameters of your choice. For instance, you can select how many rows of solar panels you want to have on each rack (I picked four because that is what Google Maps shows about the setting in PMA). Figure 4 shows the result of applying the Automatic Layout Wizard to populate the three subfields of the PMA solar farm.
Fig. 7 Monthly yields vs. tilt angle

After the layout is done, you can always revise the field. You can drag any rack to resize or move it, delete it, copy and paste it, or add a new rack. The Automatic Layout Wizard is not the only way to add solar panel arrays. It is just a super fast way to add thousands of solar panels at once -- without the wizard, it would have been too time-consuming to manually add solar panel racks one by one. The solar panel field is always editable after a layout is applied.

Let's now check how close our model is to reality. The total number of solar panels of our model is 21,064 -- only 67 more than that of the real PMA solar farm (I had no information about the exact types of solar panels deployed in PMA, so I guessed and selected two different sizes 0.99m x 1.65m and 0.99m x 1.96m for different subfields).

In terms of the annual output, Energy3D predicts approximately 9.6 GWh, about 12% higher than the estimated output of 8.5 GWh by Borrego Solar. I currently do not have access to the real operational data, though.

Having created a computer model allows us to experiment with it to study how to optimize the design. For example, we can easily change the tilt angles of the arrays and investigate how the annual yield is affected. Figure 6 shows that a tilt angle close to the latitude (42 degrees) seems to result in the highest overall annual output.

But the total annual output is not necessarily the only criterion. Sometimes, it is necessary for solar companies to consider load balancing to guarantee stable outputs throughout the year (assuming that we want to minimize the use of base load from burning fossil fuels). It is, therefore, interesting to also take a look at the outputs across 12 months of a year. Figure 7 suggests that a smaller tilt angle will produce peak power in the summer, whereas a larger tilt angle will produce peak power in early fall. If the demand of electricity in the summer is higher than that in the fall, it may be more lucrative to position solar panels at a lower tilt angle.