In Part One, we discussed the parametric design of a solar thermal collector. In this part, we’ll take a look at the HVAC modules capabilities in handling advanced radiation. Before we begin, let’s discuss the physics of radiation.
On a simple level, radiation from a heat source (any heat source) travels in waves. These waves have different wave lengths depending on the temperature of the heat source. So, a computer screen will have a different radiation spectrum than a fire or a desk lamp.
When radiation strikes an object, some of the radiation will be absorbed, a fraction will be transmitted, and the remaining radiation will be reflected. Any radiation which is absorbed turns into heat. Any radiation which is transmitted passes through the object. And reflected radiation is reflected back to the environment.
For a particular material, the ratios of absorption, transmittance and reflection are different depending on the wavelength. So, graphs of these ratios against the wavelength spectrum are required in order to get accurate radiation properties for a particular material.
Let’s relate this information to the solar collector. Radiation transmits through the solar panel glass and is absorbed by the pipes and collector’s conducting walls. The heat is then transferred to the water passing through the system.
At this point you might be wondering why we have glass on the system. A portion of the sun’s radiation will be reflected back as a result of the glass. Wouldn’t we collect more radiation on the pipes if the glass wasn’t there? The answer is YES. We would collect more radiation on the pipes if the glass wasn’t there. However, the system is more complicated than that. Every heat source radiates at a particular wave length. And, the ratios of absorption, transmittance and reflection are different depending on the wavelength. So, as it turns out, the glass transmits most of the radiation from the sun and absorbs and reflects most of the radiation from the pipes. The glass traps the heat in the system. It’s the exact same reason your car gets hot in the sun.
Now, let’s create a flow study. For this project, we’ll specify an external analysis which will have an internal fluid sub-domain for water flowing through the pipes. For the first go around, we’ll keep the model simple and exclude cavities without flow conditions. This will eliminate any convective effects from air within the box. We’ll also introduce radiation, gravity and heat conduction in solids.
Okay, this is where we’ll need to take a closer look at our solar radiation options and make some decisions. Radiation is not simple. From the general settings, we can see that there are a lot of radiation options. However, SolidWorks does a good job of making it as easy as possible to get setup.
Environmental Radiation – By selecting this option, we are considering radiation effects from the environment which we define at a specified temperature.
Solar Radiation – By selecting this option, we are considering radiation effects from a single source.
With this option selected, we can specify a radiation source by its location and intensity.
We can also take the suns radiation into account, as specified by “Location and Time.”
Let’s take a closer look at “Location and Time.” For SolidWorks to calculate the suns effects on the panel, SolidWorks needs to understand the suns orientation relative to the collector. That’s why we specify the latitude, date, time, zenith direction, angle measured from north to, and angle. The first three values seem obvious, but zenith direction, angle measured from north to, and angle might not make sense to the layman.
Zenith direction – This option specifies the highest direction in the sky. Directly above you. Up. In this case, I’ve specified a custom coordinate system and chosen the y-axis.
Angle measured from north to – This option specifies the orientation within your system which you would like to relate to north. In this case, I’ve specified a custom coordinate system and chosen the z-axis.
Angle - This specifies which way north is relative to “angle measured from north to”. In my case, I’ve kept this value at 0 radians which means my “angle measured from north to” is north as well.
For this simulation, I’ll also specify absorption in solids.
Okay, this is getting long and I’m sure your head hurts. We’ll discuss further in part Trois.
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