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Solar Power Calculation

Introduction

Obviously, it would be best to have the solar panel's normal vector pointing toward the sun at all times. But here we will assume that the solar panels lie flat on the roof of our building. That is the simplest from the design and construction standpoint. In the northern hemisphere it is obvious that having the panels on a south facing roof slope gets the panel normal closer to the sun direction at midday and conversely for the southern hemisphere. If the roof slope faces east or west then the improvement in the solar power due to the roof slope is zero. If the roof is horizontal, then the solar power is the same as if the panels were laid on the ground. If the roof slope faces north in the northern hemisphere then it actually detracts from the what the solar power would be on a flat roof.

This is an animation that shows the variation with season and planet hour of solar power as it applies to a panel on the roof of a house at any latitude and longitude on a planet. The planet tilt with respect to its orbital plane is a variable. Both the house roof orientation relative to a meridian and the house roof slope are variable by the learner. The graphics include a depiction of the roof panel as it rotates with the planet and a color plot of the intensity of the solar light Vs position on the planet. Since the tilt of the planet's rotational axis is constant with respect to its spin and orbit angle, we can invoke seasons by changing the direction of the sun with respect to the planet. The sun direction vector can cause seasons to vary from summer to fall to winter to spring with house situated in either the northern or southern hemisphere. However, with our present depiction of the planet sphere, we can only show Northern Hemisphere summer and Southern Hemisphere winter. In order to depict Northern Winter and Southern Summer we can negate the planet axis tilt angle. This facility is provided by radio buttons "Northern Summer" and "Northern Winter" Of course, to see results for Southern Summer, one has to move the solar Roof to the southern hemisphere.

Graphics Descriptions

In Canvas 1 we compute and plot the power Vs planet axis rotation angle for a full rotation. When the Start button is pressed, we compute and plot the Power Vs time or (equivalently) angle as the planet rotates about its axis. Both of these plots depend strongly on the present slider settings.

On Canvas 2 we show the 3D planet with the 3D solar roof panel applied to its surface. It appearance also depends strongly on the present slider settings.

Northern Summer
Northern Winter

Learner Control Descriptions

First we have Start and Pause buttons to initiate and pause the rotation of the planet about its axis.

Radio Buttons

After these we have radio buttons that select between Northern summer and Northern winter by changing the planet tilt angle relative to the plane in which the sun apparently rotates. Note that for purposes of power calculation, it is perfectly equivalent for the sun to rotate about the planet or to have the planet rotate about the sun which we all know is what it actually does. The advantage of this choice is that we can have a much larger planet in order to show a solar roof at a reasonable scale.

Sliders

First we have manual control of the sun angle in the orbital plane. Note that the planet illumination changes when this is adjusted.

Next we include a slider to adjust the orientation of the roof relative to south which is the optimum orientation in the northern hemisphere.

Also we have a slider to adjust the roof slope relative to horizontal at the particular latitude chosen.

We include a slider to adjust the longitude of the solar panel roof. This just affects the hours at which power starts, peaks, and stops being produced.

Of course we have a slider to adjust the latitude at which the solar panels are placed

Finally we include a slider with which to ajust the planet axis tilt angle with repect to its orbital plane

Minimum and Maximum Solar Power and Plots Vs Time of Day

We can clearly state what will be the summer maximum and winter maximum irradiance that an area `bb A` on a roof panel will see by using the equation:

`P_"summer max"=P_"normal"cos(|l|-t-s_r)`
`P_"winter max"=P_"normal"cos(|l|+t-s_r)`

d where `P_"normal"` is the solar power that would be provided if the sun direction is normal to the solar panel , `P_"summer max"` is the power at solar noon at the date of the summer solstice and `P_"winter max"` is the power at solar noon at the date of the winter solstice. The arguments of the cosines are angles in radians: `l` is the home latitude angle, `t` is the planet axis tilt angle, and `s_r` is the roof slope angle.

Canvas 3 shows the expected winter and summer power Vs hour. Note, with starting slider settings, that the sunrise time for winter is about 1.5 hours later than for summer. Also note that the sunset time for winter is about 1.5 hours earlier than for summer. This difference in day length is often just as important as the difference in peak power in determining the total energy collected.