Lesson Video: Solar Energy Physics

Video Transcript

In this video, our topic is solar power. This is power that comes from the Sun. And one of the ways we use this power is in creating electricity. Using the rays of the Sun to do this is a growing area of energy production. And as we’ll see, this whole process relies on converting one form of energy into another.

Starting out, say that we have a flat panel, this one here. Though it may look fairly ordinary, this panel actually has a special capability. It has the capacity of taking energy from the Sun, radiant energy, and then converting that into electrical energy. This means that when the panel is operating properly and Sun is shining on it, it generates electrical current. The technical name of a panel that’s able to do this, that’s able to convert energy this way, is photovoltaic cell.

By its name, we can tell that this cell somehow connects light — that’s what the prefix “photo” refers to — and voltage. And it does this by what’s called the photovoltaic effect. This is a physical and chemical phenomenon where radiant energy, light, is absorbed. And then that energy is converted to electrical energy. Fundamentally then, what this cell does is it converts one type of energy into another, radiant energy coming from the Sun into electrical energy.

So then, here we have one photovoltaic cell. These are also called solar panels. And when Sun is shining on it, this panel generates some amount of electrical current. And by the way, the type of current it generates is direct current, DC, current that’s always moving in the same direction.

Now the whole purpose of a solar panel is to take solar energy, radiant energy, and convert it to electrical energy at some rate. We’re interested not only in the electrical current the panel produces, but also in its electrical power, how many watts it generates. This output has to do with two factors. First, how much radiant energy is landing on the panel per unit time. And second, how efficient the panel is. That is, how well it’s able to convert radiant energy to electrical energy.

For example, say that, speaking in terms of power, one watt of radiant power is incident on our panel. If our panel was 100 percent efficient, that means we would generate one watt of electrical power. But at this stage of development, efficiencies for solar panels are closer to 20 percent. That means if one watt of power came in, then 0.2 watts of power would be output. Despite this low level of efficiency, the use of solar panels to generate electrical energy is expected to grow.

Knowing this, let’s think through some of the advantages as well as disadvantages of using solar panels to generate electrical energy. First, the advantages. One nice thing about solar panels is that the only energy they require is energy from the Sun. They don’t need to be powered or fueled in any other way. Another good thing about solar panels is they’re very low maintenance. Once they’re installed and working properly, nothing much needs to be done with them. And they can keep generating energy for years. Another advantage is that the type of current that solar panels naturally produce, direct current, is used by many electrical systems. This means the energy these panels produce is already in an easily consumable form.

Combining these first three advantages leads us to yet another, that solar panels are good in remote locations. They don’t need an external fuel supply other than the Sun. They don’t need to be maintained as long as they’re working. And they don’t need bulky converters to change AC to DC so that their energy is useful.

So there are quite a few advantages to solar power. But like any approach, this one also has disadvantages. Perhaps the biggest disadvantage of solar power is one we might be able to guess.

Say that we had an electrical appliance that required a constant energy supply. An example of this could be a desktop computer, which has no battery for storing up energy. This means that the moment a computer stops being supplied with electricity, say by a cord being unplugged or a power outage, the computer will shut down. To prevent this from happening, we want a very steady, reliable energy supply.

But what if our energy supply was entirely made of solar panels and we wanted the computer to be powered on at night? Well, then we might have a problem. There’s no solar energy that lands on this panel during the nighttime. So if we had an energy need then, we couldn’t meet it.

Along with this though, we know that nighttime isn’t the only time that radiant energy is blocked from reaching the Earth. What if clouds are covering the sky, obscuring the Sun’s rays? In these conditions, the solar panel’s capacity for generating electrical energy is diminished. Even in a clear sky during the daytime with no clouds, the radiant energy that reaches a panel varies over time.

Say that this is a solar panel lying flat on the ground. In the morning, the Sun’s rays will hit the panel at this angle. And because of that sharp angle, there’s not much energy to be absorbed. But then, as the morning progresses and the Sun moves, the panel is exposed to more energy. Then, at noontime, when the Sun is directly overhead, it’s getting the most radiant energy it will throughout the day. As the day wears on and the Sun eventually sets, this angle is increased. And again, the radiant energy landing on the panel is less. So the electrical energy produced by solar panels is variable. And it depends on the angle of the Sun.

From the perspective of energy demand, it would be nice if our greatest energy demand was around noontime when the supply was greatest and that it was small at the beginning and end of the day. But oftentimes energy demand doesn’t work that way.

We could sum up these advantages and disadvantages by saying that solar panels are wonderful when they receive a lot of radiant energy. But that’s not always the case. Time of day and weather has a significant effect on that. Let’s look now at a couple of example exercises on this topic.

What type of current do photovoltaic cells generate?

Okay, the first thing we can look into is just what photovoltaic cells are and what they do. A photovoltaic cell is a special sort of structure where if we shine sunlight on it, if we put radiant energy on the cell, then it’s capable of converting that radiant energy into electrical energy. In other words, if a wire is connected up to the cell, then electrical current will flow out from it. All this happens due to what’s called the photovoltaic effect. It involves taking light energy, radiant energy, and converting it to electrical energy, producing a voltage.

Our question is asking, as this process goes on, what kind of current is generated by the cell? To answer this question, it’s important to know that photovoltaic cells don’t involve any magnets in rotational motion. There are no electrical currents running through rotating armatures as in a generator. And there’s no north and south pole or up and down direction that is varied in the power generation process. All that to say, there’s never any reason for the current produced by a photovoltaic cell to change direction. It always moves the same way through the outgoing wire. The name for this kind of current, current that always moves in the same direction, is direct current. And the fact that this is the kind of current that photovoltaic cells generate is another advantage of these cells. This is because direct current is just the type of current that many electrical appliances require.

Let’s now look at a second example exercise.

Photovoltaic cells are often used to power satellites. Which of the following are reasons why photovoltaic cells are chosen as the power source for satellites?

Now, before we read these answer options, there are, even more than the four listed here. We’ll show those on the next screen. But for reasons of space, before we show them, let’s consider these four choices here.

The first possible reason says that photovoltaic cells do not require a solid or liquid fuel that needs to be resupplied in order to keep generating energy. Option B says that photovoltaic cells require very little maintenance. Option C says that photovoltaic cells provide direct current, which is what most electrical systems need to work. And then option D says that photovoltaic cells provide alternating current, which is what most electrical systems need to work. Before we get on to options E and F, Let’s evaluate these four.

So our situation is that photovoltaic cells are being used as a power source on satellites. And the question is, which of these reasons, if any, help explain why that is? To see which of these reasons are legitimate, we can recall some of the advantages of photovoltaic cells. When we have such a cell, it’s true that the only energy the cell requires is energy from the sunlight it receives. There’s no need for any other power supply or fuel source. This would come in very handy in space on a satellite where power is hard to come by. All the photovoltaic cell needs in order to keep generating electrical energy is sunlight or, specifically, radiant energy. The source could be something other than the Sun.

As we consider option A then, we see that this option does describe a reason why such cells are used as a power source for satellites. It’s true that they don’t require any other power source in order to work. So we’ll highlight option A as one of our answer choices.

On to option B, this says that photovoltaic cells require very little maintenance. This also is true. Once a cell is set up and working properly, not much is needed to keep it going. This, too, is an advantage in space where repairs are not easy to make. So we’ll also choose option B as a reason why photovoltaic cells are used as a power supply for satellites.

Option C says that photovoltaic cells provide direct current, which is what most electrical systems need to work.

Now if we look ahead to option D, we see that this says the same thing, except it claims that photovoltaic cells produce alternating current. We can tell instantly that options C and D can’t both be correct, since they claim opposite things about the current generated by photovoltaic cells. If we were to connect a wire to our photovoltaic cell and monitor the direction of the current, we would see that as this cell works, as it converts radiant energy into electrical energy, the current it produces is always running in the same direction. In other words, it’s generating DC or direct current. It’s this type of current that most electrical systems require in order to work. So we choose option C, which describes the current produced by these cells this way. And we cross off option D. These cells produce direct current but not alternating current.

Now we’re not done yet because as we mentioned there are other possible answer options, options E and F, which aren’t yet on the screen. Before we get to them, let’s record these three answer options which we have identified as correct. Summarizing these choices, we can say that option A indicates that no fuel is needed other than the sunlight. Option B says that these cells require very little maintenance. And option C says that they output direct current, which is useful for most electrical systems.

Okay, now let’s clear some space so we can write in options E and F and evaluate those. And here they are. Option E says photovoltaic cells have always been very cheap to manufacture. And option F says that there are no clouds in space to block the sunlight that the photovoltaic cells need.

Looking at choice E, we can’t say that this is an accurate statement. Even at their current state of development, if someone wanted to put photovoltaic cells on their roof to completely cover their energy needs, doing that would cost over 10,000 dollars. And this option is saying that these cells have always been very cheap to manufacture. But surely in the very early days of this technology, the costs were fairly high. So we won’t choose option E as one of the reasons these cells are used as a power source for satellites.

This brings us to our last option, option F, which says that there are no clouds in space to block the sunlight that the photovoltaic cells need. And this indeed is an outstanding reason why these cells are used as a power source for satellites. The main disadvantages of photovoltaic cells often come down to the fact that the sunlight on them isn’t steady or reliable. But this is true only for cells that are on the surface of the Earth, cells that sometimes face the Sun and sometimes face away from it. And also the cells may have clouds coming in between them and the Sun. But in space, outside of our atmosphere, there are no clouds to interfere. This means that photovoltaic cells are able to be more reliable power sources in satellites.

Altogether, we choose these four answers. Photovoltaic cells need no other fuel than sunlight to operate. They require little maintenance. They generate direct current, which many electrical systems require. And in space, there are no obstructions between these cells and the Sun, no clouds or other atmospheric conditions.

Let’s take a moment to summarize what we’ve learned about solar power. Starting off, we saw that solar power sources convert radiant energy into electrical energy. These sources are called photovoltaic cells. And it’s these cells that make up solar panels we might see on a rooftop. The output from these cells is direct current, current that always travels in the same direction. And these cells have a number of advantages as well as disadvantages.

Advantages include that they’re low maintenance, that they need no other fuel besides sunlight, and that they’re very effective in remote and bright locations, for example, on a satellite. Their disadvantages mostly have to do with these photovoltaic cells being on the surface of the Earth. In that case, their output will be dependent on the weather. If it’s cloudy or otherwise obscured, their output will be less. And it also depends on the time of day. At nighttime, with no radiant energy landing on the cells, they won’t produce electrical energy.