 What we're going to do now, we're going to take a look at a couple of videos of a Sterling fan. This Sterling fan is powered by thermal energy in the form of burning kerosene. You'll see a little lamp that is lit at the beginning or a lamp. It would be the device that would be similar to an old cold oil burning lamp. It is providing the heat for the Sterling engine. You'll also notice that I have to give it a little bit of a kickstart in order to get the fan going. Once it gets going, you can see the mechanisms. You'll see the two piston cylinder devices working within the Sterling engine as well as the regenerator within the middle. Let's take a look at that now. That's the Sterling engine operation. You could see the working mechanism. It's kind of a complex working mechanism within the Sterling engine in order to execute the process itself. What we're going to do now, let's take a look a little closer at what is going on within the two piston cylinder devices that are part of the Sterling. What I've done is I've sketched out four different steps. Those will correspond to the four steps of the process that we're looking at. The other thing you'll see in the upper left-hand corner, we have the PV diagram for our reference. We can look back to it. This here, this is our regenerator. I'm going to draw that red because it will denote the fact that it is retaining thermal energy as the gas moves from one side to the other. What we're going to do, we're going to look step-by-step through our processes. I will adjust the placement of the pistons for each of the particular steps that we're looking at. Let's begin with one through two, which we said was an expansion process that is taking place at constant temperature. Heat addition is also occurring in that process. Heat addition is coming into our hot cylinders. We have Q in here. The other thing that is happening with our cold cylinder, this is fixed during this process. It is fixed pretty much at bottom dead center. It's at the lowest point that it can go. If we're putting heat into the hot cylinder, we know that when you put heat into a cylinder, it is going to expand into the gas. What happens is we have an expansion process taking place, but we are also doing work. The net consequence of those two is that our expansion is taking place such that we maintain the temperature in an isothermal manner. The temperature is fixed at T-hot during the expansion. The next step that we'll look at in our PV is going from 2 to 3, and that will be the constant volume process that we will show here. What I'll have to do now is place my pistons within the cylinders that they're at the correct location. To begin with, the hot should be at top dead center. Let's move this guy up. It goes up the top dead center, and the cool should be down at bottom dead center, which is where it was. We're okay there. It is at top dead center, and this is at bottom dead center. What's happening? This is a constant volume process. That means that these two pistons need to move at the same rate, at the same velocity. They're moving in unison. While that is taking place, what happens is the working fluid then gets passed through the regenerator. Remember, it was hot because that's the hot cylinder. It will be depositing thermal energy into the regenerator as it goes through. That is step two through three, and we can note here heat into regenerator. That's two through three. Let's now look at three through four. This is where we're rejecting thermal energy at a constant temperature. The rejection will be from the cold cylinder. This is going to be three through four. We refer to it as a compression process. Let me adjust the location or the positions of our pistons to make sure that they're at the right location. This one should be down at bottom dead center and fixed. The hot is at bottom dead center and fixed. The cold was up close to top dead center and it's starting to come down. It's starting to come down. We're compressing and at the same time, whatever we compress a gas, it gets hot. We have Q out taking place. We're doing this in a manner that it occurs isothermally. We're exchanging enough heat such that even though we're compressing the gas, the gas is also cooling. The net consequence of those is that it occurs at a constant temperature. What we can write here is that this is occurring at T low. This here was actually occurring at T hot. I should write that in. That is the compression process. Then the final step is four through one. If we look on our PV diagram, we're going from four up to one. Again, this is another constant volume process where the gas will now come back through the regenerator but now going in the other direction. Before I show which way the gas is going, I need to adjust my pistons to make sure that they're at the right locations. The hot was near bottom dead center and it's starting to move up. The cold was near top dead center and it's moving down. They're moving again in unison for this constant volume process. What's happening is the gas or the working fluid is now coming this way through the regenerator. We can then write heat, I should say thermal energy, out of regenerator. Those are the four steps for the Stirling Engine cycle. You can see that in order to implement this with a mechanical system, that is the Stirling Engine, the crankshaft, and the connecting rods, it's a rather complex process because sometimes the pistons are moving and other times they're not moving. That's one of the tricks of the Stirling Engine is to make one that is balanced, has very low friction. It also needs to seal the gases inside. If we're dealing with helium or hydrogen, those are well known for leaking. You have to have very good seals but if you have too good of seals, you're going to have a lot of friction. Consequently, it's a bit of a mechanical engineering challenge to build the Stirling Engine. Where is the Stirling Engine found? The last thing we're going to do is close with by looking at applications. We talked about the heat engine and so you can have like the fan that I showed you where you have fuel and burning underneath, although that was a kerosene fuel and it wasn't the cleanest, it was a little smelly I must say. However, people have developed an environmentally friendly Stirling Engine whereby you use solar energy to focus down on the hot cylinder and Sandia National Labs developed one of these in New Mexico a number of years ago and they would distribute them throughout different areas in New Mexico. But environmentally friendly solar engine and this was Sandia National Labs. Another application there is a company I believe out of New Zealand called Whisper Gen and they were making a system. They still may, I'm not sure what their status is right now, but they were making a system to heat domestic water and also generate electricity and this obviously would be something that would burning methane so you have internal combustion, but again it's operating like an external fuel engine. Now another thing if you reverse this cycle and you put work in the Stirling is actually a really good refrigeration system and it's even used for cryogenic cooling so you can cool things down to very very low temperatures. Now that's not a heat power cycle but I just mentioned it because it can be used for cooling and that could be used for its late weight refrigeration essentially but it could be used for cooling electronics that are sensitive to thermal noise for example. Cooling and this obviously would be an application that would require you to have very light weight and compact refrigeration so that would be compact refrigeration but that's not what we're looking at we're looking at the power cycle but I just mentioned that as it's another application of the Stirling cycle. So that is the Stirling cycle that will conclude today's lecture. I'd like to thank you for your attention and we'll see you next time. Thank you. Bye bye.