 We're now going to take a look at a topic called Exergy or Availability. Availability in Exergy, sometimes this is referred to as looking at second law efficiencies. So what we're doing here is we're looking at processes and determining how close our process is approximating reversible process, so one without irreversibilities. And so basically what Exergy analysis enables us to do is to perform analysis of systems and determine where we can make improvements. So just to give a little bit of a review, we've looked at a number of different efficiencies thus far in the course. So we've looked at thermal efficiencies and we said that that was network output divided by total heat input. And in terms of a pictorial representation of application to a heat engine, we talked about having a source and a sink. So source is supplying the heat from a hot temperature source. The heat then flows through our heat engine and through this process we're able to extract some work out. And on the low side, we reject that heat into a sink. So that is the idea of the heat engine that we've explored thus far and we've come up with the efficiency of this. We also talked about the Carnot efficiency and the Carnot efficiencies were for reversible processes. And for the Carnot efficiency, we gave the symbol reversible after eta thermal in order to denote that this is for a reversible process. And we quantified the Carnot efficiency in terms of the temperature in Kelvin. And the last thing that we looked at with the efficiencies were the isentropic efficiencies or the adiabatic efficiencies for different components that we might have within a cycle that we're examining. And here it was things like eta t for a turbine, eta c for a compressor, eta p for a pump, and eta capital N for a nozzle. So those are some of the different efficiencies that we've looked at thus far in the course. And these are all what we can refer to as being first law efficiencies. We will now look at a different type of efficiency. And we will refer to this as being a second law efficiency. And in doing this, it will take us towards an area called exergy analysis, which enables us, like I said earlier, to compare a cycle that we might be looking at to how that cycle would be performing if it was performing without any irreversibility, so in a reversible manner.