 The last thing that we want to look at in today's lecture are the ideas of refrigerators and heat pumps and again what we will do just like we looked at heat engines we will take a look at the definition of efficiency for these devices so in this course we will be looking at later on refrigerators and heat pumps now we said that a heat engine was a device that takes heat from a high temperature source generates work and rejects the waste heat to some other sink the refrigerator and heat pump are basically the exact opposite so these are devices that take heat from a low temperature and move it to a hotter temperature medium so schematically just like we did before we looked at the heat engine what we'll do now is we'll sketch out the heat pump so for a heat pump I'll put it here in a circle and we're putting energy in or work in so we have some network coming in and then down at the bottom we have a sink which is at a low temperature and heat is flowing from the sink and it's flowing up to a source which is a high temperature source and then here we have heat going up to the source so it's going in the opposite direction that we saw for the heat engine earlier now in terms of a schematic of a cycle we would have a condenser whereby our working fluid is rejecting heat before that in the process what we would have is a compressor so the fluid is coming out of the compressor and flowing into the condenser and here is where we do our network in we have an evaporator and that's where the low temperature heat source is coming in and then after the condenser we come out and here we have an expansion valve or a throttling valve where we lower the pressure so if you recall the lecture where I was talking about the uniform flow process and I showed you a video of air coming out of a compressed tank we saw water and we also saw ice that's essentially what was going on there was this type of cycle we had a compressor it goes into a tank it's hotter than the environment so it rejects heat to the environment I then open a valve very quickly it expands it gets cold and when it's cold that's when you can absorb heat from the surrounding environment so depending on the particular application that you're looking at here if the location where the heat was coming from so your sink was the outside that would be an example of a heat pump if you've ever stayed in a hotel or a motel where they have an air conditioning unit in the window they can flip the switch from cool to heat that's basically a heat pump there they're less common in canada than they are in the united states and if it is food then that would be a refrigerator so that is the idea of a refrigeration in a cycle and a heat pump now in terms of the efficiency of this cycle work net in is equal to qh minus ql that is an expression that we will now use to determine or quantify how well the cycle is working now for a heat engine we talked about the efficiency the thermal efficiency of the cycle for a heat pump or a refrigerator what we'll be talking about is the coefficient of performance and this is written out in terms of a capital C O P and what it is is our desired output divided by the required input so looking at a refrigerator we would have the coefficient of performance of a refrigerator I'll write it as C O P subscript capital R ql is the desired and the required input is the work that we have to do which we could then rewrite that as and so that becomes an expression for the coefficient of performance of a refrigerator and similarly for a heat pump so we can substitute for the network in using qh minus ql and rearranging that what we obtain is this expression so you see it's a little different depending if you're dealing with either a refrigerator or a heat pump typically values that you'll find for coefficient of performance of heat pumps they're going to be on the range of two to three and it would depend upon the working fluid and the design of the system just like heat engines have different efficiencies you'll find a variety of different efficiencies or core efficiency of performance for heat engines so looking at this in terms of a process diagram we're sketching it out again we had our condenser now writing it in terms of a ts diagram what i'll do is map the state information to a ts diagram or sorry tv so with this process we start over here at one typically we go into a compressor and so in the compression process we increase in temperature and that takes us up to state two once we're in state two and in here we're doing work so we have work coming in once we're at state two usually we're at a hotter temperature than our surrounding environment and so we reject heat to the surrounding environment or we heat in the case of a heat pump so we're moving in this direction now and let's assume that we end at the compressed liquid line so that would be at point three we then go into a throttling valve and a throttling valve is actually a constant constant enthalpy process but it will bring us down something like that to state four and now we are at a lower temperature and consequently what we're able to do is absorb heat through this process here so that's what's going on with an evaporator so that's mapping it to a tv diagram final thing that i want to say we looked at the kelden plank statement when we were looking at heat pumps now we're going to look at the clausia statement and what the clausia statement says is that no device can transfer heat from a cooler body to a warmer one without leaving an effect on the surroundings and that effect would be the work that you have to do on the process so you need to do work in order to move heat from the cold to the hot which is the opposite way that we would normally see heat flowing so we're in a way defying nature by doing this but we have a cycle that enables us to do it so that concludes this lecture what we'll be doing next class is we'll be looking at the definition of a reversible process as well as we will be taking a look at the carno cycle which is the cycle that has the highest possible efficiency that you could have for a heat engine so thank you very much bye