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Skip to 0 minutes and 3 seconds So what type of device is a water pistol? Well it’s a pump, isn’t it? And if we look it up in the dictionary, we find that a pump is a device that raises, transfers, delivers or compresses fluids. So here’s another one. What type of device is a kettle? Well it boils something. It’s a boiler. And if we look that up in the dictionary, it describes it as a vessel used for boiling, the part of a steam generator in which water is converted into steam. So if we put these two components together, the kettle and the water pistol, then they represent half of a vapour power cycle.

Skip to 0 minutes and 39 seconds The half in which we have to deliver work via the pump, and heat via the boiler. So they’re the first part of a vapour power cycle that we can represent in this sort of manner, where we’re going to deliver some heat into the boiler. And we’re going to pump water, our working fluid round into the boiler. So we’re going to have to do a little bit of work on the pump, and we’re going to provide some heat. And the heat normally comes from an external furnace. In the case of the kettle, it comes from the electrical heating element in the bottom of the kettle. But when we build a power station, we’re trying to generate electricity.

Skip to 1 minute and 15 seconds And so normally that heat will come from a furnace. It could be a fossil fuel furnace, or it could be a nuclear reactor. Or maybe if you live in the south of France, you could use a big solar generator. So we produce some steam and we push it round into a turbine. So the steam then makes a turbine turn and we get some work out of the turbine. We can pinch a small amount– a small amount of that work, and use it to drive a pump. And the rest of it, we can use to generate electricity.

Skip to 1 minute and 47 seconds Now last week, I talked about the Clausius statement of the second law of thermodynamics. That heat does not pass from a cold body to a hot one without an accompanying change elsewhere. Around about the same time that Rudolph Clausius made that statement, Lord Kelvin recognised that heat cannot be completely converted into work. And that means that we have to dump some heat into the environment. We can’t convert all the heat we put into the steam into work in the turbine. A small amount of it needs to be used as waste, effectively, and dropped into the environment. And so we do that by taking our less energetic steam, putting it into a condenser, and converting it into water.

Skip to 2 minutes and 30 seconds And in the process, dumping heat out into the environment. And then the pump pulls the water around and pushes it back into the boiler, where we add more heat to it and round into the turbine. And so on, round and round in a cycle that’s generating electricity by pulling heat out of the furnace, using some of it to generate work, and then dumping some of it into the environment. And thereby satisfying the second law of thermodynamics.

Power cycles

Most powerstations are giant steam engines. Eann explains their steam cycle using a water pistol and kettle to represent half of the cycle.

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This video is from the free online course:

Energy: Thermodynamics in Everyday Life

University of Liverpool