Skip to 0 minutes and 12 seconds What you are looking at are two coal-fired power plants. Together, when they run at nameplate capacity, they supply about 1/7 of Dutch electricity demand. How do these plants differ and how to assess their performance? To address these questions, let’s continue and dive into some foundations of energy systems. The scheme illustrates the basic working principle of a coal-fired power plant. In a furnace, coal is combusted to deliver heat. In a steam cycle, only part of the heat is converted to electricity. The rest is carried away with cooling water. Direct cooling discharges this thermal load on the river or sea, while cooling using a cooling tower discharges the thermal load into the atmosphere.
Skip to 1 minute and 2 seconds All thermal power plants, whether they run on coal, natural gas, biomass, concentrated solar or nuclear fission, employee a steam cycle, which is formerly known as a modified Rankine cycle. So why is the performance of our two coal plants not 100%? These two plants have been built some 40 years apart. In these 40 years, the efficiency has improved from 41% to 47%. That does not seem a lot, or does it? And why has the efficiency of coal-fired power plants only increased by 6% in 40 years? Here, the laws of physics and thermodynamics come in. The conversion of heat to electricity, “work” in thermodynamics, is done with the steam cycle.
Skip to 1 minute and 53 seconds The first law of thermodynamics says that total energy inputs must equal total energy outputs. The efficiency of the system states how good the system is at delivering the wanted output per unit of input. For the entire power plant, the efficiency is the amount of electricity produced divided by the energy content of the fuel input.
Skip to 2 minutes and 21 seconds This first law efficiency is the common efficiency stated of power plants and of energy systems at large. We notice that the efficiency of the two power plants is only 41% and 47%. Now, is this inefficient? Are all energy systems this inefficient? The answer to both these questions is no. To understand this, we need the second law of thermodynamics. We all know that we can convert electricity completely into heat. This is what happens in a kettle, an electric stove, et cetera. The second law of thermodynamics implies that heat cannot be completely converted into work, in our case, electricity. Part of the heat input to the energy system will inevitably be waste heat output.
Skip to 3 minutes and 15 seconds In a thermal plant, this is carried away by cooling water.
Skip to 3 minutes and 20 seconds Carnot’s Law says how much heat input can be converted to electricity. This is only dependent on the temperature of the heat input and on the temperature of the heat output, both expressed in Kelvin. More precisely, the second law dictates that whenever we convert energy from one form into another, there must be a loss of waste heat to the environment. So whenever we use energy as food, fuel, or electricity, when running, when driving a car, when using a computer or phone, or when we make coffee, all energy input is inevitably converted and lost as waste heat. Recovery and conversion of this heat to electricity is impossible, as it is at or near ambient temperature.
Skip to 4 minutes and 9 seconds To summarise, each energy conversion implies energy loss as heat to the environment. Energy systems that involve conversion of heat to power suffer the greatest losses. Unfortunately, we have a lot of these, like electric power plants, automobiles, trucks, and airplanes. Globally, some 50% of our energy use directly ends as waste heat. The remainder we use and then becomes waste heat.
Skip to 4 minutes and 40 seconds We covered a lot of ground in this lecture. I suggest you read the accompanying article before proceeding to the next lecture, where we will see how this translates to energy system performance.
Energy system essentials / foundations
In this video, Gerard Dijkema will show differences between energy systems. Moreover, the underlying theories which define these differences will be explained. The importance of the steam cycle in our energy systems and the performance differences between power plants will be explained through some examples and the Laws of thermodynamics.
A small note. At the end of the video, Gerard mentions “an accompanying article”. This article can be found in the following step.
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