Thermodynamics is a branch of physics that deals with the transfer of energy from one place to another and from one form to another. Since it is the underpinning science for all energy systems, including renewable energy, it is important that we know it's basic principles. The four laws of thermodynamics are crucial for understanding the use of renewable energy. Here are the four laws in brief summary. The zeroth law states that heat flows from hot to cold. The first law states that energy is always conserved. The second law states that some energy is wasted in any energy conversion, and the third law states that there is an absolute zero temperature. Let's look at each of these in more detail. The zeroth law of thermodynamics states that heat flows from hot to cold. There are three principle ways that heat can flow; by conduction in a solid where vibrating molecules cause adjacent molecules to vibrate, by convection in fluids where stirring occurs due to temperature differences, and by radiation where heat energy is transferred by electromagnetic radiation, such as heat from the sun, from a hot campfire or from a hot burner as shown here. Next, the first law of thermodynamics states that energy is always conserved. Since energy and mass are equivalent by the famous equation, E equals mc squared. The first law is more correctly stated as total mass energy remains constant. But since we're not dealing with nuclear energy in this course, for our purposes, we can assume that the amount of energy never varies. Which is to say that energy is not created, energy is not destroyed or lost, and energy is only created from or converted from one form to another. As an example of energy conversion, consider a camp fire, the firewood initially is a node, but with embodied or potential chemical energy. When the camp fire is burning, chemical energy in the wood is converted to heat energy. After the campfire burns out, all that is left are ashes, which may have some residual chemical energy that was not converted to heat energy by burning. But note that most energy systems use multiple energy conversions, not just one. Here is a renewable energy example illustrating multiple energy convergence in a hydroelectric dam. We start with potential energy of stored water, which is converted to kinetic energy of flowing water, which in turn is converted to mechanical energy of a spinning turban, then to electromagnetic energy in a generator that creates electrical energy for distribution to you and me. The second law of thermodynamics states that some energy is wasted or lost in any energy conversion. First, there is some original store or a source of energy, such as fossil fuels, biomass, or energy from sunlight, wind, or a battery. This energy is converted to some form that allows it to do work, such as lifting, moving, accelerating, lighting or producing acoustic sound. Some energy is lost along the way, often as heat due to friction, heating, turbulence, or some other mechanism. So energy is conserved where work energy plus wasted energy plus residual energy must equal the original starting energy. A different way of stating the second law is that there are always energy efficiency losses in any energy transformation. All energy transformations systems have an efficiency of less than 100 percent as a consequence of the second law, there are always losses, often heats such as the cooling towers at a power plant which dump waste heat, hot exhaust from auto and truck engines, and heat dissipating from buildings and cold weather. Efficiency is defined as energy out divided by energy in. Here are some examples of energy efficiency in practice. Gasoline auto engines have an efficiency of 20-25 percent, meaning that only 20-25 percent of the embodied energy and gasoline is actually used to propel the automobile. The rest is lost as heat out of the exhaust pipe and radiated from the engine, hence, the need for a cooling radiator. Turbine engines and power plants have a 30-50 percent efficiency. Aircraft turbines, about 33 percent. Photovoltaic or PV solar cells have an efficiency of 20-25 percent, meaning that 20-25 percent of the incident sunlight on the solar cell is converted into electricity. Wind turbines have an efficiency of about 40 percent with a theoretical maximum of 56 percent. We'll talk about that later. Hydroelectric dams have an efficiency of as much as 90 percent if they will operate it. There is another example of the second law of thermodynamics. Let's consider the graph that we looked at in a previous video that shows global energy sources and uses. On the left we see that the various sources of energy used in the world, which totaled about 500 exajoules, which is a very big number. In the upper right we see conversion losses which total about 290 exajoules. In the middle of the right, we find that the remaining 210 exajoules provide useful energy services. So overall energy efficiency is about 40 percent. This means that the remaining 60 percent is lost along the way, ultimately as heat. This calculation provides a large scale example of the second law of thermodynamics in practice. Finally, the third law of thermodynamics states that there is an absolute zero temperature. This is the lowest possible temperature below which nothing can be cooled, providing a reference floor for all temperature measurement. Absolute zero is measured as zero on the Kelvin temperature scale, as minus 273 degrees on the Celsius scale, and as minus 460 on the Fahrenheit scale. Pretty chilly. As examples of what this means that the atomic level in superheated steam used in electricity generation in thermal generating plants, water molecules move fast, creating significant pressures to run turbines. At room temperature, liquid water molecules are much less vigorous with much lower kinetic energy. Near absolute zero, water molecules have very little kinetic energy and are almost inert. Summarizing; the four laws of thermodynamics are that heat flows from hot to cold via conduction, convection, and radiation. Energy is always conserved, and we note that most energy systems use multiple energy conversions. There are always energy conversion losses, so that energy efficiency is always less than 100 percent, often much less. Finally, absolute zero temperature provides a fixed baseline for temperature measurement. While all four of these laws are important for our study of renewable energy, we are most concerned with the second law, since it most directly affects the operating characteristics of renewable energy technologies. This video concludes our study of energy in general. In the next set of videos, we examine the characteristics and measurement of electrical systems. Will see you there.
