The System and Second Law – Do you stand out for that?

Kevin Kilty

Over the past few days, I have read a number of scientific papers that appear frequently in my mailbox or inbox. One recent surprise is about energy storage. It seems the storage issue is becoming almost universally recognized, but as usual, not widely understood. When I read an enthusiastic article about some new breakthrough, the description is always limited to a very narrow technical discussion of how this particular idea works. It does not go into detail on how such a device would fit into a possible system. It doesn’t help if a new technology comes with new burdens on the system it has to operate – this is a hole in the so-called renewable energy in the grid. They can add power to the distribution network, but they cannot be arbitrarily maneuvered and contribute little to voltage/frequency support. They placed the burden of backend services on other parts of the system.

Achilles’ heels

Whenever I read about a new or improved plan for storing energy, I think about two things about it. Here are two common Achilles’ heels: 1) What constraints does the Second Law of Thermodynamics impose, and 2) what other constraints would limit its usefulness in a system? I focus on the second law because the first law only deals with the conservation of energy, and this is not where most of the restrictions on energy use come from, or in fact restrictions on any human activity.[1]

Some definitions

The loss of energy for switching work is imposed by the so-called irreversible second law, or an even stranger term, “the annihilation of heretics”. This means that some of the energy converted in a system does not contribute to useful work. Useful work is always the goal everywhere. So things that contribute to this problem include: 1) heat cannot be converted 100% successfully; 2) the chemical reaction cannot be carried out to completion and results in some material being disturbed in quantity; 3) high temperature contributes to dead heat flow[2], or as Willis Eschenbach calls them parasitic losses; 4) high pressure contributes to loss of mechanical energy used to create high pressure in the first place. This work also goes to a dead state; 5) long conversion chain with little loss at each step; 6) Fast conversion rate. Be aware of these problems.

Systemic problems include: 1) insufficient quantities of critical materials; 2) inadequate terrain availability; 3) resources in the wrong places require long distance transportation; 4) the need for an impossible rotational speed (problem with time constant); 5) weight and volume constraints; 6) the impossible demand for time and the large demand for volume per unit of stored energy; and, 7) complexity.[3] Table 1 shows general examples.

Table 1.

Recent examples

Let’s look at a few examples from my recent readings. How to power airplanes is a real problem to solve for renewable energy advocates mainly because they like to fly guilt-free places. The December 2020 issue of Physics Today published an article about using hydrogen to power airplanes.[4] The problems of the system with hydrogen as fuel start with the means of hydrogen production in the first place, storage, transport, etc. However, just assuming a hydrogen supply is available, the aircraft is also limited. restrictions on weight and volume. A reader of this Physics paper today[5] wrote to explain that through a restriction on available mass, aircraft using hydrogen were limited to short flights. In fact, we are going back to the air transport system of the 1930s.

The September 2021 issue of Physics Today has an article that begins thus,

“Experts say lithium-ion batteries will be surpassed in grid-scale energy storage applications by other battery technologies and non-chemical storage.”

Experts say otherwise. An odd claim in this paper is that hydrogen is a non-chemical form of storage. Since there is no source of hydrogen, chemistry is central to its production and use.[6] Further revealed is still the artist’s drawing of a solar farm incorporating a gravity storage system like that proposed by Energy Vault (™). The storage facility was a small four-story building that looked more suitable for an administrative office. It won’t store but a few moments worth the energy. There seems to be little recognition of the material handling issues to make mass storage possible when I stated here a while ago. There are mass speed constraints of plans to lift huge blocks into a storehouse of energy, or pull railroad cars high. Pumped hydro is a better method to store potential energy, but in this case the system’s limitations are the lack of available terrain and water use problems.[7]

Recently, “Scientific Journal Table of Contents” entered my email inbox, giving me access to a brief summary of thermal energy storage.[8] Energy-storage thermal systems often suffer from the limitations of the second law of successful heat transfer. It all involves taking valuable work and converting it into heat, which is then successfully converted back to usefulness at the typical 35-40% efficiency of a thermal power plant. There are many long conversion chains involved.

This paper presents a recent innovation in thermoelectric devices (TPVs) that can absorb broad-spectrum thermal radiation from a hot mass of matter near 2,700K, which is heated by renewable energy and transforms it. it turns back into electrical energy. However, the efficiency shown here is only about 40%, equal to that of a conventional thermal power plant operating at much lower temperatures. As high temperatures lead to increasingly large parasitic losses, storing energy as heat at high temperatures is problematic not only for the second law but also for any system that requires time storage. long. The inevitable parasitic losses add up to a huge total energy loss that power generation facilities have to generate. The secondary storage battery also has this problem.

One of the most serious systemic problems for renewables to deal with is the different reaction time scales needed to create a reliable grid. First, very short time scales of fractions of a second are needed for automatic control systems to keep frequencies and voltages within specified limits. Next, a daily time scale is required to handle daily variations in load. This is followed by an indeterminate amount of storage to handle weather outages that can last 10 days or more. Finally, there’s the matter of seasonally switching energy supplies, which requires too many generational construction or huge long-term storage, or some combination.

The current grid handles the very short time scale problem by relying on the spinning KE of its turbine machine, which stores the need for seconds in the spinning volume.[9] All other time scales are included using fossil fuels stored on-site with a power factor of up to 95% of the plant. It’s not too complicated and we have almost a century of experience in systems engineering making this system more than 99.9% reliable.

Wind plants have very little rotational power to support the very short time-scale stabilization problem, and solar does not. One remedy is to add a “synchronous condenser” to the renewable energy grid to act as a device similar to the rotary turbines of thermal power plants. These solutions are parasitic that only consume energy in exchange for short-term stability. Solutions to long-term system problems rely on cascading elements of diverse energy storage and conversion schemes that require a lot of mass, a lot of ground space, exotic materials, convenience. transmission gain, demonstrated energy, excess transmit power, etc. Not only are those factors themselves unproven, but we have no systems engineering experience with them. Can they be made to work? Who knows? Take a look at their heels.

Presenter:

1. I have been teaching engineering thermodynamics for twenty years. I find the first law of thermodynamics relatively easy to understand even if people have some difficulty applying it. However, the second law is much more difficult for everyone, even chemists, physicists and engineers, to fully understand. I’m still learning about applying parts of it after 5 decades of use. It completely controls all energy usage processes. It runs the universe. Since energy costs money to convert and distribute, the second law even controls the costs of operations that people wouldn’t think involve thermodynamics. I think it even covers the 0th and 3rd laws of thermodynamics. Therefore, while Zemansky’s best textbook on Thermodynamics, Thermodynamics and Thermodynamics calls friction a third law problem, I think it belongs more to the second law.
2. Dead state is a technical concept. It is a state of matter where, despite being full of apparent energy, no energy can be used to do work. We usually consider the dead state to be the temperature of 288K, the pressure of an atmosphere, the potential of the ground, the chemical species in equilibrium at the minimum Gibbs free energy, and the saturated relative humidity. 100%.
3. Owning two VW Beetles, I have become accustomed to the characteristic of German engineers, which is to create complex designs. The rest of us aim for simplicity.
4. David Kramer, Hydrogen-powered airplanes that can be lifted, Physics Today, 73, 12, 27 (2020); doi: 10.1063 / PT.3.4632
5. Peter Rez, Hydrogen as Aviation Fuel, Physics Today, Readers’ Forum, September 2021, p. 11. While Mr. Rez thinks about hydrogen being used in fuel cells to power a turboprop or turbofan engine, hydrogen could be used as a combustion fuel. However, this would involve the second law constraints on successful heat transfer, along with all the other mass, volume, and complexity constraints for hydrogen. Specifically, hydrogen possesses a lot of energy per unit mass, but a unit mass takes up a lot of volume. It makes no sense to go this route.
6. David Kramer, Better ways to store energy are needed to achieve Biden’s carbon-free grid, Physics Today, September 2021, p. 20. Biden’s?
7. Not only have the best places for pumped hydro have been put to use, but legal battles over water ownership and use have limited it even more.
8. Robert F. Service, Thermal batteries that can efficiently store wind and solar energy in a renewable grid, Science, Vol 376, Issue 6590, online edition April 13, 2022. doi: 10.1126 /science.abq5215