What is Thermal Efficiency

As humans, we like efficiency. The most significant result at the lowest cost and effort is inherently desirable. Efficiency is often the single greatest consideration when nations are deciding how to tackle the issues of climate change. In the midst of this is thermal efficiency, one of the most critical calculations relevant to climate change.

Thermal Efficiency 101

At its core, thermal efficiency is a measurement of how efficiently a device (usually an engine) can convert heat into useful energy. Higher is, of course, better – a single percentage point can make a major difference in the amount of power a single source can generate over its lifetime.

More relevantly for climate change, engines and power plants with high thermal efficiency need less fuel to produce the same amount of electricity or force. That means less coal burned, less gasoline used per mile, and generally lower costs for everyone.

The exact thermal efficiency varies by source. Nuclear power plants have an efficiency of about 33%. This is low for thermal power plants, which can often go as high as 48%.

Why Thermal Efficiency Is Limited

The laws of thermodynamics prevent heat-based power generation from being as efficient as it could be. We lose most of the energy in other areas, like mechanical friction. The good news is that it’s not quite as bad as it sounds at first.

Many modern power plants use cogeneration, a process that aims to generate electricity and heat from a single fuel source. This is different from older power plants, which always tried to produce one type of power from their fuel source.

In cogeneration, the waste heat is captured (to the extent possible), then used for a secondary purpose like running steam generators or heating the local facility. There are limits to this – energy loss means you can’t capture waste heat over and over and while getting significant power from it – but it’s always better to get as much power as possible from any source of fuel.

The output of a mechanical energy generation system is always lower than the input. Waste is an inherent, unavoidable part of the process.

The Impact Of Other Parts

In many cases, higher thermal efficiency is theoretically possible but functionally meaningless. The issue here is the rest of the power generation system. For example, steam can only heat and expanded so much before it starts damaging containers and turbines. In cases like these, too much thermal efficiency can destroy a system because the other parts can’t keep up, and that doesn’t help anyone.

As you can see, the issue of thermal efficiency is much more than “we want to make things more efficient.” If we don’t develop the rest of a system at the same time, it doesn’t matter how efficient the generator is.

Are Any Other Methods Of Generating Power More Efficient
Than Thermal Power?

Yes. The laws of thermodynamics make thermal power generation a relatively poor choice efficiency-wise.

The most notable of these is hydropower. As noted by the U.S. Energy Information Administration, hydropower is the largest source of renewable energy currently used in the United States. When we build systems correctly, they have efficiency as high as 95% – mostly because of the way the movement of water converts into electricity.

But wait, why aren’t we using more of that? We want to! Some people have proposed setups like making thicker dams or placing more dams on rivers. The problem is that we’re already using most locations suitable for hydroelectric power, and the system won’t work if we have to expend energy to pump water up.

In light of these environmental concerns, some people have proposed alternative hydropower systems, such as new versions of tidal power generators that could be placed out in the ocean. While these plans are mostly in the experimental stage, they could replace thermal power systems in the future.

Maximizing Thermal Efficiency By Reducing Entropy

One of the most significant problems with thermal efficiency is the presence of entropy in a thermal generation system. Or, to put it another way, the very idea of thermal efficiency is an attempt to minimize the amount of entropy present in a system.

This isn’t limited to heat wasted by mechanical friction and other considerations – it comes from the fuel, too. When fuel doesn’t convert into energy very well, we need more of it to get the same result. For that matter, just getting the fuel to a power generation system takes power, and the more fuel a system needs, the higher the cost to move it any distance. We build dams on rivers for a reason.

The ideal thermal generation system can produce power while requiring as little of it as possible. This is why hydroelectric power works so well – when the water (acting as fuel) comes to the dam, we don’t have to spend energy gathering it for use. Unfortunately, most thermal power plants use limited sources of fuel.

When we’re talking about generating power, we have to look at it from a distance. A power plant can’t be judged only by the energy it produces. Every gasoline of gas used to transport a truck of coal is a net negative for energy production. That brings us right back to our first point: We have to create more power than we spend getting it to make a viable system.

highly efficient power plant can do this. A low-efficiency plant isn’t worth it, especially with so many other options available.

What Elements Can Lower Thermal Efficiency?

Many factors can reduce the thermal efficiency of a system. Here are some of the most common.

  • Friction: One of the most common inefficiencies. Friction occurs when parts need extra energy to move. In many cases, this energy moves to other mechanical parts. In a thermal generation system, friction often makes up a significant portion of the lost power.
  • Imperfect Combustion: An ideal thermal combustion system will always
    burn its materials at the right time to make the best use of the heat they generate. Imperfect combustion is more common, whether due to timing or the inherent limits of the system, resulting in a practical output far lower than the theoretical one.
  • Air Drag: Many thermal systems have an air component to them, such as heating water that rises as steam through the air. This air acts as a drag on the movement of heat (energy) through the system, reducing its overall efficiency.
  • Heat Loss In Combustion Chambers: An ideal engine would push all of the heat towards a specific destination. In reality, heat tends to expand in all directions, and the walls of the compression chamber will absorb much of it. This, too, reduces the overall effectiveness of the system.

Essentially, everything a thermal power generator does has the potential for loss. The best way to deal with this is to minimize the number of steps needed to turn fuel into useful power. By limiting the opportunities for loss, we can maximize our efficiency and reduce our impact on the climate.

What About Heat Pumps?

We’re glad you asked! Heat pumps are the reverse of a standard thermal generation system. Instead of creating heat to use, heat pumps move existing heat. This is more efficient than creating new heat from the fuel, so fuel pumps can have a coefficient of performance above 100%.

Does This Ever Apply To Consumer Devices?

Yes. We occasionally mention thermal efficiency on consumer devices, and it’s particularly popular with home heating systems. In these cases, the term refers to the system’s effectiveness at converting fuel (typically electricity or natural gas) into heat that we pump through the air.

This is almost always expressed as a percentage, as in “this unit is 95% efficient”. However, remember that this efficiency is only after we deliver the fuel to the furnace. Electricity, natural gas, and all other sources of power still need to get to you. When it comes to affecting the environment, making power generators more efficient is almost always better than making appliances using power more efficient (though both help).

What Can I Do To Help?

As a consumer, you can focus on buying energy-efficient devices. These are a good start to helping the environment. Every little bit makes a real difference in the effort to limit climate change.

Beyond that, you can support research and development into new, affordable methods of efficiently generating power. Whether you’re going to school and focusing on thermal efficiency for a Ph.D. or lobbying your representatives to increase research funding and ensure all new power generation is climate-friendly, there are things you can do to help.

What you do matters – and the sooner we design and build efficient systems, the less impact there will be on our climate.

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