Interesting article, but I do not understand why you consider solar PV to be “work energy.” Yes, wind and hydro are work energy, because they harness moving objects that can physically push other objects. A photon cannot push anything. Solar does not seem to belong in this category.
I think the contention is that solar can directly take sunlight and convert it to electrons without heat. Still, solar is not very efficient in doing this.
Thank you for writing this guys. I appreciate the work you do at RMI. The dichotomy between heat and work you outlined is useful. However, if I may add, it's a bit of an oversimplification and misses some highly relevant points to switching our current energy systems to more sustainable ones.
Heat and work are two forms of energy that can be interchanged. But not all heat and not all work have the same value. They have differing abilities to be converted to other forms based on different properties. The key variable that must be considered is exergy—or low-entropy energy. Exergy is the maximum work potential of a system in relation to a reference environment. It can be thought of as technical work capacity, usable energy, or the ability to do work.
For heat, the greater the temperature difference between something and its environment, the more exergy it contains. The efficiency of any heat engine is determined by this temperature difference. For example, 1500°C heat can be used in a natural gas powered steam turbine to produce electricity relatively efficiently, but 70°C heat cannot because it lacks sufficient exergy. However, 70°C heat can be used to heat homes and building and hot water.
Matching exergy, not energy, supply to exergy demand is an important step in transitioning to sustainable energy systems.
Renewables don't produce "work" directly, they produce electrical energy (high in exergy) that can be used to do work or produce heat. Any time energy is converted from one form to another, overall exergy decreases, while total energy remains the same. Therefore, energy always remains the same and exergy always declines towards zero. Energy always degrades into low-quality heat close to the temperature of the environment from all heat and work interactions.
Not all energy demands require a lot of exergy. Melting steel requires a lot of exergy, but heating a home does not. For example, powering a computer requires low exergy energy in the form of electricity. That electrical energy powers the circuitry in the computer to perform calculations. Then the energy ends up as heat, sound, and light—all which interact with the room and are converted to heat. 100% off the original electrical energy was converted to heat, but we got some useful computer work done in the process. If it's winter, now we've heated our house as well. Heat is always the "waste" product of "using" energy.
I think a useful framework for our sustainable energy transition is to maximize the useful work functions performed by high-exergy energy as it inevitably declines into states of zero exergy. Thinking about this in the context of renewables can allow us to design systems that require less resources overall.
I may have missed it but where does nuclear fission/fusion fit in? Isn't it also extremely inefficient and lead to large amounts of waste heat that are a form of pollution, even if that is not as urgent an issue as CO2?
I tend to agree with you conclusion that large amounts of energy are wasted converting heat into work. You may be interested in my piece here where I walk through a history of thermal efficiency: https://www.lianeon.org/p/the-engines-of-progress
It goes roughly like this:
External Combustion 1900: ~20 percent
Internal Combustion 1950: ~50 percent
Combined Cycle 2000: ~65 percent
There is also no doubting that electric motors can convert energy into work with much better efficiency ~90 percent.
The problem comes in where we get that energy. Solar may not involve heat but it’s also not efficient either, maybe 20 percent? Perhaps it’s not “wasted” in the same sense as heat is, but it’s still inefficient. Are there any direct sources of energy that exceed 65 percent efficiency?
Interesting article, but I do not understand why you consider solar PV to be “work energy.” Yes, wind and hydro are work energy, because they harness moving objects that can physically push other objects. A photon cannot push anything. Solar does not seem to belong in this category.
I think the contention is that solar can directly take sunlight and convert it to electrons without heat. Still, solar is not very efficient in doing this.
Thank you for writing this guys. I appreciate the work you do at RMI. The dichotomy between heat and work you outlined is useful. However, if I may add, it's a bit of an oversimplification and misses some highly relevant points to switching our current energy systems to more sustainable ones.
Heat and work are two forms of energy that can be interchanged. But not all heat and not all work have the same value. They have differing abilities to be converted to other forms based on different properties. The key variable that must be considered is exergy—or low-entropy energy. Exergy is the maximum work potential of a system in relation to a reference environment. It can be thought of as technical work capacity, usable energy, or the ability to do work.
For heat, the greater the temperature difference between something and its environment, the more exergy it contains. The efficiency of any heat engine is determined by this temperature difference. For example, 1500°C heat can be used in a natural gas powered steam turbine to produce electricity relatively efficiently, but 70°C heat cannot because it lacks sufficient exergy. However, 70°C heat can be used to heat homes and building and hot water.
Matching exergy, not energy, supply to exergy demand is an important step in transitioning to sustainable energy systems.
Renewables don't produce "work" directly, they produce electrical energy (high in exergy) that can be used to do work or produce heat. Any time energy is converted from one form to another, overall exergy decreases, while total energy remains the same. Therefore, energy always remains the same and exergy always declines towards zero. Energy always degrades into low-quality heat close to the temperature of the environment from all heat and work interactions.
Not all energy demands require a lot of exergy. Melting steel requires a lot of exergy, but heating a home does not. For example, powering a computer requires low exergy energy in the form of electricity. That electrical energy powers the circuitry in the computer to perform calculations. Then the energy ends up as heat, sound, and light—all which interact with the room and are converted to heat. 100% off the original electrical energy was converted to heat, but we got some useful computer work done in the process. If it's winter, now we've heated our house as well. Heat is always the "waste" product of "using" energy.
I think a useful framework for our sustainable energy transition is to maximize the useful work functions performed by high-exergy energy as it inevitably declines into states of zero exergy. Thinking about this in the context of renewables can allow us to design systems that require less resources overall.
If interested, I wrote a more about this here: https://energyresourcedynamics.substack.com/p/understanding-exergy
Thank you RMI!
I may have missed it but where does nuclear fission/fusion fit in? Isn't it also extremely inefficient and lead to large amounts of waste heat that are a form of pollution, even if that is not as urgent an issue as CO2?
I tend to agree with you conclusion that large amounts of energy are wasted converting heat into work. You may be interested in my piece here where I walk through a history of thermal efficiency: https://www.lianeon.org/p/the-engines-of-progress
It goes roughly like this:
External Combustion 1900: ~20 percent
Internal Combustion 1950: ~50 percent
Combined Cycle 2000: ~65 percent
There is also no doubting that electric motors can convert energy into work with much better efficiency ~90 percent.
The problem comes in where we get that energy. Solar may not involve heat but it’s also not efficient either, maybe 20 percent? Perhaps it’s not “wasted” in the same sense as heat is, but it’s still inefficient. Are there any direct sources of energy that exceed 65 percent efficiency?