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The author seems very confused about whether they're talking about the grid or devices. "DC power is significantly more energy efficient than AC power." -> the

by Youden 5y ago

The author seems very confused about whether they're talking about the grid or devices. "DC power is significantly more energy efficient than AC power." -> the examples go on to specify end points for electrical energy but we already use DC there, AC is mainly used in transmission, so the claimed advantages of DC are irrelevant. "DC motors and appliances have higher efficiency and power to size characteristics." -> Brushed DC motors aren't efficient, just cheap. Brushless DC motors actually require a separate circuit to turn DC into something resembling a sinusoidal current (i.e. AC). "DC is inherently compatible with renewable sources of energy such as solar and wind." -> solar generates DC but wind generates AC. "requiring storage (batteries)" -> chemical batteries require DC but other forms of storage like dams require AC to drive motors or turbines. "Most energy storage technologies are DC-based" -> at a local level (mobile phones, cordless power tools), sure. At a grid level, we're often talking about hydro. "Electronic equipment operates on DC power." -> equipment that deals with computation. Electric fans, washing machines and many industrial consumers of electricity use AC. Plus, the existing grids and existing generation infrastructure are built on AC.

jbay808 5y ago
In terms of motors, yes -- virtually all motors need to provide AC to the coils to run, so DC motors need to use an inverter. But increasingly these days, even AC motors are being run from variable-frequency drives, in order to squeeze out a bit more efficiency, because the savings from better matching the load more than makes up for the losses in the drive. Many jurisdictions are starting to incentivize or require VFDs for HVAC applications. And typically the first thing the VFD does is rectify the AC input to DC.
- fy20 5y ago
  In marketing speak, a variable frequency drive is often referred to as "inverter technology" in appliances.
- dreamcompiler 5y ago
  Indeed. There seems to be a lot of confusion about the types of AC. "AC" can mean "50 or 60 Hz sinusoid synchronized to the grid" or it can mean "a waveform that is not a constant voltage." Most motors require some form of the latter. Older motors required the former, but there are very few reasons to build motors that way today. The ones that remain in service can easily be driven by electronic inverters from a DC supply.
danhor 5y ago
AFAIK wind turbines, as opposed to conventional turbines in power plants, don't directly connect to the 60Hz Grid but go through DC and an inverter. This is done so they can efficiently work at different speeds at not just a few mechanically selectable ones
- shadowgovt 5y ago
  They have to, because the variable wind means that they need to be spinnable at a continuous range of speeds. Components connected to the AC grid need to synchronize with the grid's frequency. Since we can't force the wind to blow at a particular rate, we'd either need a lot of fancy mechanics on the turbines themselves to drop their speed (which would waste energy) or we decouple their spin rate from the grid frequency with the AC -> DC -> AC converter (which also wastes energy, but probably less and with much less cost than complicated spin-rate-stabilizing machinery).
  - sudosysgen 5y ago
    You could also have a continuously variable gearbox, which is still hugely problematic, but would waste less energy.
Reason077 5y ago
> "solar generates DC but wind generates AC." Yes, but a wind turbine is allowed to spin at variable speeds - its rotation is not synchronized to the grid frequency in the same way that hydroelectric and thermal turbines are. In order to get a wind turbine's power output to match the grid frequency, it goes through an AC -> DC -> AC conversion in a component known as a double-fed induction generator (DFIG).
- sandworm101 5y ago
  Wind turbines, the big ones, regulate themselves to turn in sync with the grid. The adjust thier blades as needed to keep pace/power. That's why huge fields of them all turn in perfect lockstep.
  - upofadown 5y ago
    From some random reading on the net, it is the smaller ones that can be directly connected. Above 2MW things are not as easy and it is better and more efficient to convert to DC.
    - Gravityloss 5y ago
      Wind turbines have frequency converters. They are semiconductor devices. In one 3 MW turbine I visited, it was at the base of the tower. The blade angle is adjusted to get optimum power extraction. Rotor RPM is completely independent of produced power frequency. References: https://www.vestas.com/en/products/offshore/V236-15MW https://www.vestas.com/en/products/offshore/V236-15MW and https://library.e.abb.com/public/bf09cdf11d234241845c79ac343dbe16/PCS6000_wind_turbine_converter_catalog_3BHS351272_RevC_EN.pdf https://library.e.abb.com/public/bf09cdf11d234241845c79ac343...
      - mlyle 5y ago
        You're talking past each other. The electronics perform two key functions: * Choosing pitch angles for efficiency and turbine safety. You can, for smaller turbines, just synchronize the turbine to the grid, but this is becoming uncommon practice. * Converting the produced AC power to DC, and then choosing the proper frequency output and voltage to feed power to the grid, and inverting the DC to make this power. This should usually be trying to "speed up" the frequency of the grid a little if it's not already way too fast and regulate the voltage appropriately. The second link you have, on page 3, shows (active) rectification (d) of the wind turbine AC power to make positive and negative DC buses, and then inversion of that DC (f) to make 3 phase AC output power.
Animats 5y ago
The author seems very confused about whether they're talking about the grid or devices. Which is surprising, given his background, a degree in electrical engineering and jobs with power companies.[1] This may be an argument for using more DC-DC converters and fewer transformers. The classic problem with shipping DC around is that voltage conversion is expensive. DC-DC converters have improved a lot. This article may be a dumbed-down version of that argument. A nice thing about large transformers is that those big hunks of copper and iron have a lifespan of 30 to 75 years. Replace those with a DC-DC converter, and it will probably have semiconductor lifespan problems. Plus someone will add on a data connection, firmware updates, a web server, and an antivirus program. [1] https://grid.pitt.edu/people/gregory-f-reed https://grid.pitt.edu/people/gregory-f-reed
upofadown 5y ago
DC is generically more efficient for transmission than AC. That is why it is used for very long lines. It used to be that you needed to stay AC to use transformers to step up the voltage but those days are long past. >Brushless DC motors actually require a separate circuit to turn DC into something resembling a sinusoidal current (i.e. AC). Typical split phase AC induction motors used in residential applications are not very efficient and have various other deficiencies. There is a tendency to do a AC>DC>AC thing to a 3 phase these days for smaller electric motors and get variable speed as a bonus. >...wind generates AC. But not at any particular frequency. So typical wind turbines have a AC>DC>AC converter to allow them to sync up with the grid.
sudosysgen 5y ago
If you wanted to efficiently make an efficient deeply variable speed modern motor run on AC, you might very well turn the AC into DC then back into AC in a brushless motor controller. Indeed, changing the frequency of an AC wave programmatically is incredibly difficult (but possible through a CVT, I guess), you're better off turning it into DC then back into AC through some some form of function generator. So indeed, the widespread use of BLDC motors is a point in favour of DC electric circuits in the home. Same goes for variable velocity generators, you will generally have an AC-DC-AC conversion in a variable speed generator. Either that, or a gearbox, those are your two options.
dreamcompiler 5y ago
> "DC power is significantly more energy efficient than AC power." -> the examples go on to specify end points for electrical energy but we already use DC there, AC is mainly used in transmission, so the claimed advantages of DC are irrelevant. DC is more efficient for transmission too. The reason the grid uses AC is that high voltage is always[1] more efficient for transmission than low voltage--regardless of whether you're talking about AC or DC--and in 1900 the only way to step voltages up or down was to use transformers, and transformers require AC. That's unfortunate because for a given voltage, it's more efficient to transmit it as DC than AC. But today we have power electronics which can step DC voltages up or down, which means we are free to convert the grid to DC. [1] The single exception is superconducting cables which can transmit low voltage power just as efficiently as high voltage power. But they are not yet cheap enough to be practical except over short distances.
- dhdc 5y ago
  I fully agree with what you said about AC/DC, just want to point out that DC benefits superconductors as well since superconductors have critical current densities where they lose their super conductivity.
kkfx 5y ago
The case for DC is named microgrids: keeping the frequency on a microgrid is hard, inverters react too slowly for most loads (that does not ramp up/does down power slowly) while DC devices can be far quicker. The case for AC is large grid and safety.
simonebrunozzi 5y ago
Youden, your comment is a great example of why I love HN so much. Thanks for sharing this, and clearing a few important things about the article.

by Youden 5y ago

jbay808 5y ago

In terms of motors, yes -- virtually all motors need to provide AC to the coils to run, so DC motors need to use an inverter. But increasingly these days, even AC motors are being run from variable-frequency drives, in order to squeeze out a bit more efficiency, because the savings from better matching the load more than makes up for the losses in the drive. Many jurisdictions are starting to incentivize or require VFDs for HVAC applications. And typically the first thing the VFD does is rectify the AC input to DC.

fy20 5y ago
In marketing speak, a variable frequency drive is often referred to as "inverter technology" in appliances.
dreamcompiler 5y ago
Indeed. There seems to be a lot of confusion about the types of AC. "AC" can mean "50 or 60 Hz sinusoid synchronized to the grid" or it can mean "a waveform that is not a constant voltage." Most motors require some form of the latter. Older motors required the former, but there are very few reasons to build motors that way today. The ones that remain in service can easily be driven by electronic inverters from a DC supply.

danhor 5y ago

AFAIK wind turbines, as opposed to conventional turbines in power plants, don't directly connect to the 60Hz Grid but go through DC and an inverter. This is done so they can efficiently work at different speeds at not just a few mechanically selectable ones

shadowgovt 5y ago
They have to, because the variable wind means that they need to be spinnable at a continuous range of speeds. Components connected to the AC grid need to synchronize with the grid's frequency. Since we can't force the wind to blow at a particular rate, we'd either need a lot of fancy mechanics on the turbines themselves to drop their speed (which would waste energy) or we decouple their spin rate from the grid frequency with the AC -> DC -> AC converter (which also wastes energy, but probably less and with much less cost than complicated spin-rate-stabilizing machinery).
- sudosysgen 5y ago
  You could also have a continuously variable gearbox, which is still hugely problematic, but would waste less energy.

Reason077 5y ago

> "solar generates DC but wind generates AC." Yes, but a wind turbine is allowed to spin at variable speeds - its rotation is not synchronized to the grid frequency in the same way that hydroelectric and thermal turbines are. In order to get a wind turbine's power output to match the grid frequency, it goes through an AC -> DC -> AC conversion in a component known as a double-fed induction generator (DFIG).

sandworm101 5y ago
Wind turbines, the big ones, regulate themselves to turn in sync with the grid. The adjust thier blades as needed to keep pace/power. That's why huge fields of them all turn in perfect lockstep.
- upofadown 5y ago
  From some random reading on the net, it is the smaller ones that can be directly connected. Above 2MW things are not as easy and it is better and more efficient to convert to DC.
  - Gravityloss 5y ago
    Wind turbines have frequency converters. They are semiconductor devices. In one 3 MW turbine I visited, it was at the base of the tower. The blade angle is adjusted to get optimum power extraction. Rotor RPM is completely independent of produced power frequency. References: https://www.vestas.com/en/products/offshore/V236-15MW https://www.vestas.com/en/products/offshore/V236-15MW and https://library.e.abb.com/public/bf09cdf11d234241845c79ac343dbe16/PCS6000_wind_turbine_converter_catalog_3BHS351272_RevC_EN.pdf https://library.e.abb.com/public/bf09cdf11d234241845c79ac343...
    - mlyle 5y ago
      You're talking past each other. The electronics perform two key functions: * Choosing pitch angles for efficiency and turbine safety. You can, for smaller turbines, just synchronize the turbine to the grid, but this is becoming uncommon practice. * Converting the produced AC power to DC, and then choosing the proper frequency output and voltage to feed power to the grid, and inverting the DC to make this power. This should usually be trying to "speed up" the frequency of the grid a little if it's not already way too fast and regulate the voltage appropriately. The second link you have, on page 3, shows (active) rectification (d) of the wind turbine AC power to make positive and negative DC buses, and then inversion of that DC (f) to make 3 phase AC output power.

Animats 5y ago

The author seems very confused about whether they're talking about the grid or devices. Which is surprising, given his background, a degree in electrical engineering and jobs with power companies.[1] This may be an argument for using more DC-DC converters and fewer transformers. The classic problem with shipping DC around is that voltage conversion is expensive. DC-DC converters have improved a lot. This article may be a dumbed-down version of that argument. A nice thing about large transformers is that those big hunks of copper and iron have a lifespan of 30 to 75 years. Replace those with a DC-DC converter, and it will probably have semiconductor lifespan problems. Plus someone will add on a data connection, firmware updates, a web server, and an antivirus program. [1] https://grid.pitt.edu/people/gregory-f-reed https://grid.pitt.edu/people/gregory-f-reed

upofadown 5y ago

DC is generically more efficient for transmission than AC. That is why it is used for very long lines. It used to be that you needed to stay AC to use transformers to step up the voltage but those days are long past. >Brushless DC motors actually require a separate circuit to turn DC into something resembling a sinusoidal current (i.e. AC). Typical split phase AC induction motors used in residential applications are not very efficient and have various other deficiencies. There is a tendency to do a AC>DC>AC thing to a 3 phase these days for smaller electric motors and get variable speed as a bonus. >...wind generates AC. But not at any particular frequency. So typical wind turbines have a AC>DC>AC converter to allow them to sync up with the grid.

sudosysgen 5y ago

If you wanted to efficiently make an efficient deeply variable speed modern motor run on AC, you might very well turn the AC into DC then back into AC in a brushless motor controller. Indeed, changing the frequency of an AC wave programmatically is incredibly difficult (but possible through a CVT, I guess), you're better off turning it into DC then back into AC through some some form of function generator. So indeed, the widespread use of BLDC motors is a point in favour of DC electric circuits in the home. Same goes for variable velocity generators, you will generally have an AC-DC-AC conversion in a variable speed generator. Either that, or a gearbox, those are your two options.

dreamcompiler 5y ago

> "DC power is significantly more energy efficient than AC power." -> the examples go on to specify end points for electrical energy but we already use DC there, AC is mainly used in transmission, so the claimed advantages of DC are irrelevant. DC is more efficient for transmission too. The reason the grid uses AC is that high voltage is always[1] more efficient for transmission than low voltage--regardless of whether you're talking about AC or DC--and in 1900 the only way to step voltages up or down was to use transformers, and transformers require AC. That's unfortunate because for a given voltage, it's more efficient to transmit it as DC than AC. But today we have power electronics which can step DC voltages up or down, which means we are free to convert the grid to DC. [1] The single exception is superconducting cables which can transmit low voltage power just as efficiently as high voltage power. But they are not yet cheap enough to be practical except over short distances.

dhdc 5y ago
I fully agree with what you said about AC/DC, just want to point out that DC benefits superconductors as well since superconductors have critical current densities where they lose their super conductivity.

kkfx 5y ago

The case for DC is named microgrids: keeping the frequency on a microgrid is hard, inverters react too slowly for most loads (that does not ramp up/does down power slowly) while DC devices can be far quicker. The case for AC is large grid and safety.

simonebrunozzi 5y ago

Youden, your comment is a great example of why I love HN so much. Thanks for sharing this, and clearing a few important things about the article.