5 ms·

For those of us that didn't study thermodynamics, what is the answer that's so stunning?

by talmand 14y ago

lutusp 14y ago
The OP didn't give enough information to produce a specific answer. For a higher voltage and lower current (same power), the wire could be thinner with the same outcome. The reason electricity utilities string million-volt lines between cities is to minimize the heating losses in the conductors, not to impress the civilians. It's the same reason electric car designs favor putting a lot of cells in series -- a high-voltage electric motor gets more of the power delivered by the batteries, less of it is wasted in the connecting wires. Also, in the car's charging circuit, the high voltage choice maximizes power transfer for a given connecting wire size.
- mbell 14y ago
  You still need to convert this high supply voltage to a lower voltage to match the cell voltage for charging. The currents involved mean this converter is still likely to dissipate a lot of heat, even if it is extremely efficient.
  - lutusp 14y ago
    > You still need to convert this high supply voltage to a lower voltage to match the cell voltage for charging. No, instead you charge the cells in series. That delivers the same power while avoiding the problem of having to produce overly high currents. > The currents involved mean this converter is still likely to dissipate a lot of heat, even if it is extremely efficient. Yes, but there are ways to minimize this loss factor. Obviously a fast-charging scheme for a car is going to involve a lot of current, but the system designers do all they can to minimize that current. The primary way they do that is by both charging and discharging the cells arranged as much in series as is practical. In the Tesla design, there are 11 modules connected in series and delivering 375 volts nominal (and requiring 375 volts to produce an optimal charging current). So there's no issue of changing individual cells at their normal cell voltage, instead the system charges modules consisting of 621 cells in an optimized series-parallel scheme: http://en.wikipedia.org/wiki/Tesla_Roadster#Battery_system http://en.wikipedia.org/wiki/Tesla_Roadster#Battery_system Quote: "Tesla Motors refers to the Roadster's battery pack as the Energy Storage System or ESS. The ESS contains 6,831 lithium ion cells arranged into 11 "sheets" connected in series; each sheet contains 9 "bricks" connected in series; each "brick" contains 69 cells connected in parallel (11S 9S 69P)"
    - mbell 14y ago
      By 'cell' I meant the overall pack, not the individual unit within the pack. From your link: "A full recharge of the battery system requires 3½ hours using the High Power Connector which supplies 70 amp, 240 volt electricity" Multiply that current by 10-100 to 'charge it faster' and you have a huge problem with thermal dissipation, not to mention using truly massive cables.
      - lutusp 14y ago
        > Multiply that current by 10-100 to 'charge it faster' ... No, that can't be done without damaging the batteries. A fast charge in this context means multiplying the default charging current by roughly three, not 10, and certainly not 100.
- cmccabe 14y ago
  Once you start getting into the thousands of volts, it becomes very dangerous. That stuff will arc through the air and kill you. There is a reason why we use a hundred or two volts in consumer appliances.
  - userulluipeste 14y ago
    Not necessarily. If the connection would be unmanaged, as having the power available for drainage all the time, then you're right! ...but we have computers and all kind of complex systems now. The cable may be "empty" of that high voltage current until the cable is safely plugged in and ready. After plugging, before unleashing energy, the car's system and the power source's system may first communicate and verify their sensors if everything's ready to go, like isolation and other things. The high voltage risk and its management it's an engineering solvable problem.
    - cmccabe 14y ago
      You have the wrong idea of how voltage works. High voltage electricity can and will kill you before the ground fault interruptor (or whatever) can kick in. It takes time to break a high voltage circuit, (on the order of milliseconds), and during that time, you will die. If the current is across your heart it's especially lethal. If the voltage is high enough, there is no way to break the circuit anyway. The electricity will just arc across the (ionized) air, even if the computer physically disconnect it. Assuming the computer itself isn't killed by the spark, which is pretty likely. This is why linemen on high voltage lines don't even try to electrically insulate themselves when they're working with it live. Ask wikipedia: For high-voltage and extra-high-voltage transmission lines, specially trained personnel use "live line" techniques to allow hands-on contact with energized equipment. In this case the worker is electrically connected to the high-voltage line but thoroughly insulated from the earth so that he is at the same electrical potential as that of the line. Since training for such operations is lengthy, and still presents a danger to personnel, only very important transmission lines are subject to maintenance while live. Outside these properly engineered situations, insulation from earth does not guarantee that no current flows to earth—as grounding or arcing to ground can occur in unexpected ways, and high-frequency currents can burn even an ungrounded person. Touching a transmitting antenna is dangerous for this reason, and a high-frequency Tesla Coil can sustain a spark with only one endpoint).
Lagged2Death 14y ago
10 gal = 37.85L 37.85L x 36MJ/L = 1,362 MJ in the tank. 1,362 MJ / 60s = 22MJ/s from the hose. J/s is Watts, so that's 22 megawatts. A cable that carried 200A at 110KV would do it. I think 000 gauge would handle 200A, it's just under 11mm in diameter.
- lutorm 14y ago
  But 110kV into the car is totally out of the question. At a more reasonable battery voltage of 1kV, you'd need 20kA. That'd be a fat cable indeed.
  - Lagged2Death 14y ago
    But 110kV into the car is totally out of the question. It would be pretty unreasonable. But burning gas at 100% efficiency is actually impossible, so it's sort of a silly scenario anyway. ...you'd need 20kA. That'd be a fat cable indeed. Maybe not as fat as you'd think; this guy figures 50KA here, and you wouldn't want to work with an extension cord like that every day, but it's not like you'd need a crane to lift it, either: http://youtu.be/uXEPy6Za6cI http://youtu.be/uXEPy6Za6cI
- EEGuy 14y ago
  Agreed, stunning it is that so common and reasonably safe a thing as a self-serve gas pump is a 22 MW chemical energy delivery system! Some reflections: * Perhaps an EV might be 10x as efficient with its input energy as a gasoline powered car. Perhaps 220 KW would do. * Perhaps a robotically mobile, rather than a counterweighted, manually lifted and connected cable is in order. The present day robotic car wash design where the car stays still while the motile, rotating washing machinery moves around it comes to mind. * Electrical connectors give me more trouble than wires and cables. A replaceable, quick-disconnect connector capable of 20KA seems more of a design challenge than the cables that would go with it. * High voltage insulation and wiring is "easier" than high current conductance (low resistance) wiring, for example: Common engineering practice with 30-volt solar panels is to wire them in series so that the total string voltage is 300 to 600 VDC at 8 Amps, rather than in parallel so that the current would be 80 to 160 Amps at 30 Volts. Air is a lousy and unsafe insulator, but there are good liquid insulators which could easily, temporarily fill a two-pole, ground-surround high-voltage connector (displacing the air) before it's allowed to be electrically energized and ramped up to a high voltage. Precise ground fault current detection could make it safe from fault currents (i.e. shocks & shorts) in the milliamp range even as the cable carries hundreds or thousands of Amps. But any HV supply requires up-conversion at the station and down-conversion in the EV; whilst these can be 95% efficient, the effect of each conversion inefficiency is multiplicative, and they add weight. * Then there's the charging station itself. If it has eight charging stations at, say 250 KW each, that's 2 MW draw from the utility (or an underground group of batteries recharged at a lower rate from utility power?) when all stations are charging cars. Not trivial; just the 500 KVA (call it 500 KW for discussion purposes) utility distribution transformer outside building where I work is the size of four refrigerators, not including its switchgear, all of which is enclosed by a 20 foot tall fence surrounding about 200 sq feet. Multiply that by four in volume to get 2 MW supply required from the utility.
VLM 14y ago
Whoa am I posting late sorry. There's a simpler way to do the analysis which doesn't require heat of combustion of typical gasoline analysis and flowrates and such. Assume it takes a KW for a horsepower (perfectly accurate for 1 sig fig) and it takes a couple horsepower (kilowatts) to go down the highway on a long road trip. To at least one sig fig this is correct. Also it takes about 3 times the chemical gasoline energy to generate electricity because IC engines are not terribly efficient. We'll call it 10 KW gas equivalent to drive on the interstate at a constant-ish high speed. Now what fraction of the time of a road trip do you spend pumping gas vs driving? Well it can't take more than 4 minutes to fill a 10 or so gallon gas tank, and on cross country trips I can get a good 8 hours or so outta one tank, so to one sig fig I spend WAY less than 1/100th the time pumping gas vs driving. So multiply 10 KW by 100 times the flow rate (conservation of energy and all that) Another way to look at it, is think of the flow rate, lets say 5 gallons per minute. The kind of engine required to burn a couple gallons per minute resembles my uncle's hospital UPS backup engines, certainly in the megawatt class. This is also in the thousand horsepower (aka megawatt) class airplane engine and race car category.

For those of us that didn't study thermodynamics, what is the answer that's so stunning?

by talmand 14y ago

For those of us that didn't study thermodynamics, what is the answer that's so stunning?

The OP didn't give enough information to produce a specific answer. For a higher voltage and lower current (same power), the wire could be thinner with the same outcome. The reason electricity utilities string million-volt lines between cities is to minimize the heating losses in the conductors, not to impress the civilians. It's the same reason electric car designs favor putting a lot of cells in series -- a high-voltage electric motor gets more of the power delivered by the batteries, less of it is wasted in the connecting wires. Also, in the car's charging circuit, the high voltage choice maximizes power transfer for a given connecting wire size.

mbell 14y ago
You still need to convert this high supply voltage to a lower voltage to match the cell voltage for charging. The currents involved mean this converter is still likely to dissipate a lot of heat, even if it is extremely efficient.
- lutusp 14y ago
  > You still need to convert this high supply voltage to a lower voltage to match the cell voltage for charging. No, instead you charge the cells in series. That delivers the same power while avoiding the problem of having to produce overly high currents. > The currents involved mean this converter is still likely to dissipate a lot of heat, even if it is extremely efficient. Yes, but there are ways to minimize this loss factor. Obviously a fast-charging scheme for a car is going to involve a lot of current, but the system designers do all they can to minimize that current. The primary way they do that is by both charging and discharging the cells arranged as much in series as is practical. In the Tesla design, there are 11 modules connected in series and delivering 375 volts nominal (and requiring 375 volts to produce an optimal charging current). So there's no issue of changing individual cells at their normal cell voltage, instead the system charges modules consisting of 621 cells in an optimized series-parallel scheme: http://en.wikipedia.org/wiki/Tesla_Roadster#Battery_system http://en.wikipedia.org/wiki/Tesla_Roadster#Battery_system Quote: "Tesla Motors refers to the Roadster's battery pack as the Energy Storage System or ESS. The ESS contains 6,831 lithium ion cells arranged into 11 "sheets" connected in series; each sheet contains 9 "bricks" connected in series; each "brick" contains 69 cells connected in parallel (11S 9S 69P)"
  - mbell 14y ago
    By 'cell' I meant the overall pack, not the individual unit within the pack. From your link: "A full recharge of the battery system requires 3½ hours using the High Power Connector which supplies 70 amp, 240 volt electricity" Multiply that current by 10-100 to 'charge it faster' and you have a huge problem with thermal dissipation, not to mention using truly massive cables.
    - lutusp 14y ago
      > Multiply that current by 10-100 to 'charge it faster' ... No, that can't be done without damaging the batteries. A fast charge in this context means multiplying the default charging current by roughly three, not 10, and certainly not 100.
cmccabe 14y ago
Once you start getting into the thousands of volts, it becomes very dangerous. That stuff will arc through the air and kill you. There is a reason why we use a hundred or two volts in consumer appliances.
- userulluipeste 14y ago
  Not necessarily. If the connection would be unmanaged, as having the power available for drainage all the time, then you're right! ...but we have computers and all kind of complex systems now. The cable may be "empty" of that high voltage current until the cable is safely plugged in and ready. After plugging, before unleashing energy, the car's system and the power source's system may first communicate and verify their sensors if everything's ready to go, like isolation and other things. The high voltage risk and its management it's an engineering solvable problem.
  - cmccabe 14y ago
    You have the wrong idea of how voltage works. High voltage electricity can and will kill you before the ground fault interruptor (or whatever) can kick in. It takes time to break a high voltage circuit, (on the order of milliseconds), and during that time, you will die. If the current is across your heart it's especially lethal. If the voltage is high enough, there is no way to break the circuit anyway. The electricity will just arc across the (ionized) air, even if the computer physically disconnect it. Assuming the computer itself isn't killed by the spark, which is pretty likely. This is why linemen on high voltage lines don't even try to electrically insulate themselves when they're working with it live. Ask wikipedia: For high-voltage and extra-high-voltage transmission lines, specially trained personnel use "live line" techniques to allow hands-on contact with energized equipment. In this case the worker is electrically connected to the high-voltage line but thoroughly insulated from the earth so that he is at the same electrical potential as that of the line. Since training for such operations is lengthy, and still presents a danger to personnel, only very important transmission lines are subject to maintenance while live. Outside these properly engineered situations, insulation from earth does not guarantee that no current flows to earth—as grounding or arcing to ground can occur in unexpected ways, and high-frequency currents can burn even an ungrounded person. Touching a transmitting antenna is dangerous for this reason, and a high-frequency Tesla Coil can sustain a spark with only one endpoint).

Lagged2Death 14y ago

10 gal = 37.85L 37.85L x 36MJ/L = 1,362 MJ in the tank. 1,362 MJ / 60s = 22MJ/s from the hose. J/s is Watts, so that's 22 megawatts. A cable that carried 200A at 110KV would do it. I think 000 gauge would handle 200A, it's just under 11mm in diameter.

lutorm 14y ago
But 110kV into the car is totally out of the question. At a more reasonable battery voltage of 1kV, you'd need 20kA. That'd be a fat cable indeed.
- Lagged2Death 14y ago
  But 110kV into the car is totally out of the question. It would be pretty unreasonable. But burning gas at 100% efficiency is actually impossible, so it's sort of a silly scenario anyway. ...you'd need 20kA. That'd be a fat cable indeed. Maybe not as fat as you'd think; this guy figures 50KA here, and you wouldn't want to work with an extension cord like that every day, but it's not like you'd need a crane to lift it, either: http://youtu.be/uXEPy6Za6cI http://youtu.be/uXEPy6Za6cI
EEGuy 14y ago
Agreed, stunning it is that so common and reasonably safe a thing as a self-serve gas pump is a 22 MW chemical energy delivery system! Some reflections: * Perhaps an EV might be 10x as efficient with its input energy as a gasoline powered car. Perhaps 220 KW would do. * Perhaps a robotically mobile, rather than a counterweighted, manually lifted and connected cable is in order. The present day robotic car wash design where the car stays still while the motile, rotating washing machinery moves around it comes to mind. * Electrical connectors give me more trouble than wires and cables. A replaceable, quick-disconnect connector capable of 20KA seems more of a design challenge than the cables that would go with it. * High voltage insulation and wiring is "easier" than high current conductance (low resistance) wiring, for example: Common engineering practice with 30-volt solar panels is to wire them in series so that the total string voltage is 300 to 600 VDC at 8 Amps, rather than in parallel so that the current would be 80 to 160 Amps at 30 Volts. Air is a lousy and unsafe insulator, but there are good liquid insulators which could easily, temporarily fill a two-pole, ground-surround high-voltage connector (displacing the air) before it's allowed to be electrically energized and ramped up to a high voltage. Precise ground fault current detection could make it safe from fault currents (i.e. shocks & shorts) in the milliamp range even as the cable carries hundreds or thousands of Amps. But any HV supply requires up-conversion at the station and down-conversion in the EV; whilst these can be 95% efficient, the effect of each conversion inefficiency is multiplicative, and they add weight. * Then there's the charging station itself. If it has eight charging stations at, say 250 KW each, that's 2 MW draw from the utility (or an underground group of batteries recharged at a lower rate from utility power?) when all stations are charging cars. Not trivial; just the 500 KVA (call it 500 KW for discussion purposes) utility distribution transformer outside building where I work is the size of four refrigerators, not including its switchgear, all of which is enclosed by a 20 foot tall fence surrounding about 200 sq feet. Multiply that by four in volume to get 2 MW supply required from the utility.

VLM 14y ago

Whoa am I posting late sorry. There's a simpler way to do the analysis which doesn't require heat of combustion of typical gasoline analysis and flowrates and such. Assume it takes a KW for a horsepower (perfectly accurate for 1 sig fig) and it takes a couple horsepower (kilowatts) to go down the highway on a long road trip. To at least one sig fig this is correct. Also it takes about 3 times the chemical gasoline energy to generate electricity because IC engines are not terribly efficient. We'll call it 10 KW gas equivalent to drive on the interstate at a constant-ish high speed. Now what fraction of the time of a road trip do you spend pumping gas vs driving? Well it can't take more than 4 minutes to fill a 10 or so gallon gas tank, and on cross country trips I can get a good 8 hours or so outta one tank, so to one sig fig I spend WAY less than 1/100th the time pumping gas vs driving. So multiply 10 KW by 100 times the flow rate (conservation of energy and all that) Another way to look at it, is think of the flow rate, lets say 5 gallons per minute. The kind of engine required to burn a couple gallons per minute resembles my uncle's hospital UPS backup engines, certainly in the megawatt class. This is also in the thousand horsepower (aka megawatt) class airplane engine and race car category.