I have a question that's been plaguing my mind. IF my my lap top runs on 3.5-5V DC possibly 12V DC why do I feed it 120V AC and on a Mac... how would one come to some saner solution than 120AC of EXPENSIVE solar system built to produce 120V to be dropped down to quite often 5-3.5V... I'm just not seeing the logic of 120V in at source power...
Thanks for your thoughts
Laptops are commonly 19.5V (5 LiIon cells in series) or something like that.
High voltages for power make the wires affordable. For each doubling of supply voltage
the wires can be 4 times less in cross-sectional area for the same resistive loss. Mains
power at 120V can carry kW power 100's of metres with relatively thin copper. At 12V the
wires would need to be an 1/2 an inch across and cost $1000's.
Power over ethernet uses 48V because that's the highest voltage that counts as "low voltage"
and thus doesn't need to pass mains-electricity safety regulations. Higher voltages would
be better, but 48V is pretty usable (5V would be hopeless over the normal length of ethernet
cabling).
FuzzyYYYY:
I have a question that's been plaguing my mind. IF my my lap top runs on 3.5-5V DC possibly 12V DC why do I feed it 120V AC and on a Mac... how would one come to some saner solution than 120AC of EXPENSIVE solar system built to produce 120V to be dropped down to quite often 5-3.5V... I'm just not seeing the logic of 120V in at source power...
Legacy - world electrical grid systems settled on 120/240 VAC a long time ago - so that's what we're stuck with.
Most solar panel installs are typically 24 volt or 48 volt from the panels to the battery bank/inverter system - which then converts things to "mains level" for the house or whatnot.
Lastly - and this is the "big one": Barring super-conductive wiring (note that room-temperature super-conductive wiring does not exist, at least outside of the lab) - the greater the voltage you can push down a wire, the smaller that wire (it's "gauge" - generally measured as the thickness or diameter of the wire - note that this will vary depending on if it is stranded vs solid core wire) that can be run for a given distance and voltage drop.
Even the best wire available has some measure of resistance for a given length of it - it may be low, but it isn't zero (otherwise, it would be a "super-conductor"!). In general, the thinner the wire, the greater resistance it has. Since according to Ohm's Law, current = voltage / resistance, the more you increase the voltage, the more current the wire can effectively carry. If you wanted to use a lower voltage, you need to lower the resistance to carry the same amount of current - meaning you need thicker wire, or wire that was more conductive (less resistance). Either way - it's going to get expensive.
Use copper or aluminium wire - but make it thicker --> $$$
Use silver or graphene wire - at normal thickness, but because it is more conductive --> $$$
Instead - up the voltage, use copper or aluminium for better conductivity (in the middle is gold - no good) - and have thinner wire over a distance (even the length of a house is enough to make lower voltages and thicker wire an issue).
Note something like the cables that run from your car battery to the starter; they are copper, but are meant to carry many hundreds of amps when starting. Since the car battery is (typically) 12 volts, the wire needs to be very thick (1-2 gauge, or thicker) to carry the current without overheating (because since wire has resistance, it will drop voltage and dissipate some of that as heat). Such wire is extremely expensive (a similar scenario is in the arc-welding world, where voltages can drop down to only a few volts, with extremely high currents - extension cables for the electrodes - even short ones - can run into the $100s USD due to the thickness needed).
What we are likely to see in the future for solar will actually be larger DC voltages (not smaller) - so to make the wiring cheaper. I could see a 96 volt system in the near future. It may also likely make the inverters less expensive. I can see the same with automobiles - a 48 volt LiPoly (or something similar) battery with thinner cables to the starter. Both of these will likely come about as copper becomes more expensive.
There may also be a switch back to aluminum wire - though I fear that we'll see similar things happen in houses as they did in the 1970s - lots of houses built during that time have either caught fire, or catch fire today in a regular manner - because of the number of aluminium to copper or brass interconnects, which have an extremely high resistance issue that doesn't tend to trip breakers. I live in a neighborhood filled with houses constructed in the 1970s - and every year, one or two houses will catch fire (usually because someone had plugged in something with a greater current than what the plug could handle - and the breaker does trip - but the dissimilar metal issue makes the problem larger). The house I own and live in already had a fire prior to when my wife and I bought it. Fortunately, the original owner refitted the entire house out with copper wire afterwards.
Oh - something else I forgot to mention, but I'll make it short:
Tesla and Edison back in the day had the "War of the Currents" - which was actually a nasty affair, and led to the development of the electric chair as a means of execution (after Edison demonstrated how AC current - as popularized by Tesla - could be used to lethal effect upon multiple dogs, and famously - a circus elephant). However, Tesla's (well, Westinghouse and GE's) system won out - due to the fact that AC could be easily boosted and lowered at each end using cheap transformers (whereas DC required a much more complex system) - and could be transmitted greater distances at higher voltages on cheaper wire (vs Edison's system, where a generating station needed to be sited every mile or so to serve only a few customers).
I seem to recall that in the AC system - frequency could play into things - that the higher the frequency, the greater the distance (or more current?) could be transmitted. Tesla wanted things running at what would be "radio frequencies" - to the point of being able to transmit power wirelessly (there's a whole 'nother story about Tesla and Marconi and the invention of radio).
*** I have to put a note here: Seriously, if you are at all interested in this - check out the history of the 19th century - particularly the mid-to-late portions. Between Babbage, Edison, Tesla, and Marconi - not to mention the countless other architects, engineers, and others (some well known - like Eiffel and Westinghouse) - the time was a magical period that extended well into the 20th century. You will learn things you never before really knew - in some cases, your mind will be changed (I used to hold the idea that "Tesla was better than Edison" - but honestly, each was a man of his era, and they both had their foibles - some worse than others, of course - but I often wonder what might've happened had Edison and Tesla worked together in a Jobs/Wozniak fashion, rather than the butting of heads that actually occurred), in other ways you will come to know why the era is much more fascinating, strange, and wondrous than even the best of Steampunk could desire.
Anyhoo...
Edison, though, is kinda getting his "comeuppance" - what with the various grid-tied solar electric panel installs (ie - an on-site generating station for each customer), which eliminates the need for a battery bank, though it can't be a base-load system. I can also see a future where houses will be mandated to have such panels installed (they might even be integrated into the roof - I do know of "solar cell shingles" which can be purchased today) by the electric utilities in coordination with local government, to reduce the need and increasing loads on the central generation stations - which they aren't making many more new ones today because of NIMBY tactics and such; those they do make are also solar or wind based (while true base-load systems like nuclear tech just keeps being vilified thanks to older designs making news as they fail - sometimes in spectacular fashion).
Finally - super-conductive DC systems are used as interconnects between grids and generation stations - depending on the distance and geography of course. These systems are very specialized, but they do have the advantage of there not needing to be any form of "synch-ing" of generators, or conversion of frequencies (depending on the systems being interconnected of course).
Ok - longer than I wanted, but shorter than it could have been! 