Renewable Energy
Thread - Renewable Energy
On 27/2/2003, Paul Williams
wrote:
Following the link below one may find a U.K. article on renewable energy
which has so many holes in it, that if one had a wind farm in back of it,
there would be no hindrance.
"If we covered a small fraction of the Sahara desert with photo-voltaic
cells, we could generate all the world's electricity requirements."
Umm...
Paul
http://news.bbc.co.uk/1/hi/sci/tech/2794169.stm
Peter
Macinnis responded:
>
"If we covered a small fraction of the Sahara desert with photo-voltaic
> cells,
we could generate all the world's electricity requirements."
Martin Green's
people at UNSW estimate that 60 km x 60 km (or maybe it was 60 km^2) of efficient
conversion would supply Australia.
Right
now, one-off units deliver around 24.5% efficiency, bulk units deliver 18-19%, but apparently
Martin Green has done an analysis, and sees no reason why it should not go
into the 90+% range -- or throwing in a few fudge factors, high 80s, and
apparently they are trying to identify the gap between theory and practice,
and what causes it, with a view to hammering it.
Meanwhile,
in the USA, conversion of sunlight to hydrogen recently topped 9% efficiency
-- the US DoE says 10% is the barrier to cross to get that off the ground.
So while the inestimable Robert Park scoffs at The Shrub's dreams of a hydrogen well,
it may be closer than anybody thinks. Personally, I would
be buying up land along the new Darwin rail line, just in case.
Imagine:
hydrogen-powered trains or electric trains, hauling in water, hauling out cylinders
of hydrogen . . .
We can
never say beforehand what a new technology will do, or where it will go, because we think in terms of
the known, a bit like people who can only see the scramjet as a way of
getting to London in two hours, or speak of horseless carriages or wireless
telegraphy.
My challenge:
what social, technical, engineering and environmental changes would YOU expect
to come from a hydrogen economy? What is needed to make it a reality?
Toby Fiander replied:
Can I get a reference to the discussion? What assumptions are involved?
Is there more to know?
BTW, assuming the cost of getting the higher efficiency is small, based
on figures of a few years ago for costs, this would make solar energy more
attractive than fossil fuels for electricity generation.... by quite a bit.
Peter
Macinnis replied:
>
Can I get a reference to the discussion? What assumptions are
> involved?
Is there more to know?
Not as yet --
gossip only, but I plan to do an interview Real Soon Now. The phrase
"Carnot analysis" {amended to "Carnot Theorem" in a
later post} or some such was used when I was being filled in, if you
mean the Martin Green stuff (I checked, it was a reference to Sadi of that
name). The hydrogen conversion I wrote up a few months back, and I can
drag it out if you wish.
>
BTW, assuming the cost of getting the higher efficiency is small,
> based
on figures of a few years ago for costs, this would make
> solar
energy more attractive than fossil fuels for electricity
> generation....
by quite a bit.
Agreed.
Hence my raising of it -- but wouldn't it be nice if, just for once, people
thought about where the new technology might go, if it came off? Also,
if the US DoE says 10% is the break-even point,
and commercial photovoltaics turn in 18% on a regular basis, could they
not break into the hydrogen economy? I assume that electrolysis is
fairly efficient, even if you have to pump the water in.
What sorts of bottles do we use to contain hydrogen?
Toby replied:
Here we go again with gases behaving badly. I had trouble last time
understanding when a gas would be anything but ideal - it is not even a genetic
trait.
I expect DEE actually understands the Carnot Cycle, and has probably even
ridden one or two of the English Machines to which Carnot referred.
Personally, I only ever thought of the Carnot cycle as a theoretical discussion
to which real machines have never had much resemblance.
The fridge that keeps the beer cold does not have to be terribly efficient
to do its job, nor does the boiler at the power station, although this is
more important than the fridge. As I understand it - and I may not -
in any process relatively small changes in efficiency make quite significant
changes to the overall power output/requirement if a process is inefficient.
The law of diminishing returns, which has some resemblance to the chemical
Principle of Le Chatelier (ie. nature is a bitch), ensures that early
success leads to less success later.
All of which is to say, I do not understand quite how the Carnot Theorem
relates in this case, but I would like to find out.
BTW, I provided a while ago based on an article prepared for the Institution
of Engineers, Australia group formed in Melbourne on hydrogen in transport.
There were said to be two competing systems for hydrogen storage in vehicles
as I understand it (and probably others). One is methanol and the other
a system of gaseous storage. I assume either would be suitable for storage
on a larger scale.
Paul Williams
responded:
>
> "If we covered a small fraction of the Sahara desert with photo-voltaic
> > cells, we could generate all the world's electricity requirements."
> Martin Green's people at UNSW estimate that 60 km x
60 km (or maybe it
> was 60 km^2) of efficient conversion would supply Australia.
<snip>
I would like to see a 3.6 sq. km array supplying a small town; or
even a 0.36 sq km array supplying a tiny town.
I know this could be done. It's the long term energy costs that appear,
to me, to be prohibitive.
I would love to see the UNSW self-sufficient in energy requirements.
Now if oil and coal became much more expensive...
When the production and maintenance of solar arrays becomes less expensive...
When the overall efficiency improves...
I realise that the Sahara desert postulation was as an example to simply
demonstrate possibilities.
Nevertheless, it misleads people as to the real world difficulties involved.
It is silly.
Transmission and storage are somewhat problematic for a start.
Recently on the 'Water on Mars' thread, it was shown in the rectenna article
that photovoltaics would have problems when weather/dust storms were factored
in.
I'm out of date, but I recall that solar cells may need to be replaced after
about 10 -15 years.
This does not factor in unusual (read probable) 'confounders'.
At the moment, my thoughts are that the economics do not stack up.
Like the 'free' wind a yacht harnesses - the 'means' to harness same and
the maintenance of this 'means' comes at a high cost.
This high cost is basically energy.
When the economics do stack up, we will find billions in venture capital
available overnight.
Martin Green is doing great work and will deservedly reap some benefits
when that time comes - as it eventually/inevitably will.
Donald Lang added:
I need to do some study before I can *really* join the discussion. I claim
'busy, busy' at the moment. I would suggest that we need to have justification
for suggestions of 90+ % efficiency. When last I looked at the means employed
to use photovoltaics they depended on releasing electrons into the conduction
band inside a semiconductor. Photons are wasted if they do not have
sufficient energy. Photons with excess energy have the excess wasted. In this
picture there is an upper limit on efficiency with 'black body' radiation
as from the sun. That upper limit is vastly short of turning 90% of the light
energy into electrical energy.
Please modernise my picture/reading as far as necessary.
If the energy is used to release and store hydrogen, life may become easier.
At the moment there is a problem that a substantial amount of use occurs at
night. Best eficiency occurs around noon. We need storage facilities. When
last heard there was some doubt about the total energy budget of a complete
system using lead storage batteries. There may be better batteries available
eventually, but lead was the nearest to economically feasible. A system that
is energy negative over its whole lifetime may still be economically viable
for an isolated location if it is manufactured elsewhere.
Sorry if all this is out of date etc. More and better, "real soon now."
Toby Fiander responded:
I did not like to display my ignorance, but since you force me into it and
I seem to be among friends... The mention of Carnot Theorem made me
think that what was being discussed was not a PV cell system, but a system
of capturing sunlight for heating water.
Martin Green made some of his original advances in the capture of heat.
Stuart White, now Director of the UTS Institute of Sustainable Futures, worked
on one of the projects for his PhD, IIRC.
OTOH, the PV cells have seemed the main game, at least to outsiders, for
some time now.
Donald Lang answered:
Colour me confused too, and that is a definite maybe. If you throw open
all matters of renewable or replacement energy, you need to specify your
ground rules. I must dig out a letter I got specifying that currently maximum
energy per dollar spent went either to much solar hot water, or nuclear.
Straight conservation is in there somewhere. Better get the ground rules
right before I sound off too much.
ttfn
DEE
P.S.
Once you incorporate Carnot and all his works you may find yourself dealing
in efficiencies where heat energy is used to produce chemical change. The
limitations imposed by thermodynamics can suprise us all. dwl.
David
Maddern added:
The problem with hydrogen is that it is an explosive gas in the presence
of oxygen - not real flash in a car smash for instance. (Petrol is not inflammable
but its vapour is with an ignition source)
The state of play is that solar cells feed into the grid from disperse locations,
when the particular owner of any array is not using all of the generated.
The converse happens at night but represents a partial call-back of
previously exported power (to the particular owner. This does away with
batteries.
Peak usage in Southern capitals anyway is air-conditioning (often 3 phase
in new buildings) so occurs in the daytime.
Such systems are available with regulators and all that equipment, or with
purpose built thin cells that sit on your roof outputting 24volts AC.
Ten of them make 240vAC synchronised with the mains so wired into any junction
box. Commercialisation of Martin Green's output
I imagine substantial losses occur with solar cells through heat generation
(seeking to capture all light infrared comes too) . Their overall performance
is said to be better in winter!
Ray enquired:
Is there anything wrong with the idea of setting up photovoltaic collectors
in heliocentric orbits and beaming the energy as transformed into laser style
EMR (microwave or other spectrum) down to collectors on Earth?
At least such collectors would have no down time, and expense would be minimal
after the initial high costs of setting up.
and in response to Toby Fiander's post :
Why would you do this? I don't understand the advantage.
Toby, the idea is to provide a solar electric source which is 100%
independant of night and day, of
weather, and of any other kind of terrestrial interference. It would
require relay stations in both heliocentric and geocentric orbits.
Pending the means used to transfer energy from an orbital position to ground
collection, which no doubt would need to be free of any geo-political
interference.
Then again, perhaps it is just
an exceptionally hare-brained scheme?
Possibly no moreso than A C Clarke's
elevator tower though.
David
Maddern responded:
Why do something that can be shot at, would cost heaps, would shade somewhere,
would cost bulk in greenhouse gasses getting it up there, some central agency
would own it, someone else would hate it, the beam might go astray, vulnerable
to space junk and by the time you get it up there some bastard has swamped
the grid with wind energy and solar power from a million roof arrays
is that enough?
(I hope so)
Anthony Morton added:
> Such systems are
available with regulators and all that equipment, or with
> purpose built thin cells that sit on your roof outputting 24volts AC.
Ten
> of them make 240vAC synchronised with the mains so wired into any junction
> box. Commercialisation of Martin Green's output
Do solar cells really exist that generate AC voltages? Photovoltaic
cells are DC sources, or so I thought.
I've often speculated about the possibility of DC buildings powered by solar
cells. The DC voltage could be electronically stepped down to 12 volts
for direct input to computers, lights and other appliances, or fed to DC-AC
inverters to drive efficient lift motors and air conditioning. No need
to synchronise every generation source with AC mains.
Toby
Fiander responded:
The brave new world so frequently described has PV cells on every roof feeding
the mains, presumably through an inverter.
I am a layman, but I am interested one and I have never been able to figure
out how this could be made to work - I can see at least three problems (synchronisation,
load does not match generation, quality control). Yet there clearly
is some way of doing it, because there are such roofs. As a layman,
I postulate that there is a sensor on the grid connection that adjusts the
inverter so that it is synchronised with the grid, but it is pure guesswork
on my part. What about the other systems of generation where an inverter
is not used?
When I looked at the IPART enquiry for an explanation of distributed generation,
what I got was effectively "small" diesel and cogeneration plants. Small
is a relative term, since the installations listed are ALL in the MW range.
At least one of them is comparable with machines operated by Pacific Power,
which is hardly small. There are also relatively few of them.
Again as a layman, I can only suppose that there are small distributed generators
at the BULK supply level. IPART mentions nothing about rooves and inverters
although it does refer to agreements to abide by quality requirements, which
would be a rather useless approach for roof-mounted PV cells and inverters,
since, by and large, there will be no "user serviceable parts".
There is also a lot of discussion on the IPART site about a complicated
system of market interaction between suppliers and retailers and those who
run cable and some people who are facilitators of one sort or another...
not a mention of a user anywhere, let alone anything technical. Like
the taxation system, the discussion makes you think there had to be a better
and more equitable way.
Zero Sum added:
> Do solar cells
really exist that generate AC voltages? Photovoltaic
> cells are DC sources, or so I thought.
You thought correctly. However
inverters are small nowadays...
> I've often speculated
about the possibility of DC buildings powered by
> solar cells. The DC voltage could be electronically stepped down
to 12
> volts for direct input to computers, lights and other appliances, or
> fed to DC-AC inverters to drive efficient lift motors and air
> conditioning. No need to synchronise every generation source
with AC
> mains.
Computers don't run on 12 volts.
Everytime I've asked someone to design or help me design a power supply they
say "get an inverter"...
Most twelve volt lights are not very bright.
There are no good reasons for a building that needs lifts in this
country.
Air conditioning? A properly designed building doesn't need it.
There are a few provocations. Maybe we can work out what power we
really need before we try to work out where we get it from.
Anthony
Morton wrote:
>
The brave new world so frequently described has PV cells on every
> roof feeding the mains, presumably through an inverter.
> I am a layman, but I am interested one and I have never been able
> to figure out how this could be made to work - I can see at least
> three problems (synchronisation, load does not match generation,
> quality control). Yet there clearly is some way of doing it,
> because there are such rooves. As a layman, I postulate that
> there is a sensor on the grid connection that adjusts the
> inverter so that it is synchronised with the grid, but it is pure
> guesswork on my part. What about the other systems of generation
> where an inverter is not used?
Power system
engineers call anything below 1000V 'low voltage'. These are the kind
of voltage levels one sees where power is actually utilised. It's also
the part of the system that power system engineers have least to do with.
When they speak of distributed generation they typically mean generators that
plug straight into the high-voltage grid, where voltage levels are typically
22000V or more and power levels are in the megawatt range.
When laypeople think about distributed generation they typically have smaller-scale
sources in mind, such as rooftop solar cells or windmills in the backyard.
I think such systems have enormous potential if they catch on the way motor
cars did in the 1950s.
At the moment, rooftop solar cells and similar systems are connected to
conventional AC mains using a sinusoidal inverter. This is an electronic
gadget that takes the DC produced by the cells and converts it to sinusoidal
AC with very low distortion. It's electronically controlled to ensure
it remains synchronised with the grid. From the point of view of the
homeowner it's no different to any other power that comes through the fuse
box.
The drawback is that these inverters cost a lot of money and have lots of
intricate components that fail and need replacing from time to time.
In the longer term I envisage appliances that could use the DC power from
the cells directly. Already there are a lot of appliances that must
convert the AC input to DC for internal use: computers, compact fluoro lamps,
'smart' washing machines and air conditioners, and so on. It's actually
these appliances that produce most of the 'power quality' problems that affect
AC grids today. In future, I could see most appliances being powered
from DC mains that interface with the AC grid at the distribution transformer.
Toby Fiander replied:
Anthony wrote:
> .... 'smart' washing
machines and air conditioners, and so on.
> It's actually these appliances that produce most of the
'power
> quality' problems that affect AC grids today. In future, I could
see
> most appliances being powered from DC mains that interface with the
AC
> grid at the distribution transformer.
Wouldn't it be logical for most electric motors to remain AC?
Does the power factor issue disappear with an interface as you describe?
> When laypeople
think about distributed generation they typically have
> smaller-scale sources in mind, such as rooftop solar cells or windmills
> in the backyard. I think such systems have enormous potential
if they
> catch on the way motor cars did in the 1950s.
Yes... but the car allowed a freedom
and privacy that was not available previously, whereas most people are not
aware of being tethered to a grid any more than they are that the milk in
the fridge comes from a cow.
A serious attempt at distributed generation would need to include a cost
advantage that made it overwhelmingly desirable to have independence of the
grid... or there needs to be the perception of imminent failure of the grid,
which is one of the reasons that water tanks have caught on a bit in Sydney
(sort of, you cannot get one at the moment and one of the company reps says
orders are related to the news and weather more than anything else).
At the moment, there is a significant case in terms of scarce national capital
NOT to build roofwater tanks or PV cells on roofs.... in the aggregate, the
money can be better spent on centralised systems. However, this values
the environmental advantage at zero. For concrete roofwater tanks, I
think it is probably doubtful that there is any significant environmental
advantage either, but there may be advantages for PV cells, I am prepared
to think there could be.
One of these threads started off with the prospect the efficiency of PV
cells (or at any rate solar collection) could be significantly increased,
which would then mean that centralised systems might not any longer have
the significant advantage in terms of cost that they currently enjoy.
Anthony
Morton responded:
>>
Do solar cells really exist that generate AC voltages? Photovoltaic
>> cells
are DC sources, or so I thought.
> You thought
correctly. However inverters are small nowadays...
One of the more
captivating applications I've come across for solar cells is the thin-film
solar window. Apparently you can even make them to vary their level
of tint according to the incident solar radiation. So on particuarly glarey
days they provide shade for those indoors while sitting there and generating
power. Unfortunately I haven't yet seen an inverter that can be embedded
in a pane of glass :-)
>
Computers don't run on 12 volts. Everytime I've asked someone to design or
> help
me design a power supply they say "get an inverter"...
Yes, computer power supplies are actually quite complicated things and can
generate up to half a dozen different voltage levels that are used on different
parts of the motherboard. 1V for the CPU, 5V for the Ethernet controller,
12V for the video card, 15V for the hard drive, not forgetting some negative
rails for good measure.....
Nonetheless you'll find that any power supply for any modern electronic
appliance will have roughly the same AC-DC front end. Usually there
are the same three basic units: a simple diode bridge, a DC-DC boost converter
(for smoothing) and a DC-DC flyback converter (for isolation). The
output of this is an isolated DC supply at some nominal voltage (24V in the
'puter I'm using now) which is then broken down further.
In laptops, this single isolated DC supply is what plugs into the mini-socket
on the computer itself. You could take any old DC power supply of the
appropriate voltage and plug it into this socket, but the manufacturer warns
you against it because it may not have the required power capability or built-in
isolation.
In principle there's no reason we couldn't run computers off a DC main:
in fact with the current designs a computer will already run off pretty
much anything between 100V and 300V, AC or DC.
> Air conditioning? A properly designed building doesn't need it.
I agree. But with the badly designed buildings we already have, there's
a lower resource cost in retrofitting energy-efficient plant than in demolishing
and rebuilding.
> There are a few provocations. Maybe we can work out what power
we
> really
> need before we try to work out where we get it from.
Couldn't agree more.
Paul Williams added:
My minimal understanding is that DC transmission is much less effiicient
than AC transmission over long distance. Would one then convert to DC at local
stations?
Aside:
I have read that Edison and Tesla had a strong disagreement about DC vrs
AC - culminating years later in the ironic awarding of the prestigious 'Edison
Medal' to Tesla.
I believe that it is folklore that Edison personally gave this medal to
Tesla.
David
Maddern posted:
> > Such systems are available with regulators and
all that equipment, or with
> > purpose built thin cells that sit on your roof outputting 24volts
AC. Ten
> > of them make 240vAC synchronised with the mains so wired into
any junction
> > box. Commercialisation of Martin Green's output
> Do solar
cells really exist that generate AC voltages? Photovoltaic
> cells are DC sources, or so I thought.
Yes I will write it again, outputting 24volts AC. They have electronics
in each of them to convert the accumulated DC to AC at 50Hz grid
Quite an innovation eh and I suspect they do it with a Trirac, and optoelectronic
number that uses light to drive the control voltage of a transistor to effectively
mirror the grid.
Pacific Power 'Plug and Power'T
AC power is heaps more efficient for transmission otherwise the wires would
be too big to hold up there with DC
And this matters over even 20 metres, so the whole building DC is a pipe
dream
>
I've often speculated about the possibility of DC buildings powered by
> solar cells. The DC voltage could be electronically stepped down
to 12
> volts for direct input to computers, lights and other appliances, or
> fed to DC-AC inverters to drive efficient lift motors and air
> conditioning. No need to synchronise every generation source
with AC
> mains.
Anthony Morton responded:
> Yes I will write
it again, outputting 24volts AC. They have
> each of them to convert the accumulated DC to AC at 50Hz grid
> Quite an innovation eh and I suspect they do it with a Trirac, and
> optoelectronic number that uses light to drive the control voltage of
a
> transistor to effectively mirror the grid.
> Pacific Power 'Plug and Power'T
Interesting - do you have a URL?
I couldn't turn up this item in a web search.
The idea I gather is to connect 10 of these devices in series to produce
a 240V AC source. Does this mean you need to connect the synchronising
signal to each individual cell? This could get unwieldy. I would
have thought it would be more economical to connect in series on the DC side
and use a single inverter operating at 240V.
> AC power is heaps
more efficient for transmission otherwise the wires would
> be too big to hold up there with DC
You're conflating AC versus DC
with high voltage versus low voltage. It would be madness to reticulate
a large commercial building with 24 volts DC, I agree, but it would be no
more sane to reticulate 24 volts AC. Ohm's Law and the voltage-times-current
relation for power hold equally well for AC and DC systems, give or take
a fudge factor.
> And this matters
over even 20 metres, so the whole building DC is a pipe dream
Here's an experiment to try (for the electrically trained). Run a 20
metre extension cord from any 240V AC power point and attach a 100W light
globe to the other end. Measure the current into the globe. Estimate
the voltage on the globe, using the fact that a 100W globe has a nominal
resistance of (240)2/100 = 576 ohms.
Next, obtain or build a 1 amp AC-DC rectifier (say a diode bridge with output
shunt capacitor). Plug this into the same power point and plug the
20 metre extension cord into the DC output. Attach the 100W globe,
measure the current and estimate the voltage as before.
Was the cable current smaller for the AC system or the DC system? In
which case are the losses smaller? Comment on your answers. For
bonus marks, what happens when you correct for the different voltage level
on AC and DC sides?
and in a later post:
> My minimal understanding
is that DC transmission is much less
> effiicient than AC transmission over long distance. Would one then
> convert to DC at local stations?
It's all a question of voltage
levels. The higher the transmission voltage, the lower the current
and the lower the transmission loss, regardless of whether it's AC or DC.
AC systems emerged as superior at the end of the 19th century because they
had the capability of using transformers to step the voltage up to a high
level for transmission and then back to a safe low level for utilisation.
The technology of the day limited DC systems to a single voltage level, so
you had to transmit at the same voltage that your appliances ran at.
This meant that DC systems were severely limited in size and extent, compared
with the huge AC grids that later emerged.
However, it's been understood since at least the 1950s that, as long as you
have a way to change the voltage level, DC transmission is actually more
efficient than AC transmission at a given voltage. Long-distance transmission
lines are now typically installed as HVDC (high-voltage DC) lines rather
than AC lines, at voltages of around a million volts DC.
Typically one would have a two-conductor line with one conductor at +500kV
(say) and the other at -500kV, giving 1MV between the two. A 1MV three-phase
AC line built with the same amount of conductor material will have the same
or higher losses (depending on operational factors) but will have a greater
voltage drop from one end to the other (due to reactive effects that don't
operate in DC systems) and will require much larger towers and a wider easement,
because the peak voltage to earth is 63% greater and the peak line-to-line
voltage 41% greater than for the DC line.
The reason one couldn't build HVDC lines in Edison and Tesla's day was that
one didn't then have the electronic technology required to efficiently convert
between AC and DC. AC transformers are still needed to provide the
high voltages for HVDC transmission, but the cost of converting high-voltage
AC to high-voltage DC is now more than low enough to make HVDC transmission
viable. And many of the advantages of DC in high-voltage transmission
translate pretty well to low-voltage systems too. So it's now quite
feasible to convert 415V three-phase AC to +/-300V DC at the entrance to
a large building and run DC cables with lower losses than conventional AC
cables.
> Aside:
> I have read that Edison and Tesla had a strong disagreement about DC
vrs
> AC - culminating years later in the ironic awarding of the prestigious
'Edison Medal' to Tesla.
> I believe that it is folklore that Edison personally gave this medal
to Tesla.
I'm not sure about the folklore,
but the 'Battle of the Systems' is certainly historical fact and even more
bitter and extensive than most of the popular accounts make out. Most
popular writers focus almost exclusively on Edison and Tesla as they were
probably the most colourful personalities involved, but in fact the battle
raged for more than 10 years over the whole of Europe and North America and
drew in most of the major scientific figures of the day. Not only Edison
but also Crompton, Siemens, Lord Kelvin and many others were lined up on
the side of DC versus the AC advocates Tesla, Westinghouse, Ferranti, Ganz
and others. Virtually every major city that electrified in the 1880s
or 1890s became host to a skirmish between rival engineers with DC and AC
systems. Melbourne had DC north of the Yarra and AC south of the Yarra
for many years.
and further, posted:
> Wouldn't it be
logical for most electric motors to remain AC?
If you mean that AC motors have
advantages over DC motors, yes. (I presume we're talking here about
induction motors and synchronous motors, not the high-speed universal motors
found in small electrical appliances - the latter are essentially DC motors
driven from an AC input.)
The tricky issue is that if you plug an AC motor into a 50Hz supply, it will
only run at one fixed speed. Most processes run more efficiently if
you can control the speed, and this requires either a mechanical gearbox
or a variable-frequency AC supply. The 'state of the art' is to use
a DC-AC inverter to provide a variable-voltage, variable-frequency supply
to the motor - so that although it's an AC motor it's effectively being operated
from a DC supply. To operate off standard AC mains requires two stages
- an AC-DC rectifier followed by the inverter.
Just to underline the changed state of affairs, technology has now reached
the point where the inverter can be embedded in the motor junction box, giving
the appearance that the motor itself takes a DC input! Industry figures
have suggested that before too long, virtually every AC motor you buy will
have an embedded inverter.
> Does the power
factor issue disappear with an interface as you
> describe?
As seen from the DC mains, yes.
With a steady DC supply free of ripple, any load that draws a constant DC
current will have power factor 1. With an inverter load the current
is contaminated by switching noise, but usually the DC bus will have sufficient
capacitance that this has little practical effect. It's certainly true
that inductance, the principal cause of poor power factors in AC systems,
has no effect in DC systems.
>> I think such
systems have enormous potential if they
>> catch on the way motor cars did in the 1950s.
>
> Yes... but the car allowed a freedom and privacy that was not
> available previously, whereas most people are not aware of being
> tethered to a grid any more than they are that the milk in the
> fridge comes from a cow.
It's in the mind to some extent.
It's not inconceivable that people could become as excited about being an
independent power producer as many are about having independent mobility.
It would certainly help if the cost of a rooftop system came down to be a
good deal less than that of a car - but this would likely come about only
if PV cells became substantially more efficient.
> At the moment,
there is a significant case in terms of scarce
> national capital NOT to build roofwater tanks or PV cells on
> rooves.... in the aggregate, the money can be better spent on
> centralised systems. However, this values the environmental
> advantage at zero. For concrete roofwater tanks, I think it is
> probably doubtful that there is any significant environmental
> advantage either, but there may be advantages for PV cells, I am
> prepared to think there could be.
The big problem with PV cells is
the embodied energy of production. I'm still not prepared to say there's
a clear environmental case for them; I think the efficiency has to increase
first. Of course, that would also improve the economic case.
David
Martin added:
>
My minimal understanding is that DC transmission is much less effiicient
> than AC
transmission over long distance. Would one then convert to DC at
> local
stations?
Hi Paul and others,
It depends what you mean by "efficient".
AC transmission is used because it can be transformed up or down in voltage
at the transmitting (generator) and receiving (e.g. household) ends respectively.
DC can't be transformed at all (details of the physics on request :-)
If you want to transmit e.g. a megawatt of power across country, then the
power, which is the product of current and voltage, must be a million. ie.
you could transmit a million amps at one volt or a thousand amps at a thousand
volts, or whatever.
Unless you use superconducting cable (which brings its own problems) there
is some power loss in the cable because of its resistance R, which is proportional
to the current *squared* (it's RI2).
So to minimise cable losses it pays to transmit power over long distances
at the highest possible voltage and hence lowest possible current. Typical
transmission line voltages are a good fraction of a million volts. Voltages
significantly higher than this cause insulator breakdown on the pylons, arcing
and so on, (can you give us some details here Anthony?).
For the same current and voltage values, then the heating of the cable is
the same for DC as AC, but there is another effect. AC generates electromagnetic
waves at whatever frequency is being used, and some additional power is lost;
making AC actually less efficient than DC. This effect is negligable at 50
or 60 Hz however.
