
San Francisco’s Secret DC Grid - coloneltcb
http://mobileorigin.spectrum.ieee.org/energy/the-smarter-grid/san-franciscos-secret-dc-grid
======
ericabiz
Interesting article. Surprised it does not mention DC use in datacenters:
[http://gigaom.com/2012/01/13/the-next-big-thing-for-data-
cen...](http://gigaom.com/2012/01/13/the-next-big-thing-for-data-centers-dc-
power/)

AC power supplies on computers are notoriously inefficient and "noisy" (as one
datacenter facilities manager described them to me--he was referring to the
voltage on the line, not sound.) Many larger datacenters have now switched
entirely to DC-powered computers to cut down on energy use.

Years ago, when I was running my hosting company, DC power supplies were more
expensive, but you could typically make up the difference in a year or so
(less than the average 2-3 year lifetime of a typical server.) I suspect costs
have come down since then.

I seem to recall at least one datacenter in SF running DC directly from the
grid, as PG&E used to sell it back then. This article makes it seem like that
may no longer be the case. Regardless, many datacenters now sell DC as an
option, and will have installed their own converters to supply DC power to
customers.

~~~
lostlogin
I wonder what is done with the heat produced by the rectification. Presumably
when you convert power for 7-10 customers at a time they aren't residential
customers and the heat must be quite great. It seems a shame to just radiate
that all away.

~~~
m4x
They don't do anything with it, judging by the photos of rectifiers mounted on
the wall without any heat exchange or recycling equipment to be seen. It's
unlikely that there's much heat to recycle though. If there was, those same
photos would show significant heat sinks to help dissipate the energy away
from the rectifiers.

We don't bother recycling any heat from roadside transformers or larger
substations either, and they carry a great deal more energy than the
rectifiers powering 7 - 10 lift motors.

~~~
pkulak
Roadside transformers are AC to AC, which is way more efficient and easy than
AC to DC. My car runs on DC, and the quick chargers that provide 60 KW are the
size of a large closet and have loud fans to dissipate heat.

~~~
dfox
Largest source of inefficiency (and heat) in common isolated SMPS designs is
output rectification, which has to run on frequencies ranging from tens kHz to
units of MHz, thus the diodes there tend to spend relatively large proportion
of operating time in various half-open-half-closed states. Active rectifiers
somewhat improves overall efficiency for high output currents, but the problem
with switching time still remains.

On the other hand for line-frequency rectifier times required for rectifier to
switch can be ignored and only significant source of heat is voltage drop on
rectification diodes (ie. 0.6 - 1.5V), so one watt of heat dissipation per one
ampere of current is good ballpark.

------
dps
Also interesting to read about the work ConEd did to move its final customers
off DC in NYC

[http://www.coned.com/newsroom/news/pr20071115.asp](http://www.coned.com/newsroom/news/pr20071115.asp)

>In January 1998, Consolidated Edison began a program to eliminate DC service
in its operating territory. At that time there were over 4,600 DC customers.
By 2006, sixty remained. Between then and today[2007], when the last customer
at 10 East 40th Street was switched to rectifiers on their side of the meter
to generate direct current to supply its building’s elevators and sprinkler
system, Con Edison has been switching DC customers to alternating current.

Since the start of the program in January 1998:

4,541 accounts in 4,288 buildings have been converted; 6,172 DC meters have
been removed from the system; 193 in-building rectifiers have been transferred
to customers; 148 250KW street rectifiers have been removed; 35,965 sections
of DC mains (5,682,337 circuit feet or 2,660,634 cable feet) has been retired;
and 545,185 circuit feet or 236,611 cable feet of DC services has been
retired.

~~~
bobdvb
If much of that cable is copper then the material recovery could be a
substantial offset to the investment.

------
amiramir
I live in a building in downtown NYC near Canal and Broadway. It was built in
the 1890s and our Otis freight elevator is supposed to be one of the oldest
operating elevators in the city. The first passenger elevator (also and Otis)
was installed in 1857 around the corner on Broome and Broadway. Our elevator
is powered by a DC motor. Until a few years ago, Consolidated Edison, who
championed DC back in the day, delivered DC to power the elevator. Over the
course of a decade or so, in an effort to get us to switch away from DC, they
raised the price and in the end it was less expensive for us to install a
rectifier in our basement to convert AC to DC to power the elevator. The
rectifier is a nondescript box about he size of a small refrigerator and emits
an audible 60Hz hum.

~~~
marincounty
I know that hum too well--and it's really irritating. For some reason I even
hate the sound of my dimmer switch.

------
Aloha
Huh. I posted this same article about two days ago.

[https://news.ycombinator.com/item?id=6810685](https://news.ycombinator.com/item?id=6810685)

[http://spectrum.ieee.org/energy/the-smarter-grid/san-
francis...](http://spectrum.ieee.org/energy/the-smarter-grid/san-franciscos-
secret-dc-grid)

------
justincormack
Reminds me of Christopher Payne's lovely book about the (DC) substations of
the New York subway
[http://www.chrispaynephoto.com/substations.html](http://www.chrispaynephoto.com/substations.html)

------
wsh
With so few customers (and no new ones allowed), it’s not surprising that the
DC distribution system in San Francisco isn’t widely known, but it’s not a
secret.

Regulated electric utilities like PG&E provide their services pursuant to
tariffs, and this is no exception. See Tariff A-15, “Direct-Current General
Service,” on the PG&E website:

[http://www.pge.com/tariffs/tm2/pdf/ELEC_SCHEDS_A-15.pdf](http://www.pge.com/tariffs/tm2/pdf/ELEC_SCHEDS_A-15.pdf)

They just raised the rates, too.

------
keithpeter
_" By the mid-20th century, large rectifiers installed at two downtown
substations supplied the current, pushing 250-V DC onto a rectangular loop of
3.6-centimeter-diameter cable."_

OK, can anyone explain why each of the drum-wind elevator motors does not have
its own small rectifier and be run from local AC mains (possibly cross-phase
for 1.4x voltage)?

As this interesting article goes on to say the local power company is
splitting the DC distribution into smaller isolated networks anyway...

~~~
asynchronous13
Remember that these were installed ~50 years before the transistor was
invented. At the time, the hardware was very expensive. It was more economical
to have a single centralized rectifier than multiple distributed rectifiers.

Since they were installed, it comes down to maintenance costs vs gutting and
installing a brand new system. Maintaining an older system was apparently the
choice these building owners made.

~~~
keithpeter
Yes, I accept your point for when the system was originally provided. But
_now_ it strikes me that the older systems could be run from a power cabinet
with a step up transformer and a rectifier running off AC mains given a supply
with sufficient current capacity.

~~~
Aloha
They could have then, but mercury vapor rectifiers are/were expensive, mogen
sets are noisy and need service - its far easier to put a big one in, than
lots of little ones.

------
amorphid
The San Francisco building in which in live has one of these drum elevators. I
had no idea it's DC powered. Fascinating.

------
eigenvector
Toronto also has a 600 V DC system that is used exclusively for supplying the
subway and street railway services.

------
stox
The article makes brief mention of DC still being used in Chicago. Anyone have
any links to such?

~~~
wsh
See Commonwealth Edison Company, Schedule of Rates for Electric Service, Ill.
C. C. No. 10, 1st Revised Sheet No. 148:

    
    
      However, in certain individual situations, certain retail customers in the
      central part of the City of Chicago are provided with direct current (DC)
      electric service. Such retail customers are provided with electric service
      through rectifiers that convert AC to DC. Such retail customers have been
      provided with DC electric service since the early years of the twentieth
      century. Beginning in the 1930's, the Company has been working toward the
      retirement of DC electric service. The Company does not serve new or
      increased electric power and energy requirements of any retail customer with
      DC electric service. For a situation in which DC electric service is retired
      at a retail customer's premises, the Company removes its rectifier and
      associated AC to DC conversion equipment that had been used to provide
      electric service to such premises. Eventually, all such rectifiers and
      associated AC to DC conversion equipment will be so removed, and all retail
      customers will be provided with AC electric service. 
    

I think the customer pays for the rectifier losses, though. According to Sheet
No. 191:

    
    
      For a situation in which DC is provided through a rectifier, meter-related
      facilities are located on the AC side of the rectifier. 
    

— [https://www.comed.com/Documents/customer-service/rates-
prici...](https://www.comed.com/Documents/customer-service/rates-
pricing/rates-information/current/Ratebook.pdf)

