[RE-wrenches] Fwd: kW/MW v. kWh/MWh

[Michael Welch]

>From Julie Haugh

> Your assessment is correct; you can't store kilowatts of power, you 
> *can* store kilowatt hours of energy.
> Author Lyn Corum might have slipped up a couple times in there and 
> dropped an 'h' from the kw, but maybe not -- because when storing energy 
> on a utility scale, the engineers are greatly concerned with how fast 
> the energy can come in, and how fast they can send it out. So when they 
> refer to a '50 kw flywheel storage unit,' that often means the max power 
> coming in or out at any given instant.
> In our (comparatively) puny end of the energy storage business, 
> batteries, we rarely have to deal with the issue. Assuming our battery 
> bank is big enough to keep the charge or discharge rate below C10, or 
> even C1, we just stack more inverters.

What you described is part of it.  The other part is that utility
scale power discussions often include units that confuse the heck
out of people.

One of my favorite confusing terms is "watts / hour" or "watts /
minute".
This can affect small scale RE folks when you look at the response
time of an inverter -- how well can it go from 0 watts out to full rated
capacity.  With utility scale discussions, it says how well a particular
generator or other resource can balance changes between supply and
demand.

So, a 100MW generator might be rated at 10MW per minute if it can
increase
or decrease output by 10MW each minute.  How many minutes it can go up
or down depends on how much power it is producing at any given minute,
but since demand rises and falls, that generator can sit there,
increasing
and decreasing its output all day long and possibly never reach 0 or
100%
of rated output, while the larger plants provide the base load.

http://en.wikipedia.org/wiki/Electric_power_transmission#Load_balancing

The relevance of this to Wrenches is that renewable sources tend to rise
and fall with the availability of the resource -- sun, water, wind.  If
a region has 100WM of solar power, the maximum increase or decrease in
output, ignoring cloud passage, etc., is when power times the rate of
change in power, is maximized.  Pre-coffee, and off the top of my
head, this happens at 9am and 3pm, local solar time, when the power is
is 70MW and the time-rate-of-change is +/- 70MW per hour.  I can show
the
maths if anyone cares, but it's related to the cosine effect (sun angle
on panels times rated panel wattage) and the first derivative of cosine,
which is -1 times sine.

This is part of why there's all the fuss about why renewables are bad
for the electric grid.  It's also why, IMHO, battery backed renewables
need to be considered more in the future -- so the regulation
requirements
can be met with the RE systems themselves.
--
Julie Haugh
Senior Design Engineer
greenHouse Computers, LLC // jfh at greenhousepc.com // greenHousePC on
Skype