(The author is a Reuters market analyst. The views expressed
are his own.)
By Gerard Wynn
LONDON, April 26 Solar power, fast catching up
with global wind capacity, presents its own problems for grid
operators balancing increasingly variable supply from renewables
with the demand fluctuations they were designed to handle.
The challenges of solar power may be less severe than those
posed by wind turbines because solar supply peaks are much more
predictable, while surges can be briefer.
Wind power variability can be forecast with reasonable
accuracy over intra-day timeframes.
Solar power variability meanwhile can be divided between,
first, a perfectly predictable element which is the position of
the sun, and second, uncertainty according to cloud cover.
A particular problem for grid balancing is posed by extreme
short-term variability, or "ramping events", which in the case
of solar can be associated with broken cloud cover, and which
can occur over shorter periods (in seconds, rather than minutes)
than with wind.
Such brief surges can be very localised, however, for
example where the shadow of a cloud may be limited to just a
part of one solar power plant, meaning balancing the grid across
a wider area can dramatically reduce variability.
Meanwhile the fact that maximum solar power generation is
known for any time of the day, as defined by clear-sky sunshine
and the position of the sun, places a limit on required stand-by
or reserve power generation.
A report from Germany's Fraunhofer Institute, published in
February, plotted actual daily, weekly and monthly solar and
wind power generation in Germany in 2012.
It shows how solar power generation is less variable on both
daily and weekly timescales compared with wind. (See Charts 1
Smaller daily solar power variation reflects the fact that
there is always some daylight over a 24-hour period,
notwithstanding peaks and troughs, and the fact that peak solar
output is ultimately limited by the power of the clear-sky sun.
That can be of use to grid operators who have a clear, daily
trajectory of maximum reserve power that they have to allocate
against a loss in solar power.
Solar power is as variable as wind over shorter timescales,
but, here too, its characteristics may be easier to handle.
Chart 1: (slides 27-28) goo.gl/YlRPO
Chart 2: (slides 21-22) goo.gl/YlRPO
Chart 3: (page 3) goo.gl/p3dFB
On a daily basis, averaged solar photovoltaic output follows
an up-turned "u", rising from sunrise, peaking around noon, and
falling back towards sunset.
But actual observations from individual power plants show
how output can fall and rise by as much as 75 percent in seconds
as generation ramps up and down between full sun and clouds.
Such ramping events are similar to wind power, but briefer,
as shown in a report published by the U.S. National Renewable
Energy Laboratory (NREL) in January, "Identifying Wind and Solar
Ramping Events". (Chart 3)
An NREL report published in 2010 used a modelling approach
to assess the grid impact of a hypothetical scenario where solar
(photovoltaic and thermal) power generation accounted for 25
percent of total generation in five western U.S. states, far
higher than presently.
The total, modelled solar power capacity was 29 gigawatts
(GW), and the model produced hourly variation in solar
generation of up to 15 GW, with serious implications for the
ability of other power generation sources, called net load, to
balance the grid.
The researchers acknowledged that they did not fully account
for the fact that such a large amount of solar power would
occupy a wider geographical area, reducing variability.
Nevertheless, the study, "Impact of High Solar Penetration
in the Western Interconnection", illustrated serious impacts
from very high levels of solar power.
Those impacts included: greater power exports to surrounding
regions; displacement of peaking gas and combined cycle gas
turbine (CCGT) generation; and more variable coal generation.
Such modelled phenomena were actually observed in Germany
last year, when solar power generation rose 44 percent compared
with 2011, reaching 5 percent of gross power generation, and
which undercut returns to gas-fired power.
A wider grid area will capture a wider range of weather and
so smooth renewable power variability, and it appears that the
benefit especially applies to solar power.
The effect was demonstrated by NREL researchers on the small
Hawaii island of Lanai, which is vulnerable to variability as an
isolated grid, in a study published last year, "PV Ramping in a
Distributed Generation Environment".
They showed how combined solar power plants always had less
variability, defined as monthly ramps in power generation at the
The ratio of ramps up or down at individual to aggregated
solar plants was always larger than the ratio of their installed
capacity, showing how aggregating plants made the ramps smaller
than otherwise expected.
They found that the smoothing effect was greater in summer,
which they attributed to more broken clouds which increased
differences across individual power plants.
NREL recommends a wider grid balancing approach to tackling
renewable power variability, for example appearing to downplay
the role for energy storage.
Balancing area is defined as the area across which grid
authorities have a responsibility to balance power supply and
demand, including the use of imports and exports, and could be a
region, whole country or countries.
The implication is that greater cooperation between grid
operators, including new operating procedures, market rules and
expanded transmission capacity, can do much to offset the
variability of high levels of solar power.
(Reporting by Gerard Wynn; Editing by Anthony Barker)