POWER April 2015 - 82

PLANT DESIGN
Leveraging Generation Synergies
with Hybrid Plants
Everyone loves efficiencies. Combining generation technologies can create a
plant that's more than the sum of its parts, but engineering challenges
mean these projects are not for the faint of heart.
Thomas W. Overton, JD
W
hen you think of " hybrids " these
days, your first thought is probably
of automobiles. But hybrids-
hybrid power plants, that is-are starting to
emerge in the power sector as well.
Co-locating different types of generation
on the same site is far from a new idea. Where
an existing plant has the space, it's typically
far simpler and less expensive to build on
the unused land rather than a greenfield site.
Many large coal- and gas-fired thermal plants
have a few combustion turbines intended for
peaking duty parked in a corner of their lot.
What is a new trend is combining generation
resources with the intent of using their
respective performance characteristics to
compensate for each other's shortcomings-
to essentially create a plant that's more than
the sum of its parts and that can do more than
the two resources acting independently.
The impetus for this new trend is the enormous
increase in wind and solar generation
over the past decade and the degree to which
the intermittency of wind and sunlight has
become an area of concern for generators
and grid operators. With other generators being
called upon to compensate for fluctuating
output from wind farms and solar plants,
plant developers have begun looking at ways
of combining resources to reduce this intermittent
output. (Note: This article will not
address examples where small amounts of renewable
generation have been added to large
fossil plants, as these are not true hybrids.)
Solar + Gas: Bringing the Heat
The combination that is probably furthest along
in terms of installed capacity is integrated solar
combined cycle (ISCC). All of the half-dozen or
so existing ISCC plants combine concentrating
solar power (CSP) with a conventional combined
cycle plant, using steam generated by the CSP
system to reduce fuel consumption from the gas
turbines during the day. The main benefit of this
combination is that the two systems employ the
same steam cycle, making for substantial cost
savings over a conventional CSP plant. It also
increases efficiency of the combined cycle plant
and reduces total emissions per megawatt.
82
The combination works best where there is
both ample solar irradiation and an economical
source of fuel for the gas turbines. Not
surprisingly, most of the completed ISCC
plants so far are in North Africa.
Spanish firm Abengoa Solar has led most
of the development in this field. Its first utilityscale
plant to commence operations was the
150-MW plant in Hassi R'Mel, Algeria, which
began construction in 2008 and came online
in mid-2011. The plant uses about 180,000
square meters (m2
) of solar collectors to add
around 25 MW of output to the gas plant.
Abengoa has also built a similar facility in
Morocco, Ain Beni Mathar, which combines
a 470-MW combined cycle plant (supplied
by Alstom) with 20 MW of CSP (Figure 1).
Ain Beni Mathar also came online in 2011.
A third plant, in Kuraymat, Egypt, developed
by the New & Renewable Energy Authority,
has a capacity of 140 MW, 20 MW of that
from CSP.
Not all ISCC plants were built as greenfield
projects. Some have involved adding
CSP to an existing combined cycle facility.
Of these, Florida Power & Light's (FPL's)
Martin Next Generation Solar Energy Center
(a 2011 POWER Top Plant) is the largest,
with 75 MW of CSP generation. Retrofitted
to the existing 3,700-MW Martin County
Power Plant and coming online in 2010, the
464,000 m2
CSP field sends its steam to the
adjacent 4 x 1 combined cycle Unit 8.
The Martin plant has experienced some
growing pains, with multiple leaks of heattransfer
fluid from the CSP system. The plant
has fallen short of its projected generation as a
result of these hiccups as well as from the challenges
of synchronizing operations with Unit 8
so that the CSP field's output is used efficiently.
FPL has been frank that the facility was meant
as a test-bed for the technology, but it believes
Martin's performance will improve over time.
Other large ISCC projects are in development.
GE announced in January that it had
been awarded the contract to supply the turbines
and steam cycle for the 600-MW Green
Duba ISCC in Saudi Arabia. The facility in
the northwest region of the country will comwww.powermag.com
POWER
| April 2015
bine a two-unit, 550-MW GE combined cycle
plant with 50 MW of CSP capacity. (Vendors
for other elements of the plant have not yet
been selected.)
It's worth pointing out that most ostensibly
" CSP-only " plants actually include a certain
amount of gas-fired capacity to augment the
CSP system. This added capacity is typically
used to reheat the system in the morning and
smooth output during cloudy periods, but is
not run continuously.
The 100-MW Shams 1 CSP plant in the
United Arab Emirates (a 2013 POWER Top
Plant), also built by Abengoa Solar, deserves
special mention. Shams uses a gas burner during
normal operations to further heat the heattransfer
fluid in between the solar collectors and
the boiler. This increases the system's overall efficiency,
but it also means that about 45% of the
plant's net electricity output comes from gas.
Solar + Biomass: Double Duty
Combining CSP with biomass generation
enables synergies similar to the CSP-gas hybrids
discussed above. The first such plant
came online in 2012 in Les Borges Blanques,
Spain (Figure 2). The 22.5-MW Termosolar
Borges plant, developed by Abantia and
Comsa Emte using components from MAN
and Siemens, integrates the CSP oil loop into
the biomass burners. Steam generated by the
CSP loop is superheated in the burners, and
1. Solar synergy. The Ain Beni Mathar integrated
solar combined cycle plant in eastern
Morocco can generate 470 MW of electricity,
with up to 20 MW of that contributed by a
concentrating solar power system. Courtesy:
Dana Smiley/World Bank
http://www.powermag.com
POWER April 2015

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