Solar Energy Perspectives - IEA

Solar Energy Perspectives: Solar thermal electricity
possibility of full dissociation in place and time of the collection of the solar energy and the
generation of electricity. These options are considered in Chapter 9.
Back-up and hybridisation
Almost all existing CSP plants use some back-up fuel to substitute or complement thermal
storage. Back-up helps to regulate production and guarantee capacity, especially in peak and
mid-peak periods. The fuel burners (which can use fossil fuel, biogas or, eventually, solar
fuels) can provide energy to the HTF or the storage medium, or directly to the power block.
Fuel burners also boost the conversion efficiency of solar heat to electricity by raising the
working temperature level; in some plants, they may be used continuously in hybrid mode.
For example, the 100-MW Shams-1 trough plant being built in the United Arab Emirates
combines hybridisation and back-up, using natural gas and two separate burners. The plant
will burn natural gas continuously during sunshine hours to raise the steam temperature (from
380°C to 540°C) for optimal turbine operation. This accounts for 18% of overall production
of this peak and mid-peak plant. The plant will also have a HTF heater which guarantees
capacity even at night. This back-up device was required by the electric utility and will be
used only at its request to run the plant at night in case of contingency.
CSP can also be hybridised by adding a solar field to fossil fuel plants, whether existing plant
or greenfield plants. Several schemes are conceivable, but only a few have been implemented.
One option is to build a small solar field adjacent to a coal plant, pre-heating the feedwater.
In coal plants, successive bleeds on the turbine subtract steam during its expansion in order
to preheat the feedwater before it enters the boiler. The solar field can replace these bleeds,
leaving more steam to be turned into electric power. One example of this is the 44-MW
Cameo coal plant in Colorado, which added a 4-MW solar trough field in 2010.
A second option is to provide high-pressure steam to the bottom cycle of a combined cycle
plant, in so-called integrated solar combined-cycle plants (ISCC). Several are in operation
today in Algeria, Egypt, and Morocco (Photo 8.4) with capacities in the tens of megawatts (in
equivalent electric capacities). The largest solar field added to an existing combined cycle
power plant was recently built in Florida, with a capacity of 75 MW equivalent electric.
A more ambitious option is to provide high-pressure superheated steam for main steam
augmentation, for example to a coal plant, possibly boosting the turbine for peak loads or
simply substituting coal when solar resource is available. Such a scheme is currently being
developed with the US Electric Power Research Institute on the 245-MW Escalante Generating
Station in Prewitt, New Mexico. If the steam flux is made very stable thanks to storage and/
or continuous hybridisation, solar heat could then benefit from the excellent conversion
efficiency of the ultra-super-critical steam turbines used in modern coal plants.
As the solar share is limited, these schemes of hybridisation are a good means to displace fossil
fuels. A positive aspect of solar fuel savers is their relatively low cost, especially when built
adjacent to an existing plant. With the steam cycle, turbine, generators, balance of plant and
connections to the grid already in place, only components specific to CSP require additional
investment. This would likely prove the cheapest and fastest way to introduce significant solar
shares into the electricity mix. Even without storage, fuel economy could be in the 20% to 30%
range. With storage, it could go up to much higher levels. As explained above, this could also
be the way to use super-efficient steam cycles to convert the solar heat into electricity.
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