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Integrated Solar Combined Cycle (ISCC)

Integrated Solar Combined Cycle (ISCC) plants are a combination of gas turbine combined cycle and solar thermal plant to add heat to the combined cycle. While solar-captured heat may be incorporated in many ways; it is typically introduced as high pressure saturated steam, mixed with HRSG HP steam, and superheated for admission to the steam turbine. THERMOFLEX can readily model this, or any other arrangement under consideration.

Medium to large scale (100 to 500 MW) ISCC plant designs have been proposed where the solar contribution is used to augment plant capacity, or to replace gas-fired duct burners to generate extra steam during peak power demand periods. In many warm locations, power demand peaks in the mid-day hours of the summer when significant air conditioning loads occur. This demand profile is well-matched to a solar field’s capacity profile.

THERMOFLEX / PEACE is ideally suited to model ISCC as described here, or in other configurations for feedwater preheating, process steam generation, etc.

THERMOFLEX in concert with GT PRO deliver rapid plant scoping capability and fully-flexible plant modeling.  In this case, GT PRO was used to create the initial plant model.  Solar heat input to GT PRO was modeled as an “external heat addition”.  Afterwards, the GT PRO design was imported to THERMOFLEX and the solar field and solar boiler were added to generate steam in the model shown below.

The plant design is derived from a heat balance provided courtesy of Siemens Industrial Turbomachinery.  It is a 2x1 ISCC with two Siemens SGT-800 gas turbines exhausting into fired single pressure HRSGs making steam at 83 bar / 565 C for admission to one condensing steam turbine.  Steam is condensed in a dry air-cooled condenser to minimize water consumption.  The plant includes a parabolic trough solar field heating thermal oil and making steam in a solar boiler.  The solar field nominally adds 50 MWth to augment HP steam generated in the HRSG.

Under desert-like ambient conditions, 35 C, 35% RH, 928 mbar, and with 49.3 MWth heat input from the solar field, the plant generates 157.6 MW gross electric output using 276.3 MWth LHV fuel input.  The solar-generated steam is about 80% of duct burner heat input at this condition, and represents about 32% of the steam flow to the steam turbine.  Considering the fuel-free solar contribution, the plant operates with a 57.1% gross LHV electric efficiency, considerably higher than typically achieved with one pressure non-reheat GTCC plants.