Solar tower ISCC: Stacking up the benefits (Part 2)
CSP Today takes an in-depth look at deploying a solar tower integrated solar combined cycle (ISCC) p
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By Juergen H. Peterseim, at the Institute for Sustainable Futures University Technology of Sydney
In the first part of this series, we looked at the multiple major benefits of hybridisation, including joint use of equipment, relatively high capacity factor without capital-intensive storage; lower DNI requirements; and lower investment costs.
In this issue, we turn our attention to actual deployment, using a 200MWe solar tower ISCC case study.
ISCC plants are ideal for sites with a high DNI, natural gas abundance and a requirement for low cost reliable power all year round. The hybridisation of CSP technologies can reduce capital expenditure significantly, enabling the construction of reliable low carbon intensity power plants, today, without significant - or indeed any - government subsidies.
Towering potential
By taking as an example a 200MWe solar tower ISCC, it is possible to demonstrate the potential for ISCC plants in Australia. At the 200MWe scale CCGT plants realise overall efficiencies of 55% resulting in a very efficient use of natural gas compared to back-up boilers in traditional CSP plants.
Larger units could even realise up to 60% conversion efficiency. Typically, ISCC plants operate in high ambient temperature environments, which reduce the gas turbine (GT) efficiency. To keep the GT efficiency high, low-temperature CSP heat could be used to chill its inlet air.
The capacity of the CSP plant is mainly driven by the part-load efficiency of the steam turbine. A 100MWe steam turbine remains efficient down to 50% part-load.
With the HRSG providing sufficient steam to generate 50MWe base-load the steam turbine is operating at a good efficiency during the night with power peaking at 100 MW at daytime through additional CSP steam.
Using thermal storage would allow a larger solar contribution when nighttime energy could be drawn from the storage tanks. However, the economic viability of thermal storage strongly depends on the value of energy dispatchability.
To optimise the heliostat field size and avoid optical losses due to mirror wobble, the heliostats are arranged in a 320° circle around the plant, with the main stack in the centre. A 360° heliostat field is not possible due to the arrangement of other plant components, such as HRSG, condenser.
The main stack in the scenario illustrated (top right) would need to be approximately 30m higher than required for a stand-alone CCGT plant to ideally locate the solar receiver. The steam turbine, cooling towers, and buildings are arranged adjacent to the main stack / solar tower.
The ISCC plant could be either air or water cooled with air-cooling being the more likely option considering water scarcity in remote sites.
Low carbon footprint
The carbon dioxide intensity of the proposed ISCC plant is 365kg/MWh, 60% lower than the 2005-07 Australian generation portfolio average. Using 15 hours of full load thermal storage has the potential to further reduce the carbon intensity to 308kg/MWh. Depending on the remoteness of the site and infrastructure availability, the
ISCC plants are a promising transition technology for Australia with abundant natural gas reserves and excellent solar conditions.
Typically, a direct normal irradiance (DNI) of ≥2,000kWh/m2/year is required for stand-alone CSP plants but due to ISCC cost reduction benefits, such as joint use of equipment, regions with a DNI of ≥1,700kWh/m2/year and natural gas resources can be considered.
In Australia, several locations in Queensland, New South Wales, Western Australia, and South Australia fulfil these criteria. Most of these sites are remote or off-grid with significantly higher electricity prices than the national average, which is one of the lowest of all OECD countries.
In addition natural gas prices are expected to increase significantly over the next years due to large LNG export facilities coming online. This will favour the use of CSP systems as fuel saver systems.
Remote sites suffering from high electricity prices stand to benefit from ISCC plants particularly when open cycle gas turbine / engine plants are the current source of power generation.
Reducing the carbon intensity of power generation by a potential 60%, compared to the 2005-07 Australian generation portfolio average, is significant and would help Australia, and other countries, to meet emission reduction targets at low cost while continuously growing a CSP industry.
From an investor perspective, coupling new technologies with conventional power generation significantly lowers risk. By increasing the financing pool, more private investment would in turn allow CSP technology suppliers to ramp up manufacturing capabilities and reduce cost through learning experiences.
To respond to this article, please write to: Juergen H. Peterseim
Or write to the Editor:
Rikki Stancich: rstancich@csptoday.com
Image credit: Institute for Sustainable Futures, University of Technology Sydney