Hybridisation: the short-term goal
Current global efforts to reduce carbon dioxide emissions, including the Australian carbon price, will inevitably place additional pressure on utilities and industrial plants to operate at competitive costs.
Many operators and proponents are looking for reliable power generation options that are green and cost effective.
When combined, renewable energy sources such as solar thermal and newly-discovered natural gas deposits across Australia can ‘tick all the boxes’ for companies. The hybridisation of open and combined-cycle gas turbine (CCGT) plants with concentrated solar power (CSP) systems is key to achieving this.
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Of particular interest is the retrofit of solar towers to reduce costs and allow independent operation of CCGT and CSP plants. Integrated solar combined-cycle (ISCC) plants are very suitable for Australia, as natural gas reserves and solar irradiance are abundant. Several existing plants are in suitable locations and could be retrofitted.
CSP technologies are considered capital-intensive to construct and implement. Hybridisation has cost reduction benefits in terms of sharing equipment and knowledge for technology providers, operators and financiers.
Integration is the key
Currently, a number of ISCC plants are in operation around the world – the largest in operation being the 75 megawatt (MW) Martin Next Generation plant in Florida, United States. There are a number of plants currently under construction in Australia, including CS Energy’s Kogan Creek and Macquarie Generation’s Liddell power stations.
All existing hybrid plants use the well-established parabolic trough solar technology, providing saturated or slightly superheated steam to the heat recovery steam generator (HRSG), where further superheating-to-steam turbine requirements take place using the hot gas turbine exhaust. This concept is proven in several plants but requires both plants to operate simultaneously.
Parabolic trough plants using thermal oil are the most mature CSP technology, however steam temperatures are limited to less than 390° Celsius, as the thermal oil used degrades very quickly above 400° Celsius. Significantly, higher steam qualities (greater than 540° Celsius and 140 bar), compared to parabolic troughs, allow simple and efficient integration into the high pressure/temperature component of the CCGT’s steam cycle. Both plants are able to operate independently when both steam generators are able to provide identical steam qualities.
With more solar plants commencing operation and construction around the world, such as Torresol’s Gemasolar project in Spain and Brightsource’s Ivanpah project in the United States, the technology is becoming more mature and bankable. Operators of ISCC plants generally understand and appreciate the cost savings gained by sharing equipment such as the steam turbine, condenser, feed water systems and auxiliary equipment.
However, an ISCC using a solar tower has the additional benefit of sharing building infrastructure. For example, the main stack can be modified to support the solar receiver.
The conversion of open cycle gas turbine to ISCC plants increases the fuel conversion efficiency and reduces investment as a significant portion of the equipment is already on site, such as the gas turbine and plant control system. Adding the HRSG and solar tower rankine cycle is in this case more cost competitive than building a greenfield ISCC plant.
The possibilities
A number of locations in Western Australia, Queensland, New South Wales and South Australia have an excellent direct normal irradiance (DNI) of sunlight, as well as access to natural gas. Typically, a DNI of 2,000 kilowatt hours per square metre (kWh/sq m) per annum is required for a stand-alone CSP plant but, due to cost-reduction benefits of ISCC, plant areas with a DNI as low as 1,600 kWh/sq m per annum could be considered.
At 200 MW, CCGT plants realise overall efficiencies of 55 per cent, resulting in a very efficient use of gas compared to back-up boilers in traditional CSP plants. Larger units could even realise up to 60 per cent conversion efficiency. ISCC plants are likely to operate in high ambient temperature environments, which reduce the gas turbine efficiency. To keep the gas turbine efficiency high, low-temperature CSP heat could be used to chill its inlet air.
The capacity of the solar plant is mainly driven by the part-load efficiency of the steam turbine. A 100 MW steam turbine remains efficient down to 50 per cent part-load. With the HRSG providing sufficient steam to generate 50 MW baseload, 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 night-time energy could be drawn from the storage tanks to keep the steam turbine operating at higher loads. The main stack in this scenario would need to be approximately 30 m higher than required for a stand-alone CCGT plant to ideally locate the solar receiver.
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. Steam turbine, cooling towers and buildings are arranged adjacent to the main stack/solar tower. To avoid thermal losses the steam turbine is placed very closely to the steam generators.
The ISCC plant could be either air or water cooled with air cooling being the more likely option, considering water scarcity in remote sites and avoidance of plume formation. The carbon dioxide intensity of the proposed ISCC plant is 365 kilograms per megawatt hour (kg/MWh), which is 60 per cent 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 308 kg/MWh. Depending on the remoteness of the site and infrastructure availability, the cost of electricity for such an ISCC plant can vary from $140 to $180/MWh.
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