Optimisation of a Low-TIT Combined Cycle Gas Turbine With Application to New Generation Solar Thermal Power Plants
Authors: Siros Frédéric , Fernández Campos Gonzalo 
Published: August 2017

The Levelized Cost of Electricity (LCOE) of Concentrated Solar Power, referred to in this document as Solar Thermal Electric (STE) is no match for that of Photovoltaic (about one third in similar conditions). However, the future electrical grids dominated by intermittent renewables (in 2030 and beyond) will value the firm capacity provided by STE that integrate a massive thermal storage. The molten salt solar tower is, beyond any doubt, the best technology to provide this firm capacity at reasonable cost for the 10–15 years to come.

We believe that the next generation of STE plants will keep the molten salt tower architecture, but with novel solar loop, storage medium and power block that will allow for higher working temperatures and conversion efficiency, thereby downsizing the solar field that accounts for ∼40% of the plant’s LCOE. Among the three power block technologies that can make good use of the high temperatures allowed by the new storage medium (probably fluidized particles) — supercritical steam Rankine cycle, supercritical CO2 Brayton cycle and combined cycle gas turbine (CCGT) — the latter is a good candidate.

In order to achieve today’s impressive efficiencies of state-of-the-art gas-fired CCGTs (up to 62%), the manufacturers relied on continuous improvements — including Turbine Inlet Temperatures (TIT) increases up to ∼1500°C — without modifying the architecture. Our problem is quite different: we consider that our TIT is limited to 1,000°C. Therefore, new architectures must be contemplated in order to reach a ∼50% efficiency that would be a significant improvement compared to the ∼42% obtained with current subcritical steam cycles.

The objective of this study is to define the optimal configuration of a CCGT working with an external heat input provided by heat exchangers that collect the heat from a storage medium. The study was conducted as follows.

The bottoming cycle’s power output per unit of GT exhaust gas mass flow was defined as a function of the exhaust gas temperature thanks to several simulations performed with Thermoflow’s GT-PRO software and a linear regression. Our efforts were then focused on the gas turbine.

Four gas turbine configurations were optimized and compared: with/without reheat, with/without intercooling. Single and double reheat were also compared. For each configuration, three TIT values were considered: 800°C, 900°C and 1,000°C. A supplementary combustion that entails a 6% pressure drop was considered after the external heat and reheat(s). These calculations were performed twice: first using an analytic model, then with the Thermoflex software. Both results were always very consistent, which is no surprise: since the gas turbine is uncooled, the analytical model is straightforward.

The single reheat cycle without intercooling is the most efficient one if TIT ≥ 900°C. The double reheat increases the efficiency for TIT = 800°C. The gross efficiencies of the combined cycle power blocks with single reheat GT with additional combustion are as follows: 46.8% if TIT = 800°C (49.7% with a double reheat and no additional combustion), 50.3% if TIT = 900°C and 53.1% if TIT = 1,000°C.

The reheat (or second reheat if applicable) poses a considerable challenge because the low working air pressure entails a high relative pressure drop of the heat exchanger that is highly detrimental to the cycle efficiency. According to expert opinion, a train of reheat exchangers with the pressure drop considered in this study (6%) and a ∼50 K final temperature difference can be manufactured at reasonable cost. The Capital Expenditure (Capex) of the CCGT plus its trains of heat exchangers must be assessed in collaboration with manufacturers but should be reasonable, lower anyway than that of a supercritical steam cycle with similar efficiency.

In conclusion, a low TIT, externally heated CCGT with single or double reheat (depending on the TIT) is a good option for the next generation of high temperature solar towers. There is no major hurdle for manufacturing such a power block right now, other than convincing a manufacturer to do it.