Gas turbine inlet air cooling has great potential for improving power plant efficiency in the Middle East. But in addition to significantly increasing plant efficiency in hot weather conditions, the use of turbine inlet cooling can also reduce variable operation and maintenance costs.
Gary Hilberg, Thomas Tillman, Turbine Air Systems, Houston, Texas, USA
Although the current power generation method of choice for new plants, gas turbines suffer from reduced capacity and heat rate during peak months on hot summer ambient days. In hot regions such as the Middle East, this degradation and overall impact can be even more significant – up to 30 per cent reduction in output and 3-4 per cent increase in fuel usage from the ISO design condition.
Figure 1. Plant unit cost in $/kW at rating point
Cooling the inlet air of the GT increases the mass flow rate and improves the GT performance. Indeed turbine inlet chilling (TIC) has been considered as an output and efficiency improvement tool for gas turbines for more than 15 years. However, several factors have changed in the past five years which have dramatically improved the economics of mechanical inlet chilling:
- Improvements in standardization and packaging of chiller systems have resulted in a 30-50 per cent reduction in first cost due to packaging economics and a 30 per cent improvement in system efficiency (Table 1 highlights these changes)
- The increased gas turbine compressor ratios have increased the dependence of the gas turbines on cool dense air
- Free market forces have placed a premium on generation when demand is highest – in most markets when the weather is warm – conversely when an unchilled gas turbine performs worst.
These factors lead to standard project measures that will evaluate the initial project investment of turbine inlet cooling as very positive on a $/kW installed and incremental heat rate (Btu/kWh). Figure 1 shows the relative $/kW for the base combined cycle GT and that with varying inlet cooling technologies. The key driver behind the value of turbine inlet mechanical chilling is the larger percentage improvement in overall generation capacity.
Figure 2. Plant output variability due to changes in ambient temperature: the difference in net plant output between 25°C and 35°C
Yet what these measures miss is the longer-term impact of turbine inlet cooling i.e. the very low incremental operations and maintenance costs and the reduced fuel usage. These savings can over-shadow the improved $/kW first cost when included in an overall net present value analysis. In addition to the improved operational cost aspect, a factor that is not considered is the value of the consistent generation when planning the operation of a gas turbine facility that utilizes mechanical turbine inlet cooling. The output of a chilled gas turbine facility will not vary by more than two per cent on any design condition (Figure 2) so whether it is 10ºC or 45ºC, the facility will generate as designed.
Figure 3 shows a typical split of costs for a gas turbine power facility. In deregulated markets most operators measure these costs on a $/MWh generated basis to allow the instantaneous determination of whether to generate in varying market conditions. This split will change based on varying gas turbine technologies, development costs and fuel prices.
Considering these factors, the components impacted by turbine inlet cooling are:
- Total plant MWh generated – 12-20 per cent increase
- Gas turbine heat rate – 0 to 4 per cent reduction
- Total debt service – no change on a per cent basis
- Major maintenance costs – no incremental addition
- Operations & routine maintenance including site labour, water & chemical usage and routine maintenance on the turbine inlet cooling system – significantly less on a per MWh basis
The major components of the cost of running a mechanical turbine inlet cooling systems are:
1. Electrical power to operate
2. Cooling tower makeup water
3. Cooling tower chemicals
4. Chiller system maintenance
For a packaged mechanical turbine inlet cooling system these non-fuel O&M costs will average $0.3 – 0.4 /MWh generated compared with F-Class gas turbine ranging from $3.0-4.0/MWh. Based on Turbine Air Systems’ (TAS) experience with over 120 systems, operators do not add incremental personnel to operate or maintain their chilling systems. Major maintenance such as tube cleaning and oil changes is outsourced.
Table 2 shows an example of how inlet chilling can reduce variable O&M costs by 1.7 per cent overall. For a 500 MW facility, operating 3000 hours per year, this saving alone is more than $850 000. This does not include the 10 per cent incremental revenue that the chilling system will deliver.
Packaged chiller systems with reliable chiller manufacturers have system availability in excess of 98 per cent. In addition to the costs, other operational considerations need to be considered.
With the ability to provide an amount of “turn-down” equivalent to 12-20 per cent, the operational flexibility will allow operation at the most efficient base load conditions for a longer period. Generation sites or companies could be able to utilize this output flexibility to significantly reduce gas turbine starts. This may seem like a small cost, but the average cost in fuel and maintenance for F-class gas turbines will range from $20 000 – $25 000 per start. For the utilities that maintain a mixed fleet of units to meet varying operational profiles, a combined cycle unit with chilling can act as a base load unit with the chilled capacity acting as a simple cycle peaker with the combined cycle heat rate.
In summary, mechanical chilling as a means of improving the operations of gas turbines has been considered in a very narrow perspective. When the overall operational benefits are considered, not only will the chilled gas turbine plant be seen as one that can generate more power during the critical warm portions of the year, but also the chilled facility will be seen as the low cost and most flexible generator.
Asset optimisation of new power projects with gas turbine inlet air cooling: Winning power plant projects in the GCC through simple and proven restructuring of generation technology”, by Christopher Landry and Thomas Tillman, Turbine Air Systems. Presented at Power-Gen Middle East, 2003, October 13-15, 2003.