In an ideal world, the most efficient equipment would also cost the least to buy and install. But in the real world that’s usually not the case.
Equipment often runs more efficiently because it’s made from higher grade materials. Higher grade materials cost more.
Anyone who has priced home air conditioning systems is aware of this. But as Energy Star labels often reveal, lower electric bills offset the higher cost to purchase the AC system over time. The efficiency pays off by making the house cheaper to operate.
A similar principle applies to turbine inlet air chilling (TIAC) systems for power plants, but on a much larger financial scale. Lower capital costs (capex) may mean higher operating expenses (opex). That’s why it’s important to consider the total lifecycle costs — and how to minimize them — when investing in a TIAC system.
This is especially true if you own and operate power plants, and seek turnkey bids for their development. Bids do not always reveal that low capital expenses can drive up operating costs.
Bidders have an incentive to keep the capital costs as low as possible to better their chances of winning the contract. So they are naturally inclined to choose the TIAC system that is the least expensive to buy and install. However, any savings achieved in capex may be lost in opex. In fact, the lower priced equipment may easily end up costing many times more than the more expensive system over its operating lifetime.
Choosing the right TIAC system
How can you ensure that you select a system that achieves the most economic balance between capex and opex?
It would be nice if a calculator existed to render a quick number. But in truth, accurately figuring life cycle costs requires an engineering analysis of your project.
Several variables must be weighed, among them:
- The cost of power supplied to the plant versus the cost of refrigerant over time
- Whether the plant relies on dry versus wet cooling
- The plant’s operating hours
Some of these variables carry uncertainty and require forecasting. For example, water may be cheap now in your region, but what will it cost in a decade?
This kind of sensitivity analysis is complex, but important to help you select a system that brings you as near as possible to the break-even point between capex and opex.
Several factors will go into your decision in choosing a TIAC system, but efficiency will be an important one. Efficiency is measured according to the kW/ton — the kilowatt hours needed to produce a ton of refrigerant delivered from the system. A system with a 0. 6kW/ton rating is highly efficient; a 1.5 kW/ton rating is inefficient. A lower rated system will usually cost more.
The compressor or chiller determines the system’s efficiency — how much work it produces from each kilowatt consumed.
Of course, it is not always possible to choose the system that is most efficient. Water-cooled systems are more efficient than dry-cooled systems. But for a power plant in the desert, a water-cooled system may not be an option.
Balancing capex and opex doesn’t end in selecting a system. After you’ve installed the TIAC system there are operating strategies you can employ to minimize lifecycle costs.
Maintenance is a major contributor to operating costs. And system maintenance requirements can vary tremendously. The same task may take anywhere from three days to just a few hours, depending on your system configuration.
Consider these two approaches to winterizing a TIAC system, and the differences they create in the capex/opex balance.
- Draining the system
Inlet chilling systems work well in the summer, spring and fall. (After all, their purpose is to reduce temperature.) Winter can be another story. TIAC systems that use water as a refrigerant are subject to freezing in cold climates. Freezing causes expansion that can damage or even rupture equipment.
To protect your system, you may choose to drain the system in autumn and then refill it in the spring. This approach creates a highly efficient system, but it is labor and time intensive. It will increase opex.
- Adding Glycol
Another option is to add glycol or anti-freeze to the system. This lowers the temperature at which the liquid will freeze. The more anti-freeze you add, the less likely it is to freeze. But the system also becomes less efficient overall. The glycol-treated liquid cannot transfer temperature as effectively as pure water.
To overcome this inefficiency, it’s necessary to install a larger system than you would need if you just used water and drained the system annually. So under this scenario, you lower opex but increase capex. You’re spared the hassle of draining the system annually since you can leave the antifreeze-treated water in the system all year.
Minimizing TIAC lifecycle costs is becoming an important discussion because of larger shifts in the power industry. TIAC was once largely the domain of simple cycle peaker plants that run only minimal hours each year, maybe 500 hours/year, usually when the grid is under strain during hot summer days.
However, TIAC is increasingly being used by baseload plants that may operate 5,000 hours a year.
Why the change?
Permitting and siting new power plants is increasingly difficult. So to maintain electric grid reliability, the industry is attempting to get more from existing plants. Adding chillers can increase a power plant’s capacity. And given the large number of hours per year these base-load plants operate, it is crucial that they minimize opex to keep costs down.
So TIAC lifecycle costs are important to the plant owner. But they have implications for the rest of us as well who count on the lights to turn on each time we flick the switch.