Polar Works-Inlet Cooling Technology Overview-Refrigeration

Refrigeration

Refrigeration-based chilling is "the big iron" of gas turbine inlet cooling. While it is significantly more expensive on a first-cost basis than inlet fogging, it is much more effective and reliable.

"Inlet fogging" has become almost synonymous with "inlet cooling," something that we intend to remedy. Polar Works is in the refrigeration-based category of chilling. In fact, if there is a spectrum of technologies that describes inlet cooling, fogging would be at one end, and Polar Works would be at the opposite end, with other refrigeration-based processes somewhere in the middle.

Economics of Refrigeration Systems

Refrigeration-based chilling technologies can produce significantly more power over the course of a year than can fogging systems. Moreover, refrigeration systems will provide at least some cooling even when the weather is humid, although most standard refrigeration systems do suffer a loss of output under very humid conditions. This is important if your plant has a Power Purchase Agreement that has a Capacity Test. Using a refrigeration-based inlet cooling system is a great way to effectively meet your power output Capacity Test requirements. Unless your plant is in a very arid location, like Nevada or Arizona, it would be foolhardy to try to perform a contract Capacity Test with inlet fogging.

For a Merchant combined cycle plant, a refrigeration system ensures that the power output will be maximized during the hottest hours of the year, when electricity spot prices are going through the roof. If you choose the meager improvements of a fogging system, you will likely be regretting that decision during those inevitable summer heat waves that have enormous revenue possibilities.

Payback

As discussed in our page on Economics, we debunk the myth of using "payback" as a financial screening tool for determining the technology for your inlet cooling project. While inlet fogging might typically have the quickest payback, the annual revenue from the project is comparatively small. On the other hand, the annual revenue from a refrigeration technology is going to be much larger. We recommend using the NPV method of financial evaluation to compare inlet fogging with refrigeration.

Refrigeration Technologies

Vapor Compression

Vapor compression systems operate like a standard air conditioner. A refrigerant is compressed, condensed, expanded in an evaporator, and re-compressed. The evaporator may be a cooling coil in the inlet air stream, or it may be a heat exchanger that makes chilled water, which in turn is pumped to a cooling coil in the inlet air stream.

Vapor compression systems are usually driven by electric motors, which have a significant parasitic load on the power output of the site. We use a rule of thumb that states that for every 7 kW incremental station power made by the refrigeration system, about 1 kW is going back to the electric vapor compression system. If that doesn't sound like too much, just check those numbers against an "F" or "G" class gas turbine with a re-heat HRSG, and you will be shocked to see that the heat rate improvement that you were expecting is exceedingly small; and that heat rate can actually suffer on most off-design hours.

Vapor compression systems can also be driven by steam turbines and natural gas engines. The choice of a gas engine makes sense for a peaking plant; the choice of a steam turbine makes very good sense for a combined cycle plant. We are strong advocates of turbine-driven systems.

Absorption Chillers

Absorption chillers usually have a higher first cost than vapor compression systems, and typically have a lower Coefficient of Performance (COP) too. We at Polar Works believe that absorption technology has largely been overlooked in inlet chilling; and we believe that this error is about to change. Several plants have already used absorption systems successfully.

Absorption systems have very low parasitic electrical load. They can be "fired" by natural gas or by low-pressure steam. Natural gas-fired absorption chillers are a good choice for peaking stations. Steam-fired absorption chillers are a good choice for combined cycle plants, in particular, older cogeneration plants, which typically have an intermediate-pressure (150 psig) steam header available to them.

When choosing between a vapor compression system and an absorption chiller, you need to look at the number of operating hours in a year, and the lost revenue associated with the electric drives of the vapor compression system. If you do, you will see that your higher electricity revenues very quickly compensate the slightly higher first cost of absorption. If the refrigeration company from which you are soliciting quotes can't give you that kind of detailed analysis, then you are likely talking to the wrong people.

One minor drawback of absorption chillers is that they are typically larger than an equivalent vapor compression system. As space is typically at a premium on most power plant sites, this can be a disadvantage to the selection of absorption technology. Most new absorption chillers, in particular the double-effect models, have a smaller footprint than older models, somewhat alleviating this concern.

Turbo-Absorption

This concept has been around for a long time, and has been championed by GRI and others. The idea is to use a turbine-driven vapor compression system for part of the chilling load. The turbine is a backpressure design, typically operating from a high pressure of at least 125 psig, and exhausting to a low pressure of 15 psig. The low-pressure steam exhaust is fed to an absorption chiller to provide the remainder of the chilling load. This steam cycle is to refrigeration what combined-cycle is to power production.

Polar Works is an advocate of this exciting technology in our projects. Most cogeneration plants have intermediate (150 - 250 psig extraction line) and low pressure (15 psig pegging steam) steam headers, making plant integration of this technology rather straightforward. One of the largest benefits of this cycle is that it relieves the increased summertime steam load on the back end of the steam turbine and condenser. In Polar Works' turbo-absorption cycle, we can still extract the mechanical energy of this intermediate pressure steam, AND use the latent heat of steam condensation to our advantage in the absorption chiller, while decreasing your existing steam turbine's backpressure. It doesn't get any better than this. Again, if your refrigeration company can't tell you what the impact of their technology will be on your steam turbine's backpressure, then you might want to talk to the power generation specialists at Polar Works.

Thermal Energy Storage

TES can only be accomplished with a refrigeration system. See our page on this important technology, which can significantly improve the economics of a peaking power plant.