M.S. Thesis Presentation by Beth Lilly
Wednesday, March 4, 1998

(Dr. Sam Shelton, advisor)

"Analysis and Performance of Gas-Electric Hybrid Chiller Systems"


Deregulation of the natural gas and electric utility industry along with the advancement of gas cooling equipment has prompted HVAC designers and building operators to consider alternatives to the typical electric chiller system. Price fluctuations between peak demand periods and off peak periods for both gas and electricity are expected to increase. These price fluctuations for gas and electricity are out of phase. This suggests studying combinations of different types of chillers.

When considering commercial building chiller systems comprised of two equal size chillers, the base loaded chiller provides 90 percent of the building's annual cooling load, while the peaking chiller provides only 10 percent.

The trend in deregulated energy rates combined with the different load characteristics for base and peaking chillers suggests looking at hybrid systems, which use a combination of electric and gas chillers. This is the primary focus of this study.

Using a building cooling load profile for a 400,000 square foot hotel or other 24 hour/day facility, actual weather data, and electric and gas rates for Atlanta, Georgia, a computer simulation model for a 1000 ton chiller/cooling tower system is developed. A 1000 ton electric chiller, a 1000 ton single stage absorption chiller, a 1000 ton double stage absorption chiller, and a 1000 ton gas engine driven chiller are first modeled. For hybrid systems, 500 ton chillers of each of these chiller types are also modeled and used in several combinations with each other. Different operating strategies are also considered.

The annual energy consumption and the annual energy costs for ten chiller systems are calculated using deregulated gas rates and hourly real-time-pricing electric rates representative of expected deregulated electric rates. Equipment capital costs and annual maintenance costs are also considered.

The results indicate that most of the multiple chiller systems have lower energy costs than the single chiller systems, primarily due to reduced cooling tower pump and fan energy use. With the real time electric rates, the total gas fired absorption systems offer little or no operating costs advantage over total electric chiller systems. The systems that use a gas engine driven chiller alone or in combination with an electric chiller show dramatically reduced energy costs. This is particularly true when their engine heat recovery option is utilized. These chillers, however, have much higher capital costs than the electric chillers.

Even though the systems using engine chillers show much lower energy costs, they have long capital cost payback periods. These engine systems also have high maintenance costs due to the high maintenance aspect of the engine.

The energy purchasing flexibility of a hybrid gas/electric system, however, may be advantageous in the future deregulated environment. Energy prices will be volatile, and predicting future gas/electric prices is problematic. Even today in Atlanta, the hourly real time pricing electric rates which will be in effect tomorrow are not known until after 4:00 PM today. This makes chiller system design optimization problematic.