Though natural gas chillers are declining in popularity, they’re still the right choice for some facilities. Typically, they replace a gas system that failed—building managers rarely choose the technology for new installations. When replacing old equipment, you can choose to stick with a gas-fired chiller or move to an electric chiller, which might be cheaper to operate but more expensive to install than the gas technology.
In the past, gas-fired chillers compensated for their high first cost by reducing the facility’s overall demand and the associated costs. However, as electric chillers become more efficient, the potential for demand savings decreases. Despite declining gas prices, gas-fired chillers haven’t grown in popularity.
One application that can still be cost-effective is in facilities that have access to waste heat or alternative energy sources, such as digester or landfill gas. If alternative gas is clean enough and cheap enough, facilities can use it instead of natural gas. However, this approach will only work for the few buildings that are near an alternative gas source. Using waste heat with hybrid direct- or indirect-fired absorption chillers can also increase a gas-fired chiller’s cost-effectiveness. The right strategy will depend on the specifics of your facility.
What are the options?
For those looking to replace a failed gas-fired chiller with another gas-fired unit, the options are vapor compression, absorption, or hybrid chiller systems.
Vapor-compression chillers are the most common type of chillers. They use the same vapor-compression cycle as electric chillers, but instead of an electric motor, they use reciprocating gas or diesel engines or a gas turbine (figure 1). If the chiller uses reciprocating engines, it uses a reciprocating compressor. These types of chillers deliver up to 125 tons cooling capacity. Gas turbines can be configured with either rotary screw compressors (which deliver about 20 to 450 tons of cooling capacity), or centrifugal screw compressors (which deliver about 350 to 5,000 tons).
Rather than using a mechanical compressor to drive a vapor-compression cycle, absorption chillers use a thermochemical compressor (figure 2). This technology relies on an absorption cycle, which uses two fluids: a refrigerant and an absorbent. The refrigerant in an absorption chiller dissolves into an absorbent solution—water is usually the absorbent and two common refrigerants are lithium bromide and ammonia. An electric pump moves the absorbent solution into a generator, where heat drives the refrigerant vapor out of the solution and into the evaporator. Because absorption chillers rely on thermal energy instead of vapor compression, they use much less electricity than vapor-compression chillers.
Absorption chillers can be direct- or indirect-fired. Direct-fired chillers contain a burner that runs on natural gas or another fuel to produce the heat required for the absorption process. Indirect-fired chillers use steam or hot water produced externally by a boiler or cogeneration system. Piping and heat exchangers transfer the heat to the chiller.
Absorption chillers can also be single- or multiple-effect. Single-effect absorption chillers run one refrigeration cycle and multiple-effect absorption chillers use two or more refrigeration cycles. In multiple-effect chillers, high-temperature thermal energy drives the first cycle, and lower-temperature energy from the previous cycle’s condenser drives the subsequent cycles. Multiple-effect chillers are more efficient than single-effect chillers, but they require a much hotter source of thermal energy. Single-effect chillers require hot water ranging from 160° to 200° Fahrenheit, but double-effect chillers use either direct heat from a gas flame or high-pressure steam. Multiple-effect chillers are also much more expensive—they can be double the initial cost of single-effect chillers. Single-effect and indirect-fired are the most common absorption chillers because of the lower first cost.
Combining electric and natural gas chillers in the same plant can help reduce equipment costs and operating expenses. Facility managers alternate between gas and electric chillers in hybrid systems to use the least-expensive energy source. For example, the electric chillers would operate only when inexpensive off-peak electric rates were available. When expensive on-peak electric rates applied, the facility would run the gas-fired equipment instead. In some instances, the facility might run its electric and natural gas chillers simultaneously to meet peak cooling loads.
How to make the best choice
Determining what to replace
Decide whether to upgrade auxiliary equipmentBefore purchasing a new unit, analyze your current chiller and auxiliary equipment to determine whether it’s most cost effective to tune or upgrade auxiliary equipment, replace the chiller unit, or do both. Though it’s more work initially to conduct this thorough analysis, you can save a substantial amount on installation and equipment costs by making an informed decision about what to change in your system. Computer simulations can help you determine which approach is best for your application by thoroughly analyzing how different equipment combinations will perform.
Decide between a natural gas and electric chiller A chiller’s operating cost depends on gas and electricity prices, the equipment’s efficiency, the facility’s cooling loads, and the operating hours. When deciding between gas and electric, you also have to consider the cost of upgrading the facility for an electric chiller. Switching from a gas to an electric chiller usually doesn’t require structural upgrades because the electric units are smaller and lighter than gas-fired equipment and have no exhaust. However, to support the higher electric load, you may have to upgrade the electrical service within the building and, in some cases, the distribution feeding the building. Sometimes the local utility will pay for this upgrade. These projects vary in cost, but a site-specific survey will help you estimate the expense.
Choosing the best natural gas chiller for your application
If you decide to go with a gas-fired chiller, consider these factors before purchasing new equipment. For more details on electric chiller selection, see the guide on centrifugal and screw chillers.
Cooling loads Before sizing your chiller, reduce cooling loads as much as possible. Load-reduction measures such as lighting retrofits not only save energy, they also reduce cooling loads. Other measures that can reduce cooling loads include retrofits that improve the efficiency of office equipment, building shell, and windows. By minimizing your cooling load, you can purchase the smallest possible equipment for your facility.
Annual chiller energy performance The performance of gas-fired chillers is usually rated in terms of coefficient of performance (COP)—the cooling output in Btu divided by the energy input in Btu (figure 3). The higher the COP, the more efficient the unit is. Because chiller efficiency varies depending on the operating load, determining annual energy performance can be tricky. Either assume the most common cooling loads and equipment efficiencies, or use building energy-simulation software for a more nuanced analysis. Running multiple simulation scenarios can help sort out which combination of chiller technologies (vapor compression, absorption, or electric), capacities, control strategies, and equipment configurations will minimize your operating costs.
Figure 3: Minimum efficiency requirements for natural gas chillers
|Equipment type||Compliance (COP)|
|Notes: COP = coefficient of performance; FL = full-load performance requirements; IPLV.IP = part-load performance requirements.||© E Source|
|Air-cooled absorption, single effect||≥0.600 FL|
|Water-cooled absorption, single effect||≥0.700 FL|
|Absorption double effect, indirect fired||≥1.000 FL
|Absorption double effect, direct fired||≥1.000 FL
Equipment cost Large scale cooling equipment is expensive, and gas-fired cooling equipment is considerably more expensive than electric. Equipment costs vary between $300 and $1,000 per ton of cooling capacity:
- Absorption chillers cost between $500 and $750 per ton
- Engine-driven chillers cost up to $1,000 per ton.
- Electric equipment costs between $300 and $500 per ton
Gas-fired systems will often also require larger cooling towers and condenser water pumps, further increasing system costs.
Electric energy and demand savings The lion’s share of the savings associated with gas-fired cooling equipment typically comes from reduced electric demand charges. Electric demand charges vary depending on the time of day, season, and the utility, and range from $0 to $25 per kilowatt per month. Because electrical demand and consumption costs vary, a hybrid system may be the most cost-effective option for some facilities. You can operate the chiller with the cheapest fuel as costs shift.
Fuel costs Even when natural gas prices are low, they often make up the majority of a gas-fired chiller’s life-cycle costs. The biggest challenge in estimating fuel costs is predicting how gas and electricity costs will vary during the equipment’s life. Natural gas costs peaked in 2008 and have fallen dramatically since then. However, some forecasts predict a slow and gradual increase in prices going forward. Another factor that influences this cost is the availability of alternative fuels, such as clean digester or landfill gas, or waste heat from an industrial process. Facilities that can use alternative fuels save on natural gas costs.
Maintenance costs It costs about $0.01 more per ton-hour to maintain vapor-compression chillers compared to electric chillers; however, facilities with on-site maintenance personnel will have lower maintenance costs. For absorption chillers, the maintenance costs are usually comparable to those for an electric chiller because they can operate with little to no maintenance for a long time. However, if you neglect the equipment, the costs can be up to one-third more than an electric chiller because they require specialized maintenance.
As gas-fired chillers become less popular, the number of personnel trained to maintain them declines. This could make it difficult to find service people when you need them. And gas-fired chiller systems are more dependent on proper maintenance than electric ones. According to manufacturers, small maintenance lapses can hinder the performance of gas-fired equipment more than they would affect the performance of electric chillers. Fortunately, most chiller manufacturers offer maintenance contracts that ensure local, factory-trained mechanics will perform all maintenance and overhauls. If you plan to use this strategy, be sure to include these expenses in the life-cycle cost analysis.
Recovered heat savings You can use thermal energy recovered from a vapor-compression chiller for space, water, or process heating. The waste heat from absorption chillers isn’t as hot, which makes it more difficult to use cost-effectively.
Costs for emissions reduction In some areas, vapor-compression chillers may require prohibitively expensive emissions controls. Absorption chillers don’t require any emissions controls in the US.
Noise-reduction costs Vapor-compresson chillers typically have noise levels ranging from 93 to 98 decibels from a distance of three feet. Manufacturers of these chillers usually sell sound-reduction enclosures, which can reduce noise levels to around 86 to 89 decibels. Absorption and electric units range from about 80 to 89 decibels. For comparison, a gas-powered lawn mower has a decibel level of about 90.
What’s on the horizon?
With the decline in US sales, development of new gas-fired chiller technology in the US has virtually stopped. However, development may continue in Asia, which currently accounts for 80% of the gas-fired chiller market.
Who are the manufacturers?
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