With the Boston Zoning Commission’s recent code amendments requiring all buildings over 50,000 square feet to meet the “certified” level of the U.S. Green Building Council’s Leadership in Energy and Environmental Design (LEED) program, as well as an overall increase in environmental awareness, more and more building owners and managers are considering environmentally friendly options for their facilities.
One aspect of a commercial building’s design that can significantly impact the level of sustainability it offers is the building’s heating and cooling system. While buildings traditionally have featured a heating and cooling system that requires large amounts of electricity to operate, in addition to chemical refrigerants and other pollutants, a new “green” alternative – called a direct-fired absorption chiller – is available and currently gaining popularity. Absorption chillers provide an environmentally friendly option and offer financial and operating advantages, too.
Ideal for buildings 100,000 square feet or larger, the direct-fired absorption machine can provide chilled water for cooling and hot water to heat a building. Absorption chillers are environmentally friendly for a variety of reasons. First, the absorption chiller uses water as a refrigerant, whereas traditional electrically driven chillers use fluorocarbon-based fluids, which have the potential to deplete ozone or promote global warming if leaked into the environment. Also, if there were a refrigerant leak in the mechanical room, the fluorocarbon-based fluid from the traditional chiller system would displace oxygen and harm workers. Due to this risk, traditional chillers necessitate the installation of refrigerant-leak detection, emergency breathing apparatus, alarm systems and mechanical room emergency exhaust systems. Alternatively, using water as a refrigerant is an environmentally friendly way to cool buildings. It is a benign, safe and readily available substance with no risk of environmental pollution or oxygen depletion.
The absorption chiller also is a green alternative because it can operate with a variety of energy sources including natural gas, oil, solar power, steam, biogas or industrial exhaust heat. Traditional chillers use mechanical compressors to operate and therefore use large amounts of electricity to produce cooling. Because electric utilities generate power by burning coal or oil, the traditional chiller can adversely impact the environment. Coal is the front-runner in environmentally unfriendly ways to generate electricity, with the burning of oil not far behind.
Absorption chillers use a heat and pressure differential to push and pull water through the system. Rather than a traditional machine that needs electricity to operate the chiller and either electricity, gas or oil to power the boiler for heating, absorption chillers require a minimal amount of electrical power to function. Direct-fired absorption machines can develop cooling by utilizing solar energy parabolic collectors, power generation or industrial waste heat (steam, hot water, exhaust) or biogas – all three of which are considered 100 percent renewable energy sources. Natural gas also is often used as a fuel in absorption chillers and is considered to be 60 percent green due to its clean burning characteristics.
The ABCs
The basic operating principles of an absorption machine using water as a refrigerant are the capture, release and recapture of water by another fluid, and the evaporation and condensing of water, under varying pressures and temperatures.
Refrigeration cycles are based on the observable fact that evaporation of a fluid produces a cooling effect, much like the cooling sensation that a small amount of alcohol brushed on the hand produces due to the absorption of heat from the skin as the alcohol evaporates. The absorption chiller generates cooling chilled water by evaporating, or boiling, water. The water is placed in a low-pressure sealed chamber so that the water boils at 40 degrees Fahrenheit, versus the 212 degrees Fahrenheit at which water boils under ordinary pressure conditions.
Heating water is produced by the addition of another heat exchanger or coil in the generator chamber. A mixture of lithium bromide and the water captured by the chiller is pumped to the generator chamber. Heat is added from an external source (solar energy, gas, steam, etc.) and the captured water is released from the lithium bromide solution in the form of a hot vapor.
The hot water vapor travels to the condenser chamber where it is cooled back to a liquid. A cooling coil inside the condenser chamber causes the vapor to condense on its outside surface, while cooling tower water passes through the inside of the coil and absorbs the heat from the hot water vapor. As with electrically driven chillers, the cooling tower water is circulated through the external cooling tower water loop, which functions as an independent heat-removal system.
The condensed liquid water is routed by way of piping from the condenser chamber to the low pressure environment of the evaporator chamber where the water is sprayed on the outside of a coil and evaporates at about 40 degrees Fahrenheit, cooling the chilled water flowing inside the coil to approximately 45 degrees Fahrenheit by the heat-removal process of the evaporation. The evaporated water vapor is then drawn back to the absorber chamber where it is recaptured by its affinity to lithium bromide to complete the cycle.
Saving Greenbacks
When calculating installation costs, building owners must consider the infrastructure required by the electric utility company to power a traditional chiller versus a direct-fired absorption system. For a typical office building of 100,000 square feet, the approximate size of the chiller would be about 250 tons. Using a traditional, electrically powered chiller, the electrical requirement would be 100 kilowatts. The present NSTAR Electric and Gas Co. cost for electrical infrastructure necessary to provide the required power is $1200 per kilowatt. In this case, the cost to the building owner to prepare the building for the installation of an electrically driven chiller would be roughly $120,000.
In contrast, the 250-ton, direct-fired absorption chiller is a heat-operated system with a much smaller electrical requirement of approximately 10 kilowatts, which brings the cost of the electric utility infrastructure down dramatically. The gas company also may offer financial incentives for building owners who use absorption chillers and generate cooling with natural gas because they can sell more gas in the summer. It is not unusual for the gas company to prepare the building for a gas-fired absorption chiller by installing the gas line from the street to the building meter with no cost to the owner.
While absorption chillers are more expensive than traditional chillers, the upfront expenditure allows building owners to save money on operating costs over time, all while lessening the impact the building makes on the environment through energy conservation and avoidance of pollutants. The direct-fired absorption cooling and heating system provides other benefits as well. An absorption machine can generate cooling and heating hot water and no additional boiler is required, so there is just one piece of equipment to maintain and, therefore, mechanical room floor space is saved.
In this age of environmental awareness due to the proven effects of global warming, the direct-fired absorption chiller provides building owners and managers with a “greener” option to heating and cooling their properties.
