One of the most efficient applications of cogeneration technology occurs where heat is provided in the winter, and cooling energy in summer. Thermal chillers are the device by which heating is turned into cooling. Wes Livingston discusses technology options, including adsorption chillers.
The average mechanical chiller consumes enough electricity in one cooling season to generate 1 million lbs (454 tonnes) of CO2 emissions from a coal power plant. Those emissions can be eliminated by using a thermal chiller with a source of hot water such as a cogeneration system. According to the EPA, in the US alone, it has estimated the potential for waste heat recovery could substitute approximately 9% of US energy usage, or 1.4 quadrillion BTU.
We all know distributing excess heat from a neighborhood power plant to homes and businesses nearby can greatly increase the efficiency of the system when there is a demand for heating. But what happens when cooling is needed instead of heating?
The answer is trigeneration with a thermal chiller. These chillers range in size from 2 tons (1.8 tonnes) of refrigeration to several thousand tons and can be installed individually at each end user’s building or centrally located at the district cooling level. The technology is proven and simple, and there are many different versions of these products available to fit almost any application. Recent advancements in adsorption chiller technology make them cost effective and reduce maintenance and upkeep costs significantly compared with other thermal chillers.
HOW THERMAL CHILLERS WORK
A thermal chiller operates by evaporating the refrigerant (usually water) over the evaporator tube bundle which contains chilled water. As the water evaporates, it chills the tubes and chills the water inside the tubes. The refrigerant water vapour migrates to an area of low pressure generated by a desiccant that readily absorbs moisture; either a liquid or a solid. The higher vapour pressure in the evaporator keeps refrigerant vapour moving to the lower pressure desiccant. The entire system is kept under vacuum so that water will boil on the evaporator tubes at temperatures as low as 35oF (1.6oC) .
As the desiccant becomes saturated with water vapour, it must be regenerated by a heating source. This can be hot water, steam, or a burner built into the chiller. Liquid desiccants are usually regenerated continuously, while solid desiccants are usually regenerated in a two-stage batch process.
The hot water used to regenerate the desiccants may come from any number of sources, including waste heat from industrial processes, prime heat from solar thermal installations or from recovered exhaust, and water jacket heat from a power generator. Process and industrial sources include food and beverage processing; chemical, plastic, rubber, paper and cement manufacturing; as well as the waste heat from steam boilers or sterilizers used in hospitals, hotels and school and corporate campuses.
The heat extracted from the system is rejected to a cooling tower which supplies 85oF (29.4oC) cooling water. Temperatures above this can severely derate the capacity of thermal chillers. Therefore, air-cooled thermal chillers are rare, but they are available for very small applications.
In summary, the heat is used to dry out the desiccant, and the dry desiccant is used to create a very low vapour pressure area to attract and hold the water vapour that evaporates off the chilled water tube bundle. As long as the vapour pressure in the desiccant is lower than the evaporator pressure, the cooling process will continue.
TYPES OF THERMAL CHILLER
Thermal chillers have been around for a long time and have proven to be reliable when controlled properly. Their construction is very robust and long lasting due to the need to maintain a vacuum.
The desiccants most commonly used are liquid salts such as lithium bromide or lithium chloride, and a few manufacturers use ammonia. These liquid desiccants lend themselves to compact, internal heat recovery heat exchangers. But these liquid desiccants are all either very corrosive or dangerous to humans. Chillers with liquid desiccants are referred to as absorption chillers.
Other chiller manufacturers use solid desiccants such as zeolites or silica gels. These are known as adsorption chillers. They operate on a batch process with usually two chambers of desiccant which alternate from adsorbing refrigerant to desorbing it by adding heat. Since one of the two chambers is always in cooling mode, continuous cooling is provided.
Adsorption chillers use low-grade waste heat to produce cooling
Adsorption chillers offer high reliability and eliminate corrosion issues. While they are usually larger than liquid desiccant chillers and have a higher first cost, recent technological advancements have made adsorption chillers more cost effective than when first introduced, and they are able to operate with lower hot water temperatures than absorption chillers. They provide other advantages as well, which will be explained shortly.
WHY RECOVER WASTE HEAT?
When a source of free hot water is available, as it is in cogeneration systems, the electrical consumption saved by using a thermal chiller instead of a mechanical chiller can be very significant. Since there is no compressor, liquid desiccant chillers simply require a circulating pump to move the desiccant through the chiller’s internal heat exchangers. The electrical consumption of absorption chillers is about 0.03 kW per ton, while the solid desiccant adsorption chillers consume even less, about 0.004 kW/ton.
Compared to mechanical chillers that typically consume about 0.65 kW/ton, the electrical savings are tremendous. As stated earlier, a typical mechanical chiller operating simply for comfort cooling will consume enough electricity to cause 1 million Ibs of carbon dioxide to be emitted from a coal power plant. Or, if the chiller were to run continuously, that figure would be 4 million lbs of carbon dioxide emissions per year.
An absorption chiller will eliminate 95% of those emissions, and an adsorption silica gel chiller will eliminate 99%. When a low cost source of hot water is available, the operating costs of a thermal chiller will be far less than a mechanical chiller as well.
SELECTING A CHILLER
When selecting a chiller, the total life cycle cost should be calculated. This will include the initial cost of the equipment, installation costs, annual maintenance costs, electrical operating costs, hot water or steam consumption costs, the overall life expectancy of the chiller, plus its final removal and disposal costs. In general, solid desiccant adsorption chillers will have a higher first cost, but lower maintenance and operating costs than liquid desiccant absorption chillers. But, only liquid desiccant chillers are currently available in double-effect or triple-effect configurations that operate with steam.
In addition, any thermal chiller may qualify for Leadership in Energy and Environmental Design (LEED) scheme points in the US, since they require no HFC refrigerants.
The design of most modern trigeneration systems today assumes all available heat should be used to produce electricity, and only the low grade waste heat should be used for building heating. This dictates any exhaust gases leaving the generator should be used to generate steam, and that steam should be used to operate a secondary steam turbine before exiting as waste hot water. which should then be used for building heating (or cooling) purposes.
By using all the available heat to generate electricity, the cost per kilowatt will be reduced. And by using a solid desiccant silica gel chiller that can operate with hot water temperatures down to 140oF (60oC) or lower, even more cooling can be obtained from a given amount of fuel. The additional latent heat of water vapour in the exhaust gases can now be captured with a hot water heat recovery coil and put to useful work.
SINGLE-EFFECT VS. DOUBLE-EFFECT
Thermal chillers are either single-effect or double-effect. Double-effect chillers require moderate temperature steam to regenerate the desiccant. In trigeneration systems, however, this steam could otherwise be used to generate electricity. Since double-effect chillers have a higher temperature fuel source, they are more efficient than single-effect chillers. As the hot refrigerant is driven off by the hot steam, it passes through a heat exchanger to regenerate a second, cooler batch of desiccant. By using the hot refrigerant as this secondary free heat source, the efficiencies of these machines can reach a coefficient of performance (COP) of 1.2, or even higher.
If the hot water source is low-temperature steam or hot water, then a single-effect chiller is the only option. Their efficiencies range from 0.4 COP for some residential units up to about 0.7 COP for high-quality, commercial-grade products with internal heat recovery devices. Absorption and adsorption single-effect chillers will have approximately the same efficiencies, but adsorption silica gel chillers can operate with lower temperature hot water.
Manufacturers are developing triple-effect chillers, though none are currently on the market. And to operate, they will require high pressure steam or a direct fuel source with their own burners. Efficiencies are expected to be around 1.5 COP.
ADSORPTION VERSUS ABSORPTION CHILLERS
Thermally driven chillers have historically all been liquid desiccant absorption systems, primarily using lithium bromide as the desiccant. They have proven to be effective, but have been burdened with significant maintenance and upkeep costs. Lithium bromide absorption chiller systems depend on a corrosive solution of lithium bromide salt that tends to corrode the internal copper tubing and steel shell of the unit in the presence of any air leakage. Additionally, absorption chillers produce hydrogen gas as a by-product, requiring a palladium cell inside the chiller unit to remove the hydrogen.
The lithium bromide solution in absorption chillers also has a tendency to solidify within the system if the regeneration temperature becomes too hot or too cold, or the conditions change too rapidly for the system to adapt. Some installations of absorption units require a dedicated caretaker to maintain them. Distilled water is used as the refrigerant. And a circulating pump continuously moves the salt solution around the various heat exchangers inside the chiller.
These factors have limited the application of absorption chillers in many trigeneration projects. But overall, absorption technology has proven itself to be reliable maintained properly.
Conversely, adsorption chiller technology is relatively new. There are just a few hundred installations of adsorption chillers worldwide, compared to several tens of thousands of absorption chillers.
Adsorption chillers have no liquid desiccants at all. By not using chemicals such as lithium bromide and ammonia, the potential for hazardous material leaks, aggressive corrosion, and chemical testing requirements are eliminated. Also, adsorption chillers use municipal tap water as the refrigerant, compared to absorption chillers that require distilled water.
An adsorption chiller significantly reduces the maintenance and upkeep costs by substituting the corrosive salt desiccant with benign silica gel or zeolite. Reliability and machine availability are therefore significantly improved. Lastly, silica gel chillers are able to start up almost immediately and shutdown without special preparation, while liquid absorption chillers can take up to 30 minutes to start and should not be shutdown without a proper shutdown cycle.
THERMAL VS CONVENTIONAL MECHANICAL CHILLERS
All thermal chillers have very few moving parts and do not require the maintenance and attention that mechanical chiller systems require. Thermal chillers also eliminate CFC and HFC refrigerants entirely. And when partnered with a waste source of heat, they can eliminate millions of pounds of carbon dioxide emissions per year.
Thermal chillers also eliminate high amperage electrical connections and operate very quietly since they do not require mechanical compression. They can be installed outdoors in a neighbourhood and will generate no noise complaints.
DISTRIBUTED COOLING VS. LOCALIZED COOLING
When designing a chilled water system, you must decide if the chiller(s) will be installed at each building or on a district scale.
For systems that constantly circulate hot water or steam all year, the distributed chiller method will probably make the most sense. The utility will provide hot water, and building owners will provide their own chiller and have control over their chilled water supply.
Alternatively, the utility could provide hot water in winter and chilled water in the summer through the same two-pipe system. The system would usually be switched seasonally, and the utility will decide when to change everyone from hot water to chilled water. This is common on some university and corporate campuses.
By alternating chilled water and hot water seasonally, the first cost is significantly reduced compared to a four-pipe system that has two supply lines and two return lines to provide constant hot and chilled water. However, occupant comfort will be reduced during the change-over period for a two-pipe system. Some days may need heating and others need cooling, but the system will usually take several hours or days to change over.
Many large campuses have two-pipe change-over systems and just accept some level of occupant discomfort. Or, they provide chilled water well into the heating season and allow tenants to use electric space heaters for a few weeks to stay comfortable. With individual control, the occupants can maintain their own peak comfort. Plus, the hot water utility can sell hot water all year.
RENEWABLE SOLAR ENERGY SYSTEMS
The latest generation solid desiccant silica gel chillers can operate with low temperature hot water obtained from low-temperature exhaust gas condensers or solar thermal collectors mounted on the roofs and sides of buildings. If the size of the adsorption chiller requires more hot water in summer than can be provided by the electric generator, solar thermal collectors can be used to provide the extra heat since they perform best on sunny days when chilled water demand will also be the highest.
Evacuated tube and quality flat panel collectors have been used on projects with hundreds of tons of chilling capacity. But concentrating parabolic or systems with mirrors are not required. By using non-concentrating, passive collectors, the systems can operate satisfactorily, even on cloudy days.
In summary, building owners and facility managers are installing trigeneration systems that run on natural gas, biomass, and diesel that use the waste heat recovered from the water jacket and exhaust gases to operate a thermal chiller.
By integrating a waste heat recovery system with on-site power generation, the system will significantly reduce carbon dioxide emissions by eliminating the chilling system’s electrical consumption, as well as eliminating heating requirements in winter. Trigeneration systems can have fuel efficiency rates of 85-95% and are becoming attractive as the new wave of thermal chillers becomes known to the market.
Wes Livingston is with Power Partners, manufacturer of ECO-MAX adsorption chillers, Athens, Georgia, US.Email: firstname.lastname@example.org
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