In last month’s column, I discussed upgrading low-temperature heat to create useful high- or medium-temperature heat for process demands (“Take a Look at Thermocompressors”). Here, I’ll take a different approach to using low-temperature heat — creating cold (chilled-water or refrigeration). When I started my career, I worked as an applications engineer for a small company (Energy Concepts) that targeted projects for effectively capturing low-temperature waste heat and using that heat to provide cooling within the plant. I was very fortunate to learn not just the concepts and theory but also the practical applications of these systems in the field. You might already have guessed it — I am talking about absorption systems.
Chilling and refrigeration almost always comes from mechanical vapor compression systems. These systems have four basic processes — evaporation, compression, condensation and throttling — that create cold and reject heat to ambient via cooling towers. The electric motor driving the refrigerant compressor uses a large amount of electrical energy to provide the cooling effect. Alternatively, there are several applications where steam backpressure or condensing turbines direct-drive the refrigeration compressor.
In absorption systems, the basic processes are evaporation, absorption, generation, condensation and throttling. The key difference between the two systems is the “driving methodology” — absorption systems use heat while vapor compression systems use shaft power.
Commercially available absorption systems fall into two main types — lithium bromide/water systems and ammonia/water systems. In lithium bromide/water systems, water is the refrigerant; however, it’s unable to provide cold below 40°F. In ammonia/water systems, ammonia is the refrigerant and, generally, temperature isn’t a constraint. Depending on the application, I have seen both types successfully applied in petrochemical plants and refineries.
Absorption systems can be very niche technologies. Applications range from simple chilled water production to complex process-integrated refrigeration for debottlenecking compressors, dropping salable product from fuel gas streams, etc. Sometimes absorption systems are used during peak operating conditions, e.g., to reduce air inlet temperature of a gas-turbine cogeneration unit during hot summer months. I touched upon this specific application in a previous column on cogeneration, see “Understand Cogeneration, Part 1,” and “Part 2.”
Because heat is the primary driver for absorption systems, the temperature at which heat is available plays a key role. You potentially can have direct-fired absorption systems but that’s not my focus here. Instead, I am targeting that area of heat (thermal energy) within the plant that’s generally rejected or more importantly, can’t be used at the temperature at which it’s available. I would like to use that waste heat to fire an absorption system and generate chilling/refrigeration.
So how do plant engineers determine if an absorption system is a good fit for their site? Often, a pinch analysis, thermal system optimization or an energy assessment identifies the need for a possible absorption system application. Nevertheless, the most common questions you should ask to determine if an absorption chiller/refrigeration system would be a good application are:
1) Do you have a need for cold? If so, at what temperature do you need this process chilling/refrigeration?
2) Is there a source of waste heat — low temperature (250–300°F or higher) from either flue/stack gas, plant processes and fluid streams, vented steam, etc.?
3) Could a process be debottlenecked and improve yield/throughput by providing extra cooling?
If the answers to the above questions are “yes,” then your first-level due-diligence should note that application of absorption systems warrants further investigation at the site. You will then get into sizing of the system, cold temperature requirements, physical proximity of where waste heat can be recovered versus where chilling/refrigeration is needed, the type of absorption system and feasibility/economics. This can sometimes become a long process due to a lack of understanding of absorption systems and their applications. Clearly, it will be a capital investment; current market-driven economics such as low electric and gas prices may make justifying an absorption system difficult. Nevertheless, from a perspective of energy efficiency, greenhouse gas emission reductions and sustainability, you can make a strong case for absorption systems.
I would encourage you to investigate and evaluate the potential applications of creating cold from waste heat within your plants and optimize their thermal energy systems.
Riyaz Papar, PE, CEM, is director, Global Energy Services, at Hudson Technologies Company, Pearl River, N.Y. He has more than 20 years of experience in industrial energy systems and with best practices. He also is a U.S. Department of Energy (DOE) Steam Best Practices senior instructor and a DOE steam energy expert. He has provided energy consulting services in 100+ industrial plants in the U.S. and internationally. You can email him at email@example.com.