Absorption Heating Technologies

A significant amount of global energy consumption is used for space heating and water heating; due to the expeditious urbanization in developing countries, energy consumption is increasing rapidly. In this chapter, the current energy usage distribution for different heating applications is presented. A review of the characteristics of conventional heating technologies are presented herein along with the proposal of a couple of advanced technologies to enhance heating efficiency. It is observed that the energy efficiency in fuel-based heating technologies could be improved by (a) upgrading/replacing the boiler, (b) condensation heat recovery from cogeneration, or (c) waste heat recovery from flue gases. Widely used vapor-compression heating systems include air source heat pumps and ground source heat pumps. However, only in a few cases could the technologies improve the energy efficiency of the heating systems and that too by a limited extent. Accordingly, there is still scope for further improvements in energy efficiency of the heating systems.

To improve the primary energy efficiencies of conventional heating systems, a cascade energy utilization principle is summarized for high-efficiency low-temperature heating systems. Absorption cycles are perfect examples of the cascade energy principle and are great options for energy-saving and emission reduction, owing to the exceptional advantages in the utilization of renewable energy and waste heat. Depending on the change in heat quantity or quality, the absorption heating technologies are classified into four main categories: heat increasing, heat shifting, temperature upgrading, and temperature adapting. The principles of various absorption heating technologies are introduced, the general characteristics of absorption working fluids are presented, the properties of the working fluids suitable for different applications are explained in detail, and the modeling methods of the absorption heating cycles are introduced, including the ideal equivalent model and the actual thermodynamic model. This chapter presents the fundamental aspects necessary to facilitate the understanding, design, analysis, and optimization of absorption heating technologies.

Three novel heating systems based on low evaporation temperature absorption heat pumps (AHPs) were proposed with an improved primary energy efficiency and reduced pollutant emissions in comparison to the conventional fuel-based heating systems. The ASAHP can be configurated as direct air source, indirect air source, spay-type air source and energy-tower air source systems, whereas the GSAHP can be configurated as buried-borehole and underground-water systems. The performance characteristics of NH₃/H₂O ASAHP under various driving source temperatures, ambient air temperatures, and produced hot water temperatures were simulated. A prototype of the low evaporation temperature AHP using NH₃/H₂O was developed to validate the energy-saving potential. Experimental investigations were conducted under standard conditions, different off-design conditions, and different adjusting methods. In addition, a comparative analysis of the heating performance (first-law and second-law of thermodynamics) and emission characteristics (CO₂ and NO_x) of the proposed heating systems to existing heating systems was conducted to determine the benefits of the modifications.

Simulations and experimental investigations on the basic absorption heat pump indicate that the heating performance (i.e., the heating capacity as well as the heating efficiency) deteriorates significantly at low ambient or low driving source temperatures. In addition, the heating efficiency stays stable or even slightly decreases when the driving temperature is higher than a certain value, leading to a waste of high-temperature heat sources. To extend the applicability and increase the efficiency, the absorption heat pumps are improved by means of advanced cycles. Different advanced cycles for theoretical improvement for lower ambient or lower driving temperatures are compared, with the compression-assisted cycle being the best solution. A prototype of compression-assisted absorption heat pump is developed and measured, indicating great potential under worse conditions. For higher driving temperatures, the generator absorber heat exchanger (GAX) cycle is studied to realize internal heat recovery and thus significantly enhance efficiency. The GAX cycle degrades to a single-effect cycle under worse conditions. Therefore, the compression-assisted GAX cycle is proposed to extend the applicable temperature range for the GAX effect. Year-round energy performance has been analyzed for all the basic and advanced cycles to identity the energy saving potentials. The advanced heat pump cycles can further improve the applicability and efficiency of the novel absorption heating technology.

For ground source electrical heat pump (GSEHP) systems used in heating-dominant regions, the heat extraction from the soil in winter is considerably higher than heat rejection into the soil in summer, which leads to underground thermal imbalance. Due to the limited temperature recovery ability of soil, the underground temperature will decrease year by year after long-term operations, causing heating performance deterioration of the heat pump unit. The existing solutions include borehole-level modification (increasing the spacing/depth/number of boreholes) and system-level modification (auxiliary heat source and energy storage), but have dissatisfactory effects (thermal balance, energy efficiency or cost-effectiveness). The novel ground source absorption heat pump (GSAHP) is proposed to address the problems. Compared to the conventional GSEHP, the GSAHP extracts less heat from the soil in winter and rejects more heat to the soil in summer, which can effectively reduce the underground thermal imbalance. In this manner, the auxiliary heat sources or heat complement devices can be reduced or eliminated. Long-term simulations indicate that the thermal imbalance ratio is kept close to 0 and the soil temperature is maintained stable in severely cold regions. In addition, the annual primary energy efficiency of the GSAHP can also be competitive due to the advantageous performance in the heating mode and the large heating load. Therefore, the novel GSAHP is quite suitable in severely cold regions.

Though the independent ground source absorption heat pump (GSAHP) is advantageous over the conventional ground source electrical heat pump (GSEHP) in extremely cold regions, some problems remain for different GSAHP applications in various climate zones. To solve these problems, the basic principles for both heat accumulation and cold accumulation occasions are established to achieve soil thermal balance and improve system performance. Based on these basic principles, we propose a series of hybrid GSAHP systems: (1) GSAHP integrated with borehole free cooling, (2) GSAHP integrated with an auxiliary cooling tower, (3) GSAHP integrated with waste heat recovery, and (4) GSAHP integrated with a conventional GSEHP. The system configurations and working principles are introduced, and the system models are built for long-term simulations. The results indicate that all propsed hybrid GSAHP systems perform well in some specific occasions. We should choose a suitable hybrid GSAHP to accommodate a certain application condition. Compared to the conventional GSEHP and independent GSAHP, the hybrid GSAHPs show better thermal balance, more stable soil temperature, higher energy efficiency, and better system economy. They can further extend the applicability of the novel GSAHP technology.

The working fluids are very important for absorption cycles. They can not only affect the heating/cooling efficiency, but also determine the operating temperature range, cycle configuration, cost-effectiveness, safety, environmental-friendliness and so on. The most widely used working fluids are H₂O/LiBr and NH₃/H₂O. H₂O/LiBr has high efficiency but suffers from crystallization and disability to operate under sub-zero evaporation temperatures. NH₃/H₂O are suitable for low evaporation temperatures, but it usually requires rectification to increase the purity of the refrigerant. In addition, the NH₃-based working fluids may not be used in some applications due to the safety concern. To address this problem, the HFC-based working fluids are alternative options. However, some HFCs need to be phased out due to their high global warming potential (GWP). Therefore, the low-GWP HFC-based and HFO-based working fluids are proposed. Advances in different novel working fluids are summarized, aiming for various objectives, including increasing efficiency, avoiding crystallization, avoiding rectification, increasing safety, as well as increasing sustainability. Noted that the introduced studies are mainly for absorption heating, with some also for absorption cooling, which can also be referenced for heating applications.

Apart from absorption cooling, absorption heating technologies are also widely used for waste heat recovery and renewable energy utilization. Waste heat recovery in district heating systems covers cogeneration heat recovery and flue gas heat recovery, which can improve the overall energy efficiency by 5–30%. Meanwhile, in industrial processes (drying, evaporation, and distillation), technologies such as closed/open absorption heat pump (AHP/OAHP), closed/open absorption heat transformers (AHT/OAHT) and absorption-compression heat pump (ACHP) are employed with various stages, effects and working fluids, to meet temperature requirements of 50–160 °C. To evaluate the efficiency of absorption heating technologies in the renewable energy field, solar heating performances between a solar air source absorption heating pump (ASAHP) and a conventional solar collector heating, are compared within ASAHP’s applicability domain (based on the temperature and radiation ranges). In addition, different geothermal heat pump systems are introduced for cascade utilization of geothermal energy. The absorption-compression heat pump (ACHP) is selected as a promising candidate, and detailed comparisons of this technology working with various novel NH₃/IL working fluids are presented. This chapter also provides many advanced prospective for a more reasonable utilization of waste heat and renewable energy sources.

In this chapter, past research on absorption heating technologies is summarized and perspectives are provided on future research and development. To further improve the performance and applicability of absorption heating technologies, future work should include: (1) the exploration and screening of more novel working fluids, (2) increasing the heat and mass transfers of key components, (3) investigating hybrid cycles that possess greater flexibility, and (4) advancing absorption thermal energy storage. These areas of research and development will be critical as absorption heating technologies play an increasingly important role in creating a low-carbon society in the future.