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District heat driven absorption chillers (AChs) contribute to a higher utilisation of the district heating network in summer and thus to a reduction of the relative heat losses. Due to higher investment but lower operating costs, AChs usually cover the base load, but compression chillers (CChs) are installed for peak load as well. For ACh and CCh, heat rejection is often realised via cooling towers (CTs). In addition, many systems provide the option of free cooling in winter by using the cooling towers to cover the cooling demand directly. Thus, absorption cooling (AC), compression cooling (CC) and free cooling (FC) represent three operating modes for cold generation in one overall refrigeration system. The concept of allocating base load and peak load is usually realised by activating CC when AC exceeds the capacity limit. FC is activated when the ambient temperature falls below a certain switchover value. In this study, extensive monitoring data from a large-scale field test (“Field test absorption chillers for CCHP systems”) are used to show that refrigeration systems controlled in such or similar ways have considerable potential to increase primary energy efficiency, even if the individual chillers are operated efficiently. Therefore, models are applied to the individual primary components (ACh, CCh, CT and auxiliary pumps). By using these models, thermal and/or electrical part-load efficiencies of AC, CC and FC and their specific dependencies on the ambient temperature and chilled water set temperature are calculated. Taking into account district heating and electricity primary energy factors allows to determine the operating mode with the lowest primary energy demand for each load condition measured in the field test within one year. The savings potential of 14% resulting from this example shows how promising the application of a model-based system control is compared to conventional strategies.

In the past years scientists of Technische Universitat Berlin, developed a new generation of single stage H 2 O/LiBr absorption chillers in the range of 50 to 500 kW nominal cooling capacity. In 16 installations 25 absorption chillers have been monitored within a broad field test ( F eldtest A bsorptions k alte fur Kraft-Warme-Kalte-Kopplung- S ysteme, FAkS, funded by BMWi as project FKZ 03ET1171a-d) in the years 2013 to 2018. Annual performance figures of thermal coefficient of performance ( C O P t h ) in the range of 0.7 kWh 0/kWh 2 and electrical coefficient of performance ( C O P e l ) superior to 20 kWh 0/kWh el have been proven, serving district heat (DH) return line temperatures in the range of 62 °C to 68 °C. While the FAkS project focused on installation and operation of absorption chillers themselves, the follow-up project called ReKs ( R egelung e nergie-aufwandsoptimierter K alteerzeugungs s ysteme, FKZ 03ET1583), concentrates on overall system control and efficiency. Controlling absorption chiller cooling supply at low district heat return line temperatures is extended to an energy efficient control of all participating components including hot water, chilled water, heat sink pump and cooling tower fans. Models and strategies have been developed on thermal science theory and implemented in industrial control software. Proof of concept has been evaluated by monitoring field test installations while in parallel particular field test related parameters have been varied. Without diminishing quality of chilled water temperature and/or cooling capacity, electrical energy efficiency can at least be doubled, compared to the figures of FAkS installations. Moreover, new strategies enable efficient absorption cooling with DH return line temperatures in the range of 50 °C during moderate weather conditions periods. Field test results from 2020 and 2021 are presented here.

Absorption chillers can use waste heat, solar heat, or excess heat by cogeneration facilities to supply chilled water. Therefore, absorption chillers are important components for poly-generation, which can improve plant utilization and efficiency of the overall energy supply system. Currently, 27 new small and medium-scale (30–160 kW cooling capacity) absorption chillers with high efficiency in full- and part-load as well as low specific size and weight are the focus of two research projects sponsored by German Federal Ministries. The chillers are located in 20 different sites, four of them in Jordan and the rest in Germany. The main objective of these projects is to enhance trigeneration and solar cooling system efficiency. The Jordan projects and more than 70% of the German installations use dry cooling towers. Combined heat and power (CHP) plants or district heating grids provide the driving heat for regeneration at the German installations. Solar thermal collectors drive one German and all Jordan installations. Within the projects, the chillers are provided with an intelligent control algorithm that allows achieving several objectives at the same time. One of the objectives, of course, is to match the desired chilled water temperature as well as the cooling capacity. Another objective is, for instance, to fit the hot water outlet temperature. This is commonly essential in cogeneration to avoid emergency shutdowns of the CHP engine. The field operation shows that the deviation of said temperatures from the set point is usually less than ±0.5 K in operation. In those operational hours in which the temperatures of the heat source or heat sink do not allow to reach all objectives, a set of hierarchical aims can be established within the algorithm so that the chiller can be controlled to match its prioritized objectives. Furthermore, the designed absorption chiller performs dynamically and autonomously in a very large range of temperatures and flow rates. These can vary from 20% up to 150% of nominal conditions. This flexibility can be used to achieve up to 80% savings of electrical power consumption using speed-controlled pumps in part load conditions. In contrast to the common opinion that small and medium-scale absorption chillers are not competitive as compared to compression chillers, this paper shows applications in which absorption chillers are advantageous because of synergies with and multiple benefits within the rest of the energy supply system.