Design of a water-ammonia heat transformer with an ejector for low-pressure steam generation in extended operating range
Absorption heat transformers (AHTs) can contribute to the decarbonization of the industrial sector thanks to their capacity to upgrade intermediate-temperature waste heat to higher temperatures, suitable for industrial processes, while rejecting part of it to a lower temperature sink. Additionally, AHTs use much less electricity than heat pumps of the same heating capacity. The European project ZIMBA aims to develop a single-stage water-ammonia AHT for steam production driven by waste heat. The system is designed to deliver 15 kW?? of steam at 1.3 bar(a) while working with a heat source at 80 °C and a heat sink at 20 °C. To extend the operability range, the integration of an ejector to decouple generator and condenser pressures is also introduced. In this work, two cycle configurations are evaluated through numerical simulations under varying cold sink temperatures to assess system performance. The most promising configuration showed that at a sink temperature of 10 °C, the system achieves an electrical COP of 43.6, thermal COP of 0.47, and 15.1 kW?? of thermal output. At 35 °C, the identified cut-off condition, performance decreases significantly, with an electrical COP of 18.5, thermal COP of 0.38, and thermal output of 9.64 kW??. This decline is mostly dependent on the increase in condensation pressure, which limits ammonia desorption in the generator. These performances result from optimal concentration operation for each sink temperature; however, near cut-off, optimal concentration shifts significantly towards higher ammonia concentration, and without separate water and ammonia storage, hardly implementable in a heat transformer cycle, cut-off performance deteriorates further. But thanks to the ejector, at 35 °C, the most promising configuration achieves an electrical COP of 21.08, a thermal COP of 0.30, and 9.37 kW?? at a limited rise of ammonia concentration.