Abstract
The present work aims at the thermodynamic analysis of different working pairs in adsorption heat transformers (AdHT) for low-temperature waste heat upgrade in industrial processes. Two different AdHT configurations have been simulated, namely with and without heat recovery between the adsorbent beds. Ten working pairs, employing different adsorbent materials and four different refrigerants, have been compared at varying working boundary conditions. The effects of heat recovery and the presence of a temperature gradient for heat transfer between sinks/sources and the AdHT components have been analyzed. The achieved results demonstrate the possibility of increasing the overall performance when internal heat recovery is implemented. They also highlight the relevant role played by the existing temperature gradient between heat transfer fluids and components, that strongly affect the real operating cycle of the AdHT and thus its expected performance. Both extremely low, i.e., 40–50 °C, and low (i.e., 80 °C) waste heat source temperatures were investigated at variable ambient temperatures, evaluating the achievable COP and specific energy. The main results demonstrate that optimal performance can be achieved when 40–50 K of temperature difference between waste heat source and ambient temperature are guaranteed. Furthermore, composite sorbents demonstrated to be the most promising adsorbent materials for this application, given their high sorption capacity compared to pure adsorbents, which is reflected in much higher achievable specific energy.
Highlights
The aim of the performed simulations is the evaluation of thermodynamic efficiency of adsorbent working pairs for adsorption heat transformer (AdHT) operation under different boundary conditions
To investigate the AdHT performance, the operating conditions under which the technology is working must be defined. These refer to the waste heat temperature, which is recovered from the process as well as to the ambient heat temperature, which is exploited as heat sink for the process
Similar considerations are obtained when 80 °C of waste heat temperature are available, Figure 5b,d. In this case, since the temperature difference between waste heat source and ambient heat is higher, the amount of refrigerant processed per each10cycle of 18 is higher as well, up to 52 K of gross temperature lift (GTL) can be achieved at a coefficient of performance (COP) of 0.3, even though with a very limited specific energy
Summary
Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. The thermochemical approach exploits the chemical reaction occurring between a working fluid (e.g., ammonia, water) as a refrigerant and a solid phase (e.g., salts) This technology is currently under investigation, with activities dedicated to the definition of selection criteria for working pairs [9] as well as design and testing of lab-scale prototypes, showing relevant performance in terms of coefficient of performance (COP) defined as the ratio between the upgraded heat and the heat spent to drive the process, and achievable temperature lift [6]. The main difference compared to the thermochemical approach is that the involved reaction is commonly a physical one, characterized by weak interactions (e.g., van der Waals) between the working fluid and the solid This guarantees much more stable processes as well as the possibility of effectively operate the system even at very low waste heat temperature (i.e., below 60 ◦ C).
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