• Energy Research

Abstract The design of adsorptive heat transformers (adsorption heat pumps and chillers and adsorption thermal energy storage) requires knowledge on the heat and mass transfer resistances in adsorbents. However, heat and mass transfer cannot be distinguished in conventional experimental setups, since only pressure data is available. In this work, we present an approach to distinguish and quantify heat and mass transfer resistances in adsorbents. For this purpose, we extended the Large-Temperature-Jump method (LTJ) with an infrared camera (IR) and combined the new IR-LTJ method with dynamic modeling. The IR camera determines the surface temperature of the adsorbent as an additional information. Subsequently, the data from the IR-LTJ setup is used in dynamic models to quantify time-resolved heat and mass transfer coefficients. We conducted experiments for one layer of granulated Fuji Siogel for use in an adsorption chiller with the temperature set 10/30/70. We show that the suggested method is able to determine heat and mass transfer coefficients.

Abstract Adsorptive drying systems aim at short drying times and small packaging. Thus, fast heat and mass transfer and high uptakes of the adsorbent are required. For this purpose, novel adsorbents are developed. To evaluate novel adsorbents for drying applications, we propose a simple two-step model-based method using equilibrium data and a dynamic process model. Based on a case study for an adsorption dishwasher, we determine the trade-off between adsorber unit size and drying times for Zeolite 13X, Silica Gel 125, SWS-1L and SBA-15 E120. For the adsorption dishwasher, SWS-1L is found to be a very promising adsorbent. Employing SWS-1L instead of Zeolite 13X in an adsorption dishwasher reduces the amount of adsorbent by 50%. Even though the adsorbent mass is lower, the drying speed is still the fastest among the studied adsorbents. The proposed two-step model-based evaluation method enables the identification of promising adsorbents for adsorptive drying systems.

Abstract Adsorption thermal energy storage is investigated for heat supply with cogeneration in industrial batch processes. The feasibility of adsorption thermal energy storage is demonstrated with a lab-scale prototype. Based on these experiments, a dynamic model is developed and successfully calibrated to measurement data. Thereby, a reliable description of the dynamic behavior of the adsorption thermal energy storage unit is achieved. The model is used to study and benchmark the performance of adsorption thermal energy storage combined with cogeneration for batch process energy supply. As benchmark, we consider both a peak boiler and latent thermal energy storage based on a phase change material. Beer brewing is considered as an example of an industrial batch process. The study shows that adsorption thermal energy storage has the potential to increase energy efficiency significantly; primary energy consumption can be reduced by up to 25%. However, successful integration of adsorption thermal storage requires appropriate integration of low grade heat: Preferentially, low grade heat is available at times of discharging and in demand when charging the storage unit. Thus, adsorption thermal energy storage is most beneficial if applied to a batch process with heat demands on several temperature levels.

Adsorption chillers provide sustainable cooling from waste or solar heat. However, adsorption chillers currently show limited performance. To increase the performance, new working pairs and adsorber geometries are constantly proposed. Evaluating the performance of new working pairs and adsorber geometries requires time and large amounts of the material. To reduce time and material needs, a method is presented to reliably predict the heat flows in the adsorber, specific cooling power (SCP), and coefficient of performance (COP) in an adsorption chiller from only 1 g of adsorbent material. For this purpose, the small‐scale Infrared‐Large‐Temperature‐Jump experiment is combined with a full‐scale adsorption chiller model. The adsorption chiller model allows determining time‐resolved heat flows, SCP, and COP. The prediction results are compared with a full‐scale experiment of an adsorption chiller. For various process conditions, the prediction is highly reliable with average deviations of 18.5% for the heat flows, 1.4% for the SCP, and 7.0% for the COP compared with the experiment. The presented method allows a comprehensive and reliable evaluation of new working pairs and adsorber designs from only small amounts of the adsorbent material, thus guiding material improvements at an early stage of development.

The metal–organic frameworks (MOFs) MIL‐101(Cr) and NH2‐MIL‐125 offer high adsorption capacities and have therefore been suggested for sustainable energy conversion in adsorption chillers. Herein, these MOFs are benchmarked to commercial Siogel. The evaluation method combines small‐scale experiments with dynamic modeling of full‐scale adsorption chillers. For the common temperature set 10/30/80 °C, it is found that MIL‐101(Cr) has the highest adsorption capacity, but considerably lower efficiency (−19%) and power density (−66%) than Siogel. NH2‐MIL‐125 increases efficiency by 18% compared with Siogel, but reduces the practically important power density by 28%. From the results, guidelines for MOF development are derived: High efficiencies are achieved by matching the shape of the isotherms to the specific operating temperatures. By only adapting shape, efficiencies are 1.5 times higher. Also, higher power density requires matching the shape of the isotherms to create high driving forces for heat and mass transfer. Second, if MOFs’ heat and mass transfer coefficients could reach the level of Siogel, their maximum power density would double. Thus, development of MOFs should go beyond adsorption capacity, and tune the structure to the application requirements. As a result, MOFs could to serve as optimal adsorbents for sustainable energy conversion.

Abstract A model-based method is presented to rigorously assess and compare adsorber-bed designs. Starting from an initial design, a dynamic model is developed. The initial design is then varied using the dynamic model to determine the Pareto frontier regarding two objectives: coefficient of performance (COP) and specific cooling power (SCP). To ensure that we assess the intrinsic properties of the design and not effects of poor control, we use dynamic optimisation to determine optimal controls for each design. The obtained Pareto frontier allows to compare the adsorber-bed designs. The proposed method is exemplified in a case study: for a finned-tube design, we assess two major design choices, namely grain size of the adsorbent pellet and fin number. The proposed method is shown to allow for a rigorous and efficient assessment of adsorber-bed designs.