• Energy Research

Metal-organic frameworks (MOFs) can be beneficial for heat transformation applications due to their potentially high water uptake and tunable working temperature levels. Although the hydrothermal stability has been assessed in some cases in terms of maximum water uptake and structural changes, there is no data on the impact of hydrothermal stress tests on adsorption dynamics. However, to maintain the designed heating or cooling power in the application, the hydrothermal stability in terms of both water uptake and adsorption dynamics is decisive. To close this gap, we present experimental data for the comprehensive evaluation of hydrothermal stability for three different MOFs and the commercially available zeotype TiAPSO. The hydrothermal stress test includes around 70,000 temperature swing cycles on aluminium sheets with a binder-based coating of different adsorbents. As a novelty of this study, adsorption dynamics are determined before and after the hydrothermal stress test using effective thermal resistances and the characteristic temperature difference. Our results show degradation in terms of a decrease in uptake around 5–10% after hydrothermal stress test for all samples. Under temperature boundary conditions relevant for the application, MIL-160(Al) shows even a drastic uptake reduction of around 35–45%. Except for CAU-10-H, none of the adsorbents show a degradation in terms of increased heat and mass transfer resistance. In case of CAU-10-H, the overall effective heat and mass transfer resistance increases by around 30–40% after the hydrothermal stress test. These results indicate that the hydrothermal stability of MOFs must be assessed in terms of both, uptake and adsorption dynamics, to ensure stable long-term performance in real-world devices. Applied Thermal Engineering, 227 ISSN:1359-4311 ISSN:1873-5606

The thermal masses of components influence the performance of many adsorption heat pump systems. However, typically when experimental adsorption systems are reported, data on thermal mass are missing or incomplete. This work provides original measurements of the thermal masses for experimental sorption heat exchanger hardware. Much of this hardware was previously reported in the literature, but without detailed thermal mass data. The data reported in this work are the first values reported in the literature to thoroughly account for all thermal masses, including heat transfer fluid. The impact of thermal mass on system performance is also discussed, with detailed calculation left for future work. The degree to which heat transfer fluid contributes to overall effective thermal mass is also discussed, with detailed calculation left for future work. This work provides a framework for future reporting of experimental thermal masses. The utilization of this framework will enrich the data available for model validation and provide a more thorough accounting of adsorption heat pumps.

The integration of Photovoltaic (PV) modules within the outer structure of commercial transporters offers a great potential as an additional source of energy that can be utilized, for example, directly by the drive train of the vehicle, or by accessory equipment. However, to understand the benefits of such a system, the effect and relevance of some thermodynamic variables introduced by the Vehicle Integrated Photovoltaic (VIPV) system must be assessed. Particularly, the share of the solar irradiance that cannot be converted into electricity by the solar cell and is instead transformed into heat must be considered for certain applications such as the transportation of goods that require a controlled temperature. A one-dimensional thermal simulation model based on a Resistance-Capacitance methodology was created and validated experimentally. The model was used to predict the thermal behavior of the box-body of a truck for a representative year in three cities in Europe (Stockholm, Freiburg, and Seville), with a known Bill of Materials (BOM), and a set of given assumptions and constraints. It was found that, under the simulated conditions laid in this study, the additional heat generated by the PV modules that manages to go through the insulation material and reach the refrigerated cargo area, may raise the temperature of the contained air by an average of 0.36 °C, 0.5 °C, and 0.67 °C, respectively for the observed cities, and as much as 3.12 °C, 2.98 °C and 2.61 °C. When forced convection caused by the movement of the truck was applied into the model (with a constant wind speed of 50 km/h), the temperature of the solar cell dropped significantly, which, for the case of Freiburg, meant an increase in the contained air temperature of a maximum of 0.6 °C. A refrigeration system was subsequently considered for a target air temperature of 2 °C and -18 °C, and it was found that for the simulated scenarios, the harvested solar energy could easily offset the additional energetic demand caused by the PV system, and, in most cases, completely balance out the total annual demand of the chiller.

The building sector is accountable for roughly one third of global energy- and process-related greenhouse gas emissions. Besides space heating, domestic hot water (DHW) heating contributes substantially to energy consumption and related greenhouse gas emissions in the building sector. Depending on the DHW system design and its required supply temperature, heat losses make up around 30…60 % of the energy required for DHW heating. To decrease energy consumption and reduce associated greenhouse gas emissions, it is essential to minimize heat losses and implement efficient DHW concepts. Heat pumps can potentially reduce energy consumption, but their efficiency strongly depends on the DHW system design and its required supply temperature. These aspects are evaluated by performing a comprehensive analysis of different DHW concepts utilizing heat pumps. Based on annual heat demands extracted from one year measurement data of a typical multi-family house in Germany, simulations of six different DHW concepts are performed. Our findings reveal that decentralized DHW systems or low system temperatures (48 °C), e.g. in combination with ultrafiltration for legionella treatment in centralized DHW systems, can lead to a substantial reduction in heat losses for DHW preparation of around 25 % and in final energy of 20 % compared to the reference case. In large systems the share of losses should be kept below 30 % by reducing pipe lengths and equipping the most distant tapping points with direct electric water heaters.

Thermally-driven heat pumps can help to mitigate CO2 emissions by enhancing the efficiency of heating systems or by driving cooling systems with waste or solar heat. In order to make the thermally-driven systems more attractive for the end consumer, these systems need a higher power density. A higher power density can be achieved by intensifying the heat and mass transfer processes within the adsorption heat exchanger. For the optimization of this key component, a numerical model of the non-isothermal adsorption dynamics can be applied. The calibration of such a model can be difficult, since heat and mass transfer processes are strongly coupled. We present a measurement and simulation procedure that makes it possible to calibrate the heat transfer part of the numerical model separately from the mass transfer part. Furthermore, it is possible to identify the parts of the model that need to be improved. For this purpose, a modification of the well-known large temperature jump method is developed. The newly-introduced measurements are conducted under an inert N2 atmosphere, and the surface temperature of the sample is measured with an infrared sensor. We show that the procedure is applicable for two completely different types of samples: a loose grains configuration and a fibrous structure that is directly crystallized.

Adsorption chillers can substitute the electric energy demand of conventional compression chillers with lowtemperature heat. The efficiency and power density of adsorption chillers depend on their adsorbent and refrigerant, which form the working pair. Novel working pairs are therefore actively developed and characterized in dynamic small-scale experiments using a characteristic time constant. However, this evaluation by only few characteristic time constants, does not resolve the trade-off between efficiency and power density of adsorption chillers and novel materials are challenging to compare due to varying size, shape, and density. To fill these gaps, we investigate the trade-off between efficiency and power density by quantifying all characteristic time constants and transferring them to specific mean powers. We perform Large-Temperature-Jump experiments with water as the refrigerant on the two metal–organic frameworks CAU-10-H and aluminum fumarate for a chilling/recooling/regeneration temperature triple of 10/30/80 °C. We compare the metal–organic frameworks’ specific mean powers to the commercially available RD type silica gels Siogel and SG123. Siogel performed best in terms of area-specific mean powers with a maximum value of 3.2 kW/m2. For low time constants up to 20 % relative loading, corresponding to high power density but lower efficiency, Siogel also provided the highest volumespecific mean power at 8.1 MW/m3. CAU-10-H had the highest mass- and volume-specific mean powers at 19.0-19.9 kW/kg and 7.2-8.6 MW/m3 for characteristic time constants of 35 % relative loading and higher. Aluminum fumarate and SG123 showed low specific mean powers for all chosen characteristic time constants. The results show that studying specific mean powers is a useful tool to benchmark adsorption working pairs as they would perform in an adsorption chiller, regardless of mass, shape, or density.