• Energy Research

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.

Enhancing the heat transfer mechanism by increasing the heat exchanger surface area is a standard way to overcome low heat transfer on the gas side of heat exchangers. Different geometrical shapes, for example, plain, wavy, or interrupted fin geometries for plate-fin or tube-fin heat exchangers, are used for this task. Wire structures with dimensions in the submillimeter range are already used in regenerators for their heat capacity, but are rarely used in recuperators as heat transfer enhancers. New textile developments enable the fabrication of adapted structures with irregular grid sizes, and purpose-built for heat exchanger application. These wire structures allow for enlarging the heat transfer surface area, decreasing material utilization, and enabling flexibility of different geometrical dimensions. Possibilities for manufacturing and design selection are studied in the project, EffiMet, and thereafter at Fraunhofer ISE for the implementation of highly efficient heat exchanger geometries based on wire structures.

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.

Abstract A main focus of recent R&D on adsorption modules for thermally driven heat pumps and chillers has been to enhance the volume specific power output while maintaining a reasonable coefficient of performance (COP). An adsorption module using a new type of heat exchanger based on aluminum sintered metal fiber structures brazed on flat fluid channels has been developed. The heat exchangers for adsorber/desorber and evaporator/condenser are identically constructed. The adsorption heat exchanger is coated with a silico-alumino phosphate (SAPO-34) by a partial support transformation direct crystallization (PST) [1]. Both components are placed in a vacuum tight housing using a valve-free configuration. Water is used as adsorptive. The experimental characterization of the module shows a high volume specific power (up to 82 W/litre module for cooling, 320 W/litre for heating). Although no heat is recovered between ad- and desorption cycle, a COP of almost 0.4 is reached for cooling and 1.4 for heating. Driving temperature differences are defined for the analysis of the heat exchanger performance. The evaporator/condenser shows extremely good performance with about 240 W/K specific evaporation power per litre of heat exchanger, while the adsorber is limiting the module performance.

In adsorption heat pumps, the adsorbent is typically combined with heat conducting structures in order to ensure high power output. A new approach for the direct integration of zeolite granules into a copper structure made of short copper fibers is presented here. Zeolite NaY granules with two different grain sizes are coated with copper fibers and powder and sintered to larger structures. The sorption dynamics of these structures were measured and evaluated in terms of heat and mass transfer resistances and compared to the loose grain configuration of the same material. We found that the thermal conductivity of such a composite structure is approximately 10 times higher than the thermal conductivity of an adsorbent bed with NaY granules. Sorption equilibrium measurements with a volumetric method indicate that the maximum uptake is not altered by the manufacturing process. Furthermore, the impact of the adsorbent–metal structure on the total thermal mass of an adsorption heat exchanger is evaluated. The price of the superior thermal conductivity is a 40% higher thermal mass of the adsorption heat exchanger compared to the loose grain configuration.

Abstract In this paper we focus on the differentiation and quantification of different heat and mass transfer phenomena governing the overall sorption dynamics, for the example of a binder-based aluminium fumarate (Alfum) coating for heat transformation applications with water as refrigerant. The methodological emphasis is on extending the volume swing frequency response (FR) method to problems with strong heat transfer limitation. The heat and mass transfer parameters are mapped to the sample temperature and loading state, in order to be able to reproduce the strongly non-linear behaviour exhibited under application conditions. Based on a model with discretised heat transfer and linear driving force (LDF)-simplified micropore diffusion, the thermal conductivity of the samples was identified as about 0.07 W/(m K), and the LDF time constant between 0.1 and 3 s–1 at 40°C with a U-shaped loading dependency and an Arrhenius-type temperature dependency. The method is validated by comparing a measured large temperature jump experiment to the results from a non-linear simulation informed solely by these parameters obtained from the new FR-based method.

Enhanced heat transfer surfaces allow more energy-efficient, compact and lightweight heat exchangers. Within this study, a method for comparing different types of enhancement and different geometries with multiple objectives is developed in order to evaluate new and existing enhancement designs. The method’s objectives are defined as energy, volume, and mass efficiency of the enhancement. They are given in dimensional and non-dimensional form and include limitations due to thermal conductivity within the enhancement. The transformation to an explicit heat transfer rate per dissipated power, volume, or mass is described in detail. The objectives are visualized for different Reynolds numbers to locate beneficial operating conditions. The multi-objective problem is further on reduced to a single-objective problem by means of weighting factors. The implementation of these factors allows a straightforward performance evaluation based on a rough estimation of the energy, volume, and mass importance set by a decision maker.