Lena Schnabel - Academia.edu (original) (raw)

Papers by Lena Schnabel

In the last years intensive research work on the topic of adsorption storage and heat pump system... more In the last years intensive research work on the topic of adsorption storage and heat pump systems has been carried out at Fraunhofer ISE. The results of this work showed the need for not only optimising the adsorber but also the evaporator. For most of the materials used in adsorptive cooling machines with solar or waste energy, water is used as refrigerant. Water is evaporated in a temperature range between 4 an 20 °C, which corresponds to a pressure of 0.81 to 2.34 kPa. Even though there is extensive literature on evaporation of water, data on the characteristics of water evaporation under vacuum conditions are still scarce. In 2006 a test rig, usable for characterisation of different heat exchanger structures under vacuum conditions, was build. Finned and tube bundle heat exchanger were characterised and compared at different evaporation rates and temperature conditions. The results of the "evaporator test rig" are influenced by internal and external heat transfer characteristics of the heat exchanger, geometrical factors and operational conditions. These results are helpful and necessary on the component level, but not detailed enough for optimisation of heat exchanger surfaces. Therefore in 2007 a complementary test rig was designed and set up. The "boiling test rig" allows the evaluation of boiling on different surfaces. Various structured surfaces can be fixed on an electrical heater, the vapour sink is realised by condensation. Design, measuring principles and first results will be reported in the paper.

Applied Thermal Engineering, 2014

ABSTRACT The present work aims at the experimental evaluation of the kinetic performance achievab... more ABSTRACT The present work aims at the experimental evaluation of the kinetic performance achievable by an innovative binder-based coating developed for application in adsorption chillers or heat pumps. It employs a commercial SAPO 34 as adsorbent material and a clay as binder. Samples with different thickness were prepared at the CNR-ITAE labs and their adsorption kinetics has been tested with a volumetric apparatus based on the Large Pressure Jump (LPJ) approach, available at the Fraunhofer ISE labs. For comparison purposes, also two representative loose adsorbent grain configurations, namely monolayer and multilayer, have been tested. The experimental evolutions have been fitted by means of a heat and mass transfer model, which allowed the determination of the effective diffusion coefficients and the effective heat conductivity as a function of the thickness of the samples. The effective diffusion coefficients showed a linear increase with the square of the coating thickness, which demonstrates that mass transfer is mainly limited by diffusion inside adsorbent grains. Finally, employing the obtained effective parameters, half-cycle simulations (adsorption after temperature jump) in real operating conditions of an adsorption chiller have been carried out for the tested adsorber configurations. A coating thickness of 0.6 mm was identified to be the most promising in terms of achievable Volumetric Specific Cooling Power (VSCP).

Energy Procedia, 2014

Solar thermal cooling has difficulty to emerge as an economically competitive solution for small ... more Solar thermal cooling has difficulty to emerge as an economically competitive solution for small systems mainly due to high investment cost and system complexity . Therefore developments of principally new system solutions are needed. This paper describes such a solution with sorption modules directly integrated into a solar thermal collector. The focus of the work has been to find key parameters influencing the module and the system efficiency. A validated model of a sorption collector has been implemented into a simplified system deck providing cooling, heating and domestic hot water to static loads in the system modelling environment TRNSYS [2]. Simulations have been conducted for different boundary conditions and at two locations with different climatic conditions. Thermal losses from the collector as well as internal thermal losses inside the collector were found to be the most important parameters influencing efficiency. The system simulations gave overall thermal cooling efficiencies in the range of 0.12-0.27 and electrical cooling efficiencies in the range of 15-45 depending on collector technology, climate and control strategy.

Applied Thermal Engineering, 2009

The performance of thick aluminophosphate molecular sieve layers for heat exchanger applications ... more The performance of thick aluminophosphate molecular sieve layers for heat exchanger applications is evaluated. The aluminophosphate AlPO-18 (AEI structure type code) molecular sieve sorbent is coated on aluminium supports prior the sorption measurements. Two AlPO-18 samples with different morphological appearance, i.e. nano-sized crystals with monomodal size distribution and micron-sized crystals of varying sizes, are used to prepare layers with thickness in the range of 80-750 lm. As a binder component, polyvinyl alcohol (PVOH) was utilized in order to prepare mechanically stable layers, which are mechanically stable over numerous measuring cycles. The sorption measurements are conducted under canonical conditions at 40°C. The AlPO-18 layers showed decreased mass flows with increasing the thickness. Additionally, the layers comprising nanosized crystals showed higher equilibrium loadings and faster kinetics compared to films based on micron-sized crystals. Following the kinetic studies of pressure, temperature and heat flow, it can be concluded that the heat transport is the rate limiting mechanism for thick aluminophosphate layers. Importantly, the diffusion limitation plays a role only for relatively thin microporous aluminophosphate layers (<200 lm). Below this thickness complete heat transfer is achieved within 2 min which allows fast heat exchanger cycles. Thus, the application of microporous aluminophosphate layers for heat transformation and storage applications is considered possible.

Applied Thermal Engineering, 2010

... Temperatures were measured with Ni 100 thin film sensors, which have short reaction times and... more ... Temperatures were measured with Ni 100 thin film sensors, which have short reaction times and relatively ... was about 1020 mg, therefore, was comparable to that of the directly crystallized zeoliteX sample. ... The results of the adsorption kinetics experiments are depicted in [Fig. ...

In the last years intensive research work on the topic of adsorption storage and heat pump system... more In the last years intensive research work on the topic of adsorption storage and heat pump systems has been carried out at Fraunhofer ISE. The results of this work showed the need for not only optimising the adsorber but also the evaporator. For most of the materials used in adsorptive cooling machines with solar or waste energy, water is used as refrigerant. Water is evaporated in a temperature range between 4 an 20 °C, which corresponds to a pressure of 0.81 to 2.34 kPa. Even though there is extensive literature on evaporation of water, data on the characteristics of water evaporation under vacuum conditions are still scarce. In 2006 a test rig, usable for characterisation of different heat exchanger structures under vacuum conditions, was build. Finned and tube bundle heat exchanger were characterised and compared at different evaporation rates and temperature conditions. The results of the "evaporator test rig" are influenced by internal and external heat transfer characteristics of the heat exchanger, geometrical factors and operational conditions. These results are helpful and necessary on the component level, but not detailed enough for optimisation of heat exchanger surfaces. Therefore in 2007 a complementary test rig was designed and set up. The "boiling test rig" allows the evaluation of boiling on different surfaces. Various structured surfaces can be fixed on an electrical heater, the vapour sink is realised by condensation. Design, measuring principles and first results will be reported in the paper.

Applied Thermal Engineering, 2014

ABSTRACT The present work aims at the experimental evaluation of the kinetic performance achievab... more ABSTRACT The present work aims at the experimental evaluation of the kinetic performance achievable by an innovative binder-based coating developed for application in adsorption chillers or heat pumps. It employs a commercial SAPO 34 as adsorbent material and a clay as binder. Samples with different thickness were prepared at the CNR-ITAE labs and their adsorption kinetics has been tested with a volumetric apparatus based on the Large Pressure Jump (LPJ) approach, available at the Fraunhofer ISE labs. For comparison purposes, also two representative loose adsorbent grain configurations, namely monolayer and multilayer, have been tested. The experimental evolutions have been fitted by means of a heat and mass transfer model, which allowed the determination of the effective diffusion coefficients and the effective heat conductivity as a function of the thickness of the samples. The effective diffusion coefficients showed a linear increase with the square of the coating thickness, which demonstrates that mass transfer is mainly limited by diffusion inside adsorbent grains. Finally, employing the obtained effective parameters, half-cycle simulations (adsorption after temperature jump) in real operating conditions of an adsorption chiller have been carried out for the tested adsorber configurations. A coating thickness of 0.6 mm was identified to be the most promising in terms of achievable Volumetric Specific Cooling Power (VSCP).

Energy Procedia, 2014

Solar thermal cooling has difficulty to emerge as an economically competitive solution for small ... more Solar thermal cooling has difficulty to emerge as an economically competitive solution for small systems mainly due to high investment cost and system complexity . Therefore developments of principally new system solutions are needed. This paper describes such a solution with sorption modules directly integrated into a solar thermal collector. The focus of the work has been to find key parameters influencing the module and the system efficiency. A validated model of a sorption collector has been implemented into a simplified system deck providing cooling, heating and domestic hot water to static loads in the system modelling environment TRNSYS [2]. Simulations have been conducted for different boundary conditions and at two locations with different climatic conditions. Thermal losses from the collector as well as internal thermal losses inside the collector were found to be the most important parameters influencing efficiency. The system simulations gave overall thermal cooling efficiencies in the range of 0.12-0.27 and electrical cooling efficiencies in the range of 15-45 depending on collector technology, climate and control strategy.

Applied Thermal Engineering, 2009

The performance of thick aluminophosphate molecular sieve layers for heat exchanger applications ... more The performance of thick aluminophosphate molecular sieve layers for heat exchanger applications is evaluated. The aluminophosphate AlPO-18 (AEI structure type code) molecular sieve sorbent is coated on aluminium supports prior the sorption measurements. Two AlPO-18 samples with different morphological appearance, i.e. nano-sized crystals with monomodal size distribution and micron-sized crystals of varying sizes, are used to prepare layers with thickness in the range of 80-750 lm. As a binder component, polyvinyl alcohol (PVOH) was utilized in order to prepare mechanically stable layers, which are mechanically stable over numerous measuring cycles. The sorption measurements are conducted under canonical conditions at 40°C. The AlPO-18 layers showed decreased mass flows with increasing the thickness. Additionally, the layers comprising nanosized crystals showed higher equilibrium loadings and faster kinetics compared to films based on micron-sized crystals. Following the kinetic studies of pressure, temperature and heat flow, it can be concluded that the heat transport is the rate limiting mechanism for thick aluminophosphate layers. Importantly, the diffusion limitation plays a role only for relatively thin microporous aluminophosphate layers (<200 lm). Below this thickness complete heat transfer is achieved within 2 min which allows fast heat exchanger cycles. Thus, the application of microporous aluminophosphate layers for heat transformation and storage applications is considered possible.

Applied Thermal Engineering, 2010

... Temperatures were measured with Ni 100 thin film sensors, which have short reaction times and... more ... Temperatures were measured with Ni 100 thin film sensors, which have short reaction times and relatively ... was about 1020 mg, therefore, was comparable to that of the directly crystallized zeoliteX sample. ... The results of the adsorption kinetics experiments are depicted in [Fig. ...