Felix Ziegler

Heat transfer enhancement in water when used as PCM in thermal energy storage

Efficient and reliable storage systems for thermal energy are an important requirement in many applications where heat demand and supply or availability do not coincide. Heat and cold stores can basically be divided in two groups. In sensible heat stores the temperature of the storage material is increased significantly. Latent heat stores, on the contrary, use a storage material that undergoes a phase change (PCM) and a small temperature rise is sufficient to store heat or cold. The major advantages of the phase change stores are their large heat storage capacity and their isothermal behavior during the charging and discharging process. However, while unloading a latent heat storage, the solid–liquid interface moves away from the heat transfer surface and the heat flux decreases due to the increasing thermal resistance of the growing layer of the molten/solidified medium. This effect can be reduced using techniques to increase heat transfer. In this paper, three methods to enhance the heat transfer in a cold storage working with water/ice as PCM are compared: addition of stainless steel pieces, copper pieces (both have been proposed before) and a new PCM-graphite composite material. The PCM-graphite composite material showed an increase in heat flux bigger than with any of the other techniques.

Thermodynamic based comparison of sorption systems for cooling and heat pumping

Abstract A comparison of thermodynamic performances of sorption systems (liquid absorption, adsorption, ammonia salts and metal hydrides) is carried out for typical applications (deep-freezing, ice making, air-conditioning and heat pumping) with either air-cooled or water-cooled heat sink. The results are given in terms of cooling coefficient of performance (COP) (heating COP or coefficient of amplification (COA) for the heat pump), cooling (heating) power versus reactor volume or weight and thermodynamic efficiency. LiBr–water systems show the best results for air-conditioning except when small units are required (metal hydride systems lead to more compact units). Other systems, however, show better results for other applications (chemical reaction with ammonia salts for deep-freezing, adsorption for heat pumping).

Heat-transfer enhancement by additives

Abstract A workshop on heat-transfer enhancement by additives in absorption processes was held last year at the Bavarian Center for Applied Energy Research (ZAE Bayern) in Garching, Germany, in collaboration with the Technical University Munchen. The workshop was the first in a series of Munchen Discussion Meetings, designed to bring together a limited number of researchers active in specific fields in order to facilitate not only presentations of research results but also discussion of related issues and problems. The specific purpose of the present workshop has been to establish the current state of knowledge in the area of transfer enhancement by additives and bring forth the outstanding problems so as to formulate research needs. During the 2 days of the workshop, 14 papers were presented and discussions were conducted by 34 participants from eight countries. At the conclusion of the workshop it was stated that well-known additives contribute a significant increase of heat and mass transfer in liquid sorption systems. However, from a fundamental point of view almost nothing is satisfactorily explained. On the one hand, in solid sorption systems the basic understanding of the combined heat- and mass-transfer processes seems to be better, on the other hand, the best means for enhancement is disputed much more controversially. This paper is a summary of the presentations and discussions during the Munchen Discussion Meeting with the main emphasis on liquid sorption systems.

Multi-effect absorption chillers

Abstract Multi-effect absorption heat pump cycles are promising to yield cooling performances which far exceed the performance of single-stage cycles. In this paper, several types of cycles are analysed. For this purpose, the respective multi-effect cycle is decomposed into elementary building blocks of single-stage absorption cycles with known performance. The multi-effect cycle is a superposition of the elementary cycles. Its performance is predicted by using the performance of the elementary cycles as parameters. The flexibility which is attained by the construction of multi-effect cycles from single-stage building blocks allows the design of highly efficient cycles according to the specific properties of competing working fluids. So, out of a wide range of possibilities, a quick choice of efficient cycles which fit the requirements of a specific application with todays working pairs can be made. A preliminary selection of the optimal cycle configuration can be made. Also, the potential of new working pairs can be validated. Out of a wide range of highly efficient absorption chiller cycles several promising machines are discussed.

Hydrothermal carbonization: Process water characterization and effects of water recirculation

Poplar wood chips were treated hydrothermally and the increase of process efficiency by water recirculation was examined. About 15% of the carbon in the biomass was dissolved in the liquid phase when biomass was treated in de-ionized water at 220 °C for 4 h. The dissolved organic matter contained oxygen and was partly aerobically biodegradable. About 30-50% of the total organic carbon originated from organic acids. A polar and aromatic fraction was extracted and a major portion of the organic load was of higher molecular weight. By process water recirculation organic acids in the liquid phase concentrated and catalyzed dehydration reactions. As a consequence, functional groups in hydrothermally synthesized coal declined and dewaterability was enhanced. Recirculated reactive substances polymerized and formed additional solid substance. As a result, carbon and energetic yields of the produced coal rose to 84% and 82%, respectively.

Coefficient of performance of multistage absorption cycles

Abstract A method is presented by which the coefficients of performance of advanced absorption cycles can be calculated very quickly if the efficiencies of single-stage heat pumps and heat transformers are known. The method provides detailed results for the heat input or output of the individual exchange units. These values can be used as a basis for comparison of different cycles, for numerical cycle calculations or for estimates of investment costs. Examples are given for heat pumps, refrigerators, heat transformers and heat pump transformers. Numerical results are presented for the working fluid pairs H2O/LiBr and NH3/H2O.

Experimental Investigation of a LiCl-Water Open Absorption System for Cooling and Dehumidification

In earlier work, a novel open absorption cycle was proposed, capable of producing both cooling and dehumidification for air conditioning, utilizing low-grade heat. The system, referred to as DER (Dehumidifier-Evaporator-Regenerator), uses ambient air in conjunction with an absorbent solution; the air is dehumidified and then employed to produce chilled water in an evaporative cooler. Alternatively, a portion of the dehumidified air may be used directly for air conditioning purposes. The system thus has the potential to provide both cooling and dehumidification in variable ratios, as required by the load. Computer simulations and theoretical investigations were carried out to analyze and predict the performance of the system. The objective of the present study has been to construct a laboratory system to test the concept, identify problems and carry out preliminary design optimization. The characteristic performance of individual components, analyzed theoretically in the simulation, was studied experimentally. Measurements have provided much-needed realistic data about heat and mass transfer coefficients. The performance of the system has been studied under varying operating conditions. The paper describes the operation of the experimental system and presents the measured data and the resulting transfer coefficients.

European research on solar-assisted air conditioning

Abstract The utilization of waste heat, on the one hand, and solar heat, on the other, in order to energize sorption chillers is an important issue for increasing the energy efficiency of cooling machines. Especially in the air-conditioning business, a tight correlation often exists between insolation and cooling requirements. Therefore, the interest in solar cooling by sorption systems has prevailed for several decades. Some hundred systems have been installed all over the world and operate satisfactorily. However, no really cost-competitive system for widespread application exists. A step forward can only be expected by innovative design of the collectors, chillers, or both. This paper reports on trends which might lead to such innovations. The sources are mainly papers and discussions at workshops held in 1994 in Dresden and 1995 in Freiburg and Garching, Germany. Although in these workshops mainly European experiences have been discussed, the conclusions are valid world-wide: generally, it can be stated that several new ideas, both in collectors and chillers, have gained ground. Cheaper collectors providing higher temperatures have become available and specially designed chillers for use with low-grade heat as input are being investigated. In this paper we highlight the most important developments reported on in the workshops mentioned above.

Absorption cycles. A review with regard to energetic efficiency

Abstract The improvement of energy conversion systems in order to decrease consumption of primary energy and to minimize negative impact on the environment which originates from industries with large cooling and heating demand is mandatory. We want to review the potential of absorption heat pump technology in this respect. Absorption systems are compared to conventional compression systems with regard to the consumption of primary energy. Different areas of application such as air-conditioning, refrigeration, heating, combined cooling and heating, and process heat recovery are treated. Absorption systems have a large potential for energy saving, especially when they are operated by heat from cogeneration systems or, of course, by waste heat. But even direct-fired systems can compete with compression machinery: highly efficient cycles which perform far better than comparable conventional machines are in development.

Heat of reaction measurements for hydrothermal carbonization of biomass

This paper presents a set of calorimetric measurements with the aim of better understanding the calorific nature of hydrothermal carbonization. Presented values so far show an inadequately high scatter to do so, preventing a well funded assessment of the energetic feasibility of this process. The heat released during hydrothermal carbonization at 240°C measured with the applied differential calorimetry setup is -1.06MJ/kg(glucose,daf) with a standard deviation of 14%, -1.07MJ/kg(cellulose,daf) with a standard deviation of 9%, and -0.76MJ/kg(wood,daf) with a standard deviation of 32%. These results are in good agreement with the theoretically derived maximum heat release. Despite the comparably high experimental standard deviation of these results, their accuracy is considerably higher than previously published results.