E. Pfender

Abstract: THIS review surveys results that have been published in various fields of heat transfer during 1975. As in the past, the number ofpapers published during that period was such that only a selection can be included in this review. A more complete listing is available in the heattransfer bibliographies published periodically in this journal. will also be published in the Journul of Heat Transfer. The 1975 International Solar Energy Congress and Exposition was organized by the International Solar Energy Society from 28 July to 1 August at Los Angeles, California. Heat-transfer topics are found interwoven in many of the papers presented at the Conference.

Abstract: Recently the synthesis of ultrafine powders has drawn considerable attention because these powders can have physical and chemical properties superior to those of bulk specimens. In particular, emphasis is on the preparation of ultrafine ceramic powders for their applications in the development of sintered products having higher density and lower sintering temperature. One such industrially important ceramic material is SiC. In many cases ultrafine SiC has been prepared by thermal plasma processes [1]. However, these processes require one or more reactive/corrosive gases. This letter describes a new method to overcome such problems. For this purpose thermally pretreated rice hull was used as the solid precursor. Rice hull, an agricultural waste product, was first used by Cutler [2] as a starting material for the production of silicon carbide. Since the rice hull route promises to be most economical, much attention has recently been paid to it [3-6]. Rice hull consists of silica in hydrated amorphous form, and cellulose which yields carbon when thermally decomposed. The relative abundance of silica with very high surface area in close proximity to active carbon in rice hull makes it amenable for SiC formation during the pyrolysis process. The possible reaction was given by Lee and Cutler [7] as

Abstract: Rice husk (RH) has now become a source for a number of silicon compounds, including silicon carbide, silica, silicon nitride, silicon tetrachloride, zeolite, and pure silicon. The applications of such materials derived from rice husks are very comprehensive. The methods of synthesizing these silicon-based materials from RHs and their applications are reviewed in this paper.

Abstract: Recently, much attention has been focused on the utilization of plant biomass to produce engineering materials, encompassing the technological/scientific aspects as well as the economic, environmental and social issues. Amongst the variety of agricultural wastes or biomasses available, rice-husk (RH) occupies a preeminent position, not only in terms of its amount produced worldwide, but also because of its unique chemistry-related features. In this sense, some attempts have been made to produce advanced materials – including SiO 2 , SiC, Si 3 N 4 , elemental Si, Mg 2 Si and more recently, active carbon –, using RH. The production of those mentioned advanced materials depends largely on the treatments used (physical and/or chemical) and the reactions involved in the process, such as pyrolysis, carbothermal and reduction processes. In this contribution, a critical review on the processing and application of rice husks (RHs) for the production of various silicon-based materials and of active carbon is presented. The review addresses the different processing methods, the effects of various process parameters on the pyrolysis stage, the influence of physical, chemical and thermal treatments, activating conditions and activated carbon consolidation mechanisms. A flow chart with all the possible routes to produce SiO 2 was purposely constructed.

Abstract: Lignocellulosics represent the organic matter produced by trees. Biopolymers such as cellulose, hemicellulose and lignin are the major macromolecular constituents of ligneous cell walls which are distinguished by a hierarchical fibrilar composite micro structure. Fundamental aspects of anatomy of wood and molecular structure of wood cell wall affecting the bioorganic–inorganic conversion process are reviewed. Basic approaches to convert the native biopolymeric materials into non-oxide as well as oxide ceramic products include: (i) pyrolytic decomposition resulting in a porous carbon replica (template) which may subsequently be reacted to form carbide phases or may be infiltrated with non-reacting sols or salts which can further be processed to yield oxide reaction products; (ii) infiltration of chemically preprocessed native lignocellulosic products with gaseous or liquid organometallic and metalorganic precursors and subsequent oxidation to remove the free carbon phase. Conversion of native (wood tissue) lignocellulosics into ceramics with a microstructure pseudomorphous to the bioorganic template anatomy offers a great potential for designing novel ceramics with anisotropic cellular morphologies. These might be of interest for applications as high temperature resistant exhaust gas filters and catalyst carriers in energy, environmental and automotive industries, bioinert and corrosion resistant immobilization supports for living cells, microbes, or enzymes in biotechnology and medicine.

Abstract: Models describing the transient behavior of phase-change energy storage (PCES) units are presented. Simulation techniques are used in conjunction with these models to determine the performance of solar heating systems utilizing PCES. Both air-based and liquid-based systems are investigated. The effects of storage capacity, storage unit heat transfer characteristics, collector area and location on the system performance are investigated for systems utilizing sodium sulfate decahydrate and paraffin wax as storage media. Optimum ranges of storage sizes are recommended on the basis of systems' thermal performance. Comparison is made between systems utilizing PCES and those using sensible heat storage, viz. rock beds in air-based systems and water tanks in liquid-based systems. The variation of the solar supplied fraction of load with storage size and collector area is given for systems utilizing both types of storage. The effects of location and collector energy loss coefficient on the relative performance of PCES and sensible heat storage are also investigated.

Abstract: Building-Integrated Concentrated Photovoltaic (BICPV) systems integrate easily into built environments, replacing building material, providing benefits of generating electricity at the point of use, allowing light efficacy within the building envelope and providing thermal management. This paper presents a novel experimental evaluation of phase change materials (PCM) to enhance performance of low-concentration BICPV system via thermal regulation. Previous studies have primarily focussed on temporal and spatial studies of PCM temperature within the BIPV systems but the current work also discusses the effect of PCM on electrical parameters of the BICPV systems. Due to the inadequacy of the earlier reported model, a new analytical model is proposed and implemented with the in-house controlled experiments. Paraffin wax based RT42 was used within an in-house designed and fabricated PCM containment. An indoor experiment was performed using highly collimated continuous light source at 1000 W m −2 . Results show an increase in relative electrical efficiency by 7.7% with PCM incorporation. An average reduction in module centre temperature by 3.8 °C was recorded in the BICPV–PCM integrated system as compared to the naturally ventilated system without PCM. Studies showed that PCM effectiveness varies with irradiance; an increase in relative electrical efficiency by 1.15% at 500 W m −2 , 4.20% at 750 W m −2 and 6.80% at 1200 W m −2 was observed.