Comparison of natural convection heat exchangers for solar water heating systems
Thermosyphon heat exchangers are used in indirect solar water heating systems to avoid using a pump to circulate water from the storage tank to the heat exchanger. In this study, the authors consider the effect of heat exchanger design on system performance. They also compare performance of a system with thermosyphon flow to the same system with a 40W pump in the water loop. In the first part of the study, the authors consider the impact of heat exchanger design on the thermal performance of both one- and two-collector solar water heaters. The comparison is based on Solar Rating and Certification Corporation (SRCC) OG300 simulations. The thermosyphon heat exchangers considered are (1) a one-pass, double wall, 0.22 m{sup 2}, four tube-in-shell heat exchanger manufactured by AAA Service and Supply, Inc., (the Quad-Rod); (2) a two-pass, double wall, 0.2 m{sup 2}, tube-in-shell made by Heliodyne, Inc., but not intended for commercial development; (3) a one-pass, single wall, 0.28 m{sup 2}, 31 tube-in-shell heat exchanger from Young Radiator Company, and (4) a one-pass single-wall, 0.61 m{sup 2}, four coil-in-shell heat exchanger made by ThermoDynamics Ltd. The authors compare performance of the systems with thermosyphon heat exchangers to a system with a 40 W pump used with the Quad-Rod heat exchanger. In the second part of the study, the effects of reducing frictional losses through the heat exchanger and/or the pipes connecting the heat exchanger to the storage tank, and increasing heat transfer area are evaluated in terms of OG300 ratings.
Solar Thermophilic Anaerobic Reactor (STAR) for Renewable Energy Production
Liquid and solid cattle manures are major waste streams in Egypt . The main objective of this research was maximising the net energy production from these wastes by using a solar energy heating system. High concentration of ammonia can strongly affect the gross methane production via inhibition of methanogenesis and reduced hydrolysis. The latter is only limited addressed so far in literature and therefore taken as a second objective of this study. To be able to design a solar thermophilic anaerobic reactor (STAR) and its pumping system, the effect of temperature on the rheological properties of liquid cattle manure was measured experimentally. The effect of temperature and temperature fluctuations on the performance of completely stirred tank reactor (CSTR) systems operated at 50 o C and 60 o C treating liquid cattle manure was studied also experimentally. The temperature fluctuations of the reactor content in a STAR system, designed without extra equipment for heating during the night, shown to be in a range, which will not significantly affect the biological activity in the reactor. Simulations were carried out to study the effect of the interaction between different reactor volumes and different insulation materials on the net thermal energy production from the STAR system at Egyptian climatic conditions. Using a flat plate solar collector, with areas equal to the reactor's cross section area, improves the efficiency for large reactors only slightly, while for small reactors a large improvement is achieved. The feasibility of a non-mixed accumulation system (AC) for treatment of solid manure was studied experimentally at 40 and 50 o C and filling time of 60 days using a 10% (V/V) inoculum placed on the reactor bottom. The results showed a distinct stratification of intermediates along the reactor height. Leachate recirculation and distribution of the inoculum with the feed could improve the system performance: higher methane production with less stratification extent. The results of a mechanistic model, describing the biological process in AC system including the dispersion of intermediates and bacteria species between the system layers showed a very well agreement with the experimental data of the methane production. Finally, a design of a STAR for solid cattle wastes was studied in a model. The simulation results show that a minimum reactor temperature of 44.5 during summer and 47.6 during winter could be achieved for a well-insulated reactor with an aspect ratio of 0.6. The effect of different ammonia concentrations on the hydrolysis constant of liquid cattle manure was also studied experimentally at 50 and 60. Negative linear relations between k h and both total and free ammonia concentrations were found at each studied temperature. From a literature review it becomes clear that next to animal manure rice straw is an important waste stream from the Egyptian agriculture. Combined composting of digested manure mixed with rice straw could produce an attractive organic fertiliser. Based on emergy analysis of the system, the STAR concept can be recommended from the sustainability point of view.
Substation automation based on IEC 61850 with new process-close technologies
New technologies in primary equipment like nonconventional instrument transformers require new interfaces to the substation automation system. The future standard IEC 61850 perfectly supports this requirement. With the introduction of the new technologies and the use of IEC 61850, a more decentralized architecture of the substation automation system will be possible. This provides several benefits but a careful system design is required to maintain the overall reliability of the system.