エネルギー工学

An important feature of small multitubes flow-through boilers is that they must be designed to be produced according to regulations such as a maximum working pressure of 1MPa and a maximum surface area of 10m2. Within these regulations, more compact and effective boilers are being designed and developed. At present, a design which has achieved a maximum equivalent steam generating rate of 2ton/h and a relatively high heat flux has been developed for industrial use. These results have been achieved due to advances in heat transfer techniques. However, several problems remain to be solved in order to make the separator and upper header, which maintain the steam dryness fraction, more compact and effective. The problems for creating a two-phase flow vapor-liquid process in this boiler are mentioned.

The multiphase technology in power plant boiler during last ten years in Japan have been reviewed. The most important events on utility boiler are the appearance of coal fired supercritical sliding pressure operation boiler, the increase in steam pressure and temperature for higher plant efficiency, and the increase in number of gas turbine combined plants. Concerning these events, some key technology were developed in multiphase field. The detailed study on boiling heat transfer in rifled tube realized the vertical tube type boiler, which had more simple furnace wall structure than the spirally wound type ones. The ultra supercritical pressure boiler achieved 5% higher efficiency than the normal pressure ones. The natural circulation waste heat boiler with horizontal evaporating tubes, which enabled low cost construction, was designed under attentive thermal and hydraulic studies, and successfully started to operate. The condensing heat transfer of ammonia-water mixture was investigated and the high efficiency of the mixture cycle was demonstrated through test plant operation. Regarding fuel, coal oil mixture, coal water mixture, and orimulsion were introduced. For clean and efficient usage of coal, PFBC and IGCC are expected as the important future power generation system.

Multiphase flows that arise in geothermal energy development have been briefly reviewed. A particular emphasis is placed on gas-liquid two-phase flows in geothermal production well. A liquid phase geothermal fluid is normally fed at the bottom of the well. However, as the fluid rises in the wellbore, it loses the pressure and eventually starts boiling; a steam-water two-phase flow results. A one-dimensional formulation of the two-phase flow for wellbore simulation is described in some detail, where the steam-water relative velocity is taken into account through empirical correlations. The presence of dissolved NaCl and noncondensable gas (CO2) is. one characteristic of geothermal fluid. The present status of two-phase flow simulators for production wellbore is also described. It is suggested that the empirical correlations to differentiate flow regimes and to evaluate the frictional pressure losses along the well, especially for large diameter pipe flows, are essential in accurate prediction of the wellbore two-phase flows.

The author of this paper has been developing a high-efficient hydrogen energy system which uses slush hydrogen as a means of transporting and storing hydrogen energy, and a refrigerant for cooling superconducting power machines. In this system, a synergetic or hybrid effect is anticipated for slush hydrogen transported in a pipeline and stored as a fuel for fuel cells, with simultaneous transmission and storage of electrical power by means of superconducting power machines using MgB2 material. In this paper, thermo-fluid phenomena of cryogenic multiphase fluid in the development of this system are reported, which consist of hydrogen liquefaction by magnetic refrigeration, nucleate boiling heat transfer to slush hydrogen and liquid hydrogen, and pressure drop reduction and heat transfer deterioration phenomena of slush fluid due to the non-Newtonian fluid behavior.

The alternative and natural refrigerants presently used to address global warming problem yield low coefficient of performance and should be studied further. Recently, new technique using two-phase ejector flow model is developed. This paper shows the theoretical analysis of energy conversion efficiency of ejector from the thermodynamic and fluid dynamics point of view. Also, two-phase flow shockwaves appearing in the nozzle of ejector are shown based on experimental results. These shockwaves are categorized into two types; one is pseudo shockwave and the other is dispersed shockwave. The theoretical analysis with momentum relaxation given in this paper provided a good representation of these shockwaves.

A general view of spray combustion is given in the present paper. Spray combustion systems are widely used in industry, but the detail is still under investigation. Atomization of fuel liquid, evaporation, mixing of fuel and air, and reaction are involved in the spray combustion, and these processes take place in turbulence and interaction each other. Examples of spray combustion and their combustion categories are given. The statistics of spray which is critical for the spray combustion is discussed. Locally-homogeneous-flow model and two phase-flow model are explained to give an insight of spray combustion. A modeling based on probability density function, which is not described in detail, is potential. Recent experimental results are also described.

Since hot-water supply has high percentage of energy consumption in residential sector, improvement in the efficiency of a gas water heater is an important issue. In order to improve thermal the efficiency of a gas water heater, an additional heat exchanger is required for latent heat recovery from the exhaust humid combustion. In this paper, the history of gas water heater, the technologies developed for latent heat recovery gas water heater and the recent progresses in the development of latent heat recovery heat exchanger are presented.

Ecotype hydraulic turbines can be utilized small and distributed water resources with little civil engineering works and without environmental burdens. In Japan, network of agricultural waterway extends 400 thousands kilometers, this abundance of waterpower has a high expectations for electric power generation to supply to agricultural facilities. This promotes local production for local consumption of electricity. Four types of turbine, Savonius turbine, waterfall turbine, jet turbine and undershot turbine, were investigated and demonstrated in some fields. We have been also investigated the standalone smart grid system which can harness a several types of ecotype turbines. It can control the amount of generation power and supplying electricity. In this manuscript, we summarize our results of the investigation of ecotype hydraulic turbines.

The Ocean Thermal Energy Conversion (OTEC) power plant is a system for generating electric power using the temperature difference between the surface and the deep water of the ocean. It is an environment-friendly source of energy. Unlike most of the renewable energy resources which are weather dependents, OTEC system provides a stable source of electricity along the year because the surface and the deep water temperatures are constant. In addition, the seawater used for electricity generation can be further used in many fields, such as desalination of seawater, agriculture, and Lithium recovery. To realize the OTEC system, various studies have been conducted for long time; one of the fields of these studies is on the use of non-azeotropic mixtures as working fluids in order to make use of the increase of the system's exergy. One of the advantages of the ammonia/water mixture is the big difference between its boiling point and its dew point. As the use of ammonia/water mixture as working fluids was in its initial stage, there has been great concern regarding the stability of the cycle because the difference between of the boiling point and the dew point of ammonia/water mixture is larger than that of CFCs. In this paper, it is introduced on the research trend and activity of Ocean Thermal Energy Conversion, especially, the challenge on ammonia/water mixture including multiphase flow research and the OTEC Road map toward 2030 by Japanese NEDO (New Energy and Industrial Technology Development Organization).

This paper summarizes the results of author's studies on solar energy resource utilization. One is the development of a micro solar power generation which was combined with micro wind power generation at urban area. The other is the solar thermal utilization by an absorption heat pump for air conditioning. Daily fluctuations of energy supply in Osaka city both by electricity and by town gas were presented precisely in order to understand the energy consumption of urban area. Then, field test results about a micro hybrid power generation system were presented at urban coastal area where there is few wind power resource. Lastly, the performance of a solar-assisted absorption chiller heater of 352 kW refrigeration capacity has been introduced through a long series of proof examination of 17 months.

A numerical forecasting model of the on-site solar spectral irradiation for photovoltaic generation is developed by coupling a meteorological model and an atmospheric irradiance model. The intensity of the solar irradiance, which mainly changes due to clouds shading and scattering, is one of the dominant parameters of the photovoltaic power generation. The coupled model developed in this study consists of three parts: First, the mesoscale meteorological model is applied to evaluate the atmospheric condition including clouds existance. Next, the optical thickness of clouds is evaluated by considering the amount of clouds and diameter distribution of their particles. Finally, the direct and scattering solar irradiance at the ground is evaluated by taking into account not only the effect of the clouds but also the atmospheric gasses, e.g. Ozone, Oxygen and Carbon Oxide, in each wave length. The direct and scattering solar spectral irradiance evaluated with this coupled model is applied to compute the electric generation of any kind of photovoltaic systems. The coupled model's forecasting can be applied to control the unstable photovoltaic output from the ordinal power generators.

Status of research and development of advanced coal-utilized power generation technologies such as pulverized coal combustion and integrated coal gasification combined cycle (IGCC) power generation systems were reviewed and discussed in this paper. It was confirmed in the pulverized coal combustion system that the advanced low NOx combustion technology with a new type low NOx burner effectively reduced NOx emission and moisture evaporated from low-rank coal significantly affected combustion characteristics of bituminous coal blended with the low-rank coal. It was also revealed in the IGCC system that simultaneous evaluation of gasification efficiency and molten slag behavior was essentially important to maintain stable operation of the system. Throughout the research activities, it is found that a numerical simulation was a key technology to assess and optimize design and operation of major components such as coal combustion boiler and coal gasifier in the systems.

Seafloor Massive Sulfides (SMSs), which were formed by deposition of precipitates from hydrothermal fluids vented from seafloor, are one of unconventional mineral resources beneath deep seafloors in the Exclusive Economic Zone (EEZ) of Japan. The authors have proposed the concept of Seafloor Mineral Processing, where useful minerals contained in SMS ores are separated from feed ores on deep seafloor to be lifted while remaining gangue is disposed on seafloor in appropriate ways. To apply column flotation, which is usually used for mineral processing on land, for seafloor mineral processing, model experiments of column flotation on deep seafloor were carried out at high pressures up to 10MPa using a large-scale high pressure vessel. In the experiments, fine bubbles suitable to flotation were generated in the flotation column at the pressures, and overflow of froth from the flotation column was observed. The content of metallic elements such as copper and zinc in the concentrates obtained from flotation at 1MPa was higher than that in the feed ores while that of silicon and calcium, which are mainly assigned to gangue, was lower than that in the feed ores. These results suggest that column flotation would be applicable for seafloor mineral processing.

When a suitable site has been found and established, tidal power and ocean current power is more predictable than wind and fluctuates less drastically than solar. It's a reliable producer of electricity. Tidal energy and ocean current energy are produced through the use of generators. These large underwater turbines are placed in areas with high tidal movements and ocean current, and are designed to capture the kinetic motion of the ebbing and surging of ocean tides in order to produce electricity. Tidal power and ocean current power have great potential for future power and electricity generation because of the massive size of the oceans. These articles explore the potential energy of tidal and ocean current power technologies.

A variety of technical methods with gas (air or oxygen) release have been used to purify water in eutrophic lakes and reservoirs. They are roughly classified into two types: 1)destratification and circulation by air bullets or bubble plumes, and 2)direct aeration in a bottom layer by micro-bubbles. Their effectiveness has been verified in many dam reservoirs, however, their further improvement and development will be needed. In this paper, a bubble plume system through the use of solar energy was presented according to a previous study, and its development was discussed including the potential for a green technology.

Nanofluid is the liquid containing colloidal dispersion of nanometer-sized solid particles. Anomalously high thermal conductivity of nanofluids compared to the base fluid was reported in 1995, and then a number of studies have been conducted so far for the nanofluids from various aspects. This paper presents a review of the researches on the critical heat flux and boiling heat transfer of nanofluids.

Alkaline water electrolysis with magnetic field is a method for increasing the efficiency of hydrogen production through the electrolysis of alkaline water solutions. In the author’s research group, experiments and simulations were conducted to elucidate the multiphase flow behavior and electrolytic efficiency in alkaline water electrolysis with magnetic field. It was demonstrated that the electrolytic efficiency can be improved by changing the bubble motion, even with electrodes that are operable with permanent magnets (without the need for magnet power) and easily scalable, and with a weak magnetic field applied vertically.