Tadayoshi Tanaka

Maximum output control of photovoltaic (PV) array

A photovoltaic (PV) system output depends on environmental parameters such as the solar insolation and the PV module temperature. If it is possible to predict the maximum power point under the outdoor environment and to operate at that point, the PV system can generate the maximum output every time. In this paper, a maximum power point control method that maximizes the output of a PV array is proposed. This method determines the maximum output operation point from the I-V characteristics introducing empirically the effects of the solar insolation and the module temperature. The authors derived two main parameters from this analysis; one is the power gain G, and another is the environmental operation parameter X. At the operation point determined by this method, G becomes larger than that of under the same environmental conditions. G becomes large with the increase of X, and the large X mainly means low solar insolation. The characteristics of PV module which will supply more power especially at large X should satisfy the following points; the fill factor of the module should be lower and the short circuit current of the module should be larger than those of arrays currently available in the market.

New proposal for photovoltaic-thermal solar energy utilization method

Abstract One of the most effective methods of utilizing solar energy is to use the sunlight and solar thermal energy such as a photovoltaic-thermal panel (PV/T panel) simultaneously. From such a viewpoint, systems using various kinds of PV panels were constructed in the world. In these panels, solar cells are set up at an absorber collecting solar thermal energy. Therefore, temperature of solar cell increases up to the prescribed temperature of thermal energy use, although it is lower than the cell temperature when using only solar cell panel. For maintaining cell conversion efficiency at the standard conditions, it is necessary to keep the cell at lower temperature. In this paper, electric and thermal energy obtained from a PV/T panel is evaluated in terms of energy. Based on this evaluation, the method of not to decrease cell conversion efficiency with collecting solar thermal energy was proposed.

Investigation of nitrate salts for solar latent heat storage

Abstract Solar thermal applications require some means of thermal energy storage. Amongst several storage concepts, latent heat storage is quite suitable because of its high storage density and almost constant temperature during charging and discharging. The temperature range between 200 and 300°C is considered to be important for solar total energy systems. In this temperature range, sodium nitrate and its mixed salts with other nitrates including eutectic and off-eutectic salts are candidates. The present paper deals with heat transfer in a latent heat storage unit utilizing these salts. A method of rough estimation of the thermal conductivity of the storage materials is described, and the temperature history of the storage material experimentally obtained is compared with numerical solutions and found to be in reasonably good agreement. It is seen that the temperature of the heat transfer surface quickly drops soon after the appearance of a solid phase due to low thermal conductivity of these salts. Ways to avoid this temperature drop are discussed.

A study on a thermally regenerative fuel cell utilizing low-temperature thermal energy

Abstract We have proposed a thermally regenerative fuel cell operated by low-temperature thermal energy such as solar thermal energy and low-temperature waste heat. It consists of the chemical reaction of 2-propanol dehydrogenation at the negative electrode and the acetone hydrogenation at the positive electrode by using the principle of a fuel cell. As the first step of this research, we investigated the acetone hydrogenation. Activity of ruthenium and platinum composite catalyst (3 wt.%) supported on a carbon plate for the reaction was much higher than that of ruthenium catalyst or platinum catalyst at 90°C. Activity of ruthenium and platinum composite catalyst was much higher when it was supported on carbon felt or cloth than a plate. We adopted ruthenium and platinum composite catalyst supported on carbon cloth as electrodes of the cell and examined its characteristics. First, we used molecular hydrogen instead of 2-propanol as a proton source. Under this condition, the open-circuit voltage and the short-circuit current were 104.6 mV and 8.98 mA, respectively. As loading of the catalyst became higher, the short-circuit current also became larger. The short-circuit currents were 11.5 and 26.7 mA when loading of the catalyst was 5 and 30 wt.%, respectively. Then we used 2-propanol as a proton source. We investigated effects of 2-propanol concentration, catalyst loading and reaction temperature on the cell efficiency. When 2-propanol was diluted with water and supplied to the negative electrode, it was shown that 2-propanol concentration of 50–70 vol% was the best for cell efficiency. The cell efficiency was improved with increasing catalyst loading. As for reaction temperature, 80°C was better to improve the efficiency.

Solar thermal electric power systems in Japan

Abstract The major subjects of this paper are to report the outline of the recent basic research and technical development for solar thermal electric power systems in Japan. Solar thermal electric power systems are presently being developed as one of the most important systems in Sunshine Projects which were initiated in 1974 to develop the utilization systems of new energy resources. Conceptional designs of solar thermal power systems were already done on the basis of the results of the supporting researches and two pilot plants of solar thermal electric power systems of a capacity of 1000 kWe are under construction on the basis of the conceptional and detailed designs and would be constructed by 1981. The present conditions of these pilot plants and the major researches which are thought to be most important subjects in the basic researches and technical developments for solar thermal electric power systems are described in this paper.

Investigation on proton exchange membrane fuel cell for solar power generation

A study is made on a proton exchange membrane (PEM) fuel cell that supports a new type of chemical coupling of 2-propanol/acetone/hydrogen to generate power. Power generation that uses this new chemical coupling of reactions requires specific types of catalysts in the fuel cell. In the present paper, investigations of a few catalysts and systematic metallic loading from different precursors in various loading ratios on carbon electrodes have been conducted to maximize current generation. Various permutations of catalysts (pure or composite) with different loading ratios were tested for their relative performance using chemical liquids as oxidants in the PEM fuel cell (reactor). The maximum electrical power obtained from a single layer of PEM fuel cell was 0.259 mW/cm2. Experimental results are promising and the new coupling of chemical reactions may be used successfully in the electrochemical reactor (fuel cell) to tap solar energy to generate power. Further work is needed to put this concept into reality for power generation from a solar assisted PEM fuel cell system.

Influence of the internal structure and temperature in the reaction layer on the electric output in a solar thermal cell

A solar thermal cell is composed of the chemical reaction of 2-propanol/acetone/hydrogen and a fuel cell. We have examined the influence of the internal structure and the temperature in the solar thermal cell on the output of the cell. As the main results, (1) with increasing the temperature of the positive electrode, the short circuit current of the cell has increased, (2) with decreasing the thickness of the electrode space, the output of the cell has increased and (3) under the condition that the electrodes are kept in good contact with the polymer electrolyte, the performance of the cell increases and the short circuit current reaches 80 mA.

Solar thermal energy storage using heat of dilution: Analysis of heat generation in multistage mixing column

Abstract Storage of solar energy is important for the future success of solar energy utilization systems such as solar thermal power plants. Various papers have described the storage methods for storing solar energy. Because of the results of assessing the storage methods proposed in these papers, the method based on heat of dilution was selected by reasons of the ease of heat transfer, the controllability of reaction and the recovery of heat stored. The effectiveness of heat recovered from heat of dilution in the case of storing solar energy in the form of heat of dilution was considered in this paper. Sulfuric acid and water solution used as the typical example of heat of dilution, since its thermodynamic properties were well known. Two liquids were mixed in two different systems of multistage mixing column, and an application of heat of dilution to multipurpose energy utilization was analyzed. The utility of heat of dilution was explained from the results of thermal characteristics of each system.

Proposal and fundamental analysis of thermally regenerative fuel cell utilizing solar heat

In Japan, it is difficult to obtain high-temperature heat continuously from the Sun. However, it is easy to obtain low-temperature solar thermal energy. In order to use such low-temperature solar energy, the authors propose the use of a thermally regenerative fuel cell operated by solar energy. It is a direct energy conversion process which consists of 2-propanol dehydrogenation at the negative electrode, acetone hydrogenation at the positive electrode and an electrolyte sandwiched by both electrodes. By means of a combination of chemical reactions and fuel cells, it can convert low temperature thermal energy (like solar thermal energy) into electrical energy directly. In order to make clear the characteristics of their proposed cell, the authors use molecular hydrogen as the proton source instead of 2-propanol. The activity of a ruthenium and platinum composite catalyst-supported carbon plate for acetone hydrogenation at 363 K is higher than that of a ruthenium or platinum mono-metallic catalyst-supported carbon plate. The activity of ruthenium and platinum composite catalyst-supported carbon felt or carbon cloth is much higher than carbon-plate-supported catalysts. The authors adopted a ruthenium and platinum composite catalyst supported carbon felt or cloth as electrodes of the cell and examined its characteristics.

Performance characteristics of barometric-type open-cycle OTEC system

Open Cycle-Ocean Thermal Energy Conversion (OC-OTEC) system has a merit to use a heat exchanger of direct contact type without a heat transfer tube. Therefore, it is expected that the capital cost of OC-OTEC is reduced by use of this simply designed heat exchanger. However, non-condensable gas is released in the system, because in a direct contact evaporator, the steam driving a turbine is generated by surface sea water including air, and existing air causes a reduction of system performance. From the above point of view, we carried out an experimental study on the operating characteristics and the effect of structure of a heat exchanger and the existence of non-condensable gas on the performance of a direct contact heat exchanger in an experimental apparatus of barometric type OC-OTEC. As a result, the performance of the direct contact evaporator can be satisfied. However, we cannot get satisfactory results in a direct contact condenser. Therefore, we conducted further experiments through the improvement of the structure of the condenser and the control of the non-condensable gas. Finally, we concluded that the major factor affecting the system performance is the non-condensable gas, but its effect can be ignored when its concentration is below 8%.