Yoshiki Nishi

Exergy analysis on solar heat collection of three–dimensional compound parabolic concentrator

This study presents a thermal model for the optimum design of the three–dimensional compound parabolic concentrator (CPC) considering the maximisation of exergy delivery. The model in this study has been validated by comparing the computed collector temperature with the measured one. A good agreement has been confirmed. The optimum temperatures of the collector and the thermal medium are calculated. Reduction of heat loss from the collector to ambient air can enhance the maximum exergy, but it is accompanied by shift of the optimum temperature. The heat extraction process has been examined by assuming that it is composed of two steps of heat flow: the first from the collector to the medium and the second from the medium to the final objective. The analysis shows how the exergy efficiency varies in response to the design change in the second step and where the overall optimised design can be found in a diagram between heat transfer coefficient and temperature.

Observation and modeling of the ice-covered ecosystem in the lagoon

A field observation was conducted in ice-covered coastal lagoon, Saroma-ko (Hokkaido, Japan) during the period from late February to mid March in 2004. The samples of the ice core and water were collected and the concentration of chlorophyll a, particulate organic matter and nutrients in both ice and water were measured. As a result, a bloom of ice algae was observed at the end of the February whereas the bio mass of pelagic phytoplankton had remained low during the observational period. In order to quantitatively clarify the behavior of material transport in the ecosystem, a coupled ice-ocean ecosystem model developed in our previous study is used. The model reproduced the observed bloom of the ice algae with good accuracy. The model results show that the organic matter accumulated in the ice by the production of the ice algae was effectively transported into the pelagic system by means of physical releasing effects, which is effective especially in the ice-melting period and that the nutrients consumed by the activity of biological production were effectively transported from the water by the above effects.