A REVIEW OF METHODS FOR SOLAR RADIATION ESTIMATION USED IN OFF-SHORE APPLICATIONS
Scientific Bulletin of Naval Academy
The importance of knowing the contribution of solar radiation is vital for adapting and implementing novel solutions for converting solar energy into thermal or electrical energy. Currently, models have been developed to estimate the solar radiation, globally or locally, that take into account inter alia latitude, certain climatic parameters, temperature differences or clouds. This paper presents an overview of the main methods used to estimate solar radiation on large water surface (seas and
... eans). A comparison is done between the measured solar radiation and empirical insolation formulas issued by Kimball, Laevastu, Reed and Tabata. Compared to the models used for land estimations, the main parameter taken into account in these empirical methods is cloud cover. The challenge consists in a good estimation of this parameter (in tenths or oktas), knowing that human error is about 10 -12% relative to satellites measurements. More precise determination of the amount of available solar radiation at sea comes in the context of refurbishment process and adoption of clean technologies shipping industry in order to reduce greenhouse gas emissions globally. Also, given the increasing need for energy, seas and oceans may become at some point due to large areas available, favorable space for installation of "green" systems. CONCLUSIONS For clear -sky conditions (CC lower than 0.3) Reed, Laevastu and Tabata formulas generate the best results with a slight underestimation of maximum 5%, which is more than acceptable. Kimball's formula on the other hand overestimates insolation in all sky condition (for CC lower than 0.3 range between 5 and 17%, for 0.3<CC<0.6 range between 10 and 27% and for CC higher than 0.6 differences can be up to 65%). Laevastu's empirical formula constantly underestimates insolation and the worst prediction is for CC higher than 0.6. It can be noticed that Tabata's formula underestimates insolation for a CC lower than 0.6 but overestimate values for a CC than 0.6. Considering results from present paper, as a recommendation, for all sky conditions, the best option it will be the use of Reed's formula, which generates satisfactory values of insolation. A better understanding of results obtained for the long -term monitoring of radiation is still required. An alternative for retrieving data is represented by satellite records, solution which allow a more detailed spatial analysis. An analysis of the differences in contribution of solar radiation for land and sea is also a key topic which will be investigated. So far, due to relative constant water vapor and low influence of industrial pollutants, is considered that, globally, the amount of solar radiation is higher on sea than on land.