Estimation of direct normal irradiance under various sky condi-tions in data sparse tropical ecological zones in Nigeria
Keywords:Decomposition Model, Equatorial Line, Direct Transmittance, Clearness Index, Extraterrestrial Solar Radiation.
In this study, models for estimating direct normal irradiance (Hb) under various sky conditions in data sparse tropical ecological zones in Nigeria were fitted. The evaluated values of clearness index indicate that the prevailing sky condition in the southern tropical zones is heavily overcast while the northern zones experience partly overcast under all sky conditions. From the analyses of the influences of clearness index on Hb, it was observed that higher values of Hb were recorded in the far north zone of Sahel savannah (FNZSS) of Sokoto while lower values were registered in the far south zone of mangrove swamp of Port Harcourt revealing an increasing trend from FSZMS to FNZSS due to the trends in cloudiness and associated atmospheric moisture with the movement through the Hadley cell calculation system along the equatorial line. The regression correlation models developed from the model performance test indicates that the proposed models could be used to estimate Hb accurately between latitude 4 to 10oN, that is, mangroves swamp and Guinea savannah tropical zones in Nigeria and other locations with comparable sky conditions.
 National Aeronautics and Space Administration (NASA) Atmospheric Science data centre, http://econsweb./arc.nasa.gov/sse/.
 Bartoli, B., Cuomo, V. Amato, U., Barone, G., Mattanelli, P. (1982). Diffuse and beam components of daily global radiation in Genoa and Macenata. Solar Energy. 28, 307 â€“ 312. https://doi.org/10.1016/0038-092X(82)90304-8.
 Benson, R.B., Paris, M.V., Sheny, J.E., Justus, C.G. (1984). Estimation of daily and monthly direct, diffuse and global solar radiation from sunshine duration measurement. solar Energy. 32, 523 â€“ 536. https://doi.org/10.1016/0038-092X(84)90267-6.
 Garison, J.D. (1989). A study of the division of global solar irradiance into direct and diffuse irradiance at thirty three U.S sites. Solar Energy. 35, 341-351. https://doi.org/10.1016/0038-092X(85)90142-2.
 Gueymard, C. (1993a). Mathematically integrated parameterization of clear-sky beam and global irradiance and its use in daily irradiance applications. Solar Energy. 50, 385 â€“ 397. https://doi.org/10.1016/0038-092X(93)90059-W.
 Gueymard, C. (1993b). Critical analysis and performance assessment of clear sky solar irradiance models using theoretical and measured data. Solar Energy, 51, 121 â€“ 138. https://doi.org/10.1016/0038-092X(93)90074-X.
 Iqbal, M. (1980). Reduction of hourly diffuse solar radiation from measured hourly global radiation on a horizontal surface. Solar Energy, 23, 491 â€“ 503. https://doi.org/10.1016/0038-092X(80)90317-5.
 Liu, B.V.H., Jordan, R.C. (1960). The interrelationship and characteristics distribution of direct, diffuse and solar radiation. Solar Energy 4, 1-9. https://doi.org/10.1016/0038-092X(60)90062-1.
 Louche, A., Notton, G. Poggi., Simonnot, G. (1991). Correlation for direct normal and global horizontal irradiance on French Mediterranean site. Solar Energy. 46, 261 â€“ 266. https://doi.org/10.1016/0038-092X(91)90072-5.
 Maduekwe, A.A.L., Chendo, M.A.C. (1993). Empirical models in predicting the direct normal solar irradiance on a horizontal surface in Lagos. International Atomic Agency and United Nations Educational Scientific and Cultural Organisation, International Centre for Theoretical physics, 1-7.
 Maxwell, A.L. (1987). A quasi-physical model for converting hourly global horizontal to direct normal insolation. Report SERI/TR-215 â€“ 3087, Solar Energy Research Institute, Golden, CO.
 Perez, R., Seals, R., Zelenka, A., Incichen, P. (1990). Climatic evaluation of models that predict hourly direct irradiance from hourly global irradiance: projects for performance improvements. Solar Energy. 47, 99 â€“ 108. https://doi.org/10.1016/0038-092X(90)90071-J.
 Vignola, F., McDaniels, D.K. (1986). Beam â€“ global correlations in the Pacific Northwest. Solar Energy. 36, 409 â€“ 418. https://doi.org/10.1016/0038-092X(86)90088-5.
 Wenxian, A. (1988). A general correlation for estimating the monthly average daily direct radiation incident on a horizontal surface in Yunnan Province, China, Solar Energy. 41, 1-3. https://doi.org/10.1016/0038-092X(88)90107-7.
 Maduekwe, A.A.L., Chendo, M.A.C. (1995). Predicting the components of the total hemispherical solar radiation from sunshine duration measurement in Lagos, Nigeria, Renewable Energy. 6 (7): 807-812 https://doi.org/10.1016/0960-1481(95)91008-2.
 ASHRAE handbook: HVAC application. Atlanta (GA): ASHRAE, 1999.
 Davies, J.A., Mckay, D.C. (1984). Estimating solar irradiance and components. Solar energy. 29, 55 â€“ 64 https://doi.org/10.1016/S0038-092X(84)80049-3.
 Sherry, T.E., Justus, C.G., (1984). A simple hourly all-sky solar-radiation model based on meteorological parameters. Solar energy. 32, 195 â€“ 204 https://doi.org/10.1016/S0038-092X(84)80036-5.
 Nfaoui, H. Buret, J. (1993). Estimation of daily and monthly direct, diffuse and global solar radiation in Rabat (Morocco). Renewable Energy, 3: 923-930 https://doi.org/10.1016/0960-1481(93)90052-I.
 Battles, F.J., Rubio, M.A., Touar, J., Olmo, J., Olmo, F.J., Alados â€“ Arboledas, L. (2002). Empirical modelling of hourly direct irradiance by means of hourly global radiance. Energy. 25, 675 â€“ 688. https://doi.org/10.1016/S0360-5442(00)00007-4.
 Nwokolo, S.C., & Ogbulezie, J.C. (2017a). A single hybrid parameter-based model for calibrating Hargreaves-Samani coefficient in Nigeria. International Journal of Physical Research, 5(2): 49-59 https://doi.org/10.14419/ijpr.v5i2.8042.
 Willmott C.J. (1981). On the validation of Models. Journal of Physics and Geography. 2, 184 -194.
 Akpabio, L.E., Etuk, S.E., (2002). Relationship between solar radiation and sunshine duration for Onne, Nigeria, Turkish Journal of Physics, 27, 161 â€“ 167.
 Augustine, C., Nnabuchi, M.N. (2009). Empirical models for the correlation of global solar radiation with meteorological data for Enugu, Nigeria â€“ Pacific Journal of Science and Technology, 10, 693 â€“ 700.
 Chineke, T.C. (2008). Equations for estimating global solar radiation in data sparse regions. Renewable Energy. 33, 827 â€“ 837. https://doi.org/10.1016/j.renene.2007.01.018.
 Dike, V.N., Chineke, T.C., Nwofor, O.K., Okoro, U.K. (2011). Evaluation of horizontal surface solar radiation levels in Southern Nigeria Journal of Reviewable and sustainable energy. 3, 93-101.
 Fagbenle, R.O. (1992). A comparative study of some simple models for global solar in radiation in Ibadan, Nigeria. International Journal of Energy Research. 16, 583 â€“ 595. https://doi.org/10.1002/er.4440160703.
 Fagbenle, R.O. (1993). Total solar radiation estimates in Nigeria using a maximum-likelihood quadratic fit. Renewable Energy. 3, 813 â€“ 817. https://doi.org/10.1016/0960-1481(93)90089-Y.
 Ituen, E.E., Esen, N.U., Nwokolo, S.C., Udo, E.G. (2012). Prediction of global solar radiation using relative humidity maximum temperature and sunshine hour in Uyo, in the Niger Delta Region, Nigeria Advances in Applied Research, 4, 1923 â€“ 1937.
 Maduekwe AAL., Chendo M.A.C. (1999) Characteristics of the monthly average hourly diffuse irradiance at Lagos and Zaira, Nigeria Renewable Energy. 17: 213-225. https://doi.org/10.1016/S0960-1481(98)00034-2.
 Sambo, A.S. (1986). Empirical models for the correlation of global solar radiation with metrological data for Northern Nigeria. Solar Wind Technology, 3, 89 â€“ 93. https://doi.org/10.1016/0741-983X(86)90019-6.
 Nwokolo, S.C. (2017). A comprehensive review of empirical models for estimating global solar radiation in Africa. Renewable and Sustainable Energy Reviews, 78: 955â€“995 https://doi.org/10.1016/j.rser.2017.04.101.
 Nwokolo, S.C., & Ogbulezie, J.C., (2017b). A Quantitative Review and Classification of Empirical Models for Predicting Global Solar Radiation in West Africa. Beni-Suef University Journal of Basic and Applied Sciences, (In Press), https://doi.org/10.1016/j.bjbas.2017.05.001.