Y.M.  Hassan, O.M.  Furawi, M.A.  Dakhil, 
    Optimum Slope for Solar Insolation on a Surface Tilted Toward the Equator in Libya,
    Research Journal of Applied Sciences,
    Volume 8,Issue 1,
    2013,
    Pages 22-26,
    ISSN 1815-932x,
    rjasci.2013.22.26,
    (https://makhillpublications.co/view-article.php?doi=rjasci.2013.22.26)
    Abstract: The determination of optimum slope angles for solar collectors 
  plays a significant role in the efficient utilization of solar energy. Monthly, 
  seasonally and yearly slope and optimum slope angles for five locations, Tripoli, 
  Shahat, Hun, Kufra and Ghat where determined using measured data of monthly 
  mean daily global solar radiation on a horizontal surface. Over a period of 
  7 years, it was found that the annual optimum slope angle is approximately equal 
  to the location latitude, φ while for the winter season it is φ+18 for 
  the coastal cities and φ+21 for cities in the desert and for sumer season 
  it is φ–23 for cosatal cities and φ–19 for desert cities, 
  respectively.
    Keywords: coastal cities;optimum slope angle;Solar radiation;utilization;Libya