Energy is an imperative contribution for social and commercial growth. In recent years the energy demand has been increased rapidly with wide-ranging activities in domestic, agricultural, industrial sectors. For various reasons, the demand is particularly more in the developing countries. As the fossil fuel is getting exhausted day by day, it has become necessary for the power and energy engineers to look out for the renewable energy sources such as sun, wind, ocean, biomass and geothermal as

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... stainable, environmental friendly and cost effective. The renewable energy resources like geothermal and ocean are not cost effective and not viable to transport the produced energy to farther distances. The possible renewable energy resources are sun and wind, but they are not available all the time-throughout the year which causes problems in generating power. In order to overcome this, we concentrate on the area of hybrid renewable energy systems which sustain lower cost and higher reliability. The main aim of this study is to review the research on the unit sizing, energy management and optimization of the hybrid renewable energy systems.