Concentrated solar power (CSP) systems use mirrors or lenses to focus sunlight onto a small area to generate heat. This heat is then used to drive a steam turbine which powers an electrical generator. CSP plants can also use molten salt storage to continue generating power after sunset. There are four main types of CSP technologies: parabolic troughs, dishes, linear Fresnel reflectors, and solar power towers. While CSP is currently more expensive than photovoltaics, global installed CSP capacity has increased nearly tenfold between 2004 and 2013, with most growth in the last five years.