Passive solar systems use solar energy directly, as happens for example in greenhouses, glass structures that allow the Sun to enter, but do not let the heat escape. In the active solar systems instead, solar energy is collected and transformed into thermal or electric energy before it is utilized. This type of system includes solar thermal power systems, solar concentrator systems used for the production of thermal energy and solar photovoltaic panels for the production of electric energy. The best technologies also enable the cogeneration of different types of energy, and it is possible to accumulate thermal energy in many ways and for different uses.