They created the first real passive solar design. In addition, Romans used the heat from the sun to warm water entering the public baths. There are three conversion systems of solar energy: the passive systems, the active systems, and the Photovoltaic systems. The passive systems collect and store solar energy without the use of any other energies. For example, people construct houses with large double-paned windows to get direct sunlight to capture and magnify the sun's warmth. Another example of effective methods of passive solar energy capture is by using the stone flooring and walls with thick insulation to keep the heat inside the buildings.
The second solar energy is called the Active energy. Active energy collect and store energy by employing another electric energy. It uses the same principal as the passive systems, except that the active systems use either air or fluid (water) to transform the heat from the collector to storage. An example of active system is the solar water heating that is placed on the roof of buildings to heat the water and then pump it through a system of pipes to heat the building. The active systems collect sunlight directly on either flat surfaces or use parabolic reflectors to reach high temperatures. Both passive and active systems use glass to collect sunlight and store the heat. The third system is called the photovoltaic systems (PV). The Photovoltaic systems is defined by Wikipedia as "A method of converting solar energy into direct current electricity using semiconducting materials that exhibit the photovoltaic effect ". Photovoltaic systems are especially useful in agriculture. The power produced by the photovoltaic panels is also used to power a purification system and clean the water. When it is not working to pump water, the electricity is saved in batteries so that pumps can be used at night. Some phone boxes are powered directly by photovoltaic system during the day, and they have batteries that are charged in the day (from sunlight) and work at night time.