Thermal solar energy is heat that is captured for heating air, water or other materials. The simplest forms of thermal solar energy is placing an object in direct sunlight to warm it. Passive solar heating captures heat by allowing sunlight to enter windows, skylights, etc. and warm the area during winter, but shading the windows in the summer by the use of roof overhangs, trees, window shades, etc.
We can capture thermal energy for more efficient use by use of collector panels through which air, water or another material flows to deliver the heat to the point of use. Heat can be stored as hot water in insulated tanks or some other storage medium.
A modern thermal collector design uses evacuated glass tubes with an internal black tube filled with a liquid having a low boiling point, such as alcohol. The vacuum between the tubes provides insulation, and the alcohol serves as the working fluid and as antifreeze. When the inner tube heats, the alcohol will evaporate or boil, creating a hot vapor that rises to a heat exchanger at the top of the tube assembly. When the alcohol vapor gives off its heat to the water in the heat exchanger, it condenses again to liquid and flows back to the bottom of the assembly to be reheated and the process repeats itself. Some of these systems incorporate the hot water tank in the heater and heat exchanger assembly, while others store the hot water in a more conventional solar/electric water heater.
To achieve higher temperatures, reflectors are used to direct sunlight from a large area onto a smaller collector, and can generate temperatures of hundreds of degrees Fahrenheit. Simple concentrating solar heating systems are solar cookers, using reflectors to reflect sunlight onto a black pot. These are routinely used in many regions of the world.
More elaborate systems include black pipes at the focus of long parabolic reflectors to heat water or other fluids, and very large systems using heliostats, or mirrors that track the Sun's movement, to focus sunlight from many acres onto a collector to produce steam. Large concentrating solar thermal systems are being developed to capture and store heat in large tanks of molten salts, which is used to generate steam and drive generators around the clock. It is hoped that these systems can overcome the shortcoming of PV systems, which is the inability to deliver electricity at night or when shaded by clouds.