Solar collector

A solar collector is a device which captures as much sunlight as possible, in order to either redistribute (focus) or absorb it into a transport medium. Solar collectors are generally used to generate heat, although in some cases a parabolic dish is used to focus sunlight on a special high-temperature solar cell. The heat generated by a solar collector can be directly used to heat another object (e.g. a kettle or a body of water) or can be indirectly used to generate electricity by driving a steam turbine (Stirling engine). Solar collectors come in a large variety of shapes, sizes and purposes. Here, we will introduce you to the most commonly used types of solar collectors. These include the solar collectors commonly seen in solar water heaters.

Flat plate solar collector

Flat plate solar collector

The most basic and most common type of solar collector is the flat plate solar collector. At the heart of this collector you will find a sheet of thermally conductive dark material (usually metal) which absorbs as much sunlight as possible. Directly below this sheet a series of water conduits is found; the heat collected by the absorber is absorbed into the water and subsequently carried away by water flow. The collector is housed in an insulating box, with a glass plate on top to further insulate and heat the system. Due to their flexibility, relatively low costs and ease of installation, flat-plate collectors are often used in solar water heating systems.

Evacuated tube collector

Evacuated tube solar collector

Evacuated tube collectors consist of a parallel row of evacuated glass tubes. Within each tube, another glass tube is placed, which is covered in a strongly absorbing material. Since the evacuated space blocks both convection and conduction, the absorbed heat has little means of escape. The temperature within the tube itself can therefore reach extreme values, with temperatures of 170 °F to 350 °F commonly achieved. An inherent advantage of the evacuated tubes, is that their cylindrical form means that the collector is always perpedicular the sun. A disadvantage to this system is that sunlight shining in between the tube sis not captured. This can be partly countered by adding a reflective film to the back of the collector. Another disadvantage is cost: evacuated tube collectors are approximately twice as expensive as their flat-plate counterparts.

Parabolic through collector

A parabolic through system

A parabolic through system consists of a long curved mirror, which focuses the sunlight on an insulated tube. This tube contains a heat transfer fluid, which transports the heat to either a generator or a water reservoir. In the former case, which is most commonly encountered, the heated transfer fluid is used to boil water. The acquired steam is passed through a sterling engine, which in turn is used to generate electricity. The advantages of a parabolic through system are efficiency (20%), ease of tracking (only one axis needs tracking) and scalability. These advantages make the system highly suitable for use in large power plants like Nevada solar one. A large number of existing and planned solar power plants are based on parabolic through technology.

There is however one major drawback to the parabolic through: it is extremely sensitive to weather. A decrease in incident solar energy will cause dramatic decreases in the system’s yield. Basically, no direct sunshine means next to no power. A parabolic through is therefore only beneficial in areas that receive plenty of sunshine. In order to bridge hours of limited sunshine, it is possible to direct the heated fluid through a tank of molten nitrate salt. Nitrate salts have a tremendously high thermal inertia and are thus very suitable for heat storage. So, when the sun gets clouded or sets under the horizon, the still hot salts will make sure the system remains operative.

Parabolic dish collector

A parabolic dish (see title image) consists of a single parabolically shaped mirror, which focuses the incident sunlight on a single point (the focal point). Temperatures in the focal point can easily reach extreme values, although performance is highly dependant on weather conditions. In the focal point, one can place any object that requires heating. Parabolic dishes are commonly used in solar cooking, but can also be used in solar photovoltaics. In such a case, a special high-yield high-temperature solar cell is placed in the focal point. The main advantage of a parabolic dish is its extreme power. Disadvantages are the requirement of a dual axis tracker and the sensitivity to weather conditions.

Towers and chimneys

More exotic approaches to solar power generation include the so-called solar tower and the solar chimney. In a solar tower system, an array of concentric mirrors (heliostats) concentrates the sunlight on a single receiving station, which is located high in a central tower. This station houses a tank of heat transfer fluid, which is used to drive a steam turbine, generating electricity. A great example of such a system is the Spanish PS10 solar tower.

The solar chimney is different in that it doesn’t directly use the heat, but rather makes use of air movement as a result of solar heating. The air is heated in a huge circular collector area. In the center of this collector area, a very high chimney is placed. Since hot air has the tendency to rise, it will forcefully expel itself through the chimney. By installing a turbine in the chimney, the air currents can be converted to electricity. An obvious disadvantage is that the system’s efficiency is strongly limited by the efficiency of the installed wind turbine. Despite poor efficiency, a solar chimney is relatively cheap to install and keep going. Note that solar chimneys are also commonly called solar updraft towers.