Solar energy originates in the nuclear reactions that take place in the core of the Sun. The radiated energy escapes in the form of sunlight and travels the vacuum of space for around 8 minutes and 20 seconds before reaching The Earth. Part of this radiation is reflected back to space by the atmosphere. The rest will impact the Earth’s surface.
This energy can then be collected by means of different technologies, either to generate electricity or to harness and use its heat in a number of applications. The following are brief descriptions of the most common solar energy technologies used nowadays.
Probably the most widely known solar energy technology, solar photovoltaics (or solar PV) bases its principle on the conversion of sunlight into direct current via the photovoltaic effect. There are two main families of PV technologies available nowadays: Silicon and Thin Film. Silicon modules are made of solar cells (Silicon wafers, often recognizable by their octagonal shape) connected in series. Thin film technologies use various semiconductor materials with the particularity that layers are considerably thinner than in Silicon, thus reducing materials and costs.
Photovoltaic modules are highly scalable, which make them suitable for a wide number of applications, ranging from small solar pico-systems to the most efficient PV modules found in the International Space Station. More commonly, PV modules can be found mounted on house rooftops, integrated on building façades, or installed forming vast PV arrays. They are also a frequent solution for off-grid systems, where energy can be temporarily stored in batteries. In this regards, many emergent nations have adopted photovoltaic solutions as part of their rural electrification strategies, sometimes as hybrid integrations of PV with wind or hydropower systems.
Solar thermal refers to a group of technologies that harness heat from sunlight, optionally store it, and finally transform this heat into a number of different applications. What all these technologies have in common is the use of solar collectors, devices that absorb the heat and transfer it to a fluid, which generally can be either water or a glycol mixture. Flat plate and evacuated tube are the most common type of solar thermal collectors. Also highly scalable, they can be used in a number of applications, such as providing domestic hot water, space heating, industrial process heating or district heating among others. Associated heat storage systems are also a matter of continuous development and research. For instance, some pilot projects are into looking for affordable seasonal heat storage (for the entire winter) for residential use.
Passive solar is related to all technologies and strategies that take advantage of solar energy to create comfortable indoor and outdoor environments, namely for residential or commercial use, and with particular focus on several design aspects of houses and buildings. As the passive term suggests, non or very little external energy, either in mechanical or electrical form, is required. This includes optimization aspects of windows, daylight, building physics (for instance insulation and thermal mass of walls), room distribution, natural ventilation, shading or microclimate. Sometimes some useful strategies can be found in traditional architectural aspects that are particular to every country and region.
As energy efficiency gains importance, especially in times of economic crisis, this is a discipline that becomes as transcendent as the other “active” solar technologies. On a final note, it is worth mentioning the so-called “passive houses” where strictly no external heating is required to keep the necessary thermal comfort throughout the year in cold climates.
Concentrating Solar Power
Concentrating Solar Power (or CSP) are solar thermal systems that make use of concentrated sunlight to generate electricity. This allows reaching higher fluid temperatures, which are required to drive many thermal cycles, and thus drive a turbine to produce electricity. The concentration of sunlight is achieved by means of different concentrating strategies, being the most common ones the parabolic trough, the array of heliostats + solar tower and the Fresnel reflectors. Dish Stirling systems are another CSP technology: they make use of dish-shaped reflectors to concentrate sunlight and use the heat to drive Stirling engines.
Some CSP power plants include thermal storage (i.e. using molten salts) that allow the production of “solar” electricity also during all or part of the night. Last not least, concentrating solar power can also be used for preheating in ISCC (Integrated Solar Combined Cycle) power plants.
Space cooling systems operated mainly with solar energy are increasingly popular nowadays. Two main families exist: photovoltaic solar cooling and solar thermal cooling systems. The first make use of PV modules to feed a conventional air conditioning system. The second family makes use of solar collectors to provide the necessary heat to either a desiccant cooling system or to an absorption / adsorption chiller. There are even some companies that have been developing solar thermal collectors capable of providing both heating and cooling (for instance to provide space cooling and domestic hot water at the same time).
The use of a solar-driven cooling system has a great advantage: the sun shines precisely at the same time when the demand is higher, thus minimizing the need of storage, as oppose to other solar applications.
Other solar applications and technologies
The above are just brief descriptions of the main solar energy technologies and applications that exist nowadays. There are, however, other domains that also deserve an honourable mention. For instance, solar energy can also be used for cooking, especially in hot emergent countries. This can be done by means of the so-called solar cookers, which are object of constant innovation – always on the look for better concentration systems or even to provide thermal storage. Solar distillation and solar crop drying are also real solutions to important challenges faced in some regions where water or food is or may be scarce in the future.
Hybridization of solar technologies are also common: for instance the use of concentrating optics in PV systems give place to concentrating PV (or CPV). Also possible is the combination of PV + solar thermal + concentrators to produce PVT (Photovoltaic + Thermal) systems, which can generate electricity and provide heat at the same time).
Suitability of each solar energy technology
Contrary to what some people may believe, solar energy technologies can be used in every corner of the world. For instance, photovoltaic modules can also contribute in colder climates. In fact Silicon PV cells perform better in sunny and cold environments. It is also important to note that Northern latitudes compensate many sun hours during summer, as days are much longer than in winter, thus contributing positively with many kWh to the annual country energy mix.
CSP, on the other hand, is an ideal solution for very hot and dry areas, with many hours of direct sunlight over the year, such as deserts. Low temperature solar thermal systems can be used virtually everywhere. Solar cooling is an ideal solution for hot and humid environments, such as tropical countries.