Ongoing government financial support and increasing support of investors have lead to innovations in solar energy’s infinite potentials.
There are three solar energy active conversions:
- solar photovoltaic,
- concentrated solar power, and
- solar heating and cooling.
Photovoltaic panels use silicon solar panels to convert the sun’s radiation to electrical energy. PV cells can be installed according to electricity needs and can power small devices like calculators and watches to a whole residential power requirement.
There are also two kinds of photovoltaic panels: crystalline silicon (c-Si) modules and thin film modules.
The majority of PV modules are based in crystalline silicon due to abundant resources. There are two main forms of c-Si: single-c-Si module and mc-Si or multi-crystalline silicon module. Commercial sc-Si converts electricity better than mc-Si while the latter is less expensive than single crystalline silicon modules.
Thin film modules
Thin film PV modules are made by depositing extremely thin layers of photosensitive materials in glass, plastic, or stainless-steel backing. The first thin film produce is a-Si or amorphous silicon. It offers several advantages including low consumption of raw materials, high production and automation efficiency, better performance in high ambient temperature, simple assembly and manufacturing integration, and more resistant to overheating. On the other hand, thin film modules have lower efficiency than crystalline silicon modules and the industry is yet to develop modules with long-term reliability.
Photovoltaic technology is a reliable source of electricity to residential and commercial users due to the effective supporting policies and remarkable cost reduction. Emerging technology include concentrating photovoltaic and organic solar cells. Concentrating PV utilizes an optical concentrator system that focuses sunlight into a small efficiency cell.
Concentrated solar power
CSP concentrates energy from the sun’s rays to heat a thermal receiver suitable to hold high temperatures. Unlike a photovoltaic system where sunlight is converted to electricity, the thermal receiver converts sunlight to heat where it will be transported to a steam generator to convert it to electricity.
Concentrated solar power has been in operation for almost two decades and is a proven solar energy technology. It is not as marketable and simple as PV modules but the government of United States and Spain supported the technology to respond to the global crisis. CSP are mostly used by large power grid companies as it can collect and generate more electricity than PV panels. There are four current CSP technologies categorized in their ability to focus the sun’ rays and receive the sun’s energy: parabolic troughs, parabolic dishes, linear Fresnel collectors, and Towers or CRS.
Parabolic troughs consist of two parallel lines of mirror or reflector curved in a single direction to focus the sun’s energy to a fluid carrying receiver placed in the heart of the curved trough mirror. The sun’s energy heats the fluid inside the tube and the generated heat energy is used to generate electricity using a steam engine or generator. The tubes or absorber collectors are generally made of stainless steel and coated with a selective coating. Both the reflector and absorber collector moves with the sun as it crosses the sky. This ensures that the sun is continually focused on the receiver pipes. Parallel lines of parabolic troughs are called collector fields.
Parabolic trough plants are hybrid. When there is insufficient sunlight, the plants burn natural gas such as coal and fuel to meet load requirements.
Parabolic dish plants use dish mirrors to concentrate sunlight and focus it to a thermal receiver. Unlike parabolic troughs, parabolic dish is a standalone unit, which is composed of a collector, thermal receiver, and an engine. The entire unit tracks the sun all throughout the day. The engine eliminates the need for heat transfer fluid and cooling water. Instead, the engine or generator is air-cooled. Parabolic dish uses dual axis collectors, allowing it to capture maximum amount of sunlight during the day. Compared to other CSP technologies, parabolic dishes offer the highest solar energy to electricity conversion as it can achieve extremely high temperatures and high efficiencies.
One of the drawbacks of small parabolic dishes is it does not work well with thermal storage. The converted electricity must be fed immediately to solar grids. However, very large parabolic dishes are found to be more compatible with thermal storage and natural gas back-up. Manufacturers as well as promoters see huge parabolic dishes to compete with larger solar thermal plants in the future.
Linear Fresnel Reflectors
This CSP technology is much like parabolic troughs as it uses a single axis collector and a fluid-carrying receiver. However, linear Fresnel collectors are made of long rows of ground mounted mirrors or reflectors to concentrate the sun’s rays. The thermal receiver is elevated and fixed atop the reflectors. Although the design has lower efficiency than parabolic troughs, it requires less land and is also more inexpensive than troughs. The current design of LFRs made possible direct steam generation by allowing water to be fed directly on the thermal receiver and be boiled at about 50 bars of atmospheric pressure. The saturated steam produced is used to power a steam cycle. LFRs produce lower optical efficiency compared to troughs and are low in compatibility with thermal storage.
Also known as central receiver systems, power towers employ thousands of field tracking reflectors, heliostats, to collect sun’s radiation. The reflectors are mounted atop a fixed tower. Sunlight is absorbed molten salt or pressurized water, working the fluid flowing through the receiver and serving as thermal storage as well. Power towers surpassed the operating temperature of parabolic troughs and linear Fresnel reflectors but not parabolic dishes. Power tower design offers more flexibility as designers can choose a wide array of heliostats, thermal storage, power blocks, and transfer fluids. This CSP technology has more potential for lower operating costs than line-focus technologies like parabolic troughs and LFRs.
PV panels or CSP
PV panels are more marketable and available to end-users. However, CSP is considered more cost-effective than photovoltaic panels. CSP can convert 60 – 80% of the sun’s rays to electricity while PV panels can only convert 10 – 15% of sunlight to grid compatible electricity. PV panels’ energy generation can be scarce as it is limited to the amount of sunlight whereas CSP reigns supreme especially in desert locations. On the other hand, the simplicity and availability of PV panels make it a more viable solution than CSP that is yet to make its mark on the market. With the progression of solar energy technology, there is a great possibility of combining PV panels with CSP technology as demand for green energy increases steadily in different parts of the world.
Solar thermal energy (STE)
Solar thermal energy is the harnessing of sunlight and utilized it in specific purposes such as heating/cooling water or building spaces. Unlike PV panels, solar thermal collectors convert heat to thermal energy so it can be used as a heating and cooling facility. Solar thermal collectors are categorized in low, medium, and high collectors. Low thermal collectors can heat swimming pools using flat plates that are similar to PV panels. Medium thermal collectors are used for both residential and commercial use and also utilize flat plates. High thermal collectors use CSP technology as it can convert heat better than flat plates. It is use in general power production. Concentrated solar power is a technique employed by solar thermal plants.
When things run smoothly, we enjoy a comfortable and convenient life. But if something happens to disrupt that grid, then what we depended on will come crashing down. When you depend on anything else but your own resources, you can’t count on it always being there for you. That’s why you always need to find a way to live off the grid.
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“A Guide to Going Off the Grid“.