Amorphous Silicon
 

Amorphous silicon cells are composed of silicon atoms in a thin homogenous layer rather than a crystal structure. Amorphous silicon absorbs light more effectively than crystalline silicon, so the cells can be thinner. For this reason, amorphous silicon is also known as a "thin film" PV technology. Amorphous silicon can be deposited on a wide range of substrates, both rigid and flexible, which makes it ideal for curved surfaces and "fold-away" modules. Amorphous cells are, however, less efficient than crystalline based cells, with typical efficiencies of around 6%, but they are easier and therefore cheaper to produce. Their low cost makes them ideally suited for many applications where high efficiency is not required and low cost is important. One characteristic of amorphous solar cells is that their power output reduces over time, particularly during the first few months, after which time they are basically stable. The quoted output of an amorphous panel should be that produced after this stabilization

10%

Cadmium Telluride 

Copper Indium Diselenide

Other Thin Films: A number of other promising materials such as Cadmium Telluride (CdTe) and Copper Indium Diselenide (CIS) are now being used for PV modules. The attraction of these technologies is that they can be manufactured by relatively inexpensive industrial processes, certainly in comparison to crystalline silicon technologies, yet they typically offer higher module efficiencies than amorphous silicon. New technologies based on the photosynthesis process are not yet on the market.

16%

19%

Polycrystalline Silicon
 

Made from cells cut from an ingot of melted and recrystallised silicon. In the manufacturing process, molten silicon is cast into ingots of polycrystalline silicon, these ingots are then saw-cut into very thin wafers and assembled into complete cells. Multi-crystalline cells are cheaper to produce than mono-crystalline ones, due to the simpler manufacturing process. However, they tend to be slightly less efficient, with average efficiencies of around 12%. They have a speckled crystal reflective appearance, and again need to be mounted in a rigid frame.

20%

Mono- crystalline Silicon

Made using cells saw-cut from a single cylindrical crystal of silicon, they are effectively a slice from a crystal. This is the most efficient of the photovoltaic (PV) technologies. The principle advantage of mono-crystalline cells are their high efficiencies, typically around 15%, although the manufacturing process required to produce mono-crystalline silicon is complicated, resulting in slightly higher costs than other technologies. In appearance, it will have a smooth texture and you will be able to see the thickness of the slice. They are also rigid and must be mounted in a rigid frame top protect them.

25%

Variable blade pitch, which gives the turbine blades the optimum angle of attack. Allowing the angle of attack to be remotely adjusted gives greater control, so the turbine collects the maximum amount of wind energy for the time of day and season.
• The tall tower base allows access to stronger wind in sites with wind shear. In some wind shear sites, the wind speed can increase by 20% and the power output by 34% for every 10 metres in elevation.
• High efficiency, since the blades always move perpendicular to the wind, receiving power through the whole rotation. In contrast, all vertical axis wind turbines, and most proposed airborne wind turbine designs, involve various types of reciprocating actions, requiring airfoil surfaces to backtrack against the wind for part of the cycle. Backtracking against the wind leads to inherently lower efficiency.
• The face of a horizontal axis blade is struck by the wind at a consistent angle regardless of the position in its rotation. This results in a consistent lateral wind loading over the course of a rotation, reducing vibration and audible noise coupled to the tower or mount.

The tall towers and blades up to 45 meters long are difficult to transport. Transportation can now amount to 20% of equipment costs.
• Tall HAWTs are difficult to install, needing very tall and expensive cranes and skilled operators.
• Massive tower construction is required to support the heavy blades, gearbox, and generator.
• Reflections from tall HAWTs may affect side lobes of radar installations creating signal clutter, although filtering can suppress it.
• Their height makes them obtrusively visible across large areas, disrupting the appearance of the landscape and sometimes creating local opposition.
• Downwind variants suffer from fatigue and structural failure caused by turbulence when a blade passes through the tower's wind shadow (for this reason, the majority of HAWTs use an upwind design, with the rotor facing the wind in front of the tower).
• HAWTs require an additional yaw control mechanism to turn the blades and nacelle toward the wind.
• In order to minimize fatigue loads due to wake turbulence, wind turbines are usually sited a distance of 5 rotor diameters away from each other, but the spacing depends on the manufacturer and the turbine model.

A massive tower structure is less frequently used, as VAWTs are more frequently mounted with the lower bearing mounted near the ground.
• Designs without yaw mechanisms are possible with fixed pitch rotor designs.
• The generator of a VAWT can be located nearer the ground, making it easier to maintain the moving parts.
• VAWTs have lower wind startup speeds than HAWTs. Typically, they start creating electricity at 6 m.p.h. (10 km/h).
• VAWTs may be built at locations where taller structures are prohibited.
• VAWTs situated close to the ground can take advantage of locations where mesas, hilltops, ridgelines, and passes funnel the wind and increase wind velocity.
• VAWTs may have a lower noise signature.

A VAWT that uses guy-wires to hold it in place puts stress on the bottom bearing as all the weight of the rotor is on the bearing. Guy wires attached to the top bearing increase downward thrust in wind gusts. Solving this problem requires a superstructure to hold a top bearing in place to eliminate the downward thrusts of gust events in guy wired models.
• The stress in each blade due to wind loading changes sign twice during each revolution as the apparent wind direction moves through 360 degrees. This reversal of the stress increases the likelihood of blade failure by fatigue.
• While VAWTs' components are located on the ground, they are also located under the weight of the structure above it, which can make changing out parts very difficult without dismantling the structure, if not designed properly.
• Having rotors located close to the ground where wind speeds are lower due to the ground's surface drag, VAWTs may not produce as much energy at a given site as a HAWT with the same footprint or height.