Technology Outline

1) Solar Cells
2) Emerging PV Technologies
3) PV Modules
4) Electrical Properties of PV Modules
5) PV Systems
6) PV Power Station
7) Balance-of-system

Solar Photovoltaic System is also called Solar Electric System. Silico- based PV technology first appeared almost half a century ago. Over the years, PV technology has progressed significantly and has become a major type of renewable energy technology.

1) Solar Cells

The basic building unit of a PV system is a PV module, which in turn in made up of solar cells. A solar cell converts the light energy in sunlight into electricity by means of the photoelectric phenomenon found in certain types of materials such as silicon and selenium. When sunlight strikes on a solar cell, electrons are "excited" to become "free electrons" which can flow through an external circuit, and hence generate an electric current.

Animation showing working principle of solar cell. The text above describes the image.

(Above: Animation showing working principle of solar cell)

Solar cells in the market can be classified into two main categories - crystalline silicon cells and thin-film cells. Crystalline silicon cells can be further divided into mono-crystalline cells and poly-crystalline cells. Thin-film cells include the amorphous silicon cells, copper indium diselenide cells (CIS) and cadmium-telluride cells (CdTe). The classification of commercial solar cells can be summarized as follows:

Monocrystalline cells - Fraunhofer Institute for Solar Energy Systems. The text above describes the image.

(Sources for above: Monocrystalline cells - This web page has hyperlinks which may transfer you to third-party website.Fraunhofer Institute for Solar Energy Systems)
                             Polycrystalline cells - This web page has hyperlinks which may transfer you to third-party website.Lanitis Solar
                             CIS cells - This web page has hyperlinks which may transfer you to third-party website.Solar World)

The performance of a solar cell is expressed in terms of its "energy conversion efficiency", i.e. the efficiency in converting the energy in sunlight into electricity. The earliest silicon solar cells had efficiencies of just a few percents. Nowadays commercial solar cells can approach almost 20% in efficiency (with some special designs exceeding 20%), while special-made cells and experimental cells can exceed 30%. (It should be noted that efficiency of a PV module is lower than that of the constituent solar cells, and efficiency of a PV system is lower than that of the constituent PV modules, meaning that cell efficiency > module efficiency > system efficiency.)

Monocrystalline cells
Monocrystalline cells are made from thin slices (wafers) cut from a single crystal of silicon, which is produced by immersing a crystal nucleus with a defined orientation into a bath of melt-silicon and very slowing drawing the crytal from the bath. The wafer is doped with impurities to form p-type areas and n-type areas. After that, electrical leads are attached to the wafers, thus forming the monocrystalline cells. The cell efficiency of monocrystalline cells is in the range of 15 - 18%.

Polycrystalline cells
Polycrystalline cells are made from thin slices (wafers) cut from a cast silicon block. After doping the wafer with impurities and attaching electrical leads to the wafer, polycrystalline cells are formed. Since crystals of various orientations are formed during block casting, the surface of a polycrystalline cell has an appearance of shattered glass. The cell efficiency of polycrystalline cells is in the range of 13 -16%.

Amorphous silicon cells
Amorphous cells are made by applying a thin layer (film) of active silicon on a solid substrate or flexible backing, typically a thin stainless steel sheet. After doping the wafer with impurities and attaching electrical leads to the wafer, polycrystalline cells are formed. The advantages of amorphous silicon cells include lower cost than that of crystalline cells, and can be applied on flexible and light-weight substrate. However, they have the disadvantage of lower efficiency and the problem of light-induced degradation. The module efficiency of amorphous silicon modules is in the range of 5 - 8 %.

Copper indium diselenide cells (CIS cells)
The active semiconductor material of CIS cells is made from copper indium diselenide alloyed with gallium and/or sulphur. The CIS cells do not have the problem of light-induced degradation but they show stability problems in hot and humid environments. The module efficiency of CIS modules is between 7.5 - 9.5%, which is the highest among all thin-film technologies.

Cadmium telluride cells (CdTe cells)
CdTe cells are thin film cells with cadmium telluride as the active semiconductor material. The cadmium telluride layer acts as the p-type absorber layer and is coated on top of an n-type cadmium sulphide layer. The module efficiency of CdTe modules is between 6-9%

2) Emerging PV Technologies

Apart from the solar cell types mentioned above, many research activities are going on to develop new types of cells, aiming to increase the energy conversion efficiency or to reduce the cost. Some examples are given below.

(a) High-concentration PV technology

High concentration PV technology is developed to reduce the amount of PV materials needed for certain power output. It makes use of mirrors or lenses to "concentrate" sunlight onto a much smaller area of active semiconductor PV cell, which produces power at a higher efficiency than at normal solar irradiance level. Actively sun tracking devices are applied in high-concentration PV system to maximize energy production.

An example of a company producing high-concentration PV power generation systems is This web page has hyperlinks which may transfer you to third-party website.AMONIX.

Working principle of high concentration PV technology. The text above describes the image.
Above: Working principle of high concentration PV technology (Source: This web page has hyperlinks which may transfer you to third-party website.AMONIX)

(b) SLIVER technology

The SLIVER technology applies the conventional moncrystalline PV technology in an innovative way to save silicon material. Thin and bifacial moncrystalline (100 mm long, 0.5 - 2 mm wide and 40-60 um thick) solar cells, called Slivers, are produced by micromachining and conventional wafer process techniques, which are used to make solar modules. The module utilizes a reflector, and has the slivers cells arranged with space between each cell. As a result, the cells occupy only half of the module surface area and hence the amount of silicon for module production can be reduced.

Working principle of a Sliver module (Source:The Australian National University). The text above describes the image.
Above: Working principle of a Sliver module (Source: This web page has hyperlinks which may transfer you to third-party website.The Australian National University)

Sliver cells (Source:The Australian National University). The text above describes the image.
Above: Sliver cells (Source: This web page has hyperlinks which may transfer you to third-party website.The Australian National University)

The Sliver technology reduces silicon consumption but demands more processing procedures than conventional solar cell fabrication. Research is being carried out to simplify the fabrication procedures.

For more information, please visit the website of the This web page has hyperlinks which may transfer you to third-party website.Centre for Sustainable Energy Systems at the Australian National University.

(c) Organic/polymer PV technology

The development of organic/polymer PV cells aims to make use of organic materials such as pentacene to replace silicon to produce flexible, light-weight PV cells with lower cost. The main issue with organic/polymer PV cells is low efficiency, and research is being carried out to improve the efficiency.

(d) Microcrystalline and micromorphous cells

Microcrystalline cells are produced by applying new thin-film technology with the deposition process occurring at a temperature between 200 - 500 degrees Celsius to produce crystalline silicon films with very fine grains. The maximum efficiency of microcrystalline cells can reach 8.5%. In order to improve their efficiency, microcrystalline cells are combined with amorphous silicon in tandem cells to better utilize the solar spectrum to give a maximum cell efficiency of 12%. The tandem cell is referred to as micromorphous cells.

(e) Hybrid HIT cells

The HIT (Heterojunction with Intrinsic Thin layer) solar cell is made up of a thin crystalline silicon wafer surrounded by ultra-thin amorphous silicon layers that is bonded with an undoped thin film (the intrinsic thin layer) in between. The HIT cells have higher efficiency than conventional crystalline cells, do not suffer from degradation by ageing like amorphous thin film cells. HIT solar cells can also achieve higher energy yield at high temperature, as compared to crystalline solar cells. More information is available in This web page has hyperlinks which may transfer you to third-party website.SANYO HIT Photovoltaic Module website.

3) PV Modules

An individual solar cell can only produce a small amount of power (typically 1 to 2 watts). To increase the electrical output, solar cells are combined in series to form PV modules. PV modules come in different forms such as the usual framed PV panels, PV glass units, flexible PV modules, PV roof tiles. Typical ratings of standard modules are from 50W to 150W, with higher ratings also available in the market. There are also many small PV modules designed for toys, calculators, etc.

Framed PV panels. The text above describes the image.
Above: Framed PV panels

PV glass units. The text above describes the image.
Above: PV glass units

Flexible PV module. The text above describes the image.
Above: Flexible PV module

PV roof tile. The text above describes the image.
Above: PV roof tile (Source: This web page has hyperlinks which may transfer you to third-party website.Premier Power Renewable Energy)

In actual systems, modules are connected in series to form "PV strings", and strings are combined in parallel to form a "PV array" (or "PV generator").

4) Electrical Properties of PV Modules

Each PV module can be characterized by its performance curve, i.e. the current-voltage curve (I-V curve). The performance of solar module is tested under standard testing conditions (STC) as defined in the IEC 60904 standards: cell temperature of 25 degrees Celsius, incident solar irradiance of 1000 W/m2, spectral distribution of the light spectrum with an air mass AM = 1.5.

I-V characteristics of a PV module. The text above describes the image.
Above: I-V characteristics of a PV module

Three points on the I-V curve are important in defining the performance of a PV module, i.e. the maximum power point, the short-circuit current and the open-circuit voltage.

  • The maximum power point (MPP) is the point on the I-V curve at which the PV module works with maximum power output.
  • The short-circuit current (Isc) is the maximum current output of a module.
  • The open-circuit voltage (Voc) is the maximum output voltage of a module.

Since PV modules in the field are not working under STC, the actual performance could be ten to fifteen percent lower than that of the STC rating.

 I-V characteristics of a PV module at various irradiance, constant temperature. The text above describes the image.
Above: I-V characteristics of a PV module at various irradiance, constant temperature

I-V characteristics of a PV module at various temperature, constant irradiance. The text above describes the image.
Above: I-V characteristics of a PV module at various temperature, constant irradiance

5) PV Systems

The function of a PV system is to generate electricity from sunlight, either in the form of DC or AC, to meet the demand of electrical loads. A PV system is made up of a PV array and the balance-of-system equipment such as charge controllers or inverters, electric cables and switchgear, surge arrestors, etc.

PV arrays at locations in the northern hemisphere such as Hong Kong are usually installed facing south with a tilt angle near to the latitude of the location, so as to maximize the amount of electricity generated over the course of a year.

PV systems in Hong Kong can be classified into two main types - stand-alone systems and grid-connected systems. These can further be divided into ordinary PV systems and building-integrated PV (BIPV) systems. For BIPV systems, the PV modules are integrated into the building envelop as part of the building structure. They replace some of the building components on the roof or on the facade, and produce electricity to meet a portion of the electricity demand of the building.

(a) Standalone PV systems operate without any interaction with the utility grid. Most standalone PV systems comprise of PV panels, a charge controller and storage batteries to supply power to DC loads. If the system has to supply power to AC loads, an inverter is needed to convert the DC power into AC power. As sunshine is intermittent in nature, storage batteries are needed to store some of the electricity generated by the PV panels, so that when sunshine is insufficient, the system can still supply power to the loads.

A standalone PV system supplying DC and AC loads. The text above describes the image.
Above: A standalone PV system supplying DC and AC loads

Standalone PV systems are usually found in locations where connection to the grid is not convenient or not economical, such as telecommunication equipment or telemetry stations in remote locations. Solar-powered lighting poles are also installed in different locations in Hong Kong. There are also some earlier BIPV systems designed to operate as standalone systems.

Standalone PV systems. The text above describes the image.

(b) Grid-connected (or grid-tied) PV systems in Hong Kong are connected to grid indirectly. The AC output of the PV system is connected to the electrical distribution system of a site or a building, and therefore the PV system operates in parallel with the electricity supply from the grid to meet the electricity consumption of the site or building. In this way, storage batteries are not required. More information is given in the Grid Connection section and Solar PV - Grid Connection section of this website.

Grid-connected (or grid-tied) PV systems. The text above describes the image.


Largest grid-connected PV system in Hong Kong - 350 kW system at EMSD Headquarters. The text above describes the image.
Above: Largest grid-connected PV system in Hong Kong - 350 kW system at EMSD Headquarters

6) PV Power Station

PV power stations are facilities constructed to generate electricity from direct sunlight with large arrays of PV panels. The generating capacities of PV power stations are usually in the range of a few hundred kilowatts to a few megawatts. PV power stations usually occupies huge amount of space and in locations with good solar resources.

7) Balance-of-system (BOS)

Apart from the PV array, the other components required to make up a PV system are referred to as the balance-of-system (BOS). The BOS includes combiner boxes (junction boxes), charge controllers and storage batteries for standalone systems, inverters (for grid-connected systems or systems supplying AC loads), mounting structures, wiring, switchgear and fuses, surge arrestors, earth-fault protection devices and so forth. The BOS may account for up to half of the capital cost of a PV system (for small systems) and most of the maintenance cost. Major components of BOS are described below.

(a) Charge controllers (for standalone systems)

A charge controller is a device used in standalone PV systems for regulating the current flowing from the PV array to the storage batteries. At high irradiance, the PV array keeps charging the batteries and when the battery is "full", the charge controller disconnects the batteries from the PV array. The configuration of a shunt-type charge controller is shown below (the other type is series-type).

 Charge controllers (for standalone systems). The text above describes the image.

A MPP (maximum power point) charge controller incorporates a DC-to-DC converter such that the PV array can operate at a voltage which can deliver maximum output power at the prevailing solar irradiance.

A MPP (maximum power point) charge controller incorporates a DC-to-DC converter. The text above describes the image.

(b) Inverters

An inverter is a device to convert electricity from DC to AC. Since PV panel output and battery output are DC, an inverter is needed in a PV system intended to supply AC loads. Standalone inverters are used for standalone PV systems, and grid-tie inverters are used in grid-connected systems.

Power loss occurs during the DC to AC conversion process. The conversion efficiencies of inverters of different designs are generally in the range of 85% to 95%. An inverter of good conversion efficiency should be used to reduce energy loss. Besides, the quality of the AC generated (such as the harmonic contents, voltage and frequency stability) and the protection features (such as short-circuit protection, overload protection, deep discharge monitoring for standalone inverters) should also be considered when choosing an inverter.

Grid-tie inverters are discussed in the Solar PV - Grid Connection section.

A wall-mounted grid-tie inverter. The text above describes the image.
Above: A wall-mounted grid-tie inverter

(c) Storage batteries

Storage batteries are essential for standalone PV systems as electricity generation is not constant and is not controllable (i.e. depends on solar irradiance). With storage batteries, electricity generated by PV panels can be stored up and used to power the electrical loads when there is no sunlight.

Lead-acid batteries, in particular the deep-cycle battery is the most popular battery type for PV systems as they are designed to be charged and discharged frequently and can handle heavy discharges time after time. The life-span of deep-cycle lead-acid batteries varies from 3 to 8 years. In PV systems, the storage capacities of battery systems are generally in the range of 0.1kWh to 100kWh.

Battery capacity is usually expressed in ampere-hour (Ah). In theory, a battery rated at 50 Ah will delivery electricity at 1 ampere for 50 hours or 2 amperes for 25 hours. (The storage capacity of a battery in kWh is equal to the Ah rating times voltage rating divided by 1000. The storage capacity of a battery system in kWh is the sum of the kWh capacities of the individual batteries.) When sizing the capacity of the battery system, an important consideration is the "days of autonomy", which is the number of days that the battery system is capable of supporting the electrical loads without being recharged in cloudy and rainy days.

The major problem of lead acid batteries is ageing, meaning an increase in internal resistance with age. Ageing of lead acid shortens its service life. Another problem caused by the use of lead acid batteries is the potential environmental hazard caused by lead upon disposal.

Storage batteries. The text above describes the image.

 
This flash illustrates working principle of solar cell. The paragraph above describes working principle of solar cell.
 
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