PV technical summary

Overview of Photovoltaics (PV)

Technical concept

Photovoltaic electrical generation (PV) refers to systems converting the energy in photons of light to the vibration of electrons, supplying electricity to an end user.  This typically refers to the sun’s light falling on semi-conductor cells to generate electricity. The sunlight has enough energy to break a bond in the semiconductor so that a free electron and hole (a bond missing an electron) can move in either p-type (where there are many free holes) or n-type silicon layer (where there are many free electrons). Due to the electric field in the junction of these layers free electrons and holes can move only in one direction. If the p-type and n-type layers are connected with an external electric circuit by the help of electrical and metal contacts, electrons move from n-type to p-type to decrease the charge imbalance. This movement is the electric current that gives up energy in the external load.

Figure 1A – The principle of a crystalline solar cell

[source: EPIA, Solar Generation 6]

In order to use the power generated by the cell for practical applications which require a particular voltage or current, a number of solar cells are connected to form a solar panel, also called a PV module. There are various different types of PV cells, which are named after the characteristics of the semiconductor material.  The most common types fall into the categories of crystalline and thin film solar cells.

Crystalline solar cells are based on bulk monocrystalline or polycrystalline silicon material as semiconductor. Monocrystalline silicon is a single crystal (i.e. well ordered atomic structure and so few barriers for movement of electrons or holes) wafer extracted from melted absolutely pure silicon by a special process called Czochralski process. Polycrystalline silicon material has crystal structures in varying sizes due to casting which results in lower efficiencies because of the boundaries between the crystals.

Silicon film of 1µm thickness can be deposited on glass or another substrate material, which is called amorphous silicon or thin film. Amorphous silicon does not have a well ordered atomic structure like crystalline silicon and so has more barriers to the movement of free electrons and holes. This leads to generally lower efficiencies than crystalline solar cells. On the other hand, copperindium gallium diselenide and cadmium telluride can be used as semiconductors instead of silicon to manufacture thin film solar cells. Since both materials have a crystalline structure, efficiencies higher than that of amorphous solar cells are achieved. 

The PV market is currently dominated by the crystalline silicon solar cells which represent around 85-90% of the world PV market[1].  The relatively high efficiency of crystalline silicon based solar cells makes them favourable compared to other types. The thin film solar cell technologies represent around 10-15% of the market1. Thin film solar cells have the second are preferred to crystalline solar cells in some cases not only because of their lower costs but also for their flexible application in different ways (especially building integrated PV applications), ease of integration and relatively simple manufacturing processes.

PV module prices (£/Wp) of thin film solar cells are lower than that of crystalline silicon solar cells[2]. Similarly the total system prices of thin film solar cells are lower than crystalline solar cells although installation costs of thin film technology are high in percentage (32% of the total system cost for roof top applications while it is 23% for the crystalline silicon[3]) due to larger surface area to generate the same amount of power.

The types of PV cells and their main characteristics

Type

Characteristic Efficiency range*

Crystalline silicon (85-90% of the market)

Monocrystalline (sc-Si) The most efficient but  more expensive and energy intensive to manufacture (cut from circular ingots)

14-20%

Multicrystallline/Polycrystalline (mc-Si/pc-Si) Lower costs and energy to manufacture (casted)than c-Si but lower efficiency

13-15%

Thin Film (10-15% of the market)

Amorphous silicon (a-Si)  Lower cost and efficiency compared to wafer silicon (sc-Si, mc-Si)

6-9%

Copper-indium-Gallium-Diselenide (CIS) The highest efficiency thin film solar cells but more complex to manufacture than the others

10-12%

Cadmiuim Telloride Manufactured by simple processes but criticised because of cadmium use

9-11%

*These efficiency ranges are given for commercially available solar cells; efficiencies have reached higher levels in labs.

Monocrystalline solar cells, as indicated above, are manufactured by cutting circular ingots of silicon which are obtained mostly by the Czochralski process ( a special method to grow single crystals from a seed crystal) or Float Zone method (a method that passes a heating coil along a pc-silicon ingot and separates a single crystalline-silicon ingot from the input ingot). Cutting circular ingots of silicon to manufacture square shaped solar cells leads to an inefficient use of material.  On the other hand, multicrystalline solar cells are manufactured by casting which requires less material and less energy per unit area of solar cell compared to monocrystalline solar cells.  However, it should be noted that supply of silicon feedstock will be a major challenge for the 1st generation solar cells, i.e. solar cells based on bulk crystalline silicon, as the demand for PV increases. Silicon material for solar cells used to be supplied from the reject or by-products of microelectronics industry, but after 2000 pure silicon is manufactured independently for PV industry which is an energy intensive process. There are developments in this area to minimise the energy use for manufacturing silicon material for solar cells.

There are also other types of solar cells developed or being developed. The developments can be summarised as follows.

Organic PV

Organic solar cells are available in two types: the hybrid dye sensitised technology and in the full organic polymeric technology (also referred to as plastic PV). The dye sensitised solar cells are based on the light absorption capabilities of the charge transfer dye layer which is attached to a nanocrystalline film. The plastic solar cells use the organic polymers to absorb the light and transfer the charges.

Organic solar cells are already in the market with less than 1% market share. So far organic PV has achieved relatively low efficiency levels of around 5%, and increasing this is the most important challenge in developing thin film solar cells.

Organic PV technology has attracted attention because of its low cost materials, low embodied energy and straightforward manufacturing and industrial production up-scaling.  Easy integration gives increased application potential, which allows greater levels of innovation to be embodied in commercial applications such as consumer electronics.

3rd Generation solar cells

3rd generation solar cells are being developed to overcome the efficiency barriers of the previous PV technologies which have a 31-41%  Shockley-Queisser limit (the maximum theoretical efficiency of a solar cell using a p-n junction by increasing the light absorption bandwidth. It is expected that with the 3rd generation solar cells, it will be possible to reach (commercial) efficiencies of over 20% at the same cost levels as thin film solar cells (referred to as 2nd generation solar cells.)

These technologies are mainly focused on nanotechnology and nanomaterials such as nanowires and nanoparticles. As an example, thin film based concepts with silicon nanowires are under development. Nanowire arrays possess excellent anti-reflection and light trapping properties that will enhance the efficiencies of the cells compared to solid thin films of the same thickness.  Another example of 3rd generation solar cells is the quantum dot solar cell which uses quantum dots as the photovoltaic material. Silicon quantum dots are small particles of semiconductor materials which are shown as one of the promising materials for this technology. Photons falling on these dots produce ‘exciton’s (loosely connected electron hole couples) and possibly more than one exciton whereas a photon falling on a conventional PV cell produces only one electron[4]. Another important property of silicon quantum dots is that by varying their size it is possible to absorb sunlight at different wavelengths.

Application

The efficiency of PV is generally greatest when the angle of the sun is perpendicular to the plane of the panels.  Large PV arrays may employ tracking devices to ensure that this is always the case.  However tracking systems are expensive and need more maintenance than fixed systems, and so in many cases fixed systems are installed at a tilt angle that will give the optimum seasonal efficiency.  Broadly speaking this is equivalent to the latitude, but adjustments need to be made to take into account the balance between direct beam radiation and diffuse radiation (resulting from cloud cover).

Tracking PV

 

 

 

 

 

 

 

Tracking (left) and fixed (roof top) PV installation

It is important to minimise shading on a PV array, as this can prevent whole modules functioning.  Therefore the impact of surrounding trees and building structures needs to be assessed.  The spacing of PV modules is also critical in preventing shading.

Spacing to prevent shading

PV systems can be installed in grid-tied or off-grid (also called stand-alone) configurations. Off-grid configuration is preferred in rural areas where power lines do not exist, and it consists of PV array, control and monitoring equipments to manipulate the energy flow, electrical fuses for safety, battery pack to store energy and a DC/AC inverter.

Grid-tied configuration is more common than off-grid configuration in places where power lines exist such as cities. The grid-tied configuration can be designed with or without electricity storage equipment such as batteries. Generally electricity storage on site is not preferred especially in small scale applications because the capital costs are high and exporting electricity to the power grid is incentivised by the recent regulations.

Following figure shows the components of a grid-tied PV system on a roof without electricity storage. The system consists of a PV array, power electronics and safety equipment and metering devices.

 

 

Residential grid-tied PV system configuration and components

Electricity generated by PV array is fed to the local distribution network or internal loads on site (in this case the appliances in a house).  The output of the PV panels is combined into one main feed by PV array circuit combiner. In order to protect the installation and the dwelling from damage, ground fault protector and fuses are connected to the circuit. Another critical component is the inverter which converts the variable Direct Current (in short DC) output of the PV array into Alternating Current (in short AC) that can be applied directly to the power grid (electricity distribution network) and/or to the internal circuit. A microcontroller situated in the inverter inverts the DC voltage generated by PV panels into AC and at the same time maximises the power output from the PV following maximum power point tracker algorithms (MPPT).  The output AC current from the inverter is fed to the external or internal circuit through the utility switch  depending on the electricity demand on site.

 Environmental

The design life of PV is typically 25 years for crystalline and thin film solar cells whereas it is estimated to be 15 years for organic solar cells.  Cells will typically last a lot longer than their design life, although there will be some degradation in the performance, and electronic components may need replacing.

Questions are often raised about the embodied energy within PV, and there have been suggestions in certain parts of the media that PV does not generate as much energy in its lifetime as is required for its production.  The ‘energy payback’ varies considerably in accordance with the type of PV, its method of installation and its location and orientation.  In the UK an energy payback of less than 3 years should be possible, and in sunnier places and with advances in technology, it should be considerably less.

PV needs to be disposed of carefully at the end of its life, and as far as possible critical materials need to be recovered.  Some of these materials are scarce and it is hoped that the transition to new types of PV such as organic will reduce dependence on materials that are significantly constrained.

Commercial

The commercial case for PV varies in accordance with type, scale and location.   In remote places where there is not  an electrical network connection PV can be economic in its own right, as the additional cost of PV generation as compared to conventional generation is outweighed by the cost of connection to the local network.  In other circumstances the commercial viability of PV may be dependent on subsidies or support mechanisms.    Recently there has been a significant reduction in the supply cost of PV, partly down to technological improvements, but largely down to greatly increased volume of manufacture.  The installation cost has not decreased at the same rate, and is a significant part of the cost for small scale (e.g. domestic) projects, which is around 50-60% of the total system cost[5].

 


[1] PV Technology Roadmap, p.8, IEA, 2010

[3] EPIA, ‘Solar Generation 6’ report, Figure 14

[5] EPIA, ‘Solar Generation 6’ report, Figure 14