Solar PV

Solar PV
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Solar PV

Solar photovoltaic, or simply photovoltaic (SPV or PV), refers to the technology of using solar cells to convert solar radiation directly into electricity. A solar cell works based on the photovoltaic effect. R&D and practical experience with photovoltaics have led to the development of three generations of solar cells: Crystalline silicon based solar cells, thin film solar cells and third generation PV. 

Responds to the following needs

  • Diversification of energy sources
  • Cleaner energy sources
  • Reduced GHG emissions

Suitable for

  • Areas with high solar irradiation
  • Residential as well as commercial buildings
  • Small off-grid systems and grid connected systems

Relevant CTCN Technical Assistance

Introduction

Solar PV has enormous energy potential and many countries have implemented policies and incentive schemes to support its growth as a source of energy. A key driver is the fact that, due to a steep learning curve and increased competition, rapid cost reductions of Solar PV systems have been experienced over recent decades. It has energy payback periods ranging from 2 to 5 years for good to moderate locations and lifecycle GHG emissions in the order of 30 to 70 gCO2e/kWh depending on panel type, solar resource, manufacturing method and installation size. This compares to emission factors for coal fired plants of more than 900 gCO2e/kWh and for gas fired power stations of more than 400 gCO2e/kWh, showing the large potential for solar PV to contribute to reductions in carbon emissions from the electricity sector.

The photovoltaic effect

The photovoltaic effect can be briefly summarised as sunlight striking a semiconductor and causing electrons to be excited due to energy in the sunlight (photons). The excited electrons become free of their atomic structure and, in moving away, they leave behind ‘holes’ of relative positive charge that can also migrate throughout the material. By placing two different semiconductors together in thin layers (or wafers) the free electrons and ‘holes’ can be separated at their interface/junction, creating a difference in charge, or voltage, across two materials. Sometimes, the term “p-n junction” is used which refers to the two different types of semiconductor used. A single such arrangement, or cell, creates only a modest voltage and current, but when arranged into larger arrays the cells can produce useful amounts of electricity which is known as solar PV electricity.

On the basis of their manufacturing process, solar cells consist basically of three main components - the semiconductor, which absorbs light and converts it into electron-hole pairs, the semiconductor junction, which separates the electrons and holes, and the electrical contacts on the front and back of the cell that allow the current to flow to the external circuit. R&D and practical experience with photovoltaics have led to the development of three generations of solar cells.

Technology

The common PV technologies can be broadly categorised into two groups – crystalline silicon and thin film. Crystalline silicon technologies account for the majority of PV cell production, whereas thin film is newer, less efficient, but growing in popularity

Crystalline silicon based solar cells

The first generation is represented by crystalline silicon (c-Si) modules, which may be single- (sc-Si) or multi-crystalline (mc-Si) depending on the manufacturing technique. They dominate the PV market with around 90% share. Improvements in efficiency have been mirrored by improvements in manufacturing techniques including thinner cells (lower material costs), larger wafers, increased automation and other factors that likewise contribute to the significant cost reductions seen in the past decades. Manufacturers are now striving to use less silver and other expensive materials (maybe replacing silver with copper), while maintaining or even extending the technical life of cells and modules.

Thin film solar cells

Second generation technologies, so called thin film (TF) solar cells are based on alternative materials such as cadmium telluride (CdTe), copper indium gallium diselenide (CIGS), amorphous silicon and micromorphous silicon set as thin films. The layer that absorbs the sunlight is only a few micrometres thick and can be deposited onto relatively large smooth surfaces such as glass, metal or plastic. This PV type has the advantage of lower labour and energy intensity compared to crystalline silicon PV but a reduced efficiency in terms of electricity generation. Thin films of various sorts now represent only about 10% of the market, down from 16% in 2009.

Multi-junction cells

Multi-junction cells, also called Third Generation, involves superposing several cells in a stack. In the case of two cells, it will form a double junction, also called a tandem cell. They were originally developed for use in space and have multiple junctions typically using more exotic semiconductors such as gallium and indium compounds. These types of cells have already crossed the maximum theoretical efficiency of single junction solar cells, and many laboratories have reported lab scale solar cells reaching efficiencies in the excess of 40%. Third generation cells are typically considered in combination with solar concentrator systems as described below and are currently being commercialised in this context. The use of concentrators allows much smaller cells to be used which in turn reduces the cost associated with these more exotic materials.

Concentrated solar PV

Solar cells have been found to operate more efficiently under concentrated light which has led to the development of a range of approaches using mirrors or lenses to focus light on a specific point of the PV cell, called concentrator systems. Specially designed cells use heat sinks, or active cooling, to dissipate the large amount of heat that is generated. This type of concentrating configuration requires a sun tracking system using either single axis or double axis tracking to make sure that the mirrors/lenses are always pointing at the correct orientation. The concentrating photovoltaics (CPV), although growing significantly, represent less than 1%.

Off-grid and grid connected PV

There is an obvious yet important qualification to the discussion above on efficiency, which is that solar panels are limited to only produce electricity in periods of sunlight, either direct light or diffuse sunlight on overcast days. During the night they will not produce power. This means that solar cells, if used for remote/off-grid generation purposes, need to be implemented in conjunction with some kind of storage system such as a battery or as a hybrid system with some other type of generator. Where solar cells are grid connected this is less of a problem. They can be used during the day to reduce the local demand from the grid (or even to export back to the grid) and then at night, or during periods of low incident light, the grid can supply the necessary power. The former kind of application, as a remote or off-grid generator, is most commonly observed in developing countries and isolated areas, while grid-connected solar PV is more common in industrialised countries which have a wider reaching grid.

Grid connected solar PV also can have differences in the approach used depending on the way in which customers purchase the electricity. If the solar array is distributed, for example over a larger number of residential houses, then the single installations are operated by the consumer directly. The advantage of this to the consumer is that the cost of electricity, that the consumer must compete with, is the distributed cost, i.e. the cost to purchase power at the location of demand which is normally significantly higher than the actual levelised production cost of electricity (that doesn’t account for transmission/distribution charges/losses and profit margins along the value chain). Solar installations can also be large and centralised but this demands that the power is sold into the common grid at market prices and must compete directly with other technologies (bearing in mind any subsidies that might be applicable for solar generation).

Decentralised systems represent approximately 60% of the global market while centralised, utility-scale systems represent close to 40%. Off-grid systems, which once dominated a much smaller market, now account for 1% at most.​

Solar Home System (SHS)

Solar Home Systems are stand-alone PV systems that offer a cost-effective mode of supplying amenity power for lighting and appliances to remote off-grid households. In rural areas, that are not connected to the grid, SHS can be used to meet a household's energy demand fulfilling basic electric needs. Globally SHS provide power to hundreds of thousands of households in remote locations where electrification by the grid is not feasible. Bangladesh is currently (2015) the world’s largest market for solar home systems.  

Feasibility of technology and operational necessities

Resource and Location

Locations closer to the tropics tend to have higher solar irradiation and hence a higher potential for solar PV electricity generation. There is a marked difference in resource levels geographically with northern Africa, for example, being exposed to more than twice the level of solar energy as northern Europe; implying that for the same size panel the electrical output could be doubled in the former location. Having said that, Germany has the largest installed capacity in the world due to domestic incentives there, illustrating that other factors particularly relating to financial aspects and incentives greatly influence the current global distribution of solar PV installations. 

Typically satellite data is used to determine the average yearly radiation level at a site for a number of reasons i) local ground based measurements are expensive and equipment must be cleaned to prevent soiling ii) satellites can provide up to 20 years of data for an average which is important given the large annual variation in solar irradiation levels and iii) the accuracy of satellite data is found to be good in correlation with ground based measurements.Based on these estimates of resource and the associated time-series/seasonal-variation it is possible to estimate the power that would be generated throughout a typical year. This allows the economics of a project to be determined and also allows other aspects of the system (for example battery size if it is an off-grid application) to be calculated.

Technical Requirements

The technical requirements for the installation of solar PV vary greatly depending on the size of the system and kind of technology used. Small off-grid systems in remote/rural areas using first generation technology, such as solar home systems, can be bought in what is effectively a ‘kit’ form and installed with relatively little local expertise. Maintenance is minimal and mainly requires the cleaning of the solar panel to ensure efficiencies are maintained. Alternately the installation of grid scale concentrating solar power with third generation technology is a highly specialised field, requiring detailed calculations for the plant layout, expected yield and economics of the project. The equipment, with the required tracking mechanisms, requires maintenance and upkeep, and the power output must be forecast for export.

Legal/Regulatory

The legal and regulatory requirements for solar PV are relatively few compared to some other renewable technologies. They have a low local environmental impact and are not very visible (for small applications they are often mounted on the roofs of buildings) typically making public/permitting acceptance high. Grid connected systems require an appropriate licence or permit to export to the grid along with the necessary metering equipment, connected by a professional, to ensure that the level of export to the grid is measured for any subsequent compensation. Larger installations obviously require the appropriate planning permissions that would accompany any moderate to large infrastructure project.

Rapid spread to new markets

Solar PV is starting to play a substantial role in electricity generation in some countries as rapidly falling costs have made unsubsidised Solar PV-generated electricity cost-competitive with fossil fuels in an increasing number of locations around the world. In 2015 investments continued to move into developing country markets as companies and investors sought higher yield, even at the expense of higher risk. Solar PV (and biofuels) provided the largest numbers of renewable energy jobs. In favourable circumstances (i.e. with good resources and a secure regulatory framework), solar PV is cost-competitive with new fossil capacity, even without accounting for externalities. Between 2014 and 2016, solar PV capacity increased from 177 to more than 300 GW. 

An estimated 22 countries had enough capacity at end-2015 to meet more than 1% of their electricity demand, with far higher shares in some countries (e.g., Italy 7.8%, Greece 6.5% and Germany 6.4%). China achieved 100% electrification, in part because of significant off-grid solar PV installed since 2012; on-grid, however, curtailment of solar generation started to become a serious challenge for China’s solar PV sector. The industry recovery of recent years strengthened further due to the rise of new markets and strong global demand, and most top-tier companies were back on their feet in 2015. Record-low bids for large-scale solar PV projects were seen in tenders from Latin America to the Middle East to India. Distributed rooftop solar PV remains more expensive than large-scale projects but has followed similar price trajectories and is competitive with retail prices in many locations. The top five countries that invested in Solar PV capacity were China, Japan, United States, UK and India.



DRE (Distributed Renewable Energy) solar PV markets also continued to flourish. Roughly 44  million off-grid pico-solar products had been sold globally by mid-2015, representing an annual market of USD 300 million. About 70 countries worldwide either had some off-grid solar PV capacity installed or had programmes in place to support off- grid solar PV applications by the end of 2015.

Product examples

[Disclaimer: Products listed here are provided by Climate Technology Network members and represent examples of products available within this technology field. The CTCN does not take responsibility for this product information and cannot guarantee its suitability in specific contexts or regions.]

Case studies

  • Brazil: Improving agricultural productivity and income generation through the use of Photo-Voltic water pumping systems in semi-arid Brazil
  • India: 100 kWp Floating Solar PV System  Floating Solar PV System, i.e. installation of solar panels on water bodies, is emerging as an alternative for conventional ground mounted PV installations, which are land intensive. Presently the technology of floating platform is limited to very few manufacturers and too expensive compared to conventional PV systems. In order to indigenously develop a cost effective floater platform, a collaborative project has been taken up by NETRA with Central Institute for Plastic Engineering and Technology (CIPET), Chennai. 

    In Phase I of the project, basic design, prototype development was done along with installation of 5 kWp pilot floating system, at NTPC Kayamkulan reservoir. The design and manufacturing process of the floaters were further revised to produce floaters for installation of the 100 kWp system as a second pilot system at the same location. 

References

  • ClimateTechWiki
  • Energypedia
  • IEA Technology Roadmap: Solar Photovoltaic Energy - 2014 edition
  • IPCC
  • REN21 Renewables- Global Status Report 2016