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An analysis of the energy efficiency of photovoltaic cells in reducing CO2 emmisions


Introduction

With advances in technology and increasingly busy lifestyles, people are becoming ever more reliant on technology, and thus the demand for energy is increasing, (237.7 Million tonnes 2001 of oil equivalent energy, compared to 213.6 Million tonnes in 1990) (Energy Consumption in the United Kingdom, n.d., chapter 1). The Graph below shows the total energy copnsumption for the UK between 1970 and 2001.


Figure 1: UK Total Energy Comsumption between 1970 and 2001. (Energy Consumption in the United Kingdom, n.d., chart 1.1)


Due to the scarcity of fossil fuels, such as lowering reserves and disruption caused by conflict, and growing concern over climate change, renewable energy is increasing. Over recent decades, solar cells have improved greatly in terms of cost and efficiency, and is quickly approaching a viable alternative to fossil fuels. Solar Grid Parity (SGP) is the point at which, if current trends continue, the lifetime cost of photovoltaic cells becomes equivilent to the cost of mains electricity (Lewis, 2009, p. 50). Often billed as 'clean energy', refering to the lack of fossil fuel burning required to produce the energy, alternative energy sources are regarded as an eco-friendly solution, however, often the environmental cost of producing the means of generating the energy is not considered. This article analyses the environmental impact of materials and processes used in the construction of photovoltaic cells, also known as solar panels, and compares the energy savings of the renewable energy produced to the energy consumed in the manufacture of the cells.

I have chosen to study this area as I have recently been researching photovoltaic cells for a job interview, and also have keen interest in polar exploration, and the effect climate change is having on the environment. In the past conducted surveys for the British Antarctic Survey (BAS) and the Kew Gardens Millenium seed bank project, measuring the effect of climate change.

Production

Photo Voltaic (PV) cells are created using purified silicon. Silicon is not natuarlly occuring in it's pure state and so must be extracted from silicion dioxide quartz, by heating the quartz in an electric arc furnace and applying a carbon arc to the substance, this process releases the oxygen as it combines with the carbon to create CO2, and leaves a molten silicon with a 1% impurity. The impurities are dragged to one end of the material using the floating zone technique and the impure section is then removed, leaving purified silicon which is then used as the basis for the PV cell ("How Solar Cell is Made", n.d. para. 8). Photovoltaic cells can make use of silicon that does not meet the requirements of the semi conductor industries, so some of the CO2 emissions can be offset as they would have been produced in the initial processing (Alsema, Frankl, Kato, 1998, p. 1). The production of PV cells also produces significant levels of Heavy Metal pollution. (Fthenakis, V,M., Chul Kim, H., Alsema, E, 2008, p. 2171).

Recently, alternative methods of producing PV cells have been discovered, which may reduce the environmental impact of PV cell production, along with the cost. These methods include, thin film cells, which are cheaper and continuously manufactured, and non-silicon based modules, which are not affected by silicon supply shortages (Lewis, 2009, p. 52).



Running Cost

Once produced and installed, the running costs of the photovoltaic cells are negligable (Alsema, Frankl, Kato, 1998, p. 1), but may not work at optimum effieciency due to climate and location. Solar Radiation from the sun occurs at 1000 Watts per day, in optimum conditions ("How Solar Cells Work" n.d. para. 3), while climate factors mean that significantly less can be used by PV cells. In the UK is typically 1,100 W per year, studies have shown that electricity costs tend to be higher in cloudy areas, such as Denmark, Germany and Belgium (Lewis, 2009, p. 52) this may be due to a higher dependancy on artificial light and heat, when compared with sunnier, warmer climes, as found in the mediteranian,with the exception of remote areas with a sunny climate, such as Hawaii, where transportation costs mean that the price of electricity is high, making solar energy a more attractive prospect. (Lewis, 2009, p. 52)

As yet there is little data regarding recycling or re-use of decomissioned systems, so for the purposes of this article it will be assumed that the equipment is scrapped after reaching the end of its usable life. (Alsema, Frankl, Kato, 1998, p. 1)

The average uk household produces 6 tonnes of C02 each year, domestic energy makig up 27% of the total UK energy consumption.

Payback Time

Reseach into the environmental cost of producing photovoltaic cells, particularly with regards to green house gas emissions is often reffered to as the "payback" or "energy Pay-back time (EBPT) of the cells. EBPT is a measure of the time taken for a PV cell to produce an equivalent amount of energy to the amount expended to produce the system. Figure 2 shows Alsema's findings, published in the year 1998, and gives estimates for 2007. that


Figure 2: Energy Pay Back time comparison for Photovoltaic Cells (Alsema, Frankl, Kato, 1998, p. 5).


Alsema (1998, p.6) also predicted that with advances in technology, these values would reduce by 1-2 years by 2010, and further by 2020. Subsequent research has shown Alsema's study, while not taking into account some factors, has proved an accurate, if cautious estimate. Colin Bankier and Steve Gale (2006) compared studies and drew the conclusion that EPBT can be shown to be between 2 and 8 years, dependant on climate conditions. Taking the worst case scenario, EPBT of 8 years is significantly less than the 25 year life cycle of most PV cells (Lewis, 2009, p.52)

Design Principles

Whilst the life cycle of photovoltaic cells is 25 -30 years, the lifetime of the batteries, charge controller and inverters are nearer to 10 years (Lewis, 2009, p. 53). If the design principle of maintaining independance of functional requirements could be applied, the components could be replaced or upgraded without the need for replacing the entire system, hence increasing the efficiency the energy needed to manufacture the system, this would reduce EBPT time and cost of production.
By minimising the information content of the deisgn, installation could be made easier, and such would reduce costs, enabling more customers to use PV cells, which would, in turn, encourage economies of scale, thereby further reducing the cost.

Conclusion

Based on the research above, it seems that although a significant amount of energy is required for the manufacturing process, the production processes do not yet justify the guilt free reputation of solar energy. Over the lifetime of the PV cell, solar energy produces less CO2 than traditional fossil fuels, and as such solar power is a more environmentally friendly option. With new technologies, the effieciency and lifespan of PV cells seems set to improve meaning solar energy will be an increasingly viable option. This, combined with the dwindling supplies of fossil fuels, will increase the appeal of renewable energy. As solar power increases in popularity, manufacturers will be able to harness economies of scale, thus bringing production costs down further. We may also see a rise in the number of solar power plants in areas which receive reliable sunlight, such as north Africa.

References

Alsema, E, A., Frankl, P., Kato, K (1998) Energy Pay-Back Time of Photovoltaic Energy Systems: Present Status and Prospects, Retrieved May 22, 2009, from New Energy India Website: http://www.newenergyindia.org/energy%20Payback%20time_Opinion%20Page.pdf

Bankier, C., Gale, S, (2006) Energy Payback of Roof Mounted Photovoltaic Cells, Retrieved May 22, 2009 from Energy Bulletin website: http://www.energybulletin.net/node/17219

Energy Consumption in the United Kingdom, (n.d.). Retrieved May 22, 2009 from Department for Business, Enterprise and Regulatory Reform website: http://www.berr.gov.uk/files/file11250.pdf

Fthenakis, V,M., Chul Kim, H., Alsema, E. Emissions from Photovoltaic Life Cycles. (2008) Environmental Science & Technology, 42 (6) P. 2168 - 2174.

How Solar Cell is Made, (n.d.). Retrieved May 22, 2009 from How Products Are Made Website: www.madehow.com/volume-1.Solar-Cell.html

How Solar Cells Work. (n.d.). Retrieved May 22, 2009 from How Stuff Works website: http://science.howstuffworks.com/solar-cell.htm

Lewis, D (2009) Solar Grid Parity, Engineering & Technology, 4 (9), 50-53.







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EBPT.jpg 2009-05-22 15:38 158.74 KB
UKEnergy.jpg 2009-05-22 13:27 374.29 KB
Total UK Energy Consumption.jpg 2009-05-22 13:12 374.29 KB