Science in Society Archive

Solar Power to the Masses

Solar cells getting better and cheaper fast as oil prices soar, soon it will cost as much to get electricity from the sun as from the grid, and distributed small scale generation is the way ahead Dr. Mae-Wan Ho

Solar tops the world’s new renewable energies

As oil prices soar, solar power has been undergoing a boom, along with other renewable energies, which attracted more than US$100 billion investment last year in new power and heating capacity, manufacturing plants, research and development [1]. Investment in solar power capacity and manufacture amounted to US$31.3 billion, while US$33. 4 billion was invested in wind.

Global capacity in photovoltaic (PV) power reached 10.6 GW in 2007, of which 7.8 GW is grid-connected. Grid-connected PV has been the fastest growing power generation, increasing by 50 percent a year in both 2006 and 2007.The Spanish PV market grew the fastest with an estimated 400 MW added in 2007, four times the 2006 additions.

Apart from rooftop installations, the growth of large-scale PV power plants also accelerated during 2006 and 2007, including many kW and MW plants. Spain now has the world’s two largest PV plants of 20 MW each in the cities of Jumilla and Beneixama in Murcia and Alicante regions respectively. There are now over 800 plants worldwide with capacity greater than 200 kW and at least 9 larger than 10 MW in Germany, Portugal, Spain and the US.

Solar hot water/heating capacity increased to an estimated 128 GW globally in 2007, up from 88 GW in 2005, reflecting an annual growth rate of 20 percent over the past two years. For comparison, the world’s total wind power capacity reached an estimated 95GW in 2007.

Concentrating solar power mega-projects not desirable nor needed

The technology for concentrating solar power (CSP) uses a parabolic array of mirrors to reflect and concentrate sunlight for heating water, turning it into pressurized steam at 800 C for driving turbines to produce electricity.

The CSP industry completed a first round of new build during 2006-2007. This included a 64 MW plant in Nevada, a 1 MW plant in Arizona and an 11 MW central receiver plant in Spain. By 2007, there were over 20 new CSP projects around the world under construction, in planning stages or undergoing feasibility studies, the majority in Spain and the US, but also in some developing countries. Chinese and German partners have agreed to develop 200 MW CSP in Inner Mongolia by 2012 as part of a broader commercial framework for 1GW of CSP in China by 2020.     

It has been estimated that an area of the Sahara desert slightly smaller than Wales in the UK would generate enough solar energy by CSP to supply all of Europe with clean electricity [2]. Arnulf Jaeger-Walden of the European Commission’s Institute for Energy said it would require the capture of just 0.3 percent of the light falling on the Sahara and Middle East deserts to meet all of Europe’s energy needs. Both Gordon Brown and Nicholas Sarkozy are supporting it, and the project is welcomed by Greenpeace and other environmental groups.

Scientists on the Sahara project admit it would take many years and a huge investment of €450 billion; and by 2050, it could produce 100 GW. One reason it is so expensive is because a ‘supergrid’ needs to be built to transport direct current (DC) electricity through high tension power lines. DC power lines have an advantage over the usual alternating current (AC) power lines, in that they lose only 3 percent of the electricity in transport rather than 7 percent or more [3].

In view of the continuing debate over the adverse health impacts of high tension power lines [4] (see Fields of Influence series, SiS 17 and SiS 24), the high costs, and not least the prospect of ruining the world’s landscapes by yet more power lines and pylons, the Sahara mega-project looks distinctly undesirable and unnecessary. The better option is distributed small to micro-scale generation of electricity that could be fed back to the existing grid, as recommended in ISIS 2006 Energy Report, Which Energy? [4]. PV technologies, in particular, are maturing fast. We do not have to wait so many years nor invest hundreds of billions. The solar age has well and truly arrived.

‘Grid-parity’ rapidly approaching for PV

PV technologies for producing electricity from sunlight have been improving by leaps and bounds while manufacturing costs are falling.

Industry leaders are saying that solar power could operate without subsidies in just a few years when electricity from the sun will be as cheap as can be bought from the grid [5]. That ‘grid-parity’ point may be reached even faster if oil prices continue to climb while manufacturing costs follow the downward trajectory.

Germany has been leading the world in PV, thanks to its ‘feed-in tariff’ support, which gives people with solar panels above-market rates for selling power back to the grid. Last year, it installed 1.1 GW, the equivalent of a large power station, and now has nearly half a million houses fitted with solar panels.

The solar market is predicted to expand another 40 percent this year for PV and solar heating.

All the companies attending the intersolar fair in Munich in June 2008 are planning big increases in production of solar panels. China-based Suntech, now the world’s largest maker of PV panels, plans to double production from 540 MW this year to 1 GW in 2009. Jerry Stokens, head of Suntech Europe, thinks parity in Germany can be reached within 5 years. But in California and Italy, where there is a lot of sun and electricity prices are high, grid parity for PV had already been achieved.

Although the price of silicon has gone up, many firms have secured long term supplies at more modest prices. Nitol, a Russian chemical company is building a new production plant in Siberia that will increase its output from 300 tonnes this year to 3 700 tonnes by 2009.

Second generation thin-film PV technologies

Companies are investing in the newer thin film PVs that are less efficient but more than make up for that in being much cheaper and easier to manufacture. These ‘second generation’ PVs include cadmium telluride (CdTe) and copper indium gallium selenide (CIGS) applied in a thin film to a supporting substrate such as glass, flexible metallic foils, high-temperature polymers or stainless steel sheets [6].. In 2007, the US-based company First Solar produced 200 MW of CdTe solar cells making it the fifth largest producer of solar cells in 2007, and first ever within the top 10 producing only second generation PV [7]. Nanosolar commercialised its CIGS technology in 2007 with a production capacity of 430 MW for 2008 in US and Germany. In 2007, CdTe production represented 4.7 percent of total market share, thin film silicon 5.2 percent and CIGS 0.5 percent. The current record efficiencies for CdTe and CIGS thin film PVs are 10 percent [8] and 19.9 percent [6] respectively.

Brighter and cheaper yet on the horizon

Third generation technologies aim to improve the efficiency of second generation thin film technologies to 30-60 percent while maintaining very low production costs. Thin-film solar cells use less than 1 percent of the raw material compared to wafer based solar cells, leading to a significant drop in price per W [7]. (The current US target is US$1/W generating power.) One of the R& D Magazine’s  prestigious R&D 100 Awards - also called the “Oscars of Invention” - for 2008 has gone to the US National Renewable Energy Laboratory hybrid CGIS cells manufactured by using ink-jet and ultrasonic technologies to precisely apply metal-organic inks in separate layers directly onto a common substrate.

Many developments are underway to increase efficiency and cut costs. I have dealt at length with the Dye Sensitized Solar Cell [9] (Organic Solar Power, SiS 29) and cells based on quantum dots [10] (Quantum Dots and Ultra-Efficient Solar Cells?. SiS 29), for example.

One current strategy to increase efficiency is to target solar concentrators, devices for increasing solar intensity. Current PV concentrators track the sun to generate high optical intensity, often by using large mobile mirrors that are expensive to make, install and maintain.  

Researchers at the Massachusetts Institute of Technology (MIT) in the US have now created a new solar concentrator based on light absorbing organic dyes. In their invention, sunlight falls on the first organic solar collector (OSC) with a dye absorbing at low wavelengths but transmits the rest to a second OSC underneath with a dye absorbing at longer wavelengths. Alternatively, solar radiation transmitted through the top OSC can be gathered by a bottom PV cell or used to heat water in a hybrid PV thermal system. The first dye re-emits light at a longer wavelength which is largely trapped inside the glass, and can be used by PV cells stuck to the sides to generate electricity. Similarly, the dye in the second OSC, on absorbing light at long wavelengths, re-emits at a still longer wavelength that ix also largely trapped within the layer so the PV cells stuck to its sides are able to generate electricity [11, 12]. This array of OSCs overlying ordinary solar cells can boost the overall efficiency by 20 to 30 percent, and bring down the cost of PV electricity substantially, perhaps by 10 fold. The MIT researchers used DCJTB (4-(dicyanomethylene)-2-t-butyl-6-(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran) and Pt(TPBP) (platinum tetraphenyltetrabenzoporphyrin) together with various host materials.

Another strategy is to use transparent thin films that are also conductors of electrical charge [7]. It allows light to pass through to the light absorbing material beneath, and also serves as an electrical contact to transport charge carriers away from the light absorbing material, thereby increasing the efficiency. These include various transparent conductive metal oxides. Physicist Bram Hoex and colleagues at Eindhoven University of Technology, together with the Fraunhofer Institute in Germany succeeded in boosting the efficiency of a crystalline silicon solar cell by 6 percent, from 21.9 to 23.3 percent. This is achieved by depositing an ultra-thin (30nm) layer of aluminium oxide at the front of the cell [13]. The layer has a high concentration of negative charges, almost entirely eliminating the energy losses through the surface of the cell. 

Article first published 31/07/08


References

  1. Global status report: keeping it clean”, Eric Martinot Renewable Energy World, 2 April 2008, http://www.renewableenergyworld.com/rea/news/reworld/story?id=52088
  2. “Solar poser from Saharan sun could provide Europe’s elecrtricity, says EU”, Alok Jha, The Guardian 23 July 2008, http://www.guardian.co.uk/environment/2008/jul/23/solarpower.windpower
  3. Electric power transmission. Wikipedia, 24 July 2008, http://en.wikipedia.org/wiki/Electric_power_transmission
  4. Ho MW. Fields of Influence series, Science in Society 17, 10-18+20; 2003; Science in Society 24, 50-53, 2004.
  5.  “Solar future brightens as oil soars”, Ashley Seager, The Guardian, 16 June 2008, http://www.guardian.co.uk/environment/2008/jun/16/renewableenergy.energy
  6. “Record makes thin-film solar cell competitive with silicon efficiency”, National Renewable Energy Laboratory News Release 24 March 2008, http://www.nrel.gov/news/press/2008/574.html
  7. Solar cell, Wikipedia, 23 July 2008, http://en.wikipedia.org/wiki/Solar_cell
  8. Fthenakis VM, Kim HC and Alsema E. Emissions from photovoltaic life cycles.  Environ Sci Technol 2008, 42, 2168-74.
  9. Ho MW. Organic solar power. Science in Society 29, 46-48, 2006; see also Ho MW. Organic solar power. In Which Energy?  ISIS 2006 Energy Report, pp. 37-39, 2006.
  10. Ho MW. Quantum dots and ultra-efficient solar cells? Science in Society 29, 48-49, 2006; see also Ho MW. Quantum dots and ultra-efficient solar cells? In Which Energy?  ISIS 2006 Energy Report, pp. 40-41, 2006.
  11. Currie MJ, Mapel JK, Heidel TD, Goffri S, and Baldo MA. Science 2008, 321, 226-8.
  12. “Concentrating sunlight cuts solar power costs” Chemistry World, RSC, 10 July 2008, http://www.rsc.org/chemistryworld/News/2008/July/10070802.asp

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