Photovoltaic cells cast new light on the world’s energy problems

Nearly 150 years after its ‘discovery’ by French physicist Antoine Becquerel, and five decades after its pioneering use in satellite and space programmes, photovoltaic solar energy is entering the mainstream. It is a proven technology that has been field-tested for many years and, due to a combination of technological innovations and larger-scale production, is becoming increasingly affordable for home use. But what is limiting its even wider adoption? What technologies are emerging? And will solar energy ever be able to compete with conventional electricity?

Introduction

Solar photovoltaic energy may account for only a fraction – around 0.001% – of world energy consumption, but the market is growing by 30% a year and is now worth around $1.5 billion. In mature markets, it may be more expensive than electricity generated from conventional sources, but costs have fallen (eight-fold) over the past 20 years. It may be true that the total power generated by all of the photovoltaic energy in the world in one year would currently keep a city the size of London going for less than a month, but some estimates suggest that by 2025 there would be enough photovoltaic energy sources installed in the world to power 100 Londons all year round.

The appeal of solar is simple: it’s clean; it’s quiet and, with no moving parts, it’s reliable. More and more people are switching on to solar, but challenges in its wide-scale adoption remain. For photovoltaic manufacturers, three are key: the costs of installed solar systems have to come down, the efficiency of the solar cell must go up, and the customer offer must improve.

How solar cells work

Solar cells, the building blocks of photovoltaic power, convert light directly into electricity. The cells are built around layers of semi-conductive material. When light strikes the surface of a cell, it triggers the movement of an electric charge from positively to negatively charged layers, which then drifts to the external contact. Cells are linked to form panels and the panels linked to form arrays. The panels or arrays are used to power electrical appliances (with energy stored in batteries where necessary) or to feed electricity into a grid.

Around 90% of today’s global market is based on the crystalline silicon solar cell. In a conventional silicon cell, metal contacts are screen printed on the silicon wafers to extract the current. Any light falling on the metal contact rather than the cell is reflected – and consequently wasted. Nevertheless efficiencies of 15% can still be achieved by this ‘conventional’ technology and this type of cell remains widely used globally.

However, some recent advances in cell design have taken this conventional silicon cell and improved efficiency by up to 3% (in other words, a further three per cent of the light falling on the cell is converted into electricity). In these high-efficiency cells, the contacts are embedded in laser-cut ultra-thin grooves, and occupy a much smaller area of the surface of the cell. The highest efficiency solar cell available commercially is the BP-manufactured Saturn cell and the efficiency of this can be as high as 18%.

Research

Research into high-efficiency buried contact cells is continuing at institutes in Germany and Australia (notably the Fraunhofer Institut Solare Energiesysteme in Freiburg and the Institut für Solarenergieforschung in Hamlin, as well as continued work at the University of New South Wales, which pioneered the development of the laser-grooved buried grid cell).

Technologists at BP Solar continue to look at other ways to enhance performance and lower costs of crystalline cells through improvements in technology and manufacturing processes. The addition of silicon nitride as anti-reflective coating results in a more efficient cell, and a 5–6% improvement in power. BP Solar is also working with the US government’s National Renewable Energy Laboratory to advance polycrystalline manufacturing ingot size, material quality and materials handling. This should result in further power and manufacturing efficiencies and improved product.

Thin films

For markets in northern Europe, where space tends to be limited and efficiency therefore critical, the Saturn cell is capturing the lion’s share of the grid connect market. But in other applications an emerging technology may offer the potential to reduce production costs for customers who have space to spare. These products are collectively known as thin films.

Thin films differ from silicon principally in the amount of active material used. Instead of wafers, the active material is deposited thinly on a large substrate such as glass or stainless steel. Thin films can absorb light in about one thousandth of a millimetre of material, and although they are less efficient, the process by which they are made has the potential to lend itself much better to automation, which in turn helps reduce costs for the customer. The choices of thin-film technologies include amorphous silicon, copper indium diselinide and cadmium telluride.

Each has its proponents. BP is marketing two kinds. The company’s amorphous silicon thin-film product, Millenia, is currently being installed at BP’s new generation of service stations, BP Connect.

But other thin films may in the long term offer higher efficiencies and are more stable products than amorphous silicon. BP’s cadmium telluride product, Apollo, currently boasts a thin-film efficiency record, achieving 10.6% in a 900 mm panel in tests carried out last year – and, in a smaller 500 mm panel, as high as 10.8%.

The future

Silicon cells are here today; commercially available thin films are beginning to have an impact: but what of the longer term? To answer this question, many would look to the emergence of so-called organic solar cells, and both Imperial College and BP along with SolarAMP, a North Carolina-based molecular solar energy solutions company, are undertaking significant research projects. Organic cells use the same principles as photosynthesis. An organic dye film is located on a titanium dioxide conductor in liquid electrolyte. The light-sensitised dye absorbs light and releases a semiconducting electron. There are some exciting predictions made about the potential for this type of cell as a low-cost, flexible technology with the ability to tailor colour in transmission and reflection. BP and SolarAMP are working on developing the first commercial solid-state molecular module.

Improving the efficiency of the cells is only one thrust of current research. Manufacturers are also facing the challenge of reducing production costs by:

• scaling up production

• improving the efficiency of the production process

• improving materials usage.

In Spain, BP has announced that it will be building the world’s largest solar plant with a five-fold expansion of its production there and investing some $100 million in the project. The new facility will be one of the largest solar plants in the world, producing some 60 MW of high-efficiency Saturn solar cells. But crucially it also aims to be one of the most efficient, striving to improve productivity by a factor of two. One priority for the team working on the project is to improve materials usage, by using ever-thinner silicon wafers.

Nevertheless, many believe that thin films have the potential to offer even greater production efficiencies. With silicon cells, it is a question of fabricating individual solar cells, electrically inter-connecting them and sealing them in a weatherproof module. Thin films offer far fewer steps – think of a production process akin to plating glass – and therefore these technologies offer the potential for mass manufacturing automation techniques. At the Apollo plant in Fairfield, California, the team is busy doing just that.

Markets

Advances in technology, improvements – and potential step-changes – to the production process will help reduce the cost to the customer. At present solar costs around $3 a watt, compared to around one dollar per watt for conventional sources of electricity. Base case projections (see Figure 4) suggest that by 2010 that will fall to $2.12, but will fall further and faster, according to a more bullish prediction from Strategies Unlimited, a Californian consultancy which monitors the industry, if five broad factors come into play. These are:

• rising fuel prices

• heightened concern over energy security, increased demand in developing countries for clean energy alternatives

• continued environmental concerns

• the continued presence of incentives and other forms of support at today’s levels.

The impact of well thought-through market incentive programmes is clear, and solar markets are thriving in several northern European countries and Japan (where two of the world’s largest solar manufacturers, Kyocera and Sharp have grown through the support of that country’s programmes). The German and Japanese governments have the most extensive support, encouraging the installation of solar energy in tens of thousands of homes. Consequently the two countries have around 60% of the world market. In the UK, the Department of Trade and Industry has recently pledged nearly $35 million over three years to a series of domestic and large-scale demonstration projects. These will provide a very real stimulus to the development of solar energy in the UK.

These are all so-called grid-connect markets, currently by far the largest of the three principal established markets for solar power. Where there is an established and extensive electricity system, such as in the UK, solar arrays are linked (via inverters, which convert direct current to alternating current) directly to the local grid. During daylight hours, the solar array is usually produces more electricity than is consumed and can ‘export’ power to the grid, while at night it will need to take electricity back from the network. One of the toughest challenges arising from solar energy use relates to these connections. In mature markets, distribution systems are based on large-scale centralised generators, whereas solar power is small-scale, distributed power with intermittent loading. Some of the support mechanisms tackle this head on: countries like the Netherlands and Germany have ‘net metering’ and in some cases even offer a premium price for the electricity generated. This market is presently the largest segment, accounting for around two-thirds of installed systems.

The second significant market is the provision of power in remote rural areas, where the cost of either building a small conventional power station in situ or transmission lines from a long distance would be prohibitive. Photovoltaic energy provides a cheaper and more reliable alternative to both, and this segment is expected to grow by more than 20% a year.

The third is the use of solar energy for off-grid industrial purposes, including power for telecommunication repeater stations, signals, and instrumentation. Marine buoys and road signs are also good examples. In this context, solar is typically more cost-effective than conventional energy sources and can be deployed on mountaintops, on the seas, and other remote areas.

All of these markets are based on established technology – the crystalline silicon cell.

Longer term, though, the as-yet untapped segment of the grid market appears to offer the most exciting prospects: this is the so-called building integrated sector. This involves replacing elements of existing buildings, typically the cladding or glass in office blocks, with solar panels so that the cost of the technology can be offset by that of the substituted product.

Conclusion

The world’s need for energy is growing steadily and solar power is already making a difference. New and improving product and technology continue to enhance performance and lower costs. While tried and tested in historical niche markets, these improvements present a whole new range of possibilities and we are just beginning to gain a glimpse of them. Stay in touch with what is happening: a few major institutions are focusing intensively and even better products are on their way.

Harry Shimp
President and Chief Executive, BP Solar

Harry Shimp is president and chief executive officer of BP Solar, a part of the BP Group. Mr Shimp joined the solar industry initially with Solarex in 1999 as president and chief executive officer. Later in 1999, when BP Solar and Solarex merged he retained the chief executive position. Previously, he spent nine years at General Electric and since forming Charon Industries in 1991, he has managed a number of successful venture partnerships specialising in the fields of glass and ceramics, optics, reinforced plastic composites, and precision metering equipment.

Mr Shimp holds a BSc in Mechanical Engineering from Lehigh University and masters degrees in both Engineering and Industrial Administration from Carnegie-Mellon University.