The previous article showed that, despite our expectations, there have been few changes in fundamental photovoltaic technology over the last 40 years. So what has driven the explosive market growth that we have seen?
The primary stimulus, of course, has been the dramatic and continuing cost reductions delivered by the solar industry.
The terrestrial industry emerged initially at a time when the only commercial applications were for space. That was a market where money was no object – space cells could command prices of the order of £1,000 per W. Early PV pioneers immediately recognised that there would be no meaningful terrestrial market at this level, so they set about finding routes to cost reduction. At Solar Power Corporation in 1975, Elliot Berman set a cost target of $10 per W.
The initial low hanging fruit was the pure semiconductor grade monocrystalline silicon wafers from which solar cells were made. Researchers realised that good cells could be fabricated on less perfect material, so early PV companies started buying reject wafers from the semiconductor industry. Some, like US-based Solarex, moved on to develop methods of growing lower cost silicon materials and started to produce polycrystalline silicon modules, which has been dominating global solar markets for years.
Another way to cut costs was to use processes better suited to volume production. One notable innovation was screen-printing the collector grid on the front of the solar cells, avoiding the high cost of depositing silver-laden solder. Screen printing technology is still state-of-the art in solar cell production.
In module assembly shops, economies were achieved by automation. Machines were introduced for testing, tabbing and stringing the solar cells. Lamination proved to be a more automatable process for assembling solar modules than the wet potting techniques that had previously been used.
The combined effect of these developments was indeed to bring solar module costs down to the order of $10 per W by the early 1980s. Even at that price, the range of applications was limited. Various studies showed that many more would be viable at around $2/Wp and in 1982 the US Department of Energy (DoE) set a cost target of $2.8/Wp. At that time, most observers believed that there was a limit to how low the costs of crystalline silicon solar cells could fall. Some experts calculated this would be of the order of $1 per watt, and most people assumed that costs would fall exponentially towards this level.
This meant that a wide range of off-grid applications would be viable but did not reach the costs that would be required to compete with wholesale electricity on the grid. Many people, even in the solar sector, doubted that such 'grid parity' would be achievable. However, early in the 21stcentury, Dick Swanson, the founder of SunPower Corp., showed that crystalline silicon solar cells could be produced at even lower cost, provided the volumes were high enough. His paper illustrated this relationship; and what became known as 'Swanson's law' suggests that cell prices can continue to decline indefinitely so long as production volumes increase.
Subsequent experience has shown dramatically how right this was, and cost reductions have continued to follow Swanson's line to today's level of under 30¢ per watt. That progress is all the more dramatic when you realise that this level equates to 20¢ in the real terms equivalent of 2000, and only 10¢ in the dollars of 1980, in which the $2.8 target was set.
The effect of these reductions is that grid parity has now been achieved in many parts of the world. As costs continue to decline it is expected that grid parity will extend to most other regions within the next decade.
The final article in the series will show how all this affects the uses to which photovoltaics has been put.
The content is based on Philip Wolfe's book on the first quarter century of the terrestrial PV sector 'The Solar Generation: Childhood and adolescence of terrestrial photovoltaics', which was published by Wiley and IEEE earlier this summer.