Concluding remarks: PV and eco-restructuring

Today, major economic constraints limit the diffusion of PV in the world energy system. However, it is argued that this renewable energy technology will play a major role in the eco-restructuring transition leading towards a sustainable energy system for the twenty first century.

First, PV is fully compatible with the long-term targets of such a sustainable system. It is environmentally benign, because it uses the sun as a fully clean source. It is fully compatible with a decarbonized system using electricity and hydrogen as energy vectors. A major part of PV systems will be installed on surfaces already occupied by buildings (the theoretical potential of PV on rooftops alone could satisfy up to one-third of world electricity demand), thus significantly limiting the main environmental impact of PV, namely the occupation of land. Other indirect environmental burdens (i.e. generated during module manufacturing) are already low and will decrease with future PV technologies. PV will be economically compatible in the long term, because module and installation costs will decrease, as a result of technological innovation and economies of scale. It is a socially compatible energy technology, because it has a wide range of applications and involves a wide set of actors and users. Owing to some unique features, such as modularity, flexibility of use, silent and clean use, it has hardly any problems of public acceptance (even fewer than for other renewables, e.g. wind). Moreover, PV is geopolitically compatible. The sun is a "shared" primary energy resource throughout the world. Most developing countries have excellent potential for the direct use of solar energy. The majority of them have higher insolation than world average values and large areas (e.g. deserts) suitable for solar panels. Today, PV modules are produced almost entirely in industrialized countries, but in the future developing countries (particularly Asian countries) will be both major users and producers of PV.

Second, PV is very open to innovation and technical change. The key feature of PV is its extraordinary flexibility in terms of technological options for different PV devices, and in terms of the wide range of applications. In contrast with other energy technologies, PV involves not only utilities but also many other interested actors, from energy distribution companies, to architects, up to final users. PV is strongly oriented towards the delivery of energy services, particularly as far as applications in buildings are concerned. In fact, building integrated PV systems can also act as energy-saving systems (e.g. as sun-shading devices) and as small co-generation systems. Furthermore, PV in buildings has great synergy with other solar-active and solar-passive energy technologies, with energy-saving measures, and also with demand-side management.

Finally, PV is fully compatible with hydrogen energy technologies. The solar-hydrogen energy technology cycle is the most likely target for a sustainable energy system for the mid-twenty-first century. The cycle includes the direct use of solar energy, the electrolytic al production of hydrogen, the use of hydrogen as a chemical energy storage means and as an energy vector, and the eventual re-electrification of hydrogen by means of fuel cells. It is a fully clean cycle that uses electricity and hydrogen as energy vectors and that is capable of providing all energy services needed. The use of hydrogen as a means of energy storage is the final solution to the intermittent nature of the solar primary energy source, and virtually eliminates the ultimate technological limit of PV.

By 2050, PV is likely to supply about 10 per cent of world electricity demand. Most importantly, however, PV is open to a lot of other benign energy technology options. PV is consistent with all sustainable energy patterns and has no real long-term limits on its exploitation potential. It will therefore play a major role in the eco-restructuring transition and in the world energy system of the second half of the twenty-first century.