Introduction

Photovoltaic (PV) cells are semiconductor devices that convert sunlight directly into electricity. Apart from using the inexhaustible, zero-cost, solar primary energy source, PV systems show several advantages, even when compared with other renewable energy technologies. These include low environmental impact, simplicity, modularity, long lifetimes, and low maintenance requirements.

Despite these advantages and despite a rapidly evolving technology, PV systems have not found large-scale use up to now. Almost all PV penetration scenarios developed in the past have proven to be overoptimistic. Certainly, the major barrier to widespread adoption of PV systems is their continuing high cost. These costs have fallen significantly, but not as rapidly as was hoped.

Given these facts, the question might be raised to what extent PV can actually play a significant role in a future sustainable energy scenario. People participating in the debate on this question are roughly divided in two opinion groups. Some people think that PV is one of the most promising energy conversion technologies for the twenty-first century. Their optimism is based mainly on three factors: the inexhaustible primary energy resource, the wide range of applications, and, above all, the enormous potential for technological improvement. PV technology is still young. As technology improves and market demand grows, PV electricity prices should fall sharply, making PV fully competitive with other conventional and renewable energy sources. Other people think that PV technology will have only a minor role in the future world energy system. These people claim that PV acceptance is impeded not only by high costs but also by other major drawbacks, namely low efficiency, the large surface areas required, and intermittent power supply source.

The aim of the present paper is to discuss these two positions in some detail and to address two fundamental questions:

- Is PV compatible with the long-term targets of a sustainable energy system?

- What roles can PV play in the eco-restructuring transition?

In order to answer these questions, the paper will first analyse the technological potential of PV (the technology "state-of-the-art" and its rate of change). Second, it will go into the details of the main barrier to the widespread use of PV, its present high cost. Then, it will identify a possible PV market diffusion strategy. Finally, it will suggest possible scenarios for PV penetration in the twenty-first-century energy system.

The paper argues that PV is fully compatible with the long-term targets of a sustainable energy system (in environmental, economic, social, and geopolitical terms, as defined by Rogner in chap. 5 of this volume). PV is compatible with a decarbonized energy system based on the delivery of energy services, with electricity and hydrogen as energy vectors. Because it is extremely flexible in terms of technological options, range of applications, synergism's with other energy technologies, and the actors involved, PV is consistent with all sustainable energy patterns open to innovation and technological change. Therefore, PV will play a major role within the twenty-first-century energy system.