Concentrator optics for the next-generation photovoltaics

P Benitez and J C Minano

Institute de Energia Solar—Universidad Politecnica de Madrid ETSI TelecomunicaciOn Ciudad Universitaria s/n—28040 Madrid, Spain

13.1 Introduction

Next-generation photovoltaic (PV) converters aim to be ultra-high efficiency devices. In order to be so efficient, it is expected that the cost of these converters will also be very high per unit area. Although it has been claimed [1] that the next-generation PV approaches should aim at high efficiency but also low-cost per unit area using thin-film technologies, perhaps it will be more probable that we first find a highly efficient next-generation solution with high cost per unit area as there will be fewer restrictions.

As an example, let us imagine that next-generation devices achieving 70% efficiency were to be invented in the near future. The electricity cost based on one – sun modules made from these devices would not be economic if their cost were to be higher than about seven times the cost of currently available commercial 15% efficient cells [2]. Such a device has not yet been found and, thus, cost analyses are obviously risky! However, the present general feeling is that, unless a new breakthrough approach appears, the relative cost per unit area of a 70% efficient device is likely to be much higher than seven times the present cost per unit area of commercial silicon cells (in fact, the present next-generation approach with the most advanced degree of development, the multi-junction tandem cell, has proven efficiencies around twice those of commercial silicon crystalline cells but their medium-term estimated cost is about 300 times higher [1]). This scenario would imply that one-sun modules based on this 70% efficient next-generation device will not be competitive with conventional modules, except in applications where other factors (like efficiency) were much more important than the cost of electricity.

Nevertheless, next-generation PV can aim to achieve the goal of competing in cost with conventional non-renewable energies, if the device cost per unit area were to be reduced by concentrating the sunlight. In addition, the potential increase in the efficiency due to the concentration also helps them to compete with conventional sources. Solar concentrators have been studied since PV cells were first applied for terrestrial applications. During these three decades, expectations of the concentrator’s capacity for cost reduction have been high but no commercial concentration product has so far proved this. The reasons for such a lack of success are multiple, as pointed out recently [2]. Let us review the following reasons:

(1) Until now, the PV market has not been driven by the PV electricity cost. Moreover, most customers have been public institutions or subsidized private ones. Furthermore, even high-cost renewable energies are gaining acceptance because sensitivity to the environmental aspects is increasing.

(2) Concentrators have not found a niche market as flat modules did. For instance, in devices integrated into buildings, tracking concentrators have had no applicability. Since tracking concentrators seem to be less reliable than non-tracking flat modules, they have not yet been accepted for automatic applications, where the maintenance cost is more important than the module cost.

The present rapidly growing PV market seems to be leading to a situation in which the subsidies will be reduced [4] and then it is likely that the lowest cost technology will start to dominate the PV market. Besides that, when the market becomes big enough, niche applications for market penetration seem to be easier to find. In this situation, concentrators and next-generation cells are expected to play an important role.