III. Renewable Energy

` R & D OF ENERGY TECHNOLOGIES ANNEX A III‐RENEWABLE ENERGY ANNEX A – RENEWABLE ENERGY 61 - TABLE OF CONTENTS - AIII‐1 Research and development needs in Photovoltaics 63 Antonio Luque AIII‐2 Storage of electric energy 76 John Ahearne AIII‐3 Space Solar Power System (SSPS) Development Status 87 and Future Perspectives Hiroshi Matsumoto and Naoki Shinohara AIII‐4 Solar Thermal Power Plants 96 Manuel Romero AIII‐5 Energy from biomass 109 Rogério Cezar de Cerqueira Leite AIII‐6 Wind Energy – Status and R&D Activities 192 Hermann‐Josef Wagner, Rodoula Tryfonidou ANNEX A – RENEWABLE ENERGY 62 Annex A‐ Section 3.1 AIII‐1 RESEARCH AND DEVELOPMENT NEEDS IN PHOTOVOLTAICS Antonio Luque Universidad Politécnica de Madrid We present here a description of the photovoltaic (PV) technology in view to determine the research needs for a cost effective mass utilisation of this technology. The paper contemplates different alternatives ranging from the present dominant silicon technology to the novel propos‐ als towards a next generation of photovoltaic concepts. AIII‐1.1 Physical grounds The photovoltaic effect allows for a silent and reliable conversion of the luminous power into electricity by means of the so called solar cells. They are made of a semiconductor with two energy bands separated by an energy gap. The basic principle of a solar cell is simple. Photons pump electrons from the lower band, called the valence band, to the upper band (the conduction band). Once there, selective contacts able to connect only (or mainly) with the conduction band can extract the electrons at a high (free) energy and render them, once they have utilised their additional (free) energy in some kind of useful work, to the valence band trough another selective contact of different type. This mechanism will (ideally) last forever as no waste, except that of the incoming photon, is produced in it. So when light shines, the electrical power can be supplied again. However, to do this, the delivery of the electrons for useful work has to compete with the natural tendency of the electrons to fall into the valence band where they have less energy. In semiconductors, chemically and structurally almost perfect (single crystals), the internal return to the valence band is hampered by the relative lack of particles to which the extra energy can be ANNEX A – RENEWABLE ENERGY 63 Annex A‐ Section 3.1 transferred. Ultimately such particles are photons emitted by the solar cell as luminescent radiation, and this is unavoidable as it is the detailed balance counterpart of the absorption. However, if the semiconductor is not perfect enough the energy can be easily delivered to vibrational states in the crystalline lattice (heat) and the electrons fall into the valence band without having produced useful work. The need of this highly perfect semiconductor to make the solar cells is one of the reasons for the relatively high cost of the present solar cells. AIII‐1.2 The PV industry Despite this high cost, the industry of solar cells fabrication is experiencing nowadays a very fast 1 growth. We present in Figure 1 the histogram of solar cells sales. Solar cells are rated in watts peak (Wp) : the maximum power that a solar cell can give at 25ºC under a solar power flux of ‐2 1000 W∙m (at the standard solar spectrum on the earth surface). The 2002 market has had an economic value of about €1.5 billion for the cell manufacturers ( €4‐5 billion for the users). The annual market has grown slowly until 1996. After this date, the growth rate has become explosive. Many PV factories are doubling their production every two years. In 2002, 92.7% of this production corresponded to crystalline (mono 36.4%, multi 56.3%) silicon cells, 6.4% to thin film amorphous silicon cells, 0.9% to cadmium telluride or copper indium diselenide thin film solar cells2. Technologies, not based on silicon, are all experiencing a relative decrease in the market. The main actors in this business³are represented in Figure 2. In 2002, they totalized 83% of the world sales. Japan is the first manufacturer followed by Europe and the USA. In Europe, Germany, Spain and France were the main manufacturers in 2002. (in 2001 the production in Spain was larger than in Germany)4. The most important and faster growing PV market is today grid connected houses5. In Japan and several European countries, they receive some kind of public subsidy, sometimes important, that adopts different modalities, depending on the country. Processes of PV modules are decreasing according to a learning curve6 of 81%. This means that every time that the cumulated ANNEX A – RENEWABLE ENERGY 64 Annex A‐ Section 3.1 cells production doubles, the prices are reduced to 19%. This is not as good as in the memory business, but better, for instance, than in the wind energy. We have developed7 a prospective model that couples the learning curve and the demand elasticity to foresee the market and price evolution. After the present period of quick growth, the model foresees a period in which the growth will be limited by the availability of capital for a product that is more expensive that the prevalent electricity. If the industrialised countries are willing to expend every year up to 0.4% of their GDP in PV installations the cost of the PV electricity would reach the costs of the prevalent electricity by 2050. They will be reached later with less investment and earlier with more investment. We consider unlikely that the needed level of investment will be produced and therefore that PV, with its present technology, can constitute an alternative for mass production of electricity in the first half of the century, but we are not sure. However the invention and commercialisation of novel technologies with a better learning curve would be the quicker way towards mass production of PV electricity. Besides the cost, the main drawback of PV electricity, is its intermittent nature, coupled to the sun availability. Some studies have determined that an electric network can deliver up to 30% of electricity8 from intermittent sources (sun and wind). Naturally, these utilities must know well the statistical consumption pattern of their final users, and so organise the dispatching. They must learn to consider them as potential negative consumers, with different consumption patterns (including this negative consumption). Of course, any storage discovery can ease the storage needs of PV electricity but probably the cheapest storage is a sophisticated electric network management. Any other alleged PV drawback we have heard of is not a real one. There are no problems of space (all the electricity for a home can be largely obtained in its roof space) nor of pollution (silicon cells manufacturing is not a specially dangerous industry, and once disposed, the silicon cells are far less poisonous than any ordinary electric equipment).However the fact is that it is necessary to wait 3‐6 years (depending on where it is installed) to recover the energy spent in manufacturing a solar cell9. But solar cells last very long. Companies guarantee them for 20 years (actually the modules are probably lasting less that the cells themselves) and most probably they can operate for much longer. Novel technologies foresee a reduction of the energy recovery time to about 1 year. To summarize it is perhaps interesting to bring here the words of Shell International, the oil company10, “Since 2025 when the first wave of renewables began to stagnate, biotechnology, materials advances and sophisticated electric network controls have enabled a new generation of renewable technologies to emerge By 2050 renewables reach a third of world primary energy and are supplying most incremental energy.” ANNEX A – RENEWABLE ENERGY 65 Annex A‐ Section 3.1 AIII‐1.3 Crystalline silicon technology Following our argumentation, the potential of the silicon cells to reach the prices of the prevalent electricity cannot be totally discarded. At least, their role is fundamental in building up a powerful photovoltaic industry, able to face energy challenges. We present in Figure 3, a schematic of the total fabrication processes leading to a solar cell. The first step, pertaining to the metallurgical industry, is the reduction of the quartz with coal to produce the 98% pure metallurgical silicon (with some production of CO2, a very small amount per PV kWh to be generated in the future). 11 From this metal, the industry produces about 1 million tm per year at a cost of about €1/kg. Part of this, is purified to produce some 20000 tm of electronic grade silicon for the use of both the electronic industry and the PV industry. The purification procedure is done in three steps. First the silicon is treated at moderate temperature (≈300 C) with HCl to produce liquid chlorosilanes that are subsequently purified by fractional distillation. Finally the pure chlorosilanes are reduced at high temperature (≈1200 C) with H2 to produce electronic grade silicon, also called polysilicon, with impurities in the range of 0.1‐1 ppb. The price of this polysilicon is of about €50/kg. Out of the 20000 tm of polysilicon, some 4000 goes to solar cells fabrication. Very often the PV industry accepts the ultrapure silicon rejected by the electronic industry at a price that may be around €5‐15/kg. The polysilicon is then molten and grown by the PV industry into single crystal (or mono‐ crystalline) ingots by the Czochralsky method or into multi‐crystalline blocs by a variety of directional solidification methods, so that big grains can be formed. These blocs, either mono‐ or multi‐crystalline, are then cut into wafers or, alternatively, the molten silicon is grown into sheets that do not require any further cutting.

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