Chapter 1 Introduction The development of solar energy conversion systems (solar power) is reaching the commercialisation phase. The growing focus at international conferences such as those of the International Solar Energy Society and Solar Power and Chemical Energy Systems (PACES) has been on large scale projects, system developments, political trends and timelines for economically competitive systems. Therefore in developing solar power, the major challenges are not in understanding the fundamental physics of the processes, but rather, in the engineering required to build such systems to create solar power that can compete economically against more established forms of power generation. Australia is blessed with an abundant solar resource. However, the possibility of large scale implementation of solar energy without government intervention is remote. This is due to the fact that Australia has some of the cheapest electricity in the world. Eighty per cent of all electricity is produced by the burning of coal (a fossil fuel) from the vast reserves which are situated close to densely populated areas. It is very difficult for renewable energy to compete against this source of base load electricity. With the growth of understanding within the civic community of the threat to the global environment from the burning of fossil fuels and the increasing awareness of the need for governments to react, the possibility of implementing solar power is now increasing. Large scale concentrating solar power systems exploit economies of scale to increase their economic viability. To commercialise such systems, some fundamental technical problems still need to be overcome. A significant aspect of this is devising an optical system that can achieve the high fluxes in the absorber plane required for such systems. This is the central focus of this thesis, which describes a computational framework to better model and solve this aspect of large scale concentration solar power systems. 1 CHAPTER 1. INTRODUCTION 2 1.1 Energy trends and the global environment Before addressing some of the logistical problems of implementing large scale solar power, it is necessary to identify the need for renewable energy, and in particular, solar power within the global community. 1.1.1 Findings of the IPCC Several international agencies have been created to monitor, advise and re- act to the perceived changes in the global climate. The most significant and widely accepted agency is the Intergovernmental Panel on Climate Change (IPCC) established from the (United Nations’) World Meteorological Or- ganisation and the United Nations Environmental Programme. The IPCC’s specific tasks are to assess scientific, technical and socio-economic informa- tion relevant to the understanding of climate change, its potential impacts and options for adaptation and mitigation1. Having recently published their major septennial report, the general con- sensus of the IPCC was that there would be an increase in average surface temperature of our planet, of between 1.4 Kelvin and 5.8 Kelvin (depending on levels of economic growth, commitment to implementing renewable en- ergy alternatives and uncertainties between global climate models) through the years of 1994 to 2100 due to an increase in levels of greenhouse gases in the atmosphere. The IPCC also concludes that: In the light of new evidence and taking into account the remaining uncertainties, most of the observed warming over the last 50 years is likely to have been due to the increase in the greenhouse gas concentrations. IPCC2. Greenhouse gases are those atmospheric gases that are responsible for trap- ping thermal radiation, driving the temperature of the planet towards warmer conditions (positive radiative forcing). The three most responsible gases for 3 the enhanced greenhouse effect are methane (CH4), nitrous oxide (N2O) and 1Mitigation is defined here as an anthropogenic intervention to reduce the sources of greenhouse gases or enhance their sinks 2Summary for Policymakers (2001), Intergovernmental Panel on Climate Change, p 10 3 Enhanced refers to anthropogenic sources only. Water vapour (H2O) is the greatest absorber of thermal energy. CHAPTER 1. INTRODUCTION 3 4 carbon dioxide (CO2). Of this list, the gas most responsible for the mea- sured increase in the average surface temperature is carbon dioxide. This is not because carbon dioxide is the greatest absorber of solar radiation, but due to the fact that carbon dioxide levels in the atmosphere have increased by 35% between 1750 and the present. Projections are that the total increase from 1750 to the year 2100 will be at least 260% leading to the conclusion of the IPCC that: Emissions of carbon dioxide due to fossil fuel burning are virtually certain to be the dominant influence on trends in atmospheric carbon dioxide levels in the 21st century. IPCC5 Anthropogenic sources of these greenhouse gases are motor vehicles run by the burning of crude oil distillates (oil), industry, heating, cooking, agricul- ture, land clearing6 and electricity generation from burning oil, natural gas and more extensively coal. In general though, the creation of greenhouse gases responsible for global warming can be directly attributed to anthro- pogenic energy consumption. 1.1.2 Global energy trends The International Energy Agency (IEA) is a working group of the Organiza- tion for Economic Cooperation and Development (OECD), set up to monitor global energy trends and advise OECD countries on energy policy. One of the statistics it monitors is fuel shares of total final consumption (FSTFC), which is literally, the consumption of energy by the different end-use sectors (Figure 1.17). It is important to point out that the FSTFC reproduced in Figure 1.1 do not directly represent greenhouse gas emissions. Electricity generation from the combustion of oil, gas and coal has a typical average efficiency of ap- proximately 33%. Systems using combustible renewables and waste burn fuel with lesser efficiency to produce an equivalent amount of end-use energy. 4 Sulfur dioxide SO2 is also influential in global climate models as a negative radiative forcer. 5Summary for Policymakers (2001), Intergovernmental Panel on Climate Change, p 11 6While land clearing is not directly responsible for greenhouse gases it is responsible for an increase in the equilibrium levels of carbon dioxide in the atmosphere. 7The units in Figure 1.1 represent millions of tonnes of oil equivalent (Mtoe) each of which represents 4.2 x 104 TJ. CHAPTER 1. INTRODUCTION 4 Figure 1.1: International Energy Agency: Fuel shares of total final consump- tion 1973 - 2001 (not in primary energy equivalent terms, reproduced with permission from the IEA (2003)) 1 Mtoe = 4.2 104 TJ. * Prior to 1994 the effects of combustible renew×able & waste final consump- tion has been estimated on the total primary energy supply. ** Others include geothermal, solar, wind, heat etc. Figure 1.2: International Energy Agency: Fuel shares of electricity generation 1973 - 2001, excluding pumped storage (reproduced with permission from the IEA (2003)). ** Others include geothermal, solar, wind, combustible renewables & waste. CHAPTER 1. INTRODUCTION 5 The statistics of FSTFC do however, represent accurate trends in the relative demand for consumable energy. Categories of end-use sectors are: oil - now almost exclusively used for trans- port; gas - used for cooking and processes heat; coal and combustible renew- ables for process heat; other: consisting of renewable thermal energy; and electricity. Investigating trends in the demand for energy, over the 28 year period be- ginning with the creation of the OECD, global consumption increased by approximately 54%, representing an average growth rate of 1.5% per annum. Of the categories of end-use sectors, the greatest change in the proportion of the FSTFC was the global demand for electricity, which increased at a rate of more than double that of the global demand for consumable energy, at 3.3% per annum. Focusing on the break up of generated electricity in 2001 (Figure 1.2), ap- proximately 64% can presently be attributed to the burning of fossil fuels producing carbon dioxide. While considerable, this does represent a marked decrease from the dependence of generated electricity on the burning of fos- sil fuels in 1973. The reasons for this reduction are that: nuclear power has made a significant impact on the global market, there has been the replace- ment of low efficiency oil burning power stations with higher efficiency gas turbines and there has been an increase in the efficiency of both coal and gas turbine generators. Electricity generated from renewable resources contributed 18.4% of the total global generation 2001. Hydroelectricity produces the greatest proportion of this, being 2569 TWh (1TWh = 3600 TJ), rising from 1285 TWh in 1973 . The remainder of electricity generated from renewable resources is from the burning of biomass (1.6%), with the remainder, in order of output, generated from: wind, solar thermal, geothermal and then solar photovoltaic. The IEA predicts that over the next 20 years the electricity generated from renewable resources will increase by 53 percent. The majority of this in- crease in capacity can be directly attributed to large-scale hydroelectric dam projects in the developing world, particularly Asia, where China8, India and other nations such as Malaysia, Nepal, and Vietnam, are already building or planning to build hydro projects that each exceed 1,000 MW of capacity. 8China is building the Three Gorges