Reducing carbon emissions to levels consistent with the stabilization of greenhouse gas concentrations in the atmosphere will require enormous amounts of clean energy supplies to displace current fossil fuel combustion. Renewable energy supplies are one source of such clean energy.
This chapter focuses on terrestrially based renewable energy options. As used here, renewable energy is defined simply as any form of energy that can be used without diminishing the resource. Some forms of renewable energy, like solar and wind, are essentially limitless. Others, like geothermal, may be depleted in the short term, but are eventually regenerated. Some, like hydroelectricity, municipal solid waste, landfill gas, and biomass must be regenerated continually. But none are totally depleted forever.
Renewable energy is generally clean. Most forms of renewable energy do not involve combustion and, therefore, do not emit gases to the atmosphere. In particular, renewables are attractive because they generally emit no net carbon dioxide. What is just as important for those countries experiencing severe local air pollution problems is that non-combusting renewables, like solar and wind, produce energy without emitting sulfur dioxide, nitrogen oxides, particulate matter, or toxic materials. In addition, renewable electric technologies, like photovoltaics and wind that do not use heat to produce electricity, require no water resources for cooling towers, and have no water effluents.
For those renewables that do employ combustion, like biomass power and municipal solid waste, most of the emissions, other than NOx, would have occurred anyway as a result of natural decomposition processes. In some cases, the natural decomposition may have produced stronger greenhouse gases, like the uncontrolled methane emissions from landfills. Certainly the mix of effluents is different for combustible renewables than for fossil fuels. For example, biomass combustion produces none of the sulfur emissions produced by coal and oil combustion. There is even evidence that cofiring a coal plant with biomass can reduce NOx emissions per unit of coal combusted. On the other hand, the combustion of municipal solid waste can produce as many particulates, volatile organic chemicals, and NOx emissions as coal burning.
Renewables provide other benefits as well. For one, they increase the diversity of energy supplies, relieving pressure on fossil fuel supplies and oil prices, and increasing energy security. In the long term, renewables are one of the few sustainable energy supply options available. Eventually they will be essential for replacing diminishing fossil fuel resources, especially given the inequitable distribution of those natural resources.
Many renewable energy technologies can be deployed in small increments. Initial increments of these modular technologies can provide economic returns while other increments are being installed, thereby reducing capital constraints. The modular nature of technologies like wind and photovoltaics also makes them candidates for deployment within an electric distribution system, bypassing the need for additional distribution lines and transformers, reducing transmission losses, and increasing the reliability of the electric supply.
For both developed and developing countries, renewables also represent an indigenous supply with significant employment opportunities in manufacturing, installation, and infrastructure development. It is not the intent of this chapter to focus on all these benefits. They are mentioned simply as co-benefits to the focus of this chapter on the ability of renewables to reduce carbon emissions worldwide.
There are vast renewable resources that can be used to reduce world fossil fuel use and carbon emissions. These resources take many forms - solar, wind, hydro, and geothermal, to name a few. They are spread widely, but non-uniformly, around the globe. While the total resource is several orders of magnitude larger than current world energy consumption, it is doubtful that all fossil fuels could be displaced by renewables. There are issues with respect to cost, access to the resources, technical integration into current energy systems, environmental factors, social barriers, and other institutional concerns.
In this chapter, we explore the potential for renewable energy technologies to address long-term global climate change. On a regional basis, we examine the full set of resources and the applications of these technologies, the issues involved in their deployment, and the prospects for the future. We end with recommendations for more rapid renewable energy deployment, prompted not by the current markets and prices for energy, but by the underlying trends in climate change, local air pollution, depletion of fossil fuel supplies, and each nation's desire to prosper in a secure world environment.
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