Renewable energy resources and technologies

Renewable energy, with its low carbon footprint, the relative speed with which it can be deployed into developed and developing communities alike, and its ability to generate new kinds of businesses and green jobs, is a key element of the transition to a "Green Economy".

Achim Steiner, UN Under-Secretary General and UN Environment Programme Executive Director (UNEP, 2009).

Assessing the renewable energy sources available in or near to a city is a crucial step before developing policies that support the deployment of renewable energy technologies. The variability of some renewable energy sources such as wind and solar can be problematic when attempting to continually match electricity supply with demand (Fig. 13). Electricity demand by season on the left (depicted as MWh per half hour intervals over 24 hour periods) shows a higher demand in winter (June/July), peaks in the evenings and troughs at night and mid-afternoons. This partly matches the local wind resource (shown on the right as mean annual wind speeds over half hour intervals) with peaks in the afternoons in spring and autumn and in evenings in late summer.

Figure 13 • The measured electricity demand of a small New Zealand community was compared with the local wind resource to attempt to match supply with demand

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Source: Murray, 2005

Source: Murray, 2005

Various technical options exist to help make the grid more flexible (IEA, 2008b). Obtaining high quality historic data showing the availability of renewable energy sources and their degree of fluctuations in space and time is required in order to apply these options most efficiently (Krewitt, 2008) and to determine the optimum mix of technologies. This includes reliable short-term weather forecasting so that the dispatching of power can be scheduled initially a day, or even just a few hours, ahead. Meteorological tools and remote sensing have been developed to aid this process so that, for example, 24-hour forecasts of wind can now have a 95% accuracy.

Long-term historic time series data indicating the average availability of renewable energy sources in a given region are useful when assessing the potential for future renewable energy project developments. For example, solar irradiation resource assessments based on satellite data can give an indication of the potential annual contribution to water heating from solar energy or the seasonal variations in power generation output from a solar PV panel. Greater shares of renewable energy generation in the supply system tend to lead to greater decentralisation. Hydro plants can be thousands of megawatts capacity each and wind, solar, bioenergy, geothermal and ocean generation can be up to hundreds of megawatts, but these technologies can also be installed at the micro-level scale of just a few kilowatts each.

This section briefly introduces the range of technologies used for generating electricity as well as for providing heating and cooling that are currently available or close to market and that might well contribute to the renewable energy supply of a city of the future (Fig. 3). For each technology discussed, there is much information in the literature and more details can also be found at the web sites of the relevant IEA Implementing Agreements. These are referenced below since most sites contain good overviews of the latest technologies based on continuing international collaboration by the members and extensive literature reviews.

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