Figure 1.8 shows the ways in which various types of energy can be converted into electricity. At present, the path generating the bulk of electricity worldwide is shown by the bold lines that lead through combustion from chemical to thermal, from thermal to mechanical and finally to electrical power conversion. The bottleneck of this path is the limited thermody-namic efficiency determined by the Carnot cycle. Older thermal generating stations have
efficiencies between 35 and 40% although in the last two decades conversion has been substantially improved to over 50% through the development of combined cycle gas turbines (CCGTs) a technology discussed in Chapter 2. It follows that when coal, oil or gas is used only 35-50% of the primary energy is successfully converted, the remaining being discharged into the environment in the form of waste heat.
One way of getting around the Carnot limit is simply to make use of the waste heat. This is the principle of combined heat and power (CHP), used extensively in Scandinavia and of growing importance elsewhere. In such schemes, the waste energy from the thermal generation of electricity is distributed through heat mains to local industry and/or housing. This requires substantial infrastructure and is therefore only viable if the power station is reasonably close to the heat users. An alternative arrangement made possible by recent developments is to transport the fuel (mainly gas) to the consumer using the existing supply infrastructure and install the CHP system at the consumer ' s premises. Such systems are known as micro - CHP and are discussed in Chapter 8.
Direct paths that bypass the Carnot bottleneck are also available. The leading example of this approach is the fuel cell, which now borders on commercial viability in a number of forms: solid oxide, molten carbonate, and proton exchange membrane (PEM) to name the main ones. Another direct path is through photovoltaics, a technology that perhaps is the most promising in the near to far future.
The conversion efficiencies of the various routes indicated in Figure 1.8 are dealt with in greater detail in Chapter 2.
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Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable. The usage of renewable energy sources is very important when considering the sustainability of the existing energy usage of the world. While there is currently an abundance of non-renewable energy sources, such as nuclear fuels, these energy sources are depleting. In addition to being a non-renewable supply, the non-renewable energy sources release emissions into the air, which has an adverse effect on the environment.