ABSTRACT

With direct solar, geothermal and biomass sources, most energy transfer is by heat rather than by mechanical or electrical processes. Heat transfer is a well-established, yet complex, subject. However, we do not need sophisticated detail, which is rarely required to understand and plan renewable energy thermal applications. For instance, as compared with fossil and nuclear fuel engineering plant, temperature differences are often smaller, geometric configurations less complicated and ‘most importantly’ energy flux densities much lower. Of course, the sophisticated detail is needed for specialised renewables design, as for instance with advanced engines powered by biofuels. This book uses a unified approach to heat transfer processes, by which

several interrelated processes are analysed as one ‘heat circuit’. For instance, the solar water heater of Figure 3.1 receives heat by solar radiation at about 10kWm−2 maximum intensity, producing surfaces about 50 C higher than the environment. Heat is lost from these surfaces by long wavelength radiation, by conduction and by convection. The useful heat is removed by mass transport. Our recommended method of analysis is to set up a heat transfer circuit of the interconnected processes, such as the one illustrated in Figure 3.2(c), and calculate each transfer process to an accuracy of about 50%. At this stage, insignificant processes can be neglected and important

transfers can be analysed to greater accuracy. Even so, it is unlikely that final accuracies will be better than ±10% of actual performance. This chapter supports subsequent chapters on individual renewable

energy systems. The main formulas needed for practical calculations are summarised in Appendix C. You may find some of this chapter particularly complicated and boring, especially convection; however, be assured, when you use the methods with real hardware, they come alive.