Heat transfer

Heat transfer

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 receives heat by solar radiation at about
1_0kWm?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 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