Flat-Plate and Evacuated Tube Collectors

Abstract

The sun has got his hat on Hip hip hip hooray! The sun has got his hat on, and he's coming out to play Gay and Butler (1932) 5.1 Overview Solar-energy collectors are devices employed to gain useful heat energy from the incident solar radiation. They can be of the concentrating or the flat-plate type. A simple flat-plate solar energy collector consists basically of an absorbing surface which absorbs the insolation and transmit it in form of heat to a working fluid commonly air or water. A general steady-state analysis for flat-plate collectors is shown in Fig. 5.1.Instantaneous efficiencies are plotted against (T fii ÀT a )/I and the intercept and the slope determined (7.1.5). The long-term performance of many solar heating collectors can be characterized by a thus determined intercept and slope as shown by the illustrative examples for different collector types given in Fig. 5.2. These parameters are not constant but depend on temperature and wind speed. Heat removed is also a weak function of overall heat loss. To account for these effects non-linear equations have been developed and together with an angular dependent "incident angle modifier" are used in various versions as the basis for rating tests. These are shown in Fig. 5.3.The absorber material in a flat-plate collector, in addition to having a high absorbance of the incident radiation should also have a low emissivity, good thermal conductivity, and be stable thermally under temperatures encountered during operation and stagnation. It should also be durable, have low weight per unit area and, most importantly, be cheap. Black coated metal sheets (in air heater) and fins in water heaters are used frequently as absorbers due to their effectiveness, ease of use, availability and relatively low cost. Wisely or not, a multitude of