Collector Solar water heating

solar thermal collectors capture , retain heat sun , use heat liquid. 2 important physical principles govern technology of solar thermal collectors:

any hot object returns thermal equilibrium environment, due heat loss conduction, convection , radiation. efficiency (the proportion of heat energy retained predefined time period) directly related heat loss collector surface. convection , radiation important sources of heat loss. thermal insulation used slow heat loss hot object. follows second law of thermodynamics (the equilibrium effect ).
heat lost more rapidly if temperature difference between hot object , environment larger. heat loss predominantly governed thermal gradient between collector surface , ambient temperatures. conduction, convection , radiation occur more rapidly on large thermal gradients (the delta-t effect).


flat-plate solar thermal collector, viewed roof-level


flat plate

flat plate collectors extension of idea place collector in oven -like box glass directly facing sun. flat plate collectors have 2 horizontal pipes @ top , bottom, called headers, , many smaller vertical pipes connecting them, called risers. risers welded (or connected) thin absorber fins. heat-transfer fluid (water or water/antifreeze mix) pumped hot water storage tank or heat exchanger collectors bottom header, , travels risers, collecting heat absorber fins, , exits collector out of top header. serpentine flat plate collectors differ harp design, , instead use single pipe travels , down collector. however, since cannot drained of water, serpentine flat plate collectors cannot used in drainback systems.

the type of glass used in flat plate collectors low-iron, tempered glass. such glass can withstand significant hail without breaking, 1 of reasons flat-plate collectors considered durable collector type.

unglazed or formed collectors similar flat-plate collectors, except not thermally insulated nor physically protected glass panel. consequently, these types of collectors less efficient. pool heating applications, water heated colder ambient roof temperature, @ point lack of thermal insulation allows additional heat drawn surrounding environment.

evacuated tube

evacuated tube solar water heater on roof

evacuated tube collectors (etc) way reduce heat loss, inherent in flat plates. since heat loss due convection cannot cross vacuum, forms efficient isolation mechanism keep heat inside collector pipes. since 2 flat glass sheets not strong enough withstand vacuum, vacuum created between 2 concentric tubes. typically, water piping in etc therefore surrounded 2 concentric tubes of glass separated vacuum admits heat sun (to heat pipe) limits heat loss. inner tube coated thermal absorber. vacuum life varies collector collector, 5 years 15 years.

flat plate collectors more efficient etc in full sunshine conditions. however, energy output of flat plate collectors reduced more etcs in cloudy or extremely cold conditions. etcs made out of annealed glass, susceptible hail, failing given golf ball -sized particles. etcs made coke glass, has green tint, stronger , less lose vacuum, efficiency reduced due reduced transparency. etcs can gather energy sun day long @ low angles due tubular shape.