Solar Water Heating

Main part of solar water heater is collector. Two types of collector Flat plate collector Concentrating/focusing collector

Flat plate collectors absorbs both direct and diffused radiations Simple and cheap Can be used to heat fluid to low temperatures(<100oC)

Concentrating Collectors Can only use direct beam Expensive and more sophisticated Require precise positioning controls Can heat fluid to high temperatures

(a) Open-container (trough) on ground; heat flows easily to ground. (b) Open trough, off ground. Clear water is not a good absorber; loses heat by evaporation. (c) Black closed container (‘tank’); large heat loss, especially to wind; no overnight storage. (d) Black tank, insulated underneath; heat losses confined to top surface, therefore only half those of (c). (e) Sheltered black tank; cheap, but materials degrade. (f) Metal tube and plate collector, and flooded plate. Standard commercial collector; fluid moves through the collector, e.g. to a separate storage tank; flooded plate more efficient than tube and plate. (g) Double glazed flat plate; better insulated version of (f); can operate up to ∼100 C; iron-free glass less absorbing than window glass. (h) Selective surface. short long , radiative losses reduced. (i) Evacuated collector. No convection losses to the cover.

Heat Balance

Uncovered Heaters (a-d) Uncovered container on the ground Uncovered, Open container off ground Enclosed black container, black tank

Uncovered container on the ground This is the simplest possible water ‘heater’ .An outdoors swimming pool is a common example of a container of water exposed to sunshine, and on, or in, the ground. On a sunny day the water is warmed, but the temperature rise is limited as heat is conducted easily to the ground and also lost by evaporation and convection. Having black surfaces would increase absorption, but obscure cleanliness.

Uncovered, open container off the ground Raising the open container off the ground reduces conductive loss, but much of the heat that is retained goes into increased evaporation, thus lessening the temperature increase.

Enclosed black container; black tank Here the water is enclosed in a shallow matt-black tank or bag So no heat is lost by evaporation. The matt-black outer surface absorbs radiation well (typically α= 09). Some of this absorbed heat is then passed to the water inside by conduction. This type of heater is cheap, easy to make and gives moderately hot water (∼20C above ambient), but may have a short lifetime. Loss of heat by forced convection from wind severely limits the performance. Despite the simplicity of construction, however, the analysis of the heating is relatively complex.

Black Container with Bottom Insulation The heat losses of the previous system can be almost halved simply by insulating the bottom of the container. Almost any material that traps air in a matrix of small volumes ≤1mm is useful as an insulator on this rear side, e.g. fiberglass, expanded polystyrene or wood shavings. The insulating volumes of air must not be too large, since otherwise the air will transfer heat by convection. Also the material must be dry, since water within the matrix is a much better conductor than air.

Improved solar water heaters Sheltered black container Metal plate collectors with moving fluid Selective surfaces Evacuated collectors

Sheltered black container Sheltering within a box with transparent cover reduces the convective losses to wind. Glass is often the chosen cover material, having small absorptance for the solar short wave irradiation

Metal Plate Collectors with moving fluid (aka tube and plate collectors) High commercial capability water is confined in parallel tubes which are attached to a black metal plate. Typically the tube diameter is ∼2cm, the tube spacing ∼20cm and the plate thickness ∼03cm. The plate and tubes are sheltered from the wind in a framework with a glass top and thick side and rear insulation. This collector has essentially the same circuit analogue as the sheltered black bag. Flooded plate collectors are potentially more efficient than tube collectors because of increased thermal contact area. The heated fluid may be used immediately, or it may be stored and/or recirculated.

Efficiency of flat plate collector P u =ηc Ap G ηc = ηspηpf ηsp = τcovαp-UL(Tp – Ta )/G which shows that as the plate gets hotter, the losses increase until ηsp decreases to zero at the ‘equilibrium’ temperature. Since the plate temp is now usually known, the collector efficiency is usually related to fluid temp as: ηsp= ηpfτcovαp-ηpfUL(Tf – Ta )/G

In a well-designed collector, the temperature difference between the plate and the fluid is small and the value of ηpf is nearly one. Typically ηpf= 085 and is almost independent of the operating conditions, and, since pipes and storage tanks should be well insulated, Tf≈Tp. The capture efficiency ηpf (and therefore also the collector efficiency ηc) would vary linearly with temperature if UL= 1/RL were constant, but in practice the radiative resistance decreases appreciably as Tp increases. Therefore a plot of ηc against operating temperature has a slight curvature.

The performance of a flat plate collector, and in particular its efficiency at high temperatures, can be substantially improved by Reducing the convective transfer between the plate and the outer glass cover by inserting an extra glass cover. Reducing the radiative loss from the plate by making its surface not simply black but selective, i.e. strongly absorbing but weakly emitting.

System with separate storage Active systems Include a pump and control unit Passive systems Involve flow by thermo siphoning

Selective Surfaces Metal–semiconductor stack Here a semiconductor (which strongly absorbs solar short wave radiation) is deposited on a metal. (which is a weak emitter of thermal long wave radiation).

Evacuated Collectors Using a selective absorbing surface substantially reduces the radiative losses from a collector. To obtain yet larger temperature differences, (e.g. to deliver heat at temperatures around or greater than 100C, for which there is substantial industrial demand), it is necessary to reduce the convective losses as well. One way is to use extra layers of glass above a flat plate collector (‘double glazing’). A method that gives better efficiency but is technically more difficult is to evacuate the space between the plate and its glass cover. This requires a very strong structural configuration to prevent the large air pressure forces breaking the glass cover; such a configuration is an outer tube of circular cross-section. Within this evacuated tube is placed the absorbing tube. Typically the tubes have outer diameter D = 5cm and inner diameter d = 4cm. By suitably connecting an array of these tubes, collectors may receive both direct and diffuse radiations