SOLAR THERMAL COLLECTORS Low Temp. (ΔT ~ 40 – 100 deg C)

Water Heating Space Heating

Cooking Liquid Flat Plate Collector Drying Flat plate Air Heater Air Conditioning & Refrigeration Distillation

Medium Temp. (ΔT ~ 100 - 350 deg C) Low pressure steam Liquid Flat Plate Collector Power Generation

High Temp. (ΔT > 350 deg C) Power Generation Cylindrical Parabolic Trough Collector Furnace for metal melting Linear Reflecting Concentrator (Heliostats – sun trackers)

Solar thermal flat plate collectors

• One of the simplest, and most cost effective renewable energy systems for small scale installation is a solar thermal collector

• Direct conversion of solar radiation into heat, stored as hot water

• Used for domestic hot water supply and space heating up to district heating scale.

Heat Gain from Solar Thermal Collection

• A simple flat plate collector consists of an absorber surface which absorbs the insolation and transmits it to a working fluid (usually air or water).

• The useful heat gained can be expressed as: Qu = m Cp (To - Ti)

Desirable Cover Material Properties

• High transmittance (τ) in the visible spectrum, e.g. glass

• Resistant to breakage, e.g. Tedlar, Mylar, Teflon film

• Low transmittance in infrared region

• Low thermal conductivity if possible.

• Thermally stable under stagnation

• UV stable, Low weight & cost

• Low heat absorption

• Use of double cover reduces convection

• Use of anti-reflection film on cover plate

Transmittance (τ) of visible light through selected cover materials

Desirable Absorber Material Properties

• High absorbtance (α) for short thermal radiation

• Good thermal conductivity, e.g. copper

• Low emissivity to long wavelengths

• Thermally stable under stagnation

• Durable & low weight

• Economic

Radiative properties of selected absorber surfaces

Minimising Absorber Losses (UL)

Absorber Insulation - Desirable Properties

• Lightweight - eases installation and minimises on-site time & costs.

• Physically robust - especially at elevated temperatures during operation and possible summer stagnation - ensures consistent thermal performance. Must also tolerate freezing temperatures in more temperate climates.

• Environmentally benign

Aerogel Insulation

SOLAR FLAT PLATE COLLECTOR-SCHEMATIC

Absorbed energy = A. I. τ. α Total heat losses = A. UL . (Tp – Ta)

HEAT TRANSFER PROCESSES IN FLAT PLATE COLLECTOR

Optimising Heat Removal

Poor heat removal Higher return temperature

increases heat loss Reduced efficiency Reduced energy consumption

of pump

Good heat removal Lower return temperature

reduces heat loss Improved efficiency Increased energy consumption

of pump

Storage capacity & Collector Inlet Temperature

All solar heated water is used. Capacity of tank ensures water

at base of tank still cool to minimise return temperature.

Also requires good heat removal.

All solar heated water is used but reduced capacity of tank

allows temperature of water at based of tank to rise minimising

heat removal capability.

Performance/Design Recommendations

Useful heat gain = Qu = A FR [I(τ.α) - UL(Tp - Ta)] (energy balance equation)

Ambient

SOLAR COLLECTORS - EFFICIENCIES

• It has been observed that flow rate does not significantly affect the

efficiency of a liquid collector but air flow rate has a significant influence

on the air collector’s performance.

• Liquid collectors are more efficient than the air collectors at the same

inlet temperature, ambient temperature, and solar radiation levels.

• Performance evaluation also requires attention to other components in the

system, mainly insulation of various components like pipes, storage tank,

etc.

ADVANTAGES OF FLAT PLATE COLLECTORS (i) Use both direct and diffused radiation

(ii) Do not require adjustment in orientation towards sun

the way concentrating collectors require

(iii) Require little maintenance

(iv) Are mechanically simpler than concentrating collectors

Active and Passive System

Active System Passive System

• More efficient • Requires more elaborate arrangement • Does not work well in low sun intensity

Evacuated Tube

Flat Curved Tubular

Evacuated Tube

• In an evacuated-tube collector, sunlight enters through the outer glass tube and strikes the absorber, where the energy is converted to heat.

• The heat is transferred to the liquid flowing through the absorber.

• The collector consists of rows of parallel transparent glass tubes, each of which contains an absorber covered with a selective coating.

• The absorber typically is of tin-tube design

• Evacuated-tube collectors are modular—tubes can be added or removed as hot-water needs change.

FP vs ET

• Evacuated-tube collectors are typically more efficient at higher temperatures than flat-plate collectors.

• ET perform well in both direct and diffuse solar radiation.

• The vacuum minimizes heat losses to the outdoors, makes these collectors particularly useful in areas with cold, cloudy winters.

• Circular shape of the evacuated tube, sunlight is perpendicular to the absorber for most of the day.

– For comparison, in a flat-plate collector that is in a fixed position, the sun is only perpendicular to the collector at noon

E-

FP vs ET

• Flat plates have longer life but Evacuated Tube systems are modular

• Evacuated-tube collectors are also more expensive.

• Flat Plate uses tempered glass while Evacuated Tube use borosilicate or soda lime glass.

• Evacuated-tube collectors are more appropriate for most commercial and industrial applications due to the extremely high temperatures they can achieve (170°-350°F).