Solar Thermal Systems Notes

Last reviewed 1 Oct 2026 · 7 min read

Solar thermal energy

Solar thermal systems convert solar radiation into heat (unlike PV, which gives electricity). They are classified by temperature:

Class	Temperature	Examples
Low	up to about 100 °C	Flat-plate water heaters, air heaters, dryers, stills, cookers, solar ponds
Medium	about 100–400 °C	Parabolic trough, linear Fresnel, dish cookers, process heat
High	above 400 °C	Solar towers and dishes for power

Flat-plate collector (FPC)

Construction

FormulaParts of a flat-plate collector

Absorber plate — copper/aluminium/steel sheet with a selective coating (high absorptance α, low emittance ε for infrared).
Fluid tubes (risers and headers) bonded to the plate; water (or an anti-freeze fluid) flows in them.
Transparent cover (glazing) — low-iron toughened glass (one or two covers) which transmits sunlight and traps infrared (greenhouse effect), and reduces convection loss.
Insulation (glass wool, mineral wool, PUF) under and around the plate.
Casing (box) — aluminium/GI, weather-sealed.

Performance

Hottel–Whillier–Bliss equation: the useful heat gain

Q_{u} = A_{c} F_{R} [S - U_{L} (T_{i} - T_{a})]

where $S = G (τ α)$ is the absorbed solar flux, $U_{L}$ the overall loss coefficient (W/m²K), $T_{i}$ the fluid inlet temperature, $T_{a}$ the ambient temperature and $F_{R}$ the collector heat-removal factor (ratio of actual to the maximum possible heat gain, 0.7–0.95).

Instantaneous efficiency:

η = F_{R} (τ α) - F_{R} U_{L} \frac{T _{i} - T _{a}}{G}

A straight-line plot of η against $(T_{i} - T_{a}) / G$ has intercept $F_{R} (τ α)$ (optical efficiency) and slope $- F_{R} U_{L}$ (loss); the point where $η = 0$ is the stagnation condition.

Worked ExampleExample — collector efficiency

A collector has $F_{R} (τ α) = 0.75$ and $F_{R} U_{L} = 5.0$ W/m²K. With $G = 800$ W/m², inlet temperature 45 °C and ambient 25 °C:

$η = 0.75 - \frac{5.0 \times ( 45 - 25 )}{800} = 0.75 - 0.125 = 0.625 (62.5 %)$ . Useful heat per m² $= 0.625 \times 800 = 500$ W/m².

Stagnation temperature (no flow) can reach about 120–200 °C in an FPC, so the materials and the tank safety valve must tolerate it.

Tilt and orientation

Fixed collectors face due south (northern hemisphere) at a tilt of about latitude + 10–15 ° for winter-biased use (water heating), and about the latitude for year-round use.

Water heating systems

System	Flow	Remarks
Natural circulation (thermosyphon)	Density difference drives water to a tank placed higher than the collector	Simple, no pump, common domestic type (capacity 100–500 LPD)
Forced circulation (pumped)	Pump and controller	Larger/commercial systems, tank anywhere
Direct	Potable water through the collector	Cheap; scaling and freezing risk
Indirect (closed loop with heat exchanger)	Anti-freeze/other fluid	Cold climates, hard water
Batch/ICS	Tank itself is the collector	Cheap, small

Worked ExampleExample — sizing a water heater

A family needs 100 litres of hot water daily at 60 °C from 25 °C. Daily insolation on the tilted collector is 5.5 kWh/m²/day; the daily average collection efficiency (including losses) is 45 %.

Energy needed $= 100 \times 4.186 \times (60 - 25) = 14, 651$ kJ $= \frac{14651}{3600} = 4.07$ kWh.
Useful energy per m² $= 5.5 \times 0.45 = 2.475$ kWh/m²/day.
Area $= \frac{4.07}{2.475} \approx 1.65 m^{2}$ — in practice about 2 m² for 100 LPD is installed to cover cloudy days and tank losses.

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