The collector efficiency curve states how a solar collector's efficiency falls as it runs hotter than its surroundings: η = η0 − a1·(Tm−Ta)/G − a2·(Tm−Ta)²/G. η0 is the optical efficiency (0.75–0.82 for good flat plates), a1 and a2 are heat loss coefficients, and G is the irradiance, referenced to 1,000 W/m². All three values come from ISO 9806 testing and appear on every Solar Keymark datasheet.
What do η0, a1 and a2 actually mean?
The three parameters split a collector's behaviour into gain and loss. η0, the optical efficiency, is the fraction of sunlight converted to heat when the collector runs at ambient temperature and loses nothing; a1 and a2 describe how fast efficiency drains away as the mean fluid temperature Tm rises above the ambient temperature Ta.
| Parameter | Meaning | Flat plate | Evacuated tube (gross) |
|---|---|---|---|
| η0 | Efficiency at ΔT = 0 (optical) | 0.75 – 0.82 | 0.45 – 0.65 |
| a1 (W/m²K) | Linear heat loss coefficient | 3.0 – 4.0 | 1.0 – 2.0 |
| a2 (W/m²K²) | Temperature-dependent loss coefficient | 0.010 – 0.020 | 0.003 – 0.008 |
A high η0 wins at low operating temperatures; low a1/a2 win at high ones. No single parameter makes a collector "the best" — the application decides which end of the curve you live on.
How is the efficiency curve measured?
An accredited laboratory measures it under EN ISO 9806, either in steady-state conditions (stable sun, fixed inlet temperatures) or with the quasi-dynamic method that extracts the parameters from varying outdoor conditions. The reference irradiance is 1,000 W/m². Since the standard's 2013 edition the parameters are reported per gross collector area — Solar Keymark datasheets completed that switch in 2016 — so older aperture-based figures look systematically higher for the identical collector; the difference is explained under aperture area. The certified values for every model are public in the Solar Keymark database, which is what makes honest comparison possible.
How do you read the curve in practice?
Insert your operating conditions and the curve returns the expected efficiency. Take a flat plate with η0 = 0.78, a1 = 3.5, a2 = 0.015, heating a store to 60 °C on a 20 °C day (ΔT = 40 K) under 800 W/m²: η = 0.78 − 3.5 × 40/800 − 0.015 × 40²/800 = 0.575. The collector converts about 58% of the available sunlight — roughly 460 W per square metre. Run the same calculation at ΔT = 70 K and efficiency drops near 40%: the same hardware, wildly different performance depending on how hot you ask it to run. This is why low return temperatures and stratified storage matter so much in system design.
Why do flat-plate and evacuated tube curves cross?
Because one starts higher and the other falls slower. The flat-plate collector opens with a superior η0; the evacuated tube collector loses efficiency far more gradually thanks to its vacuum insulation.
| ΔT (K) | Flat plate (0.78 / 3.5 / 0.015) | Evacuated tube (0.65 / 1.5 / 0.005) |
|---|---|---|
| 0 | 0.78 | 0.65 |
| 20 | 0.70 | 0.62 |
| 40 | 0.62 | 0.58 |
| 60 | 0.52 | 0.54 |
| 80 | 0.40 | 0.50 |
For these example collectors at 1,000 W/m², the curves cross between 40 and 60 K. Below the crossover — domestic hot water in mild weather — the flat plate delivers more heat per gross square metre; above it — process heat, solar cooling, deep winter — the tube collector wins.
Which part of the curve matters for your application?
Match the parameter to the temperature regime. Domestic hot water systems spend most hours at ΔT of 20–40 K, where η0 dominates — which is why flat plates rule that market. Combi systems and commercial loads at 50–70 K make a1 increasingly decisive. Process heat above 80 K is a2 territory and usually evacuated-tube or specialised collectors. Annual yield, not instantaneous efficiency, is the final arbiter: Solar Keymark datasheets state calculated annual output per module at reference locations and temperatures, translating the curve into the kWh figure a planner actually needs — the values available for each Solar Keymark-certified Solimpeks collector, from the Wunder ALS and ANSG series to the PV-T Hybrid Panel.
Frequently asked questions
What is a good η0 value for a solar collector?
For flat plates, 0.75–0.82 per gross area is the quality range; evacuated tubes certify lower per gross area (0.45–0.65) because of the gaps between tubes. Always check that the figure is gross-area based before judging it.
Why is my real annual yield lower than the efficiency curve suggests?
The curve is an instantaneous laboratory characteristic at reference conditions. A year includes nights, low sun angles, pipe and store losses and demand mismatches, so system yield lands well below peak collector efficiency. Use the datasheet's annual output figures for planning.
Are η0, a1 and a2 given per aperture or per gross area?
Current ISO 9806 testing and Solar Keymark datasheets use gross area, standard since 2016. Documents from before the switch often quote aperture-based values, which are higher for the same collector — convert to a common basis before comparing.
Does the efficiency curve apply to PVT collectors too?
Yes — the thermal side of a PVT collector is tested to the same ISO 9806 procedures and gets the same parameters. Unglazed PVT additionally carries wind-dependent coefficients, because convection dominates its heat loss.