Published： March 28, 2026
Last Modified：July 21, 2026
A common mistake when evaluating PVT collectors is focusing on only one efficiency value.
For example:
“This collector has 70% thermal efficiency.”
However, a PVT collector does not operate under only one condition.
Real systems experience changes in:
Therefore, engineers need a performance curve instead of a single number.
A PVT efficiency curve shows how thermal efficiency changes as the temperature difference between the collector and the environment increases.
It helps engineers answer:
| Question | Answer |
|---|---|
| What is a PVT efficiency curve? | A graph showing how collector thermal efficiency changes with operating conditions. |
| What does the curve represent? | The relationship between efficiency, temperature difference and solar irradiation. |
| Which parameters define the curve? | η₀ optical efficiency, a₁ first-order heat loss coefficient and a₂ second-order heat loss coefficient. |
| What evidence supports Solis PVT? | Independent laboratory thermal performance testing according to EN 12975:2022 and ISO 9806:2017. |
| Why do engineers use curves? | To select collectors, size systems and predict real operating performance. |
Evidence Source
Independent third-party laboratory test report:
Report No. 240312065GZU-001
Testing references:
The report evaluated Solis PVT collector thermal performance characteristics, including efficiency-related parameters and heat loss coefficients.
The measured coefficients allow engineers to construct and interpret thermal efficiency behaviour under different operating conditions.
A PVT efficiency curve describes:
The relationship between collector thermal efficiency and the operating temperature difference between the collector and ambient environment.
The horizontal axis usually represents:
Tm−TaG\frac{T_m-T_a}{G}
or related temperature parameters.
The vertical axis represents:
η\eta
thermal efficiency.
A typical PVT efficiency curve behaves like this:
Higher Efficiency
│\
│ \
│ \
│ \
│ \
│ \
└────────────────
Increasing Temperature DifferenceThe reason:
As temperature difference increases:
The efficiency curve is mainly determined by:
η₀ represents the initial ability of the collector to convert solar radiation into useful heat.
On the curve:
η₀ determines the starting point.
A higher η₀ generally means:
Related:
B1-T2-I01
What Is Optical Efficiency (η₀)?
a₁ describes the basic thermal losses when collector temperature rises above ambient.
On the curve:
a₁ determines the initial downward slope.
Higher a₁:
Lower a₁:
Related:
B1-T2-I02
What Does Heat Loss Coefficient a₁ Mean?
a₂ describes additional thermal losses at larger temperature differences.
On the curve:
a₂ affects the curve bending behaviour.
It becomes increasingly important when:
Related:
B1-T2-I03
What Does Heat Loss Coefficient a₂ Mean?
The thermal efficiency relationship is commonly expressed as:
η=η0−a1(Tm−Ta)G−a2(Tm−Ta)2G\eta = \eta_0 – a_1\frac{(T_m-T_a)}{G} – a_2\frac{(T_m-T_a)^2}{G}
Where:
| Parameter | Meaning |
|---|---|
| η | Thermal efficiency |
| η₀ | Optical efficiency |
| a₁ | First-order heat loss coefficient |
| a₂ | Second-order heat loss coefficient |
| Tₘ | Mean collector temperature |
| Tₐ | Ambient temperature |
| G | Solar irradiance |
Characteristics:
Important parameter:
η₀
Typical applications:
Characteristics:
Important parameters:
η₀ + a₁
Typical applications:
Characteristics:
Important parameters:
a₁ + a₂
Typical applications:
PVT collectors are often integrated with heat pumps.
The efficiency curve helps engineers understand:
How much thermal energy is available at different operating temperatures.
How many collectors are required for a specific heat demand.
How performance changes during:
ISO 9806:2025 — Solar energy — Solar thermal collectors — Test methods
defines standardized methods for evaluating solar collector thermal performance.
The Solis PVT collector thermal performance evaluation was conducted according to:
The report provides thermal performance coefficients used for engineering analysis.
The independent laboratory report provides measured thermal performance parameters, including:
ISO 9806 defines:
Engineers should select collectors based on the curve region matching the actual application.
A collector with excellent low-temperature performance may not always be the best choice for high-temperature applications.
| Parameter | Collector A | Collector B |
|---|---|---|
| η₀ | Higher | Lower |
| a₁ | Higher | Lower |
| Low temperature performance | Better initial output | Moderate |
| High temperature performance | Efficiency decreases faster | Maintains performance better |
The better collector depends on the project operating condition.
Only comparing the highest efficiency point.
Why incorrect:
The maximum value may not represent real operating conditions.
Ignoring operating temperature.
A collector used for heat pumps operates differently from one used for hot water.
Comparing curves from different test conditions.
Engineers must confirm:
Determine whether the PVT collector matches:
Use performance coefficients for:
Compare different collectors under the same operating conditions.
A PVT efficiency curve shows how collector thermal efficiency changes as operating temperature conditions change.
Because real systems operate under different temperatures and weather conditions.
Main parameters:
Through standardized thermal performance testing according to methods such as ISO 9806.
Because heat pump systems require accurate prediction of available thermal energy under different operating conditions.
A PVT efficiency curve is one of the most valuable engineering tools for evaluating collector thermal performance.
Instead of showing only one efficiency number, the curve explains how performance changes when operating conditions change.
The key parameters are:
Supported by independent laboratory testing based on recognized solar collector standards, Solis PVT provides engineering data that helps designers select and optimize PVT systems for renewable heating applications.