PVT Collector Thermal Performance Testing: ISO 9806 Efficiency & Output Guide

Published： March 6, 2026
Last Modified：July 21, 2026

PVT Collector Thermal Performance Testing: How Efficiency, Heat Loss and Thermal Output Are Measured

Introduction

Photovoltaic Thermal (PVT) collectors combine photovoltaic electricity generation with solar thermal energy recovery, allowing one solar collector to provide both electrical and thermal energy.

However, unlike conventional photovoltaic modules, PVT collectors must be evaluated not only by their electrical characteristics but also by their ability to capture, transfer, and deliver useful thermal energy under different operating conditions.

For engineers, installers, and system designers, the key question is not simply:

“How much heat can a PVT collector produce?”

The more important question is:

“How was the thermal performance measured, and can the data be trusted for system design?”

Thermal performance testing provides the engineering data required to answer these questions.

A professional PVT thermal performance evaluation should reveal:

optical efficiency
heat loss characteristics
thermal output under different operating temperatures
efficiency behaviour under different solar radiation conditions
incident angle performance
parameters required for system simulation and sizing

Solis PVT thermal performance evaluation is based on internationally recognized solar collector testing methods, supported by independent third-party laboratory testing.

Why Thermal Performance Testing Matters for PVT Collectors

Thermal Performance Determines Real System Value

The value of a PVT collector is not determined only by its peak thermal output.

In real applications, especially when integrated with heat pump systems, performance changes continuously depending on:

solar irradiation
ambient temperature
collector operating temperature
heat transfer fluid temperature
system design conditions

For example, a PVT collector operating as a heat source for a brine heat pump usually works at relatively low temperatures.

Under these conditions:

heat losses are reduced
thermal efficiency improves
renewable heat contribution increases

However, when the operating temperature rises:

temperature difference between collector and environment increases
heat losses increase
useful thermal output decreases

Therefore, engineers need complete thermal performance curves instead of a single rated value.

How PVT Thermal Energy Conversion Works

A PVT collector converts solar radiation into two forms of useful energy:

Solar Radiation

        ↓

   PVT Collector

        ↓

 ┌───────────────┐
 │               │
Electricity     Heat
 │               │
PV Output    Thermal Output
 └───────────────┘

        ↓

Heat Pump / Heating System

The photovoltaic layer generates electricity, while the thermal absorber and heat transfer circuit recover heat.

The thermal performance of the collector directly influences:

available heat source temperature
heat pump efficiency
annual renewable energy contribution
collector sizing

What Standards Are Used for PVT Thermal Performance Testing?

ISO 9806: Solar Thermal Collector Test Method

ISO 9806 is the internationally recognized test method for evaluating solar thermal collectors.

The latest edition:

ISO 9806:2025 — Solar energy — Solar thermal collectors — Test methods

defines the testing framework used to evaluate collector performance characteristics.

The standard provides methods for measuring parameters including:

thermal efficiency
heat loss behaviour
incident angle modifier
thermal capacity
pressure drop characteristics

For PVT collectors, these thermal measurement methods are particularly important because the collector operates as both:

an electricity generator
a solar thermal collector

Independent Test Verification of Solis PVT Collectors

The Solis PVT collector thermal performance data referenced in this article is supported by independent laboratory testing.

The submitted PVT collector samples were tested according to:

EN 12975:2022
ISO 9806:2017

The independent test report confirms:

The submitted samples were tested and found to comply with applicable requirements of EN 12975:2022 and ISO 9806:2017.

The test evaluation covered multiple PVT collector models, including:

PVT430
PVT450
PVT550
PVT580
PVT600
PVT670

What Is Measured During PVT Thermal Performance Testing?

A complete thermal performance evaluation includes several important measurements.

1. Thermal Efficiency Testing

Thermal efficiency testing determines how effectively the collector converts solar radiation into useful thermal energy.

The test considers:

solar irradiation
collector temperature
ambient temperature
heat transfer fluid conditions

The result is expressed through performance coefficients used to calculate thermal output under different operating conditions.

2. Performance Coefficients

The most important thermal performance parameters include:

Optical Efficiency (η0)

Represents the collector’s ability to convert incoming solar radiation into useful heat before considering thermal losses.

Heat Loss Coefficient (a1)

Describes heat loss behaviour related to the temperature difference between collector and ambient environment.

Heat Loss Coefficient (a2)

Represents the additional temperature-dependent heat loss effects at higher operating temperatures.

Together, these coefficients allow engineers to predict thermal output under different system conditions.

Understanding the Key Thermal Performance Parameters of PVT Collectors

Thermal performance testing generates several important parameters that describe how efficiently a PVT collector converts solar energy into useful heat.

For engineers designing PVT systems, three parameters are especially important:

Optical efficiency (η₀)
First-order heat loss coefficient (a₁)
Second-order heat loss coefficient (a₂)

Together, these parameters define the thermal behaviour of a PVT collector across different operating conditions.

Optical Efficiency (η₀): The Starting Point of Thermal Performance

What Does Optical Efficiency Mean?

Optical efficiency (η₀) represents the ability of a solar thermal collector to convert incoming solar radiation into useful thermal energy before considering heat losses.

In simple terms:

η₀ describes how much solar energy enters the collector and becomes available as heat at ideal operating conditions.

The optical efficiency is influenced by factors including:

solar absorber characteristics
glazing or cover properties
optical transmission losses
absorber surface quality
collector structure

A higher optical efficiency means that more solar radiation can be converted into useful thermal energy.

Why Optical Efficiency Matters for PVT Systems

For PVT systems connected with heat pumps, optical efficiency affects the available renewable heat source.

A collector with higher optical efficiency can provide:

higher thermal output under low temperature differences
better renewable energy contribution
improved annual system performance

This is particularly important for:

brine heat pump systems
ground-source heat pump applications
solar-assisted heat pump systems

These systems often operate at relatively low collector temperatures, where optical performance has a significant influence.

Heat Loss Coefficient a₁: Understanding Basic Thermal Losses

What Is a₁?

The first-order heat loss coefficient:

a₁ [W/m²K]

describes the collector’s heat loss behaviour caused by the temperature difference between the collector and the surrounding environment.

When the collector temperature increases above ambient temperature:

↓

heat begins to escape to the environment.

The a₁ coefficient describes this basic heat loss relationship.

Why a₁ Is Important

A lower a₁ value generally indicates:

better thermal insulation
reduced heat loss
improved thermal retention

For PVT collectors, thermal insulation is especially important because the collector must balance two objectives:

Recover useful heat
Maintain suitable operating temperature for the photovoltaic layer

Poor thermal design can lead to:

unnecessary thermal losses
reduced heat recovery
lower overall system efficiency

Heat Loss Coefficient a₂: Temperature-Dependent Heat Loss Behaviour

What Is a₂?

The second-order heat loss coefficient:

a₂ [W/m²K²]

describes how heat losses increase as the collector temperature rises.

At higher operating temperatures:

convection losses increase
radiation losses increase
temperature-dependent losses become more significant

Therefore, a₂ becomes important when evaluating applications requiring higher temperature operation.

Why a₂ Matters for Heat Pump Integration

Different heating systems operate at different temperature levels.

For example:

Low-temperature applications

Examples:

underfloor heating
low-temperature heat pumps

The collector operates closer to ambient temperature.

The influence of a₂ is relatively limited.

Higher-temperature applications

Examples:

domestic hot water
higher supply temperature heating systems

The collector temperature increases.

The influence of a₂ becomes more important.

Therefore, engineers should evaluate the complete thermal performance model instead of comparing only one efficiency value.

Thermal Performance Equation and Collector Behaviour

The thermal output of a solar thermal collector is commonly calculated using performance coefficients obtained from testing.

The general relationship considers:

solar irradiation
temperature difference
optical efficiency
heat loss coefficients

A simplified understanding:

Higher solar radiation

+

Lower collector temperature difference

=

Higher thermal efficiency

While:

Higher collector temperature

+

Higher temperature difference

=

Higher thermal losses

This relationship explains why the same PVT collector can produce different thermal outputs depending on the system design.

Real Thermal Performance Coefficients from Independent Testing

The Intertek test report evaluated the thermal performance of Solis PVT collector samples using ISO 9806 testing procedures.

The thermal performance testing included:

steady-state thermal performance testing
performance coefficient calculation
collector power output evaluation
incident angle modifier measurement

The report provides measured thermal performance coefficients including:

η₀
a₁
a₂
a₅

for tested collector models.

For example, the report provides performance coefficient data for models including PVT430 and PVT670.

These measured coefficients can be used by engineers for:

thermal output simulation
collector sizing
heat pump system matching
annual energy estimation

How Engineers Should Compare PVT Thermal Performance

When comparing different PVT collectors, engineers should avoid using only one parameter.

A complete evaluation should consider:

1. Optical Performance

Questions:

What is the measured η₀?
Under what test conditions?

2. Heat Loss Behaviour

Questions:

What are the a₁ and a₂ values?
How does performance change at higher temperatures?

3. Operating Conditions

Questions:

Is the collector designed for low-temperature heat pumps?
Is the application domestic hot water or space heating?

4. Independent Verification

Questions:

Are the values based on laboratory testing?
Is the test method clearly identified?

A professional comparison should always be based on measured performance data rather than marketing claims.

Thermal Performance and Heat Pump Applications

For PVT systems integrated with heat pumps, thermal performance data is one of the most important engineering inputs.

The collector acts as the heat source side of the system.

The design process typically considers:

Collector Side

thermal output
operating temperature
flow conditions

Heat Pump Side

source temperature
COP performance
seasonal efficiency

System Side

heating demand
storage requirements
collector area

Accurate thermal performance data helps engineers avoid:

oversized collector fields
insufficient heat source capacity
unrealistic energy predictions

Intertek Thermal Performance Test Results: Real Data Behind Solis PVT Collectors

A professional PVT collector evaluation requires more than theoretical calculations.

Reliable engineering design depends on measured laboratory data obtained under controlled testing conditions.

The thermal performance data presented in this section is based on an independent third-party laboratory test report for Solis PVT collector samples.

The report:

Test Report Number: 240312065GZU-001
Test method:
- EN 12975:2022
- ISO 9806:2017
Testing scope:
- Thermal performance testing
- Performance coefficient calculation
- Power output evaluation
- Incident angle modifier testing
- Pressure drop measurement

Thermal Performance Testing Conditions

The thermal performance evaluation was performed using standardized solar collector testing methods.

The test conditions included:

outdoor thermal performance testing
water as heat transfer fluid
controlled collector orientation
measured solar irradiation
measured inlet and outlet temperatures
calculated collector performance coefficients

The report records that the thermal performance testing was conducted according to ISO 9806:2017 Clause 19–27 requirements.

Tested PVT Collector Models

The independent evaluation covered multiple Solis PVT collector models:

Model	Application Type
PVT430	PVT collector series
PVT450	PVT collector series
PVT550	PVT collector series
PVT580	PVT collector series
PVT600	PVT collector series
PVT670	PVT collector series

The tested samples represent the Solis PVT collector product family evaluated for thermal performance characteristics.

Collector Performance Coefficients

Thermal performance testing generates mathematical coefficients that describe collector behaviour.

The main measured parameters include:

Parameter	Meaning	Engineering Use
η₀	Optical efficiency	Solar energy conversion capability
a₁	First-order heat loss coefficient	Basic thermal loss evaluation
a₂	Second-order heat loss coefficient	Temperature-dependent heat loss
a₅	Effective thermal capacity	Dynamic response analysis

These coefficients allow engineers to calculate thermal output under different operating conditions instead of relying only on nominal values.

Example: PVT430 Thermal Performance Data

For the PVT430 model, the test report provides measured collector performance coefficients.

The report includes values based on different reference areas, including:

gross area
aperture area
absorber area

The measured performance parameters include:

η₀
a₁
a₂
a₅

This approach follows ISO 9806 methodology, where collector performance is represented through experimentally measured coefficients rather than a single rated efficiency value.

Example: PVT670 Thermal Performance Data

The PVT670 model was also evaluated under the same thermal performance testing framework.

The report provides:

collector area information
performance coefficients
calculated power output data
incident angle modifier results

For engineering applications, larger collector models such as PVT670 can provide higher thermal output per collector unit because of their increased active area.

However, actual system performance still depends on:

operating temperature
solar conditions
heat pump design
hydraulic configuration

Understanding the PVT Thermal Output Curve

One of the most valuable results from thermal performance testing is the collector power output curve.

The curve shows how thermal output changes according to:

solar irradiation level
temperature difference between collector and ambient environment

The Intertek report provides collector power output records under different irradiation conditions, including:

400 W/m²
700 W/m²
1000 W/m²

How to Read a PVT Efficiency Curve

A typical PVT thermal performance curve contains:

X-Axis

Temperature difference:

Collector mean temperature - Ambient temperature

A larger value means:

hotter collector operation
greater heat loss potential

Y-Axis

Thermal output:

Useful thermal power output

Higher values indicate:

more available heat
better thermal performance under those conditions

Typical Performance Trend

The engineering relationship is:

Low Temperature Operation

When:

collector temperature is close to ambient
heat loss is limited

The collector achieves:

higher efficiency
higher useful heat output

This condition is common in:

brine heat pump systems
ground-source heat pump systems

High Temperature Operation

When:

collector temperature increases
temperature difference becomes larger

Heat losses increase.

The thermal efficiency decreases.

This is why PVT system design should match the collector operating temperature with the application requirements.

Incident Angle Modifier (IAM): Performance Beyond Normal Sunlight Conditions

Solar radiation does not always strike the collector surface directly.

Throughout the day and year:

sun angle changes
irradiation direction changes

The Incident Angle Modifier (IAM) describes how collector performance changes when sunlight arrives at different angles.

The Intertek report includes IAM testing results for Solis PVT collector models.

IAM data helps engineers evaluate:

annual energy yield
collector orientation
installation angle
seasonal performance

Why Thermal Test Data Is Important for Heat Pump System Design

For PVT heat pump systems, collector thermal data is not only a product specification.

It is a design input.

Engineers use thermal performance coefficients to estimate:

Available Heat Source Energy

The collector must provide sufficient thermal energy to the heat pump under expected operating conditions.

Heat Pump Operating Conditions

The PVT collector affects:

source temperature
evaporation conditions
COP performance

Collector Field Sizing

Accurate thermal data helps determine:

required collector quantity
expected seasonal contribution
system balance

Without measured thermal performance data, system calculations may rely on unrealistic assumptions.

Measured Data vs Marketing Claims

A major difference between professional engineering evaluation and basic product comparison is the evidence source.

A marketing datasheet may provide:

peak efficiency
nominal output
selected operating condition

However, independent thermal testing provides:

defined test method
measured coefficients
laboratory conditions
calculation model

For engineering decisions:

Measured laboratory data provides a more reliable foundation than isolated performance claims.

Evidence Summary

Solis PVT Thermal Performance Verification

Evidence source:

Independent third-party laboratory test report
Report Number: 240312065GZU-001
Test standards:
- EN 12975:2022
- ISO 9806:2017

Verified test areas include:

✓ Thermal performance coefficients
✓ Collector power output
✓ Incident angle modifier
✓ Pressure drop characteristics

Applying PVT Thermal Performance Data to Real System Design

Thermal performance testing is not only a way to verify collector quality.

For engineers, the most important value of test data is that it enables accurate system design.

A PVT collector does not operate independently. It works as part of a complete renewable energy system that may include:

heat pumps
circulation pumps
thermal storage
building heating systems
domestic hot water systems

Therefore, thermal performance data must be interpreted within the actual application conditions.

Using Thermal Performance Data for Heat Pump Integration

Matching PVT Collectors with Heat Pump Operating Conditions

PVT collectors are often used as renewable heat sources for:

brine heat pumps
ground-source heat pump systems
solar-assisted heat pump systems

In these applications, the collector performance influences:

source temperature available to the heat pump
heat extraction capability
seasonal system efficiency

A properly designed system should consider:

collector thermal output curve
expected ambient conditions
heat pump operating temperature
building heat demand

Low-Temperature Applications: Where PVT Performs Particularly Well

Many heat pump systems operate at relatively low source temperatures.

Examples include:

underfloor heating systems
low-temperature space heating
energy-efficient residential buildings

Under these conditions:

collector temperature remains closer to ambient
heat losses are reduced
thermal efficiency improves

The measured thermal performance coefficients from ISO 9806 testing allow engineers to evaluate this operating range more accurately.

Collector Selection Based on Real Operating Conditions

Selecting a PVT collector should not rely only on:

maximum thermal output
peak efficiency value
collector size

A professional selection process should evaluate:

1. Required Thermal Demand

Questions:

How much heat does the building require?
What percentage should PVT provide?

2. Operating Temperature

Questions:

What temperature does the heat pump require?
Will the collector operate mainly at low or high temperature?

3. Solar Conditions

Questions:

What is the annual solar radiation?
What is the installation orientation?
What is the expected seasonal variation?

4. Hydraulic Design

Questions:

What flow rate is required?
What is the pressure drop?
How should pumps be selected?

Thermal performance data should always be combined with hydraulic and system design information.

Relationship Between Thermal Performance and Hydraulic Design

A collector’s thermal performance cannot be separated from its hydraulic behaviour.

The heat transfer process depends on:

fluid flow rate
pressure drop
heat transfer conditions

The Intertek evaluation also included pressure drop measurement as part of the collector performance assessment.

For example, the report provides pressure drop testing data under controlled conditions, including:

measured flow rates
pressure difference values
pressure drop characteristics

This information helps engineers design:

circulation pump selection
collector array configuration
hydraulic balancing

Why Independent Thermal Testing Builds Engineering Confidence

For renewable energy projects, performance reliability is critical.

A PVT collector specification should ideally be supported by:

Transparent Test Method

The test method should clearly identify:

applicable standard
testing conditions
measurement approach

Measured Performance Data

The data should include:

thermal coefficients
output curves
operating parameters

Independent Verification

Third-party testing provides confidence that performance values are based on measured results rather than only manufacturer claims.

The Solis PVT thermal performance evaluation is supported by independent laboratory testing according to recognized solar collector testing procedures.

Frequently Asked Questions

What is PVT collector thermal performance testing?

PVT collector thermal performance testing evaluates how effectively a photovoltaic thermal collector converts solar radiation into useful heat.

It measures parameters such as:

optical efficiency
heat loss coefficients
thermal output
incident angle behaviour

Which standard is used for PVT thermal testing?

Solar thermal collector performance is evaluated according to ISO 9806 testing methods.

The latest edition is:

ISO 9806:2025 — Solar energy — Solar thermal collectors — Test methods.

The Solis PVT collector test report referenced in this article was tested according to:

EN 12975:2022
ISO 9806:2017

What does η₀ mean in PVT testing?

η₀ represents optical efficiency.

It indicates how effectively the collector converts incoming solar radiation into useful thermal energy before considering heat losses.

What do a₁ and a₂ represent?

a₁ and a₂ describe collector heat loss behaviour.

a₁ represents first-order heat loss
a₂ represents temperature-dependent heat loss

These parameters help engineers calculate thermal output under different operating temperatures.

Why are efficiency curves important for PVT systems?

Efficiency curves show how collector performance changes under different operating conditions.

They help engineers understand:

expected thermal output
temperature limitations
suitability for heat pump integration

Why is independent testing important for PVT collectors?

Independent testing verifies that performance values are based on controlled measurements using recognized testing methods.

It provides stronger engineering confidence than relying only on product claims.

Can PVT thermal data be used for heat pump system design?

Yes.

Thermal performance coefficients are important inputs for:

collector sizing
heat pump matching
energy simulation
seasonal performance evaluation