PVT Solar-Assisted Heat Pump System Architecture: Engineering Design Guide

1. Overview of PVT Solar-Assisted Heat Pump Architecture

A conventional heat pump extracts heat from:

• air,
• ground,
• water.

A PVT-SAHP system introduces the PVT collector as a renewable heat source.

The system combines:

Solar Radiation

        ↓

PVT Collector

        ↓

Thermal Energy

        ↓

Heat Pump Evaporator

        ↓

Compressor

        ↓

Useful Heat Output

At the same time:

Solar Radiation

        ↓

PV Cells

        ↓

Electricity Generation

The advantage of PVT integration is that the same collector area provides both:

• electrical energy;
• thermal energy.

Additionally, extracting heat from the PV module can reduce PV cell temperature, improving electrical performance.

2. The Two Fundamental Architecture Decisions

Decision 1

Direct Expansion (DX) or Indirect Expansion (IDX)?

This is the first system architecture decision.

2.1 Direct Expansion PVT-SAHP Architecture

Principle

In a DX system:

The PVT collector acts directly as the heat pump evaporator.

The refrigerant flows inside the thermal absorber of the collector and evaporates by absorbing solar-derived heat.

DX System Diagram

Solar Radiation

        ↓

PVT Collector
(Refrigerant Evaporator)

        ↓

Compressor

        ↓

Condenser

        ↓

Heating / DHW Load

Engineering Characteristics

Advantages

1. Fewer Heat Transfer Stages

Because there is no intermediate heat exchanger:

• thermal transfer losses can be reduced;
• system can be compact.

2. High Integration Level

The collector performs multiple functions:

• photovoltaic generation;
• refrigerant evaporation;
• heat extraction.

Engineering Challenges

DX systems require careful control because:

• refrigerant phase change occurs directly inside the collector;
• collector temperature changes rapidly with weather conditions;
• refrigerant distribution must remain stable.

The review identifies refrigerant selection and matching between collector and heat pump requirements as a key challenge affecting DX diffusion.

2.2 Indirect Expansion (IDX / Brine) Architecture

Principle

In IDX systems:

The refrigerant does not evaporate inside the PVT collector.

Instead:

A separate heat transfer fluid transports thermal energy from the PVT collector to the heat pump.

Brine System Diagram

Solar Radiation

        ↓

PVT Collector

        ↓

Water / Glycol / Brine Loop

        ↓

Heat Exchanger

        ↓

Heat Pump Evaporator

        ↓

Compressor

        ↓

Heating Load

Engineering Characteristics

Advantages

1. Circuit Separation

The collector loop and refrigerant loop are independent.

Benefits:

• easier system management;
• flexible heat transfer fluid selection;
• easier integration with different heat pumps.

2. Better Adaptability

Because the collector loop is separated:

• antifreeze solutions can be used;
• system operation is easier to control;
• installation flexibility increases.

3. Single Source vs Dual Source Architecture

The second major classification is:

Does the heat pump rely only on PVT?

or:

Does it combine PVT with another heat source?

3.1 Single Source PVT-SAHP

Concept

The PVT collector is the only external heat source.

Example:

PVT Collector

↓

Heat Pump

↓

Building Load

Advantages

• simpler architecture;
• fewer components;
• easier control.

Limitations

The system depends strongly on:

• solar availability;
• weather conditions;
• seasonal variation.

3.2 Dual Source PVT-SAHP

Concept

The system combines PVT with another heat source.

Examples:

• air source;
• ground source.

Example

              Air Source

                  ↓

PVT Collector → Heat Pump → Building

                  ↑

             Secondary Source

Engineering Value

Dual-source systems can overcome limitations of single-source systems.

For example:

• PVT provides solar heat when available.
• Secondary sources support operation under unfavorable conditions.

The review identifies dual-source systems as an important development direction because they improve adaptability under changing environmental conditions.

4. Solis Reference Architecture Strategy

For the Solis PVT Engineering Design Series, two reference architectures are defined.

Reference Design A

Solis Brine 450W PVT Heat Pump Architecture

Architecture Type

Single-source IDX-PVT-SAHP

(or expandable to dual-source IDX architecture)

System Concept

Solis Brine 450W

        ↓

Brine Loop

        ↓

Heat Exchanger

        ↓

Heat Pump

        ↓

Building Heating / DHW

Engineering Position

Primary advantages:

• flexible integration;
• separated circuits;
• easier project adaptation.

Reference Design B

Solis DX 450W PVT Heat Pump Architecture

Architecture Type

DX-PVT-SAHP

System Concept

Solis DX 450W

        ↓

Refrigerant Evaporation

        ↓

Compressor

        ↓

Condenser

        ↓

Building Heating / DHW

Engineering Position

Primary advantages:

• compact integration;
• direct thermal coupling.

5. Architecture Selection Guide

Engineering Insight

The Best PVT System Is Not the One With the Highest Collector Efficiency

A common design mistake is evaluating only:

• PV efficiency;
• thermal efficiency;
• collector output.

For real projects, the better question is:

Can the PVT collector and heat pump operate together efficiently throughout the entire heating season?

The architecture determines whether the system can maintain:

• stable operation;
• appropriate source temperature;
• reliable heat extraction.