DX PVT Heat Pump System Design Guide

Published： March 28, 2026
Last Modified：August 5, 2026

Designing a Direct Expansion PVT System Where the Collector Becomes the Heat Pump Evaporator

A Direct Expansion PVT (DX-PVT) heat pump system is one of the most integrated forms of photovoltaic thermal solar-assisted heat pump technology.

Unlike brine PVT systems, where a secondary heat transfer loop transfers energy between the collector and heat pump, a DX system directly connects the PVT collector with the refrigeration circuit.

In this architecture:

  • the PVT collector generates electricity;
  • the thermal absorber acts as a refrigerant evaporation surface;
  • solar-derived heat is directly transferred into the heat pump cycle.

This creates a compact solar-assisted heat pump system with fewer intermediate thermal transfer stages.

Miglioli et al. identify Direct Expansion (DX) systems as configurations where the PVT collector is directly integrated into the refrigerant circuit and acts as the evaporator of the heat pump system.

Key Engineering Takeaways

1. DX PVT integrates the collector and refrigeration cycle.

The collector is not only a solar thermal component.  It becomes:

A combined photovoltaic generator and heat pump evaporator.

2. DX architecture can reduce intermediate heat transfer losses.

Because there is no secondary fluid loop:

PVT Collector

↓

Refrigerant Evaporation

↓

Compressor
replaces:
PVT Collector

↓

Brine Loop

↓

Heat Exchanger

↓

Heat Pump

3. DX systems require higher refrigeration engineering capability.

The direct coupling between collector and refrigerant circuit creates additional design challenges:

  • refrigerant management;
  • evaporation control;
  • system matching;
  • seasonal operating stability.

1. What Is a DX PVT Heat Pump System?

A DX PVT heat pump system is a direct expansion solar-assisted heat pump architecture.

The PVT collector performs three functions:

Function 1

Electricity Generation

PV cells convert solar radiation into electrical energy.

Function 2

Thermal Energy Recovery

The thermal absorber extracts heat from the collector.

Function 3

Refrigerant Evaporation

The collector acts as the evaporator of the heat pump cycle.

The basic system concept:

 
Solar Radiation

        ↓

DX PVT Collector

        ↓

Refrigerant Evaporation

        ↓

Compressor

        ↓

Condenser

        ↓

Building Heating / DHW Load
 

Miglioli et al. describe DX-PVT-SAHP systems as architectures where the refrigerant directly exchanges heat inside the PVT collector.

2. Solis DX 450W Reference Architecture

System Concept

The Solis DX 450W Reference Design follows the DX-PVT-SAHP architecture.

 
                 Solar Radiation

                       ↓

              Solis DX 450W
              PVT Collector

                       ↓

          Refrigerant Evaporation

                       ↓

                Compressor

                       ↓

                Condenser

                       ↓

          Heating / Domestic Hot Water

                       ↓

                Building Load
 

3. DX System Main Components

3.1 DX PVT Collector

Primary Role

The DX PVT collector is the core component of the system.

It combines:

  • photovoltaic generation;
  • thermal absorption;
  • refrigerant evaporation.

Engineering Function

The collector must provide:

  • sufficient heat transfer area;
  • stable refrigerant evaporation conditions;
  • reliable operation under changing solar conditions.

3.2 Refrigerant Circuit

Unlike Brine PVT systems, the DX collector is directly connected to the refrigeration loop.

Main components include:

  • evaporating collector;
  • compressor;
  • condenser;
  • expansion device;
  • control components.

3.3 Heat Pump Cycle

The operating principle:

 
Low Temperature Solar Heat

↓

Evaporation

↓

Compression

↓

High Temperature Heat

↓

Heating Application
 

The heat pump increases the temperature level of renewable solar-derived heat so it can be used for building applications.

4. DX PVT System Engineering Advantages

4.1 Reduced Intermediate Heat Transfer

The main advantage of DX architecture is direct thermal coupling.

Compared with indirect systems:

No:

 
PVT

↓

Secondary Fluid

↓

Heat Exchanger
 

exists.

Instead:

 
PVT

↓

Refrigerant
 

directly transfers heat.

Engineering Benefit

Potential benefits include:

  • fewer heat transfer stages;
  • compact system architecture;
  • reduced temperature losses.

4.2 Higher System Integration Level

A DX PVT collector is not an independent solar thermal collector.

It is part of the heat pump cycle.

The system is designed as one integrated energy conversion device.

4.3 Compact Renewable Heating Solution

Because the collector performs multiple functions, DX systems can be attractive for:

  • limited installation space;
  • integrated building solutions;
  • projects seeking maximum system integration.

5. DX PVT System Engineering Challenges

DX technology also introduces more demanding design requirements.

5.1 Refrigerant Management

The refrigerant must:

  • circulate correctly through the collector;
  • evaporate effectively;
  • remain stable under variable solar conditions.

5.2 Solar Variation Response

The collector operating condition changes with:

  • solar radiation;
  • ambient temperature;
  • wind conditions;
  • seasonal variation.

The refrigeration system must respond to these changes.

5.3 Component Matching

The collector and heat pump cannot be designed independently.

Critical matching includes:

 
Collector

+

Evaporation Condition

+

Compressor

+

Expansion Control
 

Miglioli et al. highlight that DX systems face challenges related to refrigerant selection, collector design, and integration between PVT collectors and heat pump components.

6. Solis DX 450W Engineering Design Workflow

A practical DX system design process:

Step 1

Define Building Thermal Requirement

Determine:

  • heating demand;
  • DHW demand;
  • required operating conditions.

↓

Step 2

Select Heat Pump Operating Conditions

Define:

  • evaporation requirements;
  • heating output requirements;
  • seasonal operation strategy.

↓

Step 3

Match DX PVT Collector

Evaluate:

  • collector thermal characteristics;
  • refrigerant compatibility;
  • operating temperature range.

↓

Step 4

Design Refrigerant Circuit

Evaluate:

  • refrigerant flow;
  • evaporation behavior;
  • expansion control.

↓

Step 5

Optimize System Control

Evaluate:

  • solar variation response;
  • seasonal performance;
  • operating stability.

7. DX 450W vs Brine 450W Engineering Comparison

ItemDX 450WBrine 450W
ArchitectureDirect ExpansionIndirect Expansion
Heat Transfer MediumRefrigerantBrine / Secondary Fluid
Collector RoleEvaporator + PV sourceThermal collector
Circuit SeparationNoYes
Integration LevelHigherLower
Hydraulic FlexibilityLowerHigher
Refrigeration ComplexityHigherLower

Engineering Selection Principle

Choose DX 450W when:

  • high integration is desired;
  • refrigeration expertise is available;
  • compact system design is important.

Choose Brine 450W when:

  • flexibility is more important;
  • easier maintenance is preferred;
  • multiple heat pump configurations need to be considered.

8. DX PVT Application Scenarios

Compact Residential Systems

Suitable where:

  • roof area is limited;
  • integrated renewable heating is required.

New Building Projects

Suitable when:

  • PVT collector;
  • heat pump;
  • building energy system

can be designed together.

High Integration Renewable Systems

Suitable for projects emphasizing:

  • system compactness;
  • solar utilization;
  • integrated design.

9. DX 450W Design Checklist

Collector Side

☐ Collector thermal performance

☐ Refrigerant circuit compatibility

☐ Installation conditions

Refrigeration Side

☐ Refrigerant selection

☐ Evaporation control

☐ Compressor matching

☐ Expansion control

System Side

☐ Heating demand matching

☐ Control strategy

☐ Seasonal operation evaluation

10. Product Evidence Integration

The PVT collector testing layer supports evaluation of collector-level characteristics, including:

  • thermal performance;
  • pressure resistance;
  • durability-related tests.

 

For a complete DX heat pump system evaluation, additional system-level parameters are required, including:

  • compressor characteristics;
  • refrigerant circuit design;
  • operating conditions;
  • seasonal performance evaluation.

Therefore:

Collector testing validates the collector component.
System engineering determines the final heat pump performance.

FAQ

Q1. What is a DX PVT heat pump system?

A DX PVT system uses the PVT collector directly as the evaporator in the heat pump refrigeration cycle.

Q2. What is the main advantage of DX PVT?

The main advantage is direct integration between collector and refrigeration cycle, reducing intermediate heat transfer stages.

Q3. Is DX PVT better than Brine PVT?

Neither architecture is universally better.

DX provides higher integration, while Brine provides greater flexibility.

Q4. Why is DX PVT more technically challenging?

Because the collector becomes part of the refrigeration system and must operate reliably under changing environmental conditions.

Internal Links

Previous:

  • PVT Solar-Assisted Heat Pump System Architecture
  • Brine PVT Heat Pump System Design Guide

Next:

  • DX vs Brine PVT Engineering Selection Guide

Related:

  • PVT Collector Testing and Performance Evaluation

Need Help Designing the PVT System?

Tell us:

  • project location;
  • application;
  • heating requirements;
  • heat pump system.

Our engineering team can help evaluate the suitable PVT configuration.

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