Published： July 8, 2026
Last Modified：Sep. 21, 2026
Neither architecture is universally better.
The fundamental difference is where the refrigerant evaporates.
The refrigerant circulates directly through the PVT collector.
Solar radiation
↓
DX PVT
↓
Refrigerant evaporation
↓
Compressor
↓
Condenser
↓
BuildingA secondary fluid—typically water/glycol or another heat-transfer fluid—circulates through the PVT collector.
Solar radiation
↓
Brine PVT
↓
Brine loop
↓
Heat exchanger
↓
Refrigerant
↓
Compressor
↓
Condenser
↓
BuildingThe review by Miglioli et al. classifies PVT heat-pump systems primarily into direct-expansion (DX) and indirect-expansion (IDX) configurations, with further distinctions between single-source and dual-source systems.
For the Solis Engineering Design Series:
Brine 450W is the primary practical reference architecture; DX 450W is the advanced direct-expansion reference architecture.
They should not be presented as two versions of the same system.
The easiest way to understand DX versus brine is to follow the heat.
PVT collector
↓
Refrigerant
↓
Evaporation occurs in PVT
↓
CompressorThe collector itself functions as the evaporator.
PVT collector
↓
Brine
↓
Heat exchanger
↓
Refrigerant evaporator
↓
CompressorThe PVT collector and refrigerant circuit are separated.
That single architectural difference creates most of the downstream engineering differences.
DX = Direct Expansion.
In a DX-PVT system, the refrigerant directly enters the thermal collector and evaporates there.
Conceptually:
DX PVT
┌───────────────┐
│ Refrigerant │
│ evaporation │
└───────┬───────┘
↓
CompressorThere is no intermediate brine-to-refrigerant heat exchanger between the PVT collector and the refrigerant circuit.
This can reduce one heat-transfer step.
But it also means the collector becomes part of the refrigeration system.
In the brine architecture, the PVT collector contains a secondary heat-transfer fluid.
A simplified configuration is:
PVT
↓
Brine loop
↓
Heat exchanger
↓
Refrigerant
↓
CompressorThe brine circuit and refrigeration circuit are hydraulically separated.
The literature describes this as indirect expansion (IDX).
The intermediate heat exchanger allows the working fluid on the solar side to be selected independently from the refrigerant used by the heat pump.
The central trade-off can be summarized as:
| DX PVT | Brine PVT |
|---|---|
| Direct refrigerant evaporation | Secondary-fluid heat transfer |
| PVT is refrigerant evaporator | PVT is thermal collector |
| No intermediate HX | Requires intermediate HX |
| Potentially fewer heat-transfer steps | Additional heat-transfer step |
| Refrigeration circuit extends to roof | Refrigerant circuit can remain separated |
| More demanding refrigerant design | More flexible hydraulic design |
| Strong coupling between PVT and refrigeration | Greater separation between subsystems |
The question is therefore not simply:
Which one is more efficient?
The better question is:
Which architecture provides the best combination of thermal performance, control, reliability, installation practicality and lifecycle economics for the application?
DX can avoid the intermediate heat exchanger.
The energy pathway is:
PVT
↓
Refrigerant
↓
Compressorinstead of:
PVT
↓
Brine
↓
HX
↓
RefrigerantThis removes one heat-transfer interface.
In principle, that can reduce:
The literature review recognizes the potential performance advantages of DX configurations while also emphasizing their greater complexity and control requirements.
This is one of the most important points for the Solis website.
A shorter heat-transfer path does not automatically mean a better complete system.
DX introduces additional challenges:
The literature specifically identifies control complexity as an important issue for DX-PVT systems.
Therefore:
Potentially lower thermal resistance does not eliminate system-level engineering complexity.
At first glance, the intermediate heat exchanger looks like a disadvantage.
But it provides an important engineering boundary.
SOLAR SIDE
PVT → Brine → HX
│
│
REFRIGERANT SIDE
↓
Evaporator
↓
CompressorThe collector and heat pump can therefore be designed as two connected but relatively independent subsystems.
This can simplify:
One of the strongest advantages of IDX is fluid separation.
The literature explicitly notes that the intermediate heat exchanger permits independent selection of the solar-side and heat-pump-side working fluids.
That means the PVT loop can be designed around:
while the heat pump can use its own refrigerant circuit.
This separation is highly valuable for engineering and product design.
Freeze protection is an important practical consideration.
For a brine system:
PVT
↓
Water/glycol or suitable heat-transfer fluidThe solar-side fluid can be selected and controlled to withstand the expected outdoor temperature.
The refrigerant circuit remains separate.
For DX:
PVT
↓
RefrigerantThere is no conventional water loop to freeze.
However, that does not mean DX has no low-temperature design challenges.
The refrigeration circuit itself must be designed for:
Therefore:
DX removes the conventional water-freezing problem, but replaces it with a more tightly coupled refrigeration-system problem.
The refrigerant circuit extends to the PVT collectors.
Heat Pump
│
Refrigerant piping
│
↓
Roof
│
DX PVTThis requires the collector field to be treated as part of the refrigeration installation.
The roof carries a hydraulic circuit:
Heat Pump
│
Brine piping
│
↓
Roof
│
Brine PVTThe refrigeration circuit can remain concentrated around the heat pump.
For projects where refrigerant routing and field servicing are major concerns, this separation can be valuable.
Potential issues include:
Potential issues include:
Neither system is maintenance-free.
The maintenance philosophy is simply different.
This is one of the biggest differences.
A simplified control sequence is:
PVT temperature
↓
Pump control
↓
Brine flow
↓
HX
↓
Heat pumpThe controller manages the hydraulic side and heat-pump operation.
The controller must respond directly to changing refrigeration conditions.
Solar irradiance
↓
PVT temperature
↓
Evaporation condition
↓
Refrigerant flow
↓
CompressorThe literature identifies compressor-frequency control as particularly important for DX-PVT-SAHP systems because PVT operating conditions can change rapidly with weather.
Imagine a cloud passes over the collector.
Solar radiation ↓
↓
PVT temperature ↓
↓
Evaporation condition changesIn DX, this directly affects the refrigerant evaporating inside the collector.
In a brine system, the secondary fluid loop provides an additional thermal/hydraulic buffer.
That does not eliminate transient behavior, but it can decouple the collector dynamics from the refrigerant circuit.
This is an important reason why:
DX requires more sophisticated control.
The brine architecture is not free of disadvantages.
The intermediate HX introduces:
PVT
↓
Brine
↓
HX
↓
RefrigerantEach interface introduces a temperature difference.
Therefore, the refrigerant evaporation temperature may be lower than the PVT fluid temperature.
Conceptually:
PVT fluid temperature
↓
ΔT
↓
Refrigerant evaporation temperatureA larger temperature difference can increase the heat-pump temperature lift.
Therefore, the heat exchanger should be appropriately sized.
For a Brine 450W system, the intermediate heat exchanger should be considered part of the thermal architecture.
The design needs to consider:
A poorly selected HX can erase much of the theoretical benefit of a PVT thermal source.
For DX:
PVT absorber
↓
Refrigerant evaporationThe collector is directly constrained by the refrigerant operating condition.
For brine:
PVT absorber
↓
Brine temperature
↓
HX approach
↓
Refrigerant evaporationThe designer therefore has another degree of freedom.
This is one of the most important conceptual differences between the architectures.
The reviewed literature reports experimental average COP values broadly around:
But these figures must not be interpreted as a direct technology ranking.
The review explicitly cautions that many DX studies were based on short experiments under favorable conditions, while IDX studies more often involved longer experimental periods.
Therefore:
The literature does not justify saying that DX is universally more efficient than brine PVT.
This is an important evidence boundary.
Several factors can contribute.
Some DX studies use short test periods.
Favorable solar conditions can improve source temperature.
Collector, heat pump and controls differ between studies.
Different studies may define and calculate COP differently.
Therefore:
Literature COP
≠
Universal technology COPFor Solis content, literature values should be used as benchmarks, not product promises.
The Solis DX reference system can be represented as:
Solar
↓
DX PVT 450W
↓
Refrigerant evaporation
↓
Compressor
↓
Condenser
↓
BuildingThe collector therefore participates directly in the refrigeration cycle.
This architecture is useful for explaining:
The Solis Brine reference architecture is:
Solar
↓
Brine PVT 450W
↓
Brine loop
↓
Intermediate HX
↓
Refrigerant circuit
↓
Compressor
↓
Condenser
↓
BuildingThis architecture is particularly useful for explaining:
The DX architecture can also be expanded into a dual-source system.
DX PVT
│
│
├─────────┐
│ │
↓ ↓
PVT Evap. Air HX
│ │
└────┬────┘
↓
Compressor
↓
CondenserThe literature describes this type of configuration as a dual-source DX-PVT-SAHP.
The two evaporating sources operate in parallel, with their relative contributions depending on source conditions.
The corresponding brine architecture is:
Brine PVT
↓
Brine loop
↓
HX / Evap.
↓
│
├──────→ Compressor
│
Air HX
↑
│
Ambient AirThe PVT and air sources can be operated:
depending on the system design and control strategy.
This is the primary dual-source architecture for the Solis Engineering Design Series.
For retrofit applications, Brine PVT generally offers the cleaner engineering story.
Why?
Because:
Existing / New Heat Pump
↑
Brine HX
↑
Existing roof
↑
PVT arrayThe solar loop can be integrated without extending the refrigerant circuit throughout the collector field.
The review identifies IDX air-source integration as a flexible and cost-effective approach for retrofit applications.
This does not prove that every retrofit should use brine, but it makes the architecture particularly suitable for retrofit-oriented system design.
A new-build system provides more freedom.
DX can be attractive when the designer can optimize:
as one integrated system.
This is where DX can potentially exploit its reduced heat-transfer chain.
But the design team needs stronger refrigeration engineering capability.
Scaling can be relatively modular:
450W × 4
450W × 8
450W × 12
...The collector field feeds a common brine circuit.
The exact hydraulic configuration must be designed according to flow and pressure-drop requirements.
Scaling is more tightly linked to:
Therefore, larger DX arrays can require more careful circuit engineering.
Brine / IDX generally gives greater subsystem independence.
The designer can separately optimize:
Collector
Flow
Fluid
Hydraulicsand:
Refrigerant
Compressor
Expansion
Evaporator
CondenserThe intermediate HX connects the two.
DX has tighter coupling.
That can be an advantage for optimization, but also a constraint for independent subsystem design.
For a modular B2B product strategy, this distinction is particularly important.
A manufacturer can standardize:
while system integrators select the heat pump.
The collector is much more closely linked to the refrigeration system.
Therefore, product standardization may require tighter coordination between:
This is a major commercial as well as engineering difference.
DX places refrigerant piping directly in the collector field.
Therefore, refrigerant selection and installation requirements become part of the collector-field design.
Brine separates the refrigerant from the roof collector loop.
This can simplify the conceptual boundary between:
solar collector installation
and
refrigeration installation.
Specific refrigerant and regulatory requirements remain project-dependent and should be verified against the applicable codes and standards.
Brine systems can integrate readily with thermal storage through the hydraulic loop.
For example:
PVT
↓
Brine
↓
Thermal Storage
↓
Heat PumpThis can provide additional control flexibility.
DX systems can also be integrated with storage, but the architecture is generally more dependent on the refrigeration-side system design.
Therefore, if thermal storage is a major part of the project, brine architecture can provide a more straightforward integration pathway.
Both DX and brine PVT can be integrated into systems providing:
But the system architecture matters more than the collector alone.
For dual-source systems, an independent air heat exchanger can provide additional flexibility.
The literature identifies dual-source systems as particularly promising for combined heating, cooling and DHW applications.
| Design factor | DX PVT | Brine PVT |
|---|---|---|
| Direct refrigerant evaporation | ★★★★★ | — |
| Intermediate HX required | No | Yes |
| Solar-side fluid flexibility | Lower | Higher |
| Refrigerant circuit complexity | Higher | Lower |
| Hydraulic complexity | Lower | Higher |
| Control complexity | Higher | Moderate |
| Collector/refrigerant coupling | High | Low |
| Freeze protection of water loop | Not applicable | Required where applicable |
| Retrofit flexibility | Moderate | High |
| Thermal storage integration | Moderate | High |
| Product modularity | Moderate | High |
| Potential heat-transfer advantage | High | Moderate |
| System integration flexibility | Moderate | High |
| Best Solis reference role | Advanced | Primary |
This table is an engineering synthesis, not a claim that one architecture wins every category.
DX is worth considering when:
Brine is particularly attractive when:
Start
↓
Is direct refrigerant expansion
a design priority?
/ \
Yes No
↓ ↓
DX PVT Is flexibility/
retrofit priority?
/ \
Yes No
↓ ↓
Brine PVT Compare bothFor real projects, this screening should then be followed by:
Climate
↓
Building load
↓
PVT area
↓
Heat-pump capacity
↓
Source temperature
↓
HX / refrigerant design
↓
Controls
↓
Annual simulation
↓
Economic assessmentThe available paper should not be used to claim:
The review itself emphasizes differences in experimental conditions and study durations.
These distinctions are essential for maintaining technical credibility.
For this Engineering Design Series, the two architectures should be positioned deliberately.
Primary practical reference
PVT
↓
Brine
↓
HX
↓
Heat PumpUse it to demonstrate:
Advanced direct-expansion reference
PVT
↓
Refrigerant evaporation
↓
CompressorUse it to demonstrate:
The choice can ultimately be reduced to one principle:
DX minimizes the separation between the solar collector and refrigeration cycle; brine maximizes the separation between them.
DX therefore emphasizes:
thermal coupling and compact refrigeration integration.
Brine emphasizes:
system flexibility and subsystem independence.
Neither philosophy is inherently superior.
The correct architecture depends on the application.
direct refrigerant evaporation + integrated refrigeration design are the primary objectives.
flexibility + modularity + retrofit compatibility + hydraulic integration are more important.
For the Solis Engineering Design Series:
Brine 450W should be the default practical reference design. DX 450W should remain the advanced direct-expansion reference design.
This gives engineers two clearly differentiated architectures rather than presenting PVT as a single technology.
The strongest design framework is therefore:
SOLIS PVT
│
┌─────────┴─────────┐
↓ ↓
Brine 450W DX 450W
Practical IDX Advanced DX
│ │
↓ ↓
Flexible system Integrated
architecture refrigeration
│ │
└─────────┬─────────┘
↓
Real building load
↓
Engineering designDX circulates refrigerant directly through the PVT collector. Brine PVT uses a secondary heat-transfer fluid and an intermediate heat exchanger.
Not universally. DX can reduce one heat-transfer step, but the complete system must also account for controls, refrigerant distribution, pressure drop and operating conditions.
It can be more flexible because the roof collector is separated from the refrigeration circuit, but it introduces hydraulic equipment and an intermediate heat exchanger.
Brine PVT is generally the more flexible architecture for retrofit-oriented system design, particularly when avoiding extensive refrigerant routing is desirable.
They address freezing differently. Brine systems can use a suitable low-freezing-point heat-transfer fluid; DX has no conventional water loop but requires proper refrigerant-side low-temperature design.
Yes, in the conventional IDX architecture, an intermediate heat exchanger transfers heat from the solar-side fluid to the refrigerant circuit.
Not for circulating a brine loop through the collector, because the collector itself is part of the refrigerant circuit. Other system components may still require pumps depending on the building-side architecture.
Yes. Both can be configured as dual-source systems, although the DX configuration introduces additional refrigerant-side control complexity.
Brine 450W should be the primary practical reference. DX 450W should be used as the advanced direct-expansion reference.
Primary source
Alessandro Miglioli, Niccolò Aste, Claudio Del Pero, Fabrizio Leonforte, Photovoltaic-thermal solar-assisted heat pump systems for building applications: Integration and design methods, Energy and Built Environment, 4, 39–56.
The paper’s system classification and discussion of DX/IDX, single/dual-source configurations provide the primary technical basis for this comparison.
Its discussion of DX and IDX performance emphasizes that reported COP values must be interpreted in the context of different experimental conditions and durations.
Its discussion of IDX systems highlights the flexibility provided by the intermediate heat exchanger and independent working fluids.
This article establishes an engineering comparison framework.
It does not constitute a design calculation for a specific building.
For actual design, the next layers should calculate:
building load → PVT thermal output → source temperature → heat-exchanger capacity → refrigerant operating condition → compressor capacity → controls → seasonal performance.
That is where the Solis Brine 450W / DX 450W Reference Designs become engineering calculation cases rather than merely conceptual examples.
P5 · Comparison Center
Engineering Design
Do not choose DX or brine based on collector efficiency alone.
Start with:
building load → climate → source temperature → PVT architecture → heat exchanger/refrigerant circuit → controls → seasonal performance.
For practical system development, use:
Brine 450W → primary reference architecture
DX 450W → advanced direct-expansion reference architecture
The collector architecture determines the rest of the system.