How Is Pressure Drop Measured?
Pressure drop is not estimated from theory during product certification. It is measured under controlled laboratory conditions so that engineers can compare different collectors using standardized methods.
For PVT collectors, hydraulic testing forms part of the performance evaluation defined by ISO 9806. The objective is to establish the relationship between flow rate and pressure loss, allowing engineers to predict how the collector will behave once integrated into a real heating system.
Unlike thermal performance testing, which evaluates useful heat output, hydraulic testing focuses entirely on the movement of the heat-transfer fluid through the collector.
Standard Test Principle
The laboratory circulates a controlled heat-transfer fluid through the collector while measuring:
- Flow rate
- Inlet pressure
- Outlet pressure
- Fluid temperature
The pressure difference between the inlet and outlet is recorded for multiple flow conditions.
This produces a pressure drop curve, showing how hydraulic resistance changes as flow rate increases.
Hydraulic Test Conditions
According to the independent laboratory report for the Solis PVT collector:
| Test Parameter | Laboratory Condition |
|---|
| Test fluid | Water |
| Fluid temperature | 20.5°C |
| Test result | Pressure-drop curve generated |
| Engineering output | Pressure-drop coefficients for hydraulic calculation |
The report also provides the mathematical pressure-drop coefficients that engineers can use during system calculations.
Understanding the Pressure Drop Curve
One of the most valuable outputs of hydraulic testing is the pressure-drop curve.
Instead of giving only a single pressure value, the laboratory measures multiple operating points over different flow rates.
Conceptually, the curve looks like this:
This illustrates an important engineering principle:
As flow rate increases, pressure drop also increases.
However, the increase is not perfectly linear.
At low flow rates, pressure increases relatively slowly.
As flow velocity becomes higher, turbulence inside the absorber channels increases, causing pressure losses to rise more rapidly.
The laboratory report includes this measured pressure-drop relationship rather than relying solely on theoretical calculations.
Pressure Drop Coefficients
Rather than publishing only a graph, the laboratory also calculated pressure-drop coefficients.
For the tested PVT670 collector, the report provides:
| Parameter | Measured Value |
|---|
| Pressure-drop coefficient a | 0.3798 |
| Pressure-drop coefficient b | 0.0037 |
These coefficients allow engineers to calculate pressure losses across different operating flow rates using hydraulic design software or engineering calculations.
What Factors Affect Pressure Drop?
Pressure drop is influenced by several design variables.
1. Flow Rate
Flow rate has the greatest influence.
More water or brine moving through the collector means:
- higher velocity,
- more internal friction,
- greater pressure loss.
This is why pressure-drop curves are always plotted against flow rate rather than shown as a fixed number.
2. Internal Hydraulic Design
Different collectors have different absorber designs.
Examples include:
- serpentine flow channels,
- parallel riser tubes,
- micro-channel structures,
- different manifold layouts.
Each design creates a different hydraulic resistance.
Two collectors with identical thermal efficiency may have completely different pressure-drop characteristics.
3. Fluid Properties
Laboratory testing is commonly performed using water.
In actual installations, many heat pump systems circulate:
- glycol-water mixtures,
- brine solutions,
- antifreeze fluids.
These fluids have different viscosities, which can increase pressure losses compared with water.
Therefore, engineers often use laboratory measurements as the reference point before applying correction factors for the actual working fluid.
4. Fluid Temperature
Viscosity decreases as temperature rises.
Warmer fluids generally experience lower hydraulic resistance.
Conversely, colder brine used in winter heat pump systems may increase pressure losses.
This is one reason hydraulic calculations should always consider actual operating conditions rather than laboratory conditions alone.
Why Pressure Drop Matters in Real Projects
Pressure drop is not just a laboratory number.
It directly influences system design decisions.
Pump Selection
Every circulation pump must generate enough head to overcome the total resistance of the hydraulic circuit.
That total resistance includes:
- collector pressure drop,
- pipe friction,
- fittings,
- valves,
- heat exchangers,
- filters,
- expansion components.
If the collector contributes significant pressure loss, the required pump head increases accordingly.
Pump Energy Consumption
A larger pressure drop generally requires more pumping energy.
Although the pump consumes only a fraction of the system’s total energy, unnecessary pumping power reduces seasonal system efficiency.
Therefore, engineers seek an appropriate balance rather than simply maximizing flow rate.
Flow Distribution
Large collector arrays require balanced hydraulic flow.
If pressure losses differ significantly between parallel branches:
- some collectors receive excessive flow,
- others receive insufficient flow,
- thermal output becomes uneven.
Hydraulic testing data helps engineers size balancing valves and optimize manifold design.
System Reliability
Correct hydraulic design also reduces operational risks.
Properly matched pumps and collectors help avoid:
- insufficient circulation,
- unstable flow,
- excessive pump wear,
- unnecessary operating costs.
Engineering Comparison
Pressure Drop vs Thermal Efficiency
These two parameters describe completely different aspects of collector performance.
| Pressure Drop | Thermal Efficiency |
|---|
| Hydraulic characteristic | Thermal characteristic |
| Measured in Pa | Measured as efficiency coefficients |
| Determines pump requirements | Determines heat collection capability |
| Used for hydraulic design | Used for energy yield prediction |
| Influenced by flow resistance | Influenced by optical efficiency and heat loss |
A collector cannot be evaluated using only one of these parameters.
Good engineering requires considering both together.
Low Pressure Drop vs High Pressure Drop
| Lower Pressure Drop | Higher Pressure Drop |
|---|
| Smaller pump head | Larger pump head |
| Lower pumping electricity | Higher pumping electricity |
| Easier hydraulic balancing | Greater design attention required |
| May allow higher flow rates | Flow optimization becomes more important |
Neither condition is automatically “better.”
The correct solution depends on the complete hydraulic design of the heating system.
Common Misunderstandings
Misunderstanding 1
Lower pressure drop always means a better collector.
Not necessarily.
A collector with slightly higher hydraulic resistance may have a more effective absorber design that delivers superior thermal performance.
Misunderstanding 2
Pressure drop only matters for large commercial systems.
Incorrect.
Even residential systems require proper pump sizing.
Ignoring pressure losses can result in insufficient circulation and reduced heating performance.
Misunderstanding 3
Datasheet values are enough.
Engineering projects should always prioritize independently measured hydraulic data whenever available.
Third-party laboratory testing provides greater confidence than simplified marketing specifications because the measurements follow standardized procedures.
