Reference algae: separatus and double U 32

The reference project in Fussach, Austria, documents two 90 m deep drillings with different borehole heat exchangers: a double U-tube BHE 32 and a separatus probe. The project data provided shows design and operational differences but does not contain comparable performance, efficiency or temperature results. A statement on the thermal superiority of an embodiment is therefore not possible.

The reference project in Fussach

The single-family house was built 2021 and has a living space of 220 m2. It is heated by a modulating heat pump, the maximum of which is heating capacity 13 kW. This value denotes the heat pump and not the performance of a single probe or bore.

The substrate is described in the project as a water-saturated clay with a thermal conductivity of approximately 1,5 W/(m·K). At 15.02.2024, a bottom temperature of 4,8 °C was given in 1,0 m depth. A measurement method does not name the project source.

The heat source comprises two bores each with 90 m depth. The full text of the reference assigns a drilling diameter of 152 mm to both drillings. A fluid-assisted rotary drilling is specified as the drilling method.

Which two probe designs were installed?

In operation since · separatus probe: 2024 · double U-tube BHE 32: 2021
Drill depth · separatus probe: 90 m · double U-tube BHE 32: 90 m
Drill diameter according to project full text · separatus probe: 152 mm · double U-tube BHE 32: 152 mm
Probe dimension · separatus probe: documented contradictory in the project · double U-tube BHE 32: 32 × 3,0 mm
brine flow · separatus probe: turbulent · double U-tube BHE 32: turbulent
Connecting line for pre- and return · separatus probe: 10 m · double U-tube BHE 32: 36 m
thermal conductivity of borehole grouting · separatus probe: 2,0 W/(m·K) · double U-tube BHE 32: 1,0 W/(m·K)

As a designed volume flow the original calls 1,1 m3/h. The fragmented table does not clearly indicate whether this value applies per probe, per well or to the overall system.

Note on the dimension of the separatus probe

The project source mentions 50 × 3,1 mm, the current product specifications 51 × 3,1 mm. Since the cause of the deviation cannot be clarified, no clear diameter is shown for the installed probe.

What can be derived from the project data?

It is documented that two differently configured geothermal drillings are documented on the same reference object. They differ in probe type, start of operation, thermal conductivity, borehole grouting and length of connection lines.

separatus recommends for current planning a thermal conductivity of the borehole grouting of over 2 W/mK. The documented project value of 2,0 W/(m·K) is not subsequently changed.

What are the limits of the comparison?

No reliable efficiency or performance comparison can be derived from the available data. In particular, comparable power values, temperature profiles, measurement methods and a common measurement period are missing. The “Performance & Temperatures” section of the original reference does not contain completed results.

In addition, the following differences restrict comparability:

The double U-tube BHE has been in operation since 2021, the separatus probe since 2024.
The backfills are specified as 1,0 or 2,0 W/(m·K).
The connection lines for pre- and return are of different lengths with 36 m and 10 m respectively.
The allocation of the design volume flow of 1,1 m3/h is not clear.

General product comparisons cannot be transferred to this project: The reference project is described with 152 mm drilling and has different backfills and connection lengths.

Thus, the reference is a documentation of two different designs on the same building, but not a published proof of higher efficiency or performance of the separatus probe.