The key is this part, from page two of the brochure:
The ULOW-E2 provides enhanced convection via in-built fans. This enables heat sources to operate at their most energy efficient settings.
The "heat source" is a boiler which provides hot water. Here's an efficiency curve for a boiler relative to the return water temperature (source):
You can see that higher efficiencies occur when the water in the boiler starts at a lower temperature, in "condensing mode." Condensing boilers are designed and programmed to operate in this region:
Condensing boilers [...] achieve high efficiency (typically greater than 90% on the higher heating value) by condensing water vapour in the exhaust gases and so recovering its latent heat of vaporisation, which would otherwise have been wasted. This condensed vapour leaves the system in liquid form, via a drain.
Once in the condensing region, you can continue improving efficiency by further lowering inlet water temperature.
Thus, two factors result in making the total system more efficient with a low temperature radiator:
- The boiler temperature can be set lower, since the radiator works at a lower temperature. This will inherently lower the temperature of the water returning to the boiler.
- The radiator is able to extract more heat from the water than a traditional radiator, further lowering the temperature of the return water.
Obviously, the boiler temperature must be sufficient to provide heating for the house. But with a system like this you could also implement a hot water reset strategy to vary the boiler setpoint based on the expected need for heating.