How to defrost Dualsun hybrid panels when used as the single heat source of a brine/water heat pump?
Dualsun SPRING4 MAX panels provide the heat pump (heat pump), at its evaporator, with the thermal power necessary for heating and domestic hot water production, day and night, whether outside temperatures are positive or negative (up to -15 or -20°C at the evaporator inlet depending on the heat pump models).
When the air is cold and humid, the water vapor contained in the air can condense into droplets on the panel surfaces (front and back). If the panel temperature drops below 0 °C, this condensation freezes: frost forms on the panels. The presence of frost corresponds to additional thermal resistance on the panel surface, reducing the efficiency of thermal transfer with ambient air.
When the frost thickens, the degradation of the panel's thermal performance can exceed a limit. It is then necessary to defrost the panels, especially since beyond a certain thickness, the frost on the front could form sheets that risk sliding and falling.
Consult this FAQ for more details on the impact of frost on the performance of SPRING4 MAX panels.
According to data collected from the first projects, 0.15 to 0.25 kWh per panel are needed to melt the formed frost. Depending on the defrosting method, a defrosting cycle should last between 10 and 20 minutes.
In practice, there are three possible options, depending on the heat pump models and the hydraulic configuration:
Option n°1 - Defrosting by thermodynamic inversion of the heat pump
The heat pump reverses its thermodynamic cycle (the evaporator becomes a condenser and the condenser becomes an evaporator) to provide heat to the panels by taking energy from the buffer tank / the heating circuit bottle. For this, the heat pump must be reversible (equipped with a 4-way valve).
Examples: Arkteos and Nibe.
N.B.: the defrosting cycles of the Arkteos INVELIA heat pump have been specifically designed and optimized for Dualsun SPRING MAX solar panels.
Option n°2 - Defrosting by direct draw from the hot storage (DHW or heating buffer) via bypass
The heat pump control uses an internal bypass exchanger to draw heat from the DHW tank or heating buffer and direct it to the panels. The temperatures at the panel inlet during such a cycle are then higher than with option n°1, which can significantly reduce its duration.
Examples: MetroTherm, Ecoforest.
Option n°3 - Heat pump without defrosting control: addition of a solar station / external control
If the heat pump does not have an integrated defrosting function, it is necessary to add a solar station (solar water heater type), a 3-way valve and a dry contact controlling the heat pump. The idea is to imitate the behaviour of option n°2. When the need for defrosting is detected, the solar station controller stops the heat pump by dry contact, then circulates the heat transfer fluid from one of the DHW tank exchangers to the panels to defrost them. The tank must be equipped with a double exchanger.
Here is the technical diagram corresponding to this option in the case where the DHW and heating storage are in the same "combi" tank:
