Lets say your home is 70 degrees with 35% relative humidity for arguments sake.
That means the air has 38.13 grains of moisture per pound of air.
When I pass that over a wet panel, it will not likely pick up any moisture, it would be no better than just setting a bowl of water in the middle of the floor and blowing a fan across it. In fact it will only cool the air.
Now if I heat that same air up to 90 degrees, the humidity at that temperature (relative humidity) will be 18% RH. Dryer but not exactly parched. If wil pick up a little moisture.
If I heat that same air to 120 degrees the relative humidity of that 120 degree air is only 7% and it would suck up alot of moisture.
Think of it like a towl. A damp towl will not suck up as much mosture as a dry towl will.
Relative humidity is just that relative to the temperature.
If I cooled that 70 degree air to say 50 degrees, the humidity would be over 70%.
Again, I dont know if that explains it but it should make a little sense as to why I dont suggest evaporative humidifiers on heat pumps.
Here is a response on one humidifers FAQ sites to the question will a humidifier work on my heat pump...
"Model 400 humidifiers can be installed on heat pump systems. However, due to the fact that heat pumps deliver lower temperature air to the home than gas furnaces, evaporation will be approximately 60% of rated capacity (Note: With other Aprilaire models, hot water can be used instead of cold water to maximize evaporation. However, due to the nature of the wicking water panel in the Model 400, hot water provides no extra benefit). As such, your dealer will need to take the size and age of your home into consideration to ensure the Model 400 will provide satisfactory comfort and protection through adequate humidification"
Now I think they are being liberal with the heat output of the heat pump by giving it 60% capacity.
Another quote from a Humidifier application traning manual.
"Humidifier output is usually based on 120° duct air temperature. Older units have been tested at 140° duct temperature. Hence, warmer air has more energy resulting in higher humidifier output. So when do evaporative humidifiers perform at there rated capacity? When the forced air furnace
is in the heating cycle and is discharging 120° air. During any other operating cycle without 120° air the evaporative pad type humidifier is performing well below the rated capacity because no energy is available to change the water to a vapor. Note evaporative pad humidifiers are rated using 120° duct air for 24 hours. Actual daily output from the humidifier will vary greatly based on the furnace operating cycles. HVAC systems discharging air temperature below 120° will not have the output, for example a heat pump. Remember the energy is needed to absorb the water into the air. Heat pump systems require other humidifier technologies that can create their own source of energy, such as steam systems."