It's unlikely heaters will have much effect in the open, especially with winds blowing. On one web site I'm on there's a Swede growing tropical palms but indoors. He has quite a collection.

The garden I'm talking about is only about 400 sq. feet, and because of the fence the wind will be no problem because it is sheltered from wind from every directions. but do you still think that it would be impossible, even if the plants are sheltered and the heaters are 1-2 feet from the plants?

The part of the palm you need to protect is the crown. Unless you have dwarf varieties, they will get too tall for the heaters to have any effect.

I can't see the situation you have there so can only guess based on knowing how hot air behaves. Also, calculations based on 'yearly averages' are misleading and can lead to wrong conclusions. A number of palms will cope with moderately cold minimums if the daytime maximums are much higher. However, if daytime is also cool the palms could be dead in a few days regardless of how high the overall yearly average might be. So to get them to survive you have to concentrate on the period of time they're most at risk, not on the offset value of summer time temperatures they might not live to experience. Without doing it in a completely enclosed space, not just behind walls, you're bordering on the impossible. Heated air rises so without some barrier overhead you'll be losing it rapidly. I could imagine the cost keeping heaters going to try and compensate this would be very high. It'd be more economical to build a structure overhead and reduce the amount of heating required. I've seen a few examples (photos on the internet) where people have done that. One was a fanatical Hoya grower who posted photos of an extension he built onto the house. Snow was piled high against it on the outside while Hoyas were growing happily inside. Another was what the owner called an atrium, quite large, filled with lots of tropical plants, mainly aroids. You could do that for palms, just keeping to the smaller species of which there are many beautiful ones. Just a suggestion that could be more easily achievable.

http://youtu.be/GF9z6nRlq34

This video is a great example of a few techniques you could try that would certainly help many palm species survive through winter. Coconut Mike, the Owner of Montreal palms has a great selection of hardy palms that are surviving winters in zone 6