I thought about using the pump for distributing the heat at night, but then I read how solar collectors are usually built and I decided to put some black insulating foam mat from behind. This should increase the heat collected by the pipes during daytime.
And I don't want to remove the insulation every night, so the warm air can circulate throughout the greenhouse.
Generally water has the best heat capacity, so any "storing" of heat energy is done best with water. But as you experienced yourself, a larger water tank itself doesn't heat up that good.
The other system you're describing has a large surface which absorbs heat far better. And at night it radiates back. Here in Europe they're selling inexpensive flexible water pipes with a large diameter.
Beckmann-KG - BETA-SOLAR Wärmespeicher Sorry website only in german, but the pics will make it clear.
I've seen a very interessting system for greenhouses which has extra absorber panels, perfectly exposed to the sun, a 3,000 liter tank and seperate heating ciruit which runs along the roots of the plants. Everything is temperature controlled, like a heating system for the house.
http://www.mazdasolar.com/shop/syste...3Dposition1%26It's from a test system installed in Nicosia, Cyprus. The
company unfourtunately has no English website, but there are serveral tools for online translation of websites. Google has one itself.