DIY Solar HW systems using commercial vacuum tubes

Yesterday my students and I in Envisaj, the Mercy College Environmental Sustainability and Justice League (club),  built our second DIY Solar Hot Water system using half size (500 mm long)  vacuum tubes from 

This time we used a 4" inch fence post with 10 2" uniseals to seat the vacuum tubes.

Here is a picture of the progress we've made:

Vacuum tube solar hot water heater by T.H. Culhane and Envisaj

Is anybody else out there working with solar vacuum tubes to make their own heaters?  We'd love to share ideas with you (sucesses and failures!).