I think you will need some type of heat store so that you don't cook your plants during the day.

Something that holds a lot of water (preferably in black)

Garden Warehouse: Water Storage and Watering Accessories - 1520ltr Water Storage Tank

Good old Dickie has an idea here

Solar Greenhouse Heat Sink - Solar

Clever that the solar panel runs during the day and the heat rises at night !

Here's another idea (personally i'd paint the copper collectors mat black. Maybe line some guttering with foil as a concentrator )

DIY Solar Water Heating - Heating