So progress has been slow over the last few months but we have been figuring out the best solutions.
Sensor-wise we have found these from AliExpress: http://www.aliexpress.com/item/Protection-type-temperature-and-humidity-sensor-temperature-and-humidity-sensor-built-in-sht10/1650578403.html
For multiple on the same board we will have to multiplex the I2C connection, so we have order two and will test this with the ESP nodes when they arrive. The backpack PCB will contain the power regulator (the distributed power is 24V to get around the garden with minimal loss), the multiplexer and the FET drivers for the water valves.
Speaking of the water valves, it has been difficult finding a triple valve with 24V coils. However, AliExpress also featured one of these! And for a much cheaper price than the plumbing spares suppliers: http://www.aliexpress.com/item/MJ-P3J01-G3-4-11mm-24V-Three-output-Normally-Closed-Inlet-solenoid-valve-1pcs-lots/32345639583.html
We have now been donated a 240W solar panel from the Advanced Technology Institute at the University of Surrey, who also do a lot of research into organic and transparent solar cells, so stay tuned for future work with them! This panel will be installed in the next few weeks onto our shed roof, and we will then connect it to an MPPT charger and leisure battery. The controller/charger we are looking at allows us to monitor the voltage and current on the cell and so this will be published to a webpage which we will link here as soon as it's up!
Another job over the summer will be to install the water butt, guttering and pump system into the end of the polytunnel. This will provide us with a natural source of water for the irrigation system, and top it up when needed from the mains supply. Our latest thoughts for a pump is to use a 24V yacht deck-cleaning pump. This is both small and does not require an inverter, which is expensive for motors due to the high inrush current on starting.