I have a need to monitor the temperature of my solar panels. I occasionally notice large drops in power output that may be due to passing clouds, but may also be (partially) caused by excess temperature. Monitoring the panel temperature would be one way of seeing the effect. If temperature is found to be a problem I could arrange to switch on a sprinkler to cool the panel. [There is a thriving discussion on panel cooling on whirlpool].
I wrote about the Current Cost digital development board in my post titled Utilising the Current Cost Dev Board – Part 1 Examining the CC Dev Board. I have been looking for a suitable project for some Current Cost Analogue development boards I acquired some time ago.
Incidently, my implementation of the digital [Ideally, better called a binary ] board Utilising the Current Cost Dev Board — Part 2 Central Heating Monitor is still running. Still on breadboard unfortunately, and still on the original 2 AA batteries. For now, it is reading zero all the time – no need for central heating for the present.
In the Current Cost support forum that I visit regularly, JetJackson asked”Would it be possible to use a dev board to make an external temp sensor?” He would like to monitor the temperature of his solar panels and route the temperature through his CC bridge to PVoutput.org. SaFeHeX gave a thoughtful reply which mentioned the LM335 temperature sensor. Costs about $4 at Jaycar. He thought it might be possible to reprogram the PIC processor on the Dev board to better scale the output or to calibrate a thermistor, though he did imply it would be quite difficult.
Now I am not going to consider re-programming the PIC. I did consider front-ending the Dev board with another mcu. [The TI MSP430 is available in an evaluation kit called the Launchpad which is a combined programmer and breakout board in one for $4.30. It has a built in temperature sensor and the demo program displays it on your PC. It should be simple to output the temperature on one of the analogue pins and even calibrate it].
I don’t want to spend a lot of time getting accurate temperature measurement out of the electronics so I will do calibration downstream of the EnviR. I already have to do something downstream as the output from the Dev board is 0 – 500. Loses some accuracy reading that as o-500C.
As it happens, I have been working on another project to load data from my CC bridge to PVoutput with full calibration and combination of datastreams. A by-product of that project is a set of re-useable libraries. I’ve been intending to post on that but you know how it is with documentation! I should also point out that PVOutput now has a facility called Auto Uploader that will load data from Pachube to PVoutput, with similar features to mine, if not quite as full function. It has the advantage that no PC is involved in the process.
This then is a block diagram of my project.
Actual activities are
- Add a temperature sensor to the analoge dev board. Start with LM335.
- get Calibration factors at 0C and 100C
- Write Calibration Rules
Issues to consider:
- Temperature range – Nominal operating temperature of a solar call is about 50C at ambient 25C (source). On a 40C day, operating temp may be 70-80C – so design range should be 0-100C. [50-100 would be OK since low temperatures are not significant].
- protection of electronics. Isolate from Solar cell 70-80C not recommended . Implies separation of sensor from electronics. Protection from weather – say IP45, and heat-shrink sensor .
- wireless range dev board to EnviR.