I'm guessing most immersion heaters have a thermostatic limiter to prevent over heating of the water. So what is the behaviour of both the Mk2 and PLL diversion rigs when the immersion thermostat cuts in and prevents any more heating?
I have a rig based on the PLL code and when the immersion heater's thermostat cut it off, the unit keeps trying to divert excess solar power. This of course messes up the data, as it shows diverted power, when none has really been diverted. I'm not sure what the Mk2 software does.
Has anyone solved this problem, maybe by detecting when the diverted power equals the maximum (since my solar panels don't generate enough to run a 3kW load constantly), or is there a better way?
I know the PLL code can use a temperature sensor to limit the diversion, and I'm using this method successful now on my own system. But I need to install it for a friend who can't run temperature sensors from his hot water cylinder to his emonTx (unless there was a way to do that wirelessly).
Re: PV diversion to immersion heater with thermostatic limit
Hi Allen, I use the pll sketch but have the 3rd input enabled reporting the diverted power used rather than using the in-built calculated "power available for divertiion" and that works really well, it's accurate and simpler as well as the DHW immersion heater can just do it's own thing.
Providing the immersion heater is on it's own spur from the dist board it is just a case of changing the sketch and pluging in a 3rd CT.
If you want to know the tank temp you can just add an emonTH or similar to just report the temp as no control will need the reading because the immersion heater stat does the control.
Paul
Re: PV diversion to immersion heater with thermostatic limit
The latest Mk2 code that's posted on my website monitors the diverted power using a second CT. The total for 'today' can be displayed on a 4-digit display, or transmitted by RF for datalogging purposes. Once the stat opens, there is no further increase in the displayed/transmitted value. The earlier Mk2 code that I've posted here does not include this facility.
If you need some new hardware for your latest project, I would be happy to oblige :)
Re: PV diversion to immersion heater with thermostatic limit
Paul, that sounds ideal. Looks like I could be buying another CT. One issue I found when my system was cutting out on the immersion thermostat (before I set up the temperature sensors to cut off the diverted power), was that when the immersion cut out it seemed to take a few minutes to recover and recognise that the temperature had fallen (and could therefore start diverting again). Now that the temperature sensors shut it off before reaching the thermostatic cut-off, the code starts diverting as soon as the temperature drops again.
Robin, if you are only using 2 CTs I assume that you are only monitoring the grid input/output and the diverted power? In which case I guess you are not monitoring the generated solar power ?
Re: PV diversion to immersion heater with thermostatic limit
Robin, if you are only using 2 CTs I assume that you are only monitoring the grid input/output and the diverted power? In which case I guess you are not monitoring the generated solar power ?
That's correct. My PCB has two current sensors, only one of which is required for the diversion function. The other sensor is normally used to measure the diverted energy/power, but could equally well be used to measure/display the generated power from the inverter instead.
Re: PV diversion to immersion heater with thermostatic limit
Hi Allen,
The different switching characteristics will to be due to mechanical thermostat verses electronic sensor and probably the place(s) the sensors are located. the temp in the centre of the water will be slower to cool and the immersion heaters mechanical stat needs to reach it's lower threshold to switch back on where as the solid state sensors instantly report an actual value and the thresholds are determined by the code.
I'm sure I could improve on the basic way I've done it by adding electronic sensing (something I plan to do down the line) but the reality is as the tank is heater using "surplus" PV, I've increased the temp setting on the immersion heater, so it's current lower threshold is probably higher than it's previous higher threshold.
Obviously I wouldn't suggest you increase the tank temp without you doing your own evaluation, I have no small children or elderly people using the taps. The reason i did it was to absorb more PV when it's available and to make a tank of hot water last longer, As when being used, the hotter the water the less volume need to mix with cold to reach the same temp and the residual temperature the following day is greater so a less sunny day is less likely to result in no hot water.
For your own system you could just add the ct so you can report the correct value for "diverted power used" and continue using the electronic temp switching, but on your friends system I'm sure the immersion heater stat only solution will be sufficient given the fact that you can't run temp sensor cables.
Paul