[RE-wrenches] PV-direct electric water heating

[Luke Christy]

Hi Wrenches,
Thanks for your input. I agree that the PV water heating setup as I presented it has some obvious issues. However, as PV folks I think we all have a gut-level aversion to using PV energy for heating of any kind. As modules continue to get get cheaper I think some heating applications will emerge that make sense despite the collective instinct to gasp in horror. 


The Legionnaire's Disease point is an issue as the water in the tank will probably not be heated above 140 F very often with PV alone, (unless the homeowners are away for a few days). It seems like that could be dealt with by setting the tankless heater's delivery temp fairly high for disinfection purposes, say 140-145 F, then feeding the output to a thermostatic mixing valve to reduce it to a safer delivery temperature. There would be an efficiency penalty there, but it probably wouldn't be that significant. As Steven pointed out, tankless heaters do have a dead band where they won't add heat to preheated water, but in my experience with the right heater that band is only 10-15º F wide (below the delivery set point). 

On the other hand some of the PV-direct issues are hard to avoid, such as the array operating out of the MPPT range a lot of the time and the need for a 150VDC rated relay as David pointed out . If the idea turns into a project I may end up revisiting a diversion load arrangement with the existing battery-based system. Though I still think the PV-direct idea has potential.

Thanks again.


Luke Christy

NABCEP Certified PV Installation Professional™: Certification #031409-25 
NABCEP Certified Solar Heating Installer™: Certification #ST032611-03       
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