[RE-wrenches] PV-direct electric water heating

[frenergy]

Luke and others,

        Sorry if I've missed something in this conversation, I'm trying to 
figure out how Legionnaires Disease is an issue with hot water heaters that 
run below 140 F.  conventional water heaters come (in CA) factory set below 
140 F., I assume (there I go again) for reasons of scalding safety and 
energy conservation.

        And personally, I am very interested in PV water heating.  Yes, I 
have your same gut reaction to using PV to heat, though I have judiciously 
done this (and suggested it to others) in some certain instances.  With 
water heating in particular I appreciate the advantages of PV to do the job 
for reasons like: Simplicity, no glycol and no glycol maintenance, no leaks, 
no heat exchanger, no pumps, no failing pumps, easier to deal with 
prevention of overheating (depends on the SDHW type system employed), no air 
locks/failed air vents.

        It seems like a shoe-in for connecting a PV supply to the lower 
element of a 240vac conventional tank and have grid supplied power to the 
top element to back-up the PV.  I suppose if you're grid connected, you 
would just add 4-6-8 more PVs to the array and connect the WH like you 
always would. I just did some quick math for 6 more 250 watt PVs, racking, 
next size up inverter and you're just below the cost of a SDHW thermal 
system.

        Yes, I understand this is only possible to talk about because of the 
current PV prices but I feel it's worth pursuing.  The conversation is of 
course somewhat different if we're talking off-grid with other heat sources 
E.G.: propane, woodstove and no using the grid for excess production or 
back-up.

Thanks for any more enlightenment on this.

Bill
Feather River Solar Electric



----- Original Message ----- 
From: "Luke Christy" <sgsrenewables at gmail.com>
To: <re-wrenches at lists.re-wrenches.org>
Sent: Thursday, May 16, 2013 12:22 PM
Subject: Re: [RE-wrenches] PV-direct electric water heating


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
CoSEIA Certified PV Installer

Solar Gain Services, LLC
PO Box 531
Monte Vista, CO. 81144
SGSRenewables at Gmail.com
719.588.3044
www.sgsrenewables.com