[RE-wrenches] PV-direct electric water heating

[David Katz]

Hi Luke
I am doing this with a 1200 watt array that is AC coupled to my off grid home inverter. I am using an SMA 1800 inverter because it is 120vac.  I use a relay connected to the AC  output that connects the 4500 watt water heating element to the AC breaker panel when battery voltage reaches the bulk setting.  I use the auxiliary output of a Midnite charge controller to control the relay.  Using a grid tie inverter is more efficient because the array is MPPTracked.  Without the inverter an array connected to a water heating element, the array will spend a lot of time off of the. Maximum power point, especially n low light, and early and late  in the day.  Also the AC coupled array adds to battery charging when needed, before heating water.  I see you point, that PV is so cheap, you could add an extra 1000 watts for the price of the inverter, but be careful what you used to disconnect 150+ volts DC.  He temp switch on the water heater will arc and fail.
David Katz

Sent from my iPhone

On May 13, 2013, at 7:59 PM, "Luke Christy" <sgsrenewables at gmail.com<mailto:sgsrenewables at gmail.com>> wrote:

Hi Wrenches,
Now that the cost of modules has come down so much, has anyone out there experimented with solar electric water heating? As in: direct connecting a short series string of PV modules to a tank -style electric water heater with an element of an appropriate voltage and wattage rating…?

A off-grid customer of mine who is also an electrical engineer has a situation that seems ideal for trying this idea out: he has a gas-fired tankless water heater and a water source that is very cold year-round. The idea is to take a 30 or 40 gal electric tank heater, switch out one of the the 240V elements to something like a 96VDC, 1000W element (difficult to find, but available), and direct connect 3 or 4 60-cell modules in series (with a disconnect and high-limit control of course). The tank would then serve to preheat the cold feed to the tankless heater. We think we can get a decent daily temperature rise with this setup. Probably not enough to heat the tank to a normal DHW temperature, but certainly enough to offset a good deal of propane consumption, and all for what I predict will be considerably less cost than a small solar thermal system.

Has anyone tried this? I'd appreciate any insights or opinions.


Thanks
-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
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SGSRenewables at Gmail.com<mailto:SGSRenewables at Gmail.com>
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