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Old 12-29-09, 02:48 PM   #1
Xringer
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Default Dump the plumbing and use PV to heat water?

This simple technology (DC heating of water using PV panels)
has been tested for a few years by the government.
BFRL: Solar Photovoltaic Hot Water System


And now that PV panels are available for under one dollar a watt,

http://ecorenovator.org/forum/solar-....html#post5341

it just might be cheaper to skip all the plumbing hardware and just wire some PV to a heating element in your HW tank.?.

I'm trying to get some of those cheap panels to try my hand at making some hot water.
I'm going to start with 4 panels, into a 75 ohm resistor, making about 240 watts of heat for my 76 gallon hot water storage.

240 watts per hour = 819 BTUh. Five hours of good sun, could mean 1.2 kWh or 4094 BTUh dumped into the tank daily.

I'm not sure how much that would help keep the oil burner from coming on,
but if the heat-loss is close to 819 BTUh, it could keep the tank coasting during the sunny part of the day at least.


I like the idea of cutting out the middle-man inverter (and/or battery) losses.

And the idea of just using a 75 ohm resistor and some PV panels is very attractive to the purist in me.



Years ago, I had a good working solar hot water system working. 3 big Novan collectors on the roof, made lots of hot water!
But, after about 10 years, the plumbing starting going. The city water supply eats copper. And loves tanks and heat exchangers.
The system worked well for about 15 years and was removed after a few years of collecting dust. A new tank and exchanger
was too costly and I knew we would be needing roof shingles again before too long.

If my guess is right, a PV driven hot water system, could last 30 years and still be working. Plus, it's going to be very low maintenance, if the parts count is kept low.
Less things to go wrong, the longer they last.

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Old 12-29-09, 03:12 PM   #2
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What about efficiency?
Just the PV's are 10%-15% efficient, plus resistor efficiency.
I'm guessing that solar hot water systems can be 30% efficient (BIG ?).
On one hand, I'm an efficiency buff, on the other, there are other things to consider...
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Old 12-29-09, 04:21 PM   #3
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The efficiency is in the long term cost, IF you can get the cheap panels.

An it appears the day of the cheap panel isn't here yet..

I just got off the phone with the dealer and it turns out that Solar Panels : Solar Panels Direct $1.74 per watt
is pulling a bait-n-switch with their shipping cost.

It was:

$58.80 USD 4 $235.20 USD
Shipping and handling $101.74 USD
Total $336.94 USD

And now, they want $240 for shipping. More than the price of the panels..

I think maybe I'll wait until they have $1-a-watt panels at Lowes..
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Old 12-30-09, 08:25 AM   #4
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Yikes, yeah... that'll get people excited over nothing. Too bad too.
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Old 12-30-09, 11:42 AM   #5
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This is why when I was talking to a solar hot water installer about heat pump water heaters he got worried, 550 watts to run the heat pump, they claim with the other elements it's around 5kwh per day... seems kind of high so with those specs you would need over a kw of pv, but compared to the cost of solar hot water panels, pumps, plumbing, fluid... and they still recommend having a back up for those, this the back up would be a switch.
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Old 12-30-09, 03:10 PM   #6
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A heat pump hot-water heater is the best way to go for my setup.
I can get hot water from my boiler during the winter months,
but don't like the waste of running the boiler in the summer..

A heat pump hot-water heater would cool and dehumidify my basement during the summer. And, from what I've read, they don't use a lot of kWh,
when compared to heating elements.

What would be idea, is a 24 volt DC heat pump hot-water heater! That would
work way better than PV into resistance.. But, it would cost much more
and be more prone to breakdown.

According to the government studies that I've skimmed, the break-even for resistance hot-water heaters is the $1-a-watt panel..
Without a low priced panel, it's just something that most folks would not go for.
Except those guys who buy a new Prius with the built in solar roof..



They have mandated PV hot water on new houses and large renovations, in many areas down in Australia.

"
Hot water systems
A typical 1kW solar photovoltaic hot water system avoids around 1200-1500kWh of grid
electricity per annum, contributing to around 20% of residential sector greenhouse gas
emissions (The Nous Group 2007).
The Federal Government is currently giving consideration to a proposal to impose a
nationwide ban on electric hot water systems (HWS) in new housing in all States from 2010,
and for replacement HWS from 2012. Some States have already introduced such a ban, with
South Australia placing a ban on electric HWS for both new homes and replacement units in
existing homes on 1 July 2008, although the bans are conditional. This ban does not however
apply to homes in remote areas, multi-unit dwellings or a unit replacing one which services
only a shed or pool.
A ban on electric HWS in Queensland (implemented 2006) currently applies only to new homes
and substantial alterations and additions. It will be extended to apply to replacement units
from 2010. The ban will initially be voluntary for those without access to reticulated gas.
The NSW Building Code places an effective ban on the use of electric HWS in new housing and
for alterations and additions requiring planning approval. It is difficult to reach the BASIX
planning requirements without installing a low emissions hot water system.
"
http://www.caf.gov.au/documents/Acce...PRSReport3.pdf


I guess they are doing more than just studying PVHW Down Under
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Old 12-30-09, 03:27 PM   #7
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Vacuum tube solar heat collectors can be as much as 80% efficient!

Solar Water Heaters: Solar Hot Water Heaters For Home, Commercial & Industrial Solar Water Heating

They are used all over China, in great numbers. The latest designs AFAICT are a glass tube with a copper heatpipe inside and a (near) vacuum in between them. The top of the heatpipe has contact with the water in the header.



If one of the tubes is broken, it can be unplugged and replaced.
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Old 12-30-09, 04:33 PM   #8
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If there is more efficient technology, I wonder why the Australian government is supporting PV HW ?

Maybe they think something with no moving parts will last longer.?.
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Old 03-20-10, 09:26 AM   #9
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Xringer, I found an article on PV HW in Popular Science, February 1995, p.41.
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Old 03-20-10, 10:30 AM   #10
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Yeah, that's Fanney & Dougherty, the guys who did most of the research for the USG.

Here's a list of their papers. BFRL: Solar Photovoltaic Hot Water System

Besides getting PVWH field testing started, their big idea was Power Point Tracking.
They even took out a patent on it.


My current PVWH project idea does not use Power Point Tracking,
but does use Sun Tracking, with an old TVRO dish mount.
http://ecorenovator.org/forum/projec...r-project.html


The plan is, to track the sun, so the panels harvest the peak power
available and then feed that power into a fixed load.
The fixed load would be pre-matched to the panels..

For now, I think keeping it simple is the best choice. However, in the
coming months, I will be looking at Fanney & Dougherty's ideas about
automatic load matching. If it can be done without undue complexity, I'll do it.

For a hot-water assist heater, I'm thinking of 2 thin-film 54v panels in series.
Kaneka Thin Film Solar Panel 110w
For about 108 volts at 220 watts (750 BTUh), which might not be enough to notice..

But, I plan to pre-test the 220 watt idea, by plugging in a 250 watt heater
(using AC power).
http://ecorenovator.org/forum/applia...ad-heater.html

If 250 watts doesn't help reduce oil consumption, then I will have to think
about using a larger fixed PV array..

I have been tracking daily oil use by logging the AC power use of the oil burner and hope to install the heater pad this week.
http://ecorenovator.org/forum/projec....html#post6105

Once the 250 Watt heater is installed, and spring is here (home heating off)
I can log oil usage data that only reflects domestic hot water usage.
Maybe one week on and one week off will tell me if 250 watts makes a difference.
If I see it's working ok, early on, I'll switch to 5 hours a day (11am to 4pm)
and see how that works.

This spring, I plan to install more insulation and a new (fully functional) water mixer on the boiler. The old mixer is pretty useless and wastes a lot of energy.

Honeywell-Sparco AM100C-US-1 - Honeywell-Sparco - 1/2" Sweat Union Mixing Valve
The new mixer goes down to 80 degrees F, which might be okay in the summer time..

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