# Solar Thermal is Dead?



## semipro (Apr 24, 2012)

This is an interesting article where the author posits that PV has gotten so inexpensive that using solar to heat water directly no longer makes sense (in most cases). 

http://www.greenbuildingadvisor.com/blogs/dept/musings/solar-thermal-dead

I contend that this probably doesn't hold true for the do-it-yourselfers out there but it has got me rethinking all my plans for building and installing a solar thermal system.


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## velvetfoot (Apr 24, 2012)

Could this work by heating a storage tank, as a kind of battery?  Or, are they figuring the utility always being involved?


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## Slow1 (Apr 24, 2012)

I have to say that I came to that conclusion for myself at least in the market I live in.  With rebates currently biased toward the PV arrays it really seems to be better to build out the electric side - no wasted energy during peak production in the summer, fewer moving parts...


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## jimbom (Apr 24, 2012)

!! $10,000 for a solar electric installation to run a heat pump to heat domestic hot water.  Paging Mr. Goldberg.  Mr. Goldberg, Mr. Rube Goldberg.  A muse is horning in on your territory.


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## Slow1 (Apr 24, 2012)

jimbom said:


> !! $10,000 for a solar electric installation to run a heat pump to heat domestic hot water. Paging Mr. Goldberg. Mr. Goldberg, Mr. Rube Goldberg. A muse is horning in on your territory.


 
Reality check though - when I last received a quote for a solar DHW system for my home, I was quoted over 14K after all rebates.  Obviously I chose not to go forward with the project.  However you can see why 10K doesn't seem too shocking.  DIY clearly can be much lower, but if you are looking at retail cost of installed systems...  

I spend about $800/year heating my water.  If I am looking for a 5 year payback then that gives me about a $4K budget if it is a 100% solution - right now I'm not finding any DHW solutions that deliver that for me so I wait... as oil goes up, so does the budget though...


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## jimbom (Apr 24, 2012)

Slow1 said:


> ....right now I'm not finding any DHW solutions that deliver that for me so I wait... as oil goes up, so does the budget though...


  Very true.  I am taking the same approach.  We have natural gas and our DHW averages $20/month.  We will be waiting a long time unless I can scrounge materials and DYI this.


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## woodgeek (Apr 24, 2012)

Absolutely.  I concluded in my case that the (grid powered) HPWH would use about the same kWh as the solar thermal system would require for backup, so that the elec usage and carbon intensity of the two systems would be nearly a wash.  My HPWH install was <$4k, and my solar thermal quote for a comparable output was $15k installed, or $5k after state and federal rebates.


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## semipro (Apr 25, 2012)

My plans for tapping solar have always included thermal for water or space heating.  Now, with the availability of low interest loans, less expensive PV systems, tax rebates, and SREC sales I'm getting real serious about installing PV.  Some quick calculations indicate a 7-10 year payoff period that I find encouraging.  I'm thinking investment (and payoff) of a PV system and a plug-in electric vehicle would help set us up for retirement in 20 years. 

One potential pitfall is that my 20+ year old fiberglass shingle roof needs replacement and I'd always envisioned going with a metal replacement roof.  It seems like a waste to install the panels on the existing roof only to have to remove and reinstall them when I install the new metal roof. 
To further complicate things I'd also planned to go to a "cold" roof with the addition of XPS foam under my new metal roof with sealing of ridges and soffitts. 

It seems like I"m going to have to move the roofing mods higher on my priority list if I'm going to be able to take advantage of existing PV incentives. 

Anyone have experience with installing PV and then reinstalling after a roofing change?  It doesn't seem like it would be that much work to pull an existing array and then remount on metal but I could be underestimating what's involved.


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## GaryGary (Apr 25, 2012)

semipro said:


> My plans for tapping solar have always included thermal for water or space heating. Now, with the availability of low interest loans, less expensive PV systems, tax rebates, and SREC sales I'm getting real serious about installing PV. Some quick calculations indicate a 7-10 year payoff period that I find encouraging. I'm thinking investment (and payoff) of a PV system and a plug-in electric vehicle would help set us up for retirement in 20 years.
> 
> One potential pitfall is that my 20+ year old fiberglass shingle roof needs replacement and I'd always envisioned going with a metal replacement roof. It seems like a waste to install the panels on the existing roof only to have to remove and reinstall them when I install the new metal roof.
> To further complicate things I'd also planned to go to a "cold" roof with the addition of XPS foam under my new metal roof with sealing of ridges and soffitts.
> ...


 
Hi,
One alternative would be to work on other energy saving and efficiency projects until the roofs time has come, and then do the PV (or solar thermal).  

PV modules have dropped in price over the last couple years, but PV is still expensive and still has a poor payback period compared to a lot of other projects (at least for most people).   This is how it came out for us: http://www.builditsolar.com/Projects/PV/EnphasePV/Economics.htm  -- in the section titled "How Does PV Compare to Other Solar or Conservation Projects?"

We kept track of all of our energy and carbon reduction projects and compared them to the PV project, and the section shows how they compare on saving and payback -- a short excerpt:

"These numbers are kind of hard to believe when you first look at them.  How can a handful of simple projects (the "Best 8") that cost less than $400 to do save 3 times as much energy as a state of the art, $10,000 PV system?  Well, it may be hard to believe, but I can tell its true.  Its also very likely that if you go find your "Best 8" projects and do them, your results will be similar. 
I'm not trying to be hard on PV -- I think its a great technology, and I love my new system, but be sure you do the other stuff too.  If you do the other stuff first, you can pay for your PV system with the dollar savings from the other projects."

The difference would not be quite so dramatic today, as PV system prices are down some but the other projects still have a far better return than PV -- adjusted to around todays prices, the other projects still pay about 10.5 times better than PV.

If you do the other stuff first, you can pay for the PV with the savings -- the opposite statement is much less true.

Not trying to be preachy, and you may have already done all the high payback projects, but just thought I'd mention the possibility for people who are just starting down the energy/carbon reduction road.

Gary


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## Slow1 (Apr 25, 2012)

I would discuss with installers.  Different roof surface may change the mounting system components - depending on how long you wait between initial mount and re-mount that could be a big deal (i.e. parts availability etc).  The recommendation I received here really biased toward getting the roof done prior to the initial solar PV mount.  

In my case after looking at the cost to remove and replace the panels/mounting system which worked out to be a full day of labor for a crew on each side was well over 50% of the cost of roofing that face of roof - not even taking into consideration the possibility of any damage to mounting system that might happen and require additional parts (and delays etc).  So I opted to just go forward and have that side of the roof re-done with a roof which will outlast the panels (and me for that matter).


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## woodgeek (Apr 25, 2012)

Our local outfit does both roofing AND solar PV, so the PV mounts don't ruin the shingle warranty.  If you wait a little longer, you can go for the Dow solar shingle....


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## semipro (Apr 26, 2012)

GaryGary said:


> Hi,
> One alternative would be to work on other energy saving and efficiency projects until the roofs time has come, and then do the PV (or solar thermal).
> 
> PV modules have dropped in price over the last couple years, but PV is still expensive and still has a poor payback period compared to a lot of other projects (at least for most people). This is how it came out for us: http://www.builditsolar.com/Projects/PV/EnphasePV/Economics.htm -- in the section titled "How Does PV Compare to Other Solar or Conservation Projects?"
> ...


 
No "preachiness" taken. 
I couldn't agree more with what you say.  Its just that we've done sealing, insulation, energy-efficient lighting and appliances and, I think, reached the point of diminishing returns there.  Any further improvements in sealing and insulation will require major work like pulling all our cedar lap siding and installing additional foam sheathing.   

It appears that most of our energy costs are for water and space heating.  Teenage sons creates some challenges there.  We already use geothermal and small electric space heaters for cold-weather heating.  We have a lot of windows and even with energy saving window treatments, losses there are substantial. 

Based on how may projects I have going on around the house and the amount of time I have left to invest in DIY solar, PV may provide a way for us to go solar within a reasonable time frame.  Also, loans are relatively cheap and there are the tax and SREC incentives.  Ultimately too I think we'd strive to install some electrical storage to allow independence from the grid if possible. We have a generator but would like to avoid the associated noise and fumes.

I plan to read the article your reference on your site this weekend.  I've spent so much time on your site already I'm surprised I haven't already seen it.  Thanks again for the work you do on your site. and for your input.


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## semipro (Apr 26, 2012)

Slow1 said:


> I would discuss with installers. Different roof surface may change the mounting system components - depending on how long you wait between initial mount and re-mount that could be a big deal (i.e. parts availability etc). The recommendation I received here really biased toward getting the roof done prior to the initial solar PV mount.
> 
> In my case after looking at the cost to remove and replace the panels/mounting system which worked out to be a full day of labor for a crew on each side was well over 50% of the cost of roofing that face of roof - not even taking into consideration the possibility of any damage to mounting system that might happen and require additional parts (and delays etc). So I opted to just go forward and have that side of the roof re-done with a roof which will outlast the panels (and me for that matter).


 
I'd probably do the installation (roof and PV) myself with family and friends. I hear what you're saying and appreciate the info.  I'm now making plans to getting the roof done before PV.  Thanks.


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## semipro (Apr 26, 2012)

woodgeek said:


> Our local outfit does both roofing AND solar PV, so the PV mounts don't ruin the shingle warranty. If you wait a little longer, you can go for the Dow solar shingle....


 
And there's this option too.  http://www.amazon.com/Unisolar-Flexible-Solar-Panel-Laminate/dp/B006EP6MCU
Its a thin film laminate PV that installs in the valleys of a standing seam metal roof.  I think efficiency is lower but installation sure looks to be a breeze and they look cleaner.


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## velvetfoot (Apr 26, 2012)

I didn't realize there was such a thing as peel and stick panels.
I've noticed that Fabral also has a system for their roofs, standing seam and exposed fastener:
http://www.fabral.com/products/solar-ssr-harvest-energy-from-the-sun


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## woodgeek (Apr 27, 2012)

Going through the solar boards, it is clear that the unisolar products are considered a neat idea, but there are documented instances of them not meeting their durability guarantee.  The manufacturer is also in Chapter 11.

In fairness, the Dow shingle product is a similar thin film tech whose long-term durability has not yet been proven in the field.  Hopefully Dow's deep pockets have helped with that.


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## begreen (Apr 29, 2012)

Can you provide some links to the Unisolar issue? I was considering putting this on a camper van roof.


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## woodgeek (Apr 29, 2012)

Here you go...

http://www.wind-sun.com/ForumVB/sho...er-11-(parent-of-Unisolar)&highlight=unisolar

The critic is a bit troll-ish: http://www.wind-sun.com/ForumVB/showthread.php?8376-Uni-Solar-Laminate-Rolls&highlight=unisolar

For a camper, they could be a nice soln at the right price.


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## begreen (Apr 29, 2012)

Thanks, that was an informative read.


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## btuser (Apr 30, 2012)

Does it make sense to use PV to preheat DHW?  I don't have room on the ground for solar and don't want to do a drainback/pump system on the roof. I'd also love to head into the direction of PV for the house long term.  Can you just go direct with the juice to a electric HW tanks without a transformer + batteries contributing to loss?


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## GaryGary (Apr 30, 2012)

btuser said:


> Does it make sense to use PV to preheat DHW? I don't have room on the ground for solar and don't want to do a drainback/pump system on the roof. I'd also love to head into the direction of PV for the house long term. Can you just go direct with the juice to a electric HW tanks without a transformer + batteries contributing to loss?


 
I think you probably could.

For a Solar Electric Panel water heating system:
Here is an example
DMSolar.com sells PV panels cheap -- just as an example, pick the DMSolar 250 Watt panel: http://www.dmsolar.com/somo25.html

250 watts at STC conditions (this is full sun (1000 watt/sm) and with a panel surface temp of 20C (very optimisitic))
Cost $320, or $1.28 per watt, but more $1.50 per watt  with shipping.
Claimed efic 17.8%
Size: 65.6 inches by 39.1 inches or 17.4 sqft.
Voltage at max power with full sun is 30.6 volts.

If you hook 6 of these together (see below for why 6), then you have an array that:
- Output is  6*250 = 1500 watts under ideal conditions
- Array size is 235 inches wide by 39 inches tall, which is 104 sqft
- total collector cost $1920  (plus shipping  plus mounting rails for the 6 panels)
- If you series connect all 6, you get 184 volts at 8.2 amps  under full sun, which is the expected 1500 watts

I guess you could connect this to a resistance heating element directly in the tank -- at full (1000 watt/sm) sun, the panel produces max power at 184 volts and 8.2 amps -- this matches a resistor of  (184/8.2) = 22 oms.  
This would certainly be simple, and maybe with some creative panel selection you could match an off the shelf heating element.

At this one sun condition, you can pick a resistance that allows the panel to be at its maximum power point, but as the sun drops down in intensity, the panel current drops down.  At lower sun levels if the resistance does not change, the panel will no longer be at its max power point, but it will find some operational point -- not sure how much efficiency this costs you.  Maybe someone knows how to figure this?
The I-V curve for the panel is provided here: 
http://lib.store.yahoo.net/lib/yhst-21796510175022/DM250S2.pdf

Compare this to the Solar Thermal Collector:
Pick a 4 by 8 ft Heliodyne Gobi collector.
Under the same sort of sun conditions, but using a 50F ambient temperature, this calculator:
http://www.builditsolar.com/References/Calculators/Collector/ColEfic.htm  gives:
- efficiency 55.9%
- Output = 177 BTU/sf, or 5664 BTU for the 32 sf collector, which is 1660 watts
- Array size is 48 inches by 96 inches, or 32 sqft
- total collector cost about $800 (plus shipping and some mounting hardware)


So, for about the same power output
- The PV system takes up (104/32) = 3 times as much roof space
- The PV system collectors costs somewhat more than twice as much as the thermal collector

If you are into simple DIY projects, the same 4 by 8 panel can be built for about $5 /sf, or $160 plus zero shipping.

I picked these sizes because both of these configurations would be just large enough in a descent sun area for a family of 2 that was careful about hot water usage -- kind of a minimum size rig for a family of 2.


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For about another $200 per PV panel, you could add Enphase micro-inverters to each panel, and with just a little bit more wiring the the whole thing becomes a grid-tied PV system.  This has the advantage that the title of this thread gets into, but you are now up over $3000.  In spite of the arguments Martin makes, I doubt that it comes up a net positive, but I've not worked through all the numbers.

Gary


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## semipro (May 1, 2012)

I've looked into pre-heating water electrically using PV or wind and found that there are direct replacement water heater elements that run on 12 volts DC as opposed to 120 VAC. 
I don't think the heating element cares whether its powered by AC or DC. 
I wonder if its better to run PV panels in series using a higher voltage element versus running the panels in parallel and using the low voltage element? 
What's great about this concept is that it requires no inverters or batteries.  Cheap hot water storage can be used instead.


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## GaryGary (May 1, 2012)

semipro said:


> I've looked into pre-heating water electrically using PV or wind and found that there are direct replacement water heater elements that run on 12 volts DC as opposed to 120 VAC.
> I don't think the heating element cares whether its powered by AC or DC.
> I wonder if its better to run PV panels in series using a higher voltage element versus running the panels in parallel and using the low voltage element?
> What's great about this concept is that it requires no inverters or batteries. Cheap hot water storage can be used instead.


 

The series arrangement would allow a smaller wire size from the PV array to the tank.  But, as you say, finding an off the shelf heater whose voltage you can match up to is a big plus.

Gary


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## semipro (May 1, 2012)

I was curious so I did a few calculations which basically verify Gary's. 
I can buy 4, 230 watt, 30 volt (full load) panels for about $1k plus shipping and then connect them in series to a 120 volt heating element installed in an old water heater tank located next to my GeoSpring HP water heater.  
The tank is already insulated and corrosion protected.  I already have the tank and a heating element costs about $20 and screws right in.
My 4 panels would produce about 900 watts with an equivalent of 4.5 hours full output per day (in our area).  That's 4 kWh. 
This amount of power would raise the temp of 40 gal. (330lb) of water in my tank by about 40 degrees F so I'd end up with preheated water at about 90 degrees.
Based on the prices Gary provided for thermal heating It seems that solar thermal would be less expensive for preheating water. 
However, running piping at our house to the roof would be tough and a ground level installation won't fly.  I could mount 4 PVs very easily and run the wiring without much problem.  I also would not need to worry about the complexities of piping, valves, freezing, weight support, etc.

I've been a big proponent of using solar in the native form of heat but PV is really starting to look better.


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## webbie (May 1, 2012)

I think the lack of maintenance will eventually swing the calc toward PV - I installed a lot of solar thermal systems back when, and there are often hidden costs over the years.

Besides, right now even free PV could hardly compete with nat gas...damn stuff is almost free. But that will end.


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## GaryGary (May 1, 2012)

webbie said:


> I think the lack of maintenance will eventually swing the calc toward PV - I installed a lot of solar thermal systems back when, and there are often hidden costs over the years.
> 
> Besides, right now even free PV could hardly compete with nat gas...damn stuff is almost free. But that will end.


 
Hi,
This was what Martin put forward in his blog that stared this thread -- that maintenance on a combined PV + heat pump water heater would be less than a solar thermal system. 

I find that a bit mind boggling.
On a PV system you are likely to have to replace the several thousand dollar inverter at least once and perhaps twice in a 30 year life.  And, if you have to add the heat pump water heater to get the efficiency up to a reasonable level, you are likely to have to replace the heat pump water heater ($3000?) at least once and probably twice during the 30 year life.  You might well be talking $12,000 of maintenance over the life of the system -- that does not seem small to me?

I have two drain back water heating systems (space and domestic water) -- the oldest one being about 9 seasons old.  The only maintenance has been an early failure of the Goldline controller which cost me all of $100 to replace.  I think drain back solar thermal systems are about as maintenance free as it gets.   

Maybe I'm missing something?

Agree that if you have access to NG its getting hard to beat with anything but DIY solar thermal -- except from a carbon emissions point of view.

Gary


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## webbie (May 1, 2012)

I had to replace my $1000 stone lined solar tank in less than 10 years......

I guess the real answer, as usual, is "it depends". 

A simple solar thermal tank heater, like they use in much of the world, can't be beat. But the more you have to add in plumbing, controls, asme storage, anti-freeze and other stuff - the worse it can looks.

I'm thinking that in the future they will be able to avoid the inverter??? Couldn't they just send current direct from the panels to a heating element of sorts?

Even if not, what will inverters cost in 10 or 20 years? 

I'd say this is all a bit premature - that is, PV seems cheap due to the glut and other things. But I can imagine that someday....the idea of a single wire from remote mounted panels...instead of plumbing...will be tempting. I doubt I will see the day though!


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## woodgeek (May 2, 2012)

I share Gary's horror (despite being a nay-sayer on professionally installed solar thermal in 2012) at using PV to drive a resistive element.

In Semipro's case, he already has a Geospring, which we can assume gives a COP ~ 2 (if he is a low DHW volume user).   Thus in round numbers every solar kWh you put in only reduces your grid usage by 1/2 kWh!  Moreover, when that unit is heating a cold tank, the COP starts high (>3) when the water is cold due to lower 'lift', and then drops to <2 at the final, high lift part of the cycle.  If you are only preheating to a 'tepid' level, then you are only saving the COP=3 part of the cycle, and saving 1/3 kWh from the grid per PV kWh!  Yuck.

IOW, the Geospring (that is already installed) is so efficient that it nearly negates any savings you might get from solar preheating.

In PA, HPWH and professionally installed solar thermal can both cost similar amounts installed after rebates, (In my case quotes at ~$3k and $4.5k, respectively), and either will def use less than half of the kWh of a conventional electric, if installed properly.  Both systems are 'complex', with low unit sales hardware which leads to maintenance problem risk.  Simple payback on either system at $0.15/kWh is 5-7 yrs relative to a conventional elec tank, depending on DHW usage.  Which you pick depends on your solar site/resource and if you have a location for the HPWH.

Installing BOTH systems (each capable of saving you >50%) does not get you to zero grid kWh by a longshot, both are awesome in the summer (lots of sun and warm air) and suck in the winter (both can go to elec backup). You could still need 25% backup, and the payback could be >8-10 years.

If you want to 'go solar' and have the site for it, do conventional grid-tie PV, cash your rebate check and let the Geospring do its job.


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## jimbom (May 2, 2012)

woodgeek said:


> ...Thus in round numbers every solar kWh you put in only reduces your grid usage by 1/2 kWh!...


I haven't completely achieved a functioning state yet this morning.  My initial thought is a kWh from solar is equivalent to a kWh from the grid.  Regardless of the application that it powers.


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## btuser (May 2, 2012)

If it's ground-level passive without the need for freeze protection I don't think you can beat solar thermal.  I'm planning on working it into a greenhouse project (should get to that project by oh, say 2021) to extend the season. 

Is there any danger to harming PV panels when you're not using the current ?  Do they get "backed up" if you're not dumping into the grid or somewhere?


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## semipro (May 2, 2012)

woodgeek said:


> If you want to 'go solar' and have the site for it, do conventional grid-tie PV, cash your rebate check and let the Geospring do its job.


 
Great insight.  Point taken.  I hadn't thought of it that way. 
However, our situation is more complex than I made it sound. 
The GeoSpring HPWH in the basement is one of two water heaters. Each serves different zones.  The other WH, an electrical resistance unit is located where its hard to service and heat loss to the outside is a problem. Also, if it were to leak it would be really bad. 
I want to get rid of the 2nd WH and combine the zones for a variety of reasons. 
So basically I need to increase the capacity of my system to compensate for the loss of the 2nd WH.  I was thinking that 40 gal (or more) of water preheated to 90 degrees prior to the GeoSpring HPWH would take care of that. 
Also, my HPWH is basically "stealing" heat generated by my ground-source HP HVAC in the cold season so my HPWH COP is really lower then.
Also, we have hard water and don't want to soften it.  The only issue we've had with this is that electrical resistance WHs soften the water by removing the minerals in a way that is harmful to the WH and requires frequent maintenance.  This is one reason why we invested in the GeoSpring HPWH and why we want to get rid of the 2nd electrical resistance WH.  
And, the presence of a preheat tank or tanks in my basement allows me to recover energy from other sources if I want.  I'm thinking for example of waste water heat capture. 
I also really want a small wind generator even if its not economically practical.  I'd considered taking the power it generates and preheating water with it, again avoiding the complexities of battery storage, or inverters. 

BTW the system I made calculations on in the last post has a payoff period of 6 years assuming no financial incentives like tax rebates. It would also seem to have "zero" maintenance over 25-30 years unless the preheat tank or heating element go bad.


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## woodgeek (May 2, 2012)

Ok. So the systems to cost/compare are Solar Thermal, Solar PV straight to a conventional tank, or adding a second ASHP (or upsizing to something bigger like the AOSmith 80 gal ASHP and selling the GE). Cost will need to factor DIY-fraction, rebates, and install details like solar resource (trees) and ASHP siting (scavenging from house or not). I would still think that your best options (on a cost of ownership over >8 years basis) would be DIY or DIY-kit Solar Thermal OR ASHP, perhaps in a site different from your current one.

Oh yeah, my recent reading has suggested that the scavenging issue is not such a big deal as we might think intuitively.  IF the BTU is coming from a properly installed geo, I would say the effect may be almost negligible.


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