# If solar can work in Maine....

Posted By macmaine, Oct 23, 2011 at 11:51 AM

1. #1

### macmaine New Member 2. ```NULL ```

Sep 8, 2011
41
6
Loc:
Southern Maine
2. #2

### benjamin Minister of Fire 2. ```NULL ```

Nov 7, 2009
693
5
Loc:
SW WI
If solar can work in Maine...

It seems to me that he's taking advantage of some rate structures that may not be around for the payback period he's predicted.

Just look at Spain for what this sort of nonsense leads to.

3. #3

### DaveH9 Member 2. ```NULL ```

Jun 11, 2009
69
0
Loc:
NW CT
no one should even think about using PV for running electric resistance space heaters. But this guy is using a heat pump. Doesn't say if it's ground or air source. Interesting that he is using water storage and circulating it, I have not seen that very much. Most heat pumps use warm air for circulating heat.

4. #4

### GaryGary Feeling the Heat 2. ```NULL ```

Jul 12, 2010
291
18
Loc:
SW Montana
Hi,

If you rate thermal and PV collectors on the same basis, it comes out something like this:

PV modules are now about \$2 per peak watt.
You get this number by dividing the price of the PV module by the power the panel produces under STC conditions. STC conditions are full sun (1000 watts/sm) with the panel at 20C (which is on the optimistic side).
So, basically this is the power the PV panel might produce at solar noon on a cold day (to keep the panel temp down).

If you were to rate a solar thermal panel in the same way:
A 4 by 10 ft panel is 40/10.76 = 3.72 sq meters, and runs (typically) at about 58% efficiency, and costs about \$900.
So, in STC 1000 watts/sm sun, it produces (3.72 sm)(0.58 efic)(1000 watts/sm) = 2157 watts -- this is about 4 times more than a PV module of the same size because the PV module is only about 12% efficient.
Just to ground this in reality, the above number is what a Heliodyne Gobi 4 by 10 collector produces at solar noon on a full sun (1000 watt/sm) day with 50F ambient air and 110F water going through the collector.

The cost per peak watt is then: (\$900)/(2157 watts) = 0.42\$/peak watt.

So, bottom line:
PV module: \$2 per peak watt
Solar thermal: \$0.42 per peak watt

So, for producing heat, the commercial solar thermal panel is still about 5 times more cost efficient than the PV module.
You can bring this closer by running the PV output through a heat pump, but then you have to also add in the cost of the heat pump.
Unless you have a very good heat pump (read expensive), the thermal panel will still have better efficiency than the PV + heat pump.

If you make your own solar collector (air or water heating), the comparison gets a lot more lopsided because a quality home built panel is about 5 times less expensive than a commercial solar thermal panel (sometimes more than 5 times).

This is not the easiest comparison to make accurately, because of factors like the STC rating being optimistic, and the PV panel getting less efficient in hot weather while the thermal panel does the opposite (and viceversa), but the difference is so large that its worth making the comparison any way. This comparison also does not include the other parts of either system.

This is a nice (I think) all around house that uses the PV for everything but hot water (which is solar thermal):
http://www.builditsolar.com/Projects/SolarHomes/MAZeroEnergy/MAZeroEnergy.htm#Plans
Quiz -- what is the bottom line reason this example home is able to satisfy all of its energy needs with solar on somewhat less than half the roof?

Gary

5. #5

### Frozen Canuck Minister of Fire 2. ```NULL ```

Dec 14, 2009
858
100
Loc:
Bottom line: They focused on doing everything possible in their situation to lower the heat load (energy required) of the structure, plenty of planning before building.

Total opposite of the McMansion.

Now if we would restructure our tax systems to reward this type of build & punish the wasteful ones. Well then we would have progress.

Really when you think about it people like this who plan/prepare/execute a good energy eff build wind up subsidisizing all those who dont.

Opposite should be true, it should cost a lot to build in a wasteful manner.

6. #6

### GaryGary Feeling the Heat 2. ```NULL ```

Jul 12, 2010
291
18
Loc:
SW Montana
That's exactly my feeling -- its the good insulation, small size, efficient windows, efficient appliances and lighting that make it possible to be 100% solar with a modest size solar array.

Gary

7. #7

### DBoon Minister of Fire 2. ```NULL ```

Jan 14, 2009
1,023
149
Loc:
Central NY
My wife and I have really worked to get our electric consumption down to 400 kWh/mo - that includes electric resistance hot water heat and electric range and stove. We don't feel that we are doing without - my wife cooks and bakes a lot - we just have efficient appliances and no phantom loads.

With the installation of a hot water heat pump, we are down to ~300 kWh/mo now.

If we went 100% solar (electric and thermal), the cost of the thermal system to save 100 kWh/mo of usage for hot water (what we know use) would be ~\$5000 to \$8000 (depending on who quotes). So, solar thermal is not cheap per unit unless you need a lot of it. And if you have cut your usage a lot, then the marginal cost of a more solar panels with a heat pump water heater (which has the added benefit of dehumidifying the basement) is pretty much the same as adding solar thermal.

If I needed a lot of hot water, the economics would surely be different. I'm not sure that the math would work out for all solar electric for the installation cited in this article without the combination of incentives, credits, etc. that he no doubt receives/received.

8. #8

### woodgeek Minister of Fire 2. ```NULL ```

Jan 27, 2008
3,681
640
Loc:
SE PA
Yup. This is exactly what I found when I had solar and HPHW quoted out. The solar would use almost as much kWh in resistance backup as the HP system would in total, and the small difference made it hard to pay off the difference in up front cost, even with a 70% solar rebate in PA.

Of course, YMMV. You need a 'space' for both technologies. My solar resource is marginal, and I have a huge semi-conditioned space for the HPWH.

9. #9

### GaryGary Feeling the Heat 2. ```NULL ```

Jul 12, 2010
291
18
Loc:
SW Montana
Hi,
It is for sure true that solar water heating systems are depressingly expensive, but the same system system can be built as a nice DIY project for about \$1000. My home made system was less than \$1000 (\$500 after rebate), and has a solar fraction over 90% -- this high fraction is in part because our hot water needs are modest, but if you build the system, you can size for a very good solar fraction at very little extra cost.
Clearly not for everyone, but a good project if you like to build things and have the time.

This is my system:
http://www.builditsolar.com/Experimental/PEXColDHW/Overview.htm
This is the newer version that does both water and space heating -- about \$2000:
http://www.builditsolar.com/Projects/SpaceHeating/DHWplusSpace/Main.htm

There are also some outfits who offer a kit of commercial parts (collector, tank, controller,...), so basically all you do is mount the components and install the connection plumbing. These can be in the mid \$3Ks, and qualify for the 30% federal tax credit.

Gary

10. #10

### DBoon Minister of Fire 2. ```NULL ```

Jan 14, 2009
1,023
149
Loc:
Central NY
Hi GaryGary,

I agree that home-built solar thermal changes the equation a lot. I'm not sure my wife would trust a home-built unit on our roof, but I've thought about a breadbox solar thermal adjunct that could act as a pre-heater to feed a tank that fed my HPWH tank. So there are ways to get a good bang for the buck by home-building a solar thermal add-on, whether it is a panel, breadbox, etc.

11. #11

### GaryGary Feeling the Heat 2. ```NULL ```

Jul 12, 2010
291
18
Loc:
SW Montana
Hi,
One interesting way to work the system a bit on this is to buy a commercial collector (making your wife happy , but build the rest of the system. The advantage is that while the commercial collector adds several hundred dollars to the system cost, it also means that the whole system cost qualifies for the 30% federal rebate (and maybe other state rebates).

For this to work, the collector has to be OG-100 certified by the SRCC, but almost all of them are.

Not sure this makes much sense if you already have the heat pump water heater, but mention just because others might want to take advantage of it.

Agree that the breadbox systems are also a good way to go. In MT the state rebate would end up paying for 100% of what most batch heaters cost to build -- FREE SOLAR . They are great for warm climates, and good for 3 seasons even in cold climates.

Gary

12. #12

### DBoon Minister of Fire 2. ```NULL ```

Jan 14, 2009
1,023
149
Loc:
Central NY
I'm always bouncing ideas around in my head on this topic, but the reality is that I don't have a good location right now for solar thermal or I would take advantage of it. If we ever move to a new house with better solar orientation, I'd probably stick with a HPWH but use a solar thermal adjunct, either batch or as you described. I think that is the best bang for the buck if you need to dehumidify the basement as well.