[Greenbuilding] Solar water.....or? photovoltaic system?

[Corwyn]

On Mar 13, 2007, at 18:20, John M Dresser wrote:

Dear John,

Some thoughts:

Solar hot water runs around 50-60% efficient, PV 10-15%.  So, hot water 
has a better chance right off the bat.

> I am in the process of planning an energy efficient home to be built 
> near Kalispell, Montana (near the Canadian border) .
> Typical winter weather is very cloudy (but temperate by my standards 
> :-) ,  although my first three winters here  have been unusually 
> sunny.

8000 HDD is temperate? :-)

> I have designed and built 13 homes in the Minneapolis suburban area ( 
> 1980-1998 ),  typically featuring super insulation and passive solar 
> orientation as much as possible. All were very successful in terms of 
> energy efficiency.
>
> The cloudy winter weather here seems to discourage most builders from 
> trying active solar or PV solar.  I would like to try one or the other 
> on this next project,  even though the additional investment likely 
> will not be economically justified at current energy prices.  
> Electricity rates are very competitive here, compared to other 
> regions,  and also compared  to natural gas and propane alternatives.
>
> I am inclined to try an active hot water system that would provide 
> domestic hot water during the sunny season,  plus would charge an 
> insulated mass under my on grade slab portion (60%of the house 
> footprint) after passing through the hot water heat exchanger tank.

Why not put tubing in the slab in the basement section as well?  The 
more thermal mass you have, the more you can take advantage of the 
sunny season to offset the cloudy season.

> I would pre-temper the cold water supply to the water heater/tank by 
> coiling tubing in this warmed foundation mass (sand fill?) .

Why not just run the preheat through a solar tank,  That way you get 
the quick heat available on the day, and the rest ends up stored in the 
thermal mass.  The tank could well be 150º, while the slab will likely 
never be much above 70º (at least you hope not).

> Since I would be providing most of the labor to install the system,  I 
> am guessing my out of pocket investment for this system would be 
> roughly equal to the PV alternative system I could consider.

I would hope it would be a lot less.

> Perhaps, with PV,  I would generate a little extra electricity in the 
> summer to sell back to the utility company,  to cover some of the 
> winter electrical usage?   My gut feeling is an active water system 
> would provide more of my energy needs on an annual basis and 90% of my 
> domestic hot water in the sunny season.  With super insulation and 
> passive solar heating gains,  domestic water heating needs probably 
> consume the nearly same or more energy than space heating annually.  
> Natural cooling at night and low humidity here,  eliminate the need 
> for A/C.

> Any of you engineering experts or those with solar experiences care to 
> make a crude estimate as to which approach might make more economic 
> sense?  I might have about 2000 squ ft on main level,  (about 1300 ft 
> slab @grade/mass storage area,  plus 700+ ft of utility basement 
> area),  with 700+ ft on second level.

Sounds big.  Smaller would be cheaper to provide energy for.

>  I can have steep pitched (8/12) south facing roof,  as well as less 
> steep (4/12) shed dormer roof area as an alternative for flush mounted 
> solar panels.  The shallow roof area might be worth considering, if we 
> assume most of the solar energy will be collected when the sun is 
> higher in the warmer seasons.

4/12 would be about right for you to maximize for summer, but you might 
want to consider optimizing for any winter sun you do get (around 425 
BTU/day/ft^2 for Great Falls).  12/12 would be evenly split for winter/ 
summer.  even steeper for full winter optimization.  8/12 is still 
leaning (NPI) toward summer.

>  I'm thinking about a system with 4 solar water panels combined with a 
> 200 gal tank?  perhaps a second small standard electric water heater?

4 panels at 32ft^2 each, gives 33,000 BTUs per day in December, and 
175,000 BTUs per day in July. (extremely roughly).


> My current temporary on site abode is an 800 squ foot 
> bunkhouse/efficiency apartment with south facing glazing that works 
> well to heat the place when the sun is out. Back-up heating is by 
> woodstove (about 1 chord per winter) and electric baseboard.  My 
> electric bill is fairly steady at $45-$50 per month,  much of that is 
> water heating, running the well pump in summer replaced by some 
> heating in winter.    Thanks to all who share an opinion!!

What are you paying for electricity?

Thank You Kindly,

Corwyn


-- 
Corwyn
Kermit didn't know the half of it...
http://www.greenfret.com/
corwyn at greenfret.com