[Greenbuilding] Solar Water Heater Tilt

Mon Feb 9 13:45:11 CST 2009

Corwyn, what do you recommend for measurements to take or calculations/tools to use for PV tilt calculation?
(and if not you, someone else, what is the "rule of thumb" for PV tilt? (i think i've heard it's "your latitude" for optimizing year-round production))

I appreciate Gary's tool recommendation, and wonder if there is an equivalent tool for PV.

(just saw this from Alan A: maybe it answers somethings below
>here's a product that allows you evaluate how changes in orientation--
>including tilt--affect PV system output:

>http://rredc.nrel.gov/solar/codes_algs/PVWATTS/  )

Also thought folks might have thoughts to improve on the logic and implementation of what we have done (not sure we'll do much MAJOR to change ours, but certainly use comments for future design guidelines).

On our PV and HW system we aimed for/designed for certain goals but maybe they weren't quite right, in the goals or the implementation. We are at 39 degrees latitude and front of house is 21 degrees West of South for orientation.

Photo here: http://www.flickr.com/photos/satjiwan/2113792685/in/set-72157594505511020/

HW - We have a partially shaded (early morning) lower roof for HW so went with a "more efficient" evacuated tube system (not nearly as cost effective as Gary's DIY version).  This is tilted at about 50-60 degrees (not sure may be more). it was optimized for winter gain, since we wanted an oblique angle in summer to avoid overheating of the "more efficient" tubes.  system seems to be working well. providing close to 100% in summer and 50-75% or more in winter (though i can't (yet) meter the backup electric water heater to be sure). pump powered by small solar panel mounted on upper roof.

two usage notes:  1. for a couple hours the system does a negative heat gain if/when the solar panel have sun on it and the pump is running, but sun hasn't hit on the tubes yet.  we're working to correct this by putting a "snap switch" (?) on the pumps so they won't run until a min temp is reach or sun is on the tubes.
2. if snow collects on tubes and panel, it often comes off the tubes first (they are ready), but the pump panel takes longer to melt/slip away with lower slope so we lose some potential heat gain time. See this photo: http://www.flickr.com/photos/satjiwan/2114569720/in/set-72157594505511020/

PV - We have a 3kw grid tied, net metered system. it's providing about 100% of electricity in summer and maybe 50% in winter (need to update my log sheets to check).  Going with a "rule of thumb" that 40 degrees would optimize winter and 20 degrees optimizing summer production, when building the new second story roof we set the angle to surface mount the PV panels to lean toward summer production, 25 degrees. (is that about right?)  The intention is that we would do the most to offset the summer A/C peak demand in Wash DC area, offsetting older and dirtier, peak coal plants.  (we may also have side benefits of getting seasonal and time-of-use rates credited to our account during higher cost periods).  one observation is that even on the 3kw system, my inverters shows a consistent 2500-2600w peak instant production in middle of the day.  It's rare to see 2900 or 3000w though i have seen it briefly.  

Usage Notes:  
-It's since come to my attention that PV panels get inefficient with excess heat, so our panels may be getting hottest when we're trying to ask the most from them.  And that cooler temps (and higher efficiency) in the rest of year may be worth enough to have picked an angle closer to 40 degrees. 
- From a personal standpoint we use (a lot??) more electricity in the winter, so maybe we should have optimized for winter production or aimed a little more to the middle.

Questions: 
- For PV, does anyone know the value of efficiency loss due to high temperature vs loss due to orienation or slope? (i've heard of a solar HW system mounted behind solar slate shingles, which takes excess heat away from panels making the panels more efficient. 
- How would you design differently for similar goals?  
- I can understand how messing with mounting racks to optimize slope or orientation can be overkill, but are the claims that a (ground mounted) single axis tracking system can generate 25% more production true? and won't other optimizations be pretty worth it?
- Does that HW system behind the PV make sense (seems it would really diminish water heating efficiency in winter when most needed)?
- Anyone think that having a water trickle/mist system would be worth it to cool panels to optimum efficiciency during exceptionally hot times?

-Sat Jiwan
ps. let me know if you'd like to see my rough spreadsheet with graphs tracking 10 months PV usage/production...  After one year of daily data collection, I'll make it prettier, make some payback calculations and post it online  (it's rare to see detailed real-world case studies).

~ ~ ~ ~ please use my permanent email address: satjiwan at alumni.brandeis.edu ~ ~ ~ ~

Some of my recent projects: 
SEE: http://www.Truthful-Living.com
 Green Building Consulting Services - Green Homes Tour - (and free green home building resource and energy guide)
 Annotated green house renovation photos - Save Our Sky - corn stove cooperative - Takoma Park Green Building Group

> ----------------------------------------------------------------------
> 
> Message: 1
> Date: Mon, 09 Feb 2009 12:55:49 -0500
> From: Corwyn <corwyn at midcoast.com>
> Subject: Re: [Greenbuilding] Solar Water Heater Tilt
> To: Gennaro Brooks-Church <info at ecobrooklyn.com>
> Cc: greenbuilding at listserv.repp.org, Gary <gary at builditsolar.com>
> Message-ID: <49906E25.1050706 at midcoast.com>
> Content-Type: text/plain; charset="iso-8859-1"; Format="flowed"
> 
> 
> 
> Gennaro Brooks-Church wrote:
> > RE: The standard answer for tilting your solar water ehater collector
> > is latitude plus 15 degrees.
> >
> > Does this general rule of thumb apply to solar PV too?
> >   
> No.
> 
> However, given the availability of tools for finding the exact(ish) 
> answer, for your own particular situation, I don't see much use for the 
> rule of thumb.  If you are putting a couple of thousand dollars worth of 
> equipment way up high, it is worth some observation and calculation 
> beforehand.
> 
> Thank You Kindly,
> 
> Corwyn
> 
> -- 
> Topher Belknap
> Green Fret Consulting
> Kermit didn't know the half of it...
> http://www.greenfret.com/
> topher at greenfret.com
> (207) 882-7652
> 
> 
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> End of Greenbuilding Digest, Vol 32, Issue 24
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