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  • #16
    Originally posted by inetdog View Post

    And on the same topic, the best way to reduce the attic temperature (other than improving ventilation) is to intercept the solar energy above the roof and keep it out of the attic. That is, among other ways, with solar PV or solar thermal panels. Especially if you provide adequate air space between the PV panels and the roof.
    And, FWIW, some data:

    Between 06/14/15 and 08/1515 on 26 sets (days) of measurements under clear skies, among lots of other data, I measured the following average data based on N=26:

    GHI irradiance: 942 W/m^2, std. dev. 24.6 W/m^2
    POA irradiance: 965 W/m^2, std. dev. 16.2 W/m^2

    Ambient temp: 33.0 C. , std. dev. 3.4 C.
    Wind speed: 1.8 m/sec. std. dev. 0.38 m/sec.
    Cell temp.: 59.6 C. , std. dev. 3.6 C.
    Exposed (to irradiance) roof deck temp.: 68.4 C., std. dev., 3.8 C.
    Under panel roof deck temp.: 39.0 C., std. dev. 4.2 C.

    All temp. data for each of the 26 dates is the average of 32 separate temps., each of which are themselves averages of 4 random location temp. readings across each of 16 panels, taken immediately before and after the time of minimum incidence angle of direct beam irradiance on the array. Or, in the case of the under array roof temps., the same sampling method done at the same time. All temps. taken with IR thermometer I calibrated using an ice, bath, boiling water and my body temp.

    Thus the roof deck under the panels was, on average, ~ 29 C. lower than the same roof material exposed to direct sunlight,.
    The roof deck under the array was above the ambient temp. due to thermal radiation from the array heating that portion of e roof deck. That cell temp. to roof deck is also some information to corroborate approx. cell temp. with measurements of cell temp., although there are too many variables involved to get very quantitative about it, it can still serve as another quasi independent equation to help cover all the unknowns in getting an energy balance on the array which is the overall goal, as can the roof deck to cell temp. itself.

    About half of the array is over a cathedral ceiling in the living room. The other half is over a crawl space. The insulated ceiling temp. over the array (the south portion of the ceiling) in the living room was ~ 1.5 - 2 C. higher than the north portion, but a lot of that can probably be attributed to orientation. I don't have data on the crawl space underoof temp. diff. between those parts under the array and those parts not under the array.

    I believe array shading can help lower the temp. of an attic of a crawl space when there is no insulation directly under the roof. How that may influence the temps. in the rest of the dwelling is largely dependent of the level of insulation between an attic or crawlspace and the rest of the building space.

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    • #17
      Originally posted by paris401 View Post
      i just got an estimate (we r in ny) for a heliocor system- black tubing goes on the roof for a 18x36 inground... price installed ...9,200 bucks- WOW... thats a lot of $$$- but I figure the payback in not using my propane heater is about 4years... not a bad payback period, but a lot of upfront $$$$.... there is also another 'unique' solar heater that they place up in your attic to take advantage of the heat up there- its black tubes that suck in attic heat... and lowers the attic heat .. not sure of the pricing... trying to gather info from an installer.... downside ofcourse is u better not have a leak...
      For a few thousand more you could get a thermal system that would give you domestic hot water plus heat your pool. Or you could go with a pv system and use a heat pump pool heater and get a tax credit for the install.

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      • #18
        Originally posted by LucMan View Post

        For a few thousand more you could get a thermal system that would give you domestic hot water plus heat your pool. Or you could go with a pv system and use a heat pump pool heater and get a tax credit for the install.
        I have a PV system, and last year overproduced by 1500, and got a check from psegli for 44bucks...haha... I'm looking into a heat pump... any suggestions for a 18x36ft pool??

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        • #19
          Originally posted by paris401 View Post

          I have a PV system, and last year overproduced by 1500, and got a check from psegli for 44bucks...haha... I'm looking into a heat pump... any suggestions for a 18x36ft pool??
          1500 kWh and a heat pump w/an average C.O.P. of 4 or so will produce ~ (1500)*(3412)*(4) ~ 20,500,000 BTU/yr. to offset pool heat loss. The top heat loss of an uncovered pool that size will be about that same amount as that 20.5 MM BTU per yr. .per deg. deg. F. temp. diff., pool water to air temp. Start with a pool cover. See my 10/12/2016 post to this thread.

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          • #20
            Originally posted by J.P.M. View Post

            1500 kWh and a heat pump w/an average C.O.P. of 4 or so will produce ~ (1500)*(3412)*(4) ~ 20,500,000 BTU/yr. to offset pool heat loss. The top heat loss of an uncovered pool that size will be about that same amount as that 20.5 MM BTU per yr. .per deg. deg. F. temp. diff., pool water to air temp. Start with a pool cover. See my 10/12/2016 post to this thread.
            yea I have a pool cover at my main house, but what a PITA to uncover and then store...I hate pools, but since I don't get a vote (my wife has all the voting rites) I am screwed..

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            • #21
              Originally posted by paris401 View Post

              yea I have a pool cover at my main house, but what a PITA to uncover and then store...I hate pools, but since I don't get a vote (my wife has all the voting rites) I am screwed..
              Understood. I found a couple other, perhaps underemphasized advantages to a tight fitting and motorized pool cover besides reducing required (and more costly) heat input:

              1.) Fewer chemicals required.
              2.) Less cleaning required ( but duck guano still needs removal from the cover, otherwise the cover becomes a duck crap wrap.
              3.) If a dark and opaque cover, the cover has more than some ability to heat the water, reducing required heat addition still further (== less $$)
              4.) A lot less PITA of cover removal/storage.

              BTW, never store a wrapped/rolled up bubble wrap type cover in the sun, or shade for that matter without covering it with something opaque - otherwise it'll cook in the sun, or at least and still get pretty warm in the diffuse sunlite present in the shade.

              Motorized covers are not cheap, and not easily fitted to irregular pool shapes but pretty slick if/where they can be used. The dark brown motorized cover I had in Albuquerque covered the pool whenever to pool was not being used. Water was => 80 F. or so from mid April to mid/end of Oct. w/ no other heat addition, and uncovered more than a few nites following warm/hot days, although that increased required cleaning a bit.

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