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  • Solar fraction for solar space heating?

    I was wondering if someone knows what should the Solar Fraction for the coldest month in a year be, for solar space heating?

    I found on a number of places, that an an annual one should be 70-80%.

    But I would like to know how much would the one for the coldest month in a year be?
    100% for Mediterranean climates?

    Thank you for the reply.

  • #2
    Originally posted by bernard View Post
    I was wondering if someone knows what should the Solar Fraction for the coldest month in a year be, for solar space heating?

    I found on a number of places, that an an annual one should be 70-80%.

    But I would like to know how much would the one for the coldest month in a year be?
    100% for Mediterranean climates?

    Thank you for the reply.
    I'd suggest there is no such "should" exists. If you are referring to solar thermal for space heating, find sources that discuss solar heating and residential HVAC design. There is no simple answer. Every situation is different.

    Comment


    • #3
      Thank you for the reply J.P.M.

      English is not my mother tongue, so looks like I should have used a different term instead of "should".
      I understand a lot of factors (solar fraction being one of them) depend on the type of the swh system, collector type, heating loads, climate...
      Still, just as for solar thermal systems for domestic hot water, one could strive for 100% of solar fraction for the sunniest summer month, I was wondering if the same recommendation exists for solar thermal systems designed for space heating?

      I did checked some HVAC documentation but couldn't find an information for the month solar fraction. Only an annual one (striving towards 80%).

      Comment


      • #4
        Originally posted by bernard View Post
        Thank you for the reply J.P.M.

        English is not my mother tongue, so looks like I should have used a different term instead of "should".
        I understand a lot of factors (solar fraction being one of them) depends on the type of the swh system, collector type, heating loads, climate...
        Still, just as for solar thermal systems for domestic hot water, one could strive for 100% of solar fraction for the sunniest summer month, I was wondering if the same recommendation exists for solar thermal systems designed for space heating?

        I did checked some HVAC documentation but couldn't find an information for the month solar fraction. Only an annual one (striving towards 80%).
        I lived in a very cold, cloudy climate for many years. Solar space heating was always less cost effective than very heavy levels of insulation until the cost of the next level energy reduction via insulation equaled the cost of first level of energy (heat) supplied by alternate energy, and that R.E. source was, in that climate almost always passive solar.

        In my former climate, that solar space heating fraction for me only, was about 15-20% or so, and that was only after some gargantuan measures to reduce the space heating load by about 2/3 from that of the previous owners. Sunnier and warmer climates have a different mix as do sunny and cold climates. There is no one answer. That's why I suggest books on solar space heating design.

        A complicated subject cannot usually be answered with one rule of thumb or one number.

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        • #5
          Originally posted by J.P.M. View Post
          In my former climate, that solar space heating fraction for me only, was about 15-20% or so...
          Thank you J.P.M.

          This is an annual solar fraction (15-20%)?

          Comment


          • #6
            Originally posted by bernard View Post
            Thank you J.P.M.

            This is an annual solar fraction (15-20%)?
            You're welcome.

            Yes, pretty much. Oct. and April solar fraction for space heating might be about 30% or less. Nov. and Feb. something like 20%. Dec. and Jan. less than 10%. All real rough est. based on ~ 7 years of data from building/monitoring a south facing sunspace w/ ~182 ft^2 of vertical glazing.

            Somewhat subjectively, on average, the solar disk was visible about 60 hrs./month in Dec. and Jan., usually before about 8-10 A.M. After that, lake effect clouds often brought more snow, except on very cold days that would produce brilliant sun all day. That happened about 2 days/month.

            Again, the heating load had already been substantially reduced. in fact, the load was so low that between thermostat reductions at night and limiting the indoor temp. to 62 F. or less during the day, and heat supplied by other things - lights, cooking, hot H2O use, humans, etc., but without the sunspace contribution, I estimate the effective balance temp. - the outside temp. below which aux. furnace heat was needed to be about 53 deg. F.

            The heat was supplied by CH4. I est. the dwelling heat loss to be about 5,800 BTU/ F. degree day. The average elec. load in winter for lighting and appliances, but not much else was about 8 kWh/day. The building heat transfer time constant was about 22 hrs., giving it an effective thermal mass of about 5,300 BTU/deg. F.

            At that time, Buffalo NY had about a 7062 degree day per year (to a 65 deg. F. base) climate.

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            • #7
              Thank you J.P.M.

              That answer was more than I asked for! I am really grateful for the answer you gave.

              I am still a bit confused about the 15-20% annual solar fraction you mentioned.
              That was an annual solar fraction of your solar thermal system which was used both for domestic hot water and space heating?
              Or did I misunderstand you?

              Comment


              • #8
                Originally posted by bernard View Post
                Thank you J.P.M.

                That answer was more than I asked for! I am really grateful for the answer you gave.

                I am still a bit confused about the 15-20% annual solar fraction you mentioned.
                That was an annual solar fraction of your solar thermal system which was used both for domestic hot water and space heating?
                Or did I misunderstand you?
                You're welcome.

                The 15-20% was an estimate of my much reduced annual space heating fraction provided by a passive solar device - the sunspace. A lot of the analysis done followed the work of Doug Balcomb. I shot for cost effectiveness after some pretty heavy use reduction/conservation measures, more than a few of which are now part of std. building practice.

                I believe the largest annual space heating offset in such a cold & cloudy climate via active (usually flat plate thermal) may be about 25% or less, and that's only after and because of very heavy levels of insulation and reduction of infiltration losses.

                Recent PV and superior heat pump technology may have changed the economics some, possibly improving the solar fraction and solar economics, but I'd be surprised if use reduction, conservation and heavy levels of insulation/sealing were not still more cost effective than solar in cold, cloudy climates and therefore things to be done first. In any case, I believe the optimum economic solar fraction is still very highly dependent on local climate and building design considerations. I can design a zero net use house for any climate, and it might even be cost effective in terms of like cycle costs, but no one would want to live in it for aesthetic, or practical reasons.

                Finally, I made no serious attempt at solar domestic water heating back in the old days beyond proof of concept stuff - lots of experimentation and data gathering on DIY flat plate collectors and how diffuse and specular reflectors improve flat plate collector performance, mostly in the summer, and how liquid flow rates in the transition zone and beyond improve panel instantaneous efficiency (~ 8-10% for those who may care). That ~ 10% was later mostly confirmed after my move to CA, design/acquisition of a flat plate collector system and better instrumentation.

                Comment


                • #9
                  Originally posted by J.P.M. View Post
                  You're welcome.

                  The 15-20% was an estimate of my much reduced annual space heating fraction provided by a passive solar device - the sunspace.
                  Thank you.

                  On this side of the Atlantic (Europe) "sunspace" is a buffer zone between the inside and outside of the building.

                  So you have been using flat glazed collectors with "sunspace" zone?

                  Comment


                  • #10
                    Originally posted by bernard View Post
                    Thank you.

                    On this side of the Atlantic (Europe) "sunspace" is a buffer zone between the inside and outside of the building.

                    So you have been using flat glazed collectors with "sunspace" zone?
                    I'd use the "sunspace" term in the same way you understand it with some modifications. The way the sunspace was designed and intended for use, it is an enclosed space attached to the existing structure. However, it's purpose is primarily to provide heat to the existing structure by having a lot of glazing oriented in the most optimal way possible for the situation and limitations. Think of it as a big 3D solar air heating collector you can walk into that provides heat to the rest of the dwelling that takes no heat from the dwelling at night or during cloudy periods. It had about 6,000 lbm of H2O that was dyed black w/ food coloring contai9ned in translucent fibreglass tubes exposed to the sun lined up against the back (north wall) of the space. That stationary mass moderated the space temp. some and stored heat for release later in the day/at nite. Air was fed to the space from the existing house HVAC ducting and returned to the dwelling when needed. Simple but it worked well.

                    All that construction was in a cold, cloudy climate. As I wrote last, there was no attempt back then at domestic H2O heating beyond experimentation and some DIY collector designs.

                    I now live in what's close to a perfect (for me anyway) climate with a solar flat plate H2O heater for DHW and a PV system. A sunspace for aux. space heat is not cost effective in this climate or location.

                    There are any # of books and info on current and recent solar space heating and water heating possibilities. There's not much new under the sun. Check out a book title: "A Golden Thread" - 2,500 years of solar architecture and technology. By Ken Butti and John Perlin. ISBN #: 0-442-24005-8.

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                    • #11
                      Thank you J.P.M.

                      Comment


                      • #12
                        A little late but here goes
                        Solar fraction can be calculated for heating but is a bit more complicated than hot water. And will vary year to year depending on the weather and any additional heat source you have like a wood stove for instance.
                        It is a calculation of how many btu a system will produce in the winter months. Divided by the btu needed to heat the house. Both will vary year to year. Unlike a hot water system where the testing is done under identical conditions. ( insolation, inlet temp, target temp, and amount of daily load.
                        SRCC lists the test conditiond in the OG300 page.
                        However your conditions and system will be no where close to their results
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