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solar heating storage sizing

zacmobile
zacmobile Member Posts: 211
I've installed a few solar DHW systems and would like to offer solar heating supplement as an option to clients as well but I always like to experiment on my own systems before unleashing it on the world. So, my quandry is thus: I'm going to install a solar combi system in my place & I've read all the solar design guides I can get my hands on (the newest iDronics and Viessmanns have been the most helpful) I gather that you should take your square footage of collectors & double it for your heating storage gallons & that you should have around double to 2-1/2 times the collector area for heating over DHW. OK, now I'm really interested in <a href="http://www.buderus.us/products/storagetanks/tankssolarsystems/logaluxpl750-2s.html"> Buderus' PL750 combi-tank </a> which is 200 gal. and they say in the literature that you can connect "4-8 collectors" to it, however I figure that if I had 6X 25sq ft collectors I would need about 300 gal of storage. So how does that work? Am I missing something? I did call them and asked about this, and the guy I talked to wasn't too sure but told me to check out the schematics on the pro-site, I did but they don't have any sizing info, any solar gurus have any suggestions?

Comments

  • hot_rod
    hot_rod Member Posts: 14,552
    do a load calc

    for the building first. then run some different sizing options with a a solar sizing program like RETscreen, F-Chart, T-sol, PolySun or others. I think they all have free demos to try at their website.



    You want to determine the solar fraction SF. that being the % of the load(s) that you hope to cover with the solar. The more tank, the lower the temperature you can expect with the same collector array.



    T-sol has a small, basic sizer for free at the upper left corner of their site. Here is a quick calc for example. Showing the SF for the DHW and heating loads. www.valentin.de click on English version



    The boys at Cedar Mt in NM usually design around a SF of 30% for heating loads. Beyond that the array and cost gets a little large for most customers. Their systems have very little "wet" storage as they use the radiant mass as storage as best they can.



    Design the heat distribution to run at the lowest possible supply temperature. The efficiency of the collector array improves with lower operating temperatures. Tighten up the tube spacing, tighten up the insulation and window packages to keep the load as low as possible.



    There are some very wiiide rules of thumb, but you really need to crunch some numbers to do a proper job of sizing and bidding these jobs.



    hr
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • zacmobile
    zacmobile Member Posts: 211
    edited March 2010
    load calcs

    I guess I should have mentioned I have done a load calc (38,000 BTUH) and run a few scenarios on ESOP (same as T-Sol I believe) also been using the Valentin on-line one since you posted the link about a year ago (love that thing!) however I'm unsure which location to use in the climate section, I'm in the mountains of southern British Columbia and thre's not really anything close so I've been trying all kinds of different locations but haven't tried any US locations yet, thats a good call, maybe i'll try Montana. Anyways, I was more wondering if Buderus somehow is able to get by with less storage because their clever tank-in-tank & stratifiction lance design is somehow able to more efficiently utilize collected heat or something.
  • hot_rod
    hot_rod Member Posts: 14,552
    some consider

    storage the holy grail of solar thermal. Really it's just an insulated parking space. The amount of storage determines how long you can run without any solar input.



    There are a bunch of tank stratification concepts out there. The intent is to quickly move the warmest water to the top and provide the coolest temperature at the bottom to present the collector. I think it mainly drives the HX efficiency. Once the entire tank is warmed i suppose it doesn't really matter what "device" is inside.



    I see one Italian company has an active stratification tank. A very small recirc pump moves the stratification and the tank has a divider between the top and bottom. Lochinvar does it with a small box device at the bottom to trap the cold water down low.



    The basic BTU formula will give you the drawdown use-ability 8.33 X gallons X Tank hi- tank low temperature



    Charge a 250 gallon tank to 150F, draw it down to 90F and you have 124,950 BTUs to use. Factor in some standby loss from the tank.



    Divide your load into that to see how long that tank can supply energy to DHW and or radiant.



    I have 500 gallons of storage connected to my wood/ solar shop heat. Next time I see a deal on another 500 gallon LP. I will double the capacity. 500 gallons takes me 6- 8 hours depending on the load.



    hr



    8.33 X 250 (150-90) = t it's the drawdown time you are concerned with.



    hr
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
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