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ST collector performance parameters

  • 21-04-2010 8:57pm
    #1
    heinbloed
    Registered Users, Registered Users 2 Posts: 1,632 ✭✭✭


    Here the formula how to calculate the (winter-) performance of ST collectors:
    http://www.estif.org/solarkeymark/collector-theory.php


    Frequently we hear from people it isn't possible to heat buildings with ST energy in Ireland because the energy harvest from a ST collector aray is much less than for example on the EU continent.
    This is not true.
    To calculate the harvest this official formula should be used :

    q = A*([SIZE=-1]n[/SIZE]0*G - [SIZE=-1]a[/SIZE]1*dT - [SIZE=-1]a[/SIZE]2*dT²) [W]

    Here in Ireland with our compareable mild winter air temperature we have much better conditions to get more out of a ST collector, the

    dT: Temperature difference between collector mean fluid temperature and ambient air temperature [K] (Kelvin)

    is better suited here than for example in Austria.

    The optical efficiency to be used in this formula

    • [SIZE=-1]n[/SIZE]0: Optical efficiency
      • Combined efficiency of the transparent cover and the absorber [-]


    is -because of lesser snow on the collector surface as well as less icing and thaw - much better as in Austria as well.

    The heatloss coefficient here in Ireland for one and the same colector type is simply better,


    • [SIZE=-1]a[/SIZE]1: 1st order heat loss coefficient
      • Heat loss cofficient at collector fluid temperature equal to ambient temperature [W/K]
    • [SIZE=-1]a[/SIZE]2: 2nd order heat loss coefficient
      • Temperature dependent term of heat loss coefficient [W/K²]
      • The heat loss coefficient of a collector depends on the temperature. High values of [SIZE=-1]a[/SIZE]2 indicates large temperature influence
      • The higher the value of [SIZE=-1]a[/SIZE]2 the more "hookednosed" the power/efficiency curve)





    So get real numbers when calculating the performances of a ST heating system, not just flat statements.

    Demand a full calculation from your seller/installer, including weather compensation data like snow and icing on the pane.

    The inclination, the angle at which a ST colector should be installed to get as much as possible for winter heating must be 90 degrees (flush with the wall for example) to avoid icing, water droplets standing on it as well as snow deposits. This would give the best possible optical efficiency during winter.
    The optical efficiency describes the percentage of solar light hitting the collector's surface THROUGH the coverage, the glas pane. High quality glass is therefore used to guarantee a max optical efficiency. But this efficiency is severly hit if debris or snow, rainwater or icing covers the pane.
    Which is much less occuring if the collector stands at 90 degrees. A clean pane guarantees more light getting onto the collector.

    A roof mounted 30 degrees angled collector will hardly deliver any ST energy during the coldest months.
    An at 90 degrees mounted collector (for example on a wall) is simply better performing. It can be easily cleaned with a sponge or a hose, it insulates the wall against thermal losses like EWI (external wall insulation). And it needs hardly a heat dump, summer overheating will less like occure. Due to an intelligent mounting angle.


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