[Greenbuilding] flat plate collectors

stephen at thermotechfiberglass.com stephen at thermotechfiberglass.com
Fri Oct 14 07:54:46 CDT 2011


Collector Efficiency is not the same as System Efficiency

Back in the good 'ol days i was involved in Solar Domestic Hot Water.

Originally the thinking was higher flow rates, so that panels operated 
at lower temps, so that their efficiency was higher.  Over the day the 
the tank would 'turn over' several times getting hotter and hotter on 
each pass.  This was thermodynamically correct from a collection point 
of view.

However, if there were mid-day draws, supplemental heat was needed.

So then some started thinking of reducing the flow rate so that the 
tank turned over once per day. (Also lower pumping energy)  The idea 
being the small mid-day draws had a chance of being 100% solar. The 
result was a higher annual solar fraction and lower annual purchased 
energy.

I recognize that DHW and space heating are not the same animal.

Just wanted to point out that optimizing collector efficiency is not 
always the same as optimizing system efficiency.




On Thu, 13 Oct 2011 16:52:58 -0600, Gary BIs wrote:
> Hi,
>  This is all true, but fins as implemented on most flat plate
> collectors have high efficiencies -- its easy to get up to 95% fin
> efficiency, as this calculator shows:
> 
> http://www.builditsolar.com/References/Calculators/FinEficCalc/FinEficCalc.htm
> [1]
>
>  In a flat plate collector with a 95% efficient fin, the average
> collector temperature is about 5F hotter than it would be with a 100%
> efficient fin. Under typical collection conditions, that 5F 
> difference
> costs about 1.5% in collector efficiency. There are so many
> differences in the two collector designs that I'm not sure the 1.5%
> due to fin efficiency is going to be the biggest player -- there may
> be other things that matter more? I think the side by side test will
> tell the story.
>
>  It seems to me that once you have an efficient design (which I think
> both are), things like life and maintenance become the important
> deciding factors. The environment inside a collector is a tough one,
> and you want a good internal design that will hold up over the years.
>
>  Gary
>
>  On 12:59 PM, Clarke Olsen wrote:
>
>> The expected efficiency would be from (a) having all of the target
>> surface in contact with the fluid, and,
>> (b) eliminating the need to conduct heat through the absorber.
>>
>> Clarke
>>
>> On Oct 12, 2011, at 1:01 PM, Joe Killian wrote:
>>
>>> MTD = Modified Trickle Down - After Thomason's early designs as
>>> in the Vermont customs building which were named Trickle Down.
>>>
>>> Thomason's design trickled water down the troughs in metal
>>> roofing, with a glazing a few inches above. Many thought
>>> condensing on the glazing would render the approach useless, but
>>> it turned out to work quite well, albeit with temperatures lower
>>> than we are accustomed to in flat plate collectors. The lower
>>> temperatures contribute to increasing the collecting efficiency.
>>>
>>> There have been several designs under the MTD label, all of
>>> which capture the water between two sheets, both of which are
>>> usually behind glazing. The object being to eliminate the issue
>> of
>>> condensing water on your glazing surface and the accompanying
>>> losses. I believe all these approaches have used some material
>>> between the two sheets to help distribute the water flow to
>> better
>>> collect the heat - as Gary's piece in Build It Solar amply shows
>>> is needed.
>>>
>>> These sheets are usually (always?) plastic, not much of a heat
>>> conductor. But it's thin, with water immediately on the back
>> side,
>>> so we're conducting heat through a few mils of plastic, not along
>>> several inches of the material as in copper fins in a flat plat
>>> collector.
>>>
>>> Joe
>
>
>
> Links:
> ------
> [1]
> 
> http://www.builditsolar.com/References/Calculators/FinEficCalc/FinEficCalc.htm





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