[Greenbuilding] Solar Water Heater architecture

[Keith Winston]

Hi David!

Hmmm. I'm not quite sure about your 1st point: the additional pumping 
power is equal to the drainback distance. So yes, there will always be 
more power necessary for drainback. A really clever designer might get 
it down to some few inches of head.

Your point about siting reservoirs/solar components in existing 
construction is very true. Even in new construction, situating solar 
parts can be a pain. Depends on how things fit together, in some (rare) 
cases it might be EASIER to put the drainback reservoir in the attic.

Finally, thermosyphon systems have the major advantage that they don't 
require pumps, nor any complicated controls. But you take a considerable 
efficiency loss for that, on the order of 30%. Perhaps productivity loss 
would be a better way to think about it, since efficiency in a system 
without pump loss or any energy input is hard to define!

Good points one and all!

Keith


David Delaney wrote:
> Hi Keith
>
> It does make sense, but the fact remains that whatever the 
> drainback distance, it takes more pumping power to start the 
> collector flow than it would with a non-drainback system 
> with the collector a similar distance above the reservoir 
> and in which the up pipe and the down pipe were always 
> charged with water. Also, placing the reservoir that high 
> might well limit its size in existing houses. And require 
> pumping for space heat distribution, whereas thermosyphon
> would be possible otherwise.
>
> David
>
> At 05:12 PM 15/08/2006, Keith Winston wrote:
>   
>> Hi David,
>>
>> I'm also an Apricus dealer, in the DC area. Just to be clear, the reason 
>> Apricus collectors can stand stagnation is because the heat pipes can 
>> stand it: they are made using water as the working fluid, and are hence 
>> less prone to some problems heat pipes can develop. But there's a lot of 
>> interesting and complicated science rolled into any modern solar thermal 
>> collector.
>>
>> One thing we haven't mentioned, when talking about the head/pressure 
>> involved with drainback systems, is that you only have to drain water 
>> back into the building envelop: you could have a system with 70 vertical 
>> feet of distance between the tank and collector, and yet only "drain 
>> back" 5 feet. In which case, the pump would only have to develop 5 feet 
>> of head, plus the resistance of all the pipes and fittings.
>>
>> I hope that makes sense.
>>
>> Keith
>>
>>
>> David Delaney wrote:
>>     
>>> Under certain conditions, ordinary water makes a good 
>>> working fluid for solar water heaters, in a drainback 
>>> configuration. The evacuated tube solar water heaters made 
>>> by Apricus claim to extend these conditions. Apricus is an 
>>> Australian company. The tubes are marketed in North America.
>>>
>>> http://www.apricus.com
>>>
>>> The distinctive feature of Apricus tubes is that they are 
>>> claimed to tolerate indefinite exposure to the hot sun while 
>>> not containing water - "stagnation". This feature allows a 
>>> collector to be configured in a drainback configuration for 
>>> cold winters with short sun hours, then be run on a 
>>> short duty cycle in the summer without damage when heat is 
>>> not being removed from it by the working fluid, which is 
>>> absent when no further heat is needed for domestic hot water.
>>>
>>> One disadvantage of the drainback configuration is that it requires
>>> greater pumping power, since each time the collector has to be
>>> filled, the working fluid has to be pumped up to the collector
>>> with the full weight of the column of water being pushed up the
>>> up pipe by the pump. That weight is not balanced by fluid in the 
>>> down pipe, since the down pipe is empty until the collector has been filled.
>>>
>>> A dealer near Ottawa in Ontario: http://www.terravis.com
>>>
>>> David Delaney, Ottawa
>>>
>>>
>>>
>>>
>>>
>>>
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>>>
>>>
>>>   
>>>       
>> -- 
>> Keith Winston
>> Earth Sun Energy Systems
>> 3927 Madison St.
>> Hyattsville, MD 20781
>> 301-980-6325
>> keith at earthsunenergy.com
>> www.EarthSunEnergy.com
>>
>>
>>
>> _______________________________________________
>> Greenbuilding email list
>> List info: http://listserv.repp.org/mailman/listinfo/greenbuilding_listserv.repp.org
>> List email: Greenbuilding at listserv.repp.org
>> Managed by BuildingGreen, Inc. http://www.buildinggreen.com
>>      publisher of Environmental Building News and GreenSpec(r)
>> Hosted and archived by REPP / CREST http://www.crest.org
>>     
>
>
>
>
>
>
>   

-- 
Keith Winston
Earth Sun Energy Systems
3927 Madison St.
Hyattsville, MD 20781
301-980-6325
keith at earthsunenergy.com
www.EarthSunEnergy.com