[Greenbuilding] A Solar Hot Tub

[Keith Winston]

Is it my imagination or are you two talking past each other? If you only 
heat the hot tub to 104, then you are left to make up the energy with 
non-solar pretty regularly. With an extra storage tank that you can heat 
up quite a bit hotter, you can have a much higher solar fraction (also, 
you can put your solar system to more thorough use, instead of turning 
it off regularly in the middle of nice sunny days, potentially, once the 
tub is hot). The additional storage tank and mixing system are not going 
to be too prohibitive, IMO: they are really quite simple.

The "temperature never changes" remark is a bit obtuse, but the meaning 
is simple: if you don't overheat your hot tub, you don't have any margin 
to work with, and the instant the solar system shuts off, the 
temperature is dropping out of the comfort range. Now, maybe it takes 5 
hours to drop 1 degree, and only 5 minutes to boost it back up. Nick's 
point is simply that the additional stored energy of a second tank can, 
with rather small storage, cover several cloudy days and dramatically 
improve your solar fraction. I think you're point is that the system is 
sufficient without that. Without a better clarification of what you each 
care about (first cost, operating cost, high solar fraction at any cost, 
simplicity, etc) there's no metric to judge the better choice.

In order to provide higher solar fraction with the hot tub there has to 
be a separate tank, which would typically be called the thermal store, 
as opposed to the hot tub, which would typically be thought of as the 
load. It's a little confusing when the load is a tank...

Maybe that clarifies?

Keith


Nick Pine wrote:
> Dan writes:
>
>   
>> Like any storage system, inluding the most expensive double insulated 
>> solar storage tanks, there is a temperature loss. But even with this loss 
>> a hot tub "stores hot water.
>>     
>
>  " But a hot tub can't store any heat if its temperature never changes... "
>
> Nick, what do you mean "it's temperature never changes."  I don't follow.
>
> It's a simple matter of physics.
>
> Nick
>
>
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-- 
Keith Winston
Earth Sun Energy Systems
Hyattsville, MD 20781
301-980-6325
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keith at the company below
www.EarthSunEnergy.com