[Greenbuilding] A Solar Hot Tub

[dantonioli at earthlink.net]

Keith,

The extra storage and plumbing are easy in design but add significant
additional cost. If you want a system to be solar only to last five days in
cloudy weather in order to keep the hot tub at a temperature of at least
104, then you'll need to oversize the system. You'll need more thermal
panels, a very large tank (minimum of 120 gallons), copper, pump, etc. If
you're circulating hot tub water through a solar storage tank via a
stainless steel heat exchanger, the temperature of that water can only go up
to the amount of the temperature that's inside the tank, minus heat exchange
loss. After sunny weather the tank might be full and up to around 140
degrees, but that will drop on the first cloudy day that the hot tub
circulates cooler water through it, and so on down to day five. 

The ratings for this will depend on the specifics of the panels and tanks
used and the overall design of the system. And it also depends on how much
the tub is used. You could drain the heat source in a few hours if a lot of
people are using the tub and the water is circulating and the top is off.
Five days would be a stretch.

Remember also that in the winter the "five days of cloudy weather" also
means fewer hours of solar gain. 

You could easily spend ten thousand on an oversized system for a hot tub to
be solar only if you only want to use a thermal strategy. Better to have a
solar-based system with electric back-up and install a grid-tied pv system. 

Dan Antonioli




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
send me mail at
keith at the company below
www.EarthSunEnergy.com


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