[Greenbuilding] A Solar Hot Tub

Sat Jul 14 16:14:55 EDT 2007

Nick,

Your calculations are meaningless unless you specify what types of
components you're using: panels, storage tank, type of heater, and system
design. Your "five cloudy day" scenario simply won't work unless you
drastically oversize the system. 

Have you ever had a hot tub or worked with one?

Dan Antonioli

-----Original Message-----
From: greenbuilding-bounces at listserv.repp.org
[mailto:greenbuilding-bounces at listserv.repp.org] On Behalf Of Nick Pine
Sent: Saturday, July 14, 2007 1:02 PM
To: greenbuilding at listserv.repp.org
Subject: Re: [Greenbuilding] A Solar Hot Tub

Dan writes:

>"... a hot tub can't store any heat if its temperature never changes... " 

> Nick, what do you mean "it's temperature never changes." 

I wrote "... if its temperature never changes."

> The tub is a tank, it's heated, it's stored, it loses heat just like any
other storage. As such, it's temperature is always changing.

Not necessarily, it might be a constant 104 F, always ready for people.

> >A typical hot tub use pattern is to set a heater on a timer. At our
house, the timer is set from 6-8 pm. At six the pump and heater turn on at
it only takes about twenty minutes to bring the hot tub temperature up to
104, after which the heater turns off and the pump circulates the water
through the filter.

> " Perhaps you mean the heater initially turns off after the tub reaches
104, then cycles as needed to maintain 104 F until 8 PM. "

> No, I meant what I said, the heater turns on when the the timer turns 
> it on at 6:00.  It brings the temperature to 104 and turns on and off 
> as needed for the temperature. At 8:00, it turns off and "holds" the 
> heat. (Again, yes, there are temperature losses with heat storage.)

Sounds like the same thing, about 18% solar-heated, with a 36 minute wait
time, for the tub I described, on an average day in Sacramento. Very lame.

>> But with solar, the energy is free so all day long when the sun shines
the tub will be maintained at 104. Nothing wrong with that.
> 
> That kind of solar heating doesn't seem to save much energy. Consider 4
scenarios:
> 
> 1. "100% solar heat" with a secondary storage tank.
> 2. solar heat for 6 hours per day, with a constant 104 F tub temp 3. 
> solar heat for 6 hours per day, with electric reheat on a sunny day 4. 
> electric reheat on a cloudy day.
> 
>   Eon        Eoff          Esun          Elec          Delay
>   (Btu/day)  (Btu/day)     (kWh/day)     (kWh/day)     (minutes)
> 
> 1  5409.405   7496.244      3.782429      0             0
> 2  5409.405   7496.244      .4323834      3.350046      0
> 3  5390.759   7496.244      1.364025      2.41294       8.636646
> 4  5358.812   7496.244      0             3.767602      62.82312
> 
> In scenario 1 the sun provides all the heat for up to 5 cloudy days in 
> a row. No electricity is used for heat.
> 
> If you have an oversized, expensive system and don't mind a cold tub 
> when you get weeks of cloudy weather. You'd have to seriously oversize 
> the system in order to have a solar-only tub for five days of cloudy
weather.
> 
> In scenario 2 the sun only provides 13% of the heat, keeping the tub 
> 104 F with no reheat delay.
> 
> Solar provides most of the heat in this scenario, with electric heat 
> coming on minimally to keep it at 104. Granted, if you have a hot tub 
> party from 8:00 pm till 2:00 am it will require more conventional heat.
> 
> In scenario 3 the sun provides 56% of the heat, like a very good 
> direct gain house, with a small delay.
> 
> Same as #2.

No... If the tub is the only heat store, consider scenario 5:

410 '5 solar heat with no electric reheat 420 T5D=104'initial tub temp (F)
430 FOR DAY=1 TO 5 440 EON=22*(T5D-45.3)*(ALID+ATUB)/RV'lid-on energy
(Btu/day) 450 PW=EXP(17.863-9621/(460+T5D))'tub vapor pressure ("Hg) 460
EOFF=2*((T5D-45.3)*ATUB/RV+100*ALID*(PW-PA))'lid-off energy (Btu/day) 470
T5D=T5D-(EON+EOFF)/C'tub temp after 5 cloudy days (F) 490 NEXT DAY 500
DELAY=0 520 ELEC=0 530 PRINT 5;EON,EOFF,ESUN,ELEC,DELAY,T5D

   Eon        Eoff          Esun          Elec          Delay      Final
temp
   (Btu/day)  (Btu/day)     (kWh/day)     (kWh/day)     (minutes)  (F)

5  4535.253   5219.712      0             0             0          92.52149

Also lame. A cool tub. Bubbles would make the final temp lower...

Keith writes:

Winston <keith at earthsunenergy.com>

> 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.

I agree.

>... 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.

Dan is incorrect, according to my calculations. Where are Dan's
calculations?

Dan writes:

> The extra storage and plumbing are easy in design but add significant
additional cost. 

The tank described might cost $100.

> 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 more thermal panels

Evacuated tubes (about 13K Btu/day) are reasonably cheap and do well at high
water temps.

> a very large tank (minimum of 120 gallons), copper, pump, etc. 

Why copper? 

>If you're circulating hot tub water through a solar storage tank via a
stainless steel heat exchanger...

Why stainless steel? Why do we need a heat exchanger?

> You could easily spend ten thousand...

... 10,000 emalangeni? :-) 

Nick
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