[Greenbuilding] A Solar Hot Tub

[Nick Pine]

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