[Greenbuilding] A Solar Hot Tub

Tue Jul 10 17:23:36 EDT 2007

The tub is the storage tank. Thermosyphon potential aside, a regular solar
hot tub system uses a standard "swimming pool and spa" sensor/control that
limits the temperature to 104 degrees. When the tub temp is anywhere below
104, and the temperature of the panel is higher than the tub water, the pump
circulates the tub water. It's one big drainback system.

You still need a conventional heater, which also need to be tied to the pump
and filter assembly. If you take an evening soak and the temperature drops
you'll want to be able to keep it at a comfortable temperature. Like most
hot water systems, solar is the "pre-heat" and sometimes does all the
heating. 

The other option is to use as you suggest and run the hot tube water through
a heat exchanger. If I had the money I would have done it that way for my
tub instead of having a dedicated thermal system. It's cleaner and nicer,
just more expensive.

Dan Antonioli

-----Original Message-----
From: greenbuilding-bounces at listserv.repp.org
[mailto:greenbuilding-bounces at listserv.repp.org] On Behalf Of Nick Pine
Sent: Tuesday, July 10, 2007 2:50 AM
To: greenbuilding at listserv.repp.org
Subject: Re: [Greenbuilding] A Solar Hot Tub

dantonioli at earthlink.net writes:

> Heating a hot with solar is easy! If you're on a hill and position the
panel below the tub you might even be able to get a thermosyphon effect and
eliminate the pump altogether.

But how do we store the heat? The tub water is useless, since most people
prefer a certain fixed temperature... 104 F is too cold and 106 is too hot.
OTOH, we might store 170 F water from thermosyphoning evacuated tubes in a
heavily-insulated EPDM-lined box above them. We could keep a tub with a 5K
Btu/day heat loss exactly 105 F for 5 cloudy days in a row with
25K/(170-110) = 420 pounds (50 gallons) of 170 F water that cools to 110 F. 

Kathy Cochran writes:

>I imagine that I would line it with concrete and paint it black or dark
blue-green to absorb the sun... I live in Northern CA (Calaveras County in
the Gold Country)...

NREL says July is the warmest month in Sacramento, when 2520 Btu/ft^2 of sun
falls on the ground and 910 falls on a south wall on an average 75.7 F day
with a daily max of 93.2 and a humidity ratio wo = 0.0087 pounds of water
per pound of dry air with water vapor pressure Pa = 29.921/(1+0.62198/wo)) =
0.413 "Hg. If 1 ft^2 of hot tub at temp Tw (F) with perfect ground
insulation and no cover absorbs 2520 Btu = 24hx100(Pw-Pa), using an ASHRAE
swimming pool formula, Pw = 1.463 = e^(17.863-9621/(460+Tw)), which makes Tw
= 9621/(17.863-ln(Pw))-460 = 90.3 F on an average day, and cooler on cloudy
days. With a clear solar pool cover, it might absorb 0.9x2520 = 2268 Btu =
24h(Tw-75.7)1ft^2/R1, which makes Tw = 170.2 :-)

The worst-case month is December, when 550 Btu/ft^2 falls on the ground and
820 falls on a south wall on an average 45.3 F day with a 52.7 F daily max
and an average daytime temp of about 49. With a 2" R15 foil polyiso board
cover, the tub might lose 24h(105-45.3)1ft^2/R15 = 96 Btu/day. At 140 F (to
allow the use of a $10 plastic pipe heat exchanger) for 6 hours, a draindown
cover with an EPDM layer under R2 transparent top with 80% solar
transmission could collect 0.8x550-6h(140-49)1ft^2/R2 = 167 Btu, which seems
like a comfortable margin. We might heat the 1 ft^2 tub for 5 cloudy days
with 5x96/(140-110) = 16 pounds of 140 F water cooling to 110. For more
solar gain, we might make a lightweight lift-off transparent cover as an
equilateral pentagon hat with 4' struts, about 3' tall and 6' diameter with
2 clear polycarbonate triangles facing south and 3 reflective foil-foamboard
triangles in the other directions...   

20 PI=4*ATN(1)
50 DATA 4,8,9,10,12,18
60 FOR I=1 TO 6
70 READ L'strut length
80 ANG=36*PI/180
90 R0=L/(2*SIN(ANG))
100 D0=2*R0
110 D0R=INT(D0+.5)
120 D1=L/TAN(ANG)
130 D1R=INT(D1+.5)
140 FILMWIDTH=L*COS(30*PI/180)
150 FWR=INT(FILMWIDTH+.5)
160 H=FILMWIDTH+SQR(L^2-R0^2)
170 HR=INT(H+.5)
180 FILMLENGTH=8.5*L
190 FLR=INT(FILMLENGTH+.5)
200 FILMAREA=FILMWIDTH*FILMLENGTH
210 FAR=INT(FILMAREA+.5)
220 FCR=INT(.05*FILMAREA+.5)
230 PRINT 500+L;"'";TAB(8);D1R;TAB(16);D0R;TAB(24);HR;TAB(32);FWR;
240 PRINT TAB(40);FLR;TAB(48);FAR;TAB(56);FCR
250 FILMAREA=FILMWIDTH*FILMLENGTH
260 NEXT

L      Di      Do      H       FW      FL      FA      FC

4      6       7       6       3       34      118     6
8      11      14      11      7       68      471     24
9      12      15      13      8       77      596     30
10     14      17      14      9       85      736     37
12     17      20      17      10      102     1060    53
18     25      31      25      16      153     2385    119

Corwyn <corwyn at midcoast.com>

> PV panels are going to be VERY expensive way to heat the hot tub.  They
are about 10% efficient...

We might collect the other 85% with PVs under a shallow water-filled plastic
film pillow under under the pentagon. Plastic and water and PVs have similar
refractive indices, so adding the pillow only reduces the PV output by about
6%. Raising the PV temp lowers the output by about 0.5%/C, but we can make
up for that by concentrating more sun on the water-cooled PVs.

Nick
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