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buffalobillpatrick 03-21-14 02:26 PM

Solar hot water storage + paraffin wax?
Has anyone done this?

I have 1,000 gal. tank.

Thinking of getting 1/4 pallet of paraffin wax 1,000# $600 + shipping.

Floats on water in top of tank.

Stores/releases at phase change about 37 times more heat per unit volume than to heat/cool water, and about 30 x on a mass basis.

Paraffin wax - Wikipedia, the free encyclopedia

"Paraffin wax is an excellent material for storing heat, with a specific heat capacity of 2.14–2.9 J g−1 K−1 (joule per gram kelvin) and a heat of fusion of 200–220 J g−1."

Phase-change material - Wikipedia, the free encyclopedia

Freeze without much supercooling
Ability to melt congruently
Self nucleating properties
Compatibility with conventional material of construction
No segregation
Chemically stable
High heat of fusion
Safe and non-reactive

Low thermal conductivity in their solid state. High heat transfer rates are required during the freezing cycle
Volumetric latent heat storage capacity is low
Flammable. This can be easily alleviated by a proper container
To obtain reliable phase change points, most manufacturers use technical grade paraffins which are essentially paraffin mixture(s) and are completely refined of oil, resulting in high costs


buffalobillpatrick 03-21-14 03:43 PM

Unfortunately, "paraffin" is a somewhat general term meaning: "approximately saturated hydrocarbons of unspecified molecular weight".

"My CRC has an old table of heats of fusion. They vary, from 30.7 cal/gm for n-tricosane (C23H48) to 39.1 cal/gm for n-pentacosane (C25H52), to 59.1 cal/gm for n-eicosane (C20H42)"

"average heat of fusion to be 37 cal/gm.

37 cal/gm x 4.184 J/cal = 155 J/gm.

(CH2):=(12+1+1) gm/mol; (14 gm/mol x 24 )= 336 gm/mol.

155 J/gm * 336 gm/mol = ~52 kJ/mol."

"On this website (, I find the heat of fusion for paraffin listed as 63 BTU/lb. On this web site ( I find the chemical formula for paraffin to be C36H74.

So the gram formula weight for paraffin is 506 g/mol. Converting the units, I get the heat of fusion for paraffin to be 74 kJ/mol (Please check my math!)

Todd Clark, Office of Science
U.S. Department of Energy"

Of course, I stole all of above.


buffalobillpatrick 03-21-14 04:13 PM

So roughly 1,000# of pariffin wax, at phase change (heat of fusion 50-70*C) is about the equivalent of 3,600 gallons of liquid water.

So my 1,000 gal. will be reduced to about 750 gal. water.
And will store/release heat about like a 4,350 gal. tank of water.

Please critique my math.


jfweaver 03-21-14 05:02 PM

I do have a few questions. Will you be operating at or near the phase change point of ~133F enough to take advantage of it? How will you deal with the flammability concerns? I could see issues with it solidifying on your heat exchangers (which will likely be below the 133F where the wax solidifies. If you put your heat exchangers low enough in the water tank to keep away from the wax, you would be in cooler water due to stratification.

How about a pair of tanks inside of each other, with the paraffin on the outside, and the water in an inner tank? You would want the inner tank to be metallic or made of a material which would transfer the heat from the hot water to the cold paraffin, and then conversely from the hot paraffin to the cold water. This would alleviate any issues that you would have with paraffin solidifying on a heat exchanger, and allow you to keep your heat exchangers in the hottest area of the water. If the paraffin tank is metallic and has a cover that will close automatically (i.e. with gravity when you let it go). That should alleviate most/any of the flammability issues. A place I worked at had parts washers filled with acetone setup this way.

buffalobillpatrick 03-21-14 05:39 PM

another source:
"Heat capacity of water, Cv = 4.187 kJ/kg

"paraffin wax has high latent heat capacity of 206 kJ/kg. The phase transition temperature of the wax is 40-50*C"

Paraffin 49 x water.


buffalobillpatrick 03-21-14 06:13 PM

yes, I will need to space the pex HX's below pariffin bottom level.

Your tank within a tank would have a nice seperation benifit, but

A very big deal on solar hot water Eff. is to stratify the storage tank, pump the cool water from bottom of tank to solar panels, hot water back goes into top of tank.

To melt paraffin toward bottom would require a water circulation pump to get hot water down there.

zip 80813
Teller County Colorado
Zone 5/B dry
Design temp 2* F
AFI = 2500
Mean Annual Temp. 40* F
HDD 6415
Lat = 38.8*
Altitude 8,800'

Very good Solar, lots of UV, I can get a sunburn in 30 min.

On fire safety:

The 1,000 gal solar storage tank will be burried outside the basement.
It's temp. will vary quite a bit, possibly from 180* when there has been good sun & weather down to 35*
Insulated VERY well!!!

It will have a 300' loop of 1" pex in the top that will preheat the well water coming into a 120 gal. thermal accumulator tank on any DHW draw. well water is just over 40*

There is also a 2nd HX loop in the top of the solar tank of O2 barrier 3/4" pex that will provide heat directly to the radiant floor system, via a Taco ODR controlled TMV which provides appropriate temp. water to a Grundfos Alpha ECM pump to whatever zone of the hydronic floor that needs heat. I will use this source of heat whenever the solar tank is above 80*

When the 1,000 gal solar tank is below 80* and above 35* AND the aqustat on the 120 gal thermal accumulator tank is calling for heat, instead of firing the boiler, a new WtW heat pump will heat the 120 gal thermal accumulator tank.

EDIT: link to thermal accumulator info:

Solar tank

It is about 1/4" thick in field & 1/2" at bottom, HD fiberglass made for High Temps. It's a cylinder 10' x 4.5' made for Cray Computers as part of their water cooled super-computers 20-30 yrs. ago.

It would hold 1200 gal. but I don't want the water level up to a side port close to top thats about 8" in diameter which I will use for all pipes & sensors from tank to basement.

CL score


jfweaver 03-21-14 08:13 PM

Nice find! I didn't envision something that deep, so that will help quite a bit. How do you plan on insulating it?

Did the seller say which model Cray it was from? The older units were typically were submerged in a few tons(!) of 3M Fluorinert with chillers at the bottom of the chassis, water cooling for them would be much more modern (last 10 years). Sorry, computer nerd here. :)

buffalobillpatrick 03-21-14 08:38 PM

No info other than Cray labels on it.

Digging additional 8' x 8' x 6' deep hole in area next to where basement will go, it's already about 5' below grade there from basement excavation.

Building an 8' x 8' x 12' box of 8" thick type 2 EPS, that I also got cheap off CL.
Solar tank will be inside box with space between tank & box filled with chunk pearlite.
You can get bags of pearlite at Lowe's fairly reasonable. It will take many bags.

On your tank within a tank idea, possibly a 275 gal. HDPE tote container, they are not all HDPE some are standard PE, have to insure Food Grade & it's stamped on outside tank with a symbol that indicates HDPE.

Place this tank in bottom of solar tank, & fill with the pariffin, put lid on it that it comes with, drill small pressure equalization hole, add water to solar tank & the inner tank will float. I can add rocks to control how far it floats above water surface.

Question is will surface area of inner tank accomidate enough heat transfer?

My 240 ft2 panels (also CL score) will be ground mounted on used gas well pipe rack availible here, vertical to maximize Winter sun. I'm sure that the 1st Summer or Fall all the pariffin
will melt, if not before.


buffalobillpatrick 03-22-14 11:21 AM

This type of heat transfer mechanism is governed by the basic equation:

Q = (k A/x) (T1 – T2)
Q = Heat transfer, BTU/hr,
k = Thermal conductivity, BTU/in/ft 2/hr/F
A = Heat transfer area, ft 2
x = Wall thickness, inches
T1 = Outside temperature, F
T2 = Inside pipe temperature, F

"Note: The above equation only addresses the question of heat transfer through the PE pipe. Depending on the application (ground source heat exchange systems, snow melting, radiant heating, etc.) there are other factors that may have a significant influence on the accuracy of the heat transfer calculation including the thermal conductivity of the surrounding embedment material, the inside and outside film coefficients of the pipe and perhaps others. Therefore it is recommended that such calculations should be referred to engineers who are expert in this field."

"Badges? we ain't got no stinking badges"

I have read that inducing a slow rotation of water in the solar tank by setting the ends of all pipes to have a 90* elbow cordinated (inputs all set one direction parallel with a tangent to circumference & outputs the opposite direction)
This is suppose to not effect stratification of water vertically.

Some flow should greatly enhance thermal transfer through HDPE tank walls.


buffalobillpatrick 03-22-14 03:39 PM

Q = (k A/x) (T1 T2)


Q = Heat transfer, BTU/hr,

k = Thermal conductivity, BTU/in/ft 2/hr/F

.38 W/MK for HDPE @50*C,

convert to BTU/in/ft 2/hr/F multiply by 6.934

.38 x 6.934 = 2.635 BTU/in/ft 2/hr/F

A = Heat transfer area, in ft2

275gal IBC tote with 4 sides + bottom in contact with water = aprox. 50 ft2

x = Wall thickness, inches = .125"

50 x .125 = 400

T1 = Outside temperature, F = using 150*F example
T2 = Inside temperature, F = using 100*F example, thus 50* delta

2.635 x 400 x 50 = 52,700 btu/hr

This ignores many factors, such as thermal conductivity of paraffin in solid state & liquid state, water circulation, etc. etc.

But, I would be good with 10K btu/hr as this transfer would continue 24/7

+ I can add to water rotation with a tiny El_Sid 3gpm 28vdc pump that I have. 24/7


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