SOLAR THERMAL TANK DESIGN ?? 1

[BPLBPL]

Hi all,
I'm designing a Solar domestic hot water system consisting of solar collectors, 2 flat-plate heat-exchangers and a 3,000 gal tank which stores the hot water from the collectors.
The link below has the simple design layout and other details. Further discussion below are based on this attached drawing.

http://files.engineering.com/getfil...4a63-98f0-584653527e76&file=Piping_Layout.pdf

I want the temperature of the hotwater being drawn from point_B in the tank to be close to 140F. My design is based on the assumption that the hottest water(140F) in the tank will be at the top of the tank - close to point_A.
My justifications are i)less dense hot water will always be on the top and ii) there'll be thermal stratification i.e., hotwater(140F) at the tank top will not mix with the cold water(80F) at the tank bottom because of the large size of the tank).

Will this design work? My design would fail if the cold water at the tank bottom mixes with the hot water inputted from point_A. I guess this is unlikely to happen solely by natural convection within the tank.

Sorry about the length of the post. Please post your comments/suggestions. Thank you very much.
Baggi
===========
Storage Tank dimensions: 16' long, 4.5' dia approx, ~3000gal

 

[IRstuff]

Please do not double post:thread391-201870

TTFN

FAQ731-376

 

[BPLBPL]

Sorry, new to eng-tips.com
Thanks IRstuff for letting know.

 

[BigInch]

Whad'a know. I've been studying up on this solar stuff for the past month.

What's the max design flow from the boiler to the fixtures? W/o that you can't tell that the fixture temperatures will reach 120ºF

Also decide on the other temperatures. Don't give a 10Fº range for each flow. Pick one or the other so to state a design temperature for each flow. I'll do one problem, but I'm not going to calculate 4!*3!*2!*1! options.

Also need the efficiencies for the heat exchangers.

You might want to give the minimum heat transfer capacity of the solar collectors before the fire kicks in to heat the water, rather than the maximum heat transfer. You know you can probably get what you need on a sunny day, so the heat transfer at minimum design insolation is more important to know than the maximum.

Might want to insulate that 3000 gal tank, if the ambient temp is less than 140ºF temp, or there could be a big heat loss there. What's the heat transfer out of that to the ambient surroundings?

I take it that's 100% E-glycol.

Fill in those blanks and I'll try to help a bit more.

http://virtualpipeline.spaces.msn.com

 

[BigInch]

Ok i see its prop-glycol now. Small print there.

http://virtualpipeline.spaces.msn.com

 

[BigInch]

The toolbox only shows P-glycol properties to 60% solution.

http://www.engineeringtoolbox.com/propylene-glycol-d_363.html

Do you have a solution or is it 100%??? If its more than 60%, I'm not sure if I have the properties or not. If you have them, post'um.

http://virtualpipeline.spaces.msn.com

 

[IRstuff]

Not sure why anyone would run 100% PG...

TTFN

FAQ731-376

 

[BigInch]

found it!

C - REGRESSION FORMULA FOR DENSITY AND
HEAT CAPACITY OF PROPYLENE GLYCOL
rho = (996.5 + 152.3E-2 * x - 96.6E-4 * x2)
+ (-1.7 - 146.1E-2 * x + 76.7E-4 * X2) * 1E-2 * t
+ (-38.4 + 62.1E-2 * x - 30.8E-4 * X2) * 1E-4 * T2

Cp = (4255.5 - 958.5E-2 * x - 941.7E-4 * x2)
+ (-168.9 - 843.5E-2 *
+ (-146.5 - 79.3E-2 * x - 85.3E-4 * X2) * 1E-4 * T2

where x is the weight percentage of propylene glycol in the
solution, and t is its temperature in degrees celsius.

now just need the solution wt%

http://virtualpipeline.spaces.msn.com

 

[BigInch]

is there any worse format than *.pfd ???
make that,

C - REGRESSION FORMULA FOR DENSITY AND
HEAT CAPACITY OF PROPYLENE GLYCOL
rho = (996.5 + 152.3E-2 * x - 96.6E-4 * x^2)
+ (-1.7 - 146.1E-2 * x + 76.7E-4 * X^2) * 1E-2 * t
+ (-38.4 + 62.1E-2 * x - 30.8E-4 * x^2) * 1E-4 * t2

Cp = (4255.5 - 958.5E-2 * x - 941.7E-4 * x^2)
+ (-168.9 - 843.5 E-2 * x -0.0035*x^2)*0.01 * t
+ (-146.5 - 79.3E-2 * x - 85.3E-4 * X^2) * 1E-4 * t^2

where x is the weight percentage of propylene glycol in the
solution, and t is its temperature in degrees celsius.

http://virtualpipeline.spaces.msn.com

 

[BigInch]

finally! there it is.

http://virtualpipeline.spaces.msn.com

 

[krb]

Also chek out builditsolar.com for some additional info on solar water heating systems.

 

[BPLBPL]

Thank you all for the replies - JoeTank, IRstuff, BigInch and KRB

For IRstuff:
I thought about using an internal heatexchanger to simplify the design. But, this option was too expensive(labor+material) and not ideal for long horizontal tanks. Those internal heat exchangers may buckle the tank as they are about 200lb heavy compared to the 30lb external flatplate heat-ex. The 2 pumps on the tank are tiny (1/2 hp). I think my gpm calculations are good. The domestic hot water demand is around 6gpm. Using 40% prop. glycol soln.

For BigInch:
The design flow from boiler to heat-ex2 should be close to 6gpm.
Sorry to have given a temp range. You can select the lower-most temperatures.
The new, brazed flat-plate heatex. are highly efficient, around 90%
Max. heat transfer rate= 260,000 Btu/hr in Summer and 150,000 Btu/hr in the winter.

The 3000gal tank is being built-onsite now and will be well-insulated. So, I'd assume negligible heat loss to the surroundings. The ambient around the tank is 85F.
I will be using a 40% propylene glycol solution. Thank you for the Prop. glycol Cp graph…..very useful data.
My apologies if you had wasted your time becoz of any of these missing information.
------------------------------

The idea of this solar system is to pre-heat the cold water from street(60F) and send it to the boiler. When Solar doesn't produce enough hotwater at 120F, the boiler fires. The design does look complicated but I couldn't simplify it any further

Thanks again....BAGGI

 

[IRstuff]

If the ambient is 85ºF, then why do you need PG? The only reason for messing up the water is primarily to prevent freezing.

TTFN

FAQ731-376

 

[BPLBPL]

IRstuff:
The tank will be in the basement/indoors and the room temp. around the tank is 85F

The pipes carrying the Prop.Glycol soln will be outdoors and are exposed to freezing conditions. Thats why I need PG.

 

[BigInch]

It wasn't a waste of time at all. Very interesting too.
I'm in Spain and its 12pm, so I'll get back 2U tomorrow.

http://virtualpipeline.spaces.msn.com

 

[BigInch]

I only get 3.6 gpm of 120ºF for the fixtures
First problem is at the collector with a 152ºF outlet temp, not 160 F.
P-Glycol
Pct Solution 40%
Inlet Temp 120 F
Est Avg Temp 140 F
Temp 60 C
Heat Rate In 260000 Btu/h
Heat Rate In 72.2 Btu/s
Flowrate 17.5 gpm
Den H2O 62.4 pcf
flowrate 0.039 cfs
SG 0.996
mas rate in 2.422 lbs/s
adding heat 29.82 Btu/lb
Specific Heat 0.9228 Btu/lb-F
dT 32.3 F
Outlet Temp 152.3 F
260000 Btu

Second problem is that you are not using all the heat you are storing in the tank with your low flowrate from the tank to the street water exchanger. Theoretically, that could go to 10 gpm and still get 7 gpm of street water to a temp of 122 F (or 5 gpm to 146 F). But, you'd better count on some losses from the tank anyway.

If you have a 5 gpm flow from tank into the street Xchgr, you can only heat a 5 gpm street water flow at 60F to 103F.

Now, in the summer when there is a lot of heat in the Solar collector AND you're not using water at the fixtures, and street water may be hotter than 60F. you're going to make a whole lot of heat up in the collector, which you can't shut down or that'll melt. You'll have to bring that down into the tank and then reject it somewhere. You may need an air/radiation type heat rejector on the exterior of the tank to do that. (Or can you store it up for winter use? )

http://virtualpipeline.spaces.msn.com

 

[matsx]

I would let vessel to stand upp so that you get more hight for hot water pillow in top.
This will keep your temeratur layers more separated.

 

[zerosum]

As the schematic is drawn, forced convection is going to greatly disrupt stratification.

I worked on some of these in the early eighties. Baffling was involved to conserve the delta T, as were other tricks to inhibit mechanical mixing.

The Solar Energy Society and others produced a great amount of literature on stratification in the middle seventies to middle eighties. An associate of mine did his PHD thesis on stratification in solar ponds.

As drawn, I wouldn't count on differential temperature in the storage tank to assist the ultimate user temps.

 

[zerosum]

If you have access to ASHRAE Handbooks, look under HVAC Applications (one of four Volumes in the series), the chapter "Thermal Storage"\"Thermal StorageTechnologies", specifically 'Stratified Diffuser Design' and there are some references at the end of the Chapter. The ASHRAE HNDBK coverage is just a small piece of the knowledge.

 

[BPLBPL]

BigInch - Thanks a lot for the calculations.

MATSX - Unfortunately, vertical tank arrangement isn't possible because of low ceiling height (this tank will be indoors, in the basement). Thank you

ZOROSUM - I'll check the ASHRAE book immediately. Thank you

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The whole objective of the project is simple - to preheat the street water to 100-120F so that the boiler fires less and saves some oil. But, lots of practical constraints and $$$ issues are making the design complex! Thanks everyone for your help.