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
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Storage Tank dimensions: 16' long, 4.5' dia approx, ~3000gal

 

[JoeTank]

Just a random thought. Be sure the entry velocity of the cold water is as low as possible or use some sort of internal header so as to not promote mixing.

Joe Tank

 

[IRstuff]

Seems like a lot of complexity to get only a 60ºF rise, considering that you appear to be throwing away the remaining 40ºF delta.

I guess you also have 2? recirculation pumps, in addition, you have a glycol coolant loop? Running these pumps and maintaining a glycol coolant system would seem to be a big cost driver.

Why wouldn't you just embed the use-side exchanger inside your storage tank? You could potentially eliminate one pump.

What's the actual demand? I would think that one might expect upwards of 6 gpm on the user side. Is the 5-gpm loop enough flow to cover the demand?

TTFN

FAQ731-376

 

[davefitz]

my comments:

a) ensure the design meets the local plumbing code, espesc regarding leakage of non-potable water acors the plat frame HX into the potable water

b) need air vents and possible thermal relief valve on tanks etc,- air or gas pockets will starve the feed to your 140 F stream to the HX

c) install an internal baffle, horizontal , with small drain holes . hot water on top of baffle, cold water dishcrges and feeds under baffle. This minimizes mixing of hot vs cold, and allows you to drop the elevation of the 140 F feed to avoid entraining gas.

d) awfully complicated to heating water. 140 F is pretty warm stuff , might only need it that hot for laundry using old fashioned detergents. Can't you use a eutectic salt for heat storage?

 

[BPLBPL]

Thank you all for the replies - JoeTank, IRstuff, DaveFitz

For DAVEFITZ:
I'll definitely use double-walled heat-ex. on the potable waterside and airvents to avoid the system from getting air-bound.
The internal HORIZONTAL baffle is a good idea, thank you, will work on that.
130-140F is pretty common in solar storage tanks. Eutectic salt is an unknown territory for me and guess will complicate the project further. Also may be expensive compared to water heat storage.
-----------------------------
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]

Hmm... Your drawing doesn't really show that particular detail; it implies that the heated water bypasses the boiler.

After looking at your drawing again, it's still not clear why you need 4 loops instead of 2. Why can't the storage fluid be the same as the fluid in the solar collector? Is there problem with running the PG as the storage fluid? Then, you could get rid of P2. Also, if you got rid of the P2 loop, you'd get 20ºF back, and your mixing problem wouldn't be as signficant.

And, still, why does the P3 loop need to be external? If EX2 was inside the storage tank, you'd get rid of P3 as well.



TTFN

FAQ731-376

 

[BPLBPL]

Hi IRStuff,
a) Internal heat-ex arrangement costs $20,000(labor+material etc) compared to external heat-ex $8,000. Other issues with Internal heat-ex are chances of leaks, tank-buckling etc

b) Prop. glycol cannot be used as the storage medium in the tank because it costs $15/gallon in USA. The tank volume is 3000+ gal --> $45,000 for the heat-transfer fluid alone!!
By using PG only in the solar loop, the cost is approx. $4000.