Horizontal 1500 Gal Solar Storage Tank: Design Suggestions

Alloush

Hi All,



We will be installing 117 m2 of collector aperture area (42 AP-30 Apricus Collectors) for one of our clients in Jordan. The solar field will transfer energy to the pre-heat solar storage tank via an external PHE.



Due to the limitation of indoor space the dedicated solar storage tank will be located outdoors on the roof. Furthermore due to the loading capacity of the roof the storage tank will be mounted horizontally.



We will custom manufacture the tank, thus have the freedom to design as we please. The tank will a potable water pressure rated tank that acts as a preheat tank for the existing DHW system:



Attached is a drawing of a commercially available tank from State Water Heater, and another suggested design. I would like your input on the following:



1. In the commercial design (top), the solar supply and return piping are on the same side of the tank. Would it be better to have the return piping on the other side of the solar supply piping above the cold water inlet ? (The rational is to be mixing a larger portion of the tank and not bypassing one side)



2. To increase the effect of stratification and avoid temperature mixing we are looking to do one the the following.



a. <a href="http://www.plumbingengineer.com/feb_08/biando_feature.php">http://www.plumbingengineer.com/feb_08/biando_feature.php</a> Suggest to "To solve this problem, these tanks can have a baffle welded mid-height across the length inside of the tank, so that the cold water can flow from one end of the tank to the other before mixing with the hottest water."



b. We are also considering having an up-sized ports on the tank and having a short length larger diameter pipe on the solar return and cold water inlet that protrude into the tank with holes drilled into it. The will slow the flow down and fluid will exit the holes at different sections of its length.



Your thoughts and suggestion would be appreciated.



Cheers,

Ala

Larry Weingarten

It's not exactly...

... what you were asking for, but I'd consider not using a pressurized tank on the roof, but rather an unpressurized, site built, with a liner (or plastic) tank and run a large heat exchanger in it on the DHW side to gather up the available heat.  This is likely more durable and weighs less.  Have all connections leave via the top of the tank, so leakage is never a concern. Them's two cents.



Yours,  Larry



ps.  At a minimum, in a sunny location, I'd want about two gallons of storage per square foot of collector, to prevent overheating.  It looks like you have less than that.

Alloush

Thanks

Dear Larry,



Thank you for your input. I agree the storage tank could be larger, but for this install we actually have the demand profile on hand. Its a hotel so they have more constant draw so the 1500 Gal tank work fine.



We have considered an unpressurized tank. We found that the extra pump and hex on the DHW side, the higher flow rate required and scale formation would be a problem for this specific install.



Thanks,

Ala

Mark Eatherton

I concur, somewhat...

I agree that the tappings should be on opposing ends of the tank to increase storage utilization. However, and this is just my gut talking here, my experience with hotel systems is that when they start filling those big washing machines, it doesn't matter whose storage tanks you use, they WILL be exhausted. And as far as stratification is concerned, a BTU is a BTU is a BTU, and its effect on water is pretty straight forward. I understand why people want to get stratification and hotter water out to keep the auxiliary systems off line as much as possible, but in your case with your usage, I think the extra money for the enhanced storage system is a waste.



Speaking in general terms, keeping the storage tank cooler overall, it seems, would lessen tank standby losses, thereby increasing seasonal performance.



I guess I just don't get why people are so enamored with enhancing stratification, when in the grand scheme of things, a BTU is a BTU is a BTU, regardless. And unless there is some mechanical action keeping the tank thoroughly mixed, it is going to stratify on its own anyway.



It kind of reminds me of a customer I had who was doing is own solar experimentation using a magnifying lens and a thermocouple temperature sensor. He had the suns energy focused on a spot the size of a pin head, and was seeing a temperature of 2,000 degrees F. "Look how FANTASTICALLY hot that it gets"... The lens was only 3" diameter. It can only intercept so much energy. Although the temperature is high, the overall potential is the same, low, based on intercepted energy.



As far as collector efficiency is concerned, the cooler the entering water temperature is, the better the collectors perform.



I am a BIG fan of small solar systems. I want EVERY btu that hits the face of the collector to go to good use.



I think I'd go with the commercially off shelf available tank and not worry about the enhanced tank. Follow your gut on the tapping locations. You want to wash as much of that tank as possible with the fluids movement.



ME

Kevin_in_Denver_2

Tank Stratification is very important

Alloush,



Since Jordan doesn't have freezing issues, I agree that you shouldn't introduce a heat exchanger. What is the expected solar fraction?





Mark,



For tiny solar fractions, your logic is correct. Once temperatures and solar fractions increase, however, stratification becomes very important. (As you imply, most hotel systems have low solar fractions, so stratification is moot)



A stratified tank always has better performance because the collector inlet temperature is always lower. In addition, the standby losses of both tanks are the same because the average temperature is the same.



Here's a drawing of a "stratification wand"

http://www.heatinghelp.com/files/posts/2203/stratification%20wand.pdf



You are right that tanks tend to stratify without help, but this device "increases total collection by 8.7% ", which is worth it. However, the testing was done for a vertical stratification wand.



Therefore I'd say go with the manufacturer-designed baffle.

Mark Eatherton

Solar fraction versus system efficiency...

Kevin,



In this case, with the storage and array being kept extremely cool due to large load in comparison to available solar energy, although the "fraction" might be low, the seasonal performance of the "system" would remain relatively high, no? No BTU's left behind ;-)



As it pertains to vertical tank stratification, I've seen the long, skinny, baffled storage tanks from Europe, but how would one enhance the stratification of a tank with an immersed coil in it, like my HTP SuperStore tank? There are a LOT of this style of tank out and about. I'd think that a tank with an immersed coil would have a better chance of subtle stratification than would a tank connected to a FPHXer with a required pump, with or without baffles.



In my system, I also have the ability to exchange water between my 120 gallon PH storage tank with my 60 gallon auxiliary tank. The ENV logic is written, that if the system is in the "Unoccupied" mode, and the solar PH tank is 20 degrees F hotter than the aux tank, a small Taco 003 pump is turned on and fluid exchanged between the two tanks. This avoids the possibility of the aux. tank becoming tepid during unoccupied periods, requiring the use of paid for energy t bring it back up to its 130 degree F "Occupied" setting. The off differential of the tank to tank exchange loop is 5 degrees F, and the sensor is mid tank on the aux. tank. I have no DHW recirc losses to worry about. Just simple static losses.



Thoughts?



ME

Kevin_in_Denver_2

Sounds like a near perfect system design

Mark,



1. You said, "In this case, with the storage and array being kept extremely cool due to large load in comparison to available solar energy, although the "fraction" might be low, the seasonal performance of the "system" would remain relatively high, no? No BTU's left behind ;-)" I agree completely.



2. If you have an immersed collector loop coil in your tank, the tank water will naturally stratify. The quality of this stratification will be greatly enhanced if the flow through the coil is top down.



To quote myself from another thread:

"You want the hottest possible water at the top of the tank in order to send the hottest possible water to the auxiliary tank in order to keep the fossil fuel heat off as long as possible.



Therefore you should plumb top down.



The other happy thing about that configuration is that it sends the coldest possible water back up to the collectors AND ensures the greatest possible amount of tank temperature stratification."



Baffles don't enhance tank stratification by themselves, but they can reduce the mixing effect of the "flow jets" at the incoming ports.



3. This recirculation control logic and implementation is great. I started doing it in 1982 after listening to customer feedback. But since it requires a pump and sensors and a controller and a smart installer, I recommend one-tank systems whenever possible.