Communications with Brad @ STSS

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return Thermal Storage Design

On Fri, Mar 20, 2009 at 2:09 PM, STSS CO INC <> wrote:

OK, guys, I've got everything all set for you. I consulting with Sven, our founder/owner/president, who's been doing this for 33 years, and I've attached your quote with our recommendations.

For your solar input coils, we recommend two of the 90/90 stacked coils (all plumbed in parallel with your solar loop). Each of these is actually two 90 ft coils stacked one on top of the other (brazed together, of course, so they don't topple over). The benefit of using this coil versus one long 180 ft coil is that there is significantly less pressure loss than the single 180 ft coil but still transfers heat like 180 ft of copper. You'll need two of these stacks, so there will essentially be four "ins" and four "outs" from the 3/4" copper coils. Don't know the heat transfer assumptions. What is the capacity of each coil ...BTU/hr for what delta-T?

55k/hr for 135 degree input temp. Brad estimates 75k for 150 degree input temp

DHW is pretty straightforward. The 180 ft coil will provide 3 gpm of fully preheated water, and will transfer about 87k BTU/hour at a tank temperature of only 120F. With a tank temp of 120F, how much hot water is available at what temp. Assume the T-in of the water is 50F.

temperature assumption for the 3gpm is 47 deg in 105 degree out.

The tank connections for the geothermal heat pump is pretty straightforward, too. There will be two 1" flanges below the water line, one up high and one down low, to run the actual tank water through the heat pump in an open loop. Since it won't require heat exchanger coils, running this as an open loop will save money. Agree.

With regards to the radiant floor heating, I got your phone message, Joe, that said you'll be able to run the system to effectively heat at 90F degrees instead of 130F. That's great, because then you can use our patented copper/PEX coil. This coil does a great job at transferring heat at temperatures below 110F (is this the tank temp or the radiant floor fluid temp...or both? (However, at temperatures above 110F, the PEX acts as an insulator, so this coil is not good at all for other types of space heating or solar input.) In addition, because it isn't made of all copper, the cost is significantly lower than a full copper coil.

the 110 degree represents a threshold at which the pex looses heat transfer efficiency. Once the fluid in the pex reaches this temp, it slows down heat absorption from the tank fluid.

To cover your heat load, we recommend using six of the copper/PEX coils, plumbed in parallel. This will safely cover about 32 gpm of flow, which means the coils could fully transfer heat to as many as seven heating zones (at 4.5 gpm per zone) at one time with a delta-T of 14F degrees (which is pretty typical for a radiant floor heating setup). If all ten zones, in fact, were to call for heat at the same time, the delta-T would end up being around 10F degrees, so it really wouldn't be bad if that did occur. At a 110F tank temperature, these coils will be able to transfer about 225k BTU/hour 37.5k BTU/hr for each coil exchanger?, which is plenty for your heating load.

For tank size, we recommend the 1205 gallon tank. With a 75F degree delta-T (170F -> 95F) when does this come into play if the tank is kept at 110F?, this tank can store 753k BTU. Even with your peak heat load of 140k BTU/hour, this is almost 5.5 hours of heat storage. (i.e. If your solar collectors aren't bringing in any heat, and your geothermal heat pump isn't running, you can fully heat the building just from the tank on the coldest day of the winter for 5.5 hours.) If you charge up the storage tank at night with the geothermal heat pump, and the solar will start bringing in heat at 9am, then you would theoretically be able to shut off the geothermal heat pump with a fully-charged tank at 3:30am and be able to fully heat the building just from the tank on the coldest day of the winter until 9am.

the 2000 gal tank, which is 8'4"d x 6'h is $3653 the 2500 gal tank which is 10'd x 5'h is $4660

Based on the information provided about the office building and published data on the collectors, I estimate about a 25%-30% solar contribution. (i.e. This will cut your fossil fuel usage by 25%-30%.) Is this using our peak hourly data or our average hourly data?

With regards to seasonal mass storage that you asked about, Sven was a good resource of knowledge. He said that some places have implemented systems like this; some work well (please provide a referral on these if possible), some don't, and all of them cost a fortune. If you consider the fact that a 1205g tank that is replenished daily with solar and geothermal heat, imagine how large a storage tank you would need to hold weeks and weeks of heat for the building. The cost of that tank, shipping, assembly, installation... not to mention the logistics and labor of dealing with a tank that large... It would be astronomical. Sven can certainly go into more detail if you need specifics since he is more familiar with these setups than I am. At $9000 for our setup, I really think it's a heck of a deal. Do federal tax credits apply to the tanks? And I'm not just saying that because I work here. :)

Federal tax credits do apply when tank is used in energy saving system. see separate email from Brad.

They do not have anyone to refer us to regarding mass storage. While some buyers may have talked about it, STSS has not followed up or kept any records for tanks used for this purpose.

I'm going to check out our client list to see if I can find someone who has done similar types of designs/installations, and I'll get back to you in a later email. This would be helpful.

Thank you very much, and talk to you soon! Thank you!