thoughts on this sdhw system

bradley

I want to use a conventional electric hot water tank as my solar storage and back up heat.  I took this design from a Home Power article.  Any thoughts?

Charlie from wmass

A proper tank

will give you better control and safer water temps. Also in your drawing you forgot the Bronze pump required to circulate the stored water through the heat exchanger. The cost of this may show why proper tank would be a better choice.

bradley

thanks for the reply.

I was told that the domestic water would circulate through the heat exchanger by thermosiphon, but I have had my doubts about that. You also.  As well, I know I should have a proper tank, but, you know, money.  I do like you motto though.

NRT_Rob

I don't like thermosiphon

for solar. requires hot water, which lowers solar collection.

Charlie from wmass

Why waste when money is tight?

would it not be better to save and get the right tank then have a half working system that will not get you the best return? I can buy a $1,000.00 used work truck and I would save a ton of money. But if I can not show up to do work because my truck is broke down no one will be happy.

zacmobile

electric tank for solar

there are a few combined pumping stations on the market now that have a collector pump, tank pump, heat exchanger & controller in one neat little package for use with a plain storage tank  which could be an electric tank or a purpose built one like the new bradford white solar saver.



http://www.heatlink.ca/en/product/heatlink-heating/sol080v1/sol-80-mbh-solar-heat-exchanger-panel

http://www.bradfordwhite.com/images/shared/pdfs/specsheets/500-B.pdf

Karl_Northwind

thermosyphon

Your plan as listed will work ok, but not if you're building this as a single tank system as in the HP article.  in that case you'll need a second pump (which also means you can use a much cheaper flatplate heat exchanger than the tube in shell you have drawn)  if you can silver braze, you can build your own tube in tube HX that will thermosyphon ok.



you'll have a hard time thermosyphoning if the top half of the tank is already hot.  as a general rule, they're reasonably efficient till the tank gets hot, then the efficiency drops quite a bit, as your thermosyphon slows down as the tank temp rises.



in short: I think your design is OK for a 2 tank system (no other heat input to the solar tank) and should be changed to a double pumped system (with bronze circ and FPHE) if you're still planning on using the upper element.



also make sure your Pressure relief valve is not able to be isolated from the panel either by shutoff or by the check valve. it currently could be isolated.  I'd recommend putting it on the solar line below the heat exchanger.



good luck.

karl

bradley

update

Thanks for all the input.  To update, I ended up using a two tank system, using a conventional electric hot water tank as the solar storage, and using the bottom electric element port as the cold water in.  I also added, as recommended, a bronze pump to move the water through the heat exchanger. 

I seem to be losing some heat out of the tank at night.  Do I need a check valve on the domestic side of the heat exchanger?   I think this is where the problem lies.  If I add a check valve there, will that interfere with the vacation bypass operation around the check valve on the solar side of the heat exchanger?

bradley

pictures

all comments welcome, I have strong shoulders.  Obviously a cramped area.  The hydronic system is closest to the wall.

Kevin_in_Denver_2

Needs more pipe insulation too

And put the check valve as close as possible to the tank.



Why such a small 2nd tank?

bradley

insulation

Going to insulate once any modifications are done,  Small tank (20 gal) because there are only two of us and it was free.

Kevin_in_Denver_2

Minor system suggestion

It looks like the 20gal tank is the backup tank, since I see electricity going to it.

At times of zero call for domestic water, it could cool down and kick on the backup element, even though you have plenty of solar.



The simple solution for this is to raise the 20 gal. tank up higher in the room. Then hot water from the solar tank will thermosyphon thru the backup tank automatically whenever there is solar. In addition, it would prevent any electric heat from ever migrating to the solar tank (not a good thing).

bradley

good suggestion

Yes the 20 is the backup.  Never thought of placing it higher.  This sounds like a great idea.  How high would it have to be? I'm guessing the inlet port would have to be near the top of the solar tank.  Don't know if I can configure that with the limited space.

hot_rod

more check protection

It is a good idea to have a check valve on both lines going up to the collector. Find a solar check designed to be used for high temperature operation, especially on the hot line from the solar to the hx.



Install the pressure relief above the checks as Karl mentioned, and it should also be rated for solar applications. Installing it on the return piping as you show puts it in the lowest temperature fluid.



Ideally the expansion tank should be with the connection pointing up. Pressurize the tank a few pounds below the fill pressure so it holds a bit of fluid to prevent pressure decrease when the temperature in the solar loop drops to winter time conditions.



hr

VictoriaEnergy

Yes, you can.

Yes, you can use a standard electric HWH as a solar storage tank, but using it as a combination storage and back up will reduce the systems solar contribution during marginal conditions. 



When the solar has been inactive for a few days due to poor weather; a preheat tank will cool down to incoming water temperature (48F in my case).  Inevitably as the weather improves, there are often days when the solar gain is diffuse and intermittent.  The cold preheat tank system will start running as soon as the collector hits 50F, the  combined tank has to wait for the collector to get up to 130, and may miss out on most of the limited opportunity. 



Heat can transfer very effectively to the storage tank if the heat exchanger was intended for use with gravity flow(thermosiphon), and the heat exchanger is at the bottom, or below, the storage tank.  You save on the expense of the pump, and the energy to run it.



The Thermo Dynamics Solar Boiler uses a std HWH as a storage tank, and gravity feeds from the heat exchanger to the preheat tank. http://www.thermo-dynamics.com/solar_boiler.html  This system dates back to the late 1980s

Karl_Northwind

thermosyphon

I used to have similar thoughts to NRT rob and Charlie, then I actually sat and watched a sidearm thermosyphon tank run, and while the solar temps were slightly higher (2 identical systems running side by side, one pumped HX, one not. 

the low flow of the DHW on the tank side actually was a bonus if you used hot water during the day, because the stratification inthe tank was quite severe.  top at 130, bottom at 65.  the pumped tank may have been a little overpumped, but not much, and with a really small pump.



having that small amount of 130 F water at teh top of the tank will help when you need small draws off the tank during the day.  for most DHW setups, it doesn't matter because we don't use hot water during the day, mostly morning and evening.



something for everyone.



Karl

NRT_Rob

hmm

but that means the water coming out of the collector has to be 130 degrees.



I would prefer to see that lower, like 100 degrees. big difference in collector efficiency and BTU harvesting, right?



But then I tend to prefer a "boost" strategy so I don't care about hitting a "usability" threshold. as long as the BTUs are collected and used, if it was mixed in with a little gas or electricity I'm generally fine with that... if, over the course of the year, it means we saved more energy by using more solar... which I *think* it should, right?

Karl_Northwind

yup

Sure, you're right rob.  I don't do sidearm HX's as a general rule, except when I'm doing a high mass system, where I have 400 sf of collector and a 100 gal tank (the rest going into the sand bed)



definitely not my first choice for a system, but they were in highest vogue when there weren't many dedicated solar or boiler tanks available. 



cheers,

karl