Difference between revisions of "Conversation with Alan Rushforth on January 13, 2011"

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(New page: <small>return to Solar Thermal Collectors</small> <small>return to Mass Thermal Storage</small> Alan Rushforth from Rushforth LLC comments: === Demand Requirements === * Load Req...)
 
 
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<small>return to [[Mass Thermal Storage]]</small>
 
<small>return to [[Mass Thermal Storage]]</small>
  
Alan Rushforth from Rushforth LLC comments:
+
===Green Garage questions are numbered, responses below each are from Alan Rushforth from Rushforth LLC:===
=== Demand Requirements ===
+
* Load Requirements / Assumptions
+
** Design Heating Season = Nov 15 - March 15 ... AR: This is pretty radical to get to Nov.15 with no heat, but reportedly this was well calculated with super insulation.
+
** Total Solar Heating Load = Space Heating Load + Domestic Hot Water
+
*** Space Heating Load = 22 million BTU per heating season; 184k BTU/day - AR: Again, by normal standards this is at least an order of magnitude low, but ...we did discuss your super-insulation plan.
+
  
=== Solar Heating Schematic Overview ===
 
* '''Use Drainback w/o Heat Exchangers, Low pressure''' - I would recommend skipping the heat exchanger on the collector loop, and directly pumping the water in the 3,500 gallon tank through the collectors using a drainback design.  It will take a few more watts of pumping power, but the cost and inefficiency of the collector loop heat exchanger would be eliminated. If I were doing the job, I would want to pump the tank water directly through the collectors, no heat exchanger, all draining back to the big tank.
 
* '''Maximum tank temp = 160F''' - I would also be inclined to keep the high limit more like 160F rather than 175F for longest liner life.  Likely the only time it would get the tank to 175 would be in the summer, when you would not need the extra heat then anyway.  My feeling is the key and most useful temperature range for space heating, will be from 80F to 130F.  I doubt solar will get it much over that in the winter.  It just occured to me, if the geothermal is going to dump heat in the tank, during the winter, that will cut the efficiency of the solar collectors, maybe significantly.  This warrants more consideration.
 
* '''Eliminate Small Domestic Hot Water Tank''' - The small solar tank that preceeds the instantaneous heater, can probably be eliminated. 
 
* '''Geothermal Concerns'''
 
* Mixing the geothermal into the solar thermal tank gives me the most concern.
 
=== Solar Collector Design ===
 
* '''Use Flat Panels''' - because of longer life via simpler design.  The prices are about the same (square foot cost of discount Chinese evacs can be roughly equal to flat plates.) but the evacuated tube seals break.  He has 8 tube seals broken on one of his installations right now.
 
* '''Prevent Overheating''' - Option 1 - cover the panels...would like to see someone use the greenhouse tarp screens to rig something that could be manually controlled or controlled automatically from the ground.  Option 2 -  use a heat dump...a radiator or sauna.  You can't just leave the panels empty (drained) as the high heat build up would damage the panel. If you use a drainback, you have a little more leeway to handle occasional stagnations (no glycol to turn acidic), but I would still recommend some overheat protection - could be a good greenhouse/tarp-roll arrangement manually raised and lowered over part of the array - or a heat dump of some sort.
 
* '''Array Configuration'''
 
** Connect the panel headers/footers in series and have the water flow through the panels in parallel - this minimizes the amount of tubing on the roof. If the collectors are mounted in one straight row and connected with high temp silicone heater hose (clamped on) (no solder joints or unions) that would keep things more simple and efficient. 
 
** Low pressure - with drainback it is at atmospheric pressure +/-.  With glycol, probably 15 to 25 psi.
 
** Flow Rate - when you are dealing with a drainback with no heat exchanger on the solar loop side, I feel ''0.5 gpm/collector'' is adequate.  I feel a full 1 gpm is wasting watts and adding wear and tear on the copper collector piping)
 
** Delta T - you will probably find your T in vs. T out is under 15 especially if you have more than about .6 gpm/collector.)
 
** Panels are butted up against one another w/ very short, high temp silicon hoses (1-1/8") connecting the headers/footers on the panels.
 
** Panels can be portrait or landscape
 
** Use insulated [http://www.aqua-therm.com/pages/index.php?pID=23 pex-al-pex] to run/return between the panels and the storage.
 
** Use differential controller - Stucca (located at the storage tank) w/ $10 sensors to determine when to send the water through the panels.  Pleace the sensors in the space at the top of the collector...have it go on when it is > 95F.
 
* '''Collector Positioning'''
 
** I would recommend 5 or 8 degrees West of magnetic South to slightly favor the afternoon sun when it is warmer with lower delta T.
 
** 10 panels at 55 degrees will probably create a lot of summer overheating.  My first thought for this application is more like 65 to 70 degrees.
 
* '''Collector Manufacturers'''
 
** We are now shifting from SolarHot collectors to [http://www.solene-usa.com/chromagen.php Solene Cromagen] - slightly better numbers and equally good pricing - just under $800 for a 4x10. My local distributor, Hickory Ridge Radiant could probably help if there is no one in your area.
 
  
=== Thermal Storage ===
+
===1) What size and type of pex do you run from the tanks to the panels?===  
* '''Storage Size...Bigger is Better'''
+
** With space heating in the equation, the way I see it, the bigger the better, with the limitation being your available space and the tank cost.  On the extreme end would be seasonal storage.  3,500 gallons does not remotely approach that size.  I think 3,500 is an ok start, but if you had space and budget for 5,000 to 7,500 gallons, I would endorse the extra storage.  If the tank is site built, the extra gallons do not have to add proportionally to the cost.
+
** I agree more storage is better - especially in the shoulder seasons.
+
** Hold four days of heat (Rushforth LLC uses 2+ days... and gets much better results than one day)
+
* '''Tank Manufacturers/Construction'''
+
** Have used STSS...good tanks
+
** Build your own tank - With experience, it is possible to site-build tanks similar to STSS, but bigger and better insulated in the $2/gallon or less range  could get the plans and might be willing to help build it.
+
* '''Well Insulate the Tank...R30ish'''
+
** Insulation: R-30 around tank - Unless every inch of space is supercritical, I would suggest more insulation.
+
** This may be overkill to insulate the room the tank is in. If the tank itself is well insulated, there is very little heat loss from it.
+
* '''Use direct Thermal Exchange when possible...'''
+
** Use direct exchange when possible...lowers cost and raises efficiency.  Only use exchangers when they are needed
+
** How about having the radiant floor not have heat coils but just go directly in and out of the tank? AR: This might be a good idea but may ask the contractor to do something they are not accustom to.
+
* '''Use One Tank'''...Yes - one tank not only keeps heat loss area minimized, but it keeps plumbing simpler. With the DHW preheat coil, the instantaneous heater, and the mixing valve, I see no need for the small hot water tank.
+
* '''Questions on Connecting Geothermal'''
+
** Off-peak cooling storage for the geothermal? AR: To use the tank for cooling half the year, and heating the other half, changes everything. You loose the summer hot water. My initial feeling is to nix that idea, or have one hot tank and one cold tank - probably not practical.
+
** Heating: I have a concern about intermingling geothermal heat and solar heat in the same tank.  The solar collectors work best at low temperatures.  Any geothermal heat added into the tank will diminish the efficiency of the solar collectors. Can the geothermal heat go directly into pex lines in the slab and skip the tank entirely?
+
* '''Other Points'''
+
** Should consider a floor drain near the tank.
+
** If there is headroom to raise the floor, has Bob Ramilow's 'High Mass' slab floor storage been considered? This involves and insulated sand bed with pex solar lines, covered with a concrete slab - in his book 'Solar Water Heating
+
  
=== Radiant Floors ===
+
(I thought you ran 1" pex-al-pexAlso I remember you recommending a copper lead right near the panel)
* Though not strictly solar advice, instead of a wood floor, I would consider foam insulation and then a second concrete slab (with pex tubing) over the foam - more thermal mass, more thermal storagePossibly off peak geothermal summer cooling could integrate into the slab.
+
* What are the areas of most risk in our design?  Wood flooring on the concrete to me seems like a potential problem area – termites, rot, labor intensive, etc.  Is there ways to mitigate these risks?  I like concrete over foam – simple, sturdy, good thermal mass, good for pex.
+
  
=== Geothermal ===
+
The collector loop pipe sizing depends on the flow.  Often about .02 gallons /minute/sq.ft. flow is recommended.  That translates to about .8 gal/min for a 4’x10’ collector or about 8 gpm for 10 collectors.  If you are avoiding the efficiency loss of using a heat exchanger in the drainback loop and are taking the water directly from the bulk drainback tank and pushing it directly through the collectors and dumping it directly back in the bulk tank, it is my opinion that you can successfully get away with a bit less flow (like 6 or 7 gpm).  I feel this flow is in a borderline range where you could probably get by with ¾” pipe, or certainly 1” pipe would be plenty big.  Personally I am not one to over sizing just for the sake of it.  Larger pipes have more surface area, more heat loss, more materials, more cost…  If it were just 8 or 9 collectors I would say use ¾” (I just did a system that is working great with 9 of these exact same 4x10 panels per row fed by ¾” per row).  If it were 11 or more collectors I would  advise 1”.  If your other designers or plumbers have no strong opinions, I would recommend 1”, and that way no one can accuse anyone of undersizing.  
* Mostly sounds good to me although mixing the geothermal into the solar thermal tank gives me the most concern.
+
  
=== Suggested Reading ===
+
Regarding Pex/al/pex vs. copper, the conventional wisdom for solar hot water systems is to always use copper and never use pex.  IF (and only if) you are doing a non-pressurized bulk drainback AND IF the last 3’ of the feed and return (where connecting to the collector array) are plumbed in copper (to keep stagnation temps away from the pex/al/pex), I believe one can successfully use pex/al/pex in such a situation, and have done so repeatedly with good results.  However in your case, this is a 1 story building.  The collector pipe runs will hopefully be relatively short.  It might just be simplest (and even less expensive) to have your plumber just run these short runs in 1” copper.  You save tracking down pex/al/pex and special adapters, the plumber is happy using copper he is very familiar with, and no one will ever challenge an all 1” copper collector loop as substandard. 
* Tom Lane Lessons Learned Solar Thermal heating (Florida)
+
 
* Chuck Marken - Home Power Magazine
+
===2) About 5 - 10ft of this pex ewill be xposed on the roof, what do you use to insulate it with?===
 +
 
 +
Regardless of whether the piping is pex/al/pex or copper, it should be insulated with at least ¾” Armaflex type insulation (the better quality black stuff available from refrigeration supply houses – not the cheap grey stuff from Home Depot.).  This stuff must be protected from sunlight.  This can be done with PVC drain pipe or PVC conduit for straight runs.  On bends and odd connections one can wrap with 6” wide aluminum flashing wrap (sold at Home Depot – roofing department).  There is a newer (more expensive) version of the black foam insulation that is rated for exterior exposure to sunlight, where you can skip the wrapping.  http://www.sandsinsulation.com/HTML/pipe-insulation.html is one source for this and they will ship small quantities.  I have used it and it saves a lot of time, but I cannot say from experience if it will hold up for 20 years. 
 +
 
 +
===3) What brand / model temperature gauge do you use to sense the temp of the panels to start the flow?  You recommended starting flow at 90F, is that right?===
 +
 
 +
I normally use the basic Stecca TR0301 controller ($150ish range).  It is prewired to plug into the wall and has an outlet to plug the pump into (if you have a plug cord wired to your pump).  Often one can avoid the need for an electrical permit (for the solar system) with the prewired plug-in controller and pump.  One source to order this controller from is http://www.sunsourceproducts.com/  You can get it with 2 or 3 sensors depending on what you want.
 +
 
 +
Regarding the 90F start temp, normally I would NOT have such a start temp restriction.  Our systems are normally controlled solely by the differential controller.  In our solar tanks (used just for domestic preheating) temps can get quite low in the winter.  On poor solar weeks (dark and freezing) with incoming street water as low as 38F, and high apartment building water use, the big solar tank temp can on occasion get drawn down quite low – 60 F or even lower.  If the collectors are 12F hotter (or whatever the differential is set at) the pump will turn on and start harvesting BTUs even if the collectors are only 75F or 80F.  However I read through most of your write-up and you have some extra complications with the heat pump, dual tanks, plus you are looking for some redundancy in the controls to assure no freeze problems.  So the shut down feature below 90F may make sense.  To be honest, I would have to dig much deeper to weigh in more definitively on this.  Hopefully your other experts have this worked out reasonably well.   
 +
 
 +
===4) What is the size and type of drainback tank you recommend?=== 
 +
(We have 10 Aurora 4x10 panels.  Or give us the way you calculate it.)
 +
 
 +
I would recommend skipping the small drainback tank and using your big 2000 or 3000 gallon tank as the drainback tank.  I will do my best to relay a concept from a very talented engineer I met:  “It is easy to design something complicated, and hard to design it simple.”  Unless you have some unusual situation like a high rise building with a very long high pipe run, or a situation where the array is below the bulk storage tank, I would aim to use the bulk storage tank AS the drainback tank.  I think you should be able to do that here.  If this does not make sense, we can talk about this on Friday.   
 +
 
 +
 
 +
===5) What brand and type of pump do you typically use?  What is the normal range for the size of pump?=== 
 +
(We are lifting the water about 15 feet.  We'll have our Mechanical contractor do the final sizing...just want to make sure we're thinking about it right.)
 +
 
 +
On an atmospheric vented bulk drainback (where you will have some oxygen in the water) we will use bronze or stainless steel circulating pumps.  We have used Taco, B&G, Grundfos, Armstrong, and Laing.  Sometimes 2 will be needed in series.  I can’t really say that any brand is a whole lot better than the others (although Laing just makes small ones).  For your purposes you could probably do a lot worse than to use one bronze Taco 009.  One of these hould have plenty of head (about 32’) for your application.  My guess is it will pump about 7-8 gallons/minute through your pipe circuit (once air is pushed out and return flow stabilizes), which would be about right.   
 +
 
 +
===6) Are there any other controls/sensors you use in your design for flow rates, leak detection, heat build up, BTU's generated, etc.?===
 +
 
 +
You may not need this, but for data monitoring, charting, BTU generation for domestic hot water and internet data transfer, I use an $800ish unit made by a small local (to me) company, Emerald Electronics plus a $200 - $300 (depending on size) pulse meter from watermeters.com.  With you/your people being on site regularly, and with you having lots of other monitoring and control systems, you may not need an extra system and extra complication like this. 
 +
 
 +
 
 +
===7) Who is the supplier of your heat exchangers for your tanks?===
 +
 
 +
To get the maximum performance for the least money, I/we make our own submersed heat exchangers typically from soft ¾” x 100’ L copper coil.  The single line diagram I sent earlier showed an example of how that is plumbed.  The hard part with those copper coil heat exchangers is figuring out clean simple efficient ways to support the copper inside the big tank.  We have a few approaches plus a new variation I plan to adopt.  None are rocket science – just a little ecommon sense and ingenuity.
 +
 
 +
That's it.  With this info we can have a local mechanical engineer design it.
 +
 
 +
===Alan Rushforth's 2 additional comments are:===
 +
 
 +
I don’t see the need for the pressure alarm on the return line.  If the return line is dumping into the top of an atmospheric vented bulk drainback tank, I would predict no positive pressure in this line (maybe even a slight negative pressure) while there is flow.
 +
 
 +
I think you will find the PV covers for the thermal panels as a nifty idea, but not a practical idea.  If you can afford PV, I would put it in its own proper permanent location. 
 +
 
 +
 +
 
 +
 +
 
 +
*Alan Rushforth
 +
 
 +
*Rushforth Solar LLC
 +
 
 +
*3700 Darby Road
 +
 
 +
*Bryn Mawr, PA 19010
 +
 
 +
*P: 610 520 1968
 +
 
 +
*F: 610 520 1969
 +
 
 +
*C: 215 620 6345
 +
 
 +
*AR@RushforthSolar.com
 +
 
 +
*www.RushforthSolar.com

Latest revision as of 15:32, 14 January 2011

return to Solar Thermal Collectors

return to Mass Thermal Storage

Green Garage questions are numbered, responses below each are from Alan Rushforth from Rushforth LLC:

1) What size and type of pex do you run from the tanks to the panels?

(I thought you ran 1" pex-al-pex. Also I remember you recommending a copper lead right near the panel)

The collector loop pipe sizing depends on the flow. Often about .02 gallons /minute/sq.ft. flow is recommended. That translates to about .8 gal/min for a 4’x10’ collector or about 8 gpm for 10 collectors. If you are avoiding the efficiency loss of using a heat exchanger in the drainback loop and are taking the water directly from the bulk drainback tank and pushing it directly through the collectors and dumping it directly back in the bulk tank, it is my opinion that you can successfully get away with a bit less flow (like 6 or 7 gpm). I feel this flow is in a borderline range where you could probably get by with ¾” pipe, or certainly 1” pipe would be plenty big. Personally I am not one to over sizing just for the sake of it. Larger pipes have more surface area, more heat loss, more materials, more cost… If it were just 8 or 9 collectors I would say use ¾” (I just did a system that is working great with 9 of these exact same 4x10 panels per row fed by ¾” per row). If it were 11 or more collectors I would advise 1”. If your other designers or plumbers have no strong opinions, I would recommend 1”, and that way no one can accuse anyone of undersizing.

Regarding Pex/al/pex vs. copper, the conventional wisdom for solar hot water systems is to always use copper and never use pex. IF (and only if) you are doing a non-pressurized bulk drainback AND IF the last 3’ of the feed and return (where connecting to the collector array) are plumbed in copper (to keep stagnation temps away from the pex/al/pex), I believe one can successfully use pex/al/pex in such a situation, and have done so repeatedly with good results. However in your case, this is a 1 story building. The collector pipe runs will hopefully be relatively short. It might just be simplest (and even less expensive) to have your plumber just run these short runs in 1” copper. You save tracking down pex/al/pex and special adapters, the plumber is happy using copper he is very familiar with, and no one will ever challenge an all 1” copper collector loop as substandard.

2) About 5 - 10ft of this pex ewill be xposed on the roof, what do you use to insulate it with?

Regardless of whether the piping is pex/al/pex or copper, it should be insulated with at least ¾” Armaflex type insulation (the better quality black stuff available from refrigeration supply houses – not the cheap grey stuff from Home Depot.). This stuff must be protected from sunlight. This can be done with PVC drain pipe or PVC conduit for straight runs. On bends and odd connections one can wrap with 6” wide aluminum flashing wrap (sold at Home Depot – roofing department). There is a newer (more expensive) version of the black foam insulation that is rated for exterior exposure to sunlight, where you can skip the wrapping. http://www.sandsinsulation.com/HTML/pipe-insulation.html is one source for this and they will ship small quantities. I have used it and it saves a lot of time, but I cannot say from experience if it will hold up for 20 years.

3) What brand / model temperature gauge do you use to sense the temp of the panels to start the flow? You recommended starting flow at 90F, is that right?

I normally use the basic Stecca TR0301 controller ($150ish range). It is prewired to plug into the wall and has an outlet to plug the pump into (if you have a plug cord wired to your pump). Often one can avoid the need for an electrical permit (for the solar system) with the prewired plug-in controller and pump. One source to order this controller from is http://www.sunsourceproducts.com/ You can get it with 2 or 3 sensors depending on what you want.

Regarding the 90F start temp, normally I would NOT have such a start temp restriction. Our systems are normally controlled solely by the differential controller. In our solar tanks (used just for domestic preheating) temps can get quite low in the winter. On poor solar weeks (dark and freezing) with incoming street water as low as 38F, and high apartment building water use, the big solar tank temp can on occasion get drawn down quite low – 60 F or even lower. If the collectors are 12F hotter (or whatever the differential is set at) the pump will turn on and start harvesting BTUs even if the collectors are only 75F or 80F. However I read through most of your write-up and you have some extra complications with the heat pump, dual tanks, plus you are looking for some redundancy in the controls to assure no freeze problems. So the shut down feature below 90F may make sense. To be honest, I would have to dig much deeper to weigh in more definitively on this. Hopefully your other experts have this worked out reasonably well.

4) What is the size and type of drainback tank you recommend?

(We have 10 Aurora 4x10 panels. Or give us the way you calculate it.)

I would recommend skipping the small drainback tank and using your big 2000 or 3000 gallon tank as the drainback tank. I will do my best to relay a concept from a very talented engineer I met: “It is easy to design something complicated, and hard to design it simple.” Unless you have some unusual situation like a high rise building with a very long high pipe run, or a situation where the array is below the bulk storage tank, I would aim to use the bulk storage tank AS the drainback tank. I think you should be able to do that here. If this does not make sense, we can talk about this on Friday.


5) What brand and type of pump do you typically use? What is the normal range for the size of pump?

(We are lifting the water about 15 feet. We'll have our Mechanical contractor do the final sizing...just want to make sure we're thinking about it right.)

On an atmospheric vented bulk drainback (where you will have some oxygen in the water) we will use bronze or stainless steel circulating pumps. We have used Taco, B&G, Grundfos, Armstrong, and Laing. Sometimes 2 will be needed in series. I can’t really say that any brand is a whole lot better than the others (although Laing just makes small ones). For your purposes you could probably do a lot worse than to use one bronze Taco 009. One of these hould have plenty of head (about 32’) for your application. My guess is it will pump about 7-8 gallons/minute through your pipe circuit (once air is pushed out and return flow stabilizes), which would be about right.

6) Are there any other controls/sensors you use in your design for flow rates, leak detection, heat build up, BTU's generated, etc.?

You may not need this, but for data monitoring, charting, BTU generation for domestic hot water and internet data transfer, I use an $800ish unit made by a small local (to me) company, Emerald Electronics plus a $200 - $300 (depending on size) pulse meter from watermeters.com. With you/your people being on site regularly, and with you having lots of other monitoring and control systems, you may not need an extra system and extra complication like this.


7) Who is the supplier of your heat exchangers for your tanks?

To get the maximum performance for the least money, I/we make our own submersed heat exchangers typically from soft ¾” x 100’ L copper coil. The single line diagram I sent earlier showed an example of how that is plumbed. The hard part with those copper coil heat exchangers is figuring out clean simple efficient ways to support the copper inside the big tank. We have a few approaches plus a new variation I plan to adopt. None are rocket science – just a little ecommon sense and ingenuity.

That's it. With this info we can have a local mechanical engineer design it.

Alan Rushforth's 2 additional comments are:

I don’t see the need for the pressure alarm on the return line. If the return line is dumping into the top of an atmospheric vented bulk drainback tank, I would predict no positive pressure in this line (maybe even a slight negative pressure) while there is flow.

I think you will find the PV covers for the thermal panels as a nifty idea, but not a practical idea. If you can afford PV, I would put it in its own proper permanent location.



  • Alan Rushforth
  • Rushforth Solar LLC
  • 3700 Darby Road
  • Bryn Mawr, PA 19010
  • P: 610 520 1968
  • F: 610 520 1969
  • C: 215 620 6345
  • AR@RushforthSolar.com
  • www.RushforthSolar.com