Re: Solar Hot Water Systems


Philip Timmons
 

Drain back is the fail-safe aspect?   Gravity works and all.  

And there is no solenoid or check valve limiting return flow?

So to have a freeze failure would require that the heated space would have to go below freezing?

But these are not a fully sealed system, anyway?  Asking for the over-pressure consideration of going to steam.  Should at least have a TP (temperature - pressure) release valve, like a common water heater?   or an actual air-break would do, since there is not really cause to go over boiling (steam)?  




On Sunday, May 1, 2022, 10:44:51 AM CDT, Dan Lepinski, P.E. <dan@...> wrote:



> Anti-Freeze is for the Freezing aspect, right?

No.

Anti-freeze in a "drainback" configuration serves multiple purposes,
but not freeze prevention.  Drainback systems are inherently freeze-proof
by design.

Anti-freeze is a chemical "buffer" to protect the metals.  Collectors are made of copper tubing.  Pumps are made of brass, bronze, and steel.  Different metals in a system create a very weak "battery", with the electrical current being conducted by tap water.  Anti-freeze prevents corrosion for several years, but not forever.

Anti-freeze raises the boiling point of the water.  When our system turns on and begins circulating the water, the tubing in the collectors is often hotter than the boiling point of water.  As the water begins to circulate in the system, and it comes into contact with the very hot metal in the collectors, ordinary water without anti-freeze will boil when it comes into contact with the very hot metal.  This in turn can increase the pressure in that aspect of the solar hot water system.  These short-term pressure increases can be accommodated by proper design of the system, but this exceeds the scope of my explanation at this time.

Use of "steam distilled" water (with no minerals) alleviates all of these issues, and requires no future maintenance or service.  This is NOT "de-ionized" water or anything similar.  It's "steam distilled", water with zero minerals.  Distilled water does not conduct electricity.


> How about this . . . Distilled Water with  Anti-Freeze in it?

Use of distilled water eliminates the need for anti-freeze.  You COULD use anti-freeze with distilled water, but why do it?  It's not needed.  You'll add unnecessary expense and maintenance to the system, and will have reduced the efficiency of the system.  Pure distilled water transfers heat better than a water and anti-freeze solution.


Dan




> Or since corrosion is the issue -- use non-corroding materials?  -- eg, plastic, copper, stainless?

Pumps, valves, and other hardware have metals other than copper, and can't be eliminated.


Dissimilar metals and a conductive liquid create a battery.  It's a weak battery .. but it generates a current just the same.  Tap water conducts the weak electrical current that eventually destroys the metals.  It may take years, but the damage will eventually show up.  Since distilled water does not conduct electricity, corrosion isn't an issue.  This also eliminates the need for the anti-freeze.  When high temperatures are involved, such as in commercial hot water systems or solar concentrators, special oils and other substances are used as the heat transfer fluid.


Dan


On 5/1/22 9:45 AM, Philip Timmons via groups.io wrote:
> (sorry, hit send)
>
> Anti-Freeze is for the Freezing aspect, right?  
>
> Simple Distilled Water will still freeze -- so the equipment is at risk for mechanical (freeze) damage?
>
> How about this . . . Distilled Water with  Anti-Freeze in it?
>
> Or since corrosion is the issue -- use non-corroding materials?  -- eg, plastic, copper, stainless?
>
> Thanks (looking at doing some serious agriculture work in this domain -- so I am not so inclined to let the Solar Thermal die-off -- at least at the design pondering level)  
>
>
>
>
> On Sunday, May 1, 2022, 09:41:06 AM CDT, Philip Timmons <philiptimmons@...> wrote:
>
>
> Super Dan review -- since we gots this one open and a-going . . . 
>
> On Sunday, May 1, 2022, 08:49:46 AM CDT, Dan Lepinski, P.E. <dan@...> wrote:
>
>
>
>> On 4/30/22 8:26 PM, Jim Duncan wrote:
>> 
>> but the fluid eventually broke down like they tend to do over time
>> when they overheat.
>
>
> Since solar hot water systems aren't a commonly discussed topic, particularly among the NTREG Conversation Group members .. some explanation will help those who aren't familiar with solar hot water system construction or operation.
>
> Solar hot water systems can be constructed in several different configurations.  In all cases, the aspect of the hot water system that heats the water using sunlight is called a "solar collector".  Solar collectors are analogous to solar electric panels.  However, instead of generating electricity, solar collectors heat a fluid (typically water or water combined with anti-freeze).  The fluid is referred to as a "heat transfer fluid", or "HTF".
>
> One solar hot water system configuration that's very suitable for north Texas is called a "drainback" system.  When the system isn't operating, such as at night, the pumps are off, and the fluid "drains back" into a small storage tank.  In such cases, the solar collectors are empty of any fluid.  This makes a drainback system ideal for our area because it's freeze-proof.  When the sun is high enough in the sky to heat the collectors, pumps turn on and circulate the fluid through the collectors.  A "heat exchanger" then transfers the heat in the fluid into the water we use for washing and other purposes.  Because drainback systems are freeze-proof, no anti-freeze is needed.  More on this in a moment...
>
> In other configurations, the fluid is always present in the solar collectors, and is circulated through the solar collectors only when it's hot enough to be of use.  During freezing weather, the anti-freeze in the fluid prevents the water from turning to ice and destroying the collector plumbing.
>
> In all cases, the heated water is stored in a tank such as an ordinary water heater or other tank designed specifically for solar hot water systems.  When the water in the tank reaches a predetermined maximum temperature, the pumps are turned off to prevent overheating.  It's all electronic .. and automatic.
>
> In solar hot water systems, anti-freeze is often mixed with tap water in the heat transfer fluid to prevent degradation of the plumbing due to corrosion when different metals are present in the system, such as copper and galvanized pipe.  Anti-freeze also degrades with time and temperature, and must be replaced with fresh anti-freeze every few years.  If this isn't done, the anti-freeze breaks down and is no longer effective.  (Same reason we get the cooling systems serviced in our vehicles.)  Systems don't need to
>
> The need for anti-freeze in drainback systems can be completely eliminated by using distilled water as the heat transfer fluid.  Distilled water doesn't conduct electricity the way tap water does.  Distilled water also doesn't have any minerals in it.  Subsequently, for both of these reasons, corrosion doesn't occur.  Our home solar hot water system uses distilled water for the heat transfer fluid.  It's been there for more than 15 years, and is still as clear as the day I filled the system.  No leaks.  No corrosion.  No anti-freeze to "wear out".
>
> Solar hot water systems don't need to "overheat" to degrade the anti-freeze (when present).  The simple actions of heat and time break it down.  In a properly operating system, this takes several years.  Use exclusively distilled water, and this problem doesn't happen.
>
>
> Dan
>
>
>
> On 4/30/22 8:26 PM, Jim Duncan wrote:
>> Philip, You may recall that 4-tracker install near Glen Rose back in 2006 you worked on with me. The owner put 2 solar DHW panels on the greenhouse adjacent to the inverter room for a radiant heat floor of the green house side. There was plenty of heat but being south-facing the system was hardly run at all. It was a drainback system but the fluid eventually broke down like they tend to do over time when they overheat. Otherwise it  did keep the room warm and steamy.....Jim
>>
>> Jim Duncan
>> Solar Acres Farm
>> 817.917.0527
>> solarguy2004@... <mailto:solarguy2004@...>
>>
>> On 4/30/2022 10:29 AM, Philip Timmons via groups.io wrote:
>>> Has the general consensus become that Thermal (Solar) Collectors are not worth the time / effort? 
>>>
>>> Asking for both Water and Concrete Floor Heat.
>>>
>>> Thanks.
>>>
>>>
>>> On Saturday, April 30, 2022, 09:59:15 AM CDT, Mark Witte <witte.m@... <mailto:witte.m@...>> wrote:
>>>
>>>
>>> Folks,
>>>
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>>>
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>>>
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