size of tank for solar collectors

[Tess]

we are installing solar showers for a campsite complex. we have ordered 60 evac tubes from thermomax. my concern is the size of the hot water tank. we have already got an old milk pasturising tank which is well insulated but the capacity is around 1200l. as they are campsite showers we could get push button showers and have temp around 40 in tank as 60 tubes should heat 600l to above 80, therefore shouldn't they heat 1200l to 40? not sure whether to decrease size of tank and have cold water feed to adjust water temp or just have warm water with no cold feed. we will have additional heat exchangers connected to wood chip burner for during the winter.

[wdyasq]

"as 60 tubes should heat 600l to above 80, therefore shouldn't they heat 1200l to 40?"

I think you should be working with Kelvin rather than Celsius/Centigrade. The temperature rise will be calculated from those numbers.As you did not give a base number it is the best one can do from the information suppled.

Ron

 

[Tess]

well ok, what do mean by base number?, when calculating i use kelvin when talking to people i use celsius. the main question was can a shower system have a lower temp than a conventional domestic hot water system as it will be used for showers only. the tank could be set at 40 celsius (310K) with push button showers and use the entire tank or only use 600l and have hot water mixed with additional cold water and determind by the person in  the shower. the first being the most preferable but i am unable to find examples of this. thanks, tess

[wdyasq]

Is the temperature coming into the collectors 5C? or is it 50C? As the energy to raise the water X degrees C/K will be the same it CAN be calculated. However, with out that number I don't know anyone who can honestly calculate a number. There are still things that will need to be ignored.

Unfortunately, I left my tin hat and clairvoyance device in my other suit. There is no estimate your input water temperature without someone telling us or knowing a location. Water (ground) temperature can vary 30C over the United States.

Total energy will remain the same. Perhaps your tube supplier can give you more information. If they can't, someone there is stupid, ignorant or both.

Good luck,

Ron

[Stonebrain]

What Ron is trying to say(in his way):

What counts is not the final temp,but the temprise.

To know this,you need the temp the water comes in you system.

So,if the water comes in at 15°C and you rise t° to 80° you have

65° rise for 600l with the heat you have available.

The amount of heat you need to rise 1l water with 1 degree is constant.

So,with the same amount of, heat you can rise the t° for 1200l with32.5°

so the t° will be 15+32.5 = 47.5°C

The same story for °K °F or °C

cheers,

stonebrain

[Nando]

TESS:

You are sinning !!

When asking for comments or suggestions, One needs to supply DETAILED information for the responder to be able to get accurate suggestions or at least suggestions based on "concrete foundations".

no body here can read your mind, to determine exactly what you have and what you have done and what can be done IS YOUR JOB.

Use Kelvin or Celsius and show what you done and/or expect, this way the help will be give out intelligent information that you can use in your project.

With the information you have supplied it would be very difficult to assist you, in a practical way.

SUPPLY technical info of the Thermomax, mostly not body have or keep such info, area where you are with WATTS/M^2, tanks and connections, water basic temperature and any other facts that may assist those trying to give suggestions or solutions to your problem.

for this reason, I have not replied to your message.

Nando

[TomW]

stonebrain;

I think people get confused and think the C and F scales are linear. They are not. As in if you raise the temperature from 10 to 20 [C or F] it is not "2X the original temperature" Kind of got caught by that myself before is why I toss it out there. Other scales are linear I think but not my area of knowledge.

Cheers.

TomW

[Stonebrain]

"As in if you raise the temperature from 10 to 20 [C or F] it is not "2X the original temperature"

That's not what I said.

I meant: heating from 10° to 30° cost twice the energy it cost to rise it from

10° to 20°.This mean I think it cost the same energy to rise the t° from a certain amount of water from 10° to 20° as from 20° to 30°.

If you say the scale F or C are not lineair,I suppose you mean this is wrong.

I'll look this one up.

If you don't do the same,this means you are very sure..  

cheers,

stonebrain

 

[dinges]

Very close, Tom, but let the nitpicking begin:

It's not the linearity that is an issue here; all the main temperature scales are linear (deg. C, deg. F and Kelvin).

However, they have different 'zeros'. Both the 'zero' of the Celsius and Fahrenheit scale are completely arbitrarily chosen (for deg. C. it's the freezing point of water; could've chosen the freezing point of oxygen as well, or of...)

There are 4 different kind of scales:

• Nominal scale. For example, Female/Male would be on a nominal scale
  
• Ordinal scale. For example, soldier/sergeant/lieutenant/ would be on this scale.
  
• Interval scale. Degrees Celsius and Fahrenheit are on that scale. This is a linear scale, but with an arbitrary 'zero' or reference
  
• Ratio scale. The Kelvin temperature scale is a ratio scale.
  
  

ONLY w.r.t Ratio scales can one say that, when the value is twice as high that the thing that is being measured is twice as much.

Is 20 deg. C two times as hot as 10 deg. C ? No.

Is 100 deg. F. two times as hot as 50 deg. F? No

Is 293 K (20 deg.C) two times as hot as 147 K (-127 deg. C) ? Yes.

All the above scales are linear, but only the Kelvin scale has a non-arbitrary zero point; in nature, no temperature can be lower than 0 K. 0 K is the reference.

</lecture>

Class dismissed.

Pst, Tom, here's a link to wikipedia...

http://en.wikipedia.org/wiki/Level_of_measurement

[TomW]

Peter;

Pretty much what I was trying to point out and failed, apparently. I never intended for stonebrain to take offense. It is so darned hard to communicate sometimes because no matter what you type or how carefully you word it, someone will misunderstand and / or interpret in a way you did not intend.

Probably time to take a break again. Caught hell for telling someone deception was not my idea of the way to get things a couple days back. Not sure how that can be wrong.

Anyway, thanks for clearing up my muddled point infested with improper terminology.

Cheers.

TomW

[Tess]

ok i'm new to this, as for tech info on panels, evac tubes, says in manual as a guide 1 tube can produce 10l hot water, solar radiation ranges from 400-850 W/m2, i am in Cornwall UK (very south of UK)the water is coming from a borehole in the field so should be ground temp. absorb area for panels is 1m2 per 10 tubes so we will have 6m2. Its a bit trial and error we are all learning but any advice is well appreiciated, cheers

[Stonebrain]

Tom,

I looked it up

you're right,

It takes a slightly different amount of energy to rise the water t° from 10° to 20° then from 20° to 30°(talking about °C).

But the difference is very small.

Practically you can consider the amount of energy to rise 1kg water for 1° constant.

We're all so darned right!

cheers,

stonebrain

[Nando]

Tess:

Again the info is incomplete, 101 degrees does not say at what water volume rate flowing through the panels, also watts/m^2 variations will vary the water temperature accordingly if the flow is constant through the panels..

If you want accurate responses try to give accurate data.

What is the bore temperature ?.

Nando

[jimjjnn]

Tom, Make it a short break as we need you here to keep us inline.

[elvin1949]

AMEN

[Bruce S]

Tess;

   What is the temp of the water coming out of the ground before it hits the tubes?

Normally ours here in St.Louis, MO, US in right at 70F, which with the outside air temp sitting at 105F is pretty darn cool:-)

This will help in figuring out what out put you'll have.

Pesonally, in my travels I have had the good luck to use both types and I prefer to have the hot water mixable with cold. Got a big surprise one early morning in Jamaca when I turned on the shower and got a backside of near boiling water :-(

Since you have such a large holding tank, it might be better to have a circulating pump so the heated water can be recirculated back through the tubes, thus keeping it hot and let the showerer <-sp? chose the temp comfortable for them. A float valve to add when the water gets low or say too hot would be a good way to go also.

Best of luck with your site!!

Bruce S

PS> Tess..Not to steal from your post but, TomW, I added a little to that days old post...I agree with you BTW.

[thefinis]

Man do we like to complicate simple things but you did ask for specific numbers on water heating.

"not sure whether to decrease size of tank and have cold water feed to adjust water temp or just have warm water with no cold feed"

Get a thermo mixer valve and feed cold and hot water to the showers. They make a valve that is self adjusting for the temp you set it to. I do not know who makes them but we used them at one of the camp grounds I helped maintain years back. Pricey but it keep us from having to set the water heater to a low temp so no one could burn themselves. If we had set the temp low the hot water would have run out before all the campers finished so it was buy the valve(for a bank of showers) or buy a bigger water heater. If you go with this system just leave the setup you have as it will try and feed hot or cold to adjust temp. There is another mixer valve that you can set the flows for if your water temps hot and cold tend to remain constant but is not self adjusting. If you let the campers set the temps in regular type showers they will use all the hot water quickly.

"we will have additional heat exchangers connected to wood chip burner for during the winter."

If you do not use a mixer valve then the water from the heat exchangers could become too hot and pose a scalding threat.

Finis

[Clifford]

Lets forget the Kelvin, Centegrade, Farenheit, & etc...  the goal is to maximize the number of "comfortable" showers while minimizing waste and minimizing risk of scalding.

The risk seems that your solar water heater could vary the temp in the tank...   perhaps a lot.

Consider the difference...

Friday night, after heating all week and nobody using the water.

Monday AM, after heavy weekend use and cooling all night.

Cloudy Days

Sunny Days

Seasonal Changes

It sounds like there are a few options...

"Push Button" is the hardest to mix, but may encourage conservation.

"Hot/Cold" faucets that people can mix themselves, but risk leaving it on, or having long leisurely showers.

"Auto Mixing"

Another option might be to setup a "heat exchanger" system.

"Collector Tank" which would run from luke warm to boiling (may be pressurized, and/or use a glycol mix).

"Mixer/Exchanger" Tank.  You could thermostatically regulate that.  Pump water from the "collector" to make it warmer.  Stop pumping when it is hot enough.  You could use "conventional heat" to suppliment it if necessary.

Pumping could be passive (thermo-gradient, heat goes up), or active, depending on flow volumes needed.

Such a two-tank system would likely even out the temp fluctuations from the first to last shower, and prevent overheating and/or wasting hot water.

I'd probably configure it so my "collector" tank would act as a "heat sink", and would be the larger of the two tanks.

Say your ground water is at 60 degrees F.  Ambient temperature is 80 degrees F.

It might make sense to use an air, or shallow ground heat exchanger "pre-heat" system to bring it up to the ambient temperature before feeding it into the solar system (in the summer), possibly even tying it to a geothermal cooling system.  This would mean the last showers at least wouldn't be FRIGID.

(btw, I've been planning, but haven't been building a solar water system).

[mukunthko]

C/100 = (F-32)/180 = (K-273)/100