Solar water heater and its pipes...

[Externet]

Hi.

A solar water heating panel on a roof gets its water supply from a pipe and outputs on another pipe.

 

cold water pipe --->==============================solar

hot water pipe <---==============================panel

 

What changes in performance/efficiency if

-only the hot output pipe is insulated, or if

-both pipes are fit together inside the same insulation sleeve ?

 

(The inlet water is at ambient temperature)

[Mr Skeptic]

Think about it. What purpose would insulation serve for water at ambient heat? What would be the effect of having something with a high heat capacity (a water pipe) next to something at a different temperature?

[Externet]

Yes, I thought about it. It is not about purpose of insulation.

 

If the inlet pipe 'rides' next to the outlet pipe inside the insulation jacket, the output hot water will lose some of its heat transferred to the adjacent inlet cold pipe.

 

At the same time, the now pre-heated inlet will absorb the sun heat raising its temperature beyond. Later, at the outlet, will give up some of it.

 

What is your rationale?

[Newbies_Kid]

I think it is more efficient by just insulating the output pipe alone. Insulating inlet and outlet together would not increase the outlet temperature. It just like a heat exchanger.. it will just heat and cool then heat again and cool... bla2

Edited May 30, 2010 by Newbies_Kid

[michel123456]

The solar panel will deliver water at the same temperature independently of the temperature of the original cold input. Usually water goes from the solar panel to an insulated tank where the water is maintained at an approximatively standard temperature depending on the system.*

The purpose of the mechanism is to get hot water on the left side of the diagram, where you'll put a faucet for example. If you put both pipes into the same insulation, the water at the faucet will not have the same high temperature.

 

*most work till 60 celsius for avoiding intolerable temperature at the faucet. For other purpose such as air conditionning temperature can reach slightly lower than boiling (100 celsius), I am not aware of systems using vapor (above 100 celsius) but there might be.

[Externet]

Thanks.

The solar panel will deliver water at the same temperature independently of the temperature of the original cold input.

 

That is what am saying and believe; the rest of your post is unrelated to the question.

[Mr Skeptic]

If the inlet pipe 'rides' next to the outlet pipe inside the insulation jacket, the output hot water will lose some of its heat transferred to the adjacent inlet cold pipe.

 

At the same time, the now pre-heated inlet will absorb the sun heat raising its temperature beyond. Later, at the outlet, will give up some of it.

 

What is your rationale?

 

Right, the heat exchanger would transfer heat from your hot to your cold without doing any work. This is a waste since you want the hot output hot and the cold input cold. Essentially it is a heat engine with 0 efficiency.

 

Furthermore, the warm input will waste the solar input by warming hotter and so radiating more thermal energy, but despite the hotter temperature achieved will end up cooler. Also, the insulation will be spread more thinly as it covers two pipes.

 

Leaving the ambient temperature pipe uninsulated will have no effect since it is at the same temperature as the environment and will neither cool nor warm.

[InigoMontoya]

Leaving the ambient temperature pipe uninsulated will have no effect since it is at the same temperature as the environment and will neither cool nor warm.

Except that "cold" is a relative term. Is it colder than the outlet temp? Of course. Is it ambient? Hell no. At least in every solar water heater system I've seen, the water isn't pulled from an ambient source. It's pulled from the water heater's reservoir. Thus, while it may not be your desired temp (we'll say 120 F), it sure as heck is likely to be significantly hotter than ambient (say, 110 F).

 

Or at least, that's the situation while water is flowing.

 

But what about when water isn't flowing? What if it's freezing out? Don't want that pipe to freeze? Insulate it!

[H2Ouk]

Except that "cold" is a relative term. Is it colder than the outlet temp? Of course. Is it ambient? Hell no. At least in every solar water heater system I've seen, the water isn't pulled from an ambient source. It's pulled from the water heater's reservoir. Thus, while it may not be your desired temp (we'll say 120 F), it sure as heck is likely to be significantly hotter than ambient (say, 110 F).

 

 

Or at least, that's the situation while water is flowing.

 

 

But what about when water isn't flowing? What if it's freezing out? Don't want that pipe to freeze? Insulate it!

 

All solar heating systems should be filled with solar thermal fluid capable of frost protection down to -25 C. Insulation will never prevent freezing.