Maximizing Solar Heat Output with Vacuum Tubes: A Temperature Comparison

[pranj5]

Solar vacuum tubes are tubes covered with a vacuum jacket so that no heat can be dissipated by conduction and convection of air. By this process, we can get pretty high temperature even at very cold temperature. In this video, you can see how very temperature water can be produced by using such a tube even at -29°C temperature.
As this tube directly converts solar radiation into heat, I want to know if the input is at higher temperature, then can we get greater temperature output? Efficiency of market available solar troughs falls with rising temperature as higher temperature also means loss of more energy by means of conduction and convection of air from the outer body of the collecting tube. I just want to know whether the vacuum tube get rid of that problem or not.

 

[berkeman]

Solar vacuum tubes are tubes covered with a vacuum jacket so that no heat can be dissipated by conduction and convection of air. By this process, we can get pretty high temperature even at very cold temperature. In this video, you can see how very temperature water can be produced by using such a tube even at -29°C temperature.
As this tube directly converts solar radiation into heat, I want to know if the input is at higher temperature, then can we get greater temperature output? Efficiency of market available solar troughs falls with rising temperature as higher temperature also means loss of more energy by means of conduction and convection of air from the outer body of the collecting tube. I just want to know whether the vacuum tube get rid of that problem or not.

It would seem like it could help the efficiency some, with the added infrastructure cost. Do you have a particular solar thermal conversion installation or type of installation in mind where you would like to calculate the gain in efficiency and the increase in infrastructure cost?

 

[pranj5]

As you can see from the video that when cold water has been added, very hot water comes out. I want to know if saturated steam is added, then can we get superheated steam? In short, if the temperature of the input is high, then can we get higher temperature output.

 

[256bits]

loss of more energy by means of conduction and convection

Don't forget about radiation loss

 

[pranj5]

Don't forget about radiation loss

Radiation loss in minuscule in comparison to conduction and convection. Convection is the most energy dissipating way.

 

[256bits]

So you have been led to believe.
Every object emits radiation, the more so with increase in temperature.

 

[pranj5]

So you have been led to believe.
Every object emits radiation, the more so with increase in temperature

Anybody having basic understanding of what conduction, convection and radiation are can understand that radiation is minimal in comparison to conduction and convection. I am not saying that with increased temperature, the loss by radiation wouldn't increase but rather want to emphasis on one point that loss by radiation by very small in comparison to conduction and convection. Just go through the formulas for conduction and radiation and you yourself can see radiation loss is negligible in comparison to conduction.

 

[John Morrell]

Radiation loss is pretty minimal in standard situations where you are heating the water by means of convection or conduction (like in a gas water heater). But think about it; you're heating the water by radiation. So obviously the limit on how hot the water will get will be how quickly heat radiates out from it. Granted, no vacuum pipe is perfect so I'm sure as you get really hot a significant amount of energy is being lost by normal conduction. But still, don't ignore the radiation. It's the only thing putting energy into the water; it's not a stretch to say that it's the main source of energy loss.

 

[pranj5]

Here the radiation comes from a source having 5778°K temperature at its surface. Change in 100-200°K means just 2-3% difference in the temperature difference. How much radiation loss can be increased with such change in temperature?

 

[256bits]

Here the radiation comes from a source having 5778°K temperature at its surface. Change in 100-200°K means just 2-3% difference in the temperature difference. How much radiation loss can be increased with such change in temperature?

If you have seen solar concentrators where they use mirrors to focus the radiation from the sun to a small target area, the maximum temperature that can be achieved is that of the incoming radiation, in this case 5778 K. but that is an ideal situation.

In your case, or the case shown in the video, there is incoming radiation coming in at 5778 K. and that is a directional source.. Only the side facing the sun receives radiation and not all of it at a normal direction. The tube on the other hand will radiate completely in a 360 degree pattern to the surroundings at environmental temperature, and to the sky at a somewhat lower temperature. Of course radiation also is incoming to the tube from the sky and environment.

As always heat in = heat out at steady state.
And that should be calculable, with radiation being the predominate factor for maximum temperature obtainable for the tube.

 

[pranj5]

The radiation comes from a source at 5778°K while the radiation out is from a source just around 300°K and/or slightly above. I am curious to know without conduction or convection, when the steady state will be reached i.e. at which temperature.
In space, if things are put in open i.e. in the Sun it becomes very hot as there is now means of conducting or convecting the heat to somewhere else like Earth surface.

 

[mfb]

when the steady state will be reached i.e. at which temperature.

It depends on the solid angle the target sees the source at. With 4 pi ("sun in every direction"), it is 5778 K. Without any concentration, it is about the surface temperature of Earth. With concentration in between, it is something in between. 1000 K equilibrium temperature is not hard to achieve if your vacuum is good and if thermal conductivity of the setup holding the heated spot is low.

If you want to heat water with it, you also have to consider flow rate. A higher flow rate will lead to a lower exist temperature.

 

[pranj5]

I can understand what you want to mean. But what I want to know if I put high temperature fluid inside, do I get higher temperature fluid?

 

[sophiecentaur]

If you want to heat water with it, you also have to consider flow rate.

This is a vital consideration. You will always be limited to approximately 1kW per square metre of 'receiver', which is why people have their roofs covered with heating panels. That video is impressive at first sight but remember the temperatures that are reached are what you get with no 'heat output'. You are still only getting the Power that the tube intercepts from the Sun.

Anybody having basic understanding of what conduction, convection and radiation are can understand that radiation is minimal in comparison to conduction and convection.

"Minimal" is a quantitative description and it is entirely responsible for how the tube works. The mean surface temperature of the surface of a satellite is not far from the 300K of the Earth; that's a much less complicated system to understand than the Earth with its atmosphere (what is the Earth's actual 'surface' for such calculations?). That equilibrium temperature is entirely governed by Radiation balance (emission vs aborption).
A non directional solar vacuum tube is in the situation where it is receiving radiation from the Sun over half of its area, radiation from the Earth underneath it (about 2π solid angle at, say 270K) and the rest of space and the sky, on a clear day, at around the same (an uncertain value, it seems from this link). If you compare the environment of a satellite with the tube. The satellite is mostly radiating into outer space (just a few K temperature) with input contributions from the Sun and from the Earth at 300K (a solid angle that could be up to 2π, in very low orbit but is much smaller in geosynchronous orbit @ 40,000km) . The vacuum tube is effectively surrounded by a (4π) source of around 300K plus the Sun. It's not so surprising that its equilibrium temperature can rise to well over 450K (which is considerably higher than the isolated satellite). It's the ultimate Greenhouse Effect. It's getting an advantage of almost 300K!

 

[pranj5]

This is a vital consideration. You will always be limited to approximately 1kW per square metre of 'receiver', which is why people have their roofs covered with heating panels. That video is impressive at first sight but remember the temperatures that are reached are what you get with no 'heat output'. You are still only getting the Power that the tube intercepts from the Sun

Actually what I want to know do I always get 1 kW output irrespective of the input temperature upto a level or not.

 

[mfb]

I can understand what you want to mean. But what I want to know if I put high temperature fluid inside, do I get higher temperature fluid?

Not in equilibrium, but if you have a constant flow rate: yes. The closer you are to the equilibrium temperature the smaller the difference will be.

Actually what I want to know do I always get 1 kW output irrespective of the input temperature upto a level or not.

No, because radiative losses will increase.

 

[sophiecentaur]

Actually what I want to know do I always get 1 kW output irrespective of the input temperature upto a level or not.

If you are talking in terms of heating water, the difference will not be great because the water temperature range is not great (not as much as the smouldering rag) but the total heat output will be affected by the operating temperature. This is a general principle for heat transfer.

 

[pranj5]

No, because radiative losses will increase.

Question is how much.

 

[russ_watters]

...I want to know if the input is at higher temperature, then can we get greater temperature output? Efficiency of market available solar troughs falls with rising temperature as higher temperature also means loss of more energy by means of conduction and convection of air from the outer body of the collecting tube. I just want to know whether the vacuum tube get rid of that problem or not.

Yes, with higher input fluid temperature you can get higher output - but at a loss of capture effectiveness.

 

[russ_watters]

Anybody having basic understanding of what conduction, convection and radiation are can understand that radiation is minimal in comparison to conduction and convection.

Sorry, but "anybody having basic understanding of what conduction, convection and radiation are can understand" that what you are saying is not universally true. Since convective heat transfer is proportional to temperature difference, but radiative heat transfer is proportional to the fourth power of the temperature difference, an object doesn't need to be very hot for radiation to be the primary mode of heat transfer. For temperatures in the range of steam, it is significant and will have an impact here. It doesn't mean you can't do this (as solar thermal power plants show, you certainly can), but it will be a design consideration.

It would probably be a good idea for you to run a quick calculation of pipe heat loss to see how significant the impact of ratiation is...

 

[mfb]

Question is how much.

That depends on your setup. There is no general answer.

 

[russ_watters]

Ok, so on that note, pranj5, why don't we work out a specific example. There are a bunch of ways to do it (of varying complexity...), but my suggestion is for you to pick a diameter of collector and average ambient and fluid working temperatures, then we can assume some things about the geometry and calculate the net heat transfer rate.

 

[jbriggs444]

proportional to the fourth power of the temperature difference

Proportional to the difference of the fourth powers, not to the fourth power of the difference.