Hollow Vacuum Tubes for Air?

[beezaur]

I am in the very early stages of designing an overnight heating system using "pebble bed" packed storage. You "charge" the storage during the day using hot air, top down, then reverse flow at night to extract heat from the top. This will be for cool weather, needs to function down to say, 15 F/-10 C.

I came across some vacuum tubes that are hollow all the way through, not the standard heat pipe type. What's nice is you blow air through them and avoid an intermediate liquid stage altogether. No heat pipes. No pumps. No exchangers. It greatly simplifies the system.

solar_air_heater_collector_vacuum_tube.jpg

Found then here: http://hnchaoda.en.alibaba.com/produ...cuum_tube.html

But . . . that's about all I know about them. Anyone have any more information about something like these tubes?

[russ]

What is your benefit in using a vacuum tubes for this application?

Calculate the size of the bed you need - huge!!!!!!!!

[beezaur]

What is your benefit in using a vacuum tubes for this application?

Calculate the size of the bed you need - huge!!!!!!!!

Vacuum tubes give higher fluid temps. In liquids anyway. I'm not sure what these air tubes would do. 80 F will work in my case, 100+ F much better.

For the bed capacity, I've done some rough calcs after Duffie and Beckman's 3rd ed: http://www.amazon.com/gp/product/047...=vglnk-c934-20

With a bulk density of 1600 kg/m^3 and specific capacity of 840 J/kg-C, using a charging temperature of 90 F I get a required volume of around 550 cf. This is for about 250 MJ/65 kWh of storage.

So . . . it's a cylinder 10' diameter and about 8' tall, or some variation of that. I'm rural, have plenty of space.

Like I said, this is in the very early stages.

[inetdog]

Looking at the most detailed description I have been able to find does not provide enough about how it actually works to let me give an informed opinion.

The text and pictures at this link seem to describe a system for directly heating air in the tubes, but the manifold box at the top end of the completed panel shown has only two small pipe connections rather than the larger air connections I was expecting.
It may be designed to use a pumped air (high pressure high flow rate) circulation with very high air temperature. That is also consistent with the solar PV panel shown in one photo which would be driving the high pressure air circulator.

I have not seen this design before, although use of high temperature high velocity ducting for central heating has been around for quite a while. When done properly it can deliver heat with smaller ducts, allowing it to be fit more easily into a house, particularly an existing structure without installed air ducts.

If you want to get useful performance during cold weather, they might (might!) have an idea worth exploring. A conventional panel would rely on a dual or triple pane flat plate over the collector instead and would be mechanically simpler.

[beezaur]

The system I had in mind was a pretty simple fan system, i.e., low pressure and flow rates. I would expect it to be something on the order of 100 tubes, possibly some in series, depending in the heat transfer from collector surface to air.

Basically a soda-can system but with much better insulation and therefore higher temps.

Charging would be:

cool air from storage --> heating though collector --> downflow through rock storage bed --> around again

Discharging would be the reverse, with room air instead of the collector.

Daytime heating will be a water radiator system I'm building now.

P.S. An alternative might be to insert a divider plate into a "conventional" one-ended tube: a U-tube but for air.

[J.P.M.]

The system I had in mind was a pretty simple fan system, i.e., low pressure and flow rates. I would expect it to be something on the order of 100 tubes, possibly some in series, depending in the heat transfer from collector surface to air.

Basically a soda-can system but with much better insulation and therefore higher temps.

Charging would be:

cool air from storage --> heating though collector --> downflow through rock storage bed --> around again

Discharging would be the reverse, with room air instead of the collector.

Daytime heating will be a water radiator system I'm building now.

P.S. An alternative might be to insert a divider plate into a "conventional" one-ended tube: a U-tube but for air.

So what you envision is:

- ~550 ft.^3 of pebble bed storage.

- Air cooled solar collectors.

- Ducting between the two.

- Two heating systems, one solar air, one conventional H2O radiator.

That about it ? If so, been around a long time. Not being done much any more. I own copies of a lot of what Duffie & Beckman wrote and owe them a lot, but some of the things D & B. left out/your mother never told that might happen:

- Maldistribution in the bed lowers effective storage capacity a fair amount.
- Upping the flowrate to compensate for maldistribution increases fan noise and pumping cost.
- Flow rates tend to be high, discharge temps. lower than conventional forced air sys., leading to the feeling of cold drafts and complaints.
- Critters sometimes get in the bed, eat/crap/die, etc.
- Moisture and associated mold problems caused by partial charging of cold bed by warm, high dew point air.

Some of this and other stuff can be minimized with careful, thoughtful design. I don't want to sound negative, but this may not be the ideal DIY project.

Whether these or other things occur or not, serviceability is a problem. Whatever happens in the middle of the bed is unknown until a problem occurs and is pretty much impossible to get at/remedy. Some beds work great for years, but if they go bad, they are a PITA. That's one reason why passive solar with high building mass and high insulation levels that lead to long building thermal time constants are more common.

FWIW

[beezaur]

this may not be the ideal DIY project. . . .

Almost none of the stuff I'm after really is. I'm an engineer too (another field) and pretty patient about "measuring twice and cutting once" if you will. This stuff is quite a bit different than my normal world, so it's a pretty steep learning curve for me. But I'm not afraid to jump into the deep end of the pool. That's my point actually - to go beyond the typical DIY spectrum.

All the gory details are what I need to hear if I'm going to do a good job - fan noise, condensation problems - all that stuff. So I tremendously appreciate the practical advice.

[J.P.M.]

Almost none of the stuff I'm after really is. I'm an engineer too (another field) and pretty patient about "measuring twice and cutting once" if you will. This stuff is quite a bit different than my normal world, so it's a pretty steep learning curve for me. But I'm not afraid to jump into the deep end of the pool. That's my point actually - to go beyond the typical DIY spectrum.

All the gory details are what I need to hear if I'm going to do a good job - fan noise, condensation problems - all that stuff. So I tremendously appreciate the practical advice.

Understood. For more information, the journal " Solar energy" has many papers dealing with design and performance of packed beds. These tend to be a bit bookish but there are nuggets of wisdom now and again. Meant as friendly advise: Consider buying commercial collectors. They really do work better for a longer period. I've designed, built and bought. Bought was better for me.

[beezaur]

Understood. For more information, the journal " Solar energy" has many papers dealing with design and performance of packed beds. These tend to be a bit bookish but there are nuggets of wisdom now and again. Meant as friendly advise: Consider buying commercial collectors. They really do work better for a longer period. I've designed, built and bought. Bought was better for me.

Thanks.

I've downloaded a lot of articles, I think from that journal (?) about packed beds. Previously phase change storage was the plan (very conceptual) using wax. In other words, everything still is on the table. My schedule is to do learning and planning through the summer, and have an operational system in the fall. Although . . . it's difficult to instrument and monitor test systems for Jan/Feb use at 47 N latitude in August. I have the sneaking suspicion that I will be heating with test equipment next winter.

[J.P.M.]

Thanks.

I've downloaded a lot of articles, I think from that journal (?) about packed beds. Previously phase change storage was the plan (very conceptual) using wax. In other words, everything still is on the table. My schedule is to do learning and planning through the summer, and have an operational system in the fall. Although . . . it's difficult to instrument and monitor test systems for Jan/Feb use at 47 N latitude in August. I have the sneaking suspicion that I will be heating with test equipment next winter.

Unless you go to a library that has it and make copies, or belong to the International Solar Energy Society, it's usually not a likely download from that journal. Most large public and university libraries have it available. Most of the contained papers are not on line without buying them and for the price of one or two papers you're better off joining the ISES.

[russ]

The volume of water you mention (550ft3) would be of little use. I believe you are chasing a wild goose but if it is fun then great.

[J.P.M.]

The volume of water you mention (550ft3) would be of little use. I believe you are chasing a wild goose but if it is fun then great.

Russ: It's rocks, not water.

[russ]

Russ: It's rocks, not water.

I saw that but I have worked it out in the past for solar thermal heat collection for my hydronic system - water is far better for heat transfer than rocks. Turned out there was little hope for what I was checking on.

[beezaur]

The volume of water you mention (550ft3) would be of little use. I believe you are chasing a wild goose but if it is fun then great.

Rocks. Not water, rocks.

There's a huge flood of (very recent) technical papers and lots of engineering texts that disagree with you.

The temperature profiles in test beds have been very highly stratified. You can't do that with water - it conducts too much. Actual, measured efficiency of storage and extraction is usually around 90%, often higher. Ducting is usually the weak link.

Phase change materials can work similarly, but they don't seem to do very well below their melting point (around 120-160 F in most things I'm considering). Phase change is like stratification in a sense. The latent heat comes out at a constant temperature.

[russ]

Like I said - I think it is a wild goose chase based on what I have looked at - but have fun with Pablo.