Joined: Jun 02, 2004 Posts: 1078 Location: Bristol, UK
Posted: Tue Jun 15, 2004 11:32 am Post subject: Novel use of solar power for water heating...
I was talking about solar power with a mate a while ago and he suggested a interesting idea. We had both seen water heated in very hot countries by pumping it slowly through long lengths of black hosing. In direct sunlight with the pump running slow, 25C in would come out the other end at 60C+. We were thinking about using narrow copper pipes and an array of lenses focusing the sun’s energy from a larger area down onto the pipes. Would these intermittent very hot spots on the pipe produce a more efficient system overall?
The difficulty of course is keeping the lens focused on the pipe as the sun moves through the day!
Joined: Jun 02, 2004 Posts: 1078 Location: Bristol, UK
Posted: Tue Jun 15, 2004 1:18 pm Post subject:
Pops wrote:
A community kitchen in "Auroville , Tamil Nadu , India " has a fixed, very large parabolic dish ( multiple reflectors - 840 square metres ) built into the roof which generates steam ( 4000 Kg / day ) for the kitchen to cook with. This kitchen feeds several communities, approximately 10000 people.
Wow! Looks like impressive results can be had from passive solar! Thanks for the link.
Joined: Apr 03, 2004 Posts: 6501 Location: My Grandkids' Farm
Posted: Tue Jun 15, 2004 1:25 pm Post subject:
I've seen homebrew boilers made from older satellite dishes.
I thought it might run a small steam engine though at least one builders of the engines says it won’t work.
If you find something of interest along these lines you might post them in the “Anyone Serious?” thread which has some other ideas along the “homebrew” line. _________________ Make a plan and work it:
The house I grew up in had a solar water heater. Basically, it was just two copper plates, welded together so there was a network of openings between them. Looked like a maze. The panel went on the roof, and water was pumped through it. It came out scalding hot. Generally, it provided enough hot water for a family of four, though on cloudy days, we had to turn on the electric backup.
There's an article in the latest Discover (July, I think) about using the sun for indoor lighting. Apparently, it's fairly common in Japan. Special, high-tech fiber optic cables direct light from the roof to the inside. Supposedly, office buildings with this type of lighting cut their power use in half. And the light is much more pleasant the flourescent or incandescent lights. You do need electric lights as a backup for overcast days.
Sun boilers are fairly common in north-African countries. Usually they just take a metal container, paint it black and put it on the roof.
However I don't think it would be so effective in the northern countries, since the heat losses depend a lot on the ambient temperature.
About the lighting. I've seen a demonstration the system on a fair a while back. I believe it was by Schott Glasswerke from Germany. The idea is simple. You have a collector on the roof (basically a glass lens) and you send the light to wherever you want by glass-fiber.
At the collector-end you also have the possibility to switch between natural light and a central lamp. I was told that the central lamp idea is already commonly used in high spaces like a cinema theater, where it is difficult to change a single light-bulb.
The system is very nice, but it has a few drawbacks. Unless the system is already incorporated in the initial design of a building it is extremely expensive to install.
The second problem is the light yield. If you have a collector as big as the roof of the building and 100% efficient, you'll be able to create the same light density inside as the natural light outside. However if your building has two floors the light has to be distributed over the two floors and your light density will halve, etc.
Joined: May 20, 2004 Posts: 235 Location: Sonoma County, Northern California
Posted: Tue Jun 15, 2004 4:54 pm Post subject: High temps = low efficiency.
I spent several years designing/sizing and selling solar water heating systems for domestic hot water, home heating and industrial process water. I recently retired from Heliodyne Inc., in Richmond, CA, now the oldest manufacturer of solar water heating collectors in the U.S. You may have heard of their "Gobi" black chhrome flat plate collectors. They arethe collectors made for what is known as "low to medium" temperature applications. They will reach higher temperatures - i.e. around boiling - but efficiency drops off considerably
As solar heated water temperature rises above ambient, standby loses increase significantly. It is not practical to store very hot water, plus the effieciency of collection is a curve that drops off as the collector, plumbing, storage, etc. get hotter. Usable Btu's collected from a solar hot water system drops off significantly as you go over about 160 deg. F, and that is the price you pay for applications that require really hot water. Concentrating collectors are typically used for high temp applications.
The most "efficient" use of solar is heat is "low-grade" where you gather as many Btu's as possible at lower temperatures. This is an ideal match for a well-designed, well insulated home with radiant heat with the radiant tubing in the slab spaced closer together than normal. You can get heat into your space with water that has been depleted of heat down to about 95 deg F. Also, solar domestic hot water system that "preheats" water that is fed through a "modulating" (adjustable flame) in-line wate heater is a good solution. Most American solar hot water systems have two tanks - one solar storage and one standard water heater - that sit and loose heat all day and night The "regular" electric of gas hot water heater doesn't have to come on if it gets solar hot water, and can be bypassed in summer when the days are long and the sun is high in the sky all day.
Flat-plate collection technology is mature, and definitely not "high-tech." Since it looks like "peak-copper" is also approaching, copper tube/plate collectors may become much more expensive. There is a hell of a lot more that can be done with efficiency, conservation and design for building heating, and also for DHW applications. I worked on solar designs for "trophy houses" where people put in huge 12 - 4'x10' collector and larger systems so they could have more fenestration (windows) under California's Title 24 construction rules.
If you're interested in learning more about solar heating, get the "Solar Living Sourcebook" from Gaiam/Real Goods.
There are lots of do-it yourself solar ideas out there, and "Home Power" magazine is a good place for those who have the skills and want to build from scratch.
Kind of a ramble, but I hope this helps with understanding solar water heating.
Joined: Apr 03, 2004 Posts: 6501 Location: My Grandkids' Farm
Posted: Tue Jun 15, 2004 5:06 pm Post subject:
Thanks Dave, glad you wrote.
Right now there is still much debate here regarding the time frame and effects of the oil situation going forward and very little regarding practical methods for reducing dependence. I hope that in the future we’ll be seeing more posts along these lines. I started a thread some time ago on similar themes:
http://www.peakoil.com/postt10.html _________________ Make a plan and work it:
About the lighting. I've seen a demonstration the system on a fair a while back. I believe it was by Schott Glasswerke from Germany. The idea is simple. You have a collector on the roof (basically a glass lens) and you send the light to wherever you want by glass-fiber.
This one is at Oak Ridge National Lab in Tennessee. But it sounds like a similar system. It uses special, super-efficient silicone gel fibers from Japan.
Quote:
The second problem is the light yield. If you have a collector as big as the roof of the building and 100% efficient, you'll be able to create the same light density inside as the natural light outside. However if your building has two floors the light has to be distributed over the two floors and your light density will halve, etc.
You probably don't need the same light density as outside. The system described in the Discover article uses a movable dish and mirrors to follow the sun and direct the light to the gel fibers. They estimate that 500 square feet of floor space can be illuminated by each square yard of dish. A small, 20,000 square foot office building would require 40 dishes, each costing $4,000. They are planning to design some larger dishes to reduce this number.
As a side benefit, people are more productive when working in natural light. And customers in stores lit by natural light (via skylights) buy 30% to 50% more, though they aren't aware of the lighting. They just find the stores more pleasant, without knowing why.
Joined: May 26, 2004 Posts: 309 Location: Ontario, Canada
Posted: Wed Jun 16, 2004 10:44 am Post subject:
Also, couldn't the light density is less of an issue if you just use the light for the main areas? You light up where people spend most of the time, and leave the single hanging lightbulb for the closet that you only go into for a minute or two anyway. _________________ "Our forces are now closer to the center of Baghdad than most American commuters are to their downtown office."
--Defense Secretary Donald Rumsfeld, April 2003
Joined: Jun 05, 2004 Posts: 7 Location: Big Spring
Posted: Wed Jun 16, 2004 8:11 pm Post subject: Re: Novel use of solar power for water heating...
clv101 wrote:
We were thinking about using narrow copper pipes and an array of lenses focusing the sun’s energy from a larger area down onto the pipes. Would these intermittent very hot spots on the pipe produce a more efficient system overall?
The difficulty of course is keeping the lens focused on the pipe as the sun moves through the day!
Away back in the dark ages (the early to mid eighties), I was a student at Texas Tech University in Lubbock, Texas. At the time, the College of Engineering had an experimental Rankine cycle power plant using a solar concentrator for a heat source. The concentrator was an array of fixed mirrors that all focused on a line that moved as the sun went across the sky. A steam generator, which was basically a length of pipe with tubing wrapped around it, was mounted on a drive mechanism that held the steam generator in the focus line. The steam was used to run a turbine.
Once the OPEC, the Saudis, and everyone else started producing enough oil to bring down prices and thereby starve the old USSR of hard currency through its own oil exports (the commies' oil was more expensive to produce than than that of the Saudi's), everybody forgot all about the Ayatollah Khomeini, gas prices, and common sense. As a result, Tech ended up shutting down the project and sending the power plant to somewhere in North Africa. The last I heard (1988) about it was being used to pump water.
Later, when I was a grad student, we found two bowl facets (that's what we called the mirrors) in our wind tunnel lab and took them outside to play with them. We accidentally set a tree on fire.
Was the thing practical? Well, I don't know, but it was pretty cool engineering.
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