racookpe1978
Nuclear
- Feb 1, 2007
- 5,980
Was reading about a solar, flat plate, double-sided, fixed vertical collector a company had installed in several towns up the west coast of Greenland to provide hot water for supplemental heating through the summer months.
Lots of problems with this I can imagine, and I know the overall installation could not have operated even as well as it did - and probably would have never been tried at all! - if it were not due to the CAGW-hyped green energy funding from various governments. Regardless of its success in some of the test months, there were many installation and operational problems.
There are several design question I simply cannot figure out though, and I will raise one here first as simple a theoretical reflector problem to provide solar heating hot water. Your answers may tell me why my first-order critiques are wrong.
Long, parabolic solar concentrating troughs are commonly oriented north-south, rotating to face the sun as it travels from east-to-west to concentrate the solar energy into a very narrow collector to get very high temperature hot water (or hot fluid) to go to a power turbine or secondary boiler. Every parabolic concentrating trough I've see pictures of, and every "patent" drawing I've seen on several Google searches uses a very, very small diameter receiver that is placed at the theoretical trough focus point to heat the working fluid. Fine. it you want usably high temperature steam, you need even higher temp water/working fluid to heat the water to make the steam. Eliminatory, my dear Wattson.
But for house or apartment heating, you don't really want high temperature water, you want supplemental heating water or only 115 - 130 degrees max.
So it would seem you'd want a larger volume of slightly cooler water/working fluid out of the collector. You'd want as large a "black" receiving area of the collector as possible of course. You'd want many small pipes to pick up that heat, or some sort of a heat exchanger with as little internal volume but as large a surface area as possible exposed to the sun with as little a water volume as possible not in contact with the heating surface, right? you'd want as little non-productive area as possible - areas that are exposed to the cold Greenland air but are not actually collecting the solar energy. And, of course, you want to build the thing as cheap and as reliable as possible with as few moving parts as possible.
Question 1. So, for a parabolic trough exposed to the sun, why use a very small diameter receiver? 1 to 1-1/2 inches seems to be the maximum on any trough receiver I've seen. Why not use a larger receiver - say a double tube 6 inches OD with a 5-1/2 inch dia inner tube? The water/heating fluid would flow through the annulus between inner and outer tube.
On all current receivers I've seen, the very small "front" area absorbs almost no energy compared to the "back" of the receiver which is getting hit all of the reflected energy at the parabola's focus point. A larger dia receiver would obviously "shade" some of the reflector with its own shadow, but that same area would be hit by direct rays on its front side, so it seems there's no real loss of energy.
Question 2. If a large receiver tube is chosen, what is the equation for the reflector curve, if the "target" is not a infinitely thin long "line", but a larger, real-world diameter cylinder that is relatively large with respect to the reflector's opening size?
Question 3. Every parabolic trough I have seen has been set up for use in a desert (little rain or snow or precipitation) area with very high ambient temperatures. If a parabolic trough is used in cold climates for house heat, you need to prevent the fluid and the heat exchanger surface from the cold air itself. Is there a workable glass or plastic "greenhouse" film that could cover the front opening of the parabolic trough to trap the received heat but does not reflect too much incident inbound solar radiation?
Seems like it would require some kind of glass exactly opposite what you'd chose for a building's south wall in the Southern US.
Lots of problems with this I can imagine, and I know the overall installation could not have operated even as well as it did - and probably would have never been tried at all! - if it were not due to the CAGW-hyped green energy funding from various governments. Regardless of its success in some of the test months, there were many installation and operational problems.
There are several design question I simply cannot figure out though, and I will raise one here first as simple a theoretical reflector problem to provide solar heating hot water. Your answers may tell me why my first-order critiques are wrong.
Long, parabolic solar concentrating troughs are commonly oriented north-south, rotating to face the sun as it travels from east-to-west to concentrate the solar energy into a very narrow collector to get very high temperature hot water (or hot fluid) to go to a power turbine or secondary boiler. Every parabolic concentrating trough I've see pictures of, and every "patent" drawing I've seen on several Google searches uses a very, very small diameter receiver that is placed at the theoretical trough focus point to heat the working fluid. Fine. it you want usably high temperature steam, you need even higher temp water/working fluid to heat the water to make the steam. Eliminatory, my dear Wattson.
But for house or apartment heating, you don't really want high temperature water, you want supplemental heating water or only 115 - 130 degrees max.
So it would seem you'd want a larger volume of slightly cooler water/working fluid out of the collector. You'd want as large a "black" receiving area of the collector as possible of course. You'd want many small pipes to pick up that heat, or some sort of a heat exchanger with as little internal volume but as large a surface area as possible exposed to the sun with as little a water volume as possible not in contact with the heating surface, right? you'd want as little non-productive area as possible - areas that are exposed to the cold Greenland air but are not actually collecting the solar energy. And, of course, you want to build the thing as cheap and as reliable as possible with as few moving parts as possible.
Question 1. So, for a parabolic trough exposed to the sun, why use a very small diameter receiver? 1 to 1-1/2 inches seems to be the maximum on any trough receiver I've seen. Why not use a larger receiver - say a double tube 6 inches OD with a 5-1/2 inch dia inner tube? The water/heating fluid would flow through the annulus between inner and outer tube.
On all current receivers I've seen, the very small "front" area absorbs almost no energy compared to the "back" of the receiver which is getting hit all of the reflected energy at the parabola's focus point. A larger dia receiver would obviously "shade" some of the reflector with its own shadow, but that same area would be hit by direct rays on its front side, so it seems there's no real loss of energy.
Question 2. If a large receiver tube is chosen, what is the equation for the reflector curve, if the "target" is not a infinitely thin long "line", but a larger, real-world diameter cylinder that is relatively large with respect to the reflector's opening size?
Question 3. Every parabolic trough I have seen has been set up for use in a desert (little rain or snow or precipitation) area with very high ambient temperatures. If a parabolic trough is used in cold climates for house heat, you need to prevent the fluid and the heat exchanger surface from the cold air itself. Is there a workable glass or plastic "greenhouse" film that could cover the front opening of the parabolic trough to trap the received heat but does not reflect too much incident inbound solar radiation?
Seems like it would require some kind of glass exactly opposite what you'd chose for a building's south wall in the Southern US.
