Solar Thermal. This isn't Your Grandma's Solar Energy! II

You're so funny Shiva!

Try focusing a parabolic mirror on your pants leg vs just nothing out in the sunshine. Make sure you're close to a burn clinic and for god's sake, use protective eyewear...you're going to need to be able to see to find that burn clinic..

China of course ignores the schills of BigOil and BigNuke so prevalent at places like this and simply somersaults into the new Century, leaving dinosaur-America behind..

[ame="http://www.youtube.com/watch?v=RkucHl4GgN8"]Concentrating Solar Power Plants 1 MW- 5 MW (Fresnel technology) - YouTube[/ame]

overlay that with fresnel...

http://www.youtube.com/watch?feature=fvwp&v=eZx4XRk6rI8&NR=1

http://youtu.be/eZx4XRk6rI8


And you get something like this:

 

The current disagreement between Silhouette and Shiva boils down to the following problem.

Imagine a thermally isolated and reflective box with a hole at its top. The sun shines vertically through the hole and is completely absorbed at the bottom, heating some water. The water is stirred to quickly distribute the heat equally. Does the water heat up more quickly without a lens in the hole, with a lens, or equally fast in both cases? Why?

Here's an illustration.

 

Equally fast. In both models same amount of energy comes in from the top

 

Disagree.
You cannot store big quantities of electrical energy effectively. If you convert it to another type of energy and then back to electricity you will lose a lot of energy.

Agree.

Agree, but if we could store the energy effectively, that would not be a problem.

Today the grid must be rated according to the worst case scenario, speak the grid must support the simultaneous feed-in of all rated capacities of all solar generators, though this scenario happens very seldom.

If we could store the energy in batteries at the places where it is generated and feed it into the grid at times when it is required, we could do this without increasing of the transmission capacity of the lines and transformers.

The biggest problem that we are facing is the insufficient capacity of the grid.

We need intelligent grids and storage capacities to make the renewable energies competitive.

 

I agree with your and Shiv's' posts for the most part. BTW, I can't wait until I get a back yard nuke! Ha ha...that is a joke, well it had some truth in it. If not for the dirty bomb potential of low>mid level radioactive waste, heat from the decaying elements from the waste waste could be utilized, in fact I would guesstimate that 75% of the energy needs of a typical home could be met by a little ole pile of radioactive garbage waste heat.

That brings me to another overblown cost in the Nuclear chain; eternal storage of waste products. We should shorten the time this waste would be held in storage. Instead of tens of thousands of years now considered safe, that figure should be lowered to less than a thousand years, with money earmarked for nuclear waste disposal R&D. A thousand years should be sufficient for technologies to emerge that would solve the waste problem.

Rev A

 

yes and no - google up Vanadium Redox Batteries or flow batteries
http://en.wikipedia.org/wiki/Vanadium_redox_battery

http://www.vanadiumbattery.com/

 

There is only so much sunlight energy available per area. The term is energy density. The amount of land needed for solar and other renewable energy including bio mass etc is tremendous (comparatively speaking). I believe also one of the few reasons why solar and other renewables will not be to provide all our energy needs. Example;

Sunlight on average gives us about 1.3 KW per square meter. In one hour, that means that one square meter of sunlight generates 1.3 KWh. According to the Energy authorities (ie the dept of energy atomic and otherwise) the 2006 total world energy consumption was 472 quadrillion BTU. That's about 1.38E14 kWh/year or 3.8E11 KWh/day. In other words even using conservative figures renewables would take a area the size of Georgia to provide enough energy to energize the world. In contrast nuclear (including the mining storage) would use only 20% of that! That is not a deaths nail in renewable but its sure is a big tack.

Rev A

 

Except for the energy lost as the light passes through the lens this is almost true if we're addressing the same temperatures being reached but there is another factor involved. As the water temperature exceeds ambient temperatures it we encounter a problem of temperature differential (^T). The greater the temperature differential the greater the heat loss from the container and that is an exponential loss of energy. As the temperature of the water heated increases the amount of energy retained in the water greatly diminishes.

Yes, insulation can prevent some of this energy loss by insulation is expensive and is not totally effective. Home applications for space heating only require temperatures of about 80F, water heating of about 140F but power generation requires 600F or more. If we assumed an ambient temperature of 70F space heating has a ^T of 10F, water heating of ^T 70F but power generation has a ^T of 530F.

There is also a problem when solar thermal is compared to coal or nuclear power production because the light is only heating the water at it's maximum energy rate for a rather short period of time for solar thermal but with nuclear and coal the water is being heated 24/7. That is what is being addressed by the "capacity factor" where coal has a capacity factor of 85%, nuclear of 90% and solar thermal of only 18%. The total rated capacity of a power plant, which reflects the maximum amout of output, must be multipled by the capacity factor to find out how much power will be delivered to the customer.

Finally there is the simple problem that the sun doesn't shine every day so there will be rare days when a solar thermal plant will produce zero electricity which requires a 100% back-up system. If we require a coal fired power plant to fill in on those occations, even though they might be rare, then we have to add the cost of the entire coal fired power plant to the cost of the solar thermal power plant. This involves a huge capital investment and even if it's never used the maintenance costs continue daily even when it's shut down. It has to be on standby all of the time or we risk having no power when we need it.

 

Half the houses in Australia use Solar hot water systems - except in places like Mitchell where the bore water is so hot that they have to run a bath in the morning and hope it is cool enough by dinner to get into!!

 

Some of the problems are that Vanadium Redox batteries have a low energy storage capability requiring huge storage facilities, the systems are complex which makes them costly and vanadium is a toxic metal.

This still doesn't address the very low capacity factor of solar thermal or the cost of the back up power plant that would be required.

 

however this is an area that is getting a lot of research and although it has it's problems it offers one solution for a current problem - our current system has no storage capacity. Does it really make sense that we do not have back up capability within the system?

http://vanadiumbattery.com/index.php/applications/index/

 

We should always be researching new technology but it should be privately funded based upon potential cost/benefit analysis by enterprise. We simply don't see that happening overall with solar power where solar energy requires over 60 times the government subsidies when compared to coal. If solar thermal was cost effective it wouldn't require government subsidies so the fact that these subsidies even exist reflects that it isn't cost effective at least when it comes to large scale power production. As was previously pointed out low temperature solar thermal applications are cost effective and don't require government subsidies.

 

Don't worry the research is coming out of Australia - New South Wales University mostly

Not all costs are monetary - CO2 aside coal is a dirty industry causing a lot of deaths not just from stack fumes but also from the mining, there are substantial environmental costs in the terms of mountain top removal and water contamination

Personally I say it is worth it - but don't worry too much you can always buy the patents off of us when we have finished the research..........

 

That technology exists right now: reprocess the "waste" into nuclear fuel. It's mostly Pu-239, which is fissile. It will require a plutonium-burning reactor, but that's possible; the French use that fuel cycle today. Kill two birds with one stone.

You can only get 1.3 kW/m² above the atmosphere. Down here on the surface, count on 1 kW/m², and that's with the Sun directly overhead on a clear day. Then you also have to account for night, clouds, and the Sun not being overhead all the time. So an average square meter will get about 200 Watts year round, 24/7.

Well, not really. Land cost is a pretty small part of the total equation, especially when you consider that solar can be put in otherwise waste areas, like deserts. The real issue with solar is that it just takes a LOT of collecting area to get much energy, and that means capital costs are quite large. PV seems to be making progress with scale-up and technology improvements, but I don't see how solar-thermal can make any breakthroughs there. A mirror is still a mirror and always will be.

 

Except for "Except for the energy lost as the light passes through the lens this is almost true if we're addressing the same temperatures being reached but there is another factor involved.", I am not sure what anything else has to do with Herby's problem.
The water is stirred so heat loss from the container is the same in both figures.

 

Yes here they are called or used to be called breeder reactors. However there is still dump trucks of low level waste and less mid to high level waste. Still good you brought it up! It seems to me that these vaults could be easily if expensive to hold the waste for a thousand years. Today they want waste to be stored forever, on the order of the half lives of the raidotactive stuff, some with half lives in the order of (memory fails me but I think its over ten thousand years). In a thousand years if we are still scraching our heads about the waste we are in trouble! Ha ha~

I forget who did the study I used for reference. I will attempt to find it. Thanks for your info and opinion.

Ha, ha I see you have not experienced our EPA yet!!. The desert ain’t waste areas, according from everyone from me to greenies to federal agencies. For the most part they are protected environments. However I was making the point that solar takes too much space for energy produced. I could come up with better reasons not to try solar on a huge scale. Don’t get me wrong I love alt. energy as I said my next home is a passive solar dream, completely off grid capable if need be. But generating huge amounts of juice even if could be done would be too expensive right NOW. And we need something desperately right NOW. We could begin plowing money into renewables and R&D after we get over this crisis point. I foresee the day when we will have 100% non nuclear and other truly clean energy sources, not today, not in a decade not in twenty years, I would guess in fifty years we may have clean energy even if its not fusion!

Didn’t I say that? Ha ha energy density....eh?

I would LOVE to see a dramatic break through in PV tech! That would be a godsend for the world, even that would be an understatement! Even if it was just cost the benifits would be unforseeable especially to third world counties etc. If we could 10x increase the efficiency AND cut the costs dirt cheap then who would need fusion? Well deep space crafts would still need some kind of dense energy, fission could do that to a point right now. Another thing why couldn’t an orbiting gas generator not a geostationary orbit but rather one that circled the earth use sunlight then the deathly cold to expand a working gas and produce electricity via a turbine? It should be way cheaper than solar cells or RTG’s to generate power.

Rev A

 

The suns energy is relatively constant (about 420 btu/ft sq in outer space) and that is acknowledged but we have to take into account energy lost as well because we're concerned with the net energy that is converted into electricity and delivered to the customer and the cost of that net electrical energy to the consumer. This is where large scale high temperature solar thermal electrical power generations fails from a cost-benefit analysis.

When it's night there is just as much solar energy but it's not being collected. When it's cloudy much of the solar energy is blocked by the clouds and not collected. In the morning and afternoon the sun passes through more of the atmosphere and much of it is lost. When we have a field of parabolic or Fresnel lens collectors the energy is collected but a significant percentage is lost as radiant heat before the fluid reaches the boiler. When a heliostatic mirror system focuses on a boiler tank that tank gets very hot and radiates a significant amount of heat. Infrared images show this energy loss at solar thermal powerplants. These energy losses are significant in high temperature applications so our net energy that finally reaches the customer becomes more costly.

 

Deserts are perhaps the most fragile of all eco-systems on the planet and deserve the protections we afford to them so this is a point well noted. Hundreds of square miles of solar power plants would have a significant negative impact on our deserts.

 

http://www.politicalforum.com/scien...-solar-pv-plant-saudi-arabia.html#post4957713

New info on PV applications.....

 

While dreams of a 10X increase in PV electrical production isn't even mathmatically possible I'm actually amazed at how much progress has been made since I was involved as a solar power project manager in the 1970's. PV is now on a par with solar thermal as far as cost per kwh for electrical production even though both remain very expensive. I would not have thought that the gains made were even possible 30 years ago. The costs can't go much lower because we're almost down to the cost of the silica and energy required to process the cells today and neither are going to go any lower. We might be able to get some more efficiency from each PV cell but that becomes a matter of "cubic dollars" similiar to getting more horsepower out of a small block Chevy motor. There is a relationship between cost and efficiency where to obtain more efficiency typically costs more so there is a cost-benefit balance to it.

But it has come a long ways as we've developed about the most efficient PV cells per dollar where simplicity of design and the economics of scale has reduced costs to perhaps 60% of what they were in the 1970's. The "low hanging fruit" in reducing cost per kwh of electricity have already been picked. We'll probably see a slight reduction in cost per kwh of electricity in the future but most of the cost savings have already been realized.

 

Me neither. After following over 500 posts on this subject, I have seen little progress and a lot of repetition. My perspective on this is that the whole energy issue is, at its core, a physics and engineering problem. I'd like to break a complicated problem down into small parts that can be examined and understood separately before they're assembled into a big picture again. That's the usual approach in science and engineering. After seeing barrages of information mixed with opinion fail over and over again to achieve anything in this thread, I hope many will agree that this is a more viable approach to the issue.




So far everyone agrees that in the example above the water at the bottom is heated equally fast with or without a lens. The lens doesn't change the amount of energy contained in the light coming from the top (neglecting losses due to reflection off the lens or absorption within it). The same is true for reflectors. The concentrators increase the amount of energy per area (intensity) at their focus, but the area shrinks by the same amount, such that the total energy of the light hitting the absorber stays the same.

We now know that the energy refracted through a lens or reflected off a mirror is ideally the same as the energy of the incident radiation. The great thing about this simple statement is that we can calculate a first comparison in between coal and solar power without worrying about the details of power plant design. Let's see how much coal we need to burn to get the same amount of energy as is reflected off a mirror in the desert during one day.

The energy contained in one pound of Powder River Basin Coal is 8800 BTU or 2.58 kWh. This coal supplies around 40% of the fuel for power plants in the US and costs $12.50 per short ton (= 2000 lb). (Source: http://www.eia.gov/coal/news_markets/#spot)

The energy collected by a two-axis tracking mirror with one square meter of reflective area is 10 kWh throughout one day. This is an annual average for the most suitable regions for solar power in the US. (Source: http://rredc.nrel.gov/solar/old_data/nsrdb/1961-1990/redbook/atlas/)

This means that 3.9 lb of coal (worth 2.4 cents) need to be burned to get the amount of energy one square meter of a solar concentrator collects in one day. In one year, one square meter of two-axis tracking solar concentrator generates the same amount of thermal energy as burning coal worth $8.85 does.

Unfortunately, I haven't found any numbers yet on the cost or expected lifetime of solar concentrators for large scale projects. At the very least, the comparison to coal gives a very rough idea of what those solar installations should cost per square meter of mirror or lens in order to be competitive.

 

That is an interesting observation. But tend to disagree that turning deserts into solar energy power plants is a bad thing.

Ludwik Kowalski (see wikipedia)
.

 

http://www.politicalforum.com/scien...solar-builds-solar-pv-plant-saudi-arabia.html

Doesn't require that one damage the eco system.

 

Actually much life thrives in the desert with extra shade. Shade is crucial to creatures preserving internal body fluids during times of high heat. Increase of shade from solar parabolics and fresnel concentrators = more desert life. In fact, these solar thermal power plants can serve as mini-oases in the more vast areas of the desert.

Didn't look at it that way, did you? I grew up in the desert. In areas where there was shade, there was more life teeming around. Go figure. Solar thermal plants in the desert will increase fauna abundance.

Do you realize how vast the American desert and arid/Sunny regions are? In addition, areas that just receive sun all Winter and Summer, with occaisional rain or snow storms, like the entire Midwest for example, are capable of producing massive amounts of MW power production via solar thermal. In another thread here I proposed we build solar thermal ethanol refining plants dotted throughout the Midwest. That's where the corn and other biomass can be produced and that's how you reduce costs of production per gallon of ethanol: use solar steam instead of steam heated by gasoline!

Yes yes...why worry about a little detail that makes all the difference in the world? Why worry about how focused sunlight can cut through steel, while the same amount [in pure mathematical terms] of solar energy that isn't focused, merely shines back passively at the same temperature? Using that logic, next time you want a lot of light out of your headlight bulb, use a flat mirror, instead of a parabolic one behind the little bulb. You'd be amazed how small those little headlight bulbs are given how much light they throw out.

Besides, are we talking about thermal intensity or photons? I'm talking about thermal intensity. Get a magifying glass or parabolic mirror and shine it on your crotch vs just a flat mirror of the "same mathematical 2-dimensional area". Same "basic photo energy", yet different results. See how long it takes you to boil a kettle with a flat mirror focusing "the same amount of solar energy" as its same-size parabolic counterpart. We're talking about creating steam to run turbines, not creating a tanning booth for yuppies.. Seems the design of a power source DOES matter. You'll see what I'm talking about.

Source: http://en.wikipedia.org/wiki/Common_sense

 

It's nice to see that we're approaching the source of some disagreements, Silhouette.

Let's picture the light as equally spaced parallel rays coming from the sun. In that picture, we can see the energy transferred onto an absorber by counting the rays hitting it. An ideal lens or mirror does not change the amount of rays as it refracts or reflects the light. All energy is transferred as light. When collecting energy from the sun, the goal is to direct as many rays onto the absorber as possible. In the example below, the same number of light rays hit the black absorber in both cases and the amount of energy collected is the same.



As thermal energy Q is added to a body, there is a change in temperature ΔT given by

​

C is the heat capacity of the material and m the mass of the body to be heated.

When you focus light with a lens, the amount of rays hitting a small area is drastically increased, while the surrounding area, the shadow of the lens, gets less light. In terms of the formula above, you're sending the same amount of energy Q that was distributed on an area the size of the lens onto a much smaller area in the focus with a much smaller mass m. Since you're heating less material now, the temperature change ΔT at the focus is much bigger than without the lens.

Most importantly, however, the amount of energy redirected by the lens is proportional to its area as seen from the sun's point of view. The only way to get a lot of energy onto a small absorber is to build mirrors and/or lenses covering a big area.

By the way, the temperature is limited by losses such as thermal conduction, convection, and thermal radiation. As soon as the energy lost per time equals to the energy absorbed per time, the temperature stays the same. Those losses are proportional to the surface area of the absorber, which means that it's advantageous to build small absorbers. Thermal conduction and convection are also proportional to ΔT, while thermal radiation is proportional to T^4 (use absolute temperatures here). In other words, the losses grow in a (rather complicated) polynomial way as temperature increases.


I hope I could clarify a few things. My previous calculation based on the principle of conservation of energy still stands. I keep claiming that the amount of energy collected is proportional to the surface area of the mirrors and/or lenses (as seen from the sun).

You mentioned common sense, Silhouette. In this case, your common sense appears to be wrong. That happens in physics sometimes. Nature doesn't behave according to your or my common sense, but follows its own rules instead.

By the way, if I'm wrong somewhere, please point it out.