[livesimply]

Claims like this are frequent, and often are a way to lure venture capitalists, whether or not they actually ever pan out.

[Graeme]

This is quite an interesting introduction to the solar power industry particularly for someone like me who doesn't know much about it. I would be interested to recieve any comments from viewers who are active (or have been active) in this field of solar energy, or even those who have bought solar panels and would like to share their experience.

FAQ: A concentrated power boost for solar energy

Concentrating solar power, which has been around for decades, is one of the most promising techniques being tried today to make solar electricity more cost effective.

The concept is simply to focus light in order to boost electricity output. But there's a wide disparity in the types of solar concentrators being built, from utility-scale solar thermal projects to specialized photovoltaic solar panels that could one day go on a homeowner's roof.

In this FAQ, we will specifically discuss concentrating photovoltaics, a design being pursued by a number of solar companies seeking to lower the cost per kilowatt the sun can deliver.

What are the primary forms of solar concentrators?

Why is there interest in concentrating photovoltaics?

Why not just improve solar cells?

OK, concentrating light onto solar cells means more power output. But does that mean it's more cost-effective?

So what do these solar concentrators that use photovoltaic cells look like?

What are the tradeoffs of this approach?

Are these concentrating photovoltaic systems commercially available?

How will things look a few years from now?

CNET

[Tanada]

In general this is a viable choice because building a reflectig surface is much cheaper than building a solar array itself.

[PeakingAroundtheCorner]

I'm not actually in the industry but I do admin the website for a leading internet supplier of solar power products and, as such, regularly study solar power.

Concentrators are great for heating a glycol solution that is then used to produce steam for generating electricity. Nevada Solar One is a perfect example. The use of concentrators to boost the energy output of a solar array is a very good idea because it increases the amount of photons per square inch of panel resulting in a higher output efficiency.

There has even been talk that such a reflective concentrator could be assembled of mylar-like material that stays in a fixed orbit above a massive solar array and which is programmed to point at the array day and night like a giant flashlight. The extra hours of light on the array at night would be just that much more electricity generated and such a site would probably look pretty awesome in the deserts of Nevada, California or Arizona.

[joe1347]

SciAM Article
September 19, 2007
Sunny Outlook: Can Sunshine Provide All U.S. Electricity?
Large amounts of solar-thermal electric supply may become a reality if steam storage technology works—and new transmission infrastructure is built
By David Biello:

In the often cloudless American Southwest, the sun pours more than eight kilowatt-hours* per square meter of its energy onto the landscape. Vast parabolic mirrors in the heart of California's Mojave Desert concentrate this solar energy to heat special oil to around 750 degrees Fahrenheit (400 degrees Celsius). This hot oil transfers its heat to water, vaporizing it, and then that steam turns a turbine to produce electricity. All told, nine such mirror fields, known as concentrating solar power plants, supply 350 megawatts of electricity yearly.

"The maximum you can get into the grid is about 25 percent from solar," including photovoltaics

Scientific American has a recent article on Solar Thermal and besides the typical hype from the solar companies, I was surprised to read that solar thermal power plants (for generating electricity) are currently limited to about 25% efficiency.

I wonder what limits the overall efficiency in a solar thermal system? A quick look at figure from Schott on the evacuated glass solar thermal receivers indicates about 81% of incident (concentrated light) is converted to heat (95% absorbed - 14% emitted = 81% efficiency). I'm guessing that the concentrator optics are about 80 to 90% efficient, which then translates to about 70% conversion efficiency (to heat) for incident (unconcentrated) light. This seems to indicate that the thermal (hot oil or steam) conversion to electricity is about 33% (33% x 70% = 25% total system efficiency).

http://www.us.schott.com/solarthermal/e ... tails.html

The absorber tube (SCHOTT), which is made of steel and is located on the inside, must be capable of absorbing a lot of solar radiation without emitting significant amounts of heat. To achieve this, SCHOTT developed a coating that offers an absorption rate of 95%. At a temperature of approximately 400° Celsius, only a maximum of 14% of the total heat is emitted.

I believe that thermodynamic efficiency is increased if the temperature is increased, so wouldn't a larger parabolic or fresnel concentrator heat up the oil (or steam) to a higher temperature and thus improve efficiency? Desert land and mirrors seem to be fairly inexpensive. Or is the 400C temperature a maximum for some other reason. Such as, above 400C the heat emitted (lost) increases rapidly or does the downstream hardware fail in some way because it is unable to handle higher (>400C) temperatures.

[sicophiliac]

I would be curious to what the EROEI would be for this technology. Also how would the parabolic heat trough set up compare to the ones which are circular and focus energy in the middle like a giant magnifying glass. 25% is roughly the same as a coal fired plant isn't it? I would also imagine that the cost both energy and monetary wise of the mirror set up would be vastly lower than that of all the mining and drilling equipment that goes along with coal. The mirrors would require a hell of a lot less ore and natural resources in general compared to the thousands of tons of steel that goes into all the mining equipment used for coal mining. The costs and energy inputs in the basic steam turbine set up would be comparable to any run of the mill natural gas or coal fired plant too. Of course solar energy wont run out for billions of years where as fossil fuels have decades at best.

[sicophiliac]

Well I guess the energy invested in manufacturing the mirrors, components of the plant, wiring up the grid ect.. compared to the energy gained in the life span of the plant. I would imagine dust/sand storms over decades would degrade and chip away at the mirrors till they are no longer effective.. however that would take a long long time. It might be 10 years at least before one could even detect a difference in efficiency. Considering that the basics of the steam turbine generators are not much different than that of a conventional fossil fuel powered plant that part wouldn't change much. Certainly the energy invested in producing a few acres worth of thin relatively light weight mirrors would be far far lower than bulldozing a mountain side for coal or drilling and exploring for natural gas for years on end.

[jbeckton]

I was surprised to read that solar thermal power plants (for generating electricity) are currently limited to about 25% efficiency.

Why, after 150 years of profection a typical coal power plant is still only about 33% efficient.

Your car is probably only about 25% efficient.

If you have ever taken a thermodynamics course, you would discuss carnot, which is the theoreticical limit of efficiency for a process. It is much more useful to compare efficiency to carnot than with a percentage out of 100.

The carnot efficiency is 1- (Low temp/High temp). This is the absolute max efficiecy allowed because not all of the energy is recoverable.

Compare the ~25% to this number to see if it is really that bad.

It's been a while but there is another efficiency that compares the two,it might be called the 2nd law efficiency.

[skyemoor]

"The maximum you can get into the grid is about 25 percent from solar," including photovoltaics, Mills says. But "once you have storage, it changes from this niche thing to something that could be the big gorilla on the grid equivalent to coal."

Reading the quotes again, you will see that they were talking about the mix of energy on the grid, not the efficiency. Intermittent sources are limited because they can only provide power at certain times (i.e., tidal, solar during daytime), or when certain conditions are right (i.e., wind, and solar in partly cloudy areas). What happens during the times of low power output? There are a number of answers (i.e., spot pricing, demand side management, etc) and if those intermittent sources are coupled with energy storage, they will be able to reduce the supply intermittency, allowing them to provide a greater percentage of grid power.

[joe1347]

"The maximum you can get into the grid is about 25 percent from solar," including photovoltaics, Mills says. But "once you have storage, it changes from this niche thing to something that could be the big gorilla on the grid equivalent to coal."

Reading the quotes again, you will see that they were talking about the mix of energy on the grid, not the efficiency. Intermittent sources are limited because they can only provide power at certain times (i.e., tidal, solar during daytime), or when certain conditions are right (i.e., wind, and solar in partly cloudy areas). What happens during the times of low power output? There are a number of answers (i.e., spot pricing, demand side management, etc) and if those intermittent sources are coupled with energy storage, they will be able to reduce the supply intermittency, allowing them to provide a greater percentage of grid power.

Possibly that is what the article meant, but a little more searching around turns up similar Solar Thermal efficiency numbers (linked below) to what was originally posted. Converting the hot oil (or steam) to electricity seems to be the most inefficient part of the process. The original questions still appear to be valid.

1. I believe that thermodynamic efficiency is increased if the temperature is increased, so wouldn't a larger parabolic or fresnel concentrator heat up the oil (or steam) to a higher temperature and thus improve efficiency?


2. Desert land and mirrors seem to be fairly inexpensive. Or is the 400C temperature a maximum for some other reason. Such as, above 400C the heat emitted (lost) increases rapidly or does the downstream hardware fail in some way because it is unable to handle higher (>400C) temperatures.


3. Alternatively, is there a law of diminishing returns associated with concentrator optics?

http://www.volker-quaschning.de/article ... dex_e.html

Trough Power Plant Efficiencies

The efficiency of a solar thermal power plant is the product of the collector efficiency, field efficiency and steam-cycle efficiency. The collector efficiency depends on the angle of incidence of the sunlight and the temperature in the absorber tube, and can reach values up to 75%. Field losses are usually below 10%. Altogether, solar thermal trough power plants can reach annual efficiencies of about 15%; the steam-cycle efficiency of about 35% has the most significant influence. Central receiver systems such as solar thermal tower plants can reach higher temperatures and therefore achieve higher efficiencies.

[skyemoor]

1. I believe that thermodynamic efficiency is increased if the temperature is increased, so wouldn't a larger parabolic or fresnel concentrator heat up the oil (or steam) to a higher temperature and thus improve efficiency?

Thermodynamicor efficiency doesn't necessarily increase with temperature increase; remember that simply raising the amount of energy input does not mean that it will be consumed with less loss. A giant fresnel could be more efficient than a trough collector because there is less surface area were losses are accrued. And the larger the trough collector, the farther the collector mirror surface is from the collector tube, which means a larger percentage of heat loss due to collimation inconsistencies. Your overall question would be best answered by an engineering model (backed by empirical data from existing collector fields).

2. Desert land and mirrors seem to be fairly inexpensive. Or is the 400C temperature a maximum for some other reason. Such as, above 400C the heat emitted (lost) increases rapidly or does the downstream hardware fail in some way because it is unable to handle higher (>400C) temperatures.

I would think it would be losses that are the issue. Remember, troughs are all lined up in rows with output pipes leading to manifolds where the energy is finally extracted from the heated liquid. In a tower, all of the components are right there, not having to travel very far. And the significant lengths of pipes in the collectors themselves have quite an entropic effect.

3. Alternatively, is there a law of diminishing returns associated with concentrator optics?

The light must be focused effectively on the trough collector's tube. The larger the collector, the farther away from the tube the collector surface is and the greater the inaccuracies.

[joe1347]

Another reference seems to indicate that the maximum operating temperature for a parabolic trough solar thermal concentrator is about 400C. My interpretation is that the concentration optics are designed (sized) to ensure that the maximum operating temperature of around 400C is not exceeded, which appears to set an upper limit on the concentration ratio. Also, as others have pointed out, the carnot efficiency of the heat exchanger and steam turbine and associated hot oil and steam 'plumbing' appears to be limited to around 33% - given a 400C max 'input' temperature.

http://www.greenpeace.org/raw/content/i ... -Power.pdf

From page 13.

The use of oil-based heat transfer
media restricts operating
temperatures today to 400°C,
resulting in only moderate steam
qualities

PARABOLIC TROUGH SYSTEMS
Technology developments
Parabolic trough systems represent the most mature solar
thermal power technology, with 354 MWe connected to the
Southern California grid since the 1980s and over 2 million
square metres of parabolic trough collectors operating with a
long term availability of over 99%. Supplying an annual
924 million kWh at a generation cost of about 12 to 15 US cents/
kWh, these plants have demonstrated a maximum summer
peak efficiency of 21% in terms of conversion of direct solar
radiation into grid electricity

With these facts (observations actually), I wonder what can be done technically to improve solar thermal efficiency? Obviously, reducing the cost of the components will reduce the cost per Kw-Hr? But are there technology development opportunties associated with concentrating solar thermal that are (or are not) widely known as opposed to the fairly well known technology limitations associated with Photovoltaics?

[jbeckton]

With these facts (observations actually), I wonder what can be done technically to improve solar thermal efficiency?

I am going to assume that many of the constriants are the same a they are in a coal fueled steam cycle.

1) Materials issues. Most plants operate at about 1005 deg F and 2600psi. These factors are directly related to efficiency, if we could produce piping that could handle a hotter and more pressurized load economically, we could increace efficiency.

2) Cooling. After the steam goes through the turbine, it must be condensed back into water, otherwise you are never going to build up that kind of pressure if you start with steam.

3) Thermodynamics. You can never convert all of the heat into work.

[aahala]

I wonder if the 12-15 cost per Kwh includes the cost of land.
If so, I have a idea that should drive the cost down quite
a bit.

The US government is the largest owner of land west of the
Mississippi and the SW -- by far. Much of this federal land is
now being used for other purposes or would not be suitable
for solar thermal. What's left would still possibly consist of
several thousand square miles!! Lease it for solar thermal
production for a $1 a year per square mile.

[yesplease]

Provided solar thermal could be around 10cents/kWh w/ leased gubbermint land, the problem is sneaking that by the fossil fuel lobby IMO...

[davep]

With these facts (observations actually), I wonder what can be done technically to improve solar thermal efficiency?

There is no problem with solar thermal efficiency. The problem is conversion to electricity, as stated above. The thermal efficiency can be very high if you throw enough money at it. Ideally, in my opinion, it is better to look at a decent efficiency with low cost (for expedient home use). This involves things such as replacing black nickel selective coatings with something cheaper (such as selective paints that are easier to apply). Far cheaper reflective material that is less UV resistant is also an option.

The real problem is the conversion to electricity. The best bet for approaching 100% carnot efficiency is the stirling heat engine. This is theoretically more efficient than other heat engines due to the regenerator between the hot and cold heat exchangers, enabling reuse of the heat/coolness as the air passes through the regenerator prior to hitting the heat exchanger.

For the kind of high temperatures used with stirling engines, you are better off using two-axis parabolic dishes than single axis troughs.

[yesplease]

The fossil fuel lobby doesn't have much clout with the current congress. Many of their tax breaks, during the first 6 years of the Bush Admin, are currently at risk. The current energy bill is partially funded by allowing the oil industry tax breaks to expire. The new tax breaks are for solar and wind subsidies.

There's a world of difference between losing subsidies/tax breaks they managed to get by earlier, and actually loosing ground to a clean competitor. It's kinda like the vast sum of money that was spent opposing the CA gas tax a couple years ago. If they stand to loose out on enough cash, they'll start flexing their muscle. They have in the past...