Solar Thermal Deserves Our Support
Here’s a good article on a recently completed solar thermal tower (aka concentrated solar power or CSP) in the desert between Las Vegas and Reno, NV.
The thing to like about solar thermal, as we’ve often discussed here, is that it affords us a fairly low-cost way of storing energy and delivering it when the sun isn’t shining. This is due to the fact that in today’s world, we can store heat energy (in vats of molten salt) far less expensively than we can store electrical energy (in batteries). Thus solar thermal installations can be treated as baseload, delivering power on a consistent 24X7 basis.
That’s the good news.
The bad news is that they’re still expensive – in the neighborhood of $5 – $7 per Watt.
Yet those of us who favor solar thermal believe that costs will fall dramatically as these technologies mature. Solar thermal is really in its infancy, lagging several decades behind photovoltaics (PV) and wind in terms of its development. The plummeting cost of PV (to about $1 per Watt) wouldn’t have happened in a million years without the support of an enormous number of public and private agencies. I’m among those who hopes that solar thermal will be given the same chance.
Craig, do you see solar thermal as strictly a grid scale development when it becomes a mature technology?
I personally do not think that it has a low enough cost structure for sizing it at the home scale. Maybe it is comparable to Mao’s vision of backyard blast furnaces for steel production.
One of the major cost factors in a solar thermal installation is the cost of insulating the storage vessel. The cost of storage for both containment and insulation increases at a slower rate as the vessel gets larger. There is a definite economy of scale that is not as apparent for PV. The direct efficiency of a PV array or a wind turbine does not increase with the number of units installed. There are economies to be made by installing farms of collectors or turbines simply to cut down on serving trips but the output efficiency of one unit is the same as the output efficiency of 100.
So, a homeowner has less of a disadvantage by going with 1 turbine or one roof top system than he/she would have from trying to economically build one thermal storage unit.
Comments are welcome. LL
Solar thermal has two different meanings/applications. Solar thermal water heating is ideal at the household level, where solar thermally generated electricity doesn’t make any sense at all.
Except in remote areas of the tropics where there is no need to heat buildings but where grid power is not available. That is exactly the situation in remote areas of Fiji.
For heat storage, a tank containing fused salts (40% KNO3 and 60% NaNO3) is used. However, saying that that makes it possible to generate electricity on a 24/7 basis assumes that the weather will never be cloudy long enough to run out of thermal storage. Whether that assumption is accurate depends partly on the amount of storage that can be included in the system without excessive cost.
The power tower system may be the best way to generate large amounts of electricity from solar energy because:
1) The land need not be leveled. In fact, mirrors can even be installed on hills. Thus, environmental impact is less than for other solar systems.
2) Storing energy as heat is more practical and less costly than other storage methods except for pumped storage, which is not usually available.
3) The high temperatures generated can result in higher efficiency thereby reducing land area requirements.
4) It is probably less expensive than PV systems.
Even so, water is necessary for cleaning mirrors and for condensing steam, so the system may not be practical in areas where water is scarce. Also, it is considerably more expensive than nuclear or fossil fuel power and the high costs would create political and economic problems if attempts are made to implement it on a large scale.
I think solar thermal will always be a niche market as it is geographically linked to the availability of cheap desert land next to a large market (e.g. So Cal) that’s willing to pay above market rates for solar power.
To Larry’s point about distributed CSP, check out http://www.cyclonepower.com/whe.html. Watch the videos. I apologize if I seem to be beating the drum for Cyclone. I just think it’s a great technology. I have no connection to the firm.
Good point, but “always” is a long time. I foresee a day when power transmission in the Continental US (as well as other developed regions of the world) will be far better than it is now, and thus the location of the generator is irrelevant.
The location of the generator would never be irrelevant, even if the power transmission were 100% efficient. Transmitting power over longer distances always requires a higher investment cost. And, for really long distances, efficiency requires using DC and the equipment to convert from AC to DC and back is not inexpensive.
Regarding the Cyclone power system which uses waste heat, I don’t understand why they are using the Rankine cycle. It seems to me that the Stirling cycle would be more efficient and simpler.
Can waste heat or biomass be used to generate electricity? Of course. Is there any evidence that these people have added value in this space? Not as far as I can see. Since you feel otherwise, please feel free to call and chat about it, but come well armed. 🙂
To the extent that otherwise wasted heat can be effectively utilized, it reduces the use of fossil fuel. Until fossil fuels can be phased out, surely it is beneficial to utilize the waste heat generated by using fossil fuels.
Generating electricity from methane derived from animal manure would also result in some waste heat which could possibly be effectively utilized.
Utilizing waste heat from fossil fuel combustion can be uneconomic due to the lower temperatures involved. The Carnot cycle limits the efficiency of the process.
Degraded heat is most efficiently used by processes that require only a few degrees above ambient temperatures. Electric generation is not one of these processes.
The most effective use that I have seen involves the heating of green houses cited near power plants and the heating of water for warm water fish farming. Some cities also melt ice on sidewalks in the winter time with spent steam. I don’t think that there are many power plants right in the middle of cities anymore.
L
Waste heat is commonly used for air conditioning via an absorption cycle using lithium bromide. It is often more economical for large buildings, including office buildings, hotels, and hospitals, to generate their own power using engine driven generators and utilizing the waste heat than it is for them to buy power from the grid.
Absorption A/C systems using lithium bromide do not require high temperatures; 100 C or slightly lower is sufficient. Although such A / C systems have a COP typically 3.5, the waste heat is free so the low COP is not the only consideration.
Using waste heat for A / C probably, in most situations, makes more sense than using it to generate electricity because of the low efficiency if used to run an engine from such a low temperature source.
Craig,
The biggest advantage – in my opinion – that solar thermal has over PV is the fact that a turbine generator can easily last 40+ years, while the inverter for the PV systems will generally fail after ~10-15 years. While PV is cheaper on a capital basis, the cells degrade far more quickly than the CSP heliostats, and the eventual serious energy generation drop due to inverter failure.
Of course, as I brought up last time, there’s a time value for money, so capital cost is very important… and unless the CSP system is dry cooled then the water usage in the middle of the desert is nothing short of evil.
I do not think storage, or baseload capacity, is important. The highest value energy is that which is brought to market during the day, and the highest value time frames are between 12:00 to 6:00. The time at which energy is most worthless is between midnight and 6:00 am. Why should a CSP project spend fortunes so that some of its energy can be produced while energy is valuable, only to be sold while energy has no value?
There is no shortage of spare capacity during the middle of the night. All that should matter is whether you are producing and selling low-carbon energy (which forces fossil energy to be tamped back).
I guess that’s a good point. It’s one that David Mills, then CEO of ASRA (solar thermal company in California, later sold to French energy giant Areva) made when I interviewed him for my first book. “There is a huge correlation between the presence of the sun and human activity.”
Except that if battery electric vehicles become common, the demand for power to recharge them at night would greatly increase the need to generate power after sunset since that is when most owners would want to recharge their vehicle batteries.
Also, if fossil fuels are replaced by renewables, and a large portion of the power is generated from solar energy, then storage would become critical even though it may not be now.
Frank,
It is virtually unthinkable that there will be a significant penetration of EV’s within the next two decades. Perhaps by then there will be enough improvement and price reduction in the vehicles to make them competitive, but they are nowhere near competitive now.
Right now, extreme subsidies and gimmicks such as HOV lane passes have kept a meager sales rate of under 20,000/year, but those sales will collapse as soon as government support is lifted, and since the support is completely unjustified (coal is more polluting then oil, why are we subsidizing the replacement of oil with coal…) it will eventually be removed in an era of concern over deficit spending.
Craig,
One thing that should be noted concerning CSP is the extremely low cost of mid-grade energy steam. The inefficiency of converting mid-grade energy into electricity then leads to a higher net cost for electricity… but there are few cheaper ways of generating steam.
Originally imagined as a portion of the WindFuels system, one of the innovations we are developing is a dual-sourced organic rankine cycle that is specifically designed to couple a low grade and a mid grade energy source and convert that energy at higher efficiency. I’m not talking about breaking carnot limits… I’m just talking about getting closer to carnot limits then current generators achieve.
Using this technology, if you used either a shallow-well enhanced geothermal system or a small CSP system for low-grade energy, and a large CSP system for mid-grade energy, you could SUBSTANTIALLY lower the cost of electricity. The price point is completely unbeatable for the energy itself – we just need better technology to more efficiently convert it.
IF you do something like this, there would be some baseload power from the geothermal well even as the solar generation tapers off at night, and the generation would cycle at a much closer match to actual energy demand (very low at night, very high in the middle of the day).
As I understand it, the current focus (pardon the pun) of the industry is higher temperatures, for the reason (Carnot limit) that you express here.
Craig,
Indeed Carnot limits are important… But the theoretical limit is: (1 – exhaust temp/high temp). So in theory the limit for efficiency of a 550 K CSP system is ~45%. Most of these systems struggle to get better than ~30%. If you can get a 550 K system to achieve ~38% efficiency, you improve the power generation by 27%. In order to achieve the same efficiency improvement while maintaining the same performance (in this case the percentage of Carnot limits)… the temperature would have to be pushed to ~875 K. But pushing the temperature comes at a cost… generally the cost PER WATT of thermal energy doubles for every ~100 K that you increase the temperature.
The only reason their trying to increase temperature is the fact that current technology has much better performance at higher temperatures, and much worse performance at lower temperatures – for instance, most low-grade geothermal power achieves less than 50% of the Carnot limits, while modern gas turbines might achieve 70-80% of their Carnot limits.
But if there were technology available (which there soon will be) to push the performance of low-grade energy recovery, the price point for both CSP and geothermal energy will start dropping through the floor.
As I understand it, the Stirling engine, which is actually quite old, comes closer to achieving the Carnot efficiency than any other type of engine. Perhaps that is one reason that some solar electric systems use Stirling engines. When used with a parabolic dish, it should be possible to achieve quite high temperatures to get good efficiency.
The Stirling-solar systems I’ve read about all use numerous Stirling engines which I would think would unacceptably increase investment costs and maintenance requirements. Perhaps a solar tower system could have a Stirling engine on top of the tower instead of something to heat a liquid for the Rankine cycle.
One of the problems with Stirling engines is that quickly changing the output is awkward, but for electric power generation, it shouldn’t be difficult to work around that problem.
Frank,
The problem with the Stirling engine is cost. Cost/W is the final arbiter. If you can get a steam turbine that gets ~50-65% of Carnot limts for 1/4 the cost/W of a Stirling engine which can get 80+% of Carnot limits… you go for the cheaper option, because otherwise the capital cost for your system will eat you alive. This is especially true for lower-grade energy systems like CSP and geothermal systems where the Carnot limit isn’t going to be all that high to begin with.
But we should be able to match the $/W cost of current tech with a Rankine cycle that gets significantly better performance. At least that’s the goal.
The Stirling solar company – which coupled parabolic heliostats with Stirling engines – went bankrupt because of their cost.