Why CSP Should Not Try to be Coal  

Tom Konrad has a small group of solar thermal power related posts up at AltEnergyStocks - Why CSP Should Not Try to be Coal.

Joe Romm, at the influential Climate Progress blog, has hit on a formula for countering the coal industry's claims that we need baseload power sources. Since Concentrating Solar Power (CSP) in conjunction with thermal storage can be used to generate 24/7 or baseload power Joe has renamed it "Solar Baseload." This is win-the-battle-lose-the-war thinking. While it does neatly counter the argument we need coal or nuclear, since there are renewable power sources which can produce baseload, such as CSP, Geothermal, and Biomass. I fell into this coal-industry trap myself in a 2007 article about Geothermal, as did AltEnergyStocks Editor Charles Morand in an article on CSP.

Dispatchable Solar, not Solar Baseload.

Continuous power from solar energy was first demonstrated at the Department of Energy's (DOE) Solar Two project in the late 1990s. I recently interviewed Bill Gould, CTO of CSP company Solar Reserve. Solar Reserve is now working to commercialize the molten salt thermal storage and solar receiver technology demonstrated at Solar Two, where Bill Gould served as project manager.

According to Gould, DOE's intent at the Solar Two project was to demonstrate dispatchable power, not baseload power. Dispatchable power is power that can be called on when needed, in contrast to baseload power, which is essentially always on. As a demonstration, Gould's team throttled back the output from Solar Two to 10% of capacity, and this allowed the plant to produce power continuously for a couple weeks until it was interrupted by several consecutive days of cloudiness. But, in essence, it was a stunt: baseload power is far less valuable than dispatchable power.

The coal industry says that we need baseload power because our refrigerators still come on in the middle of the night. This is like saying we should have the water running constantly in the kitchen sink because we may get thirsty at any time and want a drink. Put in these terms, the assertion that we need baseload power is clearly nuts: what we need is controllable power that's there when we need it, but is not wasted when the lights are off and the fridge is not running.

The Problem With Baseload

Last spring, I discussed one of the problems with baseload power. The more baseload power you have, the harder it is to use variable generation such as photovolatic (PV) solar and wind power. Or, from the baseload generator's perspective, the more variable generation on the grid, the less baseload power can be added. This fact has not been lost on the UK's nuclear industry, which is fighting to get wind targets lowered.

To illustrate the incompatibility of baseload and variable energy sources, I downloaded 4 days of real demand data (January 1-4, 2008) from ERCOT's website. I then simulated production curves for two variable sources, one designed to mimic solar PV (only on during the day, with some variability due to clouds) and a more random type of generation to simulate wind. I then fixed the amount of baseload power at 25,000 MW (68% of demand) and 5,000 MW (14% of demand) in each of two scenarios, and saw how much wind and PV the remaining demand could accommodate with the constraint that total generation could not exceed demand.

As you can see, when I dropped baseload power from 68% to 14% of demand, I was able to increase the power of variable sources from 10% to 36% of demand. Almost half of the drop in baseload power was filled by variable power sources, with the balance requiring an increase in dispatchable generation. If you'd like to try your own scenarios, you can download my Excel spreadsheet here.

Better than Baseload

It should be clear that dispatchable generation is a truly premium power source. Dispatchable generation, like energy storage, long distance transmission, and demand response, all allow the grid to accommodate more variation in both power supplies and in demand. In a carbon-constrained world, where we want to use as much variable generation such as wind and PV as possible, zero carbon, dispatchable power from CSP can do far more to help us decarbonize the grid than CSP baseload.

Baseload power is part of the problem; it's not the solution. We should not denigrate CSP by pretending it is only a substitute for coal or nuclear.

Concentrating Solar Power is much better than baseload.

There are more than technical and financial barriers to development of large CSP projects. According to Aringhoff, the transmission regulatory bodies FERC and CAISO need to sort through all the interconnection applications and determine which are for otherwise viable projects. They should also create land use corridors along main transmission trunks throughout the western electrical system which can be more easily permitted for renewable energy. A full 5,000 MW of renewable energy projects are waiting on transmission upgrades.

Land use rules are also important. One of the best areas for solar development would be the Mojave Desert, given its high insolation and proximity to California's population centers. Unfortunately, the West Mojave Plan actively hinders renewable development, with only one percent of the land area set aside for renewable development. Five percent is dedicated to off road vehicle recreation.

As John White said, "It’s amazing that we can take a disturbed piece of ground where there is development across the street and the Mojave Desert commission will say 'No, we have to protect the Mojave ground squirrel.'"

Given these barriers, it's less surprising that PG&E is looking to space, where there are no endangered ground squirrels.

I'm not a space exploration expert, but solar from space seems fraught with technical risk, and Solaren seems to be planning to start with a commercial scale project (200 MW, to be scaled up to 1700 MW.) If the technical problems were solved, it would still be at risk of destruction by space debris and any country with a functioning space program. Assuming such a satellite could collect about ten times as much energy per acre as a ground-based plant, it would still need to cover 100 acres of increasingly cluttered space in order to produce 200 MW, or 850 acres for 1700 MW, making it likely to suffer regular impacts.

Would investors in any climate be willing to fund such an essentially unknowable venture? Perhaps they would if some deep-pocketed entity decided to take on much of the risk, as United Technologies Corp (UTX) is doing with Solar Reserve. But, according to Jonathan Marshall, a PG&E spokesman, "There is no risk to PG&E ratepayers for this." If there is no risk for ratepayers, there is no protection (at least from PG&E) for Solaren investors.

Of the companies that have signed PPAs with California utilities, Stirling Energy Systems' 1750 MW of projects have been most often cited to me as unlikely to be built. They have signed PPAs with San Diego Gas & Electric and Southern California Edison, but if these projects do not get built, they will probably not be alone in that.

Strategic Shifts

In contrast, Ausra, with their innovative Compact Linear Fresnel Reflector (CLFR) geometry, has not been signing PPAs they won't be able to fill. Seeing the harsh financial climate, they took the logical step and decided not to develop their own plants, but rather to sell equipment into the process heat market. I recently wrote skeptically about this while pondering the future of Concentrating Solar Power, but not because the move is foolish. The question in my mind is if the move will be enough. Can a CSP equipment manufacturer be able to ride out the storm by selling equipment to power generators or industrial customers with other, less capital intensive options that work around the clock?

Other solar developers think so. They are following this path and choosing to reduce their financial risk and need for capital by becoming equipment suppliers. Skyfuel has always been a technology and equipment provider, rather than a developer. The recent announcement from GreenVolts shows a similar shift in emphasis to selling equipment (although GreenVolts is not quite comparable to Ausra and Skyfuel, being a CPV startup that sells electricity (not heat) producing equipment.)

In addition to the financial crisis, these shifts may have been encouraged by a recent change in the Investment Tax Credit rules which allows utilities to own projects and still gain the tax benefits. But unless someone is willing to take on technical and regulatory risk, we're going to see a lot fewer of these projects built than we would like.

If we can't build new transmission, and allocate more than 1% of the Mojave to renewable development, we may just have to hope for solar electricity from space. Unfortunately, as Brett Steenbarger said in a recent interview "Hope is comforting, but ultimately is not a particularly effective coping strategy."

Hope's not a good coping strategy for climate change, either.

Dr. Arnold Leitner, CEO of Skyfuel, Inc., thinks the battle for dominance of CSP will be "winner-take-all."

The technology which can deliver power when it is needed at a reasonable price should triumph. Photovoltaic (PV) technologies are rapidly producing price reductions, and can be used almost anywhere, but only produce power when the sun is shining. In contrast, CSP is still cheaper than PV enables inexpensive thermal storage, with the promise of dispatchable power to compensate for the variability of other renewable power sources and demand. Dispatchability assures CSP with storage a place in the eventual energy mix.

Heat Transfer Fluids

The ability and efficiency of a technology to accommodate thermal storage (and provide dispatchability) is a function of the heat transfer fluid and working temperature.

Three heat transfer fluids have been demonstrated to date: Steam (in power towers and troughs) mineral oil (in most parabolic trough plants,) and molten nitrate salts (in power towers.) The working temperature for steam is limited by the potential for corrosion. Molten salts and oil break down at high temperatures, with molten salt and steam capable of achieving the highest temperatures (about 565° C for nitrate salts.)

Companies such as BrightSource and eSolar are currently working to commercialize supercritical steam in power towers.

Lower temperature steam is also the working fluid for Ausra, a company working to commercialize the Compact Linear Fresnel Reflector (CLFR) geometry. CLFR breaks up a trough into a series of narrow, nearly flat, reflectors saving on the high cost of carefully focused troughs. ...

Oil is commonly used as the heat transfer fluid in parabolic trough systems because it does not freeze at night (nitrate salts freeze at 220° C) and operates at lower pressure than steam. According to Bill Gould, Chief Technical Officer of Solar Reserve, such systems have peak operating temperatures of 375°C. Solar Reserve is working to commercialize the nitrate salt/power tower combination which was demonstrated at DOE's Solar Two in the late 1990s, for which Bill Gould was the project manager.

Thermal Storage

The best established thermal storage system is two-tank molten salt, according to Greg Glatzmaier, a Senior Engineer II on the National Renewable Energy Laboratory's (NREL) CSP research team. Pressurized steam or oil have also been used, but at higher cost per kWh. Pressurized steam is only practical for short term buffer storage, according to Greg Kolb, a Distinguished Member of Technical Staff National Solar Thermal Test Facility.

Commercial projects using oil as a heat transfer fluid and molten salt for thermal storage include Nevada Solar One and Solar Millennium's (SMLNF.PK) Andesol parabolic trough plants. Solar Millennium is currently the only pure-play publicly traded CSP company I'm aware of.)

According to Gould and Glatzmaier, the thermal storage systems systems at the Andesol plants suffer 7%-10% round-trip energy losses in heat exchange. If molten salt is also used as the heat transfer fluid, then there is no need for heat exchangers, and no such heat loss. The lower working temperature of these plants also requires much more salt and larger tanks to effectively store the same amount of electricity as for a power tower, once the lower temperatures and efficiency losses are taken into account..

Gould calculates that a trough plant will require three times as much molten salt (along with larger tanks to store it) as a power tower to store an equivalent amount of energy. With additional information from Glatzmaier, I calculate that, to store the equivalent of 1 kWh of electricity at a trough plant requires approximately $90-160 of capital cost, compared to about $30-$55 at a tower, with the variability arising from the commodity price of salt, which is mainly used as fertilizer.

The Shape of Things to Come

In terms of configuration, many experts see long term advantages in power towers. Nate Blair, a Senior Analyst at NREL says the underlying efficiency advantage of towers arising from higher working temperatures will lead to more power from a similar investment in hardware. A Rankin cycle turbine will operate at about 37% efficiency for troughs, or 41% for a tower, meaning a tower can produce approximately 8% more electricity from the same amount of heat.

The combination of energy storage using molten salt, no heat transfer losses, and the thermal efficiency of power towers, point to power towers with molten salts as the working fluid as the long-term favorite.

There are challenges. Only parabolic troughs are a proven, bankable technology. Dr. Leitner estimates that it will cost between $500-$700 million to commercialize a new technology. Solar Reserve plans to overcome this barrier with a performance guarantee from United Technologies (NYSE:UTX) up to the value of the contract, or $200 million, but in the current financial climate financing remains difficult.

SkyFuel has plans to use the innovative reflective film ReflecTech in a hybrid of parabolic trough and CLFR configuration called a Linear Power Tower (LPT). By increasing the diameter of the receiver they hope to reduce heat loss and allow the salt to stay molten for longer periods. ReflecTech enables relatively inexpensive, large parabolic mirrors to be used in the CLFR configuration, with 10 mirrors, each about 3 meters wide focused on each receiver. This should achieve 85x magnification, sufficient to reach temperatures comparable to those in a power tower.

SkyFuel hopes to commercialize the LPT incrementally, by first testing it as part of existing parabolic trough plants using oil as the heat transfer fluid. Might the parabolic trough triumph by incorporating the advantages of power towers?