September 19, 2007
Thermal Solar Stored Under Pressure For Night Usage

Using concentrating parabolic reflectors and a thermal storage system Palo Alto California company Ausra claims to have a workable way for solar power to supply electricity 24 hours per day.

Ausra claims to have solved the storage problem without using molten salts or other expensive means of conserving heat. In fact, the company estimates that the price of its electricity will drop to roughly 8 per kilowatt hour if it can store heat for 16 hours. "Thermal storage is generally considered to be quite a bit cheaper than electrical storage," says Nate Blair, a senior analyst at NREL. "There isn't a lot of power generation combined with storage systems that can take advantage of that. [Concentrated solar power] has a leg up on storage in the grid or flow batteries or even ultracapacitors."

The system will employ pressure and a steam accumulator to accomplish the trick. "You allow some of the steam to recondense," O'Donnell explains. "It flashes back to steam when you reduce the pressure just by opening the valve to the turbine."

Such long-term steam storage, however, is unproved. "Steam storage is currently feasible at small levels, for example, one hour or so," NREL's Mehos notes. "Due to large volumes and high pressures involved with steam storage, scaling up steam storage to baseload applications is very high risk."

Water boils at different temperatures at different atmospheric pressures. At high altitudes with thinner air and less atmospheric pressure water boils at lower temperatures. But put water under a sufficiently intense pressure and it will not boil into vapor. The idea is to store the water under high pressure so that it won't convert to steam and then release some of the water into a lower pressure area at night to allow the water to convert into high pressure steam and power turbines to generate electricity.

I don't know whether they can make it work. But this is an interesting approach. Store the energy as hot water rather than as hydrogen or electrochemically in a battery. Can they make this approach work? The storage container has to meet cost, pressure, and longevity goals. What sort of storage material could do this? Steel? Is insulation needed?

Share |      Randall Parker, 2007 September 19 08:53 PM  Energy Solar

Wolf-Dog said at September 19, 2007 9:06 PM:

Actually, there is no need for long term storage, but only for a few hours: the best power production of Thermal Solar systems, happens to be precisely when the air conditioners are running in the city during day time. At night power is not needed that much not only for air conditioners but also for most businesses.

There will be some loss of heat during this water storage (or molten salt storage, etc), but as the size of the hot water reservoir increases, then the loss will be very small. Large lakes stay warm or cold much longer than small ponds.

K said at September 20, 2007 12:42 AM:

Wolf-Dog is right about the load situation. I have long contended that solar generation should lead solar storage. Otherwise we get in an endless refinement mode.

The US should be heavily building comercial (not home) solar now for peak use instead of delaying it for another refinement, and then another, and then....

Cost is not just panels or thermal engines. It is licensing, site acquistion, transmission lines, integration to the existing grid, the physical plant. Only panels are likely to become much cheaper. The money is available. The real problem is massive government spending for activities we don't need, AKA Pork.

My power bill came today. Perhaps 40% was for AC between 3 and 6 PM when there is a surcharge. Another 40% for the other 10 hours of daylight. And 20% for the entire night.

Consider our options. The problem for about a decade will be in peak demand. Solve that first. Don't worry, other problems will emerged to keep us supplied in later years.

We can keep putting solar panels on roofs and dealing with tax subsidy paperwork as complicated as converting to Judaism and each istallation somewhat unique with almost no economy of scale. Or we can build another reactor outside Phoenix. Yep, that is likely to be completed quickly. Or we can decide to dig many thousands of tons of coal and move it to new generators not far from here. That promises years of fighting about the environment.

Exactly how have those options been working out for the country in recent years? Perhaps we should try committing to really large solar plants run by utilities that actually know how to supply power to millions of customers.

But I suppose we would have to stop federal subsidies for rose breeding research at the Boggs Flower Garden in Flagstaff to pay for getting something done. Can't have that.

To store solar heat for use at night I suggest trying molten iron. Trivial emissions, dirt cheap mass, few corrosion or insulation problems, and safe - something goes wrong just stop and fix your equipment.

Paul Dietz said at September 20, 2007 4:57 AM:

The safety issue from storing large amounts of pressurized water should not be underestimated. There was already a nasty explosion at a solar thermal plant down there (Luz's SEGS VIII), even though that facility did not store energy in pressurized water.

Thermal storage can already be used to shift electricity used for AC. This is demand side storage, using cheap electricity to make ice. Typically this involves shifting demand to nighttime off-peak hours, however.

$.08/kWh is still not competitive for baseload. Coal and current nuclear plants should be significantly cheaper. Even coal with sequestration is possibly cheaper than that.

Wig-Wag said at September 22, 2007 5:48 PM:

Safety issues from thermal storage are trivial compared to the issues in many industrial processes; boiler codes solved those problems long ago. The problem at the Luz plant was an *oil fire* caused by an *explosion in a gas burner*. Neither oil nor gas are used at all in Ausra's process.

While older coal-fired power plants deliver power below .08/kWh, plants now under construction will deliver power at .07-.10/kWh; construction costs for new coal fired power plants have exploded. Within the next 3 years, it's a near certainty that costs for emitting CO2 will come to the market. Since coal-fired power generation emits twice as much CO2 per MWh than gas-fired power, it is not unlikely that gas-fired power will actually wind up cheaper than coal over the next few years, and that all fossil sources will be at .10/kWh or above.

Paul Dietz said at September 23, 2007 11:19 AM:

While older coal-fired power plants deliver power below .08/kWh, plants now under construction will deliver power at .07-.10/kWh; construction costs for new coal fired power plants have exploded.

These cost estimates must involve either pessimistic assumptions or large carbon taxes. The levelized costs I've seen for coal and nuclear we well below that. $.08/kwh from this solar technology would not be competitive with nuclear for baseload electricity.

Cyril R. said at August 28, 2008 2:59 AM:

One year later, it appears Paul Dietz was wrong; US nuclear project estimates are all in the 10-20 cents/kWh total levelized cost range, very similar to current CSP levelized costs. Coal appears cheaper though but that ignores pollution and CO2.

Interesting how much things can change in just one year.

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