Here is a re-post of a write-up I submitted on the Energy From Thorium site in July 2010. It is based on the PB-AHTR, but the results for the IFR would be very similar. Note that the cost estimate (about $1/W) is much lower than what David assume up-thread.
My concern here is grids with very large renewable penetration (>20%). Note that when a nuke is used in load-following mode to backup renewables, nuclear power is essentially discarded in an amount equal to the renewable energy available. In other words, it guarantees that renewable supporters will fight nuclear power to the death.
The storage option allows nuclear and renewables to be more equal contributors (70:30?) to a post-carbon electrical system. And thermal energy storage allows every city to have local storage (which improves reliability), not just those with the right geography for pumped hydro or CAES.
www.energyfromthorium.com/forum/viewtopic.php?f=58&t=2615&p=33750#p33750 Thermal energy storage and the PB-AHTRThe high temperature heat available from the PB-AHTR (and LFTR and IFR) makes it suitable for use with thermal energy storage (TES) systems, to improve load following capability (especially for use with wind power). In particular, molten-salt systems are a good fit; particularly in combination with
helium Brayton cycle power conversion (they can load-follow with no efficiency loss, via changes in the helium inventory/pressure).
In such a system, the
power conversion system would be
upsized 20-100% above what the reactor can supply, and would be throttleable down to about 10-30% of maximum. The difference between the reactor output (which is held constant) and thermal demand would go to/from hot salt storage.
Three different
storage durations are relevant: 1 hour storage to provide regulation and spinning reserve, 4-12 hour storage for day/night load leveling (including plug-in vehicles and solar PV), 16-48 hour storage to compliment high penetration wind power. None are needed in a fossil fuel dominated energy system; all are needed for
zero-carbon electrical systems.
There are two possible arrangements: two-salt and three-salt.
With the two-salt arrangement, the secondary coolant salt is also the TES salt. This would probably be preferred for small amounts of storage. Of the salts considered in this study
nuclear.inl.gov/deliverables/docs/ornl-tm-2006-69_htl_salt.pdf (Assessment of Candidate Molten Salt Coolants for the NGNP/NHI Heat-Transfer Loop: Williams – 2006), only the chloride salts which are the major components of
sea-salt are affordable in large quantities: NaCl, KCl, and MgCl2. The KCl-MgCl2 blend is the cheapest ($0.21/kg), melts at 426C, and has 0.46 cal/cc/C for volumetric heat capacity. Adding lithium salt would decrease the freezing temperature, but at a very high cost. Similarly, fluorine based salts like flinak and NaBF4-NaF (the proposed secondary coolants from PB-AHTR and DMSR respectively) have better heat capacity, but costs fifty times as much.
After the salt cost, the next most important determinant of TES cost is the salt
temperature excursion (delta-T). If the reactor has an outlet temperature of 704C, for good freeze-margin, a reasonable salt temp excursion might be from 526 to 684C, so delta-T =158C. A higher delta-T would store more energy, so would be more cost effective; this option gets more competitive with higher reactor temperatures.
The three-salt arrangement allows the TES to use “
solar salt” (see
www.solar-reserve.com/homePage.html), a blend of NaNO3 and KNO3, which has a melting temperature of 141C and useful range of 288C to 566C, for an excellent 278C delta-T (this is also the preferred salt for the IFR, which would have about a 550C outlet temp). Solar salt also has higher heat capacity than the chloride salt (0.72 vs 0.46 cal/cc/C), so the energy storage per kg of salt is 2.75 times higher, potentially cost reducing the TES. The savings is somewhat less, since solar salt is about 50% more expensive than chloride salt, and the lower operating temperature will lead to lower power conversion efficiency; additionally, a secondary coolant to TES heat exchanger is required.
Solar Salt also gives the option to have a simple air vent on the tanks. The chloride salt would probably require an inert cover gas to reduce oxygen contamination, which could lead to corrosion problems (the secondary salt in LFTR systems always has a cover gas, at least for tritium recovery). If a cover gas system is found to be practical however, the usable temperature of solar salt may extend all the way to 650C with an oxygen cover (according to Sargent & Lundy 2003), which would greatly reduce efficiency loss caused by operating from storage.
The final concern with a nuclear-heated TES is the need for a power conversion system which can fully utilize the temperature range of the TES salt. The traditional multi-reheat Brayton cycle (e.g.
www.nuc.berkeley.edu/PB-AHTR/resources.html “A Reference 2400 MW(t) Power Conversion System Point Design for Molten-Salt-Cooled Fission and Fusion Energy Systems,”) only has 50-100C of temperature drop in each turbine stage prior to re-heat, so a requirement that the turbines cool the gas by 158 or 278C will likely involve an efficiency decrease (it would lower the average temperature of heat input to the cycle). The otherwise promising super-critical CO2 cycle is not suited to efficient load-following, as the efficiency drops steeply with output power (although a plant with multiple modular turbines could approximate load following with course steps).
A rough
estimate of the cost of the TES can be obtained by starting with a solar thermal evaluation:
www.nrel.gov/csp/troughnet/pdfs/40166.pdf NREL/SR-550-40166 Thermal Storage Commercial Plant Design Study for a 2-tank Indirect Molten Salt System, 2006.
This describes a system for a solar-trough plant, with Tcold=290C, Thot= 385C for the salt tanks, delta-T= 95C, for 35-37% gross effic at 50MWe, using Rankine steam cycle w/ reheat.
35,100 tons of salt gave 10.3 hours storage at 36.8% efficiency. Systems in the 6-12 hour range costs $30/kWht.
Scaling the cost for the higher temperature range:
$30/kWht * (385-290C) / (566-288C) =
$10.25/kWhtAssuming 40% efficiency, storage system costs for 12h, 24h, and 48h are:
$10.25/kWht / 0.40 * 12h * (1, 2, 4) = $308, $615, and $1230/kW;
plus the $281/kW to upsize the power converter (Sargent & Lundy 2003)
This is well below the cost cited for pumped hydro storage and more efficient also. The round trip energy efficiency is likely to be above 97% with the two-salt system, and around 85% with three-salts (assuming the efficiency drops from 46% to 40% when operating on stored solar salt as a result of the lower temperature).
For each kW of wind power on a grid system, only about 0.3 kW of storage is needed. Assuming 24 hour storage, this would add $271 to the cost of each kW of wind. This is about a 15% premium over the wind cost alone, and significantly reduces the fossil fuel otherwise needed to integrate the wind power.
A thermal energy storage system coupled to a high temperature reactor is therefore a promising concept, but much more detailed study is required.
