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American Nuclear Society Annual Meeting
San Antonio, Texas, 7-11 June 2015
Improving Nuclear System Economics using Firebrick Resistance-Heated Energy Storage (FIRES)
Daniel C. Stack
Charles Forsberg
Massachusetts Institute of Technology: 77 Massachusetts Avenue, Cambridge, MA 02139; dcstack@mit.edu
Massachusetts Institute of Technology: 77 Massachusetts Avenue, Cambridge, MA 02139; cforsber@mit.edu
INTRODUCTION
The introduction of renewables into the electricity grid
causes large swings in electricity supply. The motivation for
implementing these variable generation energy sources is
for the replacement of fossil fuels, presently the most easily
dispatchable and controllable energy source. The nondispatchability of renewables as well as the removal of
easily dispatchable fossil fuels from the grid creates a large
demand for storage devices to store electricity at times of
excess capacity (low prices) to provide electricity at times of
low power production (high prices).
From the free market perspective, a low carbon
electricity grid implies more hours of low and high price
electricity, which correspond to times of large sun and wind
output (surplus), and times of little or no sun and wind
(deficit), respectively. These periods will happen
predictably (midday versus night time) and unpredictably
(passing clouds and intermittent winds) [1].
The addition of significant non-dispatchable wind or
solar changes the shape of the price curve. Fig. 1 shows the
number of hours electricity can be bought in California at
different prices. It includes times of high electricity prices
and negative electricity prices.
Fig. 1. Distribution of Electrical Prices (bar chart), by
Duration, Averaged Over CAISO (California) Hubs (July
2011-June 2012) and Notational Price Curve (Red Line) for
Future Low-Carbon Grid.
The addition of a small amount of solar is beneficial
because the electricity is added at times of peak demand.
However, as additional solar is added, it drives down the
price of electricity in the middle of sunny days. Each solar
owner will sell electricity at whatever price exists above
zero. This implies that when 10 to 15% of the total annual
electricity demand is met by solar in California, the output
from solar systems during midday for parts of the year will
exceed electricity demand, the price of electricity will
collapse to near or below zero, and the revenue to all power
plants at these times will collapse to near zero. Each
incremental addition of solar at this point lowers the revenue
for existing solar electricity producers. The percentage solar
is the percentage of all electricity produced by solar—zero
in the middle of the night and exceeding electricity demand
initially in June in the middle of sunny days. Relatively
small fractions of solar have large impacts on prices in the
midday but no impact at night when there is no solar. The
same effect occurs as one adds wind capacity but wind input
is more random. As wind penetrates the market it drives the
price of electricity down on days with high wind conditions
and low electricity demand.
Recent studies have quantified this effect in the
European market [2-3]. If wind grows from providing 0% to
30% of all electricity, the average yearly price for wind
electricity in the market would drop from 73€/MWe
(79$/MWe) (first wind farm) to 18€/MWe (20$/MWe)
(30% of all electricity generated). There would be 1000
hours per year when wind could provide the total electricity
demand, the price of electricity would be near zero, and
28% of all wind energy would be sold in the market for
prices near zero.
Under such market conditions, fast-responding and high
capacity energy storage has the potential to be economic in
systems requiring large amounts of energy, enabling the
purchase of low price electricity during surplus periods to be
put to use during high price periods. Among the many
storage technologies being investigated for affordable large
scale energy storage, heat storage is an attractive option to
be used for cheap industrial heating in place of natural gas,
or for operating a nuclear power cycle for the generation of
electricity during periods of higher electricity demand.
We are developing a Firebrick Resistance-Heated
Energy Storage (FIRES) system, a system that converts and
stores electricity as heat in high-temperature firebrick to be
discharged at economically advantageous times, i.e. times of
energy deficit. FIRES is being developed alongside a
Fluoride-salt-cooled High-temperature Reactor (FHR) with
a Nuclear air-Brayton Combined Cycle (NACC), as well as
for industrial heating applications. The system is described
followed by the economic advantages it brings to investors
in storage as well as the broader low carbon energy grid.
American Nuclear Society Annual Meeting
San Antonio, Texas, 7-11 June 2015
In the context of nuclear energy, there are two
implications. First, FIRES would eliminate times of very
low or negative electricity prices caused by introduction of
renewables. This would improve the economics of base-load
nuclear plants. Second, it enables the FHR to operate at
baseload and produce variable electricity to the grid to
maximize plant revenue.
SYSTEM DESCRIPTION
FIRES consists of a firebrick storage medium of
relatively high heat capacity, density and maximum
operating temperature ~1800°C, heated by electricity at
times of low or negative electricity prices. Low electric
prices are defined as when electricity prices are less than the
competing fossil fuel—usually natural gas. For the given
application, air is blown through FIRES and is heated by hot
firebrick (firebrick that was heated by cheap electricity)
rather than natural gas. Electrically conductive firebrick
operates as both the resistance heater and the storage
medium, directly heated by electricity that is flowing
through the firebrick. The storage medium is surrounded by
insulating firebrick and insulating padding that will allow
for the thermal expansion of the firebrick over a ~1000°C
temperature range. If one allows a 1000°C from cold to hot
in temperature, the heat storage capacity is ~0.5 MWh/m3.
The firebrick, insulation systems, and most other
storage components are similar to high-temperature
firebrick industrial recuperators. The ceramic firebrick is
used because of the low cost and durability, while also
having large sensible heat storage capabilities. Firebrick
with electric heating has been used at low temperatures for
home heating in Europe—non-industrial scale. At times of
low electricity prices, the firebrick is heated. The hot
firebrick then provides hot air when needed for room
heating.
The work underway considers designs for two different
applications: FIRES for industrial heating, and FIRES
coupled with FHR and NACC. FIRES may also be coupled
with NACC of other high temperature reactor designs such
as the High Temperature Gas Cooled Reactor (HTGR).
Coupling of FIRES with LWRs is not economically viable
because of the low operating temperatures, which has a
diminished advantage of the topping heat and low roundtrip
efficiency of the energy storage.
FIRES temperature is below the temperature needed for the
furnace, natural gas heating is used to raise temperatures to
the required furnace temperature. From the perspective of
the furnace, FIRES is a substitute for a natural gas flame.
Electric heating of the firebrick may be done at the
same time FIRES is providing heat to industrial processes—
that is, it is being charged and discharged at the same time.
One wants to buy only one set of electrical heaters to take
advantage of low electricity prices.
The industrial furnace-coupled system will operate
based on the prices of electric heating versus natural gas.
During periods where electricity is cheaper than natural gas,
FIRES will be charged. It will be discharged whenever heat
is needed. If the natural gas contract has variable hourly
prices, FIRES will provide heat when there are high natural
gas prices.
FHR AND NACC APPLICATION [4]
FIRES is coupled with the NACC and provides heat to
the power cycle during peak power production to the grid.
The FHR is an advanced high temperature reactor that uses
graphite-matrix coated particle fuel (the same fuel as a
HTGR) and liquid salt coolant. Its high temperature
capabilities allow it to be coupled to a NACC.
The FHR power cycle is shown in Fig. 2. In the power
cycle external air is filtered, compressed, heated by hot salt
from the FHR while going through a coiled-tube air heat
exchanger (CTAH), sent through a turbine producing
electricity, reheated in a second CTAH to the same gas
temperature, and sent through a second turbine producing
added electricity. Warm low-pressure air flow from the gas
turbine system exhaust drives a Heat Recovery Steam
Generator (HRSG), which provides steam to either an
industrial steam distribution system for process heat sales or
a Rankine cycle for additional electricity production. The air
from the HRSG is exhausted up the stack to the atmosphere.
Added electricity can be produced by injecting fuel (natural
gas, hydrogen, etc.) or adding stored heat after nuclear
heating by the second CTAH. This boosts temperatures in
the compressed gas stream going to the second turbine and
to the HRSG.
INDUSTRIAL APPLICATIONS
FIRES is coupled to natural-gas-fired industrial
furnaces and provides hot air that replaces the burning of
natural gas. The furnaces may be producing glass, cement,
steel, or providing heat to chemical facilities including
refineries. Cold air is blown through FIRES—the firebrick
is laid in a pattern that includes air channels. If the exit air is
above the temperature limits for the furnace, the hot air is
mixed with cold air to match furnace requirements. If the
Fig. 2. FHR with NACC and FIRES
American Nuclear Society Annual Meeting
San Antonio, Texas, 7-11 June 2015
The baseload efficiency is 42%. The incremental
natural gas, hydrogen, or stored heat-to-electricity efficiency
is 66.4%--far above the best stand-alone natural gas plants
because the added heat is a topping cycle [4]. For
comparison, the same GE F7B combined cycle plant
running on natural gas has a rated efficiency of 56.9% [4].
The reason for these high incremental natural gas or stored
heat-to-electricity efficiencies is that this high temperature
heat is added on top of ―low-temperature‖ 670°C nuclearheated compressed air.
Two differences exist between the industrial
application and the NACC application. The air entering
FIRES is (1) at high pressure and (2) at a ―low‖ temperature
of ~670°C – the exit temperature and pressure of the salt-toair heat exchangers (nuclear heat). FIRES is therefore
contained within a pre-stress concrete pressure vessel and is
operated at NACC compressor exit pressure. The system
heats the air to 1065°C before it enters the second gas
turbine. This heat can be supplied entirely by FIRES or in
combination with natural gas, and provides peak power of
142 MWe on top of the 100 MWe base load of the FHR, for
a total power of 242 MWe. For a desired heat storage
capacity of 1500 MWh (charging at 250 MWe for 6 hours),
the required firebrick volume is approximately 3000 m3, or
a 14.5m cube of firebrick for heat storage.
Much of the firebrick heat storage technology in prestress concrete pressure vessels is being developed for
adiabatic compressed air storage systems; in particular, the
GE®/RWE® Adele project that will complete a
demonstration project in several years. At times of low
electricity prices air is adiabatically compressed to 70 bars
and sent through firebrick to lower its temperature from
600°C to ~40°C before being stored in an underground salt
cavern. At times of high electricity demand the compressed
air from the underground cavern goes through the firebrick,
is reheated and sent to a turbine to produce peak electricity.
The key differences between FIRES and Adele are that
FIRES operates at lower pressure and higher temperature,
and uses electric heating rather than compressed air storage.
However, the designs are similar in that both store heat in a
firebrick medium within a pressure vessel, and transfer this
heat to air for electricity generation (see Fig. 3).
As with the industrial application, the operating mode
of FIRES coupled with FHR and NACC is determined by
the price of electricity compared to the price of natural gas.
However since the product of the NACC is electricity, it is
nonsensical to simultaneously charge and discharge FIRES.
During periods when the price of electric heating is lower
than heating by natural gas, the FHR nuclear power plant
becomes a consumer of electricity rather than a supplier,
since it would not be beneficial to sell at these times. FIRES
will be charged via electric heating at a rate of 242 MWe
from the grid, equal to the total power of the FHR and
NACC. During this time, the 100 MWe base load of the
FHR is also sent to FIRES. During periods when the price
of electricity is high, the FHR-coupled system will
discharge 214 MWt to the NACC, producing the peaking
power of 142 MWe at a thermal efficiency of 66.4%, for a
total power of 242 MWe.
Fig. 3. GE®/RWE® Adele Heat Storage Firebrick contained
in Pressure Vessel (Similar Design to FIRES)
FIRES STORAGE ADVANTAGES
In the case of FIRES, the round-trip efficiency, a key
metric in evaluating storage methods, is directly related to
the thermal efficiency of the power cycle to which FIRES
supplies heat, which is primarily a function of the cycle
operating temperature and the turbine efficiency. As such,
advances in high-temperature gas turbines will continue to
improve the round-trip efficiency of FIRES.
The peak power thermal efficiency of the proposed
FHR and NACC is 66.4%. Likewise, the round-trip
efficiency (electricity to heat to electricity) of FIRES is
~66%, approximately equal to the thermal efficiency, since
the conversion of electricity to heat is ~100%. However,
advances in combined-cycle gas turbines by the time the
FHR is developed will likely increase this efficiency to
American Nuclear Society Annual Meeting
San Antonio, Texas, 7-11 June 2015
70%. This efficiency is comparable to many other electricity
storage devices [1].
While matching other storage methods in round-trip
efficiency, FIRES has major advantages over other storage
systems. First, unlike pumped or compressed storage or
batteries, which all can become fully depleted, FIRES’
integration with NACC means that natural gas can assure
peak capacity even when storage runs out, with no
interruption to supply. Natural gas burned with NACC in
the proposed system is used with a 66% thermal efficiency,
higher than the best stand-alone natural gas plants [4]. As
further strides are made in reducing carbon emissions,
natural gas can ultimately be replaced by hydrogen as a
dispatchable fuel when storage runs out. Additionally, while
most storage methods have an electrical input coupled to
electrical output, FIRES’ charge and discharge rates are
uncoupled, which allows for the design of very high charge
rates via electric heating, and, as such, the great economic
benefits of rapidly charging FIRES when the market price of
electricity is low. Finally, the ability of FIRES to consume
large quantities of electricity provides relief to the market by
creating demand when there is otherwise very little, in the
case of both the FHR-coupled and furnace-coupled systems.
CONCLUSIONS AND FUTURE WORK
As the electricity grid continues to implement unstable
and intermittent energy resources in place of conventional
dispatchable fuels, highly responsive energy storage systems
will become an essential part of grid operation, to assure
grid reliability, and reconcile electricity supply and demand.
FIRES coupled with high-temperature heat systems such as
NACC or industrial furnace enables different operating
modes capable of yielding economic advantages to those
who make the investment in storage, while also bringing
relief to the grid by responding to the highly variable supply
caused by wind and solar installations: In the case of the
industrial furnace, by varying its own demand, while in the
case of the FHR, by switching from a large supplier of
electricity to a large consumer at times of low electricity
prices. It should also benefit base-load nuclear plants by
eliminating very low electricity prices as seen in some areas
of the U.S. with significant installed wind or solar
capacity—while lowering greenhouse gas emissions.
The capital costs of firebrick heat storage materials per
unit of storage capacity are far below the costs of other
storage technologies; thus, FIRES is potentially a
replacement for other storage technologies such as batteries.
Using our earlier estimate of ~0.5 MWh/m3, for typical
materials this is ~0.12 MWh/ton. While this number
depends on the exact choice of material, it is representative
of a class of refractory materials available in large
commercial quantities with costs ~$100/ton which gives a
crude price of less than $1 per kWh for storage. For
comparison, U.S. Department of Energy goals are for
storage devices at costs between $100 and $150 per kWh [5,
6]. While FIRES does require insulation and resistance
heating components in addition to firebrick, the economic
potential is self-evident relative to other storage
technologies.
While FIRES largely employs the simple and wellunderstood ideas of resistance heating, sensible heat storage,
and power cycles to achieve storage and conversion, more
work is required to explore the many possible materials and
configurations for the system, especially determining the
storage firebrick that has the optimal energy density and
charge rate capabilities. Since FIRES is a new concept for
industrial energy storage with unique characteristics, future
work will continue to break new ground in understanding
the potential benefits and shortcomings of resistance-heated
firebrick as an energy storage method. The near-term option
is FIRES for industrial heat; the longer-term option is
FIRES as the storage component of an FHR with NACC.
ACKNOWLEDGMENTS
The authors wish to express their appreciation to the US
Department of Energy and the Idaho National Laboratory
Department of Energy support of this project. Work
supported through the INL Laboratory Directed Research &
Development (LDRD) Program under DOE Idaho
Operations Office Contract DE-AC07-05ID14517.
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