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The Electricity Journal
ELSEVIER journal homepage: www.elsevier.com/locate/tej
Converting excess low-price electricity into high-temperature stored heat for industry and high-value electricity production^
Charles W. Forsberg*, Daniel C. Stack, Daniel Curtis, Geoffrey Haratyk, Nestor Andres Sepulveda
Massachussetts Institute of Technology, Cambridge, MA, United States
ABSTRACT
The large-scale deployment of wind or solar energy results in electricity prices below the price of fossil fuels at times of high wind or solar output. Price collapse can be limited by using low-price electricity to heat firebrick to high temperatures, store the heat in firebrick, and provide hot air as needed to industrial furnaces, kilns, power plants and gas turbines. This sets a minimum price on electricity near that of fossil fuels.
1. Introduction
Most electricity is produced by burning fossil fuels. Economic variable electricity can be produced to match demand because most fossil plants have low capital costs and high operating costs. The cost of electricity does not increase rapidly for power plants operating at part load when the operating cost is the primary cost. Concerns about climate change require going to electricity generating technologies that do not emit carbon dioxide, such as nuclear, wind, and solar. These technologies have high capital costs and low operating costs (Table 1); thus, the cost of electricity increases rapidly if these capital-intensive plants are operated at part load. Because total energy costs for society are typically close to 10% of the gross national product, significant increases in energy costs implies significant decreases in the standard of living.
In deregulated markets the large-scale use of solar and wind results in electricity price collapse at times of high wind or solar input when electricity output exceeds demand. Collapsing revenue limits the economic use of solar, wind, and ultimately nuclear. A Firebrick Resistance-Heated Energy System (FIRES) is proposed (Stack et al., 2016; Stack, 2016) to limit electricity price collapse at times of high wind and solar output by converting excess low-price electricity into high-temperature stored heat that can be used as a substitute for fossil fuels by industry and to generate electricity at times of high prices. A minimum price of electricity is created near that of the price of fossil fuels used by industry. It is a mechanism to better utilize capital-intensive generating assets.
The article (1) defines and characterizes applications for FIRES, (2) describes FIRES' technical performance characteristics, (3) analyzes
implications of large-scale deployment on electricity markets, and (4) estimates capital costs. The article reports on near-term applications such as heat to industry and long-term options such as coupling FIRES to gas turbines.
2. Electricity markets
In deregulated electricity markets, electricity generators bid a day ahead on the price that they are willing to sell electricity into the market—typically for each hour of the day. The grid operator accepts electricity bids up to the expected electricity demand for each hour. The bid ($/MWh) with the highest electricity price that is accepted sets the price for that hour and everyone who bids below that price gets the same price. Historically, most electricity has been generated using fossil fuels; thus, the price set for each hour was set by the fossil fuel plant operating at that hour with the highest operating costs (Table 1). The markets have a variety of other mechanisms to assure reliable electricity and remain within the technical constraints of the electricity grid.
In a perfect market, wind and solar will bid zero dollars per megawatt-hour (Table 1)—their variable operating and maintenance costs. The Massachusetts Institute of Technology (MIT) Future of Solar Energy (Massachusetts Institute of Technology, 2015) study provides an examination of the solar option and the challenge of moving from an electricity grid dominated by fossil fuel generation to a low-carbon grid. Fig. 1 shows market income for solar plants with increased use of solar. The average price of electricity received for the first few solar plants that are built is above the average yearly electricity price because the electricity is produced in the middle of the day when there is high
☆ The authors thank Idaho National Laboratory and the U.S. Department of Energy (DOE) for their support of this work. This activity included support through the INL National Universities Consortium (NUC) program. Funding was under DOE DOE Idaho Operations Office Contract DE-AC07-05ID14517 and DOE-NE-0008285. * Corresponding author. E-mail address: cforsber@mit.edu (C.W. Forsberg).
http://dx.doi.org/10.10167j.tej.2017.06.009
1040-6190/ © 2017 The Authors. Published by Elsevier Inc. This is an open access article under the CC BY-NC-ND license (http://creativecommons.Org/licenses/BY-NC-ND/4.0/).
Please cite this article as: Forsberg, C.W., The Electricity Journal (2017), http://dx.doi.org/10.1016/j.tej.2017.06.009
CC: Combined cycle; CCS: Carbon Capture and Storage.
Table 1
U.S. Energy Information Agency Estimated Levelized Cost of Electricity (LCOE) For New Generation Resources in 2020 using 2013 $/MWh(e) (U.S. Energy Information Agency Annual Energy Outlook, 2016).
Plant Type
Capacity Factor (%)
Levelized Capital Cost (Plant and Transmission)
Fixed Operating and Maintenance
Variable O & M Including Total System Fuel LCOE
Dispatchable Technologies Conventional Coal Conventional CCa Advanced CC with CCSa Conventional Combustion
Turbine Advanced Nuclear Non-Dispatchable Technologies Wind
Wind Offshore Solar PV Solar Thermal
85 87 87 30
36 38 25 20
61.6 15.6 31.3 44.2
60.8 174.4 113.9 197.6
1.7 4.2
12.8 22.5 11.4 42.1
29.4 57.8 64.7 94.6
0.0 0.0 0.0 0.0
75.2 100.2 141.5
73.6 196.9 125.3 239.7
Solar Penetration (% Peak Demand)
Fig. 1. Solar PV Market Income and Average Wholesale Electricity Prices versus Solar PV Penetration.
demand and prices are high. As more solar plants are built, electricity prices at times of high solar output collapse; thus, solar revenue collapses as solar production increases. This limits unsubsidized solar capacity to a relatively small fraction of total electricity production even if there are large decreases in solar capital costs.
At the same time there are only small changes in the average price of electricity. Other power plants are required to provide electricity at times of low solar output—but these plants operate for fewer hours per year. Investors will not build new power plants to meet this need unless the price of electricity increases at times of low solar output to cover the costs of a power plant that operates only part of the time.
The same effect occurs with wind. Recent studies have quantified this effect in the European market (Hirth, 2013, 2015). 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 (first wind farm) to 18€/MWe (30% of all electricity generated). There would be 1000 h 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.
To use a real example, Fig. 2 shows wholesale prices for electricity in western Iowa, a state with a large installed wind capacity. One can see negative prices enabled by wind subsidies on days of high-wind conditions. When there are negative prices, the electricity generator pays the grid to take the electricity. Wind operators are willing to pay the grid to take electricity because their subsidies are tied to electricity produced. Without subsidies, prices would go to zero but not negative except under limited circumstances. In this specific example the price of electricity is less than the local industrial price of natural gas for over half the time.
Analysis (Forsberg, 2013) indicates that significant price reductions occur on a grid when solar provides over 10% of all electricity produced, wind provides over 20% of all electricity produced, and nuclear provides over 70% of all electricity produced. The different levels of
Neg'ajtive Price Electricity When Excess Wind (Subsidies)
0 5000 10000 15000
Hour number
Fig. 2. Hourly Wholesale Electricity Prices in Iowa over Two Years.
solar, wind, and nuclear penetration before significant revenue collapse reflects the relative mismatch between electricity production for each of these technologies and demand. There is a large literature on the other market effects of adding solar and wind to the grid (International Energy Agency, 2016; Nuclear Energy Agency, 2012) and limits on use of electricity storage to address this challenge (Braff et al., 2016; de Sisternes et al., 2016; Brick and Thernstrom, 2016).
The revenue collapse is a consequence of going from low-capital-cost, high-operating-cost fossil systems to high-capital-cost, low-operating cost solar, wind, and nuclear systems. Revenue collapse at times of high solar and wind input favors the use of low-capital-cost, high-operating-cost fossil fuel electricity generation at times of low wind or solar output. This expanded the use of coal in Germany and natural gas in the United States as renewables are added to the grid.
Societies can choose to subsidize particular energy systems for social reasons, but because energy is such a large fraction of the global income, this has large impacts on standards of living. What is required is low-cost methods to productively use low-operating-cost excess generating capacity when available to reduce electricity price collapse under high wind or solar conditions and thus expand use of low-carbon solar, wind and nuclear electricity generating technologies.
3. FIRES for industrial heat
3.1. Technical description
FIRES (Fig. 3) consists of a firebrick storage medium with a relatively high heat capacity, density and maximum operating temperatures up to ~ 1800 °C (Stack et al., 2016; Stack, 2016). The firebrick is "charged" by resistance heating with electricity at times of low or negative electricity prices. Low electricity prices are defined as electricity prices that are less than the competing fossil fuel—that is natural gas in
Fig. 3. Configuration of FIRES coupled with an industrial process.
the United States. Resistance heating is the lowest-cost method to use electricity.
The firebrick, insulation, and other storage components are similar to high-temperature firebrick industrial recuperators. The ceramic firebrick is used because of its low cost and durability, while also having large sensible heat storage capabilities. If one allows a 1000 °C temperature range from cold to hot temperature, the heat storage capacity is ~ 0.5-1 MWh/m3.
The heat is recovered by blowing air through channels in the brick. The output of FIRES is hot air, which is heated or cooled as needed for the given application by adding natural gas heat or cold air, respectively. The required discharge rate is determined by the hot-air requirements of the furnace to which FIRES is coupled. FIRES is designed for specific groups of industrial customers. There are three performance characteristics of interest (Fig. 4), each of which is largely independent of the other: storage capacity, charge rate, and discharge rate.
3.1.1. Storage capacity
Storage capacity of FIRES is governed by the sensible heat capable of being stored in a volume of material over a chosen temperature range (minimum and maximum temperatures). The chosen temperature range and material will be determined by the needs of the industrial process. More firebrick will store more energy.
3.1.2. Charge rate
FIRES is charged by resistance heating. The charge rate will be determined by the electricity market. If the wind or solar resource combined with market demand drives prices down for only a few hours per day, high charging capacity will be preferred to capture the lowest priced electricity. If low price electricity is available for longer periods of time, lower charging capacity with its lower costs will be preferred. The type of electric resistance heating depends upon the peak temperatures. For less than 1000 °C, traditional low-cost resistance heating elements using nichrome wire or similar materials will be used—the type of heating elements in home toasters except for industrial applications the wire is thicker and designed to operate at higher
temperatures. Typical firebrick materials are made of aluminum, magnesium, and silicon oxides, cheap high-temperature materials that are insulators. The firebrick provides the electrical insulation for the heaters. For very high-temperature operations, conductive firebrick made of materials such as SiC may be used as the resistance heaters.
3.1.3. Discharge rate
Within FIRES, the nominal discharge rate is determined by the heat transfer from the hot firebrick to the air, which is a function of air channel geometry, fan power, and other design features discussed in Section 3.4.
The heat loses in optimized systems will be below 3% per day. In addition to insulation, cold air flow into FIRES can be routed around and through the outer sections of the insulation and by electrical leads to resistance heaters to pick up heat that is leaking from the system. This type of dynamic insulation recovers most of the heat leaking though the insulation and can result in extremely low heat losses if FIRES is operating on a daily cycle.
3.2. Operations
The heat input rate would depend upon resistance heating capability, unused heat storage capacity, and day-ahead projections of electricity prices (Curtis et al., 2016). The price of electricity varies by the hour; thus, electricity to heat the firebrick would be purchased when the prices were at their lowest given the constraint to maximize total kWh of electricity bought at less than the price of the alternative fossil fuel. This creates an incentive to oversize the electrical heaters to maximize electricity purchases when the price is low.
If FIRES is fully charged and the price of electricity is less than the comparable fossil fuels, the heaters would operate at the power level of the industrial furnace—avoiding the need for a second set of electric heaters to take advantage of low-price electricity to provide heat to industrial furnaces. In locations such as Western Iowa, electricity prices are below natural gas prices for over half the time, implying electric heaters are on over half the time. With large-scale deployment, the minimum electric prices will follow fossil fuel prices much of the year. Because the industrial heat demand is larger than electricity production, it has the potential to absorb all low-price electricity. This is in contrast to other storage devices (pumped storage, batteries, etc.) that have "limited" storage capacity.
The owner of FIRES wants cheap heat—but does not care when heat is delivered. However, the electricity grid operator has a different perspective. Electricity to FIRES resistance heaters can be cut off in a fraction of a second without impacting heat delivered to the industrial furnace from the firebrick. Shutting down or turning on the FIRES resistance heaters can be used to stabilize the grid; thus, there are incentives for the grid operator to pay the FIRES owner to control when electricity is sent to FIRES resistance heaters.
Fig. 4. Independent performance aspects of FIRES.
C.W. Forsberg et al.
Fig. 5. Baseline System Designed for Constant Hot Air Output at Fixed Temperature.
3.3. Technology status
Firebrick with electric heating is used for low-temperature home heating in Europe and elsewhere. Some utilities offer a discount rate for electricity at night. At such times the firebrick is heated up to 600 °C with electric resistance heaters. The hot firebrick then provides warm air when needed for room heating by blowing air through channels in the firebrick. More than 100,000 MWh of such heat storage capacity (Mohr, 1970) has been built with heat storage capacities under 100 kWh per unit. In the last several years there have been night discount rates on electricity in parts of China. This has resulted in development of similar units to provide hot air for heating water up to 85 °C to provide hot-water heat and hot water for large apartment complexes. The larger firebrick heat storage units have capacities of 8 MWh. These units have peak firebrick temperatures of 850 °C. There are large incentives to maximize peak firebrick temperature to minimize physical size and weight and thus enable building the units in factories and delivery of assembled units by truck. Such systems could be coupled together for smaller lower-temperature industrial applications. Lower-temperature FIRES could have been developed in 1920 if there had been a market.
FIRES has two major components: firebrick and electric heaters. The concept of FIRES has a great deal in common with firebrick recuperators used today in industry that were originally developed for open-hearth steel furnaces of the early-20th century. In these large systems, hot air was blown across the surface of molten pig iron (~ 1600 °C) to convert pig iron into steel by oxidizing the carbon in the pig iron. The hot off-gas from the furnace flowed through one of two firebrick recuperators in which the hot gas flows over cold firebrick, transferring heat to the bricks before exhausting to the stacks. Later, the direction of air flow through the system was reversed, such that cold air enters and flows through the now-hot firebrick, thereby preheating the air. The hot air from the recuperator was further heated with oil before going to the furnace. The air temperatures had to be maintained above 1600 °C to avoid freezing the pig iron and freezing the liquid metal surfaces. Firebrick recuperators are used today in the steel, glass, and other high temperature industries to recover heat to lower energy costs. There is a century of experience in operating industrial recuperators up to about 1800 °C.
Large-scale low-cost electric resistance heaters are an off-the-shelf technology with peak temperatures between 1000 and 1200 °C; but there are tradeoffs between peak temperatures and heater lifetimes. There are multiple resistance-heater options at higher temperatures but the experience base is more limited and the costs are significantly higher. One option for low-cost, very-high-temperature heaters is the use of conductive firebrick. Conductive firebrick is used in electric steel making processes at very high temperatures but in a steel plant the conductive firebrick is under chemically reducing conditions and would not withstand the oxidizing environment of FIRES. There are alternative conductive firebrick options such as silicon carbide. We have initiated an experimental program to develop heaters for high temperatures using conductive firebrick as the resistance heating element, a
potentially low-cost, high-reliability heating system for this application. Unlike conventional resistance heaters, these heaters would literally be piles of conductive brick.
3.4. Design examples
We undertook parametric designs of FIRES (Stack, 2016) varying the major design variables (storage capacity, charging rate, discharge rate, height-to-diameter ratio, air pressure, alternative firebrick materials, alternative insulation systems, peak temperature, minimum temperature, allowable fan power, allowable heat losses, relative air-to-firebrick ratios, firebrick thickness to flow channel size, firebrick surface roughness, etc.). This included transient heat transfer calculations that accounted for conductivity of the brick, solid-gas heat transfer, and the changing properties of the air with temperature and heat losses through insulation—as well as associated pressure drop calculations. Fig. 5 shows the baseline system, where there is constant air inflow into the FIRES system with some fraction of the air going through FIRES and some fraction of the air bypassing FIRES. This arrangement allows FIRES to provide a constant exit temperature to the industrial furnace by varying the relative flow rates through the firebrick and the bypass duct. It is one of many possible arrangements depending upon design goals.
Fig. 6 shows the results of analyzing (Stack, 2016) one specific design using the system design in Fig. 5. Table 2 summarizes the design parameters. This system has a storage capacity of 250 MWt, a discharge rate of 50 MWt, a peak firebrick temperature of 1200 °C with a design discharge temperature of 500 °C at constant air flow to the industrial furnace. One noteworthy feature is that 92% of the heat can be delivered before the air temperature goes below 500 °C and auxiliary natural gas is required. If the design exit temperature is 1100 °C, 55% of the heat can be delivered from FIRES before the exit temperature goes below 1100 °C and auxiliary heat is required. The large number of engineering design parameters enables optimization of the design for the specific application.
3.5. Economics
Two approaches (Stack et al., 2016; Stack, 2016) were used to bound the costs of FIRES. The existing home heating variant (~ 100 kwh) of FIRES has retail prices as low as $15/kWh with a large spread in prices. Large home systems have about 100 kWh of storage capacity. For a typical industrial plant with 30 MW of heat input, FIRES would be sized to be over 100 MWh of storage capacity. Given the larger scale and avoiding markups for retail, the cost would be expected to be substantially less. The second approach was to price the components of fires from the electrical transformer to the firebrick. This yielded a price of $2.35/kWh that depending upon other factors implies an installed cost two to four times larger. These preliminary estimates result in an industrial FIRES system between 5 and 10 dollars per kWh.
Recent reviews (Brick and Thernstrom, 2016) of energy storage options have broken the cost of different energy storage systems into
Fig. 6. System Power to Industrial Furnace vs. Time and FIRES Exit Temperature vs. Time (Heat capacity: 250 MWt; Exit power: 50 MWt; Peak firebrick temperature: 1200 °C; Exit hot air temperature: 500 °C).
Table 2
Engineering Parameters for FIRES System.
Peak Temperature 1200 °C Capacity 250 MWh
Minimum Temperature 100 °C Charge Rate 75 MW
Application Operating 500 °C Discharge Rate 50 MW
Temperature
Firebrick Aluminum Peak Total Pressure 7.8 kPa
Oxide Loss
Height 18 m Peak Blower Power 636 kW
Uninsulated Diameter 4.1 Firebrick AT Max 94 °C
Insulation Thickness (JM 0.18 Firebrick AT 23 °C
23) Average
Volume Fraction Brick 0.75 Nominal Heat 2.5%
Leakage Rate
Channel Width 0.1 m Firebrick half-width 0.05 m
the cost of storage ($/kWh) and the cost of power ($/kW). The low cost of FIRES relative to other energy storage technologies is a consequence of two factors. First, firebrick is clay sent through a kiln with costs of $0.5-2/kWh of storage. For comparison, pumped hydroelectricity and underground adiabatic compressed air energy storage costs can be in the $10/kWh range but battery options are $200/kWh or more.
Second, FIRES power handling costs are low. Resistance heaters and the associated switches are cheap—a few dollars per kilowatt of power input. Resistance heaters can be designed to operate at any voltage including distribution voltages (22 kV and above) of the electricity grid and thus avoid added transformers and electrical losses. This is in contrast to other electricity storage technologies where power conversion capital costs are measured in $100s/kW. For pumped hydroelec-tricity and compressed air storage, those electrical systems must convert electricity from the grid into mechanical rotating energy and back requiring complex power control systems and motor-generators. For batteries one must take AC electricity and convert it to low-voltage DC electricity and back. Resistance electric heating is the only low-capital-cost technology for consuming electricity and thus the only cost-effective option to use excess low-price electricity if it is available for short periods of time each day.
Preliminary economic analysis (Stack et al., 2016) indicates that the most favorable locations for near-term FIRES deployment in the United States is in locations such as western Iowa (wind) where the payback is estimated to be between one and two years. The areas of FIRES economic viability will grow with additions of wind or solar.
3.6. Applications
Most of the world's energy comes from burning fossil fuels in air that creates hot air—the same product as FIRES. As a consequence,
FIRES couples to most existing energy production and use applications. FIRES economics are improved if it can operate year-round. The only two sectors of the U.S. economy capable of absorbing large quantities of energy at all times of year are the industrial sector and the market for electricity.
Fig. 7 (Ruth et al., 2014) shows energy demand and heat demand by different sectors of the U.S. economy—the industrial markets for FIRES. Table 3 (McMillan et al., 2016) describes the different markets in terms of steam production, heating of other fluids through heat exchangers, and direct heating.
The largest market is indirect heating—producing steam or heating hydrocarbons (refineries and chemical plants) where heat is transferred through metallic heat exchangers with temperature limits typically between 500 and 700 °C. Current FIRES technology can meet these requirements. The glass, cement, and steel industries require very high temperatures and traditionally use direct heating with fossil fuels. There are incentives to operate FIRES up to 1800 °C for some of these industries beyond just providing low-cost heat. For example, the production of cement and lime involves the high-temperature decomposition of CaCO3 into CaO and CO2. However, direct heating using fossil fuels creates a hot gas rich in CO2 that tends to drive the chemical reaction backward. If FIRES can provide some of the heat, the lower CO2 levels should reduce the energy required and increase the rate of decomposition of CaCO3.
FIRES can partly replace fossil fuels in various thermal electricity production systems in two different configurations. Coal, oil, and natural gas plants provide variable electricity to the electricity grid. When low-price electricity is available and the fossil plant is not providing electricity, FIRES is heated. When there is high-price electricity, FIRES heat partly replaces the burning of coal, oil, or natural gas to produce electricity. In this configuration FIRES acts as storage system. However, unlike batteries and pumped storage, if FIRES is depleted, fossil fuels can be used to assure electricity production capacity.
The alternative configuration is designing the thermal power system to couple only to FIRES; that is, FIRES is the only heat source. In this configuration FIRES is an electricity storage system equivalent to a pumped hydroelectric or battery system. Excess energy is stored as heat rather than gravitational potential or chemical energy. The round-trip efficiencies would be ~ 45% for steam cycles, ~ 40% for a simple gas turbine, and ~ 60% for a combined cycle gas turbine. For the thermal power cycles (steam, supercritical carbon dioxide, etc.) where the heat source is operating at atmospheric pressure, FIRES could be built in large sizes for storage system outputs of 100 s of MWe. The gas turbine options are discussed in Section 5.
Siemens (2016) is beginning to develop a simple FIRES heat storage system for peak electricity production. At times of low electricity prices,
Fig. 7. Energy Use by U.S. Manufacturing and Mining Industries for 2004 (Ruth et al., 2014).
Table 3
Industrial Applications of Heat and Required Temperatures (McMillan et al., 2016).
Heating Industry Application Process Temperature
Method (°C)
Steam Heating District heating; drying and 30-200
evaporation processes
Miscellaneous steam applications; 100-300
pulp and paper products; food
processing
Petrochemical refineries Distillation: 200-500
Thermal Cracking:
400-650
Indirect Inorganic minerals production Minerals Retorting:
Heating (phosphates, soda ash/sodium 350-500
hydroxide, chlorine, etc. Minerals Concentration:
150-250
Biofuel refineries (Different Distillation: 150-200
processes) Torrefaction: 250
Pyrolysis: 500
Gasification: 850-1000
Chemicals manufacturing (methanol, Distillation: 150-200
ethylene, propylene, acetic acid, Softening/Melting:
resins, etc.) 150-300
Reaction: 300-600
Hydrogen from hydrocarbons 750-900
Direct Heating Glass and fused silica; iron and steel > 1000-1500
making;
Portland cement (xCaO- yA^O3- > 1300-1800
zSiO2)
Lime (CaO/CaOH)
air is heated to 600 °C with resistance heaters and blown through a bed of crushed rock. The system is discharged by blowing cold air through the hot rock with the hot air being sent into a packaged steam boiler with the steam used to produce electricity. This storage system would be coupled to wind farms.
Last, there is potential market for FIRES coupled to photovoltaics (PV) for home or small industrial applications. PV panels are inexpensive but PV electricity for the grid is expensive because of the power conditioning and grid interface requirements (Massachusetts Institute of Technology, 2015). For areas with low-cost land, PV output could be directly coupled to the resistance heaters of FIRES without
power conditioning. Unlike other electrical devices, resistance heaters do not need power conditioning. It is an analog to the century-old Great Plains windmills used to pump water where water output was variable and uncontrolled but sent to a cheap water tank that acted as the storage system. No detailed analysis of this option has been done.
The U.S. heat market includes the industrial, commercial, and residential sectors (Lawrence Livermore National Laboratory). While FIRES could be deployed in each of these sectors, the industrial sector is economically preferred. First, the individual heat loads are two to five orders of magnitude larger with large economics of scale. Second, industrial facilities operate year round whereas most commercial and residential heat loads are seasonal. This is important in two different contexts: (1) the seasonal heat demands of commercial and residential heat loads may not match when low-price electricity is available, and (2) if FIRES operates for 300 days per year versus 100 days per year, the cost per unit of energy stored is reduced by a factor of three. The number of cycles per year strongly impacts economics. Third, the industrial heat market is larger than the electricity output of the United States (Lawrence Livermore National Laboratory). As a consequence the market for heat from FIRES is sufficient in size to consume all low-price electricity that may be produced. This is also true for other industrial countries such as Japan.
4. Implications on electrical prices
To better understand impacts of large-scale deployment of FIRES on the grid, we examined the implications of adding PV and PV with FIRES to the Tokyo electricity grid (Forsberg et al., 2016). Japan is in the process of deregulating electricity markets, has a large industrial heat demand with a commitment to maintain their industrial base, goals to decrease greenhouse gas emissions including large-scale deployment of utility PV, concerns about foreign trade deficits, and high-cost fossil fuels—a set of conditions that favor early deployment of FIRES. Previous studies (Komiyama and Fujii, 2014) showed the addition of significant PV would result in large-scale PV curtailment as significant capacity was added and electricity production exceeded demand.
To perform the analysis, we ran a simple economic dispatch model of the Tokyo electrical grid over one year (8760 h). The model takes the demand for electricity, the solar generation profile, the installed generation capacity and its marginal cost to dispatch the generation and
meet the demand at minimal cost. The price of electricity for each hour is determined by the marginal unit that serves the load. The demand is assumed to be the demand in the TEPCO area of service in 2014 (TEPCO Electric Power Company, 2015). The generation from solar PV is simulated based on sunlight exposure data from the meteorological agency of Japan and following the methodology of Esteban et al. (2010).
Perfect flexibility in operation is assumed, which leads to a linear optimization formulation of the problem, and accelerates calculation time. In other words, there is no limitation on the power ramping capabilities of the different generators. Energy storage is also neglected, which does not significantly affect the dispatch because pumped hydro is a small part of the total electricity generation. One also assumes that the availability factor of the generators is 100% when calculating the dispatch — one neglects the forced outage times of the generators. Last but not least, the grid is considered as one single node; there is no electricity transmission constraints and no imports nor exports of electricity.
The value of lost load (VOLL) is fixed at 500 $/MWh. The installed capacity and marginal generation cost of the different technologies for the base case are reported in Table 4. Most of the generation cost assumptions come from a recent paper by Komiyama from CRIEPI (Komiyama and Fijii, 2015). It assumes a natural gas price of 9.68 $/MWh. One assumes that 50% of the nuclear generation capacity of TEPCO has restarted, and that the precipitation and river flows limit the hydro generation capacity to 1891 MWe. This latter assumption leads to a fairly realistic total dispatch of hydro despite its crudeness.
The base case (3 GWe solar PV capacity installed) leads to a reference revenue of 72.1 $/MWh for the solar PV owners and $71.3 USD/MWh for nuclear power plants. The relative change in these values as solar penetration increases is illustrated in Fig. 8. The introduction of FIRES is modeled as a technology that would absorb all the excess cheap electricity (less than the price of natural gas) generated by solar, nuclear, and hydro and prevent the curtailment of the electricity generation from these technologies. One assumes that FIRES has infinite storage capacity.
The addition of 50 GWe of solar to the Tokyo electric grid would produce about one-quarter of the total electricity. It would also reduce the price of electricity in the middle of the day so the annual revenue for each solar PV system would be half of the revenue when the first PV system was installed. However, Japan imports liquefied natural gas (LNG) that is used in industrial furnaces at high costs relative to most other counties. If we assume FIRES transfers electricity as heat to the industrial sector at the price of natural gas, FIRES slows and then stops revenue decreases with increasing use of PV—enabling larger use of PV while reducing renewable subsidies, reducing imports of LNG, maintaining nuclear plant revenue and reducing greenhouse gas emissions.
In this specific case, FIRES would not be deployed until 25-30 GWe of solar is installed. It takes that much solar to drive electricity prices for a significant number of hours in the middle of the day below that of natural gas and thus make FIRES attractive. That is equivalent to producing somewhere between 10 and 15% of all electricity using solar.
Table 4
Generation costs and plant technical assumptions, unless otherwise specified.
Technology Solar PV Nuclear Coal CCGT Oil OCGT Hydro
Total capacity 3 6.31 3.2 25.352 13.155 2.317 1891
installed
Marginal 6.5 35.5 48.7 63.6 120.8 88.3 19.4
cost = vari-
generation
cost ($/MWh)
5. FIRES coupled to gas turbines
Gas turbines are heat engines where FIRES can provide heat and substitute for the use of natural gas, oil, or ultimately hydrogen. In a gas turbine air is compressed, heat is added and the hot high-temperature gas is sent to the turbine. There are major differences in the use of FIRES for these applications relative to industrial applications.
5.1. Pressure
Heat in a gas turbine is added after incoming air is compressed. If FIRES partly or completely replaces fossil fuels, FIRES must operate at gas turbine pressures and be incorporated into a pressure vessel. For large gas turbines this would be a pre-stressed concrete pressure vessel. Higher-pressure operations imply higher air densities that improve heat transfer and allow smaller air channels within FIRES that boosts heat storage per unit volume.
Temperature. FIRES temperatures are determined by the gas turbine requirements.
• FIRES minimum temperature. The minimum temperature of FIRES will correspond to the compressor exit temperature—typically between 350 and 450 °C for utility gas turbines.
• FIRES maximum temperature. In a modern gas turbine the peak temperatures before entering the turbine are up to 1400 °C with advanced turbines on the test stands with peak temperatures of 1600 °C. This creates large incentives for high peak temperatures for FIRES—temperatures to 1800 °C. FIRES exit temperatures can be reduced by mixing with colder compressed air or if coupled to a combined cycle gas turbine, by steam injection (discussed later).
• Fuel auto-ignition temperature. One gas turbine design challenge is properly mixing fuel and air to assure combustion. For many gas turbines coupled to FIRES, this challenge is reduced or disappears. Air and fuel will auto ignite above a certain temperature—295 °C for jet fuel, 580 °C for natural gas, and 500 °C for hydrogen. Once above auto-ignition temperatures, any mixture of fuel and air will burn. This simplifies assuring complete combustion if heating with a combination of FIRES and some fossil fuel because the gas temperatures leaving FIRES are above the auto-ignition temperatures.
• Unique heat storage capability. Only FIRES is capable of storing high-temperature heat for use in a gas turbine. The upper limits for practical long-lived heat exchangers made of metal is near 700 °C; thus, any heat storage system that requires a heat exchanger can't efficiently couple to a gas turbine. Gas turbine blades can withstand 1400 °C because the turbine blades have internal cooling channels that keep the blades far below these temperatures. The external surfaces of such turbine blades have ceramic coatings that act as insulators. Only FIRES can operate at gas turbine temperatures because (1) the compressed air is heated directly by the ceramic firebrick and allowable firebrick peak temperatures are far above allowable gas turbine temperatures, and (2) electric resistance heating can go to extreme temperatures.
The only studies on coupling FIRES with gas turbines has been done by us and others for Nuclear air-Brayton Combined Cycles (NACC) (Forsberg and Peterson, 2016), as discussed in Section 5.3. We discuss gas turbine applications herein from the simplest to NACC.
5.2. Simple combustion and combined cycle natural gas plants
FIRES can be coupled to simple combustion turbines (Fig. 9) with heat to electricity efficiencies near 40% or combined cycle gas turbines with efficiencies near 60%. In the near term FIRES would be a partial replacement for natural gas or other turbine fuel.
The first market locations would be areas with high-cost natural gas or liquid fuels relative to the price of electricity when high wind or solar
Fig. 8. Potential Impact of FIRES on Nuclear and Solar Plant Revenue.
output. FIRES would be charged when there was excess wind or solar with the gas turbine was not operating. The heat from FIRES would be used to reduce oil consumption by the gas turbine at times of low wind or solar output. Candidate markets include many islands (Hawaii, etc.) and some remote Arctic and Antarctic sites with high winds.
5.3. Adiabatic compressed air storage (ACAS)
Much of the firebrick heat storage technology for coupling FIRES to gas turbines is being developed for adiabatic compressed air storage systems (ACAS) as shown in Fig. 10; in particular the GE'/RWE* Adele project (Zunft et al., 2014). In an ACAS system at times of low electricity prices, air is adiabatically compressed to 70 bar and sent through a firebrick recuperator to lower the air temperature from 600 °C to ~ 40 °C before being stored in an underground salt cavern. The compressed air must be cooled before storage to avoid damaging the storage caverns. At times of high electricity demand the compressed air from the underground cavern goes through the firebrick to recovers heat from the recuperator and is sent to a turbine to produce peak electricity. The round-trip efficiency of electricity to stored energy (heat and compressed air) to electricity is about 70%.
The consortium (GE, RWE, Zublin, et al.) is investigating ACAS with and without adding natural gas after the hot air exits the firebrick recuperator and before entering the turbine to boost peak power output. The option exists to replace that natural gas with FIRES. Many alternative ACAS systems are being investigated (Luo et al., 2014; Barbour et al., 2015; Bindra et al., 2014). Fig. 11 is a schematic of the ACAS firebrick heat storage system, and laboratory experiments on the design of the concrete pressure vessel for the firebrick. Significant experimental work has also been done on the recuperator that must operate at gas turbine pressures. This project has done much of the work that
would be required to integrate FIRES into an industrial gas turbine including design of the brick recuperator system to operate at high pressures with low-pressure drop losses. The missing FIRES component is the electric resistance heaters.
5.4. Nuclear Air-Brayton Combined Cycles (NACC)
Nuclear Air-Brayton Combined Cycles are being examined for coupling to high-temperature reactors. In the 1950s and 1960s the United States had a major program to develop a nuclear-powered bomber of unlimited range by coupling a reactor to aircraft jet engines. The reactor used a molten salt coolant designed to deliver heat to the jet engine in the 600-800 °C range; that is, the salt coolant was designed to efficiently couple to a jet engine. The program was canceled with the successful development of the intercontinental ballistic missile (ICBM) that could deliver a nuclear weapon faster and cheaper.
Work continued on molten salt reactors for commercial power applications with the proposed reactors coupled to a steam cycle because of the very low efficiency of Brayton power cycles at that time. The efficiencies were near 15%. In the 50 years since then there have been extraordinary advances in gas turbines including development of combined-cycle plants so today a natural-gas-fired combined cycle plant has an efficiency of ~60%. These advances in gas turbines now make it practical to couple a reactor to a gas turbine where the salt coolant delivers heat over the temperature range of 600-700 °C.
Three types of salt-cooled reactors are being considered (Forsberg and Peterson, 2016). The near term option is the fluoride-salt-cooled high-temperature reactor (FHR) that uses a clean salt coolant and graphite-matrix coated-particle fuel developed for high-temperature gas-cooled reactors (HTGRs). The Chinese Academy of Science plans to build a 10 MWt FHR test reactor in the next decade. The somewhat
Fig. 9. Simple gas turbine with FIRES.
Fig. 10. ACAS with optional addition of natural gas or FIRES to boost turbine inlet temperatures.
longer-term option is the molten salt reactor (MSR) with the fuel dissolved in the salt. The long-term option is the salt-cooled high-magnetic-field fusion system. All would be capable of delivering heat at the required temperatures to NACC.
In the proposed power cycle (Andreades et al., 2014a,b) shown in Fig. 12, external air is filtered, compressed, heated by hot salt from the reactor 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 (FIRES) after nuclear heating after the second CTAH. This boosts temperatures in the compressed gas stream going to the second turbine and to the HRSG.
The baseload conversion efficiency from nuclear heat to electricity is 42% using a modified GE F7B gas turbine. The incremental natural gas, hydrogen, or FIRES stored heat-to-electricity efficiency is 66.4% -
Fig. 12. FHR with NACC and FIRES.
above the best standalone natural gas plants because the added heat is a thermodynamic topping cycle. For comparison, the same GE F7B combined cycle plant running on natural gas has a rated efficiency of 56.9%. 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 a "low-temperature" 670 °C nuclear-heated gas turbine system. In this specific system the exit air temperatures from the CTAH is 670 °C. The added natural gas or FIRES raises air temperatures to
Fig. 11. Project Adele system, laboratory section of pre-stress pressure vessel and schematic of the pressure vessel. Courtesy of GE, RWE and Zublin.
0-242 MWe
0-100 MWe
Brayton Power Cycle (NACC) <-
0-214 MWt 236 MWt
Storage (FIRES) High-Temperature Reactor (HTR)
0-300+MWe 0-214 MWt Natural Gas or H2 (Future)
Fig. 13. Grid perspective of high-temperature reactor with NACC, supplemental natural gas firing and FIRES with a nominal reactor base-load output of 100 MWe.
1065 °C before it enters the second gas turbine. The added heat (214 MWt) provides an additional 142 MWe on top of the 100 MWe base load of the reactor for a total power of 242 MWe. Economic analysis (Forsberg and Peterson, 2016) indicates that in markets such as Texas and California a salt-cooled reactor with NACC using natural gas peaking power will have 50% more revenue than a traditional baseload reactor because of the ability to produce peak electricity more efficiently (use less natural gas) than standalone combined cycle natural gas plants. With more advanced utility gas turbine technology coming into the market, the incremental heat-to-electricity efficiency will be above 70%.
The heat provided by natural gas can be supplied by FIRES where electricity to charge FIRES is bought whenever the price of electricity is below that of natural gas. The stored heat would be used to produce peak electricity when electricity prices are higher. This capability fundamentally changes the characteristics of the nuclear power station as shown in Fig. 13. In a market with wind or solar reducing electricity prices below that of natural gas, the addition of FIRES enables the power plant to (1) send its baseload electricity to FIRES to avoid selling low-price electricity, and (2) buy added low-price electricity from the grid. It uses the stored heat to maximize electricity production at times of higher prices. The power station now buys and sells electricity based on price. At all times the reactor operates at baseload, the mode of operation with the lowest generating costs. It is a combined power generator and energy storage system. If the power station included the reactor, NACC and FIRES but no capability to burn fossil fuels, on a daily cycle it would buy electricity whenever there was a 50% or greater difference between the daily low and high electricity prices to maximize revenue by maximizing production when prices are highest.
If we have a heat storage capacity of 1500 MWh, the required firebrick volume is approximately 3000 m3. This can provide 142 MWe of peak power for six hours and is similar in capacity to the firebrick heat storage system associated with the Adele ACAS system. With more advanced utility gas turbine technology coming into the market, the incremental heat-to-electricity will be above 70%. Because the efficiency of converting electricity to heat is very close to 100%, the round trip FIRES efficiency for electricity to heat to electricity can be above 70%. In effect, it is a power generating system with a thermal storage feature that competes with pumped storage or batteries. However, unlike other storage systems, if the auxiliary combustion chambers are maintained there is assured peak generating capacity burning fossil fuels, biofuels, or ultimately hydrogen.
The potential impacts on the electricity system (Sepulveda, 2016) of this set of capabilities were evaluated by simulating the electricity grid under a variety of different constraints with the objective of minimizing
average total electricity costs for the Texas and New England electrical grids. Under a wide variety of conditions, the new system lowers total electricity costs because NACC is more efficient in converting natural gas into electricity compared to standalone natural gas plants and FIRES acts as a battery but unlike a battery it also contains a baseload reactor that assures it does not become fully discharged during long periods of low solar or wind input. This avoids the difficulty with conventional batteries and pumped storage in that they do not eliminate the need for power generating capacity to provide electricity when there are longer periods of time with low wind or solar conditions resulting in fully discharged storage systems. There are some secondary implications.
• Heat for industry. When FIRES is coupled to Brayton cycles with HRSGs, the steam can be used to generate electricity or provide steam to industry. When peak power is being produced, steam production goes up. This provides a second route for FIRES to provide steam to industry separate from FIRES coupled to an industrial boiler.
• Hydrogen futures. Most storage technologies are only suitable for energy storage for hours to a few days. The same is true for FIRES because of the capital cost of pre-stressed concrete pressure vessel. For seasonal energy storage in a low-carbon grid, hydrogen may be attractive option. Hydrogen can be stored at low costs in large quantities using the same types of underground storage systems used for natural gas. However, the problem with hydrogen as a storage media is the round-trip electricity to hydrogen to electricity efficiency. The electricity to hydrogen efficiency is typically near 70% versus near 100% for electricity to high-temperature heat in FIRES. The heat to electricity efficiency when burning hydrogen or FIRES is the same—higher than any other technology to convert hydrogen to peak electricity. That creates large incentives to use FIRES heat storage where feasible because of the higher round-trip efficiency. Where not feasible, NACC is the preferred method to convert hydrogen to electricity because it is the most efficient method to convert hydrogen to electricity.
There is ongoing work to couple other high-temperature reactors to FIRES using closed Brayton power cycles (Forsberg, 2016) where the gas (helium, nitrogen, etc.) circulates in a closed loop. In these systems only FIRES is available for peak electricity production since natural gas can't be used in a closed Brayton power cycle.
6. Other implications
Beyond setting a minimum price for electricity that supports low-carbon generating technologies and provides heat to the industrial sector, FIRES may have other impacts on the electricity grid. Large-scale wind and solar results in large increases in grid and other electricity system costs (Massachusetts Institute of Technology, 2015; Nuclear Energy Agency, 2012) separate from the costs wind and solar generating systems. One major cost is the low capacity factors for transmission lines coupled to wind and solar because of low capacity factors for wind and solar production facilities (Table 1). In locations such as China where the major electricity load centers are far from the wind resources (Davidson et al., 2016), there is a tradeoff. The capacity factors of the transmission lines can be increased by overbuilding wind capacity, but that lowers capacity factors for the wind farms. If there are local industries that can use heat, FIRES can dump some of that excess energy to the industrial sector increasing local wind farm capacity factors while maintaining higher long-distance transmission capacity factors. Alternatively, FIRES can be coupled with a power generating system to store excess electricity to enable transport when the transmission lines have excess capacity.
7. Conclusions
The transition from a fossil-fuel-based electricity system to a low-carbon electricity system is a transition from low-capital-cost, high-operating-cost electricity generators to high-capital-cost, low-operating-cost nuclear, wind, and solar systems with low marginal generating costs. This has resulted in increasing numbers of hours in Europe and the United States with wholesale prices of electricity near zero—economically limiting the use of these low-carbon electricity sources. A low-cost technology is required to productively use excess electricity and raise the minimum prices for electricity at these times. FIRES converts low-price electricity into high-temperature hot air and stored heat to replace fossil fuels in industry—the only year-round market sufficient in size to absorb very large quantities of low-price electricity. Alternatively, FIRES can be used to store electricity in the form of thermal energy.
The basic FIRES technology for many applications could have been developed and deployed in the 1920s. It is the change in the electricity market that creates the incentive to deploy FIRES. The estimated capital costs of FIRES to provide heat to industry is $5-10/kWh, below any other technology. That is because the heat storage media is pressed dirt that has gone through a kiln (firebrick) and electric resistance heating is the lowest cost device for using electricity. For some applications, FIRES can be built today. For other applications significant development is required.
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Charles W. Forsberg is Director and Principal Investigator of the Fluoride-salt-cooled High-temperature Reactor (FHR) project (a joint effort of MIT, the University of California, the University of Wisconsin, and the University of New Mexico) with research interests in low-carbon nuclear-renewable energy systems. At MIT, he teaches the nuclear fuel cycle and nuclear chemical engineering classes. Before joining MIT, he was a Corporate Fellow at Oak Ridge National Laboratory and is a Fellow of the American Nuclear Society and the American Association for the Advancement of Science.
Daniel C. Stack is a Ph.D. graduate student in the Department of Nuclear Science and Engineering at MIT. Mr. Stack is developing high-temperature Firebrick Resistance Heated Energy Systems (FIRES).
Daniel Curtis is a Ph.D. graduate student in the Department of Nuclear Science and Engineering at MIT. Mr. Curtis is examining heat storage coupled to light-water reactors and the economic impacts on plant revenue by adding heat storage to reduce electricity production at times of low electricity prices and producing added electricity at times of high prices.
Geoffrey Haratyk is a Ph.D. graduate student in the Department of Nuclear Science and Engineering at MIT. Mr. Haratyk is examining LWR economics in the context of changing electricity markets.
Nestor Andres Sepulveda is a recent master's graduate of MIT who is modeling the electricity grid to determine minimum cost systems as a function of different technological choices and restrictions on carbon dioxide restrictions.
