Nuclear energy is faced with a number of challenges in a changing energy landscape, driven by the need to reduce carbon emissions to mitigate climate change. Renewable energy technologies are being considered as the solution to climate change and are increasingly being deployed across the world. However, renewable energy sources, particularly solar and wind, are highly variable, and deployment of these technologies has resulted in significant perturbances in the energy market, raising questions about grid stability and the adaptability of other sources to compete in a changing marketplace that prioritizes renewables. Nuclear plants, well suited for baseload operation, have demonstrated technical capability and flexibility to respond to the fluctuating demand; however, they have also discovered that the economics of such operating mode are not necessarily optimal to their financial security. On the other hand, despite contributing to the carbon emissions, the low cost of abundantly available natural gas and resultant low-cost electricity have exacerbated the economic pressure on nuclear technologies, raising questions about their survival and role in future energy systems1.
However, along with its challenges, the changing energy landscape has also opened up potential opportunities for nuclear energy. Energy systems of the future are anticipated and expected to be more than electricity generators, and provide alternative energy carriers, such as thermal energy or synthetic transportation fuels, in addition to electricity. Nuclear plants are well positioned to fill this niche, providing thermal energy directly for industrial processes, district heating, desalination, and, many other applications. Nuclear energy can combine synergistically with renewable energy in Integrated Energy Systems (IESs) to provide multiple energy outputs (e.g., electricity, thermal energy) while also promoting grid stability to ameliorate variable electricity production from renewable sources2.
Integrated Energy Systems with Energy Storage
An IES with energy-storage capabilities can operate at steady state continuously, maximizing efficiency to store energy at times of low demand and cost, relying upon the stored energy to meet consumer demands at times when they exceed power generation capacity. Energy storage is most commonly taken to mean the storage of electrical energy; however, direct storage of electrical energy is constrained by several difficulties, including limitations on capacity and parasitic losses. Indirect storage of electricity in electrochemical devices (secondary batteries) has garnered increasing attention; however, the modular nature of the storage limits the practical capacity, and the use of stored energy for industrial thermal applications is inefficient.
Thermal energy storage (TES) is a convenient, indirect energy storage option that avoids process complexity of the electrochemical and chemical energy storage alternatives. Nuclear heat can be stored directly in a thermal energy storage device at the time of low electricity demand, and then either be used directly in industrial processes or reconverted into electricity at the times of excess demand, enabling direct utilization of heat from baseload power generators for industrial applications. TESs are also important for utilization or reuse of the waste heat from many industrial processes. TESs function in one of the following three ways: sensible heat storage, latent heat storage, or thermochemical heat storage (TCS)3,4.
Sensible heat storage systems employ materials such as water, steam, oil, and graphite for short term, and concrete, crushed rock, nitrate, and chloride salts for long-term storage. Thermal energy input results in an increase in the temperature of the storage medium, and the process is reversed in the energy discharge step. Latent heat storage systems feature reversible isothermal phase transitions, typically solid-liquid transformations with minimal density changes, for energy storage and discharge. Materials used for latent heat storage include paraffin waxes, fatty acids, inorganic nitrate or carbonate salts, and alloys. TCS systems feature reversible reactions, where the decomposition of a chemical and reconstitution of the separated constituents are the energy storage and discharge steps, respectively. The energy storage step involves an endothermic reaction, typically the decomposition step with the resulting products physically separated and stored. These two products are brought together to reconstitute the original reactants of the decomposition step via the exothermic reaction. Systems based on thermochemical reactions have a potential to achieve much higher energy densities than those relying on sensible or latent heat storage.5
Chemical Heat Pumps: Thermochemical Energy Storage with Temperature Amplification Capabilities
Apart from higher energy density, the biggest advantage of TCS systems is its capability to improve the quality of thermal energy by boosting delivery temperatures. As mentioned above, TCS features an energy storage step, an endothermic reaction involving breakage of chemical bonds. Thermal energy is stored as the chemical energy of the products. The energy discharge step is the reverse of this reaction, featuring recombination of the products via an exothermic reaction. By manipulating the reaction conditions, this reverse reaction can achieve higher temperatures than that of the primary energy source driving the energy storage step—thus upgrading the quality of the thermal energy.
Chemical heat pumps can utilize heat sources directly without requiring intermediate conversion of the thermal energy to mechanical or electrical energy. As a result, CHPs are potentially more efficient than conventional vapor compression or (de)sorptions heat pump, while also offering increased versatility with respect to the range of operating temperatures6. Broadly speaking, CHPs can be classified into either sorption-based or reaction-based systems. Sorption-based systems typically involve sorption-desorption of water, ammonia, or hydrogen, while reaction-based systems involve a hydration/dehydration, carbonation/decarbonation, hydrogenation/dehydrogenation, or redox reaction couples, such as magnesium- or calcium-hydroxide/oxide and calcium-lead-carbonate/oxide.
The Ca(OH)2/CaO system utilizes abundantly available inexpensive chemicals that have minimal toxicity and pose little challenge with respect to the materials of construction. Further, high reaction enthalpy translates into high energy storage density, and the product of decomposition reaction—water—can be easily managed through condensation. These characteristics make this system considerably more attractive than other systems mentioned above.
The reversible reactions involved in the system are:
Ca(OH)2 (s) CaO(s) + H2O(g).
The forward endothermic dehydration reaction, driven by available nuclear heat, constitutes the energy storage step. The recombination of the resulting calcium oxide and water is exothermic, releasing heat that can be used for a wide variety of industrial processes. The basic principle of the process is explained by Figure 1 shown below.
The Clausius-Clapeyron diagram (Fig. 1[b]) comprises two lines, representing the equilibria for decomposition of calcium hydroxide (solid line on the left) and water vapor-liquid phase change (dashed line on the right). The energy storage step—the decomposition into calcium oxide—takes place at low pressure (hence, low temperature, represented by TM). The recombination of water vapor and calcium oxide is effected at a higher pressure PH; correspondingly, the equilibrium temperature is higher and the energy is discharged at the higher temperature TH. By manipulating the operating conditions, temperatures as high at 1,200 K are theoretically achievable . As a result, light-water reactor heat can be upgraded to provide thermal energy to various industrially significant processes. Temperature requirements of some of these processes are shown in Figure 2.10
CHP Research and Development
Recent R&D11 activities at the University of Idaho (UI) and Oregon State University (OSU) have focused on the Ca(OH)2/CaO CHP. Research at UI has successfully demonstrated multiple cycles of Ca(OH)2 decomposition, followed by the hydration of CaO. Temperature amplification in excess of 150 °C has been observed in these cycles. The overall energy efficiency of the system is 80 percent. Component and system models are being developed at OSU. Additionally, OSU conducts experimental and modeling work on a LiBr/H2O absorption pump that is proposed to be coupled to the dehydration/hydration reactor for enhanced management of water involved in the reaction. The system enables efficient storage and delivery of large amounts of energy in a small mass.
The work described herein has been made possible through grant #DE-NE0008775, awarded under the Department of Energy’s Nuclear Energy University Program.
Vivek Utgikar is a professor in the Department of Chemical and Biological Engineering at the University of Idaho; Piyush Sabharwall is a senior staff research scientist at Idaho National Laboratory; and Brian Fronk is an assistant professor of mechanical engineering at Oregon State University.