How Is Radiant Energy Stored?

Radiant energy is the energy of electromagnetic waves, which includes visible light, ultraviolet light, infrared radiation, radio waves, x-rays, and gamma rays. It is produced by the sun and emitted in all directions. Radiant energy is important because it powers life on Earth. Through the process of photosynthesis, plants convert solar radiant energy into chemical energy that provides food for themselves and animals. Solar radiant energy also warms the planet, drives weather patterns, and powers hydrologic cycles. As solar energy becomes a larger part of our energy mix, the ability to effectively collect and store radiant energy is crucial for providing reliable and consistent power.

There are several ways radiant energy can be stored for later use. Some common methods include solar thermal storage, latent heat storage, thermochemical storage, electrochemical storage, mechanical storage, and thermal mass storage. Each approach has its own advantages and applications in harnessing radiant energy.

Solar Thermal Energy Storage

Solar thermal systems utilize the sun’s radiant energy to heat up a fluid or gas, which can then be stored for later use. There are two main approaches to solar thermal storage: active and passive.

Active solar thermal systems use mechanical components like pumps and controllers to circulate a heat transfer fluid through solar collectors. The collectors absorb sunlight and convert it to thermal energy, heating up the fluid inside them. This heated fluid is then pumped to an insulated storage tank where the thermal energy is stored. This hot fluid can later be used directly for heating applications like space heating, domestic hot water and process heating. Alternatively some systems use the heated fluid to create steam to drive a turbine for electricity generation.

Passive solar thermal systems rely on natural processes like convection, conduction and radiation to collect and distribute heat without any mechanical components. Common passive techniques include orienting buildings and windows to maximize solar gain, using thermal mass materials like concrete or masonry to absorb and slowly release heat, and utilizing greenhouse spaces to trap and circulate warm air through convection. The key benefit of passive solar is its simplicity and low-cost. The main drawback is the lack of direct thermal storage. Though the structure itself acts as thermal mass to dampen temperature shifts.

Sensible Heat Storage

Sensible heat storage involves raising the temperature of a storage medium through the transfer of heat. Materials commonly used for sensible heat storage include water, rocks, sand, earth, and certain phase change materials. Water is one of the most common sensible heat storage mediums due to its availability and high specific heat capacity.

In sensible heat storage systems, energy is stored by heating the storage medium to store thermal energy. The amount of stored energy depends on the specific heat capacity of the storage material, its mass, and the temperature change it undergoes. Sensible heat storage is simple and inexpensive, but the storage density is lower compared to latent and thermochemical storage methods.

Latent Heat Storage

Latent heat storage refers to storing thermal energy by utilizing the energy absorbed or released when a storage material undergoes a phase change, typically between solid and liquid phases. The phase change allows a large amount of heat to be stored or released within a narrow temperature range.

Salt hydrates are one of the most common phase change materials used for latent heat storage. Salt hydrates are inorganic salts with varying numbers of water molecules attached to them. As heat is added, the water molecules dissociate from the salt crystals and the material transitions from solid to liquid. This endothermic process requires heat input equal to the latent heat of fusion. The process can be reversed by removing heat, allowing the water molecules to re-associate with the salt crystals. This exothermic transition from liquid back to solid state releases the stored thermal energy.

Examples of salt hydrates used for latent heat storage include sodium sulfate decahydrate, calcium chloride hexahydrate, and sodium thiosulfate pentahydrate. By selecting salts with different melting points, the phase change temperature can be tuned for particular applications. The latent heat storage capacity depends on the heat of fusion and mass of material.

Advantages of latent heat storage via phase change materials include high storage density, small temperature change during charging/discharging, and minimal heat loss if properly insulated. Challenges include supercooling of liquids below freezing point and potential corrosion issues.

Thermochemical Storage

Thermochemical heat storage relies on reversible chemical reactions to store and release heat energy. Some metal oxides and hydroxides, like magnesium oxide and calcium hydroxide, can absorb and release water in an endothermic hydration reaction and an exothermic dehydration reaction, respectively. The hydration reaction absorbs heat from the environment as water molecules bond to the salt, while the reverse dehydration reaction releases that heat back into the environment as water is driven off.

These materials have very high energy storage densities compared to sensible and latent heat storage methods. They can store heat for long periods of time without losses. The chemical reactions are also reversible, allowing the materials to be regenerated. However, the reactions often operate at high temperatures and the materials tend to be expensive. Efficient heat transfer and good heat exchanger design are needed to harness the heat during the dehydration reaction. Research is ongoing to find improved thermochemical materials that work at lower temperatures and are more cost effective.

Electrochemical Storage

Batteries and fuel cells are two types of electrochemical storage that can store electricity generated from radiant energy sources like solar panels. In both cases, the radiant energy is first converted to electrical energy by the solar panels.

Batteries store this electrical energy through electrochemical reactions between two electrodes separated by an electrolyte. As the battery charges, chemical reactions store energy in the electrodes. Then as the battery discharges, the reverse chemical reactions release energy. Lead-acid batteries are commonly paired with solar panels for home energy storage.

Fuel cells also store energy through electrochemical reactions, but instead of storing electricity directly, they convert the electrical energy into chemical bonds in a fuel like hydrogen or methane. The fuel can then be stored and run through the fuel cell later to generate electricity on demand. Fuel cells allow for larger scale and longer duration energy storage compared to batteries.

Both batteries and fuel cells allow solar panels to store the radiant energy they collect during the day for use at night or on cloudy days. The electrochemical reactions provide a battery-like functionality while avoiding some of the geographic limitations of pumped hydro storage or some thermal storage methods.

Mechanical Storage

Mechanical energy storage systems convert electricity into kinetic or gravitational potential energy that can be stored and later reconverted into electricity. Some common examples of mechanical storage include compressed air energy storage, flywheels, and pumped hydroelectric storage.

Compressed air energy storage (CAES) works by using electricity to power an air compressor, which compresses air into an airtight underground cavern or container. When electricity is needed, the compressed air is heated and expanded through a turbine to generate electricity. The main advantage of CAES is its long duration storage capability and low self-discharge losses.

Flywheels store energy in the form of rotational kinetic energy by accelerating a rotor to a very high speed. When electricity is needed, the flywheel’s rotational energy is converted back into electricity through a generator. Flywheels provide short-duration energy storage and grid stabilization services.

Pumped hydroelectric storage stores energy by pumping water from a lower reservoir to an elevated reservoir when electricity prices are low. Then when electricity demand is high, the water flows back down to the lower reservoir through a turbine, generating electricity. Pumped hydro provides large capacity, long duration energy storage useful for load shifting.

Thermal Mass Storage

Thermal mass storage refers to storing heat in the structure and materials of buildings themselves. This could include concrete, brick, tile, stone, and water tanks. The high heat capacity of these dense materials allows them to absorb, store, and later release significant amounts of thermal energy over time. This helps stabilize indoor temperatures and shift cooling loads.

During the day, sunlight and internal heat gains raise the temperature of the thermal mass. Then at night, the stored heat is gradually released as the building cools, providing passive heating. In some climates, the thermal mass of the building envelope alone is sufficient to store enough energy to maintain comfortable temperatures. Additional mass can be added by incorporating thick masonry walls, tile floors, water containers, and phase change materials.

Optimizing thermal mass requires understanding the relationship between mass and insulation. Mass stores heat, while insulation reduces heat transfer. An appropriate balance is needed to harness the benefits of both. Thoughtful zoning and ventilation is also key. Thermal mass strategies are well-suited for climates with a large day-night temperature swing.

Challenges

There are several key challenges associated with thermal energy storage technologies that need to be addressed for wider adoption and improvement of systems:

Cost – Thermal energy storage systems can involve substantial upfront capital costs for construction and installation. Bringing down these costs through technological improvements, manufacturing scaling, and process optimization is important.

Efficiency – Energy is lost during the charging and discharging processes due to heat dissipation to the surroundings. Improving insulation and the overall efficiency of storage systems is an active area of research.

Capacity – Storing large amounts of energy required for heating/cooling buildings or power generation requires scale. Increasing storage capacity in a cost-effective manner remains challenging.

Other issues like the thermal stability and degradation of storage materials, complex system integration, and competing alternative storage technologies also need to be addressed going forward.

Conclusion

As we have seen, there are a variety of ways to store radiant energy, each with their own advantages and limitations. Sensible heat storage using materials like water or molten salt is simple and cost-effective. Latent heat storage allows more energy to be stored per unit volume. Thermochemical and electrochemical storage provides high energy density storage through chemical reactions. Mechanical storage converts radiant energy into kinetic energy that can be stored. Thermal mass storage uses building materials to absorb and release heat.

While great advances have been made, continued research and innovation into thermal energy storage will be crucial as we transition to more renewable energy sources. Improving storage efficiency and capacity will allow solar and other radiant energy to become a more significant portion of our energy supply. The development of new storage materials and technologies has the potential to provide grid stability, reduce energy costs, and pave the way for a clean energy future.