Thermal energy storage

Thermal energy storage (TES) refers to technology in which a storage medium is heated or cooled to store energy for use at a later time. This could be for heating and cooling applications or power generation. TES systems are designed to store energy when production exceeds demand, making it available upon user request. They help match energy supply and demand, in particular for variable energy sources (often renewable sources); increase the overall efficiency of energy systems; and reduce waste and carbon dioxide emissions. TES systems are in use for varying scales, from industrial processes to individual homes.

TES helps to balance energy demand over both short and long time frames. Examples could be daily use, using off-peak night-time energy for hot or cold storage used to power systems throughout the day, or seasonal energy storage in which solar energy from the summer is stored for use during the winter.

TES includes a number of technologies using a range of different materials or processes to store energy, each having different thermal properties and achieving various results depending on the intended application. There are three main types of TES:

• Sensible heat storage—based on heating or cooling a liquid or solid medium, e.g. water, sand, molten salts, rocks
• Latent heat storage—the use of phase change materials (PCMs), e.g. changing material from a solid to a liquid state
• Thermo-chemical storage (TCS)—the use of chemical reactions to store and release thermal energy

A breakdown of the main types of TES.

While sensible heat storage technology is commercially available, latent heat storage is currently in the pilot phase, while TCS remains confined to the lab.

The economic performance of a TES system depends on its specific application and operational needs. Important factors include the number and frequency of storage cycles. In general, PCM and TCS systems are more expensive than sensible heat systems and are only economically viable for applications with a high number of cycles.

The most common application of TES is for solar thermal systems, helping manage intermittent solar energy in concentrated solar power (CSP) plants. Other important applications for TES are in the building sector (e.g., domestic hot water, space heating, and air-conditioning) and in the industrial sector (e.g., processes heating and cooling). The use of TES in OECD countries is limited due to the lower rate of new constructions while emerging economies have larger deployment potential.

Solar thermal energy storage tanks at the Solana generating station in Arizona.

TES systems can be installed as centralized plants or distributed devices. Centralized plants store waste heat from large industrial processes, conventional power plants, combined heat and power plants, and renewable power plants, such as CSP. The power capacity of centralized TES systems ranges from hundreds of kW to several MW. Distributed devices are generally buffer storage systems that accumulate solar heat to be used for domestic and commercial buildings (e.g., hot water, heating, and appliances). Distributed systems generally offer power supplies in the range of a few to tens of kW.

While TES encompasses a wide range of technologies and the scale and storage method used can vary considerably, the basic principle remains the same. Energy is supplied to a storage system for removal and use at a different time. It can be separated into three steps (referred to as a cycle):

1. Charging—energy transferred to the storage medium
2. Storing—energy retained by the storage medium
3. Discharging—energy transferred from the storage medium

This storage cycle applies to each of the three TES types: sensible, latent, and chemical storage.

TES aims to change the way users generate the significant amount of heating and cooling capacity they consume. According to the International Energy Agency (IEA), heat is the largest energy end‑use. Providing heating for homes, industry, and other applications accounts for roughly 50 percent of total energy consumption. Much of this energy is generated by fossil fuels that store energy, offering a consistent output when used. TES can even out the energy supply from renewable sources that typically suffer from intermittent performance, to provide consistent heating and cooling solutions without the need for fossil fuels. Research in energy storage systems has increased in importance due to the greater use of renewable energy.

Energy storage systems can be described in terms of the following characteristics:

• Capacity—the amount of energy stored in the system (depends on the storage process, the medium, and the size of the system)
• Power—how fast the energy stored in the system can be discharged (and charged)
• Efficiency—the ratio of the energy provided to the user, compared with the energy needed to charge the storage system (efficiency accounts for the energy lost during the storage period and the charging/discharging cycle)
• Storage period—how long the energy can be stored
• Charge and discharge time—the time needed to charge/discharge the system
• Cost—measured in relation to either the capacity ($/kWh) or power ($/kW) of the storage system (depends on the capital and operation costs of the storage equipment)

The capacity, power, and discharge time of an energy storage system are interdependent variables. High-energy storage density and high power capacity for charging and discharging are desirable properties of any energy storage system. The table below shows the typical range of values for the characteristics of the three types of TES systems:

Sensible heat storage, in which a liquid or solid storage medium (e.g., water, sand, molten salts, or rocks) is heated or cooled, is the simplest method of storing thermal energy. The two main advantages of sensible heat storage are (1) it is relatively inexpensive compared to PCM and TCS systems and (2) it does not require the use of toxic materials. While sensible heat storage is applicable for domestic systems, district heating, and industry, in general, it requires large volumes because of its low energy density (roughly three and five times lower than PCM and TCS systems, respectively). Sensible heat storage systems also require effective design and installation to ensure they discharge thermal energy at a constant temperature.

The linear relationship between material temperature and stored heat or energy.

Sensible heat storage takes advantage of the heat capacity and temperature change of the storage medium during charging and discharging. The amount of heat (or energy) stored is a function of the specific heat capacity of the medium, the temperature change, and the amount of storage material. The specific heat capacity of a material is not constant; however, generally speaking across the temperature ranges used in TES systems, it can be treated as constant. Therefore, the heat stored in a fixed amount of material scales linearly with temperature. The heat stored can be calculated using the following equation:

Where:

• is heat stored in Joules (J)
• is the mass of heat storage medium in kilogram (kg)
• is the specific heat of the medium in Joules per kilogram kelvin (J/kg K)
• is the initial and final temperature measured in either kelvin or celsius (K or ^oC)

A table of selected solid-liquid materials with key properties is shown in the table below:

Due to its price and high specific heat, water is the most commonly used heat storage medium, with a number of applications in residential and industrial applications. However, water is limited to below 100^oC for TES. Above this, temperature oils, molten salts, and liquid metal are used. For air heating applications, rock bed-type storage materials are commonly used.

Hot water tanks are a well-known technology for TES and serve the purpose of energy saving in water heating systems based on solar energy and in co-generation (heat and power) energy supply systems. Projects have shown that with optimal stratification in the tank and effective thermal insulation, water tank storage can be an effective storage option. Evacuated super-insulation can limit thermal loss to a rate of 0.01 W/mK at 90°C and 0.1 mbar.

A typical water tank energy storage system is shown below:

Typical water tank

Assuming the water storage unit is at a uniform temperature (fully mixed with no stratification), the total heat capacity is simply the product of the mass, specific heat, and temperature differential. The lower temperature limit of the system is typically limited by the requirements of the process and the upper limit by the vapor pressure of the liquid or the collector heat loss.

Hot water storage systems are often used as buffer storage for domestic hot water (DHW) supply usually with a volume in the range of 500 liters to several cubic meters (m³). The same storage technique is also used in solar thermal installations for DHW combined with building heating systems. Large hot water tanks can be used for seasonal storage of solar thermal heat in combination with small district heating systems; these systems have significantly larger volumes (up to several thousand cubic meters). Charging temperatures are in the range of 80-90 ^oC, and the usable temperature difference can be enhanced through the effective use of heat pumps during discharging.

Water's prevalence, high specific heat capacity, chemically stability, and low cost make it a good storage medium for low-temperature solar cooling applications (e.g., single-stage absorption chillers and desiccant systems). Water's boiling point (100^oC at 1bar) means high-temperature applications (double effect and triple effect chillers) require an increase in system pressure.

Another widely used storage technology is making use of the ground (e.g., the soil, sand, rocks, and clay) with underground thermal energy storage (UTES). The technique requires adding and removing energy to the medium through heat transfer fluids through buried pipe arrays. The rate of charging and discharging is limited by the area of the pipe array and the rate of heat transfer through the ground surrounding the pipes. UTES systems are usually not insulated, although insulation may be provided at the ground surface. The following are the main types of UTES:

• Borehole storage—This is based on vertical heat exchangers installed underground, to ensure thermal energy is transferred to and from the ground layers. Many projects using borehole storage aim for seasonal storage of solar heat in the summer to heat houses or offices during winter. Ground heat exchangers are frequently used in combination with heat pumps to extract low-temperature heat from the soil.
• Aquifer storage—This uses naturally occurring underground layers that are water-permeable as a storage medium. Thermal energy is transferred by mass transfer (i.e. extracting/re-injecting water from/into the underground layer). The primary application of aquifer storage is for winter cold to be used for the cooling of large office buildings and industrial processes in the summer. This technology is only available in locations with suitable geological formations.
• Cavern and pit storage—This is based on large underground water reservoirs created in the subsoil to serve as thermal energy storage systems. While these storage options are technically feasible, applications are limited due to high investment costs.

A packed-bed, or pebble-bed, storage unit uses the heat capacity of a bed of loosely packed particulate material (a variety of solids can be used with rock and pebble the most popular) to store energy. A fluid, generally air, is circulated through the bed to add or remove energy, with one direction (usually downward) used to add energy and the other direction for removing energy. Unlike water tanks in which heat can be simultaneously added and subtracted, pebble-bed storage systems are limited to only one at a time.

A significant advantage of packed-bed storage is the high degree of stratification. Pebbles near the entrance are heated first, with a delay in heat reaching the pebbles near the exit. A fully charged occurs when enough time has passed for heat to permeate the entire bed and the pebbles have a uniform temperature.

Latent heat creates a plateau in the energy stored as a function of temperature.

Latent heat describes energy absorbed or released during a change in physical state or phase due to an increase or decrease in temperature. It creates a plateau in stored heat as a function of temperature, with energy required to change the state of the material. Latent heat storage materials, or phase change materials (PCMs), take advantage of this property to store additional energy.

The energy storage density increases, and therefore the required volume reduces compared with sensible heat storage. Heat is primarily stored in the phase-change process with the material kept at a near-constant temperature. The use of PCMs is an effective way of storing thermal energy, offering advantages compared with sensible heat storage due to its high-energy storage density and isothermal nature.

The phase change process can occur in different modes: solid-solid, liquid-gas, and solid-liquid. In solid-solid heat is stored in the transition between different crystallization forms. Liquid-gas transitions have high latent heat but have issues in storage due to the significant volume variations during phase change. The most widely used mode is solid-liquid, which has reduced volume variation (generally <10%) and a relatively high melting latent heat. Compared with sensible heat storage, which has energy densities in the region of 25 kWh/m³, melting processes are around 100 kWh/m³.

The storage capacity of a latent heat storage system based on a solid-liquid phase change can be calculated using:

Where:

• is the storage capacity measured in Joules (J)
• is the mass of the PCM medium in kilogram (kg)
• is the initial and final temperature measured in Celsius (^oC)
• is the melting temperature in Celcius (^oC)
• is the average specific heat of the solid phase between t_i and t_m measured in units (kJ/kg K)
• is the average specific heat of the liquid phase between t_m and t_f measured in units (J/kg K)
• is the melt fraction
• is the latent heat of fusion in Joules per kilogram (J/kg)

PCMs have found use in thermal applications for decades and can be used for both short-term (daily) and long-term (seasonal) energy storage, using a variety of techniques and materials. Applications include the following:

• Implementation in wall covering materials, such as gypsum board, plaster, and concrete as part of the building structure to enhance the thermal energy storage capacity, mainly used for peak-load shifting and solar energy.
• Cold storage for cooling plants
• Warm storage for heating plants
• Hot storage for solar cooling and heating

To be effective, PCMs must have

• appropriate thermo-physical properties, such as high latent heat of transition and thermal conductivity, high density, and low volume variations during phase transition to minimize storage volume;

• kinetic and chemical properties that include super-cooling limited to a few degrees, and long-term chemical stability and compatibility with materials of construction (no toxicity, and no fire hazard); and

• economic advantages, such as low cost and large-scale availability.

PCMs for latent heat storage can be broadly classified based on their physical transformation during heat absorption. Solid-liquid PCMs are classified as follows:

• Organic—able to melt and solidify many times without phase segregation
• Inorganic—generally used in high-temperature solar applications
• Eutectic—a combination of two or more low melting materials similar melting/freezing points

PCM Classifications.

Critical heat storage properties for a select group of PCMs are shown in the table below.

TCS uses thermo-chemical materials (TCMs) that store and release energy through endothermic/exothermic reactions. TCS offers high energy densities (roughly 300 kWh/m³). While charging, heat is applied to the material producing a separation into two different parts. The two reaction products are separated and stored until the discharge (when the energy is required), when they are mixed at suitable pressure and temperature conditions and energy is released. The process requires the reaction products to be stored such that thermal losses are limited.

A range of reactions have been considered for TCS, including adsorption (i.e. adhesion of a substance to the surface of another solid or liquid) such as water vapor to silica-gel and the thermal decomposition of metal oxides such as potassium oxide. The table below shows interesting chemical reactions for TES.

Sorption refers to the physical and chemical process in which one substance accumulates within another phase or on the boundary of two phases. Generally, this refers to absorption (accumulation in a phase) and adsorption (accumulation at the phase boundary). The reverse process is called desorption.

Sorption processes make attractive candidates in TCS, as they have high storage capacity that facilitates thermal energy transportation. Applications of sorption materials include waster heat utilization. The table below shows some of the sorption materials (microporous and composite) under investigation for TCS use.

Energy storage technologies each have different properties making them suitable or unsuitable for different applications. Electrical energy storage technologies, or batteries, are also becoming a popular solution for intermittent renewable energy supply. With battery innovation driven by the growing electric vehicle industry, battery technology and production are expanding. However, many argue TES has superior performance in applications such as managing a commercial facility's energy costs. Thermal energy storage is cheaper than electricity storage.

Thermal storage mediums, such as ice, are extremely effective for storing and releasing large amounts of energy with less waste and have the potential to help companies take advantage of off-peak electricity. In many instances, companies would have to make a larger investment to match the storage and discharge capacity of a TES system using batteries. Other factors favoring TES for commercial buildings include long-term ROI (batteries only last up to fifteen years) and removing the hassle of disposing of lithium-ion waste materials. Also for HVAC applications battery storage suffers greater round-trip inefficiency losses compared to thermal energy storage.

Applications of TES systems include the building sector (DHW, space heating, air-conditioning, etc.) and the industrial sector (e.g. process heat and cold). Installing TES systems, either centralized or distributed, can improve the energy efficiency of industrial processes, residential energy use, power plants, and more.

The most common application of TES is in solar thermal systems at CSP plants. TES can help overcome intermittent supply, a major challenge of solar energy. CSP systems reflect the solar rays onto a receiver, which creates heat to generate electricity for either immediate use or storage. Through this process, CSP can be flexible or dispatchable options for providing clean energy. Multiple sensible TES technologies have been tested and implemented since 1985:

Solar thermal energy is stored in the same fluid used to collect it. Fluid is stored in two tanks—one high and one low temperature. Fluid flows from the cold tank through the receiver (where it heats) and into the hot tank for storage. During discharge, the high-temperature fluid passes through a heat exchanger, generating steam for electricity production. The fluid then returns back to the low-temperature tank.

This type of TES system was used in early parabolic trough power plants including the Solar Electric Generation Station I and the Solar Two power tower in California. The trough plants used mineral oil as the heat transfer and storage fluid and Solar Two used molten salt.

The two-tank indirect system is similar to the direct system, except different fluids are used for heat transfer and storage. Storage fluid from the low-temperature tank flows through an additional heat-exchanger where it is heated by the high-temperature heat-transfer fluid. The extra heat exchanger adds cost to this type of system. Two-tank indirect systems have been used in parabolic plants in Spain and have been proposed for parabolic plants in the US. The proposed plants would use organic oil as the heat-transfer fluid and molten salt as the storage fluid.

Single-tank thermocline systems store thermal energy in a solid medium (the most common example being silica sand) located in a single tank. During operations, a region of the medium is at a high temperature, while another region is at a low temperature. These two regions are separated by a temperature gradient (or thermocline). High-temperature heat-transfer fluid flows from the top of the thermocline to the bottom, moving the thermocline downward and adding thermal energy to the system for storage. This process is reversible, moving the thermocline upward and removing energy from the system to generate steam and electricity. Buoyancy affects produce thermal stratification of the fluid within the tank, helping stabilize and maintain the thermocline.

Using a solid storage medium and a single tank reduces the cost of this system type relative to two-tank systems. Single-tank thermocline systems were demonstrated at the Solar One power tower, where steam was used as the heat-transfer fluid and mineral oil as the storage fluid.

In 2020, the global TES market was estimated to be $188 million. Factors including the increased demand for electricity during peak hours, increased commercialization of CSP plants, and the need for heating and cooling applications in smart infrastructure, mean the market is projected to increase to $369 million by 2025 (CAGR of 14.4%).

Large companies operating in the TES market include the following:

• Climate Change Technologies
• DN Tanks
• Calmac
• Sener Group
• DC Pro Engineering
• Burns & Mcdonnel
• Baltimore Aircoil Company
• Ice Energy Technologies Inc.
• Vattenfall
• Abengoa Solar

There are also many startups operating in the space looking to commercialize TES technologies:

• Hocosto
• Nostromo
• Malta Inc
• Inficold
• Stash Energy