Carnot battery
A Carnot battery is a type of energy storage system that stores electricity in thermal energy storage. During the charging process, electricity is converted into heat and kept in heat storage. During the discharging process, the stored heat is converted back into electricity.[1][2]
Moreover, “Marguerre” patented the concept of this technology 100 years ago[3] but the development of this concept has only recently been revitalized for increasing the shares of energy delivered by renewable sources. On the other hand, “Andre Thess” invented the term of Carnot battery in 2018, before the first International Workshop on Carnot Batteries.[4]
The name "Carnot battery" comes from Carnot's theorem, which describes the maximum efficiency of converting heat into mechanical energy. The word "battery" indicates that the purpose of this technology is to store electricity. The discharge efficiency of Carnot batteries is limited by the Carnot efficiency.
The German Aerospace Center (DLR) and University of Stuttgart have been working on the concept of Carnot batteries that store electricity in high-temperature heat storage since 2014.[5] In 2018, the name "Carnot battery" was used in Hannover Messe,[6] one of the world's largest trade fairs, by DLR.[5] However, the concept of Carnot batteries covers the technologies that have been developed for years,[7] such as pumped thermal energy storage[8][9] and liquid air energy storage.
Background[]
In the transition to low-carbon energy systems, the penetration of variable renewable energy in electrical energy systems increases, and this also increases the need for energy storage. Currently, most of the new installed energy storage capacity comes from electrochemical batteries, such as lithium-ion batteries. This type of battery is suitable for short-term storage but may not be economical for longer durations due to its high energy capacity costs.[7] Thermal energy storage can store energy in inexpensive materials, such as water, rocks, and salts. Therefore, the cost for large-scale systems (e.g. gigawatt hours) can be lower than electrochemical batteries.[5]
Energy Storage Annex 36 - Carnot Batteries is a working group under the Energy Conservation and Energy Storage (ECES) programme, which is part of a Technology Collaboration Programme (TCP) under the International Energy Agency (IEA).[10]
System configuration[]
A Carnot battery system can be divided into three parts: Power to Thermal (P2T), Thermal Energy Storage (TES), and Thermal to Power (T2P).
Electricity to heat technology[]
Electricity can be converted into heat through the use of various technologies.[1]
- Resistive heating
- Heat pump is the technology to pump heat from a lower temperature reservoir to a higher temperature. It can be divided into two groups: reverse Rankine cycle and reverse Brayton cycle.
- The reverse Rankine cycle has been widely used in conventional heat pumps.
- The concept of using the Brayton cycle for charging and discharging thermal energy was proposed by Prof. Robert B. Laughlin in 2017.[11]
- Others: In liquid air energy storage systems, the Claude Cycle is utilised to liquify air. The Lamm–Honigmann process uses thermochemical cycles to covert power to heat.[12]
Thermal energy storage[]
According to the mechanism to store heat, thermal energy storage can be divided into three types: sensible heat storage, latent heat storage, and thermochemical storage. The storage materials that have been used for Carnot batteries are:
- Hot water
- Molten salt
- Packed-bed rocks
- Liquid air
- Latent heat thermal energy storage[13]
- Thermochemical materials (pairs of chemicals), such as LiBr/H2O and H2O/NH3[12]
Heat to electricity[]
Heat can be converted into power through thermodynamic cycles, such as the Rankine cycle or Brayton cycle. Some technologies use the property of semiconductor materials to convert heat into electricity, and those are not considered a Carnot battery because there are no thermodynamic cycles involved in the conversion process, such as thermoelectric materials and the "Sun in a box".[14] The typical technologies are:
- Heat engine
- Steam turbine
- Gas turbine[15]
- Organic Rankine cycle machines
- Lamm-Honigmann process can convert the stored energy in thermochemical storage into electricity.[12]
Advantages and disadvantages[]
The Carnot battery has been known by several other names such as “Pumped Thermal Electricity Storage’’ (PTES) or “Pumped Heat Electricity Storage” (PHES). This relatively new technology has become one of the most promising large-scale energy storage technology.
The main advantages of the Carnot battery are:[16]
- Free choice of site;
- Small environmental footprint;
- Life expectancies of 20–30 years;
- Optional low-cost backup capacity;
- The components of an underutilized fossil-fueled power plant can be partially reused to build the Carnot batteries unit ;
The major drawback of this technology is:[17]
- The limited roundtrip efficiency