Preliminary prospects of a Carnot-battery based on a supercritical CO2 Brayton cycle

Abstract

As a part of the change towards a higher usage of renewable energy sources, which naturally deliver the energy intermittently, the need for energy storage systems is increasing. For the compensation of the disturbance in power production due to inter-day to seasonal weather changes, a long-term energy storage is required. In the spectrum of storage systems, one out of a few geographically independent possibilities is the use of heat to store electricity, so-called Carnot-batteries. This paper presents a Pumped Thermal Energy Storage (PTES) system based on a recuperated and recompressed supercritical CO2 Brayton cycle. It is analysed if this configuration of a Brayton cycle, which is most advantageous for supercritical CO2 Brayton cycles, can be favourably integrated into a Carnot-battery and if a similar high efficiency can be achieved, despite the constraints caused by the integration. The modelled PTES operates at a pressure ratio of 3 with a low nominal pressure of 8 MPa, in a temperature range between 16 °C and 513 °C. The modelled system provides a round-trip efficiency of 38.9 % and was designed for a maximum of 3.5 MW electric power output. The research shows that an acceptable round-trip efficiency can be achieved with a recuperated and recompressed Brayton Cycle employing supercritical CO2 as the working fluid. However, a higher efficiency would be expected to justify the complexity of the configuration.

Carnot Battery Based on Brayton Supercritical CO2 Thermal Machines Using Concentrated Solar Thermal Energy as a Low-Temperature Source

“…Analysing Figure 2b, which represents the heat engine used in the present work, the CO 2 leaving the turbine (HET) takes the thermal energy from the molten salts in the molten salts/CO 2 heat exchanger (MSHE) (8)(9) and transfers it to the cold stream in the high-temperature recuperator (HTR), reaching the condition of the turbine inlet. Once the hot stream has left the HTR, it transfers thermal energy again (10)(11) in the low-temperature recuperator (LTR) to the cold stream (13)(14). It can be observed that the cold mass flow rate in the LTR (high-pressure stream) is lower than the mass flow rate of the hot stream (low-pressure steam).…”

“…The cost of the heat exchanger was determined based on the cost of a baseline heat exchanger, which was adjusted by taking into account various factors, such as different materials, pressures, and other design features. The cost, C E (USD), was estimated using Equation ( 13), supported by Equation (14). Here, C b represents the cost of the baseline heat exchanger made of a base material (carbon steel) ($/ft 2 ) designed for a specific pressure range (cost based on 1982 data).…”

“…Within the Brayton cycles, Rindt et al [14] focused on recompression with supercritical CO 2 , with a low pressure of 80 bar and a high pressure of 240 bar, ranging the temperature between 16 • C and 513 • C. They designed a unit of 3.5 MW of power output, reaching 38.9% of round-trip efficiency.…”