Optimizing Carnot batteries for renewables storage

Researchers from Portugal’s University of Coimbra have designed various versions of heat pump-organic Rankine cycle-based Carnot batteries (CBs).

Carnot batteries are systems that store electricity in the form of heat through storage media such as water or molten salt and transform the heat back to electricity when needed. This category includes liquid air energy storage (LAES) systems and Brayton or Rankine based pumped thermal energy storages (PTES) systems, as well as Lamm-Honigmann storage, which is a sorption-based technique that can be charged and discharged with both heat and electrical power, and systems based on integrated resistive heating with power cycles.

All these storage technologies allow a broad range of applications such as arbitrage business, ancillary services, or peak shaving within power networks.

The scientists simulated several systems through single-objective optimization and multi-objective optimization for energetic, exergetic, and economic efficiencies, using 16 different combinations of environmentally friendly working fluids.

“A CB is a power-to-heat-to-power energy storage technology that converts surplus electricity into thermal energy by heating or cooling a thermal energy storage (TES) system. The stored thermal energy can be later converted back into electricity when needed,” the academics explained. “The CB is divided into three thermal sectors: the TES tanks (high temperature and low temperature), the heat source, and the heat sink.”

The team suggested six HP-ORC combinations that work as a CB. System 1 is the most basic system, including a vapor compression heat pump (VCHP) and a simple ORC. System 2 adds a regenerator to the HP, system 3 adds a regenerator to the ORC, and system 4 adds regenerators to both the HP and the ORC. System 5 uses a two-stage heat pump with a flash chamber, while system 6 two-stage HP and a regenerative ORC.

Four different working fluids were tested for both the HP and the ORC sides, yielding a total of 16 possible combinations. Specifically, they were R1224yd(Z), R1234ze(Z), R1336mzz(Z), and R1233zd(E). The systems were developed in MATLAB 2024a, assuming steady-state operation for the VCHP and ORC; no heat losses and pressure losses in heat exchangers; compressor and expander constant efficiencies; water as heat transfer fluid in the heat source, storage and cold source.

Firstly, a single-objective optimization was initiated for all possible combinations of the six system configurations and the 16 working fluid pairs. Each combination went through three single-objective optimizations – energetic, exergetic, and economic. A novel scoring methodology was applied to systematically identify the optimal working fluid configuration pair.

The simulations showed that the regenerative heat pump and ORC configuration (system 4) consistently provided the best results across all fluid combinations, demonstrating its suitability for this technology compared to other systems, with R1233zd(E)-R1233zd(E), followed by R1234ze(Z)-R1224yd(Z) showing highest overall performance. The Basic heat pump and ORC configuration (system 1) with R1336mzz(Z)-R1336mzz(Z) was found to achieve the lowest performance.”

Following this analysis, the group conducted the multi-objective optimization for system 4, using R1233zd(E) on both HP and ORC sides. Per their findings, a tradeoff between round-trip efficiency and the levelized cost of storage (LCOS). The optimal design, nevertheless, achieved a 57.43% round-trip efficiency and an LCOS of €0.649 ($0.73)/kWh.

“Higher efficiencies up to 81.30% were possible without significantly compromising overall performance. Beyond this point, further improvements are not justified due to rapid performance degradation,” said the team. “The optimal design point corresponds to an LCOS of 1.093€/kWh for a small-scale experimental test rig but is expected to decrease at larger scales.”

Their findings were presented in “Multi-objective optimization and design of a Carnot Battery for energy storage applications,” published in Energy Conversion and Management: X.

In a research published in 2023, academics from the Technical University of Denmark proposed to use Carnot batteries to convert coal power plants to renewable energy production.

Another group of researchers in Denmark also investigated how Carnot batteries may be used to store renewable energy in their home country and have found that these devices may provide a significant contribution only under a certain cost threshold.