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Compressed Heat Energy Storage for Energy from Renewable sources

Periodic Reporting for period 2 - CHESTER (Compressed Heat Energy Storage for Energy from Renewable sources)

Reporting period: 2019-10-01 to 2021-03-31

Electricity storage has a key role in the transition of the European energy sector towards a neutral emissions system. Balancing the grid is mandatory in order to allow for the targeted RES penetration in the grid. Energy storage will be the key enabling technology for increasing RES production in the future.
CHESTER project aims to develop a CHEST (Compressed Heat Energy Storage) system, an innovative power-to-heat-to-power energy storage system. It delivers heat and power on demand, making use of the excess renewable and other excess energy sources available. The combination of the CHEST system with Smart District Heating leads to a very flexible and smart renewable energy management system that is able to store electric energy with a round trip efficiency of 100% or even higher, site-independent unlike pumped hydro, cyclically stable unlike batteries, able to convert power into heat, able to convert renewable low temperature heat into power, able to store and deliver independently from each other upon request both, heat and power, cost competitive.
The CHESTER project overall objectives are mainly two: to demonstrate a first-of-its-kind laboratory scale prototype system (10 kWe) and advance from TRL3 to TRL5 on system concept at real scale regarding system design and optimization, and integration into the grid and electric market.
The CHEST system is formed by a high temperature heat pump (HTHP), a high temperature thermal energy storage (HT-TES) and an ORC unit. Electricity is consumed by the HTHP and stored in the HT-TES, to be used later on by the ORC unit and produce electricity. In addition a novel heat engine (based on isobaric expansion (IE)) is being developed in order to increase cycle efficiency if integrated into the system.
The HTHP, ORC and heat engine prototypes have been already manufactured and tested. The HT-TES is being tested and experimental results are expected to be available soon. The project has entered now into its last phase. All the technologies will be integrated forming the overall CHEST system and it will be tested at laboratory. This will be carried out in the last year of the project. So far, the following results have been achieved:
• A 10kWe HTHP that uses an environmentally friendly refrigerant (R1233zd(E)) able to operate at high evaporating temperatures (70ºC to 95ºC) and high condensing temperatures (approximately 146ºC) has been built and tested. Measured COP values ranging from 3 to almost 7 were achieved.
• A novel design of a HT-TES system based on PCM has been developed and prototype built (melting temperature of the selected PCM is 133ºC). The system is formed by a latent heat storage unit (LT-TES) of 2,1 m3 and a sensible heat storage unit (SH-TES) of 2 m3.
• A novel ORC prototype has been built, commissioned and tested, including an over 10 kWe nominal power piston expander with a built-in novel variable valve timing mechanism. Experimental results demonstrate the advantageous implementation of the real time control of the expander inlet valve timing to increase the efficiency of the prototype.
• A novel isobaric expansion heat-engine pump prototype has been successfully built, commissioned and tested. Experimental results demonstrate the great potential of IE engines as a technology for conversion of heat to mechanical energy, particularly in low temperature applications.

One of the objectives of the CHESTER project is to establish the theoretical basis for the upscaling and optimization of the CHEST system. Detailed simulations are being carried out using TRNSYS and new knowledge has been generated regarding system design, operation performance and market implementation possibilities.
The CHEST system will be managed by a smart energy management system (SEMS). It will allow the system to use the energy in the most cost efficient, technically appropriate and flexible way. The performed work during the first three years of the project has allowed to develop the core part of the SEMS.
Detailed simulations have been carried out to analyze the CHEST system integrated into the energy system. Regarding technical performance, the roundtrip efficiency in CHEST is quantified as the power-to-power (P2P) ratio of the overall system. Before the CHESTER project started the available information on CHEST systems was very limited. The only reference available regarding the expected roundtrip efficiency was a P2P ratio of 0.56 expected to be achieved from the combination of Isobutane and LiNO·-NaNO3-KNO3. During the project different refrigerant and PCM combinations have been analyzed through simulations, which have shown that this P2P ratio can be doubled, and consequently reduce the costs and the required volume significantly. The assessment of different configurations shows that the configuration based on Cyclopentane and LiNO3-NaNO3-KCl has a P2P of 1.48 with 30% lower PCM volume requirement and half of the HTHP installed capacity. Moreover, due to the higher P2P, economic profitability increases, and heat demand is reduced. Due to the fact that the PCM storage is the most expensive component of the system, the reduction of storage volume has substantial effects on the economics of the system.
CHEST system:
- Before the start of the project there was not any realized system of CHEST concept and very little knowledge available regarding the concept itself. The performed work has allowed to establish the theoretical basis of the performance of full scale CHEST system on real applications and an advanced model has been developed which comprises a relevant tool for further research and development.

HT-TES based on PCM:
- Innovative heat storage design based on a cascaded unit with sensible and latent storage allowing high efficiency in condensing/evaporating processes. Prototype for 133ºC melting temperature in the range of several m3 designed and built (to be tested). PCM heat exchanger based on aluminium finned tubes. PCMs storage design for new working fluids.

- Compressor, using an alternative refrigerant with low GWP, successfully tested allowing refrigerant outlet temperatures >150ºC.
- Selection and use of the most appropriate lubricant that ensures the compressor operation and durability based on experimental characterization

- Improve standard technology of expanders, including a variable volume ratio mechanism with a sliding volume valve (never used so far in expanders)

- Novel heat engine pump based on isobaric expansion of 1 kW output designed, built and tested. Demonstration of effective operation at heat source temperature of >80ºC.

- Implementation of forecasting models into control strategies. Advanced AI-based control system that is scalable and portable. A self-learning dynamic management tool based on AI algorithms in order to control and optimize the operation of the CHEST system.

In the next 12 months (the last year of the project) all the technologies will be integrated into the overall CHEST system and it will be tested. Learnt lessons from commissioning and experimental results of CHEST prototype operation are expected by the end of the project. In addition, final conclusions regarding the economic performance of real scale CHEST systems, market implementation possibilities and business opportunities will be concluded.
Isobaric expansion engine pump experimental set-up
ORC prototype developed in CHESTER project
High temperature heat pump prototype developed in the CHESTER project
HT-TES prototype developed in CHESTER: LH-TES unit (right side) and SH-TES tanks (front left)