Thermal energy storage for Industrial applications
Although it is increasingly recognized in the domestic sector, thermal energy storage (TES) remains largely underutilized for industrial applications. Many barriers do exists, both technological but also from decision-making and integration point of view. In our work we developed a systematic approach to guide and inform decision regarding integration of TES into the industrial environment. This helps both TES developers and industrial end-users in extracting maximum value from the integration of TES across industries. Full details are available in our open access publication – link
Experimental testing of a Topology Optimization-based Latent Heat Energy Storage device
The poor thermal conductivity of most of the Phase Change Materials (PCMs) is one of the main barriers to the spread of Latent Heat Thermal Energy Storage (LHTES) technologies. In our previous work, we investigated the use of Topology Optimization (TO) as a design tool for the performance maximization of LHTES devices. In our recent study, we focused our efforts on the manufacturability and testing of such devices. Our aim is to demonstrate the use of TO as a design tool for LHTES technologies, from design specifications to device testing.
The key research questions
In this work, we investigate two different aspects of TO as a design tool for LHTES:
- The use of Additive manufacturing to fabricate a topology optimization-based design for LHTES;
- The testing of an LHTES design driven by the TO framework and the performance comparison with a commercial configuration.
Methodology description
The complex geometrical features arising from the optimization algorithm are manufactured via Selective Laser Melting (SLM). The design dimension used here is around 68 mm for adapting to a 78 mm inner diameter outside shell, while an A20XTM aluminum alloy (Al-5 % Cu) in a powder form was used as high conducting material.
A test rig for the discharge process performance evaluation is developed connecting the manufactured device with a chiller for temperature control. The discharge performance of the optimized design are investigated under different operating conditions and a CFD model is validated. By means of the high-fidelity model, we compare the performance of the TO-based design with a commercial design with the same volume fractions.
What we have found and why it is relevant
The unique element of the work is the demonstration of additive manufacturing as a viable tool to fabricate topology optimization-based designs for LHTES devices. The manufacturing of the optimized complex geometrical features is not a trivial task, and a trade-off between optimal and accessible design has to be found. The challenges we faced during the design manufacturing are presented and the adopted solution is discussed in our recent publication. The optimized design performances are compared with a commercial configuration with longitudinal fins, showing a 57.1% reduction in the discharging time.
Currently, there are limited applications of additive manufacturing techniques in the energy storage field. In the future, it is necessary to further investigate the design principles and process parameters selection during the additive manufacturing process. In addition, more topology optimised fin designs considering different hydraulic loops and charging/discharging processes can be prototyped and tested. As a perspective, it will also be useful to combine the production cost of additive manufacturing and thermal performance of topology optimised devices to evaluate the economy of this approach.
The authors of this work would like to acknowledge the financial support from the Engineering and Physical Sciences Research Council (EPSRC), United Kingdom (EP/R016402/1).
A novel optimal dispatch model for LAES including storage thermodynamic characteristics
In our previous work on Liquid Air Energy Storage (LAES), we demonstrated how different operating patterns may affect storage thermodynamic performance. Such consideration introduces the need to properly capture storage efficiency and its variations for a truthful estimation of storage performance and financial value.
Building on the outcomes from our previous investigation, in this work we propose a new methodology to economically optimize LAES power dispatch, while accounting for storage efficiency variations during operation. We use this model to put forward criteria for optimal storage sizing and market participation.
In this work we investigate three main aspects associated with LAES – or more generally storage – operation as part of the power grid.
- Optimal power dispatch profile and how this changes when storage performance variations are considered
- Optimal power and capacity sizing for the highest financial benefit
- Profitable storage participation to competing power markets and services
Model description
The novelty of the proposed approach is it includes, within a single model, technical constraints linked with multi-service grid participation, robust dispatch profile against real-time service calls and LAES thermodynamic performance maps for design and off-design conditions. These individual building blocks are integrated in the final optimization model, as shown in Figure 1. Outcome is the optimal dispatch schedule, which instructs on how to operate LAES hour by hour, to hit the highest revenues while meeting all the technical constraints associated with the energy system and the plant itself.
What we have found and why it is relevant
First of all, the results provide a feasible LAES scheduling, when compared to traditional models assuming ideal storage behaviour. LAES on average runs over fewer yearly cycles but at higher power output, closer to its rated conditions (see Figure 2). Feasible scheduling as opposed to the ideal case allows to avoid loss of revenues and ensures a better financial value from storage real operation.
Finally, independent LAES power and capacity sizing, together with the possibility of differentiating the revenue streams from entering multiple energy markets (arbitrage and reserve services) enable to create relevant business cases for LAES deployment. We showed how a suitable mix of arbitrage and reserve (STOR or FR) can lead to payback time below 20 years; this can be further enhanced by suitable storage sizing. Optimal values depend ultimately on the service portfolio provided. However, general guidelines suggest storage durations above 2-3 hour are not necessary and the liquefier power rating should be contained below 20-30% the value for the power recovery unit.
The methodology presented in this paper provides a relevant and widely applicable tool to realistically optimise LAES dispatch. Outcomes are precious for leveraging LAES financial value and ensuring its contribution to grid balancing.
This work was carried out in the framework of a joint PhD scheme between the University of Birmingham and the University of Melbourne. We gratefully thank the two universities for enabling this collaboration.
Link between liquid air energy storage performance and the balancing services provided: a techno-economic assessment
Liquid air energy storage (LAES) is a large-capacity storage technology, which is suitable for district and city scale integration. In these settings, LAES will allow to store large portions of “wrong-time” electricity in the form of liquid air, at cryogenic temperatures below -190°C, avoiding its waste and enabling its use when most needed. Such capability enables great flexibility, which will be key for transitioning towards a future low-carbon energy system with substantial integration of intermittent renewable sources.
In this piece of research, we develop a novel numerical model for LAES and we use it to elucidate the existing link between the services LAES provides to the gird and LAES techno-economic performance.
How LAES can contribute to the energy transition
LAES can provide multiple benefits to the energy system operator. By absorbing and releasing electricity “on-demand” it helps in shaving the spikes in users’ demand and shifting the load on conventional generators from peak to off-peak times. This guarantees a steadier and more energy efficient operation of generating assets. Additionally, it can also assist maintain grid frequency stable at 50 Hz through the so-called “reserve services”, by compensating unexpected variations in demand or supply. Electric power supply is safe and uninterrupted this way and the risk of black outs is mitigated (a frequency drop to 48.9 Hz due to generation outage was the main cause of the recent UK blackout on 09.08.2019). In the UK energy market, the above functionalities of LAES can be translated into the three main services we considered in our study: Arbitrage, Short Term Operating Reserve and Fast Reserve.
Even more interestingly, not only does participating to multiple balancing services mean LAES is contributing to grid stability to a greater extent, but payment from the grid operator is also received for each service provided. The economic value and return of investment for LAES operator becomes then more and more attractive. So, the key question is now: “Do different operating strategies for LAES impact on storage performance? How?”
What we are investigating in this piece of research
In this study, we look precisely at the dependency between LAES operating strategy and performance. In particular:
- We consider 3 possible combinations of the above services (we call each of them Mode)
- For each Mode, we work out a relevant daily operating cycle that will be requested to LAES for providing those services
- We build a new mathematical model of LAES, featuring a detailed description of plant components which includes relationships between the power output of LAES and the efficiency of each component
Based the steps 2 and 3, we are finally able to test LAES performance for each operating Mode, which is assessed both from a technical and an economic point of view.
What we have found and why it is relevant
Results show that LAES operation over multiple balancing services can significantly enhance LAES yearly revenues and reduce the dependence from electricity price profiles. However, this comes at the expenses of a lower thermodynamic performance for the plant.
The left-hand side figure illustrates how the liquid air in the storage tank is consumed. Higher power allocation to reserve services leads higher losses due to off-design conditions along the LAES cycle, and specifically in the turbines. As a consequence, the plant roundtrip efficiency – a common performance indicator for energy storage – will also vary according to the duty cycle requested to the plant (by up to 30% in the cases explored). Such relationship reflects the link between LAES performance and the provision of different services. The right-hand side of Figure 3 compares the economic results obtained for a constant roundtrip efficiency (Ideal) with predictions including the variable efficiency from our model. Variation in yearly revenues are significant and, crucially, higher for Mode 2 and Mode 3, which thanks to reserve provision are also the most profitable ones, and thus relevant for application.
Overall, results show that for an accurate techno-economic assessment of LAES (or similar storage technologies) it is necessary to consider such link between operating strategy and plant performance. Our research work details a new methodology to properly model and assess such interdependency, enabling to precisely characterize storage efficiency for LAES and other thermo-mechanical storage technologies.
This work was carried out in the framework of a joint PhD scheme between the University of Birmingham and the University of Melbourne. We gratefully thank the two universities for enabling this collaboration.