Development of cold thermal energy storage for industrial refrigeration applications

Abstract

Refrigeration technology is a vital part of modern society, covering multiple applications from comfort cooling and process cooling of servers in data centres, to domestic, commercial, and industrial refrigeration systems. The market for refrigeration systems is continuously expanding, and space cooling is the fastest-growing end user of electricity in buildings today. Furthermore, refrigeration is essential in the food cold chain to preserve fresh and frozen goods and to prevent the important challenge of food loss. Refrigeration technology is crucial in every link of the cold chain, all the way from the processing plants, in transport, in retail and during the final stage at the consumer. Refrigeration systems are large electricity consumers, and some of these systems face high peak refrigeration loads and the associated high electricity consumption. Some examples of applications that experience large variations in the refrigeration load during the day are process cooling for the dairy industry and industrial freezing processes in food processing plants. Thermal energy storage (TES) technology can be applied to refrigeration systems to decouple the supply of cooling from the refrigeration system and the demand for cooling from the consumer. When TES technology is applied to store thermal energy at sub-ambient temperatures, it is often called cold TES (CTES). The two methods of achieving CTES are sensible heat storage and latent heat storage. In the former method, CTES is achieved due to the change of temperature of a storage medium, such as water. In the latter method, CTES is achieved in the phase transition of a storage medium, often by melting and solidification. A substance capable of storing large quantities of thermal energy in the solid-liquid transition is often denoted as a phase change material (PCM). Common PCM for CTES application are paraffins and various salt-water solutions. An in-depth review of applications of CTES using PCMs in refrigeration systems was carried out to identify the current research gaps and establish the state-of-the art. It was found that the interest in PCMs for the temperature range relevant to CTES applications has been increasing in the last few years, and commercial PCMs have become available on the market. It was found that research on the implementation of CTES technology in refrigeration has been carried out for multiple applications, including food transport and packaging, commercial refrigeration and various other refrigeration systems. Common to many of these applications is the pressing need to conduct experimental investigations of promising concepts studied theoretically in the past, such as for large-scale CTES systems for industrial cooling and freezing processes using ammonia or CO2 as the refrigerant. Common ways to implement CTES in refrigeration systems in the past have consisted of using ice/water as the latent storage medium, particularly for space cooling and process cooling applications. These CTES systems are connected to the refrigeration plant by an intermediate heat transfer circuit with glycol. This concept reduces the efficiency of the refrigeration plant because the evaporation temperature must be reduced considerably compared to supplying the cooling directly at the consumer temperature. The cooling must be cascaded through the intermediate circuit and then to the consumer, while the charging process of the storage requires even lower supply temperatures. To improve the efficiency of CTES systems for refrigeration plants, it can be beneficial to integrate CTES units directly into the primary refrigerant circuit, effectively avoiding the secondary heat transfer circuit. However, the review revealed a research gap for CTES solutions suitable for industrial scale that can handle the operating pressure of refrigeration systems. In the present research work, a novel concept for a CTES unit suitable for integration into the primary refrigerant circuit of a CO2 refrigeration system has been developed. A lab-scale demonstration unit and an experimental test facility using CO2 as the refrigerant was constructed. The novel concept is based on a special type of welded heat exchanger (HEX) plate called pillow plate (PP). The PPs are constructed of stainless steel and are stacked together to form a PP-HEX placed into a container. The container is filled with the PCM, immersing the PP-HEX into the storage medium. The PPs have flow channels inside for the refrigerant to exchange heat with the PCM outside the PPs. During the charging process of the CTES unit, the refrigerant evaporates due to heat extraction from the PCM, which solidifies on the PP surface. During the discharging process, the heat transfer direction is reversed so that the refrigerant condenses while the PCM is melting. The CTES unit is flexible by accepting various types of PCMs and refrigerants at multiple temperature levels. The novel CTES unit has been tested experimentally by applying two types of storage media. Water/ice was first used as the PCM to provide the proof of concept, show the feasibility of operating the CTES unit in charging and discharging cycles and provide a benchmark for future PCMs. Then, a low-temperature commercial PCM with a melting temperature of - 9.6 C, suitable for the temperature requirement in food processing plants, was experimentally characterised using established methods before being tested in the CTES unit. Various refrigerant parameters and various PP-HEX geometries were tested. The experimental test campaigns on using water/ice and the commercial PCM have shown that the evaporation and condensation temperatures of the refrigerant are the most critical parameters influencing the performance of the charging and discharging cycles of the CTES unit, respectively. It was found that the charging time was mainly affected by the refrigerant evaporation temperature, while the discharging rate and discharged energy over the cycle increased with higher refrigerant condensing temperature. Furthermore, it was found that the distance between the PPs in the PP-HEX (plate pitch) influences the discharging characteristics of the CTES unit. A smaller plate pitch resulted in high discharge rates at the cost of lower thermal storage capacity. Increasing the plate pitch improved the thermal storage capacity, but the discharging cycle length increased. Hence, the average discharge rate was reduced. In summary, the flexible design of the CTES unit allows the designer to select a discharge characteristic of the CTES unit that matches the refrigeration load curve of the refrigeration plant by changing the plate pitch. The present research work establishes the foundation for further improvement of the concept and considerations for up-scaling and industrial implementation in the future.