Exergetic analysis of reverse Brayton cryocooler with different turbine arrangements for HTS power cables

Highlights

• Six novel modified configurations of reverse Brayton cycle.

• Comprehensive exergy analysis of a refrigeration system.

• The best modified configurations are PED, PES-2 and SE-1 RBC, respectively.

• The PED and PES-2 modifications have 24.72 and 24.30% exergy efficiency.

• The thermal size of heat exchangers is 76.95% larger for the PES-2 and 2.87% smaller for the PED as compared to basic RBC.

Abstract

The cooling systems of High-Temperature Superconducting (HTS) power transmission cables are unable to achieve the desired performance as prescribed in the cryogenic road map. Reverse Brayton cycle based Cryocooler (RBC) has been identified as a refrigerator that has the potential to achieve the desired performance for higher cooling load demand. In this article, an exergy analysis of the basic Reverse Brayton Cycle (RBC) and its modifications has been investigated for identification of appropriate cycle configuration that will meet the demand for practical applications of HTS cables. The modifications in the basic cycle are carried out with different arrangements of turbines such as parallel or series expansion that may increase the exergy efficiency of the cooling system with a heat load of 10 kW @ 65 K. The analysis is carried out using Aspen HYSYS V8.6, and helium is used as a working fluid. The effects of important process parameters such as pressure ratio, compressor inlet pressure, and flow diversion on the performance are investigated and are used to optimize these cycles. The best cycle configuration based on RBC is identified and recommended based on the results of the analysis.

Introduction

Globally, the recent hike in the prices of raw energy sources has led to an imbalance in the energy requirement and its delivery [1]. The major source of energy losses in the power sector is the transmission system. The losses due to weather impact on the overhead conventional power cable is another major concern than the power losses due to resistance [2]. Therefore, the world is searching for possible alternative solutions to save energy or make a more efficient energy transfer system. The use of High-Temperature Superconductors (HTS) power cable for transmission lines have a significant impact on the possible alternative solution of energy saving regarding the economical and efficient system [3]. Apart from the energy transmission efficiency, the HTS cables have other advantages over the conventional cable [4]. As these cables are undergrounded, the weather and visual impacts are insignificant [5], [6]. However, these cables require a cooling system to maintain their superconducting state.

Many large-scale experiments and test setup are established globally to evaluate the performance of HTS cable and its cooling system under practical conditions [7], [8], [9], [10]. In these projects, the cryogenic cooling system for HTS cables is a vital and significant component. If all the power losses saved by HTS cable is utilized by the cryocooler, then there is no direct benefit of HTS cables for power transmission. Thus, the demand for efficient cryocoolers is increasing to make such a system economically and practically viable.

An open-loop type cryogenic system with decompression unit based on liquid nitrogen was used in many HTS cable performance testing projects at the initial stage [1], [8], [11]. A mechanical cryocooler or refrigeration plant has advantages over the large-scale liquid nitrogen tanks for cooling HTS. Therefore, many cryocoolers such as Stirling coolers [9], Linde Hybrid refrigeration system[10], thermoacoustic [12], and reverse Brayton cryocooler [13] have been considered for cooling HTS power cable. As the HTS cable requires a cooling capacity of 2 to 5 kW per km [10], several small capacity cryocoolers such as Stirling, G-M, and pulse tube, which has a cooling capacity for few watts to 1 kW at 77 K, have been used. However, the use of several small capacity cryocoolers is not viable from the practical and economic point of view [7]. Hence, the reverse Brayton Cycle (RBC) based cryocooler is the most suitable option with many advantages over other cryocoolers [14], [15].

The RBCs were developed for HTS and LNG, and space applications with a cooling capacity of 1–10 kW at 70 K [13], [15] and 4 – 200 mW at 4 – 10 K [16], [17], respectively. Also, the Brayton cycle is studied in the regime of quantum cycles [18], [19]. A recuperative refrigerator can provide a much larger surface area for efficient cooling of liquid nitrogen [20]. For large scale applications such as HTS cooling, LNG reliquefication, Neon RBCs are commercially available [15], [21]. A helium turbo-Brayton cryocooler was developed in China [22], and some RBCs using helium as process fluid are under development for HTS power cable [13], [23]. However, the commercial RBCs for HTS cable cooling using neon as process fluid have thermodynamic efficiency (Carnot efficiency) around 16%. Studies on the effect of component performance and a mixture of helium–neon as working gas on the cryocooler performance using the exergy tool have been carried out [24], [25]. According to the cryogenic roadmap, the required Carnot efficiency is 30% at 65 K temperature[26], [27] to realize a cooling system for HTS cable economically viable. Thus, there is still a notable gap between the existing and required thermodynamic efficiency of RBC.

A neon based RBC has been developed and optimized with 2 kW cooling capacity at 65 K for the HTS power systems by Taiyo Nippon Sanso Corporation [28]. Based on their investigation, it was suggested that the neon based turbo-Brayton refrigerator needs modifications to improve the Carnot efficiency [29]. Different arrangements of turbines in parallel and series for neon RBC have also been investigated [30].

Different configurations of RBC have been analyzed for cryogenic applications using the second law of thermodynamics [31], [32]. It was suggested that the intermediate cooling improves overall thermodynamic efficiency. It was observed that the turbine is the second most vital component in the RBC after the heat exchanger that also impacts the performance of the RBC. The addition of a turbine in the process cycle reduces the irreversibilities in the heat exchangers and turbines [33]. An RBC was thermodynamically designed for liquefaction of natural gas and modified for improvement in the basic cycle [34], [35]. It was found that the thermodynamic performance can be improved significantly by employing two turbines instead of one. Therefore, to further improve the efficiency of the basic RBC, the possible strategy may be modifications of configuration through different arrangement of turbines.

The modification in the process cycle has a significant impact on the reduction of the performance deterioration of components. Hence, there is an improvement in the thermodynamic efficiency of the process cycle. It is also most important to identify the scope for the improvement of the individual components performance which leads to the enhancement of overall cycle efficiency.

In this article, the basic RBC cryocooler is modified with different arrangements of the turbine. These modifications are optimized and analyzed using the second law of thermodynamics at the component level.