**1. Introduction**

Waste heat generation is inevitable during energy utilization in an industrial process and grows evidently alongside global industrialization. Industrial waste heat is categorized into three temperature ranges, namely: low-temperature (<230 ◦C, e.g., paper, textile, food processing industry, etc.), medium-temperature (230–650 ◦C, e.g., ceramic and cement industry, etc.), and high-temperature (>650 ◦C, e.g., steel and metal processing industry, etc.) [1]. Half of input energy is lost in different forms of waste heat into its surroundings [2], wherein the low-temperature range accounted for about 66% of the total waste heat generated [3,4]. Globally, low-temperature waste heat from industrial activities was extensively observed; for example, roughly 34% was generated in Europe, 50% in China, and 60% in the United States [4]. The main application of the print circuit heat exchanger (PCHE) is in a supercritical CO2 (S-CO2) power cycle, which is promising electricity generation using waste heat recovery. This is due to the high efficiency and compact configuration of the PCHE, which reduces the system footprint area [5]. In addition, the heat exchanger can be designed using various fin geometries and working fluids to fulfill the system requirements [6]. Taiwan's rich geothermal resources have been well-developed as hot springs and are an integral part of the tourism industry. Sodium bicarbonate hot spring (pH value: 6.2~8.6; temp. 60–99 ◦C) accounts for 70% of hot springs throughout Taiwan. The heat generated from these hot springs could be recovered and shows great potential for low-temperature waste heat recovery from geothermal heat. Taking advantage of this

**Citation:** Chang, C.-Y.; Chen, W.-H.; Saw, L.H.; Arpia, A.A.; Carrera Uribe, M. Performance Analysis of a Printed Circuit Heat Exchanger with a Novel Mirror-Symmetric Channel Design. *Energies* **2021**, *14*, 4252. https:// doi.org/10.3390/en14144252

Academic Editors: Ron Zevenhoven and Mahmoud Bourouis

Received: 31 May 2021 Accepted: 8 July 2021 Published: 14 July 2021

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

waste heat from low-temperature sources, not only improves energy conversion efficiency, but also reduces harmful emissions. There are many technical applications for waste heat recovery, such as passive waste heat recovery systems for heat pipes [7] and static power generation technology by automotive exhaust with a thermoelectric generation (TEG) system [8]; however, these options are not suitable for the hot springs scenario.

Heat exchangers, such as a linchpin unit, are widely used in waste heat recovery systems, working independently or combined with other systems. Heat exchangers commonly used in the industry still have many issues that need to be addressed. For example, a shell and tube heat exchanger (STHE) and a finned-tube heat exchanger (FTE) require a large space for installation and operation and have a lower distribution of channels within a small space. In addition, for plate heat exchangers (PHE), the plates connected by welding [9] are weaker than the body. In addition, high channel distribution density has contributed to high-pressure drop and high power of the pump is needed. Increasing the area of heat transfer of the heat exchanger flow channel is a good method to improve heat recovery, but it will incur a high-pressure drop, thus, energy consumption during operation is also increased [10,11]. For low-temperature waste heat recovery, the temperature difference between the heat source and the heat collector is too small to recover, thus, low-temperature waste heat recovery is always harder than middle and high temperatures. Hence, there is less low-temperature waste heat recovery technology in industrial applications using a heat exchanger and the technology is immature compared to other types of waste heat recovery technologies.

A diffusion bonding application is viewed to be a suitable technology for the fabrication of a PCHE. It is a bonding method wherein the gap between the two materials is within an atomic level. This makes the device approximately one body without adding materials. Printed circuit heat exchangers (PCHEs) are high-integrity and density-compact heat exchangers [12]. Due to diffusion bonding technology, the strength and properties of the entire PCHE are unified, and the endurance to stress is excellent. The heat exchanger is well compacted and the channels are densely distributed. Particularly, the design using microchannels offers a better heat transfer performance [13]; however, it is accompanied by a large pressure drop. On the other hand, diffusion bonding technology can offer various advantages [14], such as the reduction of unpredictable risks, such as leakage and malfunctions in service. In addition, the lifetime of heat exchangers fabricated through diffusion bonding is longer than others. Most recent studies focused on carbon dioxide and helium as the working supercritical fluid as well as geometry designs of the channel [15–18], or applying PCHE technology as the heat exchanger for a Brayton cycle and using a nanofluid as a working fluid [19,20]. Most of the temperature conditions are medium or high temperatures and only a few examples are in low-temperature regions, as shown in Table 1 [5,21–28].




**Table 1.** *Cont.*

The above literature shows that few studies focused on heat transfer performance and efficiency of PCHEs with water as a working fluid for waste heat recovery at lowtemperatures. In view of this, this work aims to design a novel PCHE for low-temperature application fields, especially for hot springs. In this study, bezels are designed on the flow plates to increase heat exchange efficiency. Each bezel is arranged at an interval and with a mirror design for the cold and hot fluid flow plates. The current arrangement of the flow channel in the flow plates will keep the fluid flow in the special flow direction through a bezel. On the other hand, the mirror-symmetric design of the channel in the fluid flow plates is to improve heat exchange performance. Furthermore, the microchannel design is applied to the runner plates to increase the area of heat transfer and thus improve the overall efficiency of the PCHE. Although increasing the fin density will lead to an increase in pressure drop, this effect can be ignored as the current PCHE is targeted at hot-spring waste heat recovery, which does not require any pump for operation. This study investigated the relationship between the heat transfer and flow rate for the PCHE, with a particular focus on different inlet temperatures, flow rate ratio, and heat exchanger effectiveness. In addition, the performance of using precision machining manufacturing runner microchannels under various flow rates and temperature profiles is also investigated. Lastly, a comparison of the current PCHE with other heat exchangers is made, where an etching manufacturing method is used to fabricate the flow channel. This will enhance our understanding of the PCHE performance fabricated using a low-cost manufacturing method and, in return, contribute to the future development of PCHEs for special applications.

#### **2. Materials and Methods**
