**1. Introduction**

In the present time, energy saving methods and techniques in various industries are very important to prevent the global energy crisis, increasing fuel consumption and oil price and decrease the fossil fuel energy use [1]. Especially, the uses of the fossil fuels have caused the ozone layer depletion and the ecological problems including the global warming and air pollution. There are two ways to overcome these international issues. One is the replacement of the old systems with low operation efficiency for the purpose of decreasing the lost energy. Another is the re-use of the waste energy from the present systems for the purpose of improving the efficiency. To reuse a part of energy exhausted to the environment after system operation as a waste heat is a general method [2]. There is lot of research interest in utilizing waste heat to satisfy the energy demand. To utilize the waste heat effectively, the attention needs to be given to the characteristics of waste heat source and the characteristics of used place. The attention aspects are type, chemical components and basic parameters (pressure, temperature and flow), stability and continuity of waste heat source, purpose and size of application, technology and waste heat recovery system

**Citation:** Nguyen, N.-H.; Lee, D.-Y.; Garud, K.S.; Lee, M.-Y. Energy Saving and Economic Evaluations of Exhaust Waste Heat Recovery Hot Water Supply System for Resort. *Symmetry* **2021**, *13*, 624. https://doi.org/ 10.3390/sym13040624

Academic Editor: Mikhail Sheremet

Received: 22 March 2021 Accepted: 7 April 2021 Published: 8 April 2021

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diagram and economic efficiency. The waste heat source could be utilized by the direct and indirect way. If the waste heat source is clean and no-corrosive, then could be used directly otherwise indirectly through the intermediate heat exchanger. Based on the types and the physical properties of the waste heat source, the type of heat exchanger is selected. If the waste heat source is flue gas from ICE or gas turbine then the exhaust gas-to-steam heat exchanger or exhaust gas-to-hot water heat exchanger could be used [3]. If the WH source is liquid then depend on the liquid temperature, the liquid-to-steam heat exchanger or the liquid-to-hot water heat exchanger could be selected.

There are now many services as motels, hotels and resorts which were built for the purpose of travel and lodging. The tourists usually enjoy coming to the quiet places with clean environment to relax. Therefore, many resorts were built in farther places from the urban, near to the coastline or on the islands. Due to the geographic location, many resorts cannot connect with the power grid, so that the ICE for electric generation is usually chosen. Beside the power demands, hot water and steam are the important issues.

Many previous studies have focused on the waste heat recovery from the ICE to enhance the performance. The general technologies and waste heat sources of combined heat and power systems were presented in [4–6] and typical recovery of EGH from ICE were introduced in [7–9]. Separate Rankine cycle could provide additional power of 12– 16% from the waste heat of diesel engine [10,11]. The recovering of waste heat of ICE stored about 10–15% of fuel power in storage system [12,13]. The waste heat recovery from ICE of vehicle was investigated by combined thermodynamic cycles and shown good effective energy savings [14–17]. The cooling capacity of absorption refrigeration system was improved by using the exhaust gas of ICE [18,19]. The exhaust gas recovery using various bottom cycles showed improvement in the engine thermal efficiency [20,21]. The Rankine steam cycle presented the maximum energy saving potentials [22,23]. Generally, depend on the load of ICE and the applied cycle, the effective energy up to 10–20% could be achieved by recovering the waste heat from ICE. Many research show that a huge amount of energy is lost in form of heat through the exhaust gas which is about 30–40% of the combustion heat [12,13,23,24]. The generated exhaust gases are about 50–70% of the fuel input and waste heat from exhaust gas could be recovered through the engine cooling [25]. Majority of the research studies are focused on the additional power for the engine by utilizing the waste heat of ICE [10,11,14,15], some of research studies were applied in cooling [18,19] and some research was focused on the residential applications [4,9,26]. However, there are only few research studies which presented the application of waste heat in residential and small commercial buildings [4,9]. Onovwiona et al. presented the assumed data and simulated results of waste heat recovery system of ICE [26]. Jia et al. have proposed novel gas-engine driven heat pump system to overcome the limitation of insufficient engine exhaust waste heat for hot water supply [27]. Liu et al. have presented the exhaust waste heat recovery system for hot water supply which comprises of solar energy collection system, drainage system and heat pump system [28]. Butrymowicz et al. have conducted experimental study on the waste heat recovery system which uses waste heat of combustion engines for heating applications [29]. Kunal et al. have investigated the utilization of waste heat of ICE for thermoelectric power generation [30,31]. Estefania et al. have developed the energy recovery-based heat pump system which uses low grade temperature source for domestic hot water supply [32]. Ahmadi et al. have proposed solar thermal energy based parallel feed water heating system for power plant units. The net energy and exergy efficiencies of the proposed system are increased by 9.5% by using solar collectors as the replacement of high-pressure feed water heaters [33]. Ahmadi et al. have also simulated the energy and exergy performances of feed water heating repowering for steam power plan under three operating modes [34]. The repowering of steam power plant with and without integration of solar energy has been investigated based on energy, energy and environmental aspects [35].

The conducted literature review reveals that there has been no study reported until now which uses the exhaust gas of ICE for the purpose of the hot water supply as an

application of waste heat recovery. Therefore, this study focuses on the energy savings and economic evaluations of proposed hot water supply system which recovered the EGH from the ICE to produce hot water for resort. The recovered EGH from ICE is used to heat the water though the heat exchanger system. This will decrease the steam for heating water and reduces the oil consumption of the boiler for the existing system. The daily demand and volume flow rate of hot water at peak hour are surveyed. The fuel consumption and exhaust gas of ICE are investigated. The new heat exchanger system to recover EGH to hot water is proposed. The dew point temperature and the back pressure of exhaust gas are considered in design. The investment cost is calculated to evaluate the payback time of the proposed system. The findings clarify the effectiveness of utilizing exhaust gas heat of ICE to produce hot water, and simultaneously provide scientific data for constructing a new heat recovery system. The detailed methodology for designing the waste heat recovery heat exchanger is presented which could be reference guideline for the active researcher in the field of waste heat recovery application which is not cover in relevant articles in the open literature. In addition, the methodology for evaluating the economics of the waste recovery system is presented explicitly.
