A review on technologies with electricity generation potentials using liquified natural gas regasification cold energy

Abstract

In modern times, worldwide requirements to curb greenhouse gas emissions, and increment in energy demand due to the progress of humanity, have become a serious concern. In such scenarios, the effective and efficient utilization of the liquified natural gas (LNG) regasification cold energy (RCE), in the economically and environmentally viable methods, could present a great opportunity in tackling the core issues related to global warming across the world. In this paper, the technologies that are widely used to harness the LNG RCE for electrical power have been reviewed. The systems incorporating, the Rankine cycles, Stirling engines, Kalina cycles, Brayton cycles, Allam cycles, and fuel cells have been considered. Additionally, the economic and environmental studies apart from the thermal studies have also been reviewed. Moreover, the discussion regarding the systems with respect to the regassification pressure of the LNG has also been provided. The aim of this paper is to provide guidelines for the prospective researchers and policy makers in their decision making.

Graphical abstract

Keywords

liquified natural gas / cold energy / power generation

1 Introduction

Realizing the persisting energy consumption rates and environmental concerns, the requirements to find the alternatives have grown manifold. One of the methods is to harness the cold energy (CE) of liquified natural gas (LNG) [1]. LNG is supplied worldwide via special ships at approximately 101.325 kPa and around −163 °C [2]. The LNG at off-loading terminals is pressurized and delivered through pipelines before being vaporized at ambient temperatures. It has been reported that natural gas (NG) liquefaction consumes electricity about 500 kWh/t [3], or approximately from 0.45 to 0.55 kWh per kg of LNG [4]. The thermal analysis of the extraction process has been investigated by Lee et al. [5]. Besides, NG is claimed to have the advantage of being efficient, economically viable, safe, and environment friendly [6]. It has been reported by Pospíšil et al. [7] that the utilization of NG can provide economic benefits in terms of electricity production cost and costs related to the reduction of CO₂ emissions. Moreover, it can be advantageous for the economy in replacing other fossil fuels [8]. In terms of environmental benefits, LNG has been ascribed to generate 10% less particulates [9], and relative to cost, can result in a reduction of 50% for greenhouse gas emissions, in comparison to coal [10]. Furthermore, LNG is regasified using vaporizers that consumes heat from sources such as air, industrial waste, or seawater (SW) [11]. This causes the temperature of the immediate environment to decrease, which can lead to major effects pertaining to aquatic life [12]. Therefore, the exploitation of the energy that LNG regasification offers can be extremely beneficial, as in the year 2020, the estimated global trade of the LNG was reported to be 356 million tons (Mt) [13]. The list of LNG imports for the year 2020 for different regions of the world and their 2050 forecasts are provided in Tab.1 [13].

To address the above issues as well as considering the opportunities LNG regasification cold energy (RCE) presents, several methods have been studied to recover the CE of LNG, particularly, when this CE can be used as heat sinks for various power generation units [14]. Other major uses of this CE utilization include air separation [15], SW desalination [16], preservation of food [17], dry ice production [18], air conditioning [19], peak shaving [20], and CO₂ liquefaction [21]. With such importance, review regarding the various uses of the CE of LNG was conducted by Kanbur et al. [11], and the utilization of the RCE was highlighted to improve the performance of the desired processes.

With the view that electricity production contributed to approximately 36.3 Gt of CO₂ in the year 2021 [22]. From this amount of CO₂ emissions, oil and coal accounted for approximately 10.7 Gt and 15.3 Gt respective shares for the year 2021 [22]. Meanwhile, the contribution of NG was found to be around 7.5 Gt [22]. With the above-mentioned positive attributes of NG, its electricity production rate is projected to rise [24]. The LNG RCE has been used to enhance the performance of thermal systems. Realizing the importance of the utilization of LNG RCE as the heat sink for power generation, Romero Gómez et al. [25] conducted a review of Rankine cycles (RCs), Brayton cycle (BC), Kalina cycle (KC), combined cycles, their variations, and power plants with CO₂ capture from the perspective of thermal performance. Additionally, Xue et al. [26] also conducted a review of thermal cycles from the thermal standpoint. The systems evaluated by them included direct expansion expanders (DEE), RC, KC, BC, and the combined cycles comprising of the other cycles. Moreover, Mehrpooya et al. [27] presented a review of thermal cycles including RCs, BCs, and gas turbine (GT)-fuel cell (FC) units. Furthermore, considering the thermal evaluation of several proposed systems with mechanical devices, Yu et al. [28] also presented a review, which also included the RCs, BCs, and combined cycles.

In the above previously conducted reviews, the major emphasis has been limited to the thermal performance, with environmental and economic indices mentioned as being a part of literature. Keeping this in mind, to the best of the authors’ knowledge, the review particularly encompassing the economic and environmental analysis has not yet been conducted for electricity generation systems utilizing the LNG RCE. Therefore, in the present review, several technologies for power production have been presented and discussions of different technologies from the thermal, economical, and environmental aspects have been provided to aid potential researchers and the policy makers to dedicate their resources in the right direction.

The reviewed technologies, as shown in Fig.1, are DEE, Stirling engines (SE), RC-based technologies including organic RC (ORC), KC and transcritical CO₂ cycle (TRC), system having GT, closed BCs, FCs, and Allam cycle (AC). The compounding arrangement has been selected for this review. That is, the systems in Section 2, the systems with the SE as prime movers shall contain DEE, but Section 1 discussing DEE shall not contain SE. Similarly, Section 3, containing the systems with RCs as prime movers will contain SE and DEE technologies, but it will be absent in the previous sections, and the scheme continues.

2 Overview of performance indices

In this Section, a brief overview of the performance indices adopted throughout the studies ahead is presented. The commonly adopted thermal performance indices were the thermal efficiency (TE), the exergetic efficiency (EE), and the round-trip efficiency (RTE). The economic indices were, the net present value (NPV), the payback period (PP), and the levelized cost of electricity. Moreover, the methodology of the exergoeconomic technique is briefly provided. Furthermore, the environmental indicators are also described.

2.1 Thermal performance indices

In this Section, a brief overview of the recurring performance indices has been presented. Regarding the thermal aspect, two criteria exist, which are the system energy efficiency and exery efficiency. The first one being, the system energetic efficiency assigned by TE in the text, is defined mathematically as [29]

(1)TE=\frac{\mathrm{U}\mathrm{s}\mathrm{e}\mathrm{f}\mathrm{u}\mathrm{l}\mathrm{e}\mathrm{n}\mathrm{e}\mathrm{r}\mathrm{g}\mathrm{y}\mathrm{o}\mathrm{u}\mathrm{t}\mathrm{p}\mathrm{u}\mathrm{t}}{\mathrm{U}\mathrm{s}\mathrm{e}\mathrm{f}\mathrm{u}\mathrm{l}\mathrm{e}\mathrm{n}\mathrm{e}\mathrm{r}\mathrm{g}\mathrm{y}\mathrm{i}\mathrm{n}\mathrm{p}\mathrm{u}\mathrm{t}}\times 100 .

The exergy measures the potential of a system with respect to a certain state [30]. System EE in the present review is defined as [30]

(2)EE=\frac{\mathrm{E}\mathrm{x}\mathrm{e}\mathrm{r}\mathrm{g}\mathrm{y}\mathrm{r}\mathrm{e}\mathrm{c}\mathrm{o}\mathrm{v}\mathrm{e}\mathrm{r}\mathrm{e}\mathrm{d}}{\mathrm{E}\mathrm{x}\mathrm{e}\mathrm{r}\mathrm{g}\mathrm{y}\mathrm{s}\mathrm{u}\mathrm{p}\mathrm{p}\mathrm{l}\mathrm{i}\mathrm{e}\mathrm{d}}\times 100 =1-\frac{\mathrm{E}\mathrm{x}\mathrm{e}\mathrm{r}\mathrm{g}\mathrm{y}\mathrm{d}\mathrm{e}\mathrm{s}\mathrm{t}\mathrm{r}\mathrm{o}\mathrm{y}\mathrm{e}\mathrm{d}}{\mathrm{E}\mathrm{x}\mathrm{e}\mathrm{r}\mathrm{g}\mathrm{y}\mathrm{s}\mathrm{u}\mathrm{p}\mathrm{p}\mathrm{l}\mathrm{i}\mathrm{e}\mathrm{d}}\times 100 .

The EE should be preferred as it depicts the true potential that could be harnessed from external energy streams. To illustrate the utilization of the above-mentioned expression for the EE, for the case when the system supplied cooling, heating, and power, the exergy efficiency is defined as [31]

(3)EE={\displaystyle \frac{{\dot{W}}_{\mathrm{n}\mathrm{e}\mathrm{t}}+\left(1-{\displaystyle \frac{T_0}{T_{\mathrm{h}\mathrm{e}\mathrm{a}\mathrm{t}}}}\right){\dot{Q}}_{\mathrm{h}\mathrm{e}\mathrm{a}\mathrm{t}}+\left(1-{\displaystyle \frac{T_0}{T_{\mathrm{c}\mathrm{o}\mathrm{o}\mathrm{l}}}}\right){\dot{Q}}_{\mathrm{c}\mathrm{o}\mathrm{o}\mathrm{l}}}{E_{\mathrm{N}{\mathrm{G}}_{\mathrm{i}\mathrm{n}\mathrm{t}\mathrm{l}\mathrm{e}\mathrm{t}}}-E_{\mathrm{N}{\mathrm{G}}_{\mathrm{o}\mathrm{u}\mathrm{t}\mathrm{l}\mathrm{e}\mathrm{t}}}+E_{\mathrm{h}\mathrm{e}\mathrm{a}\mathrm{t}\mathrm{s}\mathrm{o}\mathrm{u}\mathrm{r}\mathrm{c}\mathrm{e}}}}.

The modifications can be made in the above-mentioned equations accordingly if the system delivers only heating or cooling utility. Similarly in the case when the hydrogen production is considered from the generated electricity, the EE can be estimated from the following mathematical expression, presented with some modifications from Ref. [32]

(4)EE=\frac{{\dot{m}}_{\mathrm{h}\mathrm{y}\mathrm{d}\mathrm{r}\mathrm{o}\mathrm{g}\mathrm{e}\mathrm{n}}\times e{x}_{\mathrm{h}\mathrm{y}\mathrm{d}\mathrm{r}\mathrm{o}\mathrm{g}\mathrm{e}\mathrm{n}}}{E_{{\mathrm{N}\mathrm{G}}_{\mathrm{i}\mathrm{n}\mathrm{t}\mathrm{l}\mathrm{e}\mathrm{t}}}-E_{{\mathrm{N}\mathrm{G}}_{\mathrm{o}\mathrm{u}\mathrm{t}\mathrm{l}\mathrm{e}\mathrm{t}}}+E_{\mathrm{h}\mathrm{e}\mathrm{a}\mathrm{t}\mathrm{s}\mathrm{o}\mathrm{u}\mathrm{r}\mathrm{c}\mathrm{e}}}.

Another countered thermal performance index is the RTE. It is defined as the amount of electricity retrieved to the amount of electricity stored [33], which can be written as

(5)RTE=\frac{\mathrm{E}\mathrm{n}\mathrm{e}\mathrm{r}\mathrm{g}\mathrm{y}\mathrm{r}\mathrm{e}\mathrm{t}\mathrm{r}\mathrm{i}\mathrm{e}\mathrm{v}\mathrm{e}\mathrm{d}}{\mathrm{E}\mathrm{x}\mathrm{e}\mathrm{r}\mathrm{g}\mathrm{y}\mathrm{s}\mathrm{t}\mathrm{o}\mathrm{r}\mathrm{e}\mathrm{d}}\times 100 .

2.2 Economic performance indices

Regarding economic evaluation, the encountered performance indices were the internal rate of return (IRR), net payback period (NPV), payback period (PP), levelized cost of electricity (LCOE), and the exergoeconomic analysis. The NPV is attributed to as an indicator which represents whether a project is economically viable or not [34]. For the project to be viable, it must be positive. It is considered a dynamic indicator as it accounts for the interest rate which is equivalent to the inflation rate, mathematically, it is defined in Ref. [34], and the unit of which is considered to be USD in the present review.

(6)NPV={\sum }_{i=1}^N\frac{c{f}_i}{{(1+r)}^i}-I,

where I is the original investment, r is the interest rate, N is the number of years, and cf_i is the annual cash flow. The cf_i is defined as [34]

(7)c{f}_i={\sum }_{t=1}^Y{(er-ex)}_t,

and

(8)i=1,2 ,\dots N,

where er is earnings, ex is the expenses, and Y is the number of time steps during a year.

Regarding the fact that the IRR is the value of the interest rate obtained by setting the value of NPV equal to zero, and the corresponding number of years is termed as the dynamic PP [35], the regular or simple PP is defined as the ratio of initial investment to average cash flow [34].

(9)PP=\frac{I}{\overline{cf}},

where I is the initial investment and \overline{cf} is the average cash flow.

Another economic indicator LCOE was also used in few instances. It represents the average income per unit of electricity to recover the investment cost [36]. The LCOE is calculated from Eq. (10) in Ref. [37], whose unit is USD/MWh in the present review.

(10)

where Z_invest is the total initial investment, Z_O&M is the yearly operations and maintenance cost, and Hr represents the yearly hours of operation. The CRF is known as the capital recovery factor which is defined as [37]

(11)CRF=\frac{r\times {(1+r\text{)}}^N}{{(1+r)}^N-1}.

Another method which combines the exergy and the economics, termed as the exergoeconomic analysis, was also mentioned in some studies. It states that the cost of all inlet streams and the cost associated with the components need to be equal to the outlet streams. The exergoeconomic equation for the jth component is written as [38]

(12)\sum {\dot{C}}_{\text{o},j}+{\dot{C}}_{\text{W},j}={\dot{C}}_{\text{Q},j}+\sum {\dot{C}}_{\text{i},j}+{\dot{Z}}_j,

where, {\dot{C}}_{\mathrm{o},j}, {\dot{C}}_{\mathrm{W},j}, {\dot{C}}_{\mathrm{Q},j}, {\dot{C}}_{\mathrm{i},j}, and {\dot{Z}}_j, are the cost rates of the outlet streams, work, heat, inlet steams, and total cost rates of the jth component. Mathematically, they are given as

(13){\dot{C}}_{\text{o},j}=c_{\text{o},j}\times {\dot{E}}_{\text{o},j},

(14){\dot{C}}_{i,j}=c_{i,j}\times {\dot{E}}_{i,j},

(15){\dot{C}}_{\text{W},j}=c_{\text{W},j}\times {\dot{W}}_j,

(16){\dot{C}}_{\mathrm{Q},j}=c_{\mathrm{Q},j}\times {\dot{E}}_{\text{q},j}.

The term, \dot{E}=\dot{m}\times e, represents the exergetic potential of the stream, c is the specific cost rate associated with the respective entity, and {\dot{Z}}_j is given as

(17)

The terms {\dot{Z}}_{j,\text{cl}} and \dot{Z}_j,O&M are the capital cost and operational and maintenance costs of the jth component. The {\dot{Z}}_j mentioned in Eq. (17) can be calculated using

(18){\dot{Z}}_j=CRF\times \frac{{\varnothing }_r}{3600\times n}\times Z_j,

where CRF is calculated from Eq. (11), {\varnothing }_r and n are the maintenance factor and the number of operating hours respectively, and Z_j is the cost of the equipment.

Following the process mentioned above, by using Eqs. (13)–(18) for all components, the unknown variables can be computed by solving Eq. (12) for all components simultaneously. Afterwards, the exergoeconomic entities, i.e., the mean unit cost per exergy for product {C_{\text{P},}}_j, the mean unit cost per exergy for fuel C_{\text{F,}j}, the exergoeconomic factor f_j, and the cost of exergy destruction rate {\dot{C}}_{\text{D},j} for each stream can be calculated using Eqs. (19)–(22) [39].

(19)C_{\text{P},j}=\frac{{\dot{C}}_{\text{P},j}}{E^{\prime }{x}_{\text{P},j}},

(20)C_{\text{F},j}=\frac{{\dot{C}}_{\text{F},j}}{E^{\prime }{x}_{\text{F},j}},

(21)f_j=\frac{{\dot{Z}}_j}{{\dot{Z}}_j+{\dot{C}}_{\text{D},j}},

(22){C_{\text{F},}}_j=\frac{{\dot{C}}_{{}_{\text{F},j}}}{E^{\prime }{x}_{\text{F},j}}.

2.3 Environmental performance indices

To evaluate the emission reduction (ER), Eq. (23) is used [40].

(23)ER=\left(\frac{{\dot{W}}_{\mathrm{n}\mathrm{e}\mathrm{t}}\times f_{\mathrm{n}\mathrm{e}\mathrm{t}}}{{\eta }_{\mathrm{c}\mathrm{o}\mathrm{n}\mathrm{v}}}+\frac{{\dot{Q}}_{\mathrm{h}\mathrm{e}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{n}\mathrm{g}}\times f_{\mathrm{N}\mathrm{G}}}{{\eta }_{\mathrm{h}\mathrm{e}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{n}\mathrm{g}}}+\frac{{\dot{Q}}_{\mathrm{c}\mathrm{o}\mathrm{o}\mathrm{l}}\times f_{\mathrm{n}\mathrm{e}\mathrm{t}}}{CO{P}_{\mathrm{c}\mathrm{o}\mathrm{o}\mathrm{l}}}\right)\times {\tau }_{\mathrm{a}\mathrm{n}\mathrm{n}\mathrm{u}\mathrm{a}\mathrm{l}},

where {\eta }_{\mathrm{c}\mathrm{o}\mathrm{n}\mathrm{v}}, {\eta }_{\mathrm{h}\mathrm{e}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{n}\mathrm{g}}, CO{P}_{\text{cool}}, and {\tau }_{\text{annual}} are the efficiency of conventional power plant, efficiency of heat pumps, coefficient of performance of cooling devices and yearly operating devices. f_{\text{net}} and f_{\text{NG}} are the emission factors of electricity and NG, having their corresponding values of 0.5246 and 0.1836 kg CO₂/kWh, respectively.

3 Systems with only direct expansion expanders (DEE)

The LNG is pumped to a higher pressure, before being distributed. The distribution pressure with respect to the application is presented in Tab.2 [41].

To harness the energetic and exergetic content before supplying NG at regular conditions, an expander or a set of expanders are directly attached at the exit to recover the pressure energy and exergy of the NG. In this paper, such a process is referred to as the DEE/expansion.

In this regard, Lee et al. [42] performed thermal evaluation of a system that incorporated the utilization of the LNG RCE for liquid air energy storage (LAES), as shown in Fig.2. The LAES is a method of storing electrical energy as liquid air/nitrogen at cryogenic levels [43], whose details can be sought from the review conducted by O’Callaghan & Donnellan [44], and Qi et al. [45]. The two designs, one with a single stage LNG pump, as shown in Fig.2 and the other with an additional LNG pump after the heat exchanger 1 (HX1), were studied by them. The stored liquid air in times of need could be directly used to generate power by allowing it to expand through the separate set of expanders.

Furthermore, Peng et al. [46] performed a thermal evaluation of the system that used the LNG RCE for aiding the LAES process via using the RCE for pressurized propane. The cold stored propane was used for enhancing the LAES process. Additionally, Barsali et al. [47] proposed a system that incorporated the LNG RCE and LAES with oxygen only. The mixture of the oxygen and the LNG was burnt as fuel. The excess electricity was used to produce liquified oxygen, which was burnt with LNG in case of higher demand and passed through the DEEs for electricity production. The CE was used to liquefy the CO₂ from the combustion products.

Apart from it, Park et al. [48] conducted a thermal and economic evaluation of the scheme involving LAES. During the off-peak time, the LNG RCE was used with intermediate CE storing liquid propane to liquefy the air. During the on-peak time, the liquified air was expanded through turbines for power generation. The overall summary of results for such systems are presented in Tab.3.

From Tab.3, it can be inferred that the exegetic efficiencies can be as high as 68.12%. The RTE of the study in Ref. [42] is high because it stores energy in the form of CE and electrical power. Such a system holds the potential to have an NPV as high as 143 million USD.

4 Systems with SE as prime movers

In this Section, an overview of the SE using the LNG RCE is provided. The mechanisms and the general configurations of the SEs can be sought from the studies conducted by Kongtragool & Wongwises [49], Sing & Kumar [50], Ahmed et al. [51], and Malik et al. [52]. The general working principle of the SE is given in Fig.3. During the process from state 1 to 2 (Fig.3(a)), the isothermal compression occurs. In the subsequent process, i.e., the process from state 2 to state 3, the isobaric heat addition takes place (Fig.3(b)). Afterwards, during the process from state 3 to state 4, the isothermal expansion happens, and the work output is obtained (Fig.3(c)). Lastly, the heat rejection at a constant pressure occurs from state 4 to state 1 (Fig.3(d)).

The thermal analysis of utilization of SE for power production using SW as HS was performed by Dong et al. [53]. Nitrogen gas was used as the working fluid (WF) of SE. Moreover, Szczygieł et al. [54] performed a theoretical exergetic evaluation of a simple piston type SE. Furthermore, the thermal investigation of utilizing SEs with the LNG regasification process including LAES was performed by Ansarinasab et al. [55]. Their study also presented the liquefaction of the NG using magnetic refrigeration. Furthermore, the effects regarding the mesh resolution and turbulence models for the α-type SE using the ANSYS platform were studied by Buliński et al. [56] with the ambient air as HS and LNG as the cold source. The influence of inclusion of the regenerator on SE was also investigated. It was reported that with the regenerator, and at lower temperatures, the increments in thermal efficiency almost remained unchanged with increasing the rotational speed, albeit the specific work increased.

A particular configuration of SE, known as the thermoacoustic SE, was investigated as the electrical power generation unit operating between a particular HS and LNG RCE. The general overview of the thermoacoustic SEs can be sought from the review conducted by Jin et al. [57].

In a study performed by Hou et al. [58], thermal evaluation of the system comprising of four thermoacoustic SEs was conducted. The impacts of gas temperature, energy, pressure, velocity, and phase angles distributions along the length were studied and displayed. In addition, the effects of the load resistance and capacitance, and regenerator dimensions were also reported. Moreover, Xu et al. [40] performed the thermal, economic, and environmental evaluation of a thermoacoustic SE combined cooling, heating and power (CCHP) system. The parametric analysis was performed considering the LNG and HS temperatures, external electrical impedances, and cooling and heating temperatures. The schematic of their proposed system is shown in Fig.4.

The results reviewed in this Section are presented in Tab.4. Displaying fewer moving parts and no phase change issues, and reaching exergetic efficiencies at around 71%, as claimed by Ansarinasab et al. [55], such systems can provide beneficial opportunities for utilizing LNG CE for power production. Furthermore, the SE systems can aide in curbing approximately 30.6 t/a of CO₂ emissions and save around 10.55 kUSD annually.

5 Systems with RC as prime movers

In this Section, the systems with Rankin cyle (RC) or its derivates are presented. The derivatives of the RC observed by the authors were found to be the ORC and the KC. The ORC is a technology that employs organic compounds as WF, which due to their intrinsic lower normal boiling point, enables the evaporation at lower temperatures [60,61]. The general working mechanism of the most basic ORC and the representative schematic of the ORC are shown in Fig.5. The WF is heated at a constant pressure from state 1 to state 2, by consuming heat from a given HS. Afterwards, the WF expands through the expander from state 2 to state 3, thereby turning the generator for electricity production. From there, the heat is rejected to any given heat sink by the WF from state 3 to state 4 at a constant pressure. Later, the WF is pumped to the boiler pressure from state 4 to state 1, thus forming a cycle. One of the important aspects of the ORC is its WF. It determines the sizing of the equipment and the technical performance [62]. Regarding the overview of the ORCs, the information can be sought from the reviews from Rahbar et al. [63], Tchanche et al. [64, 65], Bahrami et al. [66], Qyyum et al. [67], and Lecompte et al. [68].

The KC, on the other hand, uses NH₃-water as the WF. The working principle of a basic KC is shown in Fig.6. The heat is absorbed in the vapor generator from state 1 to state 2. Afterwards, the working mixture enters the separator, where the vapor gets separated from the lean liquid. The enriched vapor enters the turbine at state 3, where it expands, while delivering the mechanical energy to the generator for electricity production. Meanwhile, the lean liquid from the separator at state 12 enters the high temperature recuperator to deliver the heat to the working mixture that enters the vapor generator at state 1. The lean liquid, after passing through the recuperator enters the pressure reduction valve at state 11. The lean liquid exiting the pressure reduction valve at state 5 mixes with the rich fluid coming from the turbine at state 4, and this mixture exits the mixture at state 6. The mixture at state 6 delivers heat to the condensed working mixture in a low temperature recuperator, as it exits this recuperator at state 7. In doing so, the condensed working mixture gains heat and exits the low temperature recuperator at state 10. The working mixture at state 7, is condensed in the condenser by exchanging heat with the heat sink. This condensed working mixture exits the condenser at state 8, where its pressure is raised to the vapor generator pressure using a pump. In the low temperature recuperator, this enables a good thermal matching between the HS and the WF, and the information regarding the KCs can be retrieved from the review by Zhang et al. [69].

5.1 Systems with only single organic rankine cycle (ORC)

Considering the thermal evaluation of a ORC based system, Rao et al. [70] performed a thermal evaluation of various WFs for a solar flat plate collector powered regenerative ORC and DEE in a LNG RCE based system. Moreover, Dorosz et al. [71] evaluated the exergetic performance of installing the ORC with and without DEE. The double stage expansion in combination with ORC was found to be the best option. Apart from it, Sung & Kim [72] performed a thermal investigation of a system using two ORCs, one using the waste heat (WH) of the marine diesel engine and the other using jacket cooling water (JCW). For the ORC that used JCW as HS, the LNG RCE was used as the cooling source before it was used as fuel for the internal combustion engine. Additionally, Baldasso et al. [73] performed a thermal evaluation of using LNG RCE, either for the HVAC applications, or for condensing the WF of the ORC unit, or to pre-cool the air entering the engine. Two scenarios, i.e., a low fuel supply ferry ship, and a high fuel supply container ship were considered. Furthermore, Yu et al. [74] performed a screening of 22 potential WFs. The ambient temperature and flue gases (FGs) were considered as the HSs. The optimal LNG regasification pressures for different WFs were also determined by them.

The exergetic analysis of integrating a special ORC condenser inside the LNG regasification system that also had the DEE was performed by Sun et al. [75]. A system having an ORC, a CO₂ capture cycle, and a waste extraction system was analyzed by Tan et al. [76]. Moreover, Lim & Choi [77] performed a thermal evaluation, and sizing of shell and tube type HXs for an ORC which used the engine JCW as the HS and LNG RCE as the heat sink. Additionally, Choi et al. [78] performed a thermal analysis of an ORC that was powered by SW. They reported that priority should be given to evaporator followed by condenser, and then the trim heater in terms of size.

Apart from it, Koo et al. [79] evaluated six different configurations that were fueled by the WH from dual-fueled engines, of which, three were high pressured (about 30000 kPa) and three were medium pressured (about 1700 kPa). From their thermal and economic analysis, the ORC system depicted in Fig.7 was reported to be the best configuration.

The thermal and economic investigation of hydrogen gas production using the PEM electrolyzer powered by propane-based ORC was conducted by Musharavati et al. [80]. Six separate locations were considered for the thermoelectric generators (TEG) placement. From the evaluated schemes, the best performing schematic is presented in Fig.8. The TEG devices convert heat into electricity by a phenomenon known as the Seeback effect. The details about these devices can be sought from the review of Jaziri et al. [81]. Moreover, Mehdikhani et al. [82] performed a thermal and exergoeconomic analysis of the system that contained a geothermal flash cycle and an ORC. The electricity from the ORC was used for hydrogen production by a PEM electrolyzer.

The summary of the studies reviewed in this Section is presented in Tab.5. From Tab.5, it can be deduced that such a system can present exergetic efficiencies of around 50.4% [76]. Furthermore, the propane can be deduced as the most proper WF, in the case when the temperature of the HS was from ambient conditions [78,79], as well as for the case when the temperature was in the range from 100 to 150 °C, as can be deduced from Refs. [74,75]. However, for a temperature greater than 200 °C, R600a was shown to be the best candidate, as can be seen from Ref. [75]. Additionally, the application of the TEG was also found to enhance the thermal performance by up to 10.2%, as depicted by Musharavati et al. [80], with an added PP of 1.29 years.

Besides, the studies considering the mixtures were also performed. Since they offer non-isothermal temperature glide, they are claimed to have a better EE [83,84]. Considering mixtures as WF, Yu et al. [85] performed the thermal comparison of pure WFs with mixed WFs. A NG DEE was also an integral part of the system. From their evaluation, mixed WFs does not necessarily improve the performance of such a system for trans-critical and near critical operations and offers a slight improvement in case of a subcritical operation. Similarly, He et al. [86] reported that pure WFs were better than binary mixtures. Moreover, their study indicated the LNG regasification pressure is of significance. In addition, Zhang et al. [87] performed an advanced exergetic analysis of a system that stored transcritical CO₂ using R290 ORC. The components showing the theoretical maximum improvement in the exergetic performances were identified and reported.

Moreover, considering the economic aspects, Dutta et al. [88] compared the performance of incorporating a DEE in the ORC LNG RCE utilization system. Additionally, Park et al. [89] performed a comparative economic comparison of a simple LNG RCE system, a system containing two series recuperative ORCs utilizing LNG RCE and DEEs, and a system containing two series recuperative series ORCs with DEE and cryogenic energy storage.

Furthermore, He et al. [90] investigated the prospects of utilizing the heat from the Clathrate hydrate-based desalination for an ORC from thermal and economic viewpoints. Clathrate hydrate-based desalination is an energy intensive desalination method, whose further information can be sought from the review conducted by Babu et al. [91].

Besides, Lee & You [92] conducted a thermal and economic evaluation of a system shown in Fig.9, that used the direct as well as the indirect methods of power generations using the LNG regasification process. The WF composition was ethane/propane/isobutane/n-butane (0.4/0.4/0.1/0.1 by mole ratio). The impact of the NPV against on-peak price was discussed, and they recommended the WFs having a normal boiling point between −70 and −50 °C. A study, with addition of thermal energy storage (TES) in parallel with LAES, and with the addition of an ORC was performed by Park et al. [93]. Their proposed process stored CE using liquid propane during on-peak times. Meanwhile, during off-peak periods, both the CE from the LNG and the liquified propane can be used to enhance the process flexibility.

The summary of the results reviewed in this Section is listed in Tab.6. With TES-LAES, EE can be increased up to approximately 75.1%, as can be seen from Ref. [93]. Considering NPV as the measure of economic performance, for inserting the ORC in the regasifying process, it could be, considering minimum value, at around 2.45 million USD, and it could be increased up to 225.89 million USD. Regarding the WF selection, irrespective of the HS temperatures, the WFs with propane as the component of the mixtures were recommended in Refs. [85,90]. Apart from it, ethane can also be considered as a viable WF to be included as a component of mixture, as can be inferred from Refs. [88,90].

Another configuration of ORC that operated with multiple condensers and multiple expanders having the same WF, split after and before the evaporator, also remained among the research focus pertaining to the LNG RCE utilization. For such a system, shown in Fig.10, the thermal evaluation of several WFs was conducted by Bao et al. [94].

Additionally, Ouyang et al. [95] proposed a system that comprised of five subsystems, viz, gasoline vapor recovery, cascade Rankine power generation which consisted two different pressure level condensers and expanders, a rectisol wash which removes acidic impurities from the NG, an air conditioning system, and LAES. Apart from it, three configurations were evaluated by Badami et al. [96]. The configurations are the simple ORC with a single DEE, a simple ORC with a two stage DEE, shown in Fig.10.

Moreover, Bao et al. [97] proposed a hydrogen production system based on the dual pressure ORC and produced hydrogen via a PEM electrolyzer. The WF of the ORC was a mixture of ethane and propane with 94.85% and 5.15% by mole fraction, respectively. Furthermore, Yuan et al. [98] performed a multi-objective thermal and economic comparison between the two-stage condenser ORC and the three-stage condensation ORC for various LNG gasification pressures.

The summary of studies reviewed in this Section is presented in Tab.7. From the summary, the hydrocarbons are concluded to be the most feasible options. Moreover, the additional condensers were appealing from a thermal standpoint but were unfeasible from an economics aspect. An interesting finding was observed that for Bao et al. [97] the best results correlated with the LNG regasification pressure studied by them. Meanwhile, for Yuan et al. [98] the ideal results were depicted by the highest LNG regasification pressure studied by them. In this category, the best WF was found to be ethane and C₂H₄F₂ from the studies conducted by Bao et al. [94] and Badami et al. [96].

5.2 Systems with multiple organic Rankine cycle (ORC)

Regarding the organic Rankine cycles (ORCs) arranged in series sharing similar HS, Ma et al. [99] conducted a thermal evaluation of various refrigerants for ORCs. The schematic of their system is shown in Fig.11. The study revealed the utilization of the multi-stage ORCs was better than that of the single stage ORC. Other findings included no significant relation between the LNG regasification pressure with the power production. Moreover, the SW temperature had a major effect on the net power generation. Additionally, it was reported that the greater the number of stages, the more the power production but with an increase in capital cost. Lastly, for the subcritical pressures of the LNG, the bulk of the power generation is held by the first two stages in the series of the ORCs, but for the supercritical pressure of the LNG, it was negligible between subsequent ORCs.

In addition, Li et al. [100] performed a thermal evaluation of a system with two ORCs in series, with a DEE between the condenser and the reheater as part of the system. The ORCs harnessed the heat from the solar energy resources. Moreover, Moghimi & Khosravian [101] optimized the EE of a system that used SW as the HS for the two ORCs and the SE. Meanwhile, the CE from the LNG regasification was used in series, entering the first ORC, then the SE, and finally, the last ORC. Furthermore, Wang et al. [102] investigated the thermal and environmental performance of a system shown in Fig.12. The cascading of the condensers was done in their studies. They reported the LNG pressure increment caused the EE and the LNG CE utilization to increase. But it caused the power generation and CO₂ capture to decrease. Furthermore, the mixed WF and regenerator addition was shown to enhance the performance of the system.

Moreover, Qi et al. [103] proposed a system integrating the LAES with ORCs with DEE for enhanced flexibility. Their study revealed that the electrical round-trip efficiency could be increased by reducing the air charging pressure and by increasing the liquid air pressure. Zheng et al. [104] conducted a thermo-economic investigation of series ORCs configuration with the DEE located between the condenser and the precooler of the final ORC.

The summary of the studies considering such configurations is shown in Tab.8. From Tab.8, the maximum exergetic recovery rate was seen to be 67.70%, as displayed by the study conducted by Ma et al. [99], who also reported a 35.47% increase in exergy recovery rate, when compared to the single stage ORC. Additionally, for such configurations, the CO₂ emissions can be reduced by 0.29 t/t LNG as can be seen from the studies performed by Wang et al. [102]. Furthermore, the NPV of such systems are found to be around 45.10 million USD with integration of LAES as presented by Qi et al. [103]. For this type of configuration, considering SW as HS, propane as the first one in contact with the LNG, followed by R32, and then R600 was deemed as the best combination by Zheng et al. [104].

Considering the case where different HSs were considered for the ORCs, Atienza-Márquez et al. [105] conducted a thermal and the environmental evaluation of a system, shown in Fig.13. Several WF candidates were evaluated with an aim to supply district cooling as well.

Zhang et al. [106] evaluated the prospects of utilizing such configuration in a conventional LAES system. They found that the system displayed a positive outcome when compared with the conventional LAES system. The discharging liquid air was used as the cold source. Other findings included that the EE as well round-trip efficiency was almost independent of the liquefaction pressure of the air.

Ouyang et al. [107] studied a system where the LNG RCE was used first for the gasoline vapor condensation and then to condense the WFs for the ORCs. The heat energy for the first ORC was partially provided by the blood freezing process, and for the second ORC, via a cold warehouse. Binary mixtures were considered for the ORCs and several potential WFs were considered.

Tian et al. [108] performed a 3E (energy, exergy and economic) analysis of a system that used two ORCs. One ORC used the WH from the JCW while the other used the WH from the FGs of the engine. Their study revealed that the utilization of the mixtures was not necessarily an ideal option for LNG cold recovery.

The studies reviewed in this Section are summarized in Tab.9. From Tab.9, it can be concluded that the hydrocarbons and their mixtures displayed the best performances. Meanwhile the economic feasibility can be deduced to be in between four to six years. Furthermore, the CO₂ emissions with such a system can hold the potential to save approximately 75 kt annually. For this case, flouro-hydrocarbons and CO₂ combinations were recommended by Atienza-Marquez et al. [107]. However, the combination in series with methane, CO₂, and propane was deemed appropriate by Atienza-Márquez et al. [105]. On the other hand, R1150-R600a was reported to deliver a maximum workout, and the mixture R170/R1270 for the maximum economic benefit was reported by Tian et al. [108].

Another research arena was related to an ORC bottoming another ORC, termed in this review as the compounded configuration. Considering such an arrangement, Yu et al. [109] proposed a system utilizing both the sensible and the latent parts of the LNG regasification process and performed the thermal analysis on it. In their system, a topping ORC was powered by the FGs and had a double stage expander, and a bottoming cycle ORC that used the sensible heat from the CE provided by the LNG. In addition, Tomków & Cholewiński [110] proposed a cycle that used ethane-krypton mixture as the WF, and a bottoming cycle having a separator to separate and redirect the krypton lean and krypton rich mixtures.

The energetic, exergetic, and exergoeconomic multi objective optimization of a system containing a GW powered regenerative ORC bottoming another ORC was carried out by Mahmoudan et al. [111]. The hot water, cooling, and purified water using the RO unit were integrated to their system. Furthermore, Emadi & Mahmoudimehr [112] performed a thermo-economic analysis of a system that used GW as the HS. The residual heat from the topping ORC was used to power a LiBr-water absorption cycle and a PEM electrolyzer. Moreover, Ansarinasab & Hajabdollahi [113] conducted a multi-objective thermo-economic optimization of a system that used GW as the HS shown in Fig.14. In addition, Mehrenjani et al. [32] conducted a thermal and economic evaluation of a system that used the power generated by the ORCs to produce hydrogen. The added provisions supplied by the system were the cooling, and oxygen. The liquefaction of the hydrogen by the Claude cycle was also a part of their proposed system. The information regarding the Claude cycle can be sought from the review by Ghafri et al. [114].

The summaries regarding the configurations involving ORCs bottoming ORCs are shown in Tab.10. From Tab.10, the maximum observed EE was found to be around 52.65%. The economic evaluation on the systems with such a configuration was found to display a total cost rate of 423.5 USD/h and a PC rate of 4.35 USD/GJ in accordance with the findings from the studies conducted by Emadi & Mahmoudimehr [112] and Ansarinasab & Hajabdollahi [113], respectively.

With respect to multiple ORCs in combined series and bottoming configurations, Joy & Chowdhury [115] performed a thermal evaluation of different pressures of LNG and the temperature approaches of a triple cascade cycles in series. Their evaluations revealed that with proper temperature approaches of HXs, added power can be extracted with no change in the surface area of the HXs. Moreover, they reported that the last stage only contributes toward the 10% while the first two stages were the same from performance perspective. Additionally, they reported that by raising the pressure of LNG, the added power can be harnessed using a DEE.

Furthermore, the thermal and economic investigation of a system that comprised of an ORC bottoming ORC with another ORC in series configuration, driven by the heat from the FGs was performed by Zhang et al. [116]. Moreover, Eghtesad et al. [38], performed a thermo-economic evaluation of an array of ORCs operating between SW and LNG RCE. The heat rejected from the topping ORC and SW was used as HS for the bottoming ORC. The LNG going out of that bottoming ORC was used as the cold source for another ORC connected ahead, whose HS was SW as well. The use of the residual NG at −47.48 °C was analyzed for as cooling source for the ORC connected in series or for ice production. From their evaluation, using the residual CE from the NG for ice production was reported to be superior. Other than this, Han et al. [117] performed a thermal and economic evaluation of a system that comprised of an ORC bottoming ORC configuration consuming the heat energy from the exhaust gases of a marine engine and LNG RCE. The residual LNG RCE was used as the cold sink for another ORC attached ahead and the direct expansion was also used in the system.

Moreover, Tian et al. [118,119] performed a thermo-economic multi-objective optimization of a system that used the WH from the FGs and JCW as the HS of an ORC bottoming ORC with LNG downstream ORCs. In their studies, the WH sources were interchanged. The evaporator and the condensation saturation temperatures were optimized. Furthermore, He et al. [120] proposed a system presented in Fig.15.

The summaries of systems with combined series and parallel ORCs are provided in Tab.11. From Tab.11, it can be inferred that for the case of using the separate cycles with different cooling sources and HS, the EE can reach around 73.92% with a reported RTE of 141.88% [120].

Considering comparative studies, Sun et al. [121] compared the simple ORC, two ORCs connected in series, and an ORC bottoming another ORC. In another study, Sun et al. [122] compared the thermal performance of five configurations, namely, the regenerative ORC, the reheat ORC, the regenerative with reheat ORC, two ORCs in series using the same heat and cold source, and one ORC bottoming another. Their study indicated that modified ORCs were better than double stage ORCs in some instances. The DEE was also part of the ORC configuration described.

The thermal and life cycle cost analysis of only DEE, simple ORC, and a double and triple cascade condenser ORCs was performed by Choi et al. [123]. Furthermore, the thermal and economic evaluation of a simple ORC, an ORC with internal heat exchanger, a regenerative ORC, and an ORC bottoming configuration was performed by Mosaffa et al. [124]. Moreover, Bao et al. [125] analyzed the thermal and economic performance of eight different systems comprising of single ORC, or multiple condensers-expanders in series, or in ORC bottoming ORCs condenser configurations. From their analyses, different configuration depending on the LNG pressure was accordingly considered feasible. The ideal configurations analyzed by them are shown in Fig.16. Apart from it, Sun et al. [126] performed a 3E comparison of three various configurations. The evaporation, condensation, and the expander inlet temperature were optimized to determine both ORCs and the LNG regasification pressure for the maximum exergy. From their results, their multi condenser-expander ORC was found to be the best configuration displaying the highest EE.

The summary of the studies reviewed in this Section is provided in Tab.12. From Tab.12, it can be deduced that such a system holds the potential to have an EE of approximately 65.2%, as deduced by Ref. [123]. Furthermore, the minimum electricity production cost, and PPs for such configuration were reported to be at around 4.8 USD/GJ (USD/kWh) and around 7 years, as reported by Bao et al. [125] and Sun et al. [126] respectively.

For this type of configuration, the WF recommended by Sun et al. [121] was NH₃/ethane for all temperatures of HSs and their corresponding best considered configurations whereas ethane was the most feasible WF candidate for the configuration shown by Sun et al. [122]. Meanwhile, C₃H₈ in series was the best WF in accordance with the study performed by Choi et al. [123].

Apart from it, the WF presented by Bao et al. [125] were found to be R32 (top cycle)/R1150 (bottom cycle) for the regasification pressure of 0.6 and 2.5 MPa. Meanwhile, for the regasification pressures of 3 and 7 MPa, the WFs R41 (top cycle)/R143a (bottom cycle) was reported to be the most appropriate combination.

A system based on the solar energy input, using the CE from the LNG regasification and consisting of an SRC and an ORC was proposed by Ghorbani et al. [127]. The analysis was conducted considering the weather conditions of Tehran, Iran. The Fischer−Tropsch synthesis, used for the manufacturing of liquid fuels from the carbon monoxide and water [128], was also a part of their proposed system. The heat from the Fischer−Tropsch process combined with the solar energy was the HS for the system. The general structure of their system is shown in Fig.17. The impact of the energy output against the solar fraction, and period of return and net annual benefit against the liquid fuel price and electrical energy price were evaluated. The energetic and the exergetic efficiencies of the system were 42.36% and 64.72% respectively. Meanwhile, the period of return was found to be 2.168 years.

5.3 Systems with ammonia based Rankine and Kalina cycle (KC)

In this Section, the KC or RC derivatives that used ammonia or ammonia-water as the WF are presented. In this regard, Kim & Kim [129] and Kim et al. [130] performed a thermal evaluation of an ammonia-water ORC cooled by LNG RCE with DEE. The impact of ammonia-water mass fraction was studied. Additionally, Sadaghiani et al. [131] performed an energetic and exergetic analysis of a geothermal powered KC and ORCs based system as shown in Fig.18.

Furthermore, Ghorbani et al. [132] performed a thermal analysis of a system that produced power, refrigeration, liquid methanol, and liquid fuels. In another instance, Emadi et al. [133] performed a thermal evaluation of a geothermal powered KC system bottomed by an SE system. The cooling media for the SE was regasified LNG, and the regasification circuit also had a DEE and a cooling unit. Some part of the electricity was used to run a PEME for hydrogen production. Likewise, Ning et al. [134] proposed a system in which three R717 RC were arranged in series. The first one in the series used the HS from a freezing room and the others in the series used the heating in its evaporator from a cooling room. Additionally, Li et al. [135] proposed a system that used the steam to power a combined ammonia-water absorption combined cooling and power (CCP) system.

A thermal and economic evaluation of a GW powered system having a modified KC bottomed by an ORC was performed by Li et al. [136]. The electricity produced by the KC was used to power an electrolyzer, and the electricity produced by the ORC was used to power the compressor of the CO₂ compression refrigeration system (CCRS). A lithium-bromide absorption chiller taking and rejecting heat to the CCRS was also an integral part of their proposed system. Moreover, a 3E evaluation of an ammonia-water CCP system was conducted by Ghaebi et al. [137]. Additionally, Ghaebi et al. [138] also performed a 3E evaluation of a GW powered ammonia-water CCHP system. Furthermore, in a study conducted by Fang et al. [139], exergoeconomic optimization of a system having an exhaust gas powered KC bottomed by three ORCs was performed. The utilities provided by their systems were the CHP. Moreover, Ayou & Eveloy [140] performed a thermal and economic analysis of a CCP system. The analysis of a system that was powered by geothermal energy and having a KC and an SE was conducted by Ansarinasab et al. [141]. The heat from the GW was used by the desalination system, and then by a KC, which was bottomed by the SE. The magnetic refrigeration cycle (MRC) for providing cooling was also a part of the proposed system. The schematic of their system is presented in Fig.19.

The summary of these systems is presented in Tab.13. From Tab.13, it can be inferred that the maximum EE was found to be displayed by the system proposed by Fang et al. [139]. In addition to that, the minimum cost product of approximately 20.08 USD/GJ was shown by Ansarinasab et al. [141]. The system proposed by Ansarinasab et al. [141] also delivered additional provisions of cooling, heating, and drinking water, using a geothermal HS.

5.4 Systems with CO₂ Rankine cycle (RC)

In this Section, the systems that included CO₂ supercritical/transcritical TRC are presented. Such systems are observed to present a better thermal matching between the HS and the WF [142]. Information regarding the CO₂ supercritical Rankine, also known as transcritical CO₂ RC (TRC) can be sought from the review performed by Sarkar [142] and the studies by Wang et al. [143] and Yang et al. [144].

For such systems, Sun et al. [145] performed a thermal evaluation of a system that used three RC-based systems for a 100000 deadweight tonnage (DWT) LNG fuel powered ship. The thermal multi-objective optimization of a geothermal powered TRC along with the DEE was conducted by Wang et al. [146]. Furthermore, the optimization of the EE of a solar flat plate powered TRC was performed by Sun et al. [147]. Additionally, Zhao et al. [148] conducted a thermal evaluation of a system comprising of compressed air energy storage (CAES), TRC, and LNG regasification circuit with DEE. CAES is a method of storing the access electrical energy by compressing the air using a compressor and then using that compressed air to generate electricity in times of high demand by passing it through an expander. Further details can be found from the review conducted by Chen et al. [149]. In addition to that, Mehrpooya & Sharifzadeh [150] performed a thermal investigation of a solar concentrated power collector powered oxy-fuel TRC bottoming CO₂ RC system with CO₂ capturing. The CO₂ capturing was also incorporated in the system.

The thermo-economic optimization of a solar flat plate powered TRC system using LNG RCE was performed by Ahmadi et al. [151]. A 3E optimization of a geothermal TRC was performed by Ahmadi et al. [152]. Apart from it, Liu et al. [153] performed a thermoeconomic analysis of a CCHP system that used a CO₂ topping RC bottomed by an argon RC. Moreover, a thermal and economic evaluation of a biomass fueled TRC, cooled by the LNG RCE, with an expander in the circuit was conducted by Cao et al. [154]. The multi-effect desalination (MED) distillation to supply water was also part of the system.

A system using the biomass and coal co-gasification was proposed by Esfilar et al. [155] and is presented in Fig.20.

Another configurations involving compression-ejector refrigeration, and two low and a high pressure RCs, the likes of which, as shown in Fig.21 were proposed by Dokandari et al. [156]. Furthermore, Naseri et al. [157] performed an analysis of a system in which the TRC was bottomed by an SE. A comparative analysis with the stand alone TRC was performed. The decrease in the LNG mass flowrate was also reported. The HS for the TRC was partially from the SW and partially from the flat plate solar collectors.

A thermo-economic analysis of a system involving a TRC, an ORC, heating and cooling provision providers, a NaClO plant, and a reverse osmosis (RO) plant was performed by Tjahjono et al. [158]. The CO₂ cycle and the ORC got their heat from the same source attached in series. The NG obtained from the regasification was burnt to provide heat for the steam which was used as the HS for the prime movers.

The overview of the studies with CO₂ power cycles is presented in Tab.14. From the search of these systems, the maximum EE was reported by the system proposed by Cao et al. [154]. Considering the economical evaluations, such systems showed the PP to be around seven years as can be inferred from the studies conducted by Tjahjono et al. [158].

6 Systems with gas turbine (GT) units

In this Section, the systems that involve the GT units are presented. GTs are a well-known technology used to convert thermal energy into mechanical energy and further information can be sought from the review performed by Poullikkas et al. [159]. The general working principle of the GT is based on BC, which is described in Fig.22. The WF enters the compressor at state 1, where its pressure is increased as it exits at State 2. Afterwards, heat is added to the WF from State 2 to State 3. In case if the WF is air, mostly, the heat addition is in the form of combustion. Meanwhile, other means of heat addition can also be used, as shall be introduced shortly. The heated WF, at State 3 then enters the turbine/expander where the mechanical energy is converted into the electrical energy by using a generator, as it leaves the turbine/expander at State 4. In some occasions, when the WF specie does not change, as is the case of closed BC, the WF is cooled down from State 4 to State 1, forming a cycle.

Regarding the thermal analysis, Özen [160] performed a thermal analysis of a proposed system for the Marmara Ereglisi receiver terminal. The system comprised of a GT plant bottomed by a high pressure SRC, which was bottomed by a low pressure RC that utilized the LNG RCE.

Furthermore, Zhao et al. [31] conducted a dynamic exergetic analysis of a system that contained a GT, four RCs, and a DEE. The schematic of their system is presented in Fig.23.

Additionally, for the systems with the conventional SRC as prime movers, Moghimi et al. [161] conducted a thermal evaluation of utilizing an SRC, an argon RC, and an SE, that used the heat energy from the FGs of a GT. Furthermore, Ghorbani et al. [162] described the thermal evaluation of a system whose general scheme is presented in Fig.24.

Other than this, Bao et al. [163] compared the thermal evaluation of a GT bottomed by three pressure SRC with the post-combustion CO₂ capture (PCCC) with various alterations. The PCCC is an energy intensive process but renders lesser penalties and is usually performed by the absorption process of Monoethanolamine (MEA) solvent. To perform evaluations, 391 MW power plant PCC with different modifications, which were, the exhaust gas recirculation, the integration of a dual boiler and expander ORCs, and the integration of two-stage condensation ORC utilizing LNG RCE.

Moreover, Liang et al. [164] proposed a system that used the H₂O and CO₂ derivative from the FGs to power a N₂ BC bottomed by an ORC and a trans-critical CO₂ cycle. Two conditions, as shown in Fig.25, the O₂/CO₂ and O₂/H₂O environments were analyzed. For either of the atmospheres, their study revealed that the performance of the subsystem CO₂ trans-critical cycle is better than that of the ORC cycle.

Apart from it, considering renewable energy input, Ebrahimi et al. [165] performed a thermal evaluation of a system that incorporated a solar parabolic trough a collector powered GT system. The LAES system was used to store the LNG RCE during off-peak times. Meanwhile, the heat from the compression of the air for the LAES process was stored in the PCM tank during off-peak times. The heat and cold stored in the above mentioned processes were used during the on-peak time for the power and cooling for the KC. Additionally, She et al. [166] proposed a system that used the LAES, closed BC, and LNG regasification. Pressurized propane and methanol were used to harness the cold and heat energies acquired from the LAES process.

Additionally, several studies were conducted to assess the systems having the air separation subsystem incorporated into the power generation system. Air separation at cryogenic levels is used to primarily produce oxygen, nitrogen, and argon [167]. Further information can be sought from the review conducted by Smith and Klosek [168]. Regarding such systems, Mehrpooya et al. [169] analyzed two cryogenic air separation systems from a thermal standpoint. The comparison was between the conventional cryogenic air separation unit, and the integration of the LNG regasification, GT and CO₂ transcritical power cycles. Furthermore, Liu et al. [170] proposed an NG and oxy-fuel powered system with an air separation system and a CO₂ capturing system. The liquified oxygen from the air separation unit was burnt in the GT, which was bottomed by an SRC.

Furthermore, Krishnan et al. [171] investigated the thermal prospects of using the LNG regasification process to cool the WF at the compressor inlet within a closed loop BC. Different WFs which were argon, nitrogen, helium and air were considered for the process. In another case, Cha et al. [172] performed a thermal evaluation of a system consisting of a GT bottomed by a supercritical CO₂ RC. The compressor inlet air cooling and heat recovery system were also included in their proposed system.

The studies regarding the thermal studies conducted on the GT containing systems are summarized in Tab.15. From Tab.15, it can be observed that the best thermal performance was found to be from the Ghorbani et al. [162]. Furthermore, the systems containing GT also had several other electricity producing technologies, and in some instances, also provided additional provisions.

Considering the inclusion of economic and environmental evaluation, Ouyang et al. [173] performed a 3E evaluation of a proposed system that incorporated the LAES, and a ship wastage purification system. Moreover, Mehrpooya et al. [174] performed an advanced exergoeconomic evaluation of a GT bottoming an NH₃-H₂O cycle. The major components contributing to the exergy loss were identified and reported. Strategies to improve the process were also presented. The strategies are the improvement or replacement of the component with better equipment, the improvement in other parts, and the optimization of the complete process. Furthermore, a thermo-economic-environmental analysis of a system with the biomass fueled GT as the prime mover using LNG RCE was conducted by Cao et al. [175]. The overall system comprised of a topping cycle that used syngas as its fuel. The exhaust air from the expander of the topping GT was used to recuperate some heat for the combustor. Other than this, the heat from the combustor of the topping GT was used to provide heat to a bottoming GT. For the bottoming GT, CO₂, He and N₂ were considered as the WFs for the lower GT cycle. Meanwhile, n-heptane was selected as the WF for the ORC which used the part of the FGs from the topping GT, after they passed through the GT. The LNG RCE was used to cool the air in the intercooler. Additionally, Tang et al. [176] performed a thermal and economic evaluation of retrofitting the existing power plant located in south China with mixing the pressurized NG (after lowering its pressure) with the LNG and burning them in the combustor of the GT. A part of the LNG was used for cooling the air entering the GT. Their system comprised of a GT bottomed by an SRC which was further bottomed by an absorption chiller. Additionally, they performed a mixed integer nonlinear programming (MINLP) optimization of the retrofitting considering the total annual cost and exergy efficiency as the objective functions. The pipeline installation cost, considering the distance to deliver the cooling air conditioning to the users was also considered. Apart from it, Ozen & Uçar [177] proposed a system that used the NG to fuel a BC which was bottomed by a supercritical CO₂ cycle that was further bottomed by a propane ORC. In addition to it, Cao et al. [178] studied a system having a parabolic trough collector and biogas burner powered GT power system, a double effect absorption chiller, an SRC, an ORC, an MED system, and an LNG regasification system with an expander. The schematic of their system is shown in Fig.26.

Several potential candidates for the WFs of the ORC were evaluated in their study. Furthermore, Liu et al. [179] investigated a system that supplied cooled air to a GT bottomed by multi-stage SRC. The air-cooling system comprised of two ORCs and a vapor compression chiller powered by the electricity provided by the ORCs. The ORCs used mixed WFs having methane, ethylene, and propane. The WF of the VCC was R134a.

Additionally, Gao et al. [180] evaluated the thermal and economic performance of a GT bottomed SRC with LAES for lowering the peak demand needs. The impact of inlet temperatures of the compressor and turbines, ambient temperature, and NG pressure was determined.

Moreover, Kanbur et al. [181] performed a thermal, economic, and environmental comparative study between the recuperated GT system and a micro GT system with an SE and PCM. In addition to it, Sadeghi et al. [182] performed a thermal, economic and environmental analysis of a solar heliostat plus NG powered GT system with double stage compressors and turbines. The FGs from the GT cycle were initially used by an R134a ORC and then by a TEG. The LNG RCE was used as the heat sink of the ORC. An array of hot and cold tanks and eutectic carbonate salt was used for energy storage. A DEE was also an integral part of the overall system.

In a comparative study considering the 3E aspects, Ayou & Eveloy [183] compared a topping ORC or BC for power generation. From their evaluation, the Brayton based system was found to be better in performance but in economic terms, the ORC based system was better.

The summary of the results reviewed in this Section is presented in Tab.16. From Tab.16, it can be observed that the best thermal performance was found to be from the study conducted by Cao et al. [178]. Furthermore, the systems containing GT had several other electricity producing technologies, and in some instances, also provided additional provisions. Due to this, their thermal performance, in relative terms was better than electricity producing devices mentioned in earlier sections. On the other hand, the PP was found to be almost in the range of 4 years, as can be deduced from the studies conducted by Gao et al. [180] and Ayou & Eveloy [183].

7 Systems with gas Allam Cycle (AC) units

The AC is claimed to be an environment friendly combustion-based technology that recycles the emitted CO₂. The schematic of the traditional AC is presented in Fig.27 and the details regarding this cycle can be sought from Khallaghi et al. [184]. In general terms, the AC is a modified GT system with ASU and a post processing CO₂ separation process.

In this aspect, Li et al. [185] proposed a double condenser AC. Moreover, Yu et al. [186] studied several pathways for using the AC. The best performing schemes are presented in Fig.28, although Yu et al. [186] claimed the integration of the ORC could improve the thermal performance when replaced with the DEE but mentioned that the improvement was not significant enough to justify the investment.

The exergetic and exergoeconomic optimization of using the LNG RCE for AC CCP system was carried out by Chan et al. [187]. The schematic of their system is presented in Fig.29.

The summary of the results reviewed in this Section is shown in Tab.17. The AC is an emerging technology to be studied for utilizing the LNG CE utilization. In general, the maximum EE was found to be 80.68% with the inclusion of the ORCs by Yu et al. [186]. Furthermore, the PC for this type of technology containing systems was reported to be around 16.654%, as shown by Chan et al. [187].

8 Systems with fuel cells (FCs)

In this Section, the systems containing the FC systems for producing the electrical power are reviewed. FCs are the devices that directly transform the chemical energy of fuel into electricity [188], thereby offering an eco-friendly solution. To have a brief overview of the FC technology, Fig.30 can be referred to. The FC consumes fuel at anode and generates electrical energy via dissociation of the molecular structure of the fuel by using an electrolyte. Additional by products can be generated at the cathode. The overview of research on the FCs can be sought from the works of Sharaf et al. [188], Akinyele et al. [189], Mohapatra et al. [190], Sazali et al. [191], Jaleh et al. [192], Mehr et al. [193], Raihan et al. [194], and Yang et al. [195].

In this regard, Ahmadi et al. [196] conducted a thermal evaluation of a system having a PEMFC acting as the evaporator of a TRC. Moreover, Ebrahimi et al. [197] conducted a thermal evaluation of a system that contained molten-carbonate FC and a gas turbine (MCFC-GT) system. The block diagram of their system is shown in Fig.31.

Moreover, Ahmadi et al. [198] performed an EE of a system that used an HCFC-123 ORC, a solid oxide fuel cell (SOFC), and a GT to provide power. Water was used as the HS for the ORC which utilized the LNG RCE as the cold source. A system having coal gasification, SOFC, cryogenic air separation, CO₂ transcritical cycle, SRC, and an LNG regasification system was proposed by Mehrpooya et al. [199]. Apart from it, Liang et al. [200] proposed a system shown in Fig.32, that included an SOFC, an ORC, and a TRC. The LNG RCE was also used for the liquefaction of the captured CO₂ subsystem. Additionally, domestic heating and chilled water was also provided by the system.

Considering the inclusion of the economic studies as well, Chitgar & Moghimi [39] performed a multi-objective optimization of a system that used the SOFC-GT as the power production prime movers. The prime movers were bottomed by a KC which was further bottomed by an ORC. Apart from it, Emadi et al. [201] performed a thermal and economic evaluation of a system that used the SOFC coupled with an afterburner GT as the prime movers. The schematic of their system is shown in Fig.33.

Moreover, Mahmoudi & Ghavimi [202] performed a thermoeconomic optimization of a system with a supercritical CO₂ cycle, an MCFC, and an ORC. Additionally, Cao et al. [203] performed a multi-objective optimization of a system comprising of a geothermal flash cycle and an SOFC utilizing the regasified LNG and hot water. The exhaust gases in an afterburner were used to generate heat for partially heating the water for the SOFC and for the geothermal flash cycle. A DE was also part of the system proposed by them.

A comparative study was performed by Atsonios et al. [204] who analyzed prime movers being the SOFC, the SOFC-GT, GT bottoming SRC with a three stage turbine cycle, and a gas internal engine. Apart from it, the evaluation of the three various systems, the prospects of the district cooling, the combined cooling and power, and the cryogenic energy storage that utilized the CE from the LNG were also analyzed. Moreover, for the WH recovery, the low temperature MED system was also part of the systems. From their analysis, the system with SOFC-GT as the prime mover, with the CE storage was found to be the best solution from the thermal point of view.

The results about the systems containing FCs are presented in Tab.18. In general, in the studies on system having the FC involved other products other than the electrical power. The maximum EE in this domain was found to be 68.21% from the works of Ebrahimi et al. [197]. Additionally, the PC for such systems can be as low as 9.2 USD/GJ with a relatively reasonable EE of 51.6% as presented by Emadi et al. [201].

9 Results and discussion

For the LNG regasification process, pressure distribution plays a significant role, as can be seen from Fig.34. Therefore, the recommendations are provided, based on the highlights of the above provided review, based on the supply pressure.

For the supply pressure of approximately 600 kPa or below, the results considering the thermal, economic, and ecological specific indicators are presented in Tab.19. The results regarding the best thermal performance were shown by the system proposed by Cao et al. [154], which was powered by biomass and used the CO₂ RC as the prime mover with DEE. Apart from it, their system also provided additional products. Moreover, the system proposed by them was able to show a PP of 2.94 years. The second best in terms of EE was found to be displayed by the system presented by Wang et al. [102], with a value of 56.90%. Furthermore, the system proposed by them, powered by the FGs from the magnesite processing industry, also captured approximately 0.29 t of CO₂ per ton of LNG. Overall, the systems covered in this review showed minimum PPs to be 2.94 for the system presented by Cao et al. [154], and the second best in the range between 4.58 and 5.18 years, as presented by Tian et al. [108]. The maximum observed E{R}_{\text{C}{\text{O}}_{\text{2}}} at the presented scale is about 502.9 t/a from the study performed by Ouyang et al. [173].

The systems proposed using the pressure range above 600 kPa but within 1000 kPa were proposed by Kim & Kim [129], Kim et al. [130], Sadaghiani et al. [131], Özen [160], Ghorbani et al. [162], and Mahmoudi & Ghavimi [202]. The highest EE from this pool was observed to be 67.74%, for the system proposed by Ghorbani et al. [162]. The system contained multiple electricity generation technologies and also provided cooling, heating, and water. Regarding the economic evaluation, the system proposed by Mahmoudi & Ghavimi [202] offered a PC of approximately 12.5 USD/GJ, at an EE value of 64.7%.

For the pressure of around 2000 to 2500 kPa, the studies were found to be limited in comparison to other distribution pressures. For this range of the pressure, which is distributed to combined cycle plants, the highest EE from the studied system was shown by Tang et al. [176], with a value of 55.91%. The system included GT and also provided chilled water. The NPV of their system was shown to be approximately 57.16 million USD. Furthermore, considering the system that used the WH, the system proposed by Sun et al. [145] presented an EE of 48.06%. The system proposed by them contained CO₂ RCs and ORCs. Moreover, considering ORC systems, the LCOE was found to be 5.58 USD/GJ, for the configuration presented in Fig.12(c), as reported by Bao et al. [125].

The ideal systems with respect to various performance indices for the systems operating at 3000 kPa are provided in Tab.20. At 3000 kPa, the pressure for the local distribution of LNG, as presented in Tab.1, the best system, from the studies compiled in this paper was shown by Yu et al. [186]. Their system incorporating AC, showed an EE of 80.68%. The second best system was observed to be the one presented by Fang et al. [139], displaying an EE of around 80.49%. For the studies that included the economic evaluations as well, the system presented by Li et al. [136], reported a PP of 2.385 years, but with an EE of 26.5%. Meanwhile, the lowest LCOE was reported by Bao et al. [125], for the system shown in Fig.12(c), employing a value of 5.92 USD/GJ (0.021 USD/kWh). Furthermore, the study performed by Sun et al. [126], reported the configuration involving multiple condensers and expanders to be ideal at a regasification pressure of 3000 kPa, presenting an electricity product cost of 9 USD/GJ (0.032 USD/kWh).

For the pressures between 3000 and 7000 kPa, from the perspective of EE, from the studies compiled in this review, the best results were displayed by the system proposed by Ghorbani et al. [132], with an EE of 76.41%, at a regasification pressure of 3700 kPa. The system proposed by them contained ORC and KC as the electricity generating technologies, and provided the provisions of methanol, oxygen, and hydrogen. Meanwhile, the system proposed by He et al. [90] gave an EE of 61.11%, and the levelized cost of water was reported to be 1.946 m³/USD. The system proposed by them used ORC for power production and operated between the hydrate based desalination plant and LNG RCE, with a regasification pressure of 4400 kPa. Additionally, the cost rate of production was shown to be 9.2 USD/GJ, for the system presented by Emadi et al. [201], which incorporated SOFC, GT, and two ORCs for the electricity production, and also provided the cooling utility.

The account of two top best performing studies from the conducted review based on the performance indices, at a regasification pressure of 7000 kPa, the long-distance pressure distribution, are provided in Tab.21. At this pressure, the maximum EE from the studies considered in this paper, was observed to be reported by Mehrpooya et al. [169]. The system proposed by them had a GT and a CO₂ TRC and provided liquified oxygen and argon through an air separation unit, all with an EE of around 76%. Other than this, the second best was provided by Park et al. [93], with an EE of 75.1%. Their proposed system was based on LAES and ORC. The NPV for their proposed system was also reported to have the value of 225.89 million USD. The PP considering such a regasification pressure was observed to be approximately 4.07 years for the system proposed by Gao et al. [180], for a system having a GT, an SRC, and an LAES subsystem. Furthermore, the system presented by Park et al. [48] showed a PP of 5.6 years, which used an LAES DEE for electricity production. Furthermore, their proposed system showed an NPV of 143 million USD with an IRR of 17.7%. Considering the environmental aspects, the system proposed by Atienza-Marquez et al. [105] showed a saving of approximately 75000 t/a. Their system comprised of three ORCs and a DEE. Moreover, the system presented by Liang et al. [200], having an SOFC, ORC, and a CO₂ TRC, presented a CO₂ reduction of approximately 5219 t/a.

For an LNG regasification pressure higher than 7000 kPa, the system evaluated by Ning et al. [134] presented an EE of 85.19%. The system incorporated three R717 RCs in series with DEE, and also provided cooling, while receiving cold at an LNG regasification pressure of 12000 kPa. The second best was presented by He et al. [120], with an EE value of 73.92%. Their system having two ORCs, also provided ethylene glycol cooling, at an LNG regasification pressure of 30000 kPa. The total PC was shown to be around 1.512 USD/GJ for the system described in Ref. [111]. The system had two ORCs, and a GW powered flash cycle. A comparative study by Ayou & Eveloy [183] for the ORC system that harnessed energy from the LNG regasification CE, supplied at 8500 kPa, and for the GT powered system, for which the regasification pressure was 27000 kPa. For the GT primed system with DEE, the EE was 45.5%, with an economic savings of 8.3 million USD, with a PP of 3.9 years. The CO₂ annual reduction was reported to be 51.5 kilotons per year.

In case, if the supply and demand need to be considered, the flexibility of the system needs to be studied. For this, the round-trip evaluation is of prime importance. Considering a regasification pressure of 600 kPa, the RTE was presented to be approximately 45.44% each, by the studies conducted by Zhang et al. [106] and Ebrahimi et al. [165], but the prior system had an EE of 50.73%, in comparison to being 40.17%. Meanwhile, at 7000 kPa, the highest RTE was exhibited by Ansarinasab et al. [55], with a value of 192%, while, the second best was shown by Park et al. [93], with a value of 187.4%. On the other hand, for a higher regasification pressure, the highest RTE of approximately 141.88% was reported by He et al. [120]. The second best was reported by Lee et al. [42] to be 135.98%.

Considering systems with inclusion of renewable energy sources, from a thermal standpoint, the highest EE was displayed by the system presented by the biomass powered system proposed by Cao et al. [154] with an EE of 88.4%, and a dynamic PP of 2.94 years. Regarding solar energy input, the system presented by Ebrahimi et al. [197] had an exergetic efficiency of EE = 68.21%. The second best was presented by Ghorbani et al. [127] with an EE of 64.72% and a PP of 2.168 years. Considering geothermal sources, the system proposed by Mehdikhani et al. [82] showed the highest EE of 44.16% with a PC of 19.86 USD/GJ. Moreover, considering the study from Ansarinasab et al. [141], which used the geothermal sources, presented an EE of 42.95% with a product cost of 20.08 USD/GJ.

10 Research gaps and future recommendations

The incorporation and proper utilization of the LNG RCE holds the ability to help in generating more electrical power along with other utilities. Such incorporation can also help in assisting in curbing the environmental concerns the world is facing. There are several opportunities yet to be availed for harnessing the CE from LNG regasification.

The above-mentioned review has highlighted further research required in the following areas, in accordance with authors’ view:

• Lack of uniformity in economic indicators;

• Incorporation of renewable energy in designed systems;

• Deficient environmental studies;

• Utilization of artificial intelligence for comprehensive optimization;

• Insufficient experimental studies.

With an enhanced requirement for the energy conservation combined with the environmental concerns worldwide, there has been observed a serious lack of uniformity in the economic indicators. This causes a confusion and may lead to incorrect decisions downstream in any form. Just as a framework of definitions was setup for the net zero energy buildings by Sartori et al. [205], a similar effort is needed for this sector as well.

From the literature review conducted, there have been a relatively fewer studies conducted considering the renewable energy input, particularly the biomass renewable energy source. Regarding the modern-day scenarios involving various economic and environmental issues, the role of renewables has enhanced manifold. Considering this perspective, the research considering various polygeneration systems using solar energy as complete or partial HS were reviewed by Mohammadi et al. [206] and Kasaeian et al. [207]. A similar review, though for small scale applications for biomass driven CCHP systems, was conducted by Wegener et al. [208]. Furthermore, the integration of photovoltaic (PV) cells in such systems could shed some light on some interesting results if possible, and the information regarding the new and efficient PV cells can be sought from the review performed by Maurya et al. [209]. The incorporation of the presented systems in these reviews and those researched after their published date with the LNG RCE could present even greater opportunities for further exploration. The utilization of renewable energy sources with LNG regasification could further help in reducing the contemporary concerns related to global warming.

From the review conducted, it can be inferred that studies with environmental concerns were also very few. The recent trends on the concepts of 3E or 4E (energy, exergy, economic, and environmental) evaluation and optimization can provide clear results for any study. Detailed designing and optimization could be a cumbersome process, but for systems having numerous subsystems, the data driven techniques, such as mentioned by Tan et al. [76], Emadi & Mahmoudimehr [112], and Emadi et al. [201], could be extremely beneficial in reducing the computational effort and time. The methodology, as shown in Fig.35, is to obtain the data using thermal modeling for thermal parameters, sizing of equipment and economic evaluation, and environmental evaluation. Then the obtained data are used to train machine learning for getting the predictive models and utilizing those models to optimize the performance indices.

This method can reduce the coding efforts for making the function files for optimization using genetic algorithms or other heuristic optimization techniques, in the case of MATLAB programming environment. To state, one of the data driven method is the artificial neural network (ANN), and the schematic of back propagation ANN is presented in Fig.36, and the advantages and disadvantages are provided in Tab.22 [211].

Another aspect that was found to be extremely lacking in the authors’ opinion was that of experimental studies. Despite this, some studies were conducted with simulating the LNG RCE using liquified nitrogen or other cryogenic substances. With respect to the ORCs, the experimental investigation of a propane ORC using cryogenic CE was performed by Tian et al. [212]. For the SE, Katooli et al. [213] for gamma type SE, and Yu et al. [214] and Qiu et al. [215] studied such systems for the thermoacoustic SE. For other devices, no account was found by the authors. Thereby, leaving a considerable room in authors’ opinion for further investigations particularly considering the safety concern.

Another major concern observed by the authors was that the LNG despite being used as heat sink, still offered a lot of CE potential to be utilized, either in LNG form or in the form of regasified NG. Unfortunately, in several studies, the SW was used to soak up that CE, which can thus lead to SW pollution. The authors advise the potential researchers to be mindful of it.

11 Conclusions

A review of the electricity producing systems using the LNG RCE has been presented. The mechanical devices such as the direct expansion, RC based systems, SE, GT, and AC were reviewed. The review conducted presents the performance of several studies and the relative performance are compared. Apart from it, the future recommendations have also been presented.

The covered review indicates that the development of the right combination heavily depends on various factors, such as the condition of the LNG at offloading site, application, the capacity of the system to provide utilities other than electricity, and the operational parameters of the system. Nonetheless, for feasible and appropriate system to be designed, enhanced analysis comprising of the technical, economic, environmental, and other aspects such as safety are required.

Furthermore, from this review, it has been seen that there exists considerable opportunities for research to be conducted on the inclusion of renewable energy sources with the LNG RCE. Additionally, it was deduced that there lacks the application of the advanced computational artificial intelligence methods to be applied for the controls and dynamic optimization of the system, and there is a need for proper definition infrastructure. Last but not the least, another domain which lacks, is the experimental investigation. By addressing such concerns and researching in these lacking areas, more awareness could be reached.

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