USING AMBIENT OR RESIDUAL HEAT AND LNG COLD AS A HEAT SINK
This work is an extended summary of a recent publication by Ferreiro Garcia R. et al., aiming at the efficient extraction of electric power using the cold available as heat sink in the regasification process of LNG. Thus, an efficient power plant composed of three series Rankine cycles (RCs) combined with a direct expansion based turbine has been proposed, where the rejected heat from each cascade power unit is used to heat the liquefied natural gas in a regasification plant. As a result of the optimisation of an objective function, taking the ratio of the obtained power to the mass flow rate of LNG as the performance criteria, the cascaded Rankine cycles operating with argon and methane, combined with a direct expander working with regasified LNG, yield the highest possible exergy efficiency (approaching 300 kW/kg LNG).
In conventional regasification plants, the LNG is pumped from the storage containers to the vaporisers where heat is added at a high enough pressure (above 60-70 bar) to provide the desired gas pressure for entry into the transmission or distribution system. Nevertheless, in order to use the regasification task as heat sink to convert ambient or low temperature heat sources into electric power, the LNG must be pumped from the storage containers to the vaporisers at a sufficiently high pressure high (above 300 bar) to favour the conversion process and thus, achieve high efficiency.
Thus, in this work a cascade of three RC based transcritical power plants condensing at quasi-critical conditions and a direct expander is analysed, wherein its scheme is depicted in Fig. 1. As will be shown in this work, the proposed cascaded Rankine based power units combined with a direct expander power unit provide a higher ratio (power kW/kg LNG) than the most recent studies in the field.
The next sections are devoted to power plant modelling and analysis in order to obtain the operating conditions corresponding to the maximum attainable performance, expressed as the ratio of the net power to the mass flow rate of regasified LNG. The results of the study are compared with affine referenced previous works.
2. Description of the regasification plant structure based on three cascade RCs combined with a direct expander.
Given the characteristics of the LNG associated to the objective, consisting of the LNG regasification task to be delivered to the supply chain under the required temperature and pressure at maximum possible efficiency, it has been determined that two important design interdependent factors exert influence on the optimal design procedure: plant structure and operating conditions (operating parameters). While a feasible structure, according to the scheme depicted in Fig.1, deals with a combination of some cascade RCs cooled with the LNG being regasified, and combined with a direct expander unit installed downstream from the first two cascaded RCs, where the cold generated by the direct expander is applied as heat sink for the last RC, optimal plant parameters are conditioned by the chosen structure of the plant as well as by the results of an optimisation problem, considering the plant structure, the working fluids and the operating conditions.
Fig.1. Detailed flow diagram of the LNG regasification system equipped with three cascaded RC based PCUs, where each PCU uses the LNG cold as the heat sink and, as the heat source, seawater and residual heat, combined with a direct expander unit operating with supercritical NG
Thus, the proposed cascaded power conversion in its (PCUs) based on the three RCs, combined with a single-stage expander that is arranged as the structure of Fig. 1, where the five optimised plant options are analysed, in which the three RC based PCUs work with Ar, CH4 and, CH4 or R14 respectively, combined with a direct expander operating with NG.
The working conditions of the used working fluids are shown in Table 1, where the topping PCU (RC1) consists of an RC operating with Ar, the following downstream PCU (RC2) consists of an RC operating with CH4, the following downstream PCU consists of a direct expander operating with NG, and finally the last downstream PCU (RC3) consists of a non regenerative RC operating optionally with:
• CH4 when the demand of delivery pressure is above 30 bar and no residual heat is available, and
• R14 when under any delivery pressure within the range of 30-70 bar, residual heat is available.
Table 1. Basic data used in the analysis of the plant
In order to effectively utilise the exergy acquired by the LNG during the regasification task, a PCU based on an expander has been installed before RC3 at the distribution line. The expander favours the plant performance by means of two relevant technical contributions:
1. converts some heat of the regasified NG into electric power and
2. is responsible for cooling RC3 condenser by using the expanded NG exhausted by the expander.
Under this structure, the proposed power plant derived from the regasification process appears to be in line with the schemes depicted in Fig. 1.
2.1. A feasible implementation approach
The proposed plant is structured under the most compact format to operate with low temperature heat such as residual heat, including local environmental heat, preferably seawater heat as a stand alone power supply when residual heat is not available. The LNG regasification process is used as the necessary heat sink of the Rankine cycle based PCUs. Given that a main objective consists of achieving the highest ratio of the attained electric power to the mass flow rate of regasified LNG, several attempts to achieve the mentioned objective have been carried out. In this way, a combination of three cascade RCs with a direct expansion turbine operating with heated NG, according to the structure depicted in Fig. 1, has been chosen as a viable option.
2.2. Power plant modelling
The partial power and efficiency of the each PCU as well as the overall power and efficiency of the power plant, including the exergy analysis, is modelled following the below common methodology:
The power plant modelling task requires the optimisation of the inherent operating conditions. This task consists of finding the operating conditions that maximise the value of an objective function assumed as the performance index, which requires solving an optimisation problem ( objective function), based on the ratio of the overall net power Po (kW) to the LNG mass flow rate (kg/s). Solving this problem for every plant structure, working fluid and restrictions, will yield the optimum operating points that must be taken into account in the optimal designing task.
2.3. Main results derived from the optimisation criteria, based on the maximum specific power
In order to have an insight of the plant structure possibilities, the most relevant five case studies are considered. Two different heat sources are used:
• the seawater at ambient temperature and
• optionally, a residual heat based source at a minimum temperature of 80 ºC, assumed as capable to heat the working fluids of the Rankine cycles to approach 60 ºC.
Only case study number one operates with a single heat source (only seawater). As consequence of its lower heat flow and lower temperature, the achieved net power is lower than the rest of the carried out case studies. However, indeed, the attained net power is significant with respect to the conventional plant structures operating with a single power source (seawater) implemented to date.
Fig 2. The residual heat demand for each case study associated to its particular plant structure and operation conditions.
Fig. 3. Total heat flows (kJ/kg-LNG) for seawater and the residual heat based heat sources associated with each case study.
Fig. 4. The power plant performance (Po) for the five studied cases
The availability of heat from seawater can be assumed as unlimited in practical applications. Nevertheless, according to the availability of residual heat, it might be interesting to know the appropriate plant structure (among the structures of cases 2 to 5) as function of the heat demand and residual heat availability. In this way, Fig. 2 depicts the plant structure associated to its correspondent case study as function of the residual heat availability between 461.70 and 834.40 kJ/kg-LNG.
Table 2. Comparing the attained results with affine recent references
With regard to the residual heat demand, it follows that case study 3, according to Table 3, provides a power of 302.80 kW, when the demand of residual heat approaches 751.80 kJ/kg- LNG, also shown in Fig. 3. On the contrary, case study 5 exhibits the highest residual heat demand (834.40 kJ/kg-LNG) when the power approached only 256 kJ/ kg-LNG
Table 3. Daily revenue
The partial heat flow supplied by seawater (SEAWATER) and the residual heat is depicted in Fig. 3. As shown there, for cases 3, 4 and 5, the proportion of residual heat is greater than for the SEAWATER. This alerts us to take into account the availability of residual heat before deciding the case study associated with a plant structure.
The performance of the regasification plant is depicted in Fig. 4. It shows that with only seawater as heat source (case 1), although performance is relatively high, when compared with the cases for which residual heat is available at no economic cost (cases 2, 3, 4 and 5), performance is poor. However, the installation is far more simple and economic in terms of heat exchangers cost, since the heat exchanger for
residual heat recovery is avoided, which exerts some positive influence on the installation and maintenance cost.
2.4. Comparison with recent affine works
For cases where the heat source is cost free so that we need not pay for it, it is good practice to consider the specific power production.
Some technical contributions in the field of LNG regasification that use the environmental heat energy from seawater to obtain electric power, using the cold exergy of the LNG, are compared in Table 2 with the attained results of the five studied cases. The differences are due to the chosen plant structure associated with the different conversion strategies and the operating characteristics. While the operating characteristics are deterministically achieved by solving an optimisation problem for a particular plant structure associated with a particular working fluid, every chosen structure must satisfy economical and/or technical constraints, including physical requirements such as size and weight or environmental constraints.
The ratio of the power attained to the mass flow rate of LNG as performance index has been optimised for the five case studies carried out. Therefore, satisfactory overall results have been achieved mainly due to the taking advantage
of the optimisation of the performance index. However, the successful results obey the fact that, as has been said, the condensation of the RCs’ working fluids at quasicritical temperatures contribute considerably to avoiding superfluous heat rejection to the heat sink, thus contributing to the increasing of useful exergy. Furthermore, relevant considered characteristics are:
• the optional plant structures according to the availability of residual heat capable of admitting the proposed working fluids,
• the working fluids selection that fulfils quasi-critical condensation conditions.
• the lowest delivery pressure demand which contributes to the maximisation of the net power.
• an optimised performance index based on the ratio of the attained power to the mass flow rate of regasified LNG, which approached 302.8 kJ/kg-LNG. This quantity, compared with the most recent contributions carried out in this field, represents a significant improvement.
Ramon Ferreiro Garcia, Jose Carbia Carril, Javier Romero Gomez, Manuel Romero Gomez. Power plant based on three series Rankine cycles combined with a direct expander using LNG cold as heat sink.
Energy Conversion and Management 101 (2015) 285–294.
By Ramon Ferreiro Garcia, Jose Carbia Carril, Javier Romero Gomez, Manuel Romero Gomez
Professor Ramon Ferreiro is the head of a research group that conducts technological investigations dealing with systems engineering in the University of A Coruna, Spain. His research interest during the last decade has been focused on power plants efficiency including LNG reliquefaction, regasification and energy conversion.
JOSÉ CARBIA CARRIL
José Carbia Carril holds a PhD in Marine Engineering and is a Professor in the Department of Energy and Marine Propulsion at the University of A Coruña, Spain, and a member of the research group Energy Engineering at the university. His research focuses on the optimisation of energy systems, energy conversion and waste heat recovery.
JAVIER ROMERO GOMEZ
Javier Romero Gómez holds a PhD in Marine Engineering and is a Professor in the
Department of Energy and Marine Propulsion at the University of A Coruña, Spain. He belongs to the Energy Engineering Research Group at the university. His research is concentrated on optimising energy, energy conversion, LNG reliquefaction, refrigeration and air-conditioning systems.
MANUEL ROMERO GOMEZ
Manuel Romero Gómez holds a PhD in Marine Engineering and is a Professor in the Department of Energy and Marine Propulsion at the University of A Coruña, Spain, and a member of the research group Energy Engineering at the university.
His research focuses on the optimisation of energy systems, energy conversion and waste heat recovery.