Elsevier

Energy Conversion and Management

Volume 148, 15 September 2017, Pages 1248-1264
Energy Conversion and Management

Thermodynamic analysis of a novel pumped thermal energy storage system utilizing ambient thermal energy and LNG cold energy

https://doi.org/10.1016/j.enconman.2017.06.044Get rights and content

Highlights

Abstract

Pumped thermal energy storage (PTES) has become a hot topic on large scale energy storage technology because of the independence on geological conditions and fossil fuels. However, few of the PTES systems have higher round trip efficiencies compared with that of pumped hydro storage except for systems utilizing external heat sources. Furthermore, the used external heat sources are not available everywhere and much additional cost is required for the system integration. As an accessible and cheap heat source, ambient thermal energy is employed in the newly proposed PTES system. LNG (liquid natural gas) cold energy is also used as the heat sink based on possible combination with the natural gas distribution system in the practical operation. The charge process is based on transcritical CO2 heat pump cycle, while cascade design of transcritical CO2 Rankine cycle and subcritical NH3 Rankine cycle is employed in the discharge process. A thermodynamic model is established for energy and exergy analysis as well as the system evaluation. The analysis and evaluation of the optimized baseline case obtained by Genetic Algorithm are then carried out. In addition, the sensitivity of system performance to different variable parameters is also analyzed. Based on the analysis of optimized baseline case, the round trip efficiency can reach 139%. If for 1 MW net power output, both of the mass flow rates of CO2 and NH3 are 7.4 kg/s with LNG mass flow rate of 14.8 kg/s. Because of much higher round trip efficiency compared with other large scale energy storage systems, the proposed system is promising for future development and applications.

Introduction

Many countries worldwide turn to the exploitation and utilization of renewable energy because of severe environmental problems caused by use of fossil fuels. It is predicted that renewable energy will account for 29% of total power generation in 2040 [1]. However, because of the intermittence, fluctuation and other uncertainties, renewable power generation faces with challenges of reliability and stability [2], [3]. In terms of traditional power generation, the power demand also dramatically varies with time, which leads to high cost of power generation for frequency adjustment and peak shaving [4]. Moreover, interruption of power supply may have widespread and serious effects in case of grid destruction caused by natural or human factors [4]. To cope with the above challenges, electricity energy storage has been employed in production, transmission and utilization of electricity [2], [5], [6], [7], [8]:

  • (1)

    Peak shaving, frequency adjustment, black start and renewable power integration in production sector.

  • (2)

    Grid stability, voltage adjustment and current control in transmission sector.

  • (3)

    Uninterruptible and mobile power supply in utilization sector.

  • (4)

    Smart grid and distributed power systems.

As shown in Fig. 1, global energy storage capacity in operation has been rapidly increasing with different types of technology in the past decades according to data from Sandia National Laboratories [9]. Realized and planned energy storage capacity has been about 170 GW so far, half of which is for large scale utilization with more than 1 MW per unit. Therefore, energy storage is a prospective technology in various energy sectors.

There are many kinds of energy storage technology including pumped hydro storage [10], compressed air energy storage [11], supercapacitor energy storage [12], thermal energy storage [13], battery energy storage [14], etc.. The characteristics of energy storage, such as capacity, round trip efficiency, cycle time, energy density, capital and operation cost, lifetime, safety and environmental friendliness vary with different kinds of technology. The current commercial large scale energy storage technologies with megawatt-magnitude capacity and hours of cycle time are pumped hydro storage and compressed air energy storage [5].

Pumped hydro storage (PHS) is the earliest and most extensively used energy storage technology. Pumped hydro systems were firstly used in Italy and Switzerland in 1890. Cyclic systems based on pump and turbine were then employed to realize the efficient energy management in 1933 [15]. The global capacity of PHS has been growing from 1960s and it accounts for more than 95% of the total global energy storage capacity [9]. Traditional PHS systems are based on two water reservoirs at different altitudes, which can be alternated by available lakes and sea [10], [16]. Underground PHS systems use a water reservoir at the land surface and the aquifer [17]. The round trip efficiency is 70–85% considering water evaporation and penetration as well as the loss of mechanical machines [16]. PHS systems apply to large scale energy storage for the capacity of 100–3000 MW per unit [5]. However, the use of PHS systems are limited by the dependence on the geological and climate conditions, long construction period and high capital cost [5], [7], [16], [18].

Unlike the simple and well developed technology of PHS, the design of conventional compressed air energy storage (CAES) is based on Brayton cycle of gas turbine with underground compressed air storage, which has low round trip efficiency and is dependent on fossil fuels [19]. Higher round trip efficiency of adiabatic CAES is achieved by combination of the thermal energy storage and conventional CAES [20]. Isobaric storage of compressed air also results in higher round trip efficiency by underwater CAES [21] or integration with PHS [22]. Linde process and Rankine cycle are used in liquid air energy storage (LAES) for increasing energy density with round trip efficiency lower than 60% [23], [24]. Both of higher round trip efficiency and energy density are obtained in supercritical CAES using cryogenic technique and transcritical air cycle [25]. In compressed air storage with humidification (CASH), water vapor is induced in the compressed air for sufficient use of compression and exhaust heat [26]. In spite of lower capital cost [18], the applications of CAES are much less than that of PHS. Both of the two existing commercial CAES plants are based on conventional CAES design with dependence on fossil fuels and underground storage, while adiabatic CAES is still in R&D stage [9].

Similar to the design of CAES, compressed carbon dioxide energy storage (CCES) has been a new topic. In contrast to CAES, both high pressure and low pressure CO2 have to be stored in a closed cycle. Liquid CO2 was firstly used for high pressure and low pressure storage with organic Rankine cycle utilizing the heat of turbine exhaust, which has much higher energy density and comparable round trip efficiency to CAES without underground storage and use of fossil fuels [27]. Supercritical storage of high pressure CO2 was then proposed with higher round trip efficiency but lower energy density [28]. Both hot and cold storage are necessary for the above two methods, while the environment is used as the heat sink with hot water storage in another CCES system based on transcritical and supercritical CO2 Brayton cycles [29]. Aquifers at different depth are also employed for storage of CO2, which has both high round trip efficiency and energy density [30].

Pumped thermal energy storage (PTES) systems store electricity as thermal potential energy, where electricity converts to high temperature heat in the charge process and converts back by power cycles in the discharge process [31]. PTES is promising for future applications of large scale energy storage for its large capacity, long cycle time, low capital cost and high round trip efficiency. In addition, it is independent on geological conditions and fossil fuels in comparison with PHS and CAES.

A simplified model based on Carnot cycle and temperatures of heat source and heat sink was firstly used for calculation of round trip efficiency [32] and modifications were also proposed [33]. A more practical model based on ideal gas Brayton cycle considering thermal losses was then proposed [34], [35]. With the development of the theory and design, different power cycles apply to the discharge processes of PTES systems with various kinds of working fluids.

A thermodynamic cycle for energy storage with water as the working fluid and hot storage medium was proposed in 1924, and it is still used in solar thermal plants today [36]. On the basis of this system, specific hot storage medium was induced for storage of compression heat in 1978 [37]. The concept of conversion between electricity and heat is considered to be the origin of PTES [38]. Molten salt can be used as the hot storage medium with the environment as the heat sink. External heat source, like solar thermal energy and waste heat, is used for higher round trip efficiency [39]. The electric heater can be also used as the heat source in spite of its low efficiency [40].

Due to less volume flow rate of ammonia when evaporating at low temperatures compared with water, a water-ammonia cascade system based on subcritical cycles was proposed, whose round trip efficiency is 72.8% and can reach 94.4% when integrated with an external heat source [39]. A single subcritical ammonia cycle based system with solar thermal energy as the external heat source can achieve a round trip efficiency of more than 70% [41], [42]. Another transcritical ammonia cycle based system with exhaust heat of gas turbine as the external heat source has a round trip efficiency of 125.6% [43], [44].

Transcritical heat pump cycle and Rankine cycle are also employed in carbon dioxide based systems with hot water and ice slurry as the hot and cold storage mediums respectively [38], [45]. Multistage hot tanks are designed for good thermal matching between hot water and carbon dioxide [46]. Different system configurations have been studied including work recovery with expander, heat recuperating, superheating at the compressor inlet, intercooling and reheating between compression and expansion processes, etc. [47], [48]. According to the optimized results given by Genetic Algorithm, the round trip efficiency is at most 64% with a capital cost of $500–1000/kW [47]. Ground heat storage based system has a round trip efficiency of 50–66% with the minimum temperature difference of 1 K in the heat exchange processes [49]. Isothermal compression or expansion is realized by a liquid piston, which can be considered as a compressor or expander with an isentropic efficiency of 90%, resulting in a round trip efficiency of more than 70% [50]. CO2 heat pump cycle and water vapor Rankine cycle are integrated in a combined system with relatively lower capital cost and acceptable round trip efficiency of 60%, but CO2 does not apply to high temperature range (>300 °C) [51], [52].

Organic Rankine cycle applies to the discharge process with high temperature of 100–300 °C because of good thermal matching between the working fluid and hot storage medium. The candidate organic working fluids include HFCs, hydrocarbons and other organic solvents. The round trip efficiency is around 60% [53] and a butane based system has been analyzed in details [54].

A zeotropic mixture is another choice of the working fluid, e.g. 50% carbon dioxide and 50% butane. Due to more uniform temperature difference in the heat exchange processes, the thermal efficiency of Kalina cycle with a zeotropic mixture is at least 10% higher than that of Rankine cycle with a pure working fluid [55].

Air Brayton cycle based system is characterized by the higher operation temperature in comparison with the previous power cycles. The hot storage medium is molten salt with a temperature range of 350–700 °C, and the cold storage medium is synthetic oil with a temperature range of 100–250 °C. The round trip efficiency of this system is about 55% with turbomachinery isentropic efficiency of 90% and minimum temperature difference of 10 °C in the heat exchange processes [42]. Pseudo Ericsson cycle with less temperature variation can be achieved by multistage compression with intercoolers and multistage expansion with reheaters, but the round trip efficiency at the same conditions stated before is only 50% [42].

In contrast to air based system, the temperature range of argon Brayton cycle based system is much larger with the same pressure change. Packed bed heat exchangers filled with mineral particles or other mediums are used for hot and cold storage. Sensitivity analysis indicates that the round trip efficiency is dependent on the turbomachinery efficiency and the highest cycle temperature. The round trip efficiency is 66.7% with turbomachinery polytropic efficiency of 90% and turbine inlet temperature of 1000 °C [56]. The use of efficient reciprocating machinery leads to a round trip efficiency of more than 70% and an energy density of 200 MJ/m3 [57] as well as a capital cost of $470/kW [58].

The characteristics of the representative technology for large scale energy storage are summarized in Table 1. Due to the simple and developed technique as well as high round trip efficiency, PHS is the first choice provided that the geological conditions are available. Although several kinds of new technology have been proposed with regard to the independence on the geological conditions, few of them have a higher round trip efficiency than that of PHS except for PTES with external heat sources. However, external heat sources, like solar thermal energy, waste heat and geothermal, are not available everywhere. Much additional cost is also required for the utilization of these external heat sources. Ambient thermal energy is cheap and accessible everywhere, so it can be used as an external heat source in the PTES system to avoid the above problems. The objectives of the present study are to propose a new PTES system utilizing this type of external heat source and evaluate the thermodynamic characteristics of this system.

The organization of this paper is as follows. The conceptual design and system configuration are presented in Section 2. The thermodynamic model is established with some assumptions for energy and exergy analysis as well as the system evaluation in Section 3. The baseline case is studied specifically with different assigned or optimized parameters in Section 4. The sensitivity analysis of the system performance is then conducted within the given parameter range in Section 5. Finally, the main conclusions are drawn in Section 6.

Section snippets

Conceptual design

The conceptual design of the system is illustrated in Fig. 2. In the charge process, the electricity is stored as the thermal potential energy by the increase from ambient temperature to higher temperature than the hot storage medium through transcritical CO2 heat pump cycle. In the discharge process, the stored thermal potential energy converts back to electricity by transcritical CO2 and subcritical NH3 cascade Rankine cycles. It should be noted that ambient thermal energy and LNG (liquid

Assumptions

For further analysis and evaluation of system performance, a thermodynamic model is established with the following assumptions:

  • (1)

    Pure and dry CO2 and NH3 are used as the working fluids.

  • (2)

    The heat dissipation of system components and the pressure losses in the pipes can be neglected.

  • (3)

    The system is always at steady state.

  • (4)

    The gravitational potential energy and kinetic energy are not considered.

  • (5)

    The pressure loss rates in different heat transfer processes are considered as the same value designated with α

Setting of system parameters

As shown in Table 3, some parameters are set to be constant values in the later analysis according to the related literature with energy storage systems. For large scale energy storage systems, the time periods of charge process and discharge process should be several hours, which are only related with the volume of hot storage medium. Although these two periods are set to be equal for simplicity, they can be different from each other for the practical demand. Unit mass flow rate of CO2 is

Sensitivity analysis of system performance

The sensitivity of the round trip efficiency and net power output to the variable parameters is analyzed in this section within the given ranges in Table 4. The sensitivity analysis is carried out with one variable parameter varying while the others keeping constant at the baseline values. Additionally, adjustable parameters are adjusted according to the limitations of the maximum cycle temperature and minimum temperature differences at each side of the heat transfer processes.

Conclusions

A PTES system utilizing LNG cold energy and ambient thermal energy is proposed and analyzed in the present study. The charge process is based on transcritical CO2 heat pump cycle, while cascade design of transcritical CO2 Rankine cycle and subcritical NH3 Rankine cycle is employed in the discharge process. This design not only eliminates the dependence on specific external heat source, but also can be integrated with natural gas distribution systems. In contrast to other large scale energy

Acknowledgement

The support of National Key Research and Development Program (2016YFD0400106) and the support from Beijing Engineering Research Center of City Heat are gratefully acknowledged.

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