Elsevier

Energy Conversion and Management

Volume 156, 15 January 2018, Pages 626-638
Energy Conversion and Management

Off-design performances of gas turbine-based CCHP combined with solar and compressed air energy storage with organic Rankine cycle

https://doi.org/10.1016/j.enconman.2017.11.082Get rights and content

Highlights

  • A tri-generation and combine-cycle power system is proposed integrating with solar and CAES.

  • The off-design models of the proposed system are built.

  • The effects of the important parameters on the system performances are analyzed.

  • The energy-saving potential of the system is investigated by a case study.

Abstract

The conventional CCHP systems often operate at part load and cause power surplus under the operation mode of power following thermal load. In order to improve the energy efficiency of CCHP systems, a novel combined cooling, heating and power (CCHP) system combined with compressed air energy storage (CAES) was proposed. However, the output power of CAES was probably not high due to the low temperature of high-pressure air at air turbine inlet. In this paper, solar energy was introduced to heat the high-pressure air from air storage cavern. For the further energy utilization of the air turbine exhaust with relatively high temperature, an organic Rankine cycle (ORC) was proposed to recover the heat carried by air turbine exhaust. To obtain the off-design characteristics of the proposed system, the typical off-design models of gas turbine, compressor and air turbine were built through the performance equations of each component. The sensitivity analysis on S-CAES with ORC system was investigated to evaluate the effects of several key parameters on its performances. The results show that the energy efficiency and exergy efficiency of S-CAES with ORC system reach 98.30% and 68.94% respectively, in a whole round trip cycle. A case study of the proposed system in a typical hotel building with 180,000 m2 located in South China concludes that, compared with the conventional CCHP, the energy consumption can be reduced by 124.78 GJ, 33.82 GJ and 62.1 GJ and the average energy efficiency increased by 7.72%, 1.47% and 3.61% in summer, transition season and winter typical days, respectively.

Introduction

The energy consumption has risen greatly with the development of industry and the increment of population in the last 30 years [1]. Almost 33% of the global total final energy is consumed in buildings in 2013 for cooling, heating, cooking and other purposes [2]. Consumption of this amount of energy brings out environment and economic problems inevitably. Because the combined cooling, heating and power system (CCHP) can convert 75–80% of the fuel source into useful energy [3], CCHP is considered as one of the most promising solutions to reduce energy consumption in buildings [4].

CCHP system has obvious advantages in energy utilization, which can dramatically reduce the primary energy consumption and improve the energy efficiency [5], and produce low greenhouse gas emissions as well [6]. In China, approximately 80% of the total installed capacity of distributed energy system (DES) was driven by small or micro gas turbines by the end of 2015, according to the report from China Gas Association [7].

CCHP systems usually operate under two modes, i.e., FTL (power following thermal load) and FPL (thermal following power load). Generally, FTL mode is more helpful to increase the thermal utilization efficiency. However, the power surplus is inevitable in the case of small heat-to-electric ratio on the demand side, even in an optimal CCHP configuration in FTL mode. Therefore, many energy storage systems were introduced in CCHP system. Liu and Chen [8] proposed a CCHP system combined with ground-source heat-pumps and thermal energy storage (TES), with an advantage in reduction by 15.8% of the total installed cooling capacity and 37.5% of the total installed heating capacity of the CCHP system. Jiang and Zeng [9] established a theoretical system to evaluate the performance of CCHP systems with energy storage units. Li and Wang [10] presented an energy storage system which stored excessive energy in the form of compressed air and thermal heat in a CCHP system. The average comprehensive efficiency was found to be much higher than the conventional CCHP system, arriving at 50% and 35% in winter and summer, respectively. Many of these systems refer to store thermal energy, while few store surplus electric power directly, although the latter has obvious advantages in FTL operation strategy. Also, compressed air energy storage system (CAES) is superior to other electric storage system in longer lifetime, lower cost and higher efficiency.

So far, two commercial CAES plants have been built. One is Huntorf plant of 290 MW, which has operated since 1978 in Germany; and the other is McIntosh plant with 110 MW in America, which has operated since 1991 [11]. Japan has built a CAES test station with a certain size. In addition, France, Italy, Russia, Korea, Switzerland and other countries are also developing CAES power stations vigorously [12]. Currently, the researches about CAES mainly focus on the methods for improving its efficiency through simulation, improving the energy utilization rate through combining the CAES with other systems and proposing new compressed air energy storage technology.

Many concepts of CAES systems have been proposed and studied nowadays. Seymour [13] proposed an ocean compressed air energy storage system (OCAES) to keep compressed air at a constant high pressure. Lv and He [14] described a CAES system for tri-generation, where, the prime mover of this system was an electromotor and was different from traditional tri-generation. In a liquid air energy storage system (LAES) depicted by Morgan and Nelmes [15], the cycle efficiency was greatly improved by recycling and storing thermal energy between the charging and discharging parts of the cycle. Adriano and Li [16] developed a dynamic simulation of adiabatic compressed air energy storage (A-CAES) with integrated thermal storage system (THS). It achieved system efficiency in the range of 60–70% when the THS operated with a storage efficiency above 90%.

For the further improvement of CAES efficiency, some bottom cycle has been introduced into CAES system. Zhao and Wang [17] added a Kalina cycle to CAES system to recover the waste heat from turbine exhaust. Amin and Mohammad [18] joined an organic Rankine cycle to CAES system to recover the heat carried by turbine exhaust and produced more power. Additionally, some new energies were also introduced to CAES. Arabkoohsar and Machado [19] presented a CAES unit equipped with a solar heating system in a photovoltaic (PV) plant. Zhao and Dai [20] proposed a CAES system coupled with wind energy.

Recently, some new ideas about small-scale CAES coupled with CCHP have been put forward. Amin and Mohammad [18] developed a CCHP system coupled with wind energy and CAES to provide cooling, heating and power simultaneously, with a round trip energy efficiency of 53.94%. Yao and Wang [21] proposed a CCHP system based on small-scale CAES. The exergy efficiency of the proposed system was found to be about 51%. He and Xu [22] evaluated a hybrid CCHP system combined with CAES. They found that the fuel energy saving ratio of the proposed system was relatively 29.4% higher than that of conventional CCHP. Yan and Zhang [23] proposed a CAES-based hybrid CCHP system with renewable energies. With daily cost decreasing by 4.45%, the system obtained benefit increment in both environment and economics when compared with a battery based system in a configuration optimization.

However, most of these concepts mainly focused on the design condition of the CCHP systems in order for maximum utilization of renewable energy sources (RES), which ignored the energy waste utilization, and as a result, caused a low efficiency of gas turbine at off-design conditions [24]. Therefore, in this paper, we propose an integrated CCHP system, coupling solar energy, compressed air energy storage (S-CAES) and organic Rankine cycle (ORC), in aim to improve the part-load performances of small gas turbine based CCHP system. The air compressor in CAES of this paper is driven by the surplus power of gas turbine instead of by the RES [21], [25], which allows gas turbine operating at a higher load. In addition, the high-pressure air from cavern is heated by the solar thermal energy, rather than the gas turbine exhaust [18], [22] or the recovered heat in charge process [10]. Also, the ORC cycle is used in this paper to recover the air turbine exhaust heat. The characteristics of the proposed system are to be studied, and a case study on a typical hotel building located in South China will be taken as an example.

Section snippets

System configuration

The proposed system consists of two parts, i.e., ➀ a subsystem combined cooling, heating and power (CCHP); ➁ solar and compressed air energy storage with organic Rankine cycle system. The schematic diagrams of these two parts are shown in Fig. 1, Fig. 2 respectively, where, an ORC power cycle and an intermediate-reheat two-stage air expansion are introduced in the CAES system presented by the authors [24].

The CCHP system includes a gas turbine, a heat recovery steam generator (HRSG), a gas

Assumptions

The following assumptions are employed to simplify the analysis:

  • ➀ Air is treated as ideal gas.

  • ➁ The pressure drops in each component and pipe are neglected.

  • ➂ The isentropic efficiency of ORC turbine and pumps are fixed.

  • ➃ The temperature of air storage cavern keeps constant during a whole round trip cycle.

  • ➄ The analysis of S-CAES-ORC performances is based on a whole round trip cycle.

Gas turbine

In gas turbine, the output electric power and fuel consumption are directly determined by the output thermal

Energy efficiency

The energy efficiency of S-CAES-ORC is defined as the ratio of the total used energy of S-CAES-ORC to its energy input, and can be expressed as,ηth_s=Qheat+Wt.power+WORCWc.power+Qsolar

However, solar energy belongs to renewable energy and non fossil-fuel energy. To further analyze the utilization ratio of fossil-fuel energy, we define the other energy efficiency ignoring the grade of solar energy input. This efficiency can help to reflect the advancement of the proposed system.ηth_ws=Qheat+Wt.power+

Off-design performances of CCHP

Fig. 3 shows the off-design performances of CCHP system. We can see that, the coefficient of performance (COP) of LiBr-water absorption chiller increases at first and then decreases with the drop of gas turbine loads. This trend is related to the design of chiller. The HRGS efficiency (ηHRSG), gas turbine efficiency (ηGT) and the primary energy utilization rate of CCHP under maximum heating working condition (ηth-max.heat) and maximum cooling working condition (ηth-max.cool) all rise with the

Load distribution

To reflect the energy saving potential of the proposed CCHP over the conventional systems, this paper chooses a hotel building located in South China as a case study. The area of this building is 180,000 m2 and it works continuously for 24-h a day [37]. Fig. 17 presents the building’s load distribution of heating energy, cooling energy and power energy on a typical day in summer (from May to Oct), transition season (March, April and Nov) and winter (from Dec to Feb) respectively. Fig. 18 shows

Conclusions

A gas-turbine based CCHP combined with solar energy, compressed air energy storage (CAES) and ORC is proposed to improve the CCHP energy efficiency in this paper. The typical off-design models are built and the off-design performances of the proposed system are analyzed in charge process and discharge process as well. The sensitivity analysis on S-CAES with ORC system is conducted to evaluate the effects of several key parameters on its performances. Also, the new system is applied to a hotel

Acknowledgements

This work was financially supported by Guangdong Province Key Laboratory of Efficient and Clean Energy Utilization (2013A061401005); Key Laboratory of Efficient and Clean Energy Utilization of Guangdong Higher Education Institutes (KLB10004).

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