Elsevier

Energy

Volume 163, 15 November 2018, Pages 475-489
Energy

Adjustable performance analysis of combined cooling heating and power system integrated with ground source heat pump

https://doi.org/10.1016/j.energy.2018.08.143Get rights and content

Highlights

  • Consider the different forms of waste heat to integrate the coupling energy flows of CCHP-GSHP system.

  • Analyze adjustable areas of the coupling system without thermal energy storage to match the ratios of electricity to heat.

  • Obtain the distributions of the energy and exergy efficiencies with the adjustable areas.

  • Discuss the energy-saving and emission-reduction potentials of the coupling system.

Abstract

The difference in the electricity to cooling/heating ratio between a building and combined cooling, heating, and power (CCHP) system has a significant influence on the system configuration and performance. This paper designs a CCHP system coupled with a ground source heat pump (GSHP) to coordinate the match between system supplies and building demands. The energy flows in the cooling and heating work conditions are analyzed, and the thermodynamic models of components constructed. By means of a case study, the performances of the coupling CCHP system, under design and off-design working conditions, are evaluated and analyzed using energetic indicators, including the primary energy ratio and exergy efficiency, respectively. The adjustable areas expressed by electricity and cooling/heating, as well as the adjustable performance distributions, are obtained and discussed in order to guide operation regulation of the CCHP system integrated with the GSHP. Comparisons between the CCHP system with and without GSHP indicate that the coupling CCHP system in the specific case study can save averagely 40.6% and 39.5% of the primary energy in cooling and heating work conditions, respectively.

Introduction

Distributed energy systems have been considered to supplement the traditional central system, owing to their high overall efficiency and environmentally friendly performance [1], of which combined cooling, heating, and power system (CCHP) or combined heating and power system (CHP) is one of key forms [2]. The outstanding feature of the CCHP/CHP system is the cascading utilization of energy. Researches on distributed systems focusing on system configurations [3], performance evaluation [4], operational strategies [5], and optimization methods [6] aimed to establish optimal distributed systems in order to reduce energy consumption and greenhouse gas emissions.

Gradually, renewable energy, including solar, wind, and geothermal energy, as well as bioenergy, has been introduced into distributed energy systems to amplify the benefits gained from energy, environment, and even economy [7]. According to the different technologies and characteristics, various integrated systems have been proposed and analyzed, such as CCHP systems based on biomass gasification with air [8] or steam [9], and CCHP systems coupled with solar collectors [10] or solar fuels [11]. However, one limitation in solar or wind integrated systems is their intermittent nature. Furthermore, hybrid biomass systems may be affected by low heat density and challenges in collection and transportation. Among the available renewable energy sources, geothermal energy has attracted significant attention due to its friendly performance and relative stability, particularly for shallow underground geothermal resources from groundwater or soil coupled with a heat pump system [12], namely a ground source heat pump (GSHP) system.

Typical applications of GSHP systems consist of providing heating and/or cooling for users. Lucia et al. [12] reviewed GSHP systems that include both various GSHP technologies and thermodynamic models. The summaries demonstrated that systems with a GSHP can reduce the environmental impact of buildings. Zhai et al. [13] designed a mini-type GSHP system for a meeting room and discussed output capacities in the typical mode, which demonstrates the GSHP system applicability for building demands. Yuan et al. [14] proposed a control mode for the GSHP with a borehole-free cooling coupled system, and investigated the underground heat balance problem. This research concluded that the mode can improve energy efficiency and decrease energy consumption. In particular, the annual operational cost of the GSHP system is obviously reduced. To improve the performances of GSHP system, the exergy analysis was often used to discover the segments with more exergy destruction. Menberg et al. [15] proposed a water-based hybrid GSHP system and gas-fired boiler, and analyzed the exergy loss and destruction of the hybrid system by modeling each subsystem. Esen et al. [16,17] predicted the daily performance of GSHP in a fuzzy weighted pre-processing method with the limited experimental data and investigated the GSHP's energy and exergy performances as a function of depth trenches for heating season. The investigations shown that the energetic and exergetic efficiencies of the system increase when increasing the heat source (ground) temperature for heating season. Besides space heating/cooling provided by GSHP system, Balbay et al. [18,19] studied the using of GSHP systems for snow melting on bridge slabs and pavements, and researched the temperature distribution of slabs and pavements. Moreover, the GSHP have coupled with solar energy in recent researches [20]. Esen et al. [21] have analyzed a solar-assisted GSHP system, and obtained its coefficient of performance (COP) and other performances.

Moreover, a GSHP system for heating and cooling is coupled with a CCHP system for electricity, heating, and cooling in order to supplement each other and relieve the limitation of the fixed heat to electricity ratio of the CCHP system. The CCHP and GSHP coupling system usually consists of a prime mover, waste heat utilization system, and GSHP. The electricity produced by the prime mover is fed to the building and GSHP. The cooling and heating demands can be met by the waste heat utilization system and GSHP. Typically, the alternatives of waste heat utilization system for producing chilled water includes absorption chiller/heat pump or adsorption chiller driven by heat sources with different heating temperature. The absorption chiller or heat pump is driven by the exhausted gas from power generation unit with high temperature. Wang et al. [8] employed the absorption chiller driven by three kinds of heat sources to recover the waste heat from the exhausted gas to produce chilled water and hot water for space cooling and heating respectively and domestic hot water. Yang et al. [22] proposed and researched a new open-cycle absorption heat pump system to recover the waste heat from flue gas, and found that this system can get an excellent output even at high return water temperature. In addition, the adsorption chillers with the low temperature heat source could recover the waste heat with low grade energy level. Chorowski et al. [23] investigated an adsorption chiller which utilizes low-temperature heat from cogeneration and demonstrated that the adsorption chiller can be worked with a hot water of 65 °C, a typical cogeneration heating temperature in distributed energy systems. The performance of adsorption chiller directly influences the operation parameters on cooling capacity [24] and then determines the recovery efficiency of waste heat in CCHP system. Although the recovery efficiency of adsorption chiller is lower than the absorption chiller, it provides one alternative to utilize the waste heat with the lower temperature.

The researches on CCHP and GSHP coupling systems with the typical absorption chiller or heat pump have started to emerge in the last few years, only in performance analysis and configuration strategy. Kang et al. [25] proposed a hybrid system with four subsystems, including a power generation unit, GSHP unit, absorption chiller, and storage tank, and analyzed the energy and environmental performances in three basic modes. Similarly, Kang et al. [26] employed a comprehensive matrix approach to compare the configurations and performances of CCHP-organic Rankine cycle system with a GSHP in three energy management modes. Liu et al. [27] focused on hourly operation strategy of a CCHP system with GSHP and thermal energy storage under variable loads and discussed the economic and environmental benefits of the CCHP system with thermal energy storage. Moreover, aimed to obtain more benefits of CCHP and GSHP coupling system, optimization methodologies were applied to construct the system's configurations and determine the operation modes. Zeng et al. [28,29] employed the genetic algorithm to construct the optimization models based on energy, environment, and economy criteria in order to optimize the capacity and operation mode of the integrated system. The studies on CCHP and GSHP systems in these literatures [[25], [26], [27], [28], [29]] were only based on the general energy flows and the black-box models, and the specific configurations and flows were not considered. To reveal the mechanism to improve the performance, Kang et al. [30] present exergy analysis of a gas-turbine CHP-GSHP coupling system and investigated the key operation parameters to affect the performances. Dou and Wang [31] proposed a novel coupled CCHP-GSHP system and analyzed the thermodynamic performances of heating supply modes.

Differently to the reviewed literatures on CCHP and GSHP coupling system, this study is extended from our published literature [31] considering only heating mode and parameter's influences, and its originality lies in designing a hybrid CCHP and GSHP coupling system comprehensively integrated with the electricity output and the two forms of waste heat from an internal combustion engine (ICE) in cooling and heating operation modes, and focusing on its adjustable areas and performance distributions without adopting thermal energy storage to match the variable ratios of electricity to heating/cooling between supply and demand sides in different work conditions. Section 2 presents the flowchart and operational modes of the hybrid system; Section 3 discusses the construction of the thermodynamic models; Section 4 demonstrates the energy and exergy performances with the different adjustable ratio of electricity to heating/cooling of the hybrid system through a case study; and finally, Section 5 summarizes several conclusions.

Section snippets

System description

The flowchart of the CCHP and GSHP coupling system is displayed in Fig. 1, and consists of three subsystems: the ICE, GSHP, and dual-source powered mixed-effect LiBrsingle bondH2O absorption heat pump (AHP) subsystem. Natural gas (state 1) as fuel drives the ICE to generate electricity for the building (state 4) or GSHP system (state 5). The waste heat, including exhaust gas (state 13) and jacket water (states 6/18), is used to drive the AHP or assist the GSHP.

ICE

For the ICE driven by natural gas, the power generation efficiency (ηi) can be estimated as follows [32]:ηi=0.2808×(Nen)0.0563,where Nen is the nominal generation capacity of the ICE. The temperature of the gas exhausted from the ICE, Tex (K), is calculated as:Tex=2×105(Epgu)20.0707Epgu+758.33.

The heat ratio of the jacket water to exhausted gas differs with the various ICE types, and the ICE design parameters are listed in Table 2, according to a specific ICE from the Caterpillar company.

AHP

The

Validation of models

There are three main models including ICE, AHP and GSHP during the simulation of thermodynamic performance in the Engineering Equation Solver (EES) software, in which the ICE model, especially the power generation efficiency, is fitted according to the data from a series of engines of the AB group [32]. Herein, the simulation models of AHP and GSHP are validated as follows:

Because three are 20 states in the internal flows of the AHP, the average relative errors between our simulated parameters

Conclusions

In this study, a hybrid CCHP system integrated with a GSHP was designed to be applied into different cooling or heating work conditions, the thermodynamic models were constructed and validated, and the performance analyses were performed under design and off-design working conditions. The analysis focusing on the adjustable ability and thermodynamic performance distribution under variable work conditions achieved the following conclusions.

The waste heat of the jacket hot water and hot water

Acknowledgements

This study was supported by the National Natural Science Foundation of China (Grant No. 51876064) and the Fundamental Research Funds for the Central Universities (2018MS098).

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