Energy efficiency indicators for combined cooling, heating and power systems

Highlights

• This study conducts an innovative comparative analysis of energy indicators.

• Primary energy rate and primary energy saving rate have same optimization effect.

• Annual average comprehensive energy utilization efficiency cannot be lonely used.

• Larger value of primary energy rate means reduction of carbon dioxide emissions.

Abstract

Combined cooling, heating and power system is an effective technology for energy conservation and environmental protection, especially for buildings with multiple demands on electricity, cooling and heating load. Various energy indicators have been proposed for evaluating and optimizing the energy efficiency of combined cooling, heating and power systems, but previous studies have not yet clarified how to use them in a reasonable way that takes their specific merits into account. This paper aims to identify the underlying effects of three common energy indicators, including the primary energy rate, primary energy saving rate, and annual average comprehensive energy utilization efficiency, based on the optimized performance of combined cooling, heating and power systems with different typical configurations. This paper proposes three combined cooling, heating and power system configurations for an office building in Kunming to conduct a comparative analysis of three energy indicators. Two common operation strategy (the following thermal load and following electricity load) and the genetic algorithm are selected to performed single-objective optimization with energy indicator as target. The economic and environmental analysis is also carried out to show the impact of energy indicators on other indicators. The results show that, the indicators primary energy rate and primary energy saving rate have the same optimization effect of energy consumption when the system configuration and operation strategy are identical, as the difference between the primary energy rate values is not more than 1%, and the difference between the primary energy saving rate values is not more than 0.5%. The advantage of primary energy rate is that the difference between the primary energy rate values of combined cooling, heating and power systems and reference system could show whether carbon dioxide emissions are reduced, and the advantage of primary energy saving rate is that its value directly indicates whether the system achieves energy-saving or not. Furthermore, the annual average comprehensive energy utilization rate is not suitable for the single-objective optimization of combined cooling, heating and power systems. This study distinguishes the differences between indicators and the conclusions are of general significance which could provide valuable guidance for designers to select optimal energy indicators applying to the optimization design of combined cooling, heating and power systems.

Introduction

Combined cooling, heating and power (CCHP) system is broadly regarded as an energy-efficient and environmental-friendly technology as its capabilities of waste heat utilization and flexible arrangement close to the users [1]. A common CCHP system consists of the power generation unit (PGU), absorption chiller (AC), waste heat recovery device, auxiliary boiler (AB) and heat exchanger (HE). Differing from the conventional separation production (SP) system, in CCHP systems, user’s electricity demand is met by power generation units and the waste heat of power generation units can be simultaneously utilized for cooling and heating [2]. However, the mismatch of thermal and electrical demands of buildings limits the performance of the CCHP system [3].

The load demands vary with climate and building types and have considerable impact on the performance of CCHP system, so, the optimal design based on the given situation is essential to make sure that the CCHP system is economic, energy-saving and environmental-friendly. A good design of CCHP system covers several aspects, such as the system structure, operation strategy and equipment capacity optimization [4]. In the aspect of system structure, the compositions of CCHP systems are more and more plentiful. On the one hand, solar energy [5], wind energy [6], geothermal energy and biomass energy [7] are gradually integrated into CCHP systems, which enriches the system’s energy sources and reduces the input of fossil energy. On the other hand, many attempts have been made to combine CCHP systems with technologies such as energy storage, heat pump, electric chiller and Organic Rankine Cycle (ORC) [8]. Results show that these methods are beneficial to improve the system performance and the matching degree of system output to user requirements [9]. As for operation strategy and equipment capacity, they both have a significant influence on system energy performance, and the equipment capacity depends heavily on the operating strategy [10]. Therefore, choosing an appropriate operation strategy is critical to the optimal design of CCHP systems. There are two common operating strategies for CCHP systems which are following thermal load (FTL) and following electric load (FEL) [11]. The FTL strategy prioritizes the supply of thermal load, while the FEL strategy tries to match the electricity. However, the load demands are variable, the two basic operation strategies fail in coping with serious imbalance between electric and thermal loads and cause a waste of power and thermal energy. Some studies have proposed improved operation strategies to decrease an extra investment in energy storage devices [12]. Mago et al. [13] proposed a following hybrid electric-thermal load (FHL) operation strategy which switches between two common operation strategies in order to reduce the excess energy. Wang et al. [14] made a comparative analysis of three operation strategies (FTL, FEL and FHL) in a Beijing hotel, and found that there was poor off-design performance under FHL strtegy, although excess electricity and heat were reduced. Han et al. [15] innovatively put forward a compromised electric-thermal strategies (CET) based on the FHL strategy using the efficacy coefficient method. The whole year operation results indicated that the CET strategy had advantages in dealing with operation cost, carbon dioxide emission and exergy efficiency. A seasonal operation strategy coupled with seasonal adjustment of the cooling/heating share provided by ground source heat pump in CCHP system was developed by Ma et al. [9]. However, the output of critical equipment is constantly adjusted under above improved operation strategies, which affects the lifetime of CCHP system and increases the difficulty of control, so the FTL and FEL strategies are still the most commonly used operation strategies.

The purpose of optimization design is to obtain better performance compared to the reference system, and the optimization effect of CCHP systems is reflected by the optimization indicators [16]. The performance of CCHP systems involve a wide range of aspects, such as economy, energy, environment and society, etc. Recently, the economic, energy and environmental performance are the most important concerns of researchers and investors [17]. In terms of economic performance, annual total cost saving rate, operating cost saving rate, net present value and investment payback period are the most commonly used indicators which analyze the cost saving effect from different aspects [18]. Since CCHP systems are driven by natural gas, which is a clean fuel and the main emission of combustion is carbon dioxide, the carbon dioxide emission reduction rate is generally chosen as the environmental indicator [19]. Indicators used to evaluate the energy performance include thermal efficiency, primary energy rate (PER) [20], primary energy saving rate (PESR) [21], energy utilization factor (EUF) [22] and artificial thermal efficiency (ATE) [23]. Since the initial investment of CCHP systems is usually higher than that of the SP system, there seems to be a conflict between the system’s energy performance and economic performance. Subsequently, many researchers try to strike a balance between energy conservation and saving cost [24]. However, with the deterioration of natural environment and the sharp decrease of resources, energy conservation is an increasingly important concern compared to economic saving [25].

Primary energy rate, the ratio of the effective energy to the primary energy consumption, was introduced by Fumo et al. [26] as a new parameter to evaluate energy performance of CCHP system. Results showed that the PER takes all types of input energy in CCHP system and the difference in conversion from site-to-primary energy of the electricity and fuel into consideration and is a comprehensive energy indicators. The primary energy saving rate is a percentage of the reduction in energy consumption of the CCHP system compared to the SP system [27]. One of the advantages of PESR is that its value directly indicates whether the CCHP system saves energy or not, that is, if the value of PESR is greater than 0, the system is energy-saving. The annual average comprehensive energy utilization efficiency, a form of energy utilization factor, refers to the ratio of the system energy output to the energy input throughout the year, and was proposed by the Ministry of Housing and Urban–Rural Development of the People’s Republic of China used to judge whether a district is suitable for a CCHP system [28]. Though the three energy indicators mentioned have been frequently used in the optimization and evaluation of CCHP systems, few studies have explained the reasons for the selection of indicators, nor conducted comparative studies on the indicators. Therefore, it is not certain whether there are differences in the optimization results of three indicators. In addition, there is no authoritative selection standard for researchers and designers which results the choosing of energy indicators greatly depends on personal preferences, that may lead to the unreasonable design and optimization.

This study aims to make some improvements in terms of energy optimization indicators. Focusing on the shortcomings of the existing researches, an in-depth comparative analysis of energy optimization indicators is conducted to clarify the characteristics and application situations of the three indicators. Based on common CCHP system (consisting of the PGU, AC, waste heat recovery unit, AB and HE), CCHP system with battery under FTL strategy and CCHP system with heat storage unit under FEL strategy are also proposed. Single-objective optimization (avoiding the mutual influence between indicators) is performed on the above CCHP systems, with one of the three energy indicators as the only goal in turn and taking the genetic algorithm (GA) as optimization tool. In addition, the economic and environmental analysis is conducted to analyze the impact of energy indicators on other indicators. This study distinguishes the differences between indicators and provides valuable guidance for designers to select optimal energy indicators applying to the optimization design of CCHP systems.

This paper is organized as follows. Section 2 describes the schematics and energy flows of CCHP systems, the optimization criteria and optimization method. Section 3 introduces the details of users’ demand, operation strategies and optimized cases. Section 4 gives discussions about the optimization results and the intrinsic differences between energy indicators. Main findings are concluded in the last section.