Evaluation of a novel coupling system for various load conditions under different operating strategies

Energy Conversion and Management 109 (2016) 40–50

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Evaluation of a novel coupling system for various load conditions under different operating strategies Shushuo Kang, Hongqiang Li ⇑, Lifang Liu, Rong Zeng, Guoqiang Zhang ⇑ College of Civil Engineering, National Center for International Research Collaboration in Building Safety and Environment, Hunan University, Changsha, Hunan 410082, China

a r t i c l e

i n f o

Article history: Received 25 July 2015 Accepted 22 November 2015 Available online 10 December 2015 Keywords: Combined heating and power system Ground source heat pump Integration system Operation strategy Evaluation

a b s t r a c t In previous research, a novel coupling system, which integrates the combined heating and power system and the ground source heat pump system, is proposed and investigated. In this paper, in order to make the novel system more suitable for the buildings load demand, the performance characteristics for various load conditions are evaluated under two different operation strategies: following the thermal load and following the electricity load. For the comparison, the reference system integrated with common approach is also presented. The models for both systems are developed to examine their primary energy consumption. Energy saving ratio is selected as the evaluation criteria. Furthermore, a numerical case is given to illustrate the feasibility and availability of the novel system. The results show that the primary energy consumption of novel system is fewer than the reference system. The outlet temperature from the ground source heat pump system in novel system is the key design parameter; the lower outlet temperature can lead to a fewer primary energy consumption and a bigger energy saving ratio. Additionally, for the application of the novel system, the following the electricity load operation strategy should be preferred. Ó 2015 Elsevier Ltd. All rights reserved.

1. Introduction As an energy-efficient and environmental-friendly technology, combined heating and power system (CHP) is broadly identified as a highly efficient way to use both fossil and renewable fuels and to make a significant contribution to the sustainable energy development. The main idea of the CHP is to utilize the excess heat discharged from the power generation unit (PGU) to regenerate thermal energy [1–4]. Thus the utilization of the flue gas heat plays a significant role in the efficiency improvement of the CHP. In the CHP, the flue gas heat is always used directly by the heat exchanger (HEX) to produce heat [2,3] or generate hot water [4]. However, the flue gas heat often cannot meet the heat load demand of buildings. The additional heat needs to be generated by other equipments or systems, such as the ground source heat pump system (GSHP). The GSHP technology is paid attention from the researchers and policy makers due to its energy saving and environmental advantages [5,6]. Therefore many CHP–GSHP coupling systems are presented [7–9]. Ommen [7] gave five configurations for the CHP and GSHP system. Liu [8] proposed a coupling system which integrates CCHP and two GSHP systems. Entchev [9] presented the coupling system ⇑ Corresponding authors. Tel.: +86 731 88821040; fax: +86 731 88821004. E-mail addresses: [email protected] (H. Li), [email protected] (G. Zhang). http://dx.doi.org/10.1016/j.enconman.2015.11.045 0196-8904/Ó 2015 Elsevier Ltd. All rights reserved.

composed of GSHP and fuel cell CHP system. It is vital to find an effective system integrating method to utilize the flue gas heat and improve the system performances. Thus, based on the principle of the temperature grade counterparts and heat transfer mechanism, the novel CHP–GSHP coupling system was proposed by the authors [10]. The performance advantages and the influence characteristics of the novel system were also investigated based on the set load conditions. Because the load condition always fluctuates dramatically over time, it will be crucial to study the operation strategies and availability of the novel system under various load conditions. In general, the coupling systems are usually operated via using two basic strategies: following the electricity load (FEL) and following the thermal load (FTL) [11,12]. For the former strategy, the system can be operated according to the electricity load; while the latter one works in accordance with the thermal load. Some researchers such as Jalalzadeh-Azar [13], Mago et al. [14,15] investigated the operation of coupling systems under these operation strategies. For example, Jalalzadeh-Azar [13] analyzed the cost and primary energy consumption (PEC) of coupling system operated under FEL and FTL strategies. The results showed an 11% reduction in total energy consumption when the coupling system is operated under the FTL strategy instead of the FEL strategy. Mago et al. [14] compared FEL and FTL strategies for the coupling

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Nomenclature Abbreviations CHP combined heating and power system HEX heat exchanger GSHP ground source heat pump system FTL following the thermal load FEL following the electricity load COP coefficient of performance PEC primary energy consumption, kW ESR energy saving ratio, % E electricity load demand, kW F fuel energy, kW f part load ratio Q heat load demand, kW T temperature, °C R ratio

Subscripts chp combined heating and power system gshp ground source heat pump system b boiler pec primary energy consumption rpec PEC for the reference system npec PEC for the novel system grid grid max maximum req required energy rec heat recovery load buildings load demand f feed water pump c circulation pump w warm water el electricity

Greek symbol g efficiency

system that used an internal combustion engine as the PGU for a small office building in four different climate regions. Comparisons made are based on the PEC, cost, and carbon dioxide emissions. They concluded that, the FTL strategy generally performed better than the FEL strategy. Additionally, the energy systems are always assessed based on primary energy saving [16–18], energy consumption [13,14,18], operation cost [14,17–19] and system efficiency (electrical, thermal, and total) [20]. Mago et al. [14,15], introduced different perspectives for evaluating system performance. In their work, they demonstrated that coupling system can be evaluated based on the reduction of PEC, operational cost and carbon dioxide emission for different climate conditions. Jing et al. [18] developed a multiobjective optimization design method based on life cycle assessment, in which several objectives (energetic and environmental goals) are combined into a single objective by weighted method. Some researcher also evaluated and analyzed the benefits of systems in terms of reduction of pollutants for different applications. Some of them are: Möllersten et al. [21], Mancarella et al. [22] and Lund et al. [23], et al. In this paper, in order to make the novel system more suitable for the buildings load demand, the performance characteristics for various load conditions are evaluated under two different operation strategies. The main contributions of this paper can be outlined as follows: (1) The models for both systems (the novel system and reference system) under FEL and FTL strategies are built and calculated. The ESR is employed in order to evaluate the system performance. (2) The performance characteristics of both systems for various load conditions under two operation strategies are performed and compared. (3) The key design parameter for the novel system is revealed; and the preferred operation strategy is also suggested. The paper is structured as follows. Beside of introduction and review, the system configurations of the novel system and reference system are introduced in Section 2. In Section 3, the mathematical modeling and the evaluation criteria for both systems are presented. In Section 4, a numerical case is analyzed to illustrate the feasibility and availability of the novel system. In the last section, the main conclusions are summarized.

2. System description In this section, the novel coupling system, which integrates the CHP-subsystem and the GSHP-subsystem, is introduced. For the comparison, the reference system integrated with common approach is also presented. The distinctions between the two systems are also given. 2.1. Novel system It can be observed from Fig. 1 that: the compressed natural gas and air will be sent into combustor; after combustion the flue gas with high temperature (1040 °C) and high pressure (13 atm) flows into gas turbine to generate power; then the exit flue gas with temperature (508 °C) and pressure (1.2 atm) flows into the HEX to reheat the warm water (e.g. 35 °C) generated by the GSHPsubsystem to meet the requirement temperature (e.g. 55 °C). It should be pointed out that the temperature of warm water (e.g. 35 °C) is dropped actively from the required temperature (e.g. 55 °C), which will lead to increase the coefficient of performance (COP) of GSHP-subsystem. 2.2. Reference system A reference system is used to compare with the novel system. In the reference system, CHP-subsystem and GSHP-subsystem are operated in a parallel mode (namely in complementary pattern), in other words, both systems are operated independently. The schematic of the reference system is shown in Fig. 2. As shown in Fig. 2, the compressed natural gas and air will be sent into combustor; after combustion the flue gas with the high temperature (1040 °C) and high pressure (13 atm) flows into the gas turbine to generate power; then the exit flue gas with temperature (508 °C) and pressure (1.2 atm) flows into the HEX to generate domestic hot water at the required temperature (e.g. 55 °C). Then, the left demand of the domestic hot water at the required temperature (e.g. 55 °C) will be generated by the GSHP-subsystem. It can be found that the major differences between the systems above are as follows: First is the utilization approach of the flue gas from CHP-subsystem. Second, the GSHP-subsystem no longer generates hot water at the required temperature (e.g. 55 °C) as the reference system directly, but generates the warm water (e.g. 35 °C) instead. Third, the inlet temperature of the cool water in the HEX

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S. Kang et al. / Energy Conversion and Management 109 (2016) 40–50

Fig. 1. Schematic of the novel system.

Fig. 2. Schematic of the reference system.

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43

will be increased (e.g. from 5 °C to 35 °C). Fourth, the condenser of GSHP-subsystem and HEX need to handle the entire mass flow rate of cool water. In both systems, the electricity (Echp ) generated from the CHP-subsystem is used to fulfill the demand of buildings load and system itself (power consumption of GSHP-subsystem and pump). The electricity from the grid will be introduced when the electricity (Echp ) generated from the CHP-subsystem is not enough. The required heat (Q req ) will be met by the heat from the CHP-subsystem (Q chp ), GSHP-subsystem (Q gshp ) or the boiler (Q b ). According to the integrating mechanism of the novel system, when the total heat demand (Q req ) is given, the heat generated by GSHP-subsystem (Q gshp ) will be determined by the outlet temperature from GSHP-subsystem (T w ). For the comparison, the heat generated by the GSHP-subsystem (Q gshp ) in the reference system will be same as that in the novel system. Therefore, the analysis for reference system will be based on T w .

where Eload is the electricity load demand of buildings, kW; Egshp is the electricity consumption of GSHP-subsystem, kW; gf and gc represent the proportion that the electricity consumption from the feed water pump and circulation pump accounts for the heat transmission respectively. Therefore, the part load ratio of the CHP-subsystem f can be expressed by:

3. System model

Q rec ¼ ðF chp  Ereq Þgrec

This section presents the equations used to model both systems. The electricity efficiency of the CHP-subsystem gel can be expressed as:

For the heat load demand of buildings Q req , if ðQ req  Q gshp Þ 6 Q rec , the system PEC F pec is equal to:

E gel ¼ chp F chp

ð1Þ

where Echp is the electricity generated from the CHP-subsystem, kW; and F chp is the CHP-subsystem fuel energy, kW. The gel varies depending on the CHP-subsystem electricity load. It can be represented as [24]:

gel ¼ af 2 þ bf þ c

ð2Þ

where f is the part load ratio of the CHP-subsystem. If the maximum electricity generated from the CHP-subsystem is regarded as the Emax , f can be expressed as:

f ¼

Echp Emax

ð3Þ

The available energy Q chp from the CHP-subsystem which is not converted to electricity is

Q chp ¼ F chp  Echp

ð4Þ

Therefore, the recovered heat Q rec can be expressed as

Q rec ¼ grec ðF chp  Echp Þ ¼ grec Q chp

ð5Þ

where grec is the heat recovery system efficiency. For the GSHP-subsystem, the required electricity Egshp can be expressed as:

Egshp ¼

Q gshp COP

ð6Þ

where Q gshp is the heat generated by the GSHP-subsystem, kW; COP is the coefficient of performance of GSHP-subsystem. 3.1. Following the electricity load (FEL) For the novel system and reference system under FEL strategy, the CHP-subsystem electricity output Echp will match the required electricity load of the buildings and the system itself, that is Echp ¼ Ereq . Ereq is composed of the electricity load of buildings, the electricity consumption of GSHP-subsystem and pump, which can be expressed as:

Ereq ¼ Eload þ Egshp þ Q req gf þ Q gshp gc

ð7Þ

f ¼

Ereq Emax

ð8Þ

According to Eq. (8), the electricity efficiency of CHP-subsystem

gel can be deduced from Eq. (2).

Therefore, the CHP-subsystem fuel energy F chp can be expressed as:

F chp ¼

Ereq

ð9Þ

gel

The recovered heat Q rec , which is used to supply the heat load demand of buildings, is equal to:

F pec ¼ F chp

ð10Þ

ð11Þ

If ðQ req  Q gshp Þ > Q rec , the rest heat will be meet by the boiler Q b . Therefore,

Q b ¼ Q req  Q gshp  Q rec

ð12Þ

F b ¼ Q b =gb

ð13Þ

And the system PEC will be comprised of the fuel energy of the CHP-subsystem and the boiler.

F pec ¼ F chp þ F b

ð14Þ

For this operation strategy, the CHP-subsystem capacity Emax need to be calculated according to the following equation.

Emreq ¼ Emload þ

Q mgshp þ Q mreq gf þ Q mgshp gc COP

ð15Þ

Therefore,

Emax ¼ Emreq

ð16Þ

In Eqs. (15) and (16), Emreq is the maximum electricity load required by the buildings and the system itself, kW; Q mreq is the maximum heat load required by the buildings, kW; Q mgshp is the maximum heat generated from the GSHP-subsystem, kW. 3.2. Following the thermal load (FTL) For this operation strategy, the heat load demand of buildings must be met by the heat generated from the CHP-subsystem and GSHP-subsystem. Therefore,

Q req ¼ Q gshp þ Q rec

ð17Þ

According to Eqs. (1)-(5), the electricity generated from the CHP-subsystem Echp and the corresponding fuel energy consumption F chp can be calculated. For the electricity load demand of the buildings and system itself Ereq , it can be also verified by Eq. (7). If Ereq 6 Echp , the system PEC can be obtained, that is

F pec ¼ F chp

ð18Þ

If Ereq > Echp , the electricity from the grid has to introduced to meet the demand. Therefore,

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S. Kang et al. / Energy Conversion and Management 109 (2016) 40–50

Egrid ¼ Ereq  Echp

ð19Þ

And the system PEC can be calculated by the following equation:

F pec ¼

Echp

gel

þ

Egrid

ð20Þ

ggrid

where gel is the electricity efficiency of CHP-subsystem; and ggrid is the electricity efficiency of the plant. For this operation strategy, the CHP-subsystem capacity (Emax ) need to be calculated according to the following equations.

Q mrec ¼ Q mreq  Q mgshp

ð21Þ

ðF mchp  Emchp Þgrec ¼ Q mrec

ð22Þ

Emchp ¼ gmel F mchp

ð23Þ

By Eqs. (21)-(23), the Emchp can be obtained.

Emax ¼ Emchp

ð24Þ

In Eqs. (21)-(24), Q mreq is the maximum heat load required by the buildings, kW; Q mgshp is the maximum heat generated from the GSHP-subsystem, kW; Q mrec is the maximum heat generated from the CHP-subsystem, kW; gmel is the maximum electricity efficiency of CHP-subsystem; Emchp is the maximum electricity generated from the CHP-subsystem, kW; F mchp is the maximum CHP-subsystem fuel energy, kW. 3.3. Performance evaluation criteria In this paper, the energy-saving ratio (ESR) is employed to evaluate the performance of the both systems, which can be specified as [4,18]:

ESR ¼

F rpec  F npec  100% F rpec

ð25Þ

where ESR represents the energy saving ratio, %; F npec and F rpec represent total primary energy consumption of the novel system and reference system respectively, kW. 4. Case analysis 4.1. Calculation parameters In order to illustrate the feasibility and availability of the novel system, a numerical case analysis is given in this section. The technical parameters of both systems are listed in Table 1. It is assumed that the maximum electricity efficiency of CHP-subsystem gmel is 0.3; the heat recovery efficiency grec for the waste heat from CHP-subsystem is 0.8; the heat efficiency of the boiler is 0.9; the plant generation efficiency ggrid is 0.33. The proportion that the electricity consumption from the feed water pump gf and

circulation pump gc account for the heat transmission respectively, are assumed to be 0.05. In the novel system, the outlet temperature of GSHP-subsystem (T w ) is a key operation parameter, which play a crucial impact on the COP of GSHP-subsystem. When the outlet temperature of the GSHP at the condenser side is decreased, the condensation temperature will be lowered, the compression ratio of the refrigerant will be also reduced. Therefore, the compression power can be decreased, and thus the COP of the GSHP will be improved [25]. The corresponding relations between the outlet temperature (T w ) and the COP in the GSHP-subsystem is presented in Fig. 3, which is calculated based on the design parameters in Ref. [26]. It is assumed that the set temperature of cool water and hot water are 5 °C and 55 °C respectively; the inlet and outlet temperature of the evaporator are 15 °C and 12 °C respectively; the refrigerant of the GSHP-subsystem is R22 (formerly known as CHClF2). The maximum value of electricity load demand and heat load demand are assumed to be 600 kW and 3489 kW respectively; and thus the various load conditions is shown in Table 2, in which the line title, column title and content represent the electricity load demand, heat load demand and various load conditions labeled by 1–168 respectively. Because the load condition always fluctuates dramatically over time, the performance of both systems under all load conditions will be analyzed. 4.2. Results and discussion With the described methods and the considered data, the calculation models are developed in MATLAB software. The values of the variables used to model both systems are presented in Table 1. The various load conditions presented in Table 2 are applied for the representation of fluctuating energy demand. Therefore, this section presents the results which are obtained via using the FEL and FTL operation strategies introduced in Section 3. 4.2.1. CHP-subsystem capacity In the novel system, the heat generated by the GSHP-subsystem Q gshp and its proportion that accounts for the total heat demand Q req can be known when the T w is determined. However, the heat generated by the GSHP-subsystem Q gshp in reference system is considered to be the same as that in the novel system. Thus the T w can be seen as the key operation parameter in reference system. Therefore, when the T w are determined as 35 °C, 42.5 °C and 50 °C, the corresponding ratio Rgshp of Q gshp to Q req are 0.4, 0.25, 0.1 in the

Table 1 Technical parameters of both systems. Number

Variable

1 2 3 4 5 6 7

Electricity efficiency of CHP/kg/h Heat recovery efficiency Efficiency of the boiler Plant generation efficiency

8

Proportion of the feed water pump

9

Proportion of the circulation water pump

Symbol

Value

a b c

0.2 [24] 0.4 [24] 0.1 [24] 0.3 0.8 0.9 0.33

gmel grec gb ggrid gf gc

0.05 0.05

Fig. 3. Corresponding relations between the outlet temperature (T w ) and COP.

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S. Kang et al. / Energy Conversion and Management 109 (2016) 40–50 Table 2 Various load conditions. Electricity load demand/kW

Heat load demand /kW

3489 3198 2908 2617 2326 2035 1745 1454 1163 872 582 291 0

600

550

500

450

400

350

300

250

200

150

100

50

0

1 2 3 4 5 6 7 8 9 10 11 12 13

14 15 16 17 18 19 20 21 22 23 24 25 26

27 28 29 30 31 32 33 34 35 36 37 38 39

40 41 42 43 44 45 46 47 48 49 50 51 52

53 54 55 56 57 58 59 60 61 62 63 64 65

66 67 68 69 70 71 72 73 74 75 76 77 78

79 80 81 82 83 84 85 86 87 88 89 90 91

92 93 94 95 96 97 98 99 100 101 102 103 104

105 106 107 108 109 110 111 112 113 114 115 116 117

118 119 120 121 122 123 124 125 126 127 128 129 130

131 132 133 134 135 136 137 138 139 140 141 142 143

144 145 146 147 148 149 150 151 152 153 154 155 156

157 158 159 160 161 162 163 164 165 166 167 168 –

reference system respectively. Then, the Emax is calculated with different Rgshp in the reference system. According to the calculation models of the CHP-subsystem capacity Emax under the FEL strategy, the Emax values can be shown in Fig. 4. For the novel system, the Emax is calculated with different T w . It can be found from Fig. 4 that, the Emax decreases with the increase of T w in the novel system; accordingly, it will increase with the increase of Rgshp in the reference system. However, the Emax in the novel system is less than that in the reference system when the T w is determined. According to the calculation models of the CHP-subsystem capacity (Emax ) under the FTL strategy, the Emax value can be shown in Fig. 5. It can be found from Fig. 5 that, the Emax increases with the increase of T w in the novel system; accordingly, it will decrease with the increase of Rgshp in the reference system. However, the Emax in the novel system will be the same as that in the reference system when the T w is determined. According to the above analysis, the Emax value for the novel system under FEL and FTL strategies can be obtained respectively, which is compared in Fig. 6. When the T w is determined, the Emax value under FEL strategy is less than that under FTL strategy; namely the configuration capacity of the CHP-subsystem will be

reduced when the novel system can be operated under FEL strategy. And the Emax difference will increase with the increase of T w . 4.2.2. ESR under the FEL strategy It can be found from Figs. 7a–7c that, with the increase of T w , the ESR under the same load condition will decrease; it means that the lower T w will lead to a higher energy-saving character. The load conditions with low ESR will increase with the increase of T w . This phenomenon indicates that the T w will play a vital role on the design of the novel system. When T w is set as 35 °C, the maximum ESR (35.4%) can be obtained. In Figs. 7a–7c, it is also observed that, when the electricity load reduce gradually, the conditions (low ESR) will increase; but the maximum ESR value obtained, under the same the electricity load, will rise. The maximum ESR value can be obtained when only heat load is required, which will reduce with the increase of T w . When T w is set as 35 °C, 42.5 °C and 50 °C, the maximum ESR obtained is 35.4%, 18.3% and 11.5% respectively. Furthermore, the minimum ESR value can be got when only the electricity load is required, which will decrease with the reduction of electricity load demand. The minimum ESR obtained is 3.1%, 1.9% and 1.5% when T w is set as 35 °C, 42.5 °C and 50 °C respectively.

Fig. 4. Emax value for the novel system and reference system under FEL strategy.

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Fig. 5. Emax value for the novel system and reference system under FTL strategy.

Fig. 6. Emax value under FEL and FTL strategies for the novel system.

4.2.3. ESR under the FTL strategy It can be found from Figs. 8a–8c that, with the increase of T w , the conditions (namely ESR = 0) will increase gradually; except that, the ESR value under the same load condition will decrease gradually. Thus, the T w is also the key design parameter under FTL strategy. The maximum ESR (20.4%) can be obtained when T w is set as 35 °C, which is smaller than the one under FEL strategy. Figs. 8a–8c displays that the variation trend of ESR under FTL strategy is the same as that under FEL strategy; while when the electricity load is zero, the ESR is always zero, which will not change with the variation of heat load demand. The maximum ESR values obtained under FTL strategy (20.4%, 9.4% and 1.9%) are smaller than that under FEL strategy (35.4%, 18.3% and 11.5%) when T w is set as 35 °C, 42.5 °C and 50 °C respectively. The minimum ESR value (ESR = 0) obtained under FTL strategy is also smaller than that under FEL strategy.

4.2.4. PEC under FEL strategy for the novel system In this section, the PEC under FEL strategy for the novel system is calculated and analyzed in order to illustrate the performance character of the novel system. It can be found from Figs. 9a–9c that, with the increase of T w , the PEC will decrease under the same load condition; and under the same electricity load demand, the PEC will reduce with the decrease of heat load demand; and the reduction degree firstly increases and then decreases; while the PEC will decrease with the reduction of electricity load demand. Therefore, when T w is determined, the maximum PEC can be obtained under the maximum electricity load demand and heat load demand; the minimum PEC can be also got under the minimum electricity load demand and heat load demand. 4.2.5. Comparison In this section, in order to illustrate the advantages and performance characteristics of the novel system, the ESR value and PEC

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Fig. 7a. ESR when T w = 35 °C under FEL strategy.

Fig. 7c. ESR when T w = 50 °C under FEL strategy.

Fig. 7b. ESR when T w = 42.5 °C under FEL strategy.

Fig. 8a. ESR when T w = 35 °C under FTL strategy.

value under FEL and FTL strategies are compared and analyzed respectively.

the lower T w and the FEL operation strategy should be chosen to achieve the best system performance.

4.2.5.1. ESR difference under the FEL and FTL strategies. In order to identify the characteristics of the novel system under two operation strategies, the ESR difference under the FEL and FTL strategies is analyzed. In Fig. 10, the abscissa represents every buildings load condition points, which has been presented in Table 2; the ordinate represents the difference between the ESR under FEL and FTL strategies. It can be found from Fig. 10 that, the ESR difference is changing all the time; and the ESR under FEL strategy at almost all load conditions is higher than the ESR under FTL strategy. However, when the T w is equal to 42.5 °C, the ESR under FEL strategy at individual load conditions is smaller than the one under FTL strategy. Furthermore, the maximum ESR difference value is approximate 15.8%; the minimum value is about negative 4.6%. Therefore, for the application of the novel system, the FEL strategy should be preferred. Furthermore, it should be consider that both the load conditions and the setting of T w . Firstly, it should make sure that the building load conditions can be met. After that,

4.2.5.2. PEC difference under the FEL and FTL strategies for the novel system. For the novel system, different operation strategies and design of T w will lead to different PEC; thus their variation characteristics will be explored. It can be found from Figs. 11a–11c that, with the increase of T w , the situations (the PEC under FEL strategy is fewer than that under FTL strategy) will increase; and their PEC difference under the FEL and FTL strategies will be bigger under the same load condition. Furthermore, when the heat load demand is constant, the PEC difference will increases firstly and then reduces with the increase of electricity load demand; however, when the heat load demand is larger, the PEC difference will be always increasing, and this situation will be more obvious with the increase of T w . In Figs. 11a–11c, the maximum PEC difference appears at the minimum heat load; the minimum PEC difference can be obtained at the maximum heat load; and thus the minimum PEC difference can be got with maximum heat load demand and without electricity load demand.

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Fig. 9b. PEC for the novel system when T w = 42.5 °C. Fig. 8b. ESR when T w = 42.5 °C under FTL strategy.

Fig. 8c. ESR when T w = 50 °C under FTL strategy.

Fig. 9a. PEC for the novel system when T w = 35 °C.

Fig. 9c. PEC for the novel system when T w = 50 °C.

Fig. 10. ESR difference under the FEL and FTL strategies.

S. Kang et al. / Energy Conversion and Management 109 (2016) 40–50

Fig. 11a. PEC difference under FEL and FTL strategies when T w = 35 °C.

Fig. 11b. PEC difference under FEL and FTL strategies when T w = 42.5 °C.

Fig. 11c. PEC difference under FEL and FTL strategies when T w = 50 °C.

Fig. 12a. PEC difference under the FEL and FTL strategies when Rgshp = 0.1.

Fig. 12b. PEC difference under the FEL and FTL strategies when Rgshp = 0.25.

Fig. 12c. PEC difference under the FEL and FTL strategies when Rgshp = 0.4.

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4.2.5.3. PEC difference under the FEL and FTL strategies for the reference system. With the different heat generated by GSHPsubsystem Q gshp , the PEC under FEL and FTL strategies in the reference system is different. According to the statements in Section 4.2.1, the PEC difference under the FEL and FTL strategies will be calculated based on the ratio Rgshp of Q gshp to Q req , which is 0.1, 0.25 and 0.4 respectively. It can be found from Figs. 12a–12c that, with the increase of Rgshp , the situations (the PEC under FEL strategy is fewer than that under FTL strategy) will decrease; and their PEC difference will reduce under the same load condition. Additionally, the variation trend of the PEC difference in the reference system is same as that in the novel system.

5. Conclusions In this paper, in order to make the novel system more suitable for the buildings load demand, the performance characteristics of the novel system and reference system for various load conditions under two operation strategies are performed and compared. The main research conclusions can be outlined as follows. (1) The system models for both systems under FTL and FEL strategies are built and calculated. The ESR is employed in order to evaluate the system performance. (2) The T w (the outlet temperature from the GSHP-subsystem) for the novel system is the key design parameter. The lower T w can lead to a fewer PEC and a bigger ESR. When T w is set as 35 °C,the maximum ESR (35.4%) under FEL and the maximum ESR (20.4%) under FEL can be obtained respectively. (3) For the novel system, the CHP-subsystem capacity under FEL strategy is smaller than that under FTL strategy; the PEC under FEL strategy is also fewer than that under FTL strategy; moreover the ESR under FEL strategy is higher than that under FTL strategy. Therefore, for the application of the novel system, the FEL operation strategy should be preferred.

Acknowledgements This study is supported by the National Natural Science Foundation Project of China (No. 51541603 and No. 51408205), the International Science and Technology Cooperation Program of China (No. 2014DFE70230) and the Science and Technology Major Special Planning Project of Hunan Province (No. 2011FJ1007-1). References [1] Wu DW, Wang RZ. Combined cooling, heating and power: a review. Prog Energy Combust Sci 2006;32:459–95.

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