# Analysis and Assessments of Combined Cooling, Heating and Power Systems in Various Operation Modes for a Building in China, Dalian

^{*}

## Abstract

**:**

_{exergy}), and CO

_{2}emission reduction (CER) are defined to evaluate the performances of CCHP systems for a hypothetical building located in Dalian (China). The results indicate that: (1) a new mathematical foundation is established to find whether the recovered thermal energy and the amount of electricity generated by the power generation unit (PGU) are enough to provide the energy required; (2) the CCHP system does not always perform better than a HVAC system from an instantaneous perspective, especially in FTL mode; (3) the CCHP system in FEL operation mode can be seen as a suitable energy system in Dalian from the annual performance perspective. Furthermore, a sensitivity analysis is presented in order to show how the performances vary due to the changes of various technical variables.

## 1. Introduction

_{2}emissions reduction as a criterion to evaluate the performance of CCHP system, like Kari and Arto [16], Nelson et al. [17], Gianfranco and Pierluigi [18], Monica et al. [19], Lund et al. [20], Minciuc et al. [21], Pierluigi and Gianfranco [22], Neil and Alexander [23].

_{e}, efficiency of HVAC system ${\eta}_{e}^{HVAC}$ and the exhaust gas temperature (T

_{h}) of boiler export are introduced as a changing variable to generate a sensitivity analysis to show how the optimal operation strategy would vary.

## 2. Descriptions of HVAC System and CCHP System

#### 2.1. HVAC System

**HVAC**system used as a reference system.

_{building}is the electricity for lights and equipment in building; E

_{p}is parasitic electricity such as the energy consumption of pumps and fans; and E

_{c}is the electricity supplied to the chiller, which is employed to produce space cooling. The electricity needed by the chiller can be replaced by:

_{c}is the thermal energy for space cooling; and COP

_{e}is the coefficient of performance (COP) of the electrical chiller.

_{h}is the thermal energy for space heating and domestic hot water, which comes from gas boiler and is distributed to user through heating exchanger. The fuel energy consumption for the heating system is computed as:

_{b}is the output heat from the boiler, and ${\eta}_{b}^{HVAC}$ and ${\eta}_{h}^{HVAC}$ are the boiler efficiency and the heating coil efficiency, respectively.

#### 2.2. CCHP Model

_{2}emission reductions due to avoided electricity use in compression cooling chillers is larger than the increase due to DH (industrial excess heat associated with zero emissions) and electricity used in AC chillers [26]. Therefore in order to simplify the analysis, it is possible to calculate the Q

_{ch}values basing on the assumption to cover the entire cooling load by the absorption chiller [27,28]. This section describes the model used to calculate the energy consumption for the CCHP system [2,29,30,31].

_{grid}+ E

_{pgu}= E

_{building}+ E

_{p}= E

_{demand}

_{grid}is the electricity from grid; E

_{pgu}is the generated power by PGU; and E is the electricity consumption of lights and equipment, E

_{building}, plus the electricity consumption of pumps and fans, E

_{p}.

_{pgu}, can be estimated as:

_{e}is the fuel to electricity efficiency of the PGU.

_{r}, can be calculated to:

_{r}+ Q

_{b}= Q

_{ch}+ Q

_{hc}

_{h}is the supplementary heat from boiler. While Q

_{ch}and Q

_{hc}are the heat supplied to cooling system for space cooling and heating coil for space heating, respectively.

_{ch}represent the coefficient of performance of the absorption chiller; and η

_{h}is the efficiency of heating coil.

_{b}is the auxiliary boiler efficiency.

_{pgu}+ F

_{b}

- (1)
- The CCHP equipment can operate anywhere between 0% and 100% of its rated capacity, and ramping rate for load adjustment is not included;
- (2)
- The CCHP system is assumed to be 100% reliable;
- (3)
- The efficiency drops of CCHP equipments at part load operation are neglected to simplify the analysis and calculations.

_{grid}= 0. The total fuel energy consumption is expressed to [3,6]:

_{pgu}= ρQ

_{r}

_{req}< ρQ

_{req}, the system operates in FEL mode and the electricity generated and the heat recovered by the system can be estimated by:

_{pgu}= E

_{req}

_{req}> ρQ

_{req}, the system operates in FTL mode and the electricity generated and the heat recovered by the system can be determined by:

_{pgu}= ρQ

_{req}

_{r}= Q

_{req}

_{grid}= E

_{req}− ρQ

_{req}

## 3. Optimization Criteria

#### 3.1. Primary Energy Saving

#### 3.2. Exergy Efficiency

_{exergy}is the exergy efficiency, A

_{e}, A

_{c}, A

_{h}and A

_{f}are the exergy coefficient of electricity, cooling , heating and fuel respectively, and:

_{0}is the ambient temperature; T

_{c}and T

_{h}are the cold water temperature and the heat thermal temperature respectively; T

_{c}and T

_{h}are assumed to be constant and 280 K (7 °C) and 433 K (160 °C), respectively. T

_{0}is not constant and it changes with time.

#### 3.3. CO_{2} Emission Reduction

_{2}emission (CE) from a CCHP system depends strongly on the site energy consumption and the emission conversion factors for electricity and natural gas. It can be determined as follows [6]:

^{CCHP−FEL}= F

^{CCHP−FEL}α

_{f}

^{CCHP−FTL}= F

^{CCHP−FTL}α

_{f}+ E

^{CCHP−FTL}α

_{e}

_{f}and α

_{e}are the emission conversion factors of natural gas and the electricity from grid.

## 4. Analysis and Discussion

#### 4.1. Building Description and Energy Demand

^{2}and an average main ceiling height of 3.6 m. The hourly energy demands of the building are estimated by the DeST software. General information of this building is presented in Table 1.

Location | Dalian China | |
---|---|---|

Building type | Hotel | |

Total area | 3467 m^{2} | |

People | 2 for everyday | |

Room | Occupancy schedule | Until ^{a} (fraction) ^{b}: 7(1), 9(0.5), 11(0.3), 12(0.1), 14(0.5), 16(0.3), 18(0.1), 21(0.3), 23(1) |

Electric equipment | 20 W/m^{2} | |

Equipment schedule | Until ^{a} (fraction) ^{b}: 6(0.1), 8(0.3), 11(0), 13(0.3), 17(0), 22(1), 23(0.3) | |

Lights | 15 W/m^{2} | |

Lights schedule | Until ^{a} (fraction) ^{b}: 5(0), 6(0.2), 15(0.1), 17(0.3), 21(1), 22(0.7), 23(0.5) | |

Thermostat set point | Winter ^{c}:18–22 °C ; Summer ^{d}: 24–26 °C; Spring and autumn ^{e}: 21–24 °C |

^{a}Indicates the hour of the day until the specified fraction is considered;

^{b}indicates the fraction of the total value of the variable that is considered in the calculation for that specific period of time;

^{c}Winter: November 15—April 5;

^{d}Summer: June 1—August 31;

^{e}Spring and autumn: April 6—May 31, and 1 September—14 November.

**Figure 4.**The simulated daily energy demands based on DeST software in representative (

**a**) winter and (

**b**) summer days.

System | Variable | Symbol | Value |
---|---|---|---|

CCHP system | PGU efficiency | η_{e} | 32% |

Heat recovery system efficiency | η_{rec} | 80% | |

Heating coil efficiency | η_{h} | 80% | |

Absorption chiller coefficient of performance | COP_{ch} | 0.7 | |

Boiler efficiency | η_{b} | 80% | |

HVAC system | PGU efficiency | ${\eta}_{e}^{HVAC}$ | 25% |

Heating coil efficiency | ${\eta}_{h}^{HVAC}$ | 80% | |

Vapor compression coefficient of performance | COP_{e} | 3 | |

Boiler efficiency | ${\eta}_{b}^{HVAC}$ | 80% | |

Grid transmission efficiency | η_{grid} | 95% | |

CO_{2} emission conversion factor (g/KWh) | Electricity from the grid | α_{e} | 923 |

Gas | α_{f} | 220 |

#### 4.2. Instantaneous Performance Analysis

_{req}= E

_{pgu}

_{winter}= η

_{h}η

_{rec}(1 − η

_{e})/η

_{e}and ω

_{summer}= COP

_{ch}η

_{rec}(1 − η

_{e})/η

_{e}. From Table 2, ω

_{winter}is approximately equal and about 1.36. Following electric operation, the supplemental heat from the auxiliary boiler is needed when the ratio of heating load to electricity load is larger than 1.36. While in FTL mode, additional electricity from the grid is needed when the ratio of heating load to electricity load is less than 1.36. In summer, the variable, ω

_{summer}, is about 1.19.

**Figure 5.**Ratio of thermal demand to electricity and PES of different operation mode in representative (

**a**) winter and (

**b**) summer days based on simulated data.

Operation mode | Needing supplemental heat | Heat enough | Needing additional electricity | Electricity enough | |
---|---|---|---|---|---|

Winter | FEL | 2–7, 9–13, 15, 17–18, 24 | 1, 8, 14, 16, 19–23 | - | - |

FTL | - | - | 1, 8, 14, 16, 19–23 | 2–7, 9–13, 15, 17–18, 24 | |

Summer | FEL | 1–6, 12–15, 21–24 | 7–11, 16–20 | - | - |

FTL | - | - | 7–11, 16–20 | 1–6, 12–15, 21–24 |

_{2}emissions during most of the day, which implies FHL is not a better operation mode than FEL and FTL in CO

_{2}emissions reduction, although it offers the best performance in terms of primary energy savings. Furthermore, there are even some negative values in FTL mode, which indicates that the application of a CCHP system does not always reduce carbon dioxide emissions compared to a HVAC system from an instantaneous performance view point.

**Figure 6.**Exergy efficiencies of HVAC system and different operation modes of CCHP system in representative (

**a**) winter and (

**b**) summer days based on simulated data.

**Figure 7.**CER of different operation mode in representative (

**a**) winter and (

**b**) summer days based on simulated data.

#### 4.3. Monthly and Annually Performance Analysis

_{0}is close to T

_{c}, which could lead to low cooling exergy. It is also observed that the exergy efficiency in FHL operation mode approximately follows the electric load during heating and cooling season and follows the thermal load during the transition season, which is in agreement with the above PES research results.

**Figure 8.**The simulated electric, cooling, and heating loads based on DeST software for the reference building in Dalian, China.

**Figure 9.**Variation of the primary energy saving based on simulated data for three basic CCHP operation strategies: FEL, FTL, and FHL.

**Figure 10.**The exergy efficiency of HVAC system and CCHP system in FEL, FTL, and FHL mode based on simulated data.

_{2}emissions in China.

_{2}emissions by 42.6% and the exergy efficiency is 38.65% (see Table 4). Although FEL operation mode might not offer the best results in terms of primary energy consumption, it is really valuable for buildings that must address environmental concerns. Therefore, a CCHP system in FEL operation mode can be seen as a suitable energy system for Dalian from a annual performance perspective. The result is contrary to the result presented by Mago [4]. In Mago’s paper, the excess electricity generated by PGU can be translated into additional primary energy saving for being sold back to grid in FTL operation mode. However, usually the sale of surplus generated electricity from the CCHP system back to grid in China is not allowed when the CCHP system operates FTL mode. Therefore the excess electricity in China cannot contribute to the primary energy savings, and the results of this paper are in agreement with the results obtained by Wang, who focused on the Chinese case [6].

Operation mode | PES | η_{exergy} | CER |
---|---|---|---|

FEL | 23.1 | 38.65 | 42.6 |

FTL | 22.3 | 38.27 | 36.5 |

FHL | 23.7 | 38.94 | 38.7 |

#### 4.4. Sensitivity Analysis

_{e}) of CCHP system, efficiency of HVAC system ${\eta}_{e}^{HVAC}$ and the exhaust gas temperature (T

_{h}) of boiler export.

_{e}and ${\eta}_{e}^{HVAC}$, are used to analyze its primary energy saving ratio compared to the HVAC system. As shown in Figure 12a, the current generation efficiency range of the micro gas turbine is about 25%–32% and comparable HVAC systems with different efficiencies (35%, 38% and 40%) are provided as baseline. The above values are taken into the original calculation and the performance of CCHP system will change with the different values of the variables used. It can be integrally found that the primary energy saving increases with the increasing of η

_{e}in all operation modes and higher energy savings are obtained from FHL operation mode, mainly due to the low PGU efficiencies of the HVAC system. Moreover, with the increasing of PGU generation efficiency (η

_{e}), the PES values of the three modes become more and more close. ${\eta}_{e}^{HVAC}$, as a reference value, is another key factor when calculating primary energy savings. From Figure 12a, the PES decreases with the increase of ${\eta}_{e}^{HVAC}$. Additionally, it can be seen that when the PGU generation efficiency is 25% and the electricity efficiencies of HVAC system is 40%, the CCHP system can save at least 0.3% primary energy. However, if the generation efficiency of the HVAC system can reach above 40%, the PES of CCHP system will be negative in FEL mode and the CCHP system doesn’t really save energy when other parameters are kept constant.

_{exergy}) is related with the exhaust gas temperature (T

_{h}) of boiler effluent. It can be found that in Figure 12b that η

_{exergy}increases with the increasing of Th in all three operation modes. From Section 2, there is A

_{h}= 1 − T

_{0}/T

_{h}, and the increase of T

_{h}can make A

_{h}increase as T

_{0}is constant. Consequently the heating exergy and exergy efficiency are directly increased. Figure 12b shows that the range of fluctuation of η

_{exergy}is [35.36%, 37.17%] with the exhaust gas temperature changing from 150 °C (423 K) to 200 °C (473 K) in the three operation modes and FHL operation mode has the higher exergy efficiency than the other two operation modes.

**Figure 12.**The sensitivity analysis of CCHP system based on simulated data in FEL, FTL and FHL mode.

## 5. Conclusions

_{exergy}) and CO

_{2}emissions reductions (CER). As an illustrative example, a hypothetical hotel building located in Dalian, China, has been examined for a case study. According to the simulation results, the following conclusions can be deduced.

_{2}emissions reduction compared to FHL mode, which is really valuable for buildings in Dalian that are required to address environmental concerns. Therefore a CCHP system in FEL operation mode can be seen as a suitable energy system for Dalian from a monthly performance perspective. Additionally since the excess electricity from the CCHP system cannot be sold back to the grid, the operation of a CCHP system following electricity load is a better mode.

_{exergy}in all operation modes (FEL, FTL, FHL) and higher energy savings are obtained from operation in FHL mode, mainly due to the low PGU efficiencies of the HVAC system (${\eta}_{e}^{HVAC}$). The improvement of exhaust gas temperature T

_{h}of boiler export in CCHP system can also obviously make exergy efficiency increase.

## Acknowledgments

## Nomenclature

CCHP | Combined cooling heating and power |

HVAC | Heating, ventilating, and air conditioning |

PGU | Power generation unit |

PEC | Primary energy consumption |

PES | Primary energy saving |

FESR | primary energy saving ratio |

CE | CO _{2} emissions |

CER | CO _{2} emissions reduction ratio |

FEL | Following electric load |

FTL | Following thermal load |

FHL | Following hybrid electric-thermal load |

## Symbols

E | Electricity consumption |

EX | Exergy |

F | Fuel |

Q | Heat consumption |

T | Temperature |

A | exergy coefficient |

η | efficiency |

α | Conversion factor |

γ | the coefficient of the supplementary fuel |

δ | the coefficient of the additional electricity |

ρ | Electricity-heat constant |

## Subscripts

e | Electricity |

grid | Electricity grid |

pgu | Power generation unit |

building | Reference building |

grid | Electricity grid |

p | pump |

b | Boiler |

c | Cool |

h | heat |

ch | Absorption chiller |

hc | Heating coil |

rec | Waste heat recovery system |

r | Recovery heat |

req | require |

exergy | The exergy of each type energy |

f | fuel |

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## Share and Cite

**MDPI and ACS Style**

Li, M.; Mu, H.; Li, H.
Analysis and Assessments of Combined Cooling, Heating and Power Systems in Various Operation Modes for a Building in China, Dalian. *Energies* **2013**, *6*, 2446-2467.
https://doi.org/10.3390/en6052446

**AMA Style**

Li M, Mu H, Li H.
Analysis and Assessments of Combined Cooling, Heating and Power Systems in Various Operation Modes for a Building in China, Dalian. *Energies*. 2013; 6(5):2446-2467.
https://doi.org/10.3390/en6052446

**Chicago/Turabian Style**

Li, Miao, Hailin Mu, and Huanan Li.
2013. "Analysis and Assessments of Combined Cooling, Heating and Power Systems in Various Operation Modes for a Building in China, Dalian" *Energies* 6, no. 5: 2446-2467.
https://doi.org/10.3390/en6052446