# Exergy Analysis of a Ground-Coupled Heat Pump Heating System with Different Terminals

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## Abstract

**:**

## 1. Introduction

^{8}m

^{2}by the end of 2011 [2].

## 2. System Description

^{2}. The heat transfer coefficients of the walls, windows and roofs are all chosen to satisfy the requirements of the China Industry Standard JGJ 134-2010 [18].

## 3. Analysis for Exergy Flow and Exergy Efficiency of Each Subsystem

#### 3.1. Ground Heat Exchange System

_{g}is the exergy input rate from ground (kW); Q

_{g}is the heat extraction rate from ground (kW); T

_{0}is the reference temperature, considered as the outdoor air temperature when the GCHP system operates (K); T

_{g}is ground temperature (K); m

_{g}is the mass flow rate of circulating water in ground loop (kg/s); c

_{w}is the specific heat of water (kJ/kg·K); T

_{in}and T

_{out}are respectively the inlet and outlet temperatures of the evaporator (K).

_{g}is the exergy loss of GHE (kW).

_{g}is the exergy efficiency of ground heat exchange system (%); E

_{P}

_{1}is the mechanical exergy input rate of ground loop pumps (kW); ${\varphi}_{P1}$ is the motor efficiency of ground loop pumps; W

_{P}

_{1}is the electric power of ground loop pumps (kW).

#### 3.2. Heat Pump

_{hp}is the exergy loss of heat pump (kW); E

_{hp}is the mechanical exergy input rate of heat pump (kW); Q

_{hp}is the heat output of heat pump (kW); m

_{h}is the mass flow rate of hot water (kg/s); T

_{sup}and T

_{re}are respectively the supply and return water temperatures of the condenser (K).

_{hp}is the exergy efficiency of heat pump; ${\varphi}_{com}$ is the motor efficiency of screw compressor; W

_{hp}is the electric power of heat pump (kW).

#### 3.3. Heat Distribution System

_{hd}is the exergy loss of heat distribution (kW); γ

_{hd}is the heat loss ratio of heat distribution network (%). The total heat distribution distance is as short as 120 m. For simplicity, the heat loss ratio of heat distribution network is taken to be 1.5%.

_{hd}is the exergy efficiency of heat distribution system (%); Q

_{H}is the heating load of buildings (kW); E

_{P}

_{2}is the mechanical exergy input rate of load side pumps (kW); ${\varphi}_{P2}$ is the motor efficiency of load side pumps; W

_{P2}is the electric power of load side pumps (kW).

#### 3.4. Terminals

_{ter}is the exergy loss of terminals (kW); T

_{a}is indoor air temperature (K).

_{rf}is the exergy efficiency of radiant floors (%).

## 4. Results and Discussion

#### 4.1. Test Results

#### 4.2. Exergy Losses and Exergy Efficiencies

#### 4.3. Comparisons between Radiant Floors and FCU

#### 4.3.1. Comparison of the Energy Performance

_{i}is temperature correction factors; U

_{i}is the heat transfer coefficient of building envelope (kW/m

^{2}·K); A

_{i}is the area of building envelope (m

^{2}); N is air exchange rate of rooms (times per hour); c

_{a}is the specific heat of air (kJ/kg·K); ρ is air density (kg/m

^{3}); V is interior volume of rooms (m

^{3}). Above equation indicates that heating load is proportional to indoor and outdoor temperature difference. For the same two buildings in this case, the decrement rate of heating load (DRHL) resulting from the adoption of radiant floors can be calculated as follows:

#### 4.3.2. Comparison of the Exergetic Performance

_{FCU}is the electric power of FCU (kW); Q

_{FCU}is the heat transfer rate of FCU (kW).

_{FCU}is the exergy efficiency of FCU (%); ${\varphi}_{FCU}$ is the motor efficiency of FCU and equal to 80%. Suppose that the values of T

_{0}, T

_{sup}and T

_{re}are equal to the test results, the exergy efficiency of FCU will be 37.07%. The exergy efficiency of radiant floors is 3.24% higher than that of FCU due to the fact that the radiant floors operate without electric power. If FCU were substituted for the radiant floors in this GCHP heating system in terms of the test results, the total exergy efficiency of system would drop to 13.7%. Besides the advantages in thermal comfort and energy saving compared with FCU, the adoption of radiant floors can bring a certain increase in the total exergy efficiency of system.

## 5. Conclusions

- The heating load and exergy efficiency of the GCHP heating system with radiant floors are compared with those of the scenario system in which FCU are substituted for the radiant floors. The comparison results show that the adoption of radiant floors can result in a decrease of 12% in heating load, an increase of 3.24% in exergy efficiency of terminals and an increase of 1.18% in total exergy efficiency of system.
- The analysis results indicate that the largest exergy loss occurs in heat pump, and the second largest exergy loss occurs in terminals. The ground heat exchange system has the lowest exergy efficiency among the four subsystems. Designers of GCHP systems should pay close attention to the selection of heat pumps and terminals, especially to the design of ground heat exchange systems.
- Both load side pumping energy and source side pumping energy have obvious influence on the energy and exergetic performance of a GCHP heating system. The total exergy efficiency of a GCHP heating system will decrease with the increase of reference temperature.

## Acknowledgments

## Nomenclature

A_{i} | area of building envelope (m ^{2}) |

c | specific heat (kJ/kg·K) |

E | exergy (kW) |

I | exergy loss (kW) |

K_{i} | temperature correction factors |

m | mass flow rate (kg/s) |

N | air exchange rate of rooms (times per hour) |

Q | heat transfer rate (kW) |

T | temperature (K) |

U_{i} | heat transfer coefficient of building envelope (kW/m ^{2}·K) |

V | interior volume of rooms (m ^{3}) |

W | electric power (kW) |

Greek Letters | |
---|---|

η | exergy efficiency (%) |

ϕ | motor efficiency (%) |

ρ | air density (kg/m ^{3}) |

γ | heat loss ratio (%) |

Subscripts | |
---|---|

a | air |

com | screw compressor |

FCU | fan coil units |

g | ground |

H | heating |

h | hot water |

hd | heat distribution |

hp | heat pump |

in | inlet of the evaporator |

out | outlet of the evaporator |

P1 | ground loop pumps |

P2 | load side pumps |

re | return water |

rf | radiant floor |

ro | rooms |

sup | supply water |

ter | terminals |

w | water |

0 | reference environment condition |

## Author Contributions

## Conflicts of Interest

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Measured Parameters | Average Value |
---|---|

Inlet temperature of the evaporator (°C) | 9.2 |

Outlet temperature of the evaporator (°C) | 6.1 |

Supply water temperature of the condenser (°C) | 44.2 |

Return water temperature of the condenser (°C) | 40 |

Flow rate of the circulating water in GHE (m^{3}/h) | 41.6 |

Flow rate of the hot water (m^{3}/h) | 40.3 |

Electric power of the heat pump (kW) | 58.1 |

COP (Coeffient of Performance) of the heat pump | 3.39 |

Electric power of the ground loop pumps (kW) | 3.8 |

Electric power of the load side pumps (kW) | 4.4 |

Ground temperature (°C) | 16.9 |

Average outdoor air temperature (°C) | 2.8 |

Average indoor air temperature (°C) | 17.4 |

Parameters | Calculated Value |
---|---|

Heat output of heat pump (kW) | 195.7 |

Heat extraction rate from ground (kW) | 150 |

COP (Coeffient of Performance) of the heat pump | 3.39 |

Heating load of the buildings (kW) | 192.8 |

Heat loss rate of heat distribution network (kW) | 2.9 |

Subsystems | Exergy Efficiency (%) |
---|---|

Ground heat exchange system | 24.63 |

Heat pump | 45.66 |

Heat distribution system | 85.47 |

Terminals (Radiant floors) | 40.31 |

© 2015 by the authors; licensee MDPI, Basel, Switzerland This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution license (http://creativecommons.org/licenses/by/4.0/).

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**MDPI and ACS Style**

Chen, X.; Hao, X.
Exergy Analysis of a Ground-Coupled Heat Pump Heating System with Different Terminals. *Entropy* **2015**, *17*, 2328-2340.
https://doi.org/10.3390/e17042328

**AMA Style**

Chen X, Hao X.
Exergy Analysis of a Ground-Coupled Heat Pump Heating System with Different Terminals. *Entropy*. 2015; 17(4):2328-2340.
https://doi.org/10.3390/e17042328

**Chicago/Turabian Style**

Chen, Xiao, and Xiaoli Hao.
2015. "Exergy Analysis of a Ground-Coupled Heat Pump Heating System with Different Terminals" *Entropy* 17, no. 4: 2328-2340.
https://doi.org/10.3390/e17042328