# Experimental Study and Modeling of Ground-Source Heat Pumps with Combi-Storage in Buildings

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## Abstract

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## 1. Introduction

#### Objectives

- Presenting a novel modular heat pump testbed design that emulates a complete residential house. It includes a ground-source emulator, combi-buffer heat storage, and a building load emulator. The testbed is designed to be integrated with different heat pump types and hydraulic connections so that it can be used for standardization applications, control and optimization methods performance testing, and models validation
- Based on multiple experimental testing, the real-life optimal control criteria for a commercial, residential GSHP under the given constraints of the heat pumps manufactures have been identified.
- Demonstrating a Modelica-based heat pump model that can be easily integrated into building and district simulations due to its minimal computational requirements. The model was also validated and calibrated based on the experimental data of the presented testbed.

## 2. Experimental System Description

#### 2.1. Overview

#### 2.2. Module A: Ground-Source Emulator

#### 2.3. Module B: Combi-Storage Module

#### 2.4. Module C: Building Loads Emulator

#### 2.5. Measurement System

#### 2.6. System and Control Dynamics

## 3. Experimental Testing Procedure

## 4. Modelica Based Model

- Simplicity: the model has to be easily computable as the building modeling software such as the Modelica and TRNSYS are not yet powerful enough to solve the equations of multiple complicated dynamic systems simultaneously
- Accuracy: the model has to minimize the uncertainties of the results
- Dynamics: the model should not be concealing the dynamic behavior of the heat pump under different operating conditions.

## 5. Results

#### 5.1. Experimental Analysis

#### 5.1.1. System Performance

#### 5.1.2. Sensors Position

#### 5.1.3. Cycling Influence on the System Performance

- The long operation duration to minimize the heat pump number of starts reduces the average COP and consequently can lead to a lower seasonal performance factor (SPF)
- If the heat pump is delivering directly while minimally using the heat storage or without a heat storage, the long duration of operation has no impact on the average COP of the system

#### 5.2. Model Validation

## 6. Conclusions

- DHW/SH sensor position influence the number of starts and might lead to short cycling, yet it is not the main parameter influencing the COP
- Tank set temperature has a direct impact on COP. Thus, for the same required supply temperature, having a sensor at a higher position along with a high set temperature could be exactly equal to having a sensor at a lower position with a low set temperature
- Short cycles do not always lead to a lower COP, it can increase the average COP of the system as it maintains a lower temperature in the tank
- In case the heat pump is delivering directly to the building without storage, or once there is a consumption from storage, the long or short cycles do not have an impact on the COP
- A higher number of starts might lead to a shorter life of the compressor. Consequently, a cost of start has to be included to balance the benefit of the higher COP with short cycles. Otherwise, the EMS might tend to increase the number of starts per day of the heat pump, if no flexibility is required from the grid

## Author Contributions

## Acknowledgments

## Conflicts of Interest

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**Figure 3.**Inlet and outlet pipes positions of the storage (left) sensors position across the combi-storage (right). Technical design of the storage [34].

**Figure 4.**Starting dynamics of the heat pump testbed, (

**a**) heat pump (HP) thermal and electrical power, in addition to the brine thermal power (

**b**) brine supply temperature dynamics because of the mixer circuit.

**Figure 5.**Control dynamics of module C, (

**a**) supply and return temperature of the space heating circuit (

**b**) the flow rate of the space heating circuit.

**Figure 9.**Sensor location influence on the number of starts, average COP, and average tank temperature.

**Figure 10.**Cycling effect on the heat pump system performance, (

**a**) a constant continuous load is maintained throughout the day (

**b**) a constant return temperature is maintained throughout the day.

**Figure 12.**Temperatures and power dynamics of both the simulation model and the measurements of the testbed, (

**a**) thermal and electrical power (

**b**) supply and return temperatures.

**Figure 13.**Comparison between the measurements and the simulation model based on the heat generation and the electricity consumption.

Description | Value | Units |
---|---|---|

Heat storage Volume | 749 | l |

Diameter of heat storage | 0.79 | m |

Heat Conductance of isolation | 2 | $\frac{W}{K}$ |

Number of heat storage layers | 10 | - |

Ambient temperature | 18 | ${}^{\circ}$C |

Maximum layer temperature | 65 | ${}^{\circ}$C |

Heat transmission coefficient for neighboring layers | 465 | $\frac{W}{{m}^{2}.K}$ |

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**MDPI and ACS Style**

El-Baz, W.; Tzscheutschler, P.; Wagner, U.
Experimental Study and Modeling of Ground-Source Heat Pumps with Combi-Storage in Buildings. *Energies* **2018**, *11*, 1174.
https://doi.org/10.3390/en11051174

**AMA Style**

El-Baz W, Tzscheutschler P, Wagner U.
Experimental Study and Modeling of Ground-Source Heat Pumps with Combi-Storage in Buildings. *Energies*. 2018; 11(5):1174.
https://doi.org/10.3390/en11051174

**Chicago/Turabian Style**

El-Baz, Wessam, Peter Tzscheutschler, and Ulrich Wagner.
2018. "Experimental Study and Modeling of Ground-Source Heat Pumps with Combi-Storage in Buildings" *Energies* 11, no. 5: 1174.
https://doi.org/10.3390/en11051174