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Research on Ground Source Heat Pump VWV Control System
**Abstract:**
Ground-source heat pump technology is an energy-efficient, environmentally friendly, and sustainable solution for heating and cooling. Like other air conditioning systems, it's essential not only to meet comfort requirements but also to optimize energy savings. Applying variable flow control strategies from the HVAC and refrigeration industry can unlock significant energy-saving potential in heat pump systems. In recent years, automatic control of HVAC systems has gained increasing attention. This paper analyzes existing variable flow control methods and proposes a frequency-controlled variable flow system. It addresses the coupling between the two water loops and the heat pump units, and presents a case study on the implementation of variable flow control in a ground-source heat pump system.
**Keywords:** Ground source heat pump, variable flow, differential pressure control, variable frequency speed regulation
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**1. Introduction**
The advancement of heat pump technology plays a crucial role in the efficient use of natural resources and the reduction of environmental pollution. Energy-saving solutions are at the forefront of modern HVAC systems. However, like any other air conditioning system, it's vital to balance comfort with energy efficiency. According to current data, most air conditioning systems operate at 50–70% of their full load, and this load fluctuates over time. To match the changing demand, variable flow (VWV) control has become a standard practice in heat pump water systems.
Automatic control of HVAC systems is becoming more prevalent. From a fundamental perspective, optimal operation relies on scientifically designed control systems. While traditional VWV systems are often tailored for single-system applications, ground-source heat pump systems require centralized control of both the cold/heat source pumps and the load-side pumps. Pump frequency control is used to achieve variable flow. By integrating a laboratory-based heat pump system with a real-world project in Beijing, this paper explores the control strategies for variable flow in ground-source heat pump systems.
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**2. Pump Frequency Control Principle**
In the industry, it's widely accepted that variable frequency variable flow control systems offer substantial energy savings. Installing inverters on chilled water and cooling water pumps is popular due to mature technology, simple control, and cost-effectiveness. These systems are favored by both owners and designers because they provide quick returns on investment.
Energy-saving through frequency conversion is based on the similarity laws of water pumps:
- Flow rate is proportional to speed,
- Pressure head is proportional to the square of speed,
- Power is proportional to the cube of speed.
**2.1 Variable Pressure Variable Flow Control Concept**
Both valve control and constant pressure control methods adjust pump resistance or speed to regulate pressure and flow. However, these approaches focus only on pump speed or head, leading to limited effectiveness. They fail to account for real-time changes in water flow, resulting in inefficiencies during partial load conditions.
To address this, variable pressure variable flow control is introduced. The goal is to maintain a constant user head while adjusting the pump speed based on actual water consumption. When usage increases, the pump head rises; when usage decreases, the head drops accordingly. This ensures the pump operates efficiently under varying conditions.
**2.1.1 Basic Idea**
The core idea is to keep the user’s pressure head constant by automatically adjusting the pump speed based on water demand. This ensures that the pump head adapts to flow changes, maintaining system efficiency and reducing energy consumption.
**2.1.2 Control Method**
A key design principle of variable pressure differential flow systems is to monitor and adjust the system’s operating point according to flow changes. The pump head can be calculated using the formula:
$$ H = H_{ST} + S \cdot Q^2 $$
Where $ H_{ST} $ is the static head, $ S $ is the friction coefficient, and $ Q $ is the flow rate. By keeping $ H_{ST} $ constant, the system adjusts the pump head dynamically to compensate for flow-related losses. This approach ensures accurate and efficient control, significantly improving energy performance compared to constant pressure systems.
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**3. Ground Source Heat Pump VWV Control Program**
In a ground-source heat pump system, both the water loops and the heat pump units must be controlled simultaneously. Otherwise, the system may become unstable. A centralized control system is therefore implemented, as shown in Figure 2.
**3.1 Specific Control Program**
The control system is divided into three parts: the user-side water system, the heat pump unit, and the cold/heat source side. The user-side system maintains a constant pressure difference and temperature difference, adjusting the pump head and flow based on user load. Modern control technologies allow the evaporator and condenser flows to vary within a range of 30% to 130% of the design flow. Compressors with slide valves ensure high efficiency even at low loads.
**3.2 Control Process**
Traditional VWV systems are designed for single-system applications, but ground-source heat pump systems involve complex interactions. The goal is to maximize energy savings while ensuring stable operation. As shown in Figure 3, the control process for a ground-source heat pump system involves dynamic adjustments based on seasonal conditions, such as summer and winter loads.
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**4. Summary**
(1) Due to the complexity and high initial costs of ground-source heat pump VWV control systems, it’s important to evaluate energy savings against investment recovery on a per-project basis. These systems are best suited for large-scale projects with significant load variations.
(2) Proper selection of pressure and flow sensors, along with strategic installation locations, is essential to meet control accuracy requirements.
(3) Although still emerging in practical applications, ground-source heat pump VWV control requires collaboration between designers and operators to refine the technology, reduce operational costs, and achieve long-term energy savings.