Research on Ground Source Heat Pump VWV Control System

The development of heat pump technology is conducive to the rational use of natural resources, is conducive to the control of environmental pollution, energy-saving technology is one of the highlights. However, as with other air conditioning systems, it is necessary to consider how to not only meet the air conditioning comfort, but also maximize energy conservation. According to the current statistics, most of the time the air conditioning system is working at a load of 50-70%, and the load changes continuously with time. In order to match the cold load carried by the cold water with the frequently changing load, the water saving Conveyor power and system operating costs, the variable flow (VWV) control into the heat pump water system became a matter of course.

Automatic control of air conditioning system in the air conditioning industry more and more attention, from a fundamental point of view, the rational operation of air conditioning system is based on the system of scientific control based on. General VWV control system is designed for single-system variable flow control, and ground source heat pump VWV control system requires centralized control of cold and heat source pumps, load side pumps and heat pump units. Pump frequency control to achieve variable flow control. By combining the laboratory heat pump system with the ground-source heat pump system of a project in Beijing, we conducted a research and analysis on the control scheme of the variable-flow system of the water system of the ground-source heat pump system.

At present, people in the industry generally recognize that the design of variable frequency variable speed variable flow control system has obvious energy saving effect. In the cold water pump, cooling water pump installed inverter, due to mature technology, simple control, suppliers and more, less investment, quick returns and favored by owners and designers, as the fastest growing means of energy conservation. Energy-saving frequency conversion is mainly based on the water pump similarity law of the three comprehensive formula: ① water α speed, ② pressure head α speed 2 , ③ power α speed 3 .

2.1 variable pressure variable flow control ideas

figure 1

Both control valve method and constant pressure control method, they are by changing the pump resistance or speed to adjust the pump port pressure and flow. Control objects are directed against the pump, or pump speed n, or pump head (pump pressure) H. Therefore, the system feedback is a single physical quantity, or pump speed n, or pump head H. These control methods themselves determine that their effects can not be ideal because they do not take into account real-time changes in water flow. When the system is running at partial load, due to the pump operating characteristic curve and the system operating characteristics of the pipeline shift point, the pump speed before and after the change of the operating point for the non-similar operating conditions, as shown in Figure 1. In this case, the pump shaft power does not actually meet the pump similarity law, and it is impossible to achieve the expected energy saving purpose in actual operation. The use of variable voltage variable flow control of variable frequency speed is the only viable ideal.

2.1.1 The basic idea

To ensure that users of water remain constant head. That is based on user changes in water consumption automatically adjust the pump speed, change the pump head to adapt to changes in water flow, so water consumption, more pressure, improve the pump head; small amount of water, less pressure, reduce the pump head . Achieve automatic control between the pump head and water consumption. Improve equipment operation efficiency, save energy consumption.

2.1.2 control method

The main points of the design of the variable pressure differential flow system are to effectively monitor the operation of the system and to track and control its operating point according to the changes of the system flow rate. Pump head lift with the literature [4] proposed formula to calculate:

H = H ST + S * Q 2 (1)

Equation (1) where S is the coefficient of the friction along the pipeline and the local resistance, Q is the water flow in the pipeline (m 3 / s), H ST is the head of the user. The pump head is used in two aspects: not only to meet the needs of the user head of water (static head), but also make up for a variety of water head loss (moving head). These losses are proportional to the square of the water flow in the pipeline.

Based on the analysis of pump head expression (1), an automatic control method of variable pressure differential flow was proposed. That is: the static head H ST is set to be the control amount, and set their constant without constant pump head is provided. It is this change, we find: In the formula (1), H ST bitter to make constant, while increasing the pump flow rate, to compensate for a variety of head loss in the water pipeline, pump head must increase. When the pump flow decreases. The various head losses in the water line are also correspondingly reduced, and the head of the pump must be reduced. Otherwise equation (1) will not hold. This is exactly the ideal relationship we want. It reflects the correct relationship between pump flow and pump head, but this rule only when the user set the user's head (static head) is constant. To be established, this is the variable differential flow control theory and method of automatic control.

The theory and method of automatic control of variable differential pressure variable flow realizes the automatic adjustment of the pump speed according to the real-time changes of the user's water consumption, changes the head of the pump to adapt to the change of the water flow, makes up for the pipeline loss caused by the flow change, head H ST is constant. Achieve pump head and water adaptive control between. While saving a lot of energy. Variable differential pressure flow automatic control method to save energy than constant pressure control is much higher.

Figure 2 VWV system of centralized control schematic

In the ground source heat pump VWV system, the two water systems and heat pump units need to be controlled simultaneously and adjusted synchronously, otherwise the system will be unstable. Therefore, the centralized control system shown in FIG. 2 is adopted.

3.1 specific control program

Ground source heat pump control system can be divided into three parts: the user side of the water system, heat pump units and cold and heat source side of the water system. The user-side water system controls the user pressure difference H ST and the temperature difference ΔT 1 unchanged, and changes the pump head and flow according to the change of the user load Q 1 . With the development of control technology, the flow in the evaporator and condenser of the heat pump unit has been allowed to vary within a certain range, typically 30% to 130% of the design flow. Compressor with slide valve adjustment, the unit at low load operation still maintain a high energy efficiency ratio.

Figure 3 VWV system control flow chart

According to the principle of conservation of energy, taking the summer condition as an example, the water circulation rate W l of the cold-water source-side water system is the sum of the unit cooling capacity Q k and the power N consumed for obtaining the chilled water. When the user-side water system water circulation amount W k is changed, the water circulation amount W l of the water system on the cold heat source side is changed. Similarly, the control of underground pressure drop H HP and temperature difference â–³ T 2 unchanged. In addition, the winter conditions, should control the minimum flow of cold and heat source side, so as to avoid underground heat exchanger inlet water temperature dropped to zero, to prevent freezing.

3.2 control process

General VWV control system is for single-system variable flow control, and ground-source heat pump system needs to consider the coupling between the various systems. The purpose of control is to maximize the energy savings of each system in a stable operation. Taking summer conditions as an example, the flow chart of soil source heat pump control is shown in FIG. 3.

(1) As the ground source heat pump VWV control technology is relatively complex, the initial investment costs are high, so determine the system should be for all aspects of specific projects to balance the energy-saving effect and investment recovery. In general, the ground source heat pump VWV control technology is more suitable for large systems, air conditioning load changes in large projects.

(2) The appropriate pressure sensor and flow sensor should be selected according to the system design parameters and adjustment requirements, and the proper installation location should be selected to meet the requirements of the system's control accuracy.

(3) The ground source heat pump VWV control system is not yet common in practical engineering applications. It requires designer and operation management staff to work together to continuously improve the technical level so as to reduce the operating cost and achieve the purpose of energy conservation.