Design and application analysis of portable geothermal physical property tester

Ground source heat pump systems have outstanding advantages compared with other air conditioning systems. Because the temperature deep in the ground remains constant all year round, it is much higher than the outdoor temperature in winter, but significantly lower than the outdoor temperature in summer. Therefore, ground source heat pumps overcome the technical obstacles of air source heat pumps and greatly improve efficiency. In addition, it has many advantages such as low noise, small floor space, no pollutant emissions, no need to pump groundwater, low operation and maintenance costs, and long service life.

Designing the geothermal heat exchanger of the ground source heat pump system requires knowing the thermophysical parameters of the underground rock and soil. If the thermal physical parameters are inaccurate, the designed system may not meet the load requirements; it may also be too large in scale, thus increasing the initial investment.

The traditional method of determining the thermal physical parameters of underground geotechnical soil is to first determine the geological composition around the borehole based on samples taken from the borehole, and then determine the thermal conductivity by consulting relevant manuals. However, the underground geological composition is complex, and even with the same rock composition, the value range of its thermophysical parameters is relatively wide. Moreover, the thermal conductivity under different geological conditions can vary nearly ten times, causing the calculated buried pipe length to also vary several times, resulting in a considerable deviation in the cost of the ground source heat pump system.

In addition, different sealing materials and buried pipe methods have an impact on heat transfer. Therefore, only direct measurement on site can accurately obtain the thermophysical parameters of underground rock and soil. However, since such issues have rarely been involved in previous engineering practices, there is a lack of data accumulation in this area and a lack of ready-made testing methods.

In response to this problem, we conducted in-depth research and developed a portable geothermal physical property tester with independent intellectual property rights and applied it to actual projects.

1 Principle and composition of the tester

The thermal conductivity of underground rock and soil cannot be directly measured, and can only be inferred by measuring temperature, heat flow and other related parameters. Bury the conduit in the drilled borehole and backfill according to the design requirements. The conduit in the borehole can be used as a branch of the geothermal heat exchanger in the future. The loop is filled with water, allowing the water to circulate in the loop. From a certain Continuously heat the water for a long time (several days) from a moment, and measure the heating power, water flow rate and water temperature in the loop and the corresponding time. Finally, based on the known data, the rock surrounding the borehole is deduced. Average thermophysical parameters of soil.

This instrument consists of flow sensor, current sensor, voltage sensor, temperature sensor, pump, electric heater; pipeline and host machine, etc. The structural surface is shown in Figure 1.

In Figure 1, due to the action of the pump, the fluid enters through port A, the flow sensor collects the flow signal, and the temperature sensor collects the temperature signal (T1). After the fluid passes through the pump, it is heated by an electric heater. The heated fluid temperature signal (T2) is collected by the sensor. Then the fluid flows out from port B and is input into the conduit buried in the deep rock and soil. The heated fluid in the conduit is in contact with the deep layer. After heat exchange on the rock, it returns to the instrument from port A, forming a closed cycle. The heating power, temperature difference, and flow values ​​continuously collected within a certain period of time are used as measurement data, and then the average thermal conductivity of the rock and soil is calculated using the parameter estimation method to achieve the purpose of detection. Current sensors and voltage sensors are used to measure the heating power of the heater in real time to ensure detection accuracy.

1.1 Host hardware

As shown in Figure 2, the host computer consists of CPU AT89C52 chip, A/D conversion chip TLC2543, serial communication chip MAX232, program memory 27C128, data memory AT24C64, keyboard, LCD display, switch output, printer, power supply, etc. The main functions of each part are described as follows:

After filtering and I/V conversion, the current signals from each transmitter are converted to analog/digital by TLC2543. TLC2543 is a 12-bit analog-to-digital conversion chip with 11 channels, and the conversion of signal channels is controlled by software.

Program memory 27C128 and data memory AT24C64 are used to store part of the working program and test data. The test data stored in AT24C64 will not be lost after the system is powered off.

MAX232 is a dedicated chip for serial communication and is used to transmit test data to the host computer.

AT89C52 is a CPU with internal process memory, which controls the work of the entire system. The internal program memory stores the main work programs and parameters, and the internal RAM serves as the system's register area, flag area, printing and display buffer.

The auxiliary output of the switching value controls the power supply of the heater through a relay. When the heating temperature is too high for some reason, the power supply of the heater is disconnected to protect the equipment. Printers are used to save permanent data.

1.2 Host software

The system software is programmed using a mixture of assembly language and C language, using functional modules and subroutine structures. The main programs of the software are composed of data collection, keyboard, display, clock, communication, printing, etc.

2 test results

In order to calculate the thermophysical parameters of the surrounding rock and soil, a method of parameter estimation combined with an unsteady heat transfer model can be used. The results obtained by the heat transfer model are compared with the actual measured results, so that the variance sum f=Σ(Tcal,i-Texp,i)2 achieves the minimum value. The adjusted thermophysical parameter values ​​are the desired results. Among them, Tcal, i is the average temperature of the fluid in the conduit calculated by the model at time i; Texp, i is the average temperature of the fluid in the conduit actually measured at time i; N is the number of groups of experimental measurement data.

The following are the test results of using the geothermal physical property tester and the developed software to test the underground geothermal physical property parameters at the ground source heat pump air conditioning system project site of the Academic Lecture Hall of Shandong Institute of Architectural Engineering;

The borehole diameter is 115mm, the depth is 60m, the inner diameter of the buried pipe is 25mm, the outer diameter is 32mm, the pipe spacing is 70mm, the initial temperature of the underground rock and soil is 14.5℃, the thermal conductivity of the pipe wall is 0.33W/m℃, and the thermal conductivity of the drilling backfill material is 1.5W /m℃, heating power 48W/m.

The impact of test time on test results is shown in Figure 3. As can be seen from Figure 3, the calculated average thermal conductivity of the underground rock and soil around the borehole is also different at different test times. When the test time reaches about 50 hours, the measured thermal conductivity becomes stable and remains in the range of 1.530~1.538 W/m℃. Usually the test time can be selected to be about 60 hours, which can not only ensure the correct thermal conductivity, but also avoid the test time being too long.

Keep other conditions unchanged and only change the spacing between the riser and downcomers of the conduit. The effect on the thermal conductivity of rock and soil is shown in Figure 4. When the tube spacing changes by about 0.0lm. The calculated thermal conductivity change is about 4 to 8%. It can be seen from the figure that the larger the spacing, the smaller the calculated thermal conductivity: This is because the larger the spacing, the smaller the thermal resistance in the drill hole, under the condition that the total thermal resistance remains unchanged. The thermal resistance of the surrounding rock and soil is large. That is, the thermal conductivity is small. Therefore, how to determine the distance between tubes is an issue worthy of serious discussion in designing a ground source heat pump system.

3 Application prospects

For many years, the application of heat pump technology in our country has been in the theoretical discussion stage, and there is a lack of systematic research on ground source heat pumps. The main constraints on the application of heat pump technology in heating and air conditioning used to be insufficient power supply and low consumption levels of the people, and the market demand for heat pump air conditioning systems has not yet formed. Since the reform and opening up, with the development of my country's economy and the improvement of people's living standards, the above two constraints no longer exist. Air conditioning and heating have become the needs of ordinary people, and ground source heat pumps have technical advantages and The advantages of energy saving will make it the best choice for heating and air conditioning systems. Research, development and industrialization of ground source heat pump air conditioning systems may become a new growth point for my country's economic development.