What Is Heat Loss?

Heat loss is the amount of heat that is lost to the body or building. The heat loss of the human body refers to the heat emitted from the human body to the surrounding environment, including evaporation heat dissipation, convection heat dissipation, radiation heat dissipation, and a small amount of heat conduction. The heat loss of heating building includes the heat dissipation to the outside through the envelope structure, the heat carried by the air penetration and ventilation, the ground heat transfer, the indoor moisture evaporation and the latent heat carried by the steam penetration.

Mathematical models were established for the thermal radiation between the absorption tube and the glass tube and the thermal convection of the residual gas, the thermal convection between the glass tube and the external environment, and the thermal radiation of the glass tube to the sky in the vacuum collector tube. Comparison with experimental data validates the accuracy of the model. At the same time, several main factors affecting heat loss were analyzed using the model. The analysis results show that the higher the temperature of the absorption tube, the greater the heat loss; the lower the ambient temperature, the greater the wind speed, the greater the heat loss, but the impact is small; The emissivity of the absorption coating is the main factor affecting heat loss; the degree of vacuum also has a large effect on heat loss. ^[1]

The vacuum heat collecting tube is a key component of the concentrating collector of the trough solar thermal power generation system, and the magnitude of its heat loss is directly related to the light-heat efficiency of the system. By analyzing the heat loss of a 13m pipe composed of 6 vacuum heat collecting tubes, the results show that within the range of the temperature of the heat transfer oil 105 ~ 313 , the radiation heat loss accounts for 70% ~ 90% of the total heat loss; quasi-static equilibrium The heat loss measured by the method is larger than that measured by the steady-state equilibrium method. The heat loss measured at high temperature (290 ) is 1.18 times that of the steady-state equilibrium method. ^[2]

Heat loss experiment system and method

There are two methods to test the heat loss of the vacuum collector tube: quasi-static equilibrium method and steady-state equilibrium method. The quasi-static equilibrium method is to install a vacuum heat collecting pipe in a fluid circuit, to maintain the stability of the fluid temperature at the inlet and outlet of the vacuum heat collecting pipe, in order to achieve the purpose of constant wall temperature of the stainless steel pipe, by measuring the temperature difference and quality of the fluid inlet and outlet. Flow rate to calculate the amount of heat loss. The steady-state equilibrium method is heating by a resistance heater placed in a stainless steel tube of a vacuum collector tube to achieve a constant wall temperature of the stainless steel tube, and calculating the amount of heat loss by calculating the electric power of the heater. In the experiment, the quasi-static equilibrium method was used to measure the heat loss of the vacuum collector tube.

In the course of the experiment, first open the valve of the main circulation pipeline, start the circulation pump to drive the heat transfer oil to circulate in the pipe; turn on the heating system in the heat transfer oil tank, and stabilize the temperature of the heat transfer oil in the expansion tank (armored thermal resistance measurement) to Slightly lower than the temperature required for the test; then, adjust the automatic control valve to stabilize the display flow rate of the flow meter FM to the flow set by the test, and turn on the heat transfer oil heater to stabilize the temperature of the heat transfer oil at the inlet of the vacuum collector tube to The temperature required for the experiment; then let the system circulate stably (at least 1h) until the experiment at this stage is completed. ^[2]

Analysis of heat loss experimental results

The heat loss of the vacuum heat collecting tube increases rapidly with the increase of the temperature of the heat transfer oil. The temperature of the outer surface of the glass outer protective tube rises rapidly with the temperature of the fluid, and its convective heat dissipation also rises rapidly. The loss of 7.38% rose to 23.52%; due to the heat preservation measures and short length of stainless steel pipes and bellows, although the temperature has risen a lot, the proportion of convection heat dissipation loss accounts for a small proportion of the total heat loss, which accounts for the total heat loss. 1.20% to 5.94%; it can be seen that radiation heat loss accounts for 70% to 90% of the total heat loss. When the temperature of the heat transfer oil is low, the actual heat loss is small, and the measurement error has a large effect. As the temperature of the heat transfer oil gradually increases, the heat dissipation amount increases, and the error effect gradually decreases. ^[2]

The heat loss measured by the quasi-static equilibrium method is greater than the heat loss measured by the steady-state equilibrium method. When the temperature difference between the heat transfer oil and the environment is 83.3 ° C, the heat loss measured by the steady-state equilibrium method is 15.38 W. m ^-1 , the heat loss measured by the quasi-static equilibrium method is 26.11W ^. m ^-1 ; when the temperature difference between the heat transfer oil and the environment is 290.8 , the heat loss measured by the steady-state equilibrium method is 153.43 W. m ^-1 , the heat loss measured by the quasi-static equilibrium method is 181.54 W. m ^-1 is 1.18 times that of the steady-state equilibrium method. The main reasons are: the steady-state equilibrium method does not consider the heat loss at both ends of the vacuum heat collecting tube; it is difficult to maintain the same position of the temperature measuring elements for measuring the inner wall surface of the heat absorbing tube of the vacuum heat collecting tube, resulting in large temperature measurement errors.

The heat loss under similar temperature difference conditions is not much different. The heat loss of PTR70 vacuum heat collector tube at 98 is 26.9W ^. m ^-1 , the heat loss at 146.6 ° C is 46.4 W. m ^-1 , the heat loss at 295.2 ° C is 148.85 W. m ^-1 , and the heat loss under the similar temperature conditions of the experimental vacuum collector is 31.79, 53.29 and 181.54 W respectively ^. m ^-1 is about 1.2 times the heat loss of the PTR vacuum heat collecting tube. ^[2]