Vertical hydraulic conductivity of borehole heat exchanger systems before and after freeze-thaw cycle stress

Quality assurance is a very important aspect in the planning, implementation and operation of borehole heat exchangers (BHE). Within the research project Quality Assurance for Borehole Heat Exchangers (QEWS) the hydraulic sealing behaviour of industrial grouting materials was investigated. Important aspects therein were the capability for sealing between different aquifers and aging resistance against freeze-thaw cycle stress. For this purpose, a large-scale test rig for determining the hydraulic conductivity of borehole heat exchanger systems (BHES) was developed and put into operation. Compared to samples of the pure grouting material, the system samples exhibited significantly increased permeability, which was further increased by freeze-thaw cycle stress. It could be shown that the first freeze-thaw cycle caused the most significant aging effect. Aging caused by further freeze-thaw cycles could be partly compensated by regeneration effects. But the large-scale test rig has a substantial disadvantage: due to the length of the system sample, depending on the hydraulic permeability of the grouting material, the experimental program is very complex and time-consuming. Therefore, a downscaled test rig will be developed within the current project Quality Assurance for Borehole Heat Exchangers II (QEWS II)

A novel coupling control with decision-maker and PID controller for minimizing heating energy consumption and ensuring indoor environmental quality

Due to climate change, global energy crisis, and high-quality life requirement for people, decreasing building energy consumption and enhancing indoor environment quality through control of heating, ventilation, and air conditioning systems tend to be increasingly important. Therefore, favorable control methods for heating and ventilation systems are urgently necessary. In this work, a new coupling control with decision-maker was proposed, developed, and investigated; meanwhile, several demand controlled ventilation strategies combined with heating control method was compared considering heating energy consumption, thermal comfort, and indoor air quality. In order to properly model the service systems, the air change rates and thermal time constants have been first measured in a reference office installed with commonly applied bottom-hinged tilted windows in our low-energy building supplied by geothermal district heating. Then, simulations have been carried out across two typical winter days in the reference office. The results illustrate that the proposed combination of suitable heating and demand controlled ventilation coupling control methods with decision-maker and proportional-integral-derivative (PID) controller could greatly reduce heating consumption in the reference room during the office time: around 52.4% (4.4 kW h energy saving) per day in winter in comparison to a commonly suggested method of intensive and brief airing. At the same time, it could ensure indoor CO2 concentration to keep within the pre-set ranges (Pettenkofer limit: 1000 ppm) as well as low variations of indoor temperature (standard deviation (SD): 0.1°C)

Development of a thermal oil operated waste heat exchanger within the off-gas of an electric arc furnace at steel mills

A thermal oil operated tube bundle heat exchanger within the off-gas from an electric arc furnace (EAF) was developed for supplying heat to an electricity generating system by empirical methods. In this process both the unsteady heat flux and the dust load of the off-gas were considered. A reference heat exchanger configuration was designed and optimized by empirical methods. Therefore in-situ experiments within the off-gas channel as well as parametric studies were undertaken. It was revealed that variations of geometric parameters have a positive impact on heat exchanger performance. The impact of the dust layer thickness on heat transfer and pressure drop was quantified. A CFD model of a section of the reference heat exchanger configuration was generated using ANSYS CFX. Thus representative results for the complete heat exchanger were calculated and compared with the analytical results. The comparison of characteristic numerical results revealed a slight underestimation of heat exchange and pressure drop compared to analytical values