Development of polyolefin compound and post-polymerization treatments for ground heat exchangers

A ground source heat pump (GSHP) system can be used for both cooling and heating modes simultaneously for commercial, industrial and residential buildings virtually at any location with great flexibility to cover a wide range of demands all around the world. Polyethylene (PE) has been used as the main raw material in production of the Ground Heat Exchangers (GHE). This paper briefly reviews the history of polyethylene and development in polymerization process with emphasis on the third-generation bimodal structure. The characteristics of PE pipes used in GSHP systems are discussed. This paper is devoted to a critical review on the attempts in post-polymerization treatments of the PE, and GHEs to improve the performance of the systems. The experimental and simulated comparisons show that the enhancement of the thermal conductivity of the material can reduce significantly the overall borehole thermal resistance

Thermally Conductive High-Density Polyethylene Composites for Ground Heat Exchangers

Today, the energy used for the heating and cooling of offices, shopping centres, schools, residential areas, andother buildings is as large as 50% of the total energy consumption in the world. The climate change, globalwarming, and environmental issues have forced high requirements for energy efficiency and clean energyproduction in buildings. The general targets in all sectors are toward nearly zero energy buildings (nZEB) andthe next step solution is positive energy buildings (PEB). To achieve a net zero carbon footprint and evenpositive energy buildings, sustainable energy production systems that mainly rely on renewable energycombined with energy efficiency and integration technologies must be implemented. Geothermal energy has a unique position among the renewable energy resources. Near-surface or shallowground maintains almost a constant temperature during the seasons, and therefore ground can be used as aheat source during the winter and as a heat sink during the summer. A ground source heat pump system(GSHP) consists of a heat pump, which is connected to the delivery system and a ground heat exchanger(GHE). The majority of the ground source heat pump systems in Europe use vertical borehole heat exchangers (BHE).The key problem here is obtaining the largest possible heat flow exchanged with the rock mass. The energydemand and ground properties are the most parameters that influence the performance of a GSHP. Theamount of heat taken from the rock mass depends on the thermal conductivity of the surroundings and theborehole thermal resistance. Numerous attempts have been made to reduce the thermal resistance of theborehole in different ways but until recently, little attention has been paid to the enhancement of propertiesof the material used to produce GHEs. In light of the above considerations, a novel thermoplastic composite, to have lower thermal resistance in theGSHP systems that affects the maximum heat flow exchanged between the heat carrier and the rock mass,has been introduced. This includes the development of a state-of-the art research and developmentinfrastructure to support the use of new materials and test methods for the BHEs. High density polyethylene(HDPE) composites reinforced with inorganic fillers and their properties were investigated. The compositeswere prepared by melt blending and injection moulding. In the context of this study, the thermo-physical,mechanical, and morphological properties of these composites were studied; thereafter, using a numericalmodel simulation, the effectiveness of the new material was evaluated. The very positive finding was that theaddition of talc particulates, not only improved the thermal conductivity and thermal diffusivity of thecomposites, but it also simultaneously increased the properties of the composites regarding stiffness andimpact resistance, which are important parameters in the deep GSHP system. Thermal conductivity, thermal diffusivity, and density values of the composites increased almost linearly, butthe increase in moisture absorption in the long-term showed non-linear behaviour under the chosenexperimental conditions. The maximum thermal conductivity was up to 70% higher than for the unfilled HDPEat a talc concentration of 35 wt.-%. The numerical simulation showed that the enhancement of the thermalconductivity of the material can significantly reduce the overall borehole thermal resistance. Melt rheological investigation of the HDPE-talc blends at constant shear stress and constant shear rateindicated that the melt obeyed the power-law model and shear thinning behaviour. The results have also illustrated that the presence of talc has considerable effect on the lifetime expectancy ofthe product. It was presented that the thermal stability was enhanced, while the oxidation induction timedecreased in cooperation with the talc. Furthermore, the temperature of the α relaxation shifted towardhigher temperature and finally, the strain hardening modulus for the HDPE/talc composites was assessed andcompared to the neat HDPE as a measure of environmental stress crack resistance

Investigation of Steady-State Heat Extraction Rates for Different Borehole Heat Exchanger Configurations from the Aspect of Implementation of New TurboCollector™ Pipe System Design

When considering implementation of shallow geothermal energy as a renewable source for heating and cooling of buildings, special care should be taken in the hydraulic design of the borehole heat exchanger system. Laminar flow can occur in pipes due to the usage of glycol mixtures at low temperature or inadequate flow rates. This can lead to lower heat extraction and rejection rates of the exchanger because of higher thermal resistance. Furthermore, by increasing the flow rate to achieve turbulent flow and satisfactory heat transfer rate can lead to an increase in the pressure drop of the system and oversizing of the circulation pump which leads to impairment of the seasonal coefficient of performance at the heat pump. The most frequently used borehole heat exchanger system in Europe is a double-loop pipe system with a smooth inner wall. Lately, development is focused on the implementation of a different configuration as well as with ribbed inner walls which ensures turbulent flow in the system, even at lower flow rates. At a location in Zagreb, standard and extended thermal response tests were conducted on three different heat exchanger configurations in the same geological environment. With a standard TRT test, thermogeological properties of the ground and thermal resistance of the borehole were determined for each smooth or turbulator pipe configuration. On the other hand, extended Steady-State Thermal Response Step Test (TRST) incorporates a series of power steps to determine borehole extraction rates at the defined steady-state heat transfer conditions of 0/−3 °C. When comparing most common exchanger, 2U-loop D32 smooth pipe, with novel 1U-loop D45 ribbed pipe, an increase in heat extraction of 6.5% can be observed. Also, when the same comparison is made with novel 2U-loop D32 ribbed pipe, an increase of 18.7% is achieved. Overall results show that heat exchangers with ribbed inner pipe wall have advantages over classic double-loop smooth pipe designs, in terms of greater steady-state heat extraction rate and more favorable hydraulic conditions

Dry fermentation of manure with straw in continuous plug flow reactor : Reactor development and process stability at different loading rates

In this work, a plug flow reactor was developed for continuous dry digestion processes and its efficiency was investigated using untreated manure bedded with straw at 22% total solids content. This newly developed reactor worked successfully for 230days at increasing organic loading rates of 2.8, 4.2 and 6gVS/L/d and retention times of 60, 40 and 28days, respectively. Organic loading rates up to 4.2gVS/L/d gave a better process stability, with methane yields up to 0.163LCH4/gVSadded/d which is 56% of the theoretical yield. Further increase of organic loading rate to 6gVS/L/d caused process instability with lower volatile solid removal efficiency and cellulose degradation.[on SciFinder (R)]MEDLINE AN 2017659917(Journal; Article; (JOURNAL ARTICLE))</p