Heat Carrier Fluid Research Articles

A new version of the Large Temperature Jump method: The thermal response (T–LTJ)

In this communication, we propose a new version of the Large Temperature Jump (LTJ) method for studying the ad/desorption dynamics on representative pieces of heat exchangers (HEx) used in real adsorption chillers. This method is based on direct measurement of the temperature difference ΔT of a heat carrier at the inlet and outlet of the tested HEx fragment after a fast drop/jump of the inlet temperature. This tightly repeats the procedure used in real HExs for transformation and storage of low temperature heat. For the sake of validation, the measurements were carried out with the same adsorbent (AQSOA FAM-Z02) and HEx as well as under the same conditions already comprehensively studied in [1]. It is demonstrated that the measured ΔT-response allows studying ad/desorption dynamics, extracting the characteristic process time and heat with sufficient accuracy. The new Thermal Large Temperature Jump (T-LTJ) method gives similar information as the G-LTJ version being more simple in realization and close to the common procedure for evaluating dynamic performance of real adsorptive chillers. Moreover, the T-LTJ provides valuable information about the heat flux directly transferred to a heat carrier fluid that is not available from other LTJ versions.

Experimental thermal performance analysis of ground heat exchangers for space heating and cooling applications

In recent decades, ground source heat pump systems have become popular for space heating and cooling applications. In both modes of operation, GSHP needs to interact with the ground through GHX. The thermal performance analysis of single and double U-tube heat exchangers are discussed in this paper by focusing on its effectiveness, ground temperatures, heat extraction & injection rate and its effects on surrounding ground formations. Initially the method of calculating length of GHX for a given heat load is explained, then the effectiveness of GHX is defined. Experimental set up used to study the performance of single and double U-tube GHX at bureau of geological and mining research (BRGM), France is explained. Results obtained from the experimental study shows that the average effectiveness of single U-tube heat exchanger in heating and cooling modes are 0.34 and 0.40 respectively and for double U-tube, it is 0.46 and 0.57 respectively. In both the heat exchangers, the average effectiveness is observed to be high in cooling mode operation, the reason may be that in cooling mode operation, the temperature difference between the heat carrier fluid and the ground is found to be high compared to the difference observed during heating mode operation.

Clear-days operational performance of a hybrid experimental space heating system employing the novel mini-channel solar thermal & PV/T panels and a heat pump

The paper investigated a hybrid experimental space heating system that comprises the novel mini-channel solar thermal and PV/T panels, as well as a heat pump. The major advantage of the system lies in the multiple functions of providing energy for space heating in winter and domestic hot water in other seasons, and generating electricity annually to balance the electricity demand of the system. Compared to traditional flat-plate solar collectors, the mini-channel solar thermal and PV/T panels have the smaller cross-sectional area, higher fluid flow velocity and thus higher heat transfer rate between the heat-carrying fluid and panel walls, thus leading to enhanced solar thermal and electrical efficiencies. It is found that during the typical-day testing, the mini-channel thermal panels had average solar efficiency of 50.8%; while the mini-channel PV/T panels had average solar efficiency of 45.0% and the whole system had average Coefficient of Performance (COP) of 4.9. The system made use of solar and gas sources with the fractional solar consumption of 44%, while its operation was under the pre-set time schedule, i.e., solar panels on from 9:00 to 6:00, heat pump on from 18:00 and 20:00, and gas heater on from 22:00 to 9:00. Throughout the testing period, the room temperature was maintained at above 18 °C, thus enabling achieving the required thermal comfort. Concerning the complexity of such a system, a simplified system comprising the mini-channel thermal and PV/T panels and a heat pump was suggested. In brief, the research has produced fundamental data for the mini-channel-based solar thermal and PV/T panels and established a foundation for designing a simple and low-cost mini-channel based solar space heating system for rural houses in cold climatic regions of China, which will subsequently contribute to significant fossil fuel saving and carbon emission reduction in the global extent.

Preliminary theoretical analyses of thermal performance and available energy consumption of two-stage cascade cycle heat pump water heater

In this paper, the analysis model of thermal performance and available energy of two-stage cascade cycle heat pump water heater (HPWH) was established. For the first time, the equivalent transformation thermodynamic analysis method and available energy consumption equation were employed to carry out the theoretical analyses of thermal performance and available energy consumption for two-stage cascade cycle HPWH. And the correctness of the analysis approach proposed in this paper was verified by an example. The calculation results showed a significant agreement with the results derived by the first law of thermodynamics and energy conservation law. The available energy analysis of two-stage cascade cycle HPWH clearly revealed the available energy consumption rates for different working processes, including heat transfers of heat exchangers, flow of heat-carrying fluid, working medium cycle, output of hot water, etc. This approach could be conveniently used in the analyses of thermal performance and available energy consumption for actual complex thermal system with irreversible process. This paper provided the theoretical basis for the actual optimization design and operation analysis of cascade type heat pump systems, and furthermore, the results of the calculation instance would also have a certain reference value.

Energy performance of diaphragm walls used as heat exchangers

The possibility of equipping diaphragm walls as ground heat exchangers to meet the full or partial heating and cooling demands of overlying or adjacent buildings has been explored in recent years. In this paper, the factors affecting the energy performance of diaphragm walls equipped as heat exchangers are investigated through finite-element modelling. The numerical approach employed is first validated using available experimental data and then applied to perform parametric analyses. Parameters considered in the analysis include panel width, the ratio between the wall and excavation depths, heat transfer pipe spacing, concrete cover, heat carrier fluid velocity, concrete thermal properties and the temperature difference between the air within the excavation and the soil behind the wall. The results indicate that increasing the number of pipes by reducing their spacing is the primary route to increasing energy efficiency in the short term. However, the thermal properties of the wall concrete and the temperature excess within the excavation space are also important, with the latter becoming the most significant in the medium to long term. This confirms the benefits of exploiting the retaining walls installed for railway tunnels and metro stations where additional sources of heat are available.

A Numerical Study on the Impact of Grouting Material on Borehole Heat Exchangers Performance in Aquifers

U-pipes for ground source heat pump (GSHP) installations are generally inserted in vertical boreholes back-filled with pumpable grouts. Grout thermal conductivity is a crucial parameter, dominating the borehole thermal resistance and impacting the heat exchanger efficiency. In order to seal the borehole and prevent leakages of the heat carrier fluid, grouting materials are also hydraulically impermeable, so that groundwater flow inside the borehole is inhibited. The influence of groundwater flow on the borehole heat exchangers (BHE) performance has recently been highlighted by several authors. However groundwater impact and grouting materials influence are usually evaluated separately, disregarding any combined effect. Therefore simulation is used to investigate the role of the thermal and hydraulic conductivities of the grout when the BHE operates in an aquifer with a relevant groundwater flow. Here 3 main cases for a single U-pipe in a sandy aquifer are compared. In Case 1 the borehole is back-filled with the surrounding soil formation, while a thermally enhanced grout and a low thermal conductivity grout are considered in Case 2 and Case 3 respectively. Simulations are carried out maintaining the inlet temperature constant in order to reproduce the yearly operation of the GSHP system. For each of the 3 cases three different groundwater flow velocities are considered. The results show that a high thermal conductivity grout further enhances the effects of a significant groundwater flow. The conditions when neglecting the grout material in the numerical model does not lead to relevant errors are also identified.

Experimental efficiency of a low concentrating CPC PVT flat plate collector

PVT collectors aim to solar co-generation of electricity and heat. An new concept raises thermal efficiency by concentrating sunlight with CPC reflectors, in order to access a higher number of solar thermal applications. Losses in PV efficiency due to a higher operating temperature are accepted with regard to a higher overall collector efficiency. As a side effect, the concentration lowers the material usage of PV and enables a high efficient thermal coupling of the PV cell to the heat carrying fluid.The work focuses on the construction, as well as on the angle dependent electrical and thermal measurement of the real-size CPC PVT collector prototype (1460mm×600mm×150mm). In previous publications, calculations and experiments studied the influence of the CPC reflectors on the PV efficiency. Further, the thermal coupling between PV cell and heat carrier fluid has been measured in a lab experiment. These results, together with transient annual simulations, were considered in the design process of the CPC PVT prototype with an angular acceptance range of ±25°.The experiments were conducted on the outdoor solar test facility at ZAE Bayern in Garching, Germany. The thermal and PV efficiency has been measured with MPP tracking, as well as for open circuit voltage for fluid temperatures up to 107°C.It could be shown, that the thermal efficiency while MPP tracking is elevated to 34% compared to a glazed flat plate PVT with 17% for collector temperatures 60K over ambiance. At the same time, the electrical efficiency drops from 15% cell efficiency to an overall collector efficiency of 9%, due to the optical setup, temperature effects and a non-uniform flux distribution caused by the reflectors.

Thermal Modeling of Thermo-Active Systems in Subway Tunnels: A Chilean Case Study

Since the mid 80’s thermo-active structures have been used around the world to take advantage of shallow geothermal energy. However, these systems have just been considered as energy options in Chilean projects. In this article we explore the design, numerical modeling, and thermal behavior of a flat heat exchanger integrated to the concrete wall of a subway tunnel in Chile. The thermo-active structure is the concrete tunnel wall (energy tunnel) that has an arrangement of circular pipes with water as the heat carrier fluid. Such a system has a low-cost installation, because pipes are embedded in the concrete tunnel wall during construction. The numerical modeling considers actual environmental conditions in the tunnel and the surrounding ground properties. The considered operating condition is restricted to heat transfer from the air within the tunnel and from the surrounding ground to the heat carrier. We studied the dependency of heat absorption on the heat carrier flow rate, and on the spacing of pipe arrangements. Results predict that a 10-meter section of tunnel can capture up to 6.900W in winter and 10.300W in summer. Most heat (75%) is extracted from the air within the tunnel, which allows a reduction of the tunnel air temperature of 5°C during winter and of 8°C during summer. Thus, implementing such a system has a dual effect of cooling the tunnel (a desired outcome) and providing a source of cheap thermal energy.

Efficiency Analysis of the Main Components of a Vertical Closed-Loop System in a Borehole Heat Exchanger

In vertical closed-loop systems, it is common to use single or double U-tube heat exchangers separated by longitudinal spacers. In addition, the helical-shaped pipe is another configuration that requires lower drilling lengths but it is less used. The aim of the present research is to study the influence of these components on the total efficiency of a borehole heat exchanger (BHE). Thus, the differences between using single/double U-tubes (with or without spacers) and helical pipes are analysed in terms of efficiency. Through different laboratory tests, a small vertical closed-loop system was simulated in order to analyse all these possible configurations. The grouting materials and the temperatures of the ground were modified at the same time in these tests. Regarding the heat exchange process between the ground and the heat carrier fluid, it must be highlighted that the best results were obtained for the helical-shaped pipe configuration. Some of the improvements offered by this heat exchanger typology with respect to the vertical configuration is that a lower drilling depth is required even it requires a larger diameter. This leads to significant economic savings in the performing drilling process. Finally, it is also worth noting the importance of using spacers in vertical U-tubes and that no improvements have been found regarding the use of single or double configuration of U-tubes. Thanks to the laboratory results derived from this study it is possible to establish the optimum behaviour pattern for the entire vertical closed-loop systems.

A simulation-based analysis of variable flow pumping in ground source heat pump systems with different types of borehole heat exchangers: A case study

A simulation model of ground source heat pump systems has been used to investigate to what extent a variable flow of the heat-carrier fluid of the ground loop affects the energy efficiency of the entire system. The model contemporaneously considers the borehole heat exchangers, the heat pump, the building load, and the control strategies for the circulation pumps of the ground loop. A constant speed of the circulation pumps of the ground loop was compared with a variable flow controlled by means of a constant temperature difference across the heat pump on the ground side considering the load profile of an office building located in North Italy. The analysis was carried out for a single U-tube, double U-tube and coaxial pipe heat exchangers. The control strategies adopted to manage the flow rate of the heat-carrier fluid of the ground loop affect both the heat exchange rate of the borehole field and the heat pump’s long-term energy efficiency. The simulations show considerable differences in the system’s seasonal energy efficiency. The constant speed of the circulation pumps leads to the best results as far as the heat pump’s energy performance was concerned, but this advantage was lost because of the greater amount of electrical energy used by the circulation pumps; this, of course, affects the energy efficiency of the entire system. The optimal solution appears then to be a constant temperature difference in the heat-carrier fluid across the heat pump.

Energy performance and cost analysis of some borehole heat exchanger configurations with different heat-carrier fluids in mild climates

The popularity of ground source heat pump systems for both heating and cooling has grown significantly over recent years. Ground heat exchangers are usually buried in the ground either vertically in boreholes or horizontally in trenches.Antifreeze fluids in closed-loop systems are commonly used in these plants to protect them from freezing phenomena and also to reduce the total length of the ground exchangers. In fact, the use of antifreeze fluid allows the system to work below 0°C which implies higher temperature difference between the heat-carrier fluid and the undisturbed ground, consequently the heat flux increases and the total length of the boreholes can be reduced.This study has been set out to investigate three types of layout system: two conventional heat pumps using pure water and water-glycol respectively as the secondary fluid and an innovative heat pump with a flooded evaporator using pure water.The results demonstrated that, for conventional heat pumps with dry evaporator, the use of anti-freeze additives in mild climates is convenient only in grid-shaped borehole fields with a heating-dominant thermal load. In all other cases, the use of pure water decreases the overall operating costs. In addition, flooded evaporator heat pumps using pure water as a secondary fluid on the ground loop proved to be the most cost-effective solution.

A linear Fresnel reflector as a solar system for heating water: Theoretical and experimental study

This work is concerned with assessing the thermal performances of a solar water heating system which is dependent on a linear Fresnel receiver (LFR) as a solar energy converter. The main objective of this paper is validation the experimental work carried out in the winter of 2015 on the concentrator in the climatic conditions of Algerian city “Blida” by a numerical simulation, where the tap water used as a heat carrier fluid. This simulation was used to solution of the energy balance equations of the absorber tubes and the water, where the solution is based on the finite difference method with an implicit scheme. After the solution of nonlinear equations, the program performed by using the MATLAB language gives the thermal efficiencies, the absorber temperatures, the water temperatures at the absorber tubes outlet, and thermal losses coefficients. The thermal efficiency of the reflector is exceeded 29%. The results obtained proved the existence of substantial convergence between the experimental and the numerical results, where in all cases the water temperature exceeded 347K.

Novel Solar Thermal Collector Systems in Polymer Design – Part 3: Aging Behavior of PP Absorber Materials

A novel, accelerated aging test method was used to characterize the long-term stability of commercial black-pigmented polypropylene (PP) model materials for solar thermal absorbers at elevated temperatures. The PP model materials investigated, PP-B1 and PP-B2, are based on carbon black pigmented PP block copolymer grades. Using an automatized planning technique, sliced 100 μm thick specimens were prepared, aged in hot air and heat carrier fluid (mixture of 60 vol.-% deionized water and 40 vol.-% commercial propylene glycol) at 95 ̊C, 115 ̊C and 135 ̊C for up to 15,000 hours, and characterized in terms of various aging indicators (i.e., remaining primary stabilizer content, oxidation temperature, carbonyl index and ultimate mechanical properties). In general two major trends were discerned. First, the aging processes of the PP compounds depend on the stabilizer system, but even more heavily on the interaction of the stabilizers with the carbon black pigments and the structure and morphology of the polymer. Although the compound PP-B2 exhibited much faster stabilizer loss and an associated drop in oxidation temperature than PP-B1, mechanical investigations proved a better long-term stability for PP-B2. Second, it was shown for the compounds investigated that exposure to hot air causes harsher aging than exposure to hot heat carrier fluid. This is, presumably related to the reduced quantity of dissolved oxygen and triazole-based corrosion inhibitors used in the heat carrier fluid. While PP-B1 is use for absorbers in unglazed collectors and overheating-protected glazed collectors, the investigations clearly revealed that PP-B2 is a promising alternative.

G.POT: A quantitative method for the assessment and mapping of the shallow geothermal potential

GSHPs (Ground source heat pumps) exchange heat with the ground to provide sustainable heating or cooling. Their technological feasibility and economic viability depend on the site-specific thermal properties of the ground and on the usage profile of the plant. These parameters influence the shallow geothermal potential, which is defined as the thermal power that can be efficiently exchanged by a BHE (Borehole Heat Exchanger) of a certain depth. We present a general method (G.POT) for the determination of shallow geothermal potentials. This method was derived using a comprehensive set of analytical heat transfer simulations, performed by varying (i) the thermal properties of the ground, which comprise its thermal conductivity and capacity, (ii) the thermal properties of the borehole, and (iii) the operational and design parameters of the plant, namely, the BHE length, the threshold temperature of the heat carrier fluid, the duration of the heating/cooling season and the simulated lifetime. Therefore, the G.POT method is a simple and flexible tool that can be implemented in a wide range of different scenarios for large-scale mapping of geothermal potentials. We also assess G.POT by discussing its application to map the geothermal yield in the Province of Cuneo (Piemonte, NW Italy).

Temperature change affected groundwater quality in a confined marine aquifer during long-term heating and cooling

Global warming and urbanization together with development of subsurface infrastructures (e.g. subways, shopping complexes, sewage systems, and Ground Source Heat Pump (GSHP) systems) will likely cause a rapid increase in the temperature of relatively shallow groundwater reservoirs (subsurface thermal pollution). However, potential effects of a subsurface temperature change on groundwater quality due to changed physical, chemical, and microbial processes have received little attention. We therefore investigated changes in 34 groundwater quality parameters during a 13-month enhanced-heating period, followed by 14 months of natural or enhanced cooling in a confined marine aquifer at around 17 m depth on the Saitama University campus, Japan. A full-scale GSHP test facility consisting of a 50 m deep U-tube for circulating the heat-carrying fluid and four monitoring wells at 1, 2, 5, and 10 m from the U-tube were installed, and groundwater quality was monitored every 1–2 weeks. Rapid changes in the groundwater level in the area, especially during the summer, prevented accurate analyses of temperature effects using a single-well time series. Instead, Dual-Well Analysis (DWA) was applied, comparing variations in subsurface temperature and groundwater chemical concentrations between the thermally-disturbed well and a non-affected reference well. Using the 1 m distant well (temperature increase up to 7 °C) and the 10 m distant well (non-temperature-affected), the DWA showed an approximately linear relationships for eight components (B, Si, Li, dissolved organic carbon (DOC), Mg2+, NH4+, Na+, and K+) during the combined 27 months of heating and cooling, suggesting changes in concentration between 4% and 31% for a temperature change of 7 °C.

Lifetime modeling of polypropylene absorber materials for overheating protected hot water collectors

For the utilization of polymeric materials in high-demanding applications like solar thermal systems it is of utmost importance to define the performance requirements and to investigate the applicability of components for defined systems under service relevant conditions. This paper deals with the lifetime estimation of black-pigmented polypropylene (PP) absorber grades for overheating protected solar thermal collector systems for hot water preparation in five representative climate zones. Based on experimental aging data in hot air and heat carrier fluid at elevated temperatures (95°C, 115°C and 135°C) and climatic input data, as well as deduced loading conditions and absorber temperature distributions, the lifetime was calculated using a theoretical and an empirical extrapolation approach and assuming cumulating damages in service relevant temperature intervals. Depending on the PP grade, the extrapolation method and the location, endurance limits ranging from 8 to 50years were obtained. The PP grade with ß-spherulithic structures and less carbon black exhibited a superior performance (factor 2) compared to a well-established grade which is currently widely used for swimming pool absorbers.

Geothermal Geotechnics for Urban Undergrounds

Underground urbanization as a prerequisite for sustainable development involves the design and construction of underground spaces in difficult ground and under very restricted conditions. Additionally, it offers the possibility of using ground structures also as thermo-active heat exchangers for heating and cooling of all kinds of buildings. This keynote paper focuses on near-surface geothermal geotechnics, i.e. energy piles and other energy foundations, energy walls, energy tunnels etc. This represents an innovative technology that contributes to environmental protection and provides substantial long-term cost savings and minimized maintenance. Absorber pipes filled with a heat carrier fluid are installed within conventional structural elements (piles, barrettes, diaphragm walls, basement slabs or walls, tunnel linings), forming the primary circuit of a geothermal energy system. The natural ground temperature is used as a heat source in winter and for cooling in summer. Hence not additional elements have to be installed below surface. The primary circuit is then connected via a heat pump to a secondary circuit within the building. The paper deals with earth-contact concrete elements that are already required for structural reasons, but which simultaneously work as heat exchangers. For instance, several new metro stations in Vienna are equipped with this sustainable technology. Theoretical details, pilot research projects and case histories bridge the gap between theory and practice, and special applications reveal the wide field of geothermal geotechnics. Design recommendations are given. Finally, benefits of thermo-active ground structures and recommendations for promoting geothermal energy utilization are listed.

Possibilities of Reducing Energy Consumption by Optimization of Ground Source Heat Pump Systems in Babylon, Iraq

Iraq is located in the Middle East with an area that reaches 437,072 km2 and a population of about 36 million. This country is suffering from severe electricity shortage problems which are expected to increase with time. In this research, an attempt is made to minimize this problem by combining the borehole thermal energy storage (BTES) with a heat pump, the indoor temperature of a residential building or other facility may be increased or reduced beyond the temperature interval of the heat carrier fluid. Due to the relatively high ground temperature in Middle Eastern countries, the seasonal thermal energy storages (STES) and ground source heat pump (GSHP) systems have a remarkable potential, partly because the reduced thermal losses from the underground storage and the expected high COP (ratio of thermal energy gain to required driving energy (electricity)) of a heat pump, partly because of the potential for using STES directly for heating and cooling. In this research, groundwater conditions of Babylon city in Iraq were investigated to evaluate the possibility of using GSHP to reduce energy consumption. It is believed that such system will reduce consumed energy by about 60%.

Numerical Model of Energy Foundation Behavior: The Prototype of a Geothermal Micro-pile

This paper presents the results of a numerical study performed in the design stage of an innovative geothermal technology – the geothermal micro-piles – recently developed and currently under testing at the University of Perugia for the exploitation of low enthalpy geothermal energy in existing buildings. In this investigation, micro-piles are equipped with a primary circuit of a traditional GSHP system, where the circulation of a heat carrier fluid (i.e. glycolic water) permits a thermal interaction with the surrounding soil. The numerical study has been performed to simulate the thermal behavior of such prototype and to obtain useful information concerning its functioning under real operating conditions. The first results of the study showed that this new technology can provide a thermal flux comparable with the one provided by traditional geothermal piles.

Analysis of Vertical Ground Heat Exchangers: The New CaRM Tool

The ground source heat pump systems are worldwide used for space heating and cooling of buildings. The energy efficiency of the heat pump depends on the temperature of the heat carrier fluid on the ground side, which is affected by the annual ground load profile and the arrangement of the boreholes.This paper conducts long-term analysis of two office buildings with unbalanced load profiles in Italy. Work focuses the effects of the heat imbalance on the heat pump entering fluid temperature over ten simulated years. A detailed numerical simulation tool was used to conduct the analysis.