Constant Heat Load Research Articles

Investigating scalability of deep borehole heat exchangers: Numerical modelling of arrays with varied modes of operation

Deep Borehole Heat Exchangers (DBHEs) are a potentially important method of developing geothermal resources through closed-loop systems for carbon neutral, spatial heating. Past research has primarily focused on single-well systems, with few investigating arrays of multiple DBHEs as a method of extracting more thermal energy. In this study, a series of arrays were modelled using OpenGeoSys software, with the aim of understanding the influence of array geometry, inter-borehole spacing and the mode of operation on the thermal performance and system efficiency. OpenGeoSys software is a finite-element model which solves thermal fluxes through the wellbore and surrounding rock using the dual-continuum method. Simulations were undertaken for the lifetime of an array (20 years) with modes of operation testing 1) long-term constant heat load application and 2) intermittent operation with 6 months of extraction followed by a recovery period. Results indicate geometry and mode of operation had a significant impact on inter-borehole spacing and system performance. For long term constant heat load application of 50 kW per DBHE, the minimal spacing required for line and square arrays should be 40 and 30 m. When considering intermittent operation, recovery periods allow replenishment of heat around the borehole, meaning smaller spacing can be utilised.

Numerical Investigation of Cooling Tower Performance under Constant Heat Load

Numerical Investigation of Cooling Tower Performance under Constant Heat Load

Determination of methane emissions from biogas plants, using different quantification methods

Reliable and comparable quantification methods are needed for assessing the effectiveness of the biogas production and utilisation process in mitigating methane (CH4) emissions as well as improving the database for emission inventories. The objective of this study was to compare and validate CH4 emissions quantified at two agricultural biogas plants, for up to three days, using diverse on-site (two teams) and off-site methods (three teams), including differential absorption lidar (DIAL), tracer gas dispersion (TDM) and inverse dispersion modelling (IDM). For plant 1, with a constant combined heat and power (CHP) load, the average emission factor varied from 0.3% CH4 (on-site approaches) to 1.2% CH4 (off-site approaches). On-site approaches underestimated overall emissions due to many small (unquantified) CH4 leakages. All methods observed comparable average emission factors for plant 2, ranging between 1.9 and 2.2% CH4. In this case, the majority of emissions emanated from just a few sources. However, correcting the significant influence of the varying CHP load during the measurement campaign revealed significant differences between TDM and IDM (DIAL did not participate). It was demonstrated that TDM and IDM could recover the emission rate from a known point source (controlled release of CH4 via a small diffuser) within an accuracy of 93 ± 15% (TDM) and 92 ± 17% (IDM) under favourable and similar conditions.

Au- and Ag-Containing Contacts to GaSb-Photovoltaic Converters

The issues concerning the stability of contact systems in GaSb photovoltaic converters and their reliability enhancement have been considered. It is established that at thermally induced degradation conditions (i.e., at the constant heat load at 80, 120, and 200 °C), the contacts with a conducting Ag layer, namely, Ti-Pt-Ag, Ti-Pt-Ag-Au, Cr-Au-Ag-Au have an advantage over those of conducting Au layer, i.e., Ti-Pt-Au, Cr-Au, while silver-containing metallization Ti-Pt-Ag and Ti-Pt-Ag-Au are more stable under temperature than Cr-Au-Ag-Au one.

Effect of PCM fill ratio and heat sink orientation on the thermal management of transient power spikes in electronics

The goal of this paper is to investigate the usefulness of Phase Change Material based heat sinks in power surge operations. Experiments have been carried out on a PCM based heat sink for different fill ratios (0, 33, 66, and 99%) of the PCM and different orientations (0, 90, 180°) of the heat sink under constant and power surge heat loads. The heat sink with a fill ratio of 0% is considered as the baseline case for comparison. The heat sink with a fill ratio of 66% at 0° orientation recorded lower temperatures among all the fill ratios and orientations under both constant and power surge heat loads. Partial filling (66% fill ratio) of the PCM in the cavity is more effective than complete filling (99% fill ratio) in handling both constant and power surge heat loads.

Performance Analysis of the Developed Air Source Heat Pump System at Low-to-Medium and High Supply Temperatures for Irish Housing Stock Heat Load Applications

Decarbonization of the UK residential heating sector is crucial to cut the carbon emissions and meet the legal binding of the Climate Change Act, 2008. The current progress with residential building sector carbon neutrality is slow and, hence, acceleration in action is required. The heat pump (HP) technology was found to be a potential candidate for sustainable development instead of fossil fuel-based oil/gas boilers, but improvement in its coefficient of performance (COP) is essential to compete with the lower gas/oil unit energy cost. The number of studies found in the literature were very limited, with the customized prototype development in the context of Northern Ireland, but without considering the simultaneous impact of heat supply temperature and operating mode of control for performance improvement in different property types. It is evidenced in the literature that the variable speed capacity control approach could improve the annual performance, but the literature has not looked into the compressor efficiencies challenges. In this study, steady state testing with a range of fixed constant heat loads (3–18 KW), done by varying compressor speed and its impact on COP, compressor efficiencies, and inverter losses, was established. The HP performance was measured and evaluated at low (35 °C)-to-medium (45 °C) and high (55 °C) heat supply temperature levels under the controlled laboratory conditions over the experienced ambient temperature. According to the result the COP values varies according to heat supply temperature, ambient temperature conditions, and heating capacity. The HP annual performances with Irish housing stock were evaluated in two modes of control and three case studies (C1, C2, C3) based on the experimentally validated model. The heat load demand in five property types with four age periods were considered in the analysis. The system could meet the required heat load demand for all property types in VSM with different percentage improvements in performance in comparison to FSM depending on the considered case level of the heat supply temperature (C1, C2, C3).

Domestic Retrofit Assessment of the Heat Pump System Considering the Impact of Heat Supply Temperature and Operating Mode of Control—A Case Study

In this study, performance assessment of the variable speed compressor-based air source heat pump (ASHP) system as a domestic retrofit technology instead of fossil fuel-based heating technologies for the 1900s Mid terraced house is investigated. The assessment was conducted considering operating mode of control and heat supply temperature impact of the system. In the literature, ASHP system experimental development with variable speed mode (VSM) of control in comparison to fixed speed mode (FSM) of control at low to medium and high heat supply temperature in the context of UK was found with very limited number of studies, but without considering retrofit application. The focus of the earlier studies was on the individual components and performance improvement. The designed heat pump (HP), developed, and tested at constant heat load, simulating the real domestic heat demand under the controlled laboratory conditions and numerical modeling is utilized for the analysis purposes. The HP performance, energy demand, carbon emissions, and cost varies significantly due to changing heat supply temperature (35 °C, 45 °C, and 55 °C), control mode and accordingly the carbon emission and cost savings are achieved. The oil and gas boilers ranges from conventional to highly efficient type and evaluated in terms of annual running cost, energy consumptions, and carbon emissions in comparison with the HP system. Additionally, a comparative study with the existing retrofitted very high temperature ASHP inside the house is conducted. The developed HP at 55 °C could not defeat the very high heat supply temperature HP system (75 °C supply temperature) in performance and cost savings but become attractive at low supply temperature (35 °C). The HP system in VSM at low heat supply temperature instead of gas boiler (90% efficiency) could cut the annual carbon emissions by 59% but with additional 6% running cost for the Mid terraced test house in Belfast climatic conditions.

Experimental evaluation of zeotropic refrigerants in a dedicated mechanical subcooling system in a CO2 cycle

• Zeotropic refrigerants are evaluated experimentally in dedicated subcooling system in a CO 2 plant. • Temperature glide of mixtures allows reducing irreversibilities in the subcooler. • A CO 2 refrigeration plant is tested at three heat rejection and one evaporation temperatures. • Optimum working conditions, heat rejection pressure and subcooling degree, are determined. • R-600/R-152a [60/40%] provides 1.4% COP increment in relation to R-152a. Use of zeotropic blends in the dedicated mechanical subcooling system of a CO 2 refrigeration system was suggested as a possible improvement due to matching of evaporating temperature with CO 2 temperature profile during subcooling. This work has verified this possibility and has determined theoretically the best performing compositions of R-600, R-32 and CO 2 with the base fluid R-152a. Then, the mixtures have been tested experimentally in a lab-test bench for constant heat load temperature for three heat rejection temperatures (25.1, 30.3 and 35.1°C). Optimum conditions are measured (subcooling degree and heat rejection) and a COP increase of 1.4% has been obtained. The work, for the optimum conditions, analyses the operating parameters of the cycles and focus specially on the thermal parameters of the subcooler. It has been verified that the use of zeotropic mixtures allows to reduce irreversibilities in the cycle, as pointed out theoretically by Dai et al. (2018).

Absorption Heat Transformer for Solar Pond Energy Temperature Upgrading

Solar energy has intermittent heat collection due the atmospheric phenomena, even the clarity index is widely evaluating the approximation for modelling the solar energy always is a function of the time. Solar ponds are a technology for solar energy accumulation, this technology has evaluations for tilt angles and salt concentration temperature variation and has availability with small variation along 24 hours a day. This technology may supply constant heat load to a thermodynamic cycle for temperature upgrade from the solar pond temperature level. Absorption heat transformer is a Type II heat pump to take a heat source part of the thermal energy to increase the temperature of the half heat source, in this case, from a solar pond. This article shows the data for several operating conditions of an absorption heat pump operating whit Carrol © / water solution. The main result is the gross temperature lift from solar pond technology with an AHT operating with condensation into the same solar pond to increase the Coefficient of Performance.

Heat Transfer and Pressure Drop Characteristics of Hybrid Al2O3-SiO2

The emergence of Proton Electrolyte Membrane Fuel Cell (PEMFC) is seen as one of the greener alternatives to internal combustion engine (ICE) in automotive industry nowadays. However, the sensitivity of a PEMFC membrane needs to be further improved for the optimum performance. In this study, single and hybrid nanofluids are used as an alternative coolant in a single channel of PEMFC. The study was conducted in a channel with the adoption of single and hybrid nanofluids to observe the effect of the heat transfer and pressure drop in the set up. A heater pad was kept at constant heat load of 100 W, reflecting the actual heat load of a single cell. The study focuses on 0.5% volume concentration for single and hybrid nanofluids of Al2O3 and SiO2 in water. The numerical study was conducted via ANSYS FLUENT 16.0. The 0.5% hybrid Al2O3:SiO2 nanofluids of 10:90 and 30:70 in Re range of 300 to 1000 were compared to the base fluid. The heat transfer increament of 9.1% and 9.85%. was observed. However, the pressure drop was also increased in the range of 22 Pa to 26 Pa for 0.5 % of Al2O3:SiO2 of 10:90 and 30:70 consecutively as compared to the base fluid. The advantage ratio was then analyzed to show the feasibility of both ratios of 0.5 % hybrid nanofluid of Al2O3:SiO2 at 10:90 and 30:70. It shows that both have advantage ratio higher than 1, thus feasible for the adoption in a PEMFC.

Influence of the Fin Shape on Heat Transport in Phase Change Material Heat Sink with Constant Heat Loads

Nowadays, the heat transfer enhancement in electronic cabinets with heat-generating elements can be achieved using the phase change materials and finned heat sink. The latter allows to improve the energy transference surface and to augment the cooling effects for the heat sources. The present research deals with numerical analysis of phase change material behavior in an electronic cabinet with an energy-generating element. For an intensification of heat removal, the complex finned heat sink with overall width of 10 cm was introduced, having the complicated shape of the fins with width of 0.33 cm and height H = 5 cm. The fatty acid with melting temperature of 46 °C was considered as a phase change material. The considered two-dimensional challenge was formulated employing the non-primitive variables and solved using the finite difference method. Impacts of the volumetric heat flux of heat-generating element and sizes of the fins on phase change material circulation and energy transference within the chamber were studied. It was shown that the presence of transverse ribs makes it possible to accelerate the melting process and reduce the source temperature by more than 12 °C at a heat load of 1600 W/m. It should also be noted that the nature of melting depends on the hydrodynamics of the melt, so the horizontal partitions reduce the intensity of convective heat transfer between the upper part of the region and the lower part.

Thermal Performance of a Phase Change Material-Based Heat Sink Subject to Constant and Power Surge Heat Loads: A Numerical Study

Abstract A power surge is a frequent phenomenon that occurs in electronics. Inadequate and improper cooling during power surges results in a rapid increase in operating temperatures that may lead to failure of the electronics. In the present investigation, the thermal characteristics of a phase change material (PCM)-based heat sinks, having different configurations and orientations of fins, subject to (i) constant heat load and (ii) heat load with a power surge, are studied numerically. Preliminary investigations showed that a heat sink with PCM gets heated at a much lower temperature than an air cooled heat sink. Following this, four finned heat sinks are considered for further investigations. The heat sink with PCM, sans fins, is used for baseline comparison. The orientation of fins in the other four heat sinks is either vertical or horizontal with square and rectangular cross sections. The heat sink and fins are made of aluminum, and the PCM used is n-eicosane (C20 H42). The enthalpy-porosity method is used to model the solid–liquid phase change in the PCM. All the transient three-dimensional numerical simulations are carried out using ansys fluent 15.0. For a constant heat load of 5 W and power surges of various magnitudes at different time instants, the heat sink with vertical square fins shows superior performance. However, the performance variation among the heat sinks with different fin configurations is insignificant for constant heat load. Even so, for power surges, the location and the configuration of fins have a significant effect on the heater temperature. Cases with high power surge and shorter duration of the surge were also considered to critically examine the effect of fins in controlling the maximum temperature in the heat sink. The numerical results of the best-performing heat sink, i.e., the heat sink with vertical square fins, are finally validated against in-house experiments.

Charging Characteristics of Series Connected Insulation and No-Insulation HTS Coils by Rotary HTS Flux Pump

High-temperature superconducting (HTS) coil wounds without turn-to-turn insulation (NI coils) present excellent electrical and thermal performance compared to HTS coil wounds with Kapton tape (INS coils). However, NI HTS coils are physically connected to power supplies at room temperature, resulting in increased operating costs due to complex cooling systems and constant heat loads. To overcome this issue, recent studies have investigated alternative charging methods such as flux pump, a power source without any physical contact with the HTS coil at cryogenic temperatures. Even though the NI coil is charged using a flux pump, the disadvantageously slow charge and discharge time remain. In this study, we conducted experiments to compare the charging characteristics of INS and NI coils using a rotary HTS flux pump. A superconducting circuit comprising two coils with equal inductance magnitude is connected in series. From the experiments, we compare the charging characteristics of charging time constant and magnetic field when the same current is applied to the two coils with a power supply and a rotary HTS flux pump. In the NI coil rotary HTS flux pump case, the charging time constant converged to about 6 s in the alternating magnetic field (AMF) above 20 Hz, and the value is similar to the power supply. At 60 Hz and higher AMF, the maximum magnetic field difference from the power supply converged to less than 1%, which appears to be a difference in current and voltage source.

Evaporation heat transfer characteristic of porous wick in an open capillary evaporator

An open capillary evaporator is proposed to study the evaporation heat transfer performance of porous wick. Both temperature and evaporating mass are measured in this system. Besides, the temperature and relative height of the supplied fluid can be adjusted. A composite porous wick is tested through the constant and variable heat load operation, and it reaches a maximum heat load of 320 W, which corresponds to a heat flux of 25.5 W/cm2. Two types of the evaporator wall temperature curve are observed, and the effect of the initial condition is considered. Intermittent fluid supply appears and causes a temperature overshoot of the evaporator wall. But continuous intermittent fluid supply and small amount of fast-replenishing fluid does not cause the temperature overshoot. The influence of adding water on temperature overshoot is also studied. The boiling and evaporation occur at different heat loads, and the two heat transfer modes coexist at moderate heat load. The transition in heat transfer modes leads to that the evaporation heat transfer coefficient first increases and then decreases with the increase of heat load. This study is helpful to analyze the evaporation heat transfer performance of porous wick and understand the operating characteristics of loop heat pipe.

Coupling of engine exhaust and fuel cell exhaust with vapour absorption refrigeration/air conditioning systems for transport applications: A review

Using the residual heat from the engine exhaust and of late from the fuel cell exhaust to drive a refrigeration or air conditioning unit on-board a vehicle has been the interest of many research groups worldwide. Umpteen number of modelling studies and a few prototypes have been built in this area. In this paper, an up to date review of the heat driven absorption refrigeration/air conditioning systems specifically meant for transport applications is given. This is followed by a discussion on the major challenges involved in implementing such a technology for the transport sector, the ways in which such a technology can be developed further and why using heat driven refrigeration/air conditioning systems could be a game changer in the automotive industry. From the study carried out two things are apparent – there is currently no VARS unit that can readily be used on-board vehicles and linking VARS units with engine exhaust leads to drop in engine efficiency and thus overall vehicle performance. Fuel cells (SOFCs in particular), if used as APUs can reduce the load on the engine and also supply a constant heat load to the VARS and thus be more effective.

Energy and exergy analysis of wind farm integrated with compressed air energy storage using multi-stage phase change material

One of the most common issues associated with wind energy penetration as a viable resource is high instability under diverse amplitudes and frequencies. Compressed air storage systems (CAES) and thermal energy storages (TES) not only are promising alternative solution for renewable resources generally, but they are also used with the aim of peak shaving to provide cleaner and affordable power. Due to high storage density with low temperature gradient between storing and retrieving in comparison with sensible heat storage, phase change material (PCM) is selected considering their thermophysical properties. This work, presents a recovery approach by using PCM during 12 h’ compression periods to retrieve it in 12 h’ expansion as a case study in November for Baladeh (a city in north of Iran). It is worthy to mention that, in order to use PCM in multiple mentioned stages, auxiliary heaters facilitate system to provide constant heat load in each used stage. This concludes system to be more efficient, sustainable and can reduce irreversibilities. Temperature of PCMs and amount of heat stored and released versus time are modeled. A good agreement between PCM model and reported results of related articles was seen. Following to the energy and exergy balances calculation, round-trip efficiency (RTE) and total exergy efficiency of system, reach 70.83%, 80.71%. Exergy analysis showed that near 60% exergy destruction occurred in PCMs with major contribution of second ones in both charge and discharge times. Sensitivity analysis is also investigated assuming the effect of number of wind turbines which shows a reduction in RTE, steeper slope in increasing TES volume for PCM3 and amount of heat stored, by increasing them.

Thermal analysis and cryogenics of the Baby-IAXO magnet

Baby-IAXO is a new helioscope, a demonstration version of the proposed full-size IAXO, the International Axions Observatory. It is currently under design and once installed it will search via the Primakoff effect for axions or axion-like particles (ALPs) originating in the Sun. Axions can transform into photons in the presence of a significant magnetic field, and then be detected. The superconducting magnet for Baby-IAXO comprises two 10 m long racetrack coils, spaced by 0.8 m, operated in a quadrupole configuration and generating an average magnetic field of 2.1 T in the two 700 mm diameter free bores for axion-to-photons decay positioned in between the coils. Cool down and operational cryogenic requirements are handled by a combination of single and double stage cryocoolers. For the cool down of the 18.5 t cold mass within 22 days and for current leads cooling, the use of powerful single stage cryocoolers is of paramount importance. The cooling power is distributed across the cold mass and thermal shield using helium gas flow enforced by cryocirculators. The design of the 10 kA conduction cooled current leads is presented as well, since at 70 K these represent the dominant heat load. The cryogenic system allows for a constant heat load of 1.2 kW at the first stage and 8.5 W at the 4.2 K cold mass. Alternatively, the magnet may be operated in persistent mode, thereby reducing the heat load and the number of cryocoolers. It requires a more complex cooling circuit, implementation of a 10 kA persistent mode switch, and a delicate balance of energizing and operational heat loads.

Experimental analysis of SiO2-Distilled water nanofluids in a Polymer Electrolyte Membrane fuel cell parallel channel cooling plate

An experimental report on the thermal performance of Silicone Dioxide (SiO2) nanofluid coolants based on a PEM fuel cell cooling system is presented. The aim of this study is to evaluate the feasibility of applying these nanofluids coolants as an alternative to conventional distilled water through detailed analysis of thermofluids behaviour in a simulated cooling plate environment. SiO2 nanoparticles were dispersed in distilled water at 0.1%, 0.3% and 0.5% volume concentrations and tested in a parallel channel cooling plate system. A constant heat load was supplied to simulate a fuel cell stack thermal condition. At inlet flow conditions from 750 to 900 Reynolds number, the SiO2 nanofluids reduced the average plate temperatures by 15%–20% compared to conventional water coolant. The nanofluids also increased the cooling effectiveness by a similar margin, as well as improving the bulk heat transfer coefficient to a range between 2700 and 4400 W m−2. oC−1. However, the required pumping power was also increased due to the added viscous effect. Through the Advantage Ratio (AR) analysis, it was concluded that the enhancement in heat transfer mechanics was more significant than the penalties in fluid flow dynamics. Thus, the SiO2 nanofluids and the cooling plate design are possible options for advanced PEM fuel cell thermal management practice in future stack designs.

Experimental Investigation of Heat Transfer Enhancement of CuO–Water and ZnO–Water Nanofluids Flowing Over a Heated Plate

In this paper, experimental investigation has been performed to characterize the heat transfer behavior of CuO–water and ZnO–water nanofluids. Nanofluids containing different volume percent (vol %) of nanoparticle concentrations flowed over a flat copper plate under a constant heat load. The constant heat flux was maintained using evenly placed cartridge heaters. The heat transfer coefficients of nanofluids were measured and compared with the results obtained from identical experiments performed with de-ionized (DI) water. In order to thoroughly characterize the nanofluids, nanoparticle size was investigated to inspect for possible agglomeration. The particle size was measured by using both a transmission electron microscope (TEM) and a dynamic light scattering system (DLS). Enhancement of convective heat transfer of nanofluids was 2.5–16% depending on the nanoparticle concentrations and Reynolds number. The plausible mechanisms of the enhanced thermal performance of CuO and ZnO nanofluids will be discussed in the following paper.

Experimental characterization of a self-adaptive shape memory alloy cooling approach to regulate temperature under varying heat loads

Electronic cooling devices typically aim to provide low thermal resistance, however, for many applications, temperature non-uniformity across the cooled object is also a key parameter, as it impacts reliability. Tailored microchannels or fin layouts are examples of cooling devices that can be designed to reach uniform wall temperatures but require known and constant heat load distributions. In this work, a self-adaptive liquid cooling device is proposed to provide constant and uniform temperature distributions under spatially and time dependent heat load scenarios. The working principle is based on the use of the Two-Way Shape Memory Effect (TWSME) of trained shape memory alloy (SMA) wings that elevate into the flow to reduce the convective thermal resistance when a critical temperature is reached. The conditions for proper operation are first defined analytically. Then, the impact of SMA wings on thermal resistance is experimentally investigated. Finally, the ability of the self-adaptive concept to maintain a constant wall temperature when the heat flux increases, is demonstrated. In the studied case, for fixed inlet coolant temperature and flow rate, the liquid cooling device was able to maintain the temperature of the cooled object within an interval of 7 °C for a heat flux varying from 32.8 to 67 W/cm2. Without self-adaptation, a temperature variation over 85 °C was expected for the same cooling device, corresponding to an order of magnitude improvement.