Fluid flow and heat transfer characteristics of cross-cut heat sinks

In this study, effects of cross-cuts on the thermal performance of heat sinks under the parallel flow condition are experimentally studied. To find effects of the length, position, and number of cross-cuts, heat sinks with one or several cross-cuts ranging from 0.5 mm to 10 mm were fabricated. The pressure drop and the thermal resistance of the heat sinks are obtained in the range of 0.01 W<P_p < 1 W. Experimental results show that among the many cross-cut design parameters, the cross-cut length has the most significant influence on the thermal performance of heat sinks. The results also show that heat sinks with a cross-cut are superior to heat sinks containing several cross-cuts in the thermal performance. Based on experimental results, the friction factor and Nusselt number correlations for heat sinks with a cross-cut are suggested. Using the proposed correlations, thermal performances of cross-cut heat sinks are compared to those of optimized plate-fin and square pin-fin heat sinks under the constant pumping power condition. This comparison yields a contour map that suggests an optimum type of heat sink under the constraint of the fixed pumping power and fixed heat sink volume. The contour map shows that an optimized cross-cut heat sink outperforms optimized plate-fin and square pin-fin heat sinks when 0.04 < log L1 < 1.     

Potential of geothermal heat exchangers for office building climatisation

Low depth geothermal heat exchangers can be efficiently used as a heat sink for building energy produced during summer. If annual average ambient temperatures are low enough, direct cooling of a building is possible. Alternatively the heat exchangers can replace cooling towers in combination with active cooling systems. In the current work, the performance of vertical and horizontal geothermal heat exchangers implemented in two office building climatisation projects is evaluated.A main result of the performance analysis is that the ground coupled heat exchangers have good coefficients of performance ranging from 13 to 20 as average annual ratios of cold produced to electricity used. Best performance is reached, if the ground cooling system is used to cool down high temperature ambient air. The maximum heat dissipation per meter of ground heat exchanger measured was lower than planned and varied between 8 W m^?1 for the low depth horizontal heat exchangers up to 25 W m^?1 for the vertical heat exchangers.The experimental results were used to validate a numerical simulation model, which was then used to study the influence of soil parameters and inlet temperatures to the ground heat exchangers. The power dissipation varies by ±30% depending on the soil conductivity. The heat conductivity of vertical tube filling material influences performance by another ±30% for different materials. Depending on the inlet temperature level to the ground heat exchanger, the dissipated power increases from 2 W m^?1 for direct cooling applications at 20 °C up to 52 W m^?1 for cooling tower substitutions at 40 °C. This directly influences the cooling costs, which vary between 0.12 and 2.8€ kW h^?1.As a result of the work, planning and operation recommendations for the optimal choice of ground coupled heat exchangers for office building cooling can be given.     

An experimental investigation of a household size trigeneration

A household size trigeneration based on a small-scale diesel engine generator set is designed and realized in laboratory. Experimental tests are carried out to evaluate the performance and emissions of the original single generation (diesel engine generator); and the performances of the whole trigeneration including the diesel generator within the trigeneration system, the heat exchangers which are used to recover heat from engine exhaust, the absorption refrigerator which is driven by the exhaust heat; and the emissions from the whole trigeneration.Comparisons of the test results of two generations are also performed. The test results show that the total thermal efficiency of trigeneration reaches to 67.3% at the engine full load, comparing to that of the original single generation 22.1% only. Within the range of engine loads tested, the total thermal efficiencies of trigeneration are from 205% to 438% higher than that of the thermal efficiency of single generation.The CO₂ emission per unit (kW h) of useful energy output from trigeneration is 0.401 kg CO₂/kW h at the engine full load, compared to that of 1.22 kg CO₂/kW h from single generation at the same engine load. Within the range of engine loads tested, the reductions of CO₂ emission per unit (kW h) of trigeneration output are from 67.2% to 81.4% compared to those of single generation.The experimental results show that the idea of realizing a household size trigeneration is feasible; the design and the set-up of the trigeneration is successful. The experimental results show that the innovative small-scale trigeneration is able to generate electricity, produce heat and drive a refrigeration system, simultaneously from a single fuel (diesel) input.     

Experimental investigations on phase change material based finned heat sinks for electronic equipment cooling

This paper reports the results of an experimental investigation of the performance of finned heat sinks filled with phase change materials for thermal management of portable electronic devices. The phase change material (PCM) used in this study is n-eicosane and is placed inside a heat sink made of aluminium. Aluminium acts as thermal conductivity enhancer (TCE), as the thermal conductivity of the PCM is very low. The heat sink acts as an energy storage and a heat-spreading module. Studies are conducted for heat sinks on which a uniform heat load is applied for the unfinned and finned cases. The test section considered in all cases in the present work is a 80 × 62 mm² base with TCE height of 25 mm. A 60 × 42 mm² plate heater with 2 mm thickness is used to mimic the heat generation in electronic chips. Heat sinks with pin fin and plate fin geometries having the same volume fraction of the TCE are used. The effect of different types of fins for different power level (ranging from 2 to 7 W) in enhancing the operating time for different set point temperatures and on the duration of latent heating phase were explored in this study. The results indicate that the operational performance of portable electronic device can be significantly improved by the use of fins in heat sinks filled with PCM.     

Assessing the economic wind power potential in Austria

In the European Union, electricity production from wind energy is projected to increase by approximately 16% until 2020. The Austrian energy plan aims at increasing the currently installed wind power capacity from approximately 1 GW to 3 GW until 2020 including an additional capacity of 700 MW until 2015. The aim of this analysis is to assess economically viable wind turbine sites under current feed-in tariffs considering constraints imposed by infrastructure, the natural environment and ecological preservation zones in Austria. We analyze whether the policy target of installing an additional wind power capacity of 700 MW until 2015 is attainable under current legislation and developed a GIS based decision system for wind turbine site selection.Results show that the current feed-in tariff of 9.7 ct kW h⁻¹ may trigger an additional installation of 3544 MW. The current feed-in tariff can therefore be considered too high as wind power deployment would exceed the target by far. Our results indicate that the targets may be attained more cost-effectively by applying a lower feed-in tariff of 9.1 ct kW h⁻¹. Thus, windfall profits at favorable sites and deadweight losses of policy intervention can be minimized while still guaranteeing the deployment of additional wind power capacities.     

Experimental study of an ammonia–water bubble absorber using a plate heat exchanger for absorption refrigeration machines

The development of absorption chillers activated by renewable heat sources has increased due mainly to the increase in primary energy consumption that causes problems such as greenhouse gases and air pollution among others. These machines, which could be a good substitute for compression systems, could be used in the residential and food sectors which require a great variety of refrigeration conditions. Nevertheless, the low efficiency of these machines makes it necessary to enhance heat and mass transfer processes in the critical components, mainly the absorber, in order to reduce their large size.This study used ammonia–water as the working fluid to look at how absorption takes place in a plate heat exchanger operating under typical conditions of absorption chillers, driven by low temperature heat sources. Experiments were carried out using a corrugated plate heat exchanger model NB51, with three channels, where ammonia vapor was injected in bubble mode into the solution in the central channel. The results achieved for the absorption flux were in the range of 0.0025–0.0063 kg m^?2 s^?1, the solution heat transfer coefficient varied between 2.7 and 5.4 kW m^?2 K^?1, the absorber thermal load from 0.5 to 1.3 kW. In addition, the effect of the absorber operating conditions on the most significant efficiency parameters was analyzed. The increase in pressure, solution and cooling flow rates positively affect the absorber performance, on the other hand an increase in the concentration, cooling, and solution temperature negatively affects the absorber performance.     

Experimental study on HFC125 critical heat pump

A critical cycle heat pump with HFC125 was studied experimentally. The experimental result indicates that the heat pump with HFC125 can use the general components of the conventional heat pump well. Hot water with wide-range temperature can be conveniently got by the critical heat pump system through water flow control. The COP_h of the critical cycle drops a little when the temperature of outlet water rises from 60 °C to 75 °C. And adding heat recovering exchanger cannot improve the performance of the cycle, but can reduce the working pressure of the cycle. Comparing with the CO₂ trans-critical heat pump, HFC125 critical heat pump has a better performance of refrigeration, lower working pressure, which is especially suitable for dual-function of supplying hot water and refrigeration in the civil and industrial buildings.     

System analysis of a multifunctional PV/T hybrid solar window

The work presented in this article aims to investigate a PV/T hybrid solar window on a system level. A PV/T hybrid is an absorber on which solar cells have been laminated. The solar window is a PV/T hybrid collector with tiltable insulated reflectors integrated into a window. It simultaneously replaces thermal collectors, PV-modules and sunshade. The building integration lowers the total price of the construction since the collector utilizes the frame and the glazing in the window. When it is placed in the window a complex interaction takes place. On the positive side is the reduction of the thermal losses due to the insulated reflectors. On the negative side is the blocking of solar radiation that would otherwise heat the building passively. This limits the performance of the solar window since a photon can only be used once. To investigate the sum of such complex interaction a system analysis has to be performed. In this paper results are presented from such a system analysis showing both benefits and problems with the product. The building system with individual solar energy components, i.e. solar collector and PV modules, of the same size as the solar window, uses 1100 kW h less auxiliary energy than the system with a solar window. However, the solar window system uses 600 kW h less auxiliary energy than a system with no solar collector.     

Technical and economic assessment of grid-independent hybrid photovoltaic–diesel–battery power systems for commercial loads in desert environments

Solar photovoltaic (PV) hybrid system technology is a hot topic for R&D since it promises lot of challenges and opportunities for developed and developing countries. The Kingdom of Saudi Arabia (KSA) being endowed with fairly high degree of solar radiation is a potential candidate for deployment of PV systems for power generation. Literature indicates that commercial/residential buildings in KSA consume an estimated 10–45% of the total electric energy generated. In the present study, solar radiation data of Dhahran (East-Coast, KSA) have been analyzed to assess the techno-economic viability of utilizing hybrid PV–diesel–battery power systems to meet the load requirements of a typical commercial building (with annual electrical energy demand of 620,000 kW h). The monthly average daily solar global radiation ranges from 3.61 to 7.96 kW h/m². NREL's HOMER software has been used to carry out the techno-economic viability. The simulation results indicate that for a hybrid system comprising of 80 kWp PV system together with 175 kW diesel system and a battery storage of 3 h of autonomy (equivalent to 3 h of average load), the PV penetration is 26%. The cost of generating energy (COE, US$/kW h) from the above hybrid system has been found to be 0.149 $/kW h (assuming diesel fuel price of 0.1 $/L). The study exhibits that for a given hybrid configuration, the operational hours of diesel generators decrease with increase in PV capacity. The investigation also examines the effect of PV/battery penetration on COE, operational hours of diesel gensets for a given hybrid system. Emphasis has also been placed on unmet load, excess electricity generation, percentage fuel savings and reduction in carbon emissions (for different scenarios such as PV–diesel without storage, PV–diesel with storage, as compared to diesel-only situation), cost of PV–diesel–battery systems, COE of different hybrid systems, etc.     

α–h Diagram and principle of exergy coupling of GAX cycle

Ammonia absorption chiller systems of a single-stage cycle and a Generator Absorber heat exchanger cycle (GAX) were simulated and studied. At heat source temperatures of T_H = 120 °C, T_M = 25 °C and T_L = 5 °C, the coefficient of performances of the two cycles are 0.589 and 0.776, the GAX cycle is higher 31.8% than the single-stage cycle. And the exergy efficiencies of the two cycles are 15.4% and 27.4%, the GAX cycle is higher up to 77.9%. This paper proposes a new method that adopts the energy quality factor α as a evaluation criterion and also uses the α–h diagram as a thermodynamic analysis tool graphically, and a concept that divides absorption cycle to a heat pump subcycle and a heat engine subcycle. By means of the α–h diagram, the thermodynamic frameworks of the two cycles were illustrated. The comparison analysis indicates that the improvement of cycle performance depends on its thermodynamic perfectibility. In fact, the exergy demand of heat pump subcycle in the GAX cycle is as the same as that of the single-stage cycle, however, the energy cascading use and the exergy coupling framework of the heat engine subcycle in GAX cycle is retrofitted, so that the exergy consumption is reduced and the increased benefit is obtained from the overall cycle.     

Theoretical analysis and experimental research on transcritical CO2 two stage compression cycle with two gas coolers (TSCC + TG) and the cycle with intercooler (TSCC + IC)

As one of the natural refrigerants, CO₂ is a potential substitute for synthesized refrigerants with favorable environmental properties. In order to improve the performance of the CO₂ transcritical compression cycle, the performance of the two stage compression cycle with two gas coolers (TSCC + TG) and the two stage compression cycle with intercooler (TSCC + IC) were analyzed, respectively. Under the given calculation condition, the optimum intermediate pressure of the cycle TSCC + TG and the TSCC + IC are 7.09 MPa and 5.89 MPa, and the maximal COP are 2.77 and 3.08, respectively. Range of the given evaporating temperature and outlet temperature of gas cooler, the experimental testing shows that the performance of cycle TSCC + IC are 11.88% and 10.87% better than that of the cycle TSCC + TG, respectively. Range of the given inlet temperature and cooling water volume flow of gas cooler, the refrigeration COP (COP_c) and heat COP (COP_h) of the cycle TSCC + IC are average 10.97% and 4.39% higher than that of the cycle TSCC + TG. Range of the given inlet temperature and chilled water volume flow of evaporator, the refrigeration COP (COP_c) and heat COP (COP_h) of the cycle TSCC + IC are average 10.71% and 3.67% higher than that of the cycle TSCC + TG, respectively. The error between theoretical calculation and experimental testing is not exceeds 20%.     

Nucleate pool boiling heat transfer coefficients of pure HFC134a,HC290, HC600a and their binary and ternary mixtures

An experimental test rig for study on the pooling-boiling heat transfer performance of pure and mixed refrigerants were designed and established. With this test system, the heat transfer coefficients (HTCs) of the nucleate boiling on a smooth flat surface were measured for pure fluids of HFC134a, HC290, HC600a and their binary and ternary mixtures. Extensive experimental measures were made for those pure and mixed refrigerants at different heat fluxes from 10 kW m⁻² to 300 kW m⁻² and different pressures from 0.2 to 0.6 MPa. Comprehensive measured data are presented in this paper. From experimental results, these binary mixtures and ternary mixtures show different heat transfer features according to their vapor–liquid phase equilibria behaviors. New heat transfer correlations were regressed from the measured data with average deviations within ±15% for pure refrigerants and within ±20% for mixtures.     

Overall thermal energy and exergy analysis of hybrid photovoltaic thermal array

In this paper, overall thermal energy and exergy analysis has been carried out for different configurations of hybrid photovoltaic thermal (PVT) array. The hybrid PVT array (10.08 m × 2.16 m) is a series and parallel combinations of 36 numbers of PV modules. A one-dimensional transient model for hybrid PVT array has been developed using basic heat transfer equations. On the basis of this transient model, an attempt has been made to select an appropriate hybrid PVT array for different climatic conditions (Bangalore, Jodhpur, New Delhi, and Srinagar) of India. On the basis of high grade energy (i.e. overall exergy gain), case-III has been selected as the most appropriate configuration because overall exergy for case-III is 12.9% higher than case-II. The overall thermal energy and exergy gain for Bangalore is 4.54 × 10⁴ kW h and 2.07 × 10⁴ kW h respectively which is highest in comparison to the other cities.     

Biomass energy used in a sawmill

Biomass-energy systems are considered to be environmentally superior to traditional ones from the viewpoints of the CO₂ mitigation and the effective utilization of resources. However, the energy cost of these systems tends to be higher than that of conventional fossil-fuel systems. Furthermore, the establishment of environmental business models is expected in the near future.In this paper, the environmental improvement and the economics of a biomass-energy system in a sawmill are analyzed by a comparison of a gasification-cogeneration system with a direct-combustion system using scrap-wood material as feedstock fuel. Especially, the break-even point for marketability of the business taking the surplus electric-power into consideration is estimated under the assumption of a renewable-energy purchase system, such as the renewable portfolio standard (RPS) implemented in Japan. Consequently, when biomass-related subsidies are applied, the break-even point of the purchase price of the electric power from the gasification cogeneration becomes 7.7 → 35.7 yen/kW h. Furthermore, if the construction cost decreases by 10%, the break-even point of the purchase price will be cheaper by about 7.4 yen/kW h.     

Assessing the life cycle environmental impacts of wind power: A review of present knowledge and research needs

We critically review present knowledge of the life cycle environmental impacts of wind power. We find that the current body of life cycle assessments (LCA) of wind power provides a fairly good overall understanding of fossil energy use and associated pollution; our survey of results that appear in existing literature give mean values (± standard deviation) of, e.g., 0.060 (±0.058) kW h energy used and 19 (±13) g CO₂e emitted per kW h electricity, suggesting good environmental performance vis-à-vis fossil-based power. Total emissions of onshore and offshore wind farms are comparable. The bulk of emissions generally occur in the production of components; onshore, the wind turbine dominates, while offshore, the substructure becomes relatively more important. Strong positive effects of scale are present in the lower end of the turbine size spectrum, but there is no clear evidence for such effects for MW-sized units. We identify weaknesses and gaps in knowledge that future research may address. This includes poorly understood impacts in categories of toxicity and resource depletion, lack of empirical basis for assumptions about replacement of parts, and apparent lack of detailed considerations of offshore operations for wind farms in ocean waters. We argue that applications of the avoided burden method to model recycling benefits generally lack transparency and may be inconsistent. Assumed capacity factor values are generally higher than current mean realized values. Finally, we discuss the need for LCA research to move beyond unit-based assessments in order to address temporal aspects and the scale of impacts.     

Modeling of heat transfer and energy analysis of potato slices and cylinders during solar drying

In the present work, a method based on energy balance considering the effects of heat capacity of the food product, radiative heat transfer from food product to the drying chamber and solar radiation absorbed in the product during drying is proposed for determination of convective heat transfer coefficient, h_c. A natural convection mixed-mode solar dryer is used for performing the experiments on potato cylinders and slices of same thickness of 0.01 m with respective length and diameter of 0.05 m. The present investigation indicates that the cylindrical samples exhibit higher values of h_c and faster drying rate compared to those of slices, as expected. The h_c values for each sample shape are correlated by an equation of the form Nu = C(Ra)ⁿ. Laplace transform is applied to solve the proposed heat transfer diffusion model considering the effect of moisture transfer rate to predict the transient sample temperature. The model is validated through a close agreement between calculated and experimental results of transient sample temperature. Results of energy analysis reveal that for both the sample geometries, decreasing product moisture content during drying resulted in significant reduction in specific energy consumption. For almost similar drying conditions, a considerable amount of reduction in specific energy consumption is achieved for cylinders, as expected.     

National-scale wave energy resource assessment for Australia

A nationally consistent wave resource assessment is presented for Australian shelf (<300 m) waters. Wave energy and power were derived from significant wave height and period, and wave direction hindcast using the AusWAM model for the period 1 March 1997 to 29 February 2008 inclusive. The spatial distribution of wave energy and power is available on a 0.1° grid covering 110–156° longitude and 7–46° latitude. Total instantaneous wave energy on the entire Australian shelf is on average 3.47 PJ. Wave power is greatest on the 3000 km-long southern Australian shelf (Tasmania/Victoria, southern Western Australia and South Australia), where it widely attains a time-average value of 25–35 kW m^?1 (90th percentile of 60–78 kW m^?1), delivering 800–1100 GJ m^?1 of energy in an average year. New South Wales and southern Queensland shelves, with moderate levels of wave power (time-average: 10–20 kW m^?1; 90th percentile: 20–30 kW m^?1), are also potential sites for electricity generation due to them having a similar reliability in resource delivery to the southern margin. Time-average wave power for most of the northern Australian shelf is <10 kW m^?1. Seasonal variations in wave power are consistent with regional weather patterns, which are characterised by winter SE trade winds/summer monsoon in the north and winter temperate storms/summer sea breezes in the south. The nationally consistent wave resource assessment for Australian shelf waters can be used to inform policy development and site-selection decisions by industry.     

Investigation of coated tubes in cross-flow boiling

The boiling in cross-flow is investigated for coated tubes (low-porosity, flame-sprayed) in this paper. The effect of surface roughness on flow boiling heat transfer for a horizontal tube surface in cross-flow is studied for saturated boiling of water at atmospheric pressure. The parameters varied were for flow velocity up to 3.24 kg/s (G = 258.49 kg/m² s), heat flux from 12 to 45 kW/m², surface roughness (Ra) from 0.3296 to 4.731 μm. Nominal enhancement in heat transfer coefficient at higher mass flux may be attributed to the continued nucleation at the uppermost surfaces (in the wake region of the flow) of the rougher tubes thereby increasing the overall heat transfer rate. The flow boiling data was found to best fit the Kutateladze asymptotic equation h = h_l[1 + (h_npb/h_l)ⁿ]^1/n with the value of n = 2.258 (which is close to the value of n = 2 suggested by Kutateladze).     

Heat transfer and pressure drop properties of high viscous solutions in plate heat exchangers

Heat transfer and pressure drop characteristics of an absorbent salt solution in a commercial plate heat exchanger serving as a solution sub-cooler in the high loop of triple-effect absorption refrigeration cycle was investigated. The main objectives of this research were to establish the correlation equations to predict the heat transfer and pressure drop and to analyze and optimize the operating parameters for use in the design of absorption systems.In order to conduct above studies, a single-pass cross-corrugated ALFA-LAVAL plate heat exchanger, Model PO1-VG, with capacity of 14,650 W (50,000 Btu/h) was used. In order to evaluate the performance, hot solution inlet temperatures from 55 °C (130 °F) to 77 °C (170 °F), and inlet temperature differences from 14 °C (25 °F) to 20 °C (35 °F) were used. The cold side of the heat exchanger was operated to match the equal heat capacity rate of hot side.Based on the empirical models proposed in the literature, a program was developed and experimental data were curve fitted. From the best-fitted curves, the power-law equations for heat transfer and pressure losses were established and the performance was evaluated.In the hot salt solution side, the Reynolds number was varied from 250 to 1100 and the resulting Nusselt number varied from 7.4 to 15.8. The measured overall heat transfer coefficient U_overall varied from 970 W/m² °C (170 Btu/h ft² °F) to 2270 W/m² °C (400 Btu/h ft² °F) and the Fanning friction factor in the absorbent side of the heat exchanger varied from 5.7 to 7.6. The correlation equations developed to predict the heat transfer and friction factor perfectly agree with the experimental results. Those equations can be used to predict the performance of any solution with Prandtl numbers between 82 and 174, for heat exchangers with similar geometry.