Energy and exergy analyses of OMW solar drying process

The energy and exergy analyses of the drying process of olive mill wastewater (OMW) using an indirect type natural convection solar dryer are presented. Olive mill wastewater gets sufficiently dried at temperatures between 34 °C and 52 °C. During the experimental process, air relative humidity did not exceed 58%, and solar radiation ranged from 227 W/m² to 825 W/m². Drying air mass flow was maintained within the interval 0.036–0.042 kg/s. Under these experimental conditions, 2 days were needed to reduce the moisture content to approximately one-third of the original value, in particular from 3.153 g_water/g_{dry matter} down to 1.000 g_water/g_{dry matter}.Using the first law of thermodynamics, energy analysis was carried out to estimate the amounts of energy gained from solar air heater and the ratio of energy utilization of the drying chamber. Also, applying the second law, exergy analysis was developed to determine the type and magnitude of exergy losses during the solar drying process. It was found that exergy losses took place mainly during the second day, when the available energy was less used. The exergy losses varied from 0 kJ/kg to 0.125 kJ/kg for the first day, and between 0 kJ/kg and 0.168 kJ/kg for the second. The exergetic efficiencies of the drying chamber decreased as inlet temperature was increased, provided that exergy losses became more significant. In particular, they ranged from 53.24% to 100% during the first day, and from 34.40% to 100% during the second.     

6 years of wind data for Marsabit,Kenya average over 14 m/s at 100 m hub height; An analysis of the wind energy potential

The wind energy potential for generation of electricity and for domestic water pumping has been investigated for Marsabit, Kenya. Marsabit (37° 58′N, 2° 19′E) lies in Eastern province approximately 560 km from Nairobi. Wind data from the Kenya Meteorological department for the period 2001–2006 has been used to study the Diurnal, monthly and inter-annual variability using empirical methods including the Power law and Weibull statistics. Average wind speeds greater than11 m/s at a height of 10 m are prevalent in this area. The available power density at a height of 100 m is between 1776 W/m² and 2202 W/m² which is in the wind class range of 7 and 8. The maximum extractable power density at 100 m varied between 1417 W/m² and 1757 W/m². Values of Weibull parameters k (dimensionless) and c (m/s) ranged between 2.5–3.05 and 11.86–12.97 respectively. Wind Rose analysis revealed no marked variation in wind direction and frequency throughout the year (mean direction between 150 and 160 degrees with highest standard deviation of 33.5 degrees). From the analysis, the site was found suitable for grid connected power generation and also for other stand-alone generators that can be used for water pumping and battery charging.     

Single layer solar drying behaviour of Citrus aurantium leaves under forced convection

Convective solar drying experiments in thin layers of Citrus aurantium leaves grown in Marrakech, morocco, were conducted. An indirect forced convection solar dryer consisting of a solar air collector, an auxiliary heater, a circulation fan and a drying cabinet is used for the experiments. The air temperature was varied from 50 to 60 °C; the relative humidity from 41% to 53%; and the drying air flow rate from 0.0277 to 0.0833 m³/s. Thirteen statistical models, which are semi-theoretical and/or empirical, were tested for fitting the experimental data. A nonlinear regression analysis using a statistical computer program was used to evaluate the constants of the models. The Midilli–Kucuk drying model was found to be the most suitable for describing the solar drying curves of Citrus aurantium leaves with a correlation coefficient (r) of 0.99998, chi-square (χ²) of 4.664 × 10⁻⁶ and MBE of 4.8381 × 10⁻⁴.     

Energy and exergy of potato drying process via cyclone type dryer

This paper is concerned with the energy and exergy analyses of the single layer drying process of potato slices via a cyclone type dryer. Using the first law of thermodynamics, an energy analysis was performed to estimate the ratios of energy utilization. An exergy analysis was accomplished to determine the location, type and magnitude of the exergy losses during the drying process by applying the second law of thermodynamics. It was concluded that the exergy losses took place mostly in the 1st tray where the available energy was less utilized during the single layer drying process of potato slices. It is emphasized that the potato slices are sufficiently dried in the ranges between 60 and 80 °C and 20–10% relative humidity at 1 and 1.5 ms⁻¹ of drying air velocity during 10–12 h despite the exergy losses of 0–1.796 kJ s⁻¹.     

Evaluation of convective heat transfer coefficient of various crops in cyclone type dryer

In this paper, an attempt was made to evaluate the convective heat transfer coefficient during drying of various crops and to investigate the influences of drying air velocity and temperature on the convective heat transfer coefficient. Drying was conducted in a convective cyclone type dryer at drying air temperatures of 60, 70 and 80 °C and velocities of 1 and 1.5 m/s using rectangle shaped potato and apple slices (12.5 × 12.5 × 25 mm) and cylindrical shaped pumpkin slices (35 × 5 mm). The temperature changes of the dried crops and the temperature of the drying air were measured during the drying process. It was found that the values of convective heat transfer coefficient varied from crop to crop with a range 30.21406 and 20.65470 W/m² C for the crops studied, and it was observed that the convective heat transfer coefficient increased in large amounts with the increase of the drying air velocity but increased in small amounts with the rise of the drying air temperature.     

Assessing the influence of four bonding methods on the thermal contact resistance of open-cell aluminum foam

A thermal application of open-cell aluminum foam typically requires it to be bonded on a substrate. The resulting thermal contact resistance is investigated for four bonding methods. This is done by minimizing the difference between the calculated heat transfer via a zeroth order model and experimental data. The bonded metal foam, used to obtain the experimental data, are manufactured in-house. This allows varying pore size, porosity, aluminum alloy, foam height, air mass flow rate, air inlet temperature and bonding method. The latter is found to have an overwhelming impact. The resulting four thermal contact resistances are: 0.7 × 10^?3 m²K/W for brazing, 0.88 × 10^?3 m²K/W for co-casting, 1.25 × 10^?3 m²K/W for a single-epoxy bonding and 1.88 × 10^?3 m²K/W for a press-fit bonding. The uncertainty on these values is 11%.     

Local heat transfer coefficient for pool boiling of R-134a and R-123 on smooth and enhanced tubes

The current paper presents experimental investigation of nucleate pool boiling of R-134a and R-123 on enhanced and smooth tubes. The enhanced tubes used were TBIIHP and TBIILP for R-134a and R-123, respectively. Pool boiling data were taken for smooth and enhanced tubes in a single tube test section. Data were taken at a saturation temperature of 4.44 °C. Each test tube had an outside diameter of 19.05 mm and a length of 1 m. The test section was water heated with an insert in the water passage. The insert allowed measurement of local water temperatures down the length of the test tube. Utilizing this instrumentation, local heat transfer coefficients were determined at five locations along the test tube. The heat flux range was 2.5–157.5 kW/m² for the TBIIHP tube and 3.1–73.2 kW/m² for the TBIILP tube. The resulting heat transfer coefficient range was 4146–23255 W/m². °C and 5331–25950 W/m². °C for both tubes, respectively. For smooth tube testing, the heat flux ranges were 7.3–130.7 kW/m² and 7.5–60.7 kW/m² for R-134a and R-123, respectively; with resulting heat transfer coefficient ranges of 1798.9–11,379 W/m². °C and 535.4–3181.8 W/m². °C. The study provided one of the widest heat flux ranges ever examined for these types of tubes and showed significant structure to the pool boiling curve that had not been traditionally observed. Additionally, this paper presented an investigation of enhanced tubes pool boiling models.     

Enhancement of pool-boiling heat transfer using nanostructured surfaces on aluminum and copper

Enhanced pool-boiling critical heat fluxes (CHF) at reduced wall superheat on nanostructured substrates are reported. Nanostructured surfaces were realized using a low temperature process, microreactor-assisted-nanomaterial-deposition. Using this technique we deposited ZnO nanostructures on Al and Cu substrates. We observed pool-boiling CHF of 82.5 W/cm² with water as fluid for ZnO on Al versus a CHF of 23.2 W/cm² on bare Al surface with a wall superheat reduction of 25–38 °C. These CHF values on ZnO surfaces correspond to a heat transfer coefficient of ～23,000 W/m² K. We discuss our data and compare the behavior with conventional boiling theory.     

Permeability and thermal conductivity of compact chemical and physical adsorbents with expanded natural graphite as host matrix

Permeability and thermal conductivity test units were designed to study the heat and mass transfer performance of compact chemical and physical adsorbents, i.e. compact CaCl₂ and activated carbon (AC) using expanded natural graphite (ENG) as host matrix. The thermal conductivity was studied by steady-state heat source method and the permeability was tested with nitrogen as a gas source. For the compact CaCl₂ adsorbents, results show that permeability and thermal conductivity vary with the ratio of ENG and the density of compact adsorbents. The value of permeability is 10^-13 ～ 10^-11 m² when the density of compact sample change from 400 kg/m³ to 550 kg/m³, and it keeps increasing linearly with the less ratio of ENG. The value of thermal conductivity is 1.08 W/(m·K), which is increased by 5 times compared with granular CaCl₂ when the density is 550 kg/m³ and the ratio of ENG is the minimum value of 16.7%. The compact physical adsorbent of AC with ENG as matrix has the highest thermal conductivity of 2.61 W/(m·K) when the ratio between ENG–AC is 1.5:1. Similarly, thermal conductivity drops down with the ratio of ENG decreasing. When the ratio of ENG reaches the minimum ratio of 28.6%, the thermal conductivity is 2.08 W/(m·K), which is increased by 5.8 times if compared with the result of granular AC, and corresponding permeability is 5.16 × 10^-11 m². The thermal conductivity and permeability of compact physical adsorbent of AC are all better than the values for the compact chemical adsorbent of CaCl₂.     

Sintered porous heat sink for cooling of high-powered microprocessors for server applications

This paper experimentally investigates the sintered porous heat sink for the cooling of the high-powered compact microprocessors for server applications. Heat sink cold plate consisted of rectangular channel with sintered porous copper insert of 40% porosity and 1.44 × 10^?11 m² permeability. Forced convection heat transfer and pressure drop through the porous structure were studied at Re ? 408 with water as the coolant medium. In the study, heat fluxes of up to 2.9 MW/m² were successfully removed at the source with the coolant pressure drop of 34 kPa across the porous sample while maintaining the heater junction temperature below the permissible limit of 100 ± 5 °C for chipsets. The minimum value of 0.48 °C/W for cold plate thermal resistance (R_cp) was achieved at maximum flow rate of 4.2 cm³/s in the experiment. For the designed heat sink, different components of the cold plate thermal resistance (R_cp) from the thermal footprint of source to the coolant were identified and it was found that contact resistance at the interface of source and cold plate makes up 44% of R_cp and proved to be the main component. Convection resistance from heated channel wall with porous insert to coolant accounts for 37% of the R_cp. With forced convection of water at Re = 408 through porous copper media, maximum values of 20 kW/m² K for heat transfer coefficient and 126 for Nusselt number were recorded. The measured effective thermal conductivity of the water saturated porous copper was as high as 32 W/m K that supported the superior heat augmentation characteristics of the copper–water based sintered porous heat sink. The present investigation helps to classify the sintered porous heat sink as a potential thermal management device for high-end microprocessors.     

Solar greenhouse assisted biogas plant in hilly region – A field study

The present study was undertaken with the objective of evaluating plastic as an alternative material for biogas plant on a par with conventional brick material. The field study was carried out for one year (October, 2005–September, 2006) in a small hamlet at Nilgiris incorporating solar energy to study its influence on biogas production. During summer (April–June) the temperature reaches to the maximum of 21–25 °C and the minimum of 10–12 °C. During winter (October–December), the temperature available is maximum of 16–21 °C and minimum of 2 °C. The solar insolation in the study area ranges from 250 to 600 W/m². This study involves the control conventional Deenabandhu model (Indian standard model prevailing in most part of India made of masonry structure only) and the experimental plastic tank with greenhouse canopy of similar capacity. Our previous work [Vinoth Kumar, K., Kasturi Bai, R., 2005. Plastic biodigesters – a systematic study. Energy for Sustainable Development 9 (4), 40–49] on lab scale digester made from plastic material was compared over other materials and the results gave us much confidence to carry out further study on pilot scale. In continuation, a semi-continuous study was conducted for one year with the retention time of 55 days. The gas generated from the biogas plants was utilized for cooking (burner) and lighting (lamp) purposes. The yearly average slurry temperatures recorded during the study period was 26.3 and 22.4 °C in experimental and control biogas plants against ambient temperature of 17.0 °C. The yearly average greenhouse chamber temperature recorded was 29.1 °C in the experimental biogas plant. The yearly average gas yield from the experimental and control biogas plants were 39.1 and 34.6 l kg^?1 day^?1 respectively. Gas productions in the winter season registered lower than other months. It can be concluded that the solar greenhouse assisted plastic biogas plant can be efficiently adopted with minor modifications in hilly regions since the temperature profile plays a major role in biogas production.     

Development and tests of ammonia Miniature Loop Heat Pipes with cylindrical evaporators

Miniature Loop Heat Pipes (MLHPs) are an attractive object for development and investigation as quite a promising means for cooling powerful electronics operating in the temperature range from 50 to 100 °C. The paper generalizes and presents the results of development and tests of 15 different variants of ammonia MLHPs with cylindrical evaporators 5 and 6 mm in diameter, which have an active zone length of 20 mm and are equipped with titanium and nickel wicks. As a result of successive efforts aimed at increasing the MLHPs efficiency, it was possible to achieve values of the heat-transfer coefficient close to 162,000 W/m² °C at a value of the heat flux of about 100 × 10⁴ W/m². A maximum heat flux value of about 135 × 10⁴ W/m² was achieved at the heat-transfer coefficient equal approximately to 75,000 W/m² °C.     

Numerical modeling of heat and mass transfer during forced convection drying of rectangular moist objects

A two-dimensional analysis of heat and mass transfer during drying of a rectangular moist object is performed using an implicit finite difference method, with the convective boundary conditions at all surfaces of the moist object. The variable convective heat and mass transfer coefficients are considered during the drying process. The external flow and temperature fields are first numerically predicted through the Fluent CFD package. From these distributions, the local distributions of the convective heat transfer coefficients are determined, which are then used to predict local distributions of the convective mass transfer coefficients through the analogy between the thermal and concentration boundary layers. Also, the temperature and moisture distributions for different periods of time are obtained using the code developed to determine heat and mass transfer inside the moist material. Furthermore, the influence of the aspect ratio on the heat and mass transfer is studied. It is found that the convective heat transfer coefficient varies from 4.33 to 96.16 W/m² K, while the convective mass transfer coefficient ranges between 9.28 × 10⁻⁷ and 1.94 × 10⁻⁵ m/s at various aspect ratios. The results obtained from the present analysis are compared with the experimental data taken from the literature, and a good agreement is observed.     

Mathematical modeling of drying characteristics of tropical fruits

In this study, a heat pump dryer was designed and produced in which drying air temperature was controlled with PID. PID controlled heat pump dryer was experimentally tested in drying tropical fruits such as kiwi, avocado and banana. Drying air temperature was kept at 40 °C with the accuracy of ±0.2 °C. Drying air velocity changed between 0.03 m/s and 0.39 m/s. Initial moisture contents of the kiwi, avocado and banana were 4.31, 1.51 and 4.71 g water/g dry matter, which were dried to 0.75, 0.35 and 0.5 g water/g dry matter moisture contents in 6 h. Afterwards the moisture ratios were analyzed with “STATGRAPHIC” computer program by using semi-theoretical models and empirical values. Correlation and standard error of estimation (SEE) and R² values were achieved.     

Drying saturated zeolite pellets to assess fluidised bed performance

This work studies the drying rates of zeolite pellets (Type 1 [D.M. Ruthven, Principles of Adsorption and Adsorption Processes, John Wiley & Sons, New York, 1984]) in spouted beds and in conventional fan assisted ovens. Three temperatures were studied: 48, 85 and 133 °C for drying times up to 1 h. Heat transfer is deduced from measured mass transfer rates and literature correlations. Maximum heat transfer coefficients were found to be 115.6 W/m² K for the fluidised bed and 31.8 W/m² K for the oven using correlations from literature. Higher figures were found by experimentation and calculation, typically about 200 W/m² K. A finite element model was used to estimate the temperature profile within pellets, the predictions agree well with experimental temperature measurements.     

Experimental investigation of capillary-assisted evaporation on the outside surface of horizontal tubes

Capillary-assisted evaporation is a typical heat transfer method in heat pipes which is characterized by high evaporation coefficient due to extremely thin liquid film. This paper introduces such a micro-scale heat transfer method into normal-scale applications. A series of enhanced heat transfer tubes with circumferential rectangular micro-grooves on the outside surfaces have been experimentally investigated. The aim is to investigate the influence of the tubes’ geometries and operating parameters on the evaporation heat transfer coefficients. In the experiment, the tested tubes are hold horizontally and the bottom surfaces are immersed into a pool of liquid. The heat is added to the thin liquid film inside the micro-grooves through the heating fluid flowing inside the tubes. The factors influencing the capillary-assisted evaporation performance, such as the immersion depth, evaporation pressure, superheating degree, etc. are considered. The experimental results have indicated that there is a positive correlation between the evaporation heat transfer coefficient and evaporation pressure, and negative for the superheating and immersion depth. For water, under the evaporation saturated temperature of 5.0 ± 0.1 °C, the superheating of 4.0 ± 0.1 °C and the dimensionless liquid level of 1/2, the film side evaporation heat transfer coefficients are 3100–3500 W/m² K, which are equivalent to those of the falling film evaporator in LiBr–water absorption machine (2800–4500 W/m² K [Y.Q. Dai, Y.Q. Zheng, LiBr–water Absorption Machine, first ed., Chinese National Defence Industry Press, Beijing, China, 1980.]).     

Models of fluidized bed drying for thin-layer chopped coconut

Methods for efficiently drying agricultural products are in ever-increasing demand. Due to its thorough mixing ability, a fluidized bed technique was employed to evaluate the drying kinetics of thin-layer chopped coconut. The experiments were conducted at drying temperatures of 60–120 °C and a constant velocity of 2.5 m/s. Chopped coconut was dried from about 105% d.b. to approximately 3% d.b. The moisture transport phenomenon in fluidized bed thin-layer drying is described by immense acceleration in MR diminution in the early stage of drying, followed by considerable deceleration. Falling-rate drying, an outgrowth of restraining moisture transfer via internal mass-diffusion mechanism, thoroughly characterized chopped coconut drying. Among the 10 selected models, statistic analysis inferred that the Modified Henderson and Pabis model could predict changes in moisture content most accurately. Compared with the values of D_eff derived from Fick’s law for other food and biological materials usually dried in conventional tray dryers, the current values (5.9902 × 10^?8–2.6616 × 10^?7 m²/s) were substantially high, principally attributable to the unique characteristic of fluidized bed drying, remarkably encouraging heat and mass transfer. Activation energy was also described.     

Effects of meteorological variables on exergetic efficiency of wind turbine power plants

This present paper deals with exergy efficiency results of the Wind Turbine Power Plants (WTPPs). Effects of meteorological variables such as air density, pressure difference between state points, humidity, and ambient temperature on exergy efficiency are discussed in a satisfactory way. Some key parameters are given monthly for the three turbines. Exergy efficiency differs from 0.23 to 0.27 while temperature is changing from 268.15 K to 308.15 K with air density 1.368–1.146 (kg/m³). While pressure difference (ΔP) between inlet and outlet of the turbine differs from 100 to 1100 (Pa), exergy efficiency decreases fairly for different wind speeds. While specific humidity is changing from 0.001 to 0.015 (kg_water/kg_{dry air}), exergy efficiency decreases gently. Generally these meteorological variables are neglected while planning WTPPs, but this neglect can cause important errors in calculations and energy plans. Obtained results indicate that while planning WTPPs meteorological variables must be taken into account.     

Development of a thermoelectric refrigerator with two-phase thermosyphons and capillary lift

A thermoelectric domestic refrigerator has been developed, with a single compartment of 0.225 m³, for food preservation at 5 °C. The cooling system is made up of two equal thermoelectric devices, each composed of a Peltier module (50 W) with its hot side in contact with a two-phase and natural convection thermosyphon (TSV) and a two-phase and capillary lift thermosyphon (TPM), in contact with the cold side.The entire refrigerator has been simulated and designed using a computational model, based on the finite difference method. Subsequently an experimental optimization phase of the thermosyphons was carried out, until thermal resistance values of R_TSV = 0.256 K/W and R_TPM = 0.323 K/W were obtained. These values were lower than those obtained with finned heat sinks.Finally, a functional prototype of a thermoelectric refrigerator was built, and the results which were obtained demonstrate that it is able to maintain a thermal drop (Ambient Temperature–Inside Temperature) of 19 °C. The electric power consumption at nominal conditions was 45 W, reaching a COP value of 0.45. The study demonstrated that by incorporating these two-phase devices into thermoelectric refrigeration increases the COP by 66%, compared with those which use finned heat sinks.