Effect of phase change material on passive thermal heating of a greenhouse

In this study, a periodic analysis of a greenhouse with combination of phase change material (PCM) and insulation as a north wall has been developed for thermal heating. The thermal model is based on Fourier analysis. Effect of distribution of PCM thickness on plant and room air temperature has been studied in detail. The plant and room air temperature have been evaluated with and without north wall. Numerical computations have been carried out for a typical winter day of New Delhi. On the basis of numerical results, it is inferred that (i) there is a significant effect of PCM north wall and heat capacity of plant temperature during off‐sunshine hour due to storage effect and (ii) the rate of heat flux inside greenhouse from north wall is maximum for least thickness of PCM. Copyright © 2005 John Wiley & Sons, Ltd.     

Evaluation of solar fraction (Fn) for the north wall of a controlled environment greenhouse: an experimental validation

In this communication, an attempt has been made to define and evaluate the solar fraction (F_n) for a north wall on the basis of annual experimental studies. An even‐span greenhouse has been considered for experimental studies for the climatic conditions of Delhi. Experiments were conducted from September 1999 to August 2000. A model based on Auto‐CAD 2000 has also been developed for validation. It has been observed that the solar fraction mainly depends on angle of incidence, solar altitude angle and shape and size of the greenhouse. It is inferred that solar fraction (F_n) plays a significant role in thermal modelling of an even‐span type greenhouse. Further, it was noted that there is a fair agreement between experimental and theoretical results. Copyright © 2002 John Wiley & Sons, Ltd.     

Computer model and its validation for prediction of storage effect of water mass in a greenhouse: a transient analysis

In the present paper, an attempt has been made to develop a computer model based on transient analysis of the greenhouse. The model predicts room air temperature, storage water temperature and the thermal energy storage effect of a water mass in a low cost, passive greenhouse. Analytical expressions, based on an energy balance for each component, have been derived in terms of climatic as well as design parameters. Numerical computations have been done on typical days for the months from December 1999 to June 2000 at New Delhi. It has been observed that (i) there is a significant thermal energy storage effect of the water mass on room temperature and (ii) TLL, which is found to decrease with an increase in the mass of storage water, varies with month of year. An experimental validation of the developed model has also been demonstrated. The predicted room and water temperature show fair agreement with experimental values.     

Thermal modeling of a greenhouse integrated to an aquifer coupled cavity flow heat exchanger system

A thermal model is developed for heating and cooling of an agricultural greenhouse integrated with an aquifer coupled cavity flow heat exchanger system (ACCFHES). The ACCFHES works on the principal of utilizing deep aquifer water available at the ground surface through an irrigation tube well already installed in every agricultural field at constant year-round temperature of 24 °C. The analysis is based on the energy balance equations for different components of the greenhouse. Using the derived analytical expressions, a computer program is developed in C⁺⁺ for computing the hourly greenhouse plant and room air temperature for various design and climatic parameters. Experimental validation of the developed model is carried out using the measured plant and room air temperature data of the greenhouse (in which capsicum is grown) for the winter and summer conditions of the year 2004–2005 at Chandigarh (31°N and 78°E), Punjab, India. It is observed that the predicted and measured values are in close agreement. Greenhouse room air and plant temperature is maintained 6–7 K and 5–6 K below ambient, respectively for an extreme summer day and 7–8 K and 5–6 K above ambient, respectively for an extreme winter night. Finally, parametric studies are conducted to observe the effect of various operating parameters such as mass of the plant, area of the plant, mass flow rate of the circulating air and area of the ACCFHES on the greenhouse room air and plant temperature.     

Numerical simulation of the airflow and temperature distribution in a lean-to greenhouse

In this paper, heat transfer and flow in a lean-to passive solar greenhouse has been studied. A mathematical model based on energy equilibrium and a one-dimensional mathematical model for the unsaturated porous medium have been founded and developed to predict the temperature and moisture content in soil and the enclosed air temperature in the greenhouse. On the condition that plant and massive wall is neglected, the air is mainly heated by the soil surface in the greenhouse, which absorbs the incident solar radiation. With increase in depth, the variation of the temperature and moisture content in soil decreases on account of ambient, and the appearance of the peak temperature in soil postpone. Solar radiation absorber, heat storage and insulation are the main effects of the north massive wall on greenhouse, which is influenced by the structure and the material. The specific heat capacity and thermal conductivity of wall material have a remarkable effect on the north wall temperature. The build-up north wall with thermal insulation material may be chosen for greenhouse. The temperature distribution and gas flow in greenhouse is influenced by the cover material of the inside surface of the north wall and the inclined angle of greenhouse roof. All results should be taken into account for a better design and run of a greenhouse.     

A numerical investigation on effects of ceiling and floor surface temperatures and room dimensions on the Nusselt number for a floor heating system

In this study, the effect of ceiling and floor surface temperatures and room dimensions on the Nusselt number over the floor of a floor heating system has been investigated numerically. The variation of the Nusselt number with Rayleigh number has been analyzed under constant wall temperature condition for different ceiling temperatures (10–25 °C) and room dimensions. It has been seen that when the room dimensions and temperature difference between the ceiling and interior air are increased, the Nusselt number over the floor increases as well. The numerical results have been compared with the correlations given in the literature. It has been seen that the correlations available in the literature are valid only for given thermal conditions and room dimensions. The results calculated from the correlations which do not consider the effects of ceiling and floor surface temperatures deviate up to 35% than the results of this numerical study carried out for different ceiling and floor surface temperatures and room dimensions. Therefore, a new correlation for Nusselt number over the floor, which contain the influence of thermal conditions and all of room dimensions must be discovered.     

Energy conservation in the greenhouse system: A steady state analysis

We have evaluated the five typical shape of the greenhouse for energy conservation in winter months for a composite climate. An expression for the plant temperature has been used for steady state analysis. Numerical computation has been carried out for the climatic condition of Delhi, India. The evaluation of the shape of the greenhouse has been done for a given floor area. Additional energy required from other fuels to maintain the necessary temperature has also been considered. It has been observed that a standard peak uneven span is suitable for minimum use of liquefied petroleum gas for a given favourable plant temperature. Experimental validation of the thermal model has also been carried out.     

Evaluation of solar fraction (Fn) for quonset type empty greenhouse: an experimental validation

In this study, the experiments were conducted from December 2000 to November 2001 to evaluate solar fraction (F_n) for quonset type empty greenhouse. Solar radiation was measured at different points inside and outside the greenhouse weekly once. The model based on Auto-CAD 2000 has been used for a given solar altitude and azimuth angles. Calculations have also been carried out for a typical day of each month of the year. It is observed that the solar fraction obtained by using Auto-CAD 2000 are very close to the value determined by using experimental observations and its value is more during winter months. Copyright © 2002 John Wiley & Sons, Ltd.     

Mathematical modeling for greenhouse heating by using thermal curtain and geothermal energy

A thermal model has been developed for the heating of a greenhouse by using inner thermal curtain and natural flow of geothermal warm water through the polyethylene tube laid on its floor. The calculations were done for a typical production greenhouse with the climatic data in the central part of Argentina during winter period. From the energy conservations point of view, the greenhouse has been divided into three zones i.e., zone I (plants under thermal blanket), zone II (space under ceiling) and zone III (space between roof and ceiling). The model has been tested with the published experimental data of air temperatures in zone I and zone II of the greenhouse. From the results, it was observed that the temperatures of air surrounding the plant mass in zone I were maintained in the range of 14–23 °C during winter night and early morning resulting in the better growth of winter growing plants against the harmful freezing effects. The predicted values of air temperature both in zone I and zone II of the greenhouse obtained from the proposed model exhibited fair agreement with the published experimental values.     

Thermal modelling of hybrid photovoltaic thermal (HPVT) integrated-biogas plants

In this paper, the energy balance equations for the different components of hybrid photovoltaic thermal integrated-biogas plant have been written for quasi-steady state conditions to develop a thermal model. An analytical expression for slurry temperature has been obtained as a function of design and climatic parameters namely mass of the slurry, mass flow rate of fluid in collector, number of collectors, solar intensity, ambient temperature etc. Numerical computations have been carried out for climatic conditions of Srinagar, India. Based on mathematical computations it has been observed that the optimum slurry temperature (∼37°C) is achieved for a given set of design parameters of biogas plant and hybrid collectors (M _S = 2000, [(m)\dot]_f = 0.05 kg/s\dot m_f = 0.05 kg/s, L = 25 m). It is also observed that the peak slurry temperature decreases with increase in mass of the slurry as expected. Equivalent CO₂ credits earned by hybrid biogas plant for optimised parameters have also been evaluated.     

Numerical simulation of free convection based on experimental measured conductivity in a square cavity using Water/SiO2 nanofluid

Heat transfer enhancement has been investigated in a square cavity subject to different side wall temperatures using water/SiO₂ nanofluid. An experimental setup has been used to extract the conductivity value of nanofluid. This study has been carried out for the pertinent parameters in the following ranges: the Rayleigh number of base fluid, Ra_f = 10⁵–10⁷ and the volumetric fraction of nanoparticle between 0 and 4%. The comparisons show that the mean Nusselt number increases with volume fraction for the whole range of Rayleigh numbers. Although by using the theoretical formulations for conductivity no enhancement has been observed.     

An experimental study of flow boiling instability in a single microchannel

A simultaneous visualization and measurement study has been carried out to investigate stable and unstable flow boiling phenomena of deionized water in a single microchannel having a hydraulic diameter of 155 µm with a bottom Pyrex glass wall. Fifteen platinum serpentine microheaters, bonded on the Pyrex glass wall, were used to measure local instantaneous wall temperatures. At low mass flux, a syringe pump was used to drive the subcooled water passing through the microchannel. Stable and unstable flow boiling modes in the single microchannel are identified, and flow pattern maps in terms of heat flux and mass flux as well as in term of exit vapor quality are presented respectively. It was found that unstable flow boiling occurred in the single microchannel if the exit vapor quality x_e > 0.013.     

A passive wall design to minimize building temperature swings

It has been found in many countries with arid climates that massively walled buildings provide steady, comfortable inside temperatures even though the outside temperature fluctuations may be sizeable. The adobe houses of the American South-West, and the rondavels of southern Africa are particular examples. This phenomena is often termed the thermal flywheel effect. One explanation is that the temperature at the inside of a massive wall lags approximately out of phase with the outside, and so it partly offsets the direct, in phase, infiltration losses into the building. Thus the room temperature is kept approximately constant. In this paper the question of designing a non-homogeneous wall to optimize this effect is considered.     

Experimental study of effect of an inner thermal curtain in evaporative cooling system of a cascade greenhouse

In this paper experimental study has been carried out in a cascade greenhouse with inner thermal curtain to see the effect of thermal curtain. A thermal model has also been developed to predict the air temperature in a cascade greenhouse. The fan-pad system has been used for evaporative cooling and an inner thermal curtain has been used to divide the greenhouse in two zones. Experiments have been conducted in hot summer conditions at Solar Energy Park, IIT Delhi, New Delhi, India for empty greenhouse. Statistical analysis has been carried out to validate the agreement of experimental observations with predicted values. The values of the root mean square percent deviation and coefficient of correlation has been found out 9.0%, 0.90; 5.0%, 0.95 and 7.0%, 0.97 for April, May and June in case of evaporative cooling without curtain in greenhouse-2. The degree of freedom for the experimental work is 10.0. It is found that the use of evaporative cooling with a thermal curtain reduces the temperature of greenhouse by 5 °C and 8 °C in the second zone of greenhouse-1 and 2 in comparison to greenhouse without curtain in May.     

Entropy Generation of Natural Convection Heat Transfer in a Square Cavity Using Al2O3–Water Nanofluid

Entropy generation of an Al₂O₃–water nanofluid due to heat transfer and fluid friction irreversibility has been investigated in a square cavity subject to different side‐wall temperatures using a nanofluid for natural convection flow. This study has been carried out for the pertinent parameters in the following ranges: Rayleigh number between 10⁴ and 10⁷ and volume fraction between 0 and 0.05. Based on the obtained dimensionless velocity and temperature values, the distributions of local entropy generation, average entropy generation, and average Bejan number are determined. The results are compared for a pure fluid and a nanofluid. It is totally found that the heat transfer, and entropy generation of the nanofluid is more than the pure fluid and minimum entropy generation and Nusselt number occur in the pure fluid at any Rayleigh number. Results depict that the addition of nanoparticles to the pure fluid has more effect on the entropy generation as the Rayleigh number goes up.     

An analytical model for silicon MIS/IL solar cells to optimize cell parameters through thermal annealing

A model has been developed which simulates the effect of cell parameters in order to optimize them by controlling the fabrication conditions, namely, annealing time and annealing temperature. Calculation of the efficiency as a function of surface states density D_it, positive fixed oxide charge density Q_f and mobile charge density Q_m, that depend on anealing conditions are carried out. A compromise between D_it and Q_m for different anealing temperatures for high performance cells has been investigated.     

Thermal modelling of asymmetric overlap roof greenhouse with experimental validation

Global solar radiation availability model and thermal model for newly designed asymmetric overlap roof shape (AORS) greenhouse are presented and experimentally validated. Instantaneous solar radiation flux is utilised in a dynamic thermal model to ascertain the hourly plant and inside air temperature. The AORS is also compared with the previously developed two best greenhouse shapes. An experimental validation of both the models is carried out for the measured instantaneous solar radiation, plant and inside air temperature for a typical day in summer at Ludhiana (31°N and 77°E), Punjab, India. During the experimentation, a tomato crop was grown inside the greenhouse. From the results, it can be inferred that an east–west orientation AORS greenhouse should be preferred due to a lesser solar radiation capture in summer months. The predicted plant and air temperatures are in good agreement with the measured data having a root mean square error of 4.69 and 3.7, respectively.     

On the selection of shape and orientation of a greenhouse for composite climates

Five most commonly used single-span shapes of greenhouses such as even-span, uneven-span, vinery, modified arch and quonset type have been selected for comparison in a composite type of climate. The length, width and height (at the centre) are kept the same for all the shapes. The comparison is based on total solar radiation input (beam, diffused and ground reflected) to each shape through each wall, inclined surfaces and roofs. Total solar radiation is theoretically computed for each shape in east–west and north–south orientations and compared for each month of the year. The computed values for each shape are then introduced in a transient thermal model developed to compute the hourly inside greenhouse air temperature for the selected day of the year in each month. It is observed that the variation in greenhouse shape can cause up to 3.5–5.5 °C change in the inside air temperature during different hours of the day. Experimental validation of the model is carried out using the measured inside air temperature data (for a typical summer day) for an even-span greenhouse (in which capsicum is grown) at Ludhiana (31°N and 77°E), Punjab, India. The predicted and measured values are in close agreement. The results show that even-span or quonset shape in east–west orientation is the most suitable shape for year-round agricultural operations.     

Hazardous Emissions,Global Climate Change and Environmental Precautions

It has been recognized worldwide that the utilization of an enormous amount of fossil fuel has created various adverse effects on the environment, including acid rain and global warming. An increase in average global temperatures of approximately 0.56 K has been measured over the past century. This increase is called global climate change or global warming. The gases with three or more atoms that have higher heat capacities than those of O ₂ and N ₂ cause the greenhouse effect. Carbon dioxide (CO ₂ ) is a main greenhouse gas associated with global climate change. Nitrous oxide (N ₂ O), chlorofluorocarbons (CFCs), methane (CH ₄ ) are other important greenhouse gases. Collectively, they are projected to contribute, directly, about as much potential global warming over the next 60 years as CO ₂ . Three trace gases, HFCs, PFCs, and SF6, would be regulated under the 1997 Kyoto Protocol because of their global warming potential and for their potential growth of concentrations in the atmosphere. HFCs have been widely approved as substitutes for CFCs.     

Studies on 10kg alloy mass metal hydride based reactor for hydrogen storage

A 10 kg alloy mass metal hydride reactor, with LaNi₅ alloy was designed. Heat transfer enhacement in the reactor was achieved by including embedded cooling tubes and an external water jacket. Detailed parametric study has been carried to understand the performance of the system. The effect of both geometrical and operational parameters was studied in simulations. The optimized geometrical parameters were used for fabricating the reactor. Experimental studies were carried on the fabricated reactor. Absorption studies were carried out for different supply pressure and different cooling fluid temperatures. Storage capacity of 1.13 wt% was found in 1620 s at a supply pressure of 25 bar and with a flow rate of 20 LPM. Similarily, desorption studies were carried out for varying heat transfer fluid temperatures. Complete and fastest desorption was observed at 80 °C with the reaction completion time of 2700 s.