Galal M. Zaki

Analysis of assisted coupled solar stills

Abstract The potential of coupled solar stills as a mean for improving solar distillation yield is investigated. A model for a simple solar still assisted by an external solar collector is presented. The governing coupled heat and momentum balance equations are solved for a quasi-steady-state condition with temperature dependent physical properties. The results show that for coupled stills the fresh water productivity increases as the solar collector area of the assisting device increases. For an ideal system, neglecting the thermal inertia, the yield is linearly dependent upon the solar insolation. The thermal inertia causes a significant drop in the system yield and deviation from linearity. The net efficiency of the coupled system is higher than that of a similar simple still by a value that depends mainly upon the system configuration and independent of the meteorological conditions. Comparison of the analysis with experimental data is satisfactory.

Energy saving through intermittent evaporative roof cooling

Abstract The effect of intermittent spraying of a buildings roof on the cooling load of the building is investigated. The thermal response of the roof with intermittent stepwise spraying variation is modeled by introducing an equivalent fictitious temperature that accounts for meteorological conditions, roof-to-sky radiation and evaporative effects during roof-wetted periods. On the basis of harmonic analysis, a transfer function relating the heat transfer rate from the interior ceiling surface to the outdoor conditions is obtained. The model results show that evaporative roof cooling could reduce the cooling load by 40% for hot dry conditions. The effects of the spraying period, the mass of sprayed water and uniform or time-dependent spraying upon the energy rates are presented and discussed.

Comparative study on reduction of cooling loads by roof gravel cover

Abstract The increase in demand for power to meet the ascending energy used for air conditioning in arid areas calls for new techniques to lower the cooling loads. In this paper a simple inexpensive method is suggested, where the thermal resistance of the building roof is increased by adding a layer of loose gravel. The gravel properties as well as the intergranular trapped air reduces the heat transfer through the roof. This technique is investigated experimentally for actual outdoor weather conditions. The results show that the effect of the gravel mass intensity is more pronounced than the size of the gravel. A factor that relates the heat transfer through the gravel covered layer to that of blackened concrete roofs is suggested on the basis of the present experimental results.

ANALYSIS OF ROOF TYPE SOLAR STILLS WITH ASSISTING EXTERNAL CONDENSERS

A simple solar still coupled to an external condenser, to enhance the yield, is modeled and analytically investigated. The analysis is general in nature and not limited to a particular condenser features. The present analysis, based on actual meteorological data, shows that the yield improvement depends upon both brine level and the surface area provided for heat removal. For an ideal still with negligible thermal inertia (water depth approaches zero) an auxiliary condensing surface at a temperature below the glass temperature decreases the daily yield. The still yield is improved for brine levels up to 10 cm above this level additional heat removal from the still does not significantly improve the daily productivity. The productivity for the limiting condition when the auxiliary condensing surface is maintained at the ambient temperature is also presented and discussed.

Improving the yield of simple basin solar stills as assisted by passively cooled condensers

The performance of a simple solar still assisted by a passively cooled condenser is modeled and investigated both analytically and experimentally. The present analysis, based on actual meteorological data, shows that the yield improvement depends upon the heat removal rate by the condenser and brine depth that determines the thermal inertia of the still. For a still with negligible thermal storing capacity (brine depth approaches zero) an auxilliary condensing surface reduces the daily yield. The yield improves for stills with brine depth up to 5 cm, and increasing the condensing area helps to remove the vapor from the still and hence improves the productivity.

Application of Exergoeconomic Techniques to the Optimization of a Refrigeration Evaporator Coil With Continuous Fins

An optimization for the geometrical parameters of continuous fins on an array of tubes of a refrigeration evaporator is developed in this paper using the exergy method. The method is based on exergy, economic analysis, and optimization theory. As there are humid air and refrigerant single- and two-phase streams involved in the heat transfer process, then there are irreversibilities or exergy destruction, due to pressure losses IΔP, due to temperature difference IΔT and due to specific humidity gradient IΔω. These principal components of total irreversibility are not independent, and their relative contribution to total irreversibility of a cross-flow refrigeration evaporator is investigated. A change in geometry was obtained by varying the evaporator tube diameter for a selected evaporator capacity, and hence the evaporator tube length and total heat transfer area are calculated for a fixed evaporator face length. In this way, the effect of changes in the geometry on the total number of exergy destruction units of the heat exchange process is investigated. The optimum balance between the three components of irreversibility (IΔP,IΔT, and IΔω) is also determined, thereby giving the optimum solution for the heat exchanger area. The total cost function, which provides a measure of the contribution of the evaporator to the total cost of the refrigeration system, is expressed on the basis of annual capital and electrical energy costs. The total cost function is minimized with respect to the total heat transfer area and the total number of exergy destruction units (NI). The relationship between the operational variables, heat transfer area, refrigerant and air irreversibilities, and the total annual cost for this type of evaporator are developed, presented, and discussed. The pressure, temperature, and specific humidity irreversibilities are found to be 30.34%, 33.78%, and 35.88%, respectively, of the total irreversibility, which is 8.5% of the evaporator capacity.

Energy and exergy analysis of reverse Brayton refrigerator for Gas Turbine power boosting

In this study the use of Reverse Brayton cycle to boost up the power of gas turbine power plants operating in hot humid ambiance is analysed by the energy and exergy methods. The gas turbine inlet temperature is reduced by mixing the chilled air from Brayton refrigeration cycle and the main intake air stream reaching low intake temperatures. In this paper the effect of irreversibilities in the system components have been evaluated along with the exergetic power gain ratio, and the exergetic thermal efficiency change, of the cycle. The energy analysis results indicated that the intake temperature could be lowered below the ISO standard with maximum power increase up to 19.58% and appreciable decrease in the thermal efficiency (5.76% of the site value). Whereas, the exergy analysis approach showed that the power gain could only be 14.66% as a result of the components irreversibilities.

Exergoeconomic optimisation of the geometry of continuous fins on an array of tubes of a refrigeration air cooled condenser

An exergy method for optimising the geometrical parameters of continuous fins on an array of tubes in an air-cooled condenser is developed in this paper. The method is based on exergy, economic analysis and optimisation theory. The condenser is characterised by the single/two phase flow inside the coil and the two components of exergy destruction, IΔP and IΔT, for refrigerant and air sides are calculated for different geometrics. A change in geometry was obtained by varying the condenser tube diameter for a selected condenser capacity, and hence the condenser tube length and total heat transfer area are calculated for a fixed condenser face length. In this way the effect of changes in the geometry on the total number of exergy destruction units is investigated. Also, the optimum balance between the two components of destruction is determined thereby giving the optimum solution for the heat exchanger area. The total cost function is expressed on the basis of annual cost of capital investment and the cost of compensation for the irreversibilities. The results show that the effect of the total irreversibility is 3.885% of the condenser capacity.

Analysis of refrigerant-charged solar collectors with phase change

Abstract This paper presents a mathematical model for natural circulation refrigerant-charged solar collector plates with phase change. The model accounts for the thermal nonequilibrium vapor generation process along fully charged collector tubes. The local vapor generation rate is determined by solving the steady one-dimensional two-phase energy conservation equations in finite difference form along the flow path. The integral momentum balance around the circulation loop determines the circulation flow rate, which shows dependency on the solar flux and the pressure of the heat carrying refrigerant inside the loop. Calculations have been made for a system fully charged by R-11, with temperature-dependent thermophysical properties. The effect of solar insolation, for a controlled pressure system, on the vaquality, ad circulation rate is presented and discussed. Performance of the system is studied for a range of the independent variables, solar flux up to 1000 W m−2, and refrigerant saturations temperatures up to 80°C.

Laminar Natural Convection in a Square Enclosure with Discrete Heating of Vertical Walls

أجريت دراسة عددية لانتقال الحرارة بالحمل الحر في حيز مربع معبأ بالهواء بوضع سخان جزئي ومباشر على جدار. إن الحيز كان في وضع عمودي وقد تم إبقاء جزء من جدار عمودي في درجة حرارة ثابتة بينما بقية الجدار وكذلك الجدار المقابل في درجة حرارة أقل. وقد عزلت الجوانب العليا والسفلى من الحيز. وقد حلت المعادلات التفاضلية ذات البعدين الخاصة حفظ المادة والطاقة وكمية الحركة باستخدام طريقة الفروقات المحدودة لعدة قيم لرقم رالي تتراوح بين 210ـ 610 . ودرست التغيرات في درجة الحرارة وحقول السريان مع زيادة رقم رالي وذلك لعدة مواضع للسخان. وقد وجد من الدراسة أن المناطق الباردة المجاورة للسخان والتي تم إبقاؤها فيدرجة حرارة أقل تساعد على تكوين خلايا دوران ثانوية والتي تعتمد على رقم رالي وعلى موضع السخان. وبوجود التسخين الجزئي والمناطق الباردة على جدار واحد يتميز السريان بوجود طبقات متاخمة تحيط هذه المناطق بخلايا دوران منفصلة. وتتأثر التغيرات في رقم نوسلت المحلي بهذا النمط من السريان ويكون متوسط رقم نوسلت أكبر من ذلك عندما يكون شريط التسخين موضوعا على جدار معزول. وقد وجد أن المكان الأمثل ـ في نطاق رقم رالي ـ هو منتصف الجدار S/L=0.5. وهذه النتيجة دعمت بنتائج تجار ب سابقة.