Omar M. Al-Rabghi

Proposed energy-efficient air-conditioning system using liquid desiccant

Abstract Energy consumption due to conventional air-conditioning is very high, especially in hot, humid areas. The total air-conditioning cooling load can be divided into sensible and latent parts. Liquid desiccant solutions have the property of holding water vapor, therefore they can be used to overcome the latent part of the air-conditioning cooling load. In this work, an energy-efficient system is proposed to reduce energy consumption in air-conditioning ( A C ) systems using CaCl2 solution as a liquid desiccant. The proposed system consists mainly of two packed beds, a heat pump, air washers and a cooling coil. Mathematical modeling of various components and fluid properties of the proposed system was undertaken. A computer program was prepared to study the system performance under different conditions. Over most of the conditions at which the program was tested, the proposed system was found to be more energy-efficient than a conventional A C system.

Parametric study of an energy efficient air conditioning system using liquid desiccant

Abstract Liquid desiccant can be used effectively to reduce energy consumed in air conditioning (AC). Unlike conventional AC, the latent part of the cooling load is overcome using liquid desiccant. In a previous article, an energy efficient system utilizing CaCl2 as liquid desiccant was proposed. This study is an extension of that work. The effect of key variables on the performance of the proposed system is undertaken. The inlet temperature of the liquid desiccant to the dehumidifier, space sensible heat ratio, and heat exchanger effectivenesses are the variables studied. Variations of these parameters and their effects on system performance are presented. Good system performance and energy saving could be achieved if proper values of these variables are selected.

Utilizing transfer function method for hourly cooling load calculations

Cooling load calculations are essential in sizing air conditioning equipment. The transfer function method (TFM), which is well-known and adopted by ASHRAE, was programmed in this work to predict the hourly cooling load due to different types of wall, roof and fenestration. The developed computer code was tested against hand calculations and was found to be accurate. When comparison was done with ASHRAE examples, some differences were noticed in the cases of cooling load due to wall and roof. In other cases, the agreement was perfect. The disagreement with ASHRAE results was pin-pointed, and a revision of the ASHRAE example was made. The outputs of the program were also compared with the well-known Carrier program, and the agreement was satisfactory. Various runs were made, and the results of these runs are discussed. The effect of changing the wall color on cooling load and on sol-air temperature is studied.

Energy simulation in buildings : overview and BLAST example

Abstract Energy consumption to provide thermal comfort conditions, especially in hot and humid areas is tremendous. Ways to reduce and save energy are indeed possible and needed. Simulation of HVAC energy consumption in buildings is of considerable interest and benefit for engineers and architects. Energy simulation programs can be used to analyze cost effective energy conservation measures before the building is built or modified. There are both open type simulation programs and proprietary programs. The main components of an open type simulation program are presented, and its development is reviewed. Some of the difficulties encountered by first users of these programs are pinpointed with some suggested remedies. An annual weather data file for Jeddah, Saudi Arabia, was prepared. A three story, three zone typical school in Jeddah is considered. Building loads analysis and system thermodynamics input and output files are generated to show the capabilities of the program. Three case studies were considered.

Estimation and measurement of electric energy consumption due to air conditioning cooling load

Abstract Electric energy consumption due to air conditioning (A/C) in buildings is tremendous, especially in places having hot humid weather, such as Jeddah, Saudi Arabia. Building simulation programs, such as DOE2, are very valuable tools to study building energy consumption under different conditions. These programs can be used to investigate the effect of different building designs on their energy consumption. This paper presents the experimental and theoretical variations of electric power consumed by an A/C system. For the experimental part, a building has been chosen, and different sensors were installed to measure and record electric kW consumption at different indoor and outdoor conditions. In the theoretical part, the Visual DOE simulation program, which is based on the well known DOE2E program, has been used. Actual building data was fed to the program to generate the instantaneous electric consumption due to A/C cooling load. Comparison between measured and predicted electric consumptions is presented.

Retrofitting R-12 car air conditioner with R-134a refrigerant

Refrigerant-12 which is commonly used in air conditioning is among the substances to be phased out internationally. R-134a, which is a chlorine-free refrigerant, is the industry chosen substitute for R-12. In this study, a car air conditioned by R-12 is selected. The car is fitted with appropriate sensors and gauges to measure the car AC performance. First, the performance is measured when it is running with R-12. Careful retrbfitting is then made, including adjustment, changing parts, oil and R-12 to R-134a. The performance of the retrofitted AC with R-134a is then measured. Comparison between the two cases is made for different engine RPMs. The AC system was tested twice to see the effect of ambient temperature on the performance. The results of the comparison are very encouraging with better performance on the side of the R-12 case. The results were also compared with car manufacturers recommended performance checks, and found to be satisfactory.

Measurements of solar flux density distribution on a plane receiver due to a flat heliostat

Abstract An experimental facility is designed and manufactured to measure the solar flux density distribution on a central flat receiver due to a single flat heliostat. The tracking mechanism of the heliostat is controlled by two stepping motors, one for tilt angle control and the other for azimuth angle control. A x-y traversing mechanism is also designed and mounted on a vertical central receiver plane, where the solar flux density is to be measured. A miniature solar sensor is mounted on the platform of the traversing mechanism, where it is used to measure the solar flux density distribution on the receiver surface. The sensor is connected to a data acquisition card in a host computer. The two stepping motors of the heliostat tracking mechanism and the two stepping motors of the traversing mechanism are all connected to a controller card in the same host computer. A software “TOWER” is prepared to let the heliostat track the sun, move the platform of the traversing mechanism to the points of a preselected grid, and to measure the solar flux density distribution on the receiver plane. Measurements are carried out using rectangular flat mirrors of different dimensions at several distances from the central receiver. Two types of images were identified on the receiver plane—namely, apparent (or visible) and mirror-reflected radiation images. Comparison between measurements and a mathematical model validates the mathematical model.

Yearly-averaged daily usefulness efficiency of heliostat surfaces

Abstract An analytical expression for estimating the instantaneous usefulness efficiency of a heliostat surface is obtained. A systematic procedure is then introduced to calculate the usefulness efficiency even when overlapping of blocking and shadowing on a heliostat surface exist. For possible estimation of the reflected energy from a given field, the local yearly-averaged daily usefulness efficiency is calculated. This efficiency is found to depend on site latitude angle, radial distance from the tower measured in tower heights, heliostat position azimuth angle and the radial spacing between heliostats. Charts for the local yearly-averaged daily usefulness efficiency are presented for Φ = 0, 15, 30, and 45°N. These charts can be used in calculating the reflected radiation from a given cell. Utilization of these charts is demonstrated.

Heliostat minimum radial spacing for no blocking and no shadowing condition

In order to fully utilize the total heliostat reflected area and collect the maximum solar radiation for central receiver plants, one should minimize the heliostat shaded and blocked areas. This condition must be satisfied during the day and around the year. Based on the criterion of no blockings and no shadowing, the heliostat fields is subdivided into zones. Each zone is characterized by a predominant shadowing or blocking effect. The minimum yearly radial spacing between two adjacent heliostats for no blocking and no shadowing condition is calculated for all zones. Results for sites at latitude angles of 101°, 20°, 20° and 40°, and for daily operation periods of 4, 6 and 8 hours are obtained.

THE MONTHLY-AVERAGED AND YEARLY-AVERAGED COSINE EFFECT FACTOR OF A HELIOSTAT FIELD

Analyses are given to determine the monthly-averaged and yearly-averaged daily cosine effect factor for a heliostat field. The analyses showed that this factor depends on the latitude angle of the site, the month, and the relative position of the heliostat with respect to the tower. The position of the heliostat is defined in terms of its distance from the tower as multiples of the tower height and a position angle measured from the south direction. Calculations are carried out to determine the dependence of the monthly-averaged and the yearly-averaged daily cosine effect factor on the pertinent parameters. The results are plotted on charts for each month, and for the full year. These results cover latitude angles between 0 and 45°N, for fields with radii up to 50 tower height. In addition, the results are expressed in mathematical correlations to facilitate using them in computer applications. A procedure is outlined to use the present results to preliminary layout the heliostat field, and to predict the...