Abstract:The need for air conditioning in rural areas in which there is no source of traditional electrical power leads to look for an alternative solution by thermal solar energy. Usually the population of these areas uses typical movable house (TMH) for living. Solar thermal system drive vapor jet refrigeration cycle (VJRC) is an alternative solution replaces the vapor compression refrigeration cycle. Mathematical model will be carried out for (VJRC) using Engineering equation solver (EES) software program at various design conditions (generator, evaporator and condenser temperature). Especially attention on the performance of the cycle.
Cooling load for one TMH using TRNBuild software program will be estimated under standard design conditions for Hebron and Jericho cities.Hourly simulations for cooling load and solar thermal cooling system will be carried out over six months period (hottest months) considering climatic data for the two cities using TRANSYS software program coupling with EES. The performance of the overall system will be calculated. Energy consumption will evaluate. Effect of climate, evacuated tube solar collector tilt angle and area, hot storage tanks will study to meet the required human comfort, optimum operation conditions and parameters will be decided for the two climate regions.Introduction:The increasing demand for thermal comfort has led to a rapid increase in cooling system use and, consequently, electricity demand due to air-conditioning in buildings. The global contribution from buildings towards energy consumption, both residential and commercial, has steadily increased reaching figures between 20% and 40% in developed countries 1. Deployment of thermal energy refrigeration, using low-grade heat or solar energy, would provide a significant reduction of energy consumption.
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Among the various technologies for thermal refrigeration, heat-driven ejector seems the most promising alternative to the traditional vapor compression refrigeration cycle. 2.vapor jet refrigeration cycle (VJRC) is simple to construct, rugged, low maintenance and has few or no moving parts making it a highly reliable system with an ability to run from low temperature solar heat or other waste heat, which can result in much cheaper operation of the cooling system.Ejectors are classified into two types depending on the position of the nozzle; constant-pressure mixing ejector and constant-area mixing ejector, in the constant-pressure mixing ejector the exit plane of the nozzle is located within the suction zone upstream of the constant area section, and the static pressure throughout the mixing zone is assumed constant.
In the constant-area mixing ejector, the primary nozzle exit is located in the constant area section, where the mixing of the primary and secondary flows occurs and the pressures of the two streams are not equal. The constant-pressure mixing ejector is used more than constant-area mixing ejector because it has higher entrainment ratio and COP at the same conditions.In (VJRC), water can be used as the refrigerant. Like air, it is perfectly safe. These systems were applied successfully to refrigeration in the early years of this century.
At low temperatures the saturation pressures are low (0.008129 bar at 4 °C) and the specific volumes are high (157.3 m3/kg at 4 °C). The temperatures that can be attained using water as a refrigerant are not low enough for most refrigeration applications but are in the range which may satisfy air-conditioning, cooling, or chilling requirements 3.
Using the water in VJRC Compared with other commonly used refrigerants (Halocarbons, hydrocarbon and mixtures refrigerants), water is inexpensive, has a high latent heat, and has minimal environment impact (ODP and GWP). VJRC can operate at low generator temperature around 100 °C. It can easily be powered by high efficiency flat plate or evacuated tube solar water collectors. Since the cooling load is commonly synchronized with the availability of solar energy, this becomes an attractive application of solar energy. Palestine is located within the solar countries and considered as one of the highest solar potential energy in the world, this makes it one of the best countries to use the solar thermal energy in the solar thermal air conditioning systems, especially in rural areas where there is no or insufficient electricity and there is a sufficient lands for solar collectors.Transient system simulation program (TRNSYS) and Engineering Equation Solver (EES) coupling are used to model and analyze the solar thermal refrigeration systems. EES software program is used to build a mathematical model of the refrigeration cycle and simulate the effect of operation conditions, and TRNSYS software program is used to simulate and analysis the system using solar thermal power system under different operation conditions and parameters at various regions and climate, and find the optimum design parameters for the system.
This simulation study based on constant-pressure mixing ejector with variable geometry using water as working fluid, the source of heat in the generator is the solar energy collected by solar collectors in two different cities in West Bank in Palestine, the cooling load of the cycle is used to cover the cooling demand for a TMH that commonly used in the rural areas in this cities. Problem: The global contribution from buildings towards energy consumption due air conditioning has steadily increased reaching about 30% in summer seasons in Palestine. In addition many rural areas in Palestine suffer from insufficient electricity that used in traditional air conditioning system to achieve the human comfort. This forced the population to used diesel generator to generate electricity demand that is high cost and non-sustainable.
Palestine is located within the solar countries and considered as one of the highest solar potential energy in the world, this makes it one of the best countries to use the solar thermal energy in the solar thermal refrigeration systems. Using the solar thermal energy to operate solar cooling systems is the best solution to reduce the fuel consumption, cost, harmful emissions and handle the peak time. VJRC is one of the most solar cooling systems that can operate at low generator temperature (around 100 °C) using evacuated solar collectors. Objective: • Investigate the performance of the VJRC: Computer modeling will carried out on single VJRC. The system will demonstrated on EES software program under various operation conditions, the generator temperatures of 80-120°C, evaporator temperature of 5-15°C and condenser temperature of 25-40°C. • Weather data collection and simulation: Weather data collection and filtration for Hebron city in the south of west bank and Jericho city in the east of west bank. Hourly simulation for weather data includes ambient temperature and solar radiation of the two cities mentioned. • Cooling load simulation: Hourly simulation for cooling load demand for a typical movable house used in rural areas at the two cities mentioned.
This house built at standard constructions in Palestine. • VJRC driven by solar energy simulation: Hourly simulation for VJRC driven by solar energy to supply the cooling load demand at different climate of the two cities mentioned above, and studies the effect of the different parameters on the performance of the system and human comfort and finds the optimum design parameters for the system, the simulation attention on the following:1- Investigate the effect of tilt angle of the solar collector in summer season.2- Investigate the effect of solar collector area.
3- Investigate the effect of thermal storage tank.Literature review The (VJRC) was first developed by Le Blanc and Parson in early 1900 and was very popular in early 1930’s for air-conditioning systems for large buildings. The system was replaced with the more favorable vapor compression system. The latter system was superior in its coefficient of performance (COP), flexibility and compactness in manufacturing and operation 4. Latter several mathematical models were developed to study 1-D ejector system with analysis and fluid dynamics theories applied to the primary and secondary flow. Tashtoush et al 5 studied the performance of the ejector cooling cycle.
They investigated the model mathematically using EES software. In addition, the ejector type , it is found that constant-pressure mixing ejector generates higher backpressure than constant-area mixing ejector for the same entrainment ratio and COP. and this is the reason behind used constant-pressure mixing ejector more than constant-area mixing ejector. The 1-D model of constant-pressure mixing ejector was initially developed in 1950 by Keenan et al. 6. In this model, the pressure of primary and secondary flow is equal at the nozzle exit.
The mixing of the two fluids starts with constant pressure and reaches the inlet of the constant area section. Their theory is still considered a successful model for constant pressure mixing ejector.According thermodynamics laws.
Many theoretical studies was carried out on operation conditions of the cycle such as the generator, condenser and evaporate temperature and pressure. Al-Khalidy7 and Rani8, noted that the performance of the system increased with increasing boiler and evaporator temperatures and decreasing condenser temperature.Eames et al 9, and Chunnanond et al 10 ,experimental studies was carried out on the fixed geometry ejectors refrigerator showed that, at each setting of boiler and evaporator operating condition, the operation of the ejector can be categorized into three regions as shown in Figure 1.2 double choked flow in the mixing chamber, primary chocking flow and reversed flow.
When the ejector is operated under the “critical condenser pressure”, COP and cooling capacity remain constant. Further increase in condenser pressure above the critical condenser pressure moves the thermodynamic shock wave into the mixing chamber and prevents secondary flow from reaching sonic velocity. Upstream conditions can now be transmitted downstream, which results in a reduction in secondary flow, entrainment ratio and COP. Eventually, secondary flow drops to zero, the ejector loses its function and primary flow will reverse back into the evaporator. Figure 1.
2: Ejector flow regimes 9When the flow through the primary nozzle is choked, the mass flow rate of steam does not increase when lowering the condenser pressure. This attribute of the nozzle is there as on for the constant entrainment lines as can be seen in figure 1.2.
From this it is noted that the most economical point to operate the ejector is at the critical point. From the results published by Chunnanond et al. 9 and Eames et al.
10 using water as refrigerant, they noted that lower boiler temperature can improve the COP of the system at the expense of critical condenser, also Ruangtrakoon et al.11experimental Results reported that decreasing boiler temperature decreases the critical condenser pressure and increases the entrainment ratio .Experimental results by Xiaoli Ma 12 using the water as refrigerant, Selvaraju 13 for R134a also conclude the same results.12 noted that the increase in the boiler temperature does not always cause an increase in the efficiency of the system. In the tested boiler temperature range of 84 to 96 C, the maximum COP (0.32) were obtained at boiler temperature of 90 C.13 Noted that the COP increases with the increase in generator temperature at particular condenser and evaporator temperatures.
This trend is observed up to a certain value of generator temperature and further increase brings about a reversal of the trend.Sun 14 experimental study was carried out on the variable geometry ejectors, it is noted that, with the variable geometry ejector. There are no critical pressure value and characteristics of constant capacity for variable geometry ejectors.
Also Increasing boiler temperature results in an increase in entrainment ratio and hence system COP. There is no optimum boiler temperature which gives maximum entrainment ratio. Thus, variable geometry ejectors overcome the inefficiency of fixed geometry ejectors. Figure 1.3 show the Effect of boiler temperature on ejector and cycle performance at condenser and evaporator temperature 35 ? C and 10 ? C respectively. Figure1.
3 Effect of boiler temperature on ejector and cycle performance 14.All the theoretical and experimental study 7-14 conclude that the performance of the cycle increased with increase the evaporator temperature and decrease the condenser temperature. Figure 1.4 and figure1.
5 show the effect of condenser and evaporator temperatures on the performance of the cycle 14. Figure 1.4 Effect of condenser temperature on ejector and cycle performance 14. Figure 1.5 Effect of evaporator temperature on ejector and cycle performance 14.
Alexis 15 Study the performance of an ejector cooling system driven by solar energy with R134a as working fluid. The system operating in conjunction with intermediate temperature solar collector in Athens, is predicted along the 5 months (May–September). He found that the COP of ejector cooling system varied from 0.035 to 0.199 when the operation conditions were: generator temperature (82–92 ?C), condenser temperature (32–40 ?C) and evaporator temperature (-10–0 ?C). For solar cooling application the COP of overall system varied from 0.014 to 0.101 with the same operation conditions and total solar radiation (536– 838 W/m2) in July.
Many researchers are used TRNSYS simulation for studying solar thermal cooling systems and refrigeration systems. Pridasawas 16 studied the effect of solar collector types and operation conditions on the VJRC performance with iso-butane as a refrigerant, Tashtoush et al 5 simulate the solar collector angle and area and study the performance of VJRC for the climatic conditions of Jordan with R134a as a refrigerant, Ahmed 17, and Asim 18 used TRNSYS to simulate the absorption cooling system under climate of Egypt and Pakistan respectively. Methodology:• Literature survey on solar cooling systems, (VJRC) working principal, solar thermal systems and simulation software programs.• Computer modeling for (VJRC) using the water as refrigerant on the (EES) program. The system will demonstrate and simulated under various operation conditions such as generator, condenser, and evaporator temperatures.
• Verification of simulation results with previous study at the same operation conditions.• Decide the cooling load zone locations and specifications.• Weather data collection and filtration for Hebron and Jericho cities.• Hourly simulation for ambient temperature and solar radiation of the two cities mentioned. • Decide the design conditions and human comfort conditions according codes and standards.
• Hourly simulation for cooling load demand for a typical movable house used in rural areas at the two cities mentioned. • Build and simulate solar thermal system operate the VJRC under the solar radiation and climatic data for Hebron and Jericho cities using TRNSYS software program.• Study the effect of tilt angle of the solar collector on the collected energy during summer season and decide the optimum angle in each city.• Study the effect of the climate on the cooling capacity and performance of the system.• Study the effect of the solar collector area on the cooling capacity and performance of the system.• Study the effect of the thermal storage tank capacity on the cooling capacity and performance of the system.• Study the effect of the (heat transfer fluid) HTF flow rate on the cooling capacity and performance of the system.• Find the optimum design parameters for Hebron and Jericho cities.• Writing the final report and results of the study.• Preparing for Writing scientific paper in the same topic.Outcomes/Results• The performance of the (VJRC) driven by solar energy depends on the design conditions and climatic data such as surrounding temperature and solar radiation.• Cooling load profile depends on the climate, location and the time of the system operation.• Using (VJRC) with a storage system can be achieve more human comfort than traditional AC especially at the peak times when the solar radiations and the surrounding temperatures are maximum.• Climate, solar collector area, hot storage tanks can strongly affect on the system efficiency and cooling capacity.