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The Solar Energy Refrigeration System with Larger Heat Source Temperature Difference

Author: YinZuo
Tutor: LiuCunFang
School: Shandong University
Course: Refrigeration and cryogenic technology
Keywords: solar energy absorption refrigeration cycle thermodynamic calculation
CLC: TB65
Type: Master's thesis
Year: 2008
Downloads: 266
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Abstract

Solar energy refrigeration system is the key in solar energy usage area in recent years.However,there are many problems such as complex system,low collector efficiency,high cost etc.The solar energy refrigeration system with larger heat source temperature difference designed in the paper provides a method to solve those problems.In the paper,a new circulation principl is introduced.A mathematical model of the cycle is built.The influence of several variables(such as medium pressure,inlet temperature of the heat source,inlet temperature of cold water,inlet temperature and circulation methods of cooling water)on the cycle is analyzed with VBA program.The cycle variables and parameters are optimized.The entire solar refrigeration system model using this cycle is goven.And the collector efficiency and the system efficiency are evaluated.On the basis of analysis to existed cycles of Lithium-Bromide absorption refrigeration systems used by solar energy refrigeration,the solution cycle is improved.The solution from low-pressure absorber is mixed with the solution from high-pressure absorber first,then is sended to low-pressure generator to be partial concentrated,and finally is sended to high-pressure generator.The benefit of it lies in that the initial solution concentration entering low-pressure generator can be reduced.Therefore,the saturated pressure and the outlet temperature of the solution in low-pressure generator can be reduced.So,the temperature drop of the heat source can be increased.The inlet temperature of heat water in this kind of cycle can be reduced,and the temperature drop can be increased significantly.The temperature drop of the heat water can reach to 37℃,7℃larger than that of 1.x cycle witch has a larger temperature drop used currently.So we can take full advantage of heat water from the collector.According to the calculation results,suggested parameters are:medium pressure, 2279-2386Pa(17.1-17.9mmHg);hot water inlet temperature:85-95℃; cold water inlet temperature:15-18℃;cooling water inlet temperature: 18-32℃.The results indicate that as the medium pressure increases,the COP of the cycle increases;the total heat transfer area first decreases and then increases;the specific heat consumption and the specific cooling water consumption decrease at the same time.However,the cycle will not continue after the medium pressure increases to a certain value,the medium pressure is suggested to 2279-2386Pa (17.1-17.9mmHg)herefrom.It also indicates that as the heat source temperature increases,the COP of the cycle will increase continuously; the specific heat consumption decreases;the total heat transfer area decreases.So the higher the heat source temperature is,the better the performance of the system becomes.Based on this,we should choose as higher heat water temperature as possible.The results also indicate that as cold-water temperature increases,the COP increases;the heat transfer area decreases slightly;the specific heat consumption and specific cooling water consumption decrease slightly.So a higher cold water inlet temperature will be beneficial for the cycle.As the cooling water inlet temperature decreases,the COP increases;the total heat transfer area decreases;the specific cooling water consumption and specific heat consumption decrease too.So a lower cooling water inlet temperature will be better.Accordding to the calculation,if the cooling water enters condenser first,enters low-pressure absorber then,and enters the high-pressure absorber finally(serial flow),the performance of the cycle would be perfect.That is because the cooling water’s saturation pressure(in correspondence with its condensing temperature) determines the pressure in high-pressure generator and the initial and final generating temperature thereof.Lower condensing pressure and temperature can decrease the solution’s saturation pressure and temperature in generator.Thus we can decrease the heat water’s outlet temperature.This paper compares the cycle above with the 1.x cycle on following conditions:the heat water’s inlet temperature,the cooling water’s inlet and outlet temperature,the refrigerating output,and the cold water’s inlet and outlet temperature are same.The calculated results showed that although the COP of the cycle designed in the paper decreases(about 0.1)and the cooling water flux increases(about10%) slightly,the total transfer area of the cycle can decrease by 7%at the most,which can compact the whole structure and lower the cost,the heat water flux can decrease by 10%at the most,and the heat water outlet temperature can decrease by 7℃.So despite the cycle sacrificed partial COP,it increases the utilizable temperature differences of the heat source.So it is much more practical.Finally the paper combined the whole solar energy cycle system, introduced the solar energy system model using this cycle,and analyzed the total efficiency of entire system.The total efficiency of this solar energy refrigeration system can reach 31.4%at the most.The thermodynamic perfect degree is about 24%.Because the system can decrease the collector’s backwater temperature,the efficiency of the collector can be increased.The paper concluded that the collector used in the system can get a higher efficiency(2%larger than that of the 1.x cycle).Though the total efficiency of this cycle is a bit less than that of the 1.x cycle (about 5%),and the thermodynamic perfect degree is less than that of the 1.x cycle(about 3%),the cycle provides a possibility for increasing the heat water’s temperature differences.It has superiority in terms of collector.It has a much less total heat transfer area and heat water flux.So it can be practical used.

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