Thermoeconomic optimization of solar absorption systems

Master thesis

Trend of energy systems

Abstract

In recent years, the possibility of using solar energy for cooling and dehumidification has occupied the human mind. Solar absorption cooling systems (Solar Absorption Cycles) have advantages such as not being environmentally dangerous and low energy consumption, especially during electrical peak hours. In addition, since the cost of receiving solar energy only includes the cost of energy absorption equipment such as solar collectors and hot water storage tank, the amount of fuel consumed in this case is less than the conventional absorption cycles. In general, the optimization of heating systems is done based on thermoeconomic principles. Thermoeconomic analysis combines thermodynamic and exergetic analyzes and economic constraints to achieve the optimal practical structure of the system. In this treatise, the thermoeconomic analysis of solar absorption cycles will be investigated in the case of a sample of a common household arrangement with a cooling load of 10 kW and operated by a sample of a single-effect absorption chiller with lithium bromide-water working fluid. Due to the variable amount of solar radiation during different months and hours of the hot seasons of the year, thermal and thermodynamic analysis will be applied in a time-dependent (dynamic) manner, during the day and night hours of the hot months of the year on the desired solar absorption refrigeration cycle. In the next step, the complete arrangement of solar absorption systems will be examined in terms of exergy and the second law, and the source of system inefficiency will be determined. By combining the results of the dynamic thermal analysis of the solar absorption refrigeration cycle (determining the annual fuel consumption in the auxiliary heater and the annual investment cost of the equipment) and the exergetic analysis of the desired cycle, the annual cost rate of the input and output flows to each component of the system will be determined using thermoeconomic equations. In this treatise, it is shown that we see the most exergy losses due to the high temperature difference between the inlet and outlet streams to the condenser and absorber. It can also be seen that the annual cost rate of the product of the entire solar absorption refrigeration system is highly dependent on the temperature of the water entering the generator (this parameter will affect the annual fuel consumption in the auxiliary heater) and the levels of the solar collectors, and for both of the mentioned parameters, it reaches its lowest level at some points. Keywords: single-effect solar absorption refrigeration cycle, lithium bromide-water, collector Solar, source of hot water storage, thermoeconomics, exergy Old and early cooling cycles such as vapor compression cycles[1] have two major problems that are still being dealt with today. These two problems are [1]:

-Global increase in primary and fossil energy consumption: Old cooling cycles that operate by electricity and heat consume a large amount of fossil and electrical energy. The International Institute of Refrigeration and Refrigeration in Paris (IIF\IIR) has allocated 15% of the total amount of electrical energy produced in the world to the purposes of refrigeration and air conditioning in its various types. According to the report of this organization, 45% of the energy consumption for air conditioning is allocated to the use of residential and commercial buildings. In addition to that, in the summer, many problems in the significant increase in the consumption peak still occupy the minds of researchers in reducing it.

- Conventional cooling systems used to cause serious environmental problems: common and unnatural operating fluids [2] in former commercial systems (such as chlorofluorocarbons (CFCs), hydrochlorofluorocarbons (HCFCs) and hydrofluorocarbons (HFCs)) cause both problems of layer destruction. ozone and global warming. Since the adoption of the Montreal Protocol in 1987, international agreements have emphasized the reduction of the use of these fluids. For example, the European Union has stated that by 2015, all systems that work with HFCFs should be phased out.

After the oil crisis of the 1970s in Europe and especially in recent years, researches have been focused on the development of technologies that will cause a reduction in energy consumption, peak electricity demand and energy prices without reducing the level of the necessary conditions. For this reason, in recent years, the possibility of using solar energy for cooling and dehumidification has occupied the human mind and has led to progress in the technology of using solar energy. In the hot areas of the world where there is a serious need for cooling and air conditioning, the human mind is aware of the use of available solar energy in order to use it to provide prosperity and comfort in life. In addition, the application of solar energy is more attractive compared to other applications because when it is needed (cooling and air conditioning), the amount of solar energy is high and can be used. Solar absorption cooling systems [3] have both the advantages of not being dangerous from an environmental point of view and low energy consumption, especially during electrical peak hours.

Compared to other applications of solar energy, this application is more complex both conceptually and practically. For this reason, it has not been developed and used globally. In this method, only receiving and absorbing solar energy is not enough, but we must be able to convert this method into cold and then send it to the desired space. There must be a device that takes heat from a low temperature (ventilated space) and transfers it to a higher temperature (outside space), or in thermodynamic terms, a heat pump [4] is needed. In Figure 1, the view of a solar air conditioning cycle with all the equipment is shown in full.

The heat transfer fluid in solar collectors is heated to a temperature higher than the ambient temperature and is introduced as a stimulus and energy in a power cycle (which itself is a heat pump).

The heat transfer fluid may be air, water or another fluid. Heat can be stored for times when there is no sunlight. The heat taken from the solar cooling cycle is transferred to the surrounding environment, this is cooled by the ambient air or the water coming out of the cooling tower.

Cooling equipment may create the cooling effect in different ways. One of the methods is to produce cold water and send it to equipment that cools the environment with cold water (with the help of an air conditioner) or fans. It is also possible to cool the air directly and send it to the ventilated space.

Solar collectors[5] are an important part of any solar system that convert solar energy into heat at the right temperature, which is the power required for the cooling cycle. There are different types of collectors, from flat plates with low temperature to complex plates with very high temperature. With the increase in demand for air conditioning in recent years, especially in tropical and humid regions, the demand for energy consumption has increased. Since the demand for electric energy consumption is very high in the hot season, we are facing power outages in this season, and more demand for electric energy is a problem. Using new technologies, solar energy can be used in such situations.

Abstract

       In the recent years, the application of solar energy in refrigeration and dehumidification has amused human minds. Solar absorption systems have advantages like nature safety, minimum energy consumption in the peak of electrical hours end etc. In addition, the cost of receiving solar energy only contains the cost of solar energy equipments (solar collectors, hot water storage tank), so the fuel consumption in the solar absorption systems is less than conventional absorption systems. Generally, optimization of thermal systems is evaluated based on thermodynamic rules.         Thermoeconomic analyzes incorporate the thermodynamic analyzes and exergic and economic constraints for receiving the optimized theoretical structure of thermal system.