Transient simulation of a solar absorption cooling system

International Journal of Low-Carbon Technologies, Feb 2016

With non-renewable energy sources depleting quickly, solar energy could prove a viable option owing to its abundance and eco-friendliness. Modeling and simulation of a solar energy-driven single-stage absorption chiller was carried out using the transient simulation software ‘TRNSYS’. An evacuated tube collector coupled with an insulated tank served as heat source for the absorption chiller. Experiments were conducted to evaluate the efficiency parameters of the collector as well as the loss coefficient for the storage tank. These parameters along with standard chiller performance data were used to model the system. The influence of climatic conditions, storage capacity and various control schemes with and without auxiliary heating on the output of the system is analyzed and presented in the paper.

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Transient simulation of a solar absorption cooling system

International Journal of Low-Carbon Technologies Transient simulation of a solar absorption cooling system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . With non-renewable energy sources depleting quickly, solar energy could prove a viable option owing to its abundance and eco-friendliness. Modeling and simulation of a solar energy-driven single-stage absorption chiller was carried out using the transient simulation software 'TRNSYS'. An evacuated tube collector coupled with an insulated tank served as heat source for the absorption chiller. Experiments were conducted to evaluate the efficiency parameters of the collector as well as the loss coefficient for the storage tank. These parameters along with standard chiller performance data were used to model the system. The influence of climatic conditions, storage capacity and various control schemes with and without auxiliary heating on the output of the system is analyzed and presented in the paper. solar cooling; evacuated tube collector; thermal storage; absorption chiller; controls; simulation; TRNSYS New Delhi 110016, India Abstract 1 INTRODUCTION In tropical countries like India, which experience extreme summers in the mainland, demand for electricity shoots up due to the need for cooling. The high electricity demand not only overloads the grid but harms the environment as well due to the burning of fossil fuels, which are the primary source of power. Solar energy for cooling applications provides an opportunity to overcome this problem. The fact that cooling demand in summer is proportional to the availability of solar energy has been spurring the researchers to further exploit solar energy. In cooling applications, different types of sorption systems can be employed. Vapor absorption is a mature technology that can be integrated with solar thermal collectors. A single-effect lithium bromide – water (LiBr – H2O) absorption cooling system operates at a generator temperature in the range of 70 to 958C and requires water as cooling fluid in the absorber and the condenser [1]. A number of simulation and experimental studies [2 – 9] on various solar-powered absorption systems have been carried out by researchers to make this technology more competitive. Assilzadeh et al. [2] presented the simulation and optimization of a LiBr solar absorption cooling system with evacuated tube collectors (ETCs) for the local weather conditions of Malaysia. The simulation of the solar absorption cooling system was carried out using TRNSYS software. The results showed that for a continuous operation, a 0.8-m3 hot water storage tank is essential and the optimum design for a 3.5 kW (1 TR) system required 35-m2 evacuated tube solar collector sloped at 208. Mazloumi et al. [3] simulated a parabolic trough collector-based absorption cooling system with LiBr – H2O as absorbent refrigerant pair. The results showed that the minimum value of the required collector area was 57.6 m2, which could supply the cooling loads for weather conditions of Ahwaz, Iran, in the month of July when the maximum load reached 17.5 kW. Martinez et al. [4] simulated a hot-water-fired, double-effect LiBr – H2O absorption system using TRNSYS and also validated the model with experimental data. The model predicted 30% lower energy consumption as compared with experimental results. This difference was attributed to steady state modeling, which did not consider the transient performance. It was deduced that the simulation time steps should be lower than 1 h. Monne et al. [5] conducted a two-year experimental analysis (2007 and 2008) to study the effect of outdoor temperature of Spain on the performance of LiBr – H2O absorber cooling system. They found that the performance of the chiller was better in the year 2007 because the heat rejection temperature and the outdoor temperature were more favorable than those in 2008. Ge et al. [6] carried out simulation of a low-temperature gas-fired ammonia – water absorption chiller using TRNSYS. The chiller model was validated against experimental results obtained on a 12 kW absorption chiller and was further used to analyze the effect of important design and operating parameters on its performance. The study concluded that the increase in generator heat input from 20 to 30 kW increased the cooling capacity from 11.26 to 14.85 kW and slightly decreased the COP from 0.56 to 0.49. Florides et al. [7] modeled a solar absorption cooling system using TRNSYS for the local climate of Nicosia, Cyprus. The model predicted an optimized system consisting of a 15-m2 compound parabolic collector tilted at 308 from the horizontal and a 600-L hot water storage tank. The collector area was determined by performing the life cycle analysis of the system. The system operated with maximum performance when the auxiliary boiler thermostat was set at 878C. Ayompe et al. [8] modeled a forced circulation solar water heating system with flat plate and heat pipe-based ETCs for the climate conditions of Dublin, Ireland. Experimental study was carried out on both the systems. The comparison between modeled and measured values of heat gathered by the collector showed mean absolute error in the range of 7 to 19%. Darkwa et al. [9] evaluated the performance of integrated solar hot water absorption cooling system for Ningbo, China, a subtropical location. Results revealed an operational efficiency of 61% for the solar collector and coefficient of performance (COP) of 0.69 for the absorption chiller. Optimum slope for the collector was evaluated to be 258. It is apparent that many previous studies have aimed at optimizing the collector side efficiency and comparing experimental and simulated performance. Few studies have included the longterm influence of parameters like ambient conditions and size of storage tank, on the performance of system. The aim of this study is to employ control schemes to minimize the losses and optimize system parameters for performance enhancement. In this study, transient simulation of a hot-water-fired absorption chiller is presented. Experiments were carried out on a solar water heating system to determine the efficiency of collector and the loss coefficient of the storage tank. 2 MODELING OF SYSTEM IN TRNSYS 2.1 Absorption subsystem The absorption system was modeled using the standard absorption chiller model available in TRNSYS library (Type 107), as shown in Figure 1. This component of the software uses a normalized catalog data lookup approach to model a single-effect hot-water-fired absorption chiller. A 30 TR (105.5 kW) absorption chiller (Type 107) was modeled for which the performance curves were acquired from a chiller manufacturer’s catalog. Curve fitting was carried out using Plot digitizer (software used for data extraction from plot) to create the performance data file. Cooling water inlet temperature was fixed at 288C for simulation purposes (an approximate value for New Delhi). Chilled water outlet temperature was set to 78C based on the manufacturer’s data. Rated flow rate values and COP of the chiller are mentioned in Table 1. Simulations were performed for hot water inlet temperature range of 70– 938C. It was observed that the mean difference between the simulated values and the catalog values was less than 5%. 2.2 Collector – storage subsystem The solar collector used was an evacuated tube solar collector (TYPE-71), as shown in Figure 1. Evacuation reduces conduction and convection losses. To model the storage tank, considering the effect of stratification, a detailed stratified tank (TYPE-4a) was selected, which had an auxiliary heater responsible for providing minimum water outlet temperature (708C in the present study) to the chiller unit. In order to prevent boiling of water inside the tank, the hot water outlet temperature was set as 938C as an upper limit. The input parameters for the model such as efficiency values and area were obtained from experimentation as discussed in the next section. The efficiency parameters were calculated from the day- and night-time test results [10]. The weather data were taken from the Typical Meteorological Year data bank of TRNSYS for New Delhi. 2.3 Control subsystem A control system (Type 2b in Figure 1) was incorporated in the simulation model. This system supported dual purpose. Firstly, it saved heat losses from the collector by stopping the water flow in the collector – tank circuit when solar radiation became insufficient. It becomes necessary that when inlet water to the collector from the tank becomes hotter than what it can produce, heat Chilled water flow rate Cooling water flow rate Hot water flow rate Rated coefficient of performance Value transfer occurs from the collector to the ambient. Secondly, if auxiliary backup was absent, the control system would shut down the chiller unit if the hot water supply to the absorber fell below 708C. The presence of control system provided the flexibility to investigate system performance under different schemes, for instance with or without auxiliary. Simulation runs were carried out with three different control schemes in order to model the operation of the system. In the first type of control scheme, the water circulation was kept ON for 24 h along with the auxiliary backup. In the second scheme, the collector– storage tank circuit was switched OFF, when solar radiation became insufficient to heat the water flowing through the collector. At that time, only auxiliary heater powered the chiller. In the last scheme, absorber– storage tank water circulation was switched OFF, whenever the inlet hot water temperature to the chiller from the tank went below 708C. 3 EXPERIMENTATION ON SOLAR COLLECTOR-CUM-STORAGE SYSTEM The experimental setup (as shown in Figure 2) consisted of an evacuated tube solar collector, with a surface area of 3 m2 tilted at an angle of 198 with the horizontal, fixed on a rigid stand and lying in the north – south direction. The collector was attached to a 200-L cylindrical hot water storage tank with water circulation driven through natural convection. An overhead tank was used to provide the makeup water. The incident total global solar radiation on the inclined surface and ambient temperature Table 2. Summary of day-time tests. Total solar radiation (MJ/m2) Table 3. Summary of night-time tests. Intercept efficiency Overall heat loss coefficient Tank loss coefficient were measured using a pyranometer (Kipp-Zonen, spectral range of 310 to 2800 nm and response time of 5 s) and RTD (resistance temperature detector), respectively. The pyranometer measured radiation at an interval of 10 s to take into account the intermittent effects such as passing cloud and overcast sky. These were reflected in the simulations when the recorded radiation data were provided to the model. However, for simplification, the average values were used for other variable parameters like the storage tank overall loss coefficient. T-type thermocouple sensors were used for measuring water temperature inside the tank. All the sensors were connected to a data-acquisition system that scanned the values at an interval of 10 s and tabulated it in an excel file. Two phases of experiments were conducted, one during the day and the other during the night to evaluate various efficiency parameters and loss coefficients associated with the collector system. The standard testing procedures prescribed by Ministry of New and Renewable Energy (MNRE) in India were adopted [10]. Accordingly, the efficiency of the ETC was taken as a linear function (efficiency mode equal to 1) of the temperature difference between tank and ambient conditions. The overall efficiency is given by the following equation [10]: Usys;dX; where ho,sys is the intercept efficiency and Usys,d is the overall heat loss coefficient of the system. Usys,d includes the insulation properties of the storage tank and the piping system. The parameter ‘X’ in Equation (1) depends on the difference between the storage tank temperature and the ambient temperature as well as on the solar radiation intensity. Hence, it is the only varying parameter in the above equation. 4 RESULTS AND DISCUSSION 4.1 Collector – storage subsystem The efficiency of the system (hsys) along with the value of the constant X in Equation (1) was estimated from the experimental data collected during the day-time tests. The night-time tests gave the estimate of the loss coefficient (Usys.d), which enabled the calculation of intercept efficiency (ho,sys). The test results based on the experiments carried out on the collector to get different efficiency parameters and loss coefficients for simulations are given in Tables 2 and 3. The tables show the variation in efficiency from 0.105 to a maximum of 0.344 and the loss coefficient from a minimum of 2.84 to 5.80 W/K. It is noticeable that the resulting efficiency is higher when the initial temperature of tank is lower. This can be attributed to larger heat transfer due to higher difference in temperature between the evacuated tube walls and the fluid flowing through it. Table 4 gives the efficiency parameter and loss coefficients obtained for the collector– storage subsystem. The solar radiation data and tank average temperature were collected for different days, and modeled system was simulated for those days using TRNSYS. Ambient air temperature and solar radiation values taken during the time span of data collection were fed directly to the model. Loss coefficients and efficiency parameters were inputs to the model in TRNSYS. The remaining parameters, such as humidity, atmospheric pressure and geographical location, were taken from the standard weather data for New Delhi. The simulated mean tank temperature values were compared with the experimental data for validating the model. Figure 3 shows a comparison between the simulated and the experimental average tank temperature for two different days. The average variation came around 5%, which was considered satisfactory to predict the performance of such a system [11]. 4.1.1 Control Scheme-I In this scheme, the collector– storage tank water circulation was ON for 24 h along with auxiliary backup. Variation in total auxiliary energy required to meet a constant cooling load was studied for different values of collector area. It was observed that there exists a value of collector area for which the auxiliary energy required is minimum. This happens because initially when the collector area is increased, the effect of gain in solar energy due to a higher collector surface decreases the dependence on auxiliary heating but only to an extent. After a certain value, losses during night from the collector dominate and further increase in area requires more auxiliary backup. This scheme is obviously undesirable and needs to be improvised by Usys,n Ac (W/K) t cooling (sec.) controlling water circulation in the collector– tank circuit to prevent undesirable night losses. 4.1.2 Control Scheme-II In this scheme, the collector– storage tank water circuit was switched OFF when solar radiation became insufficient to heat water flowing through the collector. At that time, chiller was powered only by the auxiliary heater. Figure 4 shows the trends of the average cooling potential generated by the absorption unit for different months starting from March (represented by 1) till October (represented by 8).The winter months from November to February have been neglected, considering the low requirement of cooling in Delhi. The cooling demand has been assumed as constant at 10kW. The average cooling potential signifies the average cooling that can be achieved from the chiller unit in that month span. As expected, there is a significant increase in potential from March to May because of abundant solar radiation in Delhi, which increases the average temperature of hot water supplied to the absorber chiller. A dip in potential is noticed particularly in the months of July and August (5 and 6 on month axis) as these are characterized by the monsoon season in Delhi, with the sky usually covered with clouds. During autumn, the cooling potential lies somewhere in between the summer and the monsoon seasons. Figure 4 also shows the increase in cooling with increase in collector area from 100 to 200 m2. As the collector area increases, the effect of subsequent (equal) amount of increment has diminishing increase in potential. This happens because there is a cut-off for the maximum hot water temperature that can be supplied to the absorber chiller (93.338C for the selected chiller). So an increase in collector area, which otherwise would have given us greater peak temperatures inside the tank, would now only serve the purpose of maintaining the tank at the cut-off temperature for somewhat longer period of time. Also this time is not directly proportional to the collector area. Therefore, it becomes essential to increase storage tank capacity (and the cooling demand) hand in hand with the collector area to increase the cooling potential proportionately. Figure 5 shows the variation in average cooling potential over a period of 8 months (from March to October), for four Indian cities, namely New Delhi, Bikaner, Tezpur and Goa. These cities qualitatively exemplify distinctive climatic conditions of India. It is noticeable that the influence of monsoon on these regions is very different. The idea was to understand the effect of climate on the cooling of the system. It was noticed that the cooling potential is minimum in Tezpur. Tezpur is located in the heart of the mountains on the eastern part of the country and remains overcast especially during monsoons (April to June). The reason for low potential in summers can be attributed to flash rains and thunderstorms experienced by the region in the months from March to May (1 – 3 in Figure 5). However, Goa located on the western coast of the country with similar monsoonal conditions Figure 5. The variation in average cooling potential for four different Indian cities, experiencing discrete climatic conditions. shows much higher cooling potential during summer, attributed to warm clear sky with abundant solar radiation. The situation in the central part of the country is little different. It faces climatic extremes with good solar radiation during summers, marked by high cooling potential in Bikaner (located in the Thar desert in western India) and New Delhi in the central mainland. Both Delhi and Bikaner experience rain during the months of July and August (5 and 6 in Figure 5) marked by large drop in cooling potential, but Bikaner, where the overall rainfall is scanty owing to early withdrawal of monsoon, fares better than Delhi. Bikaner also shows much higher cooling potential in the month of September, when Delhi still experiences the overcast weather. The variation in the average cooling potential of the absorption system with hot water storage tank capacity under fixed load (20 kW as test value) and constant collector area of 150 m2 is shown in Figure 6. It can be observed that the cooling potential decreases as the tank volume increases. It is due to the fact that as the storage tank volume increases, the thermal capacity (mass specific heat) of water inside the tank also increases. As a result, the rate of heat input from the collector becomes insufficient to raise the temperature of water inside the tank above 708C. Thus, the cooling potential decreases. It is important to note that the minimum value of average cooling potential tends to the potential of chiller at 708C if storage tank volume is very large. Further, the effect of change in load on the system can also be seen on comparing Figure 6 with Figure 4 (at 10 kW). Higher cooling load decreases the cooling potential of the absorption system because more heat is extracted from the hot water stream (storage tank to the chiller) when the load is higher, resulting in cooler water return. 4.1.3 Control Scheme-III In this system, a control mechanism was incorporated in the absorber– storage tank water circulation channel to shut down the flow (hot water to the chiller) whenever the average tank temperature fell below 708C. Here, auxiliary back up was absent. The variation in average cooling delivered by the chiller unit with change in storage tank volume (for four different collector areas) is shown in Figure 7. It can be observed that the average cooling potential reaches a maximum value for a particular storage tank volume and then declines. The reason for an optimum storage tank volume can be attributed to a number of factors. Firstly, when the solar radiation becomes insufficient to raise the temperature of the water in the storage tank and the stored heat in tank becomes the only heat source to run the absorber chiller, the total thermal capacity of the storage tank becomes important. At low volumes, the stored heat gets utilized quickly, resulting in a steep drop in temperature. Hence, the time for which the chiller is active reduces, resulting in lower total cooling potential. On the other hand, when the storage tank volume is very large, the heat collected is not sufficient to raise the temperature of water above 708C, due to large thermal capacity. The loss of heat from the storage tank surface to the atmosphere is also higher (larger the volume, greater the surface area of tank). The cooling potential is directly proportional to the temperature of hot water received by the chiller (708C as the minimum and 93.38C as the maximum). These effects become more predominant as the storage tank volume increases, causing a decrease in total cooling potential of the machine. In the zone where these two parameters balance, a flat portion is obtained on the curve. The above-mentioned analysis was carried out for constant cooling water temperature. In practice, cooling water temperature varies with the ambient condition. Figure 8 compares the effect of variable cooling water temperature (for New Delhi conditions) with constant cooling water inlet temperature for different values of storage tank volume. The collector area was kept fixed at 250 m2. In one model, the inlet temperature of cooling water was kept constant at 288C, while in the other situation, a cooling tower to dissipate heat from the cooling water exiting from the absorber chiller has been employed. The cooling tower has crossflow geometry with a fan (1 kW) to induce force draft. Only one 3 cell is used for modeling purpose with a sump volume of 50 m . It can be noticed that the total cooling potential is greater in the Figure 7. The variation in average cooling potential (kW) with storage tank volume for different collector areas. case of a cooling tower. This is because the mean temperature of the cooling water returning from the cooling tower is ,288C and the cooling potential is more for the low values of inlet cooling water temperature. In case of locations with higher ambient wet bulb temperatures, the cooling potential may also drop. A constant cooling water temperature is possible only in a once-through system, if a large water reservoir (such as lake and ground water) is available near the site. However, even then it may not be desirable from an environmental point of view. 5 CONCLUSIONS Transient modeling of a solar-powered absorption system was carried out using TRNSYS with the collector model input parameters taken from the measured performance data of an ETC. Different control schemes were employed to predict the cooling potential of the system over a period of one year (except winter months). It was observed that the cooling potential of the given system increases with the collector area but gets limited by the storage tank volume due to an upper limit on the water temperature. The system controls play a vital role in efficient utilization of solar energy with and without auxiliary heating. When the storage tank volume became large with auxiliary heating, the average cooling potential diminished due to lower water temperature. However, without any auxiliary backup, an optimum value of storage tank volume is obtained, depending on the collector area. For collector areas of 150 and 350 m2, optimum storage tank volume was observed to be 2 and 5.8 m3, respectively. A variable cooling water temperature that depends on the ambient conditions was used for more accurate performance predictions. These results are useful in designing optimal solar absorption cooling systems subjected to different constraints. ACKNOWLEDGEMENT The authors express their sincere gratitude to the Ministry of New and Renewable Energy (MNRE) for the financial support to build the experimental facilities. [1] Duffie JA , Beckman WA. Solar Engineering of Thermal Processes . Wiley, 1991 . [2] Assilzadeh F , Kalogirou SA , Ali Y , et al. Simulation and optimization of a LiBr solar absorption cooling system with evacuated tube collectors . Renew Energ 2005 ; 30 : 1143 - 59 . [3] Mazloumi M , Naghashzadegan M , Javaherdeh K. Simulation of solar lithium bromide - water absorption cooling system with parabolic trough collector . Energ Conversi Manag 2008 ; 49 : 2820 - 32 . [4] Martinez PJ , Gracia A , Pinazo JM . Performance analysis of an air conditioning system driven by natural gas . Energy Build 2003 ; 35 : 669 - 74 . [5] Monne C , Alonso S , Palacin F , et al. Stationary analysis of a solar LiBr-H2O absorption refrigeration system . Int J Refriger 2011 ; 34 : 518 - 26 . [6] Ge YT , Tassou SA , Chaer I. Modelling and performance evaluation of a low-temperature ammonia - water absorption refrigeration system . Int J Low-Carbon Tech 2009 ; 4 : 68 - 77 . [7] Florides GA , Kalogirou SA , Tassou SA , et al. Modeling and simulation of an absorption solar cooling system for Cyprus . Sol Energ 2002 ; 72 : 43 - 51 . [8] Ayompe LM , Duffy A , McCormack SJ , et al. Validated TRNSYS model for forced circulation solar water heating systems with flat plate and heat pipe evacuated tube collectors . Appl Therm Eng 2011 ; 31 : 1536 - 42 . [9] Darkwa J , Fraser S. Performance of an integrated solar absorption cooling system in a sub-tropical region . Int J Low-Carbon Tech 2012 ; 7 : 199 - 207 . [10] MNRE (Ministry of New and Renewable Energy) . Test procedure for thermosyphon-type domestic solar hot water systems . www.mnre.gov.in/ pdf/etc_test_ procedure_stg.pdf (12 June 2011 , date last accessed). [11] Budania A , Ahmad S , Jain S , et al.. Transient simulation of an integrated evacuated tube solar collector cum storage system . In: Proceeding of International Congress on Renewable Energy , Tezpur, India, 2 - 5 November 2011 , p. 396 - 408 .


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Akshaya Budania, Suhail Ahmad, Sanjeev Jain. Transient simulation of a solar absorption cooling system, International Journal of Low-Carbon Technologies, 2016, 54-60, DOI: 10.1093/ijlct/ctt060