Commercial building sector accounts for 8% of the total electricity consumption in India. Cooling activities (HVAC) in commercial buildings consume 55% of the total energy utilized. Consequently, CO 2 emissions from conventional buildings in India were estimated to be 98 metric tonnes of CO 2 per million ft 2 in 2014. Solar thermal air conditioning can be the solution to these demands and can contribute to about 15% to 20% of India’s total oil consumption thereby reducing the dependence on fossil fuels. Hence, the main objective of the work is to model and simulate a solar absorption cooling system for GERMI office building located in Gandhinagar, Gujarat, India, using the transient simulation software ‘TRNSYS’. Cooling load estimation and comfort conditions required for the building were determined based on ASHRAE standards. Evacuated tube collectors were selected because of its market availability, ease of manufacturing and proven technology. Single effect absorption chiller was used because of its commercial availability. The effects of storage tank volume, collector area and collector slope were also investigated for parametric optimization. The results of the simulation and parametric analysis are analyzed and presented in the paper.
Air conditioners account for 45% of the total energy consumption in the residential and commercial building sector in India [
India is blessed with excellent solar radiation with an annual average global solar irradiance of 4 - 7 kWh/m2/day which can be harnessed through heat and electricity. The fact that demands for cooling in summer matches with solar radiation availability has kindled the interests of many researchers to utilize solar energy. One way of using solar radiation is to convert it into thermal energy through thermal solar collectors and use it to drive thermally driven cooling systems such as vapor absorption system. A single-effect lithium bromide-water (LiBr- H2O) absorption cooling system operates at a generator temperature in the range of 70˚C to 95˚C and requires water as cooling fluid in the absorber and the condenser [
A number of experimental and simulation studies [
At present, only a few successfully installed solar thermal air conditioning systems installations is operational in India. The first such installation was in the year 2006 at Mamata Energy Plant located in Ahmadabad, Gujarat. The system consisted of a 25TR single effect Vapour absorption chiller integrated with evacuated tube collectors. The second system was designed and commissioned by Thermax, which was installed at the Solar Energy Centre in Gurgaon, Haryana, India. The 100 kW technology demonstration project was the first of its kind in the world which involved the use of triple effect absorption chiller integrated with solar parabolic concentrators. The company claimed to have made a technological breakthrough by delivering highest Coefficient Of Performance (COP) in the global markets today. The next major installation was at Kailash Cancer Hospital and Research Centre, Muni Seva Ashram located at Goraj, Gujarat. The hospital had installed 100 Scheffler concentrators with 12.5 m2 capacity each to meet the 100 TR cooling requirements.
Besides these demonstration projects, few simulation programs have been developed to evaluate the performance of solar cooling in India. Mittal et al. [
To summarize, many previous studies have been done to simulate solar absorption system worldwide. There is a shortage on the investigation of solar cooling system performance for Indian conditions. However, to the best of our knowledge, solar absorption cooling system has not yet been investigated for climatic conditions of Gujarat. Moreover, only a few studies have included the influence of parameters like ambient conditions and size of the storage tank, on the performance of the system. Hence, the paper presents the modelling and dynamic simulation of a solar assisted air conditioning system located in an office building using TRNSYS. The effects of storage tank volume, collector area and collector slope were also investigated and presented in this paper. Sensitivity analysis was carried out to optimize system parameters for performance enhancement.
The two-story building considered in this study is situated in Gandhinagar at 72.68˚E longitude, and 23.22˚N latitude in Gujarat, India at an elevation of about 81 meters above mean sea level. The dimensions of the building that are to be air conditioned are 65 × 15 × 4 m in size.
Cooling load is the amount of heat energy that would need to be removed from a building space to maintain constant air temperature inside the premises. Cooling Load Temperature Difference method (CLTD) was adopted to determine the cooling load of the office building. We have used the standard method adopted by the American Society of Heating, Refrigerating, and Air-conditioning Engineers (ASHRAE) [
Solar based absorption cooling system was modelled using TRNSYS 17 simulation program. A TRNSYS project is typically setup by connecting components graphically in the Simulation Studio. Each Type of component is described by a mathematical model in the TRNSYS simulation engine and has a set of matching Proforma’s in the simulation studio. The proforma has a black-box description of a component: inputs, outputs, parameters, etc. The connections created between the components in the simulation studio act as paths of information flow between coupled components. The outputs of the first component are configured to be the inputs of the second component [
Evacuated Tube Collector
The collector was modeled using the standard evacuated tube collector model available in TRNSYS library (TYPE 71), as shown in
where,
Name | Value | Unit |
---|---|---|
Number in series | 8 | - |
Collector area | 35 | m2 |
Fluid specific heat | 4.19 | kJ/kg∙K |
Efficiency mode | 1 | - |
Tested flow rate | 40 | kg/hr∙m2 |
Intercept efficiency | 0.75 | - |
Efficiency slope | 6.15 | W/m2∙K |
Efficiency curvature | 0.03 | W/m2∙K2 |
tm is the collector temperature.
ta is the ambient temperature.
G is solar radiation in the collector plane.
Weather data reading and processing
For a successful simulation, accurate weather data for calculation of the incident radiation energy on the collector surface is necessary. For this purpose, a user defined weather file (Type 109 TMY2) is used to obtain the solar irradiance and various weather conditions for Gandhinagar.
Absorption chiller
The TRNSYS inbuilt Type 107 for modelling single-effect absorption chillers uses an external absorption chiller performance data file for predicting the chiller performance at the prevailing conditions of hot water supply, cooling water and chilled water inlet temperature. A 30 TR absorption chiller was modelled for which the datasheet was acquired from the manufacturer YAZAKISC30. These parameters can be found in
where,
Storage Tank
Type 4a is used to model the storage tank. This type can be used to model storage tanks with fixed inlets and uniform losses. The system also had an auxiliary heater responsible for providing minimum water outlet temperature (80˚C in the present work) to the chiller system. In order to prevent boiling of water inside the tank, the hot water outlet temperature was set as 98˚C as an upper limit.
Control Strategy
Differential Controllers (TYPE 2b) are employed to simulate real time conditions. The function of the controller (on-off) is to manage the flow through the collector depending on the fluid temperature difference across the collector array. Once the temperature of the collector exceeds the temperature at the bottom of the storage tank by 5 - 10 K, the collector pump is turned ON. On the other hand, if the temperature at the collector output is lower than that of the input by 2 K, the pump is turned OFF [
1) Temperature of inlet water to the collector is lower than temperature of outlet water of collector (T collector out > T collector in) and
2) Temperature of the outlet water of collector is less than 98˚C.
There are various parameters that affect the performance of the simulated solar absorption system. The factors that were investigated in this simulation are storage tank volume, collector slope, collector area, the mass flow rate through the collectors, useful energy gain and auxiliary energy consumption. The simulation was carried out for the entire year.
Effect of collector slope
The effect of the collector tilt angle for a south facing collector was investigated. In theory, the optimum value of collector slope for a non-tracking collector should be the latitude of the location of the collector. The collector slope was varied in the range of 3˚ to 35˚ as shown in
Chiller Model: YAZAKI WFC-SC30 | ||
---|---|---|
Cooling capacity | 105.6 KW | |
Chilled water | Inlet temperature | 12.5˚C |
Outlet temperature | 7˚C | |
Rated flow rate | 16.5 m3/hr | |
Cooling water | Inlet temperature | 31˚C |
Outlet temperature | 35˚C | |
Rated flow rate | 55.1 m3/hr | |
Hot water | Inlet temperature | 88˚C |
Outlet temperature | 83˚C | |
Inlet temperature range | 70˚C - 95˚C | |
Rated flow rate | 25.9 m3/hr |
collector energy gain is when the slope angle is equal to the local latitude. Hence, the optimum collector slope is 24˚N, which is close to the latitude of Gandhinagar 24.22˚N.
Effect of storage tank volume
The optimum storage tank is determined by examining the influence of storage volume on the system performance. For this purpose, the storage tank volume is varied from 0 m3 to 3 m3. An optimum value of storage tank would be at which minima is achieved in the trend of annual auxiliary heater power required over the parametric analysis of storage tank volume. This result was confirmed by observing the trend in
Since at the optimum storage tank volume, minima can be seen in the trend of auxiliary heater power required and useful solar energy being inversely proportional, maxima should be seen in its trend. The same is observed in
The solar fraction computes the fraction of thermal energy delivered to the absorption chiller generator that comes from solar energy. In order to achieve a high solar fraction, auxiliary heater energy should be reduced.
An optimum storage tank volume would be the one that achieves the highest solar fraction. It was observed from
From the above graphs, we can conclude that the optimum storage tank volume is 0.5m3.
Effect of collector area
The specific collector area was changed from 0 to 80 m2.
It can be observed from
collector area as shown in
The parametric analysis based on collector area could not provide an optimum value. The optimum value would be close to 35 m2 collector area as this is where the gradient of increase in solar fraction and decrease in useful solar power is observed. Thermo economic analysis is required for optimizing the solar collector area.
Effect of mass flow rate
The effect of varying the mass flow rate through the collector array is investigated. It can be observed from
300 to 10,000 kg/h did not represent a big variation in the yearly solar fraction that was in the range of 0.70 to 0.74.
The final system specifications based on the optimization study is shown in
The simulation is performed for the entire year (Jan to Dec), i.e., 8760 hours with a simulation time step of 0.125 hr. The first parameter that was taken into consideration was collector outlet temperature.
In the beginning, the system takes some time to warm up. Hence, the curve is flat as can be seen from the bottom left side of
It is evident from
As seen in
Parameter | Value | Unit |
---|---|---|
ETC collector area | 35 | m2 |
No of collectors in series | 8 | No unit |
Collector mass flow rate | 1000 | Kg/hr |
Storage tank volume | 0.5 | m3 |
Maximum heating rate of auxiliary heater | 1000 | KJ/hr |
Rated capacity of chiller | 30 | TR |
the system is working on atmospheric conditions. The outlet temperature of the collector should be below 100˚C in all cases. Hence the storage tank volume of 0.5 m3 can provide a constant temperature. The fluctuating yellow lines are noticeable during the periods of 0 - 1500 and 7300 - 8760 hours that corresponds to the months of January-March and October-December. These are the times that are considered to be the cooling periods. The solar radiation will be less intense during these periods. As the radiation is directly proportional to the tank outlet temperature, the tank outlet temperature will be less during these periods thereby the reason for fluctuations. The observation of linear trend was during the hottest periods.
A similar trend is noticeable for auxiliary heater temperature which is shown in
The next important parameter is the chilled water outlet temperature. The hot water from the storage tank goes to the absorption chiller generator and comes out as chilled water. The chiller remains in operation as long as there is hot water and a demand for chilled water. It is noticeable from
The chilled water interacts in the cooling coil with air and removes the heat from the building via an air handling unit.
The solar fraction (
Financial analysis
The cost for design, supply, installation and commissioning of solar hot water fired 30 TR single effect vapor absorption chiller = Rs. 20,62,857 (Manufacturer Quotation).
Operation and Maintenance cost = Rs. 10, 00,000.
MNRE Subsidy = Rs. 9, 18,857 (30% of capital cost).
Net cost = Rs. 30, 62,857 - Rs.9, 18,857 = Rs. 21, 44,000.
Number of sunny days in Gandhinagar = 300.
Number of working days of office = 270.
Working Hours per day = 9 hours.
Energy saving per annum = 105.5 × 270 = 28,485 kWh.
Cost of electricity per unit (2015) = Rs. 9.
Savings in energy bill per year = 28,485 × 8 = Rs. 2, 27,880.
Simple payback period =
This payback period doesn’t include accelerated depreciation benefit and fuel escalation price.
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. A sensitivity analysis was also presented, determining the approximate optimum values of a few design and operational parameters. The results obtained from the parametric optimization of the simulated solar-based absorption system indicate that an area of 35 m2 of evacuated tube collectors with an inclination of 24.2˚ and 0.5 m3 of storage tank can satisfy the cooling demand of an office building of 1000 m2 located in Gandhinagar.
Therefore, it becomes evident that the validated TRNSYS simulated model, in conjunction with Gandhinagar weather data, can be used as a useful engineering design tool to predict the long-term performance of a solar cooling system without actually performing expensive prototype or full scale testing.
Future developments of this work will include the development of a complete cost model to be installed in the office building for experimental analyses and model validation under actual climatic conditions.
Ganesh Utham,Sagarkumar M. Agravat,Bela Jani,Jignasha Bhutka, (2016) Modelling, Transient Simulation and Economic Analysis of Solar Thermal Based Air Conditioning System in Gujarat. Smart Grid and Renewable Energy,07,233-246. doi: 10.4236/sgre.2016.78018