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Thermal performance investigation of earth air tunnel heat exchanger coupled with a solar air heating duct for northwestern India
Energy and Buildings 87 (2015) 360–369
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Energy and Buildings journal homepage: www.elsevier.com/locate/enbuild
Thermal performance investigation of earth air tunnel heat exchanger coupled with a solar air heating duct for northwestern India Sanjeev Jakhar a,∗ , Rohit Misra b , Vikas Bansal c , M.S. Soni d a
Center for Renewable Energy and Environment Development, BITS – Pilani, Rajasthan, India Mechanical Engineering Department, Government Engineering College, Ajmer, India Mechanical Engineering Department, University College of Engineering, Kota, India d Department of Mechanical Engineering, BITS – Pilani, Rajasthan, India b c
a r t i c l e
i n f o
Article history: Received 26 September 2014 Received in revised form 24 November 2014 Accepted 26 November 2014 Available online 3 December 2014 Keywords: Earth air tunnel heat exchanger Solar air heating duct Heating capacity Thermal performance
a b s t r a c t In the present research thermal performance of earth air tunnel heat exchanger (EATHE) coupled with a solar air heating duct has been experimentally evaluated for arid climate of Ajmer city of northwestern India, during winter season. An attempt has been made to enhance the heating capacity of EATHE system by coupling it with a solar air heating duct at the exit end. Results show that the air which comes out of coupled EATHE system is relatively hotter than the air supplied by the stand alone EATHE system. It was found that the heating capacity of EATHE system got increased by 1217.625–1280.753 kWh when it was coupled with solar air heating duct with a substantial increase in room temperature by 1.1–3.5 ◦ C. The COP of the system also increased up to 4.57 when assisted with solar air heating duct. Therefore, the heating capacity of EATHE can be significantly increased by coupling it with solar air heating duct. © 2014 Elsevier B.V. All rights reserved.
1. Introduction With the rapid growth in population and economic growth of countries in the tropical regions, it is becoming inevitable that passive and low energy strategies must be used as suitable alternatives for heating/cooling. The utilization of geothermal energy to reduce heating and cooling needs in buildings has received increasing attention during the last several years. The potential of earth–air heat exchanger has been established in moderate climates of Europe, however not much research has been carried out in hot climates because of the claim that the potential is low. The use of passive heating techniques in the winter is advisable, with the objective of reducing energy consumption with the climatisation of spaces. It can thus be an effective tool for attenuating the growth of energy consumption for air conditioning. To ensure indoor air quality, building needs adequate ventilation. Many commercial and industrial buildings need to have high
Abbreviations: EATHE, earth air tunnel heat exchanger; PVC, poly vinyl chloride; RPM, revolution per minute; DBT, dry bulb temperature of air (◦ C); COP, coefficient of performance. ∗ Corresponding author. Tel.: +91 1596 516455. E-mail addresses: [email protected], [email protected] (S. Jakhar). http://dx.doi.org/10.1016/j.enbuild.2014.11.070 0378-7788/© 2014 Elsevier B.V. All rights reserved.
ventilation rates. During winter, fresh cold air needs to be warm up before supplying to the buildings, thus consuming more energy. Preheating of external air before entering the building can be achieved by natural means, like circulation in buried pipes [1,2]. To reduce high grade energy consumption of conventional active heating systems, numerous alternative techniques have been explored. One such proposition is the earth air tunnel heat exchanger (EATHE) system. EATHE works in principal on geothermal energy with temperature variation. Soil temperature, at a depth of about 10 feet or more, stays fairly constant throughout the year and is approximately equal to the average annual ambient air temperature. The ground can, therefore, be used as a heat source for heating in winter. Cold outdoor air is sent into the earth air tunnel heat exchanger. When air flows in the earth air tunnel, heat is transferred from the earth to the air. As a result, the air temperature at the outlet of the earth air tunnel heat exchanger is much higher than that of the ambient. The outlet air from the earth air tunnel can be directly used for space heating if its temperature is high enough. Alternatively, the outlet air may be heated further by associating air conditioning machines and solar air heating duct. Both of the above uses of earth air tunnel heat exchanger can contribute to reduction in energy consumption. The main advantages of the system are its simplicity, high preheating potential, low operational and maintenance costs, saving of fossil fuels and related emissions. Pre-heated fresh air
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supports a heat recovery system and can reduce the space heating demand in winter. The potential of earth tube air heat exchanger for space heating is well accepted in colder countries. As reported by Bansal et al. [3] not much research has been carried out in hot climates (such as in western India) because of the belief that the cooling potential of EATHE system is low due to higher soil temperature in summer. Santamouris et al. [4] investigated the impact of different ground surface boundary conditions on the efficiency of earth-to-air heat exchanger consisting of single and multiple parallel pipes and concluded that ground surface covered with short grass gives better cooling performance than bared soil condition. Hwang et al. [5] studied the performance of an innovative evaporative condenser and compared it with that of a conventional air-cooled condenser of a split heat pump system. Their experimental results showed that the evaporative condenser has a higher capacity than the air-cooled condenser by 1.8–8.1%, a higher COP by 11.1–21.6%. Hajidavalloo [6] investigated the effect of using evaporative cooler in the window-air-conditioner by injecting water on the media pad installed in front of the condenser entrance and reported 16% reduction in power consumption and 55% improvement in total performance. Most of the research has been carried out through mathematical modeling or experimental investigations. As a space cooling technology utilizing natural energy, earth–air–pipe systems have got increasing interest for energy conservation [7–9]. Kumar et al. [10] evaluated the conservation potential of an earth–air–pipe system coupled with a building with no air conditioning. The cooling power for the earth pipe was evaluated as 19 kW for pipe length of 80 m with section area of 0.53 m2 and air flow velocity of 4.9 m/s. Ajmi et al. [11] studied the cooling capacity of earth–air heat exchangers for domestic buildings in a desert (hot and arid) climate. A reduction of 30% in seasonal cooling demand was reported by using the earth-to-air heat exchanger in July. Ramirez-Davila et al. [12] performed a numerical study on the thermal behavior of an earth to air heat exchanger (EATHE) for three cities in Mexico with varying climatic conditions. Results showed that use of EAHE was appropriate for extreme and moderate temperature regions where the thermal inertia effect in soil is higher. Santamouris et al. [13] developed a new integrated method to calculate the role of earth to air heat exchangers to reduce the cooling load of the buildings based on the principle of balance point temperature. Badescu [14] developed a ground heat exchanger model based on numerical transient bi-dimensional approach for passive house space heating. Sodha et al. [15] evaluated an earth–air tunnel system for cooling/heating to provide thermal comfort inside the whole building complex at one of the hospitals in India. Their results showed that an 80 m long tunnel with a cross sectional area 0.528 m2 had a 512 kWh cooling capacity and 269 kWh heating capacity. Exergo-economics analysis was carried out by Ozgener and Ozgener [16] to determine the optimal design of a closed loop earth to air heat exchanger for greenhouse heating. Hollmuller and Lachal [17] discussed the potential and climate independent design guidelines of buried pipe systems. The design guidelines were validated using a system described by the authors. Poshtiri et al. [18] performed the feasibility study of the combined system of EAHE with a solar chimney. Similar work has been done by Maerefat and Haghighi [19]. They designed EAHE system coupled with solar chimney for both cooling and ventilation. Numerical modeling was performed for the system to optimize the diameter of pipe, which gives the minimum required number of solar chimney and EAHEs. The literature review reveals that there exists a gap to correlate the potential of solar air heating duct coupled with EAHTE system. According to Sodha et al. [15], the heating capacity of standalone EATHE is not adequate to provide necessary comfort conditions for Indian conditions. And hence to enhance the performance of
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EATHE systems, a solar air heating duct is coupled in the present study. An experimental investigation has been carried out for different modes, operating for 8 h daily incorporating the winters during January and February. An attempt has also been made to explore the validity and effectiveness of the employed EATHE coupled with a solar air heating duct system for heating in cold conditions. The heating capacity and coefficient of performance (COP) of the system were found to be in agreement for the enhancement of results.
2. Description of test facility and test unit of EATHE coupled with a solar air heating duct system The test location of the system was in the city of Ajmer, located in northwestern India, which has hot semi-arid climate having the absolute maximum and mean maximum ambient air temperatures during summer period (April to June) as 47 ◦ C and 39 ◦ C respectively. The average ambient temperature during summer ranges from 30 to 32 ◦ C. During winters (November to February), the weather remains mild and average ambient temperature ranges between 15 and 18 ◦ C. During winter, the absolute minimum and mean minimum temperatures of ambient air are close to 4 ◦ C and 9 ◦ C respectively. The annual average ambient temperature of the location is 26.7 ◦ C. The actual setup of EATHE coupled with a solar air heating duct connected to test room is shown in Fig. 1. Experimental test set up comprises of 60 m long horizontal PVC pipe of 0.10 m diameter, buried in flat land with dry soil at a depth of 3.7 m. Inlet end of EATHE pipe is connected through a vertical pipe to a 0.75 kW, single phase, variable speed motorized blower (maximum flow rate of 0.0945 m3 /s and maximum speed of 2800 rpm). A U-shaped duct (12.2 m long having 0.0645 m2 cross-sectional area) made of galvanized iron has been used as a solar air heating duct having top and lateral wall exposed to solar energy so as to have the receiving surface area of 3 m2 and 2.6 m2 respectively. Inlet of duct was connected to the outlet pipe of EATHE by means of T-socket and outlet of duct was also connected at the exit of EATHE pipe at suitable position as shown in Fig. 2. The exterior surface of the entire duct was painted black so as to absorb most of the solar radiation falling on it. Dampers were provided at the inlet and exit of solar air heating duct as well as at the exit of EATHE pipe, to regulate the flow of air. Heated air coming out of EATHE pipe can be made to flow through solar air heating duct (Mode-III) for further heating and provision was kept to supply the conditioned air directly from EATHE to test room (Mode-II) by controlling the position of dampers and valves as shown in Fig. 2. Ambient air was forced through the earth air pipe system using a centrifugal blower and air flow velocity was changed by an auto transformer (single phase, 0–270 V, 2 A maximum current, with a least count of 1 V). When the blower runs at 230 V, it corresponds to a maximum flow rate of 0.0945 m3 /s and a flow velocity of 12 m/s inside the EATHE pipe (diameter 0.1 m). However, at a certain reduced voltage, the blower supplies the air at a flow velocity of 5 m/s through EATHE pipe. The measured energy consumption of blower at this mean air velocity was 0.12 kW. The flow velocity through solar air heating duct would be different from that through EATHE pipe, but flow rate would be same through EATHE pipe as well as solar air heating duct. Some of the energy supplied to blower gets converted into heat due its inefficiency and transferred to the air passing through EATHE, which also adds to the heat imparted to the air by EATHE. Seven resistance temperature detector (RTD) (Pt-100) temperature sensors viz. T0 –T6 were mounted at a depth of 3.7 m, 3.04 m, 2.43 m, 1.82 m, 1.21 m, 0.60 m, and 0 m, respectively, from the ground surface on inlet vertical pipe to measure soil temperatures at different depths. Nine RTD (Pt-100) temperature sensors viz. T7 –T15 were also inserted at the center of EATHE pipe along the length at a horizontal
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Fig. 1. Experimental set-up of EATHE coupled with solar air heating duct.
distance of 0.2 m, 1.7 m, 4.7 m, 9.3 m, 15.1 m, 24.2 m, 34.0 m, 44.4 m, and 60.0 m respectively, from the upstream end to measure air temperature. Two RTD (Pt-100) temperature sensors were mounted at inlet and exit of U-shaped solar air heating duct to measure the temperature of air at inlet and outlet of duct, respectively. Properly calibrated, digital temperature display devices (accuracy of ±0.1 ◦ C and resolution 0.1 ◦ C) had been used. Dry bulb temperature, relative humidity of ambient air and solar radiation were recorded hourly using RTD (Pt-100) temperature sensor, capacitive transducer and pyranometer mounted on weather station. Temperature and relative humidity of air inside the test room and at the outlet of EATHE were also measured accurately with the help of calibrated thermo hygrometer (make-Fluke-971, temperature accuracy of ±0.1 ◦ C, temperature resolution of 0.1 ◦ C and relative humidity resolution of 0.1%). Air flow velocity was measured with the help of a vane probe type anemometer (makeLutron, model-AM-4201, range – 0.4–30.0 m/s and least count of 0.1 m/s). Electrical energy consumed by the entire system (centrifugal blower) was measured with the help of calibrated digital energy meter (make-Power tech measurement system, type-PTS-01, least count of 0.1 kWh and an accuracy of ±0.1 kWh). Dimensions of test room were 4.3 m × 3.8 m × 3.05 m. Room was having two windows (1.52 m × 1.22 m each, located on east and north facing walls respectively) and a door (1.82 m × 0.91 m, located on west
facing wall). Thermocol insulation was placed on the glass panes of windows to reduce the heat exchange with the surroundings and interstices around the door were also taken care of by providing packing and minimizing the leakage of conditioned air from the room.
3. Modes of EATHE coupled with a solar air heating duct and test procedure Three different arrangements of EATHE and solar air heating duct have been investigated experimentally: (1) Mode-I: Only thermal conditions of room and ambient were monitored. EATHE and solar air heating duct were not functional. This mode has been treated as base case and thermal performance of other two modes of operation is compared with this mode. (2) Mode-II: Heated air (conditioned air) from EATHE was directly supplied to the room and solar air heating duct was not functional. (3) Mode-III: Heated air (conditioned air) from EATHE was allowed to pass through solar air heating duct for further heating and then supplied to the room.
Fig. 2. Schematic diagram of the experimental setup.
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Fig. 3. Intensity of solar radiation and ambient air temperature of January and February, 2013 for different modes.
Experiments were conducted from 6th January to 27th February, 2013 for 8 h duration daily from 9 a.m. to 5 p.m. System remained closed from 6 p.m. to 9 a.m. everyday allowing the soil to get regenerated. As the cold air passes through the underground pipe, it gets warmer thus causing the soil around the pipe to cool down. This changes the thermal conductivity of soil and causes the changes in heat transfer [20]. Hence, in this study integrated EATHE was operated during the day and turned off during the night hours allowing the soil around the pipe to get regenerated. Flow velocity of air through the EATHE was maintained at 5 m/s. Measurements and recording of hourly data included the intensity of solar radiation, ambient air temperature, relative humidity, temperature and relative humidity of air at the outlet of EATHE, temperature of air in the buried pipe at nine different locations, depth-wise temperature of soil at seven points, temperature and relative humidity of air inside the conditioned room at five different locations and electrical energy consumed by air blower.
4. Performance and results of EATHE coupled with a solar air heating duct in different modes Performance of EATHE coupled with a solar air heating duct system in three different modes has been evaluated on four periods of three consecutive winter days of 2013 (14–16 January, 26–28 January, 1–3 February and 13–15 February). It is important to mention here that the experimentation continued for 2 months on all the three modes, but only those data sets have been considered in each month, for which a close agreement between the ambient air temperature, relative humidity of ambient air and solar radiation recorded on different days is observed, as shown in Figs. 3–7. It is also worth mentioning here that for first data set which comprises of the data measured on three consecutive days i.e. 14, 15 and 16 January, ambient air temperature, relative humidity of ambient air and solar radiation measured on hourly basis on 14 January were taken to be the base data and ambient air temperature, relative humidity of ambient air and solar radiation measured on 15 and
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Fig. 4. Variation of room air temperature for three different modes in January and February, 2013.
16 January were compared with the data measured on 14 January. Maximum difference between ambient air temperature, relative humidity of ambient air and solar radiation measured on 15 and 16 January and that measured on 14 January was ±1.6 ◦ C, ±3% and ±3%, respectively, therefore, the ambient conditions for all three modes were almost identical at any hour of operation. It ensures the identical outdoor conditions and thermal loading of the test room in order to evaluate the relative performance of each mode. Similar procedure was adopted to fix a base data for the rest of the three data sets by considering the weather data measured on 26 January, 1 February and 13 February as the base data. Experimental data recorded for temperature sensor T0 (which measures the temperature of soil at a depth of 3.7 m from ground surface) clearly reveals that the temperature of soil at this depth is not at all affected by the diurnal variation in ambient temperature and solar radiation, as shown in Table 1. It also reveals that the variation of temperature along the depth of tunnel follows the same trend for different modes. Results showed that significant temperature of 27.4 ◦ C was obtained at 3.7 m depth, with almost a
difference of 11 ◦ C between the temperature of the soil at that depth and ground surface. Usually severe winter is observed at Ajmer in the month of January. As mentioned earlier, the EATHE was put into use during day hours from 9.00 a.m. to 5.00 p.m., from January to February. Table 2 reveals the temperature measured along the length of pipe from the inlet of EATHE. It is observed that more than 82–85% of the total increase in the temperature of air along the EATHE pipe is obtained at a length of 34 m from inlet. The temperature growth in rest of the pipe is infinitesimally slow as it has approximately reached its steady condition by this length. It is also observed from Table 2 that at 60 m, the outlet temperature of EATHE was nearly same. Table 3 and Figs. 4–7 show that on 14, 15 and 16 January, the ambient air temperature ranges between 14.6 ◦ C and 23.4 ◦ C. Table 3 reveals that the ambient air temperature of 14, 15 and 16 January, was nearly same and had the maximum deviation of 1.6 ◦ C. It was also observed that the temperature of air in the room ranges between 17.7–21.1 ◦ C, 17.5–22.3 ◦ C and 17.6–24.1 ◦ C during Mode-I, Mode-II and Mode-III respectively. Therefore, the use of EATHE along with solar air heating duct resulted in 1.1–3.5 ◦ C
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Fig. 5. Thermal performance of Mode-I of operation in January and February, 2013.
higher temperature of air maintained inside the room as compared to Mode-I (base case). Table 3 reveals that the air temperature at the outlet of EATHE system varied very little between 24.4 and 24.8 ◦ C, during Mode-II as well as Mode-III and hence, the air temperature at the outlet of EATHE showed a lower variation (0.4 ◦ C) than the variation of 8.8 ◦ C observed for ambient air. It was also observed from Table 3 and Figs. 4–7 that relative humidity of ambient air had a variation of 83.2% (28.0–51.3%) on 14, 15 and 16 January, during the operation of different modes. Relative humidity of air inside the room varied between 43.9–46.14%, 36.12–51.8% and 32.5–53.5%
during Mode-I, Mode-II and Mode-III respectively. Table 3 reveals that the coupling of solar air heating duct with EATHE system significantly increases the temperature of air. It is observed that on 16 January, during the operation of Mode-III, solar air heating duct raised the temperature of air coming out of EATHE pipe by 6–9 ◦ C. Therefore, the heating potential of EATHE system, when coupled with solar air heating duct improves significantly. Similar results were obtained on 26, 27 and 28 January, 2013 as shown in Figs. 4–7. In Mode III significantly higher room air temperature could be maintained as compared to the Mode-I (base
Table 1 Depth wise variation in temperature of soil layers. Temperature measured at 1.00 p.m., ◦ C
Position of temperature sensor
Depth wise (from ground surface) variation of soil temperature
T0 T1 T2 T3 T4 T5 T6
at 3.7 m depth at 3.04 m depth at 2.43 m depth at 1.82 m depth at 1.21 m depth at 0.60 m depth at ground surface
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3-February
15-February
27.4 25.2 24.3 23.5 23.2 22.7 22.1
27.4 25.3 24.5 23.4 23.2 22.9 22.6
27.4 25.3 24.5 24.3 24.1 23.4 22.7
27.4 25.5 24.7 24.6 24.4 24.1 23.7
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Fig. 6. Thermal performance of Mode-II of operation in January and February, 2013.
case) and Mode-II (stand alone EATHE). Figs. 4–7 show that on 26, 27 and 28 January, the ambient air temperature ranges between 17.1 and 23 ◦ C. Temperature of air inside the room varied between 14.9–17.8 ◦ C, 14.9–19.1 ◦ C and 15.0–20.8 ◦ C during the operation of Mode-I, Mode-II and Mode-III respectively. Mode-II observed slightly higher temperature (0.9 ◦ C to 1.5 ◦ C) of air inside the room compared to the base case (Mode-I). The temperature of air at the outlet of EATHE varied from 24.4 ◦ C to 24.8 ◦ C for Mode-II and
Mode-III. However, the use of solar air heating duct in Mode-III resulted into maximum temperature of 32.2 ◦ C, up to which the air coming out of EATHE pipe could further be heated. Figs. 4–7 show that the air temperature at the outlet of EATHE was almost constant (24.4–24.8 ◦ C) irrespective of time and mode of operation. Relative humidity of air inside the room varied between 43.2–48.4%, 33.2–47.8% and 23.9–42.8% during all three modes respectively. Figs. 4–7 also reveal that during the operation of
Table 2 Temperature of air along the EATHE pipe at various distances from inlet. Temperature measured at 10.0 a.m., ◦ C
Position of temperature sensor
Temperature of air along the EATHE pipe at various distances from inlet
T7 at 0.2 m T8 at 1.7 m T9 at 4.7 m T10 at 9.3 m T11 at 15.1 m T12 at 24.2 m T13 at 34.0 m T14 at 44.4 m T15 at 60.0 m
16-January
28-January
3-February
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17.2 17.9 19.8 21.7 22.6 23.4 23.9 24.4 24.9
20.1 20.6 21.5 22.6 23.1 23.8 24.1 24.4 24.8
17.9 18.8 20.3 22.1 23.3 23.7 23.9 24.3 24.9
20.3 20.6 21.4 22.9 23.2 23.8 24.2 24.5 24.9
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Room air temperature Ambient air temperature EATHE+SAHD outlet temperature
Fig. 7. Thermal performance of Mode-III of operation in January and February, 2013.
Mode-I, Mode-II and Mode-III on 1, 2 and 3 February respectively, Mode-I resulted into a variation in room air temperature between 17.9 and 21.9 ◦ C and relative humidity varied from 46.8% to 42.1%. Mode-II had slightly higher room air temperature (0.9 ◦ C to 1.5 ◦ C)
and relative humidity 3–14% lesser than the base case (Mode-I). During Mode-II, room air temperature and relative humidity varied between 18.0–22.8 ◦ C and 49.3–34.0%, respectively. Figs. 4–7 reveal that during Mode-III, the room air temperature was 1.2 ◦ C–3.7 ◦ C
Table 3 Thermal performance of different modes of operation on 14, 15 and 16 January, 2013. Time
9:00 a.m. 10:00 a.m. 11:00 a.m. 12:00 p.m. 1:00 p.m. 2:00 p.m. 3:00 p.m. 4:00 p.m. 5:00 p.m.
Ambient air temperature (◦ C)
Room air temperature (◦ C)
Temperature of heated air entering into room (◦ C)
Mode-I (14 January)
Mode-II (15 January)
Mode-III (16 January)
Mode-I (14 January)
Mode-II (15 January)
Mode-III (16 January)
Mode-II (15 January)
Mode-III (16 January)
14.6 16.3 19.1 20 21 21.7 22.3 21.7 20.8
16 17.6 19.5 21.1 22.2 22.8 23.4 22.7 21.5
16.2 17 18.9 20.3 21.7 22.2 22.4 22 21.3
17.7 17.8 18.2 18.3 19.7 20.3 20.2 20.7 21.1
17.5 18.9 19.3 19.6 20.8 21.3 21.4 21.9 22.3
17.6 18.9 19.7 20.3 22.4 23.2 23.5 24 24.1
24.4 24.4 24.5 24.5 24.5 24.6 24.7 24.8 24.8
24.4 24.4 32.7 32.6 32.7 31.9 31 30 24.3
368
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Table 4 Heating potential calculations of EATHE and solar air heating duct. Experimental date (2013)
15-January (Mode-II) 16-January (Mode-III) 27-January (Mode-II) 28-January (Mode-III) 02-February (Mode-II) 03-February (Mode-III) 14-February (Mode-II) 15-February (Mode-III)
Average solar radiation of day (W/m2 )
381.62 378.16 422.45 430.40 451.23 453.52 481.33 487.32
Heating capacity of EATHE (kWh)
Heating capacity of solar air heating duct (kWh)
Total heating capacity of system (kWh)
a
b
c=a+b
665.529 758.395 526.232 553.318 555.252 605.554 396.609 330.830
– 1217.625 – 1260.351 – 1273.189 – 1280.753
665.5295 1976.02 526.232 1813.669 555.252 1878.743 396.609 1611.583
higher and the relative humidity was 3–17% lesser than the base case (Mode-I). Room air temperature and its relative humidity lied between 18.2–24.3 ◦ C and 47.6–23.3% respectively, during Mode-III. Figs. 4–7 show that on 13, 14 and 15 February, the ambient air temperature lies between 18.9–25.7 ◦ C. The temperature of air in the room ranges between 18.7–22.2 ◦ C, 18.8–23.5 ◦ C and 19–25.3 ◦ C during Mode-I, Mode-II and Mode-III, respectively. Mode-II observed slightly higher temperature (0.8–1.6 ◦ C) temperature of air inside the room compared to Mode-I (base case). During the operation of Mode-III, solar air heating duct raised the temperature of air coming out of EATHE outlet by 6.8–8.8 ◦ C. Relative humidity of room air varied between 45.9–54.7%, 40.2–53.5% and 41.2–59.5% during all modes respectively. During the operation of Mode-III, It is worth mentioning here that during the early hours of the day, such as at 9 a.m., when the duct surface temperature was lower than the air temperature at the outlet of EATHE, solar air heating duct was kept non functional and it was brought into function only when the duct surface temperature fell above the EATHE outlet temperature. The heating capacity (kWh) of the EATHE and solar air heating duct has been calculated by following equation.
˙ a T Qh = mC
(1)
˙ is the mass flow rate of air in kg/s, Ca is the specific heat of where m air in kJ/kg K and T is the difference temperature between ambient air temperature (inlet of EATHE) and outlet temperature of EATHE in case of Mode-II, while it is the temperature difference between duct inlet temperature and outlet temperature (in room) in case of Mode-III [15]. The coefficient of performance (COP) has been calculated as the heating capacity of EATHE (in case Mode II) or overall heating capacity of the system (in case of Mode III) divided by the blower energy consumption of the system as shown in Table 4. It is observed from the Table 4 that in case of Mode-II, the total heating capacity and COP of the system on 15 January was calculated as 665.52 kWh and 1.54 respectively. In case of Mode-III on 16 January, when solar air heating duct was coupled, the heating capacity for EATHE (758.395 kWh) was increased to 1976.02 kWh, thus increasing the total COP of the system to 4.57. A similar trend was observed on 27 January (Mode II) and 28 January (Mode III) where COP increased from 1.218 to 4.19 respectively. During the month of February, the COP of the system was further improved on 3 February (Mode III) to 4.34. Table 4 also emphasizes that the heating capacity of EATHE for 15 February during mode III, was merely 330.83 kWh, which was improved by solar air heating duct resulting in COP of 3.73. This proves that the solar air heating duct increased the heating capacity as well as the COP of the system for equal power consumption.
COP of system
1.540 4.570 1.218 4.190 1.285 4.340 0.918 3.730
5. Error analysis As per the data given in earlier sections, the minimum recorded values of temperature, flow velocity and relative humidity are 14.3 ◦ C, 5 m/s and 10.4% respectively. Least count of measuring instruments for temperature, flow velocity and relative humidity are 0.1 ◦ C, 0.1 m/s and 0.1%, respectively. Based on the analysis of errors in the experimental measurements through various instruments employed [21], the maximum error in measurement is equal to the ratio of least count of the measuring instrument and minimum recorded value of the parameter. Therefore, uncertainties in the measurement of temperature, flow velocity and relative humidity are estimated as ±1.20%, ±2.0% and ±0.96%. This error investigation explains the instrumental error during the experimentation. Since, this research work compares the performance of the various modes of operation of EATHE system on the basis of actual experimentally observed values of temperature and relative humidity parameters; therefore, error analysis given above consists of only the maximum error in measurement part only. 6. Conclusion An experimental analysis has been carried out to evaluate thermal performance of EATHE coupled with solar air heating duct meant for heating the air during winter season for an arid climate of Ajmer city, located in northwestern India. The performance of three different arrangements was compared. Experimental results showed that the heating capacity of the standalone EATHE system (Mode-II) improves when it is assisted by solar air heating duct (Mode-III, in which complete air from the EATHE is further heated by solar air heating duct and then supplied in the room). It was found that the heating capacity of EATHE system got increased by 1217.625–1280.753 kWh when it was coupled with solar air heating duct with a substantial increase in room temperature by 1.1–3.5 ◦ C higher than the base case (Mode-I). As observed in third mode of operation, proper coupling of EATHE with solar air heating duct may substantially increase the heating capacity of EATHE system. Experimental results of the study confirmed that the EATHE system coupled with solar air heating duct is quite effective for air heating during winter as it increases the COP of the system up to 4.57 with solar air heating duct for the same power consumption. It is also concluded that more than 82–85% of total increase in temperature of air along the EATHE pipe is obtained at a length of 34 m from inlet. Thus the cost of the system can be optimized by reducing the tunnel length to 34 m. Acknowledgements The authors would like to thank Late Dr. B.K. Maheshwari for his tremendous guidance and support for this work. Authors also thank Mr. Nikhil Gakkhar, Mr. Nilesh Purohit and Mr. Simarpreet singh for their valuable suggestions and help in this research.
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Contents lists available at ScienceDirect
Energy and Buildings journal homepage: www.elsevier.com/locate/enbuild
Thermal performance investigation of earth air tunnel heat exchanger coupled with a solar air heating duct for northwestern India Sanjeev Jakhar a,∗ , Rohit Misra b , Vikas Bansal c , M.S. Soni d a
Center for Renewable Energy and Environment Development, BITS – Pilani, Rajasthan, India Mechanical Engineering Department, Government Engineering College, Ajmer, India Mechanical Engineering Department, University College of Engineering, Kota, India d Department of Mechanical Engineering, BITS – Pilani, Rajasthan, India b c
a r t i c l e
i n f o
Article history: Received 26 September 2014 Received in revised form 24 November 2014 Accepted 26 November 2014 Available online 3 December 2014 Keywords: Earth air tunnel heat exchanger Solar air heating duct Heating capacity Thermal performance
a b s t r a c t In the present research thermal performance of earth air tunnel heat exchanger (EATHE) coupled with a solar air heating duct has been experimentally evaluated for arid climate of Ajmer city of northwestern India, during winter season. An attempt has been made to enhance the heating capacity of EATHE system by coupling it with a solar air heating duct at the exit end. Results show that the air which comes out of coupled EATHE system is relatively hotter than the air supplied by the stand alone EATHE system. It was found that the heating capacity of EATHE system got increased by 1217.625–1280.753 kWh when it was coupled with solar air heating duct with a substantial increase in room temperature by 1.1–3.5 ◦ C. The COP of the system also increased up to 4.57 when assisted with solar air heating duct. Therefore, the heating capacity of EATHE can be significantly increased by coupling it with solar air heating duct. © 2014 Elsevier B.V. All rights reserved.
1. Introduction With the rapid growth in population and economic growth of countries in the tropical regions, it is becoming inevitable that passive and low energy strategies must be used as suitable alternatives for heating/cooling. The utilization of geothermal energy to reduce heating and cooling needs in buildings has received increasing attention during the last several years. The potential of earth–air heat exchanger has been established in moderate climates of Europe, however not much research has been carried out in hot climates because of the claim that the potential is low. The use of passive heating techniques in the winter is advisable, with the objective of reducing energy consumption with the climatisation of spaces. It can thus be an effective tool for attenuating the growth of energy consumption for air conditioning. To ensure indoor air quality, building needs adequate ventilation. Many commercial and industrial buildings need to have high
Abbreviations: EATHE, earth air tunnel heat exchanger; PVC, poly vinyl chloride; RPM, revolution per minute; DBT, dry bulb temperature of air (◦ C); COP, coefficient of performance. ∗ Corresponding author. Tel.: +91 1596 516455. E-mail addresses: [email protected], [email protected] (S. Jakhar). http://dx.doi.org/10.1016/j.enbuild.2014.11.070 0378-7788/© 2014 Elsevier B.V. All rights reserved.
ventilation rates. During winter, fresh cold air needs to be warm up before supplying to the buildings, thus consuming more energy. Preheating of external air before entering the building can be achieved by natural means, like circulation in buried pipes [1,2]. To reduce high grade energy consumption of conventional active heating systems, numerous alternative techniques have been explored. One such proposition is the earth air tunnel heat exchanger (EATHE) system. EATHE works in principal on geothermal energy with temperature variation. Soil temperature, at a depth of about 10 feet or more, stays fairly constant throughout the year and is approximately equal to the average annual ambient air temperature. The ground can, therefore, be used as a heat source for heating in winter. Cold outdoor air is sent into the earth air tunnel heat exchanger. When air flows in the earth air tunnel, heat is transferred from the earth to the air. As a result, the air temperature at the outlet of the earth air tunnel heat exchanger is much higher than that of the ambient. The outlet air from the earth air tunnel can be directly used for space heating if its temperature is high enough. Alternatively, the outlet air may be heated further by associating air conditioning machines and solar air heating duct. Both of the above uses of earth air tunnel heat exchanger can contribute to reduction in energy consumption. The main advantages of the system are its simplicity, high preheating potential, low operational and maintenance costs, saving of fossil fuels and related emissions. Pre-heated fresh air
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supports a heat recovery system and can reduce the space heating demand in winter. The potential of earth tube air heat exchanger for space heating is well accepted in colder countries. As reported by Bansal et al. [3] not much research has been carried out in hot climates (such as in western India) because of the belief that the cooling potential of EATHE system is low due to higher soil temperature in summer. Santamouris et al. [4] investigated the impact of different ground surface boundary conditions on the efficiency of earth-to-air heat exchanger consisting of single and multiple parallel pipes and concluded that ground surface covered with short grass gives better cooling performance than bared soil condition. Hwang et al. [5] studied the performance of an innovative evaporative condenser and compared it with that of a conventional air-cooled condenser of a split heat pump system. Their experimental results showed that the evaporative condenser has a higher capacity than the air-cooled condenser by 1.8–8.1%, a higher COP by 11.1–21.6%. Hajidavalloo [6] investigated the effect of using evaporative cooler in the window-air-conditioner by injecting water on the media pad installed in front of the condenser entrance and reported 16% reduction in power consumption and 55% improvement in total performance. Most of the research has been carried out through mathematical modeling or experimental investigations. As a space cooling technology utilizing natural energy, earth–air–pipe systems have got increasing interest for energy conservation [7–9]. Kumar et al. [10] evaluated the conservation potential of an earth–air–pipe system coupled with a building with no air conditioning. The cooling power for the earth pipe was evaluated as 19 kW for pipe length of 80 m with section area of 0.53 m2 and air flow velocity of 4.9 m/s. Ajmi et al. [11] studied the cooling capacity of earth–air heat exchangers for domestic buildings in a desert (hot and arid) climate. A reduction of 30% in seasonal cooling demand was reported by using the earth-to-air heat exchanger in July. Ramirez-Davila et al. [12] performed a numerical study on the thermal behavior of an earth to air heat exchanger (EATHE) for three cities in Mexico with varying climatic conditions. Results showed that use of EAHE was appropriate for extreme and moderate temperature regions where the thermal inertia effect in soil is higher. Santamouris et al. [13] developed a new integrated method to calculate the role of earth to air heat exchangers to reduce the cooling load of the buildings based on the principle of balance point temperature. Badescu [14] developed a ground heat exchanger model based on numerical transient bi-dimensional approach for passive house space heating. Sodha et al. [15] evaluated an earth–air tunnel system for cooling/heating to provide thermal comfort inside the whole building complex at one of the hospitals in India. Their results showed that an 80 m long tunnel with a cross sectional area 0.528 m2 had a 512 kWh cooling capacity and 269 kWh heating capacity. Exergo-economics analysis was carried out by Ozgener and Ozgener [16] to determine the optimal design of a closed loop earth to air heat exchanger for greenhouse heating. Hollmuller and Lachal [17] discussed the potential and climate independent design guidelines of buried pipe systems. The design guidelines were validated using a system described by the authors. Poshtiri et al. [18] performed the feasibility study of the combined system of EAHE with a solar chimney. Similar work has been done by Maerefat and Haghighi [19]. They designed EAHE system coupled with solar chimney for both cooling and ventilation. Numerical modeling was performed for the system to optimize the diameter of pipe, which gives the minimum required number of solar chimney and EAHEs. The literature review reveals that there exists a gap to correlate the potential of solar air heating duct coupled with EAHTE system. According to Sodha et al. [15], the heating capacity of standalone EATHE is not adequate to provide necessary comfort conditions for Indian conditions. And hence to enhance the performance of
361
EATHE systems, a solar air heating duct is coupled in the present study. An experimental investigation has been carried out for different modes, operating for 8 h daily incorporating the winters during January and February. An attempt has also been made to explore the validity and effectiveness of the employed EATHE coupled with a solar air heating duct system for heating in cold conditions. The heating capacity and coefficient of performance (COP) of the system were found to be in agreement for the enhancement of results.
2. Description of test facility and test unit of EATHE coupled with a solar air heating duct system The test location of the system was in the city of Ajmer, located in northwestern India, which has hot semi-arid climate having the absolute maximum and mean maximum ambient air temperatures during summer period (April to June) as 47 ◦ C and 39 ◦ C respectively. The average ambient temperature during summer ranges from 30 to 32 ◦ C. During winters (November to February), the weather remains mild and average ambient temperature ranges between 15 and 18 ◦ C. During winter, the absolute minimum and mean minimum temperatures of ambient air are close to 4 ◦ C and 9 ◦ C respectively. The annual average ambient temperature of the location is 26.7 ◦ C. The actual setup of EATHE coupled with a solar air heating duct connected to test room is shown in Fig. 1. Experimental test set up comprises of 60 m long horizontal PVC pipe of 0.10 m diameter, buried in flat land with dry soil at a depth of 3.7 m. Inlet end of EATHE pipe is connected through a vertical pipe to a 0.75 kW, single phase, variable speed motorized blower (maximum flow rate of 0.0945 m3 /s and maximum speed of 2800 rpm). A U-shaped duct (12.2 m long having 0.0645 m2 cross-sectional area) made of galvanized iron has been used as a solar air heating duct having top and lateral wall exposed to solar energy so as to have the receiving surface area of 3 m2 and 2.6 m2 respectively. Inlet of duct was connected to the outlet pipe of EATHE by means of T-socket and outlet of duct was also connected at the exit of EATHE pipe at suitable position as shown in Fig. 2. The exterior surface of the entire duct was painted black so as to absorb most of the solar radiation falling on it. Dampers were provided at the inlet and exit of solar air heating duct as well as at the exit of EATHE pipe, to regulate the flow of air. Heated air coming out of EATHE pipe can be made to flow through solar air heating duct (Mode-III) for further heating and provision was kept to supply the conditioned air directly from EATHE to test room (Mode-II) by controlling the position of dampers and valves as shown in Fig. 2. Ambient air was forced through the earth air pipe system using a centrifugal blower and air flow velocity was changed by an auto transformer (single phase, 0–270 V, 2 A maximum current, with a least count of 1 V). When the blower runs at 230 V, it corresponds to a maximum flow rate of 0.0945 m3 /s and a flow velocity of 12 m/s inside the EATHE pipe (diameter 0.1 m). However, at a certain reduced voltage, the blower supplies the air at a flow velocity of 5 m/s through EATHE pipe. The measured energy consumption of blower at this mean air velocity was 0.12 kW. The flow velocity through solar air heating duct would be different from that through EATHE pipe, but flow rate would be same through EATHE pipe as well as solar air heating duct. Some of the energy supplied to blower gets converted into heat due its inefficiency and transferred to the air passing through EATHE, which also adds to the heat imparted to the air by EATHE. Seven resistance temperature detector (RTD) (Pt-100) temperature sensors viz. T0 –T6 were mounted at a depth of 3.7 m, 3.04 m, 2.43 m, 1.82 m, 1.21 m, 0.60 m, and 0 m, respectively, from the ground surface on inlet vertical pipe to measure soil temperatures at different depths. Nine RTD (Pt-100) temperature sensors viz. T7 –T15 were also inserted at the center of EATHE pipe along the length at a horizontal
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Fig. 1. Experimental set-up of EATHE coupled with solar air heating duct.
distance of 0.2 m, 1.7 m, 4.7 m, 9.3 m, 15.1 m, 24.2 m, 34.0 m, 44.4 m, and 60.0 m respectively, from the upstream end to measure air temperature. Two RTD (Pt-100) temperature sensors were mounted at inlet and exit of U-shaped solar air heating duct to measure the temperature of air at inlet and outlet of duct, respectively. Properly calibrated, digital temperature display devices (accuracy of ±0.1 ◦ C and resolution 0.1 ◦ C) had been used. Dry bulb temperature, relative humidity of ambient air and solar radiation were recorded hourly using RTD (Pt-100) temperature sensor, capacitive transducer and pyranometer mounted on weather station. Temperature and relative humidity of air inside the test room and at the outlet of EATHE were also measured accurately with the help of calibrated thermo hygrometer (make-Fluke-971, temperature accuracy of ±0.1 ◦ C, temperature resolution of 0.1 ◦ C and relative humidity resolution of 0.1%). Air flow velocity was measured with the help of a vane probe type anemometer (makeLutron, model-AM-4201, range – 0.4–30.0 m/s and least count of 0.1 m/s). Electrical energy consumed by the entire system (centrifugal blower) was measured with the help of calibrated digital energy meter (make-Power tech measurement system, type-PTS-01, least count of 0.1 kWh and an accuracy of ±0.1 kWh). Dimensions of test room were 4.3 m × 3.8 m × 3.05 m. Room was having two windows (1.52 m × 1.22 m each, located on east and north facing walls respectively) and a door (1.82 m × 0.91 m, located on west
facing wall). Thermocol insulation was placed on the glass panes of windows to reduce the heat exchange with the surroundings and interstices around the door were also taken care of by providing packing and minimizing the leakage of conditioned air from the room.
3. Modes of EATHE coupled with a solar air heating duct and test procedure Three different arrangements of EATHE and solar air heating duct have been investigated experimentally: (1) Mode-I: Only thermal conditions of room and ambient were monitored. EATHE and solar air heating duct were not functional. This mode has been treated as base case and thermal performance of other two modes of operation is compared with this mode. (2) Mode-II: Heated air (conditioned air) from EATHE was directly supplied to the room and solar air heating duct was not functional. (3) Mode-III: Heated air (conditioned air) from EATHE was allowed to pass through solar air heating duct for further heating and then supplied to the room.
Fig. 2. Schematic diagram of the experimental setup.
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25
500 15
400 300
10
200 5
100
25
Solar radiation (w/m2)
Solar radiation (w/m2)
20
Ambient Air Temperature (°C)
600
800 700
20
600 500
15
400 10
300 200
5
100 0
0 10
11
12
13
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15
16
0
17
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Time of day (Hr)
Mode-II,15 Jan. 2013 Mode-I,14 Jan. 2013 Mode-III,16-Jan. 2013
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5
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Mode-II,27 Jan. 2013 Mode-I,26 Jan. 2013 Mode-III,28 Jan. 2013
800
30
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Solar radiation
30
Mode-I,26 Jan. 2013 Mode-III,28 Jan. 2013 Mode-II,27 Jan. 2013
(w/m2)
700
Ambient Air Temperature (°C)
Solar radiation (w/m2)
Mode-I,14 Jan. 2013 Mode-III,16-Jan. 2013 Mode-II,15 Jan. 2013
0 9
25
600 20
500 400
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100 0
0 9
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Time of day (Hr) Mode-I,1 Feb.2013 Mode-III,3 Feb. 2013 Mode-II,2 Feb. 2013
Mode-II,2 Feb. 2013 Mode-I,1 Feb.2013 Mode-III,3 Feb. 2013
0
Ambient Air Temperature (°C)
9
Ambient Air Temperature (°C)
700
363
0 9
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Time of day (Hr) Mode-I, 13 Feb. 2013 Mode-III, 15 Feb. 2013 Mode-II, 14 Feb. 2013
Mode-II, 14 Feb. 2013 Mode-I, 13 Feb. 2013 Mode-III, 15 Feb. 2013
Fig. 3. Intensity of solar radiation and ambient air temperature of January and February, 2013 for different modes.
Experiments were conducted from 6th January to 27th February, 2013 for 8 h duration daily from 9 a.m. to 5 p.m. System remained closed from 6 p.m. to 9 a.m. everyday allowing the soil to get regenerated. As the cold air passes through the underground pipe, it gets warmer thus causing the soil around the pipe to cool down. This changes the thermal conductivity of soil and causes the changes in heat transfer [20]. Hence, in this study integrated EATHE was operated during the day and turned off during the night hours allowing the soil around the pipe to get regenerated. Flow velocity of air through the EATHE was maintained at 5 m/s. Measurements and recording of hourly data included the intensity of solar radiation, ambient air temperature, relative humidity, temperature and relative humidity of air at the outlet of EATHE, temperature of air in the buried pipe at nine different locations, depth-wise temperature of soil at seven points, temperature and relative humidity of air inside the conditioned room at five different locations and electrical energy consumed by air blower.
4. Performance and results of EATHE coupled with a solar air heating duct in different modes Performance of EATHE coupled with a solar air heating duct system in three different modes has been evaluated on four periods of three consecutive winter days of 2013 (14–16 January, 26–28 January, 1–3 February and 13–15 February). It is important to mention here that the experimentation continued for 2 months on all the three modes, but only those data sets have been considered in each month, for which a close agreement between the ambient air temperature, relative humidity of ambient air and solar radiation recorded on different days is observed, as shown in Figs. 3–7. It is also worth mentioning here that for first data set which comprises of the data measured on three consecutive days i.e. 14, 15 and 16 January, ambient air temperature, relative humidity of ambient air and solar radiation measured on hourly basis on 14 January were taken to be the base data and ambient air temperature, relative humidity of ambient air and solar radiation measured on 15 and
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Room air temperature (°C)
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Mode-II, 27 Jan. 2013
Mode-III, 28 Jan. 2013
Mode-III, 16 Jan. 2013 40
40
35
35
Room air temperature (°C)
Room air temperature (°C)
12
Time of day (Hr)
30 25 20 15
30 25 20 15
10
10 9
10
11
12
13
14
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16
17
Time of day (Hr) Mode-I, 1 Feb. 2013
Mode-II, 2 Feb. 2013
Mode-III, 3 Feb. 2013
9
10
11
12
13
14
15
16
17
Time of day (Hr) Mode-I, 13 Feb. 2013
Mode-II, 14 Feb. 2013
Mode-III, 15 Feb. 2013
Fig. 4. Variation of room air temperature for three different modes in January and February, 2013.
16 January were compared with the data measured on 14 January. Maximum difference between ambient air temperature, relative humidity of ambient air and solar radiation measured on 15 and 16 January and that measured on 14 January was ±1.6 ◦ C, ±3% and ±3%, respectively, therefore, the ambient conditions for all three modes were almost identical at any hour of operation. It ensures the identical outdoor conditions and thermal loading of the test room in order to evaluate the relative performance of each mode. Similar procedure was adopted to fix a base data for the rest of the three data sets by considering the weather data measured on 26 January, 1 February and 13 February as the base data. Experimental data recorded for temperature sensor T0 (which measures the temperature of soil at a depth of 3.7 m from ground surface) clearly reveals that the temperature of soil at this depth is not at all affected by the diurnal variation in ambient temperature and solar radiation, as shown in Table 1. It also reveals that the variation of temperature along the depth of tunnel follows the same trend for different modes. Results showed that significant temperature of 27.4 ◦ C was obtained at 3.7 m depth, with almost a
difference of 11 ◦ C between the temperature of the soil at that depth and ground surface. Usually severe winter is observed at Ajmer in the month of January. As mentioned earlier, the EATHE was put into use during day hours from 9.00 a.m. to 5.00 p.m., from January to February. Table 2 reveals the temperature measured along the length of pipe from the inlet of EATHE. It is observed that more than 82–85% of the total increase in the temperature of air along the EATHE pipe is obtained at a length of 34 m from inlet. The temperature growth in rest of the pipe is infinitesimally slow as it has approximately reached its steady condition by this length. It is also observed from Table 2 that at 60 m, the outlet temperature of EATHE was nearly same. Table 3 and Figs. 4–7 show that on 14, 15 and 16 January, the ambient air temperature ranges between 14.6 ◦ C and 23.4 ◦ C. Table 3 reveals that the ambient air temperature of 14, 15 and 16 January, was nearly same and had the maximum deviation of 1.6 ◦ C. It was also observed that the temperature of air in the room ranges between 17.7–21.1 ◦ C, 17.5–22.3 ◦ C and 17.6–24.1 ◦ C during Mode-I, Mode-II and Mode-III respectively. Therefore, the use of EATHE along with solar air heating duct resulted in 1.1–3.5 ◦ C
S. Jakhar et al. / Energy and Buildings 87 (2015) 360–369
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40
60
60
14 Jan., 2013
26 Jan., 2013 35
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25
30
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15 10 10
11
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13
14
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16
50
Temperature (°C)
30
Relative humidity (%)
Temperature (°C)
50
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0
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17
0 9
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11
Time of day (Hr)
60 50
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15 10 14
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17
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16
60
13 Feb., 2013
50 Relative humidity (%)
40
13
15
Temperature (° C)
30
12
14
35 Relave humidity (%)
Temperature (°C)
35
11
13
40
1 Feb., 2013
10
12
Time of day (Hr)
40
9
Relative humidity (%)
35
9
365
30
40
25
30
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20
10
15
10
0
10
17
0 9
Time of day (Hr)
10
11
12
13
14
15
16
17
Time of day (Hr)
Room air temperature
Relative humidity of Air inside the room
Ambient air temperature
Relative humidity of Ambient air
Fig. 5. Thermal performance of Mode-I of operation in January and February, 2013.
higher temperature of air maintained inside the room as compared to Mode-I (base case). Table 3 reveals that the air temperature at the outlet of EATHE system varied very little between 24.4 and 24.8 ◦ C, during Mode-II as well as Mode-III and hence, the air temperature at the outlet of EATHE showed a lower variation (0.4 ◦ C) than the variation of 8.8 ◦ C observed for ambient air. It was also observed from Table 3 and Figs. 4–7 that relative humidity of ambient air had a variation of 83.2% (28.0–51.3%) on 14, 15 and 16 January, during the operation of different modes. Relative humidity of air inside the room varied between 43.9–46.14%, 36.12–51.8% and 32.5–53.5%
during Mode-I, Mode-II and Mode-III respectively. Table 3 reveals that the coupling of solar air heating duct with EATHE system significantly increases the temperature of air. It is observed that on 16 January, during the operation of Mode-III, solar air heating duct raised the temperature of air coming out of EATHE pipe by 6–9 ◦ C. Therefore, the heating potential of EATHE system, when coupled with solar air heating duct improves significantly. Similar results were obtained on 26, 27 and 28 January, 2013 as shown in Figs. 4–7. In Mode III significantly higher room air temperature could be maintained as compared to the Mode-I (base
Table 1 Depth wise variation in temperature of soil layers. Temperature measured at 1.00 p.m., ◦ C
Position of temperature sensor
Depth wise (from ground surface) variation of soil temperature
T0 T1 T2 T3 T4 T5 T6
at 3.7 m depth at 3.04 m depth at 2.43 m depth at 1.82 m depth at 1.21 m depth at 0.60 m depth at ground surface
16-January
28-January
3-February
15-February
27.4 25.2 24.3 23.5 23.2 22.7 22.1
27.4 25.3 24.5 23.4 23.2 22.9 22.6
27.4 25.3 24.5 24.3 24.1 23.4 22.7
27.4 25.5 24.7 24.6 24.4 24.1 23.7
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50
35
50
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30
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20
15
10 0
10 10
11
12
13
14
15
16
9
Time of day (Hr) 60
Temperature ( °C)
Relative humidity (%)
50
Temperature ( °C)
35
12
13
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16
17
50
30
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30
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20 10
40
25
30
20
20
15
10
15
0
10
10 15
16
60
14 Feb., 2013
35
30
11 12 13 14 Time of day (Hr)
11
40
2 Feb., 2013
10
10
Time of day (Hr)
40
9
0
10
17
17
Relative humidity (%)
9
60
27 Jan., 2013
Relative humidity (%)
Temperature ( °C)
35
60
Temperature (°C)
15 Jan., 2013
Relative humidity (%)
40
0 9
10
11
12
13
14
15
16
17
Time of day (Hr)
Room air temperature
Relative humidity of room air
Ambient air temperature
Relative humidity of ambient air
EATHE outlet temperature
Relative humidity of air at EATHE outlet
Fig. 6. Thermal performance of Mode-II of operation in January and February, 2013.
case) and Mode-II (stand alone EATHE). Figs. 4–7 show that on 26, 27 and 28 January, the ambient air temperature ranges between 17.1 and 23 ◦ C. Temperature of air inside the room varied between 14.9–17.8 ◦ C, 14.9–19.1 ◦ C and 15.0–20.8 ◦ C during the operation of Mode-I, Mode-II and Mode-III respectively. Mode-II observed slightly higher temperature (0.9 ◦ C to 1.5 ◦ C) of air inside the room compared to the base case (Mode-I). The temperature of air at the outlet of EATHE varied from 24.4 ◦ C to 24.8 ◦ C for Mode-II and
Mode-III. However, the use of solar air heating duct in Mode-III resulted into maximum temperature of 32.2 ◦ C, up to which the air coming out of EATHE pipe could further be heated. Figs. 4–7 show that the air temperature at the outlet of EATHE was almost constant (24.4–24.8 ◦ C) irrespective of time and mode of operation. Relative humidity of air inside the room varied between 43.2–48.4%, 33.2–47.8% and 23.9–42.8% during all three modes respectively. Figs. 4–7 also reveal that during the operation of
Table 2 Temperature of air along the EATHE pipe at various distances from inlet. Temperature measured at 10.0 a.m., ◦ C
Position of temperature sensor
Temperature of air along the EATHE pipe at various distances from inlet
T7 at 0.2 m T8 at 1.7 m T9 at 4.7 m T10 at 9.3 m T11 at 15.1 m T12 at 24.2 m T13 at 34.0 m T14 at 44.4 m T15 at 60.0 m
16-January
28-January
3-February
15-February
17.2 17.9 19.8 21.7 22.6 23.4 23.9 24.4 24.9
20.1 20.6 21.5 22.6 23.1 23.8 24.1 24.4 24.8
17.9 18.8 20.3 22.1 23.3 23.7 23.9 24.3 24.9
20.3 20.6 21.4 22.9 23.2 23.8 24.2 24.5 24.9
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28 Jan., 2013 50
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20
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10
0
10
50
30
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25
30
20
20
15
10 10
11
12
13
14
15
16
0 9
17
10
11
12
13
60
40
50
60
40
25
30
20
20
15 10 13
14
15
16
50
35 Relative humidity (%) Temperature ( °C)
Temperature ( °C)
30
12
17
15 Feb., 2013
35
11
16
40
3 Feb., 2013
10
15
Time of day (Hr)
Time of day (Hr)
9
14
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20
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15
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10
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Relative humidity (%)
9
Temperature ( °C)
35
35
Relative humidity (%)
Temperature ( °C)
16 Jan., 2013
Relative humidity (%)
40
367
0 9
10
Time of day (Hr)
11
12
13
14
15
16
17
Time of day (Hr) Relative humidity of room air Relative humidity of ambient air Relative humidity of air at EATHE outlet
Room air temperature Ambient air temperature EATHE+SAHD outlet temperature
Fig. 7. Thermal performance of Mode-III of operation in January and February, 2013.
Mode-I, Mode-II and Mode-III on 1, 2 and 3 February respectively, Mode-I resulted into a variation in room air temperature between 17.9 and 21.9 ◦ C and relative humidity varied from 46.8% to 42.1%. Mode-II had slightly higher room air temperature (0.9 ◦ C to 1.5 ◦ C)
and relative humidity 3–14% lesser than the base case (Mode-I). During Mode-II, room air temperature and relative humidity varied between 18.0–22.8 ◦ C and 49.3–34.0%, respectively. Figs. 4–7 reveal that during Mode-III, the room air temperature was 1.2 ◦ C–3.7 ◦ C
Table 3 Thermal performance of different modes of operation on 14, 15 and 16 January, 2013. Time
9:00 a.m. 10:00 a.m. 11:00 a.m. 12:00 p.m. 1:00 p.m. 2:00 p.m. 3:00 p.m. 4:00 p.m. 5:00 p.m.
Ambient air temperature (◦ C)
Room air temperature (◦ C)
Temperature of heated air entering into room (◦ C)
Mode-I (14 January)
Mode-II (15 January)
Mode-III (16 January)
Mode-I (14 January)
Mode-II (15 January)
Mode-III (16 January)
Mode-II (15 January)
Mode-III (16 January)
14.6 16.3 19.1 20 21 21.7 22.3 21.7 20.8
16 17.6 19.5 21.1 22.2 22.8 23.4 22.7 21.5
16.2 17 18.9 20.3 21.7 22.2 22.4 22 21.3
17.7 17.8 18.2 18.3 19.7 20.3 20.2 20.7 21.1
17.5 18.9 19.3 19.6 20.8 21.3 21.4 21.9 22.3
17.6 18.9 19.7 20.3 22.4 23.2 23.5 24 24.1
24.4 24.4 24.5 24.5 24.5 24.6 24.7 24.8 24.8
24.4 24.4 32.7 32.6 32.7 31.9 31 30 24.3
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Table 4 Heating potential calculations of EATHE and solar air heating duct. Experimental date (2013)
15-January (Mode-II) 16-January (Mode-III) 27-January (Mode-II) 28-January (Mode-III) 02-February (Mode-II) 03-February (Mode-III) 14-February (Mode-II) 15-February (Mode-III)
Average solar radiation of day (W/m2 )
381.62 378.16 422.45 430.40 451.23 453.52 481.33 487.32
Heating capacity of EATHE (kWh)
Heating capacity of solar air heating duct (kWh)
Total heating capacity of system (kWh)
a
b
c=a+b
665.529 758.395 526.232 553.318 555.252 605.554 396.609 330.830
– 1217.625 – 1260.351 – 1273.189 – 1280.753
665.5295 1976.02 526.232 1813.669 555.252 1878.743 396.609 1611.583
higher and the relative humidity was 3–17% lesser than the base case (Mode-I). Room air temperature and its relative humidity lied between 18.2–24.3 ◦ C and 47.6–23.3% respectively, during Mode-III. Figs. 4–7 show that on 13, 14 and 15 February, the ambient air temperature lies between 18.9–25.7 ◦ C. The temperature of air in the room ranges between 18.7–22.2 ◦ C, 18.8–23.5 ◦ C and 19–25.3 ◦ C during Mode-I, Mode-II and Mode-III, respectively. Mode-II observed slightly higher temperature (0.8–1.6 ◦ C) temperature of air inside the room compared to Mode-I (base case). During the operation of Mode-III, solar air heating duct raised the temperature of air coming out of EATHE outlet by 6.8–8.8 ◦ C. Relative humidity of room air varied between 45.9–54.7%, 40.2–53.5% and 41.2–59.5% during all modes respectively. During the operation of Mode-III, It is worth mentioning here that during the early hours of the day, such as at 9 a.m., when the duct surface temperature was lower than the air temperature at the outlet of EATHE, solar air heating duct was kept non functional and it was brought into function only when the duct surface temperature fell above the EATHE outlet temperature. The heating capacity (kWh) of the EATHE and solar air heating duct has been calculated by following equation.
˙ a T Qh = mC
(1)
˙ is the mass flow rate of air in kg/s, Ca is the specific heat of where m air in kJ/kg K and T is the difference temperature between ambient air temperature (inlet of EATHE) and outlet temperature of EATHE in case of Mode-II, while it is the temperature difference between duct inlet temperature and outlet temperature (in room) in case of Mode-III [15]. The coefficient of performance (COP) has been calculated as the heating capacity of EATHE (in case Mode II) or overall heating capacity of the system (in case of Mode III) divided by the blower energy consumption of the system as shown in Table 4. It is observed from the Table 4 that in case of Mode-II, the total heating capacity and COP of the system on 15 January was calculated as 665.52 kWh and 1.54 respectively. In case of Mode-III on 16 January, when solar air heating duct was coupled, the heating capacity for EATHE (758.395 kWh) was increased to 1976.02 kWh, thus increasing the total COP of the system to 4.57. A similar trend was observed on 27 January (Mode II) and 28 January (Mode III) where COP increased from 1.218 to 4.19 respectively. During the month of February, the COP of the system was further improved on 3 February (Mode III) to 4.34. Table 4 also emphasizes that the heating capacity of EATHE for 15 February during mode III, was merely 330.83 kWh, which was improved by solar air heating duct resulting in COP of 3.73. This proves that the solar air heating duct increased the heating capacity as well as the COP of the system for equal power consumption.
COP of system
1.540 4.570 1.218 4.190 1.285 4.340 0.918 3.730
5. Error analysis As per the data given in earlier sections, the minimum recorded values of temperature, flow velocity and relative humidity are 14.3 ◦ C, 5 m/s and 10.4% respectively. Least count of measuring instruments for temperature, flow velocity and relative humidity are 0.1 ◦ C, 0.1 m/s and 0.1%, respectively. Based on the analysis of errors in the experimental measurements through various instruments employed [21], the maximum error in measurement is equal to the ratio of least count of the measuring instrument and minimum recorded value of the parameter. Therefore, uncertainties in the measurement of temperature, flow velocity and relative humidity are estimated as ±1.20%, ±2.0% and ±0.96%. This error investigation explains the instrumental error during the experimentation. Since, this research work compares the performance of the various modes of operation of EATHE system on the basis of actual experimentally observed values of temperature and relative humidity parameters; therefore, error analysis given above consists of only the maximum error in measurement part only. 6. Conclusion An experimental analysis has been carried out to evaluate thermal performance of EATHE coupled with solar air heating duct meant for heating the air during winter season for an arid climate of Ajmer city, located in northwestern India. The performance of three different arrangements was compared. Experimental results showed that the heating capacity of the standalone EATHE system (Mode-II) improves when it is assisted by solar air heating duct (Mode-III, in which complete air from the EATHE is further heated by solar air heating duct and then supplied in the room). It was found that the heating capacity of EATHE system got increased by 1217.625–1280.753 kWh when it was coupled with solar air heating duct with a substantial increase in room temperature by 1.1–3.5 ◦ C higher than the base case (Mode-I). As observed in third mode of operation, proper coupling of EATHE with solar air heating duct may substantially increase the heating capacity of EATHE system. Experimental results of the study confirmed that the EATHE system coupled with solar air heating duct is quite effective for air heating during winter as it increases the COP of the system up to 4.57 with solar air heating duct for the same power consumption. It is also concluded that more than 82–85% of total increase in temperature of air along the EATHE pipe is obtained at a length of 34 m from inlet. Thus the cost of the system can be optimized by reducing the tunnel length to 34 m. Acknowledgements The authors would like to thank Late Dr. B.K. Maheshwari for his tremendous guidance and support for this work. Authors also thank Mr. Nikhil Gakkhar, Mr. Nilesh Purohit and Mr. Simarpreet singh for their valuable suggestions and help in this research.
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