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Performance assessment of earth pipe cooling system for low energy buildings in a subtropical climate
Energy Conversion and Management 106 (2015) 815–825
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Energy Conversion and Management journal homepage: www.elsevier.com/locate/enconman
Performance assessment of earth pipe cooling system for low energy buildings in a subtropical climate S.F. Ahmed a,b,⇑, M.M.K. Khan a, M.T.O. Amanullah b, M.G. Rasul a, N.M.S. Hassan a a b
School of Engineering and Technology, Central Queensland University, Rockhampton Campus, Queensland 4702, Australia School of Engineering, Deakin University, Geelong Waurn Ponds Campus, Victoria 3220, Australia
a r t i c l e
i n f o
Article history: Received 9 July 2015 Accepted 10 October 2015
Keywords: Passive cooling Earth pipe cooling Energy savings Thermal comfort Subtropical climate
a b s t r a c t Energy consumption in heating and cooling around the world has been a major contributor to global warming. Hence, many studies have been aimed at finding new techniques to save and control energy through energy efficient measures. Most of this energy is used in residential, agricultural and commercial buildings. It is therefore important to adopt energy efficiency measures in these buildings through new technologies and novel building designs. These new building designs can be developed by employing various passive cooling systems. Earth pipe cooling is one of these which can assist to save energy without using any customary mechanical units. This paper investigates the earth pipe cooling performance in a hot humid subtropical climate of Rockhampton, Australia. A thermal model is developed using ANSYS Fluent for measuring its performance. Impacts of air velocity, air temperature, relative humidity and soil temperature on room cooling performance are also assessed. A temperature reduction of around 2 °C was found for the system. This temperature reduction contributed to an energy saving of a maximum of 866.54 kW (8.82%) per year for a 27.23 m3 room. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction Energy need and sustainable provision of energy has been a major concern for sustainable development worldwide. Energy consumption in different forms is steadily rising all over the world. The world energy use is projected to rise from 524 quadrillion Btu (552.85 exajoule) in 2010 to 630 quadrillion Btu (664.69 exajoule) in 2020 and 820 quadrillion Btu (865.15 exajoule) in 2040 [1]. This projection indicates that energy usage will increase by around 56% over these 30 years. During this period, more than 85% of this growing energy demand will occur within the developing nations outside the Organisation for Economic Cooperation and Development (non-OECD nations), where demand is driven by strong long-term economic growth and increasing populations. Income and population growth are the main reasons behind this significant energy demand. World population is estimated to increase to 8.3 billion from its current 7 billion by 2030, which indicates that more energy will be required for an additional 1.3 billion people [2]. Urban areas have expanded in size globally over recent decades [3] and around half of the world’s population ⇑ Corresponding author at: School of Engineering, Deakin University, Geelong Waurn Ponds Campus, Victoria 3220, Australia. E-mail addresses: [email protected], [email protected] (S.F. Ahmed). http://dx.doi.org/10.1016/j.enconman.2015.10.030 0196-8904/Ó 2015 Elsevier Ltd. All rights reserved.
(however, just 7.6% in the more developed nations) are now urban residents [4]. This has caused high energy demand. People around the world have become more energy hungry as the life style has changed significantly in recent times due to many technological advancement. About 40% of the global energy is consumed in building sector [5], where Australian buildings use 38% of the total energy consumption on cooling and heating purposes [6]. Australia is the eighteenth leading energy consumer in the world and fourteenth on a per person basis [7]. The energy demand in Australia is predicted to increase rapidly in future. Energy consumption for the Australian residential sector in 1990 was around 299 petajoules (PJ) and that by 2008 had increased to about 402 PJ and is projected to increase to 467 PJ by 2020 [8]. This signifies an increased energy consumption of 56% in this sector over this period from 1990 to 2020. According to this projection, more energy will be required for survival and achieving thermal comfort. There has been an ongoing interest in improving building energy efficiency because of ecological concerns and the high cost of energy in recent years [9]. Consumers are therefore looking for energy efficient and environmentally-friendly building designs. Energy efficient buildings can be developed by employing either passive or active energy efficient technologies. Improvements to building envelope components can be treated as passive strategies, whereas enhancements to heating, ventilating and air conditioning
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(HVAC) systems, lighting, etc., can be categorised under active strategies [9]. Passive air cooling is one of the passive strategies which can assist to reduce energy consumption in buildings for all subtropical zones. One significant outcome of incorporating passive cooling techniques is reducing air-conditioning loads or reducing heat gains in buildings to make them more sustainable. Several studies have been carried out on reducing air-conditioning loads. An experimental study was conducted using phase change material (PCM) energy storage techniques for enhancing room air quality and reducing the cooling load [10]. The experimental results showed a reduction of building energy consumption without a significant increase in the weight of the building materials with the incorporation of PCM. To reduce energy demands in buildings, different pre-cooling and ventilation methods and their overall impact on the energy use were studied by Becker and Paciuk [11]. The research involved simulations on varying building envelope characteristics for changing interior high heat loads. Their study indicated that the night pre-cooling is a productive cooling method for buildings with high interior heat loads. It reduces the inside mass temperature below the air temperature and subsequently diminishes the peak power cooling requirements. The pre-cooling system can save 115 kW/m2/month of total cooling energy demand [12]. The earth pipe cooling technique is one of the viable options to reduce the cooling loads. Thermal performance of this technology was evaluated in a subtropical climate in Queensland, Australia [13–15]. In those studies, a reduction of about 1–2 °C temperature was found for a single room. The earth pipe cooling performance was also investigated in another study to assess the influence of pipe length, pipe radius, buried pipe depth and air flow rate under different conditions [16]. The buried underground pipe depth and the pipe length turned out to influence the earth tube cooling rate, while air flow rate and pipe radius mainly affect the inlet air temperature entering the space. Many researchers found that the resulting outlet temperature at the buried pipe decreases with decreasing mass flow rate in the pipe, decreasing pipe radius, increasing pipe length and increasing depths up to 4 m [17]. To assess the impacts of these parameters on thermal performance, an implicit and transient model was developed for the earth–air– pipe systems in Southern China using PHOENICS [18]. The results demonstrated that a daily cooling capacity up to 74.6 kW h can be achieved in that region using the system. In some cases, the earth pipe cooling system is assisted by a heat pump as a heat exchanger located within the buried pipe. This is also known as an earth–pipe–air heat exchanger, which can be utilised for reducing the cooling load of buildings during summer. It can also be used for heating interior spaces in buildings in winter. These systems have been commonly implemented in Germany, Denmark, Austria and India since the 1990s, and are gradually being adopted in North America [19]. Their thermal performance was measured by Mihalakakou et al. [20] by developing a numerical model. In order to assess the cooling capacity of the ground heat exchanger system, a thermal prediction of the system is essential. Several numerical models presented in the literature [21–23] evaluate the thermal performance of the ground heat exchangers. Gallero et al. [21] developed a numerical model to simulate the thermal behaviour of a vertical borehole heat exchanger. A good agreement was observed for the borehole temperatures and modelled outlet fluid, with relative error values and root mean square smaller than 0.3% and 0.2 °C respectively. It was also found that the model is computationally fast and simple to be implemented into building thermal insulation programs. In the building the cooling capacity of the earth air pipe heat exchange was evaluated by developing a numerical model [22]. The model was developed
using the simulation program, FLUENT to investigate the thermal performance of different pipe materials and air velocities. The results demonstrated that the pipe materials have no noticeable impact on the earth air pipe heat exchanger whereas the air velocity has greater influence. The thermal performance was also investigated by a numerical model using FLUENT [23]. A theoretical model was developed for the earth–air heat exchanger for evaluating the cooling potential in a hot arid climate [24]. It was found that the earth–air heat exchanger has the potential to reduce 30% cooling energy demand in a typical house during peak summer. The cooling performance of horizontal earth pipe cooling system was assessed in an agriculture greenhouse in Thailand [25]. The study shows that the system has the potential to cool the greenhouse during daytime. There have been limited researches on earth pipe cooling techniques involving both numerical and experimental studies. Furthermore, no credible research on earth pipe cooling is seen to have been undertaken for a hot and humid climate in Australia. Therefore, the earth pipe cooling study is very timely and important for the Australian economy and environment. This study aims to assess the thermal performance of the earth pipe cooling technique in a hot humid climate in Australia by developing a thermal model. The model assists to predict the room temperatures using the pipe cooling systems. The reduction of room temperature and annual energy savings are calculated to assess the thermal performance of the system. In the following section, a comprehensive and valuable survey on the earth pipe cooling technique is presented for potential use and application in buildings to investigate all the key issues of this technique. 2. Earth pipe cooling technique Earth pipe cooling technology involves long buried pipes in which intake air comes through one end and passes through underground pipes, and finally the air cooled by the soil moves into the room through the outlet end. The pipe is laid underground at an optimum depth with the two pipe ends above the ground as shown in Fig. 1. A fan is needed to suck the fresh air from the pipe intake and to generate the air flow into the room. Earth pipe cooling technology offers several additional advantages such as protection from dust, noise, limited air infiltration, radiation and storms. It utilises the Earth’s near constant underground temperature for cooling air in industrial, residential and agricultural buildings. This fact has been studied and proven by several researchers [18,27–30]. The rationale behind this is
Fig. 1. Earth pipe cooling system [26].
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Fig. 2. Earth pipe cooling categories [35]. (a) Open loop. (b) Closed loop.
because the daily and regular temperature variation is significantly diminished in the ground below a certain depth where the soil temperature remains constant [31]. Low energy cooling techniques utilising earth became progressively popular in parts of America and Europe after the oil emergency in 1973 [31]. Typically, the soil temperature decreases in summer with increasing depth, which allows the utilisation of earth as a heat sink. It is seen from the literature that this temperature decrease continues up to a depth of 4 m underground as the soil temperature is fairly constant and stable at that depth [32]. Meanwhile, the soil temperature increases in winter with increasing depth, hence the use of earth as a heat source. Earth pipe cooling systems can be categorised in two ways: open loop and closed loop as shown in Fig. 2. In the open loop system, air comes through the buried pipes into the room and passes through a ventilation system. It provides ventilation while also cooling the interior of a house [33]. In a closed loop system, air is re-circulated from the buried underground pipes to the room [15]; this is more effective than the open loop system as it does not exchange air with the outside of the house. The latter pipe arrangement can also reduce the tunnel length since the conditioned air is re-circulated within the buried pipe [34]. The buried underground pipes may be installed in horizontal trenches or borings, vertical borings, slinky coil, or even in a surface water body. There are two main strategies for earth pipe cooling: direct and indirect earth contact [36]. Direct earth contact involves total or partial building envelope being placed in contact with the ground surface directly, and the indirect earth contact involves an earthto-air heat exchanger system through which air is circulated between the interior and exterior of the building and is subsequently brought into the building [37]. The direct earth-tobuilding contact ground cooling is a low maintenance passive cooling strategy with minimal heat gains and solar exposure. Despite the advantages of the direct-to-building contact ground cooling, it also creates environmental problems such as indoor condensation and poor indoor air quality [36]. Moreover, in some places such as desert and semi-desert countries, large excavation is not suitable due to geological conditions. Because of these disadvantages of the direct earth pipe cooling technique, the indirect earth pipe cooling technique has been considered in this study for measuring its performance.
earth pipe cooling (VEPC) system and the other to a horizontal earth pipe cooling (HEPC) system as shown in Fig. 3. Both the vertical and horizontal piping systems consist of two simple Polyvinyl Chloride (PVC) pipes of outside diameter 0.125 m, which are also known as manifolds. Intake air comes through one of the manifolds and passes through a series of buried pipes and moves into the room through another manifold. The manifolds were connected with 20 PVC pipes each of diameter 21 mm and thickness 1 mm. The 20 corrugated buried PVC pipes were aligned horizontally into one row in the HEPC system as shown in Fig. 4 and aligned vertically into 5 rows in the VEPC system as shown in Fig. 5. Each PVC pipe in each row was separated from its neighbour by 20 mm for both the piping systems. Excavations of dimensions 8.1 m 2 m were made for both the piping systems to fit the PVC pipes underground. The bottom of the excavations maintained a drop of 0.2 m down the length of the excavation to cause moisture to fall towards the sand and
Fig. 3. Experimental shed of earth pipe cooling system.
3. Experimental design and measurement The earth pipe cooling experiment was conducted in the sustainable precinct at Central Queensland University, Rockhampton, Australia. Two shipping containers, each with a dimension 5.63 m 2.14 m 2.26 m, were installed and fitted to investigate the cooling capacity and its effect. One was connected to a vertical
Fig. 4. Diagram for horizontal earth pipe cooling system.
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Pressure was discretised with a PRESTO scheme in view of its strong convergence ability. The scheme is available for all meshes such as tetrahedral, triangular, hexahedral, quadrilateral and hybrid meshes. Spatial discretisation with second-order upwind schemes was used for turbulent dissipation rate, turbulent kinetic energy and momentum as the second-order discretisation of the viscous terms is always accurate in Fluent. Moreover, the differencing scheme of second-order upwind was utilised to overcome numerical dispersion. At the end of the solver iteration, the moment coefficient, lift and drag are calculated and stored to create a convergence history. The standard initialisation in the entire domain used in this study allows setting the initial values of the flow variables and initialising the solution with these values. Fig. 5. Diagram for vertical earth pipe cooling system.
5. Results and discussion gravel drainage pit. For excess rainwater runoff, the containers were set with a 3°pitch from North to South. The sloped base ensures the flow of water to a low point, where a gravel rubble drain enables water spill over and/or provides a point from which excess water can be mechanically extracted by pump. A fan was installed inside the pipe (Figs. 4 and 5) to suck intake air from the pipe inlet and to pass the air through a series of buried pipes into the room. Air conditioners were installed to cool both the rooms, using lower energy in combination with the earth pipe cooling system. To increase the cooling effect of the system, small trees were planted to provide shade to the soil which covered the underground pipes. This was intended to reduce the amount of solar energy absorbed by the soil surface.
Experimental results were obtained through a series of experiments and measurements. The measuring tools, namely Reed Vane Anemometer, HOBO Pendant Temp, BTM-4208SD 12 CH temperature recorder and G4400 BLACKBOX fixed power quality analyser were used for measuring air velocity, air temperature and relative humidity, soil temperature at different points, and the energy consumption of the earth pipe cooling system respectively. The tests were conducted while an air conditioner of power input 1.01 kW and capacity output 5.70 kW was installed in both the earth pipe cooling and the standard room (the latter was not connected to any other system). The HEPC and VEPC measurements were conducted from April to June 2013 and July to September 2013 respectively.
4. CFD analysis 5.1. Soil temperature investigation Earth pipe cooling involves heat transfer where the earth transfers the heat to the buried underground pipes. Computational fluid dynamics (CFD) is a powerful and convenient method used for heat transfer studies over many years. A CFD model, namely the realisable k e turbulence model was used to analyse the heat transfer problem of the system. This is a reliable and more accurate model which is applicable for an extensive class of turbulent flows in heat transfer and industrial flow simulations. The turbulence model was selected for the thermal modelling as the flow passing through the buried pipes was turbulent (Reynolds Number, Re > 4000), where Reynolds number ranged from 8220 to 31,700. The model satisfies definite mathematical constraints on the Reynolds stresses and is consistent with turbulent flow physics. The problem was solved numerically by using the CFD code ‘‘Fluent in ANSYS 15.0”, which employs the finite volume method for discretisation of the computational domain. Air temperature has only been assessed using CFD as this study evaluates the thermal performance of the earth pipe cooling system.
The soil temperature analysis was carried out in April 2013, which involved the outdoor temperature and the soil temperature at different depths (0.61 m, 0.73 m, 0.85 m, 0.97 m and 1.10 m, where the PVC buried pipes were aligned). Data of April 2013 were used for this investigation as they have comparatively greater temperatures than the other months considered in this study. Average soil temperature was recorded from 10:00 am to 5:00 pm as maximum heat load occurred during this period. The lowest soil temperature distribution was observed at the depth of 1.10 m underground as shown in Fig. 6. All the underground soil temperatures were found to be lower than the ground surface and outdoor temperatures. Typically, soil temperature decreases on a hot day with increasing depth. Fig. 6
4.1. Solver approaches A two dimensional pressure-based-coupled solver was used for the numerical calculations of the model. It solves a coupled system of equations along with the momentum equations and the pressure-based continuity equation. The solver offers some additional advantages over a segregated or non-coupled approach, although it has some limitations. The coupled scheme allows a robust and efficient single phase execution for steady-state flows with high performance compared to the other solution schemes [38]. It is also necessary to use the coupled algorithm for transient flows, when the mesh quality is poor or the time step used in the solver is large. A study was carried out to check the effect of the grid variation and to establish the optimum mesh size which ensures consistent results for every mesh size.
Fig. 6. Soil temperature distribution at outdoor, ground surface and various depths.
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shows the outdoor temperature which varies from 22.4 °C to 26.4 °C, while the temperature at 1.10 m depth underground varies from 19.8 °C to 20.7 °C. The maximum temperature reduction that occurred between the outdoor and the soil at 1.10 m depth was 5.7 °C. This reduction assists to cool a room with the earth pipe cooling technology. Further work is being undertaken for the soil temperature at depths greater than 1.10 m. 5.2. Earth pipe cooling performance For measuring the performance of HEPC system, average air temperature, air velocity and energy consumption were recorded from both the rooms by turning off the VEPC system. The operation of VEPC room was turned off to consider this room as a standard room (not connected with earth pipe cooling or any other system) so that comparison can be made with the HEPC measurement and performance. Likewise, for VEPC performance, the data were recorded from both the rooms by turning off the HEPC system. Both the earth pipe cooling room temperatures were compared with the standard room temperature. The HEPC room temperature ranged from 22.61 °C to 25.32 °C, while the standard room temperature ranged from 23.27 °C to 25.72 °C as shown in Fig. 7a. The VEPC room temperature was also measured and varied from 23.48 °C to 26.39 °C, while the standard room temperature varied from 23.54 °C to 26.88 °C as shown in Fig. 7b. The maximum temperature reduction between the earth pipe cooling and the standard room was observed as 1.21 °C for the HEPC system on 14 May 2013 and 1.16 °C for the VEPC system on 16 August 2013. The velocity and temperature of air entering the earth pipe cooling room through the buried pipes were measured to evaluate their impact on room cooling performance. Cooled air from the buried pipes was supplied to the HEPC room with a velocity of 1.1 m/s and a temperature of 20.05 °C and to the VEPC room with a velocity of 3.40 m/s and a temperature of 21.50 °C at the inlet to the rooms. Relative humidity of the air also had a noticeable impact on the thermal comfort. It is still an essential variable to measure, especially in hot climates. Relative humidity of the rooms as shown in Fig. 8 was measured to assess its impact inside the rooms, which is summarised in Table 1. The average indoor relative humidity of both the HEPC and VEPC systems remained lower than 68%, which is acceptable for thermal comfort for the measured temperatures of 24.58 °C and 24.61 °C respectively. It was also found that the relative humidity of the rooms serviced by the earth pipe cooling systems was higher than that of the standard room due to the passing air entering those rooms.
Fig. 7a. Temperature profile in the HEPC and standard room.
Fig. 7b. Temperature profile in the VEPC and standard room.
Fig. 8a. Indoor relative humidity in the HEPC and standard room.
The measurements carried for both the HEPC and VEPC systems were compared with ASHRAE Standard 55-2010 using the adaptive method available in the Centre for the Built Environment (CBE) online Thermal Comfort Tool. The measured average temperature and relative humidity complies with ASHRAE Standard 55-2010 [39], ISO 7730 [40], Brown’s bioclimatic chart [41] and Givoni’s chart [42]. To validate the experimental data, one data set for daily maximum outdoor temperature was taken over the period of April 2013 and compared with the data recorded at 3 nearest bureau stations [43]. The daily maximum outdoor temperature was considered for this comparison because of the availability of the data for the same period. Moreover the data recorded at the bureau stations, namely Rockhampton Aero, Yeppoon the Esplanade, and Rosslyn Bay NTC AWS are considered as authentic as these data are collected by the Bureau of Meteorology, Australia. To compare the temperature profiles of these locations, a graph is plotted and displayed in Fig. 9. Fig. 9 shows a similar trend for each bureau station and the experimental shed at the sustainable precinct of Central Queensland University (where experiment of this study was conducted). It is seen that the temperature profile of the Rockhampton Aero is close to the profile of the sustainable precinct. The probable reason is that this station is the nearest from the precinct compared to other stations. The Rockhampton Aero (Latitude: 23.38°S, Longitude: 150.48°E) is 5.4 km away from the precinct
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Fig. 8b. Indoor relative humidity in the VEPC and standard room.
Table 1a Air temperature and relative humidity measured in the HEPC and standard room. Modelled rooms
HEPC Standard room
Indoor temperature (°C)
Outdoor temperature (°C)
Relative humidity (%)
Min
Max
Avg
Min
Max
Avg
Min
Max
Avg
22.61 23.27
25.32 25.72
24.58 24.90
20.62 –
36.19 –
29.77 –
32.12 36.72
86.37 81.95
65.24 63.14
Table 1b Air temperature and relative humidity measured in the VEPC and standard room. Modelled rooms
VEPC Standard room
Indoor temperature (°C)
Outdoor temperature (°C)
Relative humidity (%)
Min
Max
Avg
Min
Max
Avg
Min
Max
Avg
23.48 23.54
26.39 26.88
24.61 24.87
25.87 –
37.46 –
31.10 –
55.19 48.18
83.46 78.99
67.95 58.58
Fig. 9. Comparison of daily maximum outdoor temperature collected from this study and 3 bureau stations.
(Latitude: 23.32°S, Longitude: 150.52°E) while the Yeppoon the Esplanade (Latitude: 23.14°S, Longitude: 150.75°E) and Rosslyn Bay NTC AWS (Latitude: 23.16°S, Longitude: 150.79°E) are 34.2 km and 35.1 km away respectively. The maximum temperature amplitude of 3.5 °C was found between the precinct and Rosslyn Bay station whereas the minimum of 0 °C was found
between the precinct and the Rockhampton Aero. Although the experimental data of the precinct shows some variations (0–12.58%) with each station, the overall experimental results show a good agreement. The experimental data are to be compared with the numerical data in later section. Performance of the earth pipe cooling system was also calculated numerically using the simulation software Fluent. The flow and thermal variables for the boundary, and cell zone conditions were set on the boundary conditions of the model. The simulation results shown in Fig. 10 were obtained under the boundary conditions which are shown in Table 2. The average maximum room air temperature collected from the standard room was taken as the room temperature of the model. The simulation results show the average room temperatures for the earth pipe cooling system as 23.85 °C in the HEPC system and 23.05 °C in the VEPC system, which are respectively 1.06 °C and 1.82 °C lower than the standard room temperature. This indicates that the VEPC system performs better than the HEPC system. It is considered that the main reason behind this is that the buried underground pipes used in the VEPC system aligned at the depth where the soil temperature was cooler (Fig. 6) than that of the HEPC system. Moreover, the VEPC system provided sufficient air velocity (3.4 m/s) at the pipe inlet (inlet to the room) than that of the HEPC system (1.1 m/s). The higher inlet velocity in the VEPC system is because of the smaller pipe in length used in this system, where the fan can suck more air through this smaller pipe. As mentioned earlier, the VEPC system consists of 20 buried pipes each of
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Fig. 10a. Temperature distribution in the HEPC room.
Fig. 10b. Temperature distribution in the VEPC room.
Table 2 Parameters used in boundary conditions of the model. Parameters
HEPC
VEPC
Inlet velocity Inlet temperature Room temperature Air thermal conductivity Air density Specific heat of air Air viscosity
1.1 m/s 20.05 °C 24.90 °C 0.024 W/m K 1.204 kg/m3 1006.43 J/kg K 1.850387e05 kg/m s
3.40 m/s 21.01 °C 24.87 °C 0.024 W/m K 1.204 kg/m3 1006.43 J/kg K 1.850387e05 kg/m s
length 6.0 m (Fig. 5) whereas the HEPC system consists of the same number of buried pipes each of length 7.5 m (Fig. 4). The room temperatures obtained using simulation was compared with the corresponding experimental results at different heights (0.10 m, 0.25 m, 0.50 m, 0.75 m, 1.0 m, 1.25 m, 1.50 m, 1.75 m, 2.00 m and 2.25 m) along the centre of the room as summarised in Table 3.
The simulation results showed some deviations from the experimental results which are believed to be due to the measurement uncertainties in the experiments due to the instrument accuracy, reading, test planning, environment, condition, calibration, etc. Other reason is the uncertainties and errors in CFD simulations because of the auxiliary physical models [44] of turbulence model used in this study. Additionally, this may also occur due to the initial and boundary conditions, discretization and solution. However, the overall simulated results are in very good agreement with the corresponding experiments as shown in Fig. 11, and lie within 3.06–5.31% limits for HEPC system and 5.22–8.08% limits for the VEPC system.
5.3. Energy savings using earth pipe cooling technique The energy efficiency of the earth pipe cooling technique is evaluated in this section. The technique involves an 8 W fan and an air conditioner as discussed in Section 3. The air conditioners
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Table 3 Comparison in room temperature between experimental and numerical results. Height (m)
HEPC
VEPC
Experimental
Numerical
Differences (%)
Experimental
Numerical
Differences (%)
0.10 0.25 0.50 0.75 1.00 1.25 1.50 1.75 2.00 2.25
25.68 25.49 24.93 24.67 24.51 24.84 24.58 24.60 24.19 25.24
24.63 24.18 23.88 23.83 23.76 23.69 23.62 23.55 23.44 23.90
4.09 5.14 4.21 3.40 3.06 4.63 3.91 4.27 3.10 5.31
24.47 24.52 24.32 24.39 24.61 24.46 24.19 24.39 24.70 25.41
22.90 22.89 22.98 23.04 23.06 23.03 22.99 22.98 23.08 23.51
6.86 7.12 5.83 5.86 6.72 6.21 5.22 6.14 7.02 8.08
Fig. 11a. Numerical data plotted against experimental data for the HEPC room temperatures.
Fig. 12a. Energy consumption for the HEPC system.
Fig. 12b. Energy consumption for the VEPC system. Fig. 11b. Numerical data plotted against experimental data for the VEPC room temperatures.
in the earth pipe cooling system rooms and the standard rooms were set at 24 °C to provide optimal comfort and energy savings. The energy used by the fan and the air conditioning unit in both horizontal and vertical earth pipe cooling systems are displayed in Fig. 12. For measuring energy efficiency, energy data were recorded from midday to midnight from 14 June, 2013 to 22 June, 2013 for the VEPC system and from 12 July, 2013 to 20 July, 2013 for the HEPC system. The maximum energy was consumed at the hot peak hours of the day, whereas the minimum was consumed at the off peak hours at night as shown in Fig. 12.
From Figs. 12a and 12b, it can be seen that the HEPC average energy consumption ranges from 386.36 W h to 938.77 W h whereas the standard room energy consumption varies from 405.56 W h to 963.24 W h. The VEPC energy consumption ranges from 942.97.36 W h to 1637.70 W h whereas the standard room energy consumption varies from 999.08 W h to 1646.51 W h. In the HEPC room, the maximum energy consumption of 938.77 W h was observed on 20 July 2013 at 12:00 PM, while in the VEPC room the maximum energy consumption of 1637.70 W h was observed on 14 September 2013 at 12:20 PM. Energy consumption in those two days – 20 July 2013 in the HEPC and standard room, and on 14 September 2013 in the VEPC and standard room – are shown on an hourly basis in Fig. 13.
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Fig. 13a. Energy consumption on 20 July 2013 in HEPC and standard room.
Fig. 13b. Energy consumption on 14 September 2013 in VEPC and standard room.
The average maximum difference in energy consumption between the earth pipe cooling systems and standard room was observed during hot peak hours of the day whereas the minimum was observed during late night and early morning. Rockhampton weather gets warmer in the day, and cooler at late night and early morning, which is the main reason behind this trend of energy
consumption. Moreover, in the earth pipe cooling systems, the earth (soil) acts as a heat sink in warm weather to produce cooler air and as a heat source in cool weather to generate hot air into the ground. Due to the heat generation during late night and early morning, more energy is consumed by the systems, and therefore the amount of energy consumption is nearly the same as that for the standard room during this period. Typically, air conditioning energy consumption varies depending on the outdoor air temperature. Average energy consumption for the earth pipe cooling technology is summarised in Table 4. The table shows an average hourly energy savings of 24.29 W and 42.50 W using the HEPC and VEPC systems respectively. It also shows that the HEPC system has the potential to save a maximum of 66.20 W/h, whereas the VEPC system has the potential to save up to 98.92 W/h. Based on these results, energy savings of a maximum of 579.91 kW and 866.54 kW could be achieved per year by adopting the HEPC and VEPC systems respectively. A fair amount of energy cost can be saved due to the reduction of energy consumption shown in Table 4. There are several energy usage charging categories used in Australian buildings. The most common category, namely tariff 11, is used for residential customers in Australia [45]. The energy cost for tariff 11 is summarised in Table 5. It includes the cost of energy usage at a flat rate throughout the day and night, plus a daily supply charge as a service fee. The daily supply charge used in tariff 11 is a fixed cost, whereas all energy consumption is a variable cost as the energy consumption rate is varied with the energy consumers. Therefore, the total energy cost for a building can only be calculated based on the energy used in the building. According to the price of tariff 11, total energy cost savings have been calculated and are tabulated in Table 6. Table 6 shows annual average energy savings of $59.40 in the HEPC system and $103.93 in the VEPC system. It is also found that maximum annual savings of $161.89 (7.93%) and $241.90 (8.82%) can be made using the HEPC and VEPC systems respectively. The savings gained are for a single room only of dimensions 5.63 m 2.14 m 2.26 m. Hence, there is an opportunity to make substantial savings using such earth pipe cooling technology for complete buildings.
6. Error analysis Instrument selection, reading, test planning, observation, environment, condition and calibration can produce errors and uncertainties. The uncertainty analysis is needed to measure the experiment accuracy. Uncertainties in percentage of various
Table 4 Hourly energy savings using earth pipe cooling technology. Modelled rooms
HEPC VEPC
Hourly energy consumption using earth pipe cooling system (W h)
Hourly energy consumption without using earth pipe cooling system (W h)
Hourly energy savings (W h)
Min
Max
Avg
Min
Max
Avg
Min
Max
Avg
Max (%)
386.36 942.97
938.77 1637.70
651.47 1279.02
405.56 999.08
963.24 1646.51
668.11 1326.60
11.52 12.24
66.20 98.92
24.29 42.50
7.93 8.82
Table 5 Energy costs for residential customers. Tariff 11 Residential
GST inclusive from 1 July 2014 (carbon price removed)
GST inclusive from 1 July 2014 (includes carbon price)
GST inclusive from 1 July 2013 (includes carbon price)
All consumption (cents per kWh) Daily supply charge (cents per day)
27.916 91.755
30.817 91.755
29.403 55.241
824
S.F. Ahmed et al. / Energy Conversion and Management 106 (2015) 815–825
Table 6 Energy cost savings using earth pipe cooling systems (costs in Australian Dollars after removing carbon price). Modelled rooms
HEPC VEPC
Weekly
Monthly
Yearly
Min
Max
Avg
Min
Max
Avg
Min
Max
Avg
Max (%)
0.54 0.57
3.10 4.64
1.14 1.99
2.32 2.46
13.30 19.88
4.88 8.54
28.17 29.93
161.89 241.90
59.40 103.93
7.93 8.82
Table 7 Instruments used in the experiment and their accuracy, range and percentage uncertainties. Instruments
Parameters
HOBO U10-003 Reed Vane Anemometer BTM-4208SD 12 CH temperature recorder HOBO U10-003
Accuracy
Air temperature Air velocity Soil temperature Relative humidity
±0.4 °C ±(2% reading + 0.2 m/s) ±(0.4% + 0.5) ±3.5%
parameters such as air temperature, air velocity, relative humidity and energy consumption were calculated for both HEPC and VEPC systems using the instruments’ uncertainties as shown in Table 7. The uncertainty analysis in percentage is performed by the following method [46]. Total uncertainty of the experiment for the HEPC system is
¼ square root of ½ðuncertainty of room temperatureÞ
2
2
þ ðuncertainty of air velocityÞ
þ ðuncertainty of soil temperatureÞ
2
þ ðuncertainty of relative humidityÞ
¼ square root of ½ð0:02Þ2 þ ð0:2Þ2 þ ð0:02Þ2 þ ð2:28Þ2 þ ð0:01Þ2
+0.02 +0.27 +0.02 +2.38
to to to to
0.02 0.27 0.02 2.38
VEPC system. This translates to maximum annual savings of $161.89 (7.93%) using the HEPC system and $241.90 (8.82%) using the VEPC system. This investigation has established that the earth pipe cooling technique is not only energy efficient, but also cost-effective, and has less negative and environmental effects compared to other existing techniques. The findings of this study are recommended to be utilised as guidelines by the regulators, manufacturers and inhabitants for the deployment of passive cooling techniques in hot humid climates.
The authors gratefully acknowledge the support provided by Ergon Energy, Australia for supplying passive air cooling system and the installation cost of the measuring tools. The first author S F Ahmed is grateful to Central Queensland University, Australia for the international postgraduate research award (IPRA) to support this study. The authors are also grateful to Tim McSweeney for providing valuable suggestions and proofreading support that improved the quality of this paper.
¼ 2:29% Total uncertainty of the experiment for the VEPC system is
¼ square root of ½ðuncertainty of room temperatureÞ
2
2 2
þ ðuncertainty of soil temperatureÞ
þ ðuncertainty of relative humidityÞ
2
References
2
þ ðuncertainty of active energyÞ 2
VEPC
+0.02 to 0.02 +0.2 to 0.2 +0.02 to 0.02 +2.28 to 2.28
Acknowledgements
2
2
20 °C to 70 °C 0.4 m/s to 30.0 m/s 50 °C to 400 °C 25% to 95%
Percentage uncertainties HEPC
2
þ ðuncertainty of active energyÞ
þ ðuncertainty of air velocityÞ
Range
2
2
2
¼ square root of ½ð0:02Þ þ ð0:27Þ þ ð0:02Þ þ ð2:38Þ þ ð0:03Þ ¼ 2:40%
7. Conclusion Passive cooling techniques are the least expensive methodology for maximising the thermal comfort of building occupants with the lowest environmental impact. They offer a great potential for energy savings worldwide for hot and humid climates as they can reduce the air-conditioning load of many buildings while producing virtually no greenhouse gas emissions. Recent advancements on the techniques can help to enhance the quality of the indoor environment of buildings. The case study carried out on earth pipe cooling technology in a hot humid subtropical climate of Rockhampton, Australia involving two different piping layouts, namely the horizontal and vertical, reduced the room temperatures by 1.05 °C and 1.82 °C respectively. This temperature reduction saved an annual maximum energy of 579.91 kW for the HEPC system and 866.54 kW for the
[1] EIA. Energy information administration. International energy outlook; 2013. [cited on 25.03.14]. [2] Ahasan T, Ahmed SF, Rasul MG, Khan MMK, Azad AK. Performance evaluation of hybrid green roof system in a subtropical climate using fluent. J Power Energy Eng 2014;2:113. [3] Omer AM. Energy, environment and sustainable development. Renew Sustain Energy Rev 2008;12:2265–300. http://dx.doi.org/10.1016/j.rser.2007.05.001. [4] Givoni B. Climate considerations in building and urban design. John Wiley & Sons; 1998. [5] Yang L, Yan H, Lam JC. Thermal comfort and building energy consumption implications–a review. Appl Energy 2014;115:164–73. [6] Saman WY. Towards zero energy homes down under. Renew Energy 2013;49:211–5. [7] DRET. Department of Resources, Energy and Tourism. Energy in Australia; 2012. [cited on 15.09.14]. [8] Ahmed SF, Khan MMK, Amanullah MTO, Rasul MG. Selection of suitable passive cooling strategy for a subtropical climate. Int J Mech Mater Eng 2014;9:1–11. [9] Sadineni SB, Madala S, Boehm RF. Passive building energy savings: a review of building envelope components. Renew Sustain Energy Rev 2011;15:3617–31. http://dx.doi.org/10.1016/j.rser.2011.07.014. [10] Madhumathi A. Experimental study of passive cooling of building facade using phase change materials to increase thermal comfort in buildings in hot humid areas. Int J Energy Environ 2012;3:739–48.
S.F. Ahmed et al. / Energy Conversion and Management 106 (2015) 815–825 [11] Becker R, Paciuk M. Inter-related effects of cooling strategies and building features on energy performance of office buildings. Energy Build 2002;34:25–31. http://dx.doi.org/10.1016/S0378-7788(01)00081-0. [12] Chowdhury AA, Rasul MG, Khan MMK. Modelling and analysis of air-cooled reciprocating chiller and demand energy savings using passive cooling. Appl Therm Eng 2009;29:1825–30. http://dx.doi.org/10.1016/j.applthermaleng. 2008.09.001. [13] Ahmed SF, Khan MMK, Amanullah MTO., Rasul MG, Hassan NMS. Performance analysis of vertical earth pipe cooling system for subtropical climate. In: International conference on clean energy, Istanbul, Turkey; 2014. p. 691–700. [14] Ahmed SF, Khan MMK, Rasul MG, Amanullah MTO, Hassan NMS. Comparison of earth pipe cooling performance between two different piping systems. Energy Procedia 2014;61:1897–901. http://dx.doi.org/10.1016/j.egypro.2014. 12.237. [15] Ahmed SF, Khan MMK, Amanullah MTO, Rasul MG, Hassan NMS. Thermal performance analysis of earth pipe cooling system for subtropical climate. In: 12th international conference on sustainable energy technologies, Hong Kong; 2013. p. 1795–803. [16] Lee KH, Strand RK. The cooling and heating potential of an earth tube system in buildings. Energy Build 2008;40:486–94. http://dx.doi.org/10.1016/j.enbuild. 2007.04.003. [17] Ghosal M, Tiwari G. Modeling and parametric studies for thermal performance of an earth to air heat exchanger integrated with a greenhouse. Energy Convers Manage 2006;47:1779–98. http://dx.doi.org/10.1016/j.enconman.2005. 10.001. [18] Wu H, Wang S, Zhu D. Modelling and evaluation of cooling capacity of earth– air–pipe systems. Energy Convers Manage 2007;48:1462–71. http://dx.doi. org/10.1016/j.enconman.2006.12.021. [19] Bisoniya TS, Kumar A, Baredar P. Experimental and analytical studies of earth– air heat exchanger (EAHE) systems in India: a review. Renew Sustain Energy Rev 2013;19:238–46. http://dx.doi.org/10.1016/j.rser.2012.11.023. [20] Mihalakakou G, Santamouris M, Asimakopoulos D. Modelling the thermal performance of earth-to-air heat exchangers. Sol Energy 1994;53:301–5. http://dx.doi.org/10.1016/0038-092X(94)90636-X. [21] Gallero FJG, Maestre IR, Gómez PÁ, Blázquez JLF. Numerical and experimental validation of a new hybrid model for vertical ground heat exchangers. Energy Convers Manage 2015;103:511–8. [22] Bansal V, Misra R, Agrawal GD, Mathur J. Performance analysis of earth–pipe– air heat exchanger for summer cooling. Energy Build 2010;42:645–8. [23] De Paepe M, Willems N. 3D unstructured modeling technique for groundcoupled air heat exchangers. In: Proceedings of the 7th international world conference CLIMA2000, Napoli. CD-rom-uitgave, paper ID 276; 2001. 15 pp. [24] Al-Ajmi F, Loveday D, Hanby VI. The cooling potential of earth-air heat exchangers for domestic buildings in a desert climate. Build Environ 2006;41:235–44. http://dx.doi.org/10.1016/j.buildenv.2005.01.027. [25] Mongkon S, Thepa S, Namprakai P, Pratinthong N. Cooling performance assessment of horizontal earth tube system and effect on planting in tropical greenhouse. Energy Convers Manage 2014;78:225–36. http://dx.doi.org/ 10.1016/j.enconman.2013.10.076. [26] Ahmed SF, Khan MMK, Amanullah MTO, Rasul MG, Hassan NMS. Performance evaluation of hybrid earth pipe cooling with horizontal piping system. Thermofluid modeling for energy efficiency applications. USA: Elsevier; 2015. p. 1–30. http://dx.doi.org/10.1016/B978-0-12-802397-6.00001-4. [27] Mihalakakou G, Santamouris M, Asimakopoulos D, Tselepidaki I. Parametric prediction of the buried pipes cooling potential for passive cooling
[28]
[29]
[30] [31]
[32]
[33]
[34]
[35]
[36]
[37]
[38] [39] [40]
[41] [42] [43] [44] [45]
[46]
825
applications. Sol Energy 1995;55:163–73. http://dx.doi.org/10.1016/0038092X(95)00045-S. Benhammou M, Draoui B, Zerrouki M, Marif Y. Performance analysis of an earth-to-air heat exchanger assisted by a wind tower for passive cooling of buildings in arid and hot climate. Energy Convers Manage 2015;91:1–11. Zarrella A, Emmi G, De Carli M. Analysis of operating modes of a ground source heat pump with short helical heat exchangers. Energy Convers Manage 2015;97:351–61. Shen P, Lukes JR. Impact of global warming on performance of ground source heat pumps in US climate zones. Energy Convers Manage 2015;101:632–43. Krarti M, Kreider JF. Analytical model for heat transfer in an underground air tunnel. Energy Convers Manage 1996;37:1561–74. http://dx.doi.org/10.1016/ 0196-8904(95)00208-1. Santamouris M, Mihalakakou G, Balaras C, Argiriou A, Asimakopoulos D, Vallindras M. Use of buried pipes for energy conservation in cooling of agricultural greenhouses. Sol Energy 1995;55:111–24. Zaki AK, Amjad A-M, Almssad A. Cooling by underground earth tubes. In: 2nd PALENC conference and 28th AIVC conference on building low energy cooling and advanced ventilation technologies in the 21st century, Crete island, Greece; 2007. Goswami D, Biseli K. Use of underground air tunnels for heating and cooling agricultural and residential buildings. Fact sheet EES 78. Fact Sheet EES1993. [cited on 28.02.14]. Ahmed SF, Khan MMK, Amanullah MTO, Rasul MG, Hassan NMS. Numerical modelling of hybrid vertical earth pipe cooling system. In: The 19th Australasian fluid mechanics conference; 2014. Jacovides C, Mihalakakou G, Santamouris M, Lewis J. On the ground temperature profile for passive cooling applications in buildings. Sol Energy 1996;57:167–75. http://dx.doi.org/10.1016/S0038-092X(96)00072-2. Mihalakakou G, Santamouris M, Lewis J, Asimakopoulos D. On the application of the energy balance equation to predict ground temperature profiles. Sol Energy 1997;60:181–90. http://dx.doi.org/10.1016/S0038-092X(97)00012-1. ANSYS. FLUENT user’s guide; 2006. [cited on 23.01.15]. A. ASHRAE. Standard 55-2010: Thermal environmental conditions for human occupancy. ASHRAE. Atlanta USA; 2010. ISO 3370. International organization for standardization. 2005. Ergonomics of the thermal environment-analytical determination and interpretation of thermal comfort using calculation of the PMV and PPD indices and local thermal comfort criteria. 3rd ed. Brown GZ, Dekay M. Sun, wind, and light: architectural design strategies. 2nd ed. United States of America: John Willey & Sons; 2001. Givoni B. Comfort, climate analysis and building design guidelines. Energy Build 1992;18:11–23. http://dx.doi.org/10.1016/0378-7788(92)90047-K. BOM. Bureau of meteorology. Climate data online; 2015. [cited on 12.09.15]. Oberkampf WL, Helton JC, Sentz K. Mathematical representation of uncertainty. In: AIAA non-deterministic approaches forum; 2001. p. 16–9. EG. Ergon energy. General supply tariffs. Tariff 11 residential; 2014. [cited on 26.11.14]. Mani M, Nagarajan G. Influence of injection timing on performance, emission and combustion characteristics of a DI diesel engine running on waste plastic oil. Energy 2009;34:1617–23. http://dx.doi.org/10.1016/j.energy.2009.07.010.
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Energy Conversion and Management journal homepage: www.elsevier.com/locate/enconman
Performance assessment of earth pipe cooling system for low energy buildings in a subtropical climate S.F. Ahmed a,b,⇑, M.M.K. Khan a, M.T.O. Amanullah b, M.G. Rasul a, N.M.S. Hassan a a b
School of Engineering and Technology, Central Queensland University, Rockhampton Campus, Queensland 4702, Australia School of Engineering, Deakin University, Geelong Waurn Ponds Campus, Victoria 3220, Australia
a r t i c l e
i n f o
Article history: Received 9 July 2015 Accepted 10 October 2015
Keywords: Passive cooling Earth pipe cooling Energy savings Thermal comfort Subtropical climate
a b s t r a c t Energy consumption in heating and cooling around the world has been a major contributor to global warming. Hence, many studies have been aimed at finding new techniques to save and control energy through energy efficient measures. Most of this energy is used in residential, agricultural and commercial buildings. It is therefore important to adopt energy efficiency measures in these buildings through new technologies and novel building designs. These new building designs can be developed by employing various passive cooling systems. Earth pipe cooling is one of these which can assist to save energy without using any customary mechanical units. This paper investigates the earth pipe cooling performance in a hot humid subtropical climate of Rockhampton, Australia. A thermal model is developed using ANSYS Fluent for measuring its performance. Impacts of air velocity, air temperature, relative humidity and soil temperature on room cooling performance are also assessed. A temperature reduction of around 2 °C was found for the system. This temperature reduction contributed to an energy saving of a maximum of 866.54 kW (8.82%) per year for a 27.23 m3 room. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction Energy need and sustainable provision of energy has been a major concern for sustainable development worldwide. Energy consumption in different forms is steadily rising all over the world. The world energy use is projected to rise from 524 quadrillion Btu (552.85 exajoule) in 2010 to 630 quadrillion Btu (664.69 exajoule) in 2020 and 820 quadrillion Btu (865.15 exajoule) in 2040 [1]. This projection indicates that energy usage will increase by around 56% over these 30 years. During this period, more than 85% of this growing energy demand will occur within the developing nations outside the Organisation for Economic Cooperation and Development (non-OECD nations), where demand is driven by strong long-term economic growth and increasing populations. Income and population growth are the main reasons behind this significant energy demand. World population is estimated to increase to 8.3 billion from its current 7 billion by 2030, which indicates that more energy will be required for an additional 1.3 billion people [2]. Urban areas have expanded in size globally over recent decades [3] and around half of the world’s population ⇑ Corresponding author at: School of Engineering, Deakin University, Geelong Waurn Ponds Campus, Victoria 3220, Australia. E-mail addresses: [email protected], [email protected] (S.F. Ahmed). http://dx.doi.org/10.1016/j.enconman.2015.10.030 0196-8904/Ó 2015 Elsevier Ltd. All rights reserved.
(however, just 7.6% in the more developed nations) are now urban residents [4]. This has caused high energy demand. People around the world have become more energy hungry as the life style has changed significantly in recent times due to many technological advancement. About 40% of the global energy is consumed in building sector [5], where Australian buildings use 38% of the total energy consumption on cooling and heating purposes [6]. Australia is the eighteenth leading energy consumer in the world and fourteenth on a per person basis [7]. The energy demand in Australia is predicted to increase rapidly in future. Energy consumption for the Australian residential sector in 1990 was around 299 petajoules (PJ) and that by 2008 had increased to about 402 PJ and is projected to increase to 467 PJ by 2020 [8]. This signifies an increased energy consumption of 56% in this sector over this period from 1990 to 2020. According to this projection, more energy will be required for survival and achieving thermal comfort. There has been an ongoing interest in improving building energy efficiency because of ecological concerns and the high cost of energy in recent years [9]. Consumers are therefore looking for energy efficient and environmentally-friendly building designs. Energy efficient buildings can be developed by employing either passive or active energy efficient technologies. Improvements to building envelope components can be treated as passive strategies, whereas enhancements to heating, ventilating and air conditioning
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(HVAC) systems, lighting, etc., can be categorised under active strategies [9]. Passive air cooling is one of the passive strategies which can assist to reduce energy consumption in buildings for all subtropical zones. One significant outcome of incorporating passive cooling techniques is reducing air-conditioning loads or reducing heat gains in buildings to make them more sustainable. Several studies have been carried out on reducing air-conditioning loads. An experimental study was conducted using phase change material (PCM) energy storage techniques for enhancing room air quality and reducing the cooling load [10]. The experimental results showed a reduction of building energy consumption without a significant increase in the weight of the building materials with the incorporation of PCM. To reduce energy demands in buildings, different pre-cooling and ventilation methods and their overall impact on the energy use were studied by Becker and Paciuk [11]. The research involved simulations on varying building envelope characteristics for changing interior high heat loads. Their study indicated that the night pre-cooling is a productive cooling method for buildings with high interior heat loads. It reduces the inside mass temperature below the air temperature and subsequently diminishes the peak power cooling requirements. The pre-cooling system can save 115 kW/m2/month of total cooling energy demand [12]. The earth pipe cooling technique is one of the viable options to reduce the cooling loads. Thermal performance of this technology was evaluated in a subtropical climate in Queensland, Australia [13–15]. In those studies, a reduction of about 1–2 °C temperature was found for a single room. The earth pipe cooling performance was also investigated in another study to assess the influence of pipe length, pipe radius, buried pipe depth and air flow rate under different conditions [16]. The buried underground pipe depth and the pipe length turned out to influence the earth tube cooling rate, while air flow rate and pipe radius mainly affect the inlet air temperature entering the space. Many researchers found that the resulting outlet temperature at the buried pipe decreases with decreasing mass flow rate in the pipe, decreasing pipe radius, increasing pipe length and increasing depths up to 4 m [17]. To assess the impacts of these parameters on thermal performance, an implicit and transient model was developed for the earth–air– pipe systems in Southern China using PHOENICS [18]. The results demonstrated that a daily cooling capacity up to 74.6 kW h can be achieved in that region using the system. In some cases, the earth pipe cooling system is assisted by a heat pump as a heat exchanger located within the buried pipe. This is also known as an earth–pipe–air heat exchanger, which can be utilised for reducing the cooling load of buildings during summer. It can also be used for heating interior spaces in buildings in winter. These systems have been commonly implemented in Germany, Denmark, Austria and India since the 1990s, and are gradually being adopted in North America [19]. Their thermal performance was measured by Mihalakakou et al. [20] by developing a numerical model. In order to assess the cooling capacity of the ground heat exchanger system, a thermal prediction of the system is essential. Several numerical models presented in the literature [21–23] evaluate the thermal performance of the ground heat exchangers. Gallero et al. [21] developed a numerical model to simulate the thermal behaviour of a vertical borehole heat exchanger. A good agreement was observed for the borehole temperatures and modelled outlet fluid, with relative error values and root mean square smaller than 0.3% and 0.2 °C respectively. It was also found that the model is computationally fast and simple to be implemented into building thermal insulation programs. In the building the cooling capacity of the earth air pipe heat exchange was evaluated by developing a numerical model [22]. The model was developed
using the simulation program, FLUENT to investigate the thermal performance of different pipe materials and air velocities. The results demonstrated that the pipe materials have no noticeable impact on the earth air pipe heat exchanger whereas the air velocity has greater influence. The thermal performance was also investigated by a numerical model using FLUENT [23]. A theoretical model was developed for the earth–air heat exchanger for evaluating the cooling potential in a hot arid climate [24]. It was found that the earth–air heat exchanger has the potential to reduce 30% cooling energy demand in a typical house during peak summer. The cooling performance of horizontal earth pipe cooling system was assessed in an agriculture greenhouse in Thailand [25]. The study shows that the system has the potential to cool the greenhouse during daytime. There have been limited researches on earth pipe cooling techniques involving both numerical and experimental studies. Furthermore, no credible research on earth pipe cooling is seen to have been undertaken for a hot and humid climate in Australia. Therefore, the earth pipe cooling study is very timely and important for the Australian economy and environment. This study aims to assess the thermal performance of the earth pipe cooling technique in a hot humid climate in Australia by developing a thermal model. The model assists to predict the room temperatures using the pipe cooling systems. The reduction of room temperature and annual energy savings are calculated to assess the thermal performance of the system. In the following section, a comprehensive and valuable survey on the earth pipe cooling technique is presented for potential use and application in buildings to investigate all the key issues of this technique. 2. Earth pipe cooling technique Earth pipe cooling technology involves long buried pipes in which intake air comes through one end and passes through underground pipes, and finally the air cooled by the soil moves into the room through the outlet end. The pipe is laid underground at an optimum depth with the two pipe ends above the ground as shown in Fig. 1. A fan is needed to suck the fresh air from the pipe intake and to generate the air flow into the room. Earth pipe cooling technology offers several additional advantages such as protection from dust, noise, limited air infiltration, radiation and storms. It utilises the Earth’s near constant underground temperature for cooling air in industrial, residential and agricultural buildings. This fact has been studied and proven by several researchers [18,27–30]. The rationale behind this is
Fig. 1. Earth pipe cooling system [26].
S.F. Ahmed et al. / Energy Conversion and Management 106 (2015) 815–825
817
Fig. 2. Earth pipe cooling categories [35]. (a) Open loop. (b) Closed loop.
because the daily and regular temperature variation is significantly diminished in the ground below a certain depth where the soil temperature remains constant [31]. Low energy cooling techniques utilising earth became progressively popular in parts of America and Europe after the oil emergency in 1973 [31]. Typically, the soil temperature decreases in summer with increasing depth, which allows the utilisation of earth as a heat sink. It is seen from the literature that this temperature decrease continues up to a depth of 4 m underground as the soil temperature is fairly constant and stable at that depth [32]. Meanwhile, the soil temperature increases in winter with increasing depth, hence the use of earth as a heat source. Earth pipe cooling systems can be categorised in two ways: open loop and closed loop as shown in Fig. 2. In the open loop system, air comes through the buried pipes into the room and passes through a ventilation system. It provides ventilation while also cooling the interior of a house [33]. In a closed loop system, air is re-circulated from the buried underground pipes to the room [15]; this is more effective than the open loop system as it does not exchange air with the outside of the house. The latter pipe arrangement can also reduce the tunnel length since the conditioned air is re-circulated within the buried pipe [34]. The buried underground pipes may be installed in horizontal trenches or borings, vertical borings, slinky coil, or even in a surface water body. There are two main strategies for earth pipe cooling: direct and indirect earth contact [36]. Direct earth contact involves total or partial building envelope being placed in contact with the ground surface directly, and the indirect earth contact involves an earthto-air heat exchanger system through which air is circulated between the interior and exterior of the building and is subsequently brought into the building [37]. The direct earth-tobuilding contact ground cooling is a low maintenance passive cooling strategy with minimal heat gains and solar exposure. Despite the advantages of the direct-to-building contact ground cooling, it also creates environmental problems such as indoor condensation and poor indoor air quality [36]. Moreover, in some places such as desert and semi-desert countries, large excavation is not suitable due to geological conditions. Because of these disadvantages of the direct earth pipe cooling technique, the indirect earth pipe cooling technique has been considered in this study for measuring its performance.
earth pipe cooling (VEPC) system and the other to a horizontal earth pipe cooling (HEPC) system as shown in Fig. 3. Both the vertical and horizontal piping systems consist of two simple Polyvinyl Chloride (PVC) pipes of outside diameter 0.125 m, which are also known as manifolds. Intake air comes through one of the manifolds and passes through a series of buried pipes and moves into the room through another manifold. The manifolds were connected with 20 PVC pipes each of diameter 21 mm and thickness 1 mm. The 20 corrugated buried PVC pipes were aligned horizontally into one row in the HEPC system as shown in Fig. 4 and aligned vertically into 5 rows in the VEPC system as shown in Fig. 5. Each PVC pipe in each row was separated from its neighbour by 20 mm for both the piping systems. Excavations of dimensions 8.1 m 2 m were made for both the piping systems to fit the PVC pipes underground. The bottom of the excavations maintained a drop of 0.2 m down the length of the excavation to cause moisture to fall towards the sand and
Fig. 3. Experimental shed of earth pipe cooling system.
3. Experimental design and measurement The earth pipe cooling experiment was conducted in the sustainable precinct at Central Queensland University, Rockhampton, Australia. Two shipping containers, each with a dimension 5.63 m 2.14 m 2.26 m, were installed and fitted to investigate the cooling capacity and its effect. One was connected to a vertical
Fig. 4. Diagram for horizontal earth pipe cooling system.
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Pressure was discretised with a PRESTO scheme in view of its strong convergence ability. The scheme is available for all meshes such as tetrahedral, triangular, hexahedral, quadrilateral and hybrid meshes. Spatial discretisation with second-order upwind schemes was used for turbulent dissipation rate, turbulent kinetic energy and momentum as the second-order discretisation of the viscous terms is always accurate in Fluent. Moreover, the differencing scheme of second-order upwind was utilised to overcome numerical dispersion. At the end of the solver iteration, the moment coefficient, lift and drag are calculated and stored to create a convergence history. The standard initialisation in the entire domain used in this study allows setting the initial values of the flow variables and initialising the solution with these values. Fig. 5. Diagram for vertical earth pipe cooling system.
5. Results and discussion gravel drainage pit. For excess rainwater runoff, the containers were set with a 3°pitch from North to South. The sloped base ensures the flow of water to a low point, where a gravel rubble drain enables water spill over and/or provides a point from which excess water can be mechanically extracted by pump. A fan was installed inside the pipe (Figs. 4 and 5) to suck intake air from the pipe inlet and to pass the air through a series of buried pipes into the room. Air conditioners were installed to cool both the rooms, using lower energy in combination with the earth pipe cooling system. To increase the cooling effect of the system, small trees were planted to provide shade to the soil which covered the underground pipes. This was intended to reduce the amount of solar energy absorbed by the soil surface.
Experimental results were obtained through a series of experiments and measurements. The measuring tools, namely Reed Vane Anemometer, HOBO Pendant Temp, BTM-4208SD 12 CH temperature recorder and G4400 BLACKBOX fixed power quality analyser were used for measuring air velocity, air temperature and relative humidity, soil temperature at different points, and the energy consumption of the earth pipe cooling system respectively. The tests were conducted while an air conditioner of power input 1.01 kW and capacity output 5.70 kW was installed in both the earth pipe cooling and the standard room (the latter was not connected to any other system). The HEPC and VEPC measurements were conducted from April to June 2013 and July to September 2013 respectively.
4. CFD analysis 5.1. Soil temperature investigation Earth pipe cooling involves heat transfer where the earth transfers the heat to the buried underground pipes. Computational fluid dynamics (CFD) is a powerful and convenient method used for heat transfer studies over many years. A CFD model, namely the realisable k e turbulence model was used to analyse the heat transfer problem of the system. This is a reliable and more accurate model which is applicable for an extensive class of turbulent flows in heat transfer and industrial flow simulations. The turbulence model was selected for the thermal modelling as the flow passing through the buried pipes was turbulent (Reynolds Number, Re > 4000), where Reynolds number ranged from 8220 to 31,700. The model satisfies definite mathematical constraints on the Reynolds stresses and is consistent with turbulent flow physics. The problem was solved numerically by using the CFD code ‘‘Fluent in ANSYS 15.0”, which employs the finite volume method for discretisation of the computational domain. Air temperature has only been assessed using CFD as this study evaluates the thermal performance of the earth pipe cooling system.
The soil temperature analysis was carried out in April 2013, which involved the outdoor temperature and the soil temperature at different depths (0.61 m, 0.73 m, 0.85 m, 0.97 m and 1.10 m, where the PVC buried pipes were aligned). Data of April 2013 were used for this investigation as they have comparatively greater temperatures than the other months considered in this study. Average soil temperature was recorded from 10:00 am to 5:00 pm as maximum heat load occurred during this period. The lowest soil temperature distribution was observed at the depth of 1.10 m underground as shown in Fig. 6. All the underground soil temperatures were found to be lower than the ground surface and outdoor temperatures. Typically, soil temperature decreases on a hot day with increasing depth. Fig. 6
4.1. Solver approaches A two dimensional pressure-based-coupled solver was used for the numerical calculations of the model. It solves a coupled system of equations along with the momentum equations and the pressure-based continuity equation. The solver offers some additional advantages over a segregated or non-coupled approach, although it has some limitations. The coupled scheme allows a robust and efficient single phase execution for steady-state flows with high performance compared to the other solution schemes [38]. It is also necessary to use the coupled algorithm for transient flows, when the mesh quality is poor or the time step used in the solver is large. A study was carried out to check the effect of the grid variation and to establish the optimum mesh size which ensures consistent results for every mesh size.
Fig. 6. Soil temperature distribution at outdoor, ground surface and various depths.
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shows the outdoor temperature which varies from 22.4 °C to 26.4 °C, while the temperature at 1.10 m depth underground varies from 19.8 °C to 20.7 °C. The maximum temperature reduction that occurred between the outdoor and the soil at 1.10 m depth was 5.7 °C. This reduction assists to cool a room with the earth pipe cooling technology. Further work is being undertaken for the soil temperature at depths greater than 1.10 m. 5.2. Earth pipe cooling performance For measuring the performance of HEPC system, average air temperature, air velocity and energy consumption were recorded from both the rooms by turning off the VEPC system. The operation of VEPC room was turned off to consider this room as a standard room (not connected with earth pipe cooling or any other system) so that comparison can be made with the HEPC measurement and performance. Likewise, for VEPC performance, the data were recorded from both the rooms by turning off the HEPC system. Both the earth pipe cooling room temperatures were compared with the standard room temperature. The HEPC room temperature ranged from 22.61 °C to 25.32 °C, while the standard room temperature ranged from 23.27 °C to 25.72 °C as shown in Fig. 7a. The VEPC room temperature was also measured and varied from 23.48 °C to 26.39 °C, while the standard room temperature varied from 23.54 °C to 26.88 °C as shown in Fig. 7b. The maximum temperature reduction between the earth pipe cooling and the standard room was observed as 1.21 °C for the HEPC system on 14 May 2013 and 1.16 °C for the VEPC system on 16 August 2013. The velocity and temperature of air entering the earth pipe cooling room through the buried pipes were measured to evaluate their impact on room cooling performance. Cooled air from the buried pipes was supplied to the HEPC room with a velocity of 1.1 m/s and a temperature of 20.05 °C and to the VEPC room with a velocity of 3.40 m/s and a temperature of 21.50 °C at the inlet to the rooms. Relative humidity of the air also had a noticeable impact on the thermal comfort. It is still an essential variable to measure, especially in hot climates. Relative humidity of the rooms as shown in Fig. 8 was measured to assess its impact inside the rooms, which is summarised in Table 1. The average indoor relative humidity of both the HEPC and VEPC systems remained lower than 68%, which is acceptable for thermal comfort for the measured temperatures of 24.58 °C and 24.61 °C respectively. It was also found that the relative humidity of the rooms serviced by the earth pipe cooling systems was higher than that of the standard room due to the passing air entering those rooms.
Fig. 7a. Temperature profile in the HEPC and standard room.
Fig. 7b. Temperature profile in the VEPC and standard room.
Fig. 8a. Indoor relative humidity in the HEPC and standard room.
The measurements carried for both the HEPC and VEPC systems were compared with ASHRAE Standard 55-2010 using the adaptive method available in the Centre for the Built Environment (CBE) online Thermal Comfort Tool. The measured average temperature and relative humidity complies with ASHRAE Standard 55-2010 [39], ISO 7730 [40], Brown’s bioclimatic chart [41] and Givoni’s chart [42]. To validate the experimental data, one data set for daily maximum outdoor temperature was taken over the period of April 2013 and compared with the data recorded at 3 nearest bureau stations [43]. The daily maximum outdoor temperature was considered for this comparison because of the availability of the data for the same period. Moreover the data recorded at the bureau stations, namely Rockhampton Aero, Yeppoon the Esplanade, and Rosslyn Bay NTC AWS are considered as authentic as these data are collected by the Bureau of Meteorology, Australia. To compare the temperature profiles of these locations, a graph is plotted and displayed in Fig. 9. Fig. 9 shows a similar trend for each bureau station and the experimental shed at the sustainable precinct of Central Queensland University (where experiment of this study was conducted). It is seen that the temperature profile of the Rockhampton Aero is close to the profile of the sustainable precinct. The probable reason is that this station is the nearest from the precinct compared to other stations. The Rockhampton Aero (Latitude: 23.38°S, Longitude: 150.48°E) is 5.4 km away from the precinct
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Fig. 8b. Indoor relative humidity in the VEPC and standard room.
Table 1a Air temperature and relative humidity measured in the HEPC and standard room. Modelled rooms
HEPC Standard room
Indoor temperature (°C)
Outdoor temperature (°C)
Relative humidity (%)
Min
Max
Avg
Min
Max
Avg
Min
Max
Avg
22.61 23.27
25.32 25.72
24.58 24.90
20.62 –
36.19 –
29.77 –
32.12 36.72
86.37 81.95
65.24 63.14
Table 1b Air temperature and relative humidity measured in the VEPC and standard room. Modelled rooms
VEPC Standard room
Indoor temperature (°C)
Outdoor temperature (°C)
Relative humidity (%)
Min
Max
Avg
Min
Max
Avg
Min
Max
Avg
23.48 23.54
26.39 26.88
24.61 24.87
25.87 –
37.46 –
31.10 –
55.19 48.18
83.46 78.99
67.95 58.58
Fig. 9. Comparison of daily maximum outdoor temperature collected from this study and 3 bureau stations.
(Latitude: 23.32°S, Longitude: 150.52°E) while the Yeppoon the Esplanade (Latitude: 23.14°S, Longitude: 150.75°E) and Rosslyn Bay NTC AWS (Latitude: 23.16°S, Longitude: 150.79°E) are 34.2 km and 35.1 km away respectively. The maximum temperature amplitude of 3.5 °C was found between the precinct and Rosslyn Bay station whereas the minimum of 0 °C was found
between the precinct and the Rockhampton Aero. Although the experimental data of the precinct shows some variations (0–12.58%) with each station, the overall experimental results show a good agreement. The experimental data are to be compared with the numerical data in later section. Performance of the earth pipe cooling system was also calculated numerically using the simulation software Fluent. The flow and thermal variables for the boundary, and cell zone conditions were set on the boundary conditions of the model. The simulation results shown in Fig. 10 were obtained under the boundary conditions which are shown in Table 2. The average maximum room air temperature collected from the standard room was taken as the room temperature of the model. The simulation results show the average room temperatures for the earth pipe cooling system as 23.85 °C in the HEPC system and 23.05 °C in the VEPC system, which are respectively 1.06 °C and 1.82 °C lower than the standard room temperature. This indicates that the VEPC system performs better than the HEPC system. It is considered that the main reason behind this is that the buried underground pipes used in the VEPC system aligned at the depth where the soil temperature was cooler (Fig. 6) than that of the HEPC system. Moreover, the VEPC system provided sufficient air velocity (3.4 m/s) at the pipe inlet (inlet to the room) than that of the HEPC system (1.1 m/s). The higher inlet velocity in the VEPC system is because of the smaller pipe in length used in this system, where the fan can suck more air through this smaller pipe. As mentioned earlier, the VEPC system consists of 20 buried pipes each of
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Fig. 10a. Temperature distribution in the HEPC room.
Fig. 10b. Temperature distribution in the VEPC room.
Table 2 Parameters used in boundary conditions of the model. Parameters
HEPC
VEPC
Inlet velocity Inlet temperature Room temperature Air thermal conductivity Air density Specific heat of air Air viscosity
1.1 m/s 20.05 °C 24.90 °C 0.024 W/m K 1.204 kg/m3 1006.43 J/kg K 1.850387e05 kg/m s
3.40 m/s 21.01 °C 24.87 °C 0.024 W/m K 1.204 kg/m3 1006.43 J/kg K 1.850387e05 kg/m s
length 6.0 m (Fig. 5) whereas the HEPC system consists of the same number of buried pipes each of length 7.5 m (Fig. 4). The room temperatures obtained using simulation was compared with the corresponding experimental results at different heights (0.10 m, 0.25 m, 0.50 m, 0.75 m, 1.0 m, 1.25 m, 1.50 m, 1.75 m, 2.00 m and 2.25 m) along the centre of the room as summarised in Table 3.
The simulation results showed some deviations from the experimental results which are believed to be due to the measurement uncertainties in the experiments due to the instrument accuracy, reading, test planning, environment, condition, calibration, etc. Other reason is the uncertainties and errors in CFD simulations because of the auxiliary physical models [44] of turbulence model used in this study. Additionally, this may also occur due to the initial and boundary conditions, discretization and solution. However, the overall simulated results are in very good agreement with the corresponding experiments as shown in Fig. 11, and lie within 3.06–5.31% limits for HEPC system and 5.22–8.08% limits for the VEPC system.
5.3. Energy savings using earth pipe cooling technique The energy efficiency of the earth pipe cooling technique is evaluated in this section. The technique involves an 8 W fan and an air conditioner as discussed in Section 3. The air conditioners
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Table 3 Comparison in room temperature between experimental and numerical results. Height (m)
HEPC
VEPC
Experimental
Numerical
Differences (%)
Experimental
Numerical
Differences (%)
0.10 0.25 0.50 0.75 1.00 1.25 1.50 1.75 2.00 2.25
25.68 25.49 24.93 24.67 24.51 24.84 24.58 24.60 24.19 25.24
24.63 24.18 23.88 23.83 23.76 23.69 23.62 23.55 23.44 23.90
4.09 5.14 4.21 3.40 3.06 4.63 3.91 4.27 3.10 5.31
24.47 24.52 24.32 24.39 24.61 24.46 24.19 24.39 24.70 25.41
22.90 22.89 22.98 23.04 23.06 23.03 22.99 22.98 23.08 23.51
6.86 7.12 5.83 5.86 6.72 6.21 5.22 6.14 7.02 8.08
Fig. 11a. Numerical data plotted against experimental data for the HEPC room temperatures.
Fig. 12a. Energy consumption for the HEPC system.
Fig. 12b. Energy consumption for the VEPC system. Fig. 11b. Numerical data plotted against experimental data for the VEPC room temperatures.
in the earth pipe cooling system rooms and the standard rooms were set at 24 °C to provide optimal comfort and energy savings. The energy used by the fan and the air conditioning unit in both horizontal and vertical earth pipe cooling systems are displayed in Fig. 12. For measuring energy efficiency, energy data were recorded from midday to midnight from 14 June, 2013 to 22 June, 2013 for the VEPC system and from 12 July, 2013 to 20 July, 2013 for the HEPC system. The maximum energy was consumed at the hot peak hours of the day, whereas the minimum was consumed at the off peak hours at night as shown in Fig. 12.
From Figs. 12a and 12b, it can be seen that the HEPC average energy consumption ranges from 386.36 W h to 938.77 W h whereas the standard room energy consumption varies from 405.56 W h to 963.24 W h. The VEPC energy consumption ranges from 942.97.36 W h to 1637.70 W h whereas the standard room energy consumption varies from 999.08 W h to 1646.51 W h. In the HEPC room, the maximum energy consumption of 938.77 W h was observed on 20 July 2013 at 12:00 PM, while in the VEPC room the maximum energy consumption of 1637.70 W h was observed on 14 September 2013 at 12:20 PM. Energy consumption in those two days – 20 July 2013 in the HEPC and standard room, and on 14 September 2013 in the VEPC and standard room – are shown on an hourly basis in Fig. 13.
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Fig. 13a. Energy consumption on 20 July 2013 in HEPC and standard room.
Fig. 13b. Energy consumption on 14 September 2013 in VEPC and standard room.
The average maximum difference in energy consumption between the earth pipe cooling systems and standard room was observed during hot peak hours of the day whereas the minimum was observed during late night and early morning. Rockhampton weather gets warmer in the day, and cooler at late night and early morning, which is the main reason behind this trend of energy
consumption. Moreover, in the earth pipe cooling systems, the earth (soil) acts as a heat sink in warm weather to produce cooler air and as a heat source in cool weather to generate hot air into the ground. Due to the heat generation during late night and early morning, more energy is consumed by the systems, and therefore the amount of energy consumption is nearly the same as that for the standard room during this period. Typically, air conditioning energy consumption varies depending on the outdoor air temperature. Average energy consumption for the earth pipe cooling technology is summarised in Table 4. The table shows an average hourly energy savings of 24.29 W and 42.50 W using the HEPC and VEPC systems respectively. It also shows that the HEPC system has the potential to save a maximum of 66.20 W/h, whereas the VEPC system has the potential to save up to 98.92 W/h. Based on these results, energy savings of a maximum of 579.91 kW and 866.54 kW could be achieved per year by adopting the HEPC and VEPC systems respectively. A fair amount of energy cost can be saved due to the reduction of energy consumption shown in Table 4. There are several energy usage charging categories used in Australian buildings. The most common category, namely tariff 11, is used for residential customers in Australia [45]. The energy cost for tariff 11 is summarised in Table 5. It includes the cost of energy usage at a flat rate throughout the day and night, plus a daily supply charge as a service fee. The daily supply charge used in tariff 11 is a fixed cost, whereas all energy consumption is a variable cost as the energy consumption rate is varied with the energy consumers. Therefore, the total energy cost for a building can only be calculated based on the energy used in the building. According to the price of tariff 11, total energy cost savings have been calculated and are tabulated in Table 6. Table 6 shows annual average energy savings of $59.40 in the HEPC system and $103.93 in the VEPC system. It is also found that maximum annual savings of $161.89 (7.93%) and $241.90 (8.82%) can be made using the HEPC and VEPC systems respectively. The savings gained are for a single room only of dimensions 5.63 m 2.14 m 2.26 m. Hence, there is an opportunity to make substantial savings using such earth pipe cooling technology for complete buildings.
6. Error analysis Instrument selection, reading, test planning, observation, environment, condition and calibration can produce errors and uncertainties. The uncertainty analysis is needed to measure the experiment accuracy. Uncertainties in percentage of various
Table 4 Hourly energy savings using earth pipe cooling technology. Modelled rooms
HEPC VEPC
Hourly energy consumption using earth pipe cooling system (W h)
Hourly energy consumption without using earth pipe cooling system (W h)
Hourly energy savings (W h)
Min
Max
Avg
Min
Max
Avg
Min
Max
Avg
Max (%)
386.36 942.97
938.77 1637.70
651.47 1279.02
405.56 999.08
963.24 1646.51
668.11 1326.60
11.52 12.24
66.20 98.92
24.29 42.50
7.93 8.82
Table 5 Energy costs for residential customers. Tariff 11 Residential
GST inclusive from 1 July 2014 (carbon price removed)
GST inclusive from 1 July 2014 (includes carbon price)
GST inclusive from 1 July 2013 (includes carbon price)
All consumption (cents per kWh) Daily supply charge (cents per day)
27.916 91.755
30.817 91.755
29.403 55.241
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Table 6 Energy cost savings using earth pipe cooling systems (costs in Australian Dollars after removing carbon price). Modelled rooms
HEPC VEPC
Weekly
Monthly
Yearly
Min
Max
Avg
Min
Max
Avg
Min
Max
Avg
Max (%)
0.54 0.57
3.10 4.64
1.14 1.99
2.32 2.46
13.30 19.88
4.88 8.54
28.17 29.93
161.89 241.90
59.40 103.93
7.93 8.82
Table 7 Instruments used in the experiment and their accuracy, range and percentage uncertainties. Instruments
Parameters
HOBO U10-003 Reed Vane Anemometer BTM-4208SD 12 CH temperature recorder HOBO U10-003
Accuracy
Air temperature Air velocity Soil temperature Relative humidity
±0.4 °C ±(2% reading + 0.2 m/s) ±(0.4% + 0.5) ±3.5%
parameters such as air temperature, air velocity, relative humidity and energy consumption were calculated for both HEPC and VEPC systems using the instruments’ uncertainties as shown in Table 7. The uncertainty analysis in percentage is performed by the following method [46]. Total uncertainty of the experiment for the HEPC system is
¼ square root of ½ðuncertainty of room temperatureÞ
2
2
þ ðuncertainty of air velocityÞ
þ ðuncertainty of soil temperatureÞ
2
þ ðuncertainty of relative humidityÞ
¼ square root of ½ð0:02Þ2 þ ð0:2Þ2 þ ð0:02Þ2 þ ð2:28Þ2 þ ð0:01Þ2
+0.02 +0.27 +0.02 +2.38
to to to to
0.02 0.27 0.02 2.38
VEPC system. This translates to maximum annual savings of $161.89 (7.93%) using the HEPC system and $241.90 (8.82%) using the VEPC system. This investigation has established that the earth pipe cooling technique is not only energy efficient, but also cost-effective, and has less negative and environmental effects compared to other existing techniques. The findings of this study are recommended to be utilised as guidelines by the regulators, manufacturers and inhabitants for the deployment of passive cooling techniques in hot humid climates.
The authors gratefully acknowledge the support provided by Ergon Energy, Australia for supplying passive air cooling system and the installation cost of the measuring tools. The first author S F Ahmed is grateful to Central Queensland University, Australia for the international postgraduate research award (IPRA) to support this study. The authors are also grateful to Tim McSweeney for providing valuable suggestions and proofreading support that improved the quality of this paper.
¼ 2:29% Total uncertainty of the experiment for the VEPC system is
¼ square root of ½ðuncertainty of room temperatureÞ
2
2 2
þ ðuncertainty of soil temperatureÞ
þ ðuncertainty of relative humidityÞ
2
References
2
þ ðuncertainty of active energyÞ 2
VEPC
+0.02 to 0.02 +0.2 to 0.2 +0.02 to 0.02 +2.28 to 2.28
Acknowledgements
2
2
20 °C to 70 °C 0.4 m/s to 30.0 m/s 50 °C to 400 °C 25% to 95%
Percentage uncertainties HEPC
2
þ ðuncertainty of active energyÞ
þ ðuncertainty of air velocityÞ
Range
2
2
2
¼ square root of ½ð0:02Þ þ ð0:27Þ þ ð0:02Þ þ ð2:38Þ þ ð0:03Þ ¼ 2:40%
7. Conclusion Passive cooling techniques are the least expensive methodology for maximising the thermal comfort of building occupants with the lowest environmental impact. They offer a great potential for energy savings worldwide for hot and humid climates as they can reduce the air-conditioning load of many buildings while producing virtually no greenhouse gas emissions. Recent advancements on the techniques can help to enhance the quality of the indoor environment of buildings. The case study carried out on earth pipe cooling technology in a hot humid subtropical climate of Rockhampton, Australia involving two different piping layouts, namely the horizontal and vertical, reduced the room temperatures by 1.05 °C and 1.82 °C respectively. This temperature reduction saved an annual maximum energy of 579.91 kW for the HEPC system and 866.54 kW for the
[1] EIA. Energy information administration. International energy outlook; 2013.
S.F. Ahmed et al. / Energy Conversion and Management 106 (2015) 815–825 [11] Becker R, Paciuk M. Inter-related effects of cooling strategies and building features on energy performance of office buildings. Energy Build 2002;34:25–31. http://dx.doi.org/10.1016/S0378-7788(01)00081-0. [12] Chowdhury AA, Rasul MG, Khan MMK. Modelling and analysis of air-cooled reciprocating chiller and demand energy savings using passive cooling. Appl Therm Eng 2009;29:1825–30. http://dx.doi.org/10.1016/j.applthermaleng. 2008.09.001. [13] Ahmed SF, Khan MMK, Amanullah MTO., Rasul MG, Hassan NMS. Performance analysis of vertical earth pipe cooling system for subtropical climate. In: International conference on clean energy, Istanbul, Turkey; 2014. p. 691–700. [14] Ahmed SF, Khan MMK, Rasul MG, Amanullah MTO, Hassan NMS. Comparison of earth pipe cooling performance between two different piping systems. Energy Procedia 2014;61:1897–901. http://dx.doi.org/10.1016/j.egypro.2014. 12.237. [15] Ahmed SF, Khan MMK, Amanullah MTO, Rasul MG, Hassan NMS. Thermal performance analysis of earth pipe cooling system for subtropical climate. In: 12th international conference on sustainable energy technologies, Hong Kong; 2013. p. 1795–803. [16] Lee KH, Strand RK. The cooling and heating potential of an earth tube system in buildings. Energy Build 2008;40:486–94. http://dx.doi.org/10.1016/j.enbuild. 2007.04.003. [17] Ghosal M, Tiwari G. Modeling and parametric studies for thermal performance of an earth to air heat exchanger integrated with a greenhouse. Energy Convers Manage 2006;47:1779–98. http://dx.doi.org/10.1016/j.enconman.2005. 10.001. [18] Wu H, Wang S, Zhu D. Modelling and evaluation of cooling capacity of earth– air–pipe systems. Energy Convers Manage 2007;48:1462–71. http://dx.doi. org/10.1016/j.enconman.2006.12.021. [19] Bisoniya TS, Kumar A, Baredar P. Experimental and analytical studies of earth– air heat exchanger (EAHE) systems in India: a review. Renew Sustain Energy Rev 2013;19:238–46. http://dx.doi.org/10.1016/j.rser.2012.11.023. [20] Mihalakakou G, Santamouris M, Asimakopoulos D. Modelling the thermal performance of earth-to-air heat exchangers. Sol Energy 1994;53:301–5. http://dx.doi.org/10.1016/0038-092X(94)90636-X. [21] Gallero FJG, Maestre IR, Gómez PÁ, Blázquez JLF. Numerical and experimental validation of a new hybrid model for vertical ground heat exchangers. Energy Convers Manage 2015;103:511–8. [22] Bansal V, Misra R, Agrawal GD, Mathur J. Performance analysis of earth–pipe– air heat exchanger for summer cooling. Energy Build 2010;42:645–8. [23] De Paepe M, Willems N. 3D unstructured modeling technique for groundcoupled air heat exchangers. In: Proceedings of the 7th international world conference CLIMA2000, Napoli. CD-rom-uitgave, paper ID 276; 2001. 15 pp. [24] Al-Ajmi F, Loveday D, Hanby VI. The cooling potential of earth-air heat exchangers for domestic buildings in a desert climate. Build Environ 2006;41:235–44. http://dx.doi.org/10.1016/j.buildenv.2005.01.027. [25] Mongkon S, Thepa S, Namprakai P, Pratinthong N. Cooling performance assessment of horizontal earth tube system and effect on planting in tropical greenhouse. Energy Convers Manage 2014;78:225–36. http://dx.doi.org/ 10.1016/j.enconman.2013.10.076. [26] Ahmed SF, Khan MMK, Amanullah MTO, Rasul MG, Hassan NMS. Performance evaluation of hybrid earth pipe cooling with horizontal piping system. Thermofluid modeling for energy efficiency applications. USA: Elsevier; 2015. p. 1–30. http://dx.doi.org/10.1016/B978-0-12-802397-6.00001-4. [27] Mihalakakou G, Santamouris M, Asimakopoulos D, Tselepidaki I. Parametric prediction of the buried pipes cooling potential for passive cooling
[28]
[29]
[30] [31]
[32]
[33]
[34]
[35]
[36]
[37]
[38] [39] [40]
[41] [42] [43] [44] [45]
[46]
825
applications. Sol Energy 1995;55:163–73. http://dx.doi.org/10.1016/0038092X(95)00045-S. Benhammou M, Draoui B, Zerrouki M, Marif Y. Performance analysis of an earth-to-air heat exchanger assisted by a wind tower for passive cooling of buildings in arid and hot climate. Energy Convers Manage 2015;91:1–11. Zarrella A, Emmi G, De Carli M. Analysis of operating modes of a ground source heat pump with short helical heat exchangers. Energy Convers Manage 2015;97:351–61. Shen P, Lukes JR. Impact of global warming on performance of ground source heat pumps in US climate zones. Energy Convers Manage 2015;101:632–43. Krarti M, Kreider JF. Analytical model for heat transfer in an underground air tunnel. Energy Convers Manage 1996;37:1561–74. http://dx.doi.org/10.1016/ 0196-8904(95)00208-1. Santamouris M, Mihalakakou G, Balaras C, Argiriou A, Asimakopoulos D, Vallindras M. Use of buried pipes for energy conservation in cooling of agricultural greenhouses. Sol Energy 1995;55:111–24. Zaki AK, Amjad A-M, Almssad A. Cooling by underground earth tubes. In: 2nd PALENC conference and 28th AIVC conference on building low energy cooling and advanced ventilation technologies in the 21st century, Crete island, Greece; 2007. Goswami D, Biseli K. Use of underground air tunnels for heating and cooling agricultural and residential buildings. Fact sheet EES 78. Fact Sheet EES1993.