• Energy Research

Abstract One of the problems in operating earth air tunnel heat exchangers (EATHE) is the collection/rejection of heat from buried pipe to nearby sub-soil which alters the soil temperature, and, in turn, adversely affects the performance of EATHE system with time. This problem is more pronounced with the soil having poor thermal conductivity and little variation in moisture content. Extent thermal saturation and time required for self-recovery of soil around the buried pipe has great importance to ensure efficiency and usefulness of an EATHE system. However, there are hardly any studies on the self-recovery of soil during intermittent operation mode. The objective of present study is to compare the thermal saturation and self-recovery ability of soil in continuous and intermittent operation modes. The numerical results have been validated experimentally through a full scale setup. It is found that the soil temperature can be recovered in both intermittent and continuous operation modes by employing natural heat conduction (heat taken away from the pipe's immediate vicinity) and convection (heat taken away by purge air passing through EATHE in night hours when ambient is lower than soil) respectively.

AbstractThis paper presents effect of thermo-physical properties of soil on performance of an Earth Air Tunnel Heat Exchanger (EATHE). The analysis has been carried out using a validated three-dimensional, transient numerical model for three different types of soil. The governing equations, based on the k–ε model and energy equation were used to describe the turbulence and heat transfer phenomena, are solved by using finite volume method. Comparisons were made in terms of temperature drop, heat transfer rate and COP of the EATHE system by operating it continuously for 12 h duration. The study reveals that each soil exhibits different rate of heat dissipation and thermal saturation over a period of continuous operation, which adversely affects the performance of EATHE. Dissipation of heat from the EATHE pipes to its surrounding soil and subsequently to the outer subsoil region is mainly found to be depending upon the thermal conductivity of soil; even of their thermal diffusivity is of different order.

Abstract Thermal performance of Earth Air Tunnel Heat Exchanger (EATHE) system is found to be highly dependent on thermal properties of soil and soil with poor thermal properties require longer buried pipe for desired heat exchange between air and sub-soil. In densely populated country like India, EATHE systems are rarely used due to the limited ground space (higher aspect ratio in terms of length) in most of the residential and commercial applications. Therefore, in this paper, problem of space limitation for the installation of straight EATHE system is attempted by proposing new spiral shaped EATHE system. The developed EATHE systems were used to determine cooling and heating potential individually and COP were 5.94 and 6.24 in summer; 1.92 and 2.11 in winter for straight and spiral respectively. A transient numerical model has been developed to investigate the performance of both systems. The COP were 4.23 and 4.48 in summer; 5.0 and 5.16 in winter for straight and spiral respectively. The experimental and numerical results suggested that the thermal performance of these two systems are comparable can be interchanged based on available site conditions.

Abstract Energy crisis and environmental problems require a new sustainable energy supply option that uses renewable energies and is economic at the same time. Solar Thermal Power (STP) generating systems are proven renewable energy technologies and often a very cost effective way to produce electricity from solar radiation. In this paper, the potential and the cost-effectiveness of a parabolic trough collector (PTC) type solar thermal power plant for meeting the energy demand of garment zone at Sitapura industrial area, Jaipur is analyzed. In this, the present energy demand of garment zone, design of proposed solar power plant and economic evaluation of the power plant has been carried out. The estimated energy requirement of the selected garment zone was 2.21 MW for year 2011. To meet the present energy demand and expected increase in the demand a PTC type solar power plant of 2.5 MW capacity is proposed, which require about 15.76 acres of land area and has 256 solar collector elements. Looking to the scarcity and cost of the land near the city, off-site proposal of the power plant has also been considered and compared with on-site option. Four economic scenarios were analyzed for the proposed system and their financial performance is evaluated. For the onsite solar power plant internal rate of return is 19.21%, NPV at 10% discount rate is 372.77 million INR, simple payback period is 5 years and discounted payback period @10% is 7 years and 4 months, while for the off-site power plant IRR is 27.85%, NPV is 550.55 million INR, simple payback period is 3 years and 6 months and discounted payback period is 4 years and 7 months. Levelised cost of energy (LCOE) for onsite power plant is Rs 9.41/kWh and for offsite power plant is Rs 6.89/kWh.

Abstract Thermal performance of earth air tunnel heat exchanger has been investigated under transient conditions for three different soil conditions considering three operating modes. In first operating mode EATHE works continuously for 12 h, in second mode it works for 60 min and then it is off for 20 min and in the last mode EATHE runs for 60 min and remains off for 40 min. In second and third mode EATHE is operated for 12 h intermittently. CFD model was developed in GAMBIT (version 2.2.3), simulated in FLUENT (version 6.3) and then validated with experimental data. CFD analysis has been carried out using a three-dimensional transient numerical model. Air temperature drop and heat transfer between air and soil is calculated considering different thermal properties of soils. Higher thermal conductivity (1.28 W m −1 K −1 ) soil should be used in first mode of 12 h continuous operation as it improves the performance by 5.04% as compared to least thermal conductivity soil and soil with poor thermal conductivity (0.52 W m −1 K −1 ) shows performance improvement of 1.81% when operated in intermittent operation as compared to continuous running.

Abstract An adaptive thermal comfort study was carried for assessing the thermal comfort requirements of students working in naturally ventilated (NV) university workshop building in the composite climate of India over two seasons (Autumn and Winter). 1332 complete surveys, 516 and 816 from autumn and winter season, respectively were collected and analyzed. A maximum of 70% of subjects were found voting comfortable when indoor temperature varied from 28°C to 32°C. During the autumn season, the mean comfort temperature was about 5°C higher than the winter season. Also, female subjects had about 1.5°C higher comfort temperature compared to male subjects. Further, an attempt has been made to present an adaptive model for university students engaged in activity corresponding to machine work. The results were also compared with international thermal comfort standards. The results showed that students are more sensitive towards the fluctuations of outdoor conditions compared to the subjects in office and residential buildings. Moreover, subjects have shown better thermal adaptability towards prevailing mild cold climatic conditions through behavioural and clothing adjustments.

Abstract One of the problems in operating earth air tunnel heat exchangers (EATHE) for cooling in summer, with soil having high specific heat and low moisture content is accumulation of heat around the pipe. The low rate of heat dissipation due to conduction restricts the performance of EATHE over subsequent years. In the present paper, numerical simulations have been performed to investigate the thermal performance and soil temperature during summer operation in Jaipur to estimate extent of soil degradation. The simulation result indicates that by the end of summer, the soil leads to thermal saturation which in turn, may render it unusable for next summer. This scenario demands for heat removal through force convection. Three strategies namely, night purging during summer operation, day operation during winter and night operation during winter were attempted to estimate extent of soil recovery. Simulation results show that the average COPs for summer, summer with night purging, winter day and winter night operation mode are 4.23, 3.68, 5.01, and 6.65 respectively. It was found that advantage of night purging is less than energy required to run blower for night purging. However, winter day/night operation offers space heating and better soil for next summer.