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1. Thesis Presentation on Performance analysis of earth air tunnel assisted green house Department of Mechanical Engineering Motilal Nehru National Institute of Technology Allahabad, Prayagraj (U.P.) - 211004, India SUPERVISOR- PRESENTED BY- Dr. Ashwini Kumar yadav Samar Singhal Prof. Ravi Prakash (2020RME18)

2. Introduction This work presents a one-dimensional steady-state control volume method code of earth air tunnel heat exchanger (EAHE) to simulate its performance. It couples both heat and mass transfer. By taking advantage of geo-thermal energy, energy consumption for heating and cooling needs can be reduced. The code can be utilized to design and optimize the earth air tunnel heat exchanger. After obtaining the results from the code, an actual two dimensional model of a greenhouse building is simulated to study temperature distribution etc.

3. Introduction Geothermal energy is a huge reservoir of renewable energy source that can be exploited for heating and cooling purposes. An EAHE is a thermal heat exchanger with tubes buried in the ground which can extract energy from the soil. By utilising geo-thermal energy, energy consumption by conventional methods can be reduced which causes green-house emissions and other pollutants. One of the main uses of EAHE is heating or cooling of greenhouse to avoid the adverse effects on crop yield, cultivation time, quality and quantity.

4. Introduction Recently a military stand off between India and china is occurring at Ladakh, where the temperature is subjected to -30 ℃ to 40 ℃. So 50,000 troops from Indian side has been deployed. Smart facilities have been built for the troops to withstand the extreme conditions of weather provided with heating systems. Using EAHE is an effective way to meet the cooling requirements of such high altitudes structures housing. Also EAHE can be used in big commercial buildings for HVAC applications.

5. Literature Review Model proposer & year Summary Remarks Milhalakakou 1994 transient, implicit, numerical model based on coupled and simultaneous transfer of heat and mass into the soil and the pipe. The basic programme has been developed inside the TRNSYS environment.. Round single tube only Condensation not predicted De Paepe , 2002 Transient 3-D FVM Implicit No condensation Fredrik, 2002 Cartesian coordinate system Rectangular tube No condensation Wagner, 2002 Cylindrical and cartesian coordinates No condensation Athienitis , 2001 Steady state explicit Exact solution No condensation Benkert , 1998 Steady state FDM No condensation Schiller, 1982 1-D transient FDM Unrealistic prediction No condensation 5

6. Literature Review Model proposer & year Summary Remarks Santamouris , 1986 1-D FDM No condensation Levit , 1989 1-D steady state Explicit FDM No condensation Sodha et al, 1984 1-D Steady stae Explicit FDM No condensation Wagner, 2002 Cylindrical and cartesian coordinates No condensation Athienitis , 2001 Steady state explicit Exact solution No condensation Benkert , 1998 Steady state FDM No condensation Schiller, 1982 1-D transient FDM Unrealistic prediction No condensation 6

7. Literature Gap Most of the research papers ignored the latent heat part which arises due to condensation. Hence were unable to predict the humidity values of the air in EAHE. In this paper an approach is developed on how to apply the simulations to real word problems with greenhouse simulation.

8. Mathematical model of EAHE Assumptions made during the development of mathematical model: The EAHE is of uniform cross-section area and thermal characteristics. The soil properties are isotropic. There exists a perfect contact between soil and tube. Thermal resistance due to tube thickness is negligible. Air is incompressible and its thermal properties are constant. Air is well mixed in the tube with no temperature stratification.

9. Mathematical model of EAHE Fig 1 . Control volume method and energy balance in each volume

10. Mathematical model of EAHE Heat transfer for each control volume with no condensation: Heat transfer coefficient:        

11. Mathematical model of EAHE Nusselt no. for heating Nusselt no. for cooling Thermal conductivity of air:            

12. Mathematical model of EAHE With temperature at each control volume When the air temperature in a control volume drops to corresponding dew point, condensation occurs. Moisture conservation equation: Heat and mass transfer coupled :       ρ ( w a – w sat )      

13. Flowchart of Code for EAHE Fig 2 flowchart of the code

14. Validation To verify our EAHE code, the results from the code were validation with two sets of published experimental data by Benkert and Goswami. Parameters for Benkert and Goswami experiments are shown in table 1. Inlet Temperature in Celsius Diameter in meter Undisturbed soil Temperature Length in meter Relative humidity at inlet in % Velocity of air at inlet in m/s Benkert 25.3 0.25 12.8 42 50 2 Goswami 32.2 0.3 21.1 25 59 1.533 Table 1

15. Validation With Benkert’s Experiment Fig 3 Comparison of temperatures and specific humidity

16. Validation With Goswami’s Experiment Fig 4 Comparison of temperatures and specific humidity

17. Validation Our results from code agrees very well with the published results and are within acceptable limits. Deviations can be further reduced by employing 2-D or 3-D model of EAHE. But it increases computational time and complexity. A trade off between computational time and accuracy has to be considered according to practical scenario.

18. Simulation of a 2-D greenhouse With EAHE outlet temperature obtained from code, it is used to model a greenhouse of a practical dimension and simulate it for performance. Ansys Fluent Academic was used in our case to simulate greenhouse. Governing equations for cfd simulations are: Continuity :    

19. Simulation of a 2-d greenhouse Momentum Equation: Energy equation:        

20. Dimensions of Greenhouse Figure 5 Greenhouse dimensions in mm

21. Meshing Parameters Meshing information is shown in Table below. Domain Nodes Elements Air 27122 13267 Table 2 Fig 6 mesh of domain

22. Mesh Quality Skewness which should be less than 0.9. Fig 7 Showing skewness of elements

23. Mesh Quality Orthogonality which should be close to 1 as much possible. Fig 7 Showing orthogonality of elements

24. Boundary conditions Velocity m/s Temperature ℃ type EATHE inlet 1.533 25.28 Velocity inlet Outside air inlet 1 45 Velocity inlet ventilation n/a n/a Pressure-outlet Table 3

25. Results and Discussions A steady state simulation was performed on ansys for which the solution converges around 1400 iterations. Fig 8 Residuals

26. Results and discussions Temperature distribution Fig 9 Temperature

27. Results and discussions Velocity Contours Fig 10 Velocity contours

28. Results and discussions Velocity streamlines Fig 11 Streamlines

29. Results and discussions Velocity vectors Fig 9 Vectors

30. Results and discussions In the above simulation, inlet of EAHE , outlet for ventilation can be optimised for more uniform temperature distributions. Their positions can be placed as such there are very less recirculation zones to ensure proper heating or cooling. Specific humidity drops along the length of the EAHE.

31. Conclusions One-dimensional code of EAHE has been developed which is capable of predicting the performance of EAHE. Both sensible heat and latent heat was taken into account. However non-linearity was not handled in the above code as properties of air was taken constant. In cooling, the specific humidity drops if the tube surface temperature is below dew point temperature of air. Which results in increase in relative humidity at outlet. For that dehumidifier is advisable to use.

32. Scope of future work This was a one dimensional control volume method, but for more accurate results two dimensional or three dimensional modelling is required. More accurate Nusselt’s and Sherwood relations can be used to predict a higher accuracy of heat and mass transfer coefficients. Change in air properties with temperature can be taken into account for a higher accuracy. Effects of design parameters of EAHE on its performance can be studied to optimise its design. Same way design of greenhouse can be optimised for a more uniform temperature distribution.

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34. References [8] Hollmuller P. Analytical characterisation of amplitude-dampening and phase-shifting in air/soil heat-exchangers. Int J Heat Mass Transf 2003;46: 4303–17. [9] Paepe M, Janssens A. Thermo-hydraulic design of earth to air heat exchanger. Energy Build 2003;35:389–97. [10] Kumar R, Ramesh S, Kaushik SC. Performance evaluation and energy conservation potential of earth–air–tunnel system coupled with non-airconditioned building. Build Environ 2003;38:807–13. [11] Bansal V, Misra R, Agrawal GD. Performance analysis of earth–pipe–air heat exchanger for summer cooling. Energy Build 2010;42(5):645–8. [12] Zhang J, Haghighat F. Simulation of earth to air heat exchanger in hybrid ventilation systems. In: Ninth international IBPSA conference, Montreal, Canada, August 15–18; 2005. [13] Ghosal MK, Tiwari GN, Srivastava NSL. Thermal modeling of a greenhouse with an integrated earth to air heat exchanger: an experimental validation. Energy Build 2004;36:219–27. [14] Yu Y, Li H, Niu F, Yu D. Investigation of a coupled geothermal cooling system with earth tube and solar chimney. Appl Energy 2014;114:209–17. [15] ASHRAE handbook- HVAC systems and equipment. Atlanta, Geogia : American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc.; 2002. [ ch. 11]. [16] Zhao M. M. Sc. thesis: simulation of earth-to-air heat exchanger systems. Concordia University; 2004.

35. References [17] Singh SP. Optimization of earth–air tunnel system for space cooling. Energy Convers Manage 1994;35(8):721–5. [18] Lemmon EW, Jacobsen RT. Viscosity and thermal conductivity equations for nitrogen, oxygen, argon, and air. Int J Thermophys 2004;25(1):21–69. [19] Bergman TL, Lavine AS, Incropera FP, DeWitt DP. Fundamentals of heat and mass transfer. Wiley Press; 2011.

36. Appendix l = 25; %% length of the tunnel d = 0.3; %% diameter of the tunnel v = 1.533; %% velocity of the air ta = 32.2; %% dry bulb temperature of inlet air cp = 1005; %% specific heat capacity of the air k = 0.43; %% thermal conductivity of air rho = 1.23; %% density of air mu = 1.81*10^-5; %% dynamic viscosity of air f = 1.8; %% enhancement coefficent n = 100; %% no. of volumes to be divided into tt(1:n+1) = 0; rh(1:n+1) = 0; w(1:n+1) = 0; tavg (1:4) = 0; rh(1) = 59/100; %% relative humidity at inlet c1 = -5.80022006*10^3; c2 = -5.516256; c3 = -4.8640239*10^-2; c4 = 4.1764768*10^-5; c5 = -1.4452093*10^-8; c6 = 6.5459673; pt = 101.325; %% atmosphereic pressure re = (rho*v*d)/mu ; %% reynold's no. delx = l/n; ts = 21.1; %% soil surface temperature ts = ts + 273.15; mass = ((d^2)*v*rho*22)/(28); %% mass flow rate ap = (22/7)*(d); %% cross section perimeter x = 0; tt (1) = ta + 273.15;

37. Appendix pvs = exp( c1/ tt (1) + c2 + c3* tt (1) + c4* tt (1)^2 + c5* tt (1)^3 + c6*log( tt (1))); %%saturated vapur pressure pv = rh(1)* pvs ; w(1) = 0.622*( pv /( pt-pv )); %% humidity ratio wsat = 0.622*( pvs /( pt-pvs )); pvs = exp( c1/ tt (1) + c2 + c3* tt (1) + c4* tt (1)^2 + c5* tt (1)^3 + c6*log( tt (1))); %% dew point temperature dpt = (4030*(tt(1) - 273.15 + 235))/ (4030 -(tt(1) - 273.5 + 235)*log(rh(1))) - 235; disp ( dpt ); for j=1:n-1 k= 0.02442 + (0.6992* tt (j))*(10^-4); %% thermal conductivity of air pr = (mu*cp)/(k); %% prandtl no. if tt (1) < ts nus = 0.023*(re^0.8)*(pr^0.4); %% nusselt no. heating else nus = 0.023*(re^0.8)*(pr^0.3); %% nusselt no. cooling end ha = (k*nus)/d; %% heat transfer coefficent hm = ha/(rho*cp); %% mass transfer coefficent by lewis relation u = (ap*f*ha* delx )/(mass*cp); %% total heat transfer coefficent a = (mass*cp)/(ap*ha*f); for i =1:4 tavg ( i ) = ts - ( ts-tt (j))*exp(-x/a); x = x + delx /4; end temp = ( tavg (1) + tavg (2) + tavg (3) + tavg (4) )/4; if temp < ( dpt +273.15) pvss = exp( c1/(dpt+273.15) + c2 + c3*(dpt+273.15) + c4*(dpt+273.15)^2 + c5*(dpt+273.15)^3 + c6*log((dpt+273.15))); wsats = 0.622*( pvss /( pt-pvss )); w(j+1) = (mass*w(j) -ap* delx *hm*rho*(w(j) - wsats ))/mass;

38. Appendix tt (j+1) = (mass*cp* tt (j) - ap* delx *f*ha*(temp - ts ) - mass*2500*(w(j) - w(j+1)))/(mass*cp); disp ( 'activated' ); pvs = exp( c1/ tt (j+1) + c2 + c3* tt (j+1) + c4* tt (j+1)^2 + c5* tt (j+1)^3 + c6*log( tt (j+1))); wsat = 0.622*( pvs /( pt-pvs )); rh(j+1) = w(j+1)/(wsat); dpt = (4030*( tt (j+1) - 273.15 + 235))/ (4030 -( tt (j+1) - 273.5 + 235)*log(rh(j+1))) - 235; disp ( dpt ); else tt(j+1) = tt(j) - u*(temp -ts); pvs = exp( c1/ tt (j+1) + c2 + c3* tt (j+1) + c4* tt (j+1)^2 + c5* tt (j+1)^3 + c6*log( tt (j+1))); wsat = 0.622*( pvs /( pt-pvs )); w(j+1) = w(j); rh(j+1) = w(j+1)/(wsat); disp ( dpt ); end end tt(n+1) = tt(n); w(n+1) = w(n); rh(n+1) = rh(n); tt = tt - 273.15; %% celcius conversion xdata = 0:delx:l; rr = xlsread ( 'goswami.xlsx' ); rr = rr '; rrx = rr (1 , :); rry = rr (2 , :);

39. Appendix plot( rrx , rry ); title( ' Temperature vs length ' ); xlabel ( 'length m' ); ylabel ( 'Temperature celcius ' ); legend({ ' simulated' , ' experiement ' }); hold off figure; plot( xdata,w ); title( ' Specific humidity vs length ' ); xlabel ( 'length m' ); ylabel ( 'kg/kg of air' );

40. Thank You