Scholarly article on topic 'Supercritical Rankine Cycle Coupled with Ground Cooling for Low Temperature Power Generation'

Supercritical Rankine Cycle Coupled with Ground Cooling for Low Temperature Power Generation Academic research paper on "Earth and related environmental sciences"

CC BY-NC-ND
0
0
Share paper
Academic journal
Energy Procedia
Keywords
{"Low temperature power generation" / "Supercrtitical organic Rankine cycle" / Earth-air-heat-exchanger}

Abstract of research paper on Earth and related environmental sciences, author of scientific article — Rachana Vidhi, D. Yogi Goswami, Elias Stefanakos

Abstract This paper presents an application of an earth-air-heat-exchanger (EAHE) as condenser in low to medium temperature power generation plants. A supercritical Rankine cycle (SRC) utilizing organic refrigerants as working fluids was used as the power cycle for the plant. The heat source temperature was varied from 125-1750C. The condenser was coupled to an EAHE system buried at a depth of 2 m under the surface of the earth. Its effect on the power cycle efficiency over a period of six months has been studied. It was observed that the soil temperature 10cm from the surface (horizontal direction) of the underground pipe increased by almost 20C during this time. This temperature change decreased with distance from the pipe. The soil temperature profile varied with time, distance from the pipe and location along the length of the pipe. The efficiency of the SRC increased by 1% and the daily fluctuations were reduced when EAHE was used.

Academic research paper on topic "Supercritical Rankine Cycle Coupled with Ground Cooling for Low Temperature Power Generation"

(S)

CrossMark

Available online at www.sciencedirect.com

ScienceDirect

Energy Procedía 57 (2014) 524 - 532

2013 ISES Solar World Congress

Supercritical Rankine cycle coupled with ground cooling for low

temperature power generation

Rachana Vidhia, D Yogi Goswamia, Elias Stefanakosa

Clean Energy Research Center, University of South Florida, 4202 E Fowler Avenue, Tampa 33620 USA

Abstract

This paper presents an application of an earth-air-heat-exchanger (EAHE) as condenser in low to medium temperature power generation plants. A supercritical Rankine cycle (SRC) utilizing organic refrigerants as working fluids was used as the power cycle for the plant. The heat source temperature was varied from 125-1750C. The condenser was coupled to an EAHE system buried at a depth of 2 m under the surface of the earth. Its effect on the power cycle efficiency over a period of six months has been studied. It was observed that the soil temperature 10 cm from the surface (horizontal direction) of the underground pipe increased by almost 20C during this time. This temperature change decreased with distance from the pipe. The soil temperature profile varied with time, distance from the pipe and location along the length of the pipe. The efficiency of the SRC increased by 1% and the daily fluctuations were reduced when EAHE was used.

© 2014 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license

(http://creativecommons.Org/licenses/by-nc-nd/3.0/).

Selectionand/orpeer-reviewunderresponsibilityofISES.

Keywords: Low temperature power generation, Supercritical organic Rankine cycle, Earth-air-heat-exchanger

Corresponding author. Tel.: +1-813-974-0956; fax: +1-813-974-2050. E-mail address: goswami@usf.edu

1. Introduction

Low grade heat sources are available in very large amounts all over the world in various forms such as geothermal, waste heat and low to medium temperature solar thermal. These sources cannot be utilized efficiently with the traditional

1876-6102 © 2014 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license

(http://creativecommons.Org/licenses/by-nc-nd/3.0/).

Selection and/or peer-review under responsibility of ISES.

doi: 10.1016/j.egypro.2014.10.206

methods of power generation that use steam Rankine cycle. So, new methods are being investigated to improve the efficiency of power generation from such heat sources. Alternative configurations for the thermodynamic cycles have been proposed for low temperature sources. Organic Rankine cycles (ORC) are usually preferred over other configurations because of their simplicity and cost effectiveness[1].

A number of organic fluids have been examined for use as working fluids in an ORC [2-7]. A supercritical Rankine cycle (SRC) is similar to an ORC, however, the maximum pressure in a SRC is greater than the critical pressure of the fluid. Figure 1 shows the ORC and SRC cycles on a T-S diagram. It can be observed that the heating profile in the case of SRC is much straighter than that of ORC and hence better thermal match is obtained with the heat source. It was observed that organic fluids provide higher efficiency in SRC compared to a conventional ORC [2, 4, 8-13].

The efficiency of SRC depends largely on the source and sink temperatures. Since the source temperature is not very high, in order to obtain high efficiency the sink temperature must be lowered as much as possible. Using water cooling in the condenser generally provides low sink temperature, however water may not be available at all sites in which case air cooling needs to be considered. For locations where the ambient air temperature is high, ground cooling may be combined with the air cooled condenser. Since the underground temperature does not vary as much as the ambient air temperature [14, 15], passing the ambient air through underground tunnels may be used to cool the air, which can then be used in the condenser of the SRC. Earth-to-air heat exchangers have been widely used for air conditioning applications [15-27]. They have been used for cooling the ambient air in the summer and heating in the winter [14, 18, 22, 28-32]. The performance of EAHE depends chiefly on the geometric parameters (length, radius etc) and environmental conditions, such as depth and soil type. The geometric parameters can be varied depending on the load requirement. The lag between the ambient and underground temperatures increases with depth. Thus, it appears that if the system is positioned at more depth, the outlet temperature should be lower in cooling mode and hotter in heating mode. However, it was observed that this effect reaches a saturation level after certain depth and increase in depth does not provide any significant improvement in performance [22, 30, 33]. Simulation studies have shown that a depth greater than 4 m does not improve the performance [15, 25, 26, 30]. The experimental implementations EAHE have generally been done at the depths of 0.5-2 m [21, 34-36].

In this paper, a supercritical Rankine cycle having a condenser coupled with ground cooling has been investigated for low temperature power generation. Efficiency of the SRC for different working fluids under the given conditions is obtained and the effect of using ground cooling on the efficiency is studied. Finally, the variation of underground soil temperature profile due to the earth-air-heat-exchanger is analysed. Soil properties given in [37-39] are used for the analysis.

Entropy

Figure 1. T-S diagram of SRC compared to Rankine cycle.

Nomenclature

A Differential area Cp Specific heat

h Convective heat transfer coefficient

m Mass flow rate

Subscript

Element from the inlet Surface of the tube

q' Heat transfer per unit area

r Radial distance

T Temperature

U Overall heat transfer coefficient

5 Penetration depth

2. Supercritical Rankine cycle

A supercritical Rankine cycle (SRC) with organic refrigerant as the working fluid was used for studying power generation from low temperature heat sources. Figure 2 shows a schematic diagram of the cycle configuration. The working fluid is pressurized beyond its critical pressure in the pump and goes through the recuperator for pre-heating before entering the boiler. It is heated to the supercritical state in the boiler and the supercritical fluid then enters the turbine, where it is expanded and power is generated. The turbine exhaust then enters the recuperator where it transfers some of the heat to the fluid at lower temperature. Finally it is condensed to its liquid state in the condenser and the liquid is then pumped to the cycle high pressure, thus completing the cycle.

Figure 2. Schematic diagram of the SRC analyzed.

Organic fluids with low critical temperatures and pressures and zero ozone depletion potentials were selected for the analysis. Table 1 shows the critical properties of the fluids studied. The thermo-physical properties of these fluids were obtained from the NIST REFPROP database.

Table 1. Critical properties of the selected fluids

Fluids Critical temperature (0C) Critical pressure (Bars)

R32 78.11 57.8

R125 66.02 36.2

R134a 101.05 40.6

R143a 72.71 37.6

R170 32.18 48.7

R218 71.87 26.4

The heat source temperature was varied from 125-1750C while the sink temperature was kept constant at 250C. The pinch temperature in both the source and sink side heat exchangers was 50C. The low pressure was fixed to achieve complete condensation in the condenser and the high pressure was optimized for maximum efficiency. Figure 3 shows the

optimum efficiency of the SRC for each working fluid as a function of heat source temperature. It can be observed that the maximum efficiency was obtained when R134a was used as the working fluid. Efficiencies for R143a, R135 and R218 based cycles were nearly the same, while R170 produced the lowest efficiency in the temperature range considered in this analysis. R32 had lower efficiency at high heat source temperatures but its efficiency was close to that of R125 at low source temperatures.

Figure 3. Energy efficiency as a function of heat source temperature for 250C sink temperature.

3. Earth-air-heat-exchanger

An earth-air-heat-exchanger (EAHE) was coupled with the condenser of the SRC. A 25 m long pipe of 25 cm diameter is considered for the ground cooling system. Figure 4 shows the annual variation of temperature of the ambient air and soil at depths of 2, 3 and 4 m. The EAHE may be used between A and B, as shown for the 2 m graph, when the ambient air temperature is higher than the soil temperature. If the ambient temperature is colder than the soil, the ambient air can be used directly for cooling.

% 30 60 90 120 150 180 210 240 270 300 330 365

Figure 4. Annual variation of ambient air and soil temperature.

3.1 Methodology

A two dimensional model developed by Dhaliwal and Goswami was used to study heat transfer between the air and soil [37-39]. The tunnel length was divided into small segments and energy balance was performed on each segment.

• Heat transfer in air

The inlet temperature was specified and was used to find the temperature of the air leaving the first element. This value was then used as the inlet for the second element and so on. The temperature of the air leaving the nth element is given by equation 1.

f[(l - U/2)Tn_1 + UTS]

1 + U/2

(1 - U/2)Tn_1 + (^n-i - Wn) + UTs

where,

Heat transfer in soil

1 + U/2

If the air is -unsaturated If the air is saturated

U =■

Ah mC„

The temperature of the soil can be obtained using equation 2, where r is the radial distance from the center of the tunnel and t is the time.

d2T(r,t) !dT(r,t) !dT(r,t)

The solution for equation 2 was obtained using an integral method [37-39] and is given in equation 3,

q'R/kz S r\ t 1 \ i r/R \

nr,t) = 7;(t)- —(i + ---) (5/fl + 21n(1 + 5/fl))in(TT^7^). (3)

where, Te is the bulk earth temperature and 5 is the penetration depth in the soil given by equation 4. The temperature of the soil remains unchanged beyond this length.

S = V8at (4)

• Ambient air temperature

The diurnal and annual variations of average ambient air temperature are sinusoidal. A sinusoidal profile close to practical conditions was used for yearly variations; while the Erbs Model (equation 5) was selected for the variation over a day.

T(t) = Tavg +4T[0.4632cos(a - 3.805) + 0.0984 cos(2a - 0.360) + 0.0168 cos(3a - 0.822) + 0.0138 cos(4a -

3.513, (5)

2n(t - 1)

where a =-—-,and t is the hour

3.2 Underground temperature

• Effect of time and distance from the surface

The heat transfer from the air affects the temperature of the soil around the underground pipe. As expected, the temperature of the soil very close to the pipe varies more and follows a profile similar to the air temperature. As the distance from the pipe increases, the variation in soil temperature is reduced and the profile gets closer to that of the bulk soil temperature. Figures 5-7 show the variation in temperature of points located at a horizontal distance of 10, 25 and 50 cm from the surface, half way from the inlet of the tunnel for depths of 2, 3 and 4 m respectively. The undisturbed bulk temperature changes with time of the year but the temperature close to the pipe increases above the bulk temperature because of the heat transfer for the ambient air. It can be observed that for all cases the temperature of the soil 10 cm from the surface of the pipe increases by almost 10C after 2 months of operation and 1.50C after 3 months above the undisturbed bulk temperature, while the increase in soil temperature 50 cm from the surface is negligible for the first 1 month. The thermal penetration inside the soil increases with time and so the effect of heat transfer on the temperature of soil becomes

more prominent. We can observe that the soil temperature at 50 cm starts to increase above the bulk temperature after 2 months of operation and increases by 0.5°C after 6 months.

Figure 5. Variation of soil temperature over 9 months for 2 m depth.

Figure 6. Variation of soil temperature over 9 months for 3 m depth.

Figure 7. Variation of soil temperature over 9 months at 4 m depth.

• Effect of distance from inlet

The temperature of the air inside the tunnel decreases at a faster rate in the beginning as the temperature difference between the air and the soil is higher. So, the temperature of the soil increases more in the initial portion of the pipe and starts decreasing as the heat transfer from the air decreases along the length. Figure 6 shows the variation in soil temperature as a function of distance from the surface, for four corresponding positions at the pipe, for a depth of 2 m. It can be observed that the soil temperature close to the pipe surface decreases as the position is moved farther from the inlet. Also, the temperature difference between the 5 m and 10 m from the inlet is greater compared to the difference between 15 m and 20 m from the inlet. One can also notice that the temperature of the soil approximately 0.5 m from the surface converges to the undisturbed bulk earth temperature for all positions from the inlet.

—5 m —10 m 15 m -•20 m

Distance from the surface of pipe

Figure 8. Soil temperature variation with distance from the surface for different locations from the inlet.

3.2 Effect of EAHE on cycle efficiency

The air is cooled in the EAHE system and so a lower sink temperature is obtained for the SRC. With a lower sink temperature the efficiency of the SRC increases. The daily variation in air temperature is obtained by equation 5 and governs the variation in the efficiency of the SRC during the day. Figure 9 shows the variation in the efficiency of the R134a based SRC for 15 days with and without EAHE in the month of July, between points P and Q in the right hand side graph in figure 9. It can be observed that the efficiency is improved by 1% when the air coming out of the EAHE is used. We also observe that by using EAHE, the daily fluctuations in efficiency caused by the ambient temperature variation are also reduced. Figure 10 shows the daily average efficiency for 9 months at depths of 2, 3 and 4 m.

15 —Without EAHE

—Witti EAHE

Figure 9. Variation in SRC efficiency with and without EAHE.

Figure 10. Variation in SRC efficiency over 9 months for different depths.

Summary and Conclusions

A supercritical Rankine cycle was analysed for low temperature power generation and the feasibility of using an air cooled condenser coupled with ground cooling was investigated. Six organic fluids were studied and the optimum efficiency for each fluid at the given conditions is reported. The effects of earth-air-heat-exchanger on the surrounding soil and the SRC efficiency were studied.

The following conclusions can be made from this study:

• Among the working fluids considered in the analysis, the highest efficiency was obtained for R134a.

• The soil temperature around the underground pipe increased more with increasing time, as the penetration depth increased.

• The variation in soil temperature from the pipe surface was larger for positions closer to the inlet.

• A higher and more stable SRC efficiency was obtained when the EAHE was coupled with the condenser.

References

[1] Andersen, W.C. and T.J. Bruno, Rapid screening of fluids for chemical stability in organic rankine cycle applications. Industrial and Engineering Chemistry Research, 2005. 44(15): p. 5560-5566.

[2] Chen, H., D.Y. Goswami, and E.K. Stefanakos, A review of thermodynamic cycles and working fluids for the conversion of low-grade heat. Renewable and Sustainable Energy Reviews, 2010. 14(9): p. 3059-3067.

[3] Saleh, B., et al., Working fluids for low-temperature organic Rankine cycles. Energy, 2007. 32(7): p. 1210-1221.

[4] Chen, H., et al., Energetic and exergetic analysis of CO 2- and R32-based transcritical Rankine cycles for low-grade heat conversion. Applied Energy, 2011. 88(8): p. 2802-2808.

[5] Borsukiewicz-Gozdur, A. and W. Nowak, Comparative analysis of natural and synthetic refrigerants in application to low temperature Clausius-Rankine cycle. Energy, 2007. 32(4): p. 344-352.

[6] Cayer, E., N. Galanis, and H. Nesreddine, Parametric study and optimization of a transcritical power cycle using a low temperature source. Applied Energy, 2010. 87(4): p. 1349-1357.

[7] Gu, Z. and H. Sato, Optimization of cyclic parameters of a supercritical cycle for geothermal power generation. Energy Conversion and Management, 2001. 42(12): p. 1409-1416.

[8] Chen, H., et al., A supercritical Rankine cycle using zeotropic mixture working fluids for the conversion of low-grade heat into power. Energy, 2011. 36(1): p. 549-555.

[9] Chacartegui, R., et al., Alternative ORC bottoming cycles FOR combined cycle power plants. Applied Energy, 2009. 86(10): p. 2162-2170.

[10] Karellas, S. and A. Schuster, Supercritical fluid parameters in organic rankine cycle applications. International Journal of Thermodynamics, 2008. 11(3): p. 101-108.

[11] Cayer, E., et al., Analysis of a carbon dioxide transcritical power cycle using a low temperature source. Applied Energy, 2009. 86(7-8): p. 1055-1063.

[12] Ochs, T.L.A, OR, US), O'connor, William K. (Lebanon, OR, US), Energy recovery during expansion oof compressed gas using power plant low-quality heat sources. 2006, The United States of America as represented by the United States Department of Energy (Washington, DC, US): United States.

[ 13] Augustine, C., et al. Modeling and analysis of sub- and supercritical binary rankine cycles for low- to mid-temperature geothermal resources. 2009.

[ 14] Abbaspour-Fard, M.H., A. Gholami, and M. Khojastehpour, Evaluation of an earth-to-air heat exchanger for the north-east oofIran with semiarid climate. International Journal of Green Energy, 2011. 8(4): p. 499-510.

[15] Santamouris, M., et al., Use of buried pipes for energy conservation in cooling of agricultural greenhouses. Solar Energy, 1995. 55(2): p. 111124.

[16] Trombe, A. and L. Serres, Air-earth exchanger study in real site experimentation and simulation. Energy and Buildings, 1994. 21(2): p. 155162.

[17] Said, S.A.M., et al., Feasibility of using ground-coupled condensers in A/C systems. Geothermics, 2010. 39(2): p. 201-204.

[18] Bansal, N.K. and M.S. Sodha, An earth-air tunnel system for cooling buildings. Tunnelling and Underground Space Technology incorporating Trenchless, 1986. 1(2): p. 177-182.

[19] Bansal, V., et al., Performance analysis of earth-pipe-air heat exchanger for winter heating. Energy and Buildings, 2009. 41(11): p. 11511154.

[20] Bansal, V., et al., Performance analysis of earth-pipe-air heat exchanger for summer cooling. Energy and Buildings, 2010. 42(5): p. 645-648.

[21] Ghosal, M.K., G.N. Tiwari, and N.S.L. Srivastava, Thermal modeling of a greenhouse with an integrated earth to air heat exchanger: An experimental validation. Energy and Buildings, 2004. 36(3): p. 219-227.

[22] Sodha, M.S., et al., Evaluation of an earth-air tunnel system for cooling/heating of a hospital complex. Building and Environment, 1985. 20(2): p. 115-122.

[23] Jacovides, C.P. and G. Mihalakakou, An underground pipe system as an energy source for cooling/heating purposes. Renewable Energy, 1995. 6(8): p. 893-900.

[24] Lee, K.H. and R.K. Strand, The cooling and heating potential of an earth tube system in buildings. Energy and Buildings, 2008. 40(4): p. 486494.

[25] Mihalakakou, G., M. Santamouris, and D. Asimakopoulos, On the cooling potential of earth to air heat exchangers. Energy Conversion and Management, 1994. 35(5): p. 395-402.

[26] Mihalakakou, G., M. Santamouris, and D. Asimakopoulos, Use oof the ground for heat dissipation. Energy, 1994. 19(1): p. 17-25.

[27] Santamouris, M., G. Mihalakakou, and D.N. Asimakopoulos, On the coupling of thermostatically controlled buildings with ground and night ventilation passive dissipation techniques. Solar Energy, 1997. 60(3-4): p. 191-197.

[28] Ozgener, L. and O. Ozgener, An experimental study of the exergetic performance of an underground air tunnel system for greenhouse cooling. Renewable Energy, 2010. 35(12): p. 2804-2811.

[29] Ozgener, L. and O. Ozgener, Energetic performance test of an underground air tunnel system for greenhouse heating. Energy, 2010. 35(10): p. 4079-4085.

[30] Ascione, F., L. Bellia, and F. Minichiello, Earth-to-air heat exchangers for Italian climates. Renewable Energy, 2011. 36(8): p. 2177-2188.

[31] Thanu, N.M., et al., An experimental study of the thermal performance of an earth-air-pipe system in single pass mode. Solar Energy, 2001. 71(6): p. 353-364.

[32] Ozgener, O. and L. Ozgener, Exergoeconomic analysis of an underground air tunnel system for greenhouse cooling system. International Journal of Refrigeration, 2010. 33(5): p. 995-1005.

[33] Kaushik, S.C. and G.S. Kumar, Performance evaluation of an earth air tunnel for space heating of a non air-conditioned building. International Journal of Ambient Energy, 1994. 15(4): p. 205-218.

[34] Mavroyanopoulos, G.N. and S. Kyritsis, The performance of a greenhouse heated by an earth-air heat exchanger. Agricultural and Forest Meteorology, 1986. 36(3): p. 263-268.

[35] Von Zabeltitz, C., Greenhouse heating with solar energy. Energy in Agriculture, 1986. 5(2): p. 111-120.

[36] Santamouris, M., A. Argiriou, and M. Vallindras, Design and operation of a low energy consumption passive solar agricultural greenhouse. Solar Energy, 1994. 52(5): p. 371-378.

[37] Goswami, D.Y. and AS. Dhaliwal, HEAT TRANSFER ANALYSIS IN ENVIRONMENTAL CONTROL USING AN UNDERGROUND AIR TUNNEL. Journal of Solar Energy Engineering, Transactions of the ASME, 1985. 107(2): p. 141-145.

[38] Goswami, D.Y. and S. Ileslamlou, Performance analysis of a closed-loop climate control system using underground air tunnel. TRANS. ASME J. SOLAR ENERGY ENGINEERING, 1990. 112(2 , May, 1990): p. 76-81.

[39] Ileslamlou, S. and D.Y. Goswami. PERFORMANCE ANALYSIS OF A CLOSED-LOOP CLIMATE CONTROL SYSTEM FOR RESIDENTIAL AND AGRICULTURAL BUILDINGS USING UNDERGROUND AIR TUNNEL. 1987.