Hot Stream Energy Scavenging for Absorption Refrigeration

8/9/2019 Hot Stream Energy Scavenging for Absorption Refrigeration

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HOT STREAM ENERGY SCAVENGING FOR ABSORPTIONREFRIGERATION

J. C. Ordonez, H. Cortes, T. Tracy, Q. StraubDepartment of Mechanical Engineering and Center for Advanced Power Systems, Florida State University,

E-mail: [email protected]

Abstract.This paper focuses on the experimental characterization of aheat recovery system used to drive a small-scale absorption refrigerator.The experimental setup consists of a hot stream generator with flow andtemperature control and a heat exchanger that intercepts the hot streamand drives an absorption refrigeration unit. Preliminary results indicate ator sub zero (

oC) temperatures at the evaporator and refrigeration unit

cold space. The effects of hot stream mass flow rate and hot stream inlettemperature on the performance of the absorption unit and its potential

for energy recovery are studied. Test runs of the refrigerator with a hotstream mass flow rate of 0.97g/s, 1.09g/s, and 1.74 g/s and inlettemperatures of 297C, 362C, and 475C were observ ed. Minimumrefrigeration temperature and pull down time were two parameters thatwere examined. The data from these experimental tests trends wereanalyzed to determine if an optimal hot stream mass flow rate and inlettemperature exists.

Key words:Absorption Refrigeration, Heat Recovery

1. Introduction

In the context of todays energy situation, increased energy efficiency is perhaps the aspect onwhich engineering solutions are most likely to contribute in the near future. Technologies that havebeen proposed and studied for decades are now being found to be economically advantageous. Anexample of a technology that has benefit from this renewed interest is that of absorptionrefrigeration [1-7], and especially waste heat driven absorption refrigeration. Absorption refrigerationdiffers from the more conventionally used vapor compression refrigeration in that it can be poweredfrom a heat source instead of a compressor. This allows absorption refrigerators to take advantageof wasted hot stream energy sinks. This makes them economically advantageous since this energywould otherwise be lost.

2. Background

This paper examines an absorption refrigerator that utilizes a continuous cycle. The continuous-cycle absorption refrigerator is shown in Figure 1 below.


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Fig. 1. Diagram of Continuous AbsorptionRefrigeration Cycle [5] Fig. 2. Back of Absorption Fridge

First, heat is supplied by either a small gas flame inside the tube at (A), or a small electric burner at(B). The flame or burner heats a strong ammonia-water solution inside of the boiler at (C). Theentire refrigeration system is under a large enough pressure to keep the ammonia inside of theboiler in a liquid state at room temperature. The applied heat raises the temperature of the solutionto the point where the ammonia begins to boil out of the solution. The bubbles of ammonia vaporrise, pushing ahead of them small quantities of the leftover weak ammonia-water solution. Thebubbles of ammonia are carried through the vapor pipe at (E), while the left ammonia-water solutionpushed by the ammonia bubbles is left around the boiler to fill the weak solution tube at (D). Theammonia vapor that passes through vapor tube next passes through a water separator, where anyleftover water vapor attached to the ammonia bubbles is removed and passed back to the boiler forreuse.

The following steps of the absorption cycle are powered by gravity. After the dry ammonia passesthrough the separator, the vapor passes through a condenser, where heat-exchanging fins workwith the surrounding flowing air to remove heat from the ammonia vapor and condense it back to aliquid. The liquid ammonia then flows from the condenser to the evaporator. The evaporator is anarea of the refrigeration unit filled with a charge of hydrogen gas, which works off of Daltons Law ofPartial Pressures. Daltons Law states that the total pressure of gases in a fixed chamber is equal tothe pressures of the separate gases if each gas occupied the chamber separately, added together.As the liquid ammonia flows underneath the hydrogen gas, the pressure of the liquid ammonia isreduced and the ammonia begins to evaporate. As the ammonia evaporates, it draws heat from thesurrounding evaporator, which in turn draws heat from the inside of the refrigeration space. This isthe stage that creates the cooling function of the absorption cycle.

After the evaporation, the mixture of ammonia and hydrogen gas flows through a series of curvedpipes, called the absorber. In these pipes flows the leftover weak ammonia solution that waspushed out of the boiler by the ammonia vapor bubbles, at (D). As the weak solution and ammoniaflow downward through the absorber tubes, the solution absorbs the ammonia vapor and becomesa strong ammonia solution once again. The strong ammonia solution is then retained in theabsorber vessel, before flowing back into the boiler to repeat the absorption process. The hydrogencontinues to circulate around the absorber vessel and tubes to force evaporation of the liquidammonia.


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3. Experimental Methodology

The absorption refrigerator used is illustrated in Figure 2. It is a DOMETIC refrigerator model RM-

2193 with a refrigerated space volume of 0.18 m

3

. It can be operated through an electricalresistance or with LP gas. The back of the absorption unit is shown in Figure 2.

Fig. 3.Absorption Unit Layout

This absorption refrigerator was converted from its electrical resistance/LP gas system to a hotstream system. This was done by coupling a variable hot stream generator with flow control to theoriginal heat exchanger in the system. To allow for experimental analysis various temperatureswere taken throughout the system to analyze its performance. Figure 3 shows the layout of thesystem and the points at which data was collected.

A LabVIEW program running on a PC and a National Instruments PXI system were used for dataacquisition. A screen of the LabVIEW implementation can be seen in Figure 4. The PXI has theconnections to all temperature sensors indicated in Figure 3. The data acquisition system is shown

in Figures 5 and 6.

Fig. 4. Screenshot of LabVIEW programused to acquire system temperatures

Fig. 5.PXI device


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4. Results and Discussion

The modified absorption refrigerator was subjected to various input conditions. In Figures 6 through

14 three different flows at various temperature inputs were tested to see how much time it wouldtake to draw the internal refrigerator temperature to sub-zero ranges. By examining this graph wecan see they all display a similar pattern. There is an initial warm up time of about 20 minutesbefore any major temperature fluctuation is seen at the evaporator. This is because it requires timefor the apparatus to reach a stable operating temperature in which the thermal siphon pump withinthe refrigerator can start working properly. Another common occurrence within this warm up time isthat just prior to the initial temperature decrease at the evaporator there is a drastic slope change inthe generator temperature. This is an indicator that the operating fluid inside the thermal siphonpump is undergoing a phase change thus drawing the fluid up the generator and beginning thecontinuous absorption refrigeration cycle.

Fig. 6. 5.23 m/s (1.74 g/s flow) setting withhot stream inlet temperature of 475C



Fig. 9.3.26 m/s (1.09 g/s flow) setting withhot stream inlet temperature of 475C


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Fig. 11. 3.26 m/s (1.09 g/s flow) setting with hotstream inlet temperature of 297C


Fig. 13.2.91 m/s (0.97 g/s flow) setting with hotstream inlet temperature of 362C

-10

-5

0

5

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20

250 300 350 400 450 500

5.233.262.91

Inlet Temperature (C)


Fig. 15. Minimum temperatureversus inlet temperature


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Figure 15 represents the minimum evaporator temperature versus the inlet temperature for the

entire set of flow rates given above. By examining the graph it can be seen that the minimumtemperature shows signs of exponential decay as the temperature increases for each respectiveflow rate. Due to these facts one can determine that the changes in cooling effect caused by theincreased inlet temperature and flow rate diminish as the temperature and flow rates are increased.An additional point needs to be addressed, because of the design of the absorption refrigeratorthere will be a point at which an increase in temperature will cause detrimental effects to the coolingability of the refrigerator. This point was not achieved by this experiment but at that point all thesolution within the thermal siphon pump will become a gas and the thermal siphon pump will ceaseto work, fully stopping the cooling effect of this cycle.

Two separate pull down time analysis were also performed on the system. The pull down time wasstarted at 18.5C because at this point all of the setups had warmed up fully and were consistentlydecreasing in temperature. One of the stopping times was placed at 12.5C because all the setupswere capable of reaching 12.5C. A secondary time was recorded at 0C. Figure 16 shows thepull down times from 18.5C to 12.5C. This graph shows exponential decay as both inlettemperature increases and flow rate increases. As the inlet temperature increases the difference inthe pull down time from one flow rate to another also decreases. The minimum pull down timeachieved by the flow rate of 1.74 g/s (5.23 m/s) and a 475C flow is 6.81 minutes.

0

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250 300 350 400 450 500

5.23

3.26

2.91

Inlet Temperatur e (C)

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5.233.262.91

Inlet Te mperature ( C)

Fig. 16. Pull down time from18.5C to 12.5C

Fig. 17. Pull down time from18.5C to 0C

Figure 17 shows the pull down times from 18.5C to 0C. The first thing visible from this graph isthe inability for all flow rate inlet temperature combinations to reach 0C. The lowest inlettemperature setting of 297C was unable to produce evaporator temperature of 0C for all flowrates tested. This graph shows similar trends to Figure 17. The fastest measured pull down time of

53.86 minutes was achieved with an inlet temperature of 475C and a flow rate of 1.74 g/s (5.23m/s). From these three graphs we can determine that there is a maximum point at which thesystem is capable of extracting heat from a hot stream. The rate of decay of each of these systemsmay vary with each temperature and flow rate combination but all combinations are trendingtowards one maximum point. It should be noted that this experimental setup did not yield themaximum previously mentioned.


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5. Conclusion

This paper examined one possible application of an absorption refrigeration cycle in whichscavenged hot stream gases are used to drive a small absorption refrigerator. In this experiment ahot stream generator forces hot gas through a heat exchanger, which by way of heat transfer,powers the absorption refrigerator. The mass flow rate and inlet temperature of the hot stream gaswere varied and its effect on the systems operating parameters, specifically temperatures atspecific locations on the apparatus were recorded. In the domain of this experiment, the coolingcapacity increased as flow rate and inlet temperature were increased. In the system studied theminimum time of 53.86 minutes to pull the refrigerator temperature down from 18.5C to 0C wasachieved by a flow rate 1.74 g/s (5.23 m/s) and inlet temperature of 475C. This system was alsocapable of reaching a minimum temperature of -4.53C using the same parameters just previouslystated. Future testing will allow the determination of and optimal hot stream flow rate andtemperature for maximum thermodynamic performance.

Acknowledgment

J.C. Ordonez acknowledge with gratitude support from the Office of Naval Research under theElectric Ship and Research Consortium.

References

Hovsapian,R., Vargas, J.V. C., Ordonez, J.C. , Krothapalli, A., Parise, J.A. R. , Berndsen., J.C.,Thermodynamic Optimization of a Solar System for Cogeneration of Water Heating andAbsorption Cooling, International Journal of Energy Research, 2008. (In press).

M.V.A. Pereira, J.V.C. Vargas, F.G. Dias, J.C. Ordonez, J.A.R. Parise, M.C. Campos, L.S. Martins,H.Y.C. Fujii, R. Allage, Exergetic Analysis of a 5 TR Experimental Absorption Refrigeration

Unit Powered by Liquefied Petroleum Gas (GLP) and Exhaust Gases, Proceedings of theBrazilian Congress of Thermal Engineering and Sciences - ENCIT, Curitiba, Brazil, 2006

Chen J., Kim K. J., and Herold K. E. Performance enhancement of a diffusion-absorptionrefrigerator Int J Refrig (1996) Vol. 19: 208-218

Kobal A., SahinB, Ekmekci I, and Yilmaz T. Thermo economic optimization for irreversibleabsorption refrigerators and heat pumps Energy Conversion & Management 44 (2003): 109-203.

Small Ammonia Refrigerator, www.nh3tech.org

Srikhirin P., and Aphornratana S. Investigation of a diffusion absorption refrigerator Applied

Thermal Engineering 22 (2002): 1181-1193

Srikhirin P., Aphornratana S., and Chungpaibulpatana S. A review of absorption refrigerationtechnologies Renewable and Sustainable Energy Reviews 5 (2001): 343-372

Hot Stream Energy Scavenging for Absorption Refrigeration

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