ASHRAE LV-11-022-2011 Net Zero Energy Air Conditioning Using Smart Thermosiphon Arrays.pdf

1、892 ASHRAE TransactionsABSTRACTWith sufficient thermal storage capacity, it is feasible tomeet all air-conditioning and heating requirements with a triv-ial fuel or electrical input in regions with hot summers and coldwinters. Most buildings have available a large quantity ofgeologic material to sat

2、isfy the storage capacity needs, but theobstacle to using seasonal energy is an effective and inexpen-sive way to transfer heat to and from the soil. Smart thermosiphon arrays (STAs) are a way to meet thosethermal and economic benchmarks. STAs use standard passivethermosiphon mechanisms to transfer

3、energy out of soil, andcontrolled rate transfer of energy into the soil using standardmachinery. In this paper, we describe how STAs can provideseasonal energy storage to meet all climate control needs. Thepassive mode of soil freezing and the pump-assisted operationof air conditioning are modeled,

4、and the resultant simulationsare shown. When compared with conventional vertical bore-hole exchangers, simulations show the same total heat transfercan be obtained with 40% of the depth using STAs with drillingcosts per length of borehole an order of magnitude lower.Results of lab-scale smart thermo

5、siphon experimentsdemonstrate uniform temperatures, and heat fluxes can bemaintained on the inside wall of the thermosiphon pipe. Thisallows for dramatic enhancements of heat transfer rates. Thefirst pilot-scale installation of smart thermosiphons forseasonal Underground Thermal Energy Storage (UTES

6、) hasdemonstrated the ability to install the devices using inexpen-sive direct push techniques. Data indicate frozen ground in thesubsurface.INTRODUCTIONOne-quarter of the CO2production in the US is fromburning fossil fuels to meet residential energy demands,mostly for heating and air conditioning (

7、Tester et al. 2005; EIA2009). Per dollar spent, the greatest decrease in CO2isproduced from conservation. It follows that the least expensiveway to reduce residential CO2emissions is through improvedclimate control efficiencies. However, options for heating andair conditioning that do not use signif

8、icant energy derivedfrom burning fossil fuels are few.Not surprisingly, some of the greatest home heating andair-conditioning energy uses are found in climates where thewinters are cold and the summers are hot. In those climates, itis thermodynamically possible to store heat or remove heatfrom the g

9、round for use during the opposite season, whileusing minimal fossil fuel or electrical energy. As such,seasonal thermal energy storage heating and cooling canprovide near-zero-carbon heating and air conditioning. Thelimiting technical problems are related to the massive energystorage needs and the h

10、eat transfer between the building, theambient conditions, and the storage medium. However, if theheat transfer and storage problems can be solved, there is greatpotential for energy savings and CO2reductions usingseasonal thermal energy storage.This paper details the use of a new technologySmartTher

11、mosiphon Arrays (STAs)to effectively transfer heat toand from soils. Based on preliminary analysis, computermodels, and experimental data, it is clear that these systemsexchange adequate thermal energy with the ground to provideall the cooling needs of a typical house or business with mini-mal elect

12、rical or fossil fuel energy input. The goal of thisresearch is to create the engineering knowledge to design aNet Zero Energy Air Conditioning Using Smart Thermosiphon ArraysKent S. Udell, PhD Bidzina Kekelia Phil JankovichStudent Member ASHRAE Student Member ASHRAEKent S. Udell is director of the S

13、ustainability Research Center and a professor, Bidzina Kekelia is a research assistant and PhD candidate,and Phil Jankovich is a research assistant and PhD candidate in the Department of Mechanical Engineering, University of Utah, Salt LakeCity, UT.LV-11-0222011. American Society of Heating, Refrige

14、rating and Air-Conditioning Engineers, Inc. (www.ashrae.org). Published in ASHRAE Transactions, Volume 117, Part 1. For personal use only. Additional reproduction, distribution, or transmission in either print or digital form is not permitted without ASHRAES prior written permission.2011 ASHRAE 893c

15、arbon neutral heating and cooling system indistinguishablein simplicity and comfort from conventional HVAC design.SOIL FOR ENERGY STORAGEWhen soil is used as the energy storage medium, there areonly a few restrictions on storage volume, such as drillingdepth, maintenance of surface ecology, and plot

16、 size (Sanner2001; Faninger 2005; Hauer 2006; Nielsen 2003; Reu et al.2006). If heated and cooled in an optimum way, soils not onlybuffer short-term fluctuations in supply and demand, but canaccommodate a complete annual heating/cooling load andserve a seasonal balancing function. Energy storage dir

17、ectly inthe soil also reduces the cost sensitivity of reservoir depth onoptimum capacity selection since excavation is unnecessary.So, the storage system can be easily sized to maximumexpected load by a simple increase of depth in most cases.Ground-Source Heat Pumps and ThermosiphonsThe use of smart

18、 thermosiphons is similar to U-tube bore-holes typically installed with ground-source heat pumps(GSHPs) in that they facilitate heat transfer to and from thesoils. Passive thermosiphons have been used in various otherapplications (Andersland and Ladanyi 2003). However, ratherthan heating or cooling

19、the soil for future energy use, GSHPsgenerally take advantage of the earths relatively constanttemperature (Gao et al. 2009). Thus, energy dissipation, ratherthan storage, is desired with GSHPs. In contrast, STAs arebeing developed to concentrate energy for seasonal storage.Plastic (PVC and polyethy

20、lene) pipes were introduced inGSHPs for economic reasons, justified by the argument thatresistance to heat transfer is much greater in the soil than in theworking fluid. However, in (Svec et al. 1983) it is shown thatheat flows are substantially reduced (nearly half) due to highthermal resistance of

21、 the pipe walls and contact resistancebetween pipe and soil. Also, for vertical boreholes withclosed-loop tubing, “short circuiting” of heat from the hot tubeto the adjacent cold tube decreases the amount of heat that canbe transferred to the soil. This problem logically worsens asthe tube spacing d

22、ecreases. An improvement in the heat trans-fer between the ground and the heated space is of great impor-tance and economic value. Application of smart thermo-siphons to couple heat pumps with the ground seems to be asimple and effective step forward.A GSHP system can be replaced with a mostly passi

23、vesystem that uses much more effective phase-change phenom-ena for capturing/releasing heat (Udell et al. 2009). Thermo-siphon (gravity-assisted heat pipe) operation is based exactlyon the above-mentioned principle shown in Figure 1(ASHRAE 2008). If plastic piping in the ground is replacedwith an ar

24、ray of thermosiphons and connected directly to aheat exchanger in the heated or cooled space, then there wouldbe no need for intermediary heat-transfer fluids and heatexchangers used in GSHP systems. Heat in a thermosiphon-based system can be transferred from/to soil to heated/cooledmedium without a

25、 vapor-compression cycle heat pump withits electrical energy consuming compressor, intermediary heatexchangers, or liquid pumps to move water-glycol solutionthrough the plastic piping in the ground. Thermosiphon appli-cation for the net zero air-conditioning system is describedbelow.Passive Soil Coo

26、ling Mode. As discussed above, thethermosiphon operates on a heat pipe principles. Heat from thesoil vaporizes the liquid phase of the working fluid inside thesealed thermosiphon pipe (Figure 1). The vapor travels up tothe heat exchanger and condenses, thereby transferring theheat to the air. The li

27、quid condensate then drains back to thebottom of the thermosiphon and repeats the cycle. If the soilis already preheated and used as a heat source for space heating(this will work only if the soil is preheated to above 30C35C 86F95F in summer, otherwise a small “booster”heat pump will be needed to r

28、ise the working fluid temperaturein the room heat exchanger up to 35C40C 95F104F),then the above-ground heat exchanger is placed in the HVACducting of the building. If the system is designed for air-condi-tioning purposes with the winter soil precooling, needed forfuture use as a cold sink in summer

29、, then the heat exchangerwould be exposed to cold winter air.Depending on the soil temperature and temperaturedifference between the working fluid and the soil, naturalconvection of groundwater or soil vapors in the vicinity of thethermosiphon could be observed. Cooled water near the pipewalls would

30、 sink downward, bringing fresh, warmer waterFigure 1 Passive heat extraction from the ground with athermosiphon.894 ASHRAE Transactionstoward the thermosiphon, causing circulation of water near thepipe walls and increasing heat flux from the soil. Soil Heating Mode. By reversing the working fluid fl

31、owdirection in the system, heat can be removed from the cooledspace and injected into the soil (Figure 2). The smart, revers-ible thermosiphon would supply the cold liquid from itsbottom to the evaporation heat exchanger above. Liquid wouldthen evaporate and take heat from the air. Due to temperatur

32、e-induced pressure gradients, vapor would travel to the bottomof the thermosiphon and condense on the colder walls, givingup its heat to the surrounding soil. The smart thermosiphon isdesigned to regulate the supply of the working fluid conden-sate to the evaporative heat exchanger based on the mass

33、 flowrate of vapor entering into the thermosiphon. Depending onthe application (whether it is heat removal from the air-condi-tioned space or summer pre-heating of soil for future winterheating), the evaporator would be placed in HVAC ducting ofthe building or in the ambient hot summer air. Heat inj

34、ectionwould also cause ground-water circulation (convection) in theopposite direction near the thermosiphon, thus again increas-ing heat flux between the thermosiphon and soil.In case of summer soil preheating, the evaporator can alsobe coupled with solar thermal collectors or other process wastehea

35、t to be transferred into the soil. It may be possible even toincrease the thermosiphon wall temperature to over 100C(212F), which would cause water boiling on the outside wallof the thermosiphon. Under that condition, water wicksthrough the soil to the thermosiphon wall, replenishing thevaporized wa

36、ter, dramatically increasing the heat transfer inthe soils due to the heat pipe effect (Udell 1985).SMART THERMOSIPHON ARRAYSTo concentrate energy in the soil for both heating and air-conditioning purposes, two STAs would be needed: one tocreate a “cold bank” in the winter for summer air conditionin

37、g,and the other to create a “hot bank” during the summer forwinter heating. A single array cannot be used for bothpurposes. Arrays of smart thermosiphons are required toincrease the thermal efficiency of storage; a single thermosi-phon does not allow storage since the gains in one season aredissipat

38、ed before the next season arrives. Also, since the soildomain near internal thermosiphons only loses or gains heatfrom the top, the bottom, or the thermosiphons themselves, theefficiency increases with the number of thermosiphons in thearray.MODELINGAn array of seven thermosiphons, as shown in Figur

39、e 3,was modeled using the commercially available software pack-age COMSOL Multiphysics, Version 3.3. The two-dimen-sional conduction model included the passive freezing of soiland the use of that frozen soil as an air-conditioning heat sink.The geometry chosen for the analysis was an array of sixthe

40、rmosiphons placed at the corners of a regular hexagon, witha seventh thermosiphon placed at the center of the hexagon.Utilizing the symmetry of the system, a quarter-circle with a5 m (16.4 ft) radius was chosen as the domain of interest. Threethermosiphons were modeled in this domain: one positioned

41、centrally, and the other two placed 60 degrees apart, with oneof them on the axis of symmetry. The spacing between ther-mosiphons was 1.5 m (4.9 ft).Ambient Temperature ModelThe ambient temperatures were modeled by a seven-parameter empirical formula determined from 2006 hourlyFigure 2 Heat injectio

42、n into the ground with a reversiblethermosiphon.Figure 3 Plan view of thermosiphon pattern. Using thesymmetry of the pattern of seven wells, the twodashed lines are planes of temperature and heatflux symmetry. Only the top-right quadrant ismodeled and displayed.2011 ASHRAE 895weather data taken from

43、 the weather station at the Salt LakeCity International Airport. This model is a superposition oftwo sine curves, where constants A through G are the best fitto actual temperatures, as follows:(1)The independent variable, t, is time (hours). The parameters Cand F are the periods of these sine curves

44、 and were set to be2/24 to represent daily temperature fluctuations, and to 2/8760 to represent yearly seasonal temperature fluctuations,respectively. The other parameters were optimized through aleast-square difference method using the solver add-in inMicrosoft Excel. These parameters are A = 285.3

45、, B = 4.60,D = 1.62, E = 13.44, G = 3.19.Boundary ConditionsThe outer edge of the circle was set as a no heat flux, orinsulated boundary. The other two boundaries are planes ofsymmetry, also represented as insulated boundaries. Thermosiphon interactions with the soil were modeled asmodified convecti

46、ve heat flux boundaries. The heat flux wasspecified as a heat transfer coefficient multiplied by the differ-ence between the temperature in the inside wall of the ther-mosiphon and ambient temperature.(2)A highly conductive layer of aluminum, 5 mm (0.197 in.)thick, was specified at the edge of each

47、thermosiphon to modelthe wall. The thermosiphon radius (ro) was specified as 5 cm(2 in.). The heat-transfer coefficient on these boundaries wasset at 1 108W/m2K (0.17612 108Btu/hft2F) when thetemperature of the soil next to the heat pipe was larger than theoutside ambient air temperature, and was se

48、t at zero when theopposite was true. In essence, with that large of a heat-transfercoefficient, the heat flux during the winter season wasmodeled as if the temperature of the inside of the heat pipe isthe same as the external air temperature. During summer operation, the heat transfer to the soil wa

49、sspecified by a heat-transfer coefficient multiplied by thedifference between the outdoor temperature and the buildingindoor temperature (295 K = 71.3F) to define the coolingload, as shown in Equation 3.(3)The heat-transfer coefficient specified in Equation 3,28.45 W/m2K (5.01 Btu/hft2F), was chosen such that theheat transfer back to the soil was 85% of the energy taken fromthe soil during the winter months. Thus, the simulation resultsare expected to show net soil volume cooling from season toseason. MODELING RESULTSThe temperature distributions calcul