ICE FOOD DEPOT COOLED WITH A HEAT PUMP: A PRE-FEASIBILITY STUDY

Transcription

1 ICE FOOD DEPOT COOLED WITH A HEAT PUMP: A PRE-FEASIBILITY STUDY S.A. Guly, G.Z. Perlstein Nort-Eastern Researc Station o Permarost Institute, Siberian Branc o Russian Academy o Sciences. 12, Gagarin St., Magadan, , Russia [email protected] Abstract Pre-easibility studies o an ice ood depot using a eat pump or cooling were undertaken ollowing an application by a Russian native organization. Te site is on te coast o te Sea o Okotsk, about 200 km east o Magadan, witin te discontinuous permarost zone. Quantitative interrelationsips, based on analytical solutions and computer modeling, were obtained between climatic caracteristics and structural parameters. It is proposed tat te extracted eat will be mainly used to eat a greenouse. To maintain te necessary termal regime in bot te greenouse and rerigerated depot, te total eat pump power is to be in te range o kw. It is expected tat operational expenses will be compensated entirely by ood production rom te greenouse. To enance te energy and economic eiciency o te cooling-eating system, te combined sceme uses alternate eat sources wen cooling o te ice depot is not required. Introduction Te stability o engineering constructions in rozen ground tat may be subject to tawing represents a major long-standing problem in applied geocryology. In te past decade a ew attempts were made to solve tis problem using eat pump tecniques (e.g., Goodric and Plunkett, 1990). As is generally known, te eat pump is a special device tat makes it possible, by expending mecanical work, to move eat rom a cold source to a sink wit a iger temperature. Te essence o te eat pump termodynamic process is as ollows: 1. A working liquid boils in an evaporator as a result o cooling a low temperature source (water, ground or air); 2. In a compressor, te temperature o te vapor is raised to te necessary level; 3. Te warmed vapor enters a eat excanger, were it gives eat away to te consumer and condenses; 4. Te condensate returns to te evaporator, tus completing te cycle. Tereore, in principle, tere is no dierence between a eat pump and an ordinary rerigerator. Wen cooling te ground wit eat pumps, owever, te extracted eat can be utilized, tus compensating or te expense o te installation and operation o te cooling system. Teoretical investigations and initial practical applications ave sown tat te eat pump cooling metod is igly promising, especially in te context o global warming (Goodric and Plunkett, 1990; Perlstein et al., 1994). Tese researces indicate tat eat pumps can be successully used in various engineering applications in cold regions. About 50 years ago in Russia, ice ood depots (Krylov, 1951) received wide recognition. Tey are long-term structures ormed in winter o ice and covered wit soil. To increase te storage lie o perisables it is necessary to maintain a low interior temperature, wic in te warm seasons requires artiicial cooling. Heat pump cooling may requently be te most economic way to acieve te necessary termal regime. Over te past ive years, we ave prepared and disseminated inormation to government bodies and te mass media to explain and promote te ideas outlined above. In 1996, te native company ÒOyaryÓ, under te auspices o te State Committee o te Nort, requested te Nort-Eastern Researc Station o Permarost Institute to work out a design concept or constructing an ice storage cooled by eat pump. Pre-easibility studies were completed and te project approved in principle. Unortunately, inancing as been delayed or at least a year. S.A.Guly, G.Z.Perlstein 383

2 Te summer is sort and cool. Te rost-ree period averages 110 days. August is te warmest mont wit an average temperature o about 12 C. Air temperatures are iger tan 20 C or only ours trougout te wole summer. Te tawing index is 1150 C days. Te annual amount o precipitation is more tan 500 mm wit about 60% o it alling as rain. Te snow cover tickness reaces cm at a density o 0.25 g/cm 3. Over te wole year te weater is windy and winter storms are common. Annual wind velocity averages between 4.5 and 5.5 m/s. In spite o cloudy weater, te open ground surace receives a great deal o radiant energy. From mid-may to August, net radiation (sort and long wave) is W/m 2. Permarost is discontinuous. In river valley bottoms, areas o perennially rozen ground are usually ound witin wooded and swampy portions o low terraces. Te permarost tickness is m, wit ground temperatures between -1 and 0 C. Figure 1. Designed ice depot location. Neverteless, te initial work outlines important aspects o ice construction wit eat pump cooling and tese results may be o interest to permarost engineers. Project speciications and location According to te provisional speciications, te ice depot is to reeze and preserve, on a long-term basis, 150 tons o is, mainly salmon. Te site location ( ÕN, ÕE) is on te remote coast o te Sea o Okotsk, 200 km east o Magadan, te regional market center (Figure 1). Transport connections between Magadan and te site exist by sea and by winter access road (350 km). Personnel travel is by sip. Transport o te is production is planned to be mainly by trucks, rom November to January. Te temperature in te storage cambers must not be iger tan -18 C. Te major replenisment o te ice depot is to occur in te warm season between mid-june and mid-september. Te average rate o is placement equals 3 metric tons per day (t/d). It is also necessary, on 3 to 5 days o te peak ising season, to process about 15 t/d. Te climate o tis region is o a moderately cold marine caracter. Te annual air temperature is -4.0 to -4.5 C. Te winter lasts seven monts wit air temperatures o about -20 C prevailing during muc o it. Temperatures cooler tan -35 C occur rarely. Te reezing index is approximately 2650 C days. Te construction o te ice depot and accessory premises as been planned close to te mout o te Malkacan River on its let bank. Te building site is on a ig-level river terrace. Te suricial ground consists o ine-grained silty sand wit a tickness o 0.5 to 1.0 m. It is underlain by alluvial gravels wit ine-grained illing (sand and silty sand). Te seasonal taw dept ranges between 1.5 and 2.0 m. Water-bearing taliks occupy most o te lood plain. It sould be noted tat engineering explorations ave not yet been conducted at te site and te above caracteristics are taken rom reconnaissance investigations and are based on analogies wit oter well-known sites on te Sea o Okotsk coast. Heat balance o te ice depot For a designed low temperature T c in te storage cambers, all internal and external eat sources are to be absorbed by te evaporator o a cooling system. Heat inluxes rom te outside depend on te site climatic conditions, te termal resistance o structures and te T c value. Calculations o te eat pump output sould be based upon te correct presetting o boundary conditions, wic relect te real caracter o eat excange on te ground surace. Tis is o particular signiicance or a structure composed mainly o ice and cooled to a low temperature. Application o te surace temperature as a boundary condition could, or te most part, entail serious errors (Perlstein and Kapranov, 1983). Te equation o eat balance at te ground surace can be presented as 384 Te 7t International Permarost Conerence

3 Figure 2. Temperature proile troug tawed ground and ice (steady state). q = q T t were: 0 Λ s [1] ( ) q = Q ( 1 A)+ I I + α T + α e e o s a o a e a o Λ= α + α + bα r e were Q s incoming sort-wave radiation; A -albedo; Ι o Ι a are long-wave radiation o te ground surace at 0 C and te air (respectively); α coeicient o convective (sensible) eat excange; T s - ground surace temperature; T a air temperature; α e coeicient o latent eat transer; e a is water vapor pressure in air; e o is te saturated vapor pressure at 0 C; α r coeicient o linear approximation o te radiation-temperature dependence (o te Stean-Boltzman law); b is te coeicient o te saturated vapor pressure - temperature curve; q t is te eat lux into tawing ground. Te pysical meaning o q o is te total eat low rom atmospere onto te ground surace at 0 C. At te Malkacan site during te warm season, te montly average q o values vary between 145 W/m 2 (May) and 610 W/m 2 (July) and (ranges rom 40 to 45 W/(m 2 K). Correlation between climatic and constructive parameters may be obtained considering one-dimensional eat lux troug partly tawed insulation (Figure 2). At steady state, eat luxes are te same in bot te tawed and rozen zones. So te ollowing expression is true: [2] [3] λtts t λ Tc = or, taking into account (1): qo = + T Λ 1 t λ λ c were λ te conductivity; tickness; indices Ç t È and Ç È relate to te tawed and rozen layers, respectively. Te expression (4) is elpul wen coosing te insulation tickness. On te one and, te most economic way to increase te insulation is by building up ice. In doing so, te ice melting as to be eliminated and tus te maximum taw dept must be less tan te tickness o ground layer. But, as is sel-evident and as sown by equation (4), an increase o te rozen layer tickness induces an increase in te taw dept. Tereore, in neglecting te dierence between ice and rozen ground conductivities, we ave to cange equation (4) or te limiting condition: qo λt λt g i ΛT λ Λ c were i is te ice ice tickness. t Equation (5), as applied to te site conditions, can be presented in te orm: , m g i Dimensions o ice-ground barrier elements (Figure 3) were determined using equation (6). [4] [5] [6] S.A. Guly, G.Z. Perlstein 385

4 Figure 3. Plan and typical sections o te ice depot. 1 - ground; 2 - ice; 3 - boundary between tawed and rozen zones. Heat inluxes were modeled as a two-dimensional Stean problem. Te inite dierence metod was applied. Simulations o eat lows troug te wall and ceiling were ulilled or two cross-sections sown in Figures 3b and 3c. On te external surace, boundary conditions were given in te orm o equation (1). On te inner surace, a constant temperature o -20 C was maintained. Te main results o te modeling are as ollows. At te beginning o te taw season, te amount o eat inlow is equal to 24.7 kw. In July it acieves its maximum, approximately 33.5 kw. Te maximum taw dept witin te roo area varies rom 0.4 to 0.8 m, depending on distance rom te cell axis and corresponding ground insulation tickness (Figure 4). In late August, te tawed ground begins to rereeze rom below. Modeling o te temperature ield under te loor and te adjacent ground surace was conducted separately, under te ollowing simpliied boundary conditions: 0 C at te dept o 18 m; -20 C witin te store-ouse; qo Ts = variable temperature Λ on te ground surace; linearly canging temperature between T s and -20 C under te wall (at te dept 0 m). Resulting eat inlow 386 Te 7t International Permarost Conerence

5 Figure 4. Taw dept o ground insulation. Distances rom te cell axis: a - 0, b - 1.5, c m. was 2.2 kw and proved to become quasi-steady very quickly. So, te total eat inlow rom outside te storeouse varies rom 26.9 kw (mid-may) to 35.7 kw (late July). A sizable portion o eat will be released wen reezing te is. Te average rate o is storing is expected to be about 3.0 tons per day wic is a eat source o 11.6 kw. Tereore, te overall rate o eat removal is 47.3 kw. Approximately te same additional cold output (46.5 kw) must be reserved or te event o a peak salmon input to te storage. Consumption o te produced eat As is sown above, te rate o ice depot rerigeration as to range rom 26.9 to 93.8 kw to maintain an interior temperature o -20 C during te ising season. As a result, te cooling system will operate at only al o its capacity except or a ew peak days. So, it is very important to determine a sound strategy allowing te rational usage o bot produced eat and equipment capacity. Te consumption o energy or eating dwellings and or ot water supply must not exceed 15-20% o te total. On te strengt o te above, a greenouse wit a ligt syntetic cover sould be considered as te main consumer o te eat being produced rom early May up to late September. In te Magadan region, cucumbers and tomatoes are in demand and te most proitable crops. Tey are grown successully i te soil temperature is maintained at about C and te air temperature does not drop or a long time below 15 C. Planting takes place in te irst two weeks o May. Natural conditions o te site do not provide suc a termal regime outside o greenouses. In te absence o eating, even greenouses do not ave temperatures ig enoug at nigt. Te quantity o eat required to maintain te necessary interior temperatures in a ligt seasonal greenouse is given in Table 1. Wen eating a greenouse wit eat pumps, te coeicient o perormance ϕ is te most important energy caracteristic o a device. It represents te ratio below: Qt QL + ε ϕ = = ε ε were Qt te total eat output; QL is te rate o lowtemperature source eat extraction; ε electrical power consumed. Te coeicient o perormance depends on te termal parameters o te eat pump cycle: T2 ϕ = n T T 2 1 were T 1,T 2 temperatures (respectively) o boiling and condensing, measured in K; n is te ratio between real and ideal eiciencies o te termodynamic process, or modern eat pumps, usually close to 0.5. At given temperatures o is storing and vegetable growing, te ϕ -value is about 2.8. [7] [8] S.A. Guly, G.Z. Perlstein 387

6 Table 1. Greenouse eating requirements As a result rom equation (7): QL ε = ϕ 1 1 Qt = QL 1 + ϕ 1 Using equation (9) one can calculate tat, depending on eat pump operating conditions, te total eat output comprises approximately: 1) 73.6 kw under averaged conditions o illing o te storage; 2) 146 kw under peak loads wile basic and reserve eat pumps are bot running. Associated electrical powers are equal to 26.3 and 52 kw. Comparing tese numbers wit te content o Table 1, it is clear tat te total output o te basic and reserve eat pumps is quite suicient or a greenouse wit a growing area o 1000 m 2. Discussion Economic eiciency is undoubtedly te most important criterion wen comparing a new tecnical solution wit well-developed traditional metods. In tis case, a major part o revenues is obtained by selling wellpreserved and valuable sea ood. A conventional storage ouse wit an ordinary rerigeration unit could also provide te revenues rom te sale o is, but it is not under consideration in tis paper. [9] Additional advantages are taken rom te use o ice and local ground as building materials. Tis is o particular signiicance in remote places witout a developed inrastructure. An ice depot is ormed in winter, troug layer-by-layer reezing o water. Te reezing rate may be determined using te approximate expression: λi 2λiTt a i = + + Q λ α α o [10] were: is te rozen layer tickness; Q o speciic volumetric latent eat; te oter symbols are as used earlier. Calculations by equation (10) sow tat, at te Malkacan site, an ice body wit a tickness o 12 m can be ormed in tin daily layers during one winter. O course, te project scedule sould allow or a substantial contingency wic it does as te required eigt o ice structures is m. Building expenses are linearly dependent on te size o ice and ground components o construction. Te estimated cost o tis ice depot (witout cooling equipment) is about $20,000 U.S. at te present wage level in Russia. Tis is between 15 and 30% o te costs associated wit conventional storage. Te structural parameters inluence in a complicated way te economics o te cooling system. Reduction o insulation results in greater eat inlow but it leads to a rise in electrical energy consumption. Also, in te event o pure rerigeration (witout eat utilization), it is Te 7t International Permarost Conerence

7 Table 2. Comparison o alternate project concepts possible to lower te condensation temperature T 2 and decrease electricity use. Indeed, by using ground water in te condenser, T 2 may be kept at about 10 C, so ϕ = 4.1 and ε = 14.5KW. Te sceme o extracted eat usage was cosen ater taking into account operational expenses, greenouse construction costs and revenue rom selling vegetables. A comparison o tree versions was ulilled: 1. Rerigerated storage witout eat utilization; 2. Heat pump cooling system wit reserve equipment operating only in te course o peak loads; a greenouse o area o 500 m 2 serves as te extracted eat consumer; 3. Heat pump cooling system wit basic and reserve equipment being used simultaneously or eating a greenouse o area o 1000 m 2. Te results o tis comparison are given in Table 2. As would be expected, te minimum cost o electrical energy is associated wit pure rerigeration. Furter cost reduction could be acieved by increasing te insulation o te ice-ground walls and ceiling. Tis results, owever, in a iger building cost. Te tird variant is te most attractive economically. Total expenses or te ice depot cooling and greenouse running and eating are equal to $18,700, wereas te cost o greenouse production is $42,000. So, te increased revenue rom vegetable sales osets te capital and operating costs o te eat pumps almost entirely. Te low value o te coeicient o perormance represents te disadvantage o tis sceme. Tere are considerable opportunities to improve it. For example, during dayligt, wen te greenouse does not need ea-ting, ground water can be used or vapor condensation. Because o tis te T 1 value is lowered to 8-10 C, ϕ is raised to and ε amounts to 13.6 kw. Ground water sould be used as a low-potential eat source rom early May to mid-june, beore te ising season starts. Tis results in te ollowing caracteristics: T 1 =0 C, ϕ=4.8 and ε = 34.5 kw. Proposals o tis kind could signiicantly improve te energy eiciency o eat pumps in an ice depotgreenouse system. Te economic indices o te simplest operational scemes, as presented in Table 2, may be signiicantly improved as well. Tis question must be studied in detail and wit suicient accuracy at te next stage o engineering design by properly estimating capital and operating costs. Conclusions Work on tis project concept as conirmed te tecnical and economic merit o an ice ood depot cooled by eat pump tecniques, wic serve to maintain te necessary termal regime in an overall cold storagegreenouse system. Owing to te simplicity, availability and low cost, ice-ground structures are o particular interest to small companies involved in te traditional trades o te nortern native population (unting, ising, reindeer-breeding). Constructive eatures are connected wit te eat extraction rate. Tese actors inluence economic eiciency o an ice depot coupled wit a greenouse. A low coeicient o perormance value represents some demerit or te cooling-eating concept tat is under consideration. Probably tis problem is common to te use o eat pumps in permarost territories. It can be overcome wen applying a combined system tat S.A. Guly, G.Z. Perlstein 389

8 uses te eat extracted rom rozen regions (ice depot) in parallel wit oter sources, suc as ground water. At te stage o detailed design, te discussed relationsips between tecnological parameters and natural site conditions will be used as well as proposed cooling-eating scemes. Te autors tank teir colleague A.Buysky or is assistance wit computer simulations. Te initial consideration o ice structures cooled wit eat pumps was accomplised under te auspices o Russian Fund o Fundamental Researc. We igly appreciate te elp o D. McKinlay wo as taken te trouble to assist in editing tis paper. We also would like to tank te reviewers or teir job and valuable remarks. Acknowledgments Reerences Goodric, L.E. and Plunkett, J.C. (1990). Perormance o eat pump cilled oundantions. In Proceedings o te Fit Canadian Permarost Conerence, Centre dõžtudes nordiques, Universite Laval, pp Krylov, M.M. (1951). Isotermal ice storages. Academy o Sciences o te USSR (88 pp.) (In Russian). Perlstein, G.Z. and Kapranov, V.E. (1983). Metods o quantitative valuation o regional eat resources or preparation o permarost placers to mining. In Proceedings o te Fit International Conerence on Permarost, 2, Trondeim, pp Perlstein, G.Z., Vlasov, V.P. and Krustalev, L.N. (1994). Te use o eat pumps or building in permarost area. In Te 2nd International Conerence on te Arctic Margins. Abstracts vol., Magadan (91 pp.). 390 Te 7t International Permarost Conerence