The convection cooling system of the Yakutsk permafrost seed repository

Vladimir N.Panin,Georgii P.Kuzmin

Melnikov Permafrost Institute,Siberian Branch of the Russian Academy of Sciences,Yakutsk 677010,Russia

1 Introduction

Temperature is critical to maintaining seed viability under long-term storage conditions.Storage at-18 °C has been recommended by the international genebank standards for long-term storage (FAO,1994).However,the optimal temperature is species-dependent.However,the beneficial effect to seed longevity of reducing the storage temperature diminishes with a further decrease in temperature.A study by Kershengoltset al.(2013) on the impact of long-term storage on viability of legume seeds showed that temperatures of-6 °C to-10 °C are suitable for maintaining the viability and genetic integrity of seeds.Cold permafrost with such low temperatures is limited in extent and occurs mainly in the Arctic and mountainous areas (Yershov,1998).Therefore,for most of the permafrost region,underground seed vault design must incorporate cooling devices to maintain required temperatures year round.

This paper discusses the methods of maintaining optimal temperature in underground seed storage facilities by utilizing the natural cold atmosphere and reports the first-year performance of a seed repository in Yakutsk,Russia.

2 Cooling and cold accumulation methods

Cooling of the soils surrounding underground structures is commonly performed in the winter by passing cold ambient air through the entire cross section of the structure (Galkin,2000).This causes the interior temperature to drop to values close to the ambient temperature.Also,interaction of the frozen soil with air flow results in sublimation of ice,and drying and crumbling of the walls.For an underground seed repository where the storage temperature is to be maintained within-6 °C to-10 °C,the use of air ducts placed in the surrounding ground is feasible.Air motion through the cooling ducts is induced either by forced or natural convection.Forced ventilation has the advantage of the ability to control air flow over a wide range.Disadvantages are significant power or fuel consumption to operate mechanical blower systems,as well as the risk of storage temperature disturbance due to equipment failures.Utilizing a natural convective draft to create air flow through the ducts is preferable for temperature control in permafrost repositories intended for long-term seed storage.

Cold air convection through the ducts occurs due to the difference in temperature and,hence,in air density between the vertical ducts located at the ends of the horizontal cooling section.The inlet and outlet ducts are raised to different heights,one above the snow cover and the other 3–5 m above the ground surface(Kuzmin,2002).The difference in pressure between air columns of equal length at the base of the vertical ducts causes the cold air to flow through the cooling ducts.Cold air circulation is maintained as long as the mean temperature of the descending air column is colder than the air temperature in the horizontal cooling duct.Therefore,the operating season of natural convection air-cooling systems is shorter than the period of subfreezing air temperatures.In Yakutsk,for example,the period when ambient temperatures are below 0 °C is about 7 months in length,while the air convection devices are operative for only 4.5–5.0 months.

Ambient air and ground surface temperatures exhibit harmonic variation.This periodic variation gives rise to ground temperature variation with the same period and an amplitude that diminishes exponentially with depth in accordance with the Fourier law(Kudryavtsevet al.,1974).In permafrost areas,however,asymmetry of the temperature distribution is observed within the layer of annual temperature variation due to phase change.The penetration depth of annual temperature waves is determined in this case by the sum of seasonal thaw depth and depth of annual temperature fluctuations within permafrost,and may reach 15–25 m (Yershov,1998).Methods for estimating these values are well-known and described in,for example,Kudryavtsevet al.(1974).Such temperature distribution in the upper part of permafrost should be taken into account when selecting placement depth for a seed repository,because in early winter the bottom part of the layer of annual temperature variation is colder than the upper part of this layer and is always colder than the underlying permafrost.

In order to prevent warming of the repository temperature during summer months due to heat flow from the ground surface and upflow of geothermal heat,the surrounding permafrost can be cooled by accumulating the winter cold.The optimal location for air convection cooling ducts to accumulate the cold of atmospheric air is the upper part of permafrost above the repository.A row of heat exchange horizontal ducts is placed at the bottom of the active layer.Cold air circulation through the ducts during most of the winter period cools the underlying permafrost and promotes active-layer freezing from below.The depth of repository placement is determined based on the main patterns of temperature wave propagation in permafrost described in Kudryavtsevet al.(1974).

3 Description of the Yakutsk seed repository and its cooling system

In December 2012,Phase I of the Federal Permafrost Seed Repository with a capacity to store 100,000 seed samples was completed in Yakutsk by reconstructing an underground laboratory of the Permafrost Institute built in 1941 (Figure 1).The repository has one lengthwise room (1) 52.6 m2in area,and three lateral rooms (2) with areas of 11.1,10.7 and 5.8 m2,respectively,totaling 80.2 m2.

Figure 1 Seed repository plan view,storage level.1:main room;2:lateral rooms;3:main shaft;4:secondary shaft

Figure 2 shows details of the cooling system of the repository,which sits at a depth of 10 m from the ground surface.According to Pavlov (1975),this is the depth of zero annual temperature amplitude in this area.The roof and side walls are reinforced with a 20cm×20cm larch timber (15) sheathed with 8-mm-thick magnesium oxide wallboards.The floor is 20-cm-thick concrete.The main room is connected to the ground surface with two vertical shafts reinforced with a steel construction covered with sheet metal (2).The main shaft (8) used for personnel access is equipped with inclined stairs attached to the platforms.The secondary shaft (1) contains haulage equipment,as well as stairs to be used as an emergency exit.

Figure 2 Air convection duct system.(a):longitudinal section;(b):cross section.1:secondary shaft;2:metal reinforcement;3:ADS-1 inlet;4:ADS-2 inlet;5:ADS-2 horizontal cooling pipes;6:ADS-2 outlet;7:ADS-1 outlet;8:main shaft;9:soil fill;10:active layer;11:doors;12:lateral rooms;13:ADS-1 horizontal cooling ducts;14:main storage room;15:timbering;16:sheathing;17:ADS-2 manifolds

To maintain desired temperature levels in the storage area,the seed repository is equipped with two air duct systems (ADS) illustrated in figure 2.One,ADS-1,is used for winter cooling and consists of a vertical inlet (3) for downward cold air flow,horizontal cooling ducts (13),and a vertical outlet (7) for warm air.The horizontal ducts,20cm×50cm in cross section,are placed along the main room behind the timbering (15),except where they cross inside the lateral rooms (12).

The other system,ADS-2,is designed for temperature stabilization during the summer period.It consists of a vertical inlet (4),219-mm-diameter steel pipes (5) spaced in a parallel row 1.5 m apart and buried at the base of the active layer (10),manifolds(17),and a vertical outlet (6).

4 Comparison of the predicted and measured temperatures

In the climatic and ground conditions of Yakutsk,cold air circulation through ADS-1 starts in the second half of November and ceases in late March.Cold air motion in ADS-2 is maintained between mid-October and mid-March.However,the time when the air convection devices are operative may vary significantly depending on weather conditions.

The ADS parameters were selected for this study based on numerical modeling of the ground thermal regime around the repository.The heat balance modeling program developed at the Melnikov Permafrost Institute was used.The predicted temperature distributions after the first cycle of ADS operation in 2013 are shown in figure 3.

As seen in the graphs,ground temperature at repository depth in August is 1.5 times warmer with ADS-1 only (Figure 3a) than with ADS-1 and ADS-2 operated simultaneously (Figure 3b).This indicates the efficiency of ADS-2 for summer temperature stabilization in the repository.

In the early winter when permafrost temperatures are the warmest,ground temperatures at the level of the repository are lower than at any other depth (Figures 3c,d and Figure 4).This means that the depth of the repository is optimal for Yakutsk.

Figure 4 gives a comparison between the predicted and measured mean monthly temperature profiles for August and September,which show little discrepancy.The strong increase in ground temperature measured in September immediately near the repository was due to the accidental leak of water on the surface,which penetrated down the lumpy soil fill behind the service shaft timbering.

The air temperature regime in the repository for the first year of its operation is illustrated in figure 5.After the entrance of cold air into the storage rooms was discontinued,the temperature began to rise.From early May,the rate of temperature increase slowed somewhat due to the effect of cold temperature waves from ADS-2 that reached the repository level.The sharp increase to-1.0 °C in the end of August,followed by a slow decrease down to-3.0 °C in the second half of October,was caused by water from an accidental leak which infiltrated down the poorly backfilled space behind the wall of the secondary shaft.By the end of October,the temperature was decreased to-8.0 °C by running cold air through the entire section of the main room.It should be emphasized that annual temperature variation is inevitable in the repository,which relies on ambient air for cooling.However,the amplitude of temperature variations will decrease to a certain limit with an increasing number of cooling cycles.

Figure 3 Predicted ground temperature distribution around the repository.(a) with ADS-1,August;(b) with ADS-1 and ADS-2,August;(c) with ADS-1 and ADS-2,September;(d) with ADS-1 and ADS-2,October

Figure 4 Ground temperature variation with depth above the repository.1:August;2:September.solid line:measured data;dashed line:predicted data

Figure 5 Interior air temperature in the repository during the first year of operation

5 Conclusions

Based on the results of thermal modeling,the performance of the air duct systems,and the repository temperatures observed over the first year of operation,we draw the following conclusions:The air duct system for summer temperature stabilization is not capable of maintaining the required temperatures in the beginning of the winter period because of its short operating season under the climatic conditions of Yakutsk.However,storage temperatures in the range of-6 °C to-10 °C can be attained during this period by passing cold air through the main room.

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