Thermal Loads Analysis of an Underground Cold Storage Facility in Attica

Transcription

1 Thermal Loads Analysis of an Underground Cold Storage Facility in Attica Grigorios Brachos 1, Andreas Benardos 2 1 Mining Engineer, MSc, Greece 2 Mining Engineer, PhD, Lab. of Mining & Environmental Technology NTUA, Greece ABSTRACT Underground cold storage facilities present considerable advantages over conventional surface storage warehouses. Especially in climates like Greece s the energy conservation can be of extreme importance to the overall project s success. The paper focuses on the proposed development of an underground refrigerated storage facility with a special aim to calculate its thermal loads and to assess its performance and advantages over a respective surface facility. This is made possible by the comprehensive design of the underground storage and the detailed calculation of its thermal/heat loads, followed by the thermal load analysis for the case of the surface facility. 1. INTRODUCTION UNDERGROUND COLD STORAGE FACILITIES The adaptation of underground spaces for a number of uses is something that nowadays is attracting the interest of many researchers, companies and public bodies. Even though the utilisation of underground space environments has a history that can be traced back to the beginning of mankind, the subsurface has now become the provisional field for the development of modern infrastructures. Thus, underground space is becoming more and more widely used as a solution, especially in urban areas, to minimise the disturbances caused by aboveground activities (Kaliampakos, 2006). Besides the traditional uses of underground space for transport tunnels, car parks, civil defense shelters, etc., this very concept of underground space development encompasses the relocation of several surface land uses or activities, which are difficult, impractical, less profitable, or even environmentally undesirable to be installed on ground level, into subsurface built environments. A non-typical application of underground space is the development of storage warehouses for frozen goods (Unver and Agan, 2003; Choi et al., 2000; Goodall et al., 1989). This particular use takes advantage of the inherit attributes of underground space; the temperature and humidity conditions remain balanced in a yearly basis and are not affected by seasonal changes and thus resulting in considerable energy savings (Williams, 1976; Sterling, 1994). Especially, in countries like Greece, having very hot summers the adoption of similar solutions could result to considerable capital and operating expenses savings, as, both the insulation and energy consumption requirements could be limited if compared with a typical surface warehouse facility. Nevertheless, beyond qualitative expressions a detailed analysis between the two alternatives, the surface and the underground one, is needed, capable of quantitatively measure their respective thermal loads and finally translate them in financial bottom-line revenues. The paper deals with an underground cold storage facility and its main aim is to calculate the thermal loads/losses of the underground cold storage space and to provide a comparative analysis with the ones of a respective surface facility for the conditions prevailing in Greece. To facilitate this, the complete design of an underground cold storage facility and a detailed calculation of the thermal loads have been made, along with the thermal load analysis for the case of the surface storage facility. 237

2 2. DESIGN OF THE COLD STORAGE FACILITY IN ATTICA The selection of the most suitable area for the construction of the underground cold storage complex is a composite problem, as the weighting of advantages and disadvantages of each candidate area should be carefully examined. With respect to the prerequisites set by several companies, as result of a relevant market analysis, the main characteristics that an ideal establishment area should have, in order of importance, are: i. Easy access to major road transportation network ii. Low land cost iii. Easy access to commercial port iv. Small distance from the centres of distribution v. High degree of freedom in establishing land use types vi. Easy access to railway network Furthermore, with emphasis to the underground development of the complex, the geologic setting of the site should be appropriate and capable of sustaining the underground excavation with a high degree of safety. Within the Athens basin there are several areas (e.g. Shimatari, Inofita, Thriasion, etc.), that fulfil the above requirements and logistic companies have already started to swarm in there. It has been finally concluded that the most advantageous location for the siting of the complex was the Thriasion area and more particularly the area of Skalistiri hill (Fig. 1a) (Brachos, 2005). This location allows the easy access to the major road networks of Greece, the port of Piraeus and it is just a few kilometres from the Athens metropolitan area, its main consumer target. The underground complex is proposed to be constructed in the eastern part of the hill, having a maximum overburden of 60m. The geology in the area is consisted of limestone and dolomitic limestone formations (Fig. 1b), while the geotechnical classification of the formation indicates that the rockmass can be characterized as fair to good quality, having an average RMR value about 57 and average Q and GSI values about 3.9 and 64, respectively. Furthermore, based on the available hydrogeological data, the water level is found at a depth approximately 100m lower than the proposed construction level, fact that minimises the possibility of encountering any serious problems caused by underground water. Proposed construction site Fig. 1. (a) Location of the area selected for the development of the underground cold storage facility and (b) geological map of the wider area of the Skalistiri hill. The complex will have a total available area of about 11,500 m 2, with a maximum height of 6 m, thus the total excavation volume is estimated at approximately 70,000 m 3. The design is made in an attempt to facilitate the cold storage use and the dimensions of the underground excavations have been carefully selected for this task. The construction will be made following the principles of the room and pillar mining method and the development stage includes the excavation of parallel and transverse 238

3 galleries, leading to the formation of patterned pillars. The square pillars left after the excavation will have a width of 10 m, while the rooms will be 9 m in length. Using the tributary area method and the Obert and Duval (1967) pillar strength formulae, it is estimated that the safety factor of the pillars will be well over 6, ensuring the structural integrity of the complex. Fig. 2. (a) 3D view of the proposed complex developed by the room and pillar method, (b) General layout of the refrigeration compartments in the underground cold storage facility. 3. THERMAL LOADS ANALYSIS UNDERGROUND VS ABOVE GROUND FACILITY 3.1 General Outline The analysis and calculation of the thermal loads is an essential part of the design of the refrigeration system (Anastasiadis, 1989). A definition of thermal loads or thermal losses to be more exact is the amount of heat that should be provided to the facility so as to maintain its designated temperature. The thermal energy is interfused from a material of higher temperature (T 1 ) to another having a lower temperature (T 2 ) while the rate of this transfer is proportional to the magnitude of the temperature difference, temperature gradient (ΔΤ). In a typical cold storage facility the thermal losses that should be taken into account are the losses due to the energy flux from the outside environment to the refrigerated space through its structural elements (walls, etc.), the loads and thermal exchange from the stored products themselves, the lighting and ventilation systems, as well as from the thermal loads derived from the personnel. As both the underground and the above ground facility have been designed so as to have the same storage characteristics, it can be assumed that the principle differentiation between the thermal loads imposed on each facility is only linked with the different behaviour which the structural components of those two types of facilities present. Thus, for the case of underground complex the limestone rock and the installed insulation layers while for the surface facility the building structure along with its installed thermal insulation. The heat flux, heat loss for the cold storage facility, which will eventually flow through the structural element is in turn influenced by the time period of the flux, the temperature gradient, the thickness and type of the material and finally the area involved. It can be calculated using the following formulae: λ A ΔT Q =, for the case of a single material (1) d Q = k A ΔT, for the case of composite materials or multi-layer structures (2) 239

4 where, Q is the thermal transmittance or heat loss (Btu/h or W); A is the outside contact surface area (ft 2 or m 2 ); λ is the thermal conductivity of the insulation material (Btu/h. ft. F or W/m. K); k is the overall thermal transfer coefficient of the multi-layer material (Btu/h. ft 2. F or W/m 2. K); ΔΤ is the temperature gradient ( F or o C), and d is the material thickness (m or ft). The assumptions followed in the analysis are: Designated temperature in the refrigerated storage areas: -20 o C Designated temperature in the passages and utility installations: 22 o C Outside temperature (surface facility - summer): 38 o C Outside temperature temperature of host rock (underground facility): 8 o C The thermal flux from the underground cold storage to the bedrock is gradually reduced over depth and diminished after 15m (Staufer, 1976) Influence of solar radiation (only for the surface facility): 4 o C 3.1. Thermal Loads Analysis Underground storage The underground complex has three different types of sidewall insulation layers installed (Fig. 1b), namely Type A, B, and C (Brachos, 2005). The first system is used to separate the main complex from the bedrock, the second is the internal sidewall system between the refrigerated areas and the third is comprised by the rock pillars left in the facility. As far as the roof and flooring insulation is concerned, the Type A system has been selected. Their characteristics are presented in Table 1. Table 1. Characteristics of the insulation types used in the underground facility. System Layer id Material Thickness d (cm) λ (W/m. K) 1 Limestone Concrete Type A Type B Type C 3 Polyurethane Concrete Polyurethane Air space Bricks Polyurethane Concrete Limestone Concrete Polyurethane Note: 1 W/m. K = 0.86 kcal/m. h o. C = 0.58 Btu/ft. h o. F The calculation of the overall thermal transfer coefficient (k) can be made by taking into account the thermal conductivity of each one of the materials comprising the insulation layers of the site, in the sidewalls, the roof and the floor area, as well their respective thickness. Finally, in order to estimate the losses due to the convection the respective coefficients in the inner (α i =7 W/m 2. K) and outer (α a =23 W/m 2. K) parts of the sidewalls are added. The resulting formula, for layers combined in series, is as follows: 1 1 d j 1 = + +, j=number of material layers (3) k α λ α i j j a The estimated overall thermal transfer coefficient for each one of the three insulation systems is presented in Table 2. Using these results along with equation (2), the thermal losses of the underground complex due to its structural and insulation materials is estimated at approximately 59.1 kw. 240

5 Table 2. Estimation of the thermal transfer coefficient of the three insulation systems. Insulation system Overall thermal transfer coefficient (W/m 2. K) Type A Type B Type C Thermal Loads Analysis Surface storage The surface facility is designed with only one type of insulation in the sidewalls (Table 3). For simplicity reasons, and even this is a very conservative approach, it is assumed that the same type is also used in both the roof and flooring, while for the case of the exterior walls an adjustment of 6 o F is further added in the temperature gradient of Eq. (2) so as to include the effects of solar radiation. The thermal transfer coefficient (k) is estimated at W/m 2. K and the overall thermal losses of the surface cold storage facility is about kw. Table 3. Characteristics of the insulation layers used in the surface facility. Layer id Material Thickness d (cm) λ (W/m o K) 1 Concrete Polyurethane Air space Bricks Polyurethane Concrete Comparative evaluation The analysis performed indicates that there is a considerable difference in the thermal losses between these two types of cold storage facilities. More particularly, the thermal loads that should be replenished by the refrigeration system are about 3.5 times lower in the underground complex. Even though the insulation method used in the surface facility is composed of high quality, and very expensive, materials in terms of thermal conductivity, the inherit characteristics of underground space, namely the absence of solar radiation, the low temperature and the insulation performance of the host rock eventually prevailed. Although a skeptical argument might be that the initial high investment cost to set up the insulation system in the surface facility should be equally weighted with the investment needed for the underground construction, this can be challenged if one can estimate the gains from a possible sale of the aggregate excavated materials. More importantly though, are the gains from the lower operating costs achieved in the underground complex. This is made possible not only by the minimal service cost of the insulation itself, as the surrounding rock does the greater part of the job, but also with the lower electricity consumption needed for the operation of the freezer units. The electrical power which can be saved in a year s time is approximately 1,245x10 3 kwh and if the project s lifespan (i.e. 25 years) is taken into account the electrical power savings can reach up to 31,125 x10 3 kwh. A simple financial analysis for these issues is given in Table 4, where using the current electrical billing tariffs for industrial applications, an amount of 43 x10 3 can be saved every year due to lower power consumption, while this amount reaches a total of 860 x10 3 for the lifespan of the facility. Furthermore, the total reduction in CO 2 emissions for the project life, assuming a mean emission factor of 349 kg CO 2 /MWh for lignite produced electricity (Velma, 2004), reaches approximately 11,000 tons. Table 4. Comparative analysis of the expenditure for electric power in the cold storage complexes. Cold Storage complex Thermal loads structural components (kw) Electricity gains per year - underground ( ) Electricity gains in project s lifetime - underground ( ) Surface facility Underground facility , ,

6 Besides that, it is expected that with time the rock will absorb large amounts of freezing loads and its initial temperature will be further dropped. Consequently, this will result to an enhanced performance in terms of thermal losses, which it will in turn maximize the gains in electricity. 4. CONCLUSIONS The results for the case analysed in the paper, indicate that the thermal loads of the structural components in the underground site is approximately 70% lower than the ones of the surface complex. More particularly, the main reasons that lead to such results are: The initial temperature of the host rock is almost 8 o C, considerably lower than the one of the surface facility s outside environment, which in the summer period can even exceed 40 o C. The underground space environment is not influenced by the solar radiation, contrary to the surface facility that is constantly under the thermal stresses of the sun. The limestone host rock has a very low thermal conductivity and acts like a good quality insulation material, resulting to the minimization of thermal losses. Furthermore, apart from the above reasons, the insulation characteristics of the host rock minimize any requirements for the installation of any additional insulation, which is a plus in order to minimize the development cost. Also, unlike any engineered materials the rock does not sustain any gradual degradation in its insulation performance over time. Instead, its performance is enhanced and due to its large heat capacity, it can sustain the proper maintenance of the stored products even if the case of failure of the electromechanical installations, as it will transfer its thermal (freeze) load to the underground complex. This alternative freezing method could be also used as a mechanism to minimize the electricity cost of the complex, especially in peak hours. REFERENCES Anastasiadis, S.P., Calculation of freezer units, Athens. Brachos, G., Design of an underground cold storage facility. Diploma thesis. NTUA. Athens. Choi, S.O., Park, H.D., Park, Y.J., Kim, H.Y., Jang H.D., Test running of an underground food storage cavern in Korea, Tunnelling Underground Space Technology, vol.15, no.1. pp Goodall, D.C., Utheim. T., Thorbergsen, E., Back analyses of heat loads in selected thermal storages. Proc. Storage of Gases in Rock Caverns. Balkema. Kaliampakos, D., Critical remarks in urban underground development. Proc. Int. Academic Conference on Underground Space (IACUS), Beijing. Obert, L., Duvall, W.I., Rock mechanics and the design of structures in rock. John Wiley & Sons. Staufer, T., Efficiency in the use of energy has been effected through industrial use of subsurface space. Proc. Underground Utilisation: A Reference Manual of Selected Works. vol.5. pp Sterling, R.L., Utilization of underground facilities for the storage of food -why should go underground? Int. Symp. Grain Elevator and Underground Food Storage. Seoul, pp Unver, B., Agan, C., Application of heat transfer analysis for frozen food storage caverns. Tunnelling Underground Space Technology. vol.18, no1. pp Velma, I.G. (ed), Climate Change: Five years after Kyoto. Science Publishers. Williams J.L., Energy saving in underground warehouses. Proc. Underground Utilisation: A Reference Manual of Selected Works. vol.5. pp