Abstract
Computational modelling is nowadays a powerful tool for project and design of engineering systems, anticipating and/or correcting problems that may lead to inefficiencies. This chapter describes three distinct computational tools with different mathematical and numerical models. The computational tools are used with the purpose of improving the thermal and energy performance of cold stores. All tools are applied to the same agrifood company. First, Computational Fluid Dynamics is used to optimize velocity and temperature fields for the interior a cold room. Afterwards, an energy analysis and thermal load simulation is performed to the cold store facility to reduce its thermal loads. Finally, a statistical prediction model based on empirical correlations is used to predict the energy performance of the cold store and compare it to an average behaviour. The numerical results indicate the improvement of the thermal performance and consequently of food safety, as well as considerable energy savings that can be achieved in cold stores by the combined use of different modelling techniques.
TopIntroduction
It is recognized by all that energy is a major cost in the operation of many industrial engineering systems. In particular, the refrigeration industry needs to provide cold storage that meets the standards and requirements of food safety. Thus, the energy consumption in this industry reaches significant values. To reduce these high-energy consumption values it is important that the design of the cold stores be developed with special care. Computational modelling is nowadays a powerful tool for project and design of engineering systems, allowing anticipating and/or correcting problems that may lead to inefficiencies. However, to address these problems, several computational approaches can be followed.
The propose of this chapter is to describe the use of three distinct computational tools, with different mathematical and numerical models used for the same purpose which is the improvement of thermal and energy performance of cold stores. Computational Fluid Dynamics (CFD) is used to optimize conditions for the interior of cold stores, in particular the velocity and temperature fields. Additionally, energy analysis and thermal load simulation is performed in order to optimize the envelope of cold store to reduce the mass and thermal loads. Finally, a statistical prediction model based on empirical correlations is used to predict the global behaviour of the cold store as result of management actions and operation of the cold store.
All these computational approaches require the formulation of the physical-mathematical problem. A review on the physical and mathematical representation of phenomena that express fluid flow with heat and/or mass transfer is carried out. To this end, the general governing equations of motion and heat transfer in fluids and solids are described, as well as the simplifications usually considered.
The nonlinearity and complexity of the physical-mathematical models require the use of numerical models for solving the governing equations. The underlying formulations of numerical models are described. The resolution method may involve the discretization through different methods of the differential equations using finite difference or finite volume. Recurrently, the imposition of different boundary conditions is necessary to simulate the physical phenomena. In addition, it is important to highlight the application of techniques to promote the convergence of the solution.
Computational modelling through CFD allows optimizing the inner conditions of cold stores, in particular the velocity and temperature fields. CFD technique consists in predict the behaviour of physical phenomena through computational numerical calculation. The simulation of processes allows predicting the fluid flow with heat and/or mass transfer, species concentration, phase change, chemical reactions, mechanical movements, strains, among others. The mathematical models representing the laws of physics that govern the physical phenomena are described by partial differential equations, many of which without analytical solution.
Other computational tool that uses the finite difference formulation has been tested. This tool was used to perform energy analysis and thermal load calculation in order to optimize the envelope of the cold store to reduce the mass and thermal loads. The simulation accounts for the integrated simultaneous solution of building and engineering systems responses, defining time steps for interaction between thermal zones and the environment. The heat balance based solution technique for building thermal loads is used for simultaneous calculation of radiant and convective effects, while the transient heat conduction through building elements is performed using conduction transfer functions. The tool may include additional models depending on the physical process specifications.
Additionally, a statistical model was tested to predict the overall behaviour of cold stores as result of management and operation actions. The model is used to assess the energy performance of cold stores through the analysis of the energy profile based on a set of characteristic parameters (amount of raw material, annual energy consumption and volume of cold stores) obtained through empirical correlations.
The following sections will describe the study details concerning the application of each computational tool.
Key Terms in this Chapter
Energy Consumption: The energy consumption of a cold store is dependent on its internal dimensions and construction materials, product load and its arranjement inside the cold store, cooling capacity and location of the evaporator, internal and external ambient air conditions (temperature, humidity and direction and magnitude of velocity), doors area and time that are open that will contribute to the infiltration of ambient air, thermal loads of lightining, people and devices operating and generating heat inside the cold store.
Frost Formation: Frost formation occurs when humid air encounters a surface whose temperature is less than the freezing temperature of water (273 K), and is less than the dew point temperature, so that water vapour goes from a gaseous to a solid state. As the frost layer increases in the evaporator surface, the cooling capacity of refrigeration is depleted due to the extra thermal resistance to the heat transfer process and also because it increases the air pressure drop, thereby substantially reducing the fan-supplied airflow rate.
Food Safety: Food safety refers to the conditions and practices that preserve the quality of food to prevent contamination and foodborne illnesses. Ensuring the food temperature below the prescribed limits will promote their safety.
Thermal Entrainment: Dimensionless temperatures or enthalpies difference that quantifies the aero-thermodynamics blockage provided by an air curtain. This parameter varies from 0, which corresponds to no entrainment (unreachable condition) to 1, which corresponds to unblocked passage and entrainment of air between the two contiguous environments.
Heat Exchanger: Piece of equipment built for efficient heat transfer from one medium to another.
Air Infiltration Rate: Infiltration of warm moist air through doors into cold stores during the time for loading and unloading food produtcs causes increased costs for running and defrosting the refrigeration system, safety problems associated with the mist formed in the doorway; safety problems associated with ice forming around the door opening, on the floor and on the ceiling; food quality, safety and weight loss caused by temperature fluctuations.
Computational Fluid Dynamics: Technique to predict fluid flow by means of numerical methods for discretization and solution techniques of the mathematical model composed by a set of partial differential equations representing the conservation laws for the mass, momentum, and energy.
Cold Store: Cold stores have the function of storing a product at the correct temperature and to prevent quality loss. All chilled and frozen food and temperature controlled pharmaceutical products are stored in a cold store at least once from production to the consumer. Chilled stores generally maintain products at temperatures between -1 and 10 ºC whereas frozen stores generally maintain product at below -18 ºC. The cold store market varies from small stores of 10–20 m 3 up to large warehouses of >100 m 3 .
Thermal Load: Amount of heat (sensible and latent) energy to be removed from an inner environment by the refrigeration equipment to maintain that environment at the design temperature when worst case external temperature is being experienced.
Tracer Gas Concentration Decay Method: Experimental method for measuring the concentration of a given gas before and after a certain period of door opening and closing, being subsequently determined the infiltration rate. The tracer gas is injected into the sealed room/division for a short period of time. After mixing with the room air, the door is open and the tracer gas concentration is measured at regular time intervals.
Air Curtain: The Air Movement and Control Association International Inc. (AMCA International) defines air curtain as a controlled stream of air moving across the height and width of an opening with sufficient velocity and volume to reduce the infiltration or transfer of air from one side of the opening to the other and/or to inhibit insects, dust or debris from passing through.