It is well-known fact that the industrial sector consumed around 52 per cent of global delivered energy and its energy consumption grows by an average of 1.5 per cent per year over the projection. The air conditioning of warehouses and cold-storage chambers are an important part of this type of consumption in these industries. In sectors like the agricultural industry, it is necessary to carry out a rigorous control of the air temperature in warehouses, due to the sensitivity of the stored products. It is found that during summer, the ceiling and the upper strata gets warmer, whereas the cold air accumulates in the lower levels, increasing the stratification of indoor air. During winter, the ceiling gets cold due to its contact with the outdoor air, therefore, the colder, heavier air moves down to the lower strata, registering insignificant vertical temperature differences as shown in Figure 1. Air conditioning of the warehouse, besides controlling the temperature, limits the influence of the outdoor environment on the stratification of temperatures.
Temperature and relative humidity are the most important environmental factors affecting the sensory quality of fresh produce. Inadequate storage conditions may provoke undesired physicochemical changes and loss of quality in the stored products. In addition, certain humidity and temperature conditions may favour the presence of insect pests which deteriorate or spoil the product. On the other hand, poor indoor environments in industries also lead to substantial costs for health care, administration and lost productivity. Therefore, more research should be directed to these issues in order to innovate new practices and strategies to achieve indoor environments desired in industrial warehouses.
Air Conditioning of Warehouses
Warehouses may generally be operated by the help of different technologies taking into account building characteristics as well as the storage and handling equipment employed that determine storage and space utilisation. The following four most common types of warehouse technology, i.e. a) block-pallet storage as shown in Figure 2 b) wide-aisle racking c) narrowaisle racking and d) automated storage and retrieval systems. Considering benchmark figures, it becomes obvious that most material handling systems in receiving, storage, order picking and replenishment rely on conventional or conveyor-based operations; automated systems are only used in 5 per cent -9 per cent of all systems. Thus, despite potential process improvements, a future possibility for improving the warehouse performance might also be given by increasing investments in mechanisation and automation that might on the one hand improve throughput while reducing operational expenses but on the other hand influence CO2 emissions of the warehouse by affecting the overall energy consumption. Warehouse space, thus, seems to be required for dealing with continuously increasing inventories. Due to required energy consumption for lighting
or heating, cooling, ventilation and air conditioning, this directly induces a rise in warehouse-related emissions.
The annual cooling/heating demand as shown in Figure 3 may be derived as the product of heating degree days and thermal losses due to transmission and ventilation less internal heat gains. Heating degree days, defined as the sum of daily temperature difference within a year, for warehouses it varies from a low of 2000K to a high of more than 8000K a year depending on the respective climate zone. Besides, transmission heat losses are determined by the transmission coefficient (U-value) of the building envelope and its total surface.
Conventional warehouses that mainly have low insulated brick, concrete or metal panel walls and metal or synthetic roofs feature an average U-value between 0.25 and 0.30 that might be reduced to less than 0.1 by improving the insulation. Heat losses due to ventilation are determined by the number of air changes per hour that was assumed as 0.6 for the block-place store, 0.4 for the wide-aisle store and 0.3 for the narrow aisle store. The volume of the warehouse and a constant capturing air density and specific heat capacity (note that density of the air was assumed as 1.2 kg/m3 at 65 degree Fahrenheit whereas specific heat capacity was assumed as 1000 J/kg K). Beside health and safety regulations, which govern the required temperature or the number of air changes, the warehouse climate in many applications has to be controlled for specific products such as fresh, chilled or frozen goods. Thus, the climate factor is influenced by the energy efficiency of the heating and cooling system by building characteristics such as wall and roof insulation as shown in Figure 4, the state and quantity of windows and doors, the outdoor temperature and product requirements. Multiplying this factor with the warehouse size determines the aggregated HVAC energy.
In warehouse, an innovative design has been performed to reclaim the cold energy for a warm weather. Conventionally, this was done by installing vapour compression refrigeration systems, necessitating tremendous electrical power to drive the refrigerant compressor working in low temperature maintenance of warehouse as shown in Figure 5.
From the studies carried out previously by many researchers in the different parts of the world, it can be concluded that there is a strong influence of the outdoor temperature over the stratification of the air inside all the studied warehouses. The floor temperature increases as the outdoor temperature rises with the biggest vertical differences appearing during the hottest months. On the contrary, the stratification decreases rapidly when the outdoor temperature is low. Figure 6 shows different cooling principles of warehouse air-conditioning.
Comparison among Various Types of Warehouses
As shown in Figure 7, the block-place type warehouse exhibits the lowest consumption whereas narrow-aisle and AS/RS warehouses have the largest energy consumptions. This may be explained by comparing the operational performance and building requirements of the different technologies.
Assuming a given ground floor area, the more efficient narrow-aisle storage or AS/RS operate a higher amount of goods per year than the block-place or wide-aisle storage. In addition, narrow-aisle storage or AS/RS require considerably larger building heights that lead to increasing building volumes and surfaces which also increase heat losses due to transmission and ventilation.
Considering the different end-use categories, energy consumption for fixed and mobile material handling only accounts for 1.3 per cent in the case of block-place storage, around 8 per cent for the wide and narrow aisle storage and about 15 per cent of total energy consumption for the AS/RS.
Thus, an efficient means of reducing warehouse-related emissions has to take into account energy-efficient lighting and air conditioning systems while minimising heat losses due to transmission and ventilation. Considering the different types of warehouse technologies, changing the luminaries from standard incandescent lamps to fluorescents or even LEDs might reduce required lighting energy by 80 per cent to 90 per cent which leads to decreasing emissions of between 20 per cent and 34 per cent for the median warehouse. On the other hand, improving building insulation might reduce required HVAC energy by 6 per cent to 15 per cent which leads to a decrease of CO2 emissions by 4 per cent to 12 per cent for the median warehouse. As shown in Figure 8, the block-place type warehouse exhibits the lowest emissions whereas narrowaisle and AS/RS warehouses have the largest emissions.
The results of the study may be of great use for warehouses for products sensitive to temperature, which may suffer a different evolution, conservation or maturation when the temperature differences are maintained for a long time.