4 - Design Description

4 - Design Description

ME4054 – Cooler Group

4.1 Summary of the Design

The refrigerated storage facility that has been designed will utilize multiple different methods for temperature management; together they will create the required environment to preserve the desired produce for sufficient lengths of time. The major tactics employed in this design are absorption refrigeration, urethane insulation, efficient building design, interior airflow generation, and interior air exchange.

The major method of interior temperature management will be absorption refrigeration. This method of refrigeration uses an external heat source to power a thermal pump rather than using a traditional compressor-driven air conditioning system. By eliminating the compressor from the refrigeration system, the dependence on electricity was eliminated.

The next method of temperature management employed in the storage facility design was to use urethane refrigeration panels to manage the heat flow between the environment and the interior of the building. Used urethane refrigeration panels can be obtained from disassembled coolers at reduced costs that have close to the same insulating properties as new panels. The panels that will be used to insulate the walls of the warehouse have locking mechanisms built into the sides so close-fitting installation is easy and fast. The panels also yield a seal that prevents moisture ingression, which can lower the effectiveness of the insulation.

The floor plan of the warehouse was designed to create spaces that are conducive to maintaining their required temperatures. The natural temperature of the ground is not subjected to a large temperature variance; it is also near to the required temperature of the lowest room in the structure. The basement room is also split between squash storage and squash curing during the fall harvest; as soon as the curing is completed, the heated space can be converted to more storage quickly and easily. The cabbage cooling room is strategically planned to be the top room in the warehouse. Placing the coolest room in the warehouse seven feet into the ground with the remainder being built above ground will allow the room to be maintained with the natural exterior cold air of winter. Figure 1 shows the recommended building layout as previously described.

Figure 1: Warehouse Floor Plan

Lack of sufficient airflow within the warehouse is the second largest cause of produce spoilage. To prevent spoilage due to low airflow, fans will be installed within the warehouse to create sufficient air movement. Since electricity is not available for use, mechanical linkages attached directly to water turbines will power the fans. These fans will be designed to run whenever there is produce stored in the warehouse, but in the event that the water turbines cannot provide sufficient power, natural convection from the evaporator coil will provide some air movement. In addition to the designed natural convection, all produce crates will be stored on racking to promote increased airflow between the levels of crates.

Another method of increasing produce storage life is to have the interior air exchanged at least once every 24 hours. To efficiently exchange air without putting too much stress on the refrigeration system, fresh air inflow will be directed across the evaporator coil. In the warmer months when the evaporator is cool, the warm fresh air will provide both;new air and will help maintain a defrosted evaporator coil. In cooler months, interior air exchange across the evaporator coil will heat the air before it comes in contact with the produce, as the reversed absorption refrigeration system will heat the evaporator coil and cool the condenser coil. Air exchange is planned to happen for five minutes every hour to ensure adequate fresh air in the warehouse, as well as maintain a frost-free evaporator coil in warmer months.

4.2 Detailed Description

The warehouse design that will be used to preserve produce for long periods of time without electricity is a complex design that utilizes many different tactics to achieve the final goal. All of these individual systems work in unison to provide the necessary temperature and airflow regulation, to maximize the storage life of cabbage and winter squash. As described above, the systems used in the design are absorption refrigeration, urethane insulation, efficient building design, interior airflow generation, and interior air exchange.

Absorption refrigeration can be used without the use of electricity to cool the desired space. Its main method of cooling replaces the compressor that is used in traditional refrigeration with a system of a generator and absorber that act as a thermal refrigeration pump. The absorption refrigeration system is recommended for the electricity-free application is one that uses a lithium bromide salt and water in tandem to provide cooling and drive the system. A diagram of the refrigeration cycle is shown in Figure 2.

Figure 2: Absorption Refrigeration Cycle

The cycle shown in Figure 2 very closely resembles a traditional refrigeration system with the lack of a compressor to drive the system. This is where the generator and absorber come into play. Heat is added to the generator to boil off the water which then travels to the condenser, through the expansion valve, and finally to the evaporator where the water absorbs heat from the environment. The water is then returned to the absorber where the lithium bromide salt absorbs the water. Figure 2 shows a pump that would drive the lithium bromide/water combination back to the generator, but this can be done without a pump by using gravity. The expansion valve in the thermal compressor portion of the system allows the lithium bromide salt to return to the absorber once the majority of the water has been boiled from the mixture. When the lithium bromide salt returns to the absorber, it helps to drive the system by absorbing the water re-entering the thermal compressor from the evaporator. The heat needed to drive the system is added from solar panels designed to create heat from sunlight. Solar panels will not provide the necessary heat during times of low sunlight or darkness, so a reserve system is put in place to provide the necessary heat when the sunlight is not adequate. This backup system consists of a kerosene flame that is placed directly under the generator. Kerosene heatwill only be used in warm months where there is a high demand on the refrigeration system and an extended period of time without refrigeration would be detrimental to the stored produce. While this system is not considered to be as efficient as modern compressor-driven refrigeration systems, it is currently the most efficient method of refrigeration that does not use any electricity.

The urethane insulation used in the building design is a rather simple concept that yields impressive results from a relatively low investment. Insulation panels that are specified for use are constructed of a combination of metal exteriors with urethane foam sandwiched between. These panels are very common in modern refrigerated facilities so used panels are readily available from vendors who disassemble refrigeration systems. Used panels provide sufficient insulating properties at a fraction of the cost and are accepted by the Amish community as a method of insulating. Installation of the panels is made simple by cam lock mechanisms built into the sides of the panels that allow panels to lock together with a moisture blocking seal. The main enemy to urethane insulation panels is moisture. The metal exterior of the panel helps to block moisture and maintain dry panels that provide the intended insulating properties.

The floor plan of the warehouse is another passive system that makes the building as efficient as possible. The two main rooms of the building are designed to be stacked on each other to take advantage of environmental conditions that will help to maintain interior temperatures. The top room will be constructed with the majority of the room above the grade. Maintaining the temperature of the room during the warm months will be done with the absorption refrigeration system, but once the ambient air temperature becomes low enough, the refrigeration system will no longer be needed to cool the room. When the exterior air becomes too cold, heat from the refrigeration system will be used to help heat the room to prevent freezing of the produce. The bottom room of the warehouse will be situated approximately seven feet below grade. At this depth, the average ground temperature is approximately 48 degrees Fahrenheit and fluctuates by approximately 2.5 degrees Fahrenheit throughout the year. The bottom room of the warehouse needs to be maintained at 50 degrees Fahrenheit for squash storage, so very little heat will be required to raise the temperature of the room. When squash curing is required, the bottom room of the warehouse will be partitioned to allow a heated portion for curing, as well as a portion for storage. Once the curing process is completed for the season, the portion of the bottom room used for curing will be converted to storage for the remainder of the storage season.

Interior airflow is the primary concern for the squash storage portion of the warehouse. In previous years, approximately 50% of the stored crop of squash was lost due to a lack of sufficient airflow within the storage facility. In order to provide electricity-free airflow, fans will be powered using water turbines and mechanical power transmission to the fans themselves. Since airflow is required from August through March, the water turbines will be fed with a closed loop system that allows an anti-freeze/water mixture to be used that will prevent freezing of the system. This system requires two tanks: one at high elevation and one at low elevation. The high elevation tank will be placed at approximately 50 feet above the second tank. This height differential will allow for the necessary head pressure to turn the water turbines. The flow rate of water from the top tank to the bottom tank will be approximately 30 gallons per minute, which is sufficient to turn the water turbines. The water tanks will be capable of holding 100,000 gallons of the anti-freeze/water mixture to ensure a constant supply of flowing fluid.

In order to maintain a constant flow of water, the high elevation tank must be refilled when possible. To accomplish this, wind and water powered pumps will be utilized. A water turbine placed in the nearby river will provide relatively constant mechanical pumping power while the river is not frozen. In the event that the river becomes completely frozen or the flow of water is not sufficient due to environmental conditions, a wind turbine will provide pumping capacity. Wind is not nearly as consistent as the flow in a river, but the large size of the storage tanks will ensure that the fans will be capable of running for approximately one day even if the pumping systems used to refill the tank are not operational.

Since sufficient airflow is so critical to produce preservation, racking will be used within the warehouse to promote more airflow. The racking will provide more space between the storage crates than the current method of stacking the crates directly on each other. The current method of crate stacking has yielded much higher spoilage rates withinbottom crates compared to crates stacked on the top level. The racking will simulate having all crates on the top of a stack to promote the airflow between all levels of crates.

The final supplemental method of air movement within the warehouse will be employed in the cabbage storage room by using natural convection from the evaporator coil. The evaporator coil placement near the top of the storage room will take advantage of the natural flow of warm air to the top of the room and cooler air falling to the bottom of the room. A high placement of the evaporator coil will also reduce the temperature differential between the floor and ceiling of the warehouse. If the coil were placed too close to the floor of the room, the cool air would rest at the bottom of the room and warm air would stay near the ceiling if the fans were to stop running.

Interior air exchange is another very important aspect of maintaining a favorable environment for produce storage. In the case of the electricity-free warehouse, efficiency is very important because wasted energy is an unnecessary load that can be reduced. In the warmer months, the exterior air is warmer than the desired interior temperature of the warehouse. To help cool the exterior air as it is exchanged with interior air, it will be directed over the evaporator coil as it enters the building. This will not only help to cool the air as it initially enters the warehouse, but it will also serve as a means to defrost the evaporator coil. The efficiency of the evaporator coil is greatly increased by reducing the frost that can build up on the coil due to humidity in the controlled environment. Exterior air will be drawn into the building every hour for approximately five minutes. This will allow for sufficient air to be exchanged, to have full air exchange in any 24-hour period, and will also maintain a nearly frost-free evaporator coil.

In winter months when the exterior air is very cold, the air will still be directed across the evaporator coil upon entry to the warehouse. With the absorption refrigeration system set for reverse flow, the evaporator coil will become hot, so the cold exterior air will be heated before it comes in contact with any of the produce. This will reduce the required amount of heat necessary to maintain the required temperatures within the warehouse while still providing adequate fresh air to the interior of the warehouse.

4.2.1 Functional Block Diagram

Figure 3: Functional Block Diagram

4.2.2.1 Insulation

Insulation used in this design consists of an outer skin constructed of stainless steel and an inner core of urethane foam. The outside of the panel helps to protect against the ingression of moisture that can lower the R-value of the panels. The panels used in the design can be purchased as a used product, which helps to lower cost without losing much efficiency of the design.

Figure 4: Insulation Panel

4.2.2.2 Absorption Refrigeration

Absorption Refrigeration is the main method of cooling utilized in the warehouse design. The system uses an external heat source from either solar energy or a kerosene flame to drive a thermal compressor, which pumps the refrigerant to the condenser and evaporator in the system. The refrigeration will be run in a standard configuration to provide cooling to the facility and can also be reversed to provide heating for the same structure.

Figure 5: Absorption Refrigeration Cycle

4.2.2.3 Interior Air Flow

Airflow within the facility will be provided by fans, which receive rotational energy from water turbines through mechanical linkages. The water turbines are fed a sufficient flow of an anti-freeze/water mixture through a closed loop system that is fully operational through all seasonal temperatures. This system has elevated and ground level tanks which provide the necessary head height for the specified water turbines. Water is returned to the elevated tank using a water turbine driven pump, which is placed in the nearby river as well as by wind powered water pumps. The wind-powered pumps provide a means of backup pumping in the event the water flow in the river is insufficient to replenish the elevated water tank. The water tanks are designed to provide enough water to they water turbines to provide interior airflow for a full day without any pumping back to the elevated tank.

Figure 6: Water Storage System Figure 7: Water Turbine to Fan Transmission

4.2.2.4 Interior Air Exchange

Fresh air must be exchanged with the interior air periodically throughout a full day to ensure the best produce storage environment. To achieve this situation, louvers are installed near the top of the building that can be opened to allow fresh airflow in and interior airflow out. The exterior airflow into the facility will be directed over the evaporator coil in both the cold and warm months. This will allow cold outdoor air to be warmed in cold months and cooled in the warm months before it contacts the produce in storage. Drawing the warm air over the evaporator coil in the warm months will also serve as a means for defrosting the evaporator coil.