A Critical Literature Review of Defrost Technologies for Heat Pumps and Refrigeration Systems

When the operating conditions are extremely cold and humid and the surface temperature of the heat exchanger well below the freezing point (lower than the dew point temperature of the air) moisture from the air stream will freeze on the surface after condensation and the frost will start growing. The frost growth degrades the performance of the system considerably. It hinders the airflow and increases the pressure drop through the coil which means more fan power is requires for to maintain the desired flow rate. With reduced flow rate due to the increase of pressure drop, system’s capacity drops rapidly. In the case of heat pump the capacity of the evaporator decreases due to the airflow drop, which reduces the overall heating capacity and coefficient of performance of the heat pump. Additionally, the frost layer increases the thermal resistance to the heat transfer between the air and refrigerant. The reduction in airflow and increased thermal resistance reduces the heat energy extracted by the evaporator and decreases the heat pump capacity and efficiency. Similar process is observed for the cooling coils of commercial refrigeration system where the frost growth can dramatically reduce the system capacity. Once the performance reaches its minimum acceptable stage, a defrost process is introduced to remove the frost layer and to achieve the performance at the start of the cycle. The frost defrost process is repeated continuously. Overall the frost growth is highly undesired phenomena which can cause considerable reduction in performance of the system. This study overviews different procedures to counteract the frost growth. Various frost mitigation procedures have been reviewed and compared to access their feasibility. The methods such as air treatment before entering the heat exchanger are used to effectively eliminate or at least minimize the frost growth rate. Such procedures are discussed under two major categories, air treatment processes to mitigate the frost and appropriate system modification to minimize or eliminate the frost growth

Isolated Sub-Dehumidification Strategies in Large Supermarkets and Grocery Stores

The objective of this project was to determine the potential energy savings associated with reducing the relative humidity in the vicinity of refrigerated display cases in supermarkets, as compared to the widely accepted current practice of maintaining a relatively higher and uniform humidity level throughout the entire supermarket. Existing and new strategies for maintaining lower relative humidity levels near the vicinity of refrigerated display cases were analyzed to determine their effectiveness and limits of application

Energy Consumption and Product Throughput of Glass-Doored and Open Refrigerated Display Cases in Supermarkets [abstract]

Only abstract of poster available.Track III: Energy InfrastructureRefrigerated display cases are utilized by supermarkets to store and display food products in a manner that extends food shelf life and ensures food safety. Supermarkets operate their refrigeration systems continuously to maintain proper food storage conditions. Continual operation of this refrigeration equipment on a nationwide scale accounts for approximately 0.33 quads/yr of electrical energy. Infiltration accounts for over 70% of the refrigeration load in open refrigerated display cases. Clearly, an increase in the energy efficiency of refrigerated display cases will result in significantly reduced energy consumption and cost. Thus, the objective of this project is to compare a typical open refrigerated display case to a typical glass-doored refrigerated display case with the aim of quantifying the following: • The difference in overall energy consumption between the cases. • The normalized difference in food product sales between the cases. Two supermarkets are being used as test sites, which are similarly situated to ensure that climate, weather, time-of-year and economic conditions of the shoppers are comparable. One supermarket will receive an open refrigerated display case and the other will receive a doored refrigerated display case. Each refrigerated display case will be a medium temperature, self-service prepackaged deli or beverage case between 8 and 25 nominal linear feet. The thermal performance and product sales of each refrigerated display case will be monitored. Automated data logging systems will be used to measure quantities such as refrigeration load, voltage, current, temperature, humidity and pressure. Product sales for each refrigerated display case and total store product sales will be tracked using Stock Keeping Units (SKUs) and the supermarkets' computerized Point of Sale (POS) system. The resulting data will be analyzed to determine the difference in energy consumption and product sales between the two refrigerated cases

Waste Heat Dehumidification in CO2 Booster Supermarket

Carbon dioxide (CO2) being a low Global Warming Potential (GWP) refrigerant is becoming a popular choice as an efficient refrigerant in supermarket refrigeration systems, not only in moderate climates such as Northern Europe but also in the United States. Due to its low critical temperature (31.06°C), CO2 systems also offer potential for waste heat utilization. This paper, therefore, aims to uncover this potential for the US supermarket refrigeration industry through simulation. In this process, the whole building energy modeling tool, EnergyPlus, has been used to investigate the energy consumption of a supermarket utilizing packaged rooftop air conditioning units and a transcritical CO2 booster refrigeration system. The energy impact of humidity control within the supermarket, both on the HVAC and the refrigeration systems, is investigated. Also, a desiccant system incorporating waste heat for desiccant regeneration has been analyzed. Finally, the performance of the transcritical CO2 booster refrigeration system is compared with the baseline R404A multiplex direct expansion system using bin analysis in sixteen cities from eight climate zones of the United States

“From sweet potatoes to God Almighty”: Roy Rappaport on being a hedgehog

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/71618/1/ae.2007.34.3.581.pd

High Efficiency Evaporator Fan Motors for Commercial Refrigeration Applications

Evaporator fan motors used in commercial refrigeration applications are fractional horsepower in size, are responsible for moving air across the evaporator coil, and typically run at one speed. Historically, shaded-pole motors have been the most commonly used evaporator fan motors in commercial refrigeration equipment and beverage vending machines. These motors, a type of single-phase AC induction motor, are the simplest and least expensive type of fractional-horsepower motor. They are also the least efficient, with the common 9–12 W sizes exhibiting an efficiency of approximately 20%. Electronically commutated (EC) motors, also known as brushless DC motors, became widely commercialized in the late 1980s, and their use in commercial refrigeration applications has increased within the last 10 to 15 years because of economic incentives and regulatory requirements. Another motor type, the permanent split capacitor (PSC) motor, offers a mid-point between shaded-pole and EC motor price and efficiency levels. Typically for commercial refrigeration evaporator fan motor applications, EC motors are 66% efficient and PSC motors are usually about 29% efficient.  A new motor technology, a type of permanent magnet synchronous AC motor that can directly use grid-supplied AC current without the need to rectify to DC, has recently been commercialized. This new motor has the potential to significantly reduce the energy consumption of evaporator fans in commercial refrigeration equipment. Previously, synchronous motors have been prohibitively expensive for evaporator fan applications because of the high cost of the electronic control circuit that is required to bring the synchronous motor up to synchronous speed. However, this new motor technology makes use of a novel patent-pending controller that is simpler and lower in cost than previous synchronous motor controllers or EC motor controllers, making the new motor a cost-effective alternative in the commercial refrigeration market.  In this paper, the results of field demonstrations, consisting of side-by-side measurements of the power consumption of the new motor technology versus shaded-pole, PSC, or EC evaporator fan motors in identical refrigerated display cases, are presented. Measured quantities include fan motor power, current, and power factor, as well as display case discharge and return air temperatures and ambient store temperature. Currently, field demonstrations are being conducted at six supermarkets and commissaries located in Kansas City, MO, San Antonio, TX and San Diego, CA, with the duration of these tests ranging from approximately one month to over six months. Initial results from the field demonstrations indicate that the new synchronous AC motor technology is approximately 20 to 30% more energy efficient than existing EC motors and nearly 80% more energy efficient than shaded-pole motors. In addition, the new motor exhibits a power factor of approximately 0.86, which is on average 40% greater than that of existing evaporator fan motors. Analyses indicate that retrofitting the installed base of commercial refrigeration evaporator fan motors with the new motors would produce a 68% evaporator fan site energy savings, or 4.9×109 kWh/year for the installed base of commercial refrigeration evaporator fan motors. This would result in an annual cost savings of $516 million and reduce the annual CO2 equivalent emissions by 8.1×109 lb

Refrigeration Modeling Components in OpenStudio

OpenStudio, developed at the National Renewable Energy Laboratory (NREL), is a cross-platform collection of simulation tools for whole building energy modeling. EnergyPlus and Radiance are the primary simulation engines in OpenStudio and the latest addition is a collection of refrigeration objects developed at the Oak Ridge National Laboratory (ORNL) which makes the refrigeration system components of EnergyPlus available for use through OpenStudio. This paper highlights the benefits of including refrigeration modeling functionality in OpenStudio, the object model, the exposed Application Programming Interface, and discusses some of the design challenges faced. The project was implemented in four phases with each phase focusing on one aspect of the software. An initial set of EnergyPlus refrigeration components were selected and implemented, following which, additional components in decreasing order of priority were implemented. An Agile software development philosophy was employed along with modern source control tools such as GitHub. During the initial design, six main refrigeration components were determined to be critically important. These were Refrigeration System, Compressor, Display Case, Walk-In, Air-Cooled Condenser and Secondary System. Scripts were used to automatically convert various objects from the EnergyPlus Input Data Dictionary (IDD) file into C++ code. These prototypical source code files for the refrigeration objects were then modified to allow full instantiation of the objects. The objects, complete with their unit tests, were then submitted for integration into the main development branch of the software. During the second phase, the following components were implemented: Mechanical Subcooler, Liquid-Suction Subcooler, Cascade Condenser, Evaporative-Cooled Condenser, and Water-Cooled Condenser. During the third phase, the remaining refrigeration components from EnergyPlus were added: Air Chillers, Transcritical Refrigeration System, Transcritical Compressors, Air-Cooled Gas Cooler and waste heat recovery using the Air Heating Desuperheater Coil or the Water Heating Desuperheater Coil. In addition, EnergyPlus comes with metadata information of hundreds of refrigerated display cases and refrigeration compressors. These were added to the U.S. Department of Energy’s Building Component Library (BCL), which is a comprehensive online digital library of various building blocks of an energy model. During the fourth and final phase, documentation of the developed Application Programming Interface was created. In all phases, close collaboration with multi-lab partners was maintained who provided extensive support and developmental feedback to the ORNL team. It is expected that the addition of these refrigeration components fills a gap and will enhance the successful adoption of OpenStudio as a modeling tool of choice in the buildings simulation community. Furthermore, it is hoped that these refrigeration capabilities will allow retailers to assess the energy impact of a variety of supermarket and distribution facility refrigeration system design options and aid in the selection and implementation of energy efficient systems

Waste Heat Recapture from Supermarket Refrigeration Systems

The objective of this project was to determine the potential energy savings associated with improved utilization of waste heat from supermarket refrigeration systems. Existing and advanced strategies for waste heat recovery in supermarkets were analyzed, including options from advanced sources such as combined heat and power (CHP), micro-turbines and fuel cells

Laboratory Evaluation of a Commercial CO2 Booster Refrigeration System

The traditional multiplex direct expansion (DX) refrigeration system used in commercial applications is prone to significant refrigerant leakage, especially older existing systems. EPA (2012) estimates that the U.S. supermarket industry-wide average refrigerant emission rate is approximately 25%. The use of high Global Warming Potential (GWP) refrigerants in these systems, combined with high refrigerant leakage, can result in considerable direct carbon dioxide equivalent (CO2eq) emissions. In addition, commercial refrigeration systems consume a substantial amount of electrical energy, resulting in high indirect CO2eq emissions. Thus, there are ongoing efforts to reduce both the direct and indirect environmental impacts of commercial refrigeration systems through the use of leak reduction measures, refrigerant charge minimization, low GWP refrigerants and energy efficiency measures. Based on prior energy and life cycle climate performance (LCCP) analyses, it was determined that a transcritical CO2 booster refrigeration system for supermarket applications has the potential to reduce carbon emissions and increase energy efficiency. To that end, a lab-scale transcritical CO2 booster refrigeration system was fabricated and installed in the environmental test chambers at the Oak Ridge National Laboratory (ORNL). This system consists of a transcritical CO2 compressor rack, an air-cooled gas cooler/condenser, medium-temperature (MT) and low-temperature (LT) refrigerated display cases, and MT and LT “false†loads. The lab-scale refrigeration system has a low-temperature cooling capacity of approximately 9.1 kW at −30°C and a medium-temperature cooling capacity of approximately 34 kW at −6.7°C. One 4-door vertical display case, 3.0 m in length, as well as a “false†load provided by a plate heat exchanger and a glycol loop, constitutes the low-temperature load. The medium-temperature load consists of one open vertical display case, 2.4 m in length, as well as a “false†load provided by a plate heat exchanger and glycol loop. The air-cooled gas cooler/condenser is installed in a temperature and humidity controlled “outdoor†environmental chamber while the compressor rack and refrigerated display cases are installed in a separate temperature and humidity controlled “indoor†environmental chamber. For both chambers, the temperature can be controlled between −18 to 65°C and the humidity can be controlled between 30 to 90%. The performance of the transcritical CO2 booster refrigeration system was determined at four ambient temperature conditions (15.6°C, 21.1°C, 26.7°C and 32.2°C). After the refrigeration system achieved steady-state operation at each of the four ambient temperature conditions, system performance data was collected for a 24-hour period. Over the temperature range of 15.6 to 32.2°C, the total load on the system was found to remain relatively constant. In addition, the compressor power was found to increase by approximately 78% over this same temperature range. Thus, the resulting coefficient of performance (COP) of the system was found to vary from 2.2 (at 32.2°C) to 4.1 (at 15.6°C). Based on the laboratory evaluation, the transcritical CO2 booster refrigeration system demonstrates promise as a low emission, high efficiency alternative to the traditional multiplex DX systems currently in use

Impact of Surface Treatment and Geometrical Characteristics on the Condensate Retention and Frost Formation on Metal Foams

Metal foams has been considered as promising materials for multiple thermal applications including air-cooling. Their use in such an application requires a detailed analysis of the condensate retention and frost accumulation in the metal foam pores. The current study is focused on the impact of surface treatment on the performance of metal foams when they are deployed in dehumidifying and frosting environment. Aluminum and cooper foam samples have been treated to achieve hydrophilic, hydrophobic and super hydrophobic characteristics and associated condensate retention and frost accumulation on the surface has been measured at various relative humidity and base temperature conditions. High resolution imaging techniques has been used to illustrate the impact of surface treatment. Along with the surface treatment, the impact of metal foam geometry (porosity and pore size) has been highlighted as well and the findings have been summarized in form of guidelines for the application of metal foam in air cooling application where the condensation and frost formation cannot be avoided