FIELD TESTS, MODEL VALIDATION AND PERFORMANCE OF A CO2 COMMERCIAL REFRIGERATION PLANT INTEGRATED WITH HVAC SYSTEM

A fully instrumented CO2 trans-critical booster system with parallel compression, in operation in a small size supermarket in northern Italy, made available measured data that allowed monitoring its performance. The entire refrigeration system of the supermarket, i.e. the Commercial Refrigerating Unit, the refrigerated display cabinets and cold rooms, have been modelled in TRNSYS with in-house types for all components. The comparison with the field data available from monitoring the plant allowed to thoroughly calibrate the model, spanning the operation conditions of a whole year. The validated model is a reliable and powerful tool, usable to predict the plant performance in several conditions and to evaluate the feasibility and effectiveness of the integration with DHW/HVAC systems. Measurements and simulations show that, at mild climate conditions, it is possible and effective to fully provide HVAC and refrigeration duties with a single all-in-one unit properly designed and operated

Transcritical CO2 commercial refrigeration plant with adiabatic gas cooler and subcooling via HVAC: field tests and modelling

Subcooling methods at the exit of the gas cooler in transcritical CO2 commercial refrigeration systems have been studied in the recent years showing that overall remarkable improvements can be obtained. Another strategy that results efficient is the use of evaporative systems at the gas cooler (adiabatic cooling) as it allows to significantly reduce the refrigerant quality at the liquid receiver and to lower the heat rejection pressure. In this work, a fully instrumented CO2 transcritical booster system with parallel compression, in operation in a small size supermarket in northern Italy, made available measured data of its performance when subcooling and/or adiabatic cooling are active. The plant operates in a mild climate, where it suffers operation at transcritical conditions for most of the year. Subcooling in this plant is performed by coupling the refrigeration system with the HVAC system. Taking advantage of experimental measurements, a model in the TRNSYS environment is validated and allows the prediction of the annual plant performance when these strategies are adopted. The adiabatic cooling showed to allow a significant reduction (about 10%) in the energy use, and makes unnecessary the use of a parallel compressor. Subcooling by the HVAC gives rise to a reduced saving (2.9 %) due to the absence of a dedicated mechanical subcooler, however it is almost comparable to parallel compression. These trends are confirmed in two other hot and humid climates

Effect of dedicated mechanical subcooler size and gas cooler pressure control on transcritical CO2 booster systems

Dedicated Mechanical Subcooling (DMS) is one of the most investigated and effective strategies applied to increase the performance of CO2 commercial refrigeration systems in transcritical operation. Further performance benefits can be obtained by a reduction of the gas cooler pressure of the main cycle at transcritical conditions. In this work the most important parameters for the design and operation of such a system, i.e. the DMS cooling capacity, the subcooling degree and the gas cooler pressure, are considered and their effect on the annual energy use of the plant is estimated in warm and hot climate conditions by means of a validated model. DMS is also compared to the parallel compression scheme and subcooling performed through a water chiller dedicated to HVAC. DMS results to be the most effective solution among those investigated, and the choice of the best design and operating parameters allows further energy saving and cost reduction

Subcooling with AC and adiabatic gas cooling for energy efficiency improvement: field tests and modelling of CO2 booster systems

In the last decade several plant configurations and components have been proposed to increase the efficiency of CO2 refrigeration systems. Among these, subcooling is considered a simple but effective solution, together with the employment of adiabatic cooling systems at the gas cooler. In this work, a fully instrumented CO2 booster plant installed in a supermarket is considered, to compare parallel compression, subcooling and adiabatic cooling. Subcooling is performed taking advantage of chilled water available from the HVAC system. The experimental data are used to validate a model for the comparison on a yearly basis. Parallel compression and subcooling show to be almost equivalent in terms of yearly energy use, while the adiabatic cooling system gives the best performance. Comparisons reveal that the subcooler cooling capacity should be chosen carefully to avoid oversizing, while the influence of the EER for the chiller appears quite small. Subcooling performed at the expense of an HVAC plant shows to be an interesting solution, while a great benefit was experienced with the employment of an adiabatic gas cooler

Combined refrigeration, heating and air conditioning systems in supermarkets: seeking energy efficient solutions

A deep synergy between the refrigeration and the HVAC plants is a viable solution to reduce the energy use of supermarkets. Not only heat recovery can be performed from the refrigerating plant in favour of space heating and hot water production, but also provision of AC capacity. A model based on TRNSYS and in-house types, validated with field data gathered from a fully instrumented plant in an active supermarket, allows to seek the optimal coupled solution. The model has been used to predict its feasibility and energy use at different climate conditions, for a reference supermarket. An energy saving of about 9% was predicted, regardless of the climate conditions, if the energy performance quality of the building envelope is kept constant. The integration of the refrigeration and HVAC systems shows to be effective in terms of energy use. Significant reductions can also be obtained in the investment costs, space occupied by the plants and amount of refrigerant charge

Ejector characterization for refrigeration applications with natural refrigerants

Employing natural fluids in refrigerating plants at warm climate conditions sometimes impacts negatively on the system performance. Ejectors can play a key role in configurations aiming at improving the efficiency of such systems, however their geometry has to be optimized in order to gain the best benefit. Scope of this work is a numerical investigation on the geometry of the ejector in a cascade plant configuration with natural refrigerants, aiming at identifying the influence of various geometry aspects on the performance of the system. A one-dimensional model is employed for the ejector, while the performance of the refrigerating plant is evaluated in different operating conditions in order to seek the optimal configuration

Demand Side Management analysis of a commercial Water Loop Heat Pump system

Demand side management (DSM) can be defined as a set of measures adopted to modify customers\u2019 energy demand with the aim of improving the efficiency of the overall energy system. Indeed, DSM strategies can be used to reduce customers\u2019 demand at peak times, reduce energy consumption seasonally/yearly, change the timing of end-use consumption from high to low-cost periods, and increase consumption during off-peak periods. DSM strategies can be implemented by using the energy flexibility available in the final users\u2019 applications, e.g. the thermal inertia of the building mass, the presence of additional energy storage systems enabling load shaping or the use of control systems to turn on/off end-users\u2019 devices when required. Being intensive energy consumers because of a high electric energy demand (mainly for refrigeration, which accounts for about 40 % of the yearly energy consumption), supermarkets are ideal candidates for energy use optimizations obtained through a DSM approach. This work shows the results of a DSM analysis carried out for a refrigeration and HVAC plant in a supermarket coupled with a Water Loop Heat Pump (WLHP) system. The water loop is used as a heat source/sink for the refrigeration unit supplying the cooling capacity required by food preservation and for several heat pumps that provide heating/cooling inside the supermarket building. The system is modelled in TRNSYS and the role of the water loop and its thermal inertia to provide energy flexibility is investigated. The system design and control strategy are modified in order to reduce the electricity costs in presence of demand response programs based on real-time price mechanisms

Simulation of multi-deck medium temperature display cabinets with the integration of CFD and cooling coil models

This is the post-print version of the final paper published in Applied Energy. The published article is available from the link below. Changes resulting from the publishing process, such as peer review, editing, corrections, structural formatting, and other quality control mechanisms may not be reflected in this document. Changes may have been made to this work since it was submitted for publication. Copyright @ 2010 Elsevier B.V.In this paper, the model for the multi-deck medium temperature display cabinets is developed with the integration of CFD and cooling coil sub-models. The distributed method is used to develop the cooling coil model with the airside inputs from the outputs of the CFD model. Inversely, the airside outputs from the cooling coil model are used to update the boundary conditions of the CFD model. To validate this cabinet model, a multi-deck medium temperature display cabinet refrigerated with a secondary refrigerant cooling coil was selected as a prototype and mounted in an air conditioned chamber. Extensive tests were conducted at constant space air temperature and varied relative humilities. The cabinet model has been validated by comparing with the test results for the parameters of air at different locations of the flow path, and temperatures of refrigerant and food product, etc. The validated model is therefore used to explore and analyse the cabinet performance and control strategies at various operating and design conditions.DEFR

L-functions with large analytic rank and abelian varieties with large algebraic rank over function fields

The goal of this paper is to explain how a simple but apparently new fact of linear algebra together with the cohomological interpretation of L-functions allows one to produce many examples of L-functions over function fields vanishing to high order at the center point of their functional equation. The main application is that for every prime p and every integer g>0 there are absolutely simple abelian varieties of dimension g over Fp(t) for which the BSD conjecture holds and which have arbitrarily large rank.Comment: To appear in Inventiones Mathematica

Assessment of the Biocompatibility of the PLLA-PLCL Scaffold Obtained by Electrospinning

AbstractElectrospun membranes of poly (L-Lactide) / poly (L-lactide-co-caprolactone) blend were produced and evaluated by physical and mechanical tests to use as a scaffold for cell growth. The membranes were seeded with endothelial cells (HUVEC) and after culturing time it was visualized by confocal laser scanning microscopy and scanning electron microscopy. The results indicate that the process parameters were capable of producing PLLA-PLCL membranes presenting fibers with diameters in the nanometer range. The scaffolds supported cell attachment and growth, indicating the feasibility of producing scaffolds by electrospinning technique, which could be used in tissue engineering applications