C. Renno

An evaluation of R22 substitutes performances regulating continuously the compressor refrigeration capacity

Abstract This paper presents the results of an experimental analysis which compares in terms of energetic performances the refrigeration capacity control obtained by means of a variable-speed compressor with the on/off control deriving from a classical thermostatic device. The compressor considered is semi-hermetic reciprocating and is a component of a vapour compression refrigeration plant subjected to a commercially available cold store. The compressor working with the fluids R22, R507 and R407C and designed for a revolution speed corresponding to the compressor supply current nominal frequency of 50 Hz, has been tested varying the frequency in the range 30–50 Hz. In this range, the most suitable working fluids proposed as substitutes of the R22 as the R407C (R32/R125/R134a 23/25/52% in mass), the R507 (R125/R143A 50/50% in mass) and the R417A (R125/R134a/R600 46.6/50/3.4% in mass) have been tested. The results show that, using the R407C, it is possible an average an electric energy consumption about 12% smaller when an inverter is employed to control the compressor refrigeration capacity instead of the thermostatic control which imposes on/off cycles on the compressor, working at the nominal frequency of 50 Hz. So the R407C confirms its superiority in comparison with the R417A and R507; only the R22 shows a better performance.

A numerical approach to a very fast thermal transient in an air cooling evaporator

A numerical study of the performance of an air cooling evaporator inserted in a climatic chamber in the presence of a very fast thermal transient is considered. The climatic chamber is a confined environment that allows one in the presence of air, to submit for a short time a certain product to a particular thermal stress. In fact, according to some international standards, it is necessary to analyse the response of some type of products, that then will be used under severe condition, such as a sudden heating and cooling, to obtain an acceptable future operative life. Heating and cooling cycles are obtained in the confined environment respectively by means of an electrical resistance and of an air cooling evaporator linked to a vapour compression system. To study the cooling thermal cycle an air cooling evaporator numerical model has been obtained according to the hypotheses of a homogeneous model for the refrigerant and of an incompressible and one-dimensional flow for the air flowing onto the evaporator. Finally, the numerical model results have been compared with the experimental ones in terms of the evaporator outlet refrigerant temperature, the air temperature and of the evaporator outlet air relative humidity. This comparison is very satisfactory, as it shows that this study is very useful to permit proper design of the vapour compressor plant and its particular control system.

An air cooled tube-fin evaporator model for an expansion valve control law

For control purposes, a mathematical model of a tube-fin evaporator of a vapour compression plant running with R22 is analyzed. The refrigerant behavior in an evaporating region is described by a homogeneous model. The balance equations, together with the constitutive equations, determine a differential system which makes explicit the mechanism of dynamic behavior. At first, the numerical solution of the steady state is obtained, both in evaporating and superheated regions. Moreover, the numerical analysis allows us to evaluate the transition phase and to locate the interface. Subsequently, various analytical aspects are discussed. For the nonlinear two-phase flow, the dependence of the solution on the boundary data is estimated by means of a qualitative analysis. Then, a linearized model for the single-phase flow is deduced and solved explicitly. The analytical solution is compared with the numerical results and the degree of superheating is estimated in terms of the model parameters.

Life cycle assessment and economic analysis of a low concentrating photovoltaic system

ABSTRACT Many new photovoltaic (PV) applications, such as the concentrating PV (CPV) systems, are appearing on the market. The main characteristic of CPV systems is to concentrate sunlight on a receiver by means of optical devices and to decrease the solar cells area required. A low CPV (LCPV) system allows optimizing the PV effect with high increase of generated electric power as well as decrease of active surface area. In this paper, an economic analysis and a life cycle assessment (LCA) study of a particular LCPV scheme is presented and its environmental impacts are compared with those of a PV traditional system. The LCA study was performed with the software tool SimaPro 8.0.2, using the Econinvent 3.1 database. A functional unit of 1 kWh of electricity produced was chosen. Carbon Footprint, Ecological Footprint and ReCiPe 2008 were the methods used to assess the environmental impacts of the LCPV plant compared with a corresponding traditional system. All the methods demonstrated the environmental convenience of the LCPV system. The innovative system allowed saving 16.9% of CO2 equivalent in comparison with the traditional PV plant. The environmental impacts saving was 17% in terms of Ecological Footprint, and, finally, 15.8% with the ReCiPe method.

Inefficiencies analysis of a point-focus CPV∕T system

ABSTRACT In this paper, a point-focus concentrating photovoltaic and thermal (CPV∕T) system, sized for a domestic application, is presented in order to evaluate its inefficiencies and the related costs. The CPV∕T system adopts parabolic concentrator mirrors to reflect the light on triple-junction cells InGaP∕InGaAs∕Ge (indium--gallium phosphide∕indium--gallium arsenide∕germanium) placed on pipes, where the cooling fluid flows; CPV∕T system consists of three modules in parallel linked to a tank that works as hot water storage. The main maintenance problems are due to: moving parts such as the tracking system, cell wear linked to hotspot problems and optics cleaning. In particular, the model presented allows us to evaluate electric and thermal outputs, energy inefficiencies, and maintenance costs of the CPV∕T system. The analysis has been subdivided in several steps and a FMECA (Failure Mode, Effects and Criticality Analysis) approach has been considered to investigate the main failure modes and their relative effects in terms of efficiency losses and intervention costs. So, the inefficiencies analysis has allowed to identify the system malfunction causes, evaluating the impact due to the stops for each type of fault. Hence, an economic analysis has allowed to evaluate the CPV∕T system convenience when inefficiencies and maintenance incidence are also considered.

Thermal Model in ANSYS for a Comparison Between two Configurations of a Concentrating Photovoltaic System

ABSTRACT The theoretical analysis of different concentrating photovoltaic and thermal systems (CPV/T) is presented in this paper; it allows to evaluate the cooling fluid temperature in different working conditions. In particular, two CPV/T systems with different optics, line-focus and point-focus, are studied and compared theoretically by means of a model developed in ANSYS-CFX able to evaluate the theoretical temperature values of the fluid that flows in the cooling circuit, designed with the Solidworks software. The comparison in terms of the thermal performances between the point-focus and line-focus configurations of a CPV/T system has been realized considering different working conditions. As model input the uniform cell temperature has been set to different values, included between 323 K and 363 K, or determined experimentally referring to the point-focus configuration. In particular, the outlet temperatures of the cooling fluid, the fluid and cells temperatures difference at the outlet section, the time necessary to reach a steady-state condition, the fluid temperature trend in different parts of the tube and along the circuit, have been evaluated. Moreover, the fluid temperature trend has been studied varying the fluid velocity, its mass flow rate, and the insulation thickness. Finally, the point-focus CPV/T system thermal performances have been also evaluated adopting some experimental measurements as input data; the fluid temperature trend has been tested under a variable cell temperature during different hours of a sunny and a cloudy day.