Simulation and experimental studies on renewable energy-based building cooling systems

Abstract

Continuous increase in the usage of refrigeration and air-conditioning systems, rapid depletion of conventional sources of fossil fuels, and environmental concerns, necessitate the research focus towards development of efficient technologies. This work is aimed at the assessment of various building cooling systems for addressing the environmental concerns related to global warming, air quality and waste management. Here, various configurations of air-conditioning systems have been studied via simulation and experimental investigations. Based on the research gaps identified in the literature survey, the present work has been executed in the following steps. The first study assesses the possibility of integrating a “triple-hybrid” single-stage vapor absorption (VA) system for building air-conditioning using EnergyPlus simulations. The modifications required for making the absorption system energy efficient with respect to the conventional (compression-based) system are proposed. This analysis reveals the necessity of triple hybridization of the absorption system towards fulfilling its thermal energy requirement. Results show the possibility of annual electricity savings of around 18.5 % and 29.8 %, depending on the climatic condition. Furthermore, performance enhancement aspects through desiccant material-based via dedicated outdoor air system (DOAS) and multi-staging methods are investigated. In desiccant-coupled systems, the role of indirect evaporative cooling arrangement (IEC), sensible heat recovery wheel (SHRW) and cooling coil in the supply air path are mainly assessed. Results show that a double-staging system can save up to 36.4 % of annual electrical energy compared to a compression-based system. However, with singlestaging, the saving is nearly 19.5 %. Further, in a double-stage absorption system, the amount of required thermal capacity at the generator side is lesser compared to the catered cooling load, that gives the coefficient of performance (COP) greater than one. However, in the single-stage absorption system, the required thermal energy is always more than the cooling load (i.e., refrigeration effect). But, the needed heat source temperature is high (approximately 150 ºC) in double-staged absorption systems that limits the usage of the multiple-staging systems under general working conditions. In the second study, the suitability of the triple-hybrid absorption system is checked with the proven energy-efficient radiant cooling system (RCS). In this context, three configurations of RCS are compared: RCS and DOAS coupled with the compressionbased system, RCS coupled with absorption system and DOAS, and absorption-assisted RCS associated with desiccant and IEC-assisted DOAS. The simulation results reveal around 13 % of energy savings by the last configuration against the first one. This study demonstrates the possibility of coupling an absorption-based system with only RCS. However, the absorption-based system cannot be coupled to the DOAS, and compression chiller is necessary in such a situation. In the third study, to make the building cooling space fully independent of electrical energy, an analysis is performed to examine the electric grid independency potential of absorption and compression-based air-conditioning systems under different climatic conditions. Here, solar photovoltaic (PV) and solar collector systems have been installed for electricity generation and thermal energy generation, respectively. In the case of compression-based air-conditioning, the maximum area of the roof can be occupied with solar PVs, whereas, with absorption-based systems, half of the roof area is used for solar collector installation. In the case of absorption-based air-conditioning configuration, although more energy savings can be obtained compared to the compression-based system, still, this system cannot attain the target of grid independency. However, the compression-based air-conditioning system (in particular, the compression-based radiant system) effectively shows the achievement of grid independency criteria. It is revealed that 98 %, 100 %, and 74 % of the grid independency target can be attained by this configuration under composite, hot-dry, and warm-humid climates, respectively. In the fourth study, for different climatic conditions, a desiccant-assisted improved strategy of air-conditioning systems termed as variable refrigerant flow (VRF) is examined with respect to the conventional variable air volume (VAV) and VRF designs. In the proposed design, an IEC is used in the path of desiccant process air. For this purpose, an EnergyPlus simulation model is developed for a large scale office building. A loop of flat-plate solar collector with auxiliary heating element is used for the desiccant regeneration. Simulation results revealed that, for warm-humid climate, electrical energy saving potential of the proposed solar energy-assisted VRF system is around 23.9 % and 9.5 % higher than the conventional all-air VAV and VRF systems, respectively. The same savings percentage under composite climate are determined as 13.8 % and 9.4 %. For the hot-dry climate, these values are 17.5 % and 11.6 % against conventional all-air VAV and VRF systems, respectively. In the fifth study, specific outcomes of the simulation studies are used for fabricating an experimental test bench. For this purpose, a small-scale triple-hybrid-based absorption chiller (1.1 Ton or 3.85 kW capacity) is designed and fabricated. Solar thermal, waste biomass-based electricity, and biomass-based waste heat sources are used to drive the entire absorption system. Waste biomass is fed to a biomass gasifier that generates clean syngas and it is used in the electricity generator. Cold water obtained from the absorption chiller is supplied to the test chamber for air-conditioning purpose. The absorption chiller performance is assessed under different lithium bromide (LiBr)-water concentrations (54% and 58 %) and generator temperatures (60 ºC, 70 ºC, and 80 ºC). Experimental study reveals the possibility of coupling solar and biomass-based resources with an absorption-based air-conditioning system. Experimental data are further used for generating new empirical correlations. The response surface method is used to create the correlations with the coefficient of determination of 0.991 and 0.993 for evaporator load and temperature, respectively. The overall heat transfer coefficients estimated for each of the components of the absorption chiller are determined. Interestingly, the developed test bench can supply surplus energy savings. The absorption chiller can lower the room air temperature from 37 ºC to 27.5 ºC with 58 % concentration and 80 ºC generator temperature. The associated harmful emissions are found to be considerably lesser compared to the conventional modes of air-cooling.