Assessment of Variable Refrigerant Flow for Indoor Comfort Control at Masdar City Eco-villa

Abstract

The 2021 vision of the United Arab Emirates establishes a number of goals related to sustainability. One of the most energy consuming sectors in the UAE is the cooling sector. In particular, the energy consumed by air-conditioners (AC) in Abu Dhabi represents the majority of the overall energy consumption in residential and commercial buildings sector. This is mainly because of the hot and humid weather conditions of the country, which necessitate the use of AC systems most of the time. Variable Refrigerant Flow (VRF) is an advanced AC technology which promises to reduce cooling energy consumption. In this thesis, a literature review is first done in which many scientific studies are reviewed and summarized. Many studies model the VRF system on EnergyPlus software, and others evaluate the system from experimental measurements. However, both approaches agree that, when compared to other AC systems, VRF results in more energy savings. The amount of energy saved varies from a study to another, though. Also, many researchers believe that it is not an easy task to properly model VRF systems due to the variability nature of this technology, especially as the number of indoor units increase. It is worth mentioning, that only few studies found in literature covered the second law of thermodynamics in such systems. In addition, limited number of studies have been done to evaluate VRF systems in the UAE. In this thesis, the VRF system is modelled for the Eco-Villa located in Masdar City in Abu Dhabi. A thermodynamic model of the system cycle is first built on Engineering Equation Solver (EES), and then verified on IPSEpro software. The model covers both the first and second laws of thermodynamics. The results obtained from both programs are similar with minor differences that did not exceed 6 %. The coefficient of performance (COP) of the cycle as predicted by the EES model is 3.9 at an ambient temperature of 35 oC. Various parametric studies are done on the model in which the quantity of interest is varied over a suitable range and the performance of the system is assessed accordingly. The COP of the cycle is highest at higher evaporator pressure but lower condenser pressure. As for the type of refrigerant, ammonia is seen to achieve the highest COP followed by refrigerant R410a. Next, a psychrometric model is built in EES to represent the indoor VRF unit and its interaction with the cooling zone. In this analysis, the ambient weather conditions are taken into account and the effect of these conditions and the indoor conditions on energy consumption are studied. The modelling results show that ambient temperature has the highest impact on the VRF energy consumption followed by indoor set-point temperature. Finally, an effective NTU evaporator model is implemented to minimize the overall power consumption of the VRF system under various zone loads. For a given set of zone loads, the optimal refrigerant evaporating temperature is found to be 11 oC, when the sum of indoor fan power and outdoor unit power is minimized.

Date of Award	May 2017
Original language	American English
Supervisor	Mohamed Ali (Supervisor)