Abstract
The paper presents the performance assessment of a solar district cooling system with special attention to the heat rejection process. The investigation includes energetic, economic and environmental aspects. The district cooling network is driven by two-stage Li-Br absorption chillers coupled with parabolic trough solar collectors. The whole system, including solar field, storage tanks and chilled water pipeline, has been modelled in Trnsys. The focus is on the heat rejection systems, and their impact on the performance of the cooling plant. Four different types of heat rejection systems are considered: Air Cooling (AC), Evaporative Cooling Tower (ECT), Groundwater Heat Exchanger (GHE) and Geothermal Boreholes (GB). The paper presents two case studies in the Gulf region: the warm climate is compared for two condition of humidity, dry (Riyadh) and humid (Abu Dhabi). Furthermore, the work presents a multivariable optimization procedure based on GenOpt software interacting with Trnsys model under the constraint of a 70% annual solar fraction. The best option resulted to be the one based on absorption chillers coupled with Groundwater Heat Exchanger in both locations. The annual power consumption is reduced by 83% in Abu Dhabi and 82% in Riyadh compared to conventional cooling systems.
Highlights
Worldwide, power consumption for cooling systems is responsible for 10% of the global electricity usage and 14% of the peak demand [1]
A notable result is the low solar fraction achieved by the cooling plants coupled with dry cooler (0.63 in Abu Dhabi and 0.54 in Riyadh, highlighted by the red bar over the charts) due to the maximum operating temperature of the chiller, the ambient temperature is greater than 35 deg
The chart shows a solar field reduction for the layout based on the groundwater heat rejection system due to the reliability of the cooling circuit; whilst, the storage tank is similar for the four option except for the plant based on dry cooler
Summary
Power consumption for cooling systems is responsible for 10% of the global electricity usage and 14% of the peak demand [1]. Dry cooler is the most common heat rejection system for residential and industrial applications: in hot climates, air-cooled chillers exhibit a low efficiency due to high ambient air temperature [8,9]. The presence of brackish water in the subsoil, especially in the UAE region, allow to implement heat rejection systems with water circuit, preserving the freshwater resources. The use of geothermal probes as a heat rejection system can be a good alternative to avoid water consumption and keeping the cooling circuit at low temperatures but soil thermal saturation has detrimental impact on plant performance. The use of geothermal probes for air conditioning dates back many years ago and the study of these solutions is developed especially for heating. Four different types of heat rejection systems are compared: Air Cooling (AC), Evaporative Cooling Tower (ECT), Groundwater Heat Exchanger (GHE) and Geothermal Boreholes (GB)
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