Great potential for solar district cooling in Saudi Arabia
In a recent article published in Energy Conversion and Management1, a team of researchers investigates the performance of a district cooling system modelled to be used in Riyadh, Saudi Arabia. The climate of the region is known to be warm and dry, with very low humidity level all year long. In this country, "more than 70% of (...) electricity is consumed for air conditioning and cooling, and the summer demand is about twice the winter demand", according to a 2017 article2. Furthermore, Saudi Arabia still produces this electricity almost entirely using fossil fuels, despite its abundant solar resource.
Two different solar cooling technologies are compared:
- A two-stage lithium bromide absorption chiller (2sABS) driven by Parabolic Trough Collector (PTC);
- A single-stage lithium bromide absorption chiller (1sABS) driven by Evacuated Tube Collector (ETC). It is the typical system of most applications at commercial stage.
The simulation focused on a residential compound with 96 single single-family houses. The compound was modelled thanks to Google Sketch Up and Trnsys3D. The geometrical model was imported in a Trnsys deck to calculate the cooling load over 1-year period on hourly basis. The outside temperatures were modelled thanks to the Meteonorm international database.
The district cooling network was divided into 4 sub-districts of 4 houses, and the distribution network measured 16.8 km.
A solar fraction of 70% was considered, solar fraction being "the percentage of cooling load covered by the solar cooling plant". The other 30% were supposed to be covered by conventional chillers, used in case of lack or radiation or depletion of thermal storage.
The aim of the simulation was to predict the annual operation of the solar cooling systems with high level of accuracy. Cost effectiveness and energy savings and greenhouse gas emisions reduction were also taken into account.
The results of the simulation showed that the two-stage lithium bromide absorption chiller (2sABS) driven by Parabolic Trough Collector was significantly more cost effective than the single-stage solution fed by Evacuated Tube Collectors, with a 30% reduction of primary costs. The parameters taken into account were: the PTC and ETC solar field, the absorption chillers, the hot storage, the cold storage, the cooling tower, and the auxiliary chiller used in case of lack of sun. The auxiliary chiller is the same in both case. The main difference between the costs is due to a slight higher unit cost of ETCs and the higher efficiency of the two-stage absorption chiller.
The paper also reports a difference of 4% in annual CO2 emissions to the advantage of the two-stage absorption chiller, with 594 tonnes/year compared to 618 tonnes/year. The article compares those figures to the CO2 emissions of a conventional district cooling system: in that case, 2028 tonnes a year are emitted. Both single-stage and two-stage chillers therefore seem to be interesting in hot and dry climates.
1 FRANCHINI G., BRUMANA G., PERDICHIZZI A. Performance prediction of a solar district cooling system in Riyadh, Saudi Arabia - A cause study. Energy Conversion and Management. 2018, vol 499, pp.372-384. Available on Science Direct.
2 DEMIRBAS A., HASHEM A.A., BAKHSH A. A. The cost analysis of electric power generation in Saudi Arabia. Energy Sources, Part B: Economics, Planning and Policy. 2017, vol. 12, issue 6, pp.591-596.