Highly Fluctuating Electricity Prices Result in High Total Annualised Cost Savings

This study aims to simultaneously optimise reverse osmosis plant size and operation with a total annualised cost model. Highly fluctuating electricity prices result in high total annualised cost savings, as do oversizing and long shutoff durations. Reducing operating costs can be more beneficial than reducing capital costs. Researchers at the Massachusetts Institute of Technology have optimised the single-day operation and size of a seawater reverse osmosis (RO) plant subject to a half-hourly varying electricity price. A previously reported model is modified to have a variable plant size controlled by the number of modules. The objective of the optimisation is to minimise the total annualised cost of the plant. The operating and capital costs are calculated as a function of size and operation. The decision variables are the number of modules and the half-hourly varying operating frequency. The operation and size are optimised for four different electricity price functions: Constant, moderately fluctuating, highly fluctuating, and actual electricity prices from a given day in Spain. The results show that variable operation and oversizing can produce savings of up to 7% for a highly fluctuating electricity price. The plant has a higher operating frequency when electricity is cheap, and shuts off during periods of high electricity price when oversized. The size and day-by-day operation are also optimised for one year subject to the electricity price in Spain. Small savings via oversizing were obtained for day-by-day optimisation due to low fluctuations in the electricity price during the year. Source: filtsep