Document Type

Working Paper

Date of this Version

4-2014

Abstract

This paper outlines the design of a compressed air energy storage (CAES) system. The goal of this project was to develop a CAES system that could produce 1 MW of power that is both thermally efficient and financially viable. We developed four processes that each contains variations from the preceding process in order to explore different possible arrangements to maximize efficiency and feasibility.

The processes described were designed to take advantage of the price differences between off- peak and on-peak electricity. Each process compresses air for 20 hours and expands air over 4 hours, allowing for continuous daily use. The processes contain a four-stage multi-stage compressor to compress air to 1,500 psia. Multiple stages of compression allowed for the air to be cooled between stages, and the heat of compression was captured using pressurized water. In each case, the pressurized air was stored in above-ground storage vessels.

The base case process involves four stages of compression followed by four stages of expansion. Multiple stages were chosen so that preheating could take place in heat exchangers before air entered each turbine in order to maximize energy efficiency. The preheating was done with the hot water generated during the compression stages. Under our selected operating conditions, the base case had an efficiency of 34.4% and a -13.72% financial ROI.

Following the base case are three variations which explore different process modifications to determine their impacts on efficiency and financial return. The first variation re-analyzes the base case but attempts to account for heat losses from the water tank which may occur when the pressurized water is stored. This heat loss case achieves an efficiency of 28.76% and an ROI of - 13.75%. The second variation explored variation involved a single-stage turbine with preheating done by the heated water. This single stage turbine case was the most inefficient process outlined, with an efficiency of 21.39% and an ROI of -13.68%.

The last variation similarly utilizes a single-stage turbine but preheats the air using the exhaust from a gas turbine. This process was the most economical and the most efficient due to the high temperature of exhaust produced as well as the added energy production of the gas turbine. The efficiency of this gas turbine case was 33.7% with a -12.46% ROI.

Though none of our cases were profitable within the constraints of the project charter, the gas turbine case was the most optimistic. Due to the improvement in ROI in the gas turbine case in addition to the improved process feasibility associated with it, we recommend that future research into compressed air energy storage pursue a process that involves a combination of airand gas turbines to achieve maximal energy output, highest project feasibility, and greatest financial return on investment.

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Date Posted: 25 July 2014