The dynamics of integrated compressed air renewable energy systems

An integrated compressed air renewable energy system is defined here as one which harvests renewable energy directly in the form of compressed air and later converts that to the form of electrical power for transmission. There are two main motivations for considering such systems: firstly the lifetime cost per kW h exported has the potential to be substantially lower than the lifetime cost per kW h of a system generating electricity directly. Secondly these systems offer the intrinsic capability to store large amounts of energy in a very cost effective way. The only marginal costs associated with energy storage are those connected with providing some means for storing the compressed air and some means for managing heat. This paper describes an approach to simulating the performance of such systems including a controller to determine how much power to generate at a given time and it explains an appropriate rationale for the design of that controller. The simulations conducted indicate three remarkable performance measures. Specifically: (a) the marginal loss of energy associated with passing some energy through storage may be below 15% even with energy residency times in the order of months, (b) the marginal increase in total output electrical energy arising from integrating some solar heat capture can be as high as 60% of the captured solar heat for solar heat inputs up to 5% of total mechanical power and (c) the average value of the total power output may easily be raised by over 30% if power values continue to fluctuate at rates exhibited today and if the capacity for expansion-generation matches the peak input power of the primary (mechanical) energy harvesters.

► Offshore renewable energy systems which compress air directly are economically viable.
► Integration of energy storage (4 days rated power) is feasible and economic.
► Simulations indicate effective turnaround efficiencies of 85%.
► Marginal conversion efficiencies of solar heat to electricity of >60%.
► Even poor heat exchangers do not degrade performance severely.