Integrated Compressed-Air Renewable Energy Systems.
S. D. Garvey (University of Nottingham, UK), M. G. Wood (Aston University, UK) and C. N. Eastwick (University of Nottingham, UK)
The need to embrace renewable energy at large scale is now beyond contention. The major question is no longer whether to deploy renewable energy harvesting extensively but how to do it in some way which is economically acceptable and which does not create other technical problems for the distribution grid. Three key elements of concern are: (a) how to further scale-up offshore wind turbines and reduce their lifetime costs, (b) how to develop tidal power and wave power harvesters which are reliable and economic and (c) how to resolve the intrinsic difficulties associated with generating substantial fractions of the nation’s power from resources which are inherently intermittent.
One possibility currently receiving scrutiny is the concept of transforming the harvested energy directly into the form of compressed-air rather than following the approach which has been appropriate up to now wherein the harvested energy is transformed into electrical power. This concept is referred to generically as Integrated Compressed Air Renewable Energy Systems (ICARES). On the surface, there are several compelling arguments for ICARES.
The first of these arguments has to do with energy storage to alleviate intermittency issues. Compressed-air energy storage (CAES) is an established concept with some large-scale instances working successfully already. As an energy storage medium, compressed-air has shortcomings – not the least of which is the management of heat. The established CAES systems attempt to store heat produced while air is being compressed such that it can be used to advantage again when the air is expanded. ICARES mitigates these difficulties considerably through recognising that any renewable energy farm (such as an offshore wind farm) will always be exporting at least some power while the natural energy availability is high. In view of this, even if, say, 75% of the energy being produced is destined for storage, heat extracted from compressed air going into storage can be used to advantage in further raising the temperature of compressed-air about to be expanded.
A second argument is based on pure economics of the primary energy harvesters. Installing either very slow direct-drive generators or large gearbox-generator combinations at the tops of tall wind-turbine towers or inside underwater tidal-power devices is necessarily very expensive. The possibility of using much cheaper devices as primary energy harvesters and then achieving the generation of electricity through a small number of high-speed expanders coupled to high-speed generators serving the entire energy-farm has obvious attraction. Even though the net efficiency of wind-turbines, tidal turbines or wave-driven machines might be slightly lower than that of their electrical counterparts there is every reason to believe that their cost-per-kWh of output will be substantially lower.
This paper will develop the above arguments with quantitative reasoning. Laws of scaling will be applied to the case of large horizontal axis wind-turbines (HAWTs) in particular to indicate why the economics of the conventional designs changes in nature as size increase. A specific class of potential configuration for very large future HAWTs is presented which becomes more practicable and more economically feasible with increasing size. Some of the interesting new modelling challenges presented by this class of configuration are discussed. Possibilities for compressed-air storage offshore are outlined also and basic thermodynamic equations are used to substantiate the claim that heat-management is made easier with ICARES than it is for conventional CAES. Finally, an approximate economic model is used to indicate the extent of advantage expected from building at least some ICARES energy-farms offshore in place of corresponding direct-generating counterparts.