We considered a novel energy storage system based on the compression of air through pumped water. Differently from CAES on trial, the proposed indirect compression leaves the opportunity to choose the kind of compression from adiabatic to isothermal. The energy storage process could be both fast or slow leading to different configuration and applications. These novel storage system are modular and could be applied in different scales for different locations and applications, being very flexible in charge and discharge process. The system may offer an ideal energy buffer for wind and solar storage with no (or negligible) environment hazard. The main features of this novel energy storage system will be showed together with overall energy and power data.
Energy storage systems are of paramount interest at present, as they are mandatory in order to raise electricity production from uncontrollable renewable sources (e.g. solar energy, wind energy) [1][2][3][4]. Moreover, they can provide a better exploitation of existing power plants avoiding the construction of new power plants just to respond to growth in peak power demand, that is especially valuable in European countries where environmental impact is critical as best suitable sites has already been exploited.
There are a lot of different systems proposed for energy storage, right now the ones with commercial development are just hydrodynamic storage, for large systems, and lead based batteries for medium to small ones. Alternatives for mechanical storage are given by compressed air energy storage (CAES) that are under demonstrating operation from long time in some sites like Huntorf in Germany (operating since 1978) and McIntosh, Alabama, USA (operating since 1991). These systems are quite promising as they don’t have the geographical limits of hydrodynamic storage, thus being of wider use. Nonetheless they need to provide heat to air before turbine expansion, leading to a delay in activation and fuel consumption that means it has some power generation. Moreover this requires many ancillary services for the storage plant. These systems have low energy efficiency, rated 40% -75% [2], too, partly due to thermal issues and partly to mechanical issues related to air compression and air expansion with a variable pressure gap. Some evolutions of CAES systems have been proposed, trying to overcome the need for heating of air before expansion. Actually, air has to be cooled after compression, too, in order to reduce its specific volume so to increase stored mass of high pressure air in the vessel. Thus, heat storage has been proposed to avoid fuel consumption for heating in, so called, adiabatic CAES [3 -7]. This will improve energy and exergy efficiency of systems but it won’t be useful to avoid activation delay that limits the kind of service CAES systems could provide to power grid. Moreover, usual compressors and turbines are not suitable to operate with variable back pressure. So during charging phase, air is compressed up to the highest storage pressure.
While before introduction in turbine, air is expanded in a valve, lowering its pressure to the lower storage pressure. Thus, despite CAES technology has already started being exploited, a lot of improvement is possible.
In traditional CAES, compression of air takes place in the compressor, that is then moved to the storage vessel. Similarly, air is taken from the vessel and introduced in turbine for expansion. In the proposed system, air is compressed and expands directly in the storage vessel. This is done through a water piston that modifies air volume, reducing it during charge and increasing it during discharge. The water piston is used as heat storage so to absorb heat during compression and reject it during expansion, too.
The new system is thus a Hydraulic compressed air energy storage (HYCAES). It is composed of high pressure storage vessel, almost full of air when fully out of power, an atmospheric pond for water storage, a water pump and a hydraulic turbine and connecting pipes. It is not ever-new, as there are some papers illustrating similar systems [7 -10]. In present paper, thermodynamic aspects of proposed systems will be analyzed to prove its energy feasibility.
Reversible compression of air by water piston can be done through different polytropic transformations, according to heat exchange of air. Rapid compression and high volume to surface ratios provides an almost adiabatic transformation. In order to avoid limiting power to energy ratio in the system, a rapid phenomenon will be assumed, so that heat exchange through vessel is negligible. Nonetheless, a perfect mixing of water to air is assumed, so to have an almost infinite contact surface that lets any heat exchange rate be provided to air. Air and water will be assumed to the same temperature during transformation. This transformation will have the lowest possible polytropic index. Polytropic transformation will be:
introducing µ=m -1 and β=V 0 /V f in (2) it becomes:
Assuming that air behaves as an ideal gas with temperature independent specific heat, while specific heat of liquid water is almost independent of transformation, energy balance for a perfectly mixed adiabatic vessel is:
Work can be calculated straightly from polytropic equation:
Mass of air is related to initial state through equation of state:
Mass of water is related to its density:
Thus, eq. ( 4) becomes:
Substituting ( 3) in (11) it becomes:
Then:
that, by defining air initial density from (6) becomes:
So, recalling Mayer’s relation, the polytropic index with respect to adiabatic index is so expressed:
where C is the ratio of heat capacity per unit volume between water and air at initial state.
At l
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