ENERGY SAVING
others have been described in the academic and patent literature and a number have been prototyped. In some respects, there are too many technical options with no one approach suited to all applications. Systems may operate over time scales that may be hourly, daily or even seasonal. In the latter case solar summer heat maybe stored in the ground below a dwelling and subsequently recovered for winter heating perhaps using a heat pump. Although elegant engineering, are such systems economically viable?
Like any technology, energy storage has
both advantages and disadvantages. All storage methods that decouple the electric power supply from demand are inherently inefficient since energy is inevitably lost in the charging/ discharging processes. However, this narrow view of efficiency is not n ecessarily appropriate. Cold or heat storage allows a vapour recompression unit to be sized for an average, not peak, demand, minimising its cost and allowing it to run continuously under its optimum operating conditions. For air conditioning, cooling might be generated at night and when the ambient temperature is lower thus improving overall energy efficiency.
Cold or heat storage may be impractical for smaller, domestic systems because of lack of room in existing domestic buildings, especially flats, but innovative thinking might resolve this problem. Phase change materials (PCMs), familiar as the “freezer blocks” used in cool bags, h ave been developed over the past two decades as building coolth/heat stores. Typically, they contain salt hydrates or hydrocarbon-type waxes which melt/freeze at near-constant temperatures to suit various applications. Heat transfer is a fundamental problem because PCMs are relatively poor conductors, but the inclusion of metal strips helps to overcome the problem. In an interesting recent development, Riffat (University of Nottingham) has encapsulated PCMs in plastic or aluminium using bubble packaging technology producing a sheet material, called ‘ChainStore’ with a large surface area and capable of mass production. These are used to line ceilings or walls this could effectively increase the thermal mass so external temperature variations have less effect, thus reducing heat or cooling loads. Thermal energy can be stored
thermochemically by cyclically adsorbing and desorbing a fluid, notably water, CO2 or ammonia onto and from a porous solid, with
vapour being condensed by the rejecting heat to the environment, or to a room if heat pumping is required. Alternatively, evaporating liquid and absorbing the vapour on the solid provides cooling. Such systems offer scope for development offering the options of being driven by waste heat or vapour recompression units, perhaps in combination. A fundamental problem is heat transfer within the beds; in some designs the working fluid may also transfer heat to/from external heat exchangers.
An intriguing development is Dearman’s application of liquid air (or nitrogen) that potentially integrates energy storage, presently focused on low pollution delivery vehicles, and refrigeration. In my view, liquid air is the only truly ‘natural’, low temperature refrigerant that could be a long-term alternative to methane- derived, synthetic CO2 and ammonia for large scale refrigeration. Its direct GWP is zero, and in contrast to ammonia, air is nontoxic and, provided oxygen build-up in the liquid phase is avoided, does not represent a flammability hazard. Technically, I believe that we are prepared for the future. What we need is the political wisdom and entrepreneurial will to achieve it.
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