REFRIGERANTS Alternative technologies


Refrigerant Solutions (RSL) takes a broad view of potential market opportunities for innovative cooling and heat pumping technologies beyond its core business of refrigerant blends based on fluorine-containing fluids, whether based on conventional vapour- recompression or on other physical phenomena. Here, Professor Dick Powell describes ideas over the years that, while intriguing, have not been taken further.


R


SL's approach is pragmatic, responding to evolving market demands that are driven by frequently changing regulations, without


being beholden to a specific refrigerant type or political ideology. Even when exploratory work does not result in a technical advance, the insight gained helps RSL judge the chances of a new technology emerging that might compete with conventional vapour-recompression technology, which has dominated the market for the past 150 years. Despite its reservations about the intrinsic flammability hazard of the so-called ‘natural’ hydrocarbon refrigerants, this has not prevented RSL from exploring methods for reducing the risk involved in their use, in particular by minimising the inventory in small a/c units. But the Grenfell Tower tragedy emphasised all too clearly the effect of the ill-advised installation of flammable materials in buildings, even when apparently meeting official standards. RSL was not convinced that its proposed improvements to hydrocarbon air conditioning units would be compatible with future safety standards in high rise buildings, so the project was discontinued. While not doubting the good thermodynamic performance of hydrocarbons, RSL is not prepared to continue working on them, unless it has an inspiration for dealing effectively with their flammability. Although not hazard-free carbon dioxide, is


intrinsically safer and is thus a significantly more attractive refrigerant than the hydrocarbons. But its low critical temperature requires a very high pressure, transcritical compression cycle with lower efficiency than conventional fluorinated fluids operating on the Rankine cycle. RSL has explored whether effective, low pressure CO2


/micro-porous


solid cooling devices might be devised based on cyclical exothermic adsorption and endothermic desorption.


A lab demonstration rig showed that the effect was possible, but also highlighted the challenges in this approach. The temperature lift was limited by the combined heat capacity of porous carbon solid and its container, a problem avoided in the conventional Rankine cycle because only the refrigerant experiences cyclical temperature


26 March 2021


changes while the condenser and evaporator remain at essentially constant temperatures. A further drawback was that the compressor needed to be oil-free since lubricant would foul the adsorbent. A diaphragm compressor was used in the lab rig, but its operating pressure range was too limited to provide adequate cooling capacity. RSL speculated that the disadvantage of swing- bed operation might be overcome by exploiting the reverse of thermal transpiration whereby a temperature difference across a microporous solid induced a corresponding pressure difference, a phenomenon predicted and observed by engineer, Osbourne Reynolds in the 19th century. In principle, since this is a reversible process, a gas, CO2


in the RSL work, flowing through a


microporous plate under a pressure difference should establish a temperature difference across the opposite faces. RSL wonders whether ‘the reverse Reynolds


transpiration effect’, could be exploited for refrigeration. The following diagram illustrates the concept with the microporous solid also acting as part of the insulating medium for a refrigerated space. Conceivably the whole of the casing might be utilised, although only the back is shown here as providing cooling. Since the CO2


gas passing


through the porous solid absorbs incoming heat, the configuration might be described as ‘dynamic insulation’ – where the insulation as well as the refrigerant are actively involved in providing the desired cooling. In contrast to simple expansion through a nozzle, thermal transpiration is a thermodynamically reversible effect and thus potentially efficient. But it does depend upon the pore size being less than the mean free path between molecular collisions, which implies the need for ‘monolithic aerogels’, i.e. very low density, open-cell foams known to be excellent thermal insulators, although rather friable.


RSL screened various materials but did not find a solid suitable for this application. Also, if the microporous solid is also to act as the insulation, then the pressure difference across it must be modest – an elegant idea perhaps awaiting developments in other technologies to be viable.


RSL also considered the electrocaloric effect whereby the application of an electric field to a suitable material might induce a reversible temperature change, a phenomenon analogous to the better-known magnetocaloric effect, but less studied. Based on published thermodynamic data for flexible organic molecules capable of complexing with lithium and sodium ions, RSL calculated that such materials incorporated into a supercapacitor-like device might generated a temperature change of ~ 12 K by applying a potential of a ~ 3 volts, even allowing for the thermal capacity of the electrodes and case. As simple test of the hypothesis, RSL measured the temperature changes when a flat, commercial supercapacitor was charged and discharged. A reproducible cooling of ~0.2 K at room temperature was measured, which, although small, was at least proof of principle. When RSL was working on this idea in 2006,


Science published a paper reporting a ‘giant electrocaloric effect in thin-film PbZr0.95


Ti0.95 O3 ’


(0.48 K per volt at ~222°C), which sparked renewed interest in electrocaloric solid-state capacitors for cooling. A practical unit has not emerged from this work, but could the RSL supercapacitor- concept perhaps allows more scope for designing an effective system? An electrocaloric refrigerator suffers from same the heat capacity concerns about temperature lift that limits other ‘swing’ systems, such as magnetocaloric and adsorption beds. So why isn't RSL currently pursuing such alternative cooling methods further? In its exploratory project, RSL compared alternative technologies with the conventional vapour- compression cycle using wide temperature glide refrigerant blends. The company concluded that very low GWP,


nonflammable refrigerant blends, presently in development, offered the best long-term technology of delivering cooling and heat pumping. This approach has intrinsic technical advantages over alternative methods for most applications and, most importantly, still has scope for further performance improvement to minimise environmental impact.


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