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  • Edward Sabey

Future - Proofing comfort requires Decarbonised Cooling

Cooling through the use of air conditioners (ACs) and electric fans currently accounts for nearly 20% of global electricity usage in buildings. With the number of AC units expected to more than triple by 2050, coupled with a forecast 33-fold surge in AC power consumption by 2100 from current levels. The requirement for cheap, energy-efficient, and low-carbon cooling systems has never been greater.

Driving this growth are ever-increasing global temperatures, and ever-maturing developing economies which combined are expected to result in a prolific increase in the number of people who need, and can afford air conditioning. This is forecast to be particularly characteristic of equatorial regions where the hottest, most humid climates are found. Demand growth in cooling will put additional strain on the grid, with peak loads increasing and outages becoming more prevalent.


Right now, cooling in some areas of the US during hot spells can be attributed to more than 70% of peak residential electricity demand, accounting for a significant portion of household emissions. Herein lies the un-virtuous cycle of cooling whereby increased emissions result in higher temperatures that in turn encourage increased cooling.


That's not all. The impact of cooling on global temperatures is actually two-fold. Downstream generation emissions associated with the energy to power cooling devices are supplemented by the more difficult-to-quantify release of artificial refrigerants. Historically, chlorofluorocarbons (CFCs) were the norm however concerns surrounding ozone depletion led to a reform through The Montreal Protocol.


Hydrofluorocarbons (HFCs) their now ubiquitous replacement face renewed scrutiny for their high, global warming potential (GWP). As Heating, Ventilation, and Air Conditioning (HVAC) units are replaced, or reach end-of-life, these HFCs leak into the atmosphere, with 1,000+ times the GWP of regular CO2 on a mass basis. These fugitive emissions are difficult to track and wreak havoc on the atmosphere's ability to shed heat.


So there are two problems. First, the emissions related to actually powering cooling devices, and second, the warming effects of current refrigerants. Energy efficiency and the carbon intensity of the grid are the two main levers to combat the energy problem. Pragmatically, a mix of both will be required however in this article we'll just focus on efficiency as this specifically relates to HVAC. Alternate or refrigerant-free technologies address the warming concerns of current HFCs. We'll discuss both of these areas below.


The existing solution


The vapour compression cycle (VCC) is the most prominent AC refrigeration technology used globally today; however, the fundamentals of the approach have evolved little since its inception in the early 19th century. Although advancements have been made from the Ozone-depleting refrigerants once used, the most embedded refrigerant types such as R-134a still have a global warming potential (GWP) of 1430. Signifying that R-134a has 1,430 times the 100-year warming potential of carbon dioxide for the same mass.

The VCC is able to generate a cool space, by utilising the thermodynamic properties of the refrigerant to transfer heat from a room, to the environment. The refrigerant passes through a series of working components in the form of a compressor and expansion valve, resulting in the regeneration of the refrigerant. Allowing for the continuous processing of heat. This highly efficient process results in Coefficient of Performance (COP) values of approximately 3, meaning for 1kW of input power, 3kW of cooling power can be generated. This has resulted in VCC technology utilising artificial refrigerants to become truly embedded into global cooling infrastructure.


Decarbonisation routes

Caloric Materials

A novel and exciting group of materials that demonstrate caloric effects have been increasingly researched for their possible use in the refrigeration sector. Caloric materials are those which demonstrate temperature changes when subject to an external field whether that be electric, magnetic, pressure or stress.


This temperature change can be utilised to form an active regenerative cycle which allows for the transfer of energy from a heat source (E.g. Building), to a heat sink (E.g. The surrounding environment). This closely resembles the VCC cycle described earlier but with a multitude of possible upsides such as negation of an artificial refrigerant, quiet operation and very high theoretical COP values. Cyclic caloric cooling cycles are able to generate COP values of greater than 4, making the technology a viable solution to the existing artificial refrigerant systems.

Magnotherm, a company based in Germany have prevailed in taking magentocaloric materials from the lab, and through extensive R&D have produced a usable commercial product in the form of a 150-beverage capacity cooler. They have leveraged the change in temperature which magnetocaloric materials demonstrate when subject to alternating magnetic field to produce this cool space. Although currently the development of caloric materials for cooling purposes is very experimental, there is a route to commercialisation being carved out by firms such as Magnotherm.


Desiccants

Water is an excellent coolant. It is able to absorb a great amount of heat energy without vastly changing temperature, this same property means that cooling water is a very energy-intensive process.


Since humidity is effectively a measure of water content in the air, it is desirable for AC units to process air with the lowest humidity possible as this reduces the workload on mechanical components such as the compressor. Thus, water removal technologies also known as desiccants can vastly improve the COP values of AC and refrigeration systems.

Blue Frontier's AC system which utilises a liquid desiccant to generate dehumidification.

Blue Frontier, a portfolio company for Twynam have already demonstrated the impact a high-efficacy desiccant can have on electricity consumption and hence carbon emissions for AC units. Their technology is capable of reducing yearly electricity use and GHG emissions by 60% and 85% respectively. This is achieved through the use of a proprietary concentrated salt solution which decreases relative humidity before cooling.


Liquid desiccant solutions do have to overcome the difficulties associated with unit corrosion however astute selection of the construction materials can usually mitigate or limit these corrosion rates.


Refrigerant Enhancements

The most direct pathway to incremental decarbonisation of HVAC systems arises from making alterations to the VCC technology already in place. This can take the form of improvements to the GWP, and energy efficiency of the refrigerant used, implementing room level control, or increasing the performance of mechanical components.

Improving the refrigerant used in the VCC can take the form of either iterative improvements to existing artificial refrigerants, or the introduction of an entirely new natural alternative. Hydrofluorooelifins (HFOs) have been touted as possible stop-gap solutions due to their lower GWP values and relatively strong performance compared to the incumbent Hydrofluorocarbons (HFCs). However doubts still remain due to their breakdown into trifluoroacetic acid (TFA) which can be problematic as it accumulates in water systems causing long-term damage.


Natural refrigerants such as carbon dioxide and water have been considered for use due to their low GWP values. Although they do require system and cycle alterations to be made, preventing them from being as easily integrated compared to most artificial refrigerants.


Conclusion

With ever-increasing carbon dioxide emissions and strain on energy infrastructure induced from the requirement for cooling, the need to make significant changes within the HVAC industry is evident. A number of routes to decarbonisation are present, ranging from novel methods which require extensive research, or instant modifications to existing systems which can immediately improve energy usage.


At Twynam, finding highly disruptive HVAC start-ups firmly meets our decarbonisation mandate and therefore this is a sector we will continue to explore with great interest.

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