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BSEE


AIR CONDITIONING, COOLING & VENTILATION


Frozen assets: time to decarbonise cooling?


Heating has always been high on the decarbonisation agenda, but cooling less so. This, however, is changing, says Tim Mitchell, sales and marketing director of Klima­Therm. Here he examines the leading­edge technologies moving us closer to zero carbon cooling


explode over the coming 30 years. Indeed, according to the Royal Society (R Soc), it is likely to grow by around 80% over the period 2010 to 2050, even in a ‘business-as-usual’ scenario. The production of heat for industry currently accounts for an estimated 20% of global CO2 emissions, but the balance between heating and cooling is set to change.


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A briefing document published by the R Soc in mid-2021 – ‘Low carbon heating and cooling: Overcoming one of the world’s most important net zero challenges’ – predicts: “Climate change is expected to reduce the projected demand for heating and increase demand for cooling, with some estimates forecasting that space cooling will account for more global energy demand than space heating by 2060.”


This begs a fundamental question – how can we decarbonise the cooling sector? The answer is that there are several developments either on the drawing board, at prototype stage, or in production. For example, projects such as the European Union’s CryoHub is investigating storing renewable


How Zero Carbon Source works


Zero Carbon Source air conditioning from Gree significantly improves the energy efficiency of vapour-compression refrigeration systems while maximising the efficient use of natural and renewable cooling and power.


Using low-humidity air when available reduces heat and moisture load, while water is used for evaporative cooling and outdoor falling film evaporation, employing a specific type of vertically orientated heat exchanger.


Cascade heat exchange and vapour compression refrigeration cycles work in parallel. A new type of compressor, low- GWP refrigerant (R152a: GWP 124 [AR4]), dual-temperature evaporators, and evaporative condensers ensure efficient


operation and precise control of cooling and dehumidification.


Depending on the outdoor weather conditions, the system intelligently selects one or more of three modes – vapour- compression cooling, evaporative cooling, and ventilation. Photovoltaic (PV)-sourced power is employed whenever possible.


The company explained: “The vapour compression refrigeration system… uses an ingenious compressor with a low global warming potential refrigerant in conjunction with improved design of evaporator and evaporative condenser to more effectively control indoor temperature and relative humidity. “Furthermore, the PV direct-driven


10 BUILDING SERVICES & ENVIRONMENTAL ENGINEER FEBRUARY 2022


technology further lowers the overall grid electricity consumption. This climate- adaptive residential air conditioner has five times lower climate impact than today’s conventional air conditioners.” It added: “Gree’s cooling solution highlights how the upper efficiency limit of today’s predominant technology – vapour compression technology – can be largely expanded through smart, hybrid design.” Although the system uses PV power preferentially, it can seamlessly switch between PV and the grid when required. Gree’s technology was jointly awarded the Grand Winner accolade at the 2021 Global Cooling Prize, an innovation competition designed to develop climate- friendly residential cooling solutions.


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emand for heating and cooling in residential and commercial buildings is expected to


energy as a cryogenic liquid such as liquid air which can be used to refrigerate industrial facilities while being stored and boiled to generate electricity through a turbine.


The R Soc briefing paper reveals that ‘phase change materials’ (PCMs) – substances which release/absorb sufficient energy at phase transition to provide useful temperature changes – have been demonstrated for cooling as well as heating applications.


In one project for refrigerated cold chain supply, UK scientists and China’s railway rolling stock company CRRC Shijiazhuang created the world’s first shipping container using PCMs based on salt hydrates to ‘store’ coldness. Meanwhile, an influential group that includes the Carbon Trust and University of Oxford has outlined a vision that, by 2050, Net Zero cooling will be available to all. This, it says, will be achieved through a focus on three impact areas:


• Passive cooling – Widespread adoption of measures that avoid or reduce the need for mechanical cooling, including reductions in cooling loads, smart and human-centric design, and urban planning.


• Super-efficient equipment and appliances – A ‘race to the top’ S- curve transformation where the norm is super-efficient cooling


equipment and appliances powered by zero carbon energy. • Ultra-low global warming potential (GWP) refrigerants and insulation foam gases – Market domination of ultra-low (<5 GWP) refrigerants across all cooling sectors and applications.


This group has published a report called ‘Climate Action Pathway – Net-Zero Cooling’, which says that Net Zero cooling can be achieved through this ‘avoid-shift-improve’ approach applied to efficient equipment and appliances, powered by zero carbon energy, and using ultra-low GWP refrigerants. There are also initiatives from manufacturers aimed at massively improving the efficiency of cooling systems in order to decarbonise them. A prime example is the ‘Zero Carbon Source’ cooling technology under development by Chinese air conditioning giant, Gree. This integrates a range of technologies to achieve a climate impact just 20% of a standard air conditioner when tested employing the Indian seasonal energy efficiency ratio test, with


temperatures that reached a high of 36.2°C, an average of 25.6°C, and a wide range of relative humidity levels, with a high of 80.1%. Advanced vapour compression refrigeration, photovoltaic (PV) direct-driven technology, plus evaporative cooling, and ventilation


– using free cooling sources (air and water) – were combined with environmentally-friendly refrigerants to significantly reduce energy consumption and carbon emissions. Linked to energy storage, the system can easily achieve Net Zero carbon in operation.


Traditional unit air conditioners employ single-stage compression refrigeration. There is only one suction pressure for the compressor, and, during cooling, there is only one evaporation temperature at the corresponding evaporator. Using an innovative cascade heat exchange and parallel compression refrigeration cycle means there are three suction pressures while the triple-cylinder rotary compressor is working in cooling mode. The three suction inlets are connected to low- and high- temperature evaporator and flash tank discharge outlets from low to high pressure respectively. The indoor air return flow passes through both evaporators in turn to create cascade heat exchange, while the refrigerant at the condenser outlet entering the flash tank achieves sub- cooling through the first-stage throttle.


The cascade heat exchange and flash tank sub-cooling both improve the efficiency of the refrigeration cycle and, therefore, the overall energy efficiency of the air conditioning system.


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