Building air conditioning in Germany
Germany is located in a temperate climate zone where, by virtue of the country’s mean annual temperature of 10 °C, only a moderate amount of air conditioning is needed. The air conditioning in some apartment buildings needs to be used only on relatively few hot summer days. Outdoor blinds often cool off homes and apartments more efficiently and at a lower cost than is the case with air conditioning. However, this is not the case with non-residential buildings such as office buildings, where the need for air conditioning is considerably greater owing to the presence of numerous windows, a lack of shade, and elevated internal loads. Moreover, cooling such spaces with air conditioning requires around 100 times more energy than is the case with dwellings. Only around one to two percent of German dwellings versus around 50 percent of German office buildings are air conditioned (Bettgenhäuser et al. 2011).
According to one study, the carbon footprint of air conditioning is set to double by 2030, whereas the figure for non-residential buildings is 25 percent (Riviere et al. 2008).
Apart from their carbon emissions, air conditioners, which are driven by compressors, are also prone to refrigerant emissions that have a climate impact. These refrigerants are composed of hydrofluorocarbons (HFCs) that are used either in their pure form or as blends. Heat is extracted through refrigerant evaporation in a closed circuit and is released into the environment via mechanical compression and subsequent condensation. After being expanded by a butterfly valve, the liquid refrigerant is re-evaporated; this in turn closes the circuit. In Germany, stationary air conditioning units caused 405 tons of HFC emissions in 2010, or 717,400 tons CO2 equivalent.
Split air conditioners (mono and multisplit or variable refrigerant flow (VRF) air conditioners) mainly use the refrigerants R407C and R410A. These appliances are composed of an outdoor unit containing a compressor and condenser, and one or more indoor units (evaporators) that are interconnected via refrigerant pipes. Split air conditioners should only be installed by a qualified technician because the refrigerant circuit needs to be manipulated during installation. This also applies to air conditioners that are sold directly to end users. VRF air conditioners have highly ramified refrigerant piping, as well as sockets for up to 60 internal devices, and are thus prone to leakage. The amount of refrigerant needed for these types of air conditioners – with up to 300 meters of refrigerant piping – can be relatively large, i.e. up to around 50 kilograms. With an average leakage rate of seven percent, these air conditioners can release significant amounts of HFC into the environment.
Regulation 842/2006/EC stipulates that refrigerant leaks are to be eliminated by carrying out leak inspections. However, such inspections are oftentimes unfeasible due to the fact that refrigerant piping is installed in buildings in a manner that makes them difficult or impossible to access.
Mobile air conditioners contain less refrigerant owing to their compact size and are thus less prone to leakage. However, the cooling efficiency of these air conditioners is lower than for split air conditioners, since the extracted heat is conveyed outside through a tube via air flow. This creates an extraction effect whereby warm outside air flows back into the room. If the exhaust tube passes through an open window rather than a wall opening, the cooling effect is largely negated and the room temperature is only a few degrees lower than the outside temperature.
Central air conditioning systems that use R407C and R410A refrigerants and integrate cold water generators are used for spaces where 50 kW or more of cooling capacity is needed, whereby centrifugal chillers containing R134a refrigerant are used in cases where more than 250 kilowatts of cooling capacity is required. The cold water generated by such appliances extracts heat either directly via, for example, air convectors or building component activation, or indirectly via a ventilation system that pipes in cooled air via a duct system.
Air conditioning without HFCs
HFC-free solutions are available or will soon be available for all of the applications and appliances referred to above. A monosplit air conditioner with propane (R290) refrigerant has been on the market in India since 2012. Development of this product was supported by the German technical development agency known as Deutsche Gesellschaft für internationale Zusammenarbeit (GIZ), which also provided support enabling a Chinese manufacturer to begin making an R290 air conditioner – which, however, has yet to be commercialized.
For safety reasons related to the relatively large amounts of refrigerant involved, hydrocarbon refrigerants are not used in home and office multisplit and VRF systems because the outdoor and indoor units of these appliances are interconnected by refrigerant pipes. A VRF carbon dioxide (R744) refrigerant air conditioner recently came on the market, but its coefficient of performance (COP) of 2.5 is considerably lower than for the standard HFC solution. In lieu of multisplit and VRF air conditioners, central air conditioning systems can be installed which, because they generate cold indirectly via cold water, can be operated using a series of natural refrigerants such as R290 or R717 (ammonia). Owing to its positive thermodynamic properties, ammonia is mainly used for large scale air conditioning systems such as those at the Stuttgart airport or the Berlin Ostbahnhof train station. However, smaller mass produced appliances, as well as customized devices, are available for lower capacities (around 25 kW and higher). The selection of machines is much larger for capacities of 100 kilowatts or more.
Cold generators driven by thermal energy
In addition to compression chillers that run on natural refrigerants, absorption and adsorption chillers are also available. They need very little electricity as they run on thermal energy driving a thermal compression cycle. When used as air conditioners, absorption chiller appliances use water as a refrigerant. Adsorption chillers also use water as a refrigerant and can be run using a 65 °C heat source. Thus, they may run on solar heat. Various manufacturers sell systems with refrigeration capacities of around 8 kilowatts. A particularly low cost approach is running a sorption chiller using cogeneration plant waste heat. This solution combines a low cost heat source with a heat sink that is available year round. District heating is also a suitable heat source.
The UBA’s new office building in Dessau is air conditioned by a 30 kilowatt absorption chiller that runs on the heat from a solar heating installation or from district heating.
Apart from residential and non-residential buildings, other facilities and buildings such as clean rooms, labs and data centers also need air conditioning that can be provided by the technologies described above. Such facilities and buildings can be readily air conditioned using systems that run on natural refrigerants such as ammonia or hydrocarbon.
Study: Sustainable cooling supply for building air conditioning and industry in Germany
The study by the ILK Dresden (Institute of Air Handling and Refrigeration) applies the TEWI process to compare the climate impact of different refrigeration systems in the non-residential building sector and in process cooling. The study's authors used simulation calculations to prove that refrigeration units using halogen-free natural refrigerants are significantly more climate friendly than units with HFC refrigerants because they are both more energy-efficient and generate little to no direct (refrigerant) greenhouse gas emissions.
In addition to the comparison of different systems, which includes sorption cooling technology (e.g. absorption chiller systems), the study also identifies the proportion of refrigeration needs in the building and industrial sectors which could potentially be supplied by heat-driven refrigeration systems instead of compression type refrigeration systems. When solar or waste heat are used, absorption chillers prove to be outstanding in terms of energy efficiency and produce the lowest levels of greenhouse gas emissions of all systems included in the study. A potential analysis in this sector is based on a detailed assessment of refrigeration needs of different industrial sectors.