AIR QUALITY
for either absorption or rejection as there are multiple rooms to serve and therefore more than one indoor unit connected to a system. The efficiency of a system, even though regulated to conditions, is basically calculated as ‘energy out’ divided by ‘energy in’. In a typical air conditioning example, the amount of electricity consumed by the system is 10kW compared to the amount of thermal energy being transferred, 40kW (capacity of the system), resulting in an Energy Efficiency Rating (EER) of 4.0. The higher the result, the better the efficiency, and the ‘greener’ the system is.
A VRF system that can operate in a simultaneous mode of both cooling and heating, providing benefits for variation in design considerations and usability in comparison to a heat pump system. An East and West facing office is an example, where heating may be required on the West side in the morning whilst cooling is required on the East side, or an office layout whereby there are many separate rooms and many different thermal comfort requirements. But how do these systems work and why are they greener? As previously mentioned, a condenser rejects energy and an evaporator absorbs energy. In cooling mode, the condenser is outside, and the evaporator is inside. Apply
this to a VRF and there would be multiple evaporators inside, or multiple condensers when in heating. This means that a unit is classified as a condenser or evaporator, not because of its location, but because of its function. An indoor unit in a room that is in cooling is absorbing energy, and in a room that is in heating, is rejecting energy. Using an example with the mode of the system in cooling, a 3-pipe heat recovery VRF limits the amount of energy rejected to atmosphere by transferring it between indoor units via the refrigerant circuit. A room that is in heating is being supplied thermal energy by a room that is in cooling, rejecting the thermal energy from the refrigerant circuit,so further energy can continue to be absorbed – simple combination of absorb and reject. To bring this into context and towards an energy saving method, if 50% of the systems indoor units are in cooling mode and the other 50% in heating mode, then no energy is being rejected outside and the system is as efficient as it can be.
Modifying the example mentioned earlier to this case, the system cooling capacity would reduce by 50% to 20kW and the heating capacity would be 20kW, combining to meet the previous total of 40kW. The electricity consumed by the system would reduce by
approximately 50% because only half the cooling load is required. This operating pattern would see the system efficiency double to 8.0 EER and so would equate to an 800% operating efficiency (1kW in, 8kW out). Of course, these figures are just examples and some variations to this equation will exist. This type of process extends across the ratio of mixed modes so even the smallest amount of heat recovery improves system operating efficiencies.
We are quite lucky that across Europe there are independent regulatory testing bodies such as Eurovent which provide benchmarking for manufacturer test data.
However, we are still lacking in both regulated testing of heat recovery operation such as those schemes carried out by the Air conditioning, Heating and Refrigeration Institute (AHRI) in the US, and in the performance monitoring of buildings once the installation has been completed, such as is offered by the National Australian Built Environmental Rating System (NABERS). So, why should we install 3-pipe VRF, true heat recovery? The answer is quite simple; we reuse the energy of an already renewable green technology to be more flexible for design and usability, along with pushing the boundaries of system efficiencies.
www.acr-news.com
June 2019 23
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