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Reverse cycle heat pump air handling units (AHUs) are among the most energy efficient types of electric heating on the market. But their advantages don’t stop there, as Tim Mitchell, sales director of Klima-Therm, explains
he operating principles of reverse cycle heat pump AHUs are relatively simple. Essentially, they work by extracting heat from outside
air and transferring it inside, using a refrigerant to warm, or cool in summer months, the air that is being drawn inside. They also often have the facility to filter and dehumidify the air. The only difference between reverse cycle and
conventional heat pumps is that the former can be reversed to deliver controlled heating or cooling on demand rather than only operating in one mode. Traditional heat pumps typically perform only one function: they can offer recovery of the by-product of the main function, but that output cannot typically be controlled. Reverse cycle heat pumps offer a host of
advantages over conventional electric heating and cooling. They do not, for example, require an external heat or cooling supply so there is no external piping, no external heat or cooling generator, no chiller, no boiler, no external heat pump, and only duct and electrical connection is required.
In addition, they can offer a stable supply
temperature using a variable frequency drive (VFD) to change the speed of the integral scroll compressor as demand dictates. Traditional heat pumps are packaged and
therefore offer only fixed design, volumes, and so on. The latest reverse cycle models, however, can be
26 November 2021
bespoke, designed to work in customised projects. Furthermore, they can employ BACnet internet protocols which makes them visible and controllable on a building’s network. BACnet over IP also increases flexibility in terms of design and can boost installation efficiency and speed, making the contractor’s job easier and more profitable. The benefits of reverse cycle heat pump AHUs do
not, however, end there. Some models also offer a zero defrost guarantee, which itself offers a range of advantages.
“Reverse cycle heat pumps offer a host of advantages over conventional electric heating and cooling”
A conventional fully packaged heat pump AHU uses two coils to provide summer cooling and winter heating: one in the supply airstream and one in the exhaust, naturally after the heat recovery thermal wheel. Their function is reversed depending on the operating mode, with the supply air coil being the evaporator in summer and the condenser in winter, and the exhaust air coil being the condenser in summer and the evaporator in winter. The problem with defrost comes in the winter: when the fresh air temperature starts to drop, air leaving the
exhaust side of the heat wheel also gets colder and colder, and the air temperature reaching the winter cooling coil falls to a point where frost forms easily on the coil surface. This will initiate a defrost cycle. A defrost cycle means that, at best, the DX system switches off to allow the air from the heat wheel to defrost and clear the coil and, at worst, forces the refrigeration system into summer cooling mode which blows cold air into the conditioned space. Zero defrost design, on the other hand, means
that, in winter, the refrigeration system uses a third DX coil directly in the return air, positioned before the heat wheel, which guarantees a stable temperature onto the coil and a designed evaporating temperature above 1˚C to ensure no frosting on the coil surface. The heat recovery device uses the discharge air
from this coil and provides heating from as low as -7 ˚C ambient. The DX system then rejects the heat absorbed from the return air to achieve a constant supply air temperature of say 21˚C. If you marry a reverse cycle heat pump with heat
recovery, you get a very stable temperature supply – the machine will manage itself, so, for example, it can take a warm ambient temperature and cool, or cold ambient temperature and heat. During winter operation, there is no need for a boiler as the unit uses the heat recovery and DX system to provide all the necessary heating required without any external heat source, just by extracting heat from the return air. In summer, there is no need for a chiller or
external condensing unit as all the heat absorbed in cooling the air to, say, 21˚C dry bulb temperature can be exhausted to atmosphere by using the heat recovery and built-in DX system. In other words, heat recovery allows you to work with the building’s temperatures as the source rather than external temperatures, which gives huge efficiency gains all year round. Finally, reverse cycle heat pump AHUs generate
around one third the amount of CO2 from energy in operation compared to conventional systems. Indeed, it has been estimated that the most efficient units produce less than one fifth of the emissions of conventional electric heaters, which makes them particularly environmentally friendly.
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