aerothermal heat pump
Introduction
The heat pump is a thermal machine that takes heat from a cold space and transfers it to a hotter one thanks to mechanical work provided from the outside; That is, it does exactly the same thing as the refrigeration machine; the only thing that changes is the objective. In the refrigeration machine the objective is to cool and keep the cold space cold, while in the heat pump it would be the opposite, that is, to keep the hot space warm.
The reversible machine
As in the refrigeration machine, in the heat pump the most used cycle is the Rankine Cycle, see also vapor compression, which works in the same way as in the former, using the same refrigerants, the same elements and the same operating stages. Taking into account that the same machine can cool and heat, it seems coherent to use a single device for both functions.
To achieve this without having to dismantle the machine at each change of season, reversible machines appear, which although they are frequently known as reversible heat pumps, can perfectly well be called reversible refrigeration machines. It is also a widespread error to think that a heat pump is a device that produces both cold and heat, although from everything that has been said it is clear that this is not the case.
To make a reversible heat pump, the only thing you have to add to the machine is a four-way reversing valve, like the one shown in the figure. The valve is electric, that is, activated by a solenoid. This valve is inserted in the refrigeration circuit and is generally controlled with a winter-summer switch. By operating the valve, the direction of circulation of the refrigerant fluid is changed, so that the evaporator becomes a condenser "Condenser (thermodynamics)") and vice versa.[1].
During the summer, the heat exchanger located inside acts as an evaporator, and the one located on the street acts as a condenser, while in winter it is the opposite. Because both one exchanger and the other can be an evaporator and a condenser, in this type of machines they are called indoor unit and outdoor unit.
The reversibility of the machine does not pose any problem, except for the number of hours of operation, which will logically be much higher than that of any single-function machine. As for refrigerants, although there are many options, only a few such as 407C, 410A, 134a and some others are chosen by most manufacturers and the same ones are used for all types of machines, since they more than cover the temperature range of comfort facilities. The only part of the machine that will have to withstand "extraordinary" conditions compared to single-function machines will be the outdoor unit, whose exchanger, depending on the climatic zone, will have to evaporate with outdoor temperatures that can reach -10 °C in winter, and also condense with temperatures close to 40 °C in summer, which only requires good sizing of the exchange surface.