As we have previously discussed, COP is the energy efficiency ratio performance used for heating in heat pumps and EER for cooling in refrigeration cycles.
From thermodynamics, a refrigeration machine and a heat pump are essentially the same, since we have a condenser where the refrigerant transfers heat, and an evaporator where the refrigerant absorbs heat.
Sometimes, we focus on only one of this heat transfer. If we need cooling, energy is absorbed from our system and it is transferred to the environment. In the case of a heat pump, energy is transferred to our system and it is absorbed from the environment.
There is always an amount of heat that is absorbed or transferred to the environment, so that the thermodynamic cycle can work. However, in many cases, cooling and heating are required simultaneously. Therefore, there will be several opportunities to produce cooling and heating with the same vapour compression cycle.
Generally, the simplest thing is to produce with a refrigeration machine and take advantage of the heat that is transferred from the condenser to cover part of the heating demand, and the rest will be completed with an auxiliary heating equipment, such as a boiler.
In this way, we can take advantage of residual heat from our compression cycle, which would otherwise be evacuated to the atmosphere, and that same heat would have to be supplied through another heating system.
Therefore, the recovery of this heat has some great advantages, such as improving energy efficiency in our installation, obtaining some important energy and economic savings and a reduction in CO2 emissions.
After this explanation, there are two questions to be solved: how much heat can we recover? And what is the maximum temperature that I can get with this recovery? Both questions are answered with the first and second principles of thermodynamics.
The first question can be answered quite simply, with some nuances. Since energy is neither created nor destroyed, the condenser heating energy plus the energy losses are equal to the sum of the energy absorbed by the compressor and the energy absorbed in the evaporator.
Qcond + losses = Qevap + Wabs
Assuming an ideal cycle without heat losses in the system, we can estimate that the theoretical maximum condensation heat is the sum of the heat in the evaporator and the power absorbed in the compressor.
This is the maximum heat, but we must take into account another limitation, which is the maximum temperature we can reach when we heat our secondary heat transfer system or circuit. The maximum temperature will be the hottest point in the cycle, which is the compressor discharge temperature. This is because energy is always transferred from the warmer object towards the colder one, and never the other way around, even if the energy balance is right.
We can observe the behaviour of the cycle with the p-h diagram for refrigerants, paying special attention to temperature isothermal curves.
It is observed that at the compressor outlet, the discharge temperature decreases until it reaches saturation temperature (sensible heat). Then, when the phase change of the refrigerant begins, heat is transferred at a constant temperature until the whole refrigerant is condensed (latent heat). Subsequently, its temperature continues to decrease until it reaches the temperature of the environment to which the heat is transferred (sensible heat).
Therefore, it can be seen that the maximum temperature (ideally) that we can achieve with this heat recovery is the maximum temperature of the cycle, which is the discharge temperature at the compressor outlet. However, that amount of sensible heat is small compared to the total heat that is transferred from the condenser. This system is called partial heat recovery.
On the other hand, a greater amount of energy can be recovered with latent heat, but in this case, the temperature that we can reach will be limited by the saturation temperature of the refrigerant during the phase change. In this case, this is called total heat recovery.
In the following posts, we will see with more detail these two types of recovery, with some solutions to improve the recovery of heat and increase the energy efficiency of our installation.