The modelling of reverse defrosting cycles of air-to-water heat pumps with TRNSYS

Abstract

The most widespread defrosting technique adopted by Air-Source Heat Pumps (ASHPs) during the heating season is Reverse Cycle Defrosting (RCD). In this paper a dynamic model of RCD, based on performance data provided by the heat pump manufacturer, designed for TRNSYS and with a core-structure suitable for commercial units, is presented. A defrost cycle is divided in three phases. First, the unit heating capacity is reduced as a linear function of the ice layer thickness (Pre-Defrost phase). Subsequently, the reverse cycle operating mode is modelled on the basis of the performance data given by the manufacturer (Defrost phase) and, finally, the heat pump performances are altered taking into account the higher surface temperature of the external coil after the reverse mode (Post-Defrost phase). Then, the influence of defrosting energy losses on the heat pump seasonal performance factor in sites characterized by different climatic conditions has been assessed. Results point out that the ASHP seasonal efficiency decreases of about 5% taking into account defrost energy losses; in addition, the influence of defrost cycles on the internal air temperature is studied by assessing under which conditions the indoor thermal comfort can be guaranteed even in presence of frequent defrost cycles.

Defrosting of Air-Source Heat Pumps: Effect of Real Temperature Data on Seasonal Energy Performance for Different Locations in Italy

“…where ϕ is the relative humidity (in this case ϕ = 1) and p tot (Pa) is the atmospheric pressure at the altitude of the selected location. In [11], the authors simulated the RCD using time-dependent functions for electric and thermal power during the defrosting phase; a similar analysis can only be made if a huge amount of data are available from the manufacturer or if long-term experiments are conducted on a specific heat pump. Referring to the three heat pumps that are be investigated in this paper, the electric power input of the heat pump and the thermal power taken from the water tank are considered as constants along the defrost cycle.…”

“…Furthermore, additional details on the choice of both the values for the electric power and the thermal power are given in Figure 8. In detail, we assume for the same energy for the whole defrost cycle as well as the same electric energy input for each different HP, as proposed in [11]. In particular, the thermal and electric power trends along a generic defrost cycle are reported in Figure 8, according to the model proposed in [11].…”

“…In detail, we assume for the same energy for the whole defrost cycle as well as the same electric energy input for each different HP, as proposed in [11]. In particular, the thermal and electric power trends along a generic defrost cycle are reported in Figure 8, according to the model proposed in [11]. The thermal power trend is modelled with two parabolic branches, while the electric power is modeled with two linear functions.…”

“…In the same figure, the thermal and electric power considered in this analysis are reported, i.e., two constant functions during the whole defrost cycle of 300 s. In Figure 8, the surfaces below the lines are equal to both, with reference to the thermal power (a) and to the electric power (b). Moreover, the duration of the defrost cycle is taken as in [11] and corresponds to a mean time used by the manufacturers on their control systems. Table 5.…”

“…In fact, in certain weather conditions such as temperature and relative humidity, if the surface temperature of the evaporator coil is below the air dew point temperature and the water freezing point, ice deposits on it, causing a general degradation of the performance of the air-source heat pump. The reduction of HP performance is due to the reduction of the evaporator heat transfer coefficient, which is mainly for two reasons: the ice layer reduces the air flow through the heat exchanger, and the ice layer also acts as an insulant on the coil surface [11]. Ice build-up on a heat exchanger is a transient phenomenon, characterized by three different stages, as proposed by Guo et al in [12]: the frost layer growth rate in the first stage increases with time; in the second stage, depending on the relative humidity value, the frost thickness growth rate decreases with time or remains invariable; and finally, in the third stage, the frost layer thickness rapidly increases with time.…”