The impact of the work under partial load on the energy efficiency of an air-to-water heat pump

Abstract

Abstract. The energy efficiency of air-to-water heat pumps used for central heating depends significantly on the correctness of the device choice. Under given climate conditions, the temperature of the bivalent point defines the amount of energy supplied from an additional source, usually an electric heater, in periods when the power of the heat pump is too low. Commonly overlooked fact is that excess capacity is also unfavourable and has significant, negative impact on the energy efficiency of such a system. In the article, basing on the measurement data of the real installation of the air-to-water heat pump, a significant decrease in the energy efficiency of the device during periods of low energy demand was shown. The average monthly value of SCOP in the warm months was similar or even lower than in the winter months. The reason of this is the operation of the device under partial load. Although the values of COP were very high in short periods of operation, frequent switching on and off of the heat pump reduced its energy efficiency. Also, the length of operation in stand-by mode, i.e. the length of the periods between successive heating cycles, had negative influence on energy efficiency.

A Novel Data Management Methodology and Case Study for Monitoring and Performance Analysis of Large-Scale Ground Source Heat Pump (GSHP) and Borehole Thermal Energy Storage (BTES) System

“…The significant gap between the measured COP and expected (calculated) COP in summer operation-as discussed in the previous section-could probably be provoked by the HP partial load operation. Several studies [25][26][27] concluded that heat pump COP is impacted by a partial load operation and highlighted that more frequent "on/off" switching can reduce HP efficiency [25]. As mentioned previously, the ANCC system's operation is based on cascade launching of HP modules (from 1 to 9 units) and each module can be additionally adjusted to deliver a fraction between 0% and 100% of its nominal capacity.…”

Quasi-dynamic model of the energy efficiency degradation coefficient for an air-to-water heat pump

“…The value is then used to calculate the reduction of declared COP (COPd) under the same temperature conditions as for the test under part load conditions according to the equation (2). The reduced coefficient of performance is called part-load COP (COPPL).…”

“…Although both methods base on energy consumed in the stand-by mode, the reduced COP obtained on their basis often differ significantly and show great discrepancies in relation to the measured data [2,5]. E. Fuentes et al [5], on the basis of experimental research, have observed that start-up losses have significant impact on the efficiency of the device.…”

“…Taking into account the results of previous work [2] and the literature [5,7,8] which challenge the method described in the EN 14825 standard, a new definition and calculation approach to the degradation coefficient were proposed. The analytical approach proposed by the authors is aimed to test the devices in semi-dynamic conditions instead of single point measurements.…”

“…In previous works [2], the authors have already discussed the problem of inaccuracies resulting from the imprecise determination of data for the calculation of the reduction coefficient. These works has focused on possible errors related to inaccurate input data for SCOP analyzes.…”