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Received Jun 15, 2017; Accepted Jan 18, 2018
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1. Introduction
Water heating consumes nearly 20% of the total energy consumption for an average family [1]. Solar energy is a clean, inexhaustible, and abundant energy resource [2]. The development of an affordable and effective clean energy technologies such as a combination of solar energy and heat pump can have huge long-term benefits, which is the concept of solar-assisted heat pump (SAHP) [3].
The concept of direct expansion solar-assisted heat pump (DX-SAHP) was first presented by Sporn and Ambrose [4]. While the “indirect-expansion SAHP” system has an intermediate heat exchanger between the solar water circuit and a water circuit, a DX-SAHP uses a two-phase solar collector to function directly as an evaporator. The evaporator-collector configuration favourably reduces the number of components in use, which can lower the cost of the system and avoid the nighttime freeze-up problem of a traditional water collector [5].
For the SAHP system, the match between different components is critical. Liu et al. [6] found that the mass flow rate of refrigerant can match well with the thermal load of the evaporator by means of compressor frequency modulation. In this case,the compressor frequency should be adjusted according to the ambient conditions. Chaturvedi et al. [7] showed that the coefficient of performance (COP) of the system can be enhanced extensively by lowering the compressor frequency when the ambient temperatures are higher. Moreno-Rodríguez et al. [8] showed that when the condenser water flow rate is lower than 0.114 kg/s, a higher COP can be obtained by reducing the compressor frequency. However, when the water flow rate is over 0.174 kg/s, lowing the compressor frequency will drastically lead to a lower COP.
SAHP is an effective way for utilizing solar energy, but solar energy is an intermittent energy which changes greatly depending on time and weather [9]. For this reason, supplementary heat source is required to ensure the continuous and reliable operation of the SAHP. Air source heat pump (ASHP) can operate more reliably by absorbing heat from the ambient air [10]. However, the application of single ASHP...